Ubuntu

#!/bin/sh

### SET VARIABLES

echo "** setting up variables..."

IPTABLES='sudo /sbin/iptables'
IP6TABLES='sudo /sbin/ip6tables'
MODPROBE='sudo /sbin/modprobe'
INT_NET='172.16.5.24/29'
INT_INTF=eth1
EXT_INTF=eth0


### LOAD MODULES

echo "** Loading modules..."

$MODPROBE iptable_filter 
$MODPROBE iptable_nat 
$MODPROBE nf_nat
$MODPROBE nf_conntrack_ipv4
$MODPROBE nf_conntrack
$MODPROBE ip_tables 
$MODPROBE x_tables

### FLUSH EVERYTHING

echo "** Flushing existing iptables rules..."

$IPTABLES -F INPUT 
$IPTABLES -F OUTPUT 
$IPTABLES -F FORWARD
$IPTABLES -F -t nat
$IPTABLES -X

echo "** Building fw main rule..."

$IPTABLES -N block
$IPTABLES -A block -m state --state ESTABLISHED,RELATED -j ACCEPT
$IPTABLES -A block -m state --state NEW -i !eth0 -j ACCEPT
$IPTABLES -A block -j DROP

$IPTABLES -A INPUT -j block
$IPTABLES -A FORWARD -j block


iptables -t nat -A POSTROUTING -s $INT_NET -o eth1 -j MASQUERADE

sudo iptables -t nat -A POSTROUTING -o eth1 -s 172.16.5.24/29 -j MASQUERADE

sudo iptables -F block 
sudo iptables -X block