Wireshark Lab: HTTP

 
This lab manual is based on "Wireshark Lab: HTTP", version 2.0 (September 2009) by J.F. Kurose, K.W. Ross, available here. It has been prepared by Farrokh Ghani Zadegan and Niklas Carlsson, January 2012.

 

In this lab you will first need to read through "Wireshark Lab: Getting Started". This document and the excercises in that lab will prepare your for the lab itself. The appropriate HTTP traces can be found here (or locally). Second, you will be asked to answer and/or discuss a number of questions. To save time, it is important that you carefully read the instructions such that you provide answers in the desired format(s).

Also, please feel free to install wireshark on your own computer and make your own traffic captures to analyze.

Having gotten our feet wet with the Wireshark packet sniffer in the introductory lab above, we’re now ready to use Wireshark to investigate protocols in operation. In this lab, we’ll explore several aspects of the HTTP protocol: the basic GET/response interaction, HTTP message formats, retrieving large HTML files, retrieving HTML files with embedded objects, and HTTP authentication and security. Before beginning these labs, you might want to review Section 2.2 of the text.

Before you start, please consider the following:

Figure 1
Figure 1: Wireshark indicating TCP checksum errors

 

1. The Basic HTTP GET/response interaction

Let’s begin our exploration of HTTP by downloading a very simple HTML file - one that is very short, and contains no embedded objects. Do the following:

  1. Start up your web browser.
  2. Start up the Wireshark packet sniffer, as described in the Introductory lab (but don’t yet begin packet capture). Enter “http” (just the letters, not the quotation marks) in the display-filter-specification window, so that only captured HTTP messages will be displayed later in the packet-listing window. (We’re only interested in the HTTP protocol here, and don’t want to see the clutter of all captured packets).
  3. Wait a bit more than one minute (we’ll see why shortly), and then begin Wireshark packet capture.
  4. Enter the following to your browser http://gaia.cs.umass.edu/wireshark-labs/HTTP-wireshark-file1.html Your browser should display the very simple, one-line HTML file.
  5. Stop Wireshark packet capture.
Your Wireshark window should look similar to the window shown in Figure 2. If you are unable to run Wireshark on a live network connection, you can download a packet trace that was created when the steps above were followed. To do so, download the zip file http://gaia.cs.umass.edu/wireshark-labs/wireshark-traces.zip and extract the file http-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 http-ethereal-trace-1 trace file. The resulting display should look just like Figure 2 after applying the "http" filter.

 

Figure 2
Figure 2: Wireshark Display after http://gaia.cs.umass.edu/wireshark-labs/ HTTPwireshark- file1.html has been retrieved by your browser
(Note: You should ignore any HTTP GET and response for favicon.ico. If you see a reference to this file, it is your browser automatically asking the server if it (the server) has a small icon file that should be displayed next to the displayed URL in your browser. We?ll ignore references to this pesky file in this lab.).

 

The example in Figure 2 shows in the packet-listing window that two HTTP messages were captured: the GET message (from your browser to the gaia.cs.umass.edu web server) and the response message from the server to your browser. The packet-contents window shows details of the selected message (in this case the HTTP GET message, which is highlighted in the packet-listing window). Recall that since the HTTP message was carried inside a TCP segment, which was carried inside an IP datagram, which was carried within an Ethernet frame, Wireshark displays the Frame, Ethernet, IP, and TCP packet information as well. We want to minimize the amount of non-HTTP data displayed (we’re interested in HTTP here, and will be investigating these other protocols in later labs), so make sure the boxes at the far left of the Frame, Ethernet, IP and TCP information have a plus sign (which means there is hidden, undisplayed information), and the HTTP line has a minus sign (which means that all information about the HTTP message is displayed). Task A: By looking at the information in the HTTP GET and response messages, answer the following (7) questions. When answering the following questions, you should print out the GET and response messages (to include the packet data in your own report, you may export the selected packet data as a text file. To do so, use the File->Export->File... window, select file type as Plain text, choose "Selected packet", and uncheck "Packet details") and indicate where in the message you’ve found the information that answers the following questions. As for all questions in this course it is important that you clearly indicate what your answer is, how you obtained the answer, and (if applicable) discuss implications/insights regarding your answers. For example, in the questions below, can you elaborate on why you may have observed what you observed?
  1. Is your browser running HTTP version 1.0 or 1.1? What version of HTTP is the server running?
  2. What languages (if any) does your browser indicate that it can accept to the server?
  3. What is the IP address of your computer? Of the gaia.cs.umass.edu server?
  4. What is the status code returned from the server to your browser?
  5. When was the HTML file that you are retrieving last modified at the server?
  6. How many bytes of content are being returned to your browser?
  7. By inspecting the raw data in the packet content window, do you see any http headers within the data that are not displayed in the packet-listing window? If so, name one.
In your answer to question 5 above, you might have been surprised to find that the document you just retrieved was last modified within a minute before you downloaded the document. That’s because (for this particular file), the gaia.cs.umass.edu server is setting the file’s last-modified time to be the current time, and is doing so once per minute. Thus, if you wait a minute between accesses, the file will appear to have been recently modified, and hence your browser will download a “new” copy of the document.

