Forward ports through a Linux gateway with iptables

nat-fwd.sh

#!/usr/bin/env bash

i_in=en0
i_out=en1
dport=1234
dst=1.2.3.4
src=4.3.2.1

iptables -A FORWARD -i $i_in -o $i_out -p tcp --syn --dport $dport -m conntrack --ctstate NEW -j ACCEPT
iptables -A FORWARD -i $i_out -o $i_in -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
iptables -t nat -A PREROUTING -i ens5 -p tcp --dport $dport -j DNAT --to-destination $dst
iptables -t nat -A POSTROUTING -o ens5 -p tcp --dport $dport -d $dst -j SNAT --to-source $src