Ethereal_DNS - Ethereal Lab DNS Version 1.0 2005 J.F Kurose...

Ethereal Lab: DNS
Version: 1.0
© 2005 J.F. Kurose, K.W. Ross. All Rights Reserved
Computer Networking: A Top-
down Approach Featuring the
Internet, 3
rd
edition
.
As described in Section 2.5 of the textbook, the Domain Name System (DNS) translates
hostnames to IP addresses, fulfilling a critical role in the Internet infrastructure. In this
lab, we’ll take a closer look at the client side of DNS. Recall that the client’s role in the
DNS is relatively simple – a client sends a query to its local DNS server, and receives a
response back. As shown in Figures 2.26 and 2.18 in the textbook, much can go on
“under the covers,” invisible to the DNS clients, as the hierarchical DNS servers
communicate with each other to either recursively or iteratively resolve the client’s DNS
query. From the DNS client’s standpoint, however, the protocol is quite simple – a query
is formulated to the local DNS server and a response is received from that server.
Before beginning this lab, you’ll probably want to review DNS by reading Section 2.5 of
the text. In particular, you may want to review the material on local DNS servers , DNS
caching , DNS records and messages , and the TYPE field in the DNS record.
1. nslookup
In this lab, we’ll make extensive use of the nslookup tool, which is available in most
Linux/Unix and Microsoft platforms today. To run nslookup in Linux/Unix, you just type
the nslookup command on the command line. To run it in Windows, open the Command
Prompt and run nslookup on the command line.
In it is most basic operation, nslookup tool allows the host running the tool to query any
specified DNS server for a DNS record. The queried DNS server can be a root DNS
server, a top-level-domain DNS server, an authoritative DNS server, or an intermediate
DNS server (see the textbook for definitions of these terms). To accomplish this task,
nslookup sends a DNS query to the specified DNS server, receives a DNS reply from that
same DNS server, and displays the result.

The above screenshot shows the results of three independent nslookup commands
(displayed in the Windows Command Prompt). In this example, the client host is located
on the campus of Polytechnic University in Brooklyn, where the default local DNS server
is dns-prime.poly.edu. When running nslookup , if no DNS server is specified, then
nslookup sends the query to the default DNS server, which in this case is dns-
prime.poly.edu. Consider the first command:
nslookup
In words, this command is saying “please send me the IP address for the host
;. As shown in the screenshot, the response from this command provides
two pieces of information: (1) the name and IP address of the DNS server that provides
the answer; and (2) the answer itself, which is the host name and IP address of
. Although the response came from the local DNS server at Polytechnic