bittorrent-dht

Simple, robust, BitTorrent DHT implementation

Node.js implementation of the BitTorrent DHT protocol. BitTorrent DHT is the main peer discovery layer for BitTorrent, which allows for trackerless torrents. DHTs are awesome!

This module is used by WebTorrent.

features

• complete implementation of the DHT protocol in JavaScript
• follows the spec
• supports BEP44 for storing arbitrary data in the DHT
• robust and well-tested
  • Comprehensive, fully-offline test suite
  • Used by WebTorrent, peerflix, and Playback
• efficient recursive lookup algorithm minimizes UDP traffic
• supports multiple, concurrent lookups using the same routing table

Also see bittorrent-tracker.

install

npm install bittorrent-dht

example

npm install magnet-uri

import DHT from 'bittorrent-dht'
import magnet from 'magnet-uri'

const uri = 'magnet:?xt=urn:btih:e3811b9539cacff680e418124272177c47477157'
const parsed = magnet(uri)

console.log(parsed.infoHash) // 'e3811b9539cacff680e418124272177c47477157'

const dht = new DHT()

dht.listen(20000, function () {
  console.log('now listening')
})

dht.on('peer', function (peer, infoHash, from) {
  console.log('found potential peer ' + peer.host + ':' + peer.port + ' through ' + from.address + ':' + from.port)
})

// find peers for the given torrent info hash
dht.lookup(parsed.infoHash)

api

dht = new DHT([opts])

Create a new dht instance.

If opts is specified, then the default options (shown below) will be overridden.

{
  nodeId: '',      // 160-bit DHT node ID (Buffer or hex string, default: randomly generated)
  bootstrap: [],   // bootstrap servers (default: router.bittorrent.com:6881, router.utorrent.com:6881, dht.transmissionbt.com:6881)
  host: false,     // host of local peer, if specified then announces get added to local table (String, disabled by default)
  concurrency: 16, // k-rpc option to specify maximum concurrent UDP requests allowed (Number, 16 by default)
  hash: Function,  // custom hash function to use (Function, SHA1 by default),
  krpc: krpc(),     // optional k-rpc instance
  timeBucketOutdated: 900000, // check buckets every 15min
  maxAge: Infinity  // optional setting for announced peers to time out
}

To use dht_store, set opts.verify to an ed25519 supercop/ref10 implementation. opts.verify(signature, value, publicKey) should return a boolean whether the signature and value buffers were generated by the publicKey.

For example, for dht_store you can do:

import ed from 'ed25519-supercop'
const dht = new DHT({ verify: ed.verify })

dht.lookup(infoHash, [callback])

Find peers for the given info hash.

This does a recursive lookup in the DHT. Potential peers that are discovered are emitted as peer events. See the peer event below for more info.

infoHash can be a string or Buffer. callback is called when the recursive lookup has terminated, and is called with two paramaters. The first is an Error or null. The second is the number of nodes found that had peers. You usually don't need to use this info and can simply listen for peer events.

Returns an abort() function that would allow us to abort the query.

dht.listen([port], [address], [onlistening])

Make the DHT listen on the given port. If port is undefined, an available port is automatically picked.

If address is undefined, the DHT will try to listen on all addresses.

If onlistening is defined, it is attached to the listening event.

dht.address()

Returns an object containing the address information for the listening socket of the DHT. This object contains address, family and port properties.

dht.announce(infoHash, [port], [callback])

Announce that the peer, controlling the querying node, is downloading a torrent on a port.

If you omit port the implied port option will be set and other peers will use the public dht port as your announced port.

If dht.announce is called soon (< 5 minutes) after dht.lookup, then the routing table generated during the lookup can be re-used, because the "tokens" sent by each node will still be valid.

If dht.announce is called and there is no cached routing table, then a dht.lookup will first be performed to discover relevant nodes and get valid "tokens" from each of them. This will take longer.

A "token" is an opaque value that must be presented 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. This is to prevent malicious hosts from signing up other hosts for torrents. All token management is handled internally by this module.

callback will be called when the announce operation has completed, and is called with a single parameter that is an Error or null.

arr = dht.toJSON()

Returns the current state of the DHT, including DHT nodes and BEP44 values.

{
  "nodes": [],
  "values": {}
}

The DHT nodes, in particular, are useful for persisting the DHT to disk between restarts of a BitTorrent client (as recommended by the spec). Each node in the array is an object with host (string) and port (number) properties.

To restore the DHT nodes when instantiating a new DHT object, simply loop over the nodes in the array and add them with the addNode method.

const dht1 = new DHT()

// some time passes ...

// destroy the dht
const nodes = dht1.toJSON().nodes
dht1.destroy()

// some time passes ...

// initialize a new dht with the same routing table as the first
const dht2 = new DHT()

nodes.forEach(function (node) {
  dht2.addNode(node)
})

dht.addNode(node)

Manually add a node to the DHT routing table. If there is space in the routing table (or an unresponsive node can be evicted to make space), the node will be added. If not, the node will not be added. This is useful to call when a peer wire sends a PORT message to share their DHT port.

