ZTOR

Zig BitTorrent Client

A small but slightly more than non-trivial program written in Zig.

This was mostly an experiment in using zig with a c event-loop library and callback-based appraoch. A number of useful zig features, such as errors, can't be used due to the C abi boundary being pervasive.

I do not recommend using this code as a good example as there are no tests and currently I've thrown a lot of it together in one place. But if it helps then go for it.

The torrent file is for the debian net install image which was used because A. debian has seeders so we probably don't have to worry about being choked all the time, and B. it's a single file torrent so no need to deal with directories and multiple files.

Build

This program depends on a number of modules and builds both http_parser and libuv.

git clone --recursive https://github.com/iansimonson/ztor.git
cd ztor
zig build

Plan

• Connect to tracker and download peer info
  • Parse .torrent into tree
  • Find announce url
  • Connect and download peer list
• Establish connections to peers
  • Construct peer connection info per peer
  • Only download on unchoke
• Use uv_get_handle rather than the members
• Support for N .torrent files at once
  • Don't shutdown after a file is done downloading
• Implement upload and answering requests
  • Send have messages to all peers on download
  • If have send piece on request
  • provide mechanism to get non-owning immutable slice to piece
• Setup server for incoming connections
  • Upload throttled amount of data
  • Send bitfield on new incoming connections
• Add invalid threshold
  • After x amount of invalid pieces close connection to peer
• Throttle data speeds
  • Token Bucket?
  • Upload throttle
  • Download throttle
• Setup imgui (or similar) for handling torrents in GUI mode

Additional TODO:

• Build libuv in build.zig
• Move from libuv to native zig async/await
  • Abstract away libuv
• Support non-compact peer list
• Confirm peer_ids
• Support uTP protocol rather than using TCP
• Support UDP tracker protocol (BEP 015)
• SSL/TLS support?
• Watch a folder for .torrent files
• Other BEPs?
• Make the Torrent Lexer work with Reader/Writer

Relevant links

Prior to this I only knew how torrents worked conceptually. How to implement a torrent downloader was way outside what I knew. The following are relevant links when learning:

• Building a BitTorrent client from the ground up in go
• BitTorrent protocol
• BitTorrent.org
• Basics of Libuv