# 9. The DHCPv6 Server

## 9.1. Starting and Stopping the DHCPv6 Server

It is recommended that the Kea DHCPv6 server be started and stopped using `keactrl` (described in Managing Kea with keactrl); however, it is also possible to run the server directly via the `kea-dhcp6` command, which accepts the following command-line switches:

- `-c file` - specifies the configuration file. This is the only mandatory switch.

- `-d` - specifies whether the server logging should be switched to debug/verbose mode. In verbose mode, the logging severity and debuglevel specified in the configuration file are ignored; "debug" severity and the maximum debuglevel (99) are assumed. The flag is convenient for temporarily switching the server into maximum verbosity, e.g. when debugging.

- `-p server-port` - specifies the local UDP port on which the server listens. This is only useful during testing, as a DHCPv6 server listening on ports other than the standard ones is not able to handle regular DHCPv6 queries.

- `-P client-port` - specifies the remote UDP port to which the server sends all responses. This is only useful during testing, as a DHCPv6 server sending responses to ports other than the standard ones is not able to handle regular DHCPv6 queries.

- `-t file` - specifies a configuration file to be tested. `kea-dhcp6` loads it, checks it, and exits. During the test, log messages are printed to standard output and error messages to standard error. The result of the test is reported through the exit code (0 = configuration looks OK, 1 = error encountered). The check is not comprehensive; certain checks are possible only when running the server.

- `-T file` - specifies a configuration file to be tested. `kea-dhcp6` loads it, checks it, and exits. It performs extra checks beyond what `-t` offers, such as establishing database connections (for the lease backend, host reservations backend, configuration backend, and forensic logging backend), loading hook libraries, parsing hook-library configurations, etc. It does not open UNIX or TCP/UDP sockets, nor does it open or rotate files, as any of these actions could interfere with a running process on the same machine.

- `-v` - displays the Kea version and exits.

- `-V` - displays the Kea extended version with additional parameters and exits. The listing includes the versions of the libraries dynamically linked to Kea.

- `-W` - displays the Kea configuration report and exits. The report is a copy of the `config.report` file produced by `./configure`; it is embedded in the executable binary.

  The contents of the `config.report` file may also be accessed by examining certain libraries in the installation tree or in the source tree.

      # from installation using libkea-process.so
      $ strings ${prefix}/lib/libkea-process.so | sed -n 's/;;;; //p'

      # from sources using libkea-process.so
      $ strings src/lib/process/.libs/libkea-process.so | sed -n 's/;;;; //p'

      # from sources using libkea-process.a
      $ strings src/lib/process/.libs/libkea-process.a | sed -n 's/;;;; //p'

      # from sources using libcfgrpt.a
      $ strings src/lib/process/cfgrpt/.libs/libcfgrpt.a | sed -n 's/;;;; //p'

On startup, the server detects available network interfaces and attempts to open UDP sockets on all interfaces listed in the configuration file. Since the DHCPv6 server opens privileged ports, it requires root access; this daemon must be run as root.

During startup, the server attempts to create a PID file of the form: `[runstatedir]/kea/[conf name].kea-dhcp6.pid`, where:

- `runstatedir`: The value as passed into the build configure script; it defaults to `/usr/local/var/run`. Note that this value may be overridden at runtime by setting the environment variable `KEA_PIDFILE_DIR`, although this is intended primarily for testing purposes.

- `conf name`: The configuration file name used to start the server, minus all preceding paths and the file extension. For example, given a pathname of `/usr/local/etc/kea/myconf.txt`, the portion used would be `myconf`.

If the file already exists and contains the PID of a live process, the server issues a `DHCP6_ALREADY_RUNNING` log message and exits. It is possible, though unlikely, that the file is a remnant of a system crash and the process to which the PID belongs is unrelated to Kea. In such a case, it would be necessary to manually delete the PID file.

The server can be stopped using the `kill` command. When running in a console, the server can also be shut down by pressing Ctrl-c. Kea detects the key combination and shuts down gracefully.

The reconfiguration of each Kea server is triggered by the SIGHUP signal. When a server receives the SIGHUP signal it rereads its configuration file and, if the new configuration is valid, uses the new configuration. If the new configuration proves to be invalid, the server retains its current configuration; however, in some cases a fatal error message is logged indicating that the server is no longer providing any service: a working configuration must be loaded as soon as possible.

## 9.2. DHCPv6 Server Configuration

### 9.2.1. Introduction

This section explains how to configure the Kea DHCPv6 server using a configuration file.

Before DHCPv6 is started, its configuration file must be created. The basic configuration is as follows:

    {
    # DHCPv6 configuration starts on the next line
    "Dhcp6": {

    # First we set up global values
        "valid-lifetime": 4000,
        "renew-timer": 1000,
        "rebind-timer": 2000,
        "preferred-lifetime": 3000,

    # Next we set up the interfaces to be used by the server.
        "interfaces-config": {
            "interfaces": [ "eth0" ]
        },

    # And we specify the type of lease database
        "lease-database": {
            "type": "memfile",
            "persist": true,
            "name": "/var/lib/kea/dhcp6.leases"
        },

    # Finally, we list the subnets from which we will be leasing addresses.
        "subnet6": [
            {
                "id": 1,
                "subnet": "2001:db8:1::/64",
                "pools": [
                    {
                        "pool": "2001:db8:1::1-2001:db8:1::ffff"
                    }
                 ]
            }
        ]
    # DHCPv6 configuration ends with the next line
    }

    }

The following paragraphs provide a brief overview of the parameters in the above example, along with their format. Subsequent sections of this chapter go into much greater detail for these and other parameters.

The lines starting with a hash (#) are comments and are ignored by the server; they do not impact its operation in any way.

The configuration starts in the first line with the initial opening curly bracket (or brace). Each configuration must contain an object specifying the configuration of the Kea module using it. In the example above, this object is called `Dhcp6`.

The `Dhcp6` configuration starts with the `"Dhcp6": {` line and ends with the corresponding closing brace (in the above example, the brace after the last comment). Everything defined between those lines is considered to be the `Dhcp6` configuration.

In general, the order in which those parameters appear does not matter, but there are two caveats. The first one is that the configuration file must be well-formed JSON, meaning that the parameters for any given scope must be separated by a comma, and there must not be a comma after the last parameter. When reordering a configuration file, moving a parameter to or from the last position in a given scope may also require moving the comma. The second caveat is that it is uncommon — although legal JSON — to repeat the same parameter multiple times. If that happens, the last occurrence of a given parameter in a given scope is used, while all previous instances are ignored. This is unlikely to cause any confusion as there are no real-life reasons to keep multiple copies of the same parameter in the configuration file.

The first few DHCPv6 configuration elements define some global parameters. `valid-lifetime` defines how long the addresses (leases) given out by the server are valid; the default is for a client to be allowed to use a given address for 4000 seconds. (Note that integer numbers are specified as is, without any quotes around them.) The address will become deprecated in 3000 seconds, i.e. clients are allowed to keep old connections, but cannot use this address to create new connections. `renew-timer` and `rebind-timer` are values (also in seconds) that define T1 and T2 timers, which govern when the client begins the renewal and rebind procedures.

The `interfaces-config` map specifies the network interfaces on which the server should listen to DHCP messages. The `interfaces` parameter specifies a list of network interfaces on which the server should listen. Lists are opened and closed with square brackets, with elements separated by commas. To listen on two interfaces, the `interfaces-config` element should look like this:

    {
    "interfaces-config": {
        "interfaces": [ "eth0", "eth1" ]
    },
    ...
    }

The next lines define the lease database, the place where the server stores its lease information. This particular example tells the server to use memfile, which is the simplest and fastest database backend. It uses an in-memory database and stores leases on disk in a CSV (comma-separated values) file. This is a very simple configuration example; usually the lease database configuration is more extensive and contains additional parameters. Note that `lease-database` is an object and opens up a new scope, using an opening brace. Its parameters (just one in this example: `type`) follow. If there were more than one, they would be separated by commas. This scope is closed with a closing brace. As more parameters for the `Dhcp6` definition follow, a trailing comma is present.

Finally, we need to define a list of IPv6 subnets. This is the most important DHCPv6 configuration structure, as the server uses that information to process clients' requests. It defines all subnets from which the server is expected to receive DHCP requests. The subnets are specified with the `subnet6` parameter. It is a list, so it starts and ends with square brackets. Each subnet definition in the list has several attributes associated with it, so it is a structure and is opened and closed with braces. At a minimum, a subnet definition must have at least two parameters: `subnet`, which defines the whole subnet; and `pools`, which is a list of dynamically allocated pools that are governed by the DHCP server.

The example contains a single subnet. If more than one were defined, additional elements in the `subnet6` parameter would be specified and separated by commas. For example, to define two subnets, the following syntax would be used:

    {
    "subnet6": [
        {
            "id": 1,
            "pools": [ { "pool": "2001:db8:1::/112" } ],
            "subnet": "2001:db8:1::/64"
        },
        {
            "id": 2,
            "pools": [ { "pool": "2001:db8:2::1-2001:db8:2::ffff" } ],
            "subnet": "2001:db8:2::/64"
        }
    ],
    ...
    }

Note that indentation is optional and is used for aesthetic purposes only. In some cases it may be preferable to use more compact notation.

After all the parameters have been specified, there are two contexts open: `global` and `Dhcp6`; thus, two closing curly brackets must be used to close them.

### 9.2.2. Lease Storage

All leases issued by the server are stored in the lease database. There are three database backends available: memfile (the default), MySQL, PostgreSQL.

#### 9.2.2.1. Memfile - Basic Storage for Leases

The server is able to store lease data in different repositories. Larger deployments may elect to store leases in a database; Lease Database Configuration describes this option. In typical smaller deployments, though, the server stores lease information in a CSV file rather than a database. As well as requiring less administration, an advantage of using a file for storage is that it eliminates a dependency on third-party database software.

The configuration of the memfile backend is controlled through the `Dhcp6`/`lease-database` parameters. The `type` parameter is mandatory and specifies which storage for leases the server should use, through the `"memfile"` value. The following list gives additional optional parameters that can be used to configure the memfile backend.

- `persist`: controls whether the new leases and updates to existing leases are written to the file. It is strongly recommended that the value of this parameter be set to `true` at all times during the server's normal operation. Not writing leases to disk means that if a server is restarted (e.g. after a power failure), it will not know which addresses have been assigned. As a result, it may assign new clients addresses that are already in use. The value of `false` is mostly useful for performance-testing purposes. The default value of the `persist` parameter is `true`, which enables writing lease updates to the lease file.

- `name`: specifies an absolute location of the lease file in which new leases and lease updates are recorded. The default value for this parameter is `"[kea-install-dir]/var/lib/kea/kea-leases6.csv"`.

- `lfc-interval`: specifies the interval, in seconds, at which the server will perform a lease file cleanup (LFC). This removes redundant (historical) information from the lease file and effectively reduces the lease file size. The cleanup process is described in more detail later in this section. The default value of the `lfc-interval` is `3600`. A value of `0` disables the LFC.

- `max-row-errors`: specifies the number of row errors before the server stops attempting to load a lease file. When the server loads a lease file, it is processed row by row, each row containing a single lease. If a row is flawed and cannot be processed correctly the server logs it, discards the row, and goes on to the next row. This parameter can be used to set a limit on the number of such discards that can occur, after which the server abandons the effort and exits. The default value of `0` disables the limit and allows the server to process the entire file, regardless of how many rows are discarded.

An example configuration of the memfile backend is presented below:

    "Dhcp6": {
        "lease-database": {
            "type": "memfile",
            "persist": true,
            "name": "/tmp/kea-leases6.csv",
            "lfc-interval": 1800,
            "max-row-errors": 100
        }
    }

This configuration selects `/tmp/kea-leases6.csv` as the storage file for lease information and enables persistence (writing lease updates to this file). It also configures the backend to perform a periodic cleanup of the lease file every 1800 seconds (30 minutes) and sets the maximum number of row errors to 100.

#### 9.2.2.2. Why Is Lease File Cleanup Necessary?

It is important to know how the lease file contents are organized to understand why the periodic lease file cleanup is needed. Every time the server updates a lease or creates a new lease for a client, the new lease information must be recorded in the lease file. For performance reasons, the server does not update the existing client's lease in the file, as this would potentially require rewriting the entire file. Instead, it simply appends the new lease information to the end of the file; the previous lease entries for the client are not removed. When the server loads leases from the lease file, e.g. at server startup, it assumes that the latest lease entry for the client is the valid one. Previous entries are discarded, meaning that the server can reconstruct accurate information about the leases even though there may be many lease entries for each client. However, storing many entries for each client results in a bloated lease file and impairs the performance of the server's startup and reconfiguration, as it needs to process a larger number of lease entries.

Lease file cleanup (LFC) removes all previous entries for each client and leaves only the latest ones. The interval at which the cleanup is performed is configurable, and it should be selected according to the frequency of lease renewals initiated by the clients. The more frequent the renewals, the smaller the value of `lfc-interval` should be. Note, however, that the LFC takes time and thus it is possible (although unlikely) that, if the `lfc-interval` is too short, a new cleanup may be started while the previous one is still running. The server would recover from this by skipping the new cleanup when it detected that the previous cleanup was still in progress, but it implies that the actual cleanups will be triggered more rarely than the configured interval. Moreover, triggering a new cleanup adds overhead to the server, which is not able to respond to new requests for a short period of time when the new cleanup process is spawned. Therefore, it is recommended that the `lfc-interval` value be selected in a way that allows the LFC to complete the cleanup before a new cleanup is triggered.

Lease file cleanup is performed by a separate process (in the background) to avoid a performance impact on the server process. To avoid conflicts between two processes using the same lease files, the LFC process starts with Kea opening a new lease file; the actual LFC process operates on the lease file that is no longer used by the server. There are also other files created as a side effect of the lease file cleanup. The detailed description of the LFC process is located later in this Kea Administrator's Reference Manual: The LFC Process.

#### 9.2.2.3. Lease Database Configuration

Note

Lease database access information must be configured for the DHCPv6 server, even if it has already been configured for the DHCPv4 server. The servers store their information independently, so each server can use a separate database or both servers can use the same database.

Note

Kea requires the database timezone to match the system timezone. For more details, see First-Time Creation of the MySQL Database and First-Time Creation of the PostgreSQL Database.

Lease database configuration is controlled through the `Dhcp6`/`lease-database` parameters. The database type must be set to `memfile`, `mysql` or `postgresql`, e.g.:

    "Dhcp6": { "lease-database": { "type": "mysql", ... }, ... }

Next, the name of the database to hold the leases must be set; this is the name used when the database was created (see First-Time Creation of the MySQL Database or First-Time Creation of the PostgreSQL Database).

For MySQL or PostgreSQL:

    "Dhcp6": { "lease-database": { "name": "database-name" , ... }, ... }

If the database is located on a different system from the DHCPv6 server, the database host name must also be specified:

    "Dhcp6": { "lease-database": { "host": "remote-host-name", ... }, ... }

Normally, the database is on the same machine as the DHCPv6 server. In this case, set the value to the empty string:

    "Dhcp6": { "lease-database": { "host" : "", ... }, ... }

Should the database use a port other than the default, it may be specified as well:

    "Dhcp6": { "lease-database": { "port" : 12345, ... }, ... }

Should the database be located on a different system, the administrator may need to specify a longer interval for the connection timeout:

    "Dhcp6": { "lease-database": { "connect-timeout" : timeout-in-seconds, ... }, ... }

The default value of five seconds should be more than adequate for local connections. If a timeout is given, though, it should be an integer greater than zero.

The maximum number of times the server automatically attempts to reconnect to the lease database after connectivity has been lost may be specified:

    "Dhcp6": { "lease-database": { "max-reconnect-tries" : number-of-tries, ... }, ... }

If the server is unable to reconnect to the database after making the maximum number of attempts, the server will exit. A value of 0 (the default) disables automatic recovery and the server will exit immediately upon detecting a loss of connectivity (MySQL and PostgreSQL only).

The number of milliseconds the server waits between attempts to reconnect to the lease database after connectivity has been lost may also be specified:

    "Dhcp6": { "lease-database": { "reconnect-wait-time" : number-of-milliseconds, ... }, ... }

The default value for MySQL and PostgreSQL is 0, which disables automatic recovery and causes the server to exit immediately upon detecting the loss of connectivity.

    "Dhcp6": { "lease-database": { "on-fail" : "stop-retry-exit", ... }, ... }

The possible values are:

- `stop-retry-exit` - disables the DHCP service while trying to automatically recover lost connections. Shuts down the server on failure after exhausting `max-reconnect-tries`. This is the default value for MySQL and PostgreSQL.

- `serve-retry-exit` - continues the DHCP service while trying to automatically recover lost connections. Shuts down the server on failure after exhausting `max-reconnect-tries`.

- `serve-retry-continue` - continues the DHCP service and does not shut down the server even if the recovery fails.

Note

Automatic reconnection to database backends is configured individually per backend; this allows users to tailor the recovery parameters to each backend they use. We suggest that users enable it either for all backends or none, so behavior is consistent.

Losing connectivity to a backend for which reconnection is disabled results (if configured) in the server shutting itself down. This includes cases when the lease database backend and the hosts database backend are connected to the same database instance.

It is highly recommended not to change the `stop-retry-exit` default setting for the lease manager, as it is critical for the connection to be active while processing DHCP traffic. Change this only if the server is used exclusively as a configuration tool.

The host parameter is used by the MySQL and PostgreSQL backends.

Finally, the credentials of the account under which the server will access the database should be set:

    "Dhcp6": {
        "lease-database": {
            "user": "user-name",
            "password": "password",
            ...
        },
        ...
    }

If there is no password to the account, set the password to the empty string `""`. (This is the default.)

#### 9.2.2.4. Tuning Database Timeouts

In rare cases, reading or writing to the database may hang. This can be caused by a temporary network issue, or by misconfiguration of the proxy server switching the connection between different database instances. These situations are rare, but users have reported that Kea sometimes hangs while performing database IO operations. Setting appropriate timeout values can mitigate such issues.

MySQL exposes two distinct connection options to configure the read and write timeouts. Kea's corresponding `read-timeout` and `write-timeout` configuration parameters specify the timeouts in seconds. For example:

    "Dhcp6": { "lease-database": { "read-timeout" : 10, "write-timeout": 20, ... }, ... }

Setting these parameters to 0 is equivalent to not specifying them, and causes the Kea server to establish a connection to the database with the MySQL defaults. In this case, Kea waits indefinitely for the completion of the read and write operations.

MySQL versions earlier than 5.6 do not support setting timeouts for read and write operations. Moreover, the `read-timeout` and `write-timeout` parameters can only be specified for the MySQL backend; setting them for any other backend database type causes a configuration error.

To set a timeout in seconds for PostgreSQL, use the `tcp-user-timeout` parameter. For example:

    "Dhcp6": { "lease-database": { "tcp-user-timeout" : 10, ... }, ... }

Specifying this parameter for other backend types causes a configuration error.

Note

The timeouts described here are only effective for TCP connections. Please note that the MySQL client library used by the Kea servers typically connects to the database via a UNIX domain socket when the `host` parameter is `localhost`, but establishes a TCP connection for `127.0.0.1`.

### 9.2.3. Hosts Storage

Kea is also able to store information about host reservations in the database. The hosts database configuration uses the same syntax as the lease database. In fact, the Kea server opens independent connections for each purpose, be it lease or hosts information, which gives the most flexibility. Kea can keep leases and host reservations separately, but can also point to the same database. Currently the supported hosts database types are MySQL and PostgreSQL.

The following configuration can be used to configure a connection to MySQL:

    "Dhcp6": {
        "hosts-database": {
            "type": "mysql",
            "name": "kea",
            "user": "kea",
            "password": "secret123",
            "host": "localhost",
            "port": 3306
        }
    }

Depending on the database configuration, many of the parameters may be optional.

Please note that usage of hosts storage is optional. A user can define all host reservations in the configuration file, and that is the recommended way if the number of reservations is small. However, when the number of reservations grows, it is more convenient to use host storage. Please note that both storage methods (the configuration file and one of the supported databases) can be used together. If hosts are defined in both places, the definitions from the configuration file are checked first and external storage is checked later, if necessary.

Host information can be placed in multiple stores. Operations are performed on the stores in the order they are defined in the configuration file, although this leads to a restriction in ordering in the case of a host reservation addition; read-only stores must be configured after a (required) read-write store, or the addition will fail.

Note

Kea requires the database timezone to match the system timezone. For more details, see First-Time Creation of the MySQL Database and First-Time Creation of the PostgreSQL Database.

#### 9.2.3.1. DHCPv6 Hosts Database Configuration

Hosts database configuration is controlled through the `Dhcp6`/`hosts-database` parameters. If enabled, the type of database must be set to `mysql` or `postgresql`.

    "Dhcp6": { "hosts-database": { "type": "mysql", ... }, ... }

Next, the name of the database to hold the reservations must be set; this is the name used when the lease database was created (see Supported Backends for instructions on how to set up the desired database type):

    "Dhcp6": { "hosts-database": { "name": "database-name" , ... }, ... }

If the database is located on a different system than the DHCPv6 server, the database host name must also be specified:

    "Dhcp6": { "hosts-database": { "host": remote-host-name, ... }, ... }

Normally, the database is on the same machine as the DHCPv6 server. In this case, set the value to the empty string:

    "Dhcp6": { "hosts-database": { "host" : "", ... }, ... }

Should the database use a port different than the default, it may be specified as well:

    "Dhcp6": { "hosts-database": { "port" : 12345, ... }, ... }

The maximum number of times the server automatically attempts to reconnect to the host database after connectivity has been lost may be specified:

    "Dhcp6": { "hosts-database": { "max-reconnect-tries" : number-of-tries, ... }, ... }

If the server is unable to reconnect to the database after making the maximum number of attempts, the server will exit. A value of 0 (the default) disables automatic recovery and the server will exit immediately upon detecting a loss of connectivity (MySQL and PostgreSQL only).

The number of milliseconds the server waits between attempts to reconnect to the host database after connectivity has been lost may also be specified:

    "Dhcp6": { "hosts-database": { "reconnect-wait-time" : number-of-milliseconds, ... }, ... }

The default value for MySQL and PostgreSQL is 0, which disables automatic recovery and causes the server to exit immediately upon detecting the loss of connectivity.

    "Dhcp6": { "hosts-database": { "on-fail" : "stop-retry-exit", ... }, ... }

The possible values are:

- `stop-retry-exit` - disables the DHCP service while trying to automatically recover lost connections. Shuts down the server on failure after exhausting `max-reconnect-tries`. This is the default value for MySQL and PostgreSQL.

- `serve-retry-exit` - continues the DHCP service while trying to automatically recover lost connections. Shuts down the server on failure after exhausting `max-reconnect-tries`.

- `serve-retry-continue` - continues the DHCP service and does not shut down the server even if the recovery fails.

Note

Automatic reconnection to database backends is configured individually per backend. This allows users to tailor the recovery parameters to each backend they use. We suggest that users enable it either for all backends or none, so behavior is consistent.

Losing connectivity to a backend for which reconnection is disabled results (if configured) in the server shutting itself down. This includes cases when the lease database backend and the hosts database backend are connected to the same database instance.

Finally, the credentials of the account under which the server will access the database should be set:

    "Dhcp6": {
        "hosts-database": {
            "user": "user-name",
            "password": "password",
            ...
        },
        ...
    }

If there is no password to the account, set the password to the empty string `""`. (This is the default.)

The multiple-storage extension uses a similar syntax; a configuration is placed into a `hosts-databases` list instead of into a `hosts-database` entry, as in:

    "Dhcp6": { "hosts-databases": [ { "type": "mysql", ... }, ... ], ... }

If the same host is configured both in-file and in-database, Kea does not issue a warning, as it would if both were specified in the same data source. Instead, the host configured in-file has priority over the one configured in-database.

#### 9.2.3.2. Using Read-Only Databases for Host Reservations with DHCPv6

In some deployments, the user whose name is specified in the database backend configuration may not have write privileges to the database. This is often required by the policy within a given network to secure the data from being unintentionally modified. In many cases administrators have deployed inventory databases, which contain substantially more information about the hosts than just the static reservations assigned to them. The inventory database can be used to create a view of a Kea hosts database and such a view is often read-only.

Kea host-database backends operate with an implicit configuration to both read from and write to the database. If the user does not have write access to the host database, the backend will fail to start and the server will refuse to start (or reconfigure). However, if access to a read-only host database is required for retrieving reservations for clients and/or assigning specific addresses and options, it is possible to explicitly configure Kea to start in "read-only" mode. This is controlled by the `readonly` boolean parameter as follows:

    "Dhcp6": { "hosts-database": { "readonly": true, ... }, ... }

Setting this parameter to `false` configures the database backend to operate in "read-write" mode, which is also the default configuration if the parameter is not specified.

Note

The `readonly` parameter is only supported for MySQL and PostgreSQL databases.

#### 9.2.3.3. Tuning Database Timeouts for Hosts Storage

See Tuning Database Timeouts.

### 9.2.4. Interface Configuration

The DHCPv6 server must be configured to listen on specific network interfaces. The simplest network interface configuration tells the server to listen on all available interfaces:

    "Dhcp6": {
        "interfaces-config": {
            "interfaces": [ "*" ]
        },
        ...
    }

The asterisk plays the role of a wildcard and means "listen on all interfaces." However, it is usually a good idea to explicitly specify interface names:

    "Dhcp6": {
        "interfaces-config": {
            "interfaces": [ "eth1", "eth3" ]
        },
        ...
    }

It is possible to use an interface wildcard (*) concurrently with explicit interface names:

    "Dhcp6": {
        "interfaces-config": {
            "interfaces": [ "eth1", "eth3", "*" ]
        },
        ...
    }

This format should only be used when it is desired to temporarily override a list of interface names and listen on all interfaces.

As with the DHCPv4 server, binding to specific addresses and disabling re-detection of interfaces are supported. But `dhcp-socket-type` is not supported, because DHCPv6 uses only UDP/IPv6 sockets. The following example shows how to disable interface detection:

    "Dhcp6": {
        "interfaces-config": {
            "interfaces": [ "eth1", "eth3" ],
            "re-detect": false
        },
        ...
    }

The loopback interfaces (i.e. the `lo` or `lo0` interface) are not configured by default, unless explicitly mentioned in the configuration. Note that Kea requires a link-local address (which does not exist on all systems) or a specified unicast address, as in:

    "Dhcp6": {
        "interfaces-config": {
            "interfaces": [ "enp0s2/2001:db8::1234:abcd" ]
        },
        ...
    }

Kea binds the service sockets for each interface on startup. If another process is already using a port, then Kea logs the message and suppresses an error. DHCP service runs, but it is unavailable on some interfaces.

The "service-sockets-require-all" option makes Kea require all sockets to be successfully bound. If any opening fails, Kea interrupts the initialization and exits with a non-zero status. (Default is false).

    "Dhcp6": {
        "interfaces-config": {
            "interfaces": [ "eth1", "eth3" ],
            "service-sockets-require-all": true
        },
        ...
    }

Sometimes, immediate interruption isn't a good choice. The port can be unavailable only temporary. In this case, retrying the opening may resolve the problem. Kea provides two options to specify the retrying: `service-sockets-max-retries` and `service-sockets-retry-wait-time`.

The first defines a maximal number of retries that Kea makes to open a socket. The zero value (default) means that the Kea doesn't retry the process.

The second defines a wait time (in milliseconds) between attempts. The default value is 5000 (5 seconds).

    "Dhcp6": {
        "interfaces-config": {
            "interfaces": [ "eth1", "eth3" ],
            "service-sockets-max-retries": 5,
            "service-sockets-retry-wait-time": 5000
        },
        ...
    }

If "service-sockets-max-retries" is non-zero and "service-sockets-require-all" is false, then Kea retries the opening (if needed) but does not fail if any socket is still not opened.

### 9.2.5. IPv6 Subnet Identifier

The subnet identifier (subnet ID) is a unique number associated with a particular subnet. In principle, it is used to associate clients' leases with their respective subnets. The server configuration should contain unique and stable identifiers for all subnets. When a subnet identifier is not specified for a subnet, it is automatically assigned by the configuration mechanism. The identifiers are assigned starting at 1 and are monotonically increased for each subsequent subnet: 1, 2, 3, ....

