I've been running a server for years and am very familiar with it. I have my server running on a Linux machine. The folks who have been using it haven't been in in a while and they contacted me today to tell me it was "not working". I updated the server to their new client version (1.12.2), and they still couldn't connect. I checked the networking and noticed the server was behaving "normally" (that is, as it always has), by serving my server port on TCP. My client on my machine (v1.12.2 on Mac OS X) can contact it without a problem, and does so using TCP. However, the Windows client my friends are using is trying to use UDP, which I've never seen from a Minecraft client in the past. I can't figure out how to either force their client to use TCP as it has in the past, or force my server and my client to use UDP.
I have gobs of captures and screenshots if it helps, but the basic question is, how do you set the server or client protocol (TCP or UDP)?
OK, maybe that was too much text. How about this - why is the 1.12.2 Minecraft server listening on and responding to TCP, and the Mac OS X 1.12.2 client making requests on TCP, but the 1.12.2 Windows client making requests using UDP, to which the server will not respond?