Lab Assignments for Chapter 3
We have created two lab assignments for this chapter: Lab3-1 and Lab3-2. We have
also included two lab-report sheets, which means that each assignment should be
reported in a separate sheet.
Lab3-1: UDP
In this lab, we use Wireshark to capture and study UDP packets. We find the values of
different fields of a UDP user datagram header. Using the values in these fields, we also
find all the information about a user datagram and verify that the value in total length
field matches with the total number of bytes in the user datagram (header and data). We
also check to see whether a checksum has been calculated for the packet.
UDP is a connectionless protocol; no connection-establishment and connectiontermination
packets are issued by UDP. This means that UDP cannot be a source or
sink protocol in a captured frame. To analyze UDP headers, we need to use a source or
sink protocol at the application layer that uses the services of UDP. DNS is a good candidate.
However, as we learned in Chapter 2, DNS packets can be encapsulated in
either UDP or TCP packets (depending on the size). We need to be careful to select
only those DNS packets that use the service of UDP.
Since any client-server application program (except DNS itself) uses DNS as the
first step, we can open any application to capture DNS packets. We recommend to
access your favorite website for this purpose.
Assignment
■ Start your web browser and clear the browser’s cache memory, but do not access
any website yet.
■ Open Wireshark and start capturing.
■ Go back to your web browser and retrieve any file from a website. Wireshark
starts capturing packets.
■ After enough packets have been captured, stop Wireshark and save the captured
file.
■ Using the captured file, select only those DNS packets that use the service of UDP.
Type udp (lowercase) in the filter field and click Apply. The packet list pane of the
2
Wireshark window should now display a bunch of DNS messages. Each DNS message
is carried in a UDP packet.
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