Lab 4: Packet Sniffing and Wireshark




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CS 3339 Cyber Security Lab
Lab 4: Packet Sniffing and Wireshark
The first part of the lab introduces packet sniffer, Wireshark. Wireshark is a free opensource network protocol analyzer. It is used for network troubleshooting and
communication protocol analysis. Wireshark captures network packets in real time and
display them in human-readable format. It provides many advanced features including
live capture and offline analysis, three-pane packet browser, coloring rules for analysis.
This document uses Wireshark for the experiments, and it covers Wireshark installation,
packet capturing, and protocol analysis.
Figure 1: Wireshark
TCP/IP Network Stack
Figure 2: Encapsulation of Data in the TCP/IP Network Stack
This background section briefly explains the concept of TCP/IP network stack to help
you better understand the experiments. TCP/IP is the most commonly used network
model for Internet services. Because its most important protocols, the Transmission
Control Protocol (TCP) and the Internet Protocol (IP) were the first networking protocols
defined in this standard, it is named as TCP/IP. However, it contains multiple layers
including application layer, transport layer, network layer, and data link layer.
– Application Layer: The application layer includes the protocols used by most
applications for providing user services. Examples of application layer protocols are
Hypertext Transfer Protocol (HTTP), Secure Shell (SSH), File Transfer Protocol
(FTP), and Simple Mail Transfer Protocol (SMTP).
– Transport Layer: The transport layer establishes process-to-process connectivity,
and it provides end-to-end services that are independent of underlying user data. To
implement the process-to-process communication, the protocol introduces a concept
of port. The examples of transport layer protocols are Transport Control Protocol
(TCP) and User Datagram Protocol (UDP). The TCP provides flow- control,
connection establishment, and reliable transmission of data, while the UDP is a
connectionless transmission model.
– Internet Layer: The Internet layer is responsible for sending packets to across
networks. It has two functions: 1) Host identification by using IP addressing system
(IPv4 and IPv6); and 2) packets routing from source to destination. The examples of
Internet layer protocols are Internet Protocol (IP), Internet Control Message Protocol
(ICMP), and Address Resolution Protocol (ARP).
– Link Layer: The link layer defines the networking methods within the scope of the
local network link. It is used to move the packets between two hosts on the same
link. A common example of link layer protocols is Ethernet.
Packet Sniffer
Packet sniffer is a basic tool for observing network packet exchanges in a computer. As
the name suggests, a packet sniffer captures (“sniffs”) packets being sent/received
from/by your computer; it will also typically store and/or display the contents of the
various protocol fields in these captured packets. A packet sniffer itself is passive. It
observes messages being sent and received by applications and protocols running on
your computer, but never sends packets itself.
Figure 3 shows the structure of a packet sniffer. At the right of Figure 3 are the
protocols (in this case, Internet protocols) and applications (such as a web browser or
ftp client) that normally run on your computer. The packet sniffer, shown within the
dashed rectangle in Figure 3 is an addition to the usual software in your computer, and
consists of two parts. The packet capture library receives a copy of every link-layer
frame that is sent from or received by your computer. Messages exchanged by higher
layer protocols such as HTTP, FTP, TCP, UDP, DNS, or IP all are eventually
encapsulated in link-layer frames that are transmitted over physical media such as an
Ethernet cable. In Figure 1, the assumed physical media is an Ethernet, and so all
upper-layer protocols are eventually encapsulated within an Ethernet frame. Capturing
all link-layer frames thus gives you access to all messages sent/received from/by all
protocols and applications executing in your computer.
The second component of a packet sniffer is the packet analyzer, which displays the
contents of all fields within a protocol message. In order to do so, the packet analyzer
Figure 3: Packet Sniffer Structure
must “understand” the structure of all messages exchanged by protocols. For example,
suppose we are interested in displaying the various fields in messages exchanged by
the HTTP protocol in Figure 3. The packet analyzer understands the format of Ethernet
frames, and so can identify the IP datagram within an Ethernet frame. It also
understands the IP datagram format, so that it can extract the TCP segment within the
IP datagram. Finally, it understands the TCP segment structure, so it can extract the
HTTP message contained in the TCP segment. Finally, it understands the HTTP
protocol and so, for example, knows that the first bytes of an HTTP message will
contain the string “GET,” “POST,” or “HEAD”.
We will be using the Wireshark packet sniffer [] for these labs,
allowing us to display the contents of messages being sent/received from/by protocols
at different levels of the protocol stack. Wireshark is a free network protocol analyzer
that runs on Windows, Linux/Unix, and Mac computers.
Getting Wireshark
You can download Wireshark from here:
Figure 4: Download Page of Wireshar
Starting Wireshark
When you run the Wireshark program, the Wireshark graphic user interface will be
shown as Figure 5. Currently, the program is not capturing the packets.
Figure 5: Initial Graphic User Interface of Wireshark
Then, you need to choose an interface. If you are running the Wireshark on your laptop,
you need to select WiFi interface. If you are at a desktop, you need to select the
Ethernet interface being used. Note that there could be multiple interfaces. In general,
you can select any interface but that does not mean that traffic will flow through that
interface. The network interfaces (i.e., the physical connections) that your computer has
to the network are shown. The attached Figure 6 was taken from my computer.
