Computer
Science Department
Courant
Institute of Mathematical Sciences
Course Title: Data Communication &
Networks Course Number: g22.2662-001
Instructor: Jean-Claude Franchitti Session:
5
Assignment #3
Thurday March 4, 2009, at the beginning of class.
II. Objectives
III. References
IV. Software Required
V. Assignment
Preamble and Disclaimer:
As noted on the corresponding SourceForge site, the
Ethereal development team switched names from Ethereal to Wireshark in May 2006
due to trademark issues (see http://www.wireshark.org/faq.html#q1.2
for more details on this). Incidentally, some people pronounce the name
Ethereal as “ether-real,” while others pronounce it “e-thir-E-al,” as in the
English word ethereal, which means ghostly or insubstantial. The Ethereal
name’s origin comes from the Ethernet protocol, a link-level protocol that is
studied extensively in Chapter 5 of the textbook, and in the class labs.
1. Wireshark Lab - Getting Started
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. The Java applets that accompany the textbook
take the first approach. In the Wireshark labs, we’ll take the latter
approach. You’ll be running various
network applications in different scenarios using a computer on your desk, at
home, or in a lab. 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 “live” labs in this class. You’ll observe, and you’ll learn, by doing.
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 class discussions
(and corresponding material covered in the textbook used for the class) 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.
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”.
We will be using the Wireshark packet sniffer (i.e., www.wireshark.org) for this lab, 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 on your computer). Wireshark is a free network protocol
analyzer that runs on Windows, Linux/Unix, and Mac computers. It’s an ideal
packet analyzer for our labs – it is stable, has a large user base and
well-documented support that includes:
(a) A user-guide (i.e., http://www.wireshark.org/docs/wsug_html_chunked/)
(b) Man pages (i.e., http://www.wireshark.org/docs/man-pages/)
(c) A detailed FAQ (i.e., http://www.wireshark.org/faq.html)
(d) Rich functionality that includes the capability to
analyze more than 500 protocols
(e) A well-designed user interface
The Wireshark packet sniffer operates in computers using
Ethernet to connect to the Internet, as well as so-called point-to-point
protocols such as PPP.
2. Wireshark Lab – Getting Wireshark
In order to run Wireshark, you will need to have
access to a computer that supports both Wireshark and the libpcap packet capture library. If the libpcap software is not installed within your operating system, you
will need to install libpcap or have
it installed for you in order to use Wireshark.
See http://www.wireshark.org/download.html
for a list of supported operating systems and download sites.
Download and install the Wireshark and (if needed) the
libpcap software:
·
If needed,
download and install the libpcap
software. Pointers to the libpcap software are provided from the
Wireshark download pages. For Windows
machines, the libpcap software is
known as WinPCap, and can be found at
http://www.winpcap.org/install/default.htm.
·
Go to www.wireshark.org and download and
install the Wireshark binary for your computer. It is recommended to download
from http://sourceforge.net/project/showfiles.php?group_id=255
which includes a WinPCap bundle option.
·
Download the
Wireshark user guide. You will most
likely only need Chapters 1 and 3.
The Wireshark FAQ has a number of helpful hints and
interesting tidbits of information, particularly if you have trouble installing
or running Wireshark.
3. Wireshark Lab – Running Wireshark
When you run the Wireshark program, the Wireshark
graphical user interface shown in Figure 2a will be displayed. Initially, no
data will be displayed in the various windows.
Figure 2a: Wireshark Capture Options Window
Figure 2 below shows the original Ethereal graphical
user interface along with an explanation of the various areas which applies to
both Ethereal and Wireshark.
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 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 right-pointing 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 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.
4. Wireshark Lab – Taking Wireshark for a Test Run
The best way to learn about any new piece of software
is to try it out! 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, except that no packet data will
be displayed in the packet-listing, packet-header, or packet-contents window,
since Wireshark has not yet begun capturing packets.
3.
To begin packet
capture, select the Capture pull down menu and select Options. This will cause the
“Wireshark: Capture Options” window to be displayed, as shown in Figure 3.
Figure 3: Wireshark Capture Options Window
4. You can use all of the default values in this
window. The network interfaces (i.e.,
the physical connections) that your computer has to the network will be shown
in the Interface pull down menu at the top of the Capture Options window. In
case your computer has more than one active network interface (e.g., if you
have both a wireless and a wired Ethernet connection), you will need to select
an interface that is being used to send and receive packets (mostly likely the
wired interface). After selecting the network interface (or using the default
interface chosen by Wireshark), click Start. Packet capture will now begin - all packets
being sent/received from/by your computer are now being captured by Wireshark!
5. After you begin packet capture, you can select
Statistics > Protocol Hierarchy from the command menus to obtain a summary
of the number of packets of various types that are being captured as shown in
Figure 4.
Figure 4: Wireshark Protocol Hierarchy Statistics
6. While Wireshark is running, enter the URL:
http://gaia.cs.umass.edu/ethereal-labs/INTRO-ethereal-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. The Ethernet frames containing these HTTP
messages will be captured by Wireshark.
7. After your browser has displayed the
INTRO-ethereal-file1.html page, stop Wireshark packet capture by selecting
Capture > Stop in the Wireshark in the command menus. The Wireshark window will display all packets
captured since you began packet capture.
The Wireshark window should now look similar to Figure 2. 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 2). 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 select Apply (to the right of where you entered “http”). This will cause only HTTP message to be
displayed in the packet-listing window.
9. The HTTP GET message that was sent from your computer
to the gaia.cs.umass.edu HTTP server should be shown among the first few http
message shown in the packet-listing window. 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 window. 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 the material covered in class and the
corresponding material covered in the textbook. By clicking on the expansion
buttons (+ or -) to the left side of the packet details window, you can minimize
or maximize 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. Exit Wireshark
Figure 5: Wireshark Display After Step 9
Congratulations!
You’ve now completed the first lab.
5. Wireshark Lab – 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. What is the MAC
address of your Host? You can find this in the frame level information.
2. List the different protocols that appear in the
protocol column in the unfiltered packet-listing window in step 4.7 above.
3. How long did it take from when the HTTP GET message
was sent until the HTTP OK reply was received? (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.)
4. What is the Internet address of the gaia.cs.umass.edu
(also known as www-net.cs.umass.edu)?
What is the Internet address of your computer?
1. Print the two HTTP messages displayed in step 4.9
above. To do so, select Print from
the Wireshark File command menu, and
select “Selected Packet Only” under
Save your capture in a capture file named Nxxx.cap
where Nxxx is your student ID.
Submit this capture file and the answers to the
questions above.
Email
your assignment (archive) file to your TA.
VI. Deliverables
Fill in
the blank area for each field.
NOTE:
The sequence of the hardcopy submission is:
1.
Cover
sheet
2.
Assignment
Answer Sheet(s)
VII. Sample
Cover Sheet
Name ________________________ Username: ______________ Date: ____________
(last name, first name)
Section: ___________
Assignment 2
Assignment Layout (25%)
o Assignment is
neatly assembled on 8 1/2 by 11 paper.
o Cover page with your name (last name first followed by a comma then
first name), username and section number with a signed statement of independent
effort is included.
o Answers to all
assignment questions are correct.
o File name is correct.
Total in points ___________________
Professor’s Comments:
Affirmation of my Independent Effort: _____________________________
(Sign
here)