A glue record is a term for a record that’s served by a DNS server that’s not authoritative for the zone, to avoid a condition of impossible dependencies for a DNS zone.
Say I own a DNS zone for example.com
. I want to have DNS servers that’re hosting the authoritative zone for this domain so that I can actually use it – adding records for the root of the domain, www
, mail
, etc. So, I put the name servers in the registration to delegate to them – those are always names, so we’ll put in ns1.example.com
and ns2.example.com
.
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… ns1.example.com
.
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.