The Debian Free Software Guidelines (DFSG) are a short, ten-point test that the Debian project uses to decide whether a piece of software is free enough to belong in its main archive. They were adopted in 1997 as part of Debian’s Social Contract, drafted largely by Bruce Perens with input from the Debian developer community, and they remain in force today.
The guidelines are deliberately concrete rather than philosophical. As published on debian.org, they require: free redistribution (no fee or restriction on selling or giving the software away); availability of source code; permission to make and distribute derived works under the same terms; protection of the integrity of the author’s source code (which may, as a compromise, require modifications to ship as separate patch files); no discrimination against persons or groups; and no discrimination against fields of endeavor, so the software cannot be barred from, say, commercial or military use.
The remaining points govern the license itself rather than the software. The rights attached to a program must apply automatically to everyone who receives it, without requiring an additional agreement. The license must not be specific to Debian, so the same freedoms hold no matter where the software is obtained. The license must not “contaminate” other software by imposing restrictions on unrelated programs distributed alongside it. Finally, the guidelines name example licenses, such as the GPL, BSD, and Artistic licenses, that are considered to satisfy the criteria.
The DFSG’s importance reaches well beyond Debian. When the Open Source Initiative was founded in 1998 to promote the same kind of software under the new label “open source,” it needed a precise definition of what counted. The Open Source Definition was created by taking the DFSG and lightly editing it, mainly by removing the references that were specific to Debian. In effect, Debian’s internal admission test became the industry-wide standard for what “open source” means.
By writing down clear, testable rules instead of relying on case-by-case judgment, the DFSG gave the free and open source world a shared yardstick. A great deal of later license analysis, including the question of whether a new or modified license is genuinely open source, still traces back to the ten guidelines Debian set out in 1997.