@louishothot RFC 里要求这样做，因为 ns1.qq.com 在 com 的 name server 里是被认为是“非权威记录”，为了保证“权威 name server 记录有该域下所有的记录”这个原则，所以 glue records 也得添加在 ns1.qq.com 下。

http://www.rfcreader.com/#rfc1034

One of the goals of the zone structure is that any zone have all the
data required to set up communications with the name servers for any
subzones. That is, parent zones have all the information needed to
access servers for their children zones. The NS RRs that name the
servers for subzones are often not enough for this task since they name
the servers, but do not give their addresses. In particular, if the
name of the name server is itself in the subzone, we could be faced with
the situation where the NS RRs tell us that in order to learn a name
server's address, we should contact the server using the address we wish
to learn. To fix this problem, a zone contains "glue" RRs which are not
part of the authoritative data, and are address RRs for the servers.
These RRs are only necessary if the name server's name is "below" the
cut, and are only used as part of a referral response.