Debian is a computer operating system (OS) composed entirely of software which is both free and open source (FOSS). The primary form, Debian GNU/Linux, is a popular and influential Linux distribution. Debian is a multipurpose OS, which can be used as a desktop or server operating system.

 Debian is known for strict adherence to the Unix and free software philosophies. Debian is also known for an abundance of options — the current release includes over twenty-six thousand software packages for eleven computer architectures. These architectures range from the Intel/AMD 32-bit/64-bit architectures commonly found in personal computers to the ARM architecture commonly found in embedded systems and the IBM eServer zSeries mainframes. Throughout Debian's lifetime, other distributions have taken it as a basis to develop their own, including: Ubuntu, MEPIS, Dreamlinux, Damn Small Linux, Xandros, Knoppix, Linspire, sidux, Kanotix, and LinEx among others. A university's study concluded that Debian's 283 million source code lines would cost US$10 billion to develop by proprietary means.

 Prominent features of Debian are the APT package management system, its strict policies regarding its packages and the quality of its releases. These practices afford easy upgrades between releases and easy automated installation and removal of packages. Debian uses an open development and testing process. It is developed by volunteers from around the world and supported by donations through SPI, a non-profit umbrella organization for various free software projects.

 The default install provides popular programs such as: OpenOffice.org, Iceweasel (a rebranding of Firefox), Evolution mail, CD/DVD writing programs, music and video players, image viewers and editors, and PDF viewers. Only the first CD/DVD is necessary for the default install; the remaining discs contain all 26,000+ extra programs and packages currently available. If a user does not wish to download the CDs/DVDs, these extras can be downloaded and installed separately using a package manager such as APT and Synaptic. Debian also downloads updates automatically.

 History

 Debian was first announced on 16 August 1993, by Ian Murdock, who was then a student at Purdue University. Murdock initially called the system the "Debian Linux Release". Previously, Softlanding Linux System had been the first well-known attempt at making a Linux distribution from various software packages, and was a popular option among Linux users in 1994. The perceived poor maintenance and prevalence of bugs in SLS that motivated Murdock to launch a new distribution.

 In the Debian Manifesto, Murdock called for the creation of a distribution to be maintained in an open manner, in the spirit of Linux and GNU. He formed the name "Debian" as a combination of the first name of his girlfriend (later wife, now ex-wife) Debra and his own first name.

 The Debian Project grew slowly at first and released the first 0.9x versions in 1994 and 1995. The first ports to other architectures were begun in 1995, and the first 1.x version of Debian was released in 1996. In 1996, Bruce Perens replaced Ian Murdock as the project leader. In the same year, fellow developer Ean Schuessler suggested that Debian establish a social contract with its users. He distilled the resulting discussion on Debian mailing lists into the Debian Social Contract and the Debian Free Software Guidelines, defining fundamental commitments for the development of the distribution. He also initiated the creation of the legal umbrella organization Software in the Public Interest.

 Perens left in 1998 before the release of the first glibc-based Debian, 2.0. The Project elected new leaders and made two more 2.x releases, each including more ports and packages. The Advanced Packaging Tool was deployed during this time and the first port to a non-Linux kernel, Debian GNU/Hurd, was started. The first Linux distributions based on Debian, Libranet, Corel Linux and Stormix's Storm Linux, were started in 1999. Though no longer developed, these distributions were the first of many distributions based on Debian.

 In late 2000, the project made major changes to archive and release management, reorganizing software archive processes with new "package pools" and creating a testing branch as an ongoing, relatively stable staging area for the next release. Also in that year, developers began holding an annual conference called DebConf with talks and workshops for developers and technical users.

 On April 8, 2007, the project announced the official release of Debian GNU/Linux version 4.0, codenamed "Etch".

 In May 2008, security researcher Luciano Bello revealed his discovery that changes made in 2006 to the random number generator in the version of the openssl package distributed with Debian and other Debian-based distributions such as a Ubuntu, made a variety of security keys vulnerable to attack. The security weakness was caused by changes made to the openssl code by a Debian developer in response to compiler warnings of apparently redundant code. Other Linux distributions were unaffected. This security hole has been patched. See Random number generator attack.

 Development procedures

Software packages in development are either uploaded to the project branch named unstable, also known as sid, or the experimental branch. Software packages uploaded to unstable are normally versions stable enough to be released by the original upstream developer, but with the added Debian-specific packaging and other modifications introduced by Debian developers. These additions may be new and untested. Software not ready yet for the unstable branch is typically placed in the experimental branch.

