What is CSF

Config Server Firewall (or CSF) is a free and advanced firewall for most Linux distributions.

How to add NAT iptables rules with CSF Firewall

We can add custom NAT iptables rules in /etc/csf/csfpre.sh OR/AND /etc/csf/csfpost.sh
0. Create csfpre.sh or csfpost.sh

1. Change permission to make the script executable

2. Add NAT iptables rules

add iptables rules like this:

3. Restart csf and check the result

Ref

+ Google
+ TecAdmin