Linux Networking Cookbook

Enabling NAT to the outside

Connecting two local networks is useful, but these days it's more common to want to connect a local network to the Internet. The basic concept works the same, but with the necessary addition of NAT. NAT rewrites your packet headers in order to make them appear as if they come from your router, thus effectively hiding your system's address from the destination.

How to do it…

Create a third NIC (eth2) on server 1 and connect it to your cable modem or ISP's router.

1. Configure eth2 to receive an IP address via DHCP:

   auto eth2
   iface eth2 inet dhcp

2. Use iptables to enable NAT on packets heading out through eth2:

   # /sbin/iptables -t nat -A POSTROUTING -o eth2 \
     -j MASQUERADE
   # /sbin/iptables -A FORWARD -i eth2 -o eth0 -m \
    state ...