Reading E-mail Via the Web
E-mail is one of the unsung heroes of the Internet. The Web makes the Internet fun and interesting and allows me to keep up with most newspapers and magazines from the comfort of my Haifa apartment. E-mail allows me to keep in touch with friends, family and clients, as well as receive electronic newsletters in a convenient format.
I usually travel with my trusty Linux laptop, which means that with the help of a telephone line, I can dial in to my Internet provider and download the latest mail. However, on some occasions I cannot dial in to check my mail, even though I have full Internet access and a web browser. I could get an account at Hotmail, but Hotmail allows you to read mail sent to its server only, not to any mail server on the Internet.
This month, I will show you how to develop a set of CGI programs to read e-mail from any POP server. These programs do not provide a full-fledged e-mail client, but they do fill a niche and are useful in certain circumstances. The software described this month should demonstrate how relatively simple it is to create such applications and will have the added bonus of providing basic functionality for the times when you are away from the office.
Traditionally, e-mail on UNIX systems is stored on the user's computer. If you have an account on a UNIX system, e-mail sent to you is placed in a file on your computer. I receive mail on my Linux system in the file /var/spool/mail/reuven.
However, this system became inadequate over time for a variety of reasons. As users began to have their own full-fledged UNIX workstations rather than terminals connected to a central computer, system administrators wanted to centralize incoming mail on a single server.
The answer was POP, “post office protocol”. Rather than retrieving mail from a file on their own system, users would download it from the POP server, with a single POP server per work group cluster. A POP server typically stores incoming mail in a traditional UNIX-style file, but allows retrieval and deletion of individual messages via the network. Just as some cities and towns require their residents go to a central post office in order to retrieve letters and packages, POP requires users to retrieve their mail from a central server.
POP has gone through a number of updates over the years, with the most recent update named POP3. Over time, additional functionality has been added, but the basic commands have remained the same. POP allows users to check if they have mail, retrieve one or more messages and delete one or more messages.
Users are generally shielded from the underlying mechanics of POP3. Most modern e-mail programs support POP3. Indeed, e-mail programs on non-UNIX systems depend on the existence of POP3 servers, since they are rarely able to run mail servers known as “mail transport agents” or “MTAs”. Sendmail and qmail are two examples of MTAs.
Before writing a CGI program to read our mail, we must understand how the program can accomplish this feat. We could write our own software to talk to a POP3 server, but as is often the case with Perl, a module already exists to handle this for us. In this particular case, the module is Net::POP3, part of the “libnet” package of network modules available on CPAN. (For more information on CPAN and its mirrors, go to http://www.cpan.org/.)
Net::POP3 provides an object-oriented interface to POP, making it possible to connect to a POP server with only a basic understanding of how the protocol works. Import the module with
then create a new object with
my $pop = new Net::POP3($mailserver);where $mailserver is a scalar containing the name of our POP3 server. If the connection is successful, $pop will be an object with methods allowing us to read and delete messages on the mail server. If the connection is unsuccessful, $pop will be undefined. Now all methods in Net::POP3 work this way, returning undef if the call was unsuccessful. The following code checks for this condition:
die "Error connecting to $mailserver." unless (defined $pop);In order to ensure e-mail remains private, POP3 servers require users to log in with a user name and password. The login method accomplishes that, returning the number of messages waiting for the user:
my $num_messages = $pop->login($username, $password); die "Error logging in." unless (defined $num_messages);Again, notice the test to see whether $num_messages is defined. If it is undefined, then a mistake probably occurred in either the user name or password.
Each message on the POP server is identified with an index number, ranging from 1 to $num_messages. The index number should stay constant during a single POP session, but will change during future sessions. You can use the index number to read or delete a message:
my $message_ref = $pop->read($index);
If message number $index exists, the message headers and body are put into an array reference. Thus, if $index points to a message on our POP server, $message_ref is an array reference. Each element of the array contains a single line of text from the message. We can print the contents of the message by dereferencing $message_ref:
print @$message_ref, "\n";
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- Build a “Virtual SuperComputer” with Process Virtualization
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