[ Thanks to Ggarron
for this link. ]
“One of the beauties about Linux is that it is really
easy to administer it remotely, but that is also one of the major
concerns about Linux security, if you have weak passwords, you are
exposed to attacks, there are lots of ways to protect yourself, one
of the easiest is to close the firewall, and only permit access to
port 22 from some specific IPs, but this is not an option if you
travel a lot or if you do not have a fixed IP, at the place you use
to be when you access your remote server.“Another approach is to have something like DenyHosts or
fail2ban, both of them do almost the same, they block a port, (22
in the most used cases) when a threshold point of number of tries
have been reached.“Now I have found another way, you may have your firewall closed
by default, but you will be able to open it when you need to access
your server, and closed it again after use, the package that will
do the magic is knockd, and as its name says, you will knock the
door of the firewall and it will open itself for you, but you will
need a secret type of knock, it is like the “Open/Close, Simsim”
(Open/Close sesame).”