You want to check whether a particular domain name is already registered, or who has registered that domain name.
Use the whois service offered by the appropriate registration authority, or use a command-line version of whois to look up registration information about the domain name you're interested in.
The Internet Assigned Numbers Authority, or IANA, maintains a list of country-code top-level domains (ccTLDs) at http://www.iana.org/cctld/cctld-whois.htm,which includes links to the web pages of those ccTLDs registration authorities. Many of these web pages offer online whois lookups. The web site http://www.allwhois.com/also includes links to many whois lookup facilities.
If your host's operating system includes a command-line whois client, you can use that to look up to look up registration information about the domain name. Newer whois clients automatically determine which whois server to query, so you can simply run:
$ whois domain-name
Older whois clients may require you to specify the whois server to use. For these, you can try tld.whois-server.net. For example:
$ whois -h ca.whois-servers.net risq.ca
The whois output usually contains information about the registrant (the person or organization that registered the domain name). For example:
$ whois isc.org
produces output that includes:
Registrant: Internet Software Consortium (ISC2-DOM) 950 Charter Street Redwood City, CA 94062 US Domain Name: ISC.ORG Administrative Contact, Billing Contact: Conrad, David Randolph (DC396) drc@ISC.ORG Internet Software Consortium 950 Charter Street Redwood City, CA 94063 1-650-779-7061 (FAX) 1-650-779-7055 Technical Contact: Vixie, Paul (PV15) paul@VIX.COM M.I.B.H., LLC 950 Charter Street Redwood City, CA 94063 +1.650.779.7000 (FAX) +1.650.779.7055 Record last updated on 04-Mar-2002. Record expires on 05-Apr-2004. Record created on 04-Apr-1994. Database last updated on 14-Mar-2002 09:39:00 EST. Domain servers in listed order: NS-EXT.VIX.COM 220.127.116.11 NS1.GNAC.COM 18.104.22.168
If the registration authority for your prospective top-level domain doesn't offer a whois server, or you can't find it, you can look up NS records for the domain name you're interested in. For example:
$ dig ns domain-name
If the domain name has NS records, it's very likely registered. On the other hand, if a domain name lacks NS records, it may still be registered: some TLDs take a day or more to process a new registration and add the corresponding NS records.
1.5.4 See Also
"Using whois" in Chapter 3 of DNS and BIND.
BIND Name Server Configuration
BIND Name Server Operations
Delegation and Registration
Interoperability and Upgrading
Resolvers and Programming
Logging and Troubleshooting