knock

Knock on ports with specified protocol

$ knock [hostname] [port]:tcp

$ knock -u [hostname] [port]

$ knock -4 [hostname] [port]

$ knock -v [hostname] [port]

knock [options] hostname port[:protocol]...

knock is the client component of port knocking. Port knocking is a security technique where a firewall opens specific ports only after receiving a predefined sequence of connection attempts ("knocks") on closed ports.
The client sends packets to the specified ports in sequence. When the server (knockd) detects the correct sequence, it executes configured commands, typically to open a firewall port for the client's IP address.

-u, --udp

Use UDP instead of TCP

Force IPv4

Force IPv6

Display verbose information

Delay between knocks in milliseconds

Port sequence must be kept secret. Network issues can cause knock sequences to fail. Not a replacement for proper authentication but an additional security layer.

Port knocking emerged as a security technique in the early 2000s to hide services from port scanners. The knock/knockd implementation is one of the most widely used port knocking solutions.

knockd(1), iptables(8), nmap(1)