title: Loggin a user across different domains
body: <p>two years ago I had to design a system to share authentication data across multiple domains, all of them shared the same server/db. I was able to pull this off with a complex system of cookie sharing which, to date still works.</p>
  
  <p>I'm now in the process of redesigning the system and I was wondering if there are better ways to achieve this without having to write cross domain cookies.</p>
  
  <p>Basically the system MUST do this.</p>
  
  <p>Once logged in one site the user must be logged in all of the other site seamlessly, not only following a link, but even by directly writing the domain name on the address bar.</p>
  
  <p>To my knowledge the only way to achieve this are cross-domain cookies, if there are alternatives please tell me.</p>
  
  <p>Thank you very much</p>