On first sight, it doesn’t look like Nagios or Icinga can be used to monitor websites hosted on the same server. All concepts in Nagios and Icinga revolve around services and hosts. The whole “sub-part of a service”, like a VirtualHosts in the HTTP service, doesn’t seem to fit. It is perfectly possible though.

Let’s start by adding three simple commands that use check_http with various arguments to make sure the VirtualHost is returning a page correctly. Paste the following in your command config, for example in /etc/icinga/object/commands.cfg

define command {
        command_name check_http_vhost
        command_line /usr/lib/nagios/plugins/check_http -I '$HOSTADDRESS$' -k 'Host: $ARG1$'
}

define command {
        command_name check_https_vhost
        command_line /usr/lib/nagios/plugins/check_http -S -I '$ARG1$' -k 'Host: $ARG1$'
}

define command {
        command_name check_http_vhost_url
        command_line /usr/lib/nagios/plugins/check_http -I '$HOSTADDRESS$' -k 'Host: $ARG1$' -u '$ARG2$'
}

This will add three commands to your config, allowing you to check a normal VirtualHost, an https VirtualHost and a normal VirtualHost on a certain url (this can be handy if your front page redirects to a logiin page for example). You can now add a service definition to a server for every VirtualHost you want to monitor. For example:

define service {
        host_name               yourservername
        service_description     domainone.com
        check_command           check_http_vhost!domainone.com
        use                     generic-service
}

define service {
        host_name               yourservername
        service_description     domaintwo.com
        check_command           check_https_vhost!domaintwo.com
        use                     generic-service
}
define service {
        host_name               yourservername
        service_description     domainthree.com
        check_command           check_http_vhost_url!domainthree.com!/sessions/new
        use                     generic-service
}

Good luck!