OpenArena

OpenArena is an open-source fork of Quake 3 Arena.

Firewall Configuration in NixOS for Local Games

If you want to play OpenArena in a LAN setting (LAN-Party, Office Tournament) the game will not show locally hosted games. That is unless you allow the necessary UDP ports:

networking.firewall.allowedUDPPorts = [ 27960 27961 27962 27963 ];