Why the GNU Free Documentation License is not suitable for Debian main
Within the Debian community there has been a significant amount of concern about the GNU Free Documentation License (GFDL), and whether it is, in fact, a "free" license. This document attempts to explain why Debian's answer is that it is not free enough for the Debian distribution.
It should be noted that this does not imply any hostility towards the Free Software Foundation, and does not mean that GFDL documentation should not be considered "free enough" by others. Debian itself will continue distributing GFDL documentation in its "non-free" section, which does not have such strict requirements.
This document covers the GFDL version 1.2, which is the most current version at the time of writing. Earlier versions of the GFDL have similar, related problems.
What is the GFDL?
The GFDL is a license written by the Free Software Foundation, who use it as a license for their own documentation and promote it to others. Notably, it is also used as Wikipedia's license. The GFDL is a "copyleft" license in that modifications to documentation made under the GFDL must in turn be released under the GFDL, not some more restrictive license.
How does it fail to meet Debian's standards for Free Software?
The GFDL conflicts with Debian's traditional requirements for free software in a variety of ways, some of which are expanded upon below. As a copyleft license, one of the consequences of this is that it is not possible to include content from GFDL documentation directly into free software.
The major conflicts are:
The most troublesome conflict concerns the class of unmodifiable sections that, once included, may not be modified or removed from the documentation in the future. These are Cover Texts, Dedications, Acknowledgements, and Invariant Sections. Modifiability is a fundamental requirement of the DFSG, which states:
3. Derived Works The license must allow modifications and derived works, and must allow them to be distributed under the same terms as the license of the original software.
These components create particular problems in reusing small portions of the work (since any invariant sections must be included also, however large), and in making sure that documentation remains accurate and relevant.
The second conflict is related to the GFDL's requirements for "transparent copies" of documentation (that is, a copy of the documentation in a form suitable for editing). In particular, Section 3 of the GFDL requires that a transparent copy of the documentation be included with every opaque copy distributed, or that a transparent copy be made available for a year after the opaque copies are no longer being distributed.
For free software works, Debian expects that simply providing the source (or transparent copy) alongside derivative works will be sufficient, and that users need not be forced to obtain the source with every copy of the binary they download, but this does not satisfy either clause of the GFDL's requirements.
Digital Rights Management
The third conflict with the GFDL arises from the measures in Section 2 that attempt to overcome Digital Rights Management (DRM) technologies. In particular, the GFDL states that "You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute". This inhibits freedom in three ways: it limits use of the documentation as well as distribution, by covering all copies made, as well as copies distributed; it rules out distributing copies on DRM-protected media, even if done in such a way as to give users full access to a transparent copy of the work; and, as written, it also potentially disallows encrypting the documentation, or even storing it on a system that provides user restrictions or file permissions for the documentation.
Why does documentation need to be Free Software?
The question of "Why does software need free documentation?" has been addressed in the past by the Free Software Foundation in the essay Free Software and Free Manuals.
There are a number of obvious differences between programs and documentation that often inspire people to ask "why not simply have different standards for the two?" For example, books are often written by individuals, while programs are written by teams, so proper credit for a book might be more important than proper credit for a program.
On the other hand, free software is often written by a single person, and free software documentation is often written by a larger group of contributors. Even the line between what is documentation and what is a program is not always so clear, as content from one is often needed in the other (to provide online help, or to provide screenshots or interactive tutorials, or to provide a more detailed explanation by quoting some of the source code). Similarly, while not all programs demonstrate creativity or could be considered "works of art", some can, and trying to determine which is the case for all the software in Debian would be a distraction from our goals.
In practice, then, particularly for Debian's purposes, documentation simply isn't different enough to warrant different standards in the freedoms we expect for our users: we still wish to provide source code in the same manner as for programs, we still wish to be able to modify and update documentation, we still wish to be able to reuse portions of documentation elsewhere as conveniently as possible, and we still wish to be able to provide our users with exactly the documentation they want, without extraneous materials.
How can this be fixed?
What, then, can documentation authors and others do about this?
An easy first step is to not include the optional invariant sections in your documentation, since they are not required by the license, but are simply an option open to authors.
Unfortunately this alone is not enough, as other clauses of the GFDL render all GFDL documentation unsuitable for Debian. As a consequence, other licenses should be investigated; generally it is probably simplest to use the same license for the documentation as for the software it documents, or for documentation that doesn't come with a particular piece of software, to choose either the GNU General Public License (for a copyleft license) or one of the BSD or MIT licenses (for a non-copyleft license).
As most GFDL documentation is made available under "the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation", the Free Software Foundation is able to remedy these problems for a great many works by issuing a new version of the license. The problems discussed above require relatively minor changes to the GFDL -- allowing invariant sections to be removed, allowing transparent copies to be made available concurrently, and moderating the restrictions on technical measures. Unfortunately, while members of the Debian Project have been in contact with the FSF about these concerns since 2001, these negotiations have not come to any conclusion to date.