The Domain Expiry Monitor detects the expiry date of a domain via WHOIS records. WHOIS is a socket level protocol that runs over TCP port 43. A WHOIS client queries each server, starting from the root server until it receives a response containing the records.Root (whois.iana.org) > Registry (TLD specific) > Registrar
WHOIS registries can be THICK or THIN. Most registries use a THICK model which stores the WHOIS records of all the domains they're responsible for. Thin Lookups
Thin Lookups are those where the queried domain information is fetched from the Registrar's Whois Server. A few of the top level domains like .com, .net etc. implement Thin Lookups. Thin Lookup is one where you can only get to information about the registry. Given below are excerpts from a thin lookup for site24x7.com. Below are excerpts from a thin lookup for zoho.com. Thick Lookups
Thick Lookups are those where the queried domain information is fetched from the Registry Server. Whenever such a domain is registered, or the user edits the WHOIS information of the domain, the registrar pushes the information to the registry.
Most TLDs maintain thick registries which contain all the information from registrars. Below are excerpts from a thick lookup for zoho.in.
There is no registrar lookup for a thick registry. The expiration date and other details are provided by the registry itself.