Wireshark Lab DHCP v6 0 Supplement to Computer Networking A Top Down th Approach 6 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 1 In this lab we ll take a quick look at DHCP DHCP is covered in Section 4 4 2 of the text Recall that DHCP is used extensively in corporate university and home network wired and wireless LANs to dynamically assign IP addresses to hosts as well as to configure other network configuration information This lab is brief as we ll only examine the DHCP packets captured by a host If you also have administrative access to your DHCP server you may want to repeat this lab after making some configuration changes such as the lease time If you have a router at home you most likely can configure your DHCP server Because many linux Unix machines especially those that serve many users have a static IP address and because manipulating DHCP on such machines typically requires super user privileges we ll only present a Windows version of this lab below DHCP Experiment In order to observe DHCP in action we ll perform several DHCP related commands and capture the 2 DHCP messages exchanged as a result of executing these commands Do the following 1 Begin by opening the Windows Command Prompt application which can be found in your Accessories folder As shown in Figure 1 enter ipconfig release The executable for ipconfig is in C windows system32 This 1 References to figures and sections are for the 6 th edition of our text Computer Networks A Top down Approach 6th ed J F Kurose and K W Ross Addison Wesley Pearson 2012 2 If you are unable to run Wireshark live on a computer you can download the zip file http gaia cs umass edu wireshark labs wireshark traces zip and extract the file dhcp ethereal trace 1 The traces in this zip file were collected by Wireshark running on one of the author s computers while performing the steps indicated in the Wireshark lab Once you have downloaded the trace you can load it into Wireshark and view the trace using the File pull down menu choosing Open and then selecting the dhcp ethereal trace 1 trace file You can then use this trace file to answer the questions below 2 3 4 5 6 7 command releases your current IP address so that your host s IP address becomes 0 0 0 0 Start up the Wireshark packet sniffer as described in the introductory Wireshark lab and begin Wireshark packet capture Now go back to the Windows Command Prompt and enter ipconfig renew This instructs your host to obtain a network configuration including a new IP address In Figure 1 the host obtains the IP address 192 168 1 108 Wait until the ipconfig renew has terminated Then enter the same command ipconfig renew again When the second ipconfig renew terminates enter the command ipconfig release to release the previously allocated IP address to your computer Finally enter ipconfig renew to again be allocated an IP address for your computer Stop Wireshark packet capture Figure 1 Command Prompt window showing sequence of ipconfig commands that you should enter Now let s take a look at the resulting Wireshark window To see only the DHCP packets enter into the filter field bootp DHCP derives from an older protocol called BOOTP Both BOOTP and DHCP use the same port numbers 67 and 68 To see DHCP packets in the current version of Wireshark you need to enter bootp and not dhcp in the filter We see from Figure 2 that the first ipconfig renew command caused four DHCP packets to be generated a DHCP Discover packet a DHCP Offer packet a DHCP Request packet and a DHCP ACK packet Figure 2 Wireshark window with first DHCP packet the DHCP Discover packet expanded What to Hand In You should hand in a screen shot of the Command Prompt window similar to Figure 1 above Whenever possible when answering a question below you should hand in a printout of the packet s 3 within the trace that you used to answer the question asked Annotate the printout to explain your answer To print a packet use File Print choose Selected packet only choose Packet summary line and select the minimum amount of packet detail that you need to answer the question Answer the following questions 1 Are DHCP messages sent over UDP or TCP 2 Draw a timing datagram illustrating the sequence of the first four packet Discover Offer Request ACK DHCP exchange between the client and server For each packet indicated the source and destination port numbers Are the port numbers the same as in the example given in this lab assignment 3 What is the link layer e g Ethernet address of your host 4 What values in the DHCP discover message differentiate this message from the DHCP request message 5 What is the value of the Transaction ID in each of the first four Discover Offer Request ACK DHCP messages What are the values of the Transaction ID in the second set Request ACK set of DHCP messages What is the purpose of the Transaction ID field 6 A host uses DHCP to obtain an IP address among other things But a host s IP address is not confirmed until the end of the four message exchange If the IP address is not set until the end of the four message exchange then what values are used in the IP datagrams in the four message exchange For each of the four DHCP messages Discover Offer Request ACK DHCP indicate the source and destination IP addresses that are carried in the encapsulating IP datagram 7 What is the IP address of your DHCP server 8 What IP address is the DHCP server offering to your host in the DHCP Offer message Indicate which DHCP message contains the offered DHCP address 9 In the example screenshot in this assignment there is no relay agent between the host and the DHCP server What values in the trace indicate the absence of a relay agent Is there a relay agent in your experiment If so what is the IP address of the agent 10 Explain the purpose of the router and subnet mask lines in the DHCP offer message 11 In the DHCP trace file noted in footnote 2 the DHCP server offers a specific IP address to the client see also question 8 above In the client s response to the 3 What do we mean by annotate If you hand in a paper copy please highlight where in the printout you ve found the answer and add some text preferably with a colored pen noting what you found in what you ve highlight If you hand in an electronic copy it would be great if you could also highlight and annotate first server OFFER message does the client accept this