 

2. The HTTP CONDITIONAL GET/response interaction

Recall from Section 2.2.6 of the text, that most web browsers perform object caching and thus perform a conditional GET when retrieving an HTTP object. Before performing the steps below, make sure your browser’s cache is empty. (To do this under Firefox, select Tools->Clear Private Data, or for Internet Explorer, select Tools->Internet Options->Delete File. These actions will remove cached files from your browser’s cache.) Now do the following:

Practice questions (not to be answered explicitly):

  1. Inspect the contents of the first HTTP GET request from your browser to the server. Do you see an “IF-MODIFIED-SINCE” line in the HTTP GET?
  2. Inspect the contents of the server response. Did the server explicitly return the contents of the file? How can you tell?
  3. Now inspect the contents of the second HTTP GET request from your browser to the server. Do you see an “IF-MODIFIED-SINCE:” line in the HTTP GET? If so, what information follows the “IF-MODIFIED-SINCE:” header?
  4. What is the HTTP status code and phrase returned from the server in response to this second HTTP GET? Did the server explicitly return the contents of the file? Explain.
Task B: For questions 8-11, write a (combined) paragraph explaining and discussing your observations from the above practice questions. Note that your answer may benefit from explaining and/or referring to some of your observations explicitly.

 

3. Retrieving Long Documents

In our examples thus far, the documents retrieved have been simple and short HTML files. Let’s next see what happens when we download a long HTML file. Do the following:

In the packet-listing window, you should see your HTTP GET message, followed by a multiple-packet response to your HTTP GET request. This multiple-packet response deserves a bit of explanation. Recall from Section 2.2 (see Figure 2.9 in the text) that the HTTP response message consists of a status line, followed by header lines, followed by a blank line, followed by the entity body. In the case of our HTTP GET, the entity body in the response is the entire requested HTML file. In our case here, the HTML file is rather long, and at 4500 bytes is too large to fit in one TCP packet. The single HTTP response message is thus broken into several pieces by TCP, with each piece being contained within a separate TCP segment (see Figure 1.24 in the text). Each TCP segment is recorded as a separate packet by Wireshark, and the fact that the single HTTP response was fragmented across multiple TCP packets is indicated by the “TCP segment of a reassembled PDU” phrase displayed by Wireshark. We stress here that there is no “TCP segment of a reassembled PDU” message in HTTP! In this regard, Figure 3 shows a screenshot of Wireshark displaying http-ethereal-trace-3 packet trace. In the listing of the captured packets, packet No. 8 shows the HTTP GET request and packet No. 14 shows the corresponding HTTP response. It can be seen that the packets No. 10, 11 and 13 are labeled with “TCP segment of a reassembled PDU”. By clicking on the HTTP response, i.e. packet No. 14, the packet details pane shows [4 Reassembled TCP Segments (4816 bytes): #10(1460), #11(1460), #13(1460), #14(436)] (see Figure 3). Additionally, the packet bytes pane shows a new tab titled Reassembled TCP which shows the entire received HTTP response.

Figure 3
Figure 3: Wireshark display showing http-ethereal-trace-3 packet trace

A more convenient way to view the entire data (i.e. all HTTP requests and responses transported in a TCP stream) is using a Wireshark feature called "Following TCP Streams". By right-clicking on any of the TCP packets associated with a given TCP stream and selecting the "Follow TCP Stream" menu item, a new window pops up that contains the data exchanged in the selected stream. Figure 4, shows the "Follow TCP Stream" window for the GET /ethereal-labs/lab2-3.html HTTP/1.1 request and its complete associated response. In this window, the non-printable characters are replaced by dots. However, the choice of Raw or ASCII in this window, affects the way you can save the entire stream. That is, if Raw is selected, the stream is saved as a binary file preserving the non-printable characters, whereas in the case of ASCII, the stream is saved as a text file in which the non-printable characters are replaced by dots. Please note how Wireshark has changed (and applied) the display filter to show only the packets in the selected stream.

Figure 4
Figure 4: The "Follow TCP Stream" window

Practice questions (not to be answered explicitly):

  1. How many HTTP GET request messages were sent by your browser?
  2. How many data-containing TCP segments were needed to carry the single HTTP response?
  3. What is the status code and phrase associated with the response to the HTTP GET request?
  4. Are there any HTTP status lines in the transmitted data associated with TCP segmentation?
Task C: For questions 12-15, write a (combined) paragraph explaining and discussing your observations from the above practice questions. Note that your answer may benefit from explaining and/or referring to some of your observations explicitly.