If there are multiple subnets configured with auto-generated identifiers and one of them is removed, the subnet identifiers may be renumbered. For example: if there are four subnets and the third is removed, the last subnet will be assigned the identifier that the third subnet had before removal. As a result, the leases stored in the lease database for subnet 3 are now associated with subnet 4, something that may have unexpected consequences. It is one of the reasons why auto-generated subnet identifiers are deprecated starting from Kea version 2.4.0.

Note

The auto-generation of the subnet identifiers will be removed in a future release. Starting from Kea 2.4.0, a subnet without an `id` entry or with the zero value raises a warning at the configuration time.

Note

Subnet IDs must be greater than zero and less than 4294967295.

The following configuration assigns the specified subnet identifier to a newly configured subnet:

    "Dhcp6": {
        "subnet6": [
            {
                "subnet": "2001:db8:1::/64",
                "id": 1024,
                ...
            }
        ]
    }

This identifier will not change for this subnet unless the `id` parameter is removed or set to 0. The value of 0 forces auto-generation of the subnet identifier.

### 9.2.6. IPv6 Subnet Prefix

The subnet prefix is the second way to identify a subnet. Kea can accept non-canonical subnet addresses; for instance, this configuration is accepted:

    "Dhcp6": {
       "subnet6": [
           {
                "subnet": "2001:db8:1::1/64",
                ...
           }
        ]
    }

This works even if there is another subnet with the "2001:db8:1::/64" prefix; only the textual form of subnets are compared to avoid duplicates.

Note

Abuse of this feature can lead to incorrect subnet selection (see IPv6 Subnet Selection).

### 9.2.7. Unicast Traffic Support

When the DHCPv6 server starts, by default it listens to the DHCP traffic sent to multicast address ff02::1:2 on each interface that it is configured to listen on (see Interface Configuration). In some cases it is useful to configure a server to handle incoming traffic sent to global unicast addresses as well; the most common reason for this is to have relays send their traffic to the server directly. To configure the server to listen on a specific unicast address, add a slash (/) after the interface name, followed by the global unicast address on which the server should listen. The server will listen to this address in addition to normal link-local binding and listening on the ff02::1:2 address. The sample configuration below shows how to listen on 2001:db8::1 (a global address) configured on the `eth1` interface.

    "Dhcp6": {
        "interfaces-config": {
            "interfaces": [ "eth1/2001:db8::1" ]
        },
        "option-data": [
            {
                "name": "unicast",
                "data": "2001:db8::1"
            } ],
        ...
    }

This configuration will cause the server to listen on `eth1` on the link-local address, the multicast group (ff02::1:2), and 2001:db8::1.

Usually, unicast support is associated with a server unicast option which allows clients to send unicast messages to the server. The example above includes a server unicast option specification which causes the client to send messages to the specified unicast address.

It is possible to mix interface names, wildcards, and interface names/addresses in the list of interfaces. It is not possible, however, to specify more than one unicast address on a given interface.

Care should be taken to specify proper unicast addresses, as the server will attempt to bind to the addresses specified without any additional checks. This approach was selected intentionally, to allow the software to communicate over uncommon addresses if so desired.

### 9.2.8. Configuration of IPv6 Address Pools

The main role of a DHCPv6 server is address assignment. For this, the server must be configured with at least one subnet and one pool of dynamic addresses to be managed. For example, assume that the server is connected to a network segment that uses the 2001:db8:1::/64 prefix. The administrator of that network decides that addresses from the range 2001:db8:1::1 to 2001:db8:1::ffff are going to be managed by the DHCPv6 server. Such a configuration can be achieved in the following way:

    "Dhcp6": {
        "subnet6": [
           {
               "subnet": "2001:db8:1::/64",
               "pools": [
                   {
                       "pool": "2001:db8:1::1-2001:db8:1::ffff"
                   }
               ],
               ...
           }
        ]
    }

Note that `subnet` is defined as a simple string, but the `pools` parameter is actually a list of pools; for this reason, the pool definition is enclosed in square brackets, even though only one range of addresses is specified.

Each `pool` is a structure that contains the parameters that describe a single pool. Currently there is only one parameter, `pool`, which gives the range of addresses in the pool.

It is possible to define more than one pool in a subnet; continuing the previous example, further assume that 2001:db8:1:0:5::/80 should also be managed by the server. It could be written as 2001:db8:1:0:5:: to 2001:db8:1::5:ffff:ffff:ffff, but typing so many `f` characters is cumbersome. The pool can be expressed more simply as 2001:db8:1:0:5::/80. Both formats are supported by `Dhcp6` and they can be mixed in the pool list. For example, the following pools could be defined:

    "Dhcp6": {
        "subnet6": [
        {
            "subnet": "2001:db8:1::/64",
            "pools": [
                { "pool": "2001:db8:1::1-2001:db8:1::ffff" },
                { "pool": "2001:db8:1:05::/80" }
            ],
            ...
        }
        ]
    }

White space in pool definitions is ignored, so spaces before and after the hyphen are optional. They can be used to improve readability.

The number of pools is not limited, but for performance reasons it is recommended to use as few as possible.

The server may be configured to serve more than one subnet. To add a second subnet, use a command similar to the following:

    "Dhcp6": {
        "subnet6": [
        {
            "id": 1,
            "subnet": "2001:db8:1::/64",
            "pools": [
                { "pool": "2001:db8:1::1-2001:db8:1::ffff" }
            ]
        },
        {
            "id": 2,
            "subnet": "2001:db8:2::/64",
            "pools": [
                { "pool": "2001:db8:2::/64" }
            ]
        },
        ...
        ]
    }

In this example, we allow the server to dynamically assign all addresses available in the whole subnet. Although rather wasteful, it is certainly a valid configuration to dedicate the whole /64 subnet for that purpose. Note that the Kea server does not preallocate the leases, so there is no danger in using gigantic address pools.

When configuring a DHCPv6 server using prefix/length notation, please pay attention to the boundary values. When specifying that the server can use a given pool, it is also able to allocate the first (typically a network address) address from that pool. For example, for pool 2001:db8:2::/64, the 2001:db8:2:: address may be assigned as well. To avoid this, use the `min-max` notation.

### 9.2.9. Subnet and Prefix Delegation Pools

Subnets may also be configured to delegate prefixes, as defined in RFC 8415, section 6.3. A subnet may have one or more prefix delegation pools. Each pool has a prefixed address, which is specified as a prefix (`prefix`) and a prefix length (`prefix-len`), as well as a delegated prefix length (`delegated-len`). The delegated length must not be shorter than (i.e. it must be numerically greater than or equal to) the prefix length. If both the delegated and prefix lengths are equal, the server will be able to delegate only one prefix. The delegated prefix does not have to match the subnet prefix.

Below is a sample subnet configuration which enables prefix delegation for the subnet:

    "Dhcp6": {
        "subnet6": [
            {
                "id": 1,
                "subnet": "2001:d8b:1::/64",
                "pd-pools": [
                    {
                        "prefix": "3000:1::",
                        "prefix-len": 64,
                        "delegated-len": 96
                    }
                ]
            }
        ],
        ...
    }

### 9.2.10. Prefix Exclude Option

For each delegated prefix, the delegating router may choose to exclude a single prefix out of the delegated prefix as specified in RFC 6603. The requesting router must not assign the excluded prefix to any of its downstream interfaces. The excluded prefix is intended to be used on a link through which the delegating router exchanges DHCPv6 messages with the requesting router. The configuration example below demonstrates how to specify an excluded prefix within a prefix pool definition. The excluded prefix `2001:db8:1:8000:cafe:80::/72` will be sent to a requesting router which includes the Prefix Exclude option in the Option Request option (ORO), and which is delegated a prefix from this pool.

    "Dhcp6": {
        "subnet6": [
            {
                "id": 1,
                "subnet": "2001:db8:1::/48",
                "pd-pools": [
                    {
                        "prefix": "2001:db8:1:8000::",
                        "prefix-len": 56,
                        "delegated-len": 64,
                        "excluded-prefix": "2001:db8:1:8000:cafe:80::",
                        "excluded-prefix-len": 72
                    }
                ]
            }
        ]
    }

Note

Here are some liberties and limits to the values that subnets and pools can take in Kea configurations that are out of the ordinary:

Kea configuration case | Allowed | Comment
--- | --- | ---
Overlapping subnets | Yes | Administrator consideration needs to be given to how clients are matched to these subnets.
Overlapping address pools in one subnet | No | Startup error: DHCP6_PARSER_FAIL
Overlapping address pools in different subnets | Yes | Specifying the same address pool in different subnets can be used as an equivalent of the global address pool. In that case, the server can assign addresses from the same range regardless of the client's subnet. If an address from such a pool is assigned to a client in one subnet, the same address will be renewed for this client if it moves to another subnet. Another client in a different subnet will not be assigned an address already assigned to the client in any of the subnets.
Address pools that are outside the subnet they are configured under | No | Startup error: DHCP6_PARSER_FAIL
Overlapping prefix delegation pools in one subnet | No | Startup error: DHCP6_PARSER_FAIL
Overlapping prefix delegation pools in different subnets | Yes | Specifying the same prefix delegation pool in different subnets can be used as an equivalent of the global pool. In that case, the server can delegate the same prefixes regardless of the client's subnet. If a prefix from such a pool is delegated to a client in one subnet, the same prefix will be renewed for this client if it moves to another subnet. Another client in a different subnet will not be delegated a prefix already delegated to the client in any of the subnets.
Prefix delegation pools not matching the subnet prefix | Yes | It is common in many deployments to configure the prefix delegation pools not matching the subnet prefix, e.g. a prefix pool of 3000::/96 within the 2001:db8:1::/64 subnet. Such use cases are supported by the Kea DHCPv6 server.

### 9.2.11. Standard DHCPv6 Options

One of the major features of the DHCPv6 server is the ability to provide configuration options to clients. Although there are several options that require special behavior, most options are sent by the server only if the client explicitly requests them. The following example shows how to configure the addresses of DNS servers, one of the most frequently used options. Options specified in this way are considered global and apply to all configured subnets.

    "Dhcp6": {
        "option-data": [
            {
               "name": "dns-servers",
               "code": 23,
               "space": "dhcp6",
               "csv-format": true,
               "data": "2001:db8::cafe, 2001:db8::babe"
            },
            ...
        ]
    }

The `option-data` line creates a new entry in the option-data table. This table contains information on all global options that the server is supposed to configure in all subnets. The `name` line specifies the option name. (For a complete list of currently supported names, see List of standard DHCPv6 options configurable by an administrator.) The next line specifies the option code, which must match one of the values from that list. The line beginning with `space` specifies the option space, which must always be set to `dhcp6` as these are standard DHCPv6 options. For other name spaces, including custom option spaces, see Nested DHCPv6 Options (Custom Option Spaces). The following line specifies the format in which the data will be entered; use of CSV (comma-separated values) is recommended. Finally, the `data` line gives the actual value to be sent to clients. The data parameter is specified as normal text, with values separated by commas if more than one value is allowed.

Options can also be configured as hexadecimal values. If `csv-format` is set to `false`, the option data must be specified as a hexadecimal string. The following commands configure the `dns-servers` option for all subnets with the addresses 2001:db8:1::cafe and 2001:db8:1::babe.

    "Dhcp6": {
        "option-data": [
            {
               "name": "dns-servers",
               "code": 23,
               "space": "dhcp6",
               "csv-format": false,
               "data": "20 01 0D B8 00 01 00 00 00 00 00 00 00 00 CA FE \
                        20 01 0D B8 00 01 00 00 00 00 00 00 00 00 BA BE"
            },
            ...
        ]
    }

Note

The value for the setting of the `data` element is split across two lines in this example for clarity; when entering the command, the whole string should be entered on the same line.

Kea supports the following formats when specifying hexadecimal data:

- `Delimited octets` - one or more octets separated by either colons or spaces (":" or " "). While each octet may contain one or two digits, we strongly recommend always using two digits. Valid examples are "ab:cd:ef" and "ab cd ef".

- `String of digits` - a continuous string of hexadecimal digits with or without a "0x" prefix. Valid examples are "0xabcdef" and "abcdef".

Care should be taken to use proper encoding when using hexadecimal format; Kea's ability to validate data correctness in hexadecimal is limited.

It is also possible to specify data for binary options as a single-quoted text string within double quotes, as shown (note that `csv-format` must be set to `false`):

    "Dhcp6": {
        "option-data": [
            {
                "name": "subscriber-id",
                "code": 38,
                "space": "dhcp6",
                "csv-format": false,
                "data": "'convert this text to binary'"
            },
            ...
        ],
        ...
    }

Most of the parameters in the `option-data` structure are optional and can be omitted in some circumstances, as discussed in Unspecified Parameters for DHCPv6 Option Configuration. Only one of `name` or `code` is required; it is not necessary to specify both. Space has a default value of `dhcp6`, so this can be skipped as well if a regular (not encapsulated) DHCPv6 option is defined. Finally, `csv-format` defaults to `true`, so it too can be skipped, unless the option value is specified as hexstring. Therefore, the above example can be simplified to:

    "Dhcp6": {
        "option-data": [
            {
               "name": "dns-servers",
               "data": "2001:db8::cafe, 2001:db8::babe"
            },
            ...
        ]
    }

Defined options are added to the response when the client requests them, as well as any options required by a protocol. An administrator can also specify that an option is always sent, even if a client did not specifically request it. To enforce the addition of a particular option, set the `always-send` flag to `true`, as in:

    "Dhcp6": {
        "option-data": [
            {
               "name": "dns-servers",
               "data": "2001:db8::cafe, 2001:db8::babe",
               "always-send": true
            },
            ...
        ]
    }

The effect is the same as if the client added the option code in the Option Request Option (or its equivalent for vendor options), as in:

    "Dhcp6": {
        "option-data": [
            {
               "name": "dns-servers",
               "data": "2001:db8::cafe, 2001:db8::babe",
               "always-send": true
            },
            ...
        ],
        "subnet6": [
            {
               "subnet": "2001:db8:1::/64",
               "option-data": [
                   {
                       "name": "dns-servers",
                       "data": "2001:db8:1::cafe, 2001:db8:1::babe"
                   },
                   ...
               ],
               ...
            },
            ...
        ],
        ...
    }

In the example above, the `dns-servers` option respects the global `always-send` flag and is always added to responses, but for subnet `2001:db8:1::/64`, the value is taken from the subnet-level option data specification.

Contrary to `always-send`, if the `never-send` flag is set to `true` for a particular option, the server does not add it to the response. The effect is the same as if the client removed the option code in the Option Request Option (or its equivalent for vendor options):

    "Dhcp6": {
        "option-data": [
            {
               "name": "dns-servers",
               "data": "2001:db8::cafe, 2001:db8::babe"
            },
            ...
        ],
        "subnet6": [
            {
               "subnet": "2001:db8:1::/64",
               "option-data": [
                   {
                       "name": "dns-servers",
                       "never-send": true
                   },
                   ...
               ],
               ...
            },
            ...
        ],
        ...
    }

In the example above, the `dns-server` option is never added to responses on subnet `2001:db8:1::/64`. `never-send` has precedence over `always-send`, so if both are `true` the option is not added.

Note

The `always-send` and `never-send` flags are sticky, meaning they do not follow the usual configuration inheritance rules. Instead, if they are enabled at least once along the configuration inheritance chain, they are applied - even if they are disabled in other places which would normally receive a higher priority. For instance, if one of the flags is enabled in the global scope, but disabled at the subnet level, it is enabled, disregarding the subnet-level setting.

Note

The `never-send` flag is less powerful than `libdhcp_flex_option.so`; for instance, it has no effect on options managed by the server itself. Both `always-send` and `never-send` have no effect on options which cannot be requested, for instance from a custom space.

It is possible to override options on a per-subnet basis. If clients connected to most subnets are expected to get the same values of a given option, administrators should use global options; it is possible to override specific values for a small number of subnets. On the other hand, if different values are used in each subnet, it does not make sense to specify global option values; rather, only subnet-specific ones should be set.

The following commands override the global `dns-servers` option for a particular subnet, setting a single DNS server with address 2001:db8:1::3.

    "Dhcp6": {
        "subnet6": [
            {
                "option-data": [
                    {
                        "name": "dns-servers",
                        "code": 23,
                        "space": "dhcp6",
                        "csv-format": true,
                        "data": "2001:db8:1::3"
                    },
                    ...
                ],
                ...
            },
            ...
        ],
        ...
    }

In some cases it is useful to associate some options with an address or prefix pool from which a client is assigned a lease. Pool-specific option values override subnet-specific and global option values. If the client is assigned multiple leases from different pools, the server assigns options from all pools from which the leases have been obtained. However, if the particular option is specified in multiple pools from which the client obtains the leases, only one instance of this option is handed out to the client. The server's administrator must not try to prioritize assignment of pool-specific options by trying to order pool declarations in the server configuration.

The following configuration snippet demonstrates how to specify the `dns-servers` option, which will be assigned to a client only if the client obtains an address from the given pool:

    "Dhcp6": {
        "subnet6": [
            {
                "pools": [
                    {
                        "pool": "2001:db8:1::100-2001:db8:1::300",
                        "option-data": [
                            {
                                "name": "dns-servers",
                                "data": "2001:db8:1::10"
                            }
                        ]
                    }
                ]
            },
            ...
        ],
        ...
    }

Options can also be specified in class or host-reservation scope. The current Kea options precedence order is (from most important to least): host reservation, pool, subnet, shared network, class, global.

When a data field is a string and that string contains the comma (`,`; U+002C) character, the comma must be escaped with two backslashes (`\\,`; U+005C). This double escape is required because both the routine splitting of CSV data into fields and JSON use the same escape character; a single escape (`\,`) would make the JSON invalid. For example, the string "EST5EDT4,M3.2.0/02:00,M11.1.0/02:00" must be represented as:

    "Dhcp6": {
        "subnet6": [
            {
                "pools": [
                    {
                        "option-data": [
                            {
                                "name": "new-posix-timezone",
                                "data": "EST5EDT4\\,M3.2.0/02:00\\,M11.1.0/02:00"
                            }
                        ]
                    },
                    ...
                ],
                ...
            },
            ...
        ],
        ...
    }

Some options are designated as arrays, which means that more than one value is allowed. For example, the option `dns-servers` allows the specification of more than one IPv6 address, enabling clients to obtain the addresses of multiple DNS servers.

Custom DHCPv6 Options describes the configuration syntax to create custom option definitions (formats). Creation of custom definitions for standard options is generally not permitted, even if the definition being created matches the actual option format defined in the RFCs. However, there is an exception to this rule for standard options for which Kea currently does not provide a definition. To use such options, a server administrator must create a definition as described in Custom DHCPv6 Options in the `dhcp6` option space. This definition should match the option format described in the relevant RFC, but the configuration mechanism allows any option format as there is currently no way to validate it.

The currently supported standard DHCPv6 options are listed in the table below. "Name" and "Code" are the values that should be used as a name/code in the option-data structures. "Type" designates the format of the data; the meanings of the various types are given in List of standard DHCP option types.

List of standard DHCPv6 options configurable by an administrator
Name | Code | Type | Array?
--- | --- | --- | ---
preference | 7 | uint8 | false
unicast | 12 | ipv6-address | false
sip-server-dns | 21 | fqdn | true
sip-server-addr | 22 | ipv6-address | true
dns-servers | 23 | ipv6-address | true
domain-search | 24 | fqdn | true
nis-servers | 27 | ipv6-address | true
nisp-servers | 28 | ipv6-address | true
nis-domain-name | 29 | fqdn | true
nisp-domain-name | 30 | fqdn | true
sntp-servers | 31 | ipv6-address | true
information-refresh-time | 32 | uint32 | false
bcmcs-server-dns | 33 | fqdn | true
bcmcs-server-addr | 34 | ipv6-address | true
geoconf-civic | 36 | record (uint8, uint16, binary) | false
remote-id | 37 | record (uint32, binary) | false
subscriber-id | 38 | binary | false
client-fqdn | 39 | record (uint8, fqdn) | false
pana-agent | 40 | ipv6-address | true
new-posix-timezone | 41 | string | false
new-tzdb-timezone | 42 | string | false
ero | 43 | uint16 | true
lq-query (1) | 44 | record (uint8, ipv6-address) | false
client-data (1) | 45 | empty | false
clt-time (1) | 46 | uint32 | false
lq-relay-data (1) | 47 | record (ipv6-address, binary) | false
lq-client-link (1) | 48 | ipv6-address | true
v6-lost | 51 | fqdn | false
capwap-ac-v6 | 52 | ipv6-address | true
relay-id | 53 | binary | false
v6-access-domain | 57 | fqdn | false
sip-ua-cs-list | 58 | fqdn | true
bootfile-url | 59 | string | false
bootfile-param | 60 | tuple | true
client-arch-type | 61 | uint16 | true
nii | 62 | record (uint8, uint8, uint8) | false
aftr-name | 64 | fqdn | false
erp-local-domain-name | 65 | fqdn | false
rsoo | 66 | empty | false
pd-exclude | 67 | binary | false
rdnss-selection | 74 | record (ipv6-address, uint8, fqdn) | true
client-linklayer-addr | 79 | binary | false
link-address | 80 | ipv6-address | false
solmax-rt | 82 | uint32 | false
inf-max-rt | 83 | uint32 | false
dhcp4o6-server-addr | 88 | ipv6-address | true
s46-rule | 89 | record (uint8, uint8, uint8, ipv4-address, ipv6-prefix) | false
s46-br | 90 | ipv6-address | false
s46-dmr | 91 | ipv6-prefix | false
s46-v4v6bind | 92 | record (ipv4-address, ipv6-prefix) | false
s46-portparams | 93 | record(uint8, psid) | false
s46-cont-mape | 94 | empty | false
s46-cont-mapt | 95 | empty | false
s46-cont-lw | 96 | empty | false
v6-captive-portal | 103 | string | false
v6-sztp-redirect | 136 | tuple | true
ipv6-address-andsf | 143 | ipv6-address | true
v6-dnr | 144 | record (uint16, uint16, fqdn, binary) | false

Options marked with (1) have option definitions, but the logic behind them is not implemented. That means that, technically, Kea knows how to parse them in incoming messages or how to send them if configured to do so, but not what to do with them. Since the related RFCs require certain processing, the support for those options is non-functional. However, it may be useful in some limited lab testing; hence the definition formats are listed here.

Kea supports more options than those listed above. The following list is mostly useful for readers who want to understand whether Kea is able to support certain options. The following options are returned by the Kea engine itself and in general should not be configured manually.

List of standard DHCPv6 options managed by Kea on its own and not directly configurable by an administrator
Name | Code | Description
--- | --- | ---
client-id | 1 | Sent by the client; Kea uses it to distinguish between clients.
server-id | 2 | Sent by clients to request action from a specific server and by the server to identify itself. See Server Identifier in DHCPv6 for details.
ia-na | 3 | A container option that conveys IPv6 addresses (`iaddr` options). Kea receives and sends those options using its allocation engine.
ia-ta | 4 | Conveys temporary addresses. Deprecated feature, not supported.
iaaddr | 5 | Conveys addresses with lifetimes in `ia-na` and `ia-ta` options.
oro | 6 | ORO (or Option Request Option) is used by clients to request a list of options they are interested in. Kea supports it and sends the requested options back if configured with required options.
elapsed-time | 8 | Sent by clients to identify how long they have been trying to obtain a configuration. Kea uses high values sent by clients as an indicator that something is wrong; this is one of the aspects used in HA to determine if the partner is healthy or not.
relay-msg | 9 | Used by relays to encapsulate the original client message. Kea uses it when sending back relayed responses to the relay agent.
auth | 11 | Used to pass authentication information between clients and server. The support for this option is very limited.
status-code | 13 | An option that the server can attach in case of various failures, such as running out of addresses or not being configured to assign prefixes.
rapid-commit | 14 | Used to signal the client's willingness to support `rapid-commit` and the server's acceptance for this configuration. See Rapid Commit for details.
user-class | 15 | Sent by the client to self-identify the device type. Kea can use this for client classification.
vendor-class | 16 | Similar to `user-class`, but vendor-specific.
vendor-opts | 17 | A vendor-specific container that is used by both the client and the server to exchange vendor-specific options. The logic behind those options varies between vendors. Vendor options are explained in DHCPv6 Vendor-Specific Options.
interface-id | 18 | May be inserted by the relay agent to identify the interface that the original client message was received on. Kea may be told to use this information to select specific subnets. Also, if specified, Kea echoes this option back, so the relay will know which interface to use to reach the client.
ia-pd | 25 | A container for conveying Prefix Delegations (PDs)) that are being delegated to clients. See Subnet and Prefix Delegation Pools for details.
iaprefix | 26 | Conveys the IPv6 prefix in the `ia-pd` option. See Subnet and Prefix Delegation Pools for details.

### 9.2.12. Common Softwire46 Options

Softwire46 options are involved in IPv4-over-IPv6 provisioning by means of tunneling or translation, as specified in RFC 7598. The following sections provide configuration examples of these options.

#### 9.2.12.1. Softwire46 Container Options

Softwire46 (S46) container options group rules and optional port parameters for a specified domain. There are three container options specified in the "dhcp6" (top-level) option space: the MAP-E Container option, the MAP-T Container option, and the S46 Lightweight 4over6 Container option. These options only contain the encapsulated options specified below; they do not include any data fields.

To configure the server to send a specific container option along with all encapsulated options, the container option must be included in the server configuration as shown below:

    "Dhcp6": {
        "option-data": [
            {
                "name": "s46-cont-mape"
            } ],
        ...
    }

This configuration will cause the server to include the MAP-E Container option to the client. Use `s46-cont-mapt` or `s46-cont-lw` for the MAP-T Container and S46 Lightweight 4over6 Container options, respectively.

All remaining Softwire46 options described below are included in one of the container options. Thus, they must be included in appropriate option spaces by selecting a `space` name, which specifies the option where they are supposed to be included.

#### 9.2.12.2. S46 Rule Option

The S46 Rule option is used to convey the Basic Mapping Rule (BMR) and Forwarding Mapping Rule (FMR).

    {
        "space": "s46-cont-mape-options",
        "name": "s46-rule",
        "data": "128, 0, 24, 192.0.2.0, 2001:db8:1::/64"
    }

Another possible `space` value is `s46-cont-mapt-options`.

The S46 Rule option conveys a number of parameters:

- `flags` - an unsigned 8-bit integer, with currently only the most-significant bit specified. It denotes whether the rule can be used for forwarding (128) or not (0).

- `ea-len` - an 8-bit-long Embedded Address length. Allowed values range from 0 to 48.

- `IPv4 prefix length` - an 8-bit-long expression of the prefix length of the Rule IPv4 prefix specified in the `ipv4-prefix` field. Allowed values range from 0 to 32.

- `IPv4 prefix` - a fixed-length 32-bit field that specifies the IPv4 prefix for the S46 rule. The bits in the prefix after a specific number of bits (defined in `prefix4-len`) are reserved, and MUST be initialized to zero by the sender and ignored by the receiver.

- `IPv6 prefix` - a field in prefix/length notation that specifies the IPv6 domain prefix for the S46 rule. The field is padded on the right with zero bits up to the nearest octet boundary, when `prefix6-len` is not evenly divisible by 8.

#### 9.2.12.3. S46 BR Option

The S46 BR option is used to convey the IPv6 address of the Border Relay. This option is mandatory in the MAP-E Container option and is not permitted in the MAP-T and S46 Lightweight 4over6 Container options.

    {
        "space": "s46-cont-mape-options",
        "name": "s46-br",
        "data": "2001:db8:cafe::1"
    }

Another possible `space` value is `s46-cont-lw-options`.

#### 9.2.12.4. S46 DMR Option

The S46 DMR option is used to convey values for the Default Mapping Rule (DMR). This option is mandatory in the MAP-T container option and is not permitted in the MAP-E and S46 Lightweight 4over6 Container options.

    {
        "space": "s46-cont-mapt-options",
        "name": "s46-dmr",
        "data": "2001:db8:cafe::/64"
    }

This option must not be included in other containers.

#### 9.2.12.5. S46 IPv4/IPv6 Address Binding Option

The S46 IPv4/IPv6 Address Binding option may be used to specify the full or shared IPv4 address of the Customer Edge (CE). The IPv6 prefix field is used by the CE to identify the correct prefix to use for the tunnel source.

