proxy_pass directive in Nginx - Step-by-Step Execution

Process Flow - proxy_pass directive

Client sends HTTP request

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Nginx receives request

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Check location block with proxy_pass

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Rewrite request URL if needed

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Forward request to backend server

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Backend server processes request

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Backend sends response to Nginx

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Nginx sends response back to client

The proxy_pass directive tells Nginx to forward client requests to another server and then return the response back to the client.

Execution Sample

Nginx

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

This config forwards requests starting with /app/ to the backend_server, replacing the /app/ prefix with /.

Process Table

Step	Client Request URL	Location Match	proxy_pass Target	Rewritten URL	Action
1	/app/page1	/app/	http://backend_server/	/page1	Forward to http://backend_server/page1
2	/app/images/logo.png	/app/	http://backend_server/	/images/logo.png	Forward to http://backend_server/images/logo.png
3	/app	/app/	http://backend_server/	/	Forward to http://backend_server/
4	/other	No match	N/A	N/A	Serve locally or other rules
5	N/A	N/A	N/A	N/A	End of processing

💡 Requests not matching /app/ are handled by other rules or served locally.

Status Tracker

Variable	Start	After 1	After 2	After 3	Final
Client Request URL	/app/page1	/app/images/logo.png	/app	/other	/other
Location Match	Yes	Yes	Yes	No	No
proxy_pass Target	http://backend_server/	http://backend_server/	http://backend_server/	N/A	N/A
Rewritten URL	/page1	/images/logo.png	/	N/A	N/A

Key Moments - 3 Insights

Why does the URL /app/page1 become /page1 when forwarded?

Because proxy_pass with a trailing slash replaces the matching location prefix (/app/) with the slash in proxy_pass (/) as shown in execution_table rows 1 and 2.

What happens if the client requests /app without a trailing slash?

Nginx treats /app as matching /app/ location and rewrites it to /, forwarding to backend root as in execution_table row 3.

Why are requests like /other not forwarded to backend_server?

Because they do not match the location /app/ block, so proxy_pass is not applied, as shown in execution_table row 4.

Visual Quiz - 3 Questions

Look at the execution table, what is the rewritten URL for client request /app/images/logo.png?

A /logo.png

B /app/images/logo.png

C /images/logo.png

D /app/

💡 Hint

Check execution_table row 2 under 'Rewritten URL' column.

At which step does the client request not match the /app/ location?

A Step 4

B Step 3

C Step 1

D Step 2

💡 Hint

Look at execution_table row 4 under 'Location Match' column.

If proxy_pass was set without a trailing slash like 'proxy_pass http://backend_server', how would the rewritten URL for /app/page1 change?

A /page1

B /app/page1

C /backend_server/page1

D /

💡 Hint

Recall that without trailing slash proxy_pass appends full original URI after the backend URL.

Concept Snapshot

proxy_pass forwards client requests to another server.
If proxy_pass URL ends with /, Nginx replaces matching location prefix with that /.
If no trailing /, Nginx appends full original URI.
Requests not matching location block are not proxied.
Used to create reverse proxy setups easily.

Full Transcript

The proxy_pass directive in Nginx forwards incoming client requests to a backend server. When a client sends a request matching a location block with proxy_pass, Nginx rewrites the URL by replacing the matching prefix with the proxy_pass URL if it ends with a slash. Then it sends the request to the backend server. The backend processes it and returns a response, which Nginx sends back to the client. Requests not matching the location block are handled by other rules or served locally. This allows Nginx to act as a reverse proxy, forwarding requests transparently.