Ubuntu NAT setup between eth0 and eth1

etc-network-interfaces

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
auto eth0
iface eth0 inet static
address 10.1.250.179
netmask 255.254.0.0
gateway 10.1.100.63
dns-nameservers 128.49.4.2 128.49.4.3

nat.sh

#!/bin/bash

if [ $# -eq 0 ]
then
    echo "nat.sh [options] install
    install - Should be run once initially"
elif [ $1 == 'install' ]; then
    echo "Flush all the rules in filter and nat tables..."
    sudo iptables --flush
    sudo iptables --table nat --flush
    sudo iptables --delete-chain
    echo "Delete all chains that are not in default filter and nat table..."
    sudo iptables --table nat --delete-chain
    echo "Set up IP FORWARDing and Masquerading..."
    sudo iptables --table nat --append POSTROUTING --out-interface eth0 -j MASQUERADE
    sudo iptables --append FORWARD --in-interface eth1 -j ACCEPT
    sudo iptables --append FORWARD --in-interface eth1 -j ACCEPT
    echo "Enables packet forwarding by kernel..."
    sudo sh -c "echo 1 > /proc/sys/net/ipv4/ip_forward"
    echo "Apply the configuration..."
    sudo iptables-save
    echo "For List the iptables rules"
    sudo iptables -L
fi