I have SQL Server Express 2017 running on Windows Server 2016 Standard (default instance, not named). It has remote connections enabled and is listening on port 1433 and has TCP/IP and Named Pipes enabled. I have several Surface Tablets running Windows 10 and Windows 11. These tablets make a VPN connection to the server to connect to SQL Server. All of the tablets, except one of them, can connect to SQL Server. All tablets connect using the server's IP address and with SQL Server Authentication. All tablets are connected to the same WiFi router - both the ones that CAN connect and the one that CANNOT, so I believe router settings are not the problem.
The tablet that fails to connect can ping the server successfully. It cannot, however, telnet to port 1433 on the server - this times out. All other tablets can make the telnet connection. Also, using sqlcmd to connect (sqlcmd -S ip-address -U user-name -P password) works on all tablets except the one. This returns error 53. Checking the SQL Server logs after attempting to connect using sqlcmd shows no errors. So this tablet is definitely not even reaching SQL Server.
I have disabled all Windows Firewall options on the tablet with no change - still cannot telnet or connect via sqlcmd.
I have walked through multiple remote connection troubleshooting guides step by step, but most of them assume that NO remote systems can connect to SQL Server. In this case, it is just one system. So I know that the server is configured properly to allow remote connections. I just cannot determine what is different about this one tablet that is preventing it from making a connection.
What might be preventing this one system from making this connection? Any settings or other options I should be looking at?
SOLVED: After performing tracert on multiple systems that connect to this server including the problem tablet as well as attempting to telnet to various ports at the server's IP address, I discovered that the WiFi network that the tablet was on had a conflicting IP address with the server. The previous tablets that tested fine were, unbeknownst to me, on different wifi networks that did not have this conflict. As a result, this tablet was attempting to connect to a completely different device despite being properly connected to the server's network via VPN.
So the additional piece of advice to add to this troubleshooting process would be to very closely scrutinize the output of ipconfig /all. Even though you may be connected to the network of the SQL Server system you are trying to connect to remotely, if the IP of the SQL Server system is duplicated on your local network, it can be very difficult to see that all of your connection attempts are actually routing to a different system - that is why the connection is failing.
What to look for in ipconfig /all... check the client system's IP address and the default gateway that it is using. If these are using private IP addresses (as most do) most commonly starting with 192.168.x.x, and you are trying to connect to SQL Server over VPN via which the server also has a private IP address, check if your local subnet is matching the server's subnet. For instance, both the client (tablet) subnet and the server subnet were 192.168.20.x There's a chance for an IP address conflict in these conditions.
Another check that I found was helpful was, on the client, to DISCONNECT from the remote server and then try to ping the SQL Server IP address. If the ping succeeds, the server's IP address is being duplicated by another system.