Nagios is one of the most powerful network monitoring systems, which is widely used in the industry. It can actively monitor any network, and generate audio/email warnings and alerts when any problem is detected. The check types and alert timers are fully customizable.

Another incredible capability of Nagios is that it can monitor both hosts and services e.g., it can monitor IP addresses and TCP/UDP port numbers. To explain a little, let us assume that there is a web server that we want to monitor. Nagios can check whether the server is online by running ping on the IP/name of the server as well as it can be set up to provide warnings in case the round trip time (RTT) to the server increases. Further, Nagios can also check whether TCP port 80 (web server) is reachable e.g., the server is online but Apache/IIS is not responding.

There are also 3rd party monitoring tools that are based on Nagios, such as Centreon, FAN , op5 Monitor, which supplement standalone Nagios engine in terms of interface, automation, and technical support.

This tutorial explains how to install and configure Nagios on Linux.

Full article here:
How to install and configure Nagios on Linux (Xmodulo)
Related:
Install And Configure Nagios on Ubuntu 14.04 LTS (Unixmen)