What is Glue Record?

A glue record is a DNS resource record (A or AAAA) that a parent zone publishes alongside a delegation NS record when the authoritative nameserver for the child zone has a name that lies within that child zone itself. For example, if the zone "example.com" delegates authority for "sub.example.com" to a nameserver named "ns1.sub.example.com", a resolver must know the IP address of ns1.sub.example.com to query it for data about sub.example.com. But the only place that can answer queries about ns1.sub.example.com is the sub.example.com zone itself, creating a chicken-and-egg problem. The glue record breaks this cycle by providing the A or AAAA record for ns1.sub.example.com in the parent zone (example.com), so resolvers can complete the iterative lookup without entering the child zone first.

Glue records are defined in RFC 1034 and RFC 1035. They exist only in the parent zone's apex data, stored as additional information alongside the NS delegation set. When a resolver receives a referral response for a delegation, the parent nameserver includes the glue records in the Additional section of the DNS response. This allows the resolver to immediately have the IP address needed to query the delegated nameserver without performing an intermediate lookup. A glue record must match the nameserver name exactly and must be a non-CNAME record. Registrar and registry systems enforce the existence of glue records during domain delegation; if a nameserver's hostname is not an IP address literal and ends in the domain being registered, the registrant must provide corresponding glue records.

Glue records can become stale if the child zone's IP address changes but the parent zone's glue record is not updated. DNSSEC does not sign glue records in the parent zone because they are external to the child zone's trust anchor; the parent zone's delegation signer (DS) record chain does not cover them. In stealth or multi-homed configurations, operators may include "sibling glue" -- extra records for nameservers not in the child zone -- to improve resilience. Despite their name, glue records are not a separate record type; they are ordinary A or AAAA records placed in an unusual location.