Roman's Weekly SQL Server Tip - Using Windows Authentication across domains
If you try to connect to a SQL Server in another domain in the forest using Windows Authentication, you might end up getting this error:
A Connection could not be established to YourServer.
Reason: Login failed for user '(null)'. Reason: Not associated with a trusted SQL Server connection...
This is a common problem on some networks, basically TCP/IP is not passing windows credentials along and a trust relationship is not established across domains. This is a complex issue, you can get more details in this KB article.
But, you still might be able to connect with the Named Pipes protocol instead of TCP/IP, that usually does the trick for me. You can create an alias with SQL Server Client Network Utility and select Named Pipes as the protocol and then use the alias in your connection strings or when registering the server in Enterprise Manager. Another option for enforcing NP is to include “Network Library=dbnmpntw;“ in the connection string.