What is a glue record?

In the Domain Name System (DNS), a glue record is a specific type of record that helps in the resolution of domain names to IP addresses when the authoritative name servers for a domain are within the same domain itself.

Glue records are necessary when you're delegating a subdomain to a different set of name servers within the same domain. This situation often occurs when you're creating a subdomain (e.g., subdomain.example.com) and setting up its own name servers. For this new subdomain to be reachable, its name servers need to be specified within the parent domain (example.com), creating a delegation.

However, there's a bit of a circular problem when defining the name servers for the subdomain within the parent domain. The name servers for the subdomain are under the subdomain itself, so to resolve this circularity, glue records are used.

Glue records are the IP addresses associated with the name servers in the parent domain's zone file. They are provided by the domain's registrar (the entity from which the domain was purchased). These records act as a "glue" to link the subdomain's name servers to their IP addresses, enabling proper DNS resolution for the subdomain.

For example:

• If you have "subdomain.example.com" with its own name servers ns1.subdomain.example.com and ns2.subdomain.example.com, these name servers' IP addresses need to be defined in the parent domain's zone file (example.com). These IP addresses in the parent domain's zone file are the glue records.

Without these glue records, the resolution process might fail because a resolver looking up ns1.subdomain.example.com would need to know the IP address of ns1.subdomain.example.com, creating a loop as it would have to query the subdomain's name servers to find the IP address of those name servers.

Glue records help to prevent this loop by providing the necessary IP address information for the name servers of a subdomain when they are hosted within the same domain. This ensures proper resolution and functionality of the DNS delegation for the subdomain.

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