Establishing a host dependency

In this recipe, you'll learn how to establish a host dependency between two hosts. This feature can be used to control how Nagios Core checks hosts and notifies them about problems in situations where if one host is DOWN, it implies that at least one other host is necessarily DOWN.

Getting ready

First of all, it's very important to note that this is not quite the same thing as a host being UNREACHABLE, which is what the parents directive is for, as discussed in the Creating a network host hierarchy recipe in this chapter. Most of the time, a host actually being DOWN does not mean that other hosts actually go DOWN by definition. It's more typical for a child host to simply be UNREACHABLE; it might work fine, but Nagios Core can't verify this because of the DOWN host in its path.

However, there's one particularly broad category where host dependencies are definitely useful: the host/guest relationship of virtual machines. If you monitor both a host physical machine...