Quentin Torrentino

A (very) simple BitTorrent client built using GO

Idea

• Parse the .torrent file and extract the metadata & info_hash.
• Send an "announce" request with the info_hash to the tracker to recieve list of peers.
• Establish a TCP connection and perform the BitTorrent handshake with peers.
• Request file "pieces" from peers (seeding not supported yet).
• Verify each received piece using its SHA-1 hash.
• Combine all verified pieces into the final file.

tl;dr: parse -> connect -> download -> verify -> assemble

.torrent file structure

Contains the torrent metadata and the information to connect to the trackers, encoded in Bencode

Bencode format

• Strings
  • <string_len>:<string>
  • eg. 7:comment
• Integers
  • i<number>e
  • eg. i2e
• Array
  • l<items>e
  • eg. l5:helloi3ee -> ["hello", 3]
• Dict
  • d<items>e
  • eg. d3:byei57ee -> {"bye": 57}

The important contents of the .torrent file are the announce url (tracker), and a big binary blob which holds the SHA-1 hashes of all pieces.

This project uses a pre-built parser.

BitTorrent specification

https://www.bittorrent.org/beps/bep_0003.html

Inspiration

Jesse Li