How can I add header conditionally in nginx config

Question

How can I add header conditionally in nginx(1.4.6 ubuntu) configuration?

At first I tried like this.

 location / {
      add_header X-TEST-0 always-0;

      set $true 1;
      if ($true) {
          add_header X-TEST-1 only-true;
      }

      add_header X-TEST-2 always-2;
  }    

Although I expected my nginx set all header(X-TEST-0,1,2), it added header only X-TEST-1.

Next I tried like this.

 location / {
      add_header X-TEST-0 always-0;

      ##set $true 1;
      ##if ($true) {
      ##    add_header X-TEST-1 only-true;
      ##}

      add_header X-TEST-2 always-2;
  }    

My nginx added header X-TEST-1 and X-TEST-2 as I expected.

My quastions are...

1. How can I add header conditionally?

2. Why does nginx add only X-TEST-1 with my first example above?

Thanks in advance!

Answer 1

(I would write this conclusion for the people who will have the same question in the future. Thanks to @Alexey Ten for the information.)

Conclusion:

Don't use any non-rewrite directives inside if directives. Because if directive is essentially part of rewrite module.

We could get more details about it in official document named IfIsEvil.

Answer 2

I used lua nginx module to solve a similar problem:

localtion / {
  header_filter_by_lua_block {
    ngx.header["X-TEST-0"] = "always-0";
    # example of conditional header
    if ngx.req.get_method() == ngx.HTTP_POST {
      ngx.header["X-TEST-1"] = "only if POST request";
    }
    ngx.header["X-TEST-2"] = "always-2";
  }
}    

https://github.com/openresty/lua-nginx-module#header_filter_by_lua_block ```

Answer 3

This is simple if you think differently and use a map.

map $variable  $headervalue {
    1        only-true;
    default  '';
}

# ...later...
location / {
    add_header X-TEST-1   $headervalue;
}

If $variable is 1, $headervalue will be set and hence the header will be added. By default $headervalue will be empty and the header will not be added.

This is efficient because Nginx only evaluates the map when it's needed (when it gets to a point where $headervalue is referenced).

See similar: https://serverfault.com/questions/598085/nginx-add-header-conditional-on-an-upstream-http-variable