Nagios Core

Nagios Core is the long-running engine that schedules checks against hosts and services and tells you when something breaks. Its defining idea is the plugin interface: the core handles scheduling, state, and notifications, while the actual probing is delegated to small plugins, so anything you can test from a command line becomes a check. It notifies on problems and again on recovery, and flags hosts that go down or unreachable.

Out of the box it watches network services like SMTP, POP3, HTTP, and PING, and host resources such as processor load and disk usage. Event handlers can fire on state changes to attempt automatic remediation, and bundled CGI programs serve a web view of current status, history, and logs.

Written in C and released under GNU GPL v2, Nagios Core installs from source published by Nagios Enterprises. The web interface is optional rather than a hosted service.