code: func SetProxyMode(tsrv reversetunnel.Server) ServerOption {
  	return func(s *Server) error {
  		// always set proxy mode to true,
  		// because in some tests reverse tunnel is disabled,
  		// but proxy is still used without it.
  		s.proxyMode = true
  		s.proxyTun = tsrv
  		return nil
  	}
  }
docs: // SetProxyMode starts this server in SSH proxying mode