Description
PART 1: PYTHON PROGRAMMING – A SIMPLE WEB SERVER
In this lab, you will learn the basics of socket programming for TCP connections in Python: how to
create a socket, bind it to a specific address and port, as well as send and receive a HTTP packet. You
will also learn some basics of HTTP header format.
You will develop a web server that handles one HTTP request at a time. Your web server should
accept and parse the HTTP request, get the requested file from the server’s file system, create an
HTTP response message consisting of the requested file preceded by header lines, and then send the
response directly to the client. If the requested file is not present in the server, the server should send
an HTTP “404 Not Found” message back to the client.
CODE
A skeleton python code is provided with the assignment. You are to complete the skeleton code. The
places where you need to fill in code are marked with #Fill in start and #Fill in end . Each place may
require one or more lines of code.
Note:
Use the latest Python release (Python 3)
Use port number 45678 for your web server port
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RUNNING THE SERVER
Put the provided HTML files (index.html, page2.html) in the same directory that the server code is
in. Run the server program.
On the same computer open a browser and enter the URL below:
http://localhost:45678/index.html
Note the use of the port number after the colon.
The browser should then display the contents of index.html. If you omit “:45678”, the browser will
assume port 80 and you will get the web page from the server only if your server is listening at port
80.
Then try to get a file that is not present at the server (for example, click on the hyperlink Page 1 on
the Main Page). You should get a “404 Not Found” message.
SUBMISSION CONTENT
In your pdf file report, create a section called Part 1 – Python Programming Web Server. This
portion on your report shall include the following items: Python Code listing and Browser Screen
captures.
PYTHON CODE LISTING
Include as text the listing of your Python server code in your pdf report. You must use Courier New
or any equivalent monospace font such as consolas or TlwgTypewriter, size 10 for the code listing.
The use of these monospace font is to clearly show the indentations in your code.
In addition, also submit your Python server code file using the naming convention such as
yourname_p1.py to be included in your report zip file.
BROWSER SCREEN CAPTURES
Include the screen captures of your client browser, verifying that you actually receive the contents of
the HTML file from the server. Make sure the URL is clearly shown and legible.
Provide the three screen captures showing the below:
1. The Main Page
2. The 404 Not Found (when clicked on Page 1 link on the Main page)
3. The Page 2
All screen captures must be clear and legible to get full credit.
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PART 2: ANALYZING HTTP MESSAGES USING WIRESHARK
You are going to use Wireshark to capture and analyze HTTP messages.
SUBMISSION CONTENT
In your pdf file report, create a section called Part 2 – Analyzing HTTP messages using Wireshark.
Provide the answers for the 19 questions below.
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 displayfilter-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.
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Figure 1: Wireshark Display after http://gaia.cs.umass.edu/wireshark-labs/ HTTP-wiresharkfile1.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, and will be investigating these other protocols is later labs),
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.).
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ANSWER THE QUESTIONS
By looking at the information in the HTTP GET and response messages, answer the following
questions. When answering the following questions, you should include the screen capture of
the GET and response messages and indicate where in the message you’ve found the
information that answers the following questions.
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 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.
THE HTTP CONDITIONAL GET/RESPONSE INTERACTION
Recall from Section 2.2.5 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 Recent History
and check the Cache box, 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)
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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.
ANSWER THE QUESTIONS
8. 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?
9. Inspect the contents of the server response. Did the server explicitly return the contents of
the file? How can you tell?
10. 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?
11. 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.
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.
In the packet-listing window, you should see your HTTP GET message, followed by a multiplepacket TCP 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). In recent versions
of Wireshark, Wireshark indicates each TCP segment as a separate packet, and the fact that the single
HTTP response was fragmented across multiple TCP packets is indicated by the “TCP segment of a
reassembled PDU” in the Info column of the Wireshark display. Earlier versions of Wireshark used
the “Continuation” phrase to indicated that the entire content of an HTTP message was broken
across multiple TCP segments.. We stress here that there is no “Continuation” message in HTTP!
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ANSWER THE QUESTIONS
12. How many HTTP GET request messages did your browser send? Which packet number in
the trace contains the GET message for the Bill or Rights?
13. Which packet number in the trace contains the status code and phrase associated with the
response to the HTTP GET request?
14. What is the status code and phrase in the response?
15. How many data-containing TCP segments were needed to carry the single HTTP response
and the text of the Bill of Rights?
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:
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 the cover for our 5th edition (one of our favorite covers) is stored at the caite.cs.umass.edu
server. (These are two different web servers inside cs.umass.edu).
Stop Wireshark packet capture, and enter “http” in the display-filter-specification window,
so that only captured HTTP messages will be displayed.
ANSWER THE QUESTIONS
16. How many HTTP GET request messages did your browser send? To which Internet
addresses were these GET requests sent?
17. 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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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:
Make sure your browser’s cache is cleared, as discussed above, and close down your browser.
Then, start up your browser
Start up the Wireshark packet sniffer
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.
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
ANSWER THE QUESTIONS
18. What is the server’s response (status code and phrase) in response to the initial HTTP GET
message from your browser?
19. 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 (wireshark-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.motobit.com/util/base64-decoder-encoder.asp and enter the base64-encoded string
d2lyZXNoYXJrLXN0dWRlbnRz and 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.
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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!
End of Project 1
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