code: func TunnelAuthDialer(proxyAddr string, sshConfig *ssh.ClientConfig) auth.DialContext {
  	return func(ctx context.Context, network string, addr string) (net.Conn, error) {
  		// Connect to the reverse tunnel server.
  		dialer := proxy.DialerFromEnvironment(proxyAddr)
  		sconn, err := dialer.Dial("tcp", proxyAddr, sshConfig)
  		if err != nil {
  			return nil, trace.Wrap(err)
  		}
  
  		conn, err := connectProxyTransport(sconn.Conn, RemoteAuthServer)
  		if err != nil {
  			return nil, trace.Wrap(err)
  		}
  		return conn, nil
  	}
  }
docs: // TunnelAuthDialer connects to the Auth Server through the reverse tunnel.