User-mode Linux
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User-mode Linux (UML) is an architectural port of the Linux kernel to its own system call interface. A Linux kernel compiled for the um architecture can then boot as a process under another Linux kernel, entirely in user space, without affecting the host environment's configuration or stability.
Numerous things become possible through the use of UML. One can run network services from a UML environment and remain totally sequestered from the main Linux system in which the UML environment runs. Often, administrators use UML to set up honeypots, which allow one to test the security of one's computers or network. UML may also serve to test and debug new software without adversely affecting the host system. Kernel versions need not match, so it is entirely possible to test a "bleeding edge" version of Linux in User-mode on a system running a much older kernel.
Recently, several hosting providers have begun offering UML-powered virtual servers for $10 to $20 per month. Each customer has root access on what appears to be their own system, while in reality one physical computer is shared between many people.
UML was originally distributed as a patch, but as of Linux 2.6.0 it is integrated into the main kernel source tree. Another patch, called skas3, is often applied to the host kernel on systems that will be running one or more instances of UML. This patch improves performance and security. The skas3 patch is not in the official Linux kernel yet, but is included with several distributions.
External links
- User-mode Linux Kernel Home Page (http://user-mode-linux.sourceforge.net/)
- User-mode Linux Community Site (http://www.usermodelinux.org/)