A glue record is an A record or AAAA record published by a parent nameserver (typically a registry) that supplies the IP address of a child domain's NS record target when that nameserver's hostname lies within the child domain itself.
This solves a bootstrapping problem: if a domain delegates to a nameserver whose hostname is in the same domain (called "in-bailiwick"), a recursive resolver cannot look up that nameserver's IP without already knowing it. The parent zone includes the glue record so resolvers can fetch the child zone's records without circular lookups.
Example
Suppose example.com uses ns1.example.com as its nameserver. The .com registry's zone cannot just publish:
example.com. NS ns1.example.com.
and expect resolvers to work—they need the IP of ns1.example.com first. So the registry also publishes:
ns1.example.com. A 192.0.2.1
This glue record lives in the .com zone, not in example.com's zone.
When it matters
Glue records are essential when your domain's nameservers have hostnames under your own domain. They appear in the "Additional" section of DNS responses and are typically managed by your domain registrar. Most modern setups avoid this by using nameservers outside the domain (e.g., ns1.provider.com), but it remains common for organizations running their own infrastructure.
WarningGlue records are NOT authoritative data. They exist only in the parent zone to aid resolution; the authoritative copy lives on the child's own nameservers.