Jump to content
From ArchWiki

Related articles

- Active Directory Integration
- identity management

Kerberos is a network authentication system. See krb5 documentation.

## Installation

Install the krb5 package on your clients and server.

It is highly recommended to use a time synchronization daemon to keep client/server clocks in sync.

If hostname resolution has not been configured, you can manually add your clients and server to the hosts(5) file of each machine. Note that the FQDN (myclient.example.com) must be the first hostname after the IP address in the hosts file.

## Server configuration

### Domain creation

Edit `/etc/krb5.conf` to configure your domain:

    /etc/krb5.conf

    [libdefaults]
        default_realm = EXAMPLE.COM

    [realms]
        EXAMPLE.COM = {
            admin_server = $ADDRESS
            # use "kdc = ..." if the kerberos SRV records aren't in DNS (see Advanced section)
            kdc = $ADDRESS
            # This breaks krb4 compatibility but increases security
            default_principal_flags = +preauth
        }

    [domain_realm]
        example.com  = EXAMPLE.COM
        .example.com = EXAMPLE.COM

    [logging]
        kdc          = SYSLOG:NOTICE
        admin_server = SYSLOG:NOTICE
        default      = SYSLOG:NOTICE

Where `$ADDRESS` is IP address or domain name, where kerberos lives, like `10.0.0.120` or `kerberos.example.com`.

This file's format is described in the MIT Kerberos documentation

Create the database:

    # kdb5_util -r EXAMPLE.COM create -s

    Loading random data
    Initializing database '/var/lib/krb5kdc/principal' for realm 'EXAMPLE.COM',
    master key name 'K/M@EXAMPLE.COM'
    You will be prompted for the database Master Password.
    It is important that you NOT FORGET this password.
    Enter KDC database master key: ***
    Re-enter KDC database master key to verify: ***

Finally, start/enable `krb5-kdc.service` and `krb5-kadmind.service`.

### Add principals

Start the Kerberos administration tool, using local authentication

    # kadmin.local

    Authenticating as principal root/admin@EXAMPLE.COM with password.
    kadmin.local:

Add a user principal to the Kerberos database:

    kadmin.local: addprinc myuser@EXAMPLE.COM

    WARNING: no policy specified for myuser@EXAMPLE.COM; defaulting to no policy
    Enter password for principal "myuser@EXAMPLE.COM": ***
    Re-enter password for principal "myuser@EXAMPLE.COM": ***
    Principal "myuser@EXAMPLE.COM" created.

Add the KDC principal to the Kerberos database:

    kadmin.local: addprinc -randkey host/kerberos.example.com

    WARNING: no policy specified for host/kerberos.example.com@EXAMPLE.COM; defaulting to no policy
    Principal "host/kerberos.example.com@EXAMPLE.COM" created.

Finally, Add the KDC principal to the server's keytab:

    kadmin.local: ktadd host/kerberos.example.com

    Entry for principal host/kerberos.example.com with kvno 2, encryption type aes256-cts-hmac-sha1-96 added to keytab FILE:/etc/krb5.keytab.
    Entry for principal host/kerberos.example.com with kvno 2, encryption type aes128-cts-hmac-sha1-96 added to keytab FILE:/etc/krb5.keytab.

Quit the Kerberos administration tool:

    kadmin.local: quit

You should now be able to get a Kerberos ticket:

    $ kinit

    Password for myuser@EXAMPLE.COM: ***

    $ klist

    Ticket cache: FILE:/tmp/krb5cc_1000
    Default principal: myuser@EXAMPLE.COM

    Valid starting       Expires              Service principal
    08/30/2017 14:26:09  08/31/2017 14:26:09  krbtgt/EXAMPLE.COM@EXAMPLE.COM

### Firewall

Add ALLOW rules to your firewall for any applicable ports/protocols:

- 88, TCP and UDP for Kerberos v5
- 749, TCP and UDP for kadmin if you plan to configure it
- 750, TCP and UDP for Kerberos v4 if you need backwards compatibility

### DNS records

This is not necessary if you specify the kerberos and kadmin server in each machine's krb5.conf

    db.example.com

    kerberos.example.com.           A     1.2.3.4
    _kerberos.example.com.          TXT   "EXAMPLE.COM"
    _kerberos._udp.example.com.     SRV   0 0  88 kerberos.example.com.
    _kerberos-adm._udp.example.com. SRV   0 0 749 kerberos.example.com.

