From terminal, execute the following;
sudo modprobe ip_nat_ftp ports=21
sudo modprobe ip_conntrack_ftp
sudo iptables -A FORWARD -m state –state ESTABLISHED -j ACCEPT
sudo iptables -A FORWARD -p tcp -m tcp -m state –dport 21 –state NEW -j ACCEPT
sudo iptables -A FORWARD -p tcp -m tcp -m state –sport 20 –state RELATED -j ACCEPT
sudo iptables -A FORWARD -p tcp -m [...]