Open A Port For One IP With CSF Firewall

Inside /etc/csf/csf.allow

tcp:in:d=22=192.168.0.100

That will allow incoming port 22 traffic to the server for 192.168.0.100

To ensure no other traffic comes in on port 22 you have remove port 22 from the TCP_IN section of /etc/csf/csf.conf then run:

csf -r

To reload the config