Lubuntu is a promising lightweight flavor of Ubuntu that delivers a fresh and 
smooth experience even on old or ‘light’ hardware.
It still has some rough edges however (not many, this is the one I’ve discovered). Out of the box, the usual shortcut Ctrl+Alt+L doesn’t work (bug 1286686).
However an easy fix is available and it does not even require a reboot or a log-out command. In a pinch, one just needs to change the command tag using dm-tool
<command>dm-tool lock</command>
and issue a openbox –reconfigure. Thanks to Fred Schenk and others for sharing the solution.