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Next: 11. User Accounts and Up: rute Previous: 9. Processes, Environment Variables Contents
Subsections
10. Mail
Electronic Mail, or email, is the way most people first come into contact with the Internet. Although you may have used email in a graphical environment, here we show you how mail was first intended to be used on a multiuser system. To a large extent what applies here is really what is going on in the background of any system that supports mail.
A mail message is a block of text sent from one user to another,
using some mail command or mailer program. A mail
message will usually also be accompanied by a subject
explaining what the mail is about. The idea of mail is that a
message can be sent to someone even though he may not be logged
in at the time and the mail will be stored for him until he is
around to read it. An email address is probably familiar to you,
for example:
bruce@kangeroo.co.au. This means that
bruce has a user account on a computer called
kangeroo.co.au, which often means that he can log in as
bruce on that machine. The text after the
@ is always
the name of the machine. Today's Internet does not obey this
exactly, but there is always a machine that
bruce
does have an account on where mail is eventually sent.
[That machine is also usually a UNIX machine.]
Sometimes email addresses are written in a more user-friendly
form like
Bruce Wallaby <bruce@kangeroo.co.au> or
bruce@kangeroo.co.au (Bruce Wallaby). In this case,
the surrounding characters are purely cosmetic;
only
bruce@kangeroo.co.au is ever used.
When mail is received for you (from another user on the system or from a
user from another system) it is appended to the file
/var/spool/mail/<username>
called the mail file or mailbox file;
<username> is your login name. You then run some program that interprets your mail file, allowing
you to browse the file as a sequence of mail messages and read and reply to
them.
An actual addition to your mail file might look like this:
5 10 15 20 25 30 35 |
From mands@inetafrica.com Mon Jun 1 21:20:21 1998Return-Path: <mands@inetafrica.com>Received: from pizza.cranzgot.co.za (root@pizza.cranzgot.co.za [192.168.2.254]) by onion.cranzgot.co.za (8.8.7/8.8.7) with ESMTP id VAA11942 for <psheer@cranzgot.co.za>; Mon, 1 Jun 1998 21:20:20 +0200Received: from mail450.icon.co.za (mail450.icon.co.za [196.26.208.3]) by pizza.cranzgot.co.za (8.8.5/8.8.5) with ESMTP id VAA19357 for <psheer@cranzgot.co.za>; Mon, 1 Jun 1998 21:17:06 +0200Received: from smtp02.inetafrica.com (smtp02.inetafrica.com [196.7.0.140]) by mail450.icon.co.za (8.8.8/8.8.8) with SMTP id VAA02315 for <psheer@cranzgot.co.za>; Mon, 1 Jun 1998 21:24:21 +0200 (GMT)Received: from default [196.31.19.216] (fullmoon) by smtp02.inetafrica.com with smtp (Exim 1.73 #1) id 0ygTDL-00041u-00; Mon, 1 Jun 1998 13:57:20 +0200Message-ID: <357296DF.60A3@inetafrica.com>Date: Mon, 01 Jun 1998 13:56:15 +0200From: a person <mands@inetafrica.com>Reply-To: mands@inetafrica.comOrganization: privateX-Mailer: Mozilla 3.01 (Win95; I)MIME-Version: 1.0To: paul sheer <psheer@cranzgot.co.za>Subject: helloContent-Type: text/plain; charset=us-asciiContent-Transfer-Encoding: 7bitStatus: ROX-Status: A hey paulits mehow r u doingi am wellwhat u been upothows lifehope your wellamanda |
Each mail message begins with a
From at the beginning
of a line, followed by a space. Then comes the mail header,
explaining where the message was routed from to get
to your mailbox, who sent the message, where replies should
go, the subject of the mail, and various other mail header fields. Above,
the header is longer than the mail messages. Examine the header
carefully.
The header ends with the first blank line. The message itself
(or body) starts right after. The next header in the file
will once again start with a
From.
Froms on
the beginning of a line never exist within the body.
If they do, the mailbox is considered to be corrupt.
Some mail readers store their messages in a different format.
