I run ubuntu 22.04 OS which has rsync installed as backup server.

How to enable it as a service which should be started up at system reboot?

After 'atp install rsync', I have to create the config file for rsync server. The config is located at /etc/rsyncd.conf. You should have this file existing to let rsync start up as a daemon.

 sudo vi /etc/rsyncd.conf # save and close it
 sudo cp /lib/systemd/system/rsync.service /etc/systemd/system/rsync.service

Now you can start it by manual,

 sudo systemctl start rsync

But if you want to make it live after system reboot, you should enable it as a systemd service.

 sudo systemctl enable rsync

And you maybe want to edit the file '/etc/default/rsync' (it's based on your startup way) and setup this line:

 RSYNC_ENABLE=true

Now reboot the OS, and you will see rsync startup as the daemon listening on port 873.

 $ sudo netstat -ntlp
 Active Internet connections (only servers)
 Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name    
 tcp        0      0 0.0.0.0:873             0.0.0.0:*               LISTEN      833/rsync

Here are the content of my rsyncd.conf,

 use chroot = yes
 max connections = 4
 syslog facility = local5
 pid file = /var/run/rsyncd.pid
   
 [VPS]
     uid = nobody
     gid = nobody
     hosts allow = xx.xx.xx.xx yy.yy.yy.yy
     hosts deny = *
     path = /data/rsync
     comment = remote backup
     read only = no
   
 [MAC]
     uid = nobody
     gid = nobody
     path = /data/rsync
     comment = backup for mac
     auth users = authUser
     read only = no
     secrets file = /etc/rsyncd.secrets