cratetorrent

Cratetorrent is a Rust crate implementing the BitTorrent version 1 protocol.

It can be used as a library and also provides a simple example CLI torrent app. It is built on top of tokio and uses async IO for high performance.

The name is a homage to the C++ libtorrent library, from which many lessons were learned when I first wrote my torrent engine in C++.

Features

• Multiple torrent downloads or uploads, with an arbitrary number of peer connections.
• Manually specify seeds to download from.
• Get peers from HTTP trackers.
• Basic per-torrent configurability.
• Decent performance:

  On my fairly slow internet connection with peak download rates of about 9 MBps, Ubuntu 20.04 LTS (~2.8 GB) is downloaded in about 5 minutes at a download rate of 8-9 MPbs, that is, almost fully utilizing the link.

Features are continuously added, see the project milestones.

Eventually, I hope to develop cratetorrent into a full-fledged BitTorrent engine library that can be used as the engine underneath torrent clients. This means that features supported by popular clients (such as DHT, magnet links, BitTorrent protocol 2, stream encryption, and others) will be supported by cratetorrent in the future.

Download example

use cratetorrent::prelude::*;
                                                                             
#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    // spawn the engine with a default config
    let conf = Conf::new("/tmp/downloads");
    let (engine, mut alert_rx) = engine::spawn(conf)?;
                                                                             
    // parse torrent metainfo and start the download
    let metainfo = tokio::fs::read("/tmp/imaginary.torrent").await?;
    let metainfo = Metainfo::from_bytes(&metainfo)?;
    let torrent_id = engine.create_torrent(TorrentParams {
        metainfo,
        // tell the engine to assign a randomly chosen free port
        listen_addr: None,
        // here we could specify peers we knew of that we'd want
        // to connect to
        mode: Mode::Download { seeds: Vec::new() },
        conf: None,
    })?;
                                                                             
    // listen to alerts from the engine
    while let Some(alert) = alert_rx.next().await {
        match alert {
            Alert::TorrentStats { id, stats } => {
                println!("{}: {:#?}", id, stats);
            }
            Alert::TorrentComplete(id) => {
                println!("{} complete, shutting down", id);
                break;
            }
            Alert::Error(e) => {
              // this is where you'd handle recoverable errors
              println!("Engine error: {}", e);
            }
            _ => (),
        }
    }
                                                                             
    // Don't forget to call shutdown on the engine to gracefully stop all
    // entities in the engine. This will wait for announcing the client's
    // leave to all trackers of torrent, finish pending disk and network IO,
    // as well as wait for peer connections to cleanly shut down.
    engine.shutdown().await?;
                                                                             
    Ok(())
}

Project structure

The project is split up in two:

• the cratetorrent library, that defines most of the functionality,
• and a cratetorrent-cli binary for downloading torrents via the CLI. Note, however, that this is extremely simple at present and serves more as a toy for demonstration purposes.

How to run

Tested on stable Rust 1.48.

Requires Linux!

This is because file IO is done using the pwritev(2) and preadv(2) APIs for optimal performance. In the future, API shims for Windows and Darwin may be supported, but at the moment there is no capacity to do this.

Binary

The CLI binary is currently very basic, but you can perform downloads either by directly connecting to seeds or if the torrent is backed by a HTTP tracker.

Run the following from the repo root:

cargo run --release -p cratetorrent-cli -- \
    --seeds 192.168.0.10:50051,192.168.0.172:49985 \
    --metainfo path/to/mytorrent.torrent \
    --download-dir ~/Downloads

Tests

Cratetorrent is well tested to ensure correct functionality. It includes:

• an exhaustive suite of inline unit tests,
• and integration tests of various downloads and uploads, in the integration tests folder.

Design

The cratetorrent design is documented in the design doc. This mostly concerns developers of cratetorrent, as it contains fairly low-level descriptions.