I forgot to mention the other piece of my epic ADSL Bittorrent setup: my modem.
I use a weeny little Dynalink RTA1320. At first glance it probably looks a bit small and pathetic, but don’t be fooled. It has a full-blown ADSL2+ compatible modem in it, and quite a nice router system, but I’m cold and hard and don’t care about the router system. I only care that it supports PPP half bridge mode.
I’m gonna get a bit nerdy on you here. Most sane countries use PPPoE (Point to Point Protocol over Ethernet) to authenticate their ADSL clients, meaning any device that can carry an ethernet signal can carry the encapsulated authentication packets. Good idea. This means you can set up an ADSL modem in raw bridge mode, and have your router pass the authentication information down the wire. The modem only has to deal with modeming (modulating and demodulating)* the ADSL signal.
In silly old New Zealand, we use PPPoA (the A is for ATM, which is does not stand for Automated Teller Machine). I believe this is because large parts of our backend infrastructure are, ummm, crap. This causes issues because the modem has to setup the PPP connection (because routers without modems can’t talk ATM), and in most cases this means modems also have to be routers, and combined modem-routers are almost universally shite.
Enter our saviour: PPP half bridge mode. If I understand correctly (and the chances of that are relatively low), then the modem/router running in PPP half bridge mode acts as follows:
- The modem sets up the PPPoA connection using the authentication information you give it.
- It then it grabs the MAC address from the internal connection (in my case the ‘internet’ port of the WRT54GL) and exposes it to the intertr0n.
- Any packets received on that MAC address are forwarded without further consideration to the internal connection.
So once the connection is established, the ‘true’ router effectively sees all the internet traffic, and the crappy modem-router doesn’t have to do any hard work dealing with NAT or firewalling. Sweet!
So in summary, Ben’s recipe for ADSL Bittorrent successTM:
- Dynalink RTA1320 in half-bridge mode
- Linksys WRT54GL running DD-WRT, with config changes to NAT behaviour
*So why isn’t it a moddem then? Eh?