Kerberos: The Definitive Guide

Cross-Realm Authentication

All of the Kerberos discussion so far has assumed that all users and resources on your network are located in a single Kerberos realm. However, what if there are several departments, locations, or other divisions that are under different administrative control, each with their own Kerberos realm? These users want to access not only resources in their local Kerberos realm, but also resources in the other realms as well, with a minimum of hassle. Kerberos cross-realm authentication can solve this problem.

In Kerberos, cross-realm is implemented by sharing an encryption key between two realms. The key that is shared is the Ticket Granting Service principal’s key. A typical Ticket Granting Service principal for a single realm looks like:

krbtgt/[e-mail]

Note that the instance is the same as the realm name. In cross-realm, two principals are created on each participating realm. For two realms, ONE.COM and TWO.COM, these principals would be:

krbtgt/[e-mail]
krbtgt/[e-mail]

These principals have to be created on both realms, and are known as remote Ticket Granting Server principals. The Kerberos trust can be one way or both ways; since there are two separate, shared keys involved, one realm can choose to trust the other realm’s tickets, but not the other way around.

When a user who is in the ONE.COM realm wishes to communicate with a Kerberized service in TWO.COM, the client program first requests a ticket for the remote realm’s Ticket Granting ...