How to install NRPE on a OPNsense Firewall:

• pkg install nrpe3-3.2.1
• Create /etc/rc.conf.local  add nrpe3_enable=”YES”
• Edit configfile: /usr/local/etc/nrpe.cfg
• Create a Firewall Rule for Port 5666/TCP
• Start the servive: service nrpe3 start