csDoom is a source port, originally based on ZDoom version 1.22, that was created in 2000 by Sergey "Fly" Makovkin. He intended to replace ZDoom's peer-to-peer multiplayer code with a client/server system using code from QuakeWorld. Makovkin achieved his goal, and by December 2000, csDoom already had a decently stable and well-functioning client/server multiplayer mode.
The early versions of csDoom received criticism, particularly from Tom "mystican" Robinson, for being distributed only in binary form. Fly soon relented and released the source code, but in July 2000, examination of the code revealed that a hard-coded ban of Robinson's IP address was in place. Additionally, a backdoor was present that would give a player remote console access to any server if their nickname was "Fly", "fly", or "FLY".
In January 2001, Makovkin announced that he no longer had enough time to work on csDoom. While the project remained on hiatus, a new port based on csDoom, ZDaemon, surfaced, and Skulltag began using csDoom's networking code. In 2004, development of csDoom was taken over by Denis Lukianov. The last version of csDoom was 1.3, released on January 28, 2006. Active development has moved on to the project Odamex, the unofficial successor to csDoom.