Nagios XI default mornitor port 5666 and 5667

For Nagios XI to monitor remote Linux servers on default port, we need
to open 5666 and 5667 in the to be monitored linux servers, for example:

vi /etc/sysconfig/iptables

-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 5666 -s 192.168.3.75 -j ACCEPT
-A RH-Firewall-1-INPUT -m state --state NEW -m tcp -p tcp --dport 5667 -s 192.168.3.75 -j ACCEPT

192.168.3.75 is your Nagios XI server