Set up Network Address Translation

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Contents

Overview

Network address translation (NAT) is if you have a private network with private IP addresses and all these computers share access the internet via one computer. This computer, it can also be a cluster, is said to translate the network addresses between the private network and the internet.


                                               -----------------------------------------------------------
                                              |                                                           |
                                              |                          --------------                   |
                                              |                         | internal     |                  |
                                              |                        /| 172.16.0.43  |                  |
                                              |                       /  --------------                   |
                                        -------------                /                                    |
                 ---------             | NAT-gateway |              /    --------------                   |
                |internet |------------| 10.2.2.18   |             /    |  internal    |                  |
                 ---------             | 172.16.0.1  |------------------| 172.16.0.42  |                  |
                                        -------------                    --------------                   |
                                              |                                                           |
                                               -----------------------------------------------------------

Set it up

To set up NAT (network address translation) on the NAT gateway, open a console and

  • allow IP traffic forwarding on the NAT-gateway
echo 1 > /proc/sys/net/ipv4/ip_forward
  • tell the kernel on the NAT-gateway to forward traffic to the outgoing interface eth0 and masquerade it, so it will put its own IP address into each packet's header:
iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE

Test it

To test if it has worked, go to one of the internal computers and set the computer doing NAT as gateway, e.g. under Linux:

route add default gw 192.168.0.1

Then try to ping google's name server. If it works, you did it right:

ping 8.8.8.8

See also