echo 1 > /proc/sys/net/ipv4/ip_forward
iptables -A FORWARD -i eth1 -j ACCEPT
iptables -A FORWARD -o eth1 -j ACCEPT
#forward FTP
iptables -t nat -A PREROUTING -i eth0 -p tcp –dport 21 -d 202.152.14.171 -j DNAT –to 192.168.14.2:21
#Forward Web
iptables -t nat -A PREROUTING -p tcp –dport 80 -d 202.152.14.171 -j DNAT –to-destination 192.168.140.6:80
iptables -t nat -A PREROUTING -p tcp –dport 8006 -d 202.152.14.171 -j DNAT –to-destination 192.168.140.22:8006
iptables -t nat -A POSTROUTING -j MASQUERADE

Teori 1 : http://malang.linux.or.id/2012/07/belajar-iptables-dan-konfigurasi-iptables/

Teori 2 : http://www.catatanlepas.com/komputer/security/324-membuat-port-forwarding-dalam-iptables-di-centos