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A vast amount of research and development has gone into extending Unix-like systems to support security needs of various communities. For example, several Unix-like systems have been extended to support the U.S. military's desire for multilevel security. If you're developing software, you should try to design your software so that it can work within these extensions.
FreeBSD has a new system call, jail(2). The jail system call supports sub-partioning an environment into many virtual machines (in a sense, a ``super-chroot''); its most popular use has been to provide virtual machine services for Internet Service Provider environments. Inside a jail, all processes (even those owned by root) have the the scope of their requests limited to the jail. When a FreeBSD system is booted up after a fresh install, no processes will be in jail. When a process is placed in a jail, it, and any descendents of that process created will be in that jail. Once in a jail, access to the file name-space is restricted in the style of chroot(2) (with typical chroot escape routes blocked), the ability to bind network resources is limited to a specific IP address, the ability to manipulate system resources and perform privileged operations is sharply curtailed, and the ability to interact with other processes is limited to only processes inside the same jail. Note that each jail is bound to a single IP address; processes within the jail may not make use of any other IP address for outgoing or incoming connections.
Some extensions available in Linux, such as POSIX capabilities and special mount-time options, have already been discussed. Here are a few of these efforts for Linux systems for creating restricted execution environments; there are many different approaches. The U.S. National Security Agency (NSA) has developed Security-Enhanced Linux (Flask), which supports defining a security policy in a specialized language and then enforces that policy. The Medusa DS9 extends Linux by supporting, at the kernel level, a user-space authorization server. LIDS protects files and processes, allowing administrators to ``lock down'' their system. The ``Rule Set Based Access Control'' system, RSBAC is based on the Generalized Framework for Access Control (GFAC) by Abrams and LaPadula and provides a flexible system of access control based on several kernel modules. Subterfugue is a framework for ``observing and playing with the reality of software''; it can intercept system calls and change their parameters and/or change their return values to implement sandboxes, tracers, and so on; it runs under Linux 2.4 with no changes (it doesn't require any kernel modifications). Janus is a security tool for sandboxing untrusted applications within a restricted execution environment. Some have even used User-mode Linux, which implements ``Linux on Linux'', as a sandbox implementation. There are many other extensions for security on various Unix-like systems, but these are really outside the scope of this document.