IFT 201 HW7

Wireshark Lab: Getting Started v7.0

Supplement to Computer Networking: A Top-Down

Approach, 7th 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

One’s understanding of network protocols can often be greatly deepened by “seeing

protocols in action” and by “playing around with protocols” – observing the sequence of

messages exchanged between two protocol entities, delving down into the details of

protocol operation, and causing protocols to perform certain actions and then observing

these actions and their consequences. This can be done in simulated scenarios or in a

“real” network environment such as the Internet. In the Wireshark labs you’ll be doing in

this course, you’ll be running various network applications in different scenarios using

your own computer (or you can borrow a friends; let me know if you don’t have access to

a computer where you can install/run Wireshark). You’ll observe the network protocols

in your computer “in action,” interacting and exchanging messages with protocol entities

executing elsewhere in the Internet. Thus, you and your computer will be an integral

part of these “live” labs. You’ll observe, and you’ll learn, by doing.

In this first Wireshark lab, you’ll get acquainted with Wireshark, and make some simple

packet captures and observations.

The basic tool for observing the messages exchanged between executing protocol entities

is called a packet sniffer. As the name suggests, a packet sniffer captures (“sniffs”)

messages 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 messages. 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. Similarly, received

packets are never explicitly addressed to the packet sniffer. Instead, a packet sniffer

receives a copy of packets that are sent/received from/by application and protocols

executing on your machine.

Figure 1 shows the structure of a packet sniffer. At the right of Figure 1 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 1 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. Recall from the discussion from section 1.5 in

the text (Figure 1.241) that 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 all

messages sent/received from/by all protocols and applications executing in your

computer.

operating system

application

packet capture (pcap)

copy of all Ethernet frames sent/received

application (e.g., www browser, ftp client)

Transport (TCP/UDP)

Network (IP)

Link (Ethernet)

Physical

packet analyzer

packet sniffer

Figure 1: Packet sniffer structure

to/from network to/from network

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

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 1. 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,” as shown in Figure 2.8 in the text.

We will be using the Wireshark packet sniffer [http://www.wireshark.org/] for these labs,

allowing us to display the contents of messages being sent/received from/by protocols at

different levels of the protocol stack. (Technically speaking, Wireshark is a packet

analyzer that uses a packet capture library in your computer). Wireshark is a free network

protocol analyzer that runs on Windows, Mac, and Linux/Unix computer. It’s an ideal

packet analyzer for our labs – it is stable, has a large user base and well-documented

support that includes a user-guide (http://www.wireshark.org/docs/wsug_html_chunked/),

1 References to figures and sections are for the 7th edition of our text, Computer Networks, A Top-down

Approach, 7th ed., J.F. Kurose and K.W. Ross, Addison-Wesley/Pearson, 2016.

man pages (http://www.wireshark.org/docs/man-pages/), and a detailed FAQ

(http://www.wireshark.org/faq.html), rich functionality that includes the capability to

analyze hundreds of protocols, and a well-designed user interface. It operates in

computers using Ethernet, serial (PPP and SLIP), 802.11 wireless LANs, and many other

link-layer technologies (if the OS on which it's running allows Wireshark to do so).

Getting Wireshark

In order to run Wireshark, you will need to have access to a computer that supports both

Wireshark and the libpcap or WinPCap packet capture library. The libpcap software will

be installed for you, if it is not installed within your operating system, when you install

Wireshark. See http://www.wireshark.org/download.html for a list of supported

operating systems and download sites

Download and install the Wireshark software:

• Go to http://www.wireshark.org/download.html and download and install the Wireshark binary for your computer.

The Wireshark FAQ has a number of helpful hints and interesting tidbits of information,

particularly if you have trouble installing or running Wireshark.

Running Wireshark

When you run the Wireshark program, you’ll get a startup screen that looks something

like the screen below. Different versions of Wireshark will have different startup screens

– so don’t panic if yours doesn’t look exactly like the screen below! The Wireshark

documentation states “As Wireshark runs on many different platforms with many

different window managers, different styles applied and there are different versions of the

underlying GUI toolkit used, your screen might look different from the provided

screenshots. But as there are no real differences in functionality these screenshots should

still be well understandable.” Well said.

Figure 2: Initial Wireshark Screen

There’s not much interesting on this screen. But note that under the Capture section,

there is a list of so-called interfaces. The computer we’re taking these screenshots from

has just one real interface – “Wi-Fi en0,” which is the interface for Wi-Fi access. All

packets to/from this computer will pass through the Wi-Fi interface, so it’s here where we

want to capture packets. On a Mac, double click on this interface (or on another

computer locate the interface on startup page through which you are getting Internet

connectivity, e.g., mostly likely a WiFi or Ethernet interface, and select that interface.

Let’s take Wireshark out for a spin! If you click on one of these interfaces to start packet

capture (i.e., for Wireshark to begin capturing all packets being sent to/from that

interface), a screen like the one below will be displayed, showing information about the

packets being captured. Once you start packet capture, you can stop it by using the

Capture pull down menu and selecting Stop.

listing of

captured

packets

details of

selected

packet

header

packet content

in hexadecimal

and ASCII

display filter

specification

command

menus

Figure 3: Wireshark Graphical User Interface, during packet capture and analysis

This looks more interesting! 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 are 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 highest-level 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 (assuming the packet was

sent/received over an Ethernet interface) 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 plus minus boxes 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 provided.

• 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 messages.

