DNS troubleshooting is actually a bit of fun (maybe) and I had to do some of it recently for spectrumautomation.com.au. The end of the story was that the authority records were pointing to the completely wrong place. Although it was fairly easy to see, proving it was the difficult part. So here are some tools I found useful.
nslookup
One of the simplest tools out there. Does as its name suggests, DNS lookups.
dig
dig lets you grab a whole heap of DNS information, basically the entire DNS response from any server that you want. It's extremely useful to track down the DNS lookup process by basically iterating down from the root server all the way to the final authoritative servers.
dnstracer
A cool tool to automatically perform a similar trace as to what can be done by dig, a good way of getting an overall view of how the DNS system is arranged.
DNS Bajaj
DNS Bajaj generates a diagram showing the delegation and connection between the DNS servers for a domain, all the way down from the top level domain.
DNSStuff.com
DNSStuff.com has a very useful DNS report tool which will give you a wide range of information regarding the DNS information for your domain including failures and warnings with respect to RFCs. They also provide web access to standard tools such as ping, traceroute, dig and whois, although the tools are useful only for a certain amount of tests. I used DNSStuff.com as another test point to avoid caching issues.
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