Login?returnUrl=targetThatNeedsAuthentication

AuthorizeAttribute

protected override void HandleUnauthorizedRequest(AuthorizationContext filterContext)
{
    ...
    var url = // get url of resource that requires authentication
    filterContext.Result = new NewResult(url, false);
}

/membersonly/Login?redirectUrl=http://localhos:1234/targetThatNeedsAuthentication/

public NewResult(string url, bool preserveForm) : base(url)

protected override void HandleUnauthorizedRequest(AuthorizationContext filterContext)
{
    filterContext.Result = new RedirectToRouteResult(new RouteValueDictionary
    {
        { "action", "LogIn" },
        { "controller", "Account" },
        { "returnUrl", filterContext.HttpContext.Request.RawUrl}
    });
}

public ActionResult LogIn(LogInModel modelData, string returnUrl = "")
{
  // check authorization
  ...

  if (user != null) // user is authorized
  {
    if (Url.IsLocalUrl(returnUrl))
      return Redirect(returnUrl);
    else
      // return to default authorized page
  }
}