 After a version of a software package has remained in unstable for a certain length of time (depending on the urgency of the software's changes), that package is automatically migrated to the testing branch. The package's migration to testing occurs only if no serious (release-critical) bugs in the package are reported and if other software needed for package functionality qualifies for inclusion in testing.

Since updates to Debian software packages between official releases do not contain new features, some choose to use the testing and unstable branches for their newer packages. However, these branches are less tested than stable, and unstable doesn't receive timely security updates. Starting September 9, 2005[19] a testing security team provides security updates. In particular, incautious upgrades to working unstable packages can sometimes seriously break software functionality.

 After the packages in testing have matured and the goals for the next release are met, the testing branch becomes the next stable release. The latest stable release of Debian (etch) is 4.0. It was released on April 8, 2007. The forthcoming version is codenamed "lenny."

 Project organization

 The Debian Project is a volunteer organization with three foundational documents:

• The Debian Social Contract defines a set of basic principles by which the project and its developers conduct affairs.
• The Debian Free Software Guidelines define the criteria for "free software" and thus what software is permissible in the distribution, as referenced in the Social Contract. These guidelines have also been adopted as the basis of the Open Source Definition. Although it can be considered a separate document for all practical purposes, it formally is part of the Social Contract.
• The Debian Constitution describes the organizational structure for formal decision-making within the Project, and enumerates the powers and responsibilities of the Debian Project Leader, the Debian Project Secretary, and the Debian Developers generally.

 Currently, the project includes more than a thousand developers. Each of them sustains some niche in the project, be it package maintenance, software documentation, maintaining the project infrastructure, quality assurance, or release coordination. Package maintainers have jurisdiction over their own packages, although packages are increasingly co-maintained. Other tasks are usually the domain of smaller, more collaborative groups of developers.

 The project maintains official mailing lists and conferences for communication and coordination between developers. For issues with single packages or domains, a public bug tracking system is used by developers and end-users both. Informally, Internet Relay Chat channels (primarily on the OFTC and freenode networks) are used for communication among developers and users as well.

 Together, the Developers may make binding general decisions by way of a General Resolution or election. All voting is conducted by Cloneproof Schwartz Sequential Dropping, a Condorcet method of voting. A Project Leader is elected once per year by a vote of the Developers; in April 2008, Steve McIntyre was voted into this position, succeeding Sam Hocevar. The Debian Project Leader has several special powers, but this power is far from absolute and is rarely utilized. Under a General Resolution, the Developers may, among other things, recall the leader, reverse a decision by him or his delegates, and amend the constitution and other foundational documents.

 The Leader sometimes delegates authority to other developers in order for them to perform specialized tasks. Generally this means that a leader delegates someone to start a new group for a new task, and gradually a team gets formed that carries on doing the work and regularly expands or reduces their ranks as they think is best and as the circumstances allow.

 A role in Debian with a similar importance to the Project Leader's is Release Manager. Release Managers set goals for the next release, supervise the process, and make the final decision as to when to release.

 Package life cycle

 Each Debian software package has a maintainer who keeps track of releases by the "upstream" authors of the software and ensures that the package is compliant with Debian Policy, coheres with the rest of the distribution, and meets the standards of quality of Debian. In relations with users and other developers, the maintainer uses the bug tracking system to follow up on bug reports and fix bugs. Typically, there is only one maintainer for a single package, but increasingly small teams of developers "co-maintain" larger and more complex packages and groups of packages.

 Periodically, a package maintainer makes a release of a package by uploading it to the "incoming" directory of the Debian package archive (or an "upload queue" which periodically batch-transmits packages to the incoming directory). Package uploads are automatically processed to ensure that they are well-formed (all the requisite files are in place) and that the package is digitally signed by a Debian developer using OpenPGP-compatible software. All Debian developers have public keys. Packages are signed to be able to reject uploads from hostile outsiders to the project, and to permit accountability in the event that a package contains a serious bug, a violation of policy, or malicious code.

 If the package in incoming is found to be validly signed and well-formed, it is installed into the archive into an area called the "pool" and distributed every day to hundreds of mirrors worldwide. Initially, all package uploads accepted into the archive are only available in the "unstable" suite of packages, which contains the most up-to-date version of each package.

 However, new code is also untried code, and those packages are only distributed with clear disclaimers. For packages to become candidates for the next "stable" release of the Debian distribution, they first need to be included in the "testing" suite. The requirements for a package to be included in “testing” are that it:

• Must have been in unstable for the appropriate length of time (the exact duration depends on the "urgency" of the upload).
• Must not have a greater number of "release-critical" bugs filed against it than the current version in testing. Release-critical bugs are those bugs which are considered serious enough that they make the package unsuitable for release.
• Must be compiled for all release architectures the package claims to support (eg: the i386-specific package gmod can be included in "testing").
• Must be a package for an architecture that is slated to release (in other words, packages for architectures that aren't scheduled to release with all the rest are never considered for "testing").
• Must not depend on versions of any packages that do not meet the above conditions.

