title: AJAX response with status code 0
body: <p>In our web application the signup process is handled through an AJAX call. We're using the jQuery (1.7.1) library and the built in 'ajax' method.</p>
  
  <p>For some people the AJAX call is not being returned properly, generating a status code of 0. The call is certainly reaching the webserver and being processed on our end properly (we can see the results in the DB) but for some reason the client browser is seeing an error and triggering the 'error' callback function of the ajax request.</p>
  
  <p>The error response handler is:</p>
  
  <pre><code>error: function (jqXHR, textStatus, errorThrown) {...
  </code></pre>
  
  <p>and the values from the callback arguments are:</p>
  
  <pre><code>textStatus = 'error'
  jqXHR.status = 0
  jqXHR.statusText = 'error'
  jqXHR.responseText = ''
  errorThrown = ''
  </code></pre>
  
  <p>Reading through similar SO questions and various forums/blogs on the web I think this has something to do with firewalls. I've been able to confirm one case as sitting behind a corporate firewall.</p>
  
  <p>The webserver is a LAMP machine running Ubuntu. The web application is written in PHP and uses the Code Igniter framework. The website is <a href="http://thinkbinder.com" rel="nofollow">http://thinkbinder.com</a>.</p>
  
  <p>What is the specific cause of this and how can I fix it?</p>