Checking domain name availability with DNS records

Question

How accurately can a domain name's availibility be determined by checking for the existence of NS or SOA (Start of Authority) records?

If not, how can I determine this as accurately as possible without resorting to WHOIS? For example, is it worth checking for MX records if no NS records exist?

I am specifically interested in .co.za domains, but the only official, fool-proof way to check availability in the .co.za namespace is to use http://co.za/whois.shtml, which limits consecutive checking by IP address.

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

If you plan to do bulk checks you will be soon disappointed by the built-in DNS resolver - it is synchronous. There is a nice cute library called ADNS (http://www.gnu.org/software/adns/) which offers asynchronous DNS queries. It is accompanied by a small command-line tool adnshost which I am using in combination with N-DJBDNS (http://pjp.dgplug.org/ndjbdns/) as a recursive resolver.

The exact arguments are adnshost -a -tns -f

The 4th column in the output will contain the result code:

• 0 if there is at least 1 name server assigned for the domain (most registrars assign a default one until the owner assigns his own)
• 300 or 301 if there are no name servers assigned

Checking 1.2M domain names takes about 4-6 hours but this will vary with network speed/congestion.

Answer 5

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.