 

4. HTML Documents with Embedded Objects

Now that we’ve seen how Wireshark displays the captured packet traffic for large HTML files, we can look at what happens when your browser downloads a file with embedded objects, i.e., a file that includes other objects (in the example below, image files) that are stored on another server(s). Do the following:

Practice questions (not to be answered explicitly):

  1. How many HTTP GET request messages were sent by your browser? To which Internet addresses were these GET requests sent?
  2. Can you tell whether your browser downloaded the two images serially, or whether they were downloaded from the two web sites in parallel? Explain.
Task D: For questions 16-17, write a paragraph explaining and discussing your observations from the above practice questions. Note that your answer may benefit from explaining and/or referring to some of your observations explicitly.

 

5. HTTP Authentication

Finally, let’s try visiting a web site that is password-protected and examine the sequence of HTTP message exchanged for such a site. The URL http://gaia.cs.umass.edu/wireshark-labs/protected_pages/HTTP-wireshark-file5.html is password protected. The username is “wireshark-students” (without the quotes), and the password is “network” (again, without the quotes). So let’s access this “secure” password-protected site. Do the following:

Now let’s examine the Wireshark output. You might want to first read up on HTTP authentication by reviewing the easy-to-read material on “HTTP Access Authentication Framework” at http://frontier.userland.com/stories/storyReader$2159

Practice questions (not to be answered explicitly):

  1. What is the server’s response (status code and phrase) in response to the initial HTTP GET message from your browser?
  2. When your browser sends the HTTP GET message for the second time, what new field is included in the HTTP GET message?
You will not need to report answers for questions 18-19; however, please feel free to write a paragraph explaining and discussing your observations from the above practice questions.

The username (wirehsark-students) and password (network) that you entered are encoded in the string of characters (d2lyZXNoYXJrLXN0dWRlbnRzOm5ldHdvcms=) following the “Authorization: Basic” header in the client’s HTTP GET message. While it may appear that your username and password are encrypted, they are simply encoded in a format known as Base64 format. The username and password are not encrypted! To see this, go to http://gtools.org/tool/base64-encode-decode/ and enter the base64-encoded string d2lyZXNoYXJrLXN0dWRlbnRz into the "Decode from base64" text box and press "Go". Voila! You have translated from Base64 encoding to ASCII encoding, and thus should see your username! To view the password, enter the remainder of the string Om5ldHdvcms= and press decode. Since anyone can download a tool like Wireshark and sniff packets (not just their own) passing by their network adaptor, and anyone can translate from Base64 to ASCII (you just did it!), it should be clear to you that simple passwords on WWW sites are not secure unless additional measures are taken.

Fear not! As we will see in Chapter 7, there are ways to make WWW access more secure. However, we’ll clearly need something that goes beyond the basic HTTP authentication framework!

 

Demonstration and Report

For this assignment you will need to write a report that carefully answers questions 1-7 (task A), as well as provides one paragraph discussing each of the set of questions: 8-11 (task B), 12-15 (task C), and 16-17 (task D). Note that each group of questions has a theme and you are expected to convince us that you understand these aspects of HTTP.

Please structure your report such that your answers are clearly indicated for each question (and section of the assignment). It is not the TA's task to search for the answers. Both the questions themselves and the corresponding answers should be clearly stated (and indicated) in your report. Structure your report accordingly. Furthermore, your answers should be explained and supported using additional evidence, when applicable. During the demonstration the TA may ask similar questions to assess your understanding of the lab. You are expected to clearly explain and motivate your answers. As the assignments are done in groups of two, both members of the group will be asked to answer questions.

Additional instructions and information about the reports can be found here. Please take this chance to read the guidelines carefully.

 

A Note on Wireshark Protocol Dissectors

Note: You may find the following hint useful, specially when you use Wireshark to sniff data that is being exchanged between browser and the proxy server you will develop for Lab 2.

Wireshark uses protocol dissectors to extract information from packets. For example, the information shown under the "Hypertext Transfer Protocol" node in the packet details pane in Figure 2 are extracted using the HTTP protocol dissector. Wireshark, however, is not always able to choose the right dissector for a packet. This happens for example, when an uncommon port is used for a common protocol, making Wireshark not able to choose the right type of dissector. Such an example is shown in Figure 5 where a proxy server on port 60000 is used to access web pages. It can be seen from Figure 5 that although the contents of the packets are HTTP data, the protocol is not detected as HTTP.

Figure 5
Figure 5: Surfing the Web through a proxy server on port 60000

Fortunately, it is possible to instruct Wireshark what dissector to use for a given packet. By right-clicking on a packet and selecting "Decode As ...", a window opens which allows assigning the desired protocol dissector to the selected packet, see Figure 6. In the example of Figure 6, after selecting the Transport tab, one can select the HTTP protocol dissector to be used for every packet with the source port of 60000. In the same manner, one can right-click on a packet with the destination port of 60000 and assign the HTTP dissector to it, so that both outbound and inbound packets to and from the proxy server are decoded as HTTP. Please note that such user specified decodes cannot be saved and are lost upon exiting Wireshark.

Figure 6
Figure 6: The "Decode As" window