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Nagios Service Check Acceptor (NSCA)

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.[6] 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 4.0.1 has been deprecated.[7]

NRDP
 Nagios Remote Data Processor (NRDP) is a Nagios agent with a flexible data transport mechanism and processor.[8] 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 problems.

NCPA
 The Nagios Cross Platform Agent is an open source project maintained by Nagios Enterprises.[9] NCPA installs on Windows, Linux, and Mac OS X. Created as a scale-able API that allows flexibility and simplicity in monitoring hosts. NCPA allows multiple checks such as memory usage, CPU usage, disk usage, processes, services, and network usage. Active checks are queried through the API of the "NCPA Listener" service while passive checks are sent via the "NCPA Passive" service. 

 Agents
There are several agents for Nagios, generally the agents work with Nagios Core and Nagios XI but it is the ease of install and use that differentiates Nagios XI from Nagios Core. Some agents are provided by and supported by Nagios, and some are from third parties.
Here is a list of some popular agents with links more more information.
 NCPA - Nagios Cross Platform Agent		[http://support.nagios.com/kb/category.php?id=8]
 NRPE - Nagios Remote Plugin Executor	[http://support.nagios.com/kb/category.php?id=10]
**NSCA** - Nagios Service Check Acceptor	[http://support.nagios.com/kb/category.php?id=11]
There are many more available in the Nagios Exchange under Addons.