Fwknop Port Knocking Utility

fwknop is a flexible port knocking implementation that is based around iptables. Both shared knock sequences and encrypted knock sequences are supported. In addition, fwknop makes use of passive OS fingerprinting signatures derived from p0f to ensure the OS that initiates a knock sequence conforms to a specific type. This makes it possible to allow, say, only Linux systems to connect to your SSH daemon. Both the knock sequences and OS fingerprinting are completely implemented around iptables log messages, and so a separate packet capture library is not required.