Do not forget reverse DNS.

## Client configuration

Edit the client's `/etc/krb5.conf` to match your server's configuration. You can copy this file from the server, or just set the required realm information.

### Testing

You should now be able to get a Kerberos ticket on the client:

    $ kinit

    Password for myuser@EXAMPLE.COM: ***

    $ klist

    Ticket cache: FILE:/tmp/krb5cc_1000
    Default principal: myuser@EXAMPLE.COM

    Valid starting       Expires              Service principal
    08/30/2017 15:36:10  08/31/2017 15:36:10  krbtgt/EXAMPLE.COM@EXAMPLE.COM

## Configuring kadmin

You will need /etc/krb5.conf configured on the kadmin client, and the server's firewall configured for kadmin.

### Configuring kadmin ACL

Create a principal for administration:

    kadmin.local:  add_principal myuser/admin@EXAMPLE.COM

    WARNING: no policy specified for myuser/admin@EXAMPLE.COM; defaulting to no policy
    Enter password for principal "myuser/admin@EXAMPLE.COM": ***
    Re-enter password for principal "myuser/admin@EXAMPLE.COM": ***
    Principal "myuser/admin@EXAMPLE.COM" created.

Add the user to the kadmin ACL file:

    /var/lib/krb5kdc/kadm5.acl

    myuser/admin@EXAMPLE.COM *

This file's format is described in the MIT Kerberos documentation

Configure kdc.conf:

    /var/lib/krb5kdc/kdc.conf

    [kdcdefaults]
        kdc_ports = 750,88

    [realms]
        EXAMPLE.COM = {
            database_name = /var/lib/krb5kdc/principal
            acl_file = /var/lib/krb5kdc/kadm5.acl
            key_stash_file = /var/lib/krb5kdc/.k5.EXAMPLE.COM
            kdc_ports = 750,88
            max_life = 10h 0m 0s
            max_renewable_life = 7d 0h 0m 0s
        }

This file's format is described in the MIT Kerberos documentation

Restart `krb5-kdc.service` and `krb5-kadmind.service`.

You can now use kadmin as your own user, authenticating with kerberos:

    $ kadmin

    Authenticating as principal myuser/admin@EXAMPLE.COM with password.
    Password for myuser/admin@EXAMPLE.COM: ***
    kadmin:

## Service principals and keytabs

First, ensure you have configured krb5.conf on all involved machines.

A kerberos principal has three components, formatted as `primary/instance@REALM`. For user principals, the primary is your username and the instance is omitted or is a role (eg. "admin"): `myuser@EXAMPLE.COM` or `myuser/admin@EXAMPLE.COM`. For hosts, the primary is "host" and the instance is the server FQDN: `host/myserver.example.com@EXAMPLE.COM`. For services, the primary is the service abbreviation and the instance is the FQDN: `nfs/myserver.example.com@EXAMPLE.COM`. The realm can often be omitted, the local computer's default realm is usually assumed.

### With remote kadmin

This is the easier method, but requires you to have configured kadmin.

Open kadmin as root (so we can write the keytab) on the client, authenticating with your admin principal:

    client# kadmin -p myuser/admin

    Authenticating as principal myuser/admin with password.
    Password for myuser/admin@EXAMPLE.COM:
    kadmin:

Add a principal for any services you will be using, eg. "host" for SSH authentication or "nfs" for NFS:

    kadmin: addprinc -randkey host/kbclient.example.com

    WARNING: no policy specified for host/kbclient.example.com@EXAMPLE.COM; defaulting to no policy
    Principal "host/kbclient.example.com@EXAMPLE.COM" created.

