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"Owl" - a security-enhanced server platform.

"Owl" (or "Openwall GNU/*/Linux"; please, note that only the "O" is capitalized in either case) is a security-enhanced operating system with Linux and GNU software as its core, compatible with other major distributions of GNU/*/Linux. It is intended as a server platform. And, of course, it's free.


While we value quality above feature set, Owl does indeed offer a number of features besides just trying to be more secure.

Most obviously, Owl can be used as a base for installing whatever software is generally available for GNU/*/Linux systems. It offers some compatibility (read below) for software packages found in or developed for other major Linux distributions, such as Red Hat Linux.

Being a server platform, Owl includes a growing set of integrated Internet services.

Additionally, all of our official CDs and downloadable ISO images provide a complete live system usable right off the CD in multi-user mode, with networking and development tools. Of course, the same CDs and ISOs also contain packages to be installed on a hard drive (using the included installer program called "settle") and full source code.

Owl includes a complete build environment capable to re-build the entire system from source with one simple command ("make buildworld"). (This is explained in more detail below.)

Owl supports multiple architectures (currently x86, x86-64, SPARC, and Alpha) as this lets you use it in more cases and helps us catch certain classes of software bugs earlier, thus improving the reliability of Owl packages.


Owl combines several approaches to reduce the number and/or impact of flaws in its software components and impact of flaws in third-party software that one might install on the system.

The primary approach used is proactive source code review for several classes of software vulnerabilities. However, because of the large amount of code, there's a certain level of "importance" for a software component or a part thereof to be audited. Currently, only pieces of code which are typically run with privileges greater than those of a regular user and/or typically process data obtained over a network are audited before the corresponding software component is included. This covers relevant code paths in many of the system libraries, all SUID/ SGID programs, all daemons and network services. Other software may be audited when it is already a part of Owl. Potential problems found during the audit are fixed or, in some pathological cases, may prevent the software component from being included. In general, code quality and privilege management are always considered when there's a choice between implementations of a feature. As the project evolves, many of the software components will be replaced with ones of our own.

When packaged for Owl, the software components are configured or, when necessary, modified in order to provide safe defaults, apply the least privilege principle, and introduce privilege separation. The use of safe defaults, where optional and potentially dangerous features need to be turned on explicitly, lets us audit the pieces of code used in in the default configuration in a more thorough way. Extra systems administration facilities ("owl-control") are provided for managing system features such as the optional SUID/SGID binaries independently from installing the corresponding packages. Every Owl package will have its audit status documented to allow for risk assessment.

While source code review is the preferred way to deal with software vulnerabilities, it can't be applied in all cases. Typically, when insecure third-party software is installed on an otherwise secure system, "the game" is lost. The only thing an operating system can guarantee is that potential unauthorized access would be limited to those privileges granted to the software in question. However, in the recent years, a number of approaches were developed which reduce the likelihood and/or may reduce the impact of successful real-world attacks on insecure third-party software. Owl will use some of those "hardening" approaches in various parts of the system.

Owl uses "strong" cryptography within its core components, and already includes some security policy enforcement (proactive password checking with "pam_passwdqc", password and account expiration, network address- based access control) and integrity checking ("mtree") capabilities. It is one of our goals to provide a wide range of security tools with Owl, available for use "out of the box".

The build environment and package management.

Unlike most other "Linux distributions", Owl includes a complete build environment capable to re-build the entire system from source with one simple command ("make buildworld"). However, the implementation of "make buildworld" on Owl is very different from that available with *BSD. It is in fact more similar to *BSD ports/packages, covering the entire Owl userland (that is, everything but the Linux kernel).

The Owl userland source code consists of two directory trees, where each Owl package may be split between the two trees. One source tree consists of original archives as distributed by the maintainers of the corresponding software components. The other tree, which we store in a CVS repository, has the build specifications, patches, and other Owl-specific additions to the packages. Some packages were developed as a part of Owl, and thus exist entirely in the CVS repository.

Based on the two source trees, binary packages are built. They can be installed (with "make installworld") to update the system itself or to create a new Owl installation (the ROOT= setting), or they can be transferred over a network and installed elsewhere.

We're using RPM for the binary packages, as that allows for reasonable dependency handling when installing packages from or intended for Red Hat Linux and several other distributions, on an Owl system.


Except for a few cases where this conflicts with our more important goals, Owl tries to be binary- and package- compatible with several other major distributions of GNU/*/Linux. In particular, in most cases it is possible to install applications packaged for a certain version of Red Hat Enterprise Linux, CentOS, or Fedora on Owl. As of Owl 2.0 and Owl 3.0, this applies to RHEL4, CentOS 4, and FC3.

$Owl: Owl/doc/CONCEPTS,v 1.14 2010/12/14 16:37:05 solar Exp $