Apparently, I have completely misunderstood its semantics. I thought of something like this:
- A client downloads JavaScript code MyCode.js from
http://siteA
– the origin. - 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. - 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 I am 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.
Best 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:
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
'serror
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:
Accept
;Accept-Language
;Content-Language
;Content-Type
(this is only simple when its value isapplication/x-www-form-urlencoded
,multipart/form-data
, ortext/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-simpleContent-Type
value ofapplication/json
, the browser would first send a preflight request:Note that
Access-Control-Request-Method
andAccess-Control-Request-Headers
are added by the browser automatically; you do not need to add them. This OPTIONS preflight gets the successful response headers: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:
And the server sends back an
Access-Control-Allow-Origin
, just as it would for a simple request:See Understanding XMLHttpRequest over CORS for a little more information about non-simple requests.