The sys admin's daily grind: Knockd
Knock-Knock
Horror stories are full of scary characters knocking on doors at night. On Linux, we just call this port knocking, and it can actually be quite useful.
If you prefer not to have an obvious administrative port for your iptables firewall – but do need a secret one – port knocking is an interesting option that can put off script-based attacks. For the ambitious but secretive admin, the tool of choice is Knockd [1].
The package includes two components: Knock is the client that sends knocking signals, which the Knockd daemon receives.
Knocking
To monitor the process, Knock, the knocking client, only needs the port number on which to knock and a -v option.
[...]
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