There
are at least three client programs available for exploring the contents of
name servers in the Internet. The most widely available program is nslookup
; two other programs, which are a little more powerful than nslookup, are
dig and host. Lucky for us, several institutions
and individuals have made these client programs available through Web. browsers.
We stongly
encourage you to get your hands dirty and play with these programs. They
can give significant insight into how DNS works. All of these programs mimic
DNS clients. They send a DNS query message to a name server (which can often
be supplied by the user), and they receive a corresponding DNS response.
They then extract information (e.g., IP addresses, whether the response is
authoritative, etc.) and present the information to the user.
nslookup
Some of
the nslookup sites provide only the basic nslookup service, i.e., they allow
you to enter a hostname and they return an IP address. Visit some of the
nslookup sights below and try entering hostnames for popular hosts (such
as cnn.com or www.microsoft.com) as well as hostnames for the less popular
hosts. You will see that the popular hostnames typically return numerous
IP addresses, because the site is replicated in numerous servers. (See the
discussion in Section 2.5 on DNS rotation.) Some of the nslookup sites also
return the hostname and IP address of the name server that provides the information.
Also, some of the nslookup sites indicate whether the result is non-authoritative
(i.e., obtained from a cache).
Some of
the nslookup sites allow the user to supply more information. For example,
the user can request to receive the canonical hostname and IP address for
a mail server. And the user can also indicate the name server at which it
wants the chain of queries to begin.
dig
and host
The programs
dig and host allow the user to further refine the query by indicating, for
example, whether the query should be recursive or interative. There are currently
not as many Web sites that provide the dig and host service. But there are
a few:
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