Laboratory 2

Objectives

1. Make sure you understand the POP protocol you need to use for your first programming assignment
2. Get some more practice with Unix system calls
3. Implement some code that you can use for your programming assignment

Preparation

• Make sure you are familiar with the first programming assignment.
• Look through the Unix Network Programming Manual to find out how to get information about network services.
• Look through RFC 1725 and pick out some of the commands you will need to talk to a POP server.
• Do some research and find out how you can determine the user name of the logged-in user.
• Make sure you are familiar with the code at the end of the first assignment that turns off terminal echo so you can enter a password.

1. What function call will you use to retrieve the name of the currently logged in user?
2. What two functions do you use to disable and re-enable echo to the terminal, without changing any other terminal settings?
3. What function call will you use to retrieve the port number of a service given its name (in a C string)?
4. What are four different commands you will need to use to talk to a POP server to manipulate a mailbox?

In Lab Activities

For the lab, you will be creating a simple program that will perform the following steps:

1. determine the user name of the currently logged in user
2. print out the user name
3. prompt the user to enter a password
4. turn off echo and read the password, then turn echo on again
5. prompt the user to enter a service name
6. determine the port number of the specified service (specify the tcp protocol) and print it out. You should gracefully handle the case where the service name can't be found.
7. exit the program

Once you have the program running, start a typescript and run the program. Enter a password, then enter the service name pop3. It should print out 110 for the port number. Now, with the script still going, run the program again, enter a password, then enter the service name nosuchservice. The program should print out some reasonable error message. End the script.

You will also need to actually interact with a POP server, to make sure you understand how the protocol works. Here's a nifty little trick that you can use to interact with a number of different TCP-based services: you can use the telnet program to connect with any TCP port on a server. As long as the exchange with the server uses ASCII messages, which is true of the POP protocol, you can just type messages into telnet and see the responses from the server.

So try the following:

telnet esus.cs.montana.edu 110

+OK POP3 esus.cs.montana.edu v2003.83 server ready

USER your_login

+OK User name accepted, password please

You can enter POP commands and see the responses; when you are done, just type the command

QUIT

Once you feel comfortable with the POP commands, start a typescript, use telnet to connect to esus, enter the commands to get authenticated, retrieve the list of messages from the pop server, retrieve one of the messages on the list, and exit the POP session. Close the script.

Bonus: If you want to do a little more work on your lab program for some extra credit, you can do the following:

Modify your program so after you enter your password, you verify that the password string is actually the correct password for the current user. (Hint: user passwords are actually stored in an encrypted form in Unix, and you can't unencrypt them. But you can encrypt a string and see if it generates the same encrypted password.)

Lab Write-Up

• Name and lab section
• A copy of the source for the client program that you wrote for the lab
• The script of the program runs that retrieve the port for pop3 and nosuchservice
• The script of the telnet interaction with the POP3 server on esus
• If you did the bonus work, another script that shows a run of the program where you enter your password correctly, and another script that shows a run where you enter your password incorrectly.