If you have followed the steps from the previous sections carefully, then most probably, everything should be working smoothly. However, in some cases, your setup might not be working properly, and you will need to find the root cause of the problem.

The first thing that you should start with is to use the check_ssh plugin to make sure that SSH is accepting connections on the host we are checking. For example, we can run the following command:

root@ubuntu1:~# /opt/nagios/plugins/check_ssh -H 192.168.2.51
SSH OK - OpenSSH_4.7p1 Debian-8ubuntu1.2 (protocol 2.0)

Where 192.168.2.51 is the name of IP address of the remote machine we want to monitor. If no SSH server is set up on the remote host, the plugin will return Connection refused status, and if it failed to connect, the result will state No route to host. In these cases, you need to make sure SSH server is working, and that all routers and firewalls do not filter out connections for SSH — which is TCP port...