Nagios uses plugins for the host and service checks. They provide four different return values (see Table 6-1 in page 105):
2 (CRITICAL), and
The return value UNKNOWN means that the running of the plugin generally went wrong, perhaps because of wrong parameters. You can normally specify the situations in which the plugin issues a warning or a critical state when it is started.
Nagios determines the states of services and hosts from the return values of the plugin. The states for services are the same as the return values OK, WARNING, CRITICAL, and UNKNOWN. For the hosts the picture is slightly different: the UP state describes a reachable host, DOWN means that the computer is down, ...