If you are using Microsoft for your DNS servers you really should update to a version that includes this fix from March 19, 2019 monthly updates. • Addresses minor issues with unknown options (unknown OPT) in the Extension Mechanisms for DNS (EDNS) for the Windows DNS Server role. https://support.microsoft.com/en-nz/help/4489893/windows-8-1-update-kb448989... Older versions have even more egregious misbehaviour, with really old versions (pre-EDNS support) being flagged as dead by recursive servers as they do not respond consistently to EDNS queries. They respond a single time then do not respond at all for a period of time. Mark -- Mark Andrews, ISC 1 Seymour St., Dundas Valley, NSW 2117, Australia PHONE: +61 2 9871 4742 INTERNET: marka@isc.org