1. DNS/用語/glue/RFCより
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.
現れるのはreferral responseのadditional section内だけだ。