To authenticate users, the server that Cockpit is running on must be joined to a domain. This can usually be accomplished using the realm join example.com command.

The domain must be resolvable by DNS. For instance, the SRV records of the kerberos server should be resolvable:

$ host -t SRV  _kerberos._udp.example.com
_kerberos._udp.example.com has SRV record 0 100 88 dc.example.com

The server running Cockpit should have a fully qualified name that ends with the domain name.

There must be a valid Kerberos host key for the server in the /etc/krb5.keytab file. It may be necessary to create a kerberos service principal and update the keytab if it is not present. Depending on your domain type different service names are required:

The following command can be used to list the /etc/krb5.keytab:

$ sudo klist -k

Lastly accounts from the domain must be resolvable to unix accounts on the server running Cockpit. For example:

$ getent passwd user@example.com
user@example.com:*:381001109:381000513:User Name:/home/user:/bin/sh

If you wish to delegate your kerberos credentials to Cockpit, and allow Cockpit to then connect to other machines using those credentials, you should enable delegation for the hosts running Cockpit, and in some cases the HTTP service as well.

The client side, where your web browser is running, should have a valid kerberos ticket in the current user session. A command like this will get one:

$ kinit user@EXAMPLE.COM
Password for user@EXAMPLE.COM:

In addition your browser must be usually be configured to allow kerberos authentication for the domain.

{
  "AuthServerWhitelist": "*example.com"
}

Use a fully qualified server name (with the domain name at the end) to access Cockpit in your web browser.

If you wish to connect from one server to another in Cockpit using kerberos SSO, then you have to explicitly enable all sorts of things. For starters, make sure that delegated credentials are allowed by your domain (see above). Next when requesting your kerberos ticket make sure that forwardable tickets are requested:

$ kinit -f user@EXAMPLE.COM
Password for user@EXAMPLE.COM:

Make sure that the forwardable flag F is present in your ticket:

$ klist -f
Ticket cache: KEYRING:persistent:1000:1000
Default principal: user@EXAMPLE.COM

Valid starting       Expires              Service principal
18.03.2017 05:39:23  19.03.2017 05:39:20  krbtgt/EXAMPLE.COM@EXAMPLE.COM
	Flags: FIA

Lastly configure your browser to allow delegated, forwardable kerberos credentials to be sent to Cockpit:

{
  "AuthServerWhitelist": "*example.com",
  "AuthNegotiateDelegateWhitelist": "*example.com"
}