    {
        "space": "s46-cont-lw",
        "name": "s46-v4v6bind",
        "data": "192.0.2.3, 2001:db8:1:cafe::/64"
    }

This option must not be included in other containers.

#### 9.2.12.6. S46 Port Parameters

The S46 Port Parameters option specifies optional port-set information that may be provided to CEs.

    {
        "space": "s46-rule-options",
        "name": "s46-portparams",
        "data": "2, 3/4"
    }

Another possible `space` value is `s46-v4v6bind`, to include this option in the S46 IPv4/IPv6 Address Binding option.

Note that the second value in the example above specifies the PSID and PSID-length fields in the format of PSID/PSID length. This is equivalent to the values of `PSID-len=4` and `PSID=12288` conveyed in the S46 Port Parameters option.

### 9.2.13. Custom DHCPv6 Options

Kea supports custom (non-standard) DHCPv6 options. Let's say that we want to define a new DHCPv6 option called `foo`, which will have code 100 and will convey a single, unsigned, 32-bit integer value. Such an option can be defined by putting the following entry in the configuration file:

    "Dhcp6": {
        "option-def": [
            {
                "name": "foo",
                "code": 100,
                "type": "uint32",
                "array": false,
                "record-types": "",
                "space": "dhcp6",
                "encapsulate": ""
            },
            ...
        ],
        ...
    }

The `false` value of the `array` parameter determines that the option does NOT comprise an array of `uint32` values but is, instead, a single value. Two other parameters have been left blank: `record-types` and `encapsulate`. The former specifies the comma-separated list of option data fields, if the option comprises a record of data fields. The `record-types` value should be non-empty if `type` is set to `record`; otherwise it must be left blank. The latter parameter specifies the name of the option space being encapsulated by the particular option. If the particular option does not encapsulate any option space, the parameter should be left blank. Note that the `option-def` configuration statement only defines the format of an option and does not set its value(s).

The `name`, `code`, and `type` parameters are required; all others are optional. The `array` parameter default value is `false`. The `record-types` and `encapsulate` parameters default values are blank (`""`). The default `space` is `dhcp6`.

Once the new option format is defined, its value is set in the same way as for a standard option. For example, the following commands set a global value that applies to all subnets.

    "Dhcp6": {
        "option-data": [
            {
                "name": "foo",
                "code": 100,
                "space": "dhcp6",
                "csv-format": true,
                "data": "12345"
            },
            ...
        ],
        ...
    }

New options can take more complex forms than the simple use of primitives (uint8, string, ipv6-address, etc.); it is possible to define an option comprising a number of existing primitives.

For example, say we want to define a new option that will consist of an IPv6 address, followed by an unsigned 16-bit integer, followed by a boolean value, followed by a text string. Such an option could be defined in the following way:

    "Dhcp6": {
        "option-def": [
            {
                "name": "bar",
                "code": 101,
                "space": "dhcp6",
                "type": "record",
                "array": false,
                "record-types": "ipv6-address, uint16, boolean, string",
                "encapsulate": ""
            },
            ...
        ],
        ...
    }

The `type` parameter is set to `"record"` to indicate that the option contains multiple values of different types. These types are given as a comma-separated list in the `record-types` field and should be ones from those listed in List of standard DHCP option types.

The values of the options are set in an `option-data` statement as follows:

    "Dhcp6": {
        "option-data": [
            {
                "name": "bar",
                "space": "dhcp6",
                "code": 101,
                "csv-format": true,
                "data": "2001:db8:1::10, 123, false, Hello World"
            }
        ],
        ...
    }

The `csv-format` parameter is set to `true` to indicate that the `data` field comprises a comma-separated list of values. The values in `data` must correspond to the types set in the `record-types` field of the option definition.

When `array` is set to `true` and `type` is set to `"record"`, the last field is an array, i.e. it can contain more than one value, as in:

    "Dhcp6": {
        "option-def": [
            {
                "name": "bar",
                "code": 101,
                "space": "dhcp6",
                "type": "record",
                "array": true,
                "record-types": "ipv6-address, uint16",
                "encapsulate": ""
            },
            ...
        ],
        ...
    }

The new option content is one IPv6 address followed by one or more 16-bit unsigned integers.

Note

In general, boolean values are specified as `true` or `false`, without quotes. Some specific boolean parameters may also accept `"true"`, `"false"`, `0`, `1`, `"0"`, and `"1"`.

### 9.2.14. DHCPv6 Vendor-Specific Options

Vendor options in DHCPv6 are carried in the Vendor-Specific Information option (code 17). The idea behind option 17 is that each vendor has its own unique set of options with their own custom formats. The vendor is identified by a 32-bit unsigned integer called `enterprise-number` or `vendor-id`.

The standard spaces defined in Kea and their options are:

- `vendor-2495`: Internet Systems Consortium, Inc. for 4o6 options:

option code | option name | option description
--- | --- | ---
60000 | 4o6-interface | the name of the 4o6 server's client-facing interface
60001 | 4o6-source-address | the address that the 4o6 server uses to send packets to the client
60002 | 4o6-source-port | the port that the 4o6 server opens to send packets to the client

- `vendor-4491`: Cable Television Laboratories, Inc. for DOCSIS3 options:

option code | option name | option description
--- | --- | ---
1 | oro | ORO (or Option Request Option) is used by clients to request a list of options they are interested in.
2 | tftp-servers | a list of IPv4 addresses of TFTP servers to be used by the cable modem

The following examples show how to define an option `"foo"` with code 1 that consists of an IPv6 address, an unsigned 16-bit integer, and a string. The `"foo"` option is conveyed in a Vendor-Specific Information option, which comprises a single uint32 value that is set to `12345`. The sub-option `"foo"` follows the data field holding this value.

The first step is to define the format of the option:

    "Dhcp6": {
        "option-def": [
            {
                "name": "foo",
                "code": 1,
                "space": "vendor-12345",
                "type": "record",
                "array": false,
                "record-types": "ipv6-address, uint16, string",
                "encapsulate": ""
            }
        ],
        ...
    }

Note that the option space is set to `"vendor-12345"`. Once the option format is defined, the next step is to define actual values for that option:

    "Dhcp6": {
        "option-data": [
            {
                "name": "foo",
                "space": "vendor-12345",
                "data": "2001:db8:1::10, 123, Hello World"
            },
            ...
        ],
        ...
    }

We should also define a value (`"enterprise-number"`) for the Vendor-Specific Information option, to convey the option `foo`.

    "Dhcp6": {
        "option-data": [
            {
                "name": "vendor-opts",
                "data": "12345"
            },
            ...
        ],
        ...
    }

Alternatively, the option can be specified using its code.

    "Dhcp6": {
        "option-data": [
            {
                "code": 17,
                "data": "12345"
            },
            ...
        ],
        ...
    }

A common configuration is to set the `always-send` flag to `true`, so the vendor option is sent even when the client did not specify it in the query.

This is also how `kea-dhcp6` can be configured to send multiple vendor options from different vendors, along with each of their specific enterprise number. If these options need to be sent by the server regardless of whether the client specified any enterprise number, `"always-send": true` must be configured for the suboptions that will be included in the Vendor-Specific Information option (code 17).

    {
      "Dhcp6": {
        "option-data": [
          {
            "always-send": true,
            "data": "tagged",
            "name": "tag",
            "space": "vendor-2234"
          },
          {
            "always-send": true,
            "data": "https://example.com:1234/path",
            "name": "url",
            "space": "vendor-3561"
          }
        ],
        "option-def": [
          {
            "code": 22,
            "name": "tag",
            "space": "vendor-2234",
            "type": "string"
          },
          {
            "code": 11,
            "name": "url",
            "space": "vendor-3561",
            "type": "string"
          }
        ]
      }
    }

Note

The `kea-dhcp6` server is able to recognize multiple Vendor Class options (code 16) with different enterprise numbers in the client requests and to send multiple Vendor-Specific Information options (code 17) in the responses, one for each vendor.

### 9.2.15. Nested DHCPv6 Options (Custom Option Spaces)

It is sometimes useful to define a completely new option space, such as when a user creates a new option to convey sub-options that use a separate numbering scheme, such as sub-options with codes 1 and 2. Those option codes conflict with standard DHCPv6 options, so a separate option space must be defined.

Note that the creation of a new option space is not required when defining sub-options for a standard option, because one is created by default if the standard option is meant to convey any sub-options (see DHCPv6 Vendor-Specific Options).

If we want a DHCPv6 option called `container` with code 102, that conveys two sub-options with codes 1 and 2, we first need to define the new sub-options:

    "Dhcp6": {
        "option-def": [
            {
                "name": "subopt1",
                "code": 1,
                "space": "isc",
                "type": "ipv6-address",
                "record-types": "",
                "array": false,
                "encapsulate": ""
            },
            {
                "name": "subopt2",
                "code": 2,
                "space": "isc",
                "type": "string",
                "record-types": "",
                "array": false,
                "encapsulate": ""
            }
        ],
        ...
    }

Note that we have defined the options to belong to a new option space (in this case, `"isc"`).

The next step is to define a regular DHCPv6 option with the desired code and specify that it should include options from the new option space:

    "Dhcp6": {
        "option-def": [
            {
                "name": "container",
                "code": 102,
                "space": "dhcp6",
                "type": "empty",
                "array": false,
                "record-types": "",
                "encapsulate": "isc"
            },
            ...
        ],
        ...
    }

The name of the option space in which the sub-options are defined is set in the `encapsulate` field. The `type` field is set to `"empty"`, to indicate that this option does not carry any data other than sub-options.

Finally, we can set values for the new options:

    {
      "Dhcp6": {
        "option-data": [
            {
                "name": "subopt1",
                "code": 1,
                "space": "isc",
                "data": "2001:db8::abcd"
            },
            {
                "name": "subopt2",
                "code": 2,
                "space": "isc",
                "data": "Hello world"
            },
            {
                "name": "container",
                "code": 102,
                "space": "dhcp6"
            }
        ]
      }
    }

It is possible to create an option which carries some data in addition to the sub-options defined in the encapsulated option space. For example, if the `container` option from the previous example were required to carry a uint16 value as well as the sub-options, the `type` value would have to be set to `"uint16"` in the option definition. (Such an option would then have the following data structure: DHCP header, uint16 value, sub-options.) The value specified with the `data` parameter — which should be a valid integer enclosed in quotes, e.g. `"123"` — would then be assigned to the `uint16` field in the `container` option.

### 9.2.16. Unspecified Parameters for DHCPv6 Option Configuration

In many cases it is not required to specify all parameters for an option configuration, and the default values can be used. However, it is important to understand the implications of not specifying some of them, as it may result in configuration errors. The list below explains the behavior of the server when a particular parameter is not explicitly specified:

- `name` - the server requires either an option name or an option code to identify an option. If this parameter is unspecified, the option code must be specified.

- `code` - the server requires either an option name or an option code to identify an option; this parameter may be left unspecified if the `name` parameter is specified. However, this also requires that the particular option have a definition (either as a standard option or an administrator-created definition for the option using an `option-def` structure), as the option definition associates an option with a particular name. It is possible to configure an option for which there is no definition (unspecified option format). Configuration of such options requires the use of the option code.

- `space` - if the option space is unspecified it defaults to `dhcp6`, which is an option space holding standard DHCPv6 options.

- `data` - if the option data is unspecified it defaults to an empty value. The empty value is mostly used for the options which have no payload (boolean options), but it is legal to specify empty values for some options which carry variable-length data and for which the specification allows a length of 0. For such options, the data parameter may be omitted in the configuration.

- `csv-format` - if this value is not specified, the server assumes that the option data is specified as a list of comma-separated values to be assigned to individual fields of the DHCP option.

### 9.2.17. Controlling the Values Sent for T1 and T2 Times

According to RFC 8415, section 21.4, the recommended T1 and T2 values are 50% and 80% of the preferred lease time, respectively. Kea can be configured to send values that are specified explicitly or that are calculated as percentages of the preferred lease time. The server's behavior is determined by a combination of configuration parameters, of which T1 and T2 are only two.

The lease's preferred and valid lifetimes are expressed as triplets with minimum, default, and maximum values using configuration entries:

- `min-preferred-lifetime` - specifies the minimum preferred lifetime (optional).

- `preferred-lifetime` - specifies the default preferred lifetime.

- `max-preferred-lifetime` - specifies the maximum preferred lifetime (optional).

- `min-valid-lifetime` - specifies the minimum valid lifetime (optional).

- `valid-lifetime` - specifies the default valid lifetime.

- `max-valid-lifetime` - specifies the maximum valid lifetime (optional).

Since Kea 1.9.11, these values may be specified within client classes.

When the client does not specify lifetimes, the default is used. A specified lifetime - using the IAADDR or IAPREFIX sub-option with non-zero values - uses these values when they are between the configured minimum and maximum bounds. Values outside the bounds are rounded up or down as needed.

Note

As of Kea 2.3.8, if the preferred-lifetime has not been explicitly specified or the specified value is larger than the value of valid-lifetime, the server will use the value given by 0.625 * valid-lifetime.

To send specific fixed values, use the following two parameters:

- `renew-timer` - specifies the value of T1 in seconds.

- `rebind-timer` - specifies the value of T2 in seconds.

Any value greater than or equal to zero may be specified for T2. T1, if specified, must be less than T2. This flexibility allows a use case where administrators want to suppress client renewals and rebinds by deferring them beyond the lifespan of the lease. This should cause the lease to expire, rather than get renewed by clients. If T1 is specified as larger than T2, T1 is silently set to zero in the outbound IA.

In the great majority of cases, the values should follow this rule: T1 < T2 < preferred lifetime < valid lifetime. Alternatively, both T1 and T2 values can be configured to 0, which is a signal to DHCPv6 clients that they may renew at their own discretion. However, there are known broken client implementations in use that will start renewing immediately. Administrators who plan to use T1=T2=0 values should test first and make sure their clients behave rationally.

In some rare cases there may be a need to disable a client's ability to renew addresses. This is undesired from a protocol perspective and should be avoided if possible. However, if necessary, administrators can configure the T1 and T2 values to be equal or greater to the valid lifetime. Be advised that this will cause clients to occasionally lose their addresses, which is generally perceived as poor service. However, there may be some rare business cases when this is desired (e.g. when it is desirable to intentionally break long-lasting connections).

Calculation of the values is controlled by the following three parameters:

- `calculate-tee-times` - when `true`, T1 and T2 are calculated as percentages of the valid lease time. It defaults to `true`.

- `t1-percent` - the percentage of the valid lease time to use for T1. It is expressed as a real number between 0.0 and 1.0 and must be less than `t2-percent`. The default value is 0.5, per RFC 8415.

- `t2-percent` - the percentage of the valid lease time to use for T2. It is expressed as a real number between 0.0 and 1.0 and must be greater than `t1-percent`. The default value is 0.8 per RFC 8415.

Note

If both explicit values are specified and `calculate-tee-times` is `true`, the server will use the explicit values. Administrators with a setup where some subnets or shared-networks use explicit values and some use calculated values must not define the explicit values at any level higher than where they will be used. Inheriting them from too high a scope, such as global, will cause them to have values at every level underneath (both shared-networks and subnets), effectively disabling calculated values.

### 9.2.18. IPv6 Subnet Selection

The DHCPv6 server may receive requests from local (connected to the same subnet as the server) and remote (connected via relays) clients. As the server may have many subnet configurations defined, it must select an appropriate subnet for a given request.

In IPv4, the server can determine which of the configured subnets are local, as there is a reasonable expectation that the server will have a (global) IPv4 address configured on the interface. That assumption is not true in IPv6; the DHCPv6 server must be able to operate while only using link-local addresses. Therefore, an optional `interface` parameter is available within a subnet definition to designate that a given subnet is local, i.e. reachable directly over the specified interface. For example, a server that is intended to serve a local subnet over eth0 may be configured as follows:

    "Dhcp6": {
        "subnet6": [
            {
                "id": 1,
                "subnet": "2001:db8:beef::/48",
                "pools": [
                     {
                         "pool": "2001:db8:beef::/48"
                     }
                 ],
                "interface": "eth0"
            }
        ],
        ...
    }

### 9.2.19. Rapid Commit

The Rapid Commit option, described in RFC 8415, is supported by the Kea DHCPv6 server. However, support is disabled by default. It can be enabled on a per-subnet basis using the `rapid-commit` parameter as shown below:

    {
      "Dhcp6": {
        "subnet6": [
            {
                "id": 1,
                "subnet": "2001:db8:beef::/48",
                "rapid-commit": true,
                "pools": [
                     {
                         "pool": "2001:db8:beef::1-2001:db8:beef::10"
                     }
                 ]
            }
        ]
      }
    }

This setting only affects the subnet for which `rapid-commit` is set to `true`. For clients connected to other subnets, the server ignores the Rapid Commit option sent by the client and follows the 4-way exchange procedure, i.e. responds with an Advertise for a Solicit containing a Rapid Commit option.

### 9.2.20. DHCPv6 Relays

A DHCPv6 server with multiple subnets defined must select the appropriate subnet when it receives a request from a client. For clients connected via relays, two mechanisms are used:

The first uses the `linkaddr` field in the `RELAY_FORW` message. The name of this field is somewhat misleading in that it does not contain a link-layer address; instead, it holds an address (typically a global address) that is used to identify a link. The DHCPv6 server checks to see whether the address belongs to a defined subnet and, if it does, that subnet is selected for the client's request.

The second mechanism is based on `interface-id` options. While forwarding a client's message, relays may insert an `interface-id` option into the message that identifies the interface on the relay that received the message. (Some relays allow configuration of that parameter, but it is sometimes hard-coded and may range from the very simple [e.g. "vlan100"] to the very cryptic; one example seen on real hardware was "ISAM144|299|ipv6|nt:vp:1:110".) The server can use this information to select the appropriate subnet. The information is also returned to the relay, which then knows the interface to use to transmit the response to the client. For this to work successfully, the relay interface IDs must be unique within the network and the server configuration must match those values.

When configuring the DHCPv6 server, two similarly named parameters can be configured for a subnet:

- `interface` - defines which local network interface can be used to access a given subnet.

- `interface-id` - specifies the content of the `interface-id` option used by relays to identify the interface on the relay to which the response packet is sent.

The two are mutually exclusive; a subnet cannot be reachable both locally (direct traffic) and via relays (remote traffic). Specifying both is a configuration error and the DHCPv6 server will refuse such a configuration.

The following example configuration shows how to specify an `interface-id` with a value of "vlan123":

    "Dhcp6": {
        "subnet6": [
            {
                "id": 1,
                "subnet": "2001:db8:beef::/48",
                "pools": [
                     {
                         "pool": "2001:db8:beef::/48"
                     }
                 ],
                "interface-id": "vlan123"
            }
        ],
        ...
    }

### 9.2.21. Relay-Supplied Options

RFC 6422 defines a mechanism called Relay-Supplied DHCP Options. In certain cases relay agents are the only entities that may have specific information, and they can insert options when relaying messages from the client to the server. The server then does certain checks and copies those options to the response sent to the client.

There are certain conditions that must be met for the option to be included. First, the server must not provide the option itself; in other words, if both relay and server provide an option, the server always takes precedence. Second, the option must be RSOO-enabled. (RSOO is the "Relay Supplied Options option.") IANA maintains a list of RSOO-enabled options here. However, there may be cases when system administrators want to echo other options. Kea can be instructed to treat other options as RSOO-enabled; for example, to mark options 110, 120, and 130 as RSOO-enabled, the following syntax should be used:

    "Dhcp6": {
        "relay-supplied-options": [ "110", "120", "130" ],
        ...
    }

At this time, only option 65 is RSOO-enabled by IANA. This option will always be treated as RSOO-enabled, so there is no need to explicitly mark it. When enabling standard options, it is also possible to use their names rather than their option code, e.g. use `dns-servers` instead of `23`. See ref:dhcp6-std-options-list for the names. In certain cases this may also work for custom options, but due to the nature of the parser code this may be unreliable and should be avoided.

### 9.2.22. Client Classification in DHCPv6

The DHCPv6 server includes support for client classification. For a deeper discussion of the classification process, see Client Classification.

In certain cases it is useful to configure the server to differentiate between DHCP client types and treat them accordingly. Client classification can be used to modify the behavior of almost any part of DHCP message processing. Kea currently offers three mechanisms that take advantage of client classification in DHCPv6: subnet selection, address pool selection, and DHCP options assignment.

Kea can be instructed to limit access to given subnets based on class information. This is particularly useful for cases where two types of devices share the same link and are expected to be served from two different subnets. The primary use case for such a scenario is cable networks, where there are two classes of devices: the cable modem itself, which should be handed a lease from subnet A; and all other devices behind the modem, which should get leases from subnet B. That segregation is essential to prevent overly curious end-users from playing with their cable modems. For details on how to set up class restrictions on subnets, see Configuring Subnets With Class Information.

When subnets belong to a shared network, the classification applies to subnet selection but not to pools; that is, a pool in a subnet limited to a particular class can still be used by clients which do not belong to the class, if the pool they are expected to use is exhausted. The limit on access based on class information is also available at the address/prefix pool level within a subnet: see Configuring Pools With Class Information. This is useful when segregating clients belonging to the same subnet into different address ranges.

In a similar way, a pool can be constrained to serve only known clients, i.e. clients which have a reservation, using the built-in `KNOWN` or `UNKNOWN` classes. Addresses can be assigned to registered clients without giving a different address per reservation: for instance, when there are not enough available addresses. The determination whether there is a reservation for a given client is made after a subnet is selected, so it is not possible to use `KNOWN`/`UNKNOWN` classes to select a shared network or a subnet.

The process of classification is conducted in five steps. The first step is to assess an incoming packet and assign it to zero or more classes. The second step is to choose a subnet, possibly based on the class information. When the incoming packet is in the special class `DROP`, it is dropped and a debug message logged. The next step is to evaluate class expressions depending on the built-in `KNOWN`/`UNKNOWN` classes after host reservation lookup, using them for pool/pd-pool selection and assigning classes from host reservations. The list of required classes is then built and each class of the list has its expression evaluated; when it returns `true`, the packet is added as a member of the class. The last step is to assign options, again possibly based on the class information. More complete and detailed information is available in Client Classification.

There are two main methods of classification. The first is automatic and relies on examining the values in the vendor class options or the existence of a host reservation. Information from these options is extracted, and a class name is constructed from it and added to the class list for the packet. The second method specifies an expression that is evaluated for each packet. If the result is `true`, the packet is a member of the class.

Note

The new `early-global-reservations-lookup` global parameter flag enables a lookup for global reservations before the subnet selection phase. This lookup is similar to the general lookup described above with two differences:

- the lookup is limited to global host reservations

- the `UNKNOWN` class is never set

Note

Care should be taken with client classification, as it is easy for clients that do not meet class criteria to be denied all service.

#### 9.2.22.1. Defining and Using Custom Classes

The following example shows how to configure a class using an expression and a subnet using that class. This configuration defines the class named `Client_enterprise`. It is comprised of all clients whose client identifiers start with the given hex string (which would indicate a DUID based on an enterprise id of 0xAABBCCDD). Members of this class will be given an address from 2001:db8:1::0 to 2001:db8:1::FFFF and the addresses of their DNS servers set to 2001:db8:0::1 and 2001:db8:2::1.

    "Dhcp6": {
        "client-classes": [
            {
                "name": "Client_enterprise",
                "test": "substring(option[1].hex,0,6) == 0x0002AABBCCDD",
                "option-data": [
                    {
                        "name": "dns-servers",
                        "code": 23,
                        "space": "dhcp6",
                        "csv-format": true,
                        "data": "2001:db8:0::1, 2001:db8:2::1"
                    }
                ]
            },
            ...
        ],
        "subnet6": [
            {
                "id": 1,
                "subnet": "2001:db8:1::/64",
                "pools": [ { "pool": "2001:db8:1::-2001:db8:1::ffff" } ],
                "client-class": "Client_enterprise"
            }
        ],
        ...
    }

This example shows a configuration using an automatically generated `VENDOR_CLASS_` class. The administrator of the network has decided that addresses in the range 2001:db8:1::1 to 2001:db8:1::ffff are to be managed by the DHCPv6 server and that only clients belonging to the eRouter1.0 client class are allowed to use that pool.

    "Dhcp6": {
        "subnet6": [
            {
                "id": 1,
                "subnet": "2001:db8:1::/64",
                "pools": [
                     {
                         "pool": "2001:db8:1::-2001:db8:1::ffff"
                     }
                 ],
                "client-class": "VENDOR_CLASS_eRouter1.0"
            }
        ],
        ...
    }

#### 9.2.22.2. Required Classification

In some cases it is useful to limit the scope of a class to a shared network, subnet, or pool. There are two parameters which are used to limit the scope of the class by instructing the server to evaluate test expressions when required.

The first one is the per-class `only-if-required` flag, which is `false` by default. When it is set to `true`, the test expression of the class is not evaluated at the reception of the incoming packet but later, and only if the class evaluation is required.

The second is `require-client-classes`, which takes a list of class names and is valid in shared-network, subnet, and pool scope. Classes in these lists are marked as required and evaluated after selection of this specific shared network/subnet/pool and before output-option processing.

In this example, a class is assigned to the incoming packet when the specified subnet is used:

    "Dhcp6": {
        "client-classes": [
           {
               "name": "Client_foo",
               "test": "member('ALL')",
               "only-if-required": true
           },
           ...
        ],
        "subnet6": [
            {
                "subnet": "2001:db8:1::/64",
                "pools": [
                     {
                         "pool": "2001:db8:1::-2001:db8:1::ffff"
                     }
                ],
                "require-client-classes": [ "Client_foo" ],
                ...
            },
            ...
        ],
        ...
    }

Required evaluation can be used to express complex dependencies like subnet membership. It can also be used to reverse the precedence; if `option-data` is set in a subnet, it takes precedence over `option-data` in a class. If `option-data` is moved to a required class and required in the subnet, a class evaluated earlier may take precedence.

Required evaluation is also available at shared-network and pool/pd-pool levels. The order in which required classes are considered is: shared-network, subnet, and (pd-)pool, i.e. in the reverse order from the way in which `option-data` is processed.

### 9.2.23. DDNS for DHCPv6

As mentioned earlier, `kea-dhcp6` can be configured to generate requests to the DHCP-DDNS server, `kea-dhcp-ddns`, (referred to herein as "D2") to update DNS entries. These requests are known as NameChangeRequests or NCRs. Each NCR contains the following information:

1. Whether it is a request to add (update) or remove DNS entries.

2. Whether the change requests forward DNS updates (AAAA records), reverse DNS updates (PTR records), or both.

3. The Fully Qualified Domain Name (FQDN), lease address, and DHCID (information identifying the client associated with the FQDN).

DDNS-related parameters are split into two groups:

1. Connectivity Parameters

   > These are parameters which specify where and how `kea-dhcp6` connects to and communicates with D2. These parameters can only be specified within the top-level `dhcp-ddns` section in the `kea-dhcp6` configuration. The connectivity parameters are listed below:

   > - `enable-updates`

   > - `server-ip`

   > - `server-port`

   > - `sender-ip`

   > - `sender-port`

   > - `max-queue-size`

   > - `ncr-protocol`

   > - `ncr-format"`

2. Behavioral Parameters

   > These parameters influence behavior such as how client host names and FQDN options are handled. They have been moved out of the `dhcp-ddns` section so that they may be specified at the global, shared-network, and/or subnet levels. Furthermore, they are inherited downward from global to shared-network to subnet. In other words, if a parameter is not specified at a given level, the value for that level comes from the level above it. The behavioral parameters are as follows:

   > - `ddns-send-updates`

   > - `ddns-override-no-update`

   > - `ddns-override-client-update`

   > - `ddns-replace-client-name"`

   > - `ddns-generated-prefix`

   > - `ddns-qualifying-suffix`

   > - `ddns-update-on-renew`

   > - `ddns-use-conflict-resolution`

   > - `ddns-ttl-percent`

   > - `hostname-char-set`

   > - `hostname-char-replacement`

Note

For backward compatibility, configuration parsing still recognizes the original behavioral parameters specified in `dhcp-ddns`, by translating the parameter into its global equivalent. If a parameter is specified both globally and in `dhcp-ddns`, the latter value is ignored. In either case, a log is emitted explaining what has occurred. Specifying these values within `dhcp-ddns` is deprecated and support for it will be removed.

