Thanks for the tutorial, I am using this for my Synology backup destination.
However you need to edit the steps for mkdir as you missed the /etc/ folder on the third line, meaning the folder doesn't exist for the S20-init.rsyncd.conf
mkdir /boot/custom
mkdir /boot/custom/etc
mkdir /boot/custom/rc.d > mkdir /boot/custom/etc/rc.d
nano /boot/custom/etc/rsyncd.conf