Mikrotik port knocking

#!/bin/bash

# Script to perform ICMP port knocking
#
# Firewall filters for Mikrotik:
# add action=add-src-to-address-list address-list=tmp_remote address-list-timeout=3m chain=input comment="icmp port knocking" packet-size=125 protocol=icmp src-address-list=icmp_stage2
# add action=add-src-to-address-list address-list=icmp_stage2 address-list-timeout=3s chain=input packet-size=124 protocol=icmp src-address-list=icmp_stage1
# add action=add-src-to-address-list address-list=icmp_stage1 address-list-timeout=3s chain=input packet-size=123 protocol=icmp
#
# Made by: Sunn3h < m [at] sunneh.ru >
#
# Code stylized according to:
# https://google.github.io/styleguide/shell.xml

readonly DEST="$1"

if [[ -z "${DEST}" ]]; then
  echo "Usage: $0 destination"
  exit
fi

now() {
  date +'%Y-%m-%dT%H:%M:%S%z'
}

while true; do
    echo "[$(now)]: Ping..."
    ping "${DEST}" -s 95 -c1 -W1 2>&1 >/dev/null
    ping "${DEST}" -s 96 -c1 -W1 2>&1 >/dev/null
    ping "${DEST}" -s 97 -c1 -W1 2>&1 >/dev/null
    sleep 60
done