LanLink

Cross-platform file and text sharing between computers and phones on the same LAN — or across the internet. Like EasyJoin/KDE Connect, but simpler.

Features

• Auto-discovery: all instances on the same LAN find each other automatically (UDP broadcast)
• Text messaging: send text instantly between devices
• File transfer: send single files or entire directories with live progress
• Remote connect: connect to any instance over the internet by entering its IP/domain
• LAN bridging: if any device on your LAN connects to a remote device, all LAN devices can see and send to all devices on the remote's LAN — multi-hop relay with loop detection
• Auto-accept: received files save to a configurable download folder
• Drag & drop (desktop): drop files/folders onto the window to send
• Zero configuration: works out of the box, no accounts or pairing needed

Platforms

Platform	Project	UI Framework
Windows (desktop)	LanLink/	WPF (.NET 8+)
Android (mobile)	LanLink.Mobile/	.NET MAUI

Both use the same wire protocol and discover each other seamlessly — send a file from your phone to your PC or vice versa.

Building

Desktop (Windows)

dotnet build LanLink/LanLink.csproj -c Release
dotnet publish LanLink/LanLink.csproj -c Release -r win-x64 --self-contained false -o publish

The exe lands in publish/LanLink.exe. Requires .NET 8+ runtime on the target machine.

For a fully self-contained exe (no .NET install needed):

dotnet publish LanLink/LanLink.csproj -c Release -r win-x64 --self-contained true -p:PublishSingleFile=true -o publish-standalone

Installer (MSI)

Run build-msi.bat to produce LanLink-<version>.msi in the current directory. It publishes a self-contained single-file exe and packages it with WiX (installed automatically as a global dotnet tool if missing).

• Install location: C:\Program Files\LanLink (a 64-bit package; a 32-bit build would use C:\Program Files (x86)\LanLink).
• Autostart prompt: setup asks whether LanLink should start automatically when Windows starts (minimized to the tray). Setup only records that answer; LanLink applies it on first run by writing its own per-user entry — see Starting with Windows.
• Upgrades: the MSI's ProductVersion tracks the exe version, so installing a newer build automatically removes the old one. The previous install location is remembered (via HKLM\SOFTWARE\LanLink\InstallDir), so upgrades reinstall to the same folder even if you originally chose a custom path. Before replacing files it tells any running LanLink to quit via the --exit IPC command and waits for it to release the executable.

Bump the VERSION file next to build-msi.bat for each release — it is the single source of truth and flows into the exe version, the MSI ProductVersion, the APK versionName/versionCode, and the GitHub release tag.

Releasing

release-github.bat publishes a GitHub release tagged v<version> from the VERSION file, attaching the self-contained exe, the MSI, and (when it builds) the APK. It refuses to publish if that version was already released, so bump VERSION first.

Command-line flags

Flag	Effect
--minimized	Start hidden in the tray (only the tray icon shows).
--minimized-taskbar	Start minimized to the taskbar (window exists but is minimized).
--exit	Signal the already-running instance to exit cleanly, then wait for it to terminate. Used by the installer during upgrades.

The startup appearance is normally driven by the When LanLink starts setting (Show window normally / Start minimized (taskbar) / Start minimized to tray); the flags above are how the Windows-startup entry passes that choice and override the setting for a single launch.

LanLink is single-instance: launching it again brings the existing window forward and exits the new copy (a --minimized/--minimized-taskbar relaunch is ignored so autostart won't pop the window open at boot). It claims the network port exclusively, so if a stale copy is already running, a second launch surfaces that copy rather than starting a second, half-working instance.

Settings

Open ⚙ Settings from the bottom-right of the window, or Settings… on the tray icon's right-click menu (the tray route matters when LanLink starts hidden and there's no window to click). Available options:

Setting	Effect
Display name	How this device appears to other peers.
Download folder	Where received files are saved.
Port	TCP/UDP port (requires restart).
When LanLink starts	Show window normally / Start minimized (taskbar) / Start minimized to tray.
Start LanLink when Windows starts	Adds or removes the autostart entry.
Accept connections from outside the LAN	Off by default; see Remote connections.

