I'l give my vote for this application. Its available for ubuntu and debian so maybe its not that hard to repackage and maintain a package for server, client and library. Fwknop is the most feuturerich port knocker out there what i know of. Theres recently a discussion on pauldotcom.com on their latest episode talking about fwknop and SPA technology.
You can read more about it here: hXXp://pauldotcom.com/wiki/index.php/Episode221