title: Tumblr-style multiple domains
body: <p>I'm building a web app that's a little similar to Tumblr in that we'd run everything through one domain, but we'd actually have sites with other domain names running off the same app.</p>
  
  <p>That is: </p>
  
  <pre><code>    Platform (platform.dev)
      |
      |_app1 (app1.dev)
      |_app2 (app2.dev)
  </code></pre>
  
  <p>They'd share the same web server, database server, auth server.</p>
  
  <p>To keep things simple, how would I set up custom domains? On top of that, how would I handle authentication?</p>