计算机网络实验：Ethernet and ARP

Lab 7Ethernet and ARP

1. Capturing and analyzing Ethernet frames

STEPS

? First, make sure your browser’s cache is empty. (To do this under Netscape 7.0, select Edit->Preferences->Advanced->Cache and clear the memory and disk cache. For Internet Explorer, select Tools->Internet Options->Delete Files. For Firefox select Tools->Clear Private Data.

? Start up the Wireshark packet sniffer

? Enter the following URL into your browser

http://gaia.cs.umass.edu/wireshark-labs/HTTP-ethereal-lab-file3.html Your browser should display the rather lengthy US Bill of Rights.

? Stop Wireshark packet capture. First, find the packet numbers (the leftmost column in the upper Wireshark window) of the HTTP GET message that was sent from your computer to gaia.cs.umass.edu, as well as the beginning of the HTTP response message sent to your computer by gaia.cs.umass.edu. You should see a screen that looks something like this (where packet 4 in the screen shot below contains the HTTP GET message)

?Since this lab is about Ethernet and ARP, we’re not interested in IP or higherlayer protocols. So let’s change Wireshark’s “listing of captured packets” window so that it shows information only about protocols below IP. To have Wireshark do this, select Analyze->Enabled Protocols. Then uncheck the IP box and select OK.

QUESTIONS

1. What is the 48-bit Ethernet address of your computer?

The 48-bit Ethernet address of your computer is 3c : 97 : 0e : ff : 69 : 02

2. What is the 48-bit destination address in the Ethernet frame? Is this the Ethernet address of gaia.cs.umass.edu? (Hint: the answer is no). What device has this as its Ethernet address? [Note: this is an important question, and one that students

sometimes get wrong. Re-read pages 468-469 in the text and make sure you understand the answer here.]

The 48-bit destination address in the Ethernet frame is 38:22:d6:e6:0e:0d. This isn’t the Ethernet address of gaia.cs.umass.edu. It is the Ethernet address of my default gateway. That is to say, it’s the address of my router, which is the link used to get off the subnet.

3. Give the hexadecimal value for the two-byte Frame type field. What do the bit(s) whose value is 1 mean within the flag field?

The hexadecimal value for the two-byte Frame type field is 0x0800.

The bit(s) whose value is 1 within the flag fieldmean the fragment has not been fragmented.

4. How many bytes from the very start of the Ethernet frame does the ASCII “G” in “GET” appear in the Ethernet frame?

54 bytes. There are 14 bytes of Ethernet frame header, 20 bytes of IP header and 20 bytes of TCP header before HTTP data. So the answer is 54 bytes.

5. What is the hexadecimal value of the CRC field in this Ethernet frame?

There is no CRC field. Because the CRC calculated before the Wireshark packet sniffer start up.

6. What is the value of the Ethernet source address? Is this the address of your computer, or of gaia.cs.umass.edu (Hint: the answer is no). What device has this as its Ethernet address?

The value of the Ethernet source address is 38:22:d6:e6:0e:0d. This is neither the address of my computer, nor of gaia.cs.umass.edu. It is the Ethernet address of my default gateway. That is to say, it’s the address of my router, which is the link used to get off the subnet.

7. What is the destination address in the Ethernet frame? Is this the Ethernet address of your computer?

The destination address in the Ethernet frame is 3c : 97 : 0e : ff : 69 : 02. This is the Ethernet address of my computer.

8. Give the hexadecimal value for the two-byte Frame type field. What do the bit(s) whose value is 1 mean within the flag field?

The hexadecimal value for the two-byte Frame type field is 0x0800.

The bit(s) whose value is 1 within the flag fieldmean the fragment has not been fragmented.

9. How many bytes from the very start of the Ethernet frame does the ASCII “O” in “OK” (i.e., the HTTP response code) appear in the Ethernet frame?

There are 14 bytes of Ethernet frame header, 20 bytes of IP header and 20 bytes of TCP header before HTTP data.

So the HTTP data appears 54 bytes from the very start of the Ethernet frame. But before the ASCII “O” appears, there are 13 bytes. So before the ASCII “O” appears, there are 67 bytes.

10. What is the hexadecimal value of the CRC field in this Ethernet frame?

There is no CRC field. Because the CRC calculated before the Wireshark packet sniffer start up.

2. The Address Resolution Protocol

11. Write down the contents of your computer’s ARP cache. What is the meaning of

each column value?

The address of Internet column contains the IP address. The physical address column contains the MAC address. The type column tells us the information about type: dynamic or static.

STEPS

? Clear your ARP cache, as described above.

? Next, make sure your browser’s cache is empty. (To do this under Netscape 7.0, select Edit->Preferences->Advanced->Cache and clear the memory and disk cache. For Internet Explorer, select Tools->Internet Options->Delete Files.) ? Start up the Wireshark packet sniffer

? Enter the following URL into your browser http://gaia.cs.umass.edu/wireshark-labs/ HTTP-wireshark-lab-file3.html Your browser should again display the rather lengthy US Bill of Rights.

