Xymon, a network monitoring application using free software, operates under the GNU General Public License; its central server runs on Unix and Linux hosts.

[4] Xymon offers graphical monitoring, showing the status of various network services of each device, as well as a range of application and operating system metrics such as listing the number of mail messages queued after a defined level of downtime.

The web-based graphical display uses a red/yellow/green condition icon for each host/test, on top of a colored background indicating the current worst status across all hosts and tests.

Clients are available for Unix and Linux (in formats including source tarball, RPM and Debian package) from the Xymon download site at SourceForge.

Plugins extend monitoring to new types of applications and services,[7] and many extension scripts for Big Brother will run unchanged on Xymon.