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.