Nginx as reverse proxy in Flask - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a simple Flask app that returns 'Hello World'.

Flask

from flask import Flask
app = Flask(__name__)

@app.route('/')
def home():
    return [1]

Drag options to blanks, or click blank then click option'

Aprint('Hello World')

BHello World

C'Hello World'

Dreturn 'Hello World'

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting quotes around the string

Using print instead of return

The Flask route function must return a string. Using quotes around 'Hello World' returns the string correctly.

2fill in blank

medium

Complete the Nginx server block to listen on port 80.

Flask

server {
    listen [1];
    server_name example.com;

    location / {
        proxy_pass http://127.0.0.1:5000;
    }
}

Drag options to blanks, or click blank then click option'

A80

B443 ssl

C8080

D5000

Attempts:

3 left

💡 Hint

Common Mistakes

Using HTTPS port 443 without SSL configuration

Using Flask app port 5000 instead of Nginx port

Port 80 is the default HTTP port that Nginx listens on for web traffic.

3fill in blank

hard

Fix the error in the proxy_pass directive to correctly forward requests to the Flask app.

Flask

location / {
    proxy_pass [1];
}

Drag options to blanks, or click blank then click option'

Ahttp://127.0.0.1:80

Blocalhost:5000

Chttp://localhost:5000/

Dhttp://localhost:5000

Attempts:

3 left

💡 Hint

Common Mistakes

Adding trailing slash in proxy_pass

Missing http:// in URL

Using wrong port

The proxy_pass URL should not have a trailing slash to correctly forward requests to the Flask app.

4fill in blank

hard

Fill both blanks to set headers for the reverse proxy to pass the original host and client IP.

Flask

location / {
    proxy_set_header [1] $host;
    proxy_set_header [2] $remote_addr;
}

Drag options to blanks, or click blank then click option'

AHost

BX-Real-IP

CX-Forwarded-For

DServer

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Server' header incorrectly

Confusing 'X-Forwarded-For' with 'X-Real-IP'

Setting 'Host' passes the original host header, and 'X-Real-IP' passes the client IP to the Flask app.

5fill in blank

hard

Fill all three blanks to complete the Nginx config for proxy buffering and timeouts.

Flask

location / {
    proxy_buffering [1];
    proxy_connect_timeout [2]s;
    proxy_read_timeout [3]s;
}

Drag options to blanks, or click blank then click option'

Aon

B30

C60

Doff

Attempts:

3 left

💡 Hint

Common Mistakes

Setting proxy_buffering to 'on' when real-time is needed

Using wrong units for timeouts

Disabling proxy_buffering with 'off' helps with real-time responses. The connect timeout is 30 seconds, and read timeout is 60 seconds.

Practice

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

easy

A. Compile Flask Python code into machine code

B. Run the Flask application code directly

C. Forward client requests to the Flask app server

D. Store the Flask app's database

Solution

1. Step 1: Understand Nginx's role

   Nginx acts as a middleman that receives client requests first.

2. Step 2: Forward requests to Flask app

   Nginx sends these requests to the Flask app running on a backend server or port.

3. Final Answer:

   Forward client requests to the Flask app server -> Option C

4. Quick Check:

   Nginx forwards requests = D [OK]

Hint: Nginx passes requests, it does not run Flask code [OK]

Common Mistakes:

• Thinking Nginx runs Flask code
• Confusing Nginx with database storage
• Believing Nginx compiles Python code

2. Which Nginx directive is used to specify the Flask app server URL in the reverse proxy configuration?

easy

A. proxy_pass

B. server_name

C. listen

D. root

Solution

1. Step 1: Identify directive for backend URL

   The directive that tells Nginx where to send requests is proxy_pass.

2. Step 2: Confirm other directives' roles

   server_name sets domain, listen sets port, root sets file path.

3. Final Answer:

   proxy_pass -> Option A

4. Quick Check:

   Backend URL = proxy_pass [OK]

Hint: proxy_pass points to Flask app URL in Nginx config [OK]

Common Mistakes:

• Using server_name instead of proxy_pass for backend URL
• Confusing listen with proxy_pass
• Setting root for proxy backend

3. Given this Nginx snippet, what happens when a client requests http://example.com/api/data?

location /api/ {
    proxy_pass http://127.0.0.1:5000/;
}

medium

A. Request is served from Nginx's local files

B. Request is forwarded to http://127.0.0.1:5000/data

C. Request results in 404 error

D. Request is forwarded to http://127.0.0.1:5000/api/data

Solution

1. Step 1: Understand proxy_pass with trailing slash

   Since proxy_pass ends with a slash, Nginx appends the full request URI after the location prefix.

2. Step 2: Combine location and proxy_pass

   Request /api/data is forwarded as http://127.0.0.1:5000/api/data.

3. Final Answer:

   Request is forwarded to http://127.0.0.1:5000/api/data -> Option D

4. Quick Check:

   Trailing slash keeps full path = C [OK]

Hint: Trailing slash in proxy_pass keeps full path [OK]

Common Mistakes:

• Removing /api/ from forwarded URL incorrectly
• Assuming Nginx serves files locally
• Expecting 404 without backend

4. You configured Nginx as below but get a 502 Bad Gateway error. What is the likely cause?

location / {
    proxy_pass http://127.0.0.1:8000;
}

Your Flask app runs on port 5000.

medium

A. Nginx is forwarding to wrong port 8000 instead of 5000

B. proxy_pass missing trailing slash causes error

C. Flask app must run on port 8000 for Nginx

D. Nginx cannot proxy to localhost

Solution

1. Step 1: Check proxy_pass target port

   proxy_pass points to port 8000, but Flask runs on 5000, so no backend listens on 8000.

2. Step 2: Understand 502 Bad Gateway cause

   502 means Nginx can't connect to backend server at specified address and port.

3. Final Answer:

   Nginx is forwarding to wrong port 8000 instead of 5000 -> Option A

4. Quick Check:

   Port mismatch causes 502 error = A [OK]

Hint: Match proxy_pass port to Flask app port exactly [OK]

Common Mistakes:

• Blaming missing trailing slash for 502
• Thinking Flask must run on 8000
• Assuming Nginx can't proxy localhost

5. You want Nginx to forward requests from /app/ to your Flask app at http://127.0.0.1:5000/, but remove the /app prefix before forwarding. Which configuration achieves this?

hard

A. location /app/ {
proxy_pass http://127.0.0.1:5000/app/;
}

B. location /app/ {
proxy_pass http://127.0.0.1:5000;
}

C. location /app/ {
proxy_pass http://127.0.0.1:5000/;
}

D. location /app {
proxy_pass http://127.0.0.1:5000/;
}

Solution

1. Step 1: Understand proxy_pass without trailing slash

   When proxy_pass URL has no trailing slash, Nginx replaces the matching location prefix with the proxy_pass path.

2. Step 2: Apply to /app/ location

   With location /app/ and proxy_pass http://127.0.0.1:5000, the /app/ prefix is removed before forwarding.

3. Step 3: Check other options

   The configuration with proxy_pass http://127.0.0.1:5000/; keeps /app/ prefix, the one with proxy_pass http://127.0.0.1:5000/app/; adds /app/ twice, and location without trailing slash causes mismatch.

4. Final Answer:

   location /app/ { proxy_pass http://127.0.0.1:5000; } -> Option B

5. Quick Check:

   proxy_pass without slash removes location prefix = A [OK]

Hint: No trailing slash on proxy_pass removes location prefix [OK]

Common Mistakes:

• Adding trailing slash keeps prefix in forwarded URL
• Using location without trailing slash causes mismatch
• Duplicating /app/ in proxy_pass URL