A glue record is a term for a record that's served by a DNS server that's not authoritative for the zone, to avoid a condition of impossible dependencies for a DNS zone.
The primary reference is:
- RFC 1033 - Domain Administrators Operations Guide
An excerpt from the RFC explains the topic concisely:
GLUE RECORDS
If the name server host for a particular domain is itself inside the
domain, then a 'glue' record will be needed. A glue record is an A
(address) RR that specifies the address of the server. Glue records
are only needed in the server delegating the domain, not in the
domain itself.
Glue records are found in the ADDITIONAL section of a DNS packet. They are the A and/or AAAA records associated with a nameserver that control of a domain is being delegated to by a nameserver higher in the hierarchy.
Nameserver glue is frequently provided by the Top Level Domain (TLD) nameservers in order to facilitate domains that provide name servers within the same domain that they are authoritative for. Without this glue, it would be impossible for an empty resolver cache to find the IP address of the authoritative nameservers due to recursive dependencies.
TLD glue records are usually configured through a web interface provided by the domain registrar that the domain was obtained through. Non-TLD glue is typically added to the same zone file that is performing the delegation, making the inclusion of a trailing dot mandatory within the definition.
Also see: What is a glue record?
