Nginx as reverse proxy in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when Nginx is set as a reverse proxy for an Express app?

Consider an Express app running on port 3000. Nginx is configured to forward requests from port 80 to this app. What is the visible effect when a user accesses the server's IP on port 80?

Express

server {
  listen 80;
  location / {
    proxy_pass http://localhost:3000;
  }
}

A The user sees the Express app response as if directly accessing port 3000.

B The user gets a 404 error because Nginx blocks all requests.

C The user sees the Nginx default welcome page instead of the Express app.

D The user is redirected to port 3000 in the browser URL.

💡 Hint

Think about what a reverse proxy does with incoming requests.

Nginx forwards requests from port 80 to the Express app on port 3000, so the user sees the app response transparently.

📝 Syntax

intermediate

Identify the correct Nginx proxy_pass syntax for forwarding to Express

Which Nginx configuration snippet correctly forwards all requests to an Express app running on localhost port 4000?

A location / { proxy_pass http://:4000; }

B location / { proxy_pass localhost:4000; }

C location / { proxy_pass http://localhost4000; }

D location / { proxy_pass http://localhost:4000; }

💡 Hint

Remember the full URL format for proxy_pass.

The proxy_pass directive requires a full URL including the protocol (http://) and hostname with port.

🔧 Debug

advanced

Why does the Express app receive incorrect client IP behind Nginx reverse proxy?

An Express app behind Nginx reverse proxy logs the client IP as 127.0.0.1 instead of the real user IP. What is the cause?

A Nginx is not forwarding the original client IP in the X-Forwarded-For header.

B Express app is not listening on the correct port.

C Nginx is caching the responses causing IP mismatch.

D Express app has a syntax error in the logging middleware.

💡 Hint

Think about how proxies pass client IP information.

Nginx must forward the original client IP using headers like X-Forwarded-For. Without this, Express sees the proxy's IP (localhost).

❓ state_output

advanced

What is the value of req.protocol in Express behind Nginx reverse proxy with HTTPS?

Nginx terminates HTTPS and proxies requests to Express over HTTP. In Express, what is the value of req.protocol by default?

A Throws an error due to protocol mismatch.

B "https" because the original request was HTTPS.

C "http" because Express receives plain HTTP from Nginx.

D Undefined because protocol is not set.

💡 Hint

Consider what Express actually receives from Nginx.

Express sees the protocol of the direct connection, which is HTTP from Nginx, so req.protocol is "http" unless trust proxy is enabled.

🧠 Conceptual

expert

How to correctly configure Express to trust Nginx reverse proxy for secure cookies?

You want to set secure cookies in Express behind Nginx reverse proxy that terminates HTTPS. Which Express setting ensures cookies are marked secure correctly?

A Set <code>app.set('trust proxy', false)</code> to disable proxy trust.

B Set <code>app.set('trust proxy', 1)</code> to trust the first proxy.

C Set <code>app.enable('trust proxy')</code> without arguments.

D No Express setting needed; Nginx handles cookies.

💡 Hint

Think about how Express determines if a request is secure behind a proxy.

Setting app.set('trust proxy', 1) tells Express to trust the first proxy and use forwarded headers to detect HTTPS, enabling secure cookies.