#!/bin/sh
#
# masquerade: Script to masquerade an external network connection
#             (modem or ADSL) upon bringing the interface up. Name this
#             script masquerade.sh, put it in /etc/network/if-up.d/ and
#             make it executable (755).
#             Change INET_IFACE and LAN_IP_RANGE to suit your needs.
#             You will need to have packet filtering (iptables) enabled
#             in your kernel and module iptable_nat.
#
#             This script is intended for Debian systems. May not work on
#             other Linux distributions.
#
#             Author: Ricardo Yanez <ricardo.yanez@calel.org>, Mon May 30, 2011
#
# Define external interface
INET_IFACE="eth0"

# LAN IP range
LAN_IP_RANGE="192.168.0.0/24"

# Get IP number
INET_IP=`ifconfig $INET_IFACE | egrep "inet addr" | cut -d : -f2 | cut -d \  -f1`
# please note the extra space in the last cut, right after \ (very important).

# Clear all rules in POSTROUTING chain
iptables -t nat -F POSTROUTING

# Masquerade
echo -n "Masquerading interface $INET_IFACE to source $INET_IP... "
iptables -t nat -A POSTROUTING -s $LAN_IP_RANGE -o $INET_IFACE \
         -j SNAT --to-source $INET_IP
echo "done."