I use APF firewall and need to set up a port translation such that any request that comes in to destination port 21 will get re-routed to port 22777 on the same IP address. I don't understand APF's relationship to iptables, so I'm not sure where or how to make the mapping.

I notice that APF firewall has a config file called preroute.rules, but there is no mention of it in the README. Is this where I should be updating the mapping, or in a file more OS-centric? Once I make the update, is it sufficient to restart APF? If anyone can shed light on this, and perhaps provide an example iptables command to be used, it would make me very happy! Thank you!!!