Berkeley Software Distribution (BSD) is a Unix operating system derivative developed and distributed by the Computer Systems Research Group (CSRG) of the University of California, Berkeley, from 1977 to 1995. Today the term "BSD" is often used non-specifically to refer to any of the BSD descendants which together form a branch of the family of Unix-like operating systems. Operating systems derived from the original BSD code remain actively developed and widely used.
Historically, BSD has been considered a branch of Unix, Berkeley Unix, because it shared the initial codebase and design with the original AT&T Unix operating system. In the 1980s, BSD was widely adopted by vendors of workstation-class systems in the form of proprietary Unix variants such as DEC ULTRIX and Sun Microsystems SunOS. This can be attributed to the ease with which it could be licensed, and the familiarity the founders of many technology companies of the time had with it.
Although these proprietary BSD derivatives were largely superseded by the UNIX System V Release 4 and OSF/1 systems in the 1990s (both of which incorporated BSD code and are the basis of other modern Unix systems), later BSD releases provided a basis for several open source development projects, e.g. FreeBSD, OpenBSD, NetBSD, Darwin or PC-BSD, that are ongoing. These, in turn, have been incorporated in whole or in part in modern proprietary operating systems, e.g. the TCP/IP networking code in Windows NT 3.1 and most of the foundation of Apple's OS X and iOS.
The earliest distributions of Unix from Bell Labs in the 1970s included the source code to the operating system, allowing researchers at universities to modify and extend Unix. The operating system arrived at Berkeley in 1974, at the request of computer science professor Bob Fabry who had been on the program committee for the Symposium on Operating Systems Principles where Unix was first presented. A PDP-11/45 was bought to run the system, but for budgetary reasons, this machine was shared with the mathematics and statistics groups at Berkeley, who used RSTS, so that Unix only ran on the machine eight hours per day (sometimes during the day, sometimes during the night). A larger PDP-11/70 was installed at Berkeley the following year, using money from the Ingres database project.
Also in 1975, Ken Thompson took a sabbatical from Bell Labs and came to Berkeley as a visiting professor. He helped to install Version 6 Unix and started working on a Pascal implementation for the system. Graduate students Chuck Haley and Bill Joy improved Thompson's Pascal and implemented an improved text editor, ex. Other universities became interested in the software at Berkeley, and so in 1977 Joy started compiling the first Berkeley Software Distribution (1BSD), which was released on March 9, 1978. 1BSD was an add-on to Version 6 Unix rather than a complete operating system in its own right. Some thirty copies were sent out.
The Second Berkeley Software Distribution (2BSD), released in May 1979, included updated versions of the 1BSD software as well as two new programs by Joy that persist on Unix systems to this day: the vi text editor (a visual version of ex) and the C shell. Some 75 copies of 2BSD were sent out by Bill Joy. A further feature was a networking package called Berknet, developed by Eric Schmidt as part of his master's thesis work, that could connect up to twenty-six computers and provided email and file transfer.
After 3BSD (see below) had came out for the VAX line of computers, new releases of 2BSD for the PDP-11 were still issued and distributed through USENIX; for example, 1982's 2.8.1BSD included a collection of fixes for performance problems in Version 7 Unix, and later releases contained ports of changes from the VAX-based releases of BSD back to the PDP-11 architecture. 2.9BSD from 1983 included code from 4.1cBSD, and was the first release that was a full OS (a modified V7 Unix) rather than a set of applications and patches. The most recent release, 2.11BSD, was first issued in 1992. As of 2008, maintenance updates from volunteers are still continuing, with patch 448 being released on June 17, 2012.
A VAX computer was installed at Berkeley in 1978, but the port of Unix to the VAX architecture, UNIX/32V, did not take advantage of the VAX's virtual memory capabilities. The kernel of 32V was largely rewritten by Berkeley students to include a virtual memory implementation, and a complete operating system including the new kernel, ports of the 2BSD utilities to the VAX, and the utilities from 32V was released as 3BSD at the end of 1979. 3BSD was also alternatively called Virtual VAX/UNIX or VMUNIX (for Virtual Memory Unix), and BSD kernel images were normally called /vmunix until 4.4BSD.
