Proxy headers in Nginx - Step-by-Step Execution

Process Flow - Proxy headers

Client sends request

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Nginx receives request

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Nginx adds/updates proxy headers

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Nginx forwards request to backend server

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Backend server receives request with headers

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Backend processes request and responds

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Nginx sends response back to client

The flow shows how Nginx receives a client request, adds or modifies proxy headers, then forwards it to the backend server, which uses these headers for processing.

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 requests to /api/ to a backend server, adding headers that tell the backend the original host and client IP.

Process Table

Step	Action	Header Set	Value	Effect
1	Receive client request	None	None	Nginx gets request from client
2	Set Host header	Host	$host (example.com)	Backend knows original host
3	Set X-Real-IP header	X-Real-IP	$remote_addr (192.0.2.1)	Backend knows client IP
4	Set X-Forwarded-For header	X-Forwarded-For	$proxy_add_x_forwarded_for (192.0.2.1)	Tracks client IP chain
5	Forward request	All above headers	Sent to backend	Backend receives headers
6	Backend processes request	Reads headers	Uses for logging/security	Responds accordingly
7	Nginx sends response	None	None	Client receives backend response

💡 Request fully proxied with headers set, backend processed and responded

Status Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
Host	unset	example.com	example.com	example.com	example.com
X-Real-IP	unset	unset	192.0.2.1	192.0.2.1	192.0.2.1
X-Forwarded-For	unset	unset	unset	192.0.2.1	192.0.2.1

Key Moments - 3 Insights

Why do we set the Host header explicitly in proxy headers?

Because the backend server needs to know the original host requested by the client. Without setting Host, the backend might see the proxy's hostname instead, which can cause wrong responses. See execution_table step 2.

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

X-Real-IP carries the immediate client's IP address, while X-Forwarded-For can hold a chain of IPs if multiple proxies are involved. This helps backend track the original client IP. See execution_table steps 3 and 4.

What happens if proxy headers are not set correctly?

The backend may not know the real client IP or host, which can break logging, security checks, or routing. The request might still work but with incomplete info. See execution_table steps 5 and 6.

Visual Quiz - 3 Questions

Look at the execution_table, what value does the X-Real-IP header have after step 3?

A 192.0.2.1

B example.com

C unset

D proxy server IP

💡 Hint

Check the 'Header Set' and 'Value' columns at step 3 in execution_table.

At which step does Nginx forward the request to the backend with all proxy headers set?

A Step 2

B Step 4

C Step 5

D Step 6

💡 Hint

Look for the step where the action is 'Forward request' in execution_table.

If the X-Forwarded-For header was not set, what would change in the variable_tracker?

A X-Real-IP would be unset

B X-Forwarded-For would remain unset after all steps

C Host would be unset

D No variables would change

💡 Hint

Check the variable_tracker row for X-Forwarded-For and imagine skipping step 4.

Concept Snapshot

Proxy headers in Nginx add info about the original client request when forwarding.
Use proxy_set_header to set Host, X-Real-IP, and X-Forwarded-For.
Host tells backend the original domain.
X-Real-IP gives backend the client IP.
X-Forwarded-For tracks client IP chain through proxies.
Proper headers help backend logging and security.

Full Transcript

This visual execution shows how Nginx handles proxy headers. When a client sends a request, Nginx receives it and sets headers like Host, X-Real-IP, and X-Forwarded-For before forwarding to the backend server. The Host header tells the backend the original domain requested. X-Real-IP carries the client's IP address. X-Forwarded-For tracks the chain of client IPs if multiple proxies are involved. The backend uses these headers for logging and security. If headers are missing or incorrect, the backend may not get accurate client info. The execution table traces each step, showing header values and effects. The variable tracker shows how header values change step-by-step. Key moments clarify why each header matters. The quiz tests understanding of header values and flow. This helps beginners see how proxy headers work in Nginx.