Express.js: Parsing Predomain from URL works with localhost but not with 127.0.0.1

Question

I've implemented a simple Express.js application that extracts the predomain from incoming requests' hostnames. The code seems to be functioning correctly when I use localhost:3000, but it fails when I try 127.0.0.1:3000.

Here's a snippet of the code I'm using:

const express = require('express');
const app = express();

app.use((req, res, next) => {
  const parts = req.hostname.split('.');
  req.predomain = parts[0];
  next();
});

app.get('/', (req, res) => {
  res.json({ predomain: req.predomain });
});

const PORT = 3000;
app.listen(PORT, () => {
  console.log(`Server is running on port ${PORT}`);
});

When I access http://harsh.localhost:3000, the API correctly returns the predomain as expected (harsh). However, when I access http://harsh.127.0.0.1:3000 the postman or chrome cannot find the address itself. I'm curious as to why this might be happening and how I can modify my code to correctly handle the extraction of the predomain even when using 127.0.0.1 or something else.

A IP Address (127.0.0.1) is not the same as a Domain name (example.com / foo.example.com) A IP Address can not have/be treated the same as a domain/sub domain. — Marc
– Marc, Commented Nov 20, 2023 at 9:24

Mureinik · Accepted Answer · 2023-11-20 09:43:52Z

1

The issue here is conceptual, not with your code - unlike a domain that may have subdomains, an IP (v4) address is always four integers delimited by dots. You simply can't add a subdomain to an IP address.

answered Nov 20, 2023 at 9:43

Mureinik

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2 Comments

Harsh Narwariya Over a year ago

If I deploy it on example.com, will it work?

Mureinik Over a year ago

@HarshNarwariya I doubt you'd be able to deploy your app to example.com, but if you could it seems the code should work.

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