Adding response headers (add_header) in Nginx - Step-by-Step Execution

Process Flow - Adding response headers (add_header)

Start nginx config

↓

Read add_header directive

↓

Store header name and value

↓

On response generation

↓

Attach header to response

↓

Send response with added headers

↓

End

nginx reads the add_header directive in config, stores the header info, then adds it to each response before sending.

Execution Sample

Nginx

add_header X-Custom-Header "HelloWorld";

location = / {
    add_header Cache-Control "no-cache";
}

This config adds two headers: X-Custom-Header globally and Cache-Control only for / location.

Process Table

Step	Directive Read	Header Name	Header Value	Scope	Action
1	add_header X-Custom-Header "HelloWorld";	X-Custom-Header	HelloWorld	http (global)	Store header for all responses
2	add_header Cache-Control "no-cache";	Cache-Control	no-cache	location = /	Store header for / responses
3	Request to /	-	-	-	Attach X-Custom-Header and Cache-Control headers to response
4	Request to /about.html	-	-	-	Attach only X-Custom-Header header to response
5	Request to /api/data	-	-	-	Attach only X-Custom-Header header to response

💡 All requests processed; headers added according to scope.

Status Tracker

Variable	Start	After Step 1	After Step 2	Final
Headers Stored	{}	{"X-Custom-Header": "HelloWorld"}	{"X-Custom-Header": "HelloWorld", "Cache-Control": "no-cache" (location = /)}	{"X-Custom-Header": "HelloWorld" globally, "Cache-Control": "no-cache" for / location}

Key Moments - 2 Insights

Why does the Cache-Control header only appear on responses for the / location?

Because in step 2, the add_header directive for Cache-Control is inside the location = / block, so it applies only to requests matching that location, as shown in step 3 and 4 of the execution_table.

Does the global add_header directive apply to all requests?

Yes, the global add_header directive (step 1) applies to all responses unless overridden or supplemented by location-specific headers, as seen in steps 3, 4, and 5.

Visual Quiz - 3 Questions

Look at the execution_table, which headers are added to the response for a request to /api/data?

A Both X-Custom-Header and Cache-Control

B Cache-Control only

C X-Custom-Header only

D No headers added

💡 Hint

Check step 5 in the execution_table where /api/data request is handled.

At which step is the Cache-Control header stored for the / location?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look at the directive read column in the execution_table.

If you remove the global add_header directive, what happens to the X-Custom-Header in responses?

A It still appears on all responses

B It does not appear on any response

C It appears only on / location responses

D It appears only on /api/data responses

💡 Hint

Refer to variable_tracker and execution_table steps 1 and 3-5.

Concept Snapshot

add_header HEADER_NAME "VALUE";

- Adds a response header to HTTP replies.
- Can be set globally or inside location blocks.
- Location headers override or add to global headers.
- Headers appear in client responses accordingly.

Full Transcript

This visual execution shows how nginx processes add_header directives. First, it reads the global add_header directive and stores the header name and value. Then it reads location-specific add_header directives and stores those separately. When a request comes in, nginx attaches the stored headers to the response based on the request's location. For example, the global header X-Custom-Header is added to all responses, while Cache-Control is added only to responses for the / location. This step-by-step trace helps understand how headers are applied depending on configuration scope.