I use this port to play a little bit with the router, since it has a configuration board accessible. But, most of the time, I don’t use it and I don’t want anyone else to remotely use it, either.
We’re going to set a rule for the firewall to close communication in any direction on port 23. (telnet).
First, is the firewall enabled ?
sudo ufw status
If it’s not enabled, enable it :)
sudo ufw enable
sudo ufw deny in 23
sudo ufw deny out 23
sudo ufw deny in 21
sudo ufw deny out 21
sudo ufw deny in 1723
sudo ufw deny out 1723
If you scan your machine now, you won’t find anyone listening on these ports.
Moreover, I used “deny”. You could have used “reject”, but “reject” tells someone probing that you are denying traffic, whereas “deny” just ignores requests.
To limit the attempts (maximum is 6 in 30 seconds), useful to make it less easier to dictionary attacks.