Last night, I learned all about handling BitTorrent behind an IPCop box. We have multiple systems in our new apartment, and we wanted to be able to run BitTorrent clients on each system, but were having connectivity problems. Specifically, new connections to files we were seeding were getting dropped.

The problem

The basic problem is that while a firewall that uses NAT to turn one public facing IP into a network that multiple internal systems uses (one-to-many NAT) works fine for more or less stateless protocols such as HTTP, peer to peer protocols such as BitTorrent have problems. When an outside client wants to leech from my BitTorrent seed, how does it know to grab the torrent I'm seeding, rather than hitting my roommate's computer, which may or may not be seeding anything, much less the torrent in question?

The solution

I started knowing that BitTorrent typically uses ports 6881-6889 (actually, 6881-6999 after BT 3.2). After a little dredging of BitTorrent FAQs. I found that "the client starts with the lowest port in the range and sequentially tries higher ports until it can find one to which it can bind." This immediately suggested a simple and elegant solution.

Given computer 1, computer 2, ... computer N, the solution is to port-forward (on your firewall) one part of the BitTorrent port range to computer 1, another part to another computer 2, and so on until you have as many port range segregations as you have computers. Then, configure the BitTorrent clients on each machine to use the port ranges you've assigned to that particular system. Problem solved.

Here's a sample configuration:

Note that these port ranges are set in two places: in your firewall's port-forwarding table, and on the BitTorrent clients on each system.

This is a simple and obvious solution once you know that BitTorrent clients don't force a single port for the peer-to-peer connection but select from a pool. It is possible that everybody knows that, so the solution is a no-brainer, or most BitTorrent users only connect from a single client or use UPnP, so partitioning the ports is irrelevant. If not, save yourself some trouble and segment your BitTorrent port range.