Use Wireshark to Examine Ethernet Frames Topology

University:
University of Central Arkansas
Course:
CISA3300 | Introduction to Computer Architecture and Programming
Academic year:

2021
Views:

327

Pages:

7
Author:

Shamar Patton

Lab - Use Wireshark to Examine Ethernet Frames Topology Objectives Part 1: Examine the Header Fields in an Ethernet II Frame Part 2: Use Wireshark to Capture and Analyze Ethernet Frames Background / Scenario When upper layer protocols communicate with each other, data flows down the Open Systems Interconnection (OSI) layers and is encapsulated into a Layer 2 frame. The frame composition is dependent on the media access type. For example, if the upper layer protocols are TCP and IP and the media access is Ethernet, then the Layer 2 frame encapsulation will be Ethernet II. This is typical for a LAN environment. When learning about Layer 2 concepts, it is helpful to analyze frame header information. In the first part of this lab, you will review the fields contained in an Ethernet II frame. In Part 2, you will use Wireshark to capture and analyze Ethernet II frame header fields for local and remote traffic. Required Resources • 1 PC (Windows with internet access and with Wireshark installed) Instructions Part 1: Examine the Header Fields in an Ethernet II Frame In Part 1, you will examine the header fields and content in an Ethernet II frame. A Wireshark capture will be used to examine the contents in those fields. Step 1: Review the Ethernet II header field descriptions and lengths. Preamble Destination Address Source Address Frame Type Data FCS 8 Bytes 6 Bytes 6 Bytes 2 Bytes 46 – 1500 Bytes 4 Bytes Step 2: Examine the network configuration of the PC. In this example, this PC host IP address is 192.168.1.147 and the default gateway has an IP address of 192.168.1.1. C:\> ipconfig /all Lab - Use Wireshark to Examine Ethernet Frames Ethernet adapter Ethernet: Connection-specific DNS Suffix Description . . . . . . . . . . Physical Address. . . . . . . . DHCP Enabled. . . . . . . . . . Autoconfiguration Enabled . . . Link-local IPv6 Address . . . . IPv4 Address. . . . . . . . . . Subnet Mask . . . . . . . . . . Lease Obtained. . . . . . . . . Lease Expires . . . . . . . . . Default Gateway . . . . . . . . DHCP Server . . . . . . . . . . . . . . . . . . . . . . : : : : : : : : : : : : Intel(R) 82579LM Gigabit Network Connection F0-1F-AF-50-FD-C8 Yes Yes fe80::58c5:45f2:7e5e:29c2%11(Preferred) 192.168.1.147(Preferred) 255.255.255.0 Friday, September 6, 2019 11:08:36 AM Saturday, September 7, 2019 11:08:36 AM 192.168.1.1 192.168.1.1 Step 3: Examine Ethernet frames in a Wireshark capture. The screenshots of the Wireshark capture below shows the packets generated by a ping being issued from a PC host to its default gateway. A filter has been applied to Wireshark to view the ARP and ICMP protocols only. ARP stands for address resolution protocol. ARP is a communication protocol that is used for determining the MAC address that is associated with the IP address. The session begins with an ARP query and reply for the MAC address of the gateway router, followed by four ping requests and replies. This screenshot highlights the frame details for an ARP request. Lab - Use Wireshark to Examine Ethernet Frames This screenshot highlights the frame details for an ARP reply. Lab - Use Wireshark to Examine Ethernet Frames Step 4: Examine the Ethernet II header contents of an ARP request. The following table takes the first frame in the Wireshark capture and displays the data in the Ethernet II header fields. Field Value Description Preamble Not shown in capture This field contains synchronizing bits, processed by the NIC hardware. Destination Address Broadcast (ff:ff:ff:ff:ff:ff) Layer 2 addresses for the frame. Each address is 48 bits long, or 6 octets, expressed as 12 hexadecimal digits, 09,A-F. A common format is 12:34:56:78:9A:BC. Source Address Netgear_99:c5:72 (30:46:9a:99:c5:72) Frame Type 0x0806 The first six hex numbers indicate the manufacturer of the network interface card (NIC), the last six hex numbers are the serial number of the NIC. The destination address may be a broadcast, which contains all ones, or a unicast. The source address is always unicast. For Ethernet II frames, this field contains a hexadecimal value that is used to indicate the type of upper-layer protocol in the data field. There are numerous upper-layer protocols supported by Ethernet II. Two common frame types are these: Value Description 0x0800 IPv4 Protocol 0x0806 Address Resolution Protocol (ARP) Data ARP Contains the encapsulated upper-level protocol. The data field is between 46 – 1,500 bytes. FCS Not shown in capture Frame Check Sequence, used by the NIC to identify errors during transmission. The value is computed by the sending device, encompassing frame addresses, type, and data field. It is verified by the receiver. What is significant about the contents of the destination address field? All the hosts on LAN will receive the broadcast frame. The host with the IP address 192.168.1.1 will send a reply to the source.. Type your answers here. Why does the PC send out a broadcast ARP prior to sending the first ping request? A PC can't send a ping request to a host until it determines the destination MAC address. The ARP broadcast is used to request for the MAC address of the host with the IP address contained in the ARP. Type your answers here. What is the MAC address of the source in the first frame? Type your answers here. f0:1f:af:50:fd:c8 Lab - Use Wireshark to Examine Ethernet Frames What is the Vendor ID (OUI) of the Source NIC in the ARP reply? Dell. Type your answers here. What portion of the MAC address