this is only a request and not a condition, such material can still be included in OpenBSD. It should be noted that existing versions of Mach are still subject to AT&T copyrights, which prevents the general distribution of Mach sources. Apache The original Apache copyright is similar to the Berkeley copyright, except that it stipulates that products derived from the code may not have "Apache" in their name. The purpose of this clause is to avoid a situation in which another party releases a modified version of the code named in such a way to make users think that it is the "official" version. This is not an issue with OpenBSD because OpenBSD is a Compilation , and not a Derived Work . Source code published under version 2 of the Apache license cannot be included into OpenBSD. As a consequence, OpenBSD now maintains its own version of Apache based on version 1.3.29. The OpenBSD version includes many enhancements and bugfixes. ISC The ISC copyright is functionally equivalent to a two-term BSD copyright with language removed that is made unnecessary by the Berne convention. This is the preferred license for new code incorporated into OpenBSD. A sample license is included in the source tree as /usr/src/share/misc/license.template . GNU General Public License, GPL, LGPL, copyleft, etc. The GNU Public License and licenses modeled on it impose the restriction that source code must be distributed or made available for all works that are derivatives of the GNU copyrighted code. While this may