The default configuration and values would appear as follows:

    "Dhcp6": {
         "dhcp-ddns": {
            // Connectivity parameters
            "enable-updates": false,
             "server-ip": "127.0.0.1",
             "server-port":53001,
             "sender-ip":"",
             "sender-port":0,
             "max-queue-size":1024,
             "ncr-protocol":"UDP",
             "ncr-format":"JSON"
         },

         // Behavioral parameters (global)
         "ddns-send-updates": true,
         "ddns-override-no-update": false,
         "ddns-override-client-update": false,
         "ddns-replace-client-name": "never",
         "ddns-generated-prefix": "myhost",
         "ddns-qualifying-suffix": "",
         "ddns-update-on-renew": false,
         "ddns-use-conflict-resolution": true,
         "hostname-char-set": "",
         "hostname-char-replacement": "",
         ...
    }

There are two parameters which determine if `kea-dhcp6` can generate DDNS requests to D2: the existing `dhcp-ddns:enable-updates` parameter, which now only controls whether `kea-dhcp6` connects to D2; and the new behavioral parameter, `ddns-send-updates`, which determines whether DDNS updates are enabled at a given level (i.e. global, shared-network, or subnet). The following table shows how the two parameters function together:

Enabling and disabling DDNS updates
dhcp-ddns: enable-updates | Global ddns-send-updates | Outcome
--- | --- | ---
false (default) | false | no updates at any scope
false | true (default) | no updates at any scope
true | false | updates only at scopes with a local value of `true` for `ddns-enable-updates`
true | true | updates at all scopes except those with a local value of `false` for `ddns-enable-updates`

Kea 1.9.1 added two new parameters; the first is `ddns-update-on-renew`. Normally, when leases are renewed, the server only updates DNS if the DNS information for the lease (e.g. FQDN, DNS update direction flags) has changed. Setting `ddns-update-on-renew` to `true` instructs the server to always update the DNS information when a lease is renewed, even if its DNS information has not changed. This allows Kea to "self-heal" if it was previously unable to add DNS entries or they were somehow lost by the DNS server.

Note

Setting `ddns-update-on-renew` to `true` may impact performance, especially for servers with numerous clients that renew often.

The second parameter added in Kea 1.9.1 is `ddns-use-conflict-resolution`. The value of this parameter is passed by `kea-dhcp6` to D2 with each DNS update request. When `true` (the default value), D2 employs conflict resolution, as described in RFC 4703, when attempting to fulfill the update request. When `false`, D2 simply attempts to update the DNS entries per the request, regardless of whether they conflict with existing entries owned by other DHCPv6 clients.

Note

Setting `ddns-use-conflict-resolution` to `false` disables the overwrite safeguards that the rules of conflict resolution (from RFC 4703) are intended to prevent. This means that existing entries for an FQDN or an IP address made for Client-A can be deleted or replaced by entries for Client-B. Furthermore, there are two scenarios by which entries for multiple clients for the same key (e.g. FQDN or IP) can be created.

1. Client-B uses the same FQDN as Client-A but a different IP address. In this case, the forward DNS entries (AAAA and DHCID RRs) for Client-A will be deleted as they match the FQDN and new entries for Client-B will be added. The reverse DNS entries (PTR and DHCID RRs) for Client-A, however, will not be deleted as they belong to a different IP address, while new entries for Client-B will still be added.

2. Client-B uses the same IP address as Client-A but a different FQDN. In this case the reverse DNS entries (PTR and DHCID RRs) for Client-A will be deleted as they match the IP address, and new entries for Client-B will be added. The forward DNS entries (AAAA and DHCID RRs) for Client-A, however, will not be deleted, as they belong to a different FQDN, while new entries for Client-B will still be added.

Disabling conflict resolution should be done only after careful review of specific use cases. The best way to avoid unwanted DNS entries is to always ensure lease changes are processed through Kea, whether they are released, expire, or are deleted via the `lease6-del` command, prior to reassigning either FQDNs or IP addresses. Doing so causes `kea-dhcp6` to generate DNS removal requests to D2.

The DNS entries Kea creates contain a value for TTL (time to live). `kea-dhcp6` calculates that value based on RFC 4702, Section 5, which suggests that the TTL value be 1/3 of the lease's lifetime, with a minimum value of 10 minutes.

The parameter `ddns-ttl-percent`, when specified, causes the TTL to be calculated as a simple percentage of the lease's lifetime, using the parameter's value as the percentage. It is specified as a decimal percent (e.g. .25, .75, 1.00) and may be specified at the global, shared-network, and subnet levels. By default it is unspecified.

#### 9.2.23.1. DHCP-DDNS Server Connectivity

For NCRs to reach the D2 server, `kea-dhcp6` must be able to communicate with it. `kea-dhcp6` uses the following configuration parameters to control this communication:

- `enable-updates` - Enables connectivity to `kea-dhcp-ddns` such that DDNS updates can be constructed and sent. It must be `true` for NCRs to be generated and sent to D2. It defaults to `false`.

- `server-ip` - This is the IP address on which D2 listens for requests. The default is the local loopback interface at address 127.0.0.1. Either an IPv4 or IPv6 address may be specified.

- `server-port` - This is the port on which D2 listens for requests. The default value is `53001`.

- `sender-ip` - This is the IP address which `kea-dhcp6` uses to send requests to D2. The default value is blank, which instructs `kea-dhcp6` to select a suitable address.

- `sender-port` - This is the port which `kea-dhcp6` uses to send requests to D2. The default value of `0` instructs `kea-dhcp6` to select a suitable port.

- `max-queue-size` - This is the maximum number of requests allowed to queue while waiting to be sent to D2. This value guards against requests accumulating uncontrollably if they are being generated faster than they can be delivered. If the number of requests queued for transmission reaches this value, DDNS updating is turned off until the queue backlog has been sufficiently reduced. The intent is to allow the `kea-dhcp4` server to continue lease operations without running the risk that its memory usage grows without limit. The default value is `1024`.

- `ncr-protocol` - This specifies the socket protocol to use when sending requests to D2. Currently only UDP is supported.

- `ncr-format` - This specifies the packet format to use when sending requests to D2. Currently only JSON format is supported.

By default, `kea-dhcp-ddns` is assumed to be running on the same machine as `kea-dhcp6`, and all of the default values mentioned above should be sufficient. If, however, D2 has been configured to listen on a different address or port, these values must be altered accordingly. For example, if D2 has been configured to listen on 2001:db8::5 port 900, the following configuration is required:

    "Dhcp6": {
        "dhcp-ddns": {
            "server-ip": "2001:db8::5",
            "server-port": 900,
            ...
        },
        ...
    }

#### 9.2.23.2. When Does the `kea-dhcp6` Server Generate a DDNS Request?

The `kea-dhcp6` server follows the behavior prescribed for DHCP servers in RFC 4704. It is important to keep in mind that `kea-dhcp6` makes the initial decision of when and what to update and forwards that information to D2 in the form of NCRs. Carrying out the actual DNS updates and dealing with such things as conflict resolution are within the purview of D2 itself (see The DHCP-DDNS Server). This section describes when `kea-dhcp6` generates NCRs and the configuration parameters that can be used to influence this decision. It assumes that the `enable-updates` parameter is `true`.

Note

Currently the interface between `kea-dhcp6` and D2 only supports requests which update DNS entries for a single IP address. If a lease grants more than one address, `kea-dhcp6` creates the DDNS update request for only the first of these addresses.

In general, `kea-dhcp6` generates DDNS update requests when:

1. A new lease is granted in response to a DHCPREQUEST;

2. An existing lease is renewed but the FQDN associated with it has changed; or

3. An existing lease is released in response to a DHCPRELEASE.

In the second case, lease renewal, two DDNS requests are issued: one request to remove entries for the previous FQDN, and a second request to add entries for the new FQDN. In the third case, a lease release - a single DDNS request - to remove its entries will be made.

As for the first case, the decisions involved when granting a new lease are more complex. When a new lease is granted, `kea-dhcp6` generates a DDNS update request only if the DHCPREQUEST contains the FQDN option (code 39). By default, `kea-dhcp6` respects the FQDN N and S flags specified by the client as shown in the following table:

Default FQDN flag behavior
Client Flags:N-S | Client Intent | Server Response | Server Flags:N-S-O
--- | --- | --- | ---
0-0 | Client wants to do forward updates, server should do reverse updates | Server generates reverse-only request | 1-0-0
0-1 | Server should do both forward and reverse updates | Server generates request to update both directions | 0-1-0
1-0 | Client wants no updates done | Server does not generate a request | 1-0-0

The first row in the table above represents "client delegation." Here the DHCP client states that it intends to do the forward DNS updates and the server should do the reverse updates. By default, `kea-dhcp6` honors the client's wishes and generates a DDNS request to D2 to update only reverse DNS data. The parameter `ddns-override-client-update` can be used to instruct the server to override client delegation requests. When this parameter is `true`, `kea-dhcp6` disregards requests for client delegation and generates a DDNS request to update both forward and reverse DNS data. In this case, the N-S-O flags in the server's response to the client will be 0-1-1 respectively.

(Note that the flag combination N=1, S=1 is prohibited according to RFC 4702. If such a combination is received from the client, the packet will be dropped by `kea-dhcp6`.)

To override client delegation, set the following values in the configuration file:

    "Dhcp6": {
        "ddns-override-client-update": true,
        ...
    }

The third row in the table above describes the case in which the client requests that no DNS updates be done. The parameter `ddns-override-no-update` can be used to instruct the server to disregard the client's wishes. When this parameter is `true`, `kea-dhcp6` generates DDNS update requests to `kea-dhcp-ddns` even if the client requests that no updates be done. The N-S-O flags in the server's response to the client will be 0-1-1.

To override client delegation, issue the following commands:

    "Dhcp6": {
        "ddns-override-no-update": true,
        ...
    }

The `kea-dhcp6` server always generates DDNS update requests if the client request only contains the Host Name option. In addition, it includes an FQDN option in the response to the client with the FQDN N-S-O flags set to 0-1-0, respectively. The domain name portion of the FQDN option is the name submitted to D2 in the DDNS update request.

#### 9.2.23.3. `kea-dhcp6` Name Generation for DDNS Update Requests

Each NameChangeRequest must of course include the fully qualified domain name whose DNS entries are to be affected. `kea-dhcp6` can be configured to supply a portion or all of that name, based upon what it receives from the client in the DHCPREQUEST.

The default rules for constructing the FQDN that will be used for DNS entries are:

1. If the DHCPREQUEST contains the client FQDN option, take the candidate name from there.

2. If the candidate name is a partial (i.e. unqualified) name, then add a configurable suffix to the name and use the result as the FQDN.

3. If the candidate name provided is empty, generate an FQDN using a configurable prefix and suffix.

4. If the client provides neither option, then take no DNS action.

These rules can be amended by setting the `ddns-replace-client-name` parameter, which provides the following modes of behavior:

- `never` - use the name the client sent. If the client sent no name, do not generate one. This is the default mode.

- `always` - replace the name the client sent. If the client sent no name, generate one for the client.

- `when-present` - replace the name the client sent. If the client sent no name, do not generate one.

- `when-not-present` - use the name the client sent. If the client sent no name, generate one for the client.

Note

In early versions of Kea, this parameter was a boolean and permitted only values of `true` and `false`. Boolean values have been deprecated and are no longer accepted. Administrators currently using booleans must replace them with the desired mode name. A value of `true` maps to `when-present`, while `false` maps to `never`.

For example, to instruct `kea-dhcp6` to always generate the FQDN for a client, set the parameter `ddns-replace-client-name` to `always` as follows:

    "Dhcp6": {
        "ddns-replace-client-name": "always",
        ...
    }

The prefix used in the generation of an FQDN is specified by the `ddns-generated-prefix` parameter. The default value is "myhost". To alter its value, simply set it to the desired string:

    "Dhcp6": {
        "ddns-generated-prefix": "another.host",
        ...
    }

The suffix used when generating an FQDN, or when qualifying a partial name, is specified by the `ddns-qualifying-suffix` parameter. This parameter has no default value; thus, it is mandatory when DDNS updates are enabled. To set its value simply set it to the desired string:

    "Dhcp6": {
        "ddns-qualifying-suffix": "foo.example.org",
        ...
    }

When qualifying a partial name, `kea-dhcp6` constructs the name in the format:

`[candidate-name].[ddns-qualifying-suffix].`

where `candidate-name` is the partial name supplied in the DHCPREQUEST. For example, if the FQDN domain name value is "some-computer" and the `ddns-qualifying-suffix` is "example.com", the generated FQDN is:

`some-computer.example.com.`

When generating the entire name, `kea-dhcp6` constructs the name in the format:

`[ddns-generated-prefix]-[address-text].[ddns-qualifying-suffix].`

where `address-text` is simply the lease IP address converted to a hyphenated string. For example, if the lease address is 3001:1::70E, the qualifying suffix is "example.com", and the default value is used for `ddns-generated-prefix`, the generated FQDN is:

`myhost-3001-1--70E.example.com.`

#### 9.2.23.4. Sanitizing Client FQDN Names

Some DHCP clients may provide values in the name component of the FQDN option (option code 39) that contain undesirable characters. It is possible to configure `kea-dhcp6` to sanitize these values. The most typical use case is ensuring that only characters that are permitted by RFC 1035 be included: A-Z, a-z, 0-9, and "-". This may be accomplished with the following two parameters:

- `hostname-char-set` - a regular expression describing the invalid character set. This can be any valid, regular expression using POSIX extended expression syntax. Embedded nulls (0x00) are always considered an invalid character to be replaced (or omitted). The default is `"[^A-Za-z0-9.-]"`. This matches any character that is not a letter, digit, dot, hyphen, or null.

- `hostname-char-replacement` - a string of zero or more characters with which to replace each invalid character in the host name. An empty string causes invalid characters to be OMITTED rather than replaced. The default is `""`.

The following configuration replaces anything other than a letter, digit, dot, or hyphen with the letter "x":

    "Dhcp6": {
        "hostname-char-set": "[^A-Za-z0-9.-]",
        "hostname-char-replacement": "x",
        ...
    }

Thus, a client-supplied value of "myhost-$[123.org" would become "myhost-xx123.org". Sanitizing is performed only on the portion of the name supplied by the client, and it is performed before applying a qualifying suffix (if one is defined and needed).

Note

Name sanitizing is meant to catch the more common cases of invalid characters through a relatively simple character-replacement scheme. It is difficult to devise a scheme that works well in all cases. Administrators who find they have clients with odd corner cases of character combinations that cannot be readily handled with this mechanism should consider writing a hook that can carry out sufficiently complex logic to address their needs.

Do not include dots in the `hostname-char-set` expression. When scrubbing FQDNs, dots are treated as delimiters and used to separate the option value into individual domain labels that are scrubbed and then re-assembled.

If clients are sending values that differ only by characters considered as invalid by the `hostname-char-set`, be aware that scrubbing them will yield identical values. In such cases, DDNS conflict rules will permit only one of them to register the name.

Finally, given the latitude clients have in the values they send, it is virtually impossible to guarantee that a combination of these two parameters will always yield a name that is valid for use in DNS. For example, using an empty value for `hostname-char-replacement` could yield an empty domain label within a name, if that label consists only of invalid characters.

Note

It is possible to specify `hostname-char-set` and/or `hostname-char-replacement` at the global scope. This allows host names to be sanitized without requiring a `dhcp-ddns` entry. When a `hostname-char` parameter is defined at both the global scope and in a `dhcp-ddns` entry, the second (local) value is used.

### 9.2.24. DHCPv4-over-DHCPv6: DHCPv6 Side

The support of DHCPv4-over-DHCPv6 transport is described in RFC 7341 and is implemented using cooperating DHCPv4 and DHCPv6 servers. This section is about the configuration of the DHCPv6 side (the DHCPv4 side is described in DHCPv4-over-DHCPv6: DHCPv4 Side).

Note

DHCPv4-over-DHCPv6 support is experimental and the details of the inter-process communication may change; for instance, the support of port relay (RFC 8357) introduced an incompatible change. Both the DHCPv4 and DHCPv6 sides should be running the same version of Kea.

There is only one specific parameter for the DHCPv6 side: `dhcp4o6-port`, which specifies the first of the two consecutive ports of the UDP sockets used for the communication between the DHCPv6 and DHCPv4 servers. The DHCPv6 server is bound to ::1 on `port` and connected to ::1 on `port` + 1.

Two other configuration entries are generally required: unicast traffic support (see Unicast Traffic Support) and the DHCP 4o6 server address option (name "dhcp4o6-server-addr", code 88).

ISC tested the following configuration:

    {

    # DHCPv6 conf
    "Dhcp6": {

        "interfaces-config": {
            "interfaces": [ "eno33554984/2001:db8:1:1::1" ]
        },

        "lease-database": {
            "type": "memfile",
            "name": "leases6"
        },

        "preferred-lifetime": 3000,
        "valid-lifetime": 4000,
        "renew-timer": 1000,
        "rebind-timer": 2000,

        "subnet6": [ {
            "id": 1,
            "subnet": "2001:db8:1:1::/64",
            "interface": "eno33554984",
            "pools": [ { "pool": "2001:db8:1:1::1:0/112" } ]
        } ],

        "dhcp4o6-port": 6767,

        "option-data": [ {
            "name": "dhcp4o6-server-addr",
            "code": 88,
            "space": "dhcp6",
            "csv-format": true,
            "data": "2001:db8:1:1::1"
        } ],

        "loggers": [ {
            "name": "kea-dhcp6",
            "output_options": [ {
                "output": "/tmp/kea-dhcp6.log"
            } ],
            "severity": "DEBUG",
            "debuglevel": 0
        } ]
    }

    }

Note

Relayed DHCPv4-QUERY DHCPv6 messages are not supported.

### 9.2.25. Sanity Checks in DHCPv6

An important aspect of a well-running DHCP system is an assurance that the data remains consistent; however, in some cases it may be convenient to tolerate certain inconsistent data. For example, a network administrator who temporarily removes a subnet from a configuration would not want all the leases associated with it to disappear from the lease database. Kea has a mechanism to implement sanity checks for situations like this.

Kea supports a configuration scope called `sanity-checks`. A parameter, called `lease-checks`, governs the verification carried out when a new lease is loaded from a lease file. This mechanism permits Kea to attempt to correct inconsistent data.

Every subnet has a `subnet-id` value; this is how Kea internally identifies subnets. Each lease has a `subnet-id` parameter as well, which identifies the subnet it belongs to. However, if the configuration has changed, it is possible that a lease could exist with a `subnet-id` but without any subnet that matches it. Also, it is possible that the subnet's configuration has changed and the `subnet-id` now belongs to a subnet that does not match the lease.

Kea's corrective algorithm first checks to see if there is a subnet with the `subnet-id` specified by the lease. If there is, it verifies whether the lease belongs to that subnet. If not, depending on the `lease-checks` setting, the lease is discarded, a warning is displayed, or a new subnet is selected for the lease that matches it topologically.

Since delegated prefixes do not have to belong to a subnet in which they are offered, there is no way to implement such a mechanism for IPv6 prefixes. As such, the mechanism works for IPv6 addresses only.

There are five levels which are supported:

- `none` - do no special checks; accept the lease as is.

- `warn` - if problems are detected display a warning, but accept the lease data anyway. This is the default value.

- `fix` - if a data inconsistency is discovered, try to correct it. If the correction is not successful, insert the incorrect data anyway.

- `fix-del` - if a data inconsistency is discovered, try to correct it. If the correction is not successful, reject the lease. This setting ensures the data's correctness, but some incorrect data may be lost. Use with care.

- `del` - if any inconsistency is detected, reject the lease. This is the strictest mode; use with care.

This feature is currently implemented for the memfile backend. The sanity check applies to the lease database in memory, not to the lease file, i.e. inconsistent leases will stay in the lease file.

An example configuration that sets this parameter looks as follows:

    "Dhcp6": {
        "sanity-checks": {
            "lease-checks": "fix-del"
        },
        ...
    }

### 9.2.26. Storing Extended Lease Information

To support such features as DHCPv6 Reconfigure (RFC 3315) and Leasequery (RFC 5007), additional information must be stored with each lease. Because the amount of information stored for each lease has ramifications in terms of performance and system resource consumption, storage of this additional information is configurable through the `store-extended-info` parameter. It defaults to `false` and may be set at the global, shared-network, and subnet levels.

    "Dhcp6": {
        "store-extended-info": true,
        ...
    }

When set to `true`, information relevant to the DHCPv6 query (e.g. REQUEST, RENEW, or REBIND) asking for the lease is added into the lease's `user-context` as a map element labeled "ISC". Currently, the information contained in the map is a list of relays, one for each relay message layer that encloses the client query. The lease's `user-context` for a two-hop query might look something like this (shown pretty-printed for clarity):

    {
        "ISC": {
            "relay-info": [
            {
                "hop": 3,
                "link": "2001:db8::1",
                "peer": "2001:db8::2"
            },
            {
                "hop": 2,
                "link": "2001:db8::3",
                "options": "0x00C800080102030405060708",
                "peer": "2001:db8::4"
            },
            {
                "hop": 1,
                "link": "2001:db8::5",
                "options": "0x00250006010203040506003500086464646464646464",
                "remote-id": "010203040506",
                "relay-id": "6464646464646464"
            }
            ]
        }
    }

Note

Prior to Kea version 2.3.2, this entry was named `relays`; remote and relay identifier options were not decoded.

Note

It is possible that other hook libraries are already using `user-context`. Enabling `store-extended-info` should not interfere with any other `user-context` content, as long as it does not also use an element labeled "ISC". In other words, `user-context` is intended to be a flexible container serving multiple purposes. As long as no other purpose also writes an "ISC" element to `user-context` there should not be a conflict.

Extended lease information is also subject to configurable sanity checking. The parameter in the `sanity-checks` scope is named `extended-info-checks` and supports these levels:

- `none` - do no check nor upgrade. This level should be used only when extended info is not used at all or when no badly formatted extended info, including using the old format, is expected.

- `fix` - fix some common inconsistencies and upgrade extended info using the old format to the new one. It is the default level and is convenient when the Leasequery hook library is not loaded.

- `strict` - fix all inconsistencies which have an impact on the (Bulk) Leasequery hook library.

- `pedantic` - enforce full conformance to the format produced by the Kea code; for instance, no extra entries are allowed with the exception of `comment`.

Note

This feature is currently implemented only for the memfile backend. The sanity check applies to the lease database in memory, not to the lease file, i.e. inconsistent leases stay in the lease file.

### 9.2.27. Multi-Threading Settings

The Kea server can be configured to process packets in parallel using multiple threads. These settings can be found under the `multi-threading` structure and are represented by:

- `enable-multi-threading` - use multiple threads to process packets in parallel. The default is `true`.

- `thread-pool-size` - specify the number of threads to process packets in parallel. It may be set to `0` (auto-detect), or any positive number that explicitly sets the thread count. The default is `0`.

- `packet-queue-size` - specify the size of the queue used by the thread pool to process packets. It may be set to `0` (unlimited), or any positive number that explicitly sets the queue size. The default is `64`.

An example configuration that sets these parameters looks as follows:

    "Dhcp6": {
        "multi-threading": {
           "enable-multi-threading": true,
           "thread-pool-size": 4,
           "packet-queue-size": 16
        },
        ...
    }

### 9.2.28. Multi-Threading Settings With Different Database Backends

The Kea DHCPv6 server is benchmarked by ISC to determine which settings give the best performance. Although this section describes our results, they are merely recommendations and are very dependent on the particular hardware used for benchmarking. We strongly advise that administrators run their own performance benchmarks.

A full report of performance results for the latest stable Kea version can be found here. This includes hardware and benchmark scenario descriptions, as well as current results.

After enabling multi-threading, the number of threads is set by the `thread-pool-size` parameter. Results from our experiments show that the best settings for `kea-dhcp6` are:

- `thread-pool-size`: 4 when using `memfile` for storing leases.

- `thread-pool-size`: 12 or more when using `mysql` for storing leases.

- `thread-pool-size`: 6 when using `postgresql`.

Another very important parameter is `packet-queue-size`; in our benchmarks we used it as a multiplier of `thread-pool-size`. The actual setting strongly depends on `thread-pool-size`.

We saw the best results in our benchmarks with the following settings:

- `packet-queue-size`: 150 * `thread-pool-size` when using `memfile` for storing leases; in our case it was 150 * 4 = 600. This means that at any given time, up to 600 packets could be queued.

- `packet-queue-size`: 200 * `thread-pool-size` when using `mysql` for storing leases; in our case it was 200 * 12 = 2400. This means that up to 2400 packets could be queued.

- `packet-queue-size`: 11 * `thread-pool-size` when using `postgresql` for storing leases; in our case it was 11 * 6 = 66.

### 9.2.29. Lease Caching

Clients that attempt multiple renewals in a short period can cause the server to update and write to the database frequently, resulting in a performance impact on the server. The cache parameters instruct the DHCP server to avoid updating leases too frequently, thus avoiding this behavior. Instead, the server assigns the same lease (i.e. reuses it) with no modifications except for CLTT (Client Last Transmission Time), which does not require disk operations.

The two parameters are the `cache-threshold` double and the `cache-max-age` integer; they have no default setting, i.e. the lease caching feature must be explicitly enabled. These parameters can be configured at the global, shared-network, and subnet levels. The subnet level has the precedence over the shared-network level, while the global level is used as a last resort. For example:

    {
    "subnet6": [
        {
            "subnet": "2001:db8:1:1::/64",
            "pools": [ { "pool": "2001:db8:1:1::1:0/112" } ],
            "cache-threshold": .25,
            "cache-max-age": 600,
            "valid-lifetime": 2000,
            ...
        }
    ],
    ...
    }

When an already-assigned lease can fulfill a client query:

> - any important change, e.g. for DDNS parameter, hostname, or preferred or valid lifetime reduction, makes the lease not reusable.

> - lease age, i.e. the difference between the creation or last modification time and the current time, is computed (elapsed duration).

> - if `cache-max-age` is explicitly configured, it is compared with the lease age; leases that are too old are not reusable. This means that the value 0 for `cache-max-age` disables the lease cache feature.

> - if `cache-threshold` is explicitly configured and is between 0.0 and 1.0, it expresses the percentage of the lease valid lifetime which is allowed for the lease age. Values below and including 0.0 and values greater than 1.0 disable the lease cache feature.

In our example, a lease with a valid lifetime of 2000 seconds can be reused if it was committed less than 500 seconds ago. With a lifetime of 3000 seconds, a maximum age of 600 seconds applies.

In outbound client responses (e.g. DHCPV6_REPLY messages), the used preferred and valid lifetimes are the reusable values, i.e. the expiration dates do not change.

## 9.3. Host Reservations in DHCPv6

There are many cases where it is useful to provide a configuration on a per-host basis. The most obvious one is to reserve a specific, static IPv6 address or/and prefix for exclusive use by a given client (host); the returning client receives the same address and/or prefix every time, and other clients will never get that address. Host reservations are also convenient when a host has specific requirements, e.g. a printer that needs additional DHCP options or a cable modem that needs specific parameters. Yet another possible use case is to define unique names for hosts.

There may be cases when a new reservation has been made for a client for an address or prefix currently in use by another client. We call this situation a "conflict." These conflicts get resolved automatically over time, as described in subsequent sections. Once a conflict is resolved, the correct client will receive the reserved configuration when it renews.

Host reservations are defined as parameters for each subnet. Each host must be identified by either DUID or its hardware/MAC address; see MAC/Hardware Addresses in DHCPv6 for details. There is an optional `reservations` array in the `subnet6` structure; each element in that array is a structure that holds information about reservations for a single host. In particular, the structure has an identifier that uniquely identifies a host. In the DHCPv6 context, the identifier is usually a DUID, but it can also be a hardware or MAC address. One or more addresses or prefixes may also be specified, and it is possible to specify a hostname and DHCPv6 options for a given host.

Note

The reserved address must be within the subnet. This does not apply to reserved prefixes.

