Harry Harry - 1 year ago 152
Ajax Question

CORS, IIS - the strangest failure ever

I got a Microsoft IIS server (over TMG firewall). I got Apache server (different location, no firewall). My IIS server serves data files (plain text, actually - CSV).

Apache server serves simple HTML page which loads data from IIS server via

method. Or,
on IE8 and maybe 9. (Made as

Here's my web.config for directory containing data:

<?xml version="1.0" encoding="UTF-8"?>
<location path=".">
<add name="Access-Control-Allow-Origin" value="*" />
<add name="Access-Control-Allow-Methods" value="GET,POST" />
<add name="Access-Control-Allow-Headers" value="Content-Type" />

Here's the test:

<!DOCTYPE html>
<meta charset="utf-8">
<title>CORS TEST</title>
<script src="//ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js"></script>
$(function () {
var h2 = $('h2:first');
h3 = $('h3:first');
url : '//my.iis.site.net/ajax/data/test.csv',
method : 'GET'
.done(function() {
.fail(function() {
<h1>CORS TEST:</h1>

I tested it with all specified browsers supporting CORS. And a few more mobile ones. I couldn't make it fail. It just works, all by the book.

After testing this configuration with our team at work, we found it's OK to deliver our new site to a broader audience. And it's when all hell broke loose.

Most of the people say it works. But significant amount (definitely above 10%) of people say it doesn't work. I'm afraid it's way more than 10%, it could be even 50%.

And here's the worst case: 2 persons, but almost (or totally) identical software (OS, configuration, programs, ISP, browsers). One guy sees AJAX data, the other one doesn't.

Same OS, same browser (tested with 3 different browsers on both computers, exactly the same versions, and the same ISP).

On some computers CORS just doesn't work, no matter what!

And yes, they cleared all browser caches. Didn't help.

Browser doesn't throw any error in console,
method just triggers
method, instead of
. I have no idea how to test it, since on my machine it always works. Even on my all mobile devices and GSM network it works as charm. Even on quite old Android version and pretty old default browser. No matter what I can't make it fail, just like some people just can't make it work on ANY of their computers or mobile devices.

What evil magic is it? Should I just forget about CORS and switch to JSONP transport?

There's a reason I don't use JSONP. The data server will receive a huge traffic. The data is updated every 500ms and will be reloaded by thousands of users very often (even at 500ms rate). Executing script on each request on my IIS server is not the best idea. It could have quite negative effect on performance.

Any clues? Zip the data as PNG and unzip via JS? :) It's a crazy hack. CORS is not crazy, but this is what I get.

Any idea how could I start to test it? Maybe it's a bug in jQuery, but I doubt it. The guy which can't access the files via CORS can't access them even when requesting script was run from the same domain. I had to remove web.config to make AJAX work on the same domain.

Answer Source

If you place web.config file in a subdirectory of a web application, you MUST convert that subdirectory to an application in IIS Manager. Otherwise it gives error 500 for every request to this subdirectory.

Someone converted it to web application on one server, but not on the other one. That's why some users got data, and some error 500 instead of data.

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