How to get a cross-origin resource sharing (CORS)

Question

I have a machine on my local lan (machineA) that has two web servers. The first is the in-built one in XBMC (on port 8080) and displays our library. The second server is a CherryPy python script (port 8081) that I am using to trigger a file conversion on demand. The file conversion is triggered by a AJAX POST request from the page served from the XBMC server.

• Goto http://machineA:8080 which displays library
• Library is displayed
• User clicks on 'convert' link which issues the following command -

jQuery Ajax Request

$.post('http://machineA:8081', {file_url: 'asfd'}, function(d){console.log(d)})

• The browser issues a HTTP OPTIONS request with the following headers;

Request Header - OPTIONS

Host: machineA:8081
User-Agent: ... Firefox/4.01
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive: 115
Connection: keep-alive
Origin: http://machineA:8080
Access-Control-Request-Method: POST
Access-Control-Request-Headers: x-requested-with

• The server responds with the following;

Response Header - OPTIONS (STATUS = 200 OK)

Content-Length: 0
Access-Control-Allow-Headers: *
Access-Control-Max-Age: 1728000
Server: CherryPy/3.2.0
Date: Thu, 21 Apr 2011 22:40:29 GMT
Access-Control-Allow-Origin: *
Access-Control-Allow-Methods: POST, GET, OPTIONS
Content-Type: text/html;charset=ISO-8859-1

• The conversation then stops. The browser, should in theory, issue a POST request as the server responded with the correct (?) CORS headers (Access-Control-Allow-Origin: *)

For troubleshooting, I have also issued the same $.post command from http://jquery.com. This is where I am stumped, from jquery.com, the post request works, a OPTIONS request is sent following by a POST. The headers from this transaction are below;

Request Header - OPTIONS

Host: machineA:8081
User-Agent: ... Firefox/4.01
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive: 115
Connection: keep-alive
Origin: http://jquery.com
Access-Control-Request-Method: POST

Response Header - OPTIONS (STATUS = 200 OK)

Content-Length: 0
Access-Control-Allow-Headers: *
Access-Control-Max-Age: 1728000
Server: CherryPy/3.2.0
Date: Thu, 21 Apr 2011 22:37:59 GMT
Access-Control-Allow-Origin: *
Access-Control-Allow-Methods: POST, GET, OPTIONS
Content-Type: text/html;charset=ISO-8859-1

Request Header - POST

Host: machineA:8081
User-Agent: ... Firefox/4.01
Accept: */*
Accept-Language: en-us,en;q=0.5
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive: 115
Connection: keep-alive
Content-Type: application/x-www-form-urlencoded; charset=UTF-8
Referer: http://jquery.com/
Content-Length: 12
Origin: http://jquery.com
Pragma: no-cache
Cache-Control: no-cache

Response Header - POST (STATUS = 200 OK)

Content-Length: 32
Access-Control-Allow-Headers: *
Access-Control-Max-Age: 1728000
Server: CherryPy/3.2.0
Date: Thu, 21 Apr 2011 22:37:59 GMT
Access-Control-Allow-Origin: *
Access-Control-Allow-Methods: POST, GET, OPTIONS
Content-Type: application/json

I can't work out why the same request would work from one site, but not the other. I am hoping someone might be able to point out what I am missing. Thanks for your help!

Answer 1

For some reason, a question about GET requests was merged with this one, so I'll respond to it here.

This simple function will asynchronously get an HTTP status reply from a CORS-enabled page. If you run it, you'll see that only a page with the proper headers returns a 200 status if accessed via XMLHttpRequest -- whether GET or POST is used. Nothing can be done on the client side to get around this except possibly using JSONP if you just need a json object.

