Today I read about port-knocking for the first time. It has been a pretty known concept for more than ten years, but I don't know anybody who uses it for their servers.

How common is the usage of port-knocking?

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  • I've never seen it used in an enterprise. However, I have seen firewall which lay dormant and can be enabled after telnet authentication (on page 463 of this). – JZeolla Sep 19 '14 at 13:19

Port-knocking is often used in 'closed' designs to assure network integrity. In the meantime it evolved into single-packet authentication.

Check here for the source of sources on the topic :)

http://www.cipherdyne.org/fwknop/docs/SPA.html

To my knowledge, this is pretty uncommon.

What I remember about it from what I learned in college is it has a pretty big downside : it's easy to deny someone access by sending a packet to a random port using his adress, and thus breaking the "knock" sequence.

However its evolution, single-packet authorization, is much more solid (though I have no idea if it is commonly used in practice).

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