Proxy headers forwarding in Nginx - Step-by-Step Execution

Process Flow - Proxy headers forwarding

Client sends request

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Nginx receives request

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Nginx adds/forwards headers

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Nginx sends request to backend

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Backend receives request with headers

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Backend processes request

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Backend sends response

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Nginx forwards response to client

The client sends a request to Nginx, which adds or forwards headers before sending the request to the backend server. The backend receives these headers and processes the request accordingly.

Execution Sample

Nginx

location /api/ {
    proxy_pass http://backend;
    proxy_set_header Host $host;
    proxy_set_header X-Real-IP $remote_addr;
    proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
}

This Nginx config forwards the original Host, client IP, and X-Forwarded-For headers to the backend server.

Process Table

Step	Action	Header Added/Forwarded	Value	Effect
1	Client sends request	Host	example.com	Original client Host header
2	Nginx receives request	-	-	Nginx reads incoming headers
3	Nginx sets Host header	Host	$host (example.com)	Backend sees original Host
4	Nginx sets X-Real-IP header	X-Real-IP	$remote_addr (192.168.1.10)	Backend knows client IP
5	Nginx sets X-Forwarded-For header	X-Forwarded-For	$proxy_add_x_forwarded_for (192.168.1.10)	Backend tracks proxy chain
6	Nginx sends request to backend	All above headers	Forwarded	Backend receives headers
7	Backend processes request	-	-	Backend uses headers for logging/security
8	Backend sends response	-	-	Response sent back to Nginx
9	Nginx forwards response	-	-	Client receives backend response

💡 Request cycle completes after response is forwarded to client

Status Tracker

Variable	Start	After Step 3	After Step 4	After Step 5	Final
Host	example.com	example.com	example.com	example.com	example.com
X-Real-IP	-	-	192.168.1.10	192.168.1.10	192.168.1.10
X-Forwarded-For	-	-	-	192.168.1.10	192.168.1.10

Key Moments - 3 Insights

Why do we set the Host header explicitly in proxy_set_header?

Because the backend server often needs the original Host to respond correctly. Without setting it, the backend might see the proxy's hostname instead. See execution_table step 3.

What is the difference between X-Real-IP and X-Forwarded-For headers?

X-Real-IP carries the immediate client's IP, while X-Forwarded-For tracks the full chain of client IPs through proxies. See execution_table steps 4 and 5.

What happens if we don't forward these headers?

The backend won't know the original client IP or Host, which can cause logging, security, or routing issues. See execution_table step 7.

Visual Quiz - 3 Questions

Look at the execution table, what value does the X-Real-IP header have after step 4?

A example.com

B 192.168.1.10

C -

D proxy server IP

💡 Hint

Check the 'Header Added/Forwarded' and 'Value' columns at step 4 in execution_table.

At which step does Nginx add the X-Forwarded-For header?

A Step 3

B Step 4

C Step 5

D Step 6

💡 Hint

Look for the row where 'X-Forwarded-For' is set in the 'Header Added/Forwarded' column.

If we remove proxy_set_header Host, what will the backend likely see as Host?

A Nginx server hostname or IP

B Empty Host header

C Original client Host header

D Client IP address

💡 Hint

Refer to key_moments about why Host header is set explicitly and execution_table step 3.

Concept Snapshot

Nginx proxy headers forwarding:
- Use proxy_set_header to forward headers like Host, X-Real-IP, X-Forwarded-For
- Host header tells backend the original domain requested
- X-Real-IP passes client IP to backend
- X-Forwarded-For tracks client IP chain through proxies
- Proper headers help backend logging, security, and routing

Full Transcript

When a client sends a request to Nginx, Nginx receives it and adds or forwards important headers before sending the request to the backend server. These headers include Host, which tells the backend the original domain the client requested; X-Real-IP, which carries the client's IP address; and X-Forwarded-For, which tracks the chain of IP addresses if multiple proxies are involved. Nginx sets these headers using proxy_set_header directives. The backend receives these headers and can use them for logging, security checks, or routing decisions. Finally, the backend sends a response back through Nginx to the client. Forwarding these headers correctly ensures the backend knows who the client is and what domain was requested, avoiding issues with incorrect logging or routing.