The Apple solution is to buy a time machine ;) If you can get Syncthing https://syncthing.net/ https://syncthing.net/ running on NAS you even have two way sync

The Apple solution is to buy a time machine ;) If you can get Syncthing https://syncthing.net/ https://syncthing.net/ running on NAS you even have two way sync (it's like btsync but open source). rsync can be triggered by a good file monitoring tool like Watchmen https://facebook.github.io/watchman/ https://facebook.github.io/watchman/ which you should run in background as service. But running rsync on every little file change is maybe CPU and IO overkill (but you can set a trigger like an environment variable which runs rsync on a scheduled time interval). So it's all possible with a little Shell or Python magic, you can also check if the NAS is available within that script. So this is my DIY approach... would be also interesting for me if there are finished solution for that available.