Nagios offers various ways of monitoring computers, and the services offered by them. Depending on how you need to check if your services are running correctly, one of these ways will work best for you. The first and the easiest way is to set up a check that is performed from the Nagios server. This is a great way of monitoring services that work over the network. Another possibility is to run applications on one or more machines that report to the Nagios server using passive checks.

This chapter talks about another approach to service status checking. 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 a disk or memory usage. Checking if your operating system is up-to-date is also an example of such a test. This type of information is usually only available by reading special files or calling...