For some reason, ZyXEL NAS326 does not have an rsync binary. This means that you cannot copy to it via rsync, which is completely inconvenient.
Previously, you could use the installation of the RandomTools package, but now the domain zyxel.diskstation.eu it doesn’t work (and it’s unlikely to work). So I had to figure out how to do it now.
Just a few commands combined into a script (you may have a different value instead of d732885c):
cat >/i-data/d732885c/install_rsync.sh <<EOF
curl -fLO http://entware.zyxmon.org/binaries/armv7/installer/entware_install.sh
chmod +x entware_install.sh
./entware_install.sh
/opt/bin/opkg install --force-space rsync
cp /opt/bin/rsync /bin
EOF
chmod 755 /i-data/d732885c/install_rsync.sh
Only after the reboot, everything except the script disappears. You will need to repeat it:
/i-data/d732885c/install_rsync.sh
You can probably copy the /apk to /data/d732885c/rsync/ and restore it from there without the Internet, but so far it is not required.
You won’t be able to copy one rsync binary, because it depends on a lot of libraries (in theory, you can find an assembled binary somewhere that would include all libraries and not depend on external ones, but again, you don’t want to waste time on it yet - everything works and there are a lot more things that need to be done).