I like the fwknop tool , a new version has been released recently.
The tool can be used in 2 modes, SPA or Port-Knocking. SPA is a variant of Port-knocking that uses only one single knock.
- Authorization part is done by using libpcap, so there is no service and no ports to listen to.
- Access to protected service is only granted after receiving a single non-replayed encrypted packet from fwknop client.