Do a DNS query. You
don't even have to randomise the id number, just query for
something that will have a small set of results (so, not the
root) and ensure checking is disabled. For 8.8.8.8, I'm
guessing "dns.google" is probably an excellent target.
If you wanted
something generic, what about a PTR query for something in
10/8, directed at the AS112 project? That's pretty much the
sinkhole that expects that kind of unwanted traffic...
I bet that within a
gnat's crotchet you'll find systemd has adopted that as a
special "liveness" command or something. </snark>