HP TippingPoint Next Generation Firewall manual

Some BSD licenses additionally include a clause that restricts the use of the name of the project (or the names of its contributors) for endorsing or promoting derivative works.

The most basic definition of a derivative work is a product that is based on, or incorporates, one or more already existing works. This can become a complex issue, particularly with regard to software, but the primary indicator that a software program is a derivative of another program is if it includes source code from the original program, even if the source code has been modified, including improving, extending, reordering or translating it into another programming language.

Source code is the version of software (usually an application program or an operating system) as it is originally written (i.e., typed into a computer) by a human in plain text (i.e., human readable alphanumeric characters). Source code can be written in any of hundreds of programming languages, some of the most popular of which are C, C++ and Java.

Due to the extremely minimal restrictions of BSD-style licenses, software released under such licenses can be freely modified and used in proprietary (i.e., commercial) software for which the source code is kept secret.

It is possible for a product to be distributed under a BSD-style license and for some other license to apply as well. This was, in fact, the case with very early versions of BSD UNIX, which included both new code written at UCB and code from the original versions of UNIX written at Bell Labs.

BSD-style licenses have been very successful, and they are now widely used for a variety of software. Among the many products released under this class of licenses are all of the major modern descendants of the original BSD UNIX, i.e., FreeBSD, OpenBSD, NetBSD and Darwin (the foundation of the Mac OS X). BSD-licensed software is also commonly included in Linux distributions (i.e., versions) and has even been incorporated into some of the Microsoft Windows operating systems.

BSD Licenses Versus the GPL

The GPL (GNU General Public License) is by far the most widely used license for free software (i.e., software whose source code is available at no cost for anyone to use for any purpose). The Linux kernel (i.e., the core of the operating system) as well as much of the other software generally included in Linux distributions have been released under the terms of the GPL.

Although far fewer programs are released under BSD-style licenses, this class of licenses is disproportionately important because of the widespread use of BSD-licensed code in both free and proprietary operating systems.

Possibly the biggest difference between the GPL and BSD licenses is the fact that the former is a copyleft license and the latter is not. Copyleft is the application of copyright law to permit the free creation of derivative works but requiring that such works be redistributable under the same terms (i.e., the same license) as the original work.

Closely related to this is the fact that, in sharp contrast to the GPL, BSD-style licenses do not require that derivative works based on BSD-licensed software make the source code for such derivative works freely available. This allows the direct incorporation of code from open source projects (i.e., from BSD-licensed software) into closed source projects. The GPL, however, specifically states: "This General Public License does not permit incorporating your program into proprietary programs."

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