The following example shows how to reserve addresses and prefixes for specific hosts:

    {
    "subnet6": [
        {
            "id": 1,
            "subnet": "2001:db8:1::/48",
            "pools": [ { "pool": "2001:db8:1::/80" } ],
            "pd-pools": [
                {
                    "prefix": "2001:db8:1:8000::",
                    "prefix-len": 56,
                    "delegated-len": 64
                }
            ],
            "reservations": [
                {
                    "duid": "01:02:03:04:05:0A:0B:0C:0D:0E",
                    "ip-addresses": [ "2001:db8:1::100" ]
                },
                {
                    "hw-address": "00:01:02:03:04:05",
                    "ip-addresses": [ "2001:db8:1::101", "2001:db8:1::102" ]
                },
                {
                    "duid": "01:02:03:04:05:06:07:08:09:0A",
                    "ip-addresses": [ "2001:db8:1::103" ],
                    "prefixes": [ "2001:db8:2:abcd::/64" ],
                    "hostname": "foo.example.com"
                }
            ]
        }
    ],
    ...
    }

This example includes reservations for three different clients. The first reservation is for the address 2001:db8:1::100, for a client using DUID 01:02:03:04:05:0A:0B:0C:0D:0E. The second reservation is for two addresses, 2001:db8:1::101 and 2001:db8:1::102, for a client using MAC address 00:01:02:03:04:05. Lastly, address 2001:db8:1::103 and prefix 2001:db8:2:abcd::/64 are reserved for a client using DUID 01:02:03:04:05:06:07:08:09:0A. The last reservation also assigns a hostname to this client.

DHCPv6 allows a single client to lease multiple addresses and multiple prefixes at the same time. Therefore `ip-addresses` and `prefixes` are plural and are actually arrays. When the client sends multiple IA options (IA_NA or IA_PD), each reserved address or prefix is assigned to an individual IA of the appropriate type. If the number of IAs of a specific type is lower than the number of reservations of that type, the number of reserved addresses or prefixes assigned to the client is equal to the number of IA_NAs or IA_PDs sent by the client; that is, some reserved addresses or prefixes are not assigned. However, they still remain reserved for this client and the server will not assign them to any other client. If the number of IAs of a specific type sent by the client is greater than the number of reserved addresses or prefixes, the server will try to assign all reserved addresses or prefixes to the individual IAs and dynamically allocate addresses or prefixes to the remaining IAs. If the server cannot assign a reserved address or prefix because it is in use, the server will select the next reserved address or prefix and try to assign it to the client. If the server subsequently finds that there are no more reservations that can be assigned to the client at that moment, the server will try to assign leases dynamically.

Making a reservation for a mobile host that may visit multiple subnets requires a separate host definition in each subnet that host is expected to visit. It is not possible to define multiple host definitions with the same hardware address in a single subnet. Multiple host definitions with the same hardware address are valid if each is in a different subnet. The reservation for a given host should include only one identifier, either DUID or hardware address; defining both for the same host is considered a configuration error.

Adding host reservations incurs a performance penalty. In principle, when a server that does not support host reservation responds to a query, it needs to check whether there is a lease for a given address being considered for allocation or renewal. The server that does support host reservation has to perform additional checks: not only whether the address is currently used (i.e., if there is a lease for it), but also whether the address could be used by someone else (i.e., if there is a reservation for it). That additional check incurs extra overhead.

### 9.3.1. Address/Prefix Reservation Types

In a typical Kea scenario there is an IPv6 subnet defined, with a certain part of it dedicated for dynamic address allocation by the DHCPv6 server. There may be an additional address space defined for prefix delegation. Those dynamic parts are referred to as dynamic pools, address and prefix pools, or simply pools. In principle, a host reservation can reserve any address or prefix that belongs to the subnet. The reservations that specify addresses that belong to configured pools are called "in-pool reservations." In contrast, those that do not belong to dynamic pools are called "out-of-pool reservations." There is no formal difference in the reservation syntax and both reservation types are handled uniformly.

Kea supports global host reservations. These are reservations that are specified at the global level within the configuration and that do not belong to any specific subnet. Kea still matches inbound client packets to a subnet as before, but when the subnet's reservation mode is set to "global", Kea looks for host reservations only among the global reservations defined. Typically, such reservations would be used to reserve hostnames for clients which may move from one subnet to another.

Note

Global reservations, while useful in certain circumstances, have aspects that must be given due consideration when using them. Please see Conflicts in DHCPv6 Reservations for more details.

Note

Since Kea 1.9.1, reservation mode has been replaced by three boolean flags, `reservations-global`, `reservations-in-subnet` and `reservations-out-of-pool`, which allow the configuration of host reservations both globally and in a subnet. In such cases a subnet host reservation has preference over a global reservation when both exist for the same client.

### 9.3.2. Conflicts in DHCPv6 Reservations

As reservations and lease information are stored separately, conflicts may arise. Consider the following series of events: the server has configured the dynamic pool of addresses from the range of 2001:db8::10 to 2001:db8::20. Host A requests an address and gets 2001:db8::10. Now the system administrator decides to reserve address 2001:db8::10 for Host B. In general, reserving an address that is currently assigned to someone else is not recommended, but there are valid use cases where such an operation is warranted.

The server now has a conflict to resolve. If Host B boots up and requests an address, the server cannot immediately assign the reserved address 2001:db8::10. A naive approach would to be immediately remove the lease for Host A and create a new one for Host B. That would not solve the problem, though, because as soon as Host B gets the address, it will detect that the address is already in use (by Host A) and will send a DHCPDECLINE message. Therefore, in this situation, the server has to temporarily assign a different address from the dynamic pool (not matching what has been reserved) to Host B.

When Host A renews its address, the server will discover that the address being renewed is now reserved for someone else - Host B. The server will remove the lease for 2001:db8::10, select a new address, and create a new lease for it. It will send two addresses in its response: the old address, with the lifetime set to 0 to explicitly indicate that it is no longer valid; and the new address, with a non-zero lifetime. When Host B tries to renew its temporarily assigned address, the server will detect that the existing lease does not match the reservation, so it will release the current address Host B has and will create a new lease matching the reservation. As before, the server will send two addresses: the temporarily assigned one with a zero lifetime, and the new one that matches the reservation with the proper lifetime set.

This recovery will succeed, even if other hosts attempt to get the reserved address. If Host C requests the address 2001:db8::10 after the reservation is made, the server will propose a different address.

This recovery mechanism allows the server to fully recover from a case where reservations conflict with existing leases; however, this procedure takes roughly as long as the value set for `renew-timer`. The best way to avoid such a recovery is not to define new reservations that conflict with existing leases. Another recommendation is to use out-of-pool reservations; if the reserved address does not belong to a pool, there is no way that other clients can get it.

Note

The conflict-resolution mechanism does not work for global reservations. Although the global address reservations feature may be useful in certain settings, it is generally recommended not to use global reservations for addresses. Administrators who do choose to use global reservations must manually ensure that the reserved addresses are not in dynamic pools.

### 9.3.3. Reserving a Hostname

When the reservation for a client includes the `hostname`, the server assigns this hostname to the client and sends it back in the Client FQDN option, if the client included the Client FQDN option in its message to the server. The reserved hostname always takes precedence over the hostname supplied by the client (via the FQDN option) or the autogenerated (from the IPv6 address) hostname.

The server qualifies the reserved hostname with the value of the `ddns-qualifying-suffix` parameter. For example, the following subnet configuration:

    {
    "subnet6": [
        {
            "id": 1,
            "subnet": "2001:db8:1::/48",
            "pools": [ { "pool": "2001:db8:1::/80" } ],
            "ddns-qualifying-suffix": "example.isc.org.",
            "reservations": [
                {
                    "duid": "01:02:03:04:05:0A:0B:0C:0D:0E",
                    "ip-addresses": [ "2001:db8:1::100" ],
                    "hostname": "alice-laptop"
                }
            ]
        }
    ],
    "dhcp-ddns": {
        "enable-updates": true
    },
    ...
    }

will result the "alice-laptop.example.isc.org." hostname being assigned to the client using the DUID "01:02:03:04:05:0A:0B:0C:0D:0E". If the `ddns-qualifying-suffix` is not specified, the default (empty) value will be used, and in this case the value specified as a `hostname` will be treated as a fully qualified name. Thus, by leaving the `ddns-qualifying-suffix` empty it is possible to qualify hostnames for different clients with different domain names:

    {
      "subnet6": [
        {
            "id": 1,
            "subnet": "2001:db8:1::/48",
            "pools": [ { "pool": "2001:db8:1::/80" } ],
            "reservations": [
                {
                    "duid": "01:02:03:04:05:0A:0B:0C:0D:0E",
                    "ip-addresses": [ "2001:db8:1::100" ],
                    "hostname": "mark-desktop.example.org."
                }
            ]
        }
      ],
      "dhcp-ddns": {
          "enable-updates": true
      }
    }

The above example results in the assignment of the "mark-desktop.example.org." hostname to the client using the DUID "01:02:03:04:05:0A:0B:0C:0D:0E".

### 9.3.4. Including Specific DHCPv6 Options in Reservations

Kea offers the ability to specify options on a per-host basis. These options follow the same rules as any other options. These can be standard options (see Standard DHCPv6 Options), custom options (see Custom DHCPv6 Options), or vendor-specific options (see DHCPv6 Vendor-Specific Options). The following example demonstrates how standard options can be defined.

    {
    "reservations": [
    {
        "duid": "01:02:03:05:06:07:08",
        "ip-addresses": [ "2001:db8:1::2" ],
        "option-data": [
        {
            "name": "dns-servers",
            "data": "3000:1::234"
        },
        {
            "name": "nis-servers",
            "data": "3000:1::234"
        },
        ...
        ],
        ...
    },
    ...
    ],
    ...
    }

Vendor-specific options can be reserved in a similar manner:

    {
    "reservations": [
    {
        "duid": "aa:bb:cc:dd:ee:ff",
        "ip-addresses": [ "2001:db8::1" ],
        "option-data": [
        {
            "name": "vendor-opts",
            "data": 4491
        },
        {
            "name": "tftp-servers",
            "space": "vendor-4491",
            "data": "3000:1::234"
        },
        ...
        ],
        ...
    },
    ...
    ],
    ...
    }

Options defined at the host level have the highest priority. In other words, if there are options defined with the same type on global, subnet, class, and host levels, the host-specific values are used.

### 9.3.5. Reserving Client Classes in DHCPv6

Using Expressions in Classification explains how to configure the server to assign classes to a client, based on the content of the options that this client sends to the server. Host reservation mechanisms also allow for the static assignment of classes to clients. The definitions of these classes are placed in the Kea configuration file or a database. The following configuration snippet shows how to specify that a client belongs to the classes `reserved-class1` and `reserved-class2`. Those classes are associated with specific options sent to the clients which belong to them.

    {
        "client-classes": [
        {
           "name": "reserved-class1",
           "option-data": [
           {
               "name": "dns-servers",
               "data": "2001:db8:1::50"
           }
           ]
        },
        {
           "name": "reserved-class2",
           "option-data": [
           {
               "name": "nis-servers",
               "data": "2001:db8:1::100"
           }
           ]
        }
        ],
        "subnet6": [
        {
            "id": 1,
            "pools": [ { "pool": "2001:db8:1::/64" } ],
            "subnet": "2001:db8:1::/48",
            "reservations": [
            {
                "duid": "01:02:03:04:05:06:07:08",

                "client-classes": [ "reserved-class1", "reserved-class2" ]

            }
            ]
        } ]
     }

In some cases the host reservations can be used in conjunction with client classes specified within the Kea configuration. In particular, when a host reservation exists for a client within a given subnet, the "KNOWN" built-in class is assigned to the client. Conversely, when there is no static assignment for the client, the "UNKNOWN" class is assigned to the client. Class expressions within the Kea configuration file can refer to "KNOWN" or "UNKNOWN" classes using the "member" operator. For example:

    {
        "client-classes": [
            {
                "name": "dependent-class",
                "test": "member('KNOWN')",
                "only-if-required": true
            }
        ]
    }

The `only-if-required` parameter is needed here to force evaluation of the class after the lease has been allocated and thus the reserved class has been also assigned.

Note

The classes specified in non-global host reservations are assigned to the processed packet after all classes with the `only-if-required` parameter set to `false` have been evaluated. This means that these classes must not depend on the statically assigned classes from the host reservations. If such a dependency is needed, the `only-if-required` must be set to `true` for the dependent classes. Such classes are evaluated after the static classes have been assigned to the packet. This, however, imposes additional configuration overhead, because all classes marked as `only-if-required` must be listed in the `require-client-classes` list for every subnet where they are used.

Note

Client classes specified within the Kea configuration file may depend on the classes specified within the global host reservations. In such a case the `only-if-required` parameter is not needed. Refer to the Pool Selection with Client Class Reservations and Subnet Selection with Client Class Reservations for specific use cases.

### 9.3.6. Storing Host Reservations in MySQL or PostgreSQL

Kea can store host reservations in MySQL or PostgreSQL. See Hosts Storage for information on how to configure Kea to use reservations stored in MySQL or PostgreSQL. Kea provides a dedicated hook for managing reservations in a database; section libdhcp_host_cmds.so: Host Commands provides detailed information. The Kea wiki provides some examples of how to conduct common host reservation operations.

Note

In Kea, the maximum length of an option specified per-host is arbitrarily set to 4096 bytes.

### 9.3.7. Fine-Tuning DHCPv6 Host Reservation

The host reservation capability introduces additional restrictions for the allocation engine (the component of Kea that selects an address for a client) during lease selection and renewal. In particular, three major checks are necessary. First, when selecting a new lease, it is not sufficient for a candidate lease to simply not be in use by another DHCP client; it also must not be reserved for another client. Similarly, when renewing a lease, an additional check must be performed to see whether the address being renewed is reserved for another client. Finally, when a host renews an address or a prefix, the server must check whether there is a reservation for this host, which would mean the existing (dynamically allocated) address should be revoked and the reserved one be used instead.

Some of those checks may be unnecessary in certain deployments, and not performing them may improve performance. The Kea server provides the `reservation-mode` configuration parameter to select the types of reservations allowed for a particular subnet. Each reservation type has different constraints for the checks to be performed by the server when allocating or renewing a lease for the client. Allowed values are:

- `all` - enables both in-pool and out-of-pool host reservation types. This setting is the default value, and is the safest and most flexible. However, as all checks are conducted, it is also the slowest. It does not check against global reservations.

- `out-of-pool` - allows only out-of-pool host reservations. With this setting in place, the server assumes that all host reservations are for addresses that do not belong to the dynamic pool. Therefore, it can skip the reservation checks when dealing with in-pool addresses, thus improving performance. Do not use this mode if any reservations use in-pool addresses. Caution is advised when using this setting; Kea does not sanity-check the reservations against `reservation-mode` and misconfiguration may cause problems.

- `global` - allows only global host reservations. With this setting in place, the server searches for reservations for a client only among the defined global reservations. If an address is specified, the server skips the reservation checks carried out in other modes, thus improving performance. Caution is advised when using this setting; Kea does not sanity-check the reservations when `global` is set, and misconfiguration may cause problems.

- `disabled` - host reservation support is disabled. As there are no reservations, the server skips all checks. Any reservations defined are completely ignored. As checks are skipped, the server may operate faster in this mode.

Since Kea 1.9.1, the `reservation-mode` parameter is replaced by the `reservations-global`, `reservations-in-subnet` and `reservations-out-of-pool` flags. The flags can be activated independently and can produce various combinations, some of them being unsupported by the deprecated `reservation-mode`.

The `reservation-mode` parameter can be specified at:

- global level: `.Dhcp6["reservation-mode"]` (lowest priority: gets overridden by all others)

- subnet level: `.Dhcp6.subnet6[]["reservation-mode"]` (low priority)

- shared-network level: `.Dhcp6["shared-networks"][]["reservation-mode"]` (high priority)

- shared-network subnet-level: `.Dhcp6["shared-networks"][].subnet6[]["reservation-mode"]` (highest priority: overrides all others)

To decide which `"reservation-mode"` to choose, the following decision diagram may be useful:

                                  O
                                  |
                                  v
    +-----------------------------+------------------------------+
    |         Is per-host configuration needed, such as          |
    |                reserving specific addresses,               |
    |               assigning specific options or                |
    | assigning packets to specific classes on per-device basis? |
    +-+-----------------+----------------------------------------+
      |                 |
    no|              yes|
      |                 |   +--------------------------------------+
      |                 |   |         For all given hosts,         |
      +--> "disabled"   +-->+      can the reserved resources      |
                            |  be used in all configured subnets?  |
                            +--------+---------------------------+-+
                                     |                           |
    +----------------------------+   |no                         |yes
    |             Is             |   |                           |
    |  at least one reservation  +<--+               "global" <--+
    | used to reserve addresses  |
    |        or prefixes?        |
    +-+------------------------+-+
      |                        |
    no|                     yes|   +---------------------------+
      |                        |   | Is high leases-per-second |
      +--> "out-of-pool"       +-->+ performance or efficient  |
            ^                      |      resource usage       |
            |                      |  (CPU ticks, RAM usage,   |
            |                      |   database roundtrips)    |
            |                      | important to your setup?  |
            |                      +-+----------------+--------+
            |                        |                |
            |                     yes|              no|
            |                        |                |
            |          +-------------+                |
            |          |                              |
            |          |   +----------------------+   |
            |          |   | Can it be guaranteed |   |
            |          +-->+  that the reserved   |   |
            |              |  addresses/prefixes  |   |
            |              |  aren't part of the  |   |
            |              |   pools configured   |   |
            |              |  in the respective   |   |
            |              |       subnet?        |   |
            |              +-+------------------+-+   |
            |                |                  |     |
            |             yes|                no|     |
            |                |                  |     V
            +----------------+                  +--> "all"

An example configuration that disables reservations looks as follows:

    {
      "Dhcp6": {
        "subnet6": [
          {
            "id": 1,
            "pools": [
              {
                "pool": "2001:db8:1::-2001:db8:1::100"
              }
            ],
            "reservation-mode": "disabled",
            "subnet": "2001:db8:1::/64"
          }
        ]
      }
    }

An example configuration using global reservations is shown below:

    {
      "Dhcp6": {
        "reservation-mode": "global",
        "reservations": [
          {
            "duid": "00:03:00:01:11:22:33:44:55:66",
            "hostname": "host-one"
          },
          {
            "duid": "00:03:00:01:99:88:77:66:55:44",
            "hostname": "host-two"
          }
        ],
        "subnet6": [
          {
            "id": 1,
            "pools": [
              {
                "pool": "2001:db8:1::-2001:db8:1::100"
              }
            ],
            "subnet": "2001:db8:1::/64"
          }
        ]
      }
    }

The meaning of the reservation flags are:

- `reservations-global`: fetch global reservations.

- `reservations-in-subnet`: fetch subnet reservations. For a shared network this includes all subnet members of the shared network.

- `reservations-out-of-pool`: this makes sense only when the `reservations-in-subnet` flag is `true`. When `reservations-out-of-pool` is `true`, the server assumes that all host reservations are for addresses that do not belong to the dynamic pool. Therefore, it can skip the reservation checks when dealing with in-pool addresses, thus improving performance. The server will not assign reserved addresses that are inside the dynamic pools to the respective clients. This also means that the addresses matching the respective reservations from inside the dynamic pools (if any) can be dynamically assigned to any client.

The `disabled` value from the deprecated `reservation-mode` corresponds to:

    {
      "Dhcp6": {
        "reservations-global": false,
        "reservations-in-subnet": false
      }
    }

The `global` value from the deprecated `reservation-mode` corresponds to:

    {
      "Dhcp6": {
        "reservations-global": true,
        "reservations-in-subnet": false
      }
    }

The `out-of-pool` value from the deprecated `reservation-mode` corresponds to:

    {
      "Dhcp6": {
        "reservations-global": false,
        "reservations-in-subnet": true,
        "reservations-out-of-pool": true
      }
    }

And the `all` value from the deprecated `reservation-mode` corresponds to:

    {
      "Dhcp6": {
        "reservations-global": false,
        "reservations-in-subnet": true,
        "reservations-out-of-pool": false
      }
    }

To activate both `global` and `all`, the following combination can be used:

    {
      "Dhcp6": {
        "reservations-global": true,
        "reservations-in-subnet": true,
        "reservations-out-of-pool": false
      }
    }

To activate both `global` and `out-of-pool`, the following combination can be used:

    {
      "Dhcp6": {
        "reservations-global": true,
        "reservations-in-subnet": true,
        "reservations-out-of-pool": true
      }
    }

Enabling `out-of-pool` and disabling `in-subnet` at the same time is not recommended because `out-of-pool` applies to host reservations in a subnet, which are fetched only when the `in-subnet` flag is `true`.

The parameter can be specified at the global, subnet, and shared-network levels.

An example configuration that disables reservations looks as follows:

    {
      "Dhcp6": {
        "subnet6": [
          {
            "reservations-global": false,
            "reservations-in-subnet": false,
            "subnet": "2001:db8:1::/64",
            "id": 1
          }
        ]
      }
    }

An example configuration using global reservations is shown below:

    {
      "Dhcp6": {
        "reservations": [
          {
            "duid": "00:03:00:01:11:22:33:44:55:66",
            "hostname": "host-one"
          },
          {
            "duid": "00:03:00:01:99:88:77:66:55:44",
            "hostname": "host-two"
          }
        ],
        "reservations-global": true,
        "reservations-in-subnet": false,
        "subnet6": [
          {
            "pools": [
              {
                "pool": "2001:db8:1::-2001:db8:1::100"
              }
            ],
            "subnet": "2001:db8:1::/64",
            "id": 1
          }
        ]
      }
    }

For more details regarding global reservations, see Global Reservations in DHCPv6.

Another aspect of host reservations is the different types of identifiers. Kea currently supports two types of identifiers in DHCPv6: hardware address and DUID. This is beneficial from a usability perspective; however, there is one drawback. For each incoming packet Kea has to extract each identifier type and then query the database to see if there is a reservation by this particular identifier. If nothing is found, the next identifier is extracted and the next query is issued. This process continues until either a reservation is found or all identifier types have been checked. Over time, with an increasing number of supported identifier types, Kea would become slower and slower.

To address this problem, a parameter called `host-reservation-identifiers` is available. It takes a list of identifier types as a parameter. Kea checks only those identifier types enumerated in `host-reservation-identifiers`. From a performance perspective, the number of identifier types should be kept to a minimum, ideally one. If the deployment uses several reservation types, please enumerate them from most- to least-frequently used, as this increases the chances of Kea finding the reservation using the fewest queries. An example of a `host-reservation-identifiers` configuration looks as follows:

    {
    "host-reservation-identifiers": [ "duid", "hw-address" ],
    "subnet6": [
        {
            "subnet": "2001:db8:1::/64",
            ...
        }
    ],
    ...
    }

If not specified, the default value is:

    "host-reservation-identifiers": [ "hw-address", "duid" ]

Note

As soon as a host reservation is found the search is stopped so when a client has two host reservations using different enabled identifier types the first is always returned and the second ignored. In other words, this is usually a a configuration mistake. In rare cases when having two reservations for the same host makes sense, you can control which of those will be used by ordering the list of identifier types in host-reservation-identifiers.

### 9.3.8. Global Reservations in DHCPv6

In some deployments, such as mobile, clients can roam within the network and certain parameters must be specified regardless of the client's current location. To meet such a need, Kea offers a global reservation mechanism. The idea behind it is that regular host reservations are tied to specific subnets, by using a specific subnet ID. Kea can specify a global reservation that can be used in every subnet that has global reservations enabled.

This feature can be used to assign certain parameters, such as hostname or other dedicated, host-specific options. It can also be used to assign addresses or prefixes.

An address assigned via global host reservation must be feasible for the subnet the server selects for the client. In other words, the address must lie within the subnet; otherwise, it is ignored and the server will attempt to dynamically allocate an address. If the selected subnet belongs to a shared network, the server checks for feasibility against the subnet's siblings, selecting the first in-range subnet. If no such subnet exists, the server falls back to dynamically allocating the address. This does not apply to globally reserved prefixes.

Note

Prior to release 2.3.5, the server did not perform feasibility checks on globally reserved addresses, which allowed the server to be configured to hand out nonsensical leases for arbitrary address values. Later versions of Kea perform these checks.

To use global host reservations, a configuration similar to the following can be used:

    "Dhcp6": {
        # This specifies global reservations.
        # They will apply to all subnets that
        # have global reservations enabled.

        "reservations": [
        {
           "hw-address": "aa:bb:cc:dd:ee:ff",
           "hostname": "hw-host-dynamic"
        },
        {
           "hw-address": "01:02:03:04:05:06",
           "hostname": "hw-host-fixed",

           # Use of IP addresses in global reservations is risky.
           # If used outside of matching subnet, such as 3001::/64,
           # it will result in a broken configuration being handed
           # to the client.
           "ip-address": "2001:db8:ff::77"
        },
        {
           "duid": "01:02:03:04:05",
           "hostname": "duid-host"
        }
        ],
        "valid-lifetime": 600,
        "subnet4": [ {
            "subnet": "2001:db8:1::/64",
            # It is replaced by the "reservations-global",
            # "reservations-in-subnet", and "reservations-out-of-pool"
            # parameters.
            # "reservation-mode": "global",
            # Specify if the server should look up global reservations.
            "reservations-global": true,
            # Specify if the server should look up in-subnet reservations.
            "reservations-in-subnet": false,
            # Specify if the server can assume that all reserved addresses
            # are out-of-pool. It can be ignored because "reservations-in-subnet"
            # is false.
            # "reservations-out-of-pool": false,
            "pools": [ { "pool": "2001:db8:1::-2001:db8:1::100" } ]
        } ]
    }

When using database backends, the global host reservations are distinguished from regular reservations by using a `subnet-id` value of 0.

### 9.3.9. Pool Selection with Client Class Reservations

Client classes can be specified both in the Kea configuration file and/or via host reservations. The classes specified in the Kea configuration file are evaluated immediately after receiving the DHCP packet and therefore can be used to influence subnet selection using the `client-class` parameter specified in the subnet scope. The classes specified within the host reservations are fetched and assigned to the packet after the server has already selected a subnet for the client. This means that the client class specified within a host reservation cannot be used to influence subnet assignment for this client, unless the subnet belongs to a shared network. If the subnet belongs to a shared network, the server may dynamically change the subnet assignment while trying to allocate a lease. If the subnet does not belong to a shared network, once selected, the subnet is not changed once selected.

If the subnet does not belong to a shared network, it is possible to use host reservation-based client classification to select an address pool within the subnet as follows:

    "Dhcp6": {
        "client-classes": [
            {
                "name": "reserved_class"
            },
            {
                "name": "unreserved_class",
                "test": "not member('reserved_class')"
            }
        ],
        "subnet6": [
            {
                "id": 1,
                "subnet": "2001:db8:1::/64",
                "reservations": [
                    {
                        "hw-address": "aa:bb:cc:dd:ee:fe",
                        "client-classes": [ "reserved_class" ]
                    }
                ],
                "pools": [
                    {
                        "pool": "2001:db8:1::10-2001:db8:1::20",
                        "client-class": "reserved_class"
                    },
                    {
                        "pool": "2001:db8:1::30-2001:db8:1::40",
                        "client-class": "unreserved_class"
                    }
                ]
            }
        ]
    }

The `reserved_class` is declared without the `test` parameter because it may be only assigned to the client via host reservation mechanism. The second class, `unreserved_class`, is assigned to clients which do not belong to the `reserved_class`. The first pool within the subnet is only used for clients having a reservation for the `reserved_class`. The second pool is used for clients not having such a reservation. The configuration snippet includes one host reservation which causes the client with the MAC address aa:bb:cc:dd:ee:fe to be assigned to the `reserved_class`. Thus, this client will be given an IP address from the first address pool.

