The "FreeBSD license" (that is, one of them)


"The compilation of software known as FreeBSD is distributed under the
following terms:

Copyright (c) 1992-2016 The FreeBSD Project. All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
1. Redistributions of source code must retain the above copyright
   notice, this list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright
   notice, this list of conditions and the following disclaimer in the
   documentation and/or other materials provided with the distribution.

THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
ANY [...]."
/usr/src/COPYRIGHT

Complying with this license should be easy, unfortunately
there are a bunch of different licenses involved. Most of
them aren't included in distribution tarballs other than
"src.txz".