Kerberos Forest Search Order: In some situations, flat names may be used and will format the SPN request made by the client as “HTTP/ServerName” (for example) and not include the domain suffix. The domain suffix is important because the user will always go to its local domain’s KDC which uses the domain suffix to identify which Kerberos Realm it should direct the user. There is a GPO setting that can be configured either for the client or the KDC which lists out other realms where it can check for a matching SPN. A configuration that I found useful is to list the Workstation Forest in the KFSO GPO applied to the workstation. This was helpful in a situation where users have been migrated to the new AD Forest, but their workstations have yet to be migrated. Let’s watch this play out in a cartoon entitled “The Adventures of Joey Adatum!”
The beauty of this solution is that there would be no performance impact (as described in the TechNet documentation) because the workstation is just asking its local Forest and would only be relevant if the logged on user was from a different Forest.
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