Nagios' strength comes from its ability to monitor servers and the services they offer in a large number of ways. What's more interesting is that all of these ways make sure that your services are provided as functional, are external plugins, and work in quite an easy way. Many of these are even shipped with Nagios, as we mentioned in Chapter 2, Installation and Configuration. Therefore, it is possible to either use existing plugins or write your own.

The previous chapter discussed basic configuration of host and service checking. Nagios can be set up to check if your services are up and running. This chapter describes how these checks work in more detail. It also introduces some of the Nagios plugins that are developed as a part of Nagios, and as a part of the Nagios Plugins project.

Nagios performs checks by running an external command, and uses the return code, along with output from the command, as information on whether the check worked or not. It...