Licences

The various incarnations of Doom have been available under a variety of software licences.

Pre-release
A series of alphas and a press release beta version existed prior to the 1.0 release of Doom. These versions were for private testers only and were not intended for public distribution.

The alpha releases of doom carried various warnings about leaks, including the following in the Doom 0.4 menu:

HEY,MAN-IF THIS GETS OUT, YOU CAN ALL KISS YOUR AUTOGRAPHED DOOM HINTBOOKS GOODBYE.(c)1993 id Software

The alphas and press release version of doom carry the following disclaimer in their ENDOOM lumps:

This is the alpha version of DOOM. If you are not a beta tester, then you are quite the bad person. Delete your copy of DOOM right now and you will be safe from our wrath.

After the success of Doom, id Software gave Frans P. de Vries, the then-idgames archive maintainer, permission to upload the alphas and press release due to their historic interest.

Shareware episode
The episode Knee-Deep in the Dead was released as shareware on the  server on December 10, 1993.

EULA
The registered version of Doom was under a standard end-user license agreement (referred to as a "Limited Use Software License Agreement") between the purchaser and id Software, limiting rights to copy, distribute, reverse engineer, and to some extents modify the program.

It explicitly allows the user to create their own modifications for the game, with the restriction of being intended to only be used with the game. Including graphics from the game in these mods is specifically mentioned as being allowed, but no mention is made of other assets such as sounds, music, and maps.

Data Utility License
id Software introduced the Data Utility License in 1994 in an effort to protect themselves from possible issues that they felt might arise in the then-nascent editing community. Several prominent editing utility authors were asked to sign this agreement and to enforce some of its terms in turn on their end users.

Doom II
Doom II was the first traditionally commercial release of Doom. It was first sold boxed in high street stores on September 30, 1994 and no shareware or demo version was released.

Source code release
ID Software released a modified version of the Doom source code to the public on December 23, 1997. This release was under the DOOM Source Code License, described within the file DOOMLIC.TXT.

This licence permitted use the source code for educational purposes only. It explicitly forbid selling the source code or using it for commercial gain as well as distributing the source code.

Heretic and Hexen source code release
Raven Software's Heretic and Hexen were initially released under a restrictive EULA, similar to but even less permissive than the Doom source license. This was eventually remedied with a GPL re-release after an extensive community effort to petition for the change.

GNU General Public License version 2 (GPLv2)
After the loss of the source code for the glDoom source port in 1998 due to a hard drive crash, some people, including John Carmack, suggested that if the author had been required to distribute the changes he made to the code, then it would have been more likely that others would have had copies of the source. As a result, id Software re-licensed the source code for Doom under the terms of the version 2 on October 3, 1999. Many existing source ports followed suit by obtaining permission from their copyright holders to re-license their work.