CORS - How do 'preflight' an httprequest?

Question

I am trying to make a cross domain HTTP request to WCF service (that I own). I have read several techniques for working with the cross domain scripting limitations. Because my service must accommodate both GET and POST requests I cannot implement some dynamic script tag whose src is the URL of a GET request. Since I am free to make changes at the server I have begun to try to implement a workaround that involves configuring the server responses to include the "Access-Control-Allow-Origin" header and 'preflight' requests with and OPTIONS request. I got the idea from this post : Getting CORS working

At the server side, my web method is adding 'Access-Control-Allow-Origin: *' to the HTTP response. I can see that responses do include this header now. My question is: How do I 'preflight' a request (OPTIONS)? I am using jQuery.getJSON to make the GET request but the browser cancels the request right away with the infamous:

Origin http://localhost is not allowed by Access-Control-Allow-Origin

Is anyone familiar with this CORS technique? What changes need to be made at the client to preflight my request?

Thanks!

Answer 1

During the preflight request, you should see the following two headers: Access-Control-Request-Method and Access-Control-Request-Headers. These request headers are asking the server for permissions to make the actual request. Your preflight response needs to acknowledge these headers in order for the actual request to work.

For example, suppose the browser makes a request with the following headers:

Origin: http://yourdomain.com
Access-Control-Request-Method: POST
Access-Control-Request-Headers: X-Custom-Header

Your server should then respond with the following headers:

Access-Control-Allow-Origin: http://yourdomain.com
Access-Control-Allow-Methods: GET, POST
Access-Control-Allow-Headers: X-Custom-Header

Pay special attention to the Access-Control-Allow-Headers response header. The value of this header should be the same headers in the Access-Control-Request-Headers request header, and it can not be '*'.

Once you send this response to the preflight request, the browser will make the actual request. You can learn more about CORS here: http://www.html5rocks.com/en/tutorials/cors/