The success of 3BSD was a major factor in the Defense Advanced Research Projects Agency's (DARPA) decision to fund Berkeley's Computer Systems Research Group (CSRG), which would develop a standard Unix platform for future DARPA research in the VLSI Project.
4BSD (November 1980) offered a number of enhancements over 3BSD, notably job control in the previously released csh, delivermail (the antecedent of sendmail), "reliable" signals, and the Curses programming library. In a 1985 review of BSD releases, John Quarterman et al., wrote:
4BSD was the operating system of choice for VAXs from the beginning until the release of System III (1979–1982) [...] Most organizations would buy a 32V license and order 4BSD from Berkeley without ever bothering to get a 32V tape. Many installations inside the Bell System ran 4.1BSD (many still do, and many others run 4.2BSD).
4.1BSD (June 1981) was a response to criticisms of BSD's performance relative to the dominant VAX operating system, VMS. The 4.1BSD kernel was systematically tuned up by Bill Joy until it could perform as well as VMS on several benchmarks. The release would have been called 5BSD, but after objections from AT&T the name was changed; AT&T feared confusion with AT&T's UNIX System V.
4.2BSD (August 1983) would take over two years to implement and contained several major overhauls. Before its official release came three intermediate versions: 4.1a incorporated a modified version of BBN's preliminary TCP/IP implementation; 4.1b included the new Berkeley Fast File System, implemented by Marshall Kirk McKusick; and 4.1c was an interim release during the last few months of 4.2BSD's development. Back at Bell Labs, 4.1cBSD became the basis of the Eight Edition of Research Unix, and a commercially supported version was available from mtXinu.
To guide the design of 4.2BSD, Duane Adams of DARPA formed a "steering committee" consisting of Bob Fabry, Bill Joy and Sam Leffler from UCB, Alan Nemeth and Rob Gurwitz from BBN, Dennis Ritchie from Bell Labs, Keith Lantz from Stanford, Rick Rashid from Carnegie-Mellon, Bert Halstead from MIT, Dan Lynch from ISI, and Gerald J. Popek of UCLA. The committee met from April 1981 to June 1983.
Apart from the Fast File System, several features from outside contributors were accepted, including disk quotas and job control. Sun Microsystems provided testing on its Motorola 68000 machines prior to release, ensuring portability of the system.
The official 4.2BSD release came in August 1983. It was notable as the first version released after the 1982 departure of Bill Joy to co-found Sun Microsystems; Mike Karels and Marshall Kirk McKusick took on leadership roles within the project from that point forward. On a lighter note, it also marked the debut of BSD's daemon mascot in a drawing by John Lasseter that appeared on the cover of the printed manuals distributed by USENIX.
4.3BSD was released in June 1986. Its main changes were to improve the performance of many of the new contributions of 4.2BSD that had not been as heavily tuned as the 4.1BSD code. Prior to the release, BSD's implementation of TCP/IP had diverged considerably from BBN's official implementation. After several months of testing, DARPA determined that the 4.2BSD version was superior and would remain in 4.3BSD. (See also History of the Internet.)
After 4.3BSD, it was determined that BSD would move away from the aging VAX platform. The Power 6/32 platform (codenamed "Tahoe") developed by Computer Consoles Inc. seemed promising at the time, but was abandoned by its developers shortly thereafter. Nonetheless, the 4.3BSD-Tahoe port (June 1988) proved valuable, as it led to a separation of machine-dependent and machine-independent code in BSD which would improve the system's future portability.
Apart from portability, the CSRG worked on an implementation of the OSI network protocol stack, improvements to the kernel virtual memory system and (with Van Jacobson of LBL) new TCP/IP algorithms to accommodate the growth of the internet.
Until then, all versions of BSD incorporated proprietary AT&T Unix code and were, therefore, subject to an AT&T software license. Source code licenses had become very expensive and several outside parties had expressed interest in a separate release of the networking code, which had been developed entirely outside AT&T and would not be subject to the licensing requirement. This led to Networking Release 1 (Net/1), which was made available to non-licensees of AT&T code and was freely redistributable under the terms of the BSD license. It was released in June 1989.
