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 :)
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).
