Google Public DNS s a free, global Domain Name System (DNS) resolution service, that you can use as an alternative to your current DNS provider. The service’s DNS servers IP addresses are easily memorable even by end users (who the service aims to help most) and they are:

• 8.8.8.8 and
• 8.8.4.4

There are other uses for the service. Many system administrators use it for troubleshooting DNS problem in their infrastructure as an objective third party with a DNS view from “outside” their network (plus you can say to your manager that hey this is Google’s DNS view of this zone setup when nothing else helps).

Those of us who host domains, web sites and mail infrastructures have at times faced the problem that domains come and go somewhere else. However, domain owners / administrators / subcontractors / etc often neglect to inform the previous infrastructure that the domain has a new home. Then appears the phenomenon that most of the Internet knows how to access the web site (and where to route email) at the new home, with the exception of the previous ISP or hosting provider. Most of the times the previous hosting provider will find out when the contract runs out, which at times may take as long as 6 months (and I’ve seen longer times too).

In a few cases the previous hosting provider will find about the move because of complaints by its current customers who cannot reach the domain, the old customer complaining that there exist people who do not see the new site (but hey did you ever ask us to put it down?) or simply by pure chance.

In such cases as above the objectiveness of the Google Public DNS system can be of use to the DNS master who wants to maintain a clean setup. One can feed the following script with a file that contains one domain per line (the domains that you host) and ask Google who does it see as their designated DNS servers. In the old days one would ask a fellow admin at another ISP for shell access (I use SDF for similar purposes) or for query access. There is no need for that now :)

#!/usr/bin/perl
# This hack assumes that your nameservers are under the example.com domain

$ns = "8.8.8.8";
## $ns = "8.8.4.4";

while(<>) {
	next if (m/^#/);
	chop;
	$domain = $_;
	open DIG, "dig \@$ns $domain ns +short|" or die;
	while(<DIG>) {
		chop;
		next if (m/\.example\.com\.$/);
		print "$domain $_\n";
	}
	close DIG;
}

As is shown by the small script above the idea is pretty simple and can easily be customized to suite any local setup.

@dstergiou asked:

I am looking for a tool to see under which TLDs a domain is registered. E.g. check all TLDs for domains matching “test123”

One way to check is to visit registrars that cover TLDs and ccTLDs (like EuroDNS or Gandi) and submit your query to their forms. However since (at least TTBOMK) none of them offers any query API to check programmatically, one can script his way to an almost complete solution. ICANN provides a list of all available TLDs and we can iterate over it:

#!/usr/bin/perl

## To grab the TLDs: wget -c http://data.iana.org/TLD/tlds-alpha-by-domain.txt

use Net::DNS;

$check = shift or die;

open F, "< tlds-alpha-by-domain.txt" or die;
while (<F>) {
        next if (m/^#/);
        chop;
        push @tlds, $_;
}
close F;

foreach $i (@tlds) {
        my $domain = $check . "." . $i . ".";
        my $res = Net::DNS::Resolver->new;
        my $query = $res->query($domain, "NS");

        if ($query) {
                ## foreach $rr (grep { $_->type eq 'NS' } $query->answer) { print $rr->nsdname, "\n"; }
                print $domain, "\n";
        }
}

The above hack comes with three problems:

• It uses Net::DNS which makes it a little slow. There is room for improvement.
• It finds out about registered domain names that have domain name servers that answer queries about them. This is not always the case. For example in .GR one can register a domain without having it served.
• It also does not take into account strict domain hierarchies, like .co.uk, .net.uk, .org.uk but minimal changes are needed to test that too.

Somewhat incomplete as a solution, but adequate as a 5 minute hack for most purposes.

P.S. @stsimb offers a similar solution using dig:

for sfx in TLDs; do dig +short ns test123.$sfx; done