4.3BSD-Reno came in early 1990. It was an interim release during the early development of 4.4BSD, and its use was considered a "gamble", hence the naming after the gambling center of Reno, Nevada. This release was explicitly moving towards POSIX compliance, and, according to some, away from the BSD philosophy (as POSIX is very much based on System V, and Reno was quite bloated compared to previous releases). Among the new features were an NFS implementation from the University of Guelph and support for the HP 9000 range of computers, originating in the University of Utah's "HPBSD" port.
In August 2006, InformationWeek magazine rated 4.3BSD as the "Greatest Software Ever Written". They commented: "BSD 4.3 represents the single biggest theoretical undergirder of the Internet."
Net/2 and legal troubles
After Net/1, BSD developer Keith Bostic proposed that more non-AT&T sections of the BSD system be released under the same license as Net/1. To this end, he started a project to reimplement most of the standard Unix utilities without using the AT&T code. For example, vi, which had been based on the original Unix version of ed, was rewritten as nvi (new vi). Within eighteen months, all of the AT&T utilities had been replaced, and it was determined that only a few AT&T files remained in the kernel. These files were removed, and the result was the June 1991 release of Networking Release 2 (Net/2), a nearly complete operating system that was freely distributable.
Net/2 was the basis for two separate ports of BSD to the Intel 80386 architecture: the free 386BSD by William Jolitz and the proprietary BSD/386 (later renamed BSD/OS) by Berkeley Software Design (BSDi). 386BSD itself was short-lived, but became the initial code base of the NetBSD and FreeBSD projects that were started shortly thereafter.
BSDi soon found itself in legal trouble with AT&T's Unix System Laboratories (USL) subsidiary, then the owners of the System V copyright and the Unix trademark. The USL v. BSDi lawsuit was filed in 1992 and led to an injunction on the distribution of Net/2 until the validity of USL's copyright claims on the source could be determined.
The lawsuit slowed development of the free-software descendants of BSD for nearly two years while their legal status was in question, and as a result systems based on the Linux kernel, which did not have such legal ambiguity, gained greater support. Although not released until 1992, development of 386BSD predated that of Linux. Linus Torvalds has said that if 386BSD or the GNU kernel had been available at the time, he probably would not have created Linux.
4.4BSD and descendants
See also: Comparison of BSD operating systems
The lawsuit was settled in January 1994, largely in Berkeley's favor. Of the 18,000 files in the Berkeley distribution, only three had to be removed and 70 modified to show USL copyright notices. A further condition of the settlement was that USL would not file further lawsuits against users and distributors of the Berkeley-owned code in the upcoming 4.4BSD release. Marshall Kirk McKusick summarizes the lawsuit and its outcome:
Code copying and theft of trade secrets was alleged. The actual infringing code was not identified for nearly two years. The lawsuit could have dragged on for much longer but for the fact that Novell bought USL from AT&T and sought a settlement. In the end, three files were removed from the 18,000 that made up the distribution, and a number of minor changes were made to other files. In addition, the University agreed to add USL copyrights to about 70 files, with the stipulation that those files continued to be freely redistributed.
In June 1994, 4.4BSD was released in two forms: the freely distributable 4.4BSD-Lite contained no AT&T source, whereas 4.4BSD-Encumbered was available, as earlier releases had been, only to AT&T licensees.
The final release from Berkeley was 1995's 4.4BSD-Lite Release 2, after which the CSRG was dissolved and development of BSD at Berkeley ceased. Since then, several variants based directly or indirectly on 4.4BSD-Lite (such as FreeBSD, NetBSD, OpenBSD and DragonFly BSD) have been maintained.
In addition, the permissive nature of the BSD license has allowed many other operating systems, both free and proprietary, to incorporate BSD code. For example, Microsoft Windows has used BSD-derived code in its implementation of TCP/IP and bundles recompiled versions of BSD's command-line networking tools since Windows 2000. Also Darwin, the system on which Apple's Mac OS X is built, is a derivative of 4.4BSD-Lite2 and FreeBSD. Various commercial Unix operating systems, such as Solaris, also contain varying amounts of BSD code.
