From what I've seen, JJ2 from startup uses the same UDP port chosen from the ephemeral port range until the exe is closed. I'm guessing it wouldn't be too hard to force JJ2 to use a specific port, although I imagine that would have its own problems. Alternatively (and much easier to do) JJ2 could indicate to the user which UDP port is being used and thus which port should be forwarded, but that of course lacks practicality. I should really learn more about sockets.