Every plugin that is used for host and service checks is a separate and independent program that can also be used independently of Nagios. The other way round, it is not so easy: in order for Nagios to use an external program, it must obey certain rules. The most important of these concerns the return status that is returned by the program. Using this, Nagios precisely evaluates the status. Table 6-1 displays the possible values.
Table 6-1. Return values for Nagios plugins
Everything in order
Warning limit has been exceeded, but critical limit not yet reached
Critical limit exceeded or the plugin has broken off the test after a timeout
Error has occurred inside ...