code: func connectProxyTransport(sconn ssh.Conn, addr string) (net.Conn, error) {
  	channel, _, err := sconn.OpenChannel(chanTransport, nil)
  	if err != nil {
  		return nil, trace.Wrap(err)
  	}
  
  	// Send a special SSH out-of-band request called "teleport-transport"
  	// the agent on the other side will create a new TCP/IP connection to
  	// 'addr' on its network and will start proxying that connection over
  	// this SSH channel.
  	ok, err := channel.SendRequest(chanTransportDialReq, true, []byte(addr))
  	if err != nil {
  		return nil, trace.Wrap(err)
  	}
  	if !ok {
  		defer channel.Close()
  
  		// Pull the error message from the tunnel client (remote cluster)
  		// passed to us via stderr.
  		errMessage, _ := ioutil.ReadAll(channel.Stderr())
  		if errMessage == nil {
  			errMessage = []byte("failed connecting to " + addr)
  		}
  		return nil, trace.Errorf(strings.TrimSpace(string(errMessage)))
  	}
  
  	return utils.NewChConn(sconn, channel), nil
  }
docs: // connectProxyTransport opens a channel over the remote tunnel and connects
  // to the requested host.