Syncthing Ubuntu/Mint Headless
Syncthing is a free, open-source peer-to-peer file synchronization application available for Windows, Mac, Linux, Android, Solaris, Darwin, and BSD. It can sync files between devices on a local network, or between remote devices over the Internet. Data security and data safety are built into the design of the software.
This is written using Linux Mint 18.1 Serena x86_64 using kernel 4.4.0-139-generic.
I couldn't find good directions for doing this, so here we go.