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Checking domain name availability with DNS records

Answer 1

+1 A:

Whois is the canonical way to check for domain availability. The rate limiting is there deliberately to keep folks from overloading the whois servers. Existence of SOA records could be a good guess, but it isn't a certain answer.

Corey Porter 2009-11-23 00:51:03

Yes, checking for SOA isn't perfect. But is there a better way, perhaps in conjunction with other records, to improve accuracy?

FreshCode 2009-11-23 01:00:10

While it may be the canonical way, whois is also a notoriously unreliable way to check for domain availability, since each registrar operates their own databases, with their own non-uniform policies.

RickNZ 2009-11-23 01:12:18

RickNZ: it depends on the TLD. For the vast majority of the TLD, the whois server is operated by the registry, not the registrars.

bortzmeyer 2009-11-23 08:24:36

Within .com, for example, there are multiple sources / databases of whois records (the so-called "thin" model), which are located with and run by registrars. Many of those servers have a reputation for being offline regularly. Also, some TLDs don't even have whois servers (including .co.za).

RickNZ 2009-11-23 23:18:23

Answer 2

A:

In practice, the DNS lookup works 99% of the time. If you writing something like Ajaxwhois, I'd suggest going this route. If you have a million names to check, I would do a first pass with a DNS lookup, and then maybe do a second pass on the ones which you may consider purchasing.

brianegge 2009-11-23 01:26:02

DNS lookup fail for all the TLD where you can book a domain without publishing it in the DNS...

bortzmeyer 2009-11-23 08:25:07

.co.za domains require valid nameservers upon registration.

FreshCode 2009-11-23 15:21:55

Answer 3

+1 A:

The only 100% reliable way to check for domain availability is to query the registrar's database. I wouldn't trust whois.

You can use DNS to get an estimate. Instead of looking for an SOA record, I would just look to see if anything at all is listed with the TLD name server. dig is a good tool for this (runs on Windows, too), although I guess you could use nslookup too. For example:

dig co.za. NS

will provide a list of the name servers for .co.za. One of those servers is ns0.is.co.za.

Next, query that server directly to see if they have anything listed for your domain of interest:

dig ibm.co.za @ns0.is.co.za

That query returns NS records, but no SOA record since SOA records are provided by the domain's name server (which may or may not be online). The NS records indicate the domain name is in use.

The reason for going direct is that it's usually much faster than relying on recursive queries from your local name server.

RickNZ 2009-11-23 01:29:34

Hence, the existence of an NS record is a better indication that it is taken? Can you elaborate on "going direct"?

FreshCode 2009-11-23 02:20:14

Yes, NS records are a better indication. If they don't exist, you can't get the SOA record or anything else meaningful using DNS. By "going direct", I mean querying only the TLD server. The normal query process goes something like this: your app queries the local DNS, which forwards to a shared DNS (optional; such as on a router) which forwards to your ISP's DNS which queries root servers to find the right TLD server, then queries the TLD server to find the domain's server, then queries the domain server. To determine domain validity, just query the TLD server; nothing else is needed.

RickNZ 2009-11-23 07:58:27

Is there an overarching .co.za TLD nameserver? If not, which DNS server will provide the quickest answer for .co.za queries?Bulk checking for "ANY" DNS records seems to find virtually all registered .co.za domains.

FreshCode 2009-11-23 15:25:21

The first dig query above will return a list of all of the .co.za TLD servers, any one of which can handle queries for the entire domain. If you're planning a bulk query process where perf is important, you could ping all of them and see which one is topologically closest.

RickNZ 2009-11-23 23:04:11

Good suggestion. ns0.is.co.za. seems to have the least hops inbetween.

FreshCode 2009-11-24 00:51:57

Answer 4

A:

Whois is the only 100% sure way of checking for .co.za domain availability. See http://co.za/whois.shtml
My tests show that checking for the existence of NS records or ANY DNS records in bulk seems to be the most accurate way of guessing whether a .co.za domain is available for registration. SOA records work too, but not as well.
dig co.za. NS shows .co.za TLD nameservers. Ping TLD servers and use the closest ones to avoid unncessary recursive queries.

Thanks for your answers.

FreshCode 2009-11-24 01:07:45

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