Mainline DHT

Mainline DHT is the name given to the Kademlia-based Distributed Hash Table (DHT) used by BitTorrent clients to find peers via the . The idea of utilizing a DHT for distributed tracking was first implemented in Azureus 2.3.0.0 (now known as Vuze) in May 2005, from which it gained significant popularity. Unrelated but similarly timed BitTorrent, Inc. released their own similar DHT into their client, called Mainline DHT and thus popularized the use of distributed tracking in the . Recent measurement shows by 2013 users of Mainline DHT is from 10 million to 25 million, with a daily churn of at least 10 million.

Mainline DHT is based on the popular Kademlia DHT design. Previous to usage of a DHT for distributing peers, trackers were the only method of finding peers. The key feature of using the DHT over trackers is that the decentralized approach favours the nature of the BitTorrent protocol. The DHT operates by distributing lists of peers identified by the SHA-1 hash of the torrent.

The SHA-1 hash of a torrent, the infohash, is synonymous with a Kademlia key, which is used for finding peers (values) in the overlay network. To find peers in a swarm, a node sends a get_peers query with the infohash as key (equivalent to a Kademlia FIND_VALUE) to the closest known nodes (with respect to the key distance). Like Kademlia, if the node does not return the value (peers), it persists further in an iterative operation. However, after the search is exhausted, the client then also inserts the peer contact information for itself onto the responding nodes with IDs closest to the infohash of the torrent.

Nodes use an additional measure known as a token to ensure others don't sign up other hosts for torrents. The return value for a query for peers includes this opaque value. For a node to announce that its controlling peer is downloading a torrent, it must present the token received from the same queried node in a recent query for peers. When a node attempts to "announce" a torrent, the queried node checks the token against the querying node's IP address.

Mainline DHT uses the SHA1 hash of the IP address concatenated onto a secret that changes every five minutes for a token value. Tokens up to ten minutes old are accepted.

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