proxy_pass directive in Nginx - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the effect of this nginx configuration snippet?

Given the following nginx server block, what will be the URL requested by nginx when a client requests http://example.com/app/page?

Nginx

location /app/ {
    proxy_pass http://backend/;
}

A http://backend/page

B http://backend/app/page

C http://backend/app/

D http://backend/

💡 Hint

Remember how nginx replaces the matching part of the URI when proxy_pass ends with a slash.

When proxy_pass URL ends with a slash, nginx replaces the matching location prefix with the path in proxy_pass. Here, /app/ is replaced by /, so /app/page becomes /page on the backend.

💻 Command Output

intermediate

What URL does nginx request with this configuration?

Given this nginx location block, what URL will nginx request from the backend when a client requests http://example.com/app/page?

Nginx

location /app/ {
    proxy_pass http://backend;
}

A http://backend/page

B http://backend/app/

C http://backend/app/page

D http://backend/

💡 Hint

Check if the proxy_pass URL ends with a slash or not.

When proxy_pass URL does not end with a slash, nginx appends the full original request URI to the backend URL. So /app/page is appended to http://backend, resulting in http://backend/app/page.

❓ Configuration

advanced

Which configuration correctly forwards requests to backend preserving the original URI path?

You want nginx to forward requests from /service/ to http://backend/ so that /service/api becomes http://backend/service/api. Which proxy_pass directive achieves this?

A

location /service/ {
    proxy_pass http://backend/service;
}

B

location /service/ {
    proxy_pass http://backend/;
}

C

location /service/ {
    proxy_pass http://backend/service/;
}

D

location /service/ {
    proxy_pass http://backend;
}

💡 Hint

Consider how proxy_pass appends or replaces the URI based on trailing slash presence.

Option D uses proxy_pass http://backend; without a trailing slash, so nginx appends the full original URI /service/api to backend, resulting in http://backend/service/api.

Option D replaces /service/ with /, so the backend URL becomes http://backend/api, which is not desired.

Options C and D add /service again, causing duplication.

❓ Troubleshoot

advanced

Why does this nginx config cause a 404 error on backend?

Given this config, clients get 404 errors from backend when requesting /app/page. Why?

Nginx

location /app/ {
    proxy_pass http://backend/app/;
}

A Because proxy_pass URL is missing a trailing slash, causing URI to be appended incorrectly

B Because proxy_pass replaces /app/ with /app/, causing the backend to receive /app/page which may not exist

C Because proxy_pass appends the full URI, resulting in /app/app/page sent to backend

D Because proxy_pass URL ends with a slash, nginx removes /app/ from URI, sending /page to backend

💡 Hint

Check how nginx combines location and proxy_pass paths when proxy_pass has a path with trailing slash.

When proxy_pass has a URI part with trailing slash, nginx replaces the matching location prefix with that URI part. Here, location is /app/ and proxy_pass is http://backend/app/. So /app/page becomes /app/page after replacement, but nginx appends the rest of the URI after the matched prefix, causing duplication: /app/app/page.

This causes backend to receive a wrong path, leading to 404.

✅ Best Practice

expert

What is the recommended way to preserve original Host header when using proxy_pass?

You want nginx to forward requests to backend but keep the original Host header from the client. Which configuration snippet achieves this correctly?

A

proxy_set_header Host $http_host;
proxy_pass http://backend;

B

proxy_set_header Host $proxy_host;
proxy_pass http://backend;

C

proxy_set_header Host $host;
proxy_pass http://backend;

D

proxy_set_header Host backend;
proxy_pass http://backend;

💡 Hint

Check which variable holds the original Host header sent by the client.

$http_host contains the original Host header from the client request. Setting proxy_set_header Host $http_host; preserves it when forwarding.

$host is the hostname of the server handling the request, which may differ.

$proxy_host is the hostname of the proxy server, not the client.

Setting Host to a fixed string like 'backend' breaks Host preservation.

Practice

1. What is the primary purpose of the proxy_pass directive in an Nginx configuration?

easy

A. To block incoming requests based on IP address

B. To serve static files directly from Nginx

C. To forward client requests to another server

D. To rewrite URLs within the same server

Solution

1. Step 1: Understand the role of proxy_pass

   The proxy_pass directive is used to send client requests received by Nginx to another server, often called a backend or upstream server.

2. Step 2: Compare with other options

   Serving static files is done by root or alias, blocking IPs uses deny, and URL rewriting uses rewrite. These are different from proxy_pass.

