Flapping is a situation where a host or service changes states very rapidly—constantly switching between working correctly and not working at all. This can happen due to various reasons—a service might crash after a short period of operating correctly or due to some maintenance work being done by system administrators.

Nagios can detect that a host or service is flapping, if Nagios is configured to do so. It does so by analyzing previous results in terms of how many state changes have taken place within a specific period of time. Nagios keeps a history of the 21 most recent checks and analyzes changes within that history.

The following is an screenshot illustrating the 21 most recent check results, which means that Nagios can detect up to 20 state changes in the recent history of an object. It also shows how Nagios detects state transitions:

Nagios then finds all of the changes between different states, and uses them to determine if a host or service is flapping. It checks...