After you configure what hosts and services you want to monitor, Nagios schedules regular checks against that host and/or service. If it finds a problem, Nagios does the following: the Nagios site ...

Nagios remains a cornerstone of Linux system performance monitoring, offering unmatched flexibility and reliability. By mastering its setup and configuration, administrators can ensure their systems ...

Nagios Core is open source, free and has good documentation, but it could use a streamlined install process, updated Web interface and better configuration options.

From network and server monitoring to trending, graphing and even switch and router configuration backups, these utilities will see you through.