This is an intermittent problem that has plagued me for a decade, at least. I have a program that makes use of a SQL Server Express instance running on a different computer. This might work fine for a long time, then one day the computer running the program simply refuses to see the SQL Server instance.

I started playing around with WireShark to see what was going on, but when I did that, all was well. I got several dumps of the connection working as expected.

Today, it stopped working, so I started looking into it again. For one thing, I see a UDP request go out asking for information about the SQL Server instance. A reply comes back promptly with all the information. So, THAT much is working just fine. In fact, the program will (eventually) show all the available SQL Server instances, of which there are three on the target computer (for no good reason). Those three are available, but I still can't connect to them.

If I ping the other computer, the ping times out, either with the computer name or the IP address.

Looking at the WireShark traffic around the connection attempt, after the UDP request and response, the computer attempting to connect tries to establish a TCP connection (though not to the SQL Server port, which is odd). This attempt is repeated every second or two for a good long time. In among those repeated attempts, the computer trying to connect makes a few MDNS requests asking about the computer that has the SQL Server. There never seems to be a response to those requests.

At a later point in the program, once I have the SQL Server instances, I attempt to connect to one of them. Looking at the WireShark traffic around that attempt, it's just a long sequence of requests to MSDNS and other services asking for information about the other computer. No reply is ever received.

So, it appears that the other computer is sometimes responding (the UDP request for the DB is replied to promptly), but other times not responding (those TCP calls and MSDNS calls).

Does this suggest anything?