Bittorrent Go

Building a bittorent client with Go (v1.19) step by step, following the instructions on codecrafters.io.

In this project, we build a BitTorrent client that's capable of parsing a .torrent file and downloading a file from a peer. Along the way, we learn about how torrent files are structured, HTTP trackers, BitTorrent’s Peer Protocol, pipelining and more.

How to try this out

Decode bencoded strings

./your_bittorrent.sh decode 5:hello

Expected output:

"hello"

Decode bencoded integers

./your_bittorrent.sh decode i52e

Expected output:

52

Decode bencoded lists

./your_bittorrent.sh decode l5:helloi52ee

Expected output

[“hello”,52]

Decode bencoded dictionaries

./your_bittorrent.sh decode d3:foo3:bar5:helloi52ee

Expected output:

{"foo":"bar","hello":52}

Parse torrent file, calculate info hash, and get piece hashes

./your_bittorrent.sh info sample.torrent

Expected output:

Tracker URL: http://bittorrent-test-tracker.codecrafters.io/announce
Length: 92063
Info Hash: d69f91e6b2ae4c542468d1073a71d4ea13879a7f
Piece Length: 32768
Piece Hashes:
e876f67a2a8886e8f36b136726c30fa29703022d
6e2275e604a0766656736e81ff10b55204ad8d35
f00d937a0213df1982bc8d097227ad9e909acc17

Discover peers

./your_bittorrent.sh peers sample.torrent

Expected output: (exact values may be different)

167.71.141.80:51470
167.71.141.82:51482
134.209.186.165:51417

Peer handshake

./your_bittorrent.sh handshake sample.torrent 178.62.82.89:51470

Expected output: (exact value will be different as it is randomly generated)

Peer ID: 0102030405060708090a0b0c0d0e0f1011121314

Download a piece

./your_bittorrent.sh download_piece -o /tmp/test-piece-0 sample.torrent 0

Expected output:

Piece 0 downloaded to /tmp/test-piece-0.

Download a whole torrent

./your_bittorrent.sh download -o /tmp/sample.txt sample.torrent

Expected output:

Downloaded sample.torrent to /tmp/sample.txt.

Parse magnet link

./your_bittorrent.sh magnet_parse 'magnet:?xt=urn:btih:ad42ce8109f54c99613ce38f9b4d87e70f24a165&dn=magnet1.gif&tr=http%3A%2F%2Fbittorrent-test-tracker.codecrafters.io%2Fannounce'

Expected output:

Tracker URL: http://bittorrent-test-tracker.codecrafters.io/announce
Info Hash: d69f91e6b2ae4c542468d1073a71d4ea13879a7f

Announce extension support

./your_bittorrent.sh magnet_handshake 'magnet:?xt=urn:btih:ad42ce8109f54c99613ce38f9b4d87e70f24a165&dn=magnet1.gif&tr=http%3A%2F%2Fbittorrent-test-tracker.codecrafters.io%2Fannounce'

Expected output:

Peer ID: 0102030405060708090a0b0c0d0e0f1011121314