Computer Networks

Practical Exercises #4

1. When you receive mail on the departmental Unix systems, it is stored in your system mailbox. This is a file in "mailbox format". Examine your system mailbox and explain what "mailbox format" looks like. In particular, how do you tell where one message ends and the next begins? On some systems you can discover where your mailbox is by typing echo $MAIL at the shell prompt.
   
   • Note that if you don't normally read your mail on the Unix systems, don't worry too much about this exercise. On the other hand, maybe you could look at the format of the files which your particular mail agent uses to store messages.

2. The "mail spooling" system on a Unix host can be examined using the mailq command. Try it. Same proviso as previous question.

3. Investigate mail forwarding on the Unix systems. This is done by creating a .forward file in your home directory, containing the email address you want your mail forwarded to. If you normally connect to the university from an external ISP, you may wish to set a .forward containing your usual email address at your ISP. That way, mail sent to all registered users on the Unix systems will be delivered to you as well.

4. Depending on which user agent you use for reading mail, you can configure what happens when different MIME message types are received. For example, audio/basic messages probably cause a sound file player to be opened, whereas image/gif normally opens an image viewer. Discover where this behaviour is specified (possibly in a "Preferences" menu item) and customise it. Can you control the MIME types of enclosures, or attachments, which you send from your mail agent?

5. (optional) One of the nicest things about using Unix for reading your email is the range of tools available for processing incoming mail. The best of these is procmail. Look up procmail and procmailrc in the Unix man pages and see what it can do for you. Have a look at the file ~pscott/.procmailrc on the Unix systems (your lecturer's personal procmail configuration) to see how he uses it. Note same proviso as question 1.

6. The SMTP protocol can be employed in useful ways by the average network guru. For example, you can check on the validity of an email address by telnetting to the SMTP port (25) and invoking the VRFY command. Similarly, you can expand an email alias using the EXPN command. Telnet to port 25 on ironbark and VRFY the username pscott. Then see if you can EXPN the alias scott

7. OK, time for some experimental computer science. In the lecture, you saw a "by hand" SMTP session - telnetting to port 25 and typing the SMTP commands to the remote server. Try it yourself, and send a message to someone - perhaps even to yourself. Be aware that every electronic mail transaction is logged on all systems, so you would be well advised to stick to local hosts, and addresses of people you know.