Computer Networks Practical #7

INT21CN Computer Networks

Practical Exercises #7

We'll first try a slight variation of an exercise from the previous prac to observe a feature seen there, but not commented on at the time. Type the following command at the Unix command-line:
> HEAD http://ironbark.bendigo.latrobe.edu.au/
Important aspect: note the existence of an Etag: header. Note that Ironbark also sends the Last-modified: response header.

We're now going to try out the Proxy Server features of HTTP In this case, you'll have to telnet to your local proxy server. Within La Trobe Uni you do this by, as before, using telnet telnet proxy.latrobe.edu.au 8080. Once connected, the GET command is the same as usual except you request a full URL. Here's a transcript of me doing exactly this. Note that the first ">" is my Unix prompt.

> telnet proxy.latrobe.edu.au 8080
Trying 131.172.4.39...
Connected to proxy.latrobe.edu.au.
Escape character is '^]'.
GET http://www.unimelb.edu.au/ HTTP/1.0

HTTP/1.0 200 OK
Date: Tue, 25 Mar 2003 08:56:55 GMT
Server: Apache/1.3.27 (Unix) mod_perl/1.27 mod_ssl/2.8.11 OpenSSL/0.9.6e
Content-Type: text/html
Age: 9403
X-Cache: HIT from squid2.latrobe.edu.au
Proxy-Connection: close


<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>Welcome to the University of Melbourne</title>

    ...etc...
^]c

Try it. In fact, use it attempt to fetch some other external Web pages. Note that you may have to type the "Escape character" (control-right-square-bracket) and "c" to explicitly close the connection, since proxy.latrobe.edu.au seems to occasionally default to Connection: keep-alive -- although it didn't when I ran this particular example.

You can use the commandline HEAD utility to fetch pages via the La Trobe proxy server. This can be used to see which of the HTTP headers are sent. For example, try this:
> HEAD -p http://proxy.latrobe.edu.au:8080 http://www.abc.net.au/news
Note any interesting headers. Can you see any that appear to relate to cache control, but weren't mentioned in the lecture? You should.
Repeat the exercise with some other pages. For example, you might try http://www.theage.com.au/ and some of more "famous" Web site home pages.