How to get the first peer from a torrent-magnet link?
I've been trying to understand the torrent-magnet technology, but I can't seem to figure out how you get connected to the first peer when opening a magnet link.
When you get a magnet link like below, it contains no initial peer - only the BitTorrent Info Hash (btih) and the file name.
magnet:?xt=urn:btih:bbb6db69965af769f664b6636e7914f8735141b3&dn=ubuntu-12.04-desktop-i386.iso
According to BitTorrent & Magnets: How Do They Work? (MakeUseOf)
If you click a magnet link that does not specify a tracker (
tr) the first peer will be found using DHT. Once you’ve got a peer, peer exchange kicks in too.
The DHT article on Wikipedia does not specify how to find a peer, but in the Kademlia article (upon which BitTorrent DHT is based), it says
A node that would like to join the net must first go through a bootstrap process. In this phase, the joining node needs to know the IP address and port of another node—a bootstrap node (obtained from the user, or from a stored list)—that is already participating in the Kademlia network.
But where does it know that node from? I don't see an address or anything present in the magnet link. Since it's decentralized (trackerless), I wouldn't expect it to know the node in advance. Or is the DHT in fact not decentralized?
Answer
For the most part, when you start a bittorrent client, bootstrap off of:
- nodes from your last session, that were saved to disk
- other peers that you have on any of the swarms you're on
There are a few well-known bootstrap nodes which clients can use if they have no other means of finding any. Essentially the only case this happens is when you install a client for the first time, and the first torrent you download is a magnet link without a tracker.
You can then hit router.utorrent.com:6881. I believe transmission, azureus and bitcomet run similar routers, and possibly other clients as well.
By "router", I mean a node that appear to behave like any other node in the DHT, but probably has a different mechanism for determining which nodes to hand out, and probably is optimized specifically for the use case of just introducing dht nodes to each other.
UPDATE: you can run your own DHT bootstrap machine, here's the source code.