First of all: You should definitely try to ROUTE your networks, not NAT them. That's why this page's title is nat-HACK.

Why NAT-hack?

When you connect different networks you should plan ahead so that all the computers can talk together with ROUTING. But sometimes you just can't change the routing in your network: Unwilling computer department, no password to the router, etc etc.

What does the NAT-hack do? Can I use it as a universal problem solver? The NAT-hack is a way of making your openVPN server rewrite ALL TRAFFIC coming in from its VPN tunnels, sending it on to its destination but FAKING that the openVPN server is the SOURCE. This way all machines that the openVPN server is able to communicate with, can also be reached from the VPN tunnels.

It's more or less like everyone in the neighborhood using your phone. You would spend time coordinating what calls are for what neighbor. But it could work. The authorities would see all the telephone calls as coming from you. It would be better if everyone got their own phone, so the calls could be routed directly.

So the NAT-hack is NOT a problem-solver, and could be a problem-creator. But it could get you out of this following tricky situation: (See for an image) You have an openVPN server obviously sitting on a network (LAN). Now you want your machines from your VPN to be able to reach some machine on the LAN. Now, the client on your VPN tries to contact That's OK, the openVPN server forwards the packet to But doesn't know where the machine is supposed to be. So it sends it to the gateway. Now, the gateway doesn't know either, so it sends it to the internet. The internet just ignores it without warning, because 10.<something> addresses aren't allowed on the internet.

If you could you should definitely add routes to the gateway, telling it to send the packets for the VPN to the openVPN server. Or at least add that route to the machine.

But what if you don't have access to the Nor the gateway? Then you will need the NAT-hack, so that the openVPN server forwards all packets it receives from the VPN, saying "these packets are all from me". And take care of returning the answer packets to the correct destination. All traffic from the VPN will appear to come from the openVPN server. Might not be good, but might also be your only solution.

openVPN server on Linux

all these are copied from the !linnat irc "command" when you're in #openvpn on freenode:

Assuming that you have eth0 as the network card you want as "uplink" and 10.8.0.x is the vpn network:

iptables -t nat -A POSTROUTING -s -o eth0 -j MASQUERADE

Or if you want to choose what IP address to NAT as, you can use

iptables -t nat -I POSTROUTING -o eth0 -j SNAT --to <IP ADDRESS>

see for more info on NAT in linux.

NOTE you maybe need to save the iptables settings so they will be restored when you boot the system. I haven't rebooted in a long while so I don't remember the command to save iptables settings, or if it's automatically saved.

openVPN server on Windows XP

Here you've got a nice howto on how to do real NAT with windows xp. Not the NAT that messes up your LAN settings, but real NAT without touching your IP address setup.

On my openVPN server, though, the internet link was on one card, and the other links on another card.

eth0 internet  = my internet connection, 10.0.0.x
eth1 corporate = my company's connection, 172.31.25.x
vpn0 openvpn   = the openvpn network "card", 10.8.0.x

As I wanted to connect the VPN clients to the corporate network I had to add both the internet NIC and the corporate NIC as FULL:

  1. Modify Registry Entry to allow IP forwarding:
  2. net stop remoteaccess Stop the RAS Service
  3. netsh routing ip nat install
  4. netsh routing ip nat add interface "eth0 internet" full
  5. netsh routing ip nat add interface "eth1 corporate" full
  6. netsh routing ip nat add interface "vpn0 openvpn" private
  7. netsh routing ip nat add interface Internal private
  8. net start remoteaccess

(Actually I had to first open Control Panel | Administrative Tools | Services, find Remote Access Connection Manager and ENABLE it before the step #8)

Check it with

netsh routing ip nat show interface

should look something like:

NAT Internal Configuration
Mode              : Private Interface

NAT eth0 internet Configuration
Mode              : Address and Port Translation

NAT eth1 corporate Configuration
Mode              : Address and Port Translation

NAT vpn0 openvpn Configuration
Mode              : Private Interface

And of course, the classic windows apply method: Reboot the machine for the settings to take effect.

NOTE that the NAT-hack does not free you from setting up routes to the networks neither on the client nor on the server.

To set up a route on the clients

In the client config add the following:


of in the server config add the following:

push "route"
push "route"

You can keep adding more routes till you reach everything you need.

Last modified 13 years ago Last modified on 01/28/11 10:48:23