Checking domain name availability with DNS records
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
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.