A node should look like this:

{
  host: nodeHost,
  port: nodePort
}

dht.destroy([callback])

Destroy the DHT. Closes the socket and cleans up large data structure resources.

dht.put(opts, callback)

Write an arbitrary payload to the DHT. (BEP 44).

For all requests, you must specify:

• opts.v - a buffer payload to write, less than 1000 bytes

If you only specify opts.v, the content is considered immutable and the hash will just be the hash of the content.

Here is a simple example of creating some immutable content on the dht:

import DHT from 'bittorrent-dht'
const dht = new DHT()
const value = Buffer.alloc(200).fill('abc')

dht.put({ v: value }, function (err, hash) {
  console.error('error=', err)
  console.log('hash=', hash)
})

For mutable content, the hash will be the hash of the public key, opts.k. These options are available:

• opts.k - ed25519 public key buffer (32 bytes) (REQUIRED)
• opts.sign(buf) - function to generate an ed25519 signature buffer (64 bytes) corresponding to the opts.k public key (REQUIRED)
• opts.seq - optional sequence (integer), must monotonically increase (REQUIRED)
• opts.cas - optional previous sequence for compare-and-swap
• opts.salt - optional salt buffer to include (<= 64 bytes) when calculating the hash of the content. You can use a salt to have multiple mutable addresses for the same public key opts.k.

Note that bittorrent bep44 uses ed25519. You can use the ed25519-supercop package to generate the appropriate signatures, bittorrent-dht-store-keypair, bittorrent-dht-sodium or for a more convenient version.

To make a mutable update, you will need to create an elliptic key and pack values precisely according to the specification, like so:

import ed from 'bittorrent-dht-sodium'
const keypair = ed.keygen()

const value = Buffer.alloc(200).fill('whatever') // the payload you want to send
const opts = {
  k: keypair.pk,
  seq: 0,
  v: value,
  sign: function (buf) {
    return ed.sign(buf, keypair.sk)
  }
}

import DHT from 'bittorrent-dht'
const dht = new DHT()

dht.put(opts, function (err, hash) {
  console.error('error=', err)
  console.log('hash=', hash)
})

In either mutable or immutable forms, callback(error, hash, n) fires with an error if no nodes were able to store the value. n is set the amount of peers that accepted the put and hash, the location where the mutable or immutable content can be retrieved (with dht.get(hash)).

Note that you should call .put() every hour for content that you want to keep alive, since nodes may discard data nodes older than 2 hours.

If you receive a key/value pair and you want to re-add to the dht it to keep it alive you can just put it again.

import ed from 'bittorrent-dht-sodium'
const dht = new DHT({ verify: ed.verify }) // you MUST specify the "verify" param if you want to get mutable content, otherwise null will be returned

dht.get(key, function (err, res) {
  dht.put(res, function () {
    // re-added the key/value pair
  })
})

dht.get(hash, opts, callback)

Read a data record (created with .put()) from the DHT. (BEP 44)

Given hash, a hex string or buffer, lookup data content from the DHT, sending the result in callback(err, res).

These options are available:

• opts.verify - override the default ed25519 verification function supplied during DHT instantiation.
• opts.salt - optional salt buffer (if any) that was used to calculate the hash. Must be specified if included in the hash.
• opts.cache - use locally cached response value when available instead of performing a network lookup (defaults to true).

res objects are similar to the options objects written to the DHT with .put():

• res.v - the value put in
• res.id - the node that returned the content
• res.k - the public key (only present for mutable data)
• res.sig - the signature (only present for mutable data)
• res.seq - the sequence (optional, only present for mutable data)

events

dht.on('ready', function () { ... })

Emitted when the DHT is fully bootstrapped (i.e. the routing table is sufficiently populated via the bootstrap nodes). Note that it is okay to do lookups before the 'ready' event fires.

Note: If you initialize the DHT with the { bootstrap: false } option, then the 'ready' event will fire on the next tick even if there are not any nodes in the routing table. It is assumed that you will manually populate the routing table with dht.addNode if you pass this option.

dht.on('listening', function () { ... })

Emitted when the DHT is listening.

dht.on('peer', function (peer, infoHash, from) { ... })

Emitted when a potential peer is found. peer is of the form {host, port}. infoHash is the torrent info hash of the swarm that the peer belongs to. Emitted in response to a lookup(infoHash) call.

dht.on('error', function (err) { ... })

Emitted when the DHT has a fatal error.

internal events

dht.on('node', function (node) { ... })

Emitted when the DHT finds a new node.

dht.on('announce', function (peer, infoHash) { ... })

Emitted when a peer announces itself in order to be stored in the DHT.

dht.on('warning', function (err) { ... })

Emitted when the DHT gets an unexpected message from another DHT node. This is purely informational.

further reading

• BitTorrent DHT protocol
• Kademlia: A Peer-to-peer Information System Based on the XOR Metric

license

MIT. Copyright (c) Feross Aboukhadijeh, Mathias Buus, and WebTorrent, LLC.