FreeBSD distributed under "2 clause" BSD license 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. Don't claim that you wrote the code Don't blame us if the code doesn't work Apart from that, do anything you want with the code The GPL mandates that source code be disclosed The BSD License allows source code changes to be kept secret GPL is often categorised as "Copyleft", as distinct from "Copyright" If you like that nomenclature, you may think of the BSD License is "Copycentre". We actively encourage third parties to use the source code. Donating changes back is purely at the discretion of the party making the changes.