Licensing
All of the NetBSD kernel and most of the core userland source code is released under the terms of the BSD License (two, three, and four-clause variants). This essentially allows everyone to use, modify, redistribute or sell it as they wish, as long as they do not remove the copyright notice and license text (the four-clause variants also include terms relating to publicity material). Thus, the development of products based on NetBSD is possible without having to make modifications to the source code public. In contrast, the GPL stipulates that changes to source code of a product must be released to the product recipient when products derived from those changes are released.
On 20 June 2008, the NetBSD Foundation announced a transition to the two clause BSD license, citing concerns with UCB support of clause 3 and industry applicability of clause 4.
NetBSD also includes the GNU development tools and other packages, which are covered by the GPL and other open source licenses. As is the case for the other BSD projects, NetBSD separates those in its base source tree, in order to make removal of code under more restrictive licenses easier. As for packages, the installed software licenses may be controlled by modifying the list of allowed licenses in the pkgsrc configuration file (mk.conf).
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