WebSocket from Scratch (Go)

A WebSocket server implemented from scratch in Go, without any external libraries.

This is a study project — the goal is to understand how the WebSocket protocol works at a low level by implementing it manually on top of raw TCP.

What it covers

• TCP server using net.Listen
• HTTP Upgrade handshake (RFC 6455)
• Frame parsing: reading FIN, opcode, mask, and payload length (3 cases: 7-bit, 16-bit, 64-bit)
• Frame writing: building binary frames to send back to clients
• XOR unmasking of client payloads
• Opcode handling: text, close, ping/pong
• Message fragmentation (FIN=0 continuation frames)
• Broadcast to multiple connected clients using a Hub with mutex protection

Run

go run main.go

Then open a browser console and connect:

const ws = new WebSocket("ws://localhost:8180")
ws.onmessage = (e) => console.log(e.data)
ws.onopen = () => ws.send("hello")

Purpose

This project is purely for learning. The implementation intentionally avoids using Go's net/http or any WebSocket library so that every part of the protocol is written and understood by hand.