There are sites like whatsmybrowser.org and whatismybrowser.com that detect information about the technical specs of the client. As far as I can tell, this is done using the user-agent information sent by the browser on the HTTP requests.

What is puzzling me is that when I explicitly set my user-agent string to something random, in some cases it can still detect my browser and OS.

For example, whatbrowser.org knows I am using Firefox 31; and www.whatsmyua.com displays the following string:

Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:31.0) Gecko/20100101 Firefox/31.0

It looks strikingly like a user-agent.

enter image description here

But where is it being detected from? Firebug shows just fake-user-agent sent on the HTTP request headers.

If it matters, I am faking my user-agent using UAControl for Firefox (pt-br version, en-US version).


1 Answer 1


I am not sure what those sites in particular are doing, but it is possible to probe a browser with JavaScript to detect its capabilities and identification. That is how progressive enhancement is done.

  • 1
    I disabled my JavaScript. The two sites that detected my browser stopped detecting it. Sep 3, 2015 at 13:45
  • 1
    It's quite obvious the sites are using client-side java to display the UA function getUa() { return location.hash.replace('#', '') || navigator.userAgent; }
    – qasdfdsaq
    Sep 3, 2015 at 15:41
  • 6
    @qasdfdsaq What's blatantly obvious for some people might not be for others. Actually, that's kinda the point of this entire site ;-) Sep 4, 2015 at 17:54

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