Save each key to the local keytab:

    kadmin: ktadd host/kbclient.example.com

    Entry for principal host/kbclient.example.com with kvno 2, encryption type aes256-cts-hmac-sha1-96 added to keytab FILE:/etc/krb5.keytab.
    Entry for principal host/kbclient.example.com with kvno 2, encryption type aes128-cts-hmac-sha1-96 added to keytab FILE:/etc/krb5.keytab.

### Without remote kadmin

Start kadmin on the Kerberos server, using either unix or kerberos authentication:

    # kadmin.local

    Authenticating as principal root/admin@EXAMPLE.COM with password.
    kadmin.local:

Add a principal for any services you will be using, eg. "host" for SSH authentication or "nfs" for NFS:

    kadmin.local: addprinc -randkey host/kbclient.example.com

    WARNING: no policy specified for host/kbclient.example.com@EXAMPLE.COM; defaulting to no policy
    Principal "host/kbclient.example.com@EXAMPLE.COM" created.

Save each key to a new keytab to be transferred to the client:

    kadmin.local: ktadd -k kbclient.keytab host/kbclient.example.com

    Entry for principal host/kbclient.example.com with kvno 2, encryption type aes256-cts-hmac-sha1-96 added to keytab FILE:/etc/krb5.keytab.
    Entry for principal host/kbclient.example.com with kvno 2, encryption type aes128-cts-hmac-sha1-96 added to keytab FILE:/etc/krb5.keytab.

Finally, copy `kbclient.keytab` from the server to the client using SCP or similar, then put it in place with correct permissions:

    # install -b -o root -g root -m 600 kbclient.keytab /etc/krb5.keytab

Finally, delete kbclient.keytab from the server and client.

## Cross-Realm Trust

Set up a second server as shown above, then create the cross-realm principal on both KDCs. Cross-realm principals must be created with strong passwords, not `-randkey`, and the same password must be used on both KDCs. The principal must have the same key version number (kvno) in both KDCs.

To grant EXAMPLE.COM principals access to EXAMPLE.ORG resources, you would use the following principal:

    kadmin# addprinc krbtgt/EXAMPLE.ORG@EXAMPLE.COM

The `[capaths]` section of `krb5.conf` can be used to further control cross-realm trust relationships.

## SSH authentication

Use the instructions in Service principals and keytabs to create a principal for the "host" service for both client and server, then put the client's keys in the client's keytab and the server's keys in the server's keytab.

Modify your SSH server configuration to enable GSSAPI authentication:

    /etc/ssh/sshd_config

    # GSSAPI Options
    GSSAPIAuthentication yes
    GSSAPICleanupCredentials yes

And modify your client configuration to send GSSAPI requests:

    /etc/ssh/ssh_config

    Host *
      GSSAPIAuthentication yes
      GSSAPIDelegateCredentials yes

Get a ticket-granting ticket on the client before using ssh:

    $ kinit myuser@EXAMPLE.COM

    Password for myuser@EXAMPLE.COM: ***

Pass the -v option to ssh to watch what's happening:

    $ ssh sshserver.example.com -v

    debug1: Authentications that can continue: publickey,gssapi-with-mic,password
    debug1: Next authentication method: gssapi-with-mic
    debug1: Delegating credentials
    debug1: Delegating credentials
    debug1: Authentication succeeded (gssapi-with-mic).
    Authenticated to sshserver.example.com ([192.168.100.136]:22).
    debug1: channel 0: new [client-session]
    debug1: Requesting no-more-sessions@openssh.com
    debug1: Entering interactive session.
    debug1: pledge: network
    debug1: client_input_global_request: rtype hostkeys-00@openssh.com want_reply 0
    Last login: Wed Aug 30 15:52:41 2017 from 192.168.100.1

And you should now see a host ticket on the client:

    client$ klist

    Ticket cache: FILE:/tmp/krb5cc_1000
    Default principal: myuser@EXAMPLE.COM

    Valid starting       Expires              Service principal
    08/30/2017 15:37:40  08/31/2017 15:37:40  krbtgt/EXAMPLE.COM@EXAMPLE.COM
    08/30/2017 15:53:04  08/31/2017 15:37:40  host/sshserver.example.com@EXAMPLE.COM

