Team Fortress 2 has recently gone free to play, which I am guessing has resulted in a big spike in players (it is currently the most played game on Steam, with a peak of nearly 100 000 players today – no doubt it was busier at the weekend). As a result, my old post on fixing the “lost connection to server” issue has been very popular in the last few days.
I find it surprising this problem has persisted for over three years without apparent resolution. The fix that worked for me on my old BeBox was to forward to following ports to my PC:
TCP 27030 – 27039
UDP 1200
UDP 27000 – 27015
If this doesn’t fix the problem, there may be complicating factors. I’ve seen suggestions that the TCP timeout on a router may be set too low. I have a feeling that this may be what is causing connection problems a friend of mine is having, but since it does not affect me I haven’t investigated further. Wireshark would probably be useful if you wanted to probe further.
You can also take a look at my old post if you want more information.