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So, my situation is, I have a PC with two network cards, one is a physical ethernet adapter, the other a wireless adapter. Both network cards are on the same subnet. I need to be able to forward IP packets from one adapter to the other.

I have already attempted to use a bridge and this does allow the packet forwarding to function. The problem with the bridge is that it changes the MAC address of packets that go through it. There are devices on the network that cannot handle this.

I have also just tried setting the IPEnableRouter registry parameter, since apparently that does what I need. However it seems to have no effect.

I suspect the issue here may be that they are both on the same subnet, so the routing logic in windows just sends it back out on the same adapter.

Anyway to connect these two NICs?

Cheers

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2 Answers

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Bridging can be made transparent (i.e. not change the MAC adresses) if the network adapter supports sending frames with arbitrary source MACs. With two ethernet cards it would have probably been fine.

However, because of the way Wifi operates in plain Infrastructure mode (the Access Point coordinates traffic between stations identified with their MAC), you can only do transparent bridging at the AP itself.

If your AP supports WDS, you can still achieve transparent bridging, if your wifi card is able to act as a WDS client. You could also buy a cheap wireless AP that does WDS, and configure it as a WDS client to your main AP.

If you decide to go with routing, you may have to reassign adresses in two distinct blocks for the two sides, this will split your broadcast domain (some protocols may not like it) and you will have to set up manual routes on all hosts that need to communicate across the border (or set up routes on the default gateway only, and bear with inefficient routes)

It is very weird, however, that a device cannot tolerate the MAC being changed by the bridge. Can we get more details on that ?

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The devices in question are custom hardware devices with an embedded TCP/IP stack. Basically, the target network is the same either side of the bridge, it's just that the option is there to connect either through cable, or through wireless. It's the change between the two that confuses everything, because they end up with the same IP address having a different MAC address (because it now goes through the bridge). Additionally, plugging in the cable while the wireless is connected results in some horrific IP conflicts and loops (due to that MAC Address change on the bridge). – Kazar Jul 13 '10 at 17:49
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I'm afraid you will have to change the subnet of one of your cards. Routing and bridging and so on is all about connecting different networks/subnets, they most probably will not work with the same subnet since there is no way the operating system can find a specific route if you can't differentiate between the routes and networks.

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Ok, interesting, might there a way to change the subnet of a card, and yet still force all packets for a specific IP address on a different subnet to still use that card? Static routing perhaps? (Not holding my breath though) – Kazar Jul 13 '10 at 10:52
If you, let's say, set 192.168.1.1 on the wifi card and 192.168.0.1 on the wired card, you only have to start "connection sharing" and Windows will create for you a NAT, meaning that computers behind your computer over the wired will get out to the Internet over your wifi. Of course you can do this either way. Although you may read about the "route" command for Windows and use it to set up your specific settings. – Patkos Csaba Jul 13 '10 at 16:43
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