<div align="center"> <h1>EchoLink - Sync With Devices</h1>

Secure, SSH-based device-to-device connectivity and sharing across Windows, Linux, and Android—powered by a self-hosted Headscale tailnet.

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What is EchoLink?

TL;DR: EchoLink connects your devices into a private mesh network and uses SSH/SFTP to enable seamless clipboard sync, file transfers, remote actions, and system monitoring between any two online devices—keeping your data entirely off third-party clouds.

The Problem

You have a Windows desktop, a Linux laptop, and an Android phone. You want to:

Copy text on your phone and paste it on your desktop
Send a file to your laptop without uploading it to some cloud first
Lock your desktop from your phone when you're away
Use your phone as a microphone for your PC
Control your PC's mouse from your phone

Existing solutions either require cloud services, work only on specific platforms, or need complex network configuration.

The Solution

EchoLink creates a private mesh network between your devices using Tailscale (self-hosted via Headscale). Once connected, devices communicate directly over SSH and a custom TCP protocol—no cloud relays, no third parties, no port forwarding.

┌──────────────┐                    ┌──────────────┐
│   Windows    │◄──────SSH─────────►│    Linux     │
│   Desktop    │◄────Port 55555────►│   Laptop     │
└──────┬───────┘                    └──────┬───────┘
       │                                   │
       │         ┌──────────────┐          │
       └────────►│   Android    │◄─────────┘
                 │    Phone     │
                 └──────────────┘

       All traffic flows over Tailscale mesh (100.x.y.z)
       No internet required after initial auth

✨ Features

Core

🔐 Private Mesh Network - Self-hosted Tailscale via Headscale, zero cloud dependency
📱 Cross-Platform - Windows, Linux, Android with consistent UX
🔑 NodeId-Based Identity - Devices maintain identity across IP changes
🛡️ SSH Security - ed25519/RSA key exchange, no passwords
🔄 Smart Device Discovery - Automatic IP tracking, no duplicate devices

File & Clipboard

📋 Clipboard Sync - Auto-broadcast clipboard changes or manual push (SnapShare)
📁 File Transfer - SFTP-based transfer with progress tracking, remote browsing
📤 Multi-Device Support - Send to multiple devices simultaneously

Remote Control & Monitoring

🖱️ Mouse Control - Trackpad-style remote mouse movement with left/right/middle click buttons
⚡ System Actions - Lock, restart, shutdown remote devices
📊 System Monitor - Real-time CPU, RAM, disk, battery, temperature
🎤 Audio Streaming - Two-way audio (use phone as mic, or stream PC audio)

Automation

⚙️ Macros - Custom command buttons synced across devices
🔔 File Watchers - Auto-reload macros on changes

🛠️ Tech Stack

| Component | Technology | |-----------|------------| | UI Framework | Avalonia UI 11 (cross-platform XAML) | | Language | C# (.NET 10), Go (Android native bridge) | | Mesh Network | Tailscale + Headscale (self-hosted) | | File Transfer | SSH.NET (SFTP over SSH) | | Audio Codec | Opus (48kHz, low-latency) | | Audio I/O | NAudio (Windows), Native bridges (Linux/Android) | | Protocol | Custom TCP on port 55555 + SOCKS5 proxy |

📑 Project Status

Completed ✅

[x] Headscale control server integration (control.echo-link.app)
[x] Google OIDC authentication with middleware PIN exchange
[x] Tailscale daemon with userspace networking + SOCKS5 proxy (port 1055)
[x] Cross-platform UI with dashboard, device discovery, QR/PIN pairing
[x] SSH key pair generation and bidirectional exchange (port 44444)
[x] SFTP file transfer with remote browsing, progress tracking, Android path handling
[x] Clipboard sync with MirrorClip (auto), SnapShare (manual), GhostPaste (remote apply)
[x] Remote mouse control with trackpad + click buttons (left/right/middle)
[x] System commands (lock/restart/shutdown)
[x] System monitoring (CPU/RAM/disk/battery/temp) with 10s polling
[x] Two-way audio streaming with Opus encoding, VB-Audio Cable routing
[x] Macro system with mesh sync and file watcher auto-reload
[x] Android Go bridge with tsnet, SSH server on port 2222, native audio capture
[x] Unified TCP protocol (port 55555) for all non-SSH features
[x] Guest device support with time-limited PINs
[x] NodeId-based device identity (no duplicates on IP change)
[x] Automatic device refresh with selection preservation