### 9.3.10. Subnet Selection with Client Class Reservations

There is one specific use case when subnet selection may be influenced by client classes specified within host reservations: when the client belongs to a shared network. In such a case it is possible to use classification to select a subnet within this shared network. Consider the following example:

    "Dhcp6": {
        "client-classes": [
            {
                "name": "reserved_class"
            },
            {
                "name": "unreserved_class",
                "test": "not member('reserved_class')"
            }
        ],
        "reservations": [
            {
                "hw-address": "aa:bb:cc:dd:ee:fe",
                "client-classes": [ "reserved_class" ]
            }
        ],
        # It is replaced by the "reservations-global",
        # "reservations-in-subnet", and "reservations-out-of-pool" parameters.
        # Specify if the server should look up global reservations.
        "reservations-global": true,
        # Specify if the server should look up in-subnet reservations.
        "reservations-in-subnet": false,
        # Specify if the server can assume that all reserved addresses
        # are out-of-pool. It can be ignored because "reservations-in-subnet"
        # is false, but if specified, it is inherited by "shared-networks"
        # and "subnet6" levels.
        # "reservations-out-of-pool": false,
        "shared-networks": [
            {
            "subnet6": [
                {
                    "id": 1,
                    "subnet": "2001:db8:1::/64",
                    "pools": [
                        {
                            "pool": "2001:db8:1::10-2001:db8:1::20",
                            "client-class": "reserved_class"
                        }
                    ]
                },
                {
                    "id": 2,
                    "subnet": "2001:db8:2::/64",
                    "pools": [
                        {
                            "pool": "2001:db8:2::10-2001:db8:2::20",
                            "client-class": "unreserved_class"
                        }
                    ]
                }
            ]
            }
        ]
    }

This is similar to the example described in the Pool Selection with Client Class Reservations. This time, however, there are two subnets, each of which has a pool associated with a different class. The clients that do not have a reservation for the `reserved_class` are assigned an address from the subnet 2001:db8:2::/64. Clients with a reservation for the `reserved_class` are assigned an address from the subnet 2001:db8:1::/64. The subnets must belong to the same shared network. In addition, the reservation for the client class must be specified at the global scope (global reservation) and `reservations-global` must be set to `true`.

In the example above, the `client-class` could also be specified at the subnet level rather than the pool level, and would yield the same effect.

### 9.3.11. Multiple Reservations for the Same IP

Host reservations were designed to preclude the creation of multiple reservations for the same IP address or delegated prefix within a particular subnet, to avoid having two different clients compete for the same lease. When using the default settings, the server returns a configuration error when it finds two or more reservations for the same lease within a subnet in the Kea configuration file. `libdhcp_host_cmds.so` returns an error in response to the `reservation-add` command when it detects that the reservation exists in the database for the lease for which the new reservation is being added.

Similar to DHCPv4 (see Multiple Reservations for the Same IP), the DHCPv6 server can also be configured to allow the creation of multiple reservations for the same IPv6 address and/or delegated prefix in a given subnet. This is supported since Kea release 1.9.1 as an optional mode of operation enabled with the `ip-reservations-unique` global parameter.

The `ip-reservations-unique` is a boolean parameter that defaults to `true`, which forbids the specification of more than one reservation for the same lease in a given subnet. Setting this parameter to `false` allows such reservations to be created both in the Kea configuration file and in the host database backend, via `libdhcp_host_cmds.so`.

Setting `ip-reservations-unique` to `false` when using memfile, MySQL or PostgreSQL is supported. This setting is not supported when using Host Cache (see libdhcp_host_cache.so: Host Cache Reservations for Improved Performance), and the RADIUS backend (see libdhcp_radius.so: RADIUS Server Support). These reservation backends simply do not support multiple reservations for the same IP. If either of these hooks are loaded and `ip-reservations-unique` is set to `false`, then a configuration error will be emitted and the server will fail to start.

Note

When `ip-reservations-unique` is set to `true` (the default value), the server ensures that IP reservations are unique for a subnet within a single host backend and/or Kea configuration file. It does not guarantee that the reservations are unique across multiple backends.

The following is an example configuration with two reservations for the same IPv6 address but different MAC addresses:

    "Dhcp6": {
        "ip-reservations-unique": false,
        "subnet6": [
            {
                "id": 1,
                "subnet": "2001:db8:1::/64",
                "reservations": [
                    {
                        "hw-address": "1a:1b:1c:1d:1e:1f",
                        "ip-address": "2001:db8:1::11"
                    },
                    {
                        "hw-address": "2a:2b:2c:2d:2e:2f",
                        "ip-address": "2001:db8:1::11"
                    }
                ]
            }
        ]
    }

It is possible to control the `ip-reservations-unique` parameter via the Configuration Backend in DHCPv6. If the new setting of this parameter conflicts with the currently used backends (i.e. backends do not support the new setting), the new setting is ignored and a warning log message is generated. The backends continue to use the default setting, expecting that IP reservations are unique within each subnet. To allow the creation of non-unique IP reservations, the administrator must remove the backends which lack support for them from the configuration file.

Administrators must be careful when they have been using multiple reservations for the same IP address and/or delegated prefix and later decide to return to the default mode in which this is no longer allowed. They must make sure that at most one reservation for a given IP address or delegated prefix exists within a subnet, prior to switching back to the default mode. If such duplicates are left in the configuration file, the server reports a configuration error. Leaving such reservations in the host databases does not cause configuration errors but may lead to lease allocation errors during the server's operation, when it unexpectedly finds multiple reservations for the same IP address or delegated prefix.

Note

Currently the Kea server does not verify whether multiple reservations for the same IP address and/or delegated prefix exist in MySQL and/or PostgreSQL) host databases when `ip-reservations-unique` is updated from `true` to `false`. This may cause issues with lease allocations. The administrator must ensure that there is at most one reservation for each IP address and/or delegated prefix within each subnet, prior to the configuration update.

The `reservations-lookup-first` is a boolean parameter which controls whether host reservations lookup should be performed before lease lookup. This parameter has effect only when multi-threading is disabled. When multi-threading is enabled, host reservations lookup is always performed first to avoid lease-lookup resource locking. The `reservations-lookup-first` parameter defaults to `false` when multi-threading is disabled.

### 9.3.12. Host Reservations as Basic Access Control

Starting with Kea 2.3.5, it is possible to define a host reservation that contains just an identifier, without any address, options, or values. In some deployments this is useful, as the hosts that have a reservation belong to the KNOWN class while others do not. This can be used as a basic access control mechanism.

The following example demonstrates this concept. It indicates a single IPv6 subnet and all clients will get an address from it. However, only known clients (those that have reservations) will get their default DNS server configured. Empty reservations i.e. reservations that only have the identification criterion, can be specifically useful in this regard of making the clients known.

    "Dhcp6": {
        "client-classes": [
            {
                "name": "KNOWN",
                "option-data": [
                    {
                        "name": "dns-servers",
                        "data": "2001:db8::1"
                    }
                ]
            }
        ],
        "reservations": [
            // Clients on this list will be added to the KNOWN class.
            { "duid": "01:02:03:04:05:0A:0B:0C:0D:0E" },
            { "duid": "02:03:04:05:0A:0B:0C:0D:0E:0F" }
        ],
        "reservations-in-subnet": true,

        "subnet6": [
            {
                "id": 1,
                "subnet": "2001:db8:1::/48",
                "pools": [
                    {
                        "pool": "2001:db8:1:1::/64"
                    }
                ]
            }
        ]
    }

This concept can be extended further. A good real-life scenario might be a situation where some customers of an ISP have not paid their bills. A new class can be defined to use an alternative default DNS server that, instead of giving access to the Internet, redirects those customers to a captive portal urging them to bring their accounts up to date.

    "Dhcp6": {
        "client-classes": [
            {
                "name": "blocked",
                "option-data": [
                    {
                        "name": "dns-servers",
                        "data": "2001:db8::2"
                    }
                ]
            }
        ],
        "reservations": [
            // Clients on this list will be added to the KNOWN class. Some
            // will also be added to the blocked class.
            { "duid": "01:02:03:04:05:0A:0B:0C:0D:0E",
              "client-classes": [ "blocked" ] },
            { "duid": "02:03:04:05:0A:0B:0C:0D:0E:0F" }
        ],
        "reservations-in-subnet": true,

        "subnet6": [
            {
                "id": 1,
                "subnet": "2001:db8:1::/48",
                "pools": [
                    {
                        "pool": "2001:db8:1:1::/64"
                    }
                ],
                "option-data": [
                    {
                        "name": "dns-servers",
                        "data": "2001:db8::1"
                    }
                ]
            }
        ]
    }

## 9.4. Shared Networks in DHCPv6

DHCP servers use subnet information in two ways. It is used to both determine the point of attachment, i.e. where the client is connected to the network, and to group information pertaining to a specific location in the network. Sometimes it is useful to have more than one logical IP subnet being deployed on the same physical link. Understanding that two or more subnets are used on the same link requires additional logic in the DHCP server. This capability is called "shared networks" in Kea, and sometimes also "shared subnets"; in Microsoft's nomenclature it is called "multinet."

There are many cases where the shared networks feature is useful; here we explain just a handful of the most common ones. The first and by far most common use case is an existing IPv4 network that has grown and is running out of available address space. This is less common in IPv6, but shared networks are still useful: for example, with the exhaustion of IPv6- delegated prefixes within a subnet, or the desire to experiment with an addressing scheme. With the advent of IPv6 deployment and a vast address space, many organizations split the address space into subnets, deploy it, and then after a while discover that they want to split it differently. In the transition period, they want both the old and new addressing to be available: thus the need for more than one subnet on the same physical link.

Finally, the case of cable networks is directly applicable in IPv6. There are two types of devices in cable networks: cable modems and the end-user devices behind them. It is a common practice to use different subnets for cable modems to prevent users from tinkering with them. In this case, the distinction is based on the type of device, rather than on address-space exhaustion.

A client connected to a shared network may be assigned a lease (address or prefix) from any of the pools defined within the subnets belonging to the shared network. Internally, the server selects one of the subnets belonging to a shared network and tries to allocate a lease from this subnet. If the server is unable to allocate a lease from the selected subnet (e.g., due to pool exhaustion), it uses another subnet from the same shared network and tries to allocate a lease from this subnet. The server typically allocates all leases available in a given subnet before it starts allocating leases from other subnets belonging to the same shared network. However, in certain situations the client can be allocated a lease from another subnet before the pools in the first subnet get exhausted; this sometimes occurs when the client provides a hint that belongs to another subnet, or the client has reservations in a subnet other than the default.

Note

Deployments should not assume that Kea waits until it has allocated all the addresses from the first subnet in a shared network before allocating addresses from other subnets.

To define a shared network, an additional configuration scope is introduced:

    {
    "Dhcp6": {
        "shared-networks": [
            {
            # Name of the shared network. It may be an arbitrary string
            # and it must be unique among all shared networks.
            "name": "ipv6-lab-1",

            # The subnet selector can be specified on the shared network
            # level. Subnets from this shared network will be selected
            # for clients communicating via relay agent having
            # the specified IP address.
            "relay": {
                "ip-addresses": [ "2001:db8:2:34::1" ]
            },

            # This starts a list of subnets in this shared network.
            # There are two subnets in this example.
            "subnet6": [
                {
                    "id": 1,
                    "subnet": "2001:db8::/48",
                    "pools": [ { "pool":  "2001:db8::1 - 2001:db8::ffff" } ]
                },
                {
                    "id": 2,
                    "subnet": "3ffe:ffe::/64",
                    "pools": [ { "pool":  "3ffe:ffe::/64" } ]
                }
            ]
            }
        ],
        # end of shared-networks

        # It is likely that in the network there will be a mix of regular,
        # "plain" subnets and shared networks. It is perfectly valid
        # to mix them in the same configuration file.
        #
        # This is a regular subnet. It is not part of any shared-network.
        "subnet6": [
            {
                "id": 3,
                "subnet": "2001:db9::/48",
                "pools": [ { "pool":  "2001:db9::/64" } ],
                "relay": {
                    "ip-addresses": [ "2001:db8:1:2::1" ]
                }
            }
        ]
    }
    }

As demonstrated in the example, it is possible to mix shared and regular ("plain") subnets. Each shared network must have a unique name. This is similar to the ID for subnets, but gives administrators more flexibility. It is used for logging, but also internally for identifying shared networks.

In principle it makes sense to define only shared networks that consist of two or more subnets. However, for testing purposes, an empty subnet or a network with just a single subnet is allowed. This is not a recommended practice in production networks, as the shared network logic requires additional processing and thus lowers the server's performance. To avoid unnecessary performance degradation, shared subnets should only be defined when required by the deployment.

Shared networks provide an ability to specify many parameters in the shared network scope that apply to all subnets within it. If necessary, it is possible to specify a parameter in the shared-network scope and then override its value in the subnet scope. For example:

    {
    "shared-networks": [
        {
            "name": "lab-network3",
            "relay": {
                 "ip-addresses": [ "2001:db8:2:34::1" ]
            },

            # This applies to all subnets in this shared network, unless
            # values are overridden on subnet scope.
            "valid-lifetime": 600,

            # This option is made available to all subnets in this shared
            # network.
            "option-data": [ {
                "name": "dns-servers",
                "data": "2001:db8::8888"
            } ],

            "subnet6": [
                {
                    "id": 1,
                    "subnet": "2001:db8:1::/48",
                    "pools": [ { "pool":  "2001:db8:1::1 - 2001:db8:1::ffff" } ],

                    # This particular subnet uses different values.
                    "valid-lifetime": 1200,
                    "option-data": [
                    {
                        "name": "dns-servers",
                        "data": "2001:db8::1:2"
                    },
                    {
                        "name": "unicast",
                        "data": "2001:abcd::1"
                    } ]
                },
                {
                     "id": 2,
                     "subnet": "2001:db8:2::/48",
                     "pools": [ { "pool":  "2001:db8:2::1 - 2001:db8:2::ffff" } ],

                     # This subnet does not specify its own valid-lifetime value,
                     # so it is inherited from shared network scope.
                     "option-data": [
                     {
                         "name": "dns-servers",
                         "data": "2001:db8:cafe::1"
                     } ]
                }
            ]
        }
        ],
        ...
    }

In this example, there is a `dns-servers` option defined that is available to clients in both subnets in this shared network. Also, the valid lifetime is set to 10 minutes (600s). However, the first subnet overrides some of the values (the valid lifetime is 20 minutes, there is a different IP address for `dns-servers`), but also adds its own option (the unicast address). Assuming a client asking for server unicast and `dns-servers` options is assigned a lease from this subnet, it will get a lease for 20 minutes and `dns-servers`, and be allowed to use server unicast at address 2001:abcd::1. If the same client is assigned to the second subnet, it will get a 10-minute lease, a `dns-servers` value of 2001:db8:cafe::1, and no server unicast.

Some parameters must be the same in all subnets in the same shared network. This restriction applies to the `interface` and `rapid-commit` settings. The most convenient way is to define them on the shared-network scope, but they can be specified for each subnet. However, each subnet must have the same value.

Note

There is an inherent ambiguity when using clients that send multiple IA options in a single request, and shared-networks whose subnets have different values for options and configuration parameters. The server sequentially processes IA options in the order that they occur in the client's query; if the leases requested in the IA options end up being fulfilled from different subnets, which parameters and options should apply? Currently, the code uses the values from the last subnet of the last IA option fulfilled.

We view this largely as a site configuration issue. A shared network generally means the same physical link, so services configured by options from subnet A should be as easily reachable from subnet B and vice versa. There are a number of ways to avoid this situation:

- Use the same values for options and parameters for subnets within the shared network.

- Use subnet selectors or client class guards that ensure that for a single client's query, the same subnet is used for all IA options in that query.

- Avoid using shared networks with clients that send multiple IA options per query.

### 9.4.1. Local and Relayed Traffic in Shared Networks

It is possible to specify an interface name at the shared-network level, to tell the server that this specific shared network is reachable directly (not via relays) using the local network interface. As all subnets in a shared network are expected to be used on the same physical link, it is a configuration error to attempt to define a shared network using subnets that are reachable over different interfaces. In other words, all subnets within the shared network must have the same value for the `interface` parameter. The following configuration is an example of what NOT to do:

    {
    "shared-networks": [
        {
            "name": "office-floor-2",
            "subnet6": [
                {
                    "id": 1,
                    "subnet": "2001:db8::/64",
                    "pools": [ { "pool":  "2001:db8::1 - 2001:db8::ffff" } ],
                    "interface": "eth0"
                },
                {
                     "id": 2,
                     "subnet": "3ffe:abcd::/64",
                     "pools": [ { "pool":  "3ffe:abcd::1 - 3ffe:abcd::ffff" } ],
                     ...
                     # Specifying a different interface name is a configuration
                     # error. This value should rather be "eth0" or the interface
                     # name in the other subnet should be "eth1".
                     # "interface": "eth1"
                }
            ]
        }
    ],
    ...
    }

To minimize the chance of configuration errors, it is often more convenient to simply specify the interface name once, at the shared-network level, as shown in the example below.

    {
    "shared-networks": [
        {
            "name": "office-floor-2",

            # This tells Kea that the whole shared network is reachable over a
            # local interface. This applies to all subnets in this network.
            "interface": "eth0",

            "subnet6": [
                {
                    "id": 1,
                    "subnet": "2001:db8::/64",
                    "pools": [ { "pool":  "2001:db8::1 - 2001:db8::ffff" } ]
                },
                {
                     "id": 2,
                     "subnet": "3ffe:abcd::/64",
                     "pools": [ { "pool":  "3ffe:abcd::1 - 3ffe:abcd::ffff" } ]
                }
            ]
        }
    ],
    ...
    }

With relayed traffic, subnets are typically selected using the relay agents' addresses. If the subnets are used independently (not grouped within a shared network), a different relay address can be specified for each of these subnets. When multiple subnets belong to a shared network they must be selected via the same relay address and, similarly to the case of the local traffic described above, it is a configuration error to specify different relay addresses for the respective subnets in the shared network. The following configuration is another example of what NOT to do:

    {
    "shared-networks": [
        {
            "name": "kakapo",
            "subnet6": [
                {
                    "id": 1,
                    "subnet": "2001:db8::/64",
                    "relay": {
                        "ip-addresses": [ "2001:db8::1234" ]
                    },
                    "pools": [ { "pool":  "2001:db8::1 - 2001:db8::ffff" } ]
                },
                {
                    "id": 2,
                    "subnet": "3ffe:abcd::/64",
                    "pools": [ { "pool":  "3ffe:abcd::1 - 3ffe:abcd::ffff" } ],
                    "relay": {
                        # Specifying a different relay address for this
                        # subnet is a configuration error. In this case
                        # it should be 2001:db8::1234 or the relay address
                        # in the previous subnet should be 3ffe:abcd::cafe.
                        "ip-addresses": [ "3ffe:abcd::cafe" ]
                    }
                }
            ]
        }
    ],
    ...
    }

Again, it is better to specify the relay address at the shared-network level; this value will be inherited by all subnets belonging to the shared network.

    {
    "shared-networks": [
        {
            "name": "kakapo",
            "relay": {
                # This relay address is inherited by both subnets.
                "ip-addresses": [ "2001:db8::1234" ]
            },
            "subnet6": [
                {
                    "id": 1,
                    "subnet": "2001:db8::/64",
                    "pools": [ { "pool":  "2001:db8::1 - 2001:db8::ffff" } ]
                },
                {
                     "id": 2,
                     "subnet": "3ffe:abcd::/64",
                     "pools": [ { "pool":  "3ffe:abcd::1 - 3ffe:abcd::ffff" } ]
                }
            ]
        }
    ],
    ...
    }

Even though it is technically possible to configure two (or more) subnets within the shared network to use different relay addresses, this will almost always lead to a different behavior than what the user would expect. In this case, the Kea server will initially select one of the subnets by matching the relay address in the client's packet with the subnet's configuration. However, it MAY end up using the other subnet (even though it does not match the relay address) if the client already has a lease in this subnet or has a host reservation in this subnet, or simply if the initially selected subnet has no more addresses available. Therefore, it is strongly recommended to always specify subnet selectors (interface or relay address) at the shared-network level if the subnets belong to a shared network, as it is rarely useful to specify them at the subnet level and may lead to the configuration errors described above.

### 9.4.2. Client Classification in Shared Networks

Sometimes it is desirable to segregate clients into specific subnets based on certain properties. This mechanism is called client classification and is described in Client Classification. Client classification can be applied to subnets belonging to shared networks in the same way as it is used for subnets specified outside of shared networks. It is important to understand how the server selects subnets for clients when client classification is in use, to ensure that the appropriate subnet is selected for a given client type.

If a subnet is associated with a class, only the clients belonging to this class can use this subnet. If there are no classes specified for a subnet, any client connected to a given shared network can use this subnet. A common mistake is to assume that the subnet that includes a client class is preferred over subnets without client classes. Consider the following example:

    {
        "client-classes": [
            {
                "name": "b-devices",
                "test": "option[1234].hex == 0x0002"
            }
        ],
        "shared-networks": [
            {
                "name": "galah",
                "relay": {
                    "ip-address": [ "2001:db8:2:34::1" ]
                },
                "subnet6": [
                    {
                        "id": 1,
                        "subnet": "2001:db8:1::/64",
                        "pools": [ { "pool": "2001:db8:1::20 - 2001:db8:1::ff" } ]
                    },
                    {
                        "id": 2,
                        "subnet": "2001:db8:3::/64",
                        "pools": [ { "pool": "2001:db8:3::20 - 2001:db8:3::ff" } ],
                        "client-class": "b-devices"
                    }
                ]
            }
        ]
    }

If the client belongs to the "b-devices" class (because it includes option 1234 with a value of 0x0002), that does not guarantee that the subnet 2001:db8:3::/64 will be used (or preferred) for this client. The server can use either of the two subnets, because the subnet 2001:db8:1::/64 is also allowed for this client. The client classification used in this case should be perceived as a way to restrict access to certain subnets, rather than as a way to express subnet preference. For example, if the client does not belong to the "b-devices" class, it may only use the subnet 2001:db8:1::/64 and will never use the subnet 2001:db8:3::/64.

A typical use case for client classification is in a cable network, where cable modems should use one subnet and other devices should use another subnet within the same shared network. In this case it is necessary to apply classification on all subnets. The following example defines two classes of devices, and the subnet selection is made based on option 1234 values.

    {
        "client-classes": [
            {

                "name": "a-devices",
                "test": "option[1234].hex == 0x0001"
            },
            {
                "name": "b-devices",
                "test": "option[1234].hex == 0x0002"
            }
        ],
        "shared-networks": [
            {
                "name": "galah",
                "relay": {
                    "ip-addresses":  [ "2001:db8:2:34::1" ]
                },
                "subnet6": [
                    {
                        "id": 1,
                        "subnet": "2001:db8:1::/64",
                        "pools": [ { "pool": "2001:db8:1::20 - 2001:db8:1::ff" } ],
                        "client-class": "a-devices"
                    },
                    {
                        "id": 2,
                        "subnet": "2001:db8:3::/64",
                        "pools": [ { "pool": "2001:db8:3::20 - 2001:db8:3::ff" } ],
                        "client-class": "b-devices"
                    }
                ]
            }
        ]
    }

In this example each class has its own restriction. Only clients that belong to class "a-devices" are able to use subnet 2001:db8:1::/64 and only clients belonging to "b-devices" are able to use subnet 2001:db8:3::/64. Care should be taken not to define too-restrictive classification rules, as clients that are unable to use any subnets will be refused service. However, this may be a desired outcome if one wishes to provide service only to clients with known properties (e.g. only VoIP phones allowed on a given link).

It is possible to achieve an effect similar to the one presented in this section without the use of shared networks. If the subnets are placed in the global subnets scope, rather than in the shared network, the server will still use classification rules to pick the right subnet for a given class of devices. The major benefit of placing subnets within the shared network is that common parameters for the logically grouped subnets can be specified once, in the shared network scope, e.g. the `interface` or `relay` parameter. All subnets belonging to this shared network will inherit those parameters.

### 9.4.3. Host Reservations in Shared Networks

Subnets that are part of a shared network allow host reservations, similar to regular subnets:

    {
        "shared-networks": [
        {
            "name": "frog",
            "relay": {
                "ip-addresses": [ "2001:db8:2:34::1" ]
            },
            "subnet6": [
                {
                    "subnet": "2001:db8:1::/64",
                    "id": 100,
                    "pools": [ { "pool": "2001:db8:1::1 - 2001:db8:1::64" } ],
                    "reservations": [
                    {
                        "duid": "00:03:00:01:11:22:33:44:55:66",
                        "ip-addresses": [ "2001:db8:1::28" ]
                    }
                    ]
                },
                {
                    "subnet": "2001:db8:3::/64",
                    "id": 101,
                    "pools": [ { "pool": "2001:db8:3::1 - 2001:db8:3::64" } ],
                    "reservations": [
                        {
                            "duid": "00:03:00:01:aa:bb:cc:dd:ee:ff",
                            "ip-addresses": [ "2001:db8:2::28" ]
                        }
                    ]
                }
            ]
        }
        ]
    }

It is worth noting that Kea conducts additional checks when processing a packet if shared networks are defined. First, instead of simply checking whether there is a reservation for a given client in its initially selected subnet, Kea looks through all subnets in a shared network for a reservation. This is one of the reasons why defining a shared network may impact performance. If there is a reservation for a client in any subnet, that particular subnet is picked for the client. Although it is technically not an error, it is considered bad practice to define reservations for the same host in multiple subnets belonging to the same shared network.

While not strictly mandatory, it is strongly recommended to use explicit "id" values for subnets if database storage will be used for host reservations. If an ID is not specified, the values for it are auto generated, i.e. Kea assigns increasing integer values starting from 1. Thus, the auto-generated IDs are not stable across configuration changes.

## 9.5. Server Identifier in DHCPv6

The DHCPv6 protocol uses a "server identifier" (also known as a DUID) to allow clients to discriminate between several servers present on the same link. RFC 8415 currently defines four DUID types: DUID-LLT, DUID-EN, DUID-LL, and DUID-UUID.

The Kea DHCPv6 server generates a server identifier once, upon the first startup, and stores it in a file. This identifier is not modified across restarts of the server and so is a stable identifier.

Kea follows the recommendation from RFC 8415 to use DUID-LLT as the default server identifier. However, ISC has received reports that some deployments require different DUID types, and that there is a need to administratively select both the DUID type and/or its contents.

The server identifier can be configured using parameters within the `server-id` map element in the global scope of the Kea configuration file. The following example demonstrates how to select DUID-EN as a server identifier:

    "Dhcp6": {
        "server-id": {
            "type": "EN"
        },
        ...
    }

Currently supported values for the `type` parameter are: "LLT", "EN", and "LL", for DUID-LLT, DUID-EN, and DUID-LL respectively.

When a new DUID type is selected, the server generates its value and replaces any existing DUID in the file. The server then uses the new server identifier in all future interactions with clients.

Note

If the new server identifier is created after some clients have obtained their leases, the clients using the old identifier are not able to renew their leases; the server will ignore messages containing the old server identifier. Clients will continue sending RENEW until they transition to the rebinding state. In this state, they will start sending REBIND messages to the multicast address without a server identifier. The server will respond to the REBIND messages with a new server identifier, and the clients will associate the new server identifier with their leases. Although the clients will be able to keep their leases and will eventually learn the new server identifier, this will be at the cost of an increased number of renewals and multicast traffic due to a need to rebind. Therefore, it is recommended that modification of the server-identifier type and value be avoided if the server has already assigned leases and these leases are still valid.

There are cases when an administrator needs to explicitly specify a DUID value rather than allow the server to generate it. The following example demonstrates how to explicitly set all components of a DUID-LLT.

    "Dhcp6": {
        "server-id": {
            "type": "LLT",
            "htype": 8,
            "identifier": "A65DC7410F05",
            "time": 2518920166
        },
        ...
    }

where:

- `htype` is a 16-bit unsigned value specifying hardware type,

- `identifier` is a link-layer address, specified as a string of hexadecimal digits, and

- `time` is a 32-bit unsigned time value.

The hexadecimal representation of the DUID generated as a result of the configuration specified above is:

     00:01:00:08:96:23:AB:E6:A6:5D:C7:41:0F:05
    |type |htype|   time    |   identifier    |

A special value of "0" for `htype` and `time` is allowed, which indicates that the server should use ANY value for these components. If the server already uses a DUID-LLT, it will use the values from this DUID; if the server uses a DUID of a different type or does not yet use any DUID, it will generate these values. Similarly, if the `identifier` is assigned an empty string, the value of the `identifier` will be generated. Omitting any of these parameters is equivalent to setting them to those special values.

