Nagios, 2nd Edition

Chapter 5. Service Checks and How They Are Performed

To test services, Nagios makes use of external programs called plugins. In the simplest case this involves testing an Internet service, for example, SMTP. Here the service can be addressed directly over the network, so it is sufficient to call a program locally on the Nagios server that tests the mail server on the remote host.

Not everything you might want to test can be reached so easily over the network, however; there is no network protocol for checking free capacity on a hard drive, for example. Then you must either start a plugin on the remote host via a remote shell (but first this has to be installed on the remote computer), or you use other methods, such as the Simple Network Management ...