proxy_pass directive in Nginx - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the proxy_pass directive in nginx?

The proxy_pass directive tells nginx to forward client requests to another server or service, acting as a middleman to pass requests and responses.

beginner

How do you use proxy_pass to forward requests to http://localhost:8080?

Inside an nginx location block, write:

proxy_pass http://localhost:8080;

This sends matching requests to the server running on port 8080 on the same machine.

intermediate

What happens if you omit the trailing slash in proxy_pass URL?

If the URL in proxy_pass does not end with a slash, nginx replaces the matching part of the request URI with the URL path. This can change the request path sent to the backend.

beginner

Can proxy_pass be used to forward requests to an HTTPS backend?

Yes, you can use proxy_pass https://backend-server; to forward requests securely to an HTTPS backend server.

intermediate

What directive is commonly used with proxy_pass to set the original host header?

The directive proxy_set_header Host $host; is often used with proxy_pass to pass the original host header to the backend server.

What does the proxy_pass directive do in nginx?

A Forwards client requests to another server

B Blocks client requests

C Caches static files

D Starts the nginx server

proxy_pass forwards requests to another server or service, acting as a reverse proxy.

Which of the following is a correct proxy_pass usage to forward to http://example.com?

A proxy_pass http://example.com;

B proxy_pass example.com;

C proxy_pass https://example.com

D proxy_pass //example.com;

The URL must include the scheme like http:// or https://.

What is the effect of adding a trailing slash to the URL in proxy_pass?

A It disables proxying

B It changes how the request URI is forwarded

C It enables caching

D It restarts nginx

A trailing slash changes how nginx modifies the request URI when forwarding.

Can proxy_pass forward requests to an HTTPS backend?

A Only if SSL is disabled

B No, only HTTP backends are supported

C Yes, by specifying <code>https://</code> in the URL

D Only with extra modules

Using https:// in the proxy_pass URL forwards requests securely.

Which directive is used to pass the original host header to the backend when using proxy_pass?

A proxy_cache on;

B proxy_redirect off;

C proxy_buffering off;

D proxy_set_header Host $host;

proxy_set_header Host $host; passes the original host header to the backend server.

Explain how the proxy_pass directive works in nginx and why it is useful.

Think about nginx acting like a middleman between clients and backend servers.

Describe the difference in behavior when using a trailing slash versus no trailing slash in the proxy_pass URL.

Focus on how nginx changes the request path sent to the backend.

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