Nagios is an opensource software used for network and infrastructure monitoring. Nagios will monitor servers, switches, applications and services. It alerts the System Administrator when something went wrong and also alerts back when the issues has been rectified.
Install Nagios on CentOS 6.4
Though the steps provided here are tested in CentOS 6.4, it may work on RHEL 6.4, Scientific Linux 6.4 and other RPM based distros.