Reserved DNS header flag is echoed/dropped/formerr

[marka63]

[vttale]

Specifically the Z bit. Mark's been doing tests that show that if it is set in the request then various misbehaviours occur with regard to the response. Per his message to dns-operations on 10 May 2019:

"I m currently testing the handling of the last remaining DNS header flag by DNS servers for the Alexa Top 1 Million sites. At this stage I m approaching 200000 sites and have a 11% misbehaviour rate where the servers echo back the flag. If we ever want to use this flag in the future we need to fix this issue now well before we start using the flag."