DNS

DNS Servers Open to Public Query

This is not an exhaustive list of publicly accessible DNS servers. There are more, those listed below are a subset, and may be better or worse than others.

IP Address	Provider	Location
8.8.8.8	Google	Global (routed via Anycast)
8.8.4.4	Google	Global (routed via Anycast)
156.154.70.1	DNSadvantage	Global
156.154.71.1	DNSadvantage	Global
208.67.220.220	OpenDNS	Global, mainly US (routed via Anycast)
208.67.220.222	OpenDNS	Global, mainly US (routed via Anycast)
4.2.2.1	Level 3	Probably global (routed via Anycast)
4.2.2.2	Level 3	Probably global (routed via Anycast)
194.72.9.34	BT	UK
194.72.9.34	BT	UK

Sources: http://theos.in/windows-xp/free-fast-public-dns-server-list/

Troubleshooting

NSLookup

NSLookup is a Name Server Lookup/Query tool which exists in both Unix and Windows distributions, and generally speaking, functionaly the same regardless of which version you're using. To start, run nslookup from the command line.

Command	Description
server	Displays the DNS server(s) currently being used by nslookup
server 8.8.4.4	Changes the DNS server being used/queried to 8.8.4.4
host.domain.com	Attempts to resolve host.domain.com against current DNS server
109.123.76.85	Attempts a reverse lookup for IP 109.123.76.85 against current DNS server
set q=mx	Set query type to MX (subsequent queries will look for Message Exchange (email) records)
set q=all	Set query type back to all records
exit	Quit nslookup

For example, to find the email exchange servers for a particular domain...

> set q=mx
> microsoft.com
Server:  [8.8.4.4]
Address:  8.8.4.4

Non-authoritative answer:
microsoft.com   MX preference = 10, mail exchanger = mail.messaging.microsoft.com
>

Propagation

One of the features of DNS is the way that records get cached around the globe. Meaning that any changes to existing records have to ripple around. Generally changes take up to 72 hrs (3 days) to take effect.

However, you can change the TTL of record to ensure that it needs to be refreshed from its authoritative name-server more frequently than normal, ensuring that any changes apply more quickly.

To view the propagation of a record around the world see http://www.whatsmydns.net

General

The following site has a number of tools that allow you to investigate how a domain is viewed externally/publicly...

http://enc.com.au/itools/