CORS Access-Control-Allow-Origin despite correct headers

Question

I am trying to set up simple Cross-Origin Resource Sharing using jQuery (1.7.1) powered ajax on the client and apache served python (django) server. According to all the instructions I have read my headers are set correctly, but I keep getting the following error:

XMLHttpRequest cannot load http://myexternaldomain.com/get_data. Origin http://localhost:8080 is not allowed by Access-Control-Allow-Origin.

The header being I am trying to (I am not sure it is even getting past the browser) send is:

Request URL:http://myexternaldomain.com/get_data
Accept:application/json, text/javascript, */*; q=0.01
Origin:http://localhost:8080
Referer:http://localhost:8080/static/js/test-zetta.html
User-Agent:Mozilla/5.0 (Macintosh; Intel Mac OS X 10_6_8) AppleWebKit/535.11 (KHTML, like Gecko) Chrome/17.0.963.66 Safari/535.11

The javascript code is

    var request = $.ajax({
        url : "http://myexternaldomain.com/get_data",
        type : "POST",
        dataType : "json",
        crossDomain : true
    });

Note that origin is set correctly. The server adds the header Access-Control-Allow-Origin = * using the following python code

def process_response(self, response):
    if response.has_header('Access-Control-Allow-Origin'):
            return response

    response['Access-Control-Allow-Origin'] = '*'
    return response

def get_orders(request):
    """ Tell worker what to do """
    response_data = {}
    response_data['action'] = 'probe'
    response = process_response(HttpResponse(json.dumps(response_data), mimetype="application/json"))
    return response

If I visit the address directly, it appears to confirm that the header is being set correctly

Access-Control-Allow-Origin:*
Content-Type:application/json
Date:Thu, 08 Mar 2012 05:06:25 GMT
Server:Apache/2.2.20 (Ubuntu)
Transfer-Encoding:chunked

However in the cross domain setting it always fails (tried both chrome and firefox). I've tried implementing the code exactly as per the selected answer to this question, but get the same error

Update

I am quite sure that the problem is server side, as I have managed to get my ajax calls working with a different public CORS enabled server. When I compare the headers coming back from this public server, and the ones returned from mine (when I test from same domain), I cannot see any major difference which could account for difference (see below).

One subtlety that I excluded, which may or may be important is that the actual domain is an amazon domain of multiple subdomains. The real address is http://ec2-23-20-27-108.compute-1.amazonaws.com/get_orders , feel free to probe it to see what I am doing wrong.

From Public server

Access-Control-Allow-Origin:*
Connection:Keep-Alive
Content-Encoding:gzip
Content-Length:622
Content-Type:text/html
Date:Thu, 08 Mar 2012 15:33:20 GMT
Keep-Alive:timeout=15, max=99
Server:Apache/2.2.14 (Ubuntu)
Vary:Accept-Encoding
X-Powered-By:Perl/5.8.7, PHP/4.4.0

From my server - (not working cross domain)

Access-Control-Allow-Origin:*
Content-Encoding:gzip
Content-Type:text/plain
Date:Thu, 08 Mar 2012 15:32:24 GMT
Server:Apache/2.2.20 (Ubuntu)
Transfer-Encoding:chunked
Vary:Accept-Encoding

Answer 1

So I was being mislead by the response from going to the URL, and in fact the problem was that when doing the ajax request, I was getting a 403 (only revealed in firefox not chrome) error due to csrf protection.