Want to do rsync to/from a Synology (perhaps to do backups with rsnapshot) and you want to use SSH keys? 

====== Synology GUI ======
Make an rsync user with read and possibly write permissions to the desired share(s). Can use an existing user at your discretion.

====== Synology SSH ======
SSH to the box with a sudo-capable account.

Enable SSH keys in general:
  sudo vim /etc/ssh/sshd_config

Uncomment each of:
  * ''PubkeyAuthentication yes''
  * ''AuthorizedKeysFile .ssh/authorized_keys''
  * ''ChallengeResponseAuthentication no''

If enabling rsync on a dedicated account, become that account: ''sudo -u rsync /bin/bash''

Put the key into ''.ssh/authorized_keys''.

Constrain permissions appropriately. From homedir:
  chmod 700 . .ssh
  chmod 600 .ssh/authorized_keys

[[https://community.synology.com/enu/forum/1/post/136213|Credit here]] for the above.

====== Rsync as root? ======

Generally, rsync'ing as root is bad. However, if you're pulling backups, it's the only way to read all files and preserve their permissions (necessary if you're using the NAS in a UNIX-like way). To do this, the above directions work, but with one issue. On rsync, you'll get the error "ERROR: user has disabled/expired". This is coming from the rsync layer, not ssh, and it's because the root account is "disabled".

Surprisingly, this is fixed from the GUI of Synology -- find the account called "admin" and enable it, and that will fix rsync-as-root. 

NOTE: Secure your SSH key fastidiously, as it now is an admin credential to the whole NAS box.