What Are Plugins?
Plugins are compiled executables or scripts (Perl, shell, etc.) that can be run from a command line to check the status or a host or service. Nagios uses the results from plugins to determine the current status or hosts and services on your network. No, you can't get away without using plugins - Nagios is useless without them.
Plugin development for Nagios is being done at SourceForge. The Nagios plugin development project page (where the latest version of by plugins can always be found) is located at http://sourceforge.net/projects/nagiosplug/.
How Do I Use Plugin X?
Documentation on how to use individual plugins is not supplied with the core Nagios distribution. You should refer to the latest plugin distribution for information on using plugins. Karl DeBisschop, lead plugin developer/maintainer points out the following:
For example, if you want to know how the check_http plugin works or what options it accepts, you should try executing one of the following commands:
Command Definition Examples For Services
It is important to note that command definitions found in sample config files in the core Nagios distribution are probably not accurate as to command line parameters, etc when it comes to the plugins. They are simply provided as examples of how to define commands.
Creating Custom Plugins
Creating your own plugins to perform custom host or service checks is easy. You can find information on how to write plugins at http://sourceforge.net/projects/nagiosplug/. The developer guidelines can be found at http://nagiosplug.sourceforge.net/developer-guidelines.html.