As glue records are called A or AAAA records, which are stored in non-traditional authoritative DNS servers to the upper level, 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 the authoritative DNS servers are everywhere especially given by its name, but to be able to connect with them, we need the corresponding IP address. To find out IP addresses of the name “ns.tld.cz” we must first find out cz domain through the authoritative DNS servers for the domain “tld.cz”, but we will at first need to connect to the DNS servers domain “cz”, but it is (among other things) “ns.tld.cz” so we trapped in a loop and the usual way we would not know anything.
In these cases, where DNS server contains your domain name for which is authoritative, must be the DNS server to upper level include its IP address. This is called a glue record. It is therefore evident that the A record for “ns.tld.cz” you must know all the root DNS servers, and indeed in the direct request on “ns.tld.cz” root server returns A “18.104.22.168” to us.
Glue records may be present for NS records, which do not require it. It accelerates the work with the DNS system (otherwise it is necessary to do some additional steps to obtain DNS server IP addresses). On the other side glue records cause some duplicate information (because out there the same glue A record in the zone of the appropriate domain), so you can easily make a mistake by changing DNS server IP addresses will change it only in one place and on the other forget about. Therefore today’s glue records where that is not necessary, not using.