A few days ago I wrote up a test that passed on Windows and failed on Linux, and one of its three lessons was: give every negative probe a control arm. A probe that asserts "nothing is listening on this port" cannot, on its own, tell "the listener is gone" from "I built the probe wrong" — so you stand up a second listener you know is running, and check the probe reports it . That advice is right. It is also not enough, and I know because my control arm lied to me. This is the follow-up: t...