in reply to Server sends knock-knock, client does something.

Consider looking into snmp. It's for this purpose.

s$$([},&%#}/&/]+}%&{})*;#$&&s&&$^X.($'^"%]=\&(|?*{%
+.+=%;.#_}\&"^"-+%*).}%:##%}={~=~:.")&e&&s""`$''`"e

In Section Seekers of Perl Wisdom