Linux: Open ssh port access using a combination... (knocking technique)

# debian and derived distros... install knockd: sudo apt-get install knockd # we edit /etc/default/knockd: (knockd confif file) sudo nano /etc/default/knockd # and set: START_KNOCKD=0 # to START_KNOCKD=1 # let's create our ports sequence: let's say 3030,5050,7070 (open) and 7070,5050,3030 (close) # for this we edit /etc/knockd.conf: sudo nano /etc/knockd.conf: [options] UseSyslog [openSSH] sequence = 3030,5050,7070 seq_timeout = 5 # add our input access to iptables command = /sbin/iptables -A INPUT -s %IP% -p tcp --dport 22 -j DROP tcpflags = syn [closeSSH] sequence = 7070,5050,3030 seq_timeout = 5 # delete our input access to iptables command = /sbin/iptables -D INPUT -s %IP% -p tcp --dport 22 -j DROP tcpflags = syn # we start service: sudo /etc/init.d/knockd start # That's all, we're done. # .. and now... How can I open my host's ssh port (22) from remote location? # ... just like this (using nmap): # OPEN: nmap -sT -p3030,5050,7070 <ipaddress> # and then we CLOSE it: nmap -sT -p7070,5050,3030 <ipaddress> # you'll this output (example with 192.168.1.33): knockd: 192.168.1.33: openSSH: Stage 1 knockd: 192.168.1.33: closeSSH: Stage 2 knockd: 192.168.1.33: closeSSH: Stage 3 knockd: 192.168.1.33: closeSSH: OPEN SESAME knockd: closeSSH: running command: /sbin/iptables -D INPUT -s 192.168.1.33...
This is a technique called "knocking" (knock knock) to open ssh port only when we need it in a secure way, just as if it was a "safe box" (using a combination of access to specific ports and specific order).

It's very secure because only opens access to the ip address which get connected and give correct combination order.

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