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New York University
Computer Science Department
Courant Institute of Mathematical Sciences
Course Title: Data Communication & Networks Course Number: g22.2662- Instructor: Jean-Claude Franchitti Session: 6
Assignment
I. Due
Tuesday November 15, 2012, at the beginning of class.
II. Objectives
- See protocols in action.
III. References
- Slides and handouts posted on the course Web site
- Textbook chapters as applicable
IV. Software Required
- Wireshark Packet Sniffer and Packet Capture Library (see section V below).
- Microsoft Word.
- Win Zip as necessary.
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.
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.
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
- Start up your favorite web browser, which will display your selected homepage.
- 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.
- 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
- 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!
- 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.
now, you should just be aware that there is often much more going on than “meet’s the eye”!
- 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.
- 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).
- 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.
- What is the MAC address of your Host? You can find this in the frame level information.
- List the different protocols that appear in the protocol column in the unfiltered packet-listing window in step 4.7 above.
VII. Sample Cover Sheet
Name ________________________ Date: ____________ (last name, first name) Section: ___________
Assignment 6
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 Question 5 are correct. o File name is correct.
Answers to Individual Questions:
(100 points total, all questions weighted equally)
o Assumptions provided when required.
Total in points (100 points total): ___________________
Professor’s Comments:
