Simple iptables matching question

[Jarjar]

Long stort short: I've got two external interfaces, two different ISPs. I want to use one (eth0) for all traffic except bittorrent. The bittorrent traffic needs to go on the other external interface, eth2.
I think I have everything set up except the packet marking, so...

All bittorrent traffic will be on port 49000. How do I mark all such traffic (using -j MARK --set-mark)? I'm clueless when it comes to prerouting, postrouting et. al, so this should be easy.

Edit: Doohh... I figured it out, using a packet sniffer. The packets were of course leaving using random ports, so I didn't catch any of the outgoing traffic.
I solved it using L7-filter, now everything matches up just fine. Onto the next problem...
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