Understanding checks in Nagios

Nagios is a monitoring tool. It 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 is the command's responsibility to verify if a host or service is working at the time the command is invoked.

Nagios itself handles all of the internals such as scheduling the commands to be run, storing their results, and determining what the status is for each host and service.

It is required that all plugins follow a specific behavior in order for them to work smoothly. These rules are common for both host and service checks. It requires that each command returns specific result codes, which are outlined in the following table:

Exit code	Status	Description
0	OK	Working correctly
1	WARNING	Working, but needs attention (for example, low resources)
2	CRITICAL	Not working correctly or requires attention
3	UNKNOWN	Plugin was unable to determine the status...