#!/bin/bash

# program to knock (telnet or netcat)
prog="telnet"
# must be equal to time period on knocking server
time_period=100
# period between knocking sequence and ssh connect
sleep_period=2
# ssh user
username=$1
# destination ip
ip=$2

if [ $# -ne 2 ]; then
    echo "usage: ./clientname username ip_address"
    exit
fi

read -p "enter base port of knocking: " -s port0
echo
read -p "enter knocking password: " -s pass
echo

# calculate secure sequence of ports
time=`date +%s`
time_stamp=$(($time/$time_period))
sum=`echo $pass$time_stamp | md5sum`

i=0
ports=""
while [ $i -lt 16 ]
do
    j=${sum:$i*2:2}
    port=$(($port0+0x$j*16+$i))
    ports="$ports $port"
    i=$((i+1))
done

# start knocking
(
for port in $ports
do
    $prog $ip $port &
done
pkill $prog
) >/dev/null 2>&1
echo "knocking done"
sleep $sleep_period

echo "trying to ssh ..."
ssh -l $username $ip