btpeer 0.1.0

Simple CLI tool to get peers from BitTorrent trackers
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mod config;

use clap::Parser;
use rand::RngExt;
use std::{
    net::{Ipv4Addr, Ipv6Addr, SocketAddr, SocketAddrV4, SocketAddrV6, UdpSocket},
    time::Duration,
};

fn main() -> Result<(), Box<dyn std::error::Error>> {
    // Setup environment...
    let config = config::Config::parse();

    let socket = UdpSocket::bind(if config.udp_tracker.is_ipv4() {
        SocketAddr::V4(SocketAddrV4::new(Ipv4Addr::UNSPECIFIED, config.port))
    } else {
        SocketAddr::V6(SocketAddrV6::new(Ipv6Addr::UNSPECIFIED, config.port, 0, 0))
    })?;
    socket.set_read_timeout(Some(Duration::from_secs(config.udp_read_timeout)))?;

    let mut rng = rand::rng();
    let transaction_id: u32 = rng.random();

    let mut c = Vec::with_capacity(16);
    c.extend_from_slice(&0x41727101980u64.to_be_bytes()); // magic const
    c.extend_from_slice(&0u32.to_be_bytes());
    c.extend_from_slice(&transaction_id.to_be_bytes());

    // Initial connection...
    socket.send_to(&c, config.udp_tracker)?;

    let mut buf = [0u8; 2048];
    let (amt, _) = socket.recv_from(&mut buf)?;

    if amt < 16 {
        return Err("Tracker response too short".into());
    }

    let action = u32::from_be_bytes(buf[0..4].try_into()?);
    let res_transaction_id = u32::from_be_bytes(buf[4..8].try_into()?);

    if res_transaction_id != transaction_id {
        return Err("Transaction ID missmatch".into());
    }

    if action == 3 {
        return Err(format!("Tracker error: `{}`", String::from_utf8_lossy(&buf[8..amt])).into());
    }

    let connection_id = u64::from_be_bytes(buf[8..16].try_into()?);

    // Sending announce request...
    let mut a = Vec::with_capacity(98);
    let announce_trans_id: u32 = rng.random();
    let mut peer_id = [0u8; 20];
    rng.fill(&mut peer_id);

    a.extend_from_slice(&connection_id.to_be_bytes());
    a.extend_from_slice(&1u32.to_be_bytes()); // action = 1 (announce)
    a.extend_from_slice(&announce_trans_id.to_be_bytes());
    a.extend_from_slice(&config.info_hash);
    a.extend_from_slice(&peer_id);
    a.extend_from_slice(&0u64.to_be_bytes()); // downloaded
    a.extend_from_slice(&1024u64.to_be_bytes()); // left
    a.extend_from_slice(&0u64.to_be_bytes()); // uploaded
    a.extend_from_slice(&0u32.to_be_bytes()); // event = 0 (none)
    a.extend_from_slice(&0u32.to_be_bytes()); // IP address = 0 (default)
    a.extend_from_slice(&0u32.to_be_bytes()); // key
    a.extend_from_slice(&(-1i32).to_be_bytes()); // num want = -1
    a.extend_from_slice(&config.port.to_be_bytes()); // port

    println!("Sending announce...");
    socket.send_to(&a, config.udp_tracker)?;

    let (amt, _) = socket.recv_from(&mut buf)?;

    let res_action = u32::from_be_bytes(buf[0..4].try_into()?);
    let res_announce_trans_id = u32::from_be_bytes(buf[4..8].try_into()?);

    if res_announce_trans_id != announce_trans_id {
        return Err("Announce Transaction ID does not match.".into());
    }

    if res_action == 3 {
        return Err(format!(
            "Tracker announce declined: {}",
            String::from_utf8_lossy(&buf[8..amt])
        )
        .into());
    }

    if amt < 20 {
        return Err("Unexpected prefix len".into());
    }

    // Extract peers info...
    let interval = u32::from_be_bytes(buf[8..12].try_into()?);
    let leechers = u32::from_be_bytes(buf[12..16].try_into()?);
    let seeders = u32::from_be_bytes(buf[16..20].try_into()?);

    println!("Update interval: {interval}s",);
    println!("L: {leechers}, S: {seeders}",);

    let peers_data = &buf[20..amt];

    if peers_data.is_empty() {
        println!("No peers.");
    } else if config.udp_tracker.is_ipv6() {
        for chunk in peers_data.chunks_exact(18) {
            let ip_bytes: [u8; 16] = chunk[0..16].try_into()?;
            let ip = std::net::Ipv6Addr::from(ip_bytes);
            let port = u16::from_be_bytes(chunk[16..18].try_into()?);
            println!("[{ip}]:{port}")
        }
    } else {
        for chunk in peers_data.chunks_exact(6) {
            let ip_bytes: [u8; 4] = chunk[0..4].try_into()?;
            let ip = std::net::Ipv4Addr::from(ip_bytes);
            let port = u16::from_be_bytes(chunk[4..6].try_into()?);
            println!("{ip}:{port}")
        }
    }

    Ok(())
}