M6 Wireshark HTTPIn this lab, we’ll explore several aspects of the HTTP protocol: the basic GET/response
interaction, HTTP message formats, and retrieving HTML files with embedded objects.
Before beginning these labs, you might want to review Section 10.3.1 of the text.
For each of the following tasks, you can try the steps listed in the instructions to create
your own trace file to practice the Wireshark on your own computer, but you need to
answer the questions using the provided trace files.
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 but don’t yet begin packet capture. Enter
“http” (just the letters, not the quotation marks, and in lower case) in the displayfilter-specification window, so that only captured HTTP messages will be
displayed later in the packet-listing window.
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 1. You can
download a packet trace file http-wireshark-trace1-1 that was created when the steps
above were followed. 1
1
Once you’ve downloaded a trace file, you can load it into Wireshark and view the trace using the File
pull down menu, choosing Open, and then selecting the trace file name. The resulting display should look
similar to Figure 1 (for the http-wireshark-trace1-1 trace file for this HTTP lab). The Wireshark user
interface displays just a bit differently on different operating systems, and in different versions of
Wireshark.
Figure 1: Wireshark Display after http://gaia.cs.umass.edu/wireshark-labs/ HTTPwireshark-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 OK 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), so make sure the boxes at the far left of the
Frame, Ethernet, IP and TCP information have a plus sign or a right-pointing triangle
(which means there is hidden, undisplayed information), and the HTTP line has a minus
sign or a down-pointing triangle (which means that all information about the HTTP
message is displayed).
(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.).
By looking at the information in the HTTP GET and response messages, answer the
following questions using the provided trace file http-wireshark-trace1-1.
1. Is your browser running HTTP version 1.0, 1.1, or 2? 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 status code returned from the server to your browser?
4. When was the HTML file that you are retrieving last modified at the server?
5. How many bytes of content are being returned to your browser?
2. The HTTP CONDITIONAL GET/response interaction
Recall from Section 10.3.1 of the text, that most web browsers perform object caching
and thus often perform a conditional GET when retrieving an HTTP object. Before
performing the steps below, make sure your browser’s cache is empty 2. 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” (again, in lower case without the
quotation marks) in the display-filter-specification window, so that only captured
HTTP messages will be displayed later in the packet-listing window.
You can download a packet trace http-wireshark-trace2-1 that was created when the steps
above were followed.
Answer the following questions using the provided trace file http-wireshark-trace2-1:
6. 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?
7. Inspect the contents of the server response to this . Did the server explicitly return
the contents of the file? How can you tell?
8. 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?
9. 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. HTML Documents with Embedded Objects
We now 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).
See https://www.howtogeek.com/304218/how-to-clear-your-history-in-any-browser/ for instructions on
clearing your browser cache.
2
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-file4.html
Your browser should display a short HTML file with two images. These two
images are referenced in the base HTML file. That is, the images themselves are
not contained in the HTML; instead the URLs for the images are contained in the
downloaded HTML file. As discussed in the textbook, your browser will have to
retrieve these logos from the indicated web sites. Our publisher’s logo is
retrieved from the gaia.cs.umass.edu web site. The image of our 8th edition
cover (one of our favorite covers) is stored at a server in France.
• Stop Wireshark packet capture, and enter “http” in the display-filter-specification
window, so that only captured HTTP messages will be displayed.
You can download a packet trace http-wireshark-trace4-1 that was created when the steps
above were followed.
Answer the following questions using the provided trace file http-wireshark-trace4-1:
10. How many HTTP GET request messages did your browser send? To which
Internet addresses were these GET requests sent?
11. Can you tell whether your browser downloaded the two images serially, or
whether they were downloaded from the two web sites in parallel? Explain.
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