Table 2. Plugin Return Codes

Numeric Value	Service Status	Status Description
0	OK	The plugin was able to check the service and it appeared to be functioning properly
1	Warning	The plugin was able to check the service, but it appeared to be above some "warning" threshold or did not appear to be working properly
2	Critical	The plugin detected that either the service was not running or it was above some "critical" threshold
3	Unknown	Invalid command line arguments were supplied to the plugin or low-level failures internal to the plugin (such as unable to fork, or open a tcp socket) that prevent it from performing the specified operation. Higher-level errors (such as name resolution errors, socket timeouts, etc) are outside of the control of plugins and should generally NOT be reported as UNKNOWN states.

link: http://nagiosplug.sourceforge.net/developer-guidelines.html#AEN35