3. Final Answer:

   To forward client requests to another server -> Option C

4. Quick Check:

   proxy_pass forwards requests [OK]

Hint: Remember: proxy_pass sends requests to another server [OK]

Common Mistakes:

• Confusing proxy_pass with static file serving
• Thinking proxy_pass blocks requests
• Mixing proxy_pass with URL rewriting

2. Which of the following is the correct syntax to forward all requests from /app/ to http://backend.local/ using proxy_pass inside a location /app/ block?

easy

A. proxy_pass http://backend.local/;

B. proxy_pass http://backend.local/app/;

C. proxy_pass backend.local;

D. proxy_pass http://backend.local

Solution

1. Step 1: Understand proxy_pass URL format

   The URL in proxy_pass must include the scheme (http://) and a trailing slash if you want to replace the matching part of the request URI.

2. Step 2: Analyze options

   proxy_pass http://backend.local/; uses http://backend.local/ with trailing slash, which correctly forwards requests replacing /app/ with /. proxy_pass http://backend.local/app/; appends /app/ again, causing duplication. proxy_pass backend.local; lacks scheme, invalid syntax. proxy_pass http://backend.local misses trailing slash, causing different URI handling.

3. Final Answer:

   proxy_pass http://backend.local/; -> Option A

4. Quick Check:

   Correct proxy_pass URL includes scheme and trailing slash [OK]

Hint: Always include scheme and trailing slash for correct URI mapping [OK]

Common Mistakes:

• Omitting http:// scheme
• Missing trailing slash causing URI errors
• Appending location path twice

3. Given this Nginx config snippet:

location /api/ {
    proxy_pass http://backend.local/;
}

If a client requests /api/users, what will be the proxied request URI sent to http://backend.local?

medium

A. /api/users

B. /users

C. /

D. /api/

Solution

1. Step 1: Understand URI replacement with trailing slash

   When proxy_pass URL ends with a slash, Nginx replaces the matching part of the request URI (/api/) with the path in proxy_pass (which is / here).

2. Step 2: Apply to example

   Request URI /api/users has /api/ replaced by /, resulting in /users sent to backend.

3. Final Answer:

   /users -> Option B

4. Quick Check:

   Trailing slash in proxy_pass replaces matching location path [OK]

Hint: Trailing slash means replace matching URI part [OK]

Common Mistakes:

• Assuming original URI is sent unchanged
• Confusing with proxy_pass without trailing slash
• Thinking /api/ prefix remains in proxied URI

4. You have this Nginx config:

location /service/ {
    proxy_pass http://backend.local/service/;
}

Clients report 404 errors from backend for requests like /service/data. What is the likely cause?

medium

A. proxy_pass URL duplicates the /service/ path causing wrong backend URI

B. Missing trailing slash in proxy_pass URL

C. Backend server is down

D. Nginx location block syntax error

Solution

1. Step 1: Analyze proxy_pass with matching location

   When proxy_pass URL includes the same path as location, Nginx appends the full request URI after it, causing duplication.

2. Step 2: Understand effect on backend URI

   Request /service/data becomes /service/service/data at backend, likely causing 404 errors.

3. Final Answer:

   proxy_pass URL duplicates the /service/ path causing wrong backend URI -> Option A

4. Quick Check:

   Duplicated path in proxy_pass causes 404 [OK]

Hint: Avoid repeating location path in proxy_pass URL [OK]

Common Mistakes:

• Ignoring duplicated path in proxy_pass
• Assuming backend down without checking URI
• Confusing missing slash with duplicated path

5. You want to forward requests from /app/ to http://backend.local/ but keep the original /app/ prefix in the proxied URI. Which proxy_pass configuration achieves this?

hard

A. proxy_pass http://backend.local/;

B. proxy_pass http://backend.local/app/

C. proxy_pass http://backend.local/app;

D. proxy_pass http://backend.local;

Solution

1. Step 1: Understand proxy_pass without trailing slash

   If proxy_pass URL has no trailing slash, Nginx appends the full original request URI after the host.

2. Step 2: Apply to example

   Using proxy_pass http://backend.local; inside location /app/ sends requests like /app/data as http://backend.local/app/data, preserving the prefix.

3. Final Answer:

   proxy_pass http://backend.local; -> Option D

4. Quick Check:

   No trailing slash keeps original URI prefix [OK]

Hint: No trailing slash keeps full original URI [OK]

Common Mistakes:

• Adding trailing slash removes original prefix
• Appending /app/ twice causing duplication
• Missing scheme in proxy_pass URL