javascript - How does Access-Control-Allow-Origin header work?

Apparently, I have completely misunderstood its semantics. I thought of something like this:

1. A client downloads javascript code MyCode.js from http://siteA - the origin.


2. The response header of MyCode.js contains Access-Control-Allow-Origin: http://siteB, which I thought meant that MyCode.js was allowed to make cross-origin references to the site B.


3. The client triggers some functionality of MyCode.js, which in turn make requests to http://siteB, which should be fine, despite being cross-origin requests.

Well, I am wrong. It does not work like this at all. So, I have read Cross-origin resource sharing and attempted to read Cross-Origin Resource Sharing in w3c recommendation

One thing is sure - I still do not understand how am I supposed to use this header.

I have full control of both site A and site B. How do I enable the javascript code downloaded from the site A to access resources on the site B using this header?

P.S.

I do not want to utilize JSONP.

Answer

Access-Control-Allow-Origin is a CORS (Cross-Origin Resource Sharing) header.

When Site A tries to fetch content from Site B, Site B can send an Access-Control-Allow-Origin response header to tell the browser that the content of this page is accessible to certain origins. (An origin is a domain, plus a scheme and port number.) By default, Site B's pages are not accessible to any other origin; using the Access-Control-Allow-Origin header opens a door for cross-origin access by specific requesting origins.

For each resource/page that Site B wants to make accessible to Site A, Site B should serve its pages with the response header:

Access-Control-Allow-Origin: http://siteA.com

Modern browsers will not block cross-domain requests outright. If Site A requests a page from Site B, the browser will actually fetch the requested page on the network level and check if the response headers list Site A as a permitted requester domain. If Site B has not indicated that Site A is allowed to access this page, the browser will trigger the XMLHttpRequest's error event and deny the response data to the requesting JavaScript code.

What happens on the network level can be slightly more complex than explained above. If the request is a "non-simple" request, the browser first sends a data-less "preflight" OPTIONS request, to verify that the server will accept the request. A request is non-simple when either (or both):

• using an HTTP verb other than GET or POST (e.g. PUT, DELETE)


• using non-simple request headers; the only simple requests headers are:
  
  
  
  
  • Accept
  
  
  • Accept-Language
  
  
  
  • Content-Language
  
  
  • Content-Type (this is only simple when its value is application/x-www-form-urlencoded, multipart/form-data, or text/plain)
  
  

If the server responds to the OPTIONS preflight with appropriate response headers (Access-Control-Allow-Headers for non-simple headers, Access-Control-Allow-Methods for non-simple verbs) that match the non-simple verb and/or non-simple headers, then the browser sends the actual request.

Supposing that Site A wants to send a PUT request for /somePage, with a non-simple Content-Type value of application/json, the browser would first send a preflight request:

OPTIONS /somePage HTTP/1.1

Origin: http://siteA.com
Access-Control-Request-Method: PUT
Access-Control-Request-Headers: Content-Type

Note that Access-Control-Request-Method and Access-Control-Request-Headers are added by the browser automatically; you do not need to add them. This OPTIONS preflight gets the successful response headers:

Access-Control-Allow-Origin: http://siteA.com
Access-Control-Allow-Methods: GET, POST, PUT
Access-Control-Allow-Headers: Content-Type

When sending the actual request (after preflight is done), the behavior is identical to how a simple request is handled. In other words, a non-simple request whose preflight is successful is treated the same as a simple request (i.e., the server must still send Access-Control-Allow-Origin again for the actual response).

The browsers sends the actual request:

PUT /somePage HTTP/1.1
Origin: http://siteA.com
Content-Type: application/json


{ "myRequestContent": "JSON is so great" }

And the server sends back an Access-Control-Allow-Origin, just as it would for a simple request:

Access-Control-Allow-Origin: http://siteA.com

See Understanding XMLHttpRequest over CORS for a little more information about non-simple requests.