We will use
knockd to hide SSH from scripting brute-force tools, wandering around the Internet.
Operating system used:
- Ubuntu 18.04
sudo apt install knockd sudo systemctl enable knockd sudo service knockd start
After install, default open knock sequence is
7000,8000,9000. Default manually invited closing sequence is
We will remove these lines, and add our own setup.
Do choose your own
This is my
[options] UseSyslog [SSH] sequence = 23800,9258,29015 seq_timeout = 5 start_command = ufw insert 1 allow from %IP% to any port 22 tcpflags = syn cmd_timeout = 10 stop_command = ufw delete allow from %IP% to any port 22
This configures knockd to listen for connections on the 4 specified ports, within 5 seconds after each other. Once the sequence is completed, a hole is opened for 10 seconds using the given ufw commands.
nc -w 1 server.example.com 23800 nc -w 1 server.example.com 9258 nc -w 1 server.example.com 29015 ssh server.example.com
Alternatively, instead of
netcat, you can use
knock client at your laptop:
knock -d 300 server.example.com 23800 9258 29015 ssh server.example.com
This process can be easily followed in
Jan 30 13:40:17 test9 knockd: 188.8.131.52: SSH: Stage 1 Jan 30 13:40:18 test9 knockd: 184.108.40.206: SSH: Stage 2 Jan 30 13:40:19 test9 knockd: 220.127.116.11: SSH: Stage 3 Jan 30 13:40:19 test9 knockd: 18.104.22.168: SSH: OPEN SESAME Jan 30 13:40:19 test9 knockd: SSH: running command: ufw insert 1 allow from 22.214.171.124 to any port 22 Jan 30 13:40:19 test9 knockd: Rule inserted Jan 30 13:40:29 test9 knockd: 126.96.36.199: SSH: command timeout Jan 30 13:40:29 test9 knockd: SSH: running command: ufw delete allow from 188.8.131.52 to any port 22 Jan 30 13:40:29 test9 knockd: Rule deleted
Disable existing OpenSSH rule, if any
If everything is OK with previous test, especially if you see line
SSH: OPEN SESAME, you can disable existing rule:
sudo ufw delete allow OpenSSH
After these steps, you will be able to connect to your server, anytime you like, but will keep port 22 closed for any Internet-wide brute-force attack on your SSH.
All you need to do is to remember the port sequence!