Edit the file /etc/systemimager/rsyncd.conf and put something like this into it:
#hosts allow = 127.0.0.0/24 MY_NET/NETMASK MY_CLIENT/32
hosts allow = 192.168.0.0/24 126.96.36.199/32
hosts deny = *
This will allow access from the subnet 192.168.0.0 and from the single machine 188.8.131.52; all other systems will be blocked.
Restart your SystemImager server afterwards: