Get response headers cross domain

Get response headers cross domain

How can I get the response headers when making a cross domain request? xhr.getAllResponseHeaders() just returns null.

  2. $.ajax({
  3. url: 'http://search.twitter.com/search.atom?q=foo',
  4. success: function(data, textStatus, xhr){
  5. alert("Success:\ntextStatus: " + textStatus + "\nxhr: " + xhr.getAllResponseHeaders());
  6. },
  7. error: function(xhr, textStatus, errorThrown) {
  8. alert("Error:\nxhr:" + xhr.getAllResponseHeaders() + "\ntextStatus: " + textStatus + '\nerrorThrown: ' + errorThrown);
  9. }        
  10. });

I especially need to get the value of the "Retry-After" response header when twitter issues an HTTP 420 response code. See more on twitter's rate limiting here:  http://dev.twitter.com/pages/rate-limiting

Thanks, hope you can help :)