Cross-subdomain ajax request denied even when document.domain is set correctly

document.domain doesn't work with AJAX. It is intended for cross domain iframe and window communication. In your case you are violating the same origin policy (last line of the table) so you need to use either JSONP or server side bridge.

Here's a very nice guide which illustrates different techniques for achieving cross domain AJAX requests.

the same origin policy is one of the most frustrating browser related topics I have had to deal with. Silly to me that 2 servers on the same domain can not communicate. Unfortunately the same origin policy considers even 2 requests to the same server but on a different port a violation of the same origin policy. I think this will get better with future browsers:

http://www.html5rocks.com/en/tutorials/file/xhr2/

search for : Cross Origin Resource Sharing (CORS)

basically your server just has to set a response header saying "yeah it is ok to allow cross domain or cross subdomain calls to server xyz".

It will be some time before all browsers support this Im sure (and hell I have to support ie8 till most our users are off it anyway) - but at least there is light at the end of the tunnel.

You need to add document.domain = 'u413.com to your other sub domain aswell.