In addition to being useful as a standalone monitoring framework, Nagios Core has a modular design that allows both interaction with and extension by other programs and tools, predominantly using its external command file for controlling the behavior of the server.

One of the most useful ways of interacting with the Nagios Core server in this way is through the use of passive checks: submitting check results to the server directly, rather than as the result of the server's own active checks.

The simplest application of the idea of passive checks is for monitoring some process that might take an indeterminate amount of time to run, and hence resists active checking; instead of the service making active checks of its own, it accepts a check result submitted by another application, perhaps something like a backup script after it has completed its run. These can be sent and accepted via an addon called the Nagios Service Check Acceptor (NSCA). Similarly, just as plugins and notifications...