All you need to know about Linux whois command (domain lookup)

  • Domain names (e.g.
  • IP address (e.g.
  • ASN (Autonomous System Numbers) (e.g. AS15169)

Registries and Registrars

When you lookup a domain name (e.g. using the command “whois”, you will probably end up with the following result:

Two-level WHOIS record organization
Thick WHOIS record for
  1. Get the thin record from the registry.
  2. Extract the registrar’s whois server from the thin record and get the thick record from registrar’s who is server.
  • Sometimes, thick record does not mention the whois server to lookup. You may need to use your wit to find the whois server based on the registrar’s name.
  • Some registrars explicitly block whois lookup from command line (e.g. They provide web based lookup with CAPTCHA protection — you need to manually look it up — no automation. It makes sense have such rate limitations to prevent DoS attacks on whois servers maintained by registrars.



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