After you select the interface, you can click start to capture the packets as shown in
Figure 6.
Figure 6: Capture Interfaces in Wireshark
Figure 7: Capturing Packets in Wireshark
Figure 8: Wireshark Graphical User Interface on Microsoft Windows
The Wireshark interface has five major components:
The command menus are standard pulldown menus located at the top of the window.
Of interest to us now is the File and Capture menus. The File menu allows you to save
captured packet data or open a file containing previously captured packet data, and exit
the Wireshark application. The Capture menu allows you to begin packet capture.
The packet-listing window displays a one-line summary for each packet captured,
including the packet number (assigned by Wireshark; this is not a packet number
contained in any protocol’s header), the time at which the packet was captured, the
packet’s source and destination addresses, the protocol type, and protocol-specific
information contained in the packet. The packet listing can be sorted according to any of
these categories by clicking on a column name. The protocol type field lists the highestlevel protocol that sent or received this packet, i.e., the protocol that is the source or
ultimate sink for this packet.
The packet-header details window provides details about the packet selected
(highlighted) in the packet-listing window. (To select a packet in the packet-listing
window, place the cursor over the packet’s one-line summary in the packet-listing
window and click with the left mouse button.). These details include information about
the Ethernet frame and IP datagram that contains this packet. The amount of Ethernet
and IP-layer detail displayed can be expanded or minimized by clicking on the rightpointing or down-pointing arrowhead to the left of the Ethernet frame or IP datagram line
in the packet details window. If the packet has been carried over TCP or UDP, TCP or
UDP details will also be displayed, which can similarly be expanded or minimized.
Finally, details about the highest-level protocol that sent or received this packet are also
The packet-contents window displays the entire contents of the captured frame, in
both ASCII and hexadecimal format.
Towards the top of the Wireshark graphical user interface, is the packet display filter
field, into which a protocol name or other information can be entered in order to filter
the information displayed in the packet-listing window (and hence the packet-header
and packet-contents windows). In the example below, we’ll use the packet-display filter
field to have Wireshark hide (not display) packets except those that correspond to HTTP
Capturing Packets
After downloading and installing Wireshark, you can launch it and click the name of an
interface under Interface List to start capturing packets on that interface. For example, if
you want to capture traffic on the wireless network, click your wireless interface.
Test Run
Do the following steps:
1. Start up the Wireshark program (select an interface and press start to capture
2. Start up your favorite browser
3. In your browser, go to
4. After your browser has displayed the SMU webpage, stop Wireshark packet capture
by selecting stop in the Wireshark capture window.
5. Color Coding: You’ll probably see packets highlighted in green, blue, and black.
Wireshark uses colors to help you identify the types of traffic at a glance. By default,
green is TCP traffic, dark blue is DNS traffic, light blue is UDP traffic, and black
identifies TCP packets with problems — for example, they could have been
delivered out-of-order.
6. You now have live packet data that contains all protocol messages exchanged
between your computer and other network entities! However, as you will notice the
HTTP messages are not clearly shown because there are many other packets
included in the packet capture. Even though the only action you took was to open
your browser, there are many other programs in your computer that communicate
via the network in the background. To filter the connections to the ones we want to
focus on, we have to use the filtering functionality of Wireshark by typing “http” in the
filtering field as shown below:
Notice that we now view only the packets that are of protocol HTTP. However, we also
still do not have the exact communication we want to focus on because using HTTP as
a filter is not descriptive enough. We need to be more precise if we want to capture the
correct set of packets.
7. To further filter packets in Wireshark, we need to use a more precise filter. By setting
the we are restricting the view to packets that have as
an http host the website. Notice that we need two equal signs to
perform the match “==” not just one. See the screenshot below:
8. Now, we can try another protocol. Let’s use Domain Name System (DNS) protocol
as an example here.
9. Let’s try now to find out what those packets contain by following one of the
conversations (also called network flows), select one of the packets and press the
right mouse button (if you are on a Mac use the command button and click), you
should see something similar to the screen below:
Click on Follow and then on UDP Stream. You will see following screen.
10. If we close this window and change the filter back to “”
and then follow a packet from the list of packets that match that filter, we should get
the something similar to the following screen. Note that we click on Follow -> TCP
Stream this time.
Questions for the Lab
1. Carefully read the lab instructions and finish all tasks above. Attach screenshots to
your report for steps 7, 8 and 10
2. If a packet is highlighted by black, what does it mean for the packet?
3. What is the filter command for listing all outgoing http traffic?
4. Why does DNS use Follow UDP Stream while HTTP use Follow TCP Stream?
5. Using Wireshark to capture the FTP password. Explain how you found the password
and attach a screenshot of the password packet. How could we have prevented
sending the FTP login credentials in plain text over the network?
Use a terminal to log into the FTP server and use Wireshark to capture the
password. The FTP host is: and the username and password
are below. You will use the username and password to login into the FTP server
while Wireshark is running. Have fun!
USERNAME: smucs3339