While OP5 Monitor does not currently support CentOS 8, you can still run remote checks on nodes running CentOS 8 by calling the NRPE daemon. If you want to run checks remotely over NRPE, you will likely have to install the NRPE daemon on your remote node, as it is often not included by default.
1. Add the EPEL repository:
# dnf install epel-release
2. Install nrpe
# dnf install nrpe
3. Install the nagios-plugins you want to use. Do a search to list available options:
# dnf search nagios-plugins
4. Install the plugins of your choice:
# dnf install nagios-plugins-disk nagios-plugins-load (...)
5. Enable and start nrpe:
# systemctl enable --now nrpe
6. If you have firewalld enabled, open up the standard NRPE port 5666 over tcp:
# firewall-cmd --add-port=5666/tcp --permanent
# firewall-cmd --reload
7. Configure your nrpe daemon by editing /etc/nagios/nrpe.cfg or adding files to /etc/nrpe.d/. Inspect the default configuration and make sure you're happy with it. Remember to set your "allowed_hosts" to whitelist your OP5 Monitor node, as remote checks may be denied otherwise.