is the OUI? The first three octets of the MAC address is the OUI. Type your answers here. What is the NIC serial number of the source? Type your99:c5:72 answers here. Part 2: Use Wireshark to Capture and Analyze Ethernet Frames In Part 2, you will use Wireshark to capture local and remote Ethernet frames. You will then examine the information that is contained in the frame header fields. Step 1: Determine the IP address of the default gateway on your PC. Open a Windows command prompt. Open a command prompt window and issue the ipconfig command. What is the IP address of the PC default gateway? Type your answers here. 192.168.1.254 Close a Windows command prompt. Step 2: Start capturing traffic on your PC NIC. a. Open Wireshark to start data capture. b. Observe the traffic that appears in the packet list window. Step 3: Filter Wireshark to display only ICMP traffic. You can use the filter in Wireshark to block visibility of unwanted traffic. The filter does not block the capture of unwanted data; it only filters what you want to display on the screen. For now, only ICMP traffic is to be displayed. In the Wireshark Filter box, type icmp. The box should turn green if you typed the filter correctly. If the box is green, click Apply (the right arrow) to apply the filter. Step 4: From the command prompt window, ping the default gateway of your PC. Open a Windows command prompt. From the command window, ping the default gateway using the IP address that you recorded in Step 1. Close Windows command prompt. Step 5: Stop capturing traffic on the NIC. Click the Stop Capturing Packets icon to stop capturing traffic. Step 6: Examine the first Echo (ping) request in Wireshark. The Wireshark main window is divided into three sections: the packet list pane (top), the Packet Details pane (middle), and the Packet Bytes pane (bottom). If you selected the correct interface for packet capturing previously, Wireshark should display the ICMP information in the packet list pane of Wireshark. a. In the packet list pane (top section), click the first frame listed. You should see Echo (ping) request under the Info heading. The line should now be highlighted. b. Examine the first line in the packet details pane (middle section). This line displays the length of the frame. c. The second line in the packet details pane shows that it is an Ethernet II frame. The source and destination MAC addresses are also displayed. Lab - Use Wireshark to Examine Ethernet Frames Questions: What is the MAC address of the PC NIC? f8:5e:a0:de:8c:6c Type your answers here. What is the default gateway’s MAC address? Type yourb0:be:76:d5:44:9f answers here. d. You can click the greater than (>) sign at the beginning of the second line to obtain more information about the Ethernet II frame. Question: What type of frame is displayed? IPv4 (0x0800) Type your answers here. e. The last two lines displayed in the middle section provide information about the data field of the frame. Notice that the data contains the source and destination IPv4 address information. Questions: What is the source IP address? Type your answers here. Src: 192.168.1.157 What is the destination IP address? Dst: 192.168.1.254 Type your answers here. f. You can click any line in the middle section to highlight that part of the frame (hex and ASCII) in the Packet Bytes pane (bottom section). Click the Internet Control Message Protocol line in the middle section and examine what is highlighted in the Packet Bytes pane. Question: What do the last two highlighted octets spell? hi Type your answers here. g. Click the next frame in the top section and examine an Echo reply frame. Notice that the source and destination MAC addresses have reversed, because this frame was sent from the default gateway router as a reply to the first ping. Question: What device and MAC address is displayed as the destination address? Type yourf8:5e:a0:de:8c:6c answers here. Step 7: Capture packets for a remote host. a. Click the Start Capture icon to start a new Wireshark capture. You will receive a popup window asking if you would like to save the previous captured packets to a file before starting a new capture. Click Continue without Saving. Open a Windows command prompt. b. In a command prompt window, ping www.cisco.com. Close a Windows command prompt. c. Stop capturing packets. d. Examine the new data in the packet list pane of Wireshark. Questions: In the first echo (ping) request frame, what are the source and destination MAC addresses? Source: f8:5e:a0:de:8c:6c Type your answers here. Destination: b0:be:76:d5:44:9f Type your answers here. What are the source and destination IP addresses contained in the data field of the frame? Lab - Use Wireshark to Examine Ethernet Frames Source: 192.168.1.157 Type your answers here. Destination: 23.6.241.49 Type your answers here. Compare these addresses to the addresses you received in Step 6. The only address that changed is the destination IP address. Why has the destination IP address changed, while the destination MAC address remained the same? Thats because the Layer 2 frames never leave the LAN. The source will use the MAC default gateway for the frame destination. The gateway receives the packet to remove the frame information from the packet and create a new frame with the MAC address. It will continue until the packt reach the destination IP. Type your answers here. Reflection Question Wireshark does not display the preamble field of a frame header. What does the preamble contain? Type your answers here. End of Document It contains 7 octets with alternating 1010 sequences, and one that signal the beginning frame.

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