Relationship to Research Unix
Starting with the 8th Edition, versions of Research Unix at Bell Labs had a close relationship to BSD. This began when 4.1cBSD for the VAX was used as the basis for Research Unix 8th Edition. This continued in subsequent versions, such as the 9th Edition, which incorporated source code and improvements from 4.3BSD. The result was that these later versions of Research Unix were closer to BSD than they were to System V. In a Usenet posting from 2000, Dennis Ritchie described this relationship between BSD and Research Unix:
Research Unix 8th Edition started from (I think) BSD 4.1c, but with enormous amounts scooped out and replaced by our own stuff. This continued with 9th and 10th. The ordinary user command-set was, I guess, a bit more BSD-flavored than SysVish, but it was pretty eclectic.
Relationship to System V
Eric S. Raymond summarizes the longstanding relationship between System V and BSD, stating, "The divide was roughly between longhairs and shorthairs; programmers and technical people tended to line up with Berkeley and BSD, more business-oriented types with AT&T and System V."
In 1989, David A. Curry wrote about the differences between BSD and System V. He characterized System V as being often regarded as the "standard Unix." However, he described BSD as more popular among university and government computer centers, due to its advanced features and performance:
Most university and government computer centers that use UNIX use Berkeley UNIX, rather than System V. There are several reasons for this, but perhaps the two most significant are that Berkeley UNIX provides networking capabilities that until recently (Release 3.0) were completely unavailable in System V, and that Berkeley UNIX is much more suited to a research environment, which requires a faster file system, better virtual memory handling, and a larger variety of programming languages.
Berkeley's Unix was the first Unix to include libraries supporting the Internet Protocol stacks: Berkeley sockets. A Unix implementation of IP's predecessor, the ARPAnet's NCP, with FTP and Telnet clients, had been produced at U. Illinois in 1975, and was available at Berkeley. However, the memory scarcity on the PDP-11 forced a complicated design and performance problems.
By integrating sockets with the Unix operating system's file descriptors, it became almost as easy to read and write data across a network as it was to access a disk. The AT&T laboratory eventually released their own STREAMS library, which incorporated much of the same functionality in a software stack with a different architecture, but the wide distribution of the existing sockets library reduced the impact of the new API. Early versions of BSD were used to form Sun Microsystems' SunOS, founding the first wave of popular Unix workstations.
BSD operating systems can run much native software of several other operating systems on the same architecture, using a binary compatibility layer. Much simpler and faster than emulation, this allows, for instance, applications intended for Linux to be run at effectively full speed. This makes BSDs not only suitable for server environments, but also for workstation ones, given the increasing availability of commercial or closed-source software for Linux only. This also allows administrators to migrate legacy commercial applications, which may have only supported commercial Unix variants, to a more modern operating system, retaining the functionality of such applications until they can be replaced by a better alternative.
Current BSD operating system variants support many of the common IEEE, ANSI, ISO, and POSIX standards, while retaining most of the traditional BSD behavior. Like AT&T Unix, the BSD kernel is monolithic, meaning that device drivers in the kernel run in privileged mode, as part of the core of the operating system.
Significant BSD descendants
BSD has been the base of a large number of operating systems. Most notable among these today are perhaps the major open source BSDs: FreeBSD, NetBSD and OpenBSD, which are all derived from 386BSD and 4.4BSD-Lite by various routes. Both NetBSD and FreeBSD started life in 1993, initially derived from 386BSD, but in 1994 migrating to a 4.4BSD-Lite code base. OpenBSD was forked in 1995 from NetBSD. The three most notable descendants in current use—sometimes known as the BSDs—have themselves spawned a number of children, including DragonFly BSD, FreeSBIE, MirOS BSD, DesktopBSD, and PC-BSD. They are targeted at an array of systems for different purposes and are common in government facilities, universities and in commercial use. A number of commercial operating systems are also partly or wholly based on BSD or its descendants, including Sun's SunOS and Apple Inc.'s Mac OS X.
Most of the current BSD operating systems are open source and available for download, free of charge, under the BSD License, the most notable exception being Mac OS X. They also generally use a monolithic kernel architecture, apart from Mac OS X and DragonFly BSD which feature hybrid kernels. The various open source BSD projects generally develop the kernel and userland programs and libraries together, the source code being managed using a single central source repository.
