WHOIS Protocol
The WHOIS protocol (RFC 3912) is a query-response protocol that provides registration data about domain names, IP addresses, and autonomous systems over TCP port 43. Originally plain-text and unauthenticated, WHOIS has largely been superseded by [[rdap|RDAP]] for structured, access-controlled lookups. GDPR-driven redaction has significantly reduced the personal data visible in WHOIS responses since 2018, with [[icann|ICANN]]'s SSAD framework governing access to non-public data.
Example
Running `whois example.com` on the command line queries a WHOIS server on port 43 and returns registration information in unstructured plain text.