req.ip and req.hostname in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

Understanding req.ip in Express

What will be the value of req.ip when a client makes a request to an Express server running locally?

Express

app.get('/', (req, res) => {
  res.send(req.ip);
});

A "localhost"

B "::1"

C "127.0.0.1"

D "example.com"

💡 Hint

Think about how IPv6 localhost address is represented.

When running locally, Express sets req.ip to the client's IP address. For localhost, this is usually the IPv6 loopback address ::1.

❓ component_behavior

intermediate

Behavior of req.hostname in Express

If a client accesses an Express app via http://myapp.example.com, what will req.hostname return?

Express

app.get('/', (req, res) => {
  res.send(req.hostname);
});

A "myapp.example.com"

B "example.com"

C "localhost"

D "undefined"

💡 Hint

Consider what part of the URL req.hostname extracts.

req.hostname returns the hostname from the Host HTTP header, which includes subdomains like myapp.example.com.

❓ component_behavior

advanced

Accessing non-existent req.ipAddress

What happens when executing this code to get the client's IP address in Express?

Express

app.get('/', (req, res) => {
  const clientIp = req.ipAddress;
  res.send(clientIp);
});

A Returns the client's IP address as a string

B ReferenceError because req.ipAddress is undefined

C Returns undefined without error

D SyntaxError due to invalid property access

💡 Hint

Check what happens when accessing a non-existent property on an object.

req.ipAddress does not exist. Accessing it returns undefined without error. The correct property is req.ip.

❓ state_output

advanced

Output of req.hostname with port in Host header

What will req.hostname return if the client sends a request with the Host header set to myapp.example.com:3000?

Express

app.get('/', (req, res) => {
  res.send(req.hostname);
});

A "localhost"

B "myapp.example.com:3000"

C "example.com"

D "myapp.example.com"

💡 Hint

Remember how Express parses the Host header for hostname.

req.hostname excludes the port number and returns only the hostname part of the Host header.

🔧 Debug

expert

Debugging incorrect req.ip behind a proxy

An Express app is behind a proxy. The developer notices req.ip always shows the proxy's IP, not the client's. What is the correct fix?

Express

app.set('trust proxy', false);
app.get('/', (req, res) => {
  res.send(req.ip);
});

A Set app.set('trust proxy', true) to trust the proxy headers

B Use req.hostname instead of req.ip to get client IP

C Disable the proxy to get the real client IP

D Manually parse req.headers['x-forwarded-for'] without changing trust proxy

💡 Hint

Express needs to trust the proxy to get the real client IP.

Setting trust proxy to true tells Express to use the X-Forwarded-For header to get the real client IP behind proxies.

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