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If you missed Part 1 you can find it here
Onwards we go! The next step will be to extract the relevant data from this .torrent file, create a HTTP GET request and retrieve the list of peers.
Step 0
Before doing anything I need to read the torrent file into my node app. To do this I simply run the file with the .torrent file location as an arg. Like a mini-cli. I set this up so I could just run this and it works.
node cli.ts ../torrents/test.torrent
Step 1
The goal of this step is to decode the "announce" and "info" objects. This can be done using our handy bencoding functions we did in part 1.
I pulled out separately the whole decoded message and the info section (I found out I needed the raw version of this later on).
const searchString = "4:info";
const position = buffer.indexOf(searchString);
const infoStart = position + searchString.length;
const decoded = decode(buffer, 0);
const infoSection = decode(buffer, infoStart);
This got me in a place where I could start to build out the http request and assembler the headers.
Step 2
This step deals with getting the right data to build the header object.
In this part I created a function to help pull out relevant parts of the torrent file so the resulting object looked like this:
return {
url: announceURL.toString("utf8"),
info_hash: hashedInfo,
peer_id: createPeerId(),
port: 6881,
uploaded: 0,
downloaded: 0,
left: length,
compact: 1,
};
I followed the BitTorrent spec to find out what these should be:
URL: a utf-8 string of the announce URL pulled from the decoded bencoded torrent object.
info_hash: this is the raw info section I pulled from earlier as "infoSection", the info had to be hashed as per the spec:
const hashedInfo = crypto.createHash("sha1").update(rawInfoBytes).digest();
peer_id: The spec said this needed to be a random 12 bytes prefixed with an ID you give your client. The AS001 could be anything, this is how I handled it:
const createPeerId = () => {
const prefix = Buffer.from("-AS0001-");
const random = crypto.randomBytes(12);
return Buffer.concat([prefix, random]);
};
port: defaulted to 6881
uploaded: defaulted to 0
left: Calculated from the info.length
compact: Defaulted to 1
With that the all the pieces are in place to assemble the URL to fetch the peer information required.
Step 3
The goal of this section is to get a peer list of IPs and PORTS.
This is how the end URL looked:
const assembledURL = `${trackerUrl}?info_hash=${info_hash_converted}&peer_id=${peer_id_converted}&port=${port}&uploaded=${uploaded}&downloaded=${downloaded}&left=${left}&compact=${compact}`;
Out of all these values that need to be slotted in, two need to be modified before we do send the request
- info_hash
- peer_id
These two need to be modified by converting buffer to hex and adding a % to the start. Which I did like so:
const bufferToHex = (buffer: Buffer) => {
return [...new Uint8Array(buffer)]
.map((x) => "%" + x.toString(16).padStart(2, "0"))
.join("");
};
How did I find this out? I tried all the wrong ways first! I directly put in the url - didn't work, missed out the '%' - didn't work, etc etc.
This will return a response, which I converted to a buffer and decoded with the newly created bencoder functions (turning out to be very helpful!)
The peers IP and port are given per 6 bytes of the buffer, so loop through every 6 bytes and split the first 4 to become the IP and the last 2 to become PORT.
Something like:
const parsePeer = (peerBuffer: Buffer) => {
const IP = peerBuffer.subarray(0, 4);
const PORT = peerBuffer.subarray(4, 6);
const stringIP = Array.from(IP).join(".");
const stringPORT = PORT.readUInt16BE(0);
return { ip: stringIP, port: stringPORT };
};
Just a side note, I had no idea what any of these Buffer methods (.subarray(), .readUInt16BE()) did before exploring this project. So I had to read documentation, and Google to find out what does what.
When all is said and done (with tweaks and fixes here and there) the result appeared in the console:
STATUS: 200
Got 50 peers
Peer 0 is ip: 192.99.233.115 port: 6130
Peer 1 is ip: 67.169.223.61 port: 6889
Peer 2 is ip: 71.161.110.90 port: 60000
Peer 3 is ip: 185.111.109.15 port: 42933
Peer 4 is ip: 71.126.144.178 port: 6881
Peer 5 is ip: 85.119.221.147 port: 6001
...
Part 3 will look into the peer encoding and decoding and performing the handshake. I suspect this will be a more complicated job but I'm ready for the challenge!