For example, the following configuration:

    "Dhcp6": {
        "server-id": {
            "type": "LLT",
            "htype": 0,
            "identifier": "",
            "time": 2518920166
        },
        ...
    }

indicates that the server should use ANY link-layer address and hardware type. If the server is already using DUID-LLT, it will use the link-layer address and hardware type from the existing DUID. If the server is not yet using any DUID, it will use the link-layer address and hardware type from one of the available network interfaces. The server will use an explicit value of time; if it is different than a time value present in the currently used DUID, that value will be replaced, effectively modifying the current server identifier.

The following example demonstrates an explicit configuration of a DUID-EN:

    "Dhcp6": {
        "server-id": {
            "type": "EN",
            "enterprise-id": 2495,
            "identifier": "87ABEF7A5BB545"
        },
        ...
    }

where:

- `enterprise-id` is a 32-bit unsigned value holding an enterprise number, and

- `identifier` is a variable- length identifier within DUID-EN.

The hexadecimal representation of the DUID-EN created according to the configuration above is:

     00:02:00:00:09:BF:87:AB:EF:7A:5B:B5:45
    |type |  ent-id   |     identifier     |

As in the case of the DUID-LLT, special values can be used for the configuration of the DUID-EN. If the `enterprise-id` is "0", the server will use a value from the existing DUID-EN. If the server is not using any DUID or the existing DUID has a different type, the ISC enterprise ID will be used. When an empty string is entered for `identifier`, the identifier from the existing DUID-EN will be used. If the server is not using any DUID-EN, a new 6-byte-long `identifier` will be generated.

DUID-LL is configured in the same way as DUID-LLT except that the `time` parameter has no effect for DUID-LL, because this DUID type only comprises a hardware type and link-layer address. The following example demonstrates how to configure DUID-LL:

    "Dhcp6": {
        "server-id": {
            "type": "LL",
            "htype": 8,
            "identifier": "A65DC7410F05"
        },
        ...
    }

which will result in the following server identifier:

     00:03:00:08:A6:5D:C7:41:0F:05
    |type |htype|   identifier    |

The server stores the generated server identifier in the following location: `[kea-install-dir]/var/lib/kea/kea-dhcp6-serverid`.

In some uncommon deployments where no stable storage is available, the server should be configured not to try to store the server identifier. This choice is controlled by the value of the `persist` boolean parameter:

    "Dhcp6": {
        "server-id": {
            "type": "EN",
            "enterprise-id": 2495,
            "identifier": "87ABEF7A5BB545",
            "persist": false
        },
        ...
    }

The default value of the `persist` parameter is `true`, which configures the server to store the server identifier on a disk.

In the example above, the server is configured not to store the generated server identifier on a disk. But if the server identifier is not modified in the configuration, the same value is used after server restart, because the entire server identifier is explicitly specified in the configuration.

## 9.6. DHCPv6 Data Directory

The Kea DHCPv6 server puts the server identifier file and the default memory lease file into its data directory. By default this directory is `prefix/var/lib/kea` but this location can be changed using the `data-directory` global parameter, as in:

    "Dhcp6": {
        "data-directory": "/var/tmp/kea-server6",
        ...
    }

## 9.7. Stateless DHCPv6 (INFORMATION-REQUEST Message)

Typically DHCPv6 is used to assign both addresses and options. These assignments (leases) have a state that changes over time, hence their description as "stateful." DHCPv6 also supports a "stateless" mode, where clients request only configuration options. This mode is considered lightweight from the server perspective, as it does not require any state tracking.

The Kea server supports stateless mode. When clients send INFORMATION-REQUEST messages, the server sends back answers with the requested options, if they are available in the server configuration. The server attempts to use per-subnet options first; if that fails, it then tries to provide options defined in the global scope.

Stateless and stateful mode can be used together. No special configuration directives are required to handle this; simply use the configuration for stateful clients and the stateless clients will get only the options they requested.

It is possible to run a server that provides only options and no addresses or prefixes. If the options have the same value in each subnet, the configuration can define the required options in the global scope and skip subnet definitions altogether. Here's a simple example of such a configuration:

    "Dhcp6": {
        "interfaces-config": {
            "interfaces": [ "ethX" ]
        },
        "option-data": [ {
            "name": "dns-servers",
            "data": "2001:db8::1, 2001:db8::2"
        } ],
        "lease-database": {
            "type": "memfile"
        }
     }

This very simple configuration provides DNS server information to all clients in the network, regardless of their location. The memfile lease database must be specified, as Kea requires a lease database to be specified even if it is not used.

## 9.8. Support for RFC 7550 (now part of RFC 8415)

RFC 7550 introduced some changes to the previous DHCPv6 specifications, RFC 3315 and RFC 3633, to resolve issues with the coexistence of multiple stateful options in the messages sent between clients and servers. Those changes were later included in the most recent DHCPv6 protocol specification, RFC 8415, which obsoleted RFC 7550. Kea supports RFC 8415 along with these protocol changes, which are briefly described below.

When a client, such as a requesting router, requests an allocation of both addresses and prefixes during the 4-way (SARR) exchange with the server, and the server is not configured to allocate any prefixes but can allocate some addresses, it will respond with the IA_NA(s) containing allocated addresses and the IA_PD(s) containing the NoPrefixAvail status code. According to the updated specifications, if the client can operate without prefixes it should accept allocated addresses and transition to the "bound" state. When the client subsequently sends RENEW/REBIND messages to the server to extend the lifetimes of the allocated addresses, according to the T1 and T2 times, and if the client is still interested in obtaining prefixes from the server, it may also include an IA_PD in the RENEW/REBIND to request allocation of the prefixes. If the server still cannot allocate the prefixes, it will respond with the IA_PD(s) containing the NoPrefixAvail status code. However, if the server can allocate the prefixes, it allocates and sends them in the IA_PD(s) to the client. A similar situation occurs when the server is unable to allocate addresses for the client but can delegate prefixes: the client may request allocation of the addresses while renewing the delegated prefixes. Allocating leases for other IA types while renewing existing leases is by default supported by the Kea DHCPv6 server, and the server provides no configuration mechanisms to disable this behavior.

The following are the other behaviors first introduced in RFC 7550 (now part of RFC 8415) and supported by the Kea DHCPv6 server:

- Set T1/T2 timers to the same value for all stateful (IA_NA and IA_PD) options to facilitate renewal of all of a client's leases at the same time (in a single message exchange).

- Place NoAddrsAvail and NoPrefixAvail status codes in the IA_NA and IA_PD options in the ADVERTISE message, rather than as the top-level options.

## 9.9. Using a Specific Relay Agent for a Subnet

The DHCPv6 server follows the same principles as the DHCPv4 server to select a subnet for the client, with noticeable differences mainly for relays.

Note

When the selected subnet is a member of a shared network, the whole shared network is selected.

A relay must have an interface connected to the link on which the clients are being configured. Typically the relay has a global IPv6 address configured on that interface, which belongs to the subnet from which the server assigns addresses. Normally, the server is able to use the IPv6 address inserted by the relay (in the `link-addr` field in the RELAY-FORW message) to select the appropriate subnet.

However, that is not always the case; the relay address may not match the subnet in certain deployments. This usually means that there is more than one subnet allocated for a given link. The two most common examples of this are long-lasting network renumbering (where both the old and new address spaces are still being used) and a cable network. In a cable network, both cable modems and the devices behind them are physically connected to the same link, yet they use distinct addressing. In such a case, the DHCPv6 server needs additional information (the value of the `interface-id` option or the IPv6 address inserted in the `link-addr` field in the RELAY-FORW message) to properly select an appropriate subnet.

The following example assumes that there is a subnet 2001:db8:1::/64 that is accessible via a relay that uses 3000::1 as its IPv6 address. The server is able to select this subnet for any incoming packets that come from a relay that has an address in the 2001:db8:1::/64 subnet. It also selects that subnet for a relay with address 3000::1.

    "Dhcp6": {
        "subnet6": [
            {
                "id": 1,
                "subnet": "2001:db8:1::/64",
                "pools": [
                     {
                         "pool": "2001:db8:1::1-2001:db8:1::ffff"
                     }
                ],
                "relay": {
                    "ip-addresses": [ "3000::1" ]
                }
            }
        ]
    }

If `relay` is specified, the `ip-addresses` parameter within it is mandatory. The `ip-addresses` parameter supports specifying a list of addresses.

## 9.10. Segregating IPv6 Clients in a Cable Network

In certain cases, it is useful to mix relay address information (introduced in Using a Specific Relay Agent for a Subnet) with client classification (explained in Client Classification). One specific example is in a cable network, where modems typically get addresses from a different subnet than all the devices connected behind them.

Let us assume that there is one Cable Modem Termination System (CMTS) with one CM MAC (a physical link that modems are connected to). We want the modems to get addresses from the 3000::/64 subnet, while everything connected behind the modems should get addresses from the 2001:db8:1::/64 subnet. The CMTS that acts as a relay uses address 3000::1. The following configuration can serve that situation:

    "Dhcp6": {
        "subnet6": [
            {
                "id": 1,
                "subnet": "3000::/64",
                "pools": [
                    { "pool": "3000::2 - 3000::ffff" }
                ],
                "client-class": "VENDOR_CLASS_docsis3.0",
                "relay": {
                    "ip-addresses": [ "3000::1" ]
                }
            },
            {
                "id": 2,
                "subnet": "2001:db8:1::/64",
                "pools": [
                     {
                         "pool": "2001:db8:1::1-2001:db8:1::ffff"
                     }
                ],
                "relay": {
                    "ip-addresses": [ "3000::1" ]
                }
            }
        ]
    }

## 9.11. MAC/Hardware Addresses in DHCPv6

MAC/hardware addresses are available in DHCPv4 messages from clients, and administrators frequently use that information to perform certain tasks like per-host configuration and address reservation for specific MAC addresses. Unfortunately, the DHCPv6 protocol does not provide any completely reliable way to retrieve that information. To mitigate that issue, a number of mechanisms have been implemented in Kea. Each of these mechanisms works in certain cases, but may not in others. Whether the mechanism works in a particular deployment is somewhat dependent on the network topology and the technologies used.

Kea allows specification of which of the supported methods should be used and in what order, via the `mac-sources` parameter. This configuration may be considered a fine tuning of the DHCP deployment.

Here is an example:

    "Dhcp6": {
        "mac-sources": [
            "method1",
            "method2",
            "method3",
            ...
        ],

        "subnet6": [
            {
                ...
            },
            ...
        ],
        ...
    }

When not specified, a value of "any" is used, which instructs the server to attempt to try all the methods in sequence and use the value returned by the first one that succeeds. In a typical deployment the default value of "any" is sufficient and there is no need to select specific methods. Changing the value of this parameter is most useful in cases when an administrator wants to disable certain methods; for example, if the administrator trusts the network infrastructure more than the information provided by the clients themselves, they may prefer information provided by the relays over that provided by clients.

If specified, `mac-sources` must have at least one value.

Supported methods are:

- `any` - this is not an actual method, just a keyword that instructs Kea to try all other methods and use the first one that succeeds. This is the default operation if no `mac-sources` are defined.

- `raw` - in principle, a DHCPv6 server could use raw sockets to receive incoming traffic and extract MAC/hardware address information. This is currently not implemented for DHCPv6 and this value has no effect.

- `duid` - DHCPv6 uses DUID identifiers instead of MAC addresses. There are currently four DUID types defined, and two of them (DUID-LLT, which is the default, and DUID-LL) convey MAC address information. Although RFC 8415 forbids it, it is possible to parse those DUIDs and extract necessary information from them. This method is not completely reliable, as clients may use other DUID types, namely DUID-EN or DUID-UUID.

- `ipv6-link-local` - another possible acquisition method comes from the source IPv6 address. In typical usage, clients are sending their packets from IPv6 link-local addresses. There is a good chance that those addresses are based on EUI-64, which contains a MAC address. This method is not completely reliable, as clients may use other link-local address types. In particular, privacy extensions, defined in RFC 4941, do not use MAC addresses. Also note that successful extraction requires that the address's u-bit must be set to "1" and its g-bit set to "0", indicating that it is an interface identifier as per RFC 2373, section 2.5.1.

- `client-link-addr-option` - one extension defined to alleviate missing MAC issues is the client link-layer address option, defined in RFC 6939. This is an option that is inserted by a relay and contains information about a client's MAC address. This method requires a relay agent that supports the option and is configured to insert it. This method is useless for directly connected clients. The value `rfc6939` is an alias for `client-link-addr-option`.

- `remote-id` - RFC 4649 defines a `remote-id` option that is inserted by a relay agent. Depending on the relay agent configuration, the inserted option may convey the client's MAC address information. The value `rfc4649` is an alias for `remote-id`.

- `subscriber-id` - Defined in RFC 4580, `subscriber-id` is somewhat similar to `remote-id`; it is also inserted by a relay agent. The value `rfc4580` is an alias for `subscriber-id`. This method is currently not implemented.

- `docsis-cmts` - Yet another possible source of MAC address information are the DOCSIS options inserted by a CMTS that acts as a DHCPv6 relay agent in cable networks. This method attempts to extract MAC address information from sub-option 1026 (cm mac) of the vendor-specific option with `vendor-id=4491`. This vendor option is extracted from the Relay-forward message, not the original client's message.

- `docsis-modem` - The final possible source of MAC address information are the DOCSIS options inserted by the cable modem itself. This method attempts to extract MAC address information from sub-option 36 (`device-id`) of the vendor-specific option with `vendor-id=4491`. This vendor option is extracted from the original client's message, not from any relay options.

An empty `mac-sources` parameter is not allowed. Administrators who do not want to specify it should either simply omit the `mac-sources` definition or specify it with the "any" value, which is the default.

## 9.12. Duplicate Addresses (DHCPDECLINE Support)

The DHCPv6 server is configured with a certain pool of addresses that it is expected to hand out to DHCPv6 clients. It is assumed that the server is authoritative and has complete jurisdiction over those addresses. However, for various reasons such as misconfiguration or a faulty client implementation that retains its address beyond the valid lifetime, there may be devices connected that use those addresses without the server's approval or knowledge.

Such an unwelcome event can be detected by legitimate clients (using Duplicate Address Detection) and reported to the DHCPv6 server using a DHCPDECLINE message. The server does a sanity check (to see whether the client declining an address really was supposed to use it), then conducts a clean-up operation, and confirms the DHCPDECLINE by sending back a REPLY message. Any DNS entries related to that address are removed, the event is logged, and hooks are triggered. After that is complete, the address is marked as declined (which indicates that it is used by an unknown entity and thus not available for assignment) and a probation time is set on it. Unless otherwise configured, the probation period lasts 24 hours; after that time, the server will recover the lease (i.e. put it back into the available state) and the address will be available for assignment again. It should be noted that if the underlying issue of a misconfigured device is not resolved, the duplicate-address scenario will repeat. If reconfigured correctly, this mechanism provides an opportunity to recover from such an event automatically, without any system administrator intervention.

To configure the decline probation period to a value other than the default, the following syntax can be used:

    "Dhcp6": {
        "decline-probation-period": 3600,
        "subnet6": [
            {
                ...
            },
            ...
        ],
        ...
    }

The parameter is expressed in seconds, so the example above instructs the server to recycle declined leases after one hour.

There are several statistics and hook points associated with the decline handling procedure. The `lease6_decline` hook point is triggered after the incoming DHCPDECLINE message has been sanitized and the server is about to decline the lease. The `declined-addresses` statistic is increased after the hook returns (both the global and subnet-specific variants). (See Statistics in the DHCPv6 Server and Hook Libraries for more details on DHCPv6 statistics and Kea hook points.)

Once the probation time elapses, the declined lease is recovered using the standard expired-lease reclamation procedure, with several additional steps. In particular, both `declined-addresses` statistics (global and subnet-specific) are decreased. At the same time, `reclaimed-declined-addresses` statistics (again in two variants, global and subnet-specific) are increased.

A note about statistics: The Kea server does not decrease the `assigned-nas` statistics when a DHCPDECLINE message is received and processed successfully. While technically a declined address is no longer assigned, the primary usage of the `assigned-nas` statistic is to monitor pool utilization. Most people would forget to include `declined-addresses` in the calculation, and would simply use `assigned-nas`/`total-nas`. This would cause a bias towards under-representing pool utilization. As this has a potential to cause serious confusion, ISC decided not to decrease `assigned-nas` immediately after receiving DHCPDECLINE, but to do it later when Kea recovers the address back to the available pool.

## 9.13. Statistics in the DHCPv6 Server

The DHCPv6 server supports the following statistics:

DHCPv6 statistics
Statistic | Data Type | Description
--- | --- | ---
pkt6-received | integer | Number of DHCPv6 packets received. This includes all packets: valid, bogus, corrupted, rejected, etc. This statistic is expected to grow rapidly.
pkt6-receive-drop | integer | Number of incoming packets that were dropped. The exact reason for dropping packets is logged, but the most common reasons may be: an unacceptable or not supported packet type is received, direct responses are forbidden, the server-id sent by the client does not match the server's server-id, or the packet is malformed.
pkt6-parse-failed | integer | Number of incoming packets that could not be parsed. A non-zero value of this statistic indicates that the server received a malformed or truncated packet. This may indicate problems in the network, faulty clients, faulty relay agents, or a bug in the server.
pkt6-solicit-received | integer | Number of SOLICIT packets received. This statistic is expected to grow; its increase means that clients that just booted started their configuration process and their initial packets reached the Kea server.
pkt6-advertise-received | integer | Number of ADVERTISE packets received. ADVERTISE packets are sent by the server and the server is never expected to receive them. A non-zero value of this statistic indicates an error occurring in the network. One likely cause would be a misbehaving relay agent that incorrectly forwards ADVERTISE messages towards the server, rather than back to the clients.
pkt6-request-received | integer | Number of DHCPREQUEST packets received. This statistic is expected to grow. Its increase means that clients that just booted received the server's response (DHCPADVERTISE) and accepted it, and are now requesting an address (DHCPREQUEST).
pkt6-reply-received | integer | Number of REPLY packets received. This statistic is expected to remain zero at all times, as REPLY packets are sent by the server and the server is never expected to receive them. A non-zero value indicates an error. One likely cause would be a misbehaving relay agent that incorrectly forwards REPLY messages towards the server, rather than back to the clients.
pkt6-renew-received | integer | Number of RENEW packets received. This statistic is expected to grow; its increase means that clients received their addresses and prefixes and are trying to renew them.
pkt6-rebind-received | integer | Number of REBIND packets received. A non-zero value indicates that clients did not receive responses to their RENEW messages (through the regular lease-renewal mechanism) and are attempting to find any server that is able to take over their leases. It may mean that some servers' REPLY messages never reached the clients.
pkt6-release-received | integer | Number of RELEASE packets received. This statistic is expected to grow when a device is being shut down in the network; it indicates that the address or prefix assigned is reported as no longer needed. Note that many devices, especially wireless, do not send RELEASE packets either because of design choice or due to the client moving out of range.
pkt6-decline-received | integer | Number of DECLINE packets received. This statistic is expected to remain close to zero. Its increase means that a client leased an address, but discovered that the address is currently used by an unknown device in the network. If this statistic is growing, it may indicate a misconfigured server or devices that have statically assigned conflicting addresses.
pkt6-infrequest-received | integer | Number of INFORMATION-REQUEST packets received. This statistic is expected to grow if there are devices that are using stateless DHCPv6. INFORMATION-REQUEST messages are used by clients that request stateless configuration, i.e. options and parameters other than addresses or prefixes.
pkt6-dhcpv4-query-received | integer | Number of DHCPv4-QUERY packets received. This statistic is expected to grow if there are devices that are using DHCPv4-over-DHCPv6. DHCPv4-QUERY messages are used by DHCPv4 clients on an IPv6-only line which encapsulates the requests over DHCPv6.
pkt6-dhcpv4-response-received | integer | Number of DHCPv4-RESPONSE packets received. This statistic is expected to remain zero at all times, as DHCPv4-RESPONSE packets are sent by the server and the server is never expected to receive them. A non-zero value indicates an error. One likely cause would be a misbehaving relay agent that incorrectly forwards DHCPv4-RESPONSE message towards the server rather than back to the clients.
pkt6-unknown-received | integer | Number of packets received of an unknown type. A non-zero value of this statistic indicates that the server received a packet that it was unable to recognize; either it had an unsupported type or was possibly malformed.
pkt6-sent | integer | Number of DHCPv6 packets sent. This statistic is expected to grow every time the server transmits a packet. In general, it should roughly match pkt6-received, as most incoming packets cause the server to respond. There are exceptions (e.g. server receiving a REQUEST with server-id matching another server), so do not worry if it is less than pkt6-received.
pkt6-advertise-sent | integer | Number of ADVERTISE packets sent. This statistic is expected to grow in most cases after a SOLICIT is processed. There are certain uncommon, but valid, cases where incoming SOLICIT packets are dropped, but in general this statistic is expected to be close to pkt6-solicit-received.
pkt6-reply-sent | integer | Number of REPLY packets sent. This statistic is expected to grow in most cases after a SOLICIT (with rapid-commit), REQUEST, RENEW, REBIND, RELEASE, DECLINE, or INFORMATION-REQUEST is processed. There are certain cases where there is no response.
pkt6-dhcpv4-response-sent | integer | Number of DHCPv4-RESPONSE packets sent. This statistic is expected to grow in most cases after a DHCPv4-QUERY is processed. There are certain cases where there is no response.
subnet[id].total-nas | big integer | Total number of NA addresses available for DHCPv6 management for a given subnet; in other words, this is the count of all addresses in all configured pools. This statistic changes only during configuration changes. It does not take into account any addresses that may be reserved due to host reservation. The id is the subnet-id of a given subnet. This statistic is exposed for each subnet separately, and is reset during a reconfiguration event.
subnet[id].pool[pid].total-nas | big integer | Total number of NA addresses available for DHCPv6 management for a given subnet pool; in other words, this is the count of all addresses in configured subnet pool. This statistic changes only during configuration changes. It does not take into account any addresses that may be reserved due to host reservation. The id is the subnet-id of a given subnet. The pid is the pool-id of a given pool. This statistic is exposed for each subnet pool separately, and is reset during a reconfiguration event.
cumulative-assigned-nas | integer | Cumulative number of NA addresses that have been assigned since server startup. It is incremented each time a NA address is assigned and is not reset when the server is reconfigured.
subnet[id].cumulative-assigned-nas | integer | Cumulative number of NA addresses in a given subnet that were assigned. It increases every time a new lease is allocated (as a result of receiving a REQUEST message) and is never decreased. The id is the subnet-id of a given subnet. This statistic is exposed for each subnet separately, and is reset during a reconfiguration event.
subnet[id].pool[pid].cumulative-assigned-nas | integer | Cumulative number of NA addresses in a given subnet pool that were assigned. It increases every time a new lease is allocated (as a result of receiving a REQUEST message) and is never decreased. The id is the subnet-id of a given subnet. The pid is the pool-id of a given pool. This statistic is exposed for each subnet pool separately, and is reset during a reconfiguration event.
subnet[id].assigned-nas | integer | Number of NA addresses in a given subnet that are assigned. It increases every time a new lease is allocated (as a result of receiving a REQUEST message) and is decreased every time a lease is released (a RELEASE message is received) or expires. The id is the subnet-id of a given subnet. This statistic is exposed for each subnet separately, and is reset during a reconfiguration event.
subnet[id].pool[pid].assigned-nas | integer | Number of NA addresses in a given subnet pool that are assigned. It increases every time a new lease is allocated (as a result of receiving a REQUEST message) and is decreased every time a lease is released (a RELEASE message is received) or expires. The id is the subnet-id of a given subnet. The pid is the pool-id of the pool. This statistic is exposed for each subnet pool separately, and is reset during a reconfiguration event.
subnet[id].total-pds | big integer | Total number of PD prefixes available for DHCPv6 management for a given subnet; in other words, this is the count of all prefixes in all configured pools. This statistic changes only during configuration changes. Note it does not take into account any prefixes that may be reserved due to host reservation. The id is the subnet-id of a given subnet. This statistic is exposed for each subnet separately, and is reset during a reconfiguration event.
subnet[id].pd-pool[pid].total-pds | big integer | Total number of PD prefixes available for DHCPv6 management for a given subnet pool; in other words, this is the count of all prefixes in configured subnet pd-pool. This statistic changes only during configuration changes. It does not take into account any prefixes that may be reserved due to host reservation. The id is the subnet-id of a given subnet. The pid is the pool-id of a given pool. This statistic is exposed for each subnet pd-pool separately, and is reset during a reconfiguration event.
cumulative-assigned-pds | integer | Cumulative number of PD prefixes that have been assigned since server startup. It is incremented each time a PD prefix is assigned and is not reset when the server is reconfigured.
subnet[id].cumulative-assigned-pds | integer | Cumulative number of PD prefixes in a given subnet that were assigned. It increases every time a new lease is allocated (as a result of receiving a REQUEST message) and is never decreased. The id is the subnet-id of a given subnet. This statistic is exposed for each subnet separately, and is reset during a reconfiguration event.
subnet[id].pd-pool[pid].cumulative-assigned-pds | integer | Cumulative number of PD prefixes in a given subnet pd-pool that were assigned. It increases every time a new lease is allocated (as a result of receiving a REQUEST message) and is never decreased. The id is the subnet-id of a given subnet. The pid is the pool-id of a given pd-pool. This statistic is exposed for each subnet pd-pool separately, and is reset during a reconfiguration event.
subnet[id].assigned-pds | integer | Number of PD prefixes in a given subnet that are assigned. It increases every time a new lease is allocated (as a result of receiving a REQUEST message) and is decreased every time a lease is released (a RELEASE message is received) or expires. The id is the subnet-id of a given subnet. This statistic is exposed for each subnet separately, and is reset during a reconfiguration event.
subnet[id].pd-pool[pid].assigned-pds | integer | Number of PD prefixes in a given subnet pd-pool that are assigned. It increases every time a new lease is allocated (as a result of receiving a REQUEST message) and is decreased every time a lease is released (a RELEASE message is received) or expires. The id is the subnet-id of a given subnet. The pid is the pool-id of the pd-pool. This statistic is exposed for each subnet pd-pool separately, and is reset during a reconfiguration event.
reclaimed-leases | integer | Number of expired leases that have been reclaimed since server startup. It is incremented each time an expired lease is reclaimed (counting both NA and PD reclamations). This statistic never decreases. It can be used as a long-term indicator of how many actual leases have been reclaimed. This is a global statistic that covers all subnets.
subnet[id].reclaimed-leases | integer | Number of expired leases associated with a given subnet that have been reclaimed since server startup. It is incremented each time an expired lease is reclaimed (counting both NA and PD reclamations). The id is the subnet-id of a given subnet. This statistic is exposed for each subnet separately.
subnet[id].pool[pid].reclaimed-leases | integer | Number of expired NA addresses associated with a given subnet pool that have been reclaimed since server startup. It is incremented each time an expired lease is reclaimed. The id is the subnet-id of a given subnet. The pid is the pool-id of the pool. This statistic is exposed for each subnet pool separately, and is reset during a reconfiguration event.
subnet[id].pd-pool[pid].reclaimed-leases | integer | Number of expired PD prefixes associated with a given subnet pd-pool that have been reclaimed since server startup. It is incremented each time an expired lease is reclaimed. The id is the subnet-id of a given subnet. The pid is the pool-id of the pd-pool. This statistic is exposed for each subnet pd-pool separately, and is reset during a reconfiguration event.
declined-addresses | integer | Number of IPv6 addresses that are currently declined; a count of the number of leases currently unavailable. Once a lease is recovered, this statistic will be decreased; ideally, this statistic should be zero. If this statistic is non-zero or increasing, a network administrator should investigate whether there is a misbehaving device in the network. This is a global statistic that covers all subnets.
subnet[id].declined-addresses | integer | Number of IPv6 addresses that are currently declined in a given subnet; a count of the number of leases currently unavailable. Once a lease is recovered, this statistic will be decreased; ideally, this statistic should be zero. If this statistic is non-zero or increasing, a network administrator should investigate whether there is a misbehaving device in the network. The id is the subnet-id of a given subnet. This statistic is exposed for each subnet separately.
subnet[id].pool[pid].declined-addresses | integer | Number of IPv6 addresses that are currently declined in a given subnet pool; a count of the number of leases currently unavailable. Once a lease is recovered, this statistic will be decreased; ideally, this statistic should be zero. If this statistic is non-zero or increasing, a network administrator should investigate whether there is a misbehaving device in the network. The id is the subnet-id of a given subnet. The pid is the pool-id of the pool. This statistic is exposed for each subnet pool separately.
reclaimed-declined-addresses | integer | Number of IPv6 addresses that were declined, but have now been recovered. Unlike declined-addresses, this statistic never decreases. It can be used as a long-term indicator of how many actual valid declines were processed and recovered from. This is a global statistic that covers all subnets.
subnet[id].reclaimed-declined-addresses | integer | Number of IPv6 addresses that were declined, but have now been recovered. Unlike declined-addresses, this statistic never decreases. It can be used as a long-term indicator of how many actual valid declines were processed and recovered from. The id is the subnet-id of a given subnet. This statistic is exposed for each subnet separately.
subnet[id].pool[pid].reclaimed-declined-addresses | integer | Number of IPv6 addresses that were declined, but have now been recovered. Unlike declined-addresses, this statistic never decreases. It can be used as a long-term indicator of how many actual valid declines were processed and recovered from. The id is the subnet-id of a given subnet. The pid is the pool-id of the pool. This statistic is exposed for each subnet pool separately.
v6-allocation-fail | integer | Number of total address allocation failures for a particular client. This consists in the number of lease allocation attempts that the server made before giving up and was unable to use any of the address pools. This is a global statistic that covers all subnets.
subnet[id].v6-allocation-fail | integer | Number of total address allocation failures for a particular client. This consists in the number of lease allocation attempts that the server made before giving up and was unable to use any of the address pools. The id is the subnet-id of a given subnet. This statistic is exposed for each subnet separately.
v6-allocation-fail-shared-network | integer | Number of address allocation failures for a particular client connected to a shared network. This is a global statistic that covers all subnets.
subnet[id].v6-allocation-fail-shared-network | integer | Number of address allocation failures for a particular client connected to a shared network. The id is the subnet-id of a given subnet. This statistic is exposed for each subnet separately.
v6-allocation-fail-subnet | integer | Number of address allocation failures for a particular client connected to a subnet that does not belong to a shared network. This is a global statistic that covers all subnets.
subnet[id].v6-allocation-fail-subnet | integer | Number of address allocation failures for a particular client connected to a subnet that does not belong to a shared network. The id is the subnet-id of a given subnet. This statistic is exposed for each subnet separately.
v6-allocation-fail-no-pools | integer | Number of address allocation failures because the server could not use any configured pools for a particular client. It is also possible that all of the subnets from which the server attempted to assign an address lack address pools. In this case, it should be considered misconfiguration if an operator expects that some clients should be assigned dynamic addresses. This is a global statistic that covers all subnets.
subnet[id].v6-allocation-fail-no-pools | integer | Number of address allocation failures because the server could not use any configured pools for a particular client. It is also possible that all of the subnets from which the server attempted to assign an address lack address pools. In this case, it should be considered misconfiguration if an operator expects that some clients should be assigned dynamic addresses. The id is the subnet-id of a given subnet. This statistic is exposed for each subnet separately.
v6-allocation-fail-classes | integer | Number of address allocation failures when the client's packet belongs to one or more classes. There may be several reasons why a lease was not assigned. One of them may be a case when all pools require packet to belong to certain classes and the incoming packet didn't belong to any of them. Another case where this information may be useful is to point out that the pool reserved to a given class has ran out of addresses. This is a global statistic that covers all subnets.
subnet[id].v6-allocation-fail-classes | integer | Number of address allocation failures when the client's packet belongs to one or more classes. There may be several reasons why a lease was not assigned. One of them may be a case when all pools require packet to belong to certain classes and the incoming packet didn't belong to any of them. Another case where this information may be useful is to point out that the pool reserved to a given class has ran out of addresses. The id is the subnet-id of a given subnet. This statistic is exposed for each subnet separately.
v6-ia-na-lease-reuses | integer | Number of times an IA_NA lease had its CLTT increased in memory and its expiration time left unchanged in persistent storage as part of the lease caching feature. This is referred to as a lease reuse. This statistic is global.
subnet[id].v6-ia-na-lease-reuses | integer | Number of times an IA_NA lease had its CLTT increased in memory and its expiration time left unchanged in persistent storage as part of the lease caching feature. This is referred to as a lease reuse. This statistic is on a per-subnet basis. The id is the subnet-id of a given subnet.
v6-ia-pd-lease-reuses | integer | Number of times an IA_PD lease had its CLTT increased in memory and its expiration time left unchanged in persistent storage as part of the lease caching feature. This is referred to as a lease reuse. This statistic is global.
subnet[id].v6-ia-pd-lease-reuses | integer | Number of times an IA_PD lease had its CLTT increased in memory and its expiration time left unchanged in persistent storage as part of the lease caching feature. This is referred to as a lease reuse. This statistic is on a per-subnet basis. The id is the subnet-id of a given subnet.

