Best method: Access-Control-Allow-Origin Multiple Origin Domains
This question has been asked on here before and given an array of good answers, mainly: Access-Control-Allow-Origin Multiple Origin Domains?
However there seems to be a gap in explanation in terms of the approved method that should be undertaken. Reading through the W3 documentation we have what appears to me to be a conflict of guidance.
Firstly we see the answer given as the right way to do it in a lot of the previous answers which dictates that the host server must dynamically echo back the the given 'Origin' if it appears on a predefined 'whitelist'. http://www.w3.org/TR/cors/#resource-implementation
However a lot of the answers and methods used also allude to a space delimited list which can also be used as a method of passing multiple 'Origins' to allow. If we take a look at another piece of W3 documentation at http://www.w3.org/wiki/CORS_Enabled we see an example of this in the first section of the page as:
Access-Control-Allow-Origin: http://example.com:8080 http://blah.example.com http://foo.example.com
Of these two methods I would be equally happy to incorparate either however there may be a large list of URLs which will need to be whilelisted and so I wanted to ensure that I am doing this corrently first time round. If anyone has any insight into the two methods mentioned above I would be very grateful to hear the decision in your choices and if there is a definitive guide to the recommended method I may have missed.
Answer
The documentation on this seems to imply that it allows multiple origins with a space separated list, but that's not what it actually means. Here's what I could gather as the most definitive answer to your question: the Access-Control-Allow-Origin header should be the same value as the Origin header as long as you want to allow it.
The reason it's not a whitelist that you send back to the client is because technically the client can send a space separated list of origins so that the server can validate the request. The purpose of origin list then is because the request could've come from multiple origins (ie. the request was redirected across domains). A test suite makes it easy to observe this behavior with varying redirect possibilities, even though a space separated list is never generated (by Firefox at least).
This is illustrated lower in the first linked W3C document you provided:
The Access-Control-Allow-Origin header indicates whether a resource can be shared based by returning the value of the Origin request header, "*", or "null" in the response. ABNF:
Access-Control-Allow-Origin = "Access-Control-Allow-Origin" ":" origin-list-or-null | "*"
In practice the origin-list-or-null production is more constrained. Rather than allowing a space-separated list of origins, it is either a single origin or the string "null".
And again in the definition of the origin list. In addition it shows if you do want to allow the string "null" as an origin, it wouldn't be able to be embedded in an origin list anyways.
So stick with the dynamically generated header based on the client's Origin header and whether that matches your whitelist.