However the above format (called the mbox format) is the
most common for UNIX. Of interest is a format called Maildir,
which is one format that does not store mail messages
in a single contiguous file. Instead, Maildir stores each message
as a separate file within a directory. The name of the directory is
then considered to be the mailbox ``file''; by default Maildir uses a
directory
Maildir within the user's home directory.
10.1 Sending and Reading Mail
The simplest way to send mail is to use the
mail command.
Type
mail
-s "hello there" <username>. The
mail
program will then wait for you to type out your message. When you are
finished, enter a
. on its own on a single line. The
user name will be another user on your system. If no one else is
on your system, then send mail to
root with
mail -s "Hello there" root or
mail -s "Hello there" root@localhost (if
the @ is not present, then the local machine,
localhost, is implied).
Sending files over email is discussed in Section 12.6.
You can use
mail to view your mailbox. This is a
primitive utility in comparison with modern graphical mail readers
but is probably the only mail reader that can handle arbitrarily
sized mailboxes. Sometimes you may get a mailbox that is over a
gigabyte in size, and
mail is the only way to delete
messages from it. To view your mailbox, type
mail, and then
z to read your next window of messages, and
z- to view
the previous window. Most commands work like command
message_number, for example,
delete 14 or
reply 7.
The message number is the left column with an
N next to it (for a
New message).
For the state of the art in terminal-based mail readers (also called
mail clients), try
mutt and
pine. [
pine's license is not Free.]
There are also some graphical mail readers in various stages of
development. At the time I am writing this, I have been using
balsa for a few months, which was the best mail reader
I could find.
10.2 The SMTP Protocol -- Sending Mail Raw to Port
25
To send mail, you need not use a mail client at all. The mail client just follows SMTP (Simple Mail Transfer Protocol), which you can type in from the keyboard.
For example, you can send mail by
telneting to
port 25 of a machine that has an
MTA (Mail Transfer Agent--also called the mailer daemon
or mail server)
running. The word daemon denotes programs that
run silently without user intervention.
This is, in fact, how so-called anonymous mail or
spam mail [Spam is a term used to indicate
unsolicited email--that is, junk mail that is posted in bulk to
large numbers of arbitrary email addresses. Sending spam is considered
unethical Internet practice.] is sent on the Internet. A mailer
daemon runs in most small institutions in the world and has the
simple task of receiving mail requests and relaying them on to other
mail servers. Try this, for example (obviously substituting
mail.cranzgot.co.za for the name of a mail server that you
normally use):
5 10 15 20 |
[root@cericon]# Trying 192.168.2.1...Connected to 192.168.2.1.Escape character is '^]'.220 onion.cranzgot.co.za ESMTP Sendmail 8.9.3/8.9.3; Wed, 2 Feb 2000 14:54:47 +0200250 onion.cranzgot.co.za Hello cericon.ctn.cranzgot.co.za [192.168.3.9], pleased to meet yo250 psheer@cranzgot.co.za... Sender ok250 mands@inetafrica.com... Recipient ok354 Enter mail, end with "." on a line by itself 250 OAA04620 Message accepted for delivery221 onion.cranzgot.co.za closing connectionConnection closed by foreign host.[root@cericon]# |
The above causes the message ``
hi there heres a short
message'' to be delivered to
mands@inetafrica.com (the
Re
Ci
Pien
T).
Of course, I can enter any address that I like as the sender, and it
can be difficult to determine who sent the message. In this
example, the
Subject: is the only header field, although
I needn't have supplied a header at all.
Now, you may have tried this and gotten a rude error message. This might be because the MTA is configured not to relay mail except from specific trusted machines--say, only those machines within that organization. In this way anonymous email is prevented.
On the other hand, if you are connecting to the user's very own mail
server, it has to necessarily receive the mail, regardless of who sent it. Hence, the above is a
useful way to supply a bogus
FROM address and thereby
send mail almost anonymously. By ``almost'' I mean that the mail
server would still have logged the machine from which you
connected and the time of connection--there is no perfect anonymity
for properly configured mail servers.
The above technique is often the only way to properly test a mail server, and should be practiced for later.
Next: 11. User Accounts and Up: rute Previous: 9. Processes, Environment Variables Contents