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<TITLE>Developer.com - Online Reference Library - 0672311739:RED HAT LINUX 2ND EDITION:SMTP and POP</TITLE>
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<!-- ISBN=0672311739 //-->
<!-- TITLE=RED HAT LINUX 2ND EDITION //-->
<!-- AUTHOR=DAVID PITTS ET AL //-->
<!-- PUBLISHER=MACMILLAN //-->
<!-- IMPRINT=SAMS PUBLISHING //-->
<!-- PUBLICATION DATE=1998 //-->
<!-- CHAPTER=07 //-->
<!-- PAGES=0097-0130 //-->
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<P><CENTER>
<a href="0101-0102.html">Previous</A> | <a href="../ewtoc.html">Table of Contents</A> | <a href="0106-0108.html">Next</A>
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<A NAME="PAGENUM-103"><P>Page 103</P></A>
<P>Ted, and Ben, without inspecting the addresses in the memo itself. As far as the Gonzo
and Whizzer mail clerks are concerned, the memo itself might be addressed to the Pope; they
care only about the envelope addresses.
</P>
<P>SMTP clients and servers work in much the same way. Suppose that
joe@gonzo.gov sends mail to his colleagues
betty@zippy.gov and fred@whizzer.com. The recipient list in the letter's
headers may look like this:
</P>
<!-- CODE SNIP //-->
<PRE>
To: betty@zippy.gov, fred@whizzer.com
</PRE>
<!-- END CODE SNIP //-->
<P>The SMTP client at gonzo.gov connects to the
whizzer.com mailer to deliver Fred's copy. When it's ready to list the recipients (the envelope address), what should it say? If it gives both
recipients as they are listed in the preceding To: line (the
header address), Betty will get two copies of the letter because the
whizzer.com mailer will forward a copy to
zippy.gov. The same problem occurs if the
gonzo.gov SMTP client connects to zippy.gov and lists both Betty and Fred as
recipients. The zippy.gov mailer will forward a second copy of Fred's letter.
</P>
<P>The solution is the same one that Alphonse and his fellow mail clerks used. The
gonzo.gov SMTP client puts an envelope around the letter that contains only the names of the
recipients on each host. The complete recipient list is still in the letter's headers, but they are inside
the envelope, and the SMTP servers at gonzo.gov and
whizzer.com don't look at them. In this example, the envelope for the
whizzer.com mailer would list only fred, and the envelope
for zippy.gov would list only betty.
</P>
<P>Aliases illustrate another reason that header and envelope addresses differ. Suppose that
you send mail to the alias homeboys, which includes the names
alphonse, joe, betty, and george. In your letter, you write
To: homeboys. However, sendmail expands the alias and constructs
an envelope that includes all the recipients. Depending on whether the names are also
aliases, perhaps on other hosts, the original message might be put into as many as four different
envelopes and delivered to four different hosts. In each case, the envelope will contain only the
name of the recipients, but the original message will contain the alias
homeboys (expanded to homeboys@your.host.domain so replies will work).
</P>
<P>A final example shows another way in which envelope addresses might differ from header
addresses. sendmail allows you to specify recipients on the command line. Suppose that you
have a file named letter that looks like this:
</P>
<!-- CODE SNIP //-->
<PRE>
$ cat letter
To: null recipient <>
Subject: header and envelope addresses
testing
</PRE>
<!-- END CODE SNIP //-->
<P>You send this letter with the following command (substituting your own login name
for yourlogin):
</P>
<!-- CODE SNIP //-->
<PRE>
$ sendmail yourlogin < letter
</PRE>
<!-- END CODE SNIP //-->
<A NAME="PAGENUM-104"><P>Page 104</P></A>
<P>You will receive the letter even though your login name doesn't appear in the letter's
headers because your address was on the envelope. Unless told otherwise (with the
-t flag), sendmail constructs envelope addresses from the recipients you specify on the command line, and a
correspondence doesn't necessarily exist between the header addresses and the envelope addresses.
</P>
<H4><A NAME="ch07_ 7">
sendmail's Jobs
</A></H4>
<P>To better understand how to set up sendmail, you need to know what different jobs it
does and how these jobs fit into the scheme of MUAs, MTAs, mail routers, final delivery
agents, and SMTP clients and servers. sendmail can act as a mail router, an SMTP client, and an
SMTP server. However, it does not do final delivery of mail.
</P>
<H4><A NAME="ch07_ 8">
sendmail as a Mail Router
</A></H4>
<P>sendmail is primarily a mail router, meaning it takes a letter, inspects the recipient
addresses, and decides the best way to send it. How does
sendmail perform this task?
