Glue record is an useful DNS (Domain Name System) record to break circular dependency in name resolution while using a subdomain as a nameserver of the domain itself. It glue or stick an IP address with a sub-domain in Top Level Domain (TLD) registry and return it when quarried for a primary domain, using additional section of DNS reply. Starting from root name-server DNS name resolution proceeds through TLDs’ name-server then register’s nameserver and downwards. So, in name resolution hierarchy a sub-domain can’t be resolved before the primary domain, unless specified otherwise.
Consider a scenario where ns.serverlog.net is the nameserver of serverlog.net. To resolve serverlog.net or any of it’s sub-domain browser will ask root and then .net’s nameserver.
- Browser: “What is the IP address of serverlog.net?”
- TLD’s Nameserver: “ns.serverlog.net is the authoritative nameserver of serverlog.net. Ask him you’ll find the IP.”
- Browser: “Fine..!! What is the IP of “ns.serverlog.net?”
- TLD’s Nameserver: “ns.serverlog.net is a sub-domain of serverlog.net, ask the authoritative nameserver of serverlog.net, you’ll find the IP”
- Browser: “ns.serverlog.net is the authorative nameserver of serverlog.net and it’s a subdomain of serverlog.net means ns.serverlog.net only know the IP address of ns.serverlog.net. Then from where I’ll know the IP of eigther serverlog.net ot ns.serverlog.net…!! ? ? “
Glue record comes into this play
- Browser: What is the IP address of serverlog.net
- TLD’s Nameserver: “ns.serverlog.net with IP address 192.168.0.1 is the authoritative nameserver of serverlog.net. Ask him you’ll find the IP.”
- Browser: ? ?
Adding Glue Record: Domain registrar can add a glue record in TLDs’ registry. Most of the registrar provide web based tool for adding and maintaining glue record in their customer’s control panel. I’ve tested Namecheap.com, Bigrock.in, Namesilo.com, all worked file.