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

Challenge - 5 Problems

🧠 Conceptual

intermediate

What is the main role of Nginx when used as a reverse proxy for a Flask app?

Imagine you have a Flask app running on port 5000. You want users to access it via port 80 using Nginx. What does Nginx do in this setup?

A Nginx forwards incoming requests on port 80 to the Flask app on port 5000 and sends back the Flask app's response to the client.

B Nginx replaces the Flask app and handles all Python code execution.

C Nginx runs the Flask app internally and serves static files only.

D Nginx blocks all requests except those from localhost.

💡 Hint

Think about how Nginx acts as a middleman between the user and the Flask app.

Nginx listens on port 80 and forwards requests to the Flask app running on port 5000. It then sends the Flask app's response back to the user, acting as a reverse proxy.

❓ component_behavior

intermediate

What happens if Nginx is not configured to forward the Host header to the Flask app?

You configured Nginx as a reverse proxy but forgot to pass the Host header. How does this affect your Flask app?

A The Flask app will crash immediately with a server error.

B The Flask app may receive incorrect Host information, causing URL generation or routing issues.

C Nginx will automatically add the Host header, so nothing changes.

D The Flask app will ignore the Host header and work normally.

💡 Hint

Consider how Flask uses the Host header for URL building and routing.

If Nginx does not forward the Host header, Flask may generate URLs with wrong hosts or fail to route requests properly because it relies on this header.

📝 Syntax

advanced

Which Nginx configuration snippet correctly sets up a reverse proxy to a Flask app on localhost:5000?

Choose the Nginx server block that properly proxies requests to a Flask app running on port 5000.

Flask

server {
    listen 80;
    server_name example.com;

    location / {
        proxy_pass http://localhost:5000;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header X-Forwarded-Proto $scheme;
    }
}

A

location / {
    proxy_pass http://localhost:5000;
    proxy_set_header Host $host;
    proxy_set_header X-Real-IP $remote_addr;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    proxy_set_header X-Forwarded-Proto $scheme;
}

B

location / {
    proxy_pass http://localhost:5000/
    proxy_set_header Host $host;
    proxy_set_header X-Real-IP $remote_addr;
}

C

location / {
    proxy_pass http://localhost:5000
    proxy_set_header Host $host
}

D

location / {
    proxy_pass http://localhost:5000;
    proxy_set_header Host;
    proxy_set_header X-Real-IP;
}

💡 Hint

Look for correct syntax including semicolons and proper proxy headers.

Option A correctly sets proxy_pass with a semicolon and includes necessary headers with proper syntax. Other options miss semicolons or headers.

❓ state_output

advanced

What is the visible effect if Nginx reverse proxy is misconfigured with an incorrect upstream address?

You set proxy_pass to http://localhost:6000 but your Flask app runs on port 5000. What will a user see when accessing your site?

A Nginx serves a default welcome page instead of the Flask app.

B The Flask app loads normally without any issues.

C The browser shows a 502 Bad Gateway error because Nginx cannot connect to the Flask app.

D The browser shows a 404 Not Found error from Flask.

💡 Hint

Think about what happens when Nginx tries to forward requests to a non-listening port.

If Nginx cannot reach the upstream server, it returns a 502 Bad Gateway error to the client indicating a bad connection.

🔧 Debug

expert

Why does your Flask app behind Nginx reverse proxy always see the client IP as 127.0.0.1?

After setting up Nginx as a reverse proxy, your Flask app logs show all client IPs as 127.0.0.1. What is the cause?

A Nginx is blocking the client IP and replacing it with 127.0.0.1 for security.

B Flask is ignoring all headers and always uses 127.0.0.1 by default.

C The client is actually connecting from localhost, so IP is correct.

D Nginx is not forwarding the original client IP in the X-Real-IP or X-Forwarded-For headers, so Flask sees the proxy's IP.

💡 Hint

Check how Nginx passes client IP information to backend apps.

By default, Nginx proxies requests from itself, so Flask sees Nginx's IP (127.0.0.1). To get real client IP, Nginx must forward headers like X-Real-IP and Flask must read them.