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BitTorrent-Client

A small Linux terminal-based BitTorrent client.

The main purpose of this project was to learn about asynchronous networking and the BitTorrent protocol. The bulk of the project is implemented in peerwireclient.cpp, where most of the networking work is carried out using the ASIO networking library.

Some info about how the BitTorrent protocol works:

  • Files are downloaded in a distributed manner. Your client will connect to multiple peers, each of which will serve the client "pieces" of the full file.
  • Info about the file you're trying to download is stored in a .torrent file, encoded using the Bencode format.
  • The client gets info about the file and a special tracker URL from the .torrent file. It gets peer info by sending an HTTP GET request to the tracker.

This client consequently works in 3 stages:

  • First, the .torrent file is opened and the file info is scraped from it using a custom Bencode parser.
  • Second, the client makes a request to the tracker using the cURL library to format a GET request.
  • Third, once the tracker has responded with peer info, a PeerWireClient object is created for each peer. All peers share a single ASIO io_context object which handles asychronous messaging between the client and peers. This is where the actual downloading happens.

Currently the client only supports downloading a single-file torrent from the .torrent file. Some future features may include:

  • Multi-file downloads
  • Multiple concurrent downloads
  • Magnet links
  • Pausing/restarting torrents

More info on how the BitTorrent protocol works can be found on the wiki: https://wiki.theory.org/BitTorrentSpecification

Building

On Ubuntu you can install libcurl with

sudo apt-get install libcurl4-openssl-dev

After that just run 'make' in the root directory to build the client, and it should automatically build the other dependencies.

Running

From the bin/ directory, you can run the client with

./client <file.torrent>

to start downloading. The file will be created in the current directory.