Glue records are A or AAAA records, which are non-traditionally stored in authoritative DNS servers one level higher than the usual. Earlier. it was mentioned that root DNS servers already contain the IP address for the domain name “ns.tld.cz”. This is because authoritative DNS servers are listed everywhere mainly by their name, but in order to connect to them, we need an appropriate IP address. To find out the IP address to the name “ns.tld.cz”, we must first find out the authoritative DNS servers for the domain “tld.cz” via the cz domain, but to do that we first need to connect to the DNS servers of the “cz” domain. This, however, is (among others) “ns.tld.cz”, so we are trapped in a loop and we are not likely to learn anything in this usual way.

In these cases, when the DNS server contains in its name a domain for which it is authoritative, its IP address must also be listed on the DNS server one level above. This is called a glue record. It is therefore obvious that the A record for “ns.tld.cz” must directly know all root DNS servers, and indeed when directly querying “ns.tld.cz”, root server A returns “217.31.196.10” to us.

A glue record may be present for NS records, which do not require it. It accelerates the work with the DNS system (otherwise it is necessary to carry out some additional steps to obtain the IP addresses of a DNS server). On the other hand, glue records cause some duplicate information (because outside the glue record, the same A record exists directly in the zone of the respective domain), so you can easily make the mistake of changing the IP address of the DNS server only in one place and forgetting about the other. This is why glue records are not used today where it is not necessary.