Wireless client isolation - how does it work, and can it be bypassed?

Question

Many SOHO routers these days support a feature called "wireless client isolation", or similar. What this is supposed to do, in principle, is to limit the connectivity between wireless clients connected to the AP. Wireless clients can talk to the LAN, and reach the Internet if such connection is available, but they cannot communicate with one another.

How is this achieved? Are there any particular weaknesses which would allow this to be easily bypassed?

Answer 1

The implementation that I've seen of this is done by fiddling with the MAC forwarding table on the access point. Since the access point simply acts as a network bridge, it is fairly well suited to this kind of task. At the switching layer it is already collecting all of the heard (sometimes called learned) MACs and which interface it can be found on.

The logic looks kind of like this:

1. Access Point receives a packet over the wireless interface
2. Bridging subsystem examines packet for destination MAC
3. If destination MAC is in the learned switching table for wireless interface -> DROP
4. Otherwise forward packet via wired interface

Because of the way network bridges work I see this being fairly difficult to trick the access point into forwarding a packet to a client in spite of the isolation. Your best bet would be to attempt to talk directly to the other client, as if you were operating with an ad-hoc network.