《计算机网络 Computer Networking》课程课程实验讲稿（Wireshark Lab，6th edition）Getting Started Wireshark Lab.pdf_大学文库

Wireshark Lab: Computer Networking Getting Started v6.0 Top-Down Approach Supplement to Computer Networking:A Top-Down Approach,ed..J.F.Kurose and K.W.Ross "Tell me and I forget.Show me and I remember.Involve me andI mdersiand.Chinese proverb KUROSE ROSS 2005-21012,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 I is an addition to the usual software in your computer,and consists

Wireshark Lab: Getting Started v6.0 Supplement to Computer Networking: A Top-Down Approach, 6th 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 © 2005-21012, 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 the text(Figure 1.24)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. packet sniffer application apoeene8ew Transport(TCP/UDP) Network (IP) Link (Ethernet) Physical Figure Packet sniffer structure The second cc ent of a packet sniffer is the packet analyzer,which displays the nts rde analyz stan str f all d by prote ols Fo este hag the rious y the et analyz nds of Etp erne rames,and so can identify the datagram thin an Etheret f so understands the IP datagram format,so that it can extract the TCP segment with 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 Figure2.8 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 you omnute )Wireshark is a free network protocol analyzer that runs on Windows.Linux/Unix,and Mac computers.It's an ideal acket analyer for our absit has a lare ser hase and y support that includes user-guide (http:/www.wireshark org/docs/wsug html_chunked/ hehd:

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, 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 user-guide (http://www.wireshark.org/docs/wsug_html_chunked/), 1 References to figures and sections are for the 6th edition of our text, Computer Networks, A Top-down Approach, 6th ed., J.F. Kurose and K.W. Ross, Addison-Wesley/Pearson, 2012

The packet-listing window displays a one-line summary for each packet captured,including the packet number(assigned by Wireshark:this is not a BecputTeekcsnaoenacahmateciepohbet and protocol-specific information contained in the packet The packet listin be sorted according to any of these categories by clicking on a colu that sent or received this sink for this s packet tocol tha is the e or ulti ket The packet-header details owprovides details about th (mg lighted)in t e packet-listing wi e listing window,place the cursor over the packet's one ine summary in th 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.Finallv.details about the highest-level protocol that sent or received this packet are also provided. Th plays the entire contents of the captured frame al fo Towa ds he top of the Wireshark graphical e entered ir order to filter t displayed in the packet-isting wind w(and hence the packet- eader and packet-contents windows).In the example elow,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 canture select the capture pull down menu and select interfaces This will cause the"Wireshark:Capture Interfaces"window to be displaved.as shown in Figure 4

• 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 packetlisting 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 and select Interfaces. This will cause the “Wireshark: Capture Interfaces” window to be displayed, as shown in Figure 4

Wireshark:Capture Interfaces Description Packets Packets/s Stop Intel(R)82567LM t Ne 8441239eb1974 0 Start Options Detail Microsoft 192.1681100 Help Close Figure 4:Wireshark Capture Interface Window 4.You'll see a list of the interfaces on your computer as well as a count of the packets that have been observed on that interface so far.Click on Start for the interface on which you want to begin packet capture(in the case,the Gigabit network Connection).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 ar This window shows the packets being captured by selecting capture ldown memu and selecting Son you can ston packet canture But don't stor packet canture vet let's capture some interesting g packets first.To do so.wel erate some netw ork traffic.Let's do sing a weh bro which will use the HTTP otocol that we will study in detail i 6 While Wireshark is running,ente the URI http://gaia cs umass.edu/wireshark-labs/INTRO-wireshark-filel.html and have th at page dis played in your browser.In order to display this page,your browser will contact the HTTP server at gaia.cs.umass.edu and exchange HTTP ges with the server in order to download this page,as discussed in section he The Ethemet rmes comtaning thesewell as all other frames passing through your Ethernet adapter)will be captured by Wireshark 7.After your browser has displayed the INTRO-wireshark-filel.html page(it is a simple one line of congratulations).stop Wireshark packet capture by selecting ston in the wireshark canture window The main wireshark window should now look similar to Figure 3.You now have live packet data that contains all protocol messages exchange ed hetween vour and other network entities!The HTTP me ha ith the edu we should pe the list th will h S ckets dis yed as ured re ny othe type na nt protoco ure nough the 00 K wa do wn eb pag the many oth running on you computer that 1 are unseen by the learn much m ore ab thes as we p rogress through th text! w,yo should just be aware that there is often much more going on than"meet's the eye

Figure 4: Wireshark Capture Interface Window 4. You’ll see a list of the interfaces on your computer as well as a count of the packets that have been observed on that interface so far. Click on Start for the interface on which you want to begin packet capture (in the case, the Gigabit network Connection). 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”!