WHOIS.

WHOIS is a query-and-response protocol from the early internet: connect to port 43, send a domain name, receive a block of text. It was standardised in 2004, but the standard covered the transport and pointedly not the format of the reply, which is the root of every problem it has.

Because nothing defines the response layout, every registry answers differently. One labels the expiry as Registry Expiry Date, another as expires, another as paid-till, another as renewal date, in a different date format, in a different order, sometimes in a different language. Anything consuming WHOIS is therefore parsing prose with regular expressions and hoping. When a domain tool reports an expiry date that disagrees with your registrar, this is almost always where it went wrong.

There is a second subtlety. .com and .net run on a thin registry model: Verisign holds only the sponsoring registrar, the status codes and the dates, while contact details sit with the registrar. So a registry WHOIS query gives you a partial picture and you have to follow the referral to the registrar for the rest. Most other gTLDs use a thick model and answer everything from one place. Since GDPR, the contact half of the answer is mostly redacted anyway, whether or not you have bought WHOIS privacy.

For gTLDs, WHOIS is now on the way out: the contractual requirement to operate it ended in January 2025, and RDAP is the supported replacement. The word will stick around for years because everyone types it, but if you are building anything that depends on registration data, build it on RDAP and keep WHOIS as a fallback for the ccTLDs that have not moved. Our bulk lookup tool does exactly that.