“Failed to join domain” trying to add Windows 2008 R2 Core to domain

Today I was using SCONFIG (if you don’t know what SCONFIG is, check out these posts to add a Windows 2008 R2 core server to an existing domain. I had set the IP address and ensured that the domain name and credentials are correct, but I kept getting ‘failed to join domain’.

I tried pre-creating the computer objects in the AD but to no avail. Finally after some troubleshooting it turned out to be a most basic issue: the primary DNS IP was not set correctly. After setting that (again with SCONFIG) I was able to add the Windows core box into the domain.