Nagios

Nagios Core watches hosts, services, and networks, telling you when something breaks and again when it recovers. It has done that since 1999, written in C under version 2 of the GNU General Public License, which is a large part of why so much monitoring tooling still speaks its plugin format.

Checks are external programs. The core schedules them, reads their exit status, and acts on the result, so a new kind of check means writing a small program rather than extending the daemon. Hosts form a hierarchy through parent relationships, which lets the software distinguish a host that is down from one merely unreachable behind a failed router. CGI programs bundled with the release provide the optional web interface.

The functionality has held the same shape for a long time.

• Service checks: network services monitored over SMTP, POP3, HTTP, PING, and more.
• Host resources: processor load, disk usage, and similar local measurements.
• Plugin interface: user-developed monitoring methods plug in without touching the core.
• Parent hosts: a hierarchy that separates down from unreachable.
• Notifications: alerts on problem and recovery by email, pager, or a method you define.
• Event handlers: commands run automatically to attempt resolution before a person is paged.
• Web interface: current status, notification and problem history, and log files in a browser.

Log files rotate and archive on their own. Quickstart guides, a user guide, and a documentation library are published alongside the releases, support runs through community forums, and security issues go to a dedicated address, not the public tracker.