 Thus, a release-critical bug in a package on which many packages depend, such as a shared library, may prevent many packages from entering the "testing" area, because that library is considered deficient.

 Periodically, the Release Manager publishes guidelines to the developers in order to ready the release, and in accordance with them eventually decides to make a release. This occurs when all important software is reasonably up-to-date in the release-candidate suite for all architectures for which a release is planned, and when any other goals set by the Release Manager have been met. At that time, all packages in the release-candidate suite ("testing") become part of the released suite ("stable").

 It is possible for a package -- particularly an old, stable, and seldom-updated one -- to belong to more than one suite at the same time. The suites are simply collections of pointers into the package "pool" mentioned above.

 Releases

 As of April 2007, the latest stable release is version 4.0, code name etch. When a new version is released, the previous stable is labeled oldstable; currently, this is version 3.1, code name sarge.

 The Debian security team releases security updates for the latest stable major release, as well as for the previous stable release for one year. Version 4.0 Etch was released on 8 April 2007, and the security team supported version 3.1 Sarge until March 31 2008. For most uses it is strongly recommended to run a system which receives security updates. The testing version also receives security updates.

Debian has made nine major stable releases:

 Branches, official and unofficial repositories

 There are stable releases about every 18 months, therefore packages in this branch can become too old for users' tastes. This is countered by the testing and unstable branches which include recent package versions. The repositories update older packages in stable and oldstable with more modern ones or include packages which do not follow Debian guidelines.

 Branches

• Old stable, presently aliased sarge, is the previous stable release. It is supported until 1 year after a new stable is released. Debian recommends to update to the new stable once it has been released.
• Stable, presently aliased etch, is the current release that has stable and well tested software. Stable is made by freezing testing for a few months where bugs are fixed in order to make the distribution as stable as possible; then the resulting system is released as stable. It is updated only if major security or usability fixes are incorporated. There are stable releases about every 18 months. Stable's CDs and DVDs can be found in the Debian web site.
• Testing, presently aliased lenny, is what the next major release will be and is currently being tested. The packages included in this branch have had some testing in unstable but they may not be completely fit for release yet. It contains more modern packages than stable but older than unstable. This branch is updated continuously. Stable can be upgraded to testing by modifying the /etc/apt/sources.list file but the reverse is not possible. It does not get regular security updates and scrutiny from the Debian team. Testing's CDs and DVDs can be found on the Debian web site.
• Unstable, permanently aliased sid, repository contains packages currently under development; it is updated continuously. Packages can be retrieved from the unstable repository on any Debian site and use them to upgrade a system at any time, but it should not be expected that the system is as usable or as stable as more well tested branches. This repository is for Debian developers who participate in a project and need the latest libraries available. There are Debian Live CDs available and it can be added as a repository.

 Official repositories

 The Debian Project has rigorous requirements for software to be considered free. This is why not all software and documentation is available in the official Debian software repository. For example, documents using the GNU Free Documentation License with sections that the author does not permit to be altered or removed are considered non-free by Debian. Some in the free software community have criticized the Debian Project for providing a non-free repository, rather than excluding proprietary software entirely. Others have criticized Debian for separating non-free packages from the main repository. These rigorous requirements are countered by the existence of official and unofficial repositories.

 These repositories can be used by modifying the /etc/apt/sources.list file.

• Non-free: Repositories have license conditions restricting use or redistribution of the software.
• contrib: repository are freely licensed by the copyright holder but depend on other software that is not free.
• Experimental: is not actually a full (self-contained) development branch, it is meant to be a temporary staging area for highly experimental software. Dependencies missing are most likely found in unstable. Debian warns that these packages are likely unstable or buggy and are to be used at the user's own risk.
• Volatile project: repository contains updates to the stable and oldstable release for programs whose functionality requires frequent updates. Some packages aim at fast moving targets, such as spam filtering and virus scanning, and even when using updated data patterns, they do not really work for the full time of a stable release. The main goal of volatile is allowing system administrators to update their systems in a nice, consistent way, without getting the drawbacks of using unstable, even without getting the drawbacks for the selected packages. So debian-volatile will only contain updates to programs that are necessary to keep them functional.

