Suppose you reside in the realm `MY.REALM', how do you authenticate to a server in `OTHER.REALM'? Having valid tickets in `MY.REALM' allows you to communicate with Kerberised services in that realm. However, the computer in the other realm does not have a secret key shared with the Kerberos server in your realm.
It is possible to share keys between two realms that trust each other. When a client program, such as telnet or ssh, finds that the other computer is in a different realm, it will try to get a ticket granting ticket for that other realm, but from the local Kerberos server. With that ticket granting ticket, it will then obtain service tickets from the Kerberos server in the other realm.
For a two way trust between `MY.REALM' and `OTHER.REALM' add the following principals to each realm. The principals should be `krbtgt/OTHER.REALM@MY.REALM' and `krbtgt/MY.REALM@OTHER.REALM' in `MY.REALM', and `krbtgt/MY.REALM@OTHER.REALM' and `krbtgt/OTHER.REALM@MY.REALM'in `OTHER.REALM'.
In Kerberos 5 the trust can be configured to be one way. So that users from `MY.REALM' can authenticate to services in `OTHER.REALM', but not the opposite. In the example above, the `krbtgt/MY.REALM@OTHER.REALM' then should be removed.
The two principals must have the same key, key version number, and the same set of encryption types. Remember to transfer the two keys in a safe manner.
vr$ klist Credentials cache: FILE:/tmp/krb5cc_913.console Principal: lha@E.KTH.SE Issued Expires Principal May 3 13:55:52 May 3 23:55:54 krbtgt/E.KTH.SE@E.KTH.SE vr$ telnet -l lha hummel.it.su.se Trying 2001:6b0:5:1095:250:fcff:fe24:dbf... Connected to hummel.it.su.se. Escape character is '^]'. Waiting for encryption to be negotiated... [ Trying mutual KERBEROS5 (host/hummel.it.su.se@SU.SE)... ] [ Kerberos V5 accepts you as ``lha@E.KTH.SE'' ] Encryption negotiated. Last login: Sat May 3 14:11:47 from vr.l.nxs.se hummel$ exit vr$ klist Credentials cache: FILE:/tmp/krb5cc_913.console Principal: lha@E.KTH.SE Issued Expires Principal May 3 13:55:52 May 3 23:55:54 krbtgt/E.KTH.SE@E.KTH.SE May 3 13:55:56 May 3 23:55:54 krbtgt/SU.SE@E.KTH.SE May 3 14:10:54 May 3 23:55:54 host/hummel.it.su.se@SU.SE