BitTorrent Client in JavaScript

This is an implementation of a BitTorrent client in JavaScript that supports downloading files from both .torrent files and magnet links.

Table of Contents

• Features
• Project Structure
• Implementation Details
• Limitations and Unsupported Features
• Installation and Setup
• Usage
• Testing
• License

Features

• Parsing of .torrent files
• Extracting tracker URL, info hash, piece length, and piece hashes from .torrent files
• Discovering peers via HTTP trackers
• Establishing TCP connections with peers and performing handshakes
• Exchanging peer messages to download file pieces
• Integrity checking of downloaded pieces using SHA-1 hashes
• Saving downloaded pieces to disk and combining them to assemble the complete file
• Parsing of magnet links
• Fetching torrent metadata from peers using the BitTorrent extension protocol
• Downloading files from magnet links by requesting pieces from peers

Project Structure

The project is structured as follows:

bittorrent-client-js/
├── README.md
├── codecrafters.yml
├── package.json
├── sample.torrent
├── your_bittorrent.sh
└── app/
    ├── main.js
    ├── utility.js
    ├── bencode/
    │   ├── decode_bencode.js
    │   └── encode_bencode.js
    ├── magnet/
    │   ├── download_complete.js
    │   ├── download_piece.js
    │   ├── extension_handshake.js
    │   ├── magnet_parse.js
    │   ├── metadata_exchange.js
    │   └── peer_handshake.js
    ├── peerMessage/
    │   ├── read_piece_message.js
    │   ├── send_interested.js
    │   ├── send_request.js
    │   ├── wait_for_bitfield.js
    │   └── wait_for_unchoke.js
    └── torrent/
        ├── download_complete.js
        ├── download_piece.js
        ├── handshake.js
        ├── info.js
        └── peers.js

Implementation Details

The BitTorrent client is implemented in JavaScript and consists of several modules:

• bencode/: Contains functions for encoding and decoding data in the Bencode format, which is used in .torrent files and peer communication.
• magnet/: Contains functions specific to handling magnet links, including parsing magnet links, fetching torrent metadata from peers, and downloading files using magnet links.
• peerMessage/: Contains functions for sending and receiving peer messages, such as requesting pieces, sending interest, and waiting for unchoke messages.
• torrent/: Contains functions for handling .torrent files, including extracting information, discovering peers, performing handshakes, and downloading pieces.
• utility.js: Contains utility functions used across different modules.
• main.js: The entry point of the application that handles command-line arguments and invokes the appropriate functions.

Limitations and Unsupported Features

Please note the following limitations and unsupported features of this BitTorrent client:

• Only single-file torrents are supported. Multi-file torrents are not handled.
• Tracker communication is limited to HTTP trackers. UDP trackers are not supported.
• Peer selection strategy is basic and does not implement advanced algorithms for optimizing download speeds.
• Seeding functionality is not implemented. The client can only download files and does not upload pieces to other peers.
• DHT (Distributed Hash Table) and PEX (Peer Exchange) are not supported.

Installation and Setup

To run this BitTorrent client on your machine:

1. Ensure you have Node.js installed (version 12 or higher)

2. Clone this repository:

   git clone https://github.com/yourusername/bittorrent-client-js.git

   cd bittorrent-client-js

3. Install dependencies:

   npm install

4. Make the shell script executable:

   chmod +x your_bittorrent.sh

I'll reorganize the Usage section to group related commands together, starting with basic operations, then .torrent operations, and finally magnet operations:

Usage

To run the BitTorrent client, ensure you have Node.js installed. Then, execute the your_bittorrent.sh script with the appropriate command and arguments. Here are all available commands:

Basic Operations

• To decode a bencoded string:

  ./your_bittorrent.sh decode <bencoded_string>
  

  Try it with a simple bencoded string:

  ./your_bittorrent.sh decode "d3:bar4:spam3:fooi42ee"

Torrent File Operations

• To parse a .torrent file and print the tracker URL, info hash, piece length, and piece hashes:

  ./your_bittorrent.sh info <path/to/file.torrent>
  

  Try it yourself! Run this command to examine the sample.torrent file included in the repository:

  ./your_bittorrent.sh info sample.torrent

• To discover peers for a .torrent file:

  ./your_bittorrent.sh peers <path/to/file.torrent>
  

  Want to see the available peers? Try this for the sample.torrent file included in the repository:

  ./your_bittorrent.sh peers sample.torrent

• To perform a handshake with a specific peer:

  ./your_bittorrent.sh handshake <path/to/file.torrent> <peer_address>
  

  First get a peer address using the peers command, then try connecting to one:

  ./your_bittorrent.sh handshake sample.torrent 165.232.41.73:51556

• To download a single piece from a .torrent file:

  ./your_bittorrent.sh download_piece -o <output_directory/output_filename> <path/to/file.torrent> <piece_index>
  

  Try downloading a piece of the file pointed by the sample.torrent file included in the repository:

