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.
Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:31.0) Gecko/20100101 Firefox/31.0
It looks strikingly like a user-agent.
But where is it being detected from? Firebug shows just
fake-user-agent sent on the HTTP request headers.