We introduced an upgrade test that verifies that DNS remains always available during the upgrade: https://teamcity.mesosphere.io/project.html?projectId=ClosedSource_Dcos_IntegrationTests_CloudIntegrationTests&testNameId=-1166573179581081236&tab=testDetails
We are observing a consistent failure with DNS unavailability during upgrade. The test is currently muted (without a open JIRA ticket). So, raising this ticket to track it and we might unmute the test to get more details on failure.
There were some DNS unavailability related failures previously: