Knock: A Linux kernel patch for NAT-compatible, stealthy port knocking
The point of a port knock tool is to make TCP servers respond to TCP SYN request only after a pre-defined sequence of packets have been sent and received. It’s like having a club house whose doors open only after a set number and sequence of knocks. There have been tools like in the past, but as far as I know, all have been in user space. Knock is thus likely the first port knock tool for Linux in kernel space.