Two attempts at writing a simple BitTorrent client for CS 396 at Reed College
e08e6292 — Thalia Wright 2 years ago
Sourcehut doesn't support org mode files 😞
051755a4 — Thalia Wright 2 years ago
Actually finished
68ea7045 — Thalia Wright 2 years ago
Finished writeup


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#Bitclient: A Simple BitTorrent Client

This repository contains two attempts at my final project for CS386 (Computer Networks) at Reed College.

  • v1-broken/ contains a mostly non-functional BitTorrent client written (mostly) from scratch in C.
  • v2-working/ contains a fully-functional BitTorrent client written in C++ that offloads all the interesting parts to LibTorrent.

#The Broken C Version

This version lives in the v1-broken/ directory.

  1. Download and build this bencode library. (git submodule init; cd bencode; make).
  2. In the unlikely event that it isn't pre-installed, install libcurl.
  3. To build the program just run make.

My initial plan was to use C to write a program that would take a .torrent file, decode it, ask one of the specified trackers for a list of peers, and then download the file or files from them. I got it to the point where I was able to send a HTTP request to a tracker, but they all responded with errors along the lines of "torrent not valid". Turns out that the bencode library I was using (and another I tired), were unable to re-encode the info dictionary into bencode that would produce a valid checksum.

At this point I gave up on metainfo files and rewrote it to use the more modern magnet URI scheme. Unfortunately, all the magnet URIs I found preferred UDP trackers. I attempted to write a client for the UDP tracker protocol, but was unable to figure out how to pack together a valid packet.

Therefore, in order to actually get a list of peers, we need a hand-crafted magnet URI that uses HTTP trackers. At this point I was running rather short on time and chose to abandon this program in favour of a LibTorrent wrapper discussed below. The code in the old project is organised in the v1-broken/ directory as follows:

  • bitclient.c Contains the programs main; its responsibility is to build the torrent_t structure with information about the file we want to download, then spawning off a pair of threads to concurrently download and upload chunks to and from peers.
  • bitclient.h Contains a few macros and definition of the central torrent structure.
  • magnet.(c,h) Exposes a pair of functions to main, the first of which parses the magnet URI and the second of which uses that information to contact trackers.
  • leecher.(c,h) Exposes the function to main which is responsible for downloading the file from peers.
  • seeder.(c,h) Exposes the function to main which uploads pieces of the file to peers.

The bak/ directory also contains extract.(c,h) and tracker.(c,h), which, in the earlier iteration of the program, were responsible for decoding the metainfo file and requesting information from a HTTP tracker.

The existing program takes two flags, one to print a help message and exit (-h), and one to print debugging information (-v). I strongly recommend running it with the latter flag. For the sake of simplicity I only chose to support downloading one torrent at a time, concurrency can be achieved with an external tool like xargs(1).

#The Boring But Working LibTorrent Version

The source and Makefile live in v2-working/.

  1. Install LibTorrent. Weirdly, on Arch GNU/Linux the libtorrent package didn't contain the C++ header files, so I had to install the libtorrent-rasterbar package. I believe it depends on Boost.
  2. Just run make.

Upon realising that I was on a trajectory to not complete this assignment in time, I investigated LibTorrent, a BitTorrent client implementation in a C++ library, and threw together a basic program.

I was initially resistant to using any kind of library because my primary goal was to learn how BitTorrent, and peer-to-peer protocols more generally, work. I believe I achieved this goal while trying to write the first version. Although I didn't gain any knowledge writing second version (other than how to use LibTorrent and a reminder of how much I dislike C++), I produced a program that could successfully download /ahem/ Linux ISOs from magnet links.

This program lives in a single C++ file and doesn't take any options other than a magnet link. It does, however, contain a log_verbosely boolean which you can set to true if you want to see something more interesting that the number of bytes downloaded. By default I also have it set to exit when the torrent is fully downloaded, thought this can be changed by commenting out the marked lines.