title: ASP NET MVC 5 adding only external authentication to existing project
body: <p>I recently started learning ASP NET MVC 5.
  I want to make a website were people upload files.
  You can upload only when you are logged in.
  I don't want to store passwords so I want people to login using only google, microsoft, facebook, etc accounts</p>
  
  <p>The code for how the webpage looks is done.
  The controllers, models and the database part is still going, no problems here because of good tutorials</p>
  
  <p>but I am stuck, writing the code in the controller that adds a file, I need to add it with the email of the uploader so I need to understand how to use an external autentication.
  The external authentication code that comes with the VS 2013 MVC project with individual authentication is hard to understand.</p>
  
  <p>for example this code</p>
  
  <pre><code>@Html.ActionLink("Log in", "Login", "Account", routeValues: null, htmlAttributes: new { id = "loginLink" })&lt;/li&gt;
  
  //
  // GET: /Account/Login
  [AllowAnonymous]
  public ActionResult Login(string returnUrl)
  {
      ViewBag.ReturnUrl = returnUrl;
      return View();
  }
  </code></pre>
  
  <p>where does "returnUrl" come from?</p>
  
  <p>Apart form that I can't find a good complete tutorial on adding only external authentication to my existing  project.
  I started my project from an empty one with only the mvc checkbox checked, it doesn't has the template that comes with an MVC project with individual authentication, did it this way for learning and clean purpouses</p>