Torrent files contain the metadata required to start downloading data. Underneath an encoding layer, they contain a dictionary structure. The goal of this document is to retrieve a list of peers from a .torrent file. Let's start by decoding the dictionary.

If you don't have a .torrent file, there's one available on the Qubes OS download page.

Decoding the .torrent file

Torrent files are BEncoded (pronounced "b-encoded"). Most languages have a package available for encoding and decoding the format.

For Python, you can use the tool bencode.py, as follows:

import bencode

with open('./your-file.torrent') as f: raw_data = f.read()
data = bencode.decode(raw_data)

There are three important keys in the data dictionary that we will deal with: info and announce / announce-list.

info contains details on the contents of the torrent, but more importantly it is used to compute what is known as the "info hash"

announce and announce-list contain the URLs of servers which will help us to find peers.

Computing the info hash

The info hash is used to identify the torrent on the network. You can compute it as follows:

import hashlib
info_hash = hashlib.sha1(bencode.bencode(data['info'])).hexdigest()

Finding peers

There are two types of announce URLs: UDP and HTTP. The protocols for these URLs are different. HTTP URLs are documented here and UDP URLs are documented here. The spec ends up being quite complicated; an easier route is to use an already implemented library.

We've had success using BTDHT, "an implementation of the BitTorrent Distributed Hash Table"

After computing the info hash, you can use the BTDHT library to fetch a list of peers as follows:

import btdht
import binascii
from time import sleep
dht = btdht.DHT()
dht.start()
sleep(15) # wait for the DHT to build

while True:
    print(dht.get_peers(binascii.a2b_hex(info_hash)))
    sleep(1)

You'll notice that this list will continue to grow as more peers are discovered.

The list will contain an IP address and a port for each peer.