I have just been through this so that I can use my libreelec box (a droidbox) as an rsync server. I can now backup to its HDD from my Synology NAS.
As already stated the rsync command is in: (once you've installed from the repo)
/storage/.kodi/addons/virtual.network-tools/bin/rsync
In that folder (since the root filesystem is read-only) create an rsyncd.conf file with the following contents (but change the path you want the backup folders to go into)
pid file = /var/run/rsyncd.pid
lock file = /var/run/rsync.lock
log file = /var/log/rsync.log
port = 873
uid = nobody
[files]
path = /media/HDD/NAS_Backup
comment = RSYNC FILES
read only = false
timeout = 300
The [files] entry here creates a 'module' called files, that will be available to the backup app on your NAS.
You can change the port number here, but you must match it in the entry in the backup app (eg Hyper Backup)
In the directory where the rsync binary is located, start the rsync daemon (do all of this via ssh using PUTTY or similar)
rsync --daemon --config=./rsyncd.conf
Check that it is runing: (following command should return 2 lines, the first being the' rsync --daemon....' command)
ps | grep rsync
Now it is useful to keep an eye on the log file before continuing..
tail -f /var/log/rsync.log
Now you can create a new backup config on your NAS. The example below is using Hyper Backup (Synology)
When you create a new backup session and get to the page below, enter the ip address of your libreelec box, the port number, and other options similar to those below.
The dropdown for 'Backup Module' should present you with a list of the modules as per the rsyncd.conf file. - Note that this step will only work once the rsync daemon is running on your libreelec box.
Then enter a name for the backup dir that will be generated
On the following screens select what you want to backup, provide a sheduled time, and 'APPLY'. You get asked if you want to run the backup now. Select yes to test it out.
Keep an eye on the rsync log file, and in the directory that you are expecting the backup files to appear in (/media/HDD/NAS_Backup in my case).
Hope this works for you