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I have found several domains on the internet that have no nameservers, however they have WHOIS records and domain registrars say they are taken.

My understanding of DNS on the internet was that the presence of a domain's nameserver showed that is was registered, but apparently this is incorrect.

So, how can a domain not have a nameserver, and what determines if a domain is available or not? WHOIS?

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The only way to determine if a domain is available or not is to query the registry database for that domain. Usually you can do that by performing a WHOIS query on the registry WHOIS interface.

There are several reasons to explain why a DNS check is not safe.

The most simple explanation is because you can register a domain without pointing it to any location. It's like a house. You can buy a house, but you are not forced to build a road to reach it.

The most part of available domain providers force you to point a DNS, otherwise they set a default one. But you're not forced, it's not a kind of mandatory domain requirement.

The second reason is because there are actually cases when a domain is registered but not available. One of this cases is the period often known as redemption period. This period lasts for some days after the domain is expired and the current owner don't renew it. Several registries disable the domain, it means it is no longer reachable via DNS, but the domain still exists and can be redeemed (usually) only by the original owner.

The DNS check is cheap and sometimes faster. If you need to check whether the domain exists, then you can try a DNS query first and, in case of failure, fallback to the WHOIS query. In other words, if the DNS query succeed the WHOIS query is almost pointless. But you cannot do the opposite because if a DNS query fails, it doesn't mean the domain is not registered.

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  • whois is not a DAS (Domain Availability Service) so using whois is not always sufficient. You can have rules like "one-letter names are forbidden". – bortzmeyer Jul 12 '17 at 11:52
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Yes, you are incorrect. A domain is only available if it has not been registered; that it is not available via DNS does not indicate that it has not been registered, merely that it is not able to be looked up on the Internet.

This stack overflow question gives more useful details on the whole process.

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As stated in this StackOverflow question "Checking domain name availability with DNS records" registrar databases are the only definitive way to determine if a domain name is owned or not. DNS is only necessary if you plan on making your domain resolvable.

(Answer stolen, with permission, from Zoredache's comment as per this Meta thread.)

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Whois records are only valid for some domains, usually delegations from some TLDs (including odd ones like *.co.uk.), where some registrars handle the administrative tasks of delegating subdomains.

So technically, you are right: a domain is free if there is no delegation and no resource: it is not in use.

On the administrative side, whois records are the most convenient way to go, but lots of rules may apply: some names may be judged as offensive, some TLDs are reserved to some countries, ... even if whois records say the domain is free.

Some domains operate without any registrars nor whois records: eu.org is a good example.

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  • I see a whois record for eu.org, so I'm not sure what your point is. Or are you saying that eu.org doesn't run a whois for its subdomains? – MadHatter Jun 2 '17 at 11:28
  • There is a whois record for eu.org, but not (to my knowledge) for all its subdomains, which anyone can register for free. – Xavier Nicollet Jun 14 '17 at 12:54
  • But eu.org isn't a registry; they're simply the owner of a domain who are renting usage rights to subdomains thereof. That's really the point of the question, and it seems to me that you're missing it - whois records are indeed authoritative, but you have to know where you should reasonably expect them - and soi-disant registrars like the operators of eu.org and uk.com aren't the right place. – MadHatter Jun 14 '17 at 13:50
  • You're right, the question is a bit ambiguous: if it's about the registries only, then Whois is the only official source of truth. If it's about the existence of a domain, then NS records are authoritative. The absence of a registry does not make it less official from a technical point of view. The DNS protocol works without any knowledge of any whois database. – Xavier Nicollet Jun 28 '17 at 17:01
  • Well, not entirely - the glue records have to come from somewhere, and the registrars populate them from the whois data. – MadHatter Jun 29 '17 at 15:55

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