iptables-route-masquerade-traffic.sh

#!/usr/bin/env /bin/sh
#
# This script uses Linux iptables to route and masquerade ipv4 traffic 
# from a network interface to another.
#
# License: MIT
#
# Author: Asher256
# Github: https://github.com/Asher256
# Website: http://www.asher256.com/
#

set -e
set -u

in_nic=eth0
out_nic=eth1

# Flush old iptables rules
iptables -F; iptables -X
iptables -t nat -F; iptables -t nat -X
iptables -t mangle -F; iptables -t mangle -X

# Iptables policies (accept OUTPUT only)
iptables -P INPUT DROP
iptables -P FORWARD DROP
iptables -P OUTPUT ACCEPT

# Route and masquerade traffic
iptables -A FORWARD -i "$in_nic" -o "$out_nic" -j ACCEPT
iptables -A FORWARD -i "$out_nic" -o "$in_nic" -m state --state RELATED,ESTABLISHED -j ACCEPT
iptables -t nat -A POSTROUTING -o "$out_nic" -j MASQUERADE