  ./your_bittorrent.sh download_piece -o ./test-piece sample.torrent 1

• To download the entire file from a .torrent file:

  ./your_bittorrent.sh download -o <output_directory/output_filename> <path/to/file.torrent>
  

  Try downloading the file pointed out by the sample.torrent file included in the repository:

  ./your_bittorrent.sh download -o ./test sample.torrent

Magnet Link Operations

• To parse a magnet link and print the info hash and tracker URL:

  ./your_bittorrent.sh magnet_parse <magnet_link>
  

  Try parsing one of our test magnet links:

  ./your_bittorrent.sh magnet_parse "magnet:?xt=urn:btih:ad42ce8109f54c99613ce38f9b4d87e70f24a165&dn=magnet1.gif&tr=http%3A%2F%2Fbittorrent-test-tracker.codecrafters.io%2Fannounce"

• To perform a handshake using a magnet link:

  ./your_bittorrent.sh magnet_handshake <magnet_link>
  

  Try it with one of our test magnet links:

  ./your_bittorrent.sh magnet_handshake "magnet:?xt=urn:btih:ad42ce8109f54c99613ce38f9b4d87e70f24a165&dn=magnet1.gif&tr=http%3A%2F%2Fbittorrent-test-tracker.codecrafters.io%2Fannounce"

• To retrieve metadata from a magnet link:

  ./your_bittorrent.sh magnet_info <magnet_link>
  

  Try getting info from one of our test magnet links:

  ./your_bittorrent.sh magnet_info "magnet:?xt=urn:btih:ad42ce8109f54c99613ce38f9b4d87e70f24a165&dn=magnet1.gif&tr=http%3A%2F%2Fbittorrent-test-tracker.codecrafters.io%2Fannounce"

• To download a specific piece from a magnet link:

  ./your_bittorrent.sh magnet_download_piece -o <output_directory/output_filename> <magnet_link> <piece_index>
  

  Try downloading piece #1 from one of our test magnet links:

  ./your_bittorrent.sh magnet_download_piece -o ./test-piece "magnet:?xt=urn:btih:ad42ce8109f54c99613ce38f9b4d87e70f24a165&dn=magnet1.gif&tr=http%3A%2F%2Fbittorrent-test-tracker.codecrafters.io%2Fannounce" 1

• To download the entire file from a magnet link:

  ./your_bittorrent.sh magnet_download -o <output_directory/output_filename> <magnet_link>
  

  Here are a few examples of downloading GIF files from magnet links to your current directory:

  ./your_bittorrent.sh magnet_download -o ./test1.gif "magnet:?xt=urn:btih:ad42ce8109f54c99613ce38f9b4d87e70f24a165&dn=magnet1.gif&tr=http%3A%2F%2Fbittorrent-test-tracker.codecrafters.io%2Fannounce"

  ./your_bittorrent.sh magnet_download -o ./test2.gif "magnet:?xt=urn:btih:3f994a835e090238873498636b98a3e78d1c34ca&dn=magnet2.gif&tr=http%3A%2F%2Fbittorrent-test-tracker.codecrafters.io%2Fannounce"

  ./your_bittorrent.sh magnet_download -o ./test3.gif "magnet:?xt=urn:btih:c5fb9894bdaba464811b088d806bdd611ba490af&dn=magnet3.gif&tr=http%3A%2F%2Fbittorrent-test-tracker.codecrafters.io%2Fannounce"

Important Notes

• When using the -o flag, you must provide a filename, not just a directory path. If you only specify a directory like -o./, the download will fail because no filename was given.
• Always replace <output_directory/output_filename> with the directory where you want the file saved and the exact filename you want it to have.
• When downloading binary files (images, videos, etc.), include the appropriate file extension (like .mp4, .jpg, .zip, etc.) so your computer knows how to open it properly.
• For text files, the extension is optional. If you don't provide one, the file will be saved without an extension.

Testing

The BitTorrent client has been tested using various .torrent files and magnet links.

The following magnet links have been used to successfully download GIF files:

Magnet Link 1:

magnet:?xt=urn:btih:ad42ce8109f54c99613ce38f9b4d87e70f24a165&dn=magnet1.gif&tr=http%3A%2F%2Fbittorrent-test-tracker.codecrafters.io%2Fannounce

Magnet Link 2:

magnet:?xt=urn:btih:3f994a835e090238873498636b98a3e78d1c34ca&dn=magnet2.gif&tr=http%3A%2F%2Fbittorrent-test-tracker.codecrafters.io%2Fannounce

Magnet Link 3:

magnet:?xt=urn:btih:c5fb9894bdaba464811b088d806bdd611ba490af&dn=magnet3.gif&tr=http%3A%2F%2Fbittorrent-test-tracker.codecrafters.io%2Fannounce

However, please note that not all types of magnet links are guaranteed to work flawlessly due to the limitations mentioned above.

License

This project is licensed under the MIT License.