Are there open source software that let OS X see a generic NAS as a Time Machine destination? Is it possible to use "Time-Machine" with an external hard-drive through an IP?

I have an external HD connected to a Raspberry Pi and I have installed SSH. What I am hoping to achieve is this:

I have a Mac Mini and a Macbook Pro, I want their back-ups to be in-sync but, through the user of an IP that is directed to the raspberry pi that connects the hard-drive.  I know/read that this is possible using Airport extreme but, I do not have that, instead a standard router but I am able to port forward. 

Can anyone recommend any open source solutions rather than needing a Time Capsule or OS X Server running on the NAS?
I think what you really want is Netatalk. It is excellent software, and if run with a zfs backend, it is simply perfect as a time machine target.