Doom Source License
The Doom Source License (or DSL) is the original source code license under which the Doom source code was released in late 1997. The Doom source was later re-released under the GNU GPL. As the license cannot be revoked, the Doom source is essentially dual licensed under the DSL and the GPL.
The DSL is significantly more restrictive than the GNU GPL, and as such is incompatible with it. For example, any form of commercial exploitation is forbidden. Heavy restrictions apply to the distribution of copies of the source code; the license grants permission only for the distribution of portions of the code for "educational use". The DSL is not an Open Source license (under the Open Source Definition) or a Free Software license.
Despite these restrictions, some source port authors nonetheless choose the DSL over the GPL for their ports. One reason for this is that the DSL does not include the copyleft provisions that the GPL includes, which requires the source code of any derivative version to be released. This is used as justification by the authors of source ports such as ZDaemon, who keep their source code secret.
In the past, another reason to use the DSL was that the GNU GPL was incompatible with the Raven source code license under which the Heretic and Hexen sources were originally released, while the DSL is compatible. By using the DSL, it was possible to use the Raven source code while avoiding infringing upon the GPL. This is no longer an issue as the Raven source code was re-released under the GPL.
Confusingly, source ports which use the DSL are not Open Source, even if the source code is still available; this is because the DSL does not meet the requirements for what is considered "open" under the Open Source Definition. Similarly, DSL source ports are also non-Free Software.
 License text
This is a verbatim copy of the Doom license text for reference purposes.
LIMITED USE SOFTWARE LICENSE AGREEMENT This Limited Use Software License Agreement (the "Agreement") is a legal agreement between you, the end-user, and Id Software, Inc. ("ID"). By downloading or purchasing the software material, which includes source code (the "Source Code"), artwork data, music and software tools (collectively, the "Software"), you are agreeing to be bound by the terms of this Agreement. If you do not agree to the terms of this Agreement, promptly destroy the Software you may have downloaded or copied. ID SOFTWARE LICENSE 1. Grant of License. ID grants to you the right to use the Software. You have no ownership or proprietary rights in or to the Software, or the Trademark. For purposes of this section, "use" means loading the Software into RAM, as well as installation on a hard disk or other storage device. 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