This is a fairly specific issue, but I'm at a loss for reasons to what might be happening.
We have SQL Server 2008 running on what we'll call ServerA. We can RDP in ServerA, we can ping ServerA. If we RDP into ServerA, we can open up SQL Server Management Studio and connect to all of the database instances running on ServerA just fine.
We can't connect to any of those instances from anywhere else on the network. I don't think its a network problem, because if I can RDP and ping ServerA, then the network is fine. And I don't think its a permission thing, because if I'm RDP'd in, I can connect to all the instances just fine. It's almost like I've got some secret safety setting turned on that's preventing connections from any IP other than localhost, or something like that.
Does such a safety setting exist? I wouldn't know how to get into this state even I wanted to, so I have no idea how to get out of it.