odamex (Online Doom Multiplayer Experience)

Odamex (Online Doom Multiplayer Experience) is a modification of DOOM
to allow players to compete with each other over the Internet using
the client/server architecture. It also plays single-player Doom
(and Doom 2) nicely.

You'll also need the game data (WAD file) from one of the following:

Shareware Doom v1.9 (DOOM1.WAD, aka 'doom_shareware_data' SBo build)
Ultimate Doom v1.9 (DOOM.WAD)
Doom II v1.9 (DOOM2.WAD)
Final Doom: TNT Evilution v1.9 (TNT.WAD)
Final Doom: The Plutonia Experiment v1.9 (PLUTONIA.WAD)
FreeDoom >=0.6.2 (FREEDOOM.WAD, aka 'freedoom' SBo build)
Chex Quest (CHEX.WAD, aka 'chexquest' SBo build)

Place the WAD file(s) in /usr/share/games/doom. The filenames can be
all lowercase (doom.wad) or all uppercase (DOOM.WAD).

Note: starting with odamex-12.3.0, SDL Mixer MIDI support is disabled
in this build. The default is now OPL emulation via the bundled ADL,
and the only other choice is PortMidi (also bundled). This means the
music may sound different when upgrading from an older version.

Note for 32-bit users: This requires SSE and SSE2. If your CPU is
newer than about 15 or 20 years old, you should be fine. If you get
'Illegal Instruction' errors at runtime, your CPU is too old (sorry).