We'll try to monitor a maximum via check_by_ssh

 ssh-keygen

Copy the key

ssh-copy-id nagios@dns.fortier-family.com
ssh nagios@dns.fortier-family.com

useradd -m -d /home/nagios -s /bin/bash -p nagios nagios
passwd nagios

check /etc/ssh/sshd_config for:

PasswordAuthentication yes

References:

• https://assets.nagios.com/downloads/nagioscore/docs/nagioscore/3/en/monitoring-publicservices.html
• https://linux.101hacks.com/unix/check-http/
• https://www.digitalocean.com/community/tutorials/how-to-install-nagios-4-and-monitor-your-servers-on-ubuntu-16-04