SOA

mnameにはNSレコードのネームサーバの値が入っているように見えるがなんなのか

It is quite clear in the specifications, yet seems to have been widely ignored, that the MNAME field of the SOA record should contain the name of the primary (master) server for the zone identified by the SOA.

It should not contain the name of the zone itself. That information would be useless, as to discover it, one needs to start with the domain name of the SOA record - that is the name of the zone.

dynamic updateはmnameめがけて飛んでくる

普通のクエリは来ないらしい