CSF is very helpful to allow or deny certain IP address with port in your Linux server.

Deny :

Some users sending spam email's from the server with ports and IP address, need to block the IP address and port in  csf.deny file,
assume that the port is 1774 and IP 192.168.1.200

  # vim /etc/csf/csf.deny
tcp|in|d=1774|d=192.168.1.200


Allow :

If you want to allow port with IP address add below line to csf.allow
 
# vim /etc/csf/csf.allow


tcp|in|d=1774|d=192.168.1.200

tcp|in|d=2774|d=192.168.1.201