title: Nagios - how does native service such as check_ping monitor remote host without the remote host not having nrpe client installed?
body: <p>When I checked the dashboard from Nagios server(linux platform), I noticed that one remote host(linux platform) that the server monitors has many services with status as critical and some are ok. When I did initial investigation, I found out that this remote host is being monitored does not have nrpe installed or plugins such as check_ping and etc. However from the dashboard, Service called Ping which is tied to check_ping (I assume) runs and I can see that status is ok. I wonder how this happens. I looked at commands.cfg file and check_ping is defined twice:</p>
  
  <pre><code>define command{
      command_name    check-host-alive
      command_line    $USER1$/check_ping -H $HOSTADDRESS$ -w 3000.0,80% -c 5000.0,100% -p 5
  }
  define command{
      command_name    check_ping
      command_line    $USER1$/check_ping -H $HOSTADDRESS$ -w $ARG1$ -c $ARG2$ -p 5
  }
  </code></pre>
  
  <p>My question is if indeed Ping service is running on the remote host, how does the nagios server is able to do it with defined commands without the remote host not having nrpe installed nor plugins. </p>
  
  <p>My limited understanding of the Nagios is check_nrpe asks nrpe (proxy agent installed on remote host) to execute a program (check_ping or check_ssh) installed on the remote host.</p>