Well, there are multiple ways to monitor remote services - all dependant on what OS the Host you want to monitor is running.
If you are running a Windows Server - I recoomend using NRPE to execute WMI plugins.
The setup is something like this:
- You will need to download and install the NRPE addon for nagios from Nagios's Site.
- You will need to download a Windows "Agent" for NRPE.
- You will need to have the "Agent" loaded on at least one of your Windows Servers"
- You will need to download, Install, Configure WMI Check Plugins on the Agent Server.
- You will need to add some commands to Nagios.
Google the following:
Nagios WMI Monitoring
Nagios windows Monitoring WMI
Nagios windows Monitoring NRPE
You should find tons of more detailed how-to's.
Also check www.nagiosexchange.org for tons of plugins for all kinds of checks.
For Linux Servers, you could monitor them via SSH.
Copy the plugins nagios uses to check itself from the libexec folder to your remote server and execute them via SSH.
Search www.nagiosexchange.org for plugins that allow you to execute nagios plugins on remote linux servers via ssh. You will also have to authorize your nagios server to connect to the server you want to monitor without needing to enter a password. Authenticate it via Public/Private DSA/RSA Keys.