The ports SRCDS officially requires are: 27015 TCP/UDP (game transmission, pings and RCON) Can be changed using -port on startup 27020 UDP (SourceTV transmission) Can be changed using +tv_port on startup 27005 UDP (Client Port) Can be changed using -clientport on startup 26900 UDP (Steam Port, outgoing) Can be changed using -sport on startup SRCDS has also been spotted opening connections on 27005 and 51840 UDP, but these may be outbound only.
https://developer.valvesoftware.com/wiki/Source_Dedicated_Server
sudo ufw allow 27015/tcp sudo ufw allow 27015/udp sudo ufw allow 27020/udp