I only get to work with Active Directory trusts every so often. I think multi-domain forests seem to be falling out of fashion. At least for those of us who've heard of federation. Regardless, here's an interesting issue I ran into the other day:
So I have this domain, contoso.com. In contoso.com, there is a one-way forest trust established with fabrikam.com, such that contoso.com trusts fabrikam.com. This way, users in fabrikam.com can access resources in contoso.com.
On pc1.contoso.com, I create a network file share. I add permissions for my own user account, contoso\ryan, to it. Then, I add permissions for fabrikam\steve to access the file share also. The operation was successful and the file share appears to be set correctly. However, when I go back and view the permissions for the file share again, it takes a very long time, as if it were waiting on something to time out, and then eventually, this is what I see:
So why is fabrikam\steve showing up as <Unknown Contact> when viewed from the contoso.com domain? What we have here, is a SID translation failure. But why? First, a little background. Here is what Microsoft says about users in your forest who are members of another forest:
"When a trust is established between a domain in a forest and a domain outside of that forest, security principals from the external domain can access resources in the internal domain. Active Directory creates a foreign security principal object in the internal domain to represent each security principal from the trusted external domain. These foreign security principals can become members of domain local groups in the internal domain. Directory objects for foreign security principals are created by Active Directory and should not be manually modified. You can view foreign security principal objects from Active Directory Users and Computers by enabling advanced features." [ Source ]
So if you go look in the ForeignSecurityPrincipals container in contoso.com, you'll see an object that represents the user account of fabrikam\steve, but his friendly name or samAccountName is not part of that record. It's just a SID. When we pull up a permissions file dialog box like the one above, Windows attempts a SID to name translation... but it fails. There's a little bit of technical documentation on how SID translation occurs:
"LSA on the computer that the call is sent to (using the LSA RPC interface) will resolve the SIDs it can map and send on the remaining unresolved SIDs to a domain controller in the primary domain. The domain controller will resolve additional SIDs to account names from the local database, including SIDs found in SidHistory on a global catalog.
If SIDs cannot be resolved there, the domain controller will send remaining SIDs to domain controllers in a trusted domain where the domain part of the SID matches the trust information." [ Source ]
There are many functions regarding SID lookups, and I don't know exactly which ones are used at each location, but the general concept is the same and you can see how this procedure could take a while to time out. And when it fails, you see <Unknown Contact>. Or maybe just the unresolved SID of security principal. Depends on which version of Windows you're using and exactly which dialog box you're looking at.
The reason it happens in our scenario is because of the one-way trust. Contoso.com cannot call upon fabrikam to translate SIDs from its forest, because fabrikam does not trust contoso.
To fix it, we could allow anonymous SID translation in fabrikam... but for many that is an unacceptable security risk. Or we could make the trust two-way. Or, if you're unable to do either of those things, you could at least create a security group in contoso, add the individuals from fabrikam to that group, and just assign the group to the network share ACL. The functionality would be the same but at least you wouldn't have to look at "<Unknown Contact>" every time you opened that dialog box.
For more information on this, see the ServerFault question that I answered here, as well as the much better Ask the Directory Services team blog post here.