https://www.linuxquestions.org/questions/linux-networking-3/iptables-forward-all-ftp-to-other-server-692717/ - Forward All FTP to Other Server

$ modprobe nf_nat_ftp
$ iptables -t nat -A PREROUTING -p tcp -d serverA –dport 21 -j DNAT –to-destination serverB
$ iptables -t nat -A PREROUTING -p tcp -d serverB –dport 21 -j SNAT –to-source serverA