req.ip and req.hostname in Express

Introduction

We use req.ip and req.hostname to find out where a web request comes from and which website address was used. This helps us understand and control how users connect to our server.

When you want to log the visitor's IP address for security or analytics.

When you need to customize responses based on the website domain used.

When you want to restrict access to certain IP addresses.

When you want to detect if a request is coming from a trusted domain.

When you want to show different content depending on the hostname.

Syntax

Express

req.ip
req.hostname

req.ip gives the IP address of the client making the request.

req.hostname gives the domain name used in the request, without the port number.

Examples

This example prints the visitor's IP address in the console and sends it back in the response.

Express

app.get('/', (req, res) => {
  console.log(req.ip);
  res.send('Your IP is ' + req.ip);
});

This example prints the hostname used to visit the site and sends it back in the response.

Express

app.get('/', (req, res) => {
  console.log(req.hostname);
  res.send('You visited ' + req.hostname);
});

This example checks the hostname and sends a special message if it matches 'example.com'.

Express

app.get('/', (req, res) => {
  if (req.hostname === 'example.com') {
    res.send('Welcome to example.com!');
  } else {
    res.send('Unknown host');
  }
});

Sample Program

This simple Express server listens on port 3000. When you visit the root URL, it shows your IP address and the hostname you used to visit.

Express

import express from 'express';

const app = express();
const port = 3000;

app.get('/', (req, res) => {
  const clientIp = req.ip;
  const hostName = req.hostname;
  res.send(`Hello! Your IP is ${clientIp} and you visited ${hostName}`);
});

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

Important Notes

req.ip may show IPv6 format like ::1 for localhost.

req.hostname does not include the port number, only the domain.

If your app is behind a proxy, you may need to set app.set('trust proxy', true) to get the real client IP.

Summary

req.ip tells you the visitor's IP address.

req.hostname tells you the domain name used in the request.

Use these to customize responses or log visitor info in Express apps.

Practice

1. What does req.ip represent in an Express app?

easy

A. The visitor's IP address

B. The domain name used in the request

C. The server's IP address

D. The HTTP method of the request

Solution

1. Step 1: Understand the purpose of req.ip

   req.ip provides the IP address of the client making the request.

2. Step 2: Differentiate from req.hostname

   req.hostname gives the domain name, not the IP address.

3. Final Answer:

   The visitor's IP address -> Option A

4. Quick Check:

   req.ip = visitor's IP [OK]

Hint: Remember: IP is address, hostname is domain name [OK]

Common Mistakes:

• Confusing req.ip with req.hostname
• Thinking req.ip is server IP
• Assuming req.ip gives HTTP method

2. Which of the following is the correct way to access the hostname in an Express route handler?

easy

A. req.host

B. req.domain

C. req.hostname

D. req.hostName

Solution

1. Step 1: Recall Express request object properties

   The correct property for the domain name is req.hostname.

2. Step 2: Check other options for validity

   req.host, req.domain, and req.hostName are not valid Express properties.

3. Final Answer:

   req.hostname -> Option C

4. Quick Check:

   Hostname property = req.hostname [OK]

Hint: Hostname is all lowercase: req.hostname [OK]

Common Mistakes:

• Using req.host instead of req.hostname
• Capitalizing property names incorrectly
• Assuming req.domain exists

3. Consider this Express route handler: <pre>app.get('/', (req, res) => { res.send(`IP: ${req.ip}, Hostname: ${req.hostname}`); }); If a user visits http://example.com from IP 192.168.1.5, what will be the response text?

medium

A. IP: undefined, Hostname: example.com

B. IP: example.com, Hostname: 192.168.1.5

C. IP: 127.0.0.1, Hostname: localhost

D. IP: 192.168.1.5, Hostname: example.com

Solution

1. Step 1: Identify req.ip and req.hostname values

   req.ip gives the visitor's IP, which is 192.168.1.5. req.hostname gives the domain, which is example.com.

2. Step 2: Substitute values into the response string

   The response string becomes: IP: 192.168.1.5, Hostname: example.com.

3. Final Answer:

   IP: 192.168.1.5, Hostname: example.com -> Option D

4. Quick Check:

   IP and hostname match request info [OK]

Hint: Match IP to visitor, hostname to domain in response [OK]

Common Mistakes:

• Swapping IP and hostname values
• Assuming localhost IP instead of visitor IP
• Using undefined or wrong properties

4. You wrote this Express middleware: <pre>app.use((req, res, next) => { console.log(req.ipAddress); next(); }); What is the problem with this code?

medium

A. Middleware must be declared after routes

B. req.ipAddress is not a valid property; should use req.ip

C. Logging should be done with res.ip instead

D. Missing call to next() to continue middleware chain

Solution

1. Step 1: Check property names on req

   req.ipAddress does not exist in Express; the correct property is req.ip.

2. Step 2: Verify middleware correctness

   The middleware calls next(), so chain continues correctly.

3. Final Answer:

   req.ipAddress is not a valid property; should use req.ip -> Option B

4. Quick Check:

   Use req.ip for IP address [OK]

Hint: Use exact property names: IP is req.ip [OK]

Common Mistakes:

• Using req.ipAddress instead of req.ip
• Forgetting to call next()
• Trying to log IP from res object

5. You want to restrict access to your Express app only to requests coming from the domain trusted.com. Which code snippet correctly uses req.hostname to allow this?

hard

A. app.use((req, res, next) => { if (req.hostname === 'trusted.com') next(); else res.status(403).send('Forbidden'); });

B. app.use((req, res, next) => { if (req.ip === 'trusted.com') next(); else res.status(403).send('Forbidden'); });

C. app.use((req, res, next) => { if (req.host === 'trusted.com') next(); else res.status(403).send('Forbidden'); });

D. app.use((req, res, next) => { if (req.hostname.includes('trusted.com')) next(); else res.status(403).send('Forbidden'); });

Solution

1. Step 1: Use req.hostname to check exact domain

   app.use((req, res, next) => { if (req.hostname === 'trusted.com') next(); else res.status(403).send('Forbidden'); }); correctly compares req.hostname to the string 'trusted.com'.

2. Step 2: Evaluate other options for correctness

   app.use((req, res, next) => { if (req.ip === 'trusted.com') next(); else res.status(403).send('Forbidden'); }); wrongly compares req.ip to a domain string. app.use((req, res, next) => { if (req.host === 'trusted.com') next(); else res.status(403).send('Forbidden'); }); uses invalid req.host. app.use((req, res, next) => { if (req.hostname.includes('trusted.com')) next(); else res.status(403).send('Forbidden'); }); uses includes which may allow subdomains, not exact match.

3. Final Answer:

   Option A code snippet -> Option A

4. Quick Check:

   Exact match with req.hostname = app.use((req, res, next) => { if (req.hostname === 'trusted.com') next(); else res.status(403).send('Forbidden'); }); [OK]

Hint: Check exact domain with req.hostname === 'domain' [OK]

Common Mistakes:

• Comparing IP to domain string
• Using invalid property req.host
• Using partial match instead of exact domain