Multihomed WLANs from Intel

accessdeniednsp writes: "El Reg gives us some insight on Intel Labs' new software to let your wireless LAN card hop between various networks (802.3, 802.11, and 'fixed Ethernet' they call it). Perfect for us snoopers to walk by college frat houses and hopping on the 'net with our linux ipaq's :)" First company to come out with a "universal connectivity" PCMCIA card wins all the marbles.

Comments

What we really need

[vlag]

What we all really need is a wireless nic that can be upgraded to new wireless standards à la 802.11x via firmware. The argument exists that we don't have the components to build these cards but I feel that this is where research should move. As it stands now, we are poised for a new standard roll-out every 8 months for the next few years. Anyone know about this type of tech being develloped?

Re:Frats with LANs?

[garcia]

I am not in a frat, but I do have a wireless LAN in my apartment. Playing MP3s to the stereo from a laptop (and using it to surf from the couch) is far easier and safer (for drunken idiots) than running ethernet to the living room from the hub in my room.

I do phone support and I hear a lot of people getting into wireless routers b/c of the ease of having it run to all the computers in the house. Why not do it for frats?

And unfortunatly I do have a WLAN but no kegerator ;(

you can do much of this now

[The Pim]

I do part of this (hopping between wired and wireless) now with ordinary hardware and software, although a few bits are missing. The key piece is proxy ARP, a nifty trick I had never heard of until I tried to solve this problem.

The basic idea is to set up a gateway on both the wired and wireless networks, and proxies ARPs on both networks, so that hosts on the different network see each other as if they were on the same LAN. This is a little like bridging--except that only a tiny bit of traffic (the ARP's) needs to "bridge" the two networks. The rest is taken care of by normal routing.

The trick is switching a host from wired to wireless without changing its IP addresses (so it doesn't drop any connections). Note this implies that the gateway's routing table has a host route (specifying the interface) to every address that is allowed to switch networks: you can't tell from the address which side its on, so the usual subnet mask routing won't work.

Pulling off the switch requires that the gateway be able to detect the switch, and then do two things: One, change its routing table, so that traffic for the address goes out on the right interface. Two, send "gratuitous ARPs" to other hosts, forcing them to update their ARP tables (since, if the host moved to the other network, traffic to it now needs to be routed through the gateway).

I think the most straightforward way to detect the switch is to have the gateway run a DHCP server, and have the mobile hosts renegotiate a lease when they switch networks. Then, add a hook to the DHCP server to do the magic whenever it notices a host renegotiate on a different network. For the mobile hosts to be identifyable across networks, they need to send the same client-identifier on both networks. Since the default client-identifier is usually the MAC address, this requires configuration on the clients (I edit /etc/dhclient.conf and pick one MAC address to use as the client-identifier). Of course, the DHCP server needs to be configured to give out the same address range on both interfaces.

Unfortunately, on the network I care about, my gateway is not the DHCP server. Instead, I run a DHCP relay. This mostly works--except the ISC DHCP relay doesn't have any hooks, and I haven't hacked it to add them. But it should be easy.

Another way to solve this might be for the gateway simply to monitor ARPs and do something when it notices a host switch networks. I haven't found a clean way to do this, and I think it might be less than perfect, because the host wouldn't get switched until it initiated an ARP transaction.

The last problem is that different systems seem to respond differently to gratuitous ARPs. For example, Linux systems don't seem to require them at all, because they (apparently) issue a new ARP pretty quickly after the old MAC address stops answering. But I can't get Solaris systems to listen to gratuitious ARPs at all, and they don't time out for minutes.

Also, gratuitiously ARPing the whole network is ugly. Ideally, we would would only send an ARP when we notice another host using a MAC that we know has moved to the other network. I have no idea how to do this.

Despite all the glitches, it's quite fun to switch to the wireless for mobility and back to the ethernet for speed, without losing my ssh connections. Improvements on this setup would be welcome!