The following can be easily modified to get the data held in the xmlHttpRequestObject object:

function checkCorsSource(source) {
  var xmlHttpRequestObject;
  if (window.XMLHttpRequest) {
    xmlHttpRequestObject = new XMLHttpRequest();
    if (xmlHttpRequestObject != null) {
      var sUrl = "";
      if (source == "google") {
        var sUrl = "https://www.google.com";
      } else {
        var sUrl = "https://httpbin.org/get";
      }
      document.getElementById("txt1").innerHTML = "Request Sent...";
      xmlHttpRequestObject.open("GET", sUrl, true);
      xmlHttpRequestObject.onreadystatechange = function() {
        if (xmlHttpRequestObject.readyState == 4 && xmlHttpRequestObject.status == 200) {
          document.getElementById("txt1").innerHTML = "200 Response received!";
        } else {
          document.getElementById("txt1").innerHTML = "200 Response failed!";
        }
      }
      xmlHttpRequestObject.send();
    } else {
      window.alert("Error creating XmlHttpRequest object. Client is not CORS enabled");
    }
  }
}

<html>
<head>
  <title>Check if page is cors</title>
</head>
<body>
  <p>A CORS-enabled source has one of the following HTTP headers:</p>
  <ul>
    <li>Access-Control-Allow-Headers: *</li>
    <li>Access-Control-Allow-Headers: x-requested-with</li>
  </ul>
  <p>Click a button to see if the page allows CORS</p>
  <form name="form1" action="" method="get">
    <input type="button" name="btn1" value="Check Google Page" onClick="checkCorsSource('google')">
    <input type="button" name="btn1" value="Check Cors Page" onClick="checkCorsSource('cors')">
  </form>
  <p id="txt1" />
</body>
</html>

Answer 2

I had the exact same issue where jquery ajax only gave me cors issues on post requests where get requests worked fine - I tired everything above with no results. I had the correct headers in my server etc. Changing over to use XMLHTTPRequest instead of jquery fixed my issue immediately. No matter which version of jquery I used it didn't fix it. Fetch also works without issues if you don't need backward browser compatibility.

        var xhr = new XMLHttpRequest()
        xhr.open('POST', 'https://mywebsite.com', true)
        xhr.withCredentials = true
        xhr.onreadystatechange = function() {
          if (xhr.readyState === 2) {// do something}
        }
        xhr.setRequestHeader('Content-Type', 'application/json')
        xhr.send(json)

Hopefully this helps anyone else with the same issues.

Answer 3

REQUEST:

 $.ajax({
            url: "http://localhost:8079/students/add/",
            type: "POST",
            crossDomain: true,
            data: JSON.stringify(somejson),
            dataType: "json",
            success: function (response) {
                var resp = JSON.parse(response)
                alert(resp.status);
            },
            error: function (xhr, status) {
                alert("error");
            }
        });

RESPONSE:

response = HttpResponse(json.dumps('{"status" : "success"}'))
response.__setitem__("Content-type", "application/json")
response.__setitem__("Access-Control-Allow-Origin", "*")

return response

Answer 4

I finally stumbled upon this link "A CORS POST request works from plain javascript, but why not with jQuery?" that notes that jQuery 1.5.1 adds the

 Access-Control-Request-Headers: x-requested-with

header to all CORS requests. jQuery 1.5.2 does not do this. Also, according to the same question, setting a server response header of

Access-Control-Allow-Headers: *

does not allow the response to continue. You need to ensure the response header specifically includes the required headers. ie:

Access-Control-Allow-Headers: x-requested-with 

Answer 5

Took me some time to find the solution.

In case your server response correctly and the request is the problem, you should add withCredentials: true to the xhrFields in the request:

$.ajax({
    url: url,
    type: method,
    // This is the important part
    xhrFields: {
        withCredentials: true
    },
    // This is the important part
    data: data,
    success: function (response) {
        // handle the response
    },
    error: function (xhr, status) {
        // handle errors
    }
});

Note: jQuery >= 1.5.1 is required

Answer 6

Well I struggled with this issue for a couple of weeks.

The easiest, most compliant and non hacky way to do this is to probably use a provider JavaScript API which does not make browser based calls and can handle Cross Origin requests.

E.g. Facebook JavaScript API and Google JS API.

In case your API provider is not current and does not support Cross Origin Resource Origin '*' header in its response and does not have a JS api (Yes I am talking about you Yahoo ),you are struck with one of three options-

1. Using jsonp in your requests which adds a callback function to your URL where you can handle your response. Caveat this will change the request URL so your API server must be equipped to handle the ?callback= at the end of the URL.

2. Send the request to your API server which is controller by you and is either in the same domain as the client or has Cross Origin Resource Sharing enabled from where you can proxy the request to the 3rd party API server.

3. Probably most useful in cases where you are making OAuth requests and need to handle user interaction Haha! window.open('url',"newwindowname",'_blank', 'toolbar=0,location=0,menubar=0')