If your machines started exhibiting behavior you didn't expect or intend from a configuration you believe to be proper, then something has gone wrong. It may be

If your machines started exhibiting behavior you didn't expect or intend from a configuration you believe to be proper, then something has gone wrong. It may be the system, it may be the configuration, and it may be your understanding of either. Or any combination. In this particular case it sounds like your resolver was set up to try an external resolution first and then append an internal domain if the external resolution failed. And as you say, this clearly worked just fine for a long, long time. Then it suddenly started failing one day, for reasons unrelated to anything you changed. At this point, most people would find it reasonable to blame the external change for breaking their fully functional, correctly working, "properly configured" setup. Some, perhaps contrarian or perhaps more cautious, would note that the "properly configured" approach only worked so long as external systems played ball. I think it might be the case that you were bitten by this assumption that seemed safe at the time, leading to the awkward and uncomfortable conclusion that your systems were indeed misconfigured.