code: def start(self):
          
          if self.is_alive:
              self.logger.warning('Already started!')
              return
          self._create_tunnels()
          if not self.is_active:
              self._raise(BaseSSHTunnelForwarderError,
                          reason='Could not establish session to SSH gateway')
          for _srv in self._server_list:
              thread = threading.Thread(
                  target=self._serve_forever_wrapper,
                  args=(_srv, ),
                  name='Srv-{0}'.format(address_to_str(_srv.local_port))
              )
              thread.daemon = self.daemon_forward_servers
              thread.start()
              self._check_tunnel(_srv)
          self.is_alive = any(self.tunnel_is_up.values())
          if not self.is_alive:
              self._raise(HandlerSSHTunnelForwarderError,
                          'An error occurred while opening tunnels.')
docs: Start the SSH tunnels