P2P LAN File Transfer
Patterns and approaches for direct device-to-device file transfer over a local network. No cloud, no internet required.
When to Use
| Scenario |
Best Approach |
| Same WiFi network |
Direct TCP over LAN |
| Mobile hotspot |
Direct TCP (both on hotspot) |
| Ethernet + WiFi |
Direct TCP (same subnet) |
| Different subnets |
UPnP / NAT traversal or relay |
| No network |
WiFi Direct or USB tethering |
Architecture Options
1. Direct TCP (Simplest)
Device A (server) ←→ TCP connection ←→ Device B (client)
- One device acts as TCP server
- Other connects directly
- Fastest possible speed (full LAN bandwidth)
- No dependencies beyond Python stdlib
2. WebSocket
Device A ←→ WebSocket ←→ Device B
- Built-in browser support
- Framing protocol built in
- Slightly more overhead than raw TCP
3. WebRTC
Device A ←→ WebRTC ←→ Device B
- NAT traversal built in
- Browser-compatible
- Works across some subnets
- Complex setup
Recommended: Direct TCP
For Python-to-Python transfers on the same LAN, direct TCP is fastest and simplest:
# Server
import asyncio
async def handle_client(reader, writer):
# Receive file metadata
data = await reader.readexactly(4)
# ... receive file
async def main():
server = await asyncio.start_server(handle_client, "0.0.0.0", 8877)
await server.serve_forever()
# Client
async def send_file(host, port, filepath):
reader, writer = await asyncio.open_connection(host, port)
# Send file
writer.write(data)
await writer.drain()
Discovery Methods
| Method |
Library |
Requires |
| mDNS |
zeroconf |
Same network, UDP 5353 open |
| Broadcast ping |
UDP broadcast |
Same subnet |
| Manual IP |
User input |
User knows the IP |
| QR code |
qrcode |
One device has a screen |
Speed Optimization
| Technique |
Gain |
| Chunk size 1MB |
Reduces syscall overhead |
| Parallel streams (2-4) |
2-4x on high-latency links |
| TCP_NODELAY |
Reduces Nagle algorithm delay |
| Large socket buffers |
256KB+ for fast networks |
| Async I/O |
Non-blocking, efficient |
Security
- Same network = same trust domain (typically)
- Add pairing/PIN to prevent unauthorized access
- TLS for encryption over LAN (self-signed certs)
- Rate limit connection attempts to prevent scanning
Pitfalls
- Windows Firewall blocks inbound connections by default
- Mobile networks isolate devices (carrier NAT)
- Company WiFi may have client isolation (AP isolation)
- VPNs route traffic externally, defeating LAN transfer
LoopyLuci