#!/usr/bin/env zsh

# load module to parse command line arguments
zmodload zsh/zutil
zparseopts -D -E -A opts -- h x

# load module to avoid use of GNU sleep
zmodload zsh/zselect

# enable XTRACE shell option for full debugging output of scripts
if (( ${+opts[-x]} )); then
	set -x
fi

if [[ -z "${2}" ]] || (( ${+opts[-h]} )); then
	echo "usage: ${0:t} [ -h ] [ -x ] host port [ knock_port ] .." >&2
	echo -e '\n\t-h\tshow this help\n\t-x\tenable shell debugging' >&2
	echo -e '\thost\tdestination host name' >&2
	echo -e '\tport\tdestination service port\n' >&2
	echo -e 'Specifying no knock_port(s) will use 12345 23456 34567 45678 by default.\n' >&2
	exit 1
fi

host="${1}"
port="${2}"
shift 2
knock_ports="${@:-12345 23456 34567 45678}"
attempts=1

function check_service_port {
	if nc -w1 ${host} ${port} &> /dev/null <& -; then
		exit 0
	fi
}

#check_service_port

while [[ ${attempts} -lt 9 ]]; do

	for knock_port in ${=knock_ports}; do
		nc -w1 ${host} ${knock_port} &> /dev/null <& - &
		zselect -t ${attempts}0
	done

	check_service_port
	((attempts+=1))

done

exit 1