I'm trying to serve a maintenance page to clients making request to my application when it is under maintenance. Following is my nginx configuration for that purpose.

server {
  recursive_error_pages on;
  listen 80;
   ...

  if (-f $document_root/maintenance.html) {
    return 503;
  }

  error_page 404 /404.html;
  error_page 500 502 504 /500.html;
  error_page 503 @503;

  location = /404.html {
    root $document_root;
  }
  location = /500.html {
    root $document_root;
  }
  location @503 {
    error_page 405 =/maintenance.html;
    if (-f $request_filename) {
      break;
    }
    rewrite ^(.*)$ /maintenance.html break;
  }
}

Lets say I have enabled maintenance of my site by creating a $document_root/maintenance.html. This file, correctly, is served when a user makes a request with with Accept header of text/html.

$ curl http://server.com/ -i -v -X GET -H "Accept: text/html"
* Adding handle: conn: 0xf89420
* Adding handle: send: 0
* Adding handle: recv: 0
* Curl_addHandleToPipeline: length: 1
* - Conn 0 (0xf89420) send_pipe: 1, recv_pipe: 0
* About to connect() to server.com port 80 (#0)
*   Trying xxx.xxx.xxx.xxx...
* Connected to server.com (xxx.xxx.xxx.xxx) port 80 (#0)
> GET / HTTP/1.1
> User-Agent: curl/7.33.0
> Host: server.com
> Accept: text/html
> 
< HTTP/1.1 503 Service Temporarily Unavailable
HTTP/1.1 503 Service Temporarily Unavailable
* Server nginx/1.1.19 is not blacklisted
< Server: nginx/1.1.19
Server: nginx/1.1.19
< Date: Thu, 14 Nov 2013 11:16:16 GMT
Date: Thu, 14 Nov 2013 11:16:16 GMT
< Content-Type: text/html
Content-Type: text/html
< Content-Length: 27
Content-Length: 27
< Connection: keep-alive
Connection: keep-alive

< 
This is under maintenance.
* Connection #0 to host server.com left intact

Now some clients set Accept header to application/json. How do I send them a JSON response instead of maintenance.html?

Following is the response that I get when setting Accept to application/json.

$ curl http://server.com/ -i -v -X GET -H "Accept: application/json"
* Adding handle: conn: 0x190c430
* Adding handle: send: 0
* Adding handle: recv: 0
* Curl_addHandleToPipeline: length: 1
* - Conn 0 (0x190c430) send_pipe: 1, recv_pipe: 0
* About to connect() to server.com port 80 (#0)
*   Trying xxx.xxx.xxx.xxx...
* Connected to server.com (xxx.xxx.xxx.xxx) port 80 (#0)
> GET / HTTP/1.1
> User-Agent: curl/7.33.0
> Host: server.com
> Accept: application/json
> 
< HTTP/1.1 503 Service Temporarily Unavailable
HTTP/1.1 503 Service Temporarily Unavailable
* Server nginx/1.1.19 is not blacklisted
< Server: nginx/1.1.19
Server: nginx/1.1.19
< Date: Thu, 14 Nov 2013 11:15:50 GMT
Date: Thu, 14 Nov 2013 11:15:50 GMT
< Content-Type: text/html
Content-Type: text/html
< Content-Length: 27
Content-Length: 27
< Connection: keep-alive
Connection: keep-alive

< 
This is under maintenance.
* Connection #0 to host server.com left intact