I had to do a lot of fiddling before I got nagios over ssh working. I used this article as source, mostly, even though I did it differently.

First add some commands to commands.cfg: 

define command{
        command_name    check_remote_disk
        command_line    /usr/lib/nagios/plugins/check_by_ssh -p $ARG1$ -l nagios -t 30 -o StrictHostKeyChecking=no -i /etc/nagios3/id_rsa -H $HOSTADDRESS$ -C '/usr/lib/nagios/plugins/check_disk -w $ARG2$ -c $ARG3$ -p $ARG4$'
}
 
define command{
        command_name    check_remote_load
        command_line    /usr/lib/nagios/plugins/check_by_ssh -p $ARG1$ -l nagios -t 30 -o StrictHostKeyChecking=no -i /etc/nagios3/id_rsa -H $HOSTADDRESS$ -C '/usr/lib/nagios/plugins/check_load -w $ARG2$ -c $ARG3$'
}
 
define command{
        command_name    check_remote_swap
        command_line    /usr/lib/nagios/plugins/check_by_ssh -p $ARG1$ -l nagios -t 30 -o StrictHostKeyChecking=no -i /etc/nagios3/id_rsa -H $HOSTADDRESS$ -C '/usr/lib/nagios/plugins/check_swap -w $ARG2$ -c $ARG3$'
}

The file referenced by -i is an SSH secret key. You can create this key by running ssh-keygen and giving the right path. You can’t store anything in the nagios home dir, because that is in /var/run, which is cleared after a reboot. So, you can’t use the default key file.

The -t 30 is necessary because sometimes there is network lag, causing the service to be reported as failure. The default of 10 is not enough…

Next you need to create a user nagios on the target machine and add the .pub file of the key to the authorized_keys. Creating the user should be done like: 

useradd --system --shell /bin/bash nagios

Then install the nagios plugins on the target host: 

aptitude -P install nagios-plugins-basic

The nagios host needs to be able to log in with user nagios. On Debian, the user that runs nagios (also called ‘nagios’) doesn’t have a shell by default. So, give it a shell.

Then you can create a hostgroup (for example). You can do: 

define hostgroup {
        hostgroup_name  nagios-enabled
        alias           Nagios enabled
        members         host1, host2
}

Then create services: 

define service {
        hostgroup_name                  nagios-enabled
        service_description             Root partition space
        check_command                   check_remote_disk!22!20%!10%!/
        use                             generic-service
        notification_interval           0
}
 
define service {
        hostgroup_name                  nagios-enabled
        service_description             Swap space
        check_command                   check_remote_swap!22!50%!30%
        use                             generic-service
        notification_interval           0
}
 
define service {
        hostgroup_name                  nagios-enabled
        service_description             Load
        check_command                   check_remote_load!22!5.0,4.0,3.0!10.0,6.0,4.0
        use                             generic-service
        notification_interval           0
}

This will check load, swap and root space on all your standard nagios enabled hosts. Next you can define custom services: 

define service {
        host_name                       piet
        service_description             Some partition
        check_command                   check_remote_disk!22!40%!30%!/mnt/dinklefat
        use                             generic-service
        notification_interval           0
}