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Wireshark Lab: DNS
v7.0
Supplement to
Computer Networking: A Top-Down
Approach, 7
th
ed.,
J.F. Kurose and K.W. Ross
“Tell me and I forget. Show me and I remember. Involve me
and I understand.”
Chinese proverb
© 2005-2016, J.F Kurose and K.W. Ross, All Rights Reserved
As described in Section 2.4 of the text
1
, 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 2.19 and 2.20 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 2.4 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.
Note: For Mac users, please use the following Google or similar query to find equivalent
Window commands for your Mac computer:
nslookup command for Mac
ipconfig example for Mac
ipconfig/displaydns example for Mac
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.
1 References to figures and sections are for the 7
th
edition of our text,
Computer Networks, A Top-down
Approach, 7
th
ed.,
J.F. Kurose and K.W. Ross, Addison-Wesley/Pearson, 2016.
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 www.mit.edu
In words, this command is saying “please send me the IP address for the host
www.mit.edu”. 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
www.mit.edu. Although the response came from the local DNS server at Polytechnic
University, it is quite possible that this local DNS server iteratively contacted several
other DNS servers to get the answer, as described in Section 2.4 of the textbook.
Now consider the second command:
nslookup –type=NS mit.edu
In this example, we have provided the option “-type=NS” and the domain “mit.edu”. This
causes
nslookup
to send a query for a type-NS record to the default local DNS server. In
words, the query is saying, “please send me the host names of the authoritative DNS for
mit.edu”. (When the –type option is not used,
nslookup
uses the default, which is to query
for type A records.) The answer, displayed in the above screenshot, first indicates the
DNS server that is providing the answer (which is the default local DNS server) along
with three MIT nameservers. Each of these servers is indeed an authoritative DNS server
for the hosts on the MIT campus. However,
nslookup
also indicates that the answer is
“non-authoritative,” meaning that this answer came from the cache of some server rather
than from an authoritative MIT DNS server. Finally, the answer also includes the IP
addresses of the authoritative DNS servers at MIT. (Even though the type-NS query
generated by
nslookup
did not explicitly ask for the IP addresses, the local DNS server
returned these “for free” and
nslookup
displays the result.)
Now finally consider the third command:
nslookup www.aiit.or.kr bitsy.mit.edu
In this example, we indicate that we want to the query sent to the DNS server
bitsy.mit.edu rather than to the default DNS server (dns-prime.poly.edu). Thus, the query
and reply transaction takes place directly between our querying host and bitsy.mit.edu. In
this example, the DNS server bitsy.mit.edu provides the IP address of the host
www.aiit.or.kr, which is a web server at the Advanced Institute of Information
Technology (in Korea).
Now that we have gone through a few illustrative examples, you are perhaps wondering
about the general syntax of
nslookup
commands. The syntax is:
nslookup –option1 –option2 host-to-find dns-server
In general,
nslookup
can be run with zero, one, two or more options. And as we have seen
in the above examples, the dns-server is optional as well; if it is not supplied, the query is
sent to the default local DNS server.
Now that we have provided an overview of
nslookup
, it is time for you to test drive it
yourself.
Based on your Wireshark exercises and practices, open the “DNS Review
Questions” in the same module, and based on the similar screenshots provided in
the file set, submit your answers accordingly as well.
Your preview ends here
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