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.
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.
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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Copyright 1996–1999 Keith W. Ross and James F. Kurose