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日期:2020-11-16 11:09

Wireshark Lab:

Ethernet and ARP v8.0

Supplement to Computer Networking: A Top-Down

“Tell me and I forget. Show me and I remember. Involve me

and I understand.” Chinese proverb

In this lab, we’ll investigate the Ethernet protocol and the ARP protocol. Before

beginning this lab, you’ll probably want to review sections 6.4.1 (Link-layer addressing

and ARP) and 6.4.2 (Ethernet) in the text1. RFC 826 (ftp://ftp.rfc-editor.org/innotes/std/std37.txt)

contains the gory details of the ARP protocol, which is used by an IP

device to determine the IP address of a remote interface whose Ethernet address is

known.

1. Capturing and analyzing Ethernet frames

Let’s begin by capturing a set of Ethernet frames to study. Do the following2

:

? First, make sure your browser’s cache is empty. To do this under Mozilla Firefox

V3, select Tools->Clear Recent History and check the box for 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-ethereal-lab-file3.html

Your browser should display the rather lengthy US Bill of Rights.

1 References to figures and sections are for the 8th edition of our text, Computer Networks, A Top-down

Approach, 8th ed., J.F. Kurose and K.W. Ross, Addison-Wesley/Pearson, 2020. 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 ethernet--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

ethernet-ethereal-trace-1 trace file. You can then use this trace file to answer the questions below.

? 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.

You should now see a Wireshark window that looks like:

In order to answer the following questions, you’ll need to look into the packet details and

packet contents windows (the middle and lower display windows in Wireshark).

Select the Ethernet frame containing the HTTP GET message. (Recall that the HTTP

GET message is carried inside of a TCP segment, which is carried inside of an IP

datagram, which is carried inside of an Ethernet frame; reread section 1.5.2 in the text if

you find this encapsulation a bit confusing). Expand the Ethernet II information in the

packet details window. Note that the contents of the Ethernet frame (header as well as

payload) are displayed in the packet contents window.

Answer the following questions, based on the contents of the Ethernet frame containing

the HTTP GET message. Whenever possible, when answering a question you should

hand in a printout of the packet(s) within the trace that you used to answer the question

asked. Annotate the printout3 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.

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

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.]

3. Give the hexadecimal value for the two-byte Frame type field. What upper layer

protocol does this correspond to?

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

“GET” appear in the Ethernet frame?

Next, answer the following questions, based on the contents of the Ethernet frame

containing the first byte of the HTTP response message.

5. 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?

6. What is the destination address in the Ethernet frame? Is this the Ethernet address

of your computer?

7. Give the hexadecimal value for the two-byte Frame type field. What upper layer

protocol does this correspond to?

8. 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?

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.

2. The Address Resolution Protocol

In this section, we’ll observe the ARP protocol in action. We strongly recommend that

you re-read section 6.4.1 in the text before proceeding.

ARP Caching

Recall that the ARP protocol typically maintains a cache of IP-to-Ethernet address

translation pairs on your comnputer The arp command (in both MSDOS and

Linux/Unix) is used to view and manipulate the contents of this cache. Since the arp

command and the ARP protocol have the same name, it’s understandably easy to confuse

them. But keep in mind that they are different - the arp command is used to view and

manipulate the ARP cache contents, while the ARP protocol defines the format and

meaning of the messages sent and received, and defines the actions taken on message

transmission and receipt.

Let’s take a look at the contents of the ARP cache on your computer:

? MS-DOS. The arp command is in c:\windows\system32, so type either “arp” or

“c:\windows\system32\arp” in the MS-DOS command line (without quotation

marks).

? Linux/Unix/MacOS. The executable for the arp command can be in various

places. Popular locations are /sbin/arp (for linux) and /usr/etc/arp (for some Unix

variants).

The Windows arp command with no arguments will display the contents of the ARP

cache on your computer. Run the arp command.

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

each column value?

In order to observe your computer sending and receiving ARP messages, we’ll need to

clear the ARP cache, since otherwise your computer is likely to find a needed IP-Ethernet

address translation pair in its cache and consequently not need to send out an ARP

message.

? MS-DOS. The MS-DOS arp –d * command will clear your ARP cache. The –d

flag indicates a deletion operation, and the * is the wildcard that says to delete all

table entries.

? Linux/Unix/MacOS. The arp –d * will clear your ARP cache. In order to run

this command you’ll need root privileges. If you don’t have root privileges and

can’t run Wireshark on a Windows machine, you can skip the trace collection part

of this lab and just use the trace discussed in the earlier footnote.

Observing ARP in action

Do the following4

:

? Clear your ARP cache, as described above.

? Next, make sure your browser’s cache is empty. To do this under Mozilla Firefox

V3, select Tools->Clear Recent History and check the box for 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:

4 The ethernet-ethereal-trace-1 trace file in http://gaia.cs.umass.edu/wireshark-labs/wireshark-traces.zip

was created using the steps below (in particular after the ARP cache had been flushed).

In the example above, the first two frames in the trace contain ARP messages (as does the

6th message). The screen shot above corresponds to the trace referenced in footnote 1.

Answer the following questions:

10. What are the hexadecimal values for the source and destination addresses in the

Ethernet frame containing the ARP request message?

11. Give the hexadecimal value for the two-byte Ethernet Frame type field. What

upper layer protocol does this correspond to?

12. Download the ARP specification from

ftp://ftp.rfc-editor.org/in-notes/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?

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?

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

d) Where in the ARP request does the “question” appear – the Ethernet

address of the machine whose corresponding IP address is being queried?

13. 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?

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?

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?

14. What are the hexadecimal values for the source and destination addresses in the

Ethernet frame containing the ARP reply message?

15. 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 indicated 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?

Extra Credit

EX-1. The arp command:

arp -s InetAddr EtherAddr

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?

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.


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