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DNS: Glue records

DNS: Glue records


and more specifically


Circular dependencies and glue records

Name servers in delegations appear listed by name, rather than by IP address. This means that a resolving name server must issue another DNS request to find out the IP address of the server to which it has been referred. Since this can introduce a circular dependency if the nameserver referred to is under the domain that it is authoritative of, it is occasionally necessary for the nameserver providing the delegation to also provide the IP address of the next nameserver. This record is called a glue record.

For example, the has the following authoritative name servers

[m7h@trigger]$ dig NS +short

[note: intentionally omitted to emphasize a point]
A computer trying to resolve will thus first have to resolve,, or But how can the computer resolve one of those names if in order to lookup the name, it needs to know the DNS server IP? In this case, the dependency is broken by the glue record in the nameservers for .edu. This provides the IP address of, and to the requester, enabling it to bootstrap the process by figuring out where the nameservers are located.

As of 4/22/2009, here are the glue records for adns1,2, as seen in the zone file.            IN  NS            IN  NS            IN  NS      IN  A      IN  A      IN  A
If the IP address of adns1,2,3 ever changes, it would need to be updated in the zone as well as in the zone, where the glue record is kept. If you omit the final step, adns1,2,3 will be unresolvable for ALL recursive nameservers.

Glue records and determining authority

A glue record doesn't provide any authority, it just tells a recursive nameserver where to find an authoritative answer. Ultimately, the authoritative nameservers provide this information.

For example, lets say that only the following records for the domain existed in            IN  NS      IN  A
A dig might still reveal the following.
[m7h@trigger named]$ dig NS +short | sort
Where did adns2 and adns3 come from? The recursive client is given the adns1 glue record for the zone from CS, but adns1 reports that itself, as well as adns2 and adns3, are authoritative.

Glue records for adns2 and adns3 are critical. If they are not in place, and adns1 goes offline, the zone can not be queried because a recursive DNS server will not know to ask adns2 or adns3

Keywords:DNS: Glue records   Doc ID:8959
Owner:Tim C.Group:DNS, DHCP, and IPAM
Created:2009-01-29 18:00 CSTUpdated:2020-08-04 10:12 CST
Sites:DNS, DHCP, and IPAM, Network Services, Systems & Network Control Center, University of Wisconsin System Network
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