Port Knocking is an approach that helps protecting your services ports from attacks the most famous brute forced would be SSH , the port knocking method depends that the client trying to connect will first attempt to connect to a predefined ports which will enable connection to your secured service port for 5 seconds , here is a simple script to set this kind of protection

#!/bin/sh
#
# Netfilter/IPtables - example of multiple-port knocking
# Note: Knock ports 3456,2345,1234 to open SSH and MySQL ports for 5 seconds.
# Nice thing to knock TCP with is `nc' program:
# $> nc -w 1 <ip> 3456 ; nc -w 1 <ip> 2345 ; nc -w 1 <ip> 1234 ,ssh <ip>
#
# Change this to the name of the interface that provides your "uplink"
# (connection to the Internet) or connection you want to protect.
UPLINK="eth0"
#
# Comma seperated list of ports to protect with no spaces.
SERVICES="22,3306"
#
# Location of iptables command
IPTABLES='/sbin/iptables'
#
${IPTABLES} -N stage1
${IPTABLES} -A stage1 -m recent --remove --name knock
${IPTABLES} -A stage1 -p tcp --dport 3456 -m recent --set --name knock2

${IPTABLES} -N stage2
${IPTABLES} -A stage2 -m recent --remove --name knock2
${IPTABLES} -A stage2 -p tcp --dport 2345 -m recent --set --name heaven

${IPTABLES} -N door
${IPTABLES} -A door -m recent --rcheck --seconds 5 --name knock2 -j stage2
${IPTABLES} -A door -m recent --rcheck --seconds 5 --name knock -j stage1
${IPTABLES} -A door -p tcp --dport 1234 -m recent --set --name knock

${IPTABLES} -A INPUT -m --state ESTABLISHED,RELATED -j ACCEPT
${IPTABLES} -A INPUT -p tcp --match multiport --dport ${SERVICES}  -i ${UPLINK} -m recent --rcheck --seconds 5 --name heaven -j ACCEPT
${IPTABLES} -A INPUT -p tcp --syn -j door

Some known pitfalls of port knocking are :

1- using consecutive port numbers like 100,200,300 which would cause the secured service port to be opened while using a port scanner like Nmap.

2- port knocking is not effective against replay attacks

3- using port knocking as your sole line of defense, it has to be one of many .

Some other famous scripts used for port knocking are fwknop and knockd