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.