When to create a realm trust
You can establish a realm trust between any non-Windows Kerberos version 5 (V5) realm and a Windows Server 2008 or a Windows Server 2008 R2 domain. This trust relationship allows cross-platform interoperability with security services that are based on other versions of the Kerberos V5 protocol, for example, UNIX and MIT implementations. Realm trusts can switch from nontransitive to transitive and back. Realm trusts can also be either one-way or two-way.
For information about how to create a realm trust, see Create a Realm Trust.