title: How to set traefik with OAuth2 authentication
body: <p>I'm using traefik as a reverse proxy. I want to set OAuth2 authentication for a entry point.
  In the document, I found the <a href="https://docs.traefik.io/middlewares/forwardauth/" rel="nofollow noreferrer">Forward Authentication</a> which I think may be useful for this. But the document is just too simple</p>
  <blockquote>
  <p>This configuration will first forward the request to <a href="http://authserver.com/auth" rel="nofollow noreferrer">http://authserver.com/auth</a>.</p>
  <p>If the response code is 2XX, access is granted and the original request is performed. Otherwise, the response from the authentication server is returned.</p>
  </blockquote>
  <p>I've no idea how can I achieve authentication OAuth2 within a forwarding?
  I've tried <a href="https://github.com/bitly/oauth2_proxy" rel="nofollow noreferrer">oauth2_proxy</a> but didn't find a solution.<br />
  In this <a href="https://github.com/containous/traefik/issues/593#issuecomment-282717328" rel="nofollow noreferrer">issue/comment</a> guybrush provided a solution. But that, in fact, was a double reverse proxys.</p>