Note:
This is a copy of an article posted by Valery Letroye, on May 16, 2020, at https://www.beatificabytes.be/backup-synology-to-unraid/, which is now defunct somehow.
So this is just posted as a backup and not my own writing, but it was helpful learning about how to use rsync with Unraid and another NAS.
I've posted it here for reference only. Keep in mind it's geared towards a Synology NAS, and Unraid 6.8.
On my QNAP I found none of this necessary, but only found out much later after a lot of tinkering.

Backup Synology to Unraid

The easiest way to backup a Synology NAS to Unraid Server is to use Hyper Backup on Synology and rsync on Unraid.

Click to Close
First, enable rsync on your Unraid Server. It is preinstalled but not running as a daemon.

Create a file /boot/custom/etc/rsyncd.conf with the following content:

uid             = root
gid             = root
use chroot      = no
max connections = 4
pid file        = /var/run/rsyncd.pid
timeout         = 600


    path = /mnt/user/backups
    comment = Backups
    read only = FALSE

Here above:

• The name "backups" between brackets will be visible as "backup module" from the Synology. You can create several blocks like this one.
• The "path" (here /mnt/user/backups) must exist on your Unraid server.
• Notice: the folder /boot should exist. But you could possibly have to create the subfolders /custom/etc

Next, create a file /boot/custom/etc/rc.d/S20-init.rsyncd with the following content:

#!/bin/bash

if ! grep ^rsync /etc/inetd.conf > /dev/null ; then
cat <<-EOF >> /etc/inetd.conf
rsync   stream  tcp     nowait  root    /usr/sbin/tcpd  /usr/bin/rsync --daemon
EOF
read PID < /var/run/inetd.pid
kill -1 ${PID}
fi

cp /boot/custom/etc/rsyncd.conf /etc/rsyncd.conf

Finally, add the following line in the file /boot/config/go :

#!/bin/bash

bash /boot/custom/etc/rc.d/S20-init.rsyncd

Now, either reboot or execute: bash /boot/custom/etc/rc.d/S20-init.rsyncd

Go now on your Synology and open "Hyper Backup" to Create a new Data Backup Task:

Select rsync as Backup Destination:

And Create the backup Task with "rsync-compatible server" as Server Type: