What is a Leecher?
A leecher is a peer that is downloading a file from the network. It connects to other peers (seeds or other leechers) to request and download file pieces. Key characteristics:
- Starts with no file pieces (empty bitfield)
- Connects to an initial peer using the
--peer flag
- Downloads pieces from available peers
- Becomes a seed automatically after completing the download
- Can share downloaded pieces with other leechers even before completion
Starting a Leecher Node
Identify an initial peer
You need the host and port of at least one seed or leecher that has the file. For example: 127.0.0.1:6881
Start the leecher using npm script
Use the leech npm script to start downloading:npm run leech -- --port 6882 --file ./output.ext --peer 127.0.0.1:6881
npm start -- --port 6882 --file "/ruta/de/salida.ext" --peer 127.0.0.1:6881
Monitor download progress
The leecher will display real-time progress:Conectando con peer inicial 127.0.0.1:6881...
Meta de archivo recibida: "video.mkv" (150000000 bytes, 2286 piezas). Iniciando descarga...
Progreso: 10.00% (15000000/150000000 bytes). Velocidad: 800.0 KB/s
Pieza 0 recibida (65536 bytes). Piezas restantes: 2285.
Wait for completion
When all pieces are downloaded, the leecher verifies the file integrity:¡Descarga completada! Archivo "video.mkv" descargado completamente.
Verificación de integridad: OK (hash coincide).
El nodo continuará corriendo como seed para compartir el archivo con otros peers.
Command-Line Usage
The basic syntax for starting a leecher is:
node src/peer.js --port <puerto> --file <rutaArchivo> --peer <host:puerto>
Example: Downloading from a local seed
node src/peer.js --port 6882 --file "./downloads/video.mkv" --peer 127.0.0.1:6881
Example: Downloading from a remote peer
node src/peer.js --port 6883 --file "./movie.mp4" --peer 192.168.1.100:6881
Console Output
Here’s what you’ll see when running as a leecher:
Initial connection:
Conectando con peer inicial 127.0.0.1:6881...
Meta de archivo recibida: "video.mkv" (150000000 bytes, 2286 piezas). Iniciando descarga...
Progreso: 10.00% (15000000/150000000 bytes). Velocidad: 800.0 KB/s
Pieza 0 recibida (65536 bytes). Piezas restantes: 2285.
¡Descarga completada! Archivo "video.mkv" descargado completamente.
Verificación de integridad: OK (hash coincide).
El nodo continuará corriendo como seed para compartir el archivo con otros peers.
How Leechers Become Seeds
When a leecher completes the download, it automatically transitions to seed mode. This happens in src/node.js:368-394:
if (this.missingPieces.size === 0) {
// Descarga terminada
console.log(`¡Descarga completada! Archivo "${this.fileName}" descargado completamente.`);
// Detener intervalo de progreso
if (this.progressInterval) {
clearInterval(this.progressInterval);
this.progressInterval = null;
}
// Verificar integridad del archivo comparando hash
if (this.fileHash) {
try {
const downloadedHash = await this.fileManager.computeHash();
if (downloadedHash === this.fileHash) {
console.log('Verificación de integridad: OK (hash coincide).');
} else {
console.warn('Advertencia: el hash del archivo descargado difiere del esperado.');
}
} catch (err) {
console.error('No se pudo verificar el hash del archivo descargado:', err);
}
}
console.log('El nodo continuará corriendo como seed para compartir el archivo con otros peers.');
// Marcar estado como seeder a partir de ahora
this.isSeed = true;
}
- The node continues accepting connections
- It can serve pieces to other leechers
- It participates in peer exchange (PEX)
Progress Tracking
The leecher displays download progress every second, showing:
- Percentage complete:
(bytesDone / fileSize) * 100
- Bytes downloaded: Total bytes written to disk
- Download speed: Average KB/s since start
This is implemented in src/node.js:506-524:
_startProgressInterval() {
if (this.progressInterval) return;
this.progressInterval = setInterval(() => {
if (!this.fileSize) return;
const bytesDone = this.fileSize - (this.missingPieces.size * this.pieceSize);
const percent = ((bytesDone / this.fileSize) * 100).toFixed(2);
let speedStr = '';
if (this.startTime) {
const elapsed = (Date.now() - this.startTime) / 1000;
if (elapsed > 0) {
const speed = bytesDone / elapsed; // bytes por segundo
const speedKB = speed / 1024;
speedStr = `Velocidad: ${speedKB.toFixed(1)} KB/s`;
}
}
console.log(`Progreso: ${percent}% (${bytesDone}/${this.fileSize} bytes). ${speedStr}`);
}, 1000);
}
Peer Exchange (PEX)
Leechers automatically discover additional peers through Peer Exchange. When you connect to one seed, it shares addresses of other connected peers, allowing you to download from multiple sources simultaneously.
- Initial connection: You connect to one peer using
--peer
- Peer list received: The initial peer sends a list of other connected peers
- Automatic connections: Your node connects to newly discovered peers
- Anti-collision rule: Only the peer with the higher ID initiates the connection (prevents duplicate connections)
This is handled in src/node.js:414-433:
_handlePeers(socket, message) {
const peersList = message.peers;
for (let peer of peersList) {
const { id: peerId, host, port } = peer;
if (peerId === this.id) continue;
if (!this.knownPeers.has(peerId)) {
this.knownPeers.set(peerId, {
id: peerId, host: host, port: port,
socket: null, availablePieces: new Set(), busy: false
});
// Solo el peer con ID mayor inicia la conexión
if (this._shouldInitiateConnection(peerId)) {
console.log(`Descubierto peer ${peerId} en ${host}:${port}, iniciando conexión...`);
this.connectToPeer(host, port);
}
}
}
}
File Verification
After downloading all pieces, the leecher calculates the SHA-1 hash of the complete file and compares it with the hash received from the seed during the handshake.
If hashes match:
Verificación de integridad: OK (hash coincide).
Advertencia: el hash del archivo descargado difiere del esperado. El archivo podría estar corrupto.
If the integrity check fails, the downloaded file may be corrupted. This could happen due to network errors or if peers are sharing different versions of the file.