Starting with Windows

Exactly one autostart entry exists, and only LanLink writes it: a per-user HKCU\...\Run value whose command-line flag matches the chosen start mode.

That matters because Windows launches an app once per Run entry. If a machine-wide HKLM entry existed alongside the per-user one, each launch would carry its own mode flag and whichever instance won the single-instance race would decide how the app appears — so setup's mode could silently override the one picked in Settings, and turning autostart off would look like it did nothing. Removing an HKLM entry also needs administrator rights the app doesn't have.

So the installer never writes a Run entry. Its autostart checkbox writes HKLM\SOFTWARE\LanLink\AutostartRequest = <version>|<1 if ticked>, and LanLink applies that on its first run after the install, creating or removing the per-user entry to match. The version stamp means each install's answer is applied exactly once: change the setting afterwards and it sticks, because the stamp hasn't changed.

Installs from 1.0.3.0 and earlier did create a machine-wide entry. Upgrading removes it. To check or clear one by hand, from an elevated prompt:

reg query  "HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run" /v LanLink
reg delete "HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Run" /v LanLink /f

Saving Settings also drops such a leftover entry when LanLink happens to be elevated, and warns in the log when it can't.

Note that the checkbox reflects the actual registry state (either hive), not a saved flag, so it stays honest about an entry an old installer created.

Android

Requires the MAUI Android workload:

dotnet workload install maui-android

Then build the APK:

dotnet publish LanLink.Mobile/LanLink.Mobile.csproj -c Release -f net9.0-android

The APK is at LanLink.Mobile/bin/Release/net9.0-android/publish/com.lanlink.mobile-Signed.apk. Sideload it via ADB or copy to your phone.

A convenience script is included: run build-apk.bat.

How it works

Network

• Port 37656 — TCP for data, UDP for discovery (same port number, different protocols)
• Discovery: every 3 seconds each instance sends a UDP broadcast on every active network interface's subnet broadcast address
• Connection: after discovery, the node with the lexicographically lower ID initiates a TCP connection; both exchange Hello messages
• Protocol: binary framed — [4B header len][4B payload len][UTF-8 JSON header][raw binary payload]
• File transfer: files are chunked at 256 KB and streamed; progress is reported ~2x/second

Bridging (the interesting part)

When device A on LAN 1 connects to device B on LAN 2 over the internet:

1. A and B exchange their peer lists over TCP
2. Each propagates the remote peers to its own LAN connections
3. Every device on LAN 1 learns routes to LAN 2 devices (next-hop = A)
4. Every device on LAN 2 learns routes to LAN 1 devices (next-hop = B)
5. Messages to non-local peers are wrapped in relay envelopes and forwarded hop-by-hop
6. Loop detection via hop-list tracking; max relay depth = 4 hops

This means: connect one device on each LAN and everything else is automatic.

Firewall

On first launch (Windows), the app creates inbound firewall rules for TCP/UDP port 37656 via an elevated netsh command (you'll see one UAC prompt). If you decline, LAN discovery won't work until you manually allow the port.

On Android, no firewall setup is needed — the manifest declares the required permissions and the app acquires a WiFi MulticastLock for reliable broadcast reception.

Remote connections

To connect two instances over the internet:

1. Forward port 37656 (TCP) on the remote router to the target machine
2. On the target machine, enable Accept connections from outside the LAN in Settings (off by default — inbound non-LAN connections are rejected for safety). Connections you initiate with the "Remote address" field always work regardless of this setting.
3. On the connecting side, enter the public IP or domain in the "Remote address" field
4. Once connected, all LAN peers on both sides can see each other (bridging)

Download folder

Platform	Default location
Windows	%USERPROFILE%\Downloads\LanLink\
Android	/storage/emulated/0/Android/data/com.lanlink.mobile/files/LanLink/

Configurable in Settings on both platforms. Files with name conflicts are saved as file (2).ext, file (3).ext, etc.

License

Do whatever you want with it.