Image by Fenng(dbanotes)
How Passive Checks Work
Here’s how passive checks work in more detail…
1. An external application checks the status of a host or service.
2. The external application writes the results of the check to the external command file.
3. The next time Nagios reads the external command file it will place the results of all passive checks into a queue for later processing. The same queue that is used for storing results from active checks is also used to store the results from passive checks.
4. Nagios will periodically execute a check result reaper event and scan the check result queue. Each service check result that is found in the queue is processed in the same manner – regardless of whether the check was active or passive. Nagios may send out notifications, log alerts, etc. depending on the check result information.
The processing of active and passive check results is essentially identical. This allows for seamless integration of status information from external applications with Nagios.