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Is this for internal facing servers? Not much more than CIS and the usual Best Practices (no root for SSH, etc)
For a DMZ node, minimal software (ie Arch) and automated defenses like fail2ban, key authentication, etc...
Firewalls with Geo-IP blocking also help, but that's not technically what you're asking for.
Port knocking is a cool technique
I have used this with second port for handshake (with no info in heading and 20 second times) and then the final port opens with key exchanged from handshake.
Would you use that on internal LAN connections or only external internet facing connections? I'm not aware (not checked) if any firewalls support it... not sure why?
With knockd you can execute arbitrary commands upon a port knocking sequence. So any application that is configurable via terminal is eligible. Here's a tutorial of knockd+iptables (1). Alternativly there's (2) that achieves the same effect in a different way.
You can use it wherever, as part of security in depth. It's essentially a pre-shared secret.
It'll have it's largest effect on publicly facing interfaces. It does not replace having a proper ssh setup (disabling root, disabling password login, etc).
Thanks for the links, I'll take a look as I've never actually played with port knocking.
I'm probably in minority here. My setup is simple, I chose a good OS like Debian or Alpine to run things on, make sure it's always patched, move sshd to a nonstandard port and harden it.