title: Set custom referer
body: <p>just for the purpose of study, I see that some sites offer services that allow the user to change the referrer for custom links. Services like spoofr and hitleap for example allow the User to define a particular access such as the twiiter.com domain, facebook.com, pinterest or even a custom domain.</p>
  
  <p>Again I say, it is a curiosity that came to me and not for other purposes. My question is as follows:</p>
  
  <p>How could this sites change referer to make fake referer?</p>
  
  <p>I do not need scripts or other information, I just want to know how they do it in textual form.</p>