A record used to resolve an authoritative DNS server's own name, avoiding a circular reference.
A record that holds an IP address directly in the parent zone to avoid a circular reference when the authoritative server itself resides within the zone it serves. For example, if example.com's name server is ns1.example.com -- itself inside that zone -- the parent .com servers hold a glue record with ns1.example.com's IP address directly, sidestepping the circular lookup that would otherwise result.
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