Taking Wireshark for a Test Run

The best way to learn about any new piece of software is to try it out! We’ll assume that

your computer is connected to the Internet via a wired Ethernet interface. Indeed, I

recommend that you do this first lab on a computer that has a wired Ethernet connection,

rather than just a wireless connection. Do the following

1. Start up your favorite web browser, which will display your selected homepage.

2. Start up the Wireshark software. You will initially see a window similar to that shown in Figure 2. Wireshark has not yet begun capturing packets.

3. To begin packet capture, select the Capture pull down menu.

Figure 4: Wireshark Capture Interface Window

4. Click on Start, making sure that the interface on which you are using the internet is selected below (Wi-Fi in Figure 4) as shown in Figure 4. Packet capture will

now begin - Wireshark is now capturing all packets being sent/received from/by

your computer!

5. Once you begin packet capture, a window similar to that shown in Figure 3 will appear. This window shows the packets being captured. By selecting Capture

pulldown menu and selecting Stop, you can stop packet capture. But don’t stop

packet capture yet. Let’s capture some interesting packets first. To do so, we’ll

need to generate some network traffic. Let’s do so using a web browser, which

will use the HTTP protocol that we will study in detail in class to download

content from a website.

6. While Wireshark is running, enter the URL: http://gaia.cs.umass.edu/wireshark-labs/INTRO-wireshark-file1.html

and have that page displayed in your browser. In order to display this page, your

browser will contact the HTTP server at gaia.cs.umass.edu and exchange HTTP

messages with the server in order to download this page, as discussed in section

2.2 of the text. The Ethernet frames containing these HTTP messages (as well as

all other frames passing through your Ethernet adapter) will be captured by

Wireshark.

7. After your browser has displayed the INTRO-wireshark-file1.html page (it is a simple one line of congratulations), stop Wireshark packet capture by selecting

stop in the Wireshark capture window. The main Wireshark window should now

look similar to Figure 3. You now have live packet data that contains all protocol

messages exchanged between your computer and other network entities! The

HTTP message exchanges with the gaia.cs.umass.edu web server should appear

somewhere in the listing of packets captured. But there will be many other types

of packets displayed as well (see, e.g., the many different protocol types shown in

the Protocol column in Figure 3). Even though the only action you took was to

download a web page, there were evidently many other protocols running on your

computer that are unseen by the user. We’ll learn much more about these

protocols as we progress through the text! For now, you should just be aware that

there is often much more going on than “meet’s the eye”!

8. Type in “http” (without the quotes, and in lower case – all protocol names are in lower case in Wireshark) into the display filter specification window at the top of

the main Wireshark window. Then hit Enter. This will cause only HTTP

message to be displayed in the packet-listing window.

9. Find the HTTP GET message that was sent from your computer to the gaia.cs.umass.edu HTTP server. (Look for an HTTP GET message in the “listing

of captured packets” portion of the Wireshark window (see Figure 3) that shows

“GET” followed by the resource requested: /wireshark-labs/INTRO-

wireshark-file1.html. When you select the HTTP GET message, the Ethernet

frame, IP datagram, TCP segment, and HTTP message header information will be

displayed in the packet-header window2. By clicking on right-pointing and down-

pointing arrowheads to the left side of the packet details window, minimize the

amount of Frame, Ethernet, Internet Protocol, and Transmission Control Protocol

information displayed. Maximize the amount information displayed about the

HTTP protocol. Your Wireshark display should now look roughly as shown in

Figure 5. (Note, in particular, the minimized amount of protocol information for

all protocols except HTTP, and the maximized amount of protocol information for

HTTP in the packet-header window).

10. Similarly, find the HTTP OK response (which says something like ‘HTTP/1.1 200 OK’ in Info).

Note: If you can’t seem to find either or both the GET and OK responses (if you

get something like ‘304 not modified’ instead of the HTTP OK response or HTTP

HEAD instead of HTTP GET, open the link http://gaia.cs.umass.edu/wireshark-

2 Recall that the HTTP GET message that is sent to the gaia.cs.umass.edu web server is contained within a

TCP segment, which is contained (encapsulated) in an IP datagram, which is encapsulated in an Ethernet

frame. If this process of encapsulation isn’t quite clear yet, review section 1.5 in the text

labs/INTRO-wireshark-file1.html in Private Browsing/Incognito mode in your

browser ).

11. Exit Wireshark

Congratulations! You’ve now completed the first lab.

Figure 5: Wireshark window after step 9

What to hand in

The goal of this first lab was primarily to introduce you to Wireshark. The following

questions will demonstrate that you’ve been able to get Wireshark up and running, and

have explored some of its capabilities. Answer the following questions, based on your

Wireshark experimentation:

1. List 3 different protocols that appear in the protocol column in the unfiltered packet-listing window in step 7 above.

2. How long did it take from when the HTTP GET message was sent until the HTTP OK reply was received? Make sure you mention the unit of time in your answer.

(By default, the value of the Time column in the packet-listing window is the

amount of time, in seconds, since Wireshark tracing began. To display the Time

field in time-of-day format, select the Wireshark View pull down menu, then

select Time Display Format, then select Time of Day.)

3. What is the Internet address (IP address) of the gaia.cs.umass.edu (also known as www-net.cs.umass.edu)? What is the Internet address of your computer?

4. Print the two HTTP messages (GET and OK) referred to in question 2 above into a PDF file. To do so, select Print from the Wireshark File command menu, and

select the “Selected Packet Only” and “Print as displayed” radial buttons, and

then click OK.

Submit the PDF(s) along with a document containing answers to questions 1-3.