A Glue Record is the IP Address of a name server at a domain name registry. Glue Records are core parts of DNS records because they help to resolve DNS servers at a core level. If you would like to change the name servers for a site, you'll have to provide the Glue Records for the new name serves. Without this, a domain name will not work because anyone requiring the DNS information will be stuck in a loop. There is a cyclic dependency of circular referencing. Circular references exist when the name servers for a domain can't be resolved without resolving the domain they're responsible for. So, a Glue Record is an additional A record that allow the DNS client to locate name servers.
Let's say you own a DNS zone for
example.com. You'd want to have DNS servers that are hosting the authoritative zone for this domain - adding records for the root of the domain,
There's the trick. The TLD's servers will delegate to the DNS servers in the whois record - but they're within
example.com. They try to find
ns1.example.com, ask the
.com servers, and get referred back to...
What glue records do is to allow the TLD's servers to send extra information in their response to the query for the
example.com zone - to send the IP address that's configured for the name servers, too. It's not authoritative, but it's a pointer to the authoritative servers, allowing for the loop to be resolved.
Until next time!