handyfloss

Maybe you have two computers at home, with no router, and no wireless, and want both to share an Internet connection. Or maybe you want to set up a home LAN with non-public addresses. If so, read on.

I have set up my laptop to use my desktop computer as a NAT to connect to the Internet.

Requirements and setup

Your NAT computer needs two Ethernet cards: one to connect it to the Internet, and another one to connect it to the laptop. You also need a crossover cable, to connect the laptop to the desktop computer.

The physical setup is easy: make the connections as below:

Desktop computer setup

You need to set eth1 (second NIC) of your comp to connect to the LAN, and leave the eth0 as it was (to connect to the internet). Edit /etc/network/interfaces (for Debian. Other distros, edit the corresponding file), and add (suposing the network you want to create is 192.168.10.0):

iface eth1 inet static
  address 192.168.10.1
  netmask 255.255.255.0
  broadcast 192.168.10.255
  gateway 192.168.10.1
  post-up route del -net 0.0.0.0 gw 192.168.10.1 eth1
  post-up route add -net 192.168.10.0 netmask 255.255.255.0 gw 192.168.10.1 eth1

The last two lines (man interfaces) remove the default routing path that bringing up eth1 sets, because we want eth0 to still be the default, with only signals going to 192.168.10.0 network being routed through eth1. The latter is set by the last line.

Laptop setup

We have to modify /etc/network/interfaces, too. Here it’s eth0 that we set up:

iface eth0 inet static
  address 192.168.10.2
  netmask 255.255.255.0
  broadcast 192.168.10.255
  gateway 192.168.10.1