Description:

Nagios is a program that will monitor hosts and services on your
network. It has the ability to email or page you when a problem arises
and when a problem is resolved. Nagios is written in C and is designed
to run under Linux (and some other *NIX variants) as a background
process, intermittently running checks on various services that you
specify.

The actual service checks are performed by separate "plugin" programs
which return the status of the checks to Nagios. The plugins are
available in nagios-plugins packages.

Nagios is successor to NetSaint.