Nagios offers various ways of monitoring the computers and services. The previous chapter talked about passive checks and how they can be used to submit results to Nagios. It also discussed NSCA, which can be used to send check results from other machines to the Nagios server.

This chapter talks about another approach to check the service status. It uses Nagios active checks that run the actual check commands on different hosts. This approach is most useful in cases where resources local to a particular machine are to be checked. A typical example is monitoring disk or memory usage. Checking if your operating system is up-to-date is also an example of such a test. This type of data cannot be checked without running commands on the target computer.

Remote checks are usually used in combination with the Nagios plugins package that use either SSH or Nagios Remote Plugin Executor (NRPE) to run the plugins on the remote machine. This makes monitoring remote systems...