gistfile1.txt

#!/bin/bash
internal_ip='10.0.0.0/8'
external_ip='172.16.0.0/12'
iptables -F
iptables -t nat -F

iptables -P INPUT ACCEPT
iptables -P OUTPUT ACCEPT

iptables -A FORWARD -i eth0 -o eth1 -s $internal_ip -j ACCEPT
iptables -A FORWARD -i eth1 -o eth0 -s $external_ip -j ACCEPT
iptables -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT
iptables -P FORWARD DROP

iptables -t nat -A POSTROUTING -o eth1 -s $internal_ip -j MASQUERADE
iptables -t nat -A POSTROUTING -o eth0 -s $external_ip -j MASQUERADE