In the past, BSD was also used as a basis for several proprietary versions of Unix, such as Sun's SunOS, Sequent's Dynix, NeXT's NeXTSTEP, DEC's Ultrix and OSF/1 AXP (now Tru64 UNIX). Parts of NeXT's software became the foundation for Mac OS X, among the most commercially successful BSD variants in the general market.
A selection of significant Unix versions and Unix-like operating systems that descend from BSD includes:
- FreeBSD, an open source general purpose operating system.
- NeXT NEXTSTEP and OpenStep, based on the Mach kernel and 4BSD; the ancestor of Mac OS X
- Apple Inc.'s Darwin, the core of OS X and iOS; built on the XNU kernel (part Mach, part FreeBSD, part Apple-derived code) and a userland much of which comes from FreeBSD
- PC-BSD, GhostBSD and DesktopBSD, distributions of FreeBSD with emphasis on ease of use and user friendly interfaces for the desktop/laptop PC user.
- MidnightBSD, another fork of FreeBSD
- DragonFly BSD, a fork of FreeBSD to follow an alternative design, particularly related to SMP.
- NextBSD, new BSD distribution derived from FreeBSD 10.1 and various OS X components.
- FreeNAS a free network-attached storage server based on a minimal version of FreeBSD.
- NAS4Free fork of 0.7 FreeNAS version, Network attached storage server.
- Nokia IPSO (IPSO SB variant), the FreeBSD-based OS used in Nokia Firewall Appliances.
- The OS for the Netflix Open Connect Appliance.
- Junos, the operating system for Juniper routers, a customized version of FreeBSD, and a variety of other embedded operating systems
- Isilon Systems' OneFS, the operating system used on Isilon IQ-series clustered storage systems, is a heavily customized version of FreeBSD.
- NetApp's Data ONTAP, the operating system for NetApp filers, is a customized version of FreeBSD with the ONTAP architecture built on top.
- m0n0wall, a FreeBSD distribution tweaked for usage as a firewall.
- Pfsense open firewall and router.
- Coyote Point Systems EQ/OS, a hardened high-performance runtime for server load balancing.
- NeXT NEXTSTEP and OpenStep, based on the Mach kernel and 4BSD; the ancestor of Mac OS X
- NetBSD, an open source BSD focused on clean design and portability.
- OpenBSD, a 1995 fork of NetBSD, focused on security.
- MirOS BSD, a fork of OpenBSD with additional features such as European localizations.
- ekkoBSD, a discontinued fork of OpenBSD, now partially merged with MirOS.
- MicroBSD, an OpenBSD-based operating system with a small memory footprint.
- Force10 FTOS, the operating system for Force 10 and Dell datacenter network switches.
- Dell DNOS version 9 and above, the successor to FTOS.
- OpenBSD, a 1995 fork of NetBSD, focused on security.
- F5 Networks, F5 BIGIP Appliances used a BSD OS as the management OS until version 9.0 was released, which is built on top of Linux.
- DEC's Ultrix, the official version of Unix for its PDP-11, VAX, and DECstation systems
- Sony NEWS-OS, a BSD-based operating system for their network engineering workstations
- OSF/1, a microkernel-based Unix developed by the Open Software Foundation, incorporating the Mach kernel and parts of 4BSD
- Tru64 UNIX (formerly DEC OSF/1 AXP or Digital UNIX), the port of OSF/1 for DEC Alpha-based systems from DEC, Compaq and HP.
- Pre-5.0 versions of Sun Microsystems SunOS, an enhanced version of 4BSD for the Sun Motorola 68k-based Sun-2 and Sun-3 systems, SPARC-based systems, and x86-based Sun386i systems (SunOS 5.0 and later versions are System V Release 4-based)
- 386BSD, the first open source BSD-based operating system and the ancestor of most current BSD systems
- DEMOS, a Soviet BSD clone
- BSD/OS, a (now defunct) proprietary BSD for PCs
- RetroBSD, a fork of BSD 2.11 designed to run on microcontrollers such as the PIC32
- BSD Daemon
- BSD licenses
- Comparison of BSD operating systems
- List of BSD operating systems