43
Domain and IP registration information lookup (WHOIS)
WHOIS Protocol
What is port 43?
Port 43 is used by the WHOIS protocol to query domain name and IP address registration information: registrar, registrant, registration and expiry dates, nameservers. It works via simple plaintext TCP queries to authoritative WHOIS servers. Many WHOIS lookups today use the HTTPS-based RDAP (Registration Data Access Protocol) as a modern replacement, but port 43 remains in active use.
Quick checks
$
nc -zv example.com 43
$
nmap -p 43 example.com
$
telnet example.com 43