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What is a Torrent Tracker

A tracker is an HTTP(s) service which helps peers of a torrent discover one another to facilitate file sharing. Torrent (metainfo) files or magnet links can contain a list of trackers that the swarm (the entire network of people connected to a single torrent) should use.

Examples of torrent trackers include https://torrent.ubuntu.com/announce and http://tracker.archlinux.org:6969/announce.

There are a few variants of torrent trackers out there. Common ones include UDP trackers and Websocket trackers. There are even “trackerless” torrents which use a Distributed Hash Table (DHT) to find peers without a centralized tracker.

Often, torrents will be tracked by multiple different trackers and the DHT to be redundant to failures of any one tracker.

How does a Tracker work

When starting a download, your torrent client connects to any trackers listed in the .torrent file or magnet link and asks for peers for that torrent. The tracker then responds with the IP addresses and ports of other peers who have recently connected to that same tracker. Your client will periodically ping the tracker to ensure that the list of peers is up to date.

If a peer closes their client or stops checking in with the tracker, it will be assumed dead and will not be handed out to other peers 1.

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BitTorrent Spec

The BitTorrent protocol is quite simple. Instead of being standardized with RFCs, as most open protocols are, the BitTorrent protocol is defined with BEPs– BitTorrent Enhancement Proposals. BEPs are modeled after the Python Enhancement Proposal (PEP) process2.

The BEP 3 specification covers the BitTorrent protocol. This will be referenced in describing the tracker protocol.

Tracker HTTP Protocol

HTTP torrent trackers speak a protocol as defined in BEP 3. Torrent clients periodically issue HTTP GET requests to a tracker to discover peers.

GET requests have the following URL query parameters.

  • info_hash
    • In the case of a .torrent file a SHA1 hash of an encoding of the “info” section of the torrent file.
    • Listed as a parameter of a Magnet link.
  • peer_id
    • Client-created 20 byte self-identifier. Used to identify peers.
  • ip
    • Optional IP address (or DNS name) that the peer should be reached at.
  • port
    • Port the peer is listening on.
  • uploaded
    • The total amount uploaded, so far.
  • downloaded
    • The total amount downloaded, so far.
  • left
    • The number of bytes this peer still has to download.
  • event
    • Optional key representing the state of the download: started, complete, or stopped.

This can be represented as an HTTP request with curl:

# The info_hash must be specified as URL encoded hex.
curl --get https://torrent.ubuntu.com/announce \
  --data 'info_hash=%D1%10%1A%2B%9D%20%28%11%A0%5E%8C%57%C5%57%A2%0B%F9%74%DC%8A' \
  --data-urlencode 'peer_id=01234567890123456789' \
  --data-urlencode 'port=12345' \
  --data-urlencode 'uploaded=0' \
  --data-urlencode 'downloaded=0' \
  --data-urlencode 'left=0'

In response, the tracker will return to you a bencoded dictionary containing two keys: interval how long this client should wait before contacting the tracker again and peers a list of peers to connect to. In event of an error, a single key failure will be populated instead.

Optional keys: complete and incomplete can be returned representing the number of seeders and peers in the swarm respectively.

# Continue reading for how to decode response from the tracker to this.
{
    "complete": 2068,
    "incomplete": 177,
    "interval": 1800,
    "peers": [
        {
            "ip": "1.2.3.4",
            "peer id": "-TR2940-0890apv0oznf",
            "port": 51413
        }
    ]
}

Armed with a list of peers, your torrent client will connect to them and negotiate downloads of pieces of the file. I’ll write up how this works at a later time.

This response from the tracker is bencoded, an encoding method used by BitTorrent. There aren’t many bencoders out there, but this simple Go Playground can parse the response for you if you provide a hexdump of the output.

curl --get https://torrent.ubuntu.com/announce \
  --data 'info_hash=%D1%10%1A%2B%9D%20%28%11%A0%5E%8C%57%C5%57%A2%0B%F9%74%DC%8A' \
  --data-urlencode 'peer_id=01234567890123456789' \
  --data-urlencode 'port=12345' \
  --data-urlencode 'uploaded=0' \
  --data-urlencode 'downloaded=0' \
  --data-urlencode 'left=0' | hexdump -ve '1/1 "%.2x"'