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.

Non-simple requests

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.