 Semi-official repositories

• Backports.org repository are recompiled packages from testing (mostly) and unstable (in a few cases only, e.g. security updates), therefore they will run without new libraries, wherever it is possible, on stable or oldstable. This repository's packages are listed along with the Official repositories and Branches in debian.org but the packages are hosted at backports.org.
  

Unofficial repositories

 These repositories contain packages that are either more modern than the ones found in stable or include packages that are not included in the official Debian repositories for a variety of reasons such as: e.g. alleged possible patent infringement, binary-only/no sources, or special too restrictive licenses. These repositories can be used by modifying the /etc/apt/sources.list file and by installing a keyring. Their use requires precise configuration of the priority of the repositories to be merged, otherwise these packages may not integrate correctly into the system, and may cause problems upgrading or conflicts between packages from different sources. The Debian Project discourages the use of these repositories as they are not part of the project. Some well-known unofficial repositories include:

• apt-get.org
• debian-multimedia.org
• debian-unofficial.org
  

Ports

 Architectures

 As of the current stable release, the official ports are:

• i386 – x86-32 architecture designed for Intel/AMD 32-bit PCs and various other systems
• amd64 – x86-64 architecture designed for Intel/AMD 64-bit PCs and various other systems
• alpha – DEC Alpha architecture
• sparc – Sun SPARC architecture on Sun-4m, Sun-4u, and Sun-4v systems
• arm – ARM architecture on Risc PC and various embedded systems (little-endian)
• powerpc – PowerPC architecture
• hppa – HP PA-RISC architecture
• ia64 – Intel Itanium (IA-64) architecture
• mips, mipsel – MIPS architecture (big-endian and little-endian)
• s390 – IBM ESA/390 architecture and z/Architecture
  

 The m68k port was the second official port in Debian, and has been part of five stable Debian releases. Due to its failure to meet the release criteria, it has been dropped before the release of etch. Still, it continues to be available as part of the unstable distribution:

• m68k – Motorola 68k architecture on Amiga, Atari, Macintosh, and various embedded VME systems

 Ongoing efforts include ports to Hitachi SuperH (sh) and Renesas M32R (m32r) architectures, big-endian ARM port (armeb), little-endian EABI ARM port (armel), and 64-bit-only PowerPC port (ppc64).

 Kernels

 The Project describes itself as creating a "Universal Operating System" and several ports of all userland software to various operating system kernels are under development:

• Debian GNU/Linux, on the Linux kernel — the original, officially released port
• Debian GNU/Hurd, on GNU Hurd. Debian GNU/Hurd has been in development for years, but still has not been officially released. Roughly half of the software packaged for Debian GNU/Linux has been ported to the GNU Hurd. However, the Hurd itself remains under development, and as such is not ready for use in production systems. Most Debian users run Debian GNU/Linux, rather than Debian GNU/Hurd. The current version of Debian GNU/Hurd is K16 (released 2007-12-21). It works on i386 and amd64 PCs.
• Debian GNU/kFreeBSD, on the FreeBSD kernel, for i486-compatible computer architectures. It is a distribution of GNU with Debian package management and the kernel of FreeBSD. The k in kFreeBSD refers to the fact that only the kernel of the complete FreeBSD operating system is used. One such Debian GNU/kFreeBSD Live CD is Ging.
• Debian GNU/NetBSD, on the NetBSD kernel, currently available for i386 and DEC Alpha architectures.

 Although these are official Debian projects, there have been no official releases of the non-Linux ports yet, so currently Debian is exclusively a Linux distribution.

 Debian Installer

 Starting Debian 4.0 Etch, a graphical version of the installer is available for i386 and amd64. For PowerPC the graphical installer is only available as a separate, experimental image. For most installation images, the graphical installer can be started by typing "installgui" at the boot screen. The graphical version allows the user to use the mouse rather than just the keyboard.

 Desktop environments

 Debian offers stable and testing CDs/DVDs for each major desktop environment: GNOME, the default, along with KDE and Xfce as alternatives.

 Debian Live

 A Debian Live system is a version of Debian that can be booted directly from removable media (CDs, DVDs, USB keys) or via netboot without having to install it on the hard drive. This allows the user to try out Debian before installing it or use it as a boot-disk. There are prebuilt Debian Live CD Images for etch, lenny, and sid for all three major desktop environments: GNOME, KDE and Xfce. Etch is available in both i386 and amd64 while lenny and sid are only available in i386. A hard disk installation can be achieved using the Debian Installer included in the CD. Customized CD Images can be built using live-helper. Live-helper can not only generate CD Images, but also bootable DVDs, images for USB thumb drives, or netboot images. Live-magic is a GUI for live-helper.