CastServer

A lightweight, self-hosted WebRTC-based screen casting server. Share your screen (or a window/tab) to multiple viewers in real-time with sub-second latency — no plugins, no accounts, just a browser.

Architecture

flowchart LR
    P[Publisher<br/>Browser] -->|WebRTC<br/>getDisplayMedia| S[CastServer SFU<br/>Go + Pion]
    S -->|WebRTC| V1[Subscriber 1]
    S -->|WebRTC| V2[Subscriber 2]
    S -->|WebRTC| VN[Subscriber N]

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CastServer acts as a Selective Forwarding Unit (SFU): the publisher's browser captures the screen via getDisplayMedia, streams it to the Go server over WebRTC, and the server relays RTP packets to all connected viewers without re-encoding.

Features

• One-to-many streaming with sub-second latency
• Multiple quality presets: Low Latency (480p), Balanced (720p), High Quality (1080p), Native (passthrough)
• Configurable FPS: 10, 15, 20, 30, or 60
• Codec selection: Auto, H.264, VP9, or VP8
• System audio sharing alongside screen capture
• Keyframe caching for instant playback when a new viewer joins
• RTCP forwarding for adaptive bitrate and smooth playback
• Auto-reconnection — viewers reconnect automatically when the stream comes back
• Diagnostics HUD — live bitrate, FPS, encode/decode latency, RTT, jitter buffer, dropped frames
• QR codes per network interface for easy mobile access
• Dark/light theme toggle
• Optional TURN server for NAT traversal
• Single self-contained binary — static assets embedded via Go's embed

Quick Start

Prerequisites

• Go 1.25+
• Node.js and npm (for building the frontend)

Build

# Clone the repo
git clone https://github.com/LETHEVIET/casting_things.git
cd casting_things

# Build the frontend
cd server/cmd/castserver/frontend
npm install
npm run build
cd ../../../..

# Build the server
cd server
go build -o castserver ./cmd/castserver
cd ..

Run

./server/castserver --listen :1108

Then open:

• Admin UI (publish your screen): http://localhost:1108/
• Viewer (watch the stream): http://localhost:1108/web

CLI Flags

Flag	Default	Description
--listen	:1108	HTTP listen address
--turn-url	(none)	TURN server URL for NAT traversal
--turn-user	(none)	TURN username
--turn-pass	(none)	TURN password

TURN Server Example

For streaming outside your local network, configure a TURN server:

./server/castserver --listen :1108 --turn-url turn:your-turn-server:3478 --turn-user user --turn-pass pass

Setup Scenarios

Cast to devices on the same network

The admin UI shows a list of local network IPs with QR codes. Viewers on the same WiFi / LAN just open the URL on any browser — desktops, tablets, phones:

You publish from...	Viewers open...	Use case
Desktop/laptop	http://192.168.1.x:1108/web	Cast slides to a tablet used as a second screen
Desktop/laptop	http://192.168.1.x:1108/web	Watch a design mockup live on your phone

No NAT traversal needed — WebRTC uses host candidates within the same subnet.

Cast remotely with Tailscale (no TURN needed)

Tailscale creates a mesh VPN with direct peer-to-peer connections. Install Tailscale on the CastServer host and viewer devices, then just start the server normally:

./server/castserver --listen :1108

The server binds to all interfaces, so it's automatically reachable via the Tailscale IP. The admin UI detects network interfaces and will display the Tailscale URL alongside local IPs.

Now any device on your tailnet can open:

• Admin UI: http://<tailscale-ip>:1108/
• Viewer: http://<tailscale-ip>:1108/web

Tailscale handles NAT traversal at the network layer, so WebRTC host candidates work end-to-end without configuring a TURN server.

Cast publicly with a TURN server

For public internet streaming, deploy a TURN server (e.g. coturn) and pass credentials:

./server/castserver --listen :1108 \
  --turn-url turn:turn.example.com:3478 \
  --turn-user myuser \
  --turn-pass mypassword

API

Endpoint	Method	Description
/	GET	Admin UI (publisher)
/web	GET	Viewer UI
/health	GET	Health check ({"status":"ok"})
/presets	GET	List quality presets
/stats	GET	Real-time streaming stats (JSON)
/stats/stream	GET	Server-Sent Events stream of stats
/interfaces	GET	List local network IPs
/webrtc/publish	WebSocket	Publisher signaling
/webrtc/subscribe	WebSocket	Subscriber signaling
/webrtc/stop	POST	Force-stop active session

Tech Stack

Layer	Technology
Backend	Go (Pion WebRTC, Gorilla WebSocket)
Frontend	Svelte 4 + TypeScript
Build	Vite 5
Styling	Tailwind CSS 3 + Flowbite

Project Structure

server/
├── cmd/castserver/
│   ├── main.go          # Entry point
│   ├── static/           # Embedded frontend build output
│   └── frontend/         # Svelte source
│       └── src/
│           ├── App.svelte
│           ├── Admin.svelte    # Publisher UI
│           ├── Viewer.svelte   # Viewer UI
│           └── lib/
│               ├── api.ts      # TypeScript types
│               └── qrcode.ts   # QR code generator
└── internal/
    ├── control/          # HTTP handlers, stats, presets
    └── sfu/              # WebRTC peer management, keyframe cache

License

MIT