Nagios Core , formerly known as Nagios, is a free and open-source computer-software application that monitors systems, networks and infrastructure. Nagios offers monitoring and alerting services for servers, switches, applications and services. It alerts users when things go wrong and alerts them a second time when the problem has been resolved. Ethan Galstad and a group of developers originally wrote Nagios as NetSaint. they actively maintain both the official and unofficial plugins. Nagios is a recursive acronym: "Nagios Ain't Gonna Insist On Sainthood" – "sainthood" makes reference to the original name NetSaint, which changed in response to a legal challenge by owners of a similar trademark. "Agios" (or "hagios") also transliterates the Greek word άγιος, which means "saint". Nagios was originally designed to run under Linux, but it also runs on other Unix variants. It is free software licensed under the terms of the GNU General Public License version 2 as published by the Free Software Foundation. Nagios agents NRPE Nagios Remote Plugin Executor (NRPE) is a Nagios agent that allows remote system monitoring using scripts that are hosted on the remote systems. It allows for monitoring of resources such as disk usage, system load or the number of users currently logged in. Nagios periodically polls the agent on remote system using the check_nrpe plugin. NRPE allows you to remotely execute Nagios plugins on other Linux/Unix machines. This allows you to monitor remote machine metrics (disk usage, CPU load, etc.). NRPE can also communicate with some of the Windows agent add-ons, so you can execute scripts and check metrics on remote Windows machines, as well. As of 28 Jan 2020, NRPE has been deprecated. NRDP Nagios Remote Data Processor (NRDP) is a Nagios agent with a flexible data transport mechanism and processor. It is designed with an architecture that allows it to be easily extended and customized. NRDP uses standard ports and protocols (HTTP and XML) and can be implemented as a replacement for Nagios Service Check Acceptor (NSCA). NSClient++ This program is mainly used to monitor Windows machines. Being installed on a remote system NSClient++ listens to port TCP 12489. The Nagios plugin that is used to collect information from this addon is called check_nt. As NRPE, NSClient++ allows to monitor the so-called 'private services' (memory usage, CPU load, disk usage, running processes, etc.) Nagios is a host and service monitor which is designed to inform you of network