Note

The pool ID can be configured on each pool by explicitly setting the `pool-id` parameter in the pool parameter map. If not configured, `pool-id` defaults to 0. The statistics related to pool ID 0 refer to all the statistics of all the pools that have unconfigured `pool-id`. The pool ID does not need to be unique within the subnet or across subnets. The statistics regarding a specific pool ID within a subnet will be combined with the other statistics of all other pools with the same pool ID in the respective subnet.

Note

This section describes DHCPv6-specific statistics. For a general overview and usage of statistics, see Statistics.

The DHCPv6 server provides two global parameters to control the default sample limits of statistics:

- `statistic-default-sample-count` - determines the default maximum number of samples which are kept. The special value of 0 indicates that a default maximum age should be used.

- `statistic-default-sample-age` - determines the default maximum age in seconds of samples which are kept.

For instance, to reduce the statistic-keeping overhead, set the default maximum sample count to 1 so only one sample is kept:

    "Dhcp6": {
        "statistic-default-sample-count": 1,
        "subnet6": [
            {
                ...
            },
            ...
        ],
        ...
    }

Statistics can be retrieved periodically to gain more insight into Kea operations. One tool that leverages that capability is ISC Stork. See Monitoring Kea With Stork for details.

## 9.14. Management API for the DHCPv6 Server

The management API allows the issuing of specific management commands, such as statistics retrieval, reconfiguration, or shutdown. For more details, see Management API. Currently, the only supported communication channel type is the UNIX stream socket. By default there are no sockets open; to instruct Kea to open a socket, the following entry in the configuration file can be used:

    "Dhcp6": {
        "control-socket": {
            "socket-type": "unix",
            "socket-name": "/path/to/the/unix/socket"
        },

        "subnet6": [
            {
                ...
            },
            ...
        ],
        ...
    }

The length of the path specified by the `socket-name` parameter is restricted by the maximum length for the UNIX socket name on the administrator's operating system, i.e. the size of the `sun_path` field in the `sockaddr_un` structure, decreased by 1. This value varies on different operating systems, between 91 and 107 characters. Typical values are 107 on Linux and 103 on FreeBSD.

Communication over the control channel is conducted using JSON structures. See the Control Channel section in the Kea Developer's Guide for more details.

The DHCPv6 server supports the following operational commands:

- `build-report`

- `config-get`

- `config-hash-get`

- `config-reload`

- `config-set`

- `config-test`

- `config-write`

- `dhcp-disable`

- `dhcp-enable`

- `leases-reclaim`

- `list-commands`

- `shutdown`

- `status-get`

- `version-get`

as described in Commands Supported by Both the DHCPv4 and DHCPv6 Servers. In addition, it supports the following statistics-related commands:

- `statistic-get`

- `statistic-reset`

- `statistic-remove`

- `statistic-get`-all

- `statistic-reset`-all

- `statistic-remove`-all

- `statistic-sample-age-set`

- `statistic-sample-age-set`-all

- `statistic-sample-count-set`

- `statistic-sample-count-set`-all

as described in Commands for Manipulating Statistics.

## 9.15. User Contexts in IPv6

Kea allows the loading of hook libraries that can sometimes benefit from additional parameters. If such a parameter is specific to the whole library, it is typically defined as a parameter for the hook library. However, sometimes there is a need to specify parameters that are different for each pool.

See Comments and User Context for additional background regarding the user-context idea. See User Contexts in Hooks for a discussion from the hooks perspective.

User contexts can be specified at global scope; at the shared-network, subnet, pool, client-class, option-data, or definition level; and via host reservation. One other useful feature is the ability to store comments or descriptions.

Let's consider an example deployment of lightweight 4over6, an IPv6 transition technology that allows mapping IPv6 prefixes into full or partial IPv4 addresses. In the DHCP context, these are specific parameters that are supposed to be delivered to clients in the form of additional options. Values of these options are correlated to delegated prefixes, so it is reasonable to keep these parameters together with the prefix delegation (PD) pool. On the other hand, lightweight 4over6 is not a commonly used feature, so it is not a part of the base Kea code. The solution to this problem is to specify a user context. For each PD pool that is expected to be used for lightweight 4over6, a user context with extra parameters is defined. Those extra parameters will be used by a hook library and loaded only when dynamic calculation of the lightweight 4over6 option is actually needed. An example configuration looks as follows:

    "Dhcp6": {
        "subnet6": [ {
            "pd-pools": [
            {
                "prefix": "2001:db8::",
                "prefix-len": 56,
                "delegated-len": 64,

                # This is a pool specific context.
                "user-context": {
                    "threshold-percent": 85,
                    "v4-network": "192.168.0.0/16",
                    "v4-overflow": "10.0.0.0/16",
                    "lw4over6-sharing-ratio": 64,
                    "lw4over6-v4-pool": "192.0.2.0/24",
                    "lw4over6-sysports-exclude": true,
                    "lw4over6-bind-prefix-len": 56
                }
            } ],
            "id": 1,
            "subnet": "2001:db8::/32",

            # This is a subnet-specific context. Any type of
            # information can be entered here as long as it is valid JSON.
            "user-context": {
                "comment": "Those v4-v6 migration technologies are tricky.",
                "experimental": true,
                "billing-department": 42,
                "contacts": [ "Alice", "Bob" ]
            }
        } ]
    }

Kea does not interpret or use the user-context information; it simply stores it and makes it available to the hook libraries. It is up to each hook library to extract that information and use it. The parser translates a `comment` entry into a user context with the entry, which allows a comment to be attached inside the configuration itself.

## 9.16. Supported DHCPv6 Standards

The following standards are currently supported in Kea:

- Dynamic Host Configuration Protocol for IPv6, RFC 3315: Supported messages are SOLICIT, ADVERTISE, REQUEST, RELEASE, RENEW, REBIND, INFORMATION-REQUEST, CONFIRM, DECLINE and REPLY. The only unsupported message is RECONFIGURE.

- Dynamic Host Configuration Protocol (DHCPv6) Options for Session Initiation Protocol (SIP) Servers, RFC 3319: All defined options are supported.

- IPv6 Prefix Options for Dynamic Host Configuration Protocol (DHCP) version 6, RFC 3633: Supported options are IA_PD and IA_PREFIX. Also supported is the status code NoPrefixAvail.

- DNS Configuration options for Dynamic Host Configuration Protocol for IPv6 (DHCPv6), RFC 3646: All defined options are supported.

- Stateless Dynamic Host Configuration Protocol (DHCP) Service for IPv6, RFC 3736: Server operation in stateless mode is supported. Kea is currently server-only, so the client side is not implemented.

- Information Refresh Time Option for Dynamic Host Configuration Protocol for IPv6 (DHCPv6), RFC 4242: The sole defined option (`information-refresh-time`) is supported.

- The Dynamic Host Configuration Protocol for IPv6 (DHCPv6) Relay Agent Remote-ID Option, RFC 4649: The REMOTE-ID option is supported.

- Resolution of Fully Qualified Domain Name (FQDN) Conflicts among Dynamic Host Configuration Protocol (DHCP) Clients, RFC 4703: The DHCPv6 server uses the DHCP-DDNS server to resolve conflicts.

- The Dynamic Host Configuration Protocol for IPv6 (DHCPv6) Client Fully Qualified Domain Name (FQDN) Option, RFC 4704: The supported option is CLIENT_FQDN.

- Dynamic Host Configuration Protocol for IPv6 (DHCPv6) Option for Dual-Stack Lite, RFC 6334: The AFTR-Name DHCPv6 Option is supported.

- Relay-Supplied DHCP Options, RFC 6422: The full functionality is supported: OPTION_RSOO; the ability of the server to echo back the options; verification of whether an option is RSOO-enabled; the ability to mark additional options as RSOO-enabled.

- Prefix Exclude Option for DHCPv6-based Prefix Delegation, RFC 6603: The Prefix Exclude option is supported.

- Client Link-Layer Address Option in DHCPv6, RFC 6939: The supported option is the client link-layer address option.

- Issues and Recommendations with Multiple Stateful DHCPv6 Options, RFC 7550: All recommendations related to the DHCPv6 server operation are supported.

- DHCPv6 Options for Configuration of Softwire Address and Port-Mapped Clients, RFC 7598: All options indicated in this specification are supported by the DHCPv6 server.

- Generalized UDP Source Port for DHCP Relay, RFC 8357: The Kea server is able to handle Relay Source Port option in a received Relay-forward message, remembers the UDP port and sends back Relay-reply with a copy of the option to the relay agent using this UDP port.

- Dynamic Host Configuration Protocol for IPv6 (DHCPv6), RFC 8415: This new DHCPv6 protocol specification obsoletes RFC 3315, RFC 3633, RFC 3736, RFC 4242, RFC 7083, RFC 7283, and RFC 7550. All features, with the exception of the RECONFIGURE mechanism and the now-deprecated temporary addresses (IA_TA) mechanism, are supported.

- Captive-Portal Identification in DHCP and Router Advertisements (RAs), RFC 8910: The Kea server can configure both v4 and v6 versions of the captive portal options.

## 9.17. DHCPv6 Server Limitations

These are the current known limitations of the Kea DHCPv6 server software. Most of them are reflections of the current stage of development and should be treated as “not implemented yet”, rather than actual limitations.

- The server will allocate, renew, or rebind a maximum of one lease for a particular IA option (IA_NA or IA_PD) sent by a client. RFC 8415 allows for multiple addresses or prefixes to be allocated for a single IA.

- Temporary addresses are not supported. There is no intention to ever implement this feature, as it is deprecated in RFC 8415.

- Client reconfiguration (RECONFIGURE) is not yet supported.

## 9.18. Kea DHCPv6 Server Examples

A collection of simple-to-use examples for the DHCPv6 component of Kea is available with the source files, located in the `doc/examples/kea6` directory.

## 9.19. Configuration Backend in DHCPv6

In the Kea Configuration Backend section we have described the Configuration Backend (CB) feature, its applicability, and its limitations. This section focuses on the usage of the CB with the DHCPv6 server. It lists the supported parameters, describes limitations, and gives examples of DHCPv6 server configurations to take advantage of the CB. Please also refer to the corresponding section Configuration Backend in DHCPv4 for DHCPv4-specific usage of the CB.

### 9.19.1. Supported Parameters

The ultimate goal for the CB is to serve as a central configuration repository for one or multiple Kea servers connected to a database. In currently supported Kea versions, only a subset of the DHCPv6 server parameters can be configured in the database. All other parameters must be specified in the JSON configuration file, if required.

All supported parameters can be configured via `libdhcp_cb_cmds.so`. The general rule is that scalar global parameters are set using `remote-global-parameter6-set`; shared-network-specific parameters are set using `remote-network6-set`; and subnet-level and pool-level parameters are set using `remote-subnet6-set`. Whenever there is an exception to this general rule, it is highlighted in the table. Non-scalar global parameters have dedicated commands; for example, the global DHCPv6 options (`option-data`) are modified using `remote-option6-global-set`. Client classes, together with class-specific option definitions and DHCPv6 options, are configured using the `remote-class6-set` command.

The Configuration Sharing and Server Tags section explains the concept of shareable and non-shareable configuration elements and the limitations for sharing them between multiple servers. In the DHCP configuration (both DHCPv4 and DHCPv6), the shareable configuration elements are subnets and shared networks. Thus, they can be explicitly associated with multiple server tags. The global parameters, option definitions, and global options are non-shareable and can be associated with only one server tag. This rule does not apply to the configuration elements associated with `all` servers. Any configuration element associated with `all` servers (using the `all` keyword as a server tag) is used by all servers connecting to the configuration database.

The following table lists DHCPv6-specific parameters supported by the Configuration Backend, with an indication of the level of the hierarchy at which it is currently supported.

List of DHCPv6 parameters supported by the Configuration Backend
Parameter | Global | Client Class | Shared Network | Subnet | Pool | Prefix Delegation Pool
--- | --- | --- | --- | --- | --- | ---
allocator | yes | n/a | yes | yes | n/a | n/a
cache-max-age | yes | n/a | no | no | n/a | n/a
cache-threshold | yes | n/a | no | no | n/a | n/a
calculate-tee-times | yes | n/a | yes | yes | n/a | n/a
client-class | n/a | n/a | yes | yes | yes | yes
ddns-send-update | yes | n/a | yes | yes | n/a | n/a
ddns-override-no-update | yes | n/a | yes | yes | n/a | n/a
ddns-override-client-update | yes | n/a | yes | yes | n/a | n/a
ddns-replace-client-name | yes | n/a | yes | yes | n/a | n/a
ddns-generated-prefix | yes | n/a | yes | yes | n/a | n/a
ddns-qualifying-suffix | yes | n/a | yes | yes | n/a | n/a
decline-probation-period | yes | n/a | n/a | n/a | n/a | n/a
delegated-len | n/a | n/a | n/a | n/a | n/a | yes
dhcp4o6-port | yes | n/a | n/a | n/a | n/a | n/a
excluded-prefix | n/a | n/a | n/a | n/a | n/a | yes
excluded-prefix-len | n/a | n/a | n/a | n/a | n/a | yes
hostname-char-set | no | n/a | no | no | n/a | n/a
hostname-char-replacement | no | n/a | no | no | n/a | n/a
interface | n/a | n/a | yes | yes | n/a | n/a
interface-id | n/a | n/a | yes | yes | n/a | n/a
max-preferred-lifetime | yes | yes | yes | yes | n/a | n/a
max-valid-lifetime | yes | yes | yes | yes | n/a | n/a
min-preferred-lifetime | yes | yes | yes | yes | n/a | n/a
min-valid-lifetime | yes | yes | yes | yes | n/a | n/a
option-data | yes (via remote-option6-global-set) | yes | yes | yes | yes | yes
option-def | yes (via remote-option-def6-set) | yes | n/a | n/a | n/a | n/a
pd-allocator | yes | n/a | yes | yes | n/a | n/a
preferred-lifetime | yes | yes | yes | yes | n/a | n/a
prefix | n/a | n/a | n/a | n/a | n/a | yes
prefix-len | n/a | n/a | n/a | n/a | n/a | yes
rapid-commit | yes | n/a | yes | yes | n/a | n/a
rebind-timer | yes | n/a | yes | yes | n/a | n/a
relay | n/a | n/a | yes | yes | n/a | n/a
renew-timer | yes | n/a | yes | yes | n/a | n/a
require-client-classes | n/a | n/a | yes | yes | yes | yes
reservation-mode | yes | n/a | yes | yes | n/a | n/a
reservations-global | yes | n/a | yes | yes | n/a | n/a
reservations-in-subnet | yes | n/a | yes | yes | n/a | n/a
reservations-out-of-pool | yes | n/a | yes | yes | n/a | n/a
t1-percent | yes | n/a | yes | yes | n/a | n/a
t2-percent | yes | n/a | yes | yes | n/a | n/a
valid-lifetime | yes | yes | yes | yes | n/a | n/a

- `yes` - indicates that the parameter is supported at the given level of the hierarchy and can be configured via the Configuration Backend.

- `no` - indicates that a parameter is supported at the given level of the hierarchy but cannot be configured via the Configuration Backend.

- `n/a` - indicates that a given parameter is not applicable at the particular level of the hierarchy or that the server does not support the parameter at that level.

### 9.19.2. Enabling the Configuration Backend

Consider the following configuration snippet, which uses a MySQL configuration database:

    {
        "Dhcp6": {
        "server-tag": "my DHCPv6 server",
            "config-control": {
                "config-databases": [
                    {
                        "type": "mysql",
                        "name": "kea",
                        "user": "kea",
                        "password": "kea",
                        "host": "2001:db8:1::1",
                        "port": 3302
                    }
                ],
                "config-fetch-wait-time": 20
            },
            "hooks-libraries": [
                {
                    "library": "/usr/local/lib/kea/hooks/libdhcp_mysql_cb.so"
                },
                {
                    "library": "/usr/local/lib/kea/hooks/libdhcp_cb_cmds.so"
                }
            ]
        }
    }

The configuration structure is almost identical to that of the DHCPv4 server (see Enabling the Configuration Backend for the detailed description).

## 9.20. Kea DHCPv6 Compatibility Configuration Parameters

ISC's intention is for Kea to follow the RFC documents to promote better standards compliance. However, many buggy DHCP implementations already exist that cannot be easily fixed or upgraded. Therefore, Kea provides an easy-to-use compatibility mode for broken or non-compliant clients. For that purpose, the compatibility option must be enabled to permit uncommon practices:

    {
      "Dhcp6": {
        "compatibility": {
        }
      }
    }

### 9.20.1. Lenient Option Parsing

By default, DHCPv6 option 16's `vendor-class-data` field is parsed as a set of length-value pairs. Same for tuple fields defined in custom options.

With `"lenient-option-parsing": true`, if a length ever exceeds the rest of the option's buffer, previous versions of Kea returned a log message `unable to parse the opaque data tuple, the buffer length is x, but the tuple length is y` with `x < y`; this no longer occurs. Instead, the value is considered to be the rest of the buffer, or in terms of the log message above, the tuple length `y` becomes `x`.

Enabling this flag is expected to improve compatibility with devices such as RAD MiNID.

    {
      "Dhcp6": {
        "compatibility": {
          "lenient-option-parsing": true
        }
      }
    }

## 9.21. Allocation Strategies in DHCPv6

A DHCP server follows a complicated algorithm to select a DHCPv6 lease for a client. It prefers assigning specific addresses or delegated prefixes requested by the client and the ones for which the client has reservations.

When the client requests a specific delegated prefix, there are a few steps that `kea-dhcp6` goes through to try to satisfy the request, in the following order:

1. It searches for a lease that matches the requested prefix and prefix length.

2. It searches for a lease that matches the prefix length.

3. It searches for a lease with a larger address space (smaller prefix length).

4. It searches for a lease with a smaller address space (larger prefix length).

If the client requests no particular lease and has no reservations, or other clients are already using any requested leases, the server must find another available lease within the configured pools. A server function called an "allocator" is responsible in Kea for finding an available lease in such a case.

The Kea DHCPv6 server provides configuration parameters to select different allocators at the global, shared-network, and subnet levels. It also allows for selecting different allocation strategies for address assignments and prefix delegation.

Consider the following example:

    {
        "Dhcp6": {
            "allocator": "iterative",
            "pd-allocator": "random",
            "subnet6": [
                {
                    "id": 1,
                    "subnet": "2001:db8:1::/64",
                    "allocator": "random"
                },
                {
                    "id": 2,
                    "subnet": "2001:db8:2::/64",
                    "pd-allocator": "iterative"
                }
            ]
        }
    }

The iterative allocator is globally selected for address assignments, while the random allocator is globally selected for prefix delegation. These settings are selectively overridden at the subnet level.

The following sections describe the supported allocators and their recommended uses.

### 9.21.1. Allocators Comparison

In the table below, we briefly compare the supported allocators. The detailed allocators' descriptions are in later sections.

Comparison of the lease allocators supported by Kea DHCPv6
Allocator | Low Utilization Performance | High Utilization Performance | Lease Randomization | Startup/Configuration | Memory Usage
--- | --- | --- | --- | --- | ---
Iterative | very high | low | no | very fast | low
Random | high | low | yes | very fast | high (varying)
Free Lease Queue | high | high | yes | slow (depends on pool sizes) | high (varying)

### 9.21.2. Iterative Allocator

This is the default allocator used by the Kea DHCPv6 server. It remembers the last offered lease and offers the following sequential lease to the next client. For example, it may offer addresses in this order: `2001:db8:1::10`, `2001:db8:1::11`, `2001:db8:1::12`, and so on. Similarly, it offers the next sequential delegated prefix after the previous one to the next client. The time to find and offer the next lease is very short; thus, this is the most performant allocator when pool utilization is low and there is a high probability that the next selected lease is available.

The iterative allocation underperforms when multiple DHCP servers share a lease database or are connected to a cluster. The servers tend to offer and allocate the same blocks of addresses to different clients independently, which causes many allocation conflicts between the servers and retransmissions by clients. A random allocation addresses this issue by dispersing the allocation order.

### 9.21.3. Random Allocator

The random allocator uses a uniform randomization function to select offered addresses and delegated prefixes from subnet pools. It is suitable in deployments where multiple servers are connected to a shared database or a database cluster. By dispersing the offered leases, the servers minimize the risk of allocating the same lease to two different clients at the same or nearly the same time. In addition, it improves the server's resilience against attacks based on allocation predictability.

The random allocator is, however, slightly slower than the iterative allocator. Moreover, it increases the server's memory consumption because it must remember randomized leases to avoid offering them repeatedly. Memory consumption grows with the number of offered leases; in other words, larger pools and more clients increase memory consumption by random allocation.

### 9.21.4. Free Lease Queue Allocator (Prefix Delegation Only)

This is a sophisticated allocator whose use should be considered in subnets with highly utilized delegated prefix pools. In such cases, it can take a considerable amount of time for the iterative or random allocator to find an available prefix, because they must repeatedly check whether there is a valid lease for a prefix they will offer. The number of checks can be as high as the number of delegated prefixes in the subnet when the subnet pools are exhausted, which can have a direct negative impact on the DHCP response time for each request.

The Free Lease Queue (FLQ) allocator tracks lease allocations and de-allocations and maintains a running list of available delegated prefixes for each pool. It allows an available lease to be selected within a constant time, regardless of the subnet pools' utilization. The allocator continuously updates the list of free leases by removing any allocated leases and adding released or reclaimed ones.

The following configuration snippet shows how to select the FLQ allocator for prefix delegation in a subnet:

    {
        "Dhcp6": {
            "subnet6": [
                {
                    "id": 1,
                    "subnet": "2001:db8:1::/64",
                    "pd-allocator": "flq"
                }
            ]
        }
    }

Note

The Free Lease Queue allocator can only be used for DHCPv6 prefix delegation. An attempt to use this allocator for address assignment (with the `allocator` parameter) will cause a configuration error. DHCPv6 address pools are typically very large and their utilization is low; in these situation, the benefits of using the FLQ allocator diminish. The amount of time required for the allocator to populate the free lease queue would cause the server to freeze upon startup.

There are several considerations that the administrator should take into account before using this allocator for prefix delegation. The FLQ allocator can heavily impact the server's startup and reconfiguration time, because the allocator has to populate the list of free leases for each subnet where it is used. These delays can be observed both during the configuration reload and when the subnets are created using `libdhcp_subnet_cmds.so`. The allocator increases the memory consumption to hold the list of free leases, proportional to the total size of the pools for which this allocator is used. Finally, lease reclamation must be enabled with a low value of the `reclaim-timer-wait-time` parameter, to ensure that the server frequently collects expired leases and makes them available for allocation via the free lease queue. Expired leases are not considered free by the allocator until they are reclaimed by the server. See Lease Reclamation for more details about the lease reclamation process.

We recommend that the FLQ allocator be selected only after careful consideration. The server puts no restrictions on the delegated prefix pool sizes used with the FLQ allocator, so we advise users to test how long it takes for the server to load the pools before deploying the configuration using the FLQ allocator in production. We also recommend specifying another allocator type in the global configuration settings and overriding this selection at the subnet or shared-network level, to use the FLQ allocator only for selected subnets. That way, when a new subnet is added without an allocator specification, the global setting is used, thus avoiding unnecessary impact on the server's startup time.

Like the random allocator, the FLQ allocator offers leases in random order, which makes it suitable for use with a shared lease database.