What is it with computers? No matter how many years you’ve been using them they can still surprise you as to just how awkward they can be. This weekend, I tried to bridge my upstairs and downstairs networks using wireless....fat chance.
You’d think that getting a relatively obscure wireless ethernet card working on Linux would be a pain. It’s not supported directly in the kernel, so I had to grab some additional drivers. I was not holding out much hope, to be honest — but amazingly it all worked first time. Within a few minutes of downloading the driver, it was all up and running. However, this lulled me into a false sense of security…
Upstairs I have a DLink 604+ ADSL router which acts as my NAT and modem. There’s currently a long self-installed RJ45 cable winding its way down the stairs to the kitchen where my 3 PCs live (don’t ask). I was hoping to use the wireless card to replace the cable with an invisible link and thus win the favour of my girlfriend.
So, I duly configured downstairs as being 192.168.1. and upstairs as 192.168.0.. I added the relevant routes to each subnet, with the linux box (with both a normal ethernet connection and the wireless) as the gateway between them. I rejoiced as I disconnected the upstairs/downstairs lead and yet I was still able to ping the router from my windows boxes downstairs — all was working!
Until…I realised I couldn’t access the Internet from the windows boxes. Try as I might, the router was rejecting packets from the downstairs subnet, giving me ‘network unreachable’ ICMP replies. It would seem that the router only NATs outgoing packets from its local subnet — 192.168.1.* it wouldn’t touch with a bargepole. No amount of misconfiguring netmasks got me anywhere — except one moment when I thought I had it working only to discover I’d plugged the up/downstairs cable back in…outgoing packets were getting upstairs via wireless, but back down the wire. Arse.
I also tried bridging the two segments with the ethernet bridge kernel configuration…but sadly that caused massive loopback problems; effectively the router itself (which has both RJ45 and wireless) is already an ethernet bridge it would seem — whenever I bridged the two nets downstairs I’d get a slew of packets pinging back and forth upstairs/downstairs and still no access to the internet.
The only way I could think of doing things was to re-NAT on the linux box so all the packets appear to come from it — then the router will happily NAT outbound again — but that’s a right old pain that I don’t want just yet.
All in all, a bit of a nightmare. Something that seemed simple, turned out to be a right pain. Don’t you just love computers?