addheaderredefinition.md

[add_header_redefinition] Redefining of response headers by "add_header" directive

Unfortunately, many people don't know how the inheritance of directives works. Most often this leads to misuse of the add_header directive while trying to add a new response header on the nested level. This feature is mentioned in Nginx docs:

There could be several add_header directives. These directives are inherited from the previous level if and only if there are no add_header directives defined on the current level.

The logic is quite simple: if you set headers at one level (for example, in server section) and then at a lower level (let's say location) you set some other headers, then the first headers will discarded.

It's easy to check:

• Configuration:

server {
  listen 80;
  add_header X-Frame-Options "DENY" always;
  location / {
      return 200 "index";
  }

  location /new-headers {
    # Add special cache control
    add_header Cache-Control "no-cache, no-store, max-age=0, must-revalidate" always;
    add_header Pragma "no-cache" always;

    return 200 "new-headers";
  }
}

• Request to location / (X-Frame-Options header is in server response):

GET / HTTP/1.0

HTTP/1.1 200 OK
Server: nginx/1.10.2
Date: Mon, 09 Jan 2017 19:28:33 GMT
Content-Type: application/octet-stream
Content-Length: 5
Connection: close
X-Frame-Options: DENY

index

• Request to location /new-headers (headers Cache-Control and Pragma are present, but there's no X-Frame-Options):

GET /new-headers HTTP/1.0


HTTP/1.1 200 OK
Server: nginx/1.10.2
Date: Mon, 09 Jan 2017 19:29:46 GMT
Content-Type: application/octet-stream
Content-Length: 11
Connection: close
Cache-Control: no-cache, no-store, max-age=0, must-revalidate
Pragma: no-cache

new-headers

What can I do?

There are several ways to solve this problem:

• duplicate important headers;
• set all headers at one level (server section is a good choice)
• use ngx_headers_more module.