Proxy headers in Nginx - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the effect of this Nginx proxy header configuration?

Given the following Nginx snippet inside a location block, what will be the value of the X-Forwarded-For header sent to the upstream server if the client IP is 192.168.1.10 and the original X-Forwarded-For header was 10.0.0.1?

Nginx

proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;

A X-Forwarded-For: 10.0.0.1, 192.168.1.10

B X-Forwarded-For: 192.168.1.10

C X-Forwarded-For: 192.168.1.10, 10.0.0.1

D X-Forwarded-For: 10.0.0.1

💡 Hint

Think about how $proxy_add_x_forwarded_for appends the client IP to the existing header.

The variable $proxy_add_x_forwarded_for appends the client IP to the existing X-Forwarded-For header, separated by a comma. So the original header 10.0.0.1 becomes 10.0.0.1, 192.168.1.10.

🧠 Conceptual

intermediate

Why is setting the Host header important in Nginx proxying?

When proxying requests with Nginx, why should you explicitly set the Host header using proxy_set_header Host $host;?

A To disable caching on the upstream server.

B To hide the client IP address from the upstream server.

C To encrypt the request headers between Nginx and the upstream.

D To ensure the upstream server receives the original requested hostname for proper routing or virtual hosting.

💡 Hint

Think about how web servers use the Host header to decide which site to serve.

The Host header tells the upstream server which hostname the client requested. This is critical for servers hosting multiple sites (virtual hosts) to respond correctly.

❓ Troubleshoot

advanced

Why is the client IP not appearing correctly in the upstream logs?

You configured Nginx as a reverse proxy with this header:
proxy_set_header X-Real-IP $remote_addr;
But the upstream server logs show the proxy server's IP instead of the client IP. What is the most likely cause?

A The <code>proxy_set_header</code> directive is syntactically incorrect.

B The upstream server is not configured to log the <code>X-Real-IP</code> header and logs the connection IP instead.

C The client IP is blocked by Nginx and replaced with the proxy IP.

D The <code>X-Real-IP</code> header is overwritten by the client.

💡 Hint

Check how the upstream server obtains the client IP for logging.

Nginx sets the X-Real-IP header correctly, but the upstream server must be configured to read this header for logging. Otherwise, it logs the IP of the proxy connection.

✅ Best Practice

advanced

Which proxy header configuration best preserves the original client IP chain?

You want to forward the full chain of client IPs through Nginx to the upstream server. Which configuration is best?

A proxy_set_header X-Forwarded-For '';

B proxy_set_header X-Forwarded-For $remote_addr;

C proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;

D proxy_set_header X-Forwarded-For $http_x_forwarded_for;

💡 Hint

Consider how to append the new client IP to existing forwarded IPs.

The variable $proxy_add_x_forwarded_for appends the current client IP to the existing X-Forwarded-For header, preserving the full chain.

🔀 Workflow

expert

Order the steps to correctly configure Nginx to forward client IPs to an upstream server behind a load balancer.

Arrange these steps in the correct order to ensure the upstream server receives the real client IP when Nginx is behind a load balancer.

A 2,1,3,4

B 3,2,1,4

C 2,3,1,4

D 1,2,3,4

💡 Hint

Think about the data flow from client to load balancer, then Nginx, then upstream.

First, the load balancer must pass the client IP in headers. Then Nginx adds or appends headers before proxying. The upstream server must trust and extract the IP from headers. Finally, verify logs.