? Stop Wireshark packet capture. Again, we’re not interested in IP or higher-layer protocols, so change Wireshark’s “listing of captured packets” window so that it shows information only about protocols below IP. To have Wireshark do this, select Analyze->Enabled Protocols. Then uncheck the IP box and select OK. You should now see an Wireshark window that looks like:

QUESTIONS

(The following answers are based on the ethernet-ethereal-trace-1 trace file)

12.What are the hexadecimal values for the source and destination addresses in the Ethernet frame containing the ARP request message?

The hexadecimal values for the source addresses in the Ethernet frame is 00:d0:59:a9:3d:68.The hexadecimal values for the destination addresses in the Ethernet frame is ff :ff : ff : ff : ff : ff.

13. Give the hexadecimal value for the two-byte Ethernet Frame type field. What do the bit(s) whose value is 1 mean within the flag field?

The hexadecimal value for the two-byte Ethernet Frame type field is 0x0806. There is no flag field.

14.Download the ARP specification from ftp://ftp.rfc-editor.org/innotes/std/std37.txt. A readable, detailed discussion of ARP is also at http://www.erg.abdn.ac.uk/users/gorry/course/inet-pages/arp.html.

a) How many bytes from the very beginning of the Ethernet frame does the ARP opcode field begin?

There are 14 bytes of Ethernet frame header. In the ARP data, before the ARP opcode field begin, there are 6 bytes. So there are 20 bytes before the ARP opcode field begin.

b) What is the value of the opcode field within the ARP-payload part of the Ethernet frame in which an ARP request is made?

The value of the opcode field within the ARP-payload part of the Ethernet framein which an ARP request is made is 1.

c) Does the ARP message contain the IP address of the sender?

Yes, it contains. In here, the IP address of the sender is 192.168.1.105.

d) Where in the ARP request does the “question” appear – the Ethernet address of the machine whose corresponding IP address is being queried?

The field “Target MAC address” is set to 00:00:00:00:00:00 to question the machine’s Ethernet addresswhose corresponding IP address.

15.Now find the ARP reply that was sent in response to the ARP request.

a) How many bytes from the very beginning of the Ethernet frame does the ARP opcode field begin?

There are 14 bytes of Ethernet frame header. In the ARP data, before the ARP opcode field begin, there are 6 bytes. So there are 20 bytes before the ARP opcode field begin.

b) What is the value of the opcode field within the ARP-payload part of the Ethernet frame in which an ARP response is made?

The value of the opcode field within the ARP-payload part of the Ethernet frame in which an ARP response is made is 2

c) Where in the ARP message does the “answer” to the earlier ARP request appear – the IP address of the machine having the Ethernet address whose corresponding IP address is being queried?

“Sender MAC address” is the answer to the earlier ARP request. In here, it contain the MAC address of 192.168.1.1, which is 00:06:25:da:af:73

16.What are the hexadecimal values for the source and destination addresses in the Ethernet frame containing the ARP reply message?

The hexadecimal values for the source addresses in the Ethernet frame containing the ARP reply message is 00:06:25:da:af:73.

The hexadecimal values for thedestination addresses in the Ethernet frame containing the ARP reply message is 00:d0:59:a9:3d;68.

17. Open the ethernet-ethereal-trace-1 trace file in http://gaia.cs.umass.edu/wireshark-labs/wireshark-traces.zip. The first and second ARP packets in this trace correspond to an ARP request sent by the computer running Wireshark, and the ARP reply sent to the computer running Wireshark by the computer with the ARP-requested Ethernet address. But there is yet another computer on this network, as indiated by packet 6 – another ARP request. Why is there no ARP reply (sent in response to the ARP request in packet 6) in the packet trace?

This a broadcastARP packet sent by 192.168.1.104. So every host in the same subnet will receive the packet. But the ARP is to try find the MAC address of 192.168.1.117. So only the host which have the IP address of 192.168.1.117 will reply. Now “my” host’s IP address is 192.168.1.105. So “my” host won’t reply. And “my” host didn’t receive the reply packet. So there is no ARP reply (sent in response to the ARP request in packet 6) in the packet trace.

Extra Credit

EX-1. The arp command: arp -s InetAddrEtherAddr allows you to manually add an entry to the ARP cache that resolves the IP address InetAddr to the physical address EtherAddr. What would happen if, when you manually added an entry, you entered the correct IP address, but the wrong Ethernet address for that remote interface?

If I entered the correct IP address, but the wrong Ethernet address for that remote

interface, then I will get the wrong MAC address when I try to connect with the IP address that I entered. This is just like what happened in ARP spoofing.

EX-2. What is the default amount of time that an entry remains in your ARP cache before being removed.You can determine this empirically (by monitoring the cache contents) or by looking this up in your operation system documentation. Indicate how/where you determined this value.

In default, in Windows Server 2003 and Windows XP, the entry in the ARP cache will store 2 minutes. If an entry is used in this 2 minutes, the deadline will delay another 2 minutes until it have past 10 minutes. When an entry is stored for 10 minutes, then it must be removed.

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