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Wireshark Lab: HTTP
Version: 2.0 (June 2007) © 2007 J.F. Kurose, K.W. Ross. All Rights Reserved
Computer Networking: A Top- down Approach, 4 th^ edition.
Having gotten our feet wet with the Wireshark packet sniffer in the introductory lab, 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.
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:
- Start up your web browser.
- 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).
- Wait a bit more than one minute (we’ll see why shortly), and then begin Wireshark packet capture.
- 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.
- Stop Wireshark packet capture.
Your Wireshark window should look similar to the window shown in Figure 1. 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. 1
(^1) 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
Figure 1: Wireshark Display after http://gaia.cs.umass.edu/wireshark-labs/ HTTP- wireshark-file1.html has been retrieved by your browser
The example in Figure 1 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
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 1.
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 Netscape 7.0, select Edit->Preferences->Advanced->Cache and clear the memory and disk cache. For 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:
- Start up your web browser, and make sure your browser’s cache is cleared, as discussed above.
- Start up the Wireshark packet sniffer
- Enter the following URL into your browser http://gaia.cs.umass.edu/wireshark-labs/HTTP-wireshark-file2.html Your browser should display a very simple five-line HTML file.
- Quickly enter the same URL into your browser again (or simply select the refresh button on your browser)
- Stop Wireshark packet capture, and enter “http” in the display-filter-specification window, so that only captured HTTP messages will be displayed later in the packet-listing window.
- ( Note: If you are unable to run Wireshark on a live network connection, you can use the http-ethereal-trace-2 packet trace to answer the questions below; see footnote 1. This trace file was gathered while performing the steps above on one of the author’s computers.)
Answer the following questions:
- 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?
- Inspect the contents of the server response. Did the server explicitly return the contents of the file? How can you tell?
- 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?
- 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.
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:
- Start up your web browser, and make sure your browser’s cache is cleared, as discussed above.
- Start up the Wireshark packet sniffer
- Enter the following URL into your browser http://gaia.cs.umass.edu/wireshark-labs/HTTP-wireshark-file3.html Your browser should display the rather lengthy US Bill of Rights.
- Stop Wireshark packet capture, and enter “http” in the display-filter-specification window, so that only captured HTTP messages will be displayed.
- ( Note: If you are unable to run Wireshark on a live network connection, you can use the http-ethereal-trace-3 packet trace to answer the questions below; see footnote 1. This trace file was gathered while performing the steps above on one of the author’s computers.)
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 1.20 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 “Continuation” phrase displayed by Wireshark. We stress here that there is no “Continuation” message in HTTP!
Answer the following questions:
- How many HTTP GET request messages were sent by your browser?
- How many data-containing TCP segments were needed to carry the single HTTP response?
- What is the status code and phrase associated with the response to the HTTP GET request?
- Are there any HTTP status lines in the transmitted data associated with a TCP- induced “Continuation”?
- Enter the following URL into your browser http://gaia.cs.umass.edu/wireshark-labs/protected_pages/HTTP-wireshark- file5.html Type the requested user name and password into the pop up box.
- Stop Wireshark packet capture, and enter “http” in the display-filter-specification window, so that only captured HTTP messages will be displayed later in the packet-listing window.
- ( Note: If you are unable to run Wireshark on a live network connection, you can use the http-ethereal-trace-5 packet trace to answer the questions below; see footnote 1. This trace file was gathered while performing the steps above on one of the author’s computers.)
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$
Answer the following questions:
- What is the server’s response (status code and phrase) in response to the initial HTTP GET message from your browser?
- When your browser’s sends the HTTP GET message for the second time, what new field is included in the HTTP GET message?
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://www.securitystats.com/tools/base64.php and enter the base64-encoded string d2lyZXNoYXJrLXN0dWRlbnRz and press decode. 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 8, there are ways to make WWW access more secure. However, we’ll clearly need something that goes beyond the basic HTTP authentication framework!
