Running a service on all hosts on a group

You should have a working Nagios Core 4.0 or better server running with a web interface. You should be familiar with adding services to individual hosts.

You should also have at least one hostgroup defined, with at least one host in it; we'll use a group called webservers with the sparta.example.net and athens.example.net hosts defined in it.

For reference, here is the hostgroup definition and the definitions for the two hosts in it:

define hostgroup {
    hostgroup_name  webservers
    alias           Webservers
}
define host {
    use         linux-server
    host_name   athens.example.net
    alias       athens
    address     192.0.2.22
    hostgroups  webservers
}
define host {
    use         linux-server
    host_name   sparta.example.net
    alias       sparta
    address     192.0.2.21
    hostgroups  webservers
}

We can create the service definition for the webservers group as follows:

It's important to note that if we were already monitoring these hosts with a per-host service of the same name, we will need to remove the definitions as well; Nagios Core will not start if a service of the same description is already defined on the same host.

Adding a service to a hostgroup works exactly in the same way as adding it to an individual host, except it only requires one definition, which is then individually applied to all the hosts in the group. This means that it's a very good way to keep a Nagios Core configuration tidier. If we have a group with 50 different web servers in it and we need to monitor their HTTP services on the same basis for every one of them, we don't need to create 50 service definitions as well; we can just create one for its hostgroup, which amounts to a smaller and more easily updated configuration.

Like the host_name directive for services, hostgroup_name can actually have several hostgroups defined, separated by commas. This means that we can apply the same service to not just one group but several of them. For services that we would want to run on several different groups, for example basic PING monitoring, this can amount to a much more flexible configuration.