LanLink

Drop a file on one device, watch it land on another — no cloud, no accounts, just your LAN.

LanLink is a cross-platform Flutter app for sending files between devices on the same local network. One device discovers the other over mDNS/Bonjour, then the file moves directly over a raw TCP socket — nothing leaves the LAN, nothing touches a server.

Features

• Send — pick any file, see the devices currently in Receive mode on your LAN, send with live progress.
• Receive — turns this device into a discoverable LAN endpoint. Receiving is continuous: one file completes, it's saved, and the device keeps listening for the next one without restarting.
• Received Files — browse everything this device has ever received, open a file with the OS's default handler, or delete it (with a confirmation step — deletion is permanent).

How it works

The UI never touches a socket or the filesystem directly — everything goes through one API surface, Core (lib/src/Core.dart): discoverDevices(), establishServer(), sendFile(), showReceivedFiles(), openReceivedFile(), deleteReceivedFile(), and a handful more. Pages call Core; Core owns the transport.

Wire protocol (lib/src/features/protocol.dart): every message on the socket starts with a fixed 64-byte ASCII header holding the exact UTF-8 byte length of what follows, right-padded with spaces — no delimiter, just a byte count you can always read up front. A file transfer is two such messages: a small JSON metadata frame ({"name": ..., "size": ...}), then the raw file bytes. The receiving side (lib/src/features/server.dart) reads exactly size bytes off the wire — it isn't reading "until the connection closes." The connection's eventual close (a DISCONNECT sentinel) only means "no more files this session," and is explicitly checked for so it's never mistaken for the next file's metadata.

Discovery runs over mDNS/Bonjour via bonsoir: a receiving device broadcasts itself as <hostname>-lanlink under the service type _lanlink._tcp on TCP port 5050; a sending device browses for that same service type and resolves each hit into a host/port pair to connect to.

Storage: received files land in <ApplicationDocumentsDirectory>/LanLink/. The receiver only ever takes the basename of the sender-supplied filename before writing it, so a crafted path can't write outside that folder.

Receive status is a broadcast stream (waiting → incoming → completed) that the Receive screen consumes to drive its UI and a "received this session" list — the underlying server stays bound and keeps accepting connections across that whole cycle, which is what makes continuous receiving possible without any polling.

Tech stack

• Flutter (Material 3) / Dart
• bonsoir — mDNS/Bonjour discovery and broadcast
• file_picker — native file picker for Send
• path_provider — locating the app's documents directory
• dart:io Socket / ServerSocket — the actual file transport, hand-rolled rather than pulled from a package

Build & run

flutter pub get

# pick one, per platform:
flutter run -d macos
flutter run -d windows
flutter run -d linux
flutter run -d <android-device-id>
flutter run -d <ios-device-id>

flutter test

Use flutter devices to list available targets. Android, iOS, and macOS builds carry the local-network and Bonjour permissions LanLink needs (AndroidManifest.xml, both platforms' Info.plist, macOS sandbox entitlements); Windows and Linux run on the standard Flutter desktop scaffolding.

Prior art

Tools like LocalSend already solve LAN file transfer well. LanLink exists as a from-the-socket-layer-up implementation — the discovery, the wire framing, and the transfer state machine are all written here rather than pulled in as a transfer library, which is the part of this project actually worth looking at.