</P>
<P>sendmail determines some of the information it needs on its own, such as the current time
and the name of the host on which it's running, but most of its brains are supplied by you,
the postmaster, in the form of a configuration file,
sendmail.cf. This somewhat cryptic file tells
sendmail exactly how you want various kinds of mail handled. It is extremely flexible
and powerful, and at first glance seemingly inscrutable. However, one of the strengths of V8
sendmail is its set of modular configuration file building blocks. Most sites can easily construct
their configuration files from these modules, and many examples are included. Writing a
configuration file from scratch is a daunting task, so you should avoid it if you can.
</P>
<H4><A NAME="ch07_ 9">
sendmail as an MTA—Client (Sender) and Server (Receiver) SMTP
</A></H4>
<P>As mentioned before, sendmail can function as an MTA because it understands the
SMTP protocol (V8 sendmail also understands ESMTP). Because SMTP is a
connection-oriented protocol, a client and a server (also known as a sender and a receiver) always exist. The
SMTP client delivers a letter to an SMTP server, which listens continuously on its computer's
SMTP port. sendmail can be an SMTP client or an SMTP server. When run by an MUA, it
becomes an SMTP client and speaks client-side SMTP to an SMTP server (not necessarily
another sendmail program). When your system boots and it starts in daemon mode, it runs
continuously, listening on the SMTP port for incoming mail.
</P>
<H4><A NAME="ch07_ 10">
sendmail as a Final Delivery Agent (NOT!)
</A></H4>
<P>One thing sendmail doesn't do is final delivery.
sendmail's author wisely chose to leave this task to other programs.
sendmail is a big, complicated program that runs with superuser
privileges, an almost guaranteed recipe for security problems, and quite a few have occurred
in sendmail's past. The additional complexity of final mail delivery is the last thing
sendmail needs.
</P>
<A NAME="PAGENUM-105"><P>Page 105</P></A>
<H4><A NAME="ch07_ 11">
sendmail's Auxiliary Files
</A></H4>
<P>sendmail depends on a number of auxiliary files to do its job. The most important are the
aliases file and the configuration file,
sendmail.cf. The statistics file, sendmail.st, can be created
or not, depending on whether you want the statistics.
sendmail.hf, which is the SMTP help file, should be installed if you intend to run
sendmail as an SMTP server (most sites do). That's
all that needs to be said about sendmail.st and
sendmail.hf. (Other auxiliary files are covered in the
Sendmail Installation and Operating Guide, or SIOG for short.) The aliases and
sendmail.cf files, on the other hand, are important enough to be covered in their own sections.
</P>
<H4><A NAME="ch07_ 12">
The Aliases File
</A></H4>
<P>sendmail always checks recipient addresses for aliases, which are alternative names for
recipients. For example, each Internet site is required to have a valid address postmaster to whom
mail problems can be reported. Most sites don't have an actual account of that name but divert
the postmaster's mail to the person or persons responsible for e-mail administration. For
example, at the mythical site gonzo.gov, the users
joe and betty are jointly responsible for e-mail administration, and the aliases file has the following entry:
</P>
<!-- CODE SNIP //-->
<PRE>
postmaster: joe, betty
</PRE>
<!-- END CODE SNIP //-->
<P>This line tells sendmail that mail to
postmaster should instead be delivered to the login
names joe and betty. In fact, these names could also be aliases:
</P>
<!-- CODE SNIP //-->
<PRE>
postmaster: firstshiftops, secondshiftops, thirdshiftops
firstshiftops: joe, betty
secondshiftops: lou, emma
thirdshiftops: ben, mark, clara
</PRE>
<!-- END CODE SNIP //-->
<P>In all these examples, the alias names are on the left side of the colon, and the aliases for
those names are on the right side. sendmail repeatedly evaluates aliases until they resolve to a
real user or a remote address. In the preceding example, to resolve the alias
postmaster, sendmail first expands it into the list of recipients
firstshiftops, secondshiftops, and thirdshiftops and then expands each of these aliases into the final
list—joe, betty, lou, emma, ben, mark, and
clara.
</P>
<P>Although the right side of an alias can refer to a remote host, the left side cannot. The
alias joe: joe@whizzer.com is legal, but joe@gonzo.gov:
joe@whizzer.com is not.
</P>
<!-- CODE SNIP //-->
<PRE>
Reading Aliases from a File—The :include: Directive
</PRE>
<!-- END CODE SNIP //-->
<P>Aliases can be used to create mailing lists. (In the example shown in the preceding section,
the alias postmaster is in effect a mailing list for the local postmasters.) For big or frequently
changing lists, you can use the :include: alias form to direct
sendmail to read the list members from a file. If the aliases file contains the line
</P>
<!-- CODE SNIP //-->
<PRE>
homeboys: :include:/home/alphonse/homeboys.aliases
</PRE>
<!-- END CODE SNIP //-->
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