Nagios is by many considered to be the godfather of host and service monitoring on Linux. It is without doubt the best known monitoring tool out there, and is available by default in all major Linux distributions. Nagios version 3.0 was released in March 2008, but will only be picked up by most distributions in the next 6-12 months. In the meantime, version 2.11 is the most common in today's distributions. Nagios was created by Ethan Galstad and is licensed under the GPL v2. Many others contributed to Nagios since its conception through plugins, add-ons, bugfixes, suggestions, testing, bug reporting, ideas and feature requests. Nagios was created by Ethan Galstad and licensed under the GPL v2. But different other people have been contributing plugins to nagios. Where as Nagios was incepted in 1999 Ethan only recently started Nagios Enterprises to commercially support the Open Source project.

One of the greatest features of Nagios is the great scriptability of its configuration. Nagios is fully configured with text files, which can easily be generated on the fly to perfectly fit into your network. Everything from hosts, services, contacts to groups and alert escalation plans can be configured in this way. Configuring Nagios comes with a pretty steep learning curve though for first-time users. At frequent intervals the Nagios monitoring deamon will run checks on hosts and services you specify via a mechanism of external plugins that then return status information.

Problems can be reported to the administrators by means of SMS, email, instant messages, or a variety of other ways. Escalation is also supported, so that when the first contact doesn't acknowledge the problem within a predefined time frame, another person or group can be notified in order to get the problem resolved. Nagios works by running predefined checks on a configurable interval through a plethora of external plugins that return status information to the Nagios service itself. Since Nagios is solely a alerting tool, and lacks a fancy GUI with graphs, trending, and other monitoring features, it is usually accompanied by a separate tool that handles those features, such as Cacti. The Nagios/Cacti combo is the most popular one. Several independent projects also try to improve and extend the pretty rudimentary and boring looks of the Nagios web interface by implementing a new web based frontend on top of the Nagios data and configuration, thus merging the reporting and monitoring/trending features into 1 single rich web interface.

Nagios by itself is pretty lightweight, with a c-based backend and a cgi-based frontend ( web gui ), and requires hardly any dependencies to get it in a working state, and can be used on a very modest (virtual) machine.