The BSD operating systems are not clones, but open source derivatives of AT&T's Research UNIX® operating system, which is also the ancestor of the modern UNIX® System V. This may surprise you. How could that happen when AT&T has never released its code as open source?

It is true that AT&T UNIX® is not open source, and in a copyright sense BSD is very definitely not UNIX®, but on the other hand, AT&T has imported sources from other projects, noticeably the Computer Sciences Research Group (CSRG) of the University of California in Berkeley, CA. Starting in 1976, the CSRG started releasing tapes of their software, calling them Berkeley Software Distribution or BSD.

Initial BSD releases consisted mainly of user programs, but that changed dramatically when the CSRG landed a contract with the Defense Advanced Research Projects Agency (DARPA) to upgrade the communications protocols on their network, ARPANET. The new protocols were known as the Internet Protocols, later TCP/IP after the most important protocols. The first widely distributed implementation was part of 4.2BSD, in 1982.

In the course of the 1980s, a number of new workstation companies sprang up. Many preferred to license UNIX® rather than developing operating systems for themselves. In particular, Sun Microsystems licensed UNIX® and implemented a version of 4.2BSD, which they called SunOS™. When AT&T themselves were allowed to sell UNIX® commercially, they started with a somewhat bare-bones implementation called System III, to be quickly followed by System V. The System V code base did not include networking, so all implementations included additional software from the BSD, including the TCP/IP software, but also utilities such as the csh shell and the vi editor. Collectively, these enhancements were known as the Berkeley Extensions.

The BSD tapes contained AT&T source code and thus required a UNIX® source license. By 1990, the CSRG's funding was running out, and it faced closure. Some members of the group decided to release the BSD code, which was Open Source, without the AT&T proprietary code. This finally happened with the Networking Tape 2, usually known as Net/2. Net/2 was not a complete operating system: about 20% of the kernel code was missing. One of the CSRG members, William F. Jolitz, wrote the remaining code and released it in early 1992 as 386BSD. At the same time, another group of ex-CSRG members formed a commercial company called Berkeley Software Design Inc. and released a beta version of an operating system called BSD/386, which was based on the same sources. The name of the operating system was later changed to BSD/OS.

386BSD never became a stable operating system. Instead, two other projects split off from it in 1993: NetBSD and FreeBSD. The two projects originally diverged due to differences in patience waiting for improvements to 386BSD: the NetBSD people started early in the year, and the first version of FreeBSD was not ready until the end of the year. In the meantime, the code base had diverged sufficiently to make it difficult to merge. In addition, the projects had different aims, as we will see below. In 1996, OpenBSD split off from NetBSD, and in 2003, DragonFlyBSD split off from FreeBSD.