How is it possible to enable CORS from server-side?

问题

From my little experience in web API, same origin policy is a policy of browsers i.e, browser doesn't allow to make requests to other hosts rather than the origin. I wonder how it is possible to enable CORS from server side(talking about ASP.net Web API)?

This is how i enable CORS in webAPI

namespace WebService.Controllers
{
    [EnableCors(origins: "*", headers: "*", methods: "*")]
    public class TestController : ApiController
    {
        // Controller methods ...
    }
}

If CORS is a browser thing, isn't it more logical to enable it from client side. Can anybody clear this out

回答1:

Here’s an attempt at a short summary of how it works: The browser is where the same-origin policy and cross-origin restrictions are enforced. Specifically, browsers block frontend JavaScript code from being able to access responses from cross-origin requests—unless the servers the requests are made to send the response header Access-Control-Allow-Origin in responses.

In other words, the way for getting browsers to relax the same-origin policy is for servers to use the Access-Control-Allow-Origin header to indicate they’re opting in to cross-origin requests.

So, browsers are the place where any cross-origin restrictions are either being applied or relaxed.

One case that helps to illustrate how it works is a simple cross-origin POST. As long as a cross-origin POST doesn’t have any custom request headers that will trigger browsers to do a CORS preflight OPTIONS request, a browser will go ahead and make the request, even cross-origin. And the server that POST is sent to will go ahead and accept it and then send a response.

What happens then is where the cross-origin restrictions from browsers kick in—because if that POST request was sent from frontend JavaScript code using XHR or the Fetch API or an Ajax method from some JavaScript library, then unless the response includes the Access-Control-Allow-Origin header, browsers won’t allow the frontend code to access the response (even though the server accepted the POST and it succeeded).

Anyway, I hope the above helps to clarify what enabling CORS support in servers actually means, and what effects it has, and that the actual policy enforcement is performed by browsers.

Of course all of the above just describes the simplest case, where there are no characteristics of the request that will trigger browsers to do a CORS preflight OPTIONS request.

But still in that case, the policy enforcement is all performed by the browser—in fact even more so, in that, for example, browsers won’t allow a POST with custom headers to even be sent to a server to begin with unless the server explicitly indicates (in its response to the preflight OPTIONS) that the server has opted in to receiving cross-origin requests which include that custom header.

来源：https://stackoverflow.com/questions/45807478/how-is-it-possible-to-enable-cors-from-server-side