In Progress / Needs Polish 🚧

[ ] Automated VB-Audio Cable driver installation (Windows)
[ ] Linux audio capture bridge (PulseAudio/PipeWire integration)
[ ] macOS full support (audio, remote control implementations)
[ ] Hotkey registration system (UI exists, backend incomplete)
[ ] Mesh topology visualization
[ ] Quickstart documentation

Planned 📋

[ ] iOS client
[ ] LAN-only fallback mode (offline operation)
[ ] End-to-end encryption beyond SSH
[ ] Network graph visualization

🏗️ Architecture

How It Works

1. Authentication

User Login → Google OIDC → JWT → Middleware → Tailscale Auth Key → Daemon Starts

2. Device Pairing

Generate PIN (6-digit) → Middleware API → Other device claims PIN →
SSH key exchange (port 44444) → Keys added to authorized_keys → Paired

3. Communication

┌─────────────────────────────────────────────────────┐
│  App Layer (Clipboard, File Transfer, Remote, etc.) │
└───────────────────┬─────────────────────────────────┘
                    │
         ┌──────────┴──────────┐
         │                     │
   ┌─────▼─────┐       ┌──────▼──────┐
   │   SSH     │       │  Unified    │
   │  (Port 22)│       │ Protocol    │
   │  SFTP     │       │ (Port 55555)│
   └──────────┘       └──────┬──────┘
         │                    │
         └───────────────────┘
                    │
         ┌──────────▼──────────┐
         │   SOCKS5 Proxy      │
         │   (Port 1055)       │
         │   Tailscale Mesh    │
         └─────────────────────┘

Ports & Protocols

| Port | Protocol | Purpose | |------|----------|---------| | 1055 | SOCKS5 | Tailscale userspace proxy (localhost only) | | 22 | SSH/SFTP | Standard SSH (Windows/Linux) | | 2222 | SSH/SFTP | Android SSH server | | 44444 | TCP | SSH key exchange handshake | | 55555 | Custom TCP | Unified protocol (clipboard, remote, audio, monitoring) |

Platform-Specific Notes

Android:

Uses Go tsnet library for userspace networking (no TUN device)
All connections must route through SOCKS5 proxy at 127.0.0.1:1055
SSH runs on port 2222 (not 22)
Files saved to /storage/emulated/0/Download/
Audio capture: microphone only (no system loopback)

Windows:

Full WASAPI loopback for system audio capture
VB-Audio Cable for virtual microphone routing
OpenSSH server auto-install with admin rights
Best overall support

Linux:

Uses system OpenSSH server
Audio requires manual PulseAudio/PipeWire routing to EchoLink_Sink
Telemetry reads from /proc and /sys/class/thermal
System commands via loginctl and systemctl

📖 Documentation

Quickstart Guide - Get up and running in 10 minutes
Developer Guide - Architecture deep-dive, building, debugging
Contributing - How to contribute, coding standards

🚀 Quick Start

# Clone
git clone https://github.com/uganthan2005/EchoLink
cd EchoLink

# Build (requires .NET 10 SDK)
dotnet build

# Run
dotnet run --project EchoLink.csproj

See docs/SETUP.md for detailed setup instructions, Headscale configuration, and first-time pairing guide.

🤝 Contributing

Contributions welcome! See docs/CONTRIBUTING.md for:

Development setup
Coding standards
Pull request process
Areas that need help (Linux audio, macOS support, iOS client)

📄 License

MIT License - see LICENSE file.

🏆 Built With

Avalonia UI - Cross-platform XAML UI
Tailscale - Mesh networking
Headscale - Self-hosted control server
SSH.NET - SSH/SFTP library
Opus - Low-latency audio codec
NAudio - Windows audio I/O

Questions? Open an issue