A Synology is basically a linux system, in a small case and with a nice web interface to do most basic tasks. For the tasks which do not run by default from the web interface, SSH can be used. This tutorial demonstrates how to set up passwordless SSH between two (or more) Synology boxes. This is very useful for automated tasks, such as automated backups. In this tutorial we will have a local Synology and a remote Synology. The local Synology will be able to connect over SSH without a password, to the remote Synology.
Out of the box, Mac OS High Sierra ships with a 12 year old version of Rsync. The reason for this is that Apple doesn’t include anything released under GPLv3 or similar licenses. Luckily, it's relatively quick and simple to update Rsync using Homebrew. Homebrew is a package manager not dissimilar to Yum on Redhat or Apt on Debian. You can follow the instructions in the above link, or just copy and paste the commands documented as follows.