Nagios is an elaborate piece of software to monitor hosts and services. I will explain a bit how you can configure nagios to monitor an HTTP service. I’m assuming your nagios setup already has the default config files generic-host_nagios2.cfg and generic-service_nagios2.cfg, which tell nagios how to monitor hosts and services.

Most configuration is done in /etc/nagios3/conf.d. For some reason, the standard config files all end with _nagios2.cfg, so I guess this is old syntax. But, I don’t really know why these files are named that way.

Nagios comes with a bunch of default files to which you can add your hosts, services, etc.

First you have to define a host. If you’re monitoring on the machine itself, you could add a host to localhost_nagios2.cfg. Using the default localhost doesn’t work, because you need to access the machine using the address of the virtual host. 

define host{
        use                     generic-host            ; Name of host template to use
        host_name               my-site
        address                 www.halfgaar.net
}

Then you need to define a hostgroup for your HTTP servers. A default HTTP hostgroup is probably already defined, so you can add your host to http-servers in hostgroups_nagios2.cfg 

define hostgroup {
        hostgroup_name  http-servers        
        members         localhost, my-site # comma separated
}

Lastly, you need to configure a service. Nagios comes with a default one for the hostgroup http-servers so you should be done, but just in case: 

define service {
        hostgroup_name                  http-servers
        service_description             HTTP
        check_command                   check_http
        use                             generic-service
        notification_interval           0 ; set > 0 if you want to be renotified
}