knockr is a port-knocking utility potentially more convenient to use than a general purpose tool like nmap or netcat. Written in Go, the utility is a single binary, installable on any platform Go supports including Linux, BSD/Unix, Windows and Mac.

go install github.com/mwyvr/knockr@latest

Linux without glibc: The Go net package includes CGO bindings; Linux distributions not based on glibc such as Alpine Linux, Chimera Linux or Void Linux (musl variant) can install a statically linked version with:

CGO_ENABLED=0 go install github.com/mwyvr/knockr@latest

Pre-built binary for Linux:

The releases page provides a link to a non CGO-based binary that will run on various Linux distributions.

The default timeout and delay durations should be sufficient for most use cases.

Usage: knockr [OPTIONS] address port1,port2...

-d duration
        delay between knocks (default 100ms)
-n string
        network protocol (default "tcp")
-s	silent: suppress all but error output
-t duration
        timeout for each knock (default 1.5s)

Example:

# knock on three ports using the default protocol (tcp) and delays
knockr my.host.name 1234,8923,1233

Tip: Include the port(s) you expect to be unlocked as the first and last port in the chain to observe status before and after. For example, if intending to unlock port 22 (ssh) on a specific host:

# 22 last to demonstrate it has been opened
knockr my.host.name 1234,18923,1233,22

Port-knocking is a network access method that opens ports normally left closed to the outside world, but only when the right sequence of ports has been visited and within time frames determined by your network access configuration. That sequence of ports acts as a key.

knockr is the remote side of the solution; a network access device like a router must be configured.

Port-knocking can be configured on hosts and many routers including some low-cost, high functionality devices accessible to technical consumers such as Mikrotik RouterOS devices.

Typically the solution will be configured such that the target port (not necessarily specified in the port-knocking requests) are only opened to the IP address issuing the correct knock sequence, further improving security and resiliency to exploit, and reducing port-scanning log burden.

See also: Wikipedia - port-knocking.