Have you read the Contributing Guidelines on issues?

• I have read the Contributing Guidelines on issues.

Prerequisites

• I'm using the latest version of Docusaurus.
• I have tried the npm run clear or yarn clear command.
• I have tried rm -rf node_modules yarn.lock package-lock.json and re-installing packages.
• I have tried creating a repro with https://new.docusaurus.io.
• I have read the console error message carefully (if applicable).

Description

On macOS, consider the following script server.js:

const http = require("http");

const port = 3000;

const requestListener = function (req, res) {
    res.writeHead(200);
    res.end("My first server!");
};

const server = http.createServer(requestListener);

server.listen(port,  () => {
    console.log(`Server is running on http://localhost:${port}`);
});

Run it using node server.js.
Then start a Docusaurus project (running on v2.1.0 in my case). The documentation server will start on port 3000, although it is busy. If you then open Docusaurus in Chrome:

however, if I use curl, I get the docusaurus page!

If my server passes a host:

- server.listen(port,  () => {
+ server.listen(port, "localhost",  () => {

then Docusaurus will correctly identify that the port is busy and asks if I want to choose another port. Nonetheless I see it as a bug in Docusaurus or one of it's dependencies.

Possible solution

Although used by less people, this logic for finding a free port works now in Remotion without bug reports coming in: https://github.com/remotion-dev/remotion/blob/main/packages/renderer/src/get-port.ts

I am happy to chat on Discord how to investigate it together!

Reproducible demo

No response

Steps to reproduce

As above

Expected behavior

Busy port detection kicks in and asks if a different port should be used

Actual behavior

Server gets started and a conflict is created

Your environment

• Docusaurus version used: v2.1.0
• Environment name and version (e.g. Chrome 89, Node.js 16.4): Node 16.7
• Operating system and version (e.g. Ubuntu 20.04.2 LTS): macOS

Self-service

• I'd be willing to fix this bug myself.