### Authorize other principals

To allow a different kerberos principal to authenticate to a user account, add the principal name to the target account's `.k5login` file. For example, to allow `robert@EXAMPLE.COM` to SSH to alice's account:

    /home/alice/.k5login

    robert@EXAMPLE.COM

## NFS security

First, configure your NFS server. Also see NFS/Troubleshooting. Configuring a time synchronization daemon on both the clients and the server is strongly recommended. Clock drift will cause this to break, and the error message will not be helpful.

Use the instructions in #Service principals and keytabs to create a principal for the "nfs" service for both client and server, then put the client's keys in the client's keytab and the server's keys in the server's keytab.

### NFS server

Add a Kerberos export option. If necessary, multiple options can be specified using a colon as a delimiter with the preferred setting first, e.g. `sec=krb5p:krb5i`.

- `sec=krb5p` uses kerberos for authentication, integrity, and encryption.
- `sec=krb5i` uses kerberos for authentication and integrity checking, but still transmits data unencrypted.
- `sec=krb5` uses kerberos for authentication only, and transmits the data unauthenticated and unencrypted.
- `sec=sys` is the default and does not provide any cryptographic security.

    /etc/exports

    /srv/export *(rw,async,no_subtree_check,no_root_squash,sec=krb5p)

And reload the exports:

    # exportfs -arv

### NFS client

Mount the exported directory:

    # mount nfsserver:/srv/export /mnt/

You can add `-vv` for verbose information, and may need `-t nfs4` and `-o sec=krb5p` or your chosen security option.

Check that it worked with the `mount` command:

    mount | grep nfs

    nfsserver:/srv/export on /mnt type nfs4 (rw,relatime,vers=4.1,rsize=131072,wsize=131072,namlen=255,hard,proto=tcp,port=0,timeo=600,retrans=2,sec=krb5,clientaddr=192.168.100.139,local_lock=none,addr=192.168.100.136)

## Browsers

Some browsers have support for Kerberos protocol but disable it by default. Here are the instructions how to enable it:

### Chromium

Chromium needs to be run with a command line parameter that specifies a list of sites where Kerberos authentication is allowed. The easiest way is to add persistent flag to the configuration file:

    /etc/chromium/policies/managed/test_policy.json

    {
      "AuthServerAllowlist": "*.mycompany.com",
      "DisableAuthNegotiateCnameLookup": true
    }

### Firefox

To configure Firefox with trusted sites visit `about:config` and set `network.negotiate-auth.trusted-uris` property to FOO.COM (Note: for Firefox there is no "*."; for Chrome, there is).

## Troubleshooting

### Cannot set GSSAPI authentication names

    Cannot set GSSAPI authentication names, aborting

Your realm is missing either the `kadmin/admin` or `kadmin/changepw` principal.

For clients, invalid arguments/options may happen on first setup if rpc-gssd is not loaded. Loading it is usually acomplished by enabling and starting `nfs-client.target`, but after first setup this target will need a restart.

### SSH authentication fails while connecting to a server requiring GSSAPI with KeyExchange

If any of the following errors are encountered:

    $ ssh -v -o GSSAPIDelegateCredentials=yes -o GSSAPIAuthentication=yes <user>@<IP address>
    Unable to negotiate with <IP address> port 22: no matching key exchange method found. Their offer: gss-group14-sha1-...

    $ ssh -v -o GSSAPIDelegateCredentials=yes -o GSSAPIKeyExchange=yes -o GSSAPIAuthentication=yes <user>@<IP address>
    command-line: line 0: Bad configuration option: gssapikeyexchange

it means that package openssh is not configured with GSSAPI patch for OpenSSH. You can install openssh-gssapiAUR or follow this method.

## See also

- RHEL7: Configure a Kerberos KDC
- RHEL7: Configure a system to authenticate using Kerberos

Categories:

- Networking
- Authentication