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<!-- TITLE=RED HAT LINUX 2ND EDITION //-->
<!-- AUTHOR=DAVID PITTS ET AL //-->
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<!-- PUBLICATION DATE=1998 //-->
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<A NAME="PAGENUM-97"><P>Page 97</P></A>
<H3><A NAME="ch07_ 1">
CHAPTER 7
</A></H3>
<H2>
SMTP and POP
</H2>
<B>
by Jeff Smith, Chris Byers, and Steve
Shah
</B>
<H3><A NAME="ch07_ 2">
IN THIS CHAPTER
</A></H3>
<UL>
<LI> SMTP 98
<LI> POP 124
</UL>
<A NAME="PAGENUM-98"><P>Page 98</P></A>
<P>Electronic mail (e-mail) is arguably the most used application of any data network such as
the Internet. (Yes, even more so than the World Wide Web!) Early in the Internet's life, there
were many proposed standards for e-mail between systems. These standards changed often, and
the corresponding software changes were significant.
</P>
<P>During this time of rapid change in e-mail protocols, a package emerged as a standard for
mail transfer—sendmail. sendmail, written by Eric Allman of UC Berkeley, was an unusual
program for its time because it thought of the e-mail problem in a different light. Instead of
rejecting e-mail from different networks using so-called incorrect protocols, it massaged the
message and fixed it so that the message could be passed on to its destination. The side effect of
this level of configurability has been complexity. Several books have been written on the
subject (the authoritative texts have reached over 1000 pages); however, for most administrators
those are overkill.
</P>
<P>One of the key features of sendmail that differentiated it from other
mail transfer agents (MTAs) during the '80s was the separation of mail routing, mail delivery, and mail readers.
sendmail performed mail routing functions only, leaving delivery to local agents that the
administrator could select. This also allowed users to select their preferred mail readers as long as they
could read the format of the messages written by the delivery software.
</P>
<P>With the advent of larger heterogeneous networks, the need for
mail readers that worked on network clients and connected to designated mail servers to send and receive mail gave way
to the Post Office Protocol (POP). POP mail readers have since flourished with client
software available for every imaginable platform and server software available for not only various
implementations of UNIX (including Linux) but for other less robust operating systems as well.
</P>
<P>This chapter covers the most recent work on
sendmail, its underlying protocol SMTP, and the
qpopper package which implements POP support for Linux.
</P>
<H3><A NAME="ch07_ 3">
SMTP
</A></H3>
<P>The Simple Mail Transfer Protocol (SMTP) is the established standard way
of transferring mail over the Internet. The
sendmail program provides the services needed to support SMTP
connections for Linux.
</P>
<P>This section covers the various details needed to understand the
sendmail package, install it, and configure it. Before getting into the details, however, I'll take a moment to discuss
the SMTP protocol in better detail and how the Domain Name System (DNS) (see Chapter
13, "TCP/IP Network Management," for details on DNS configuration) interacts with e-mail
across the Internet.
</P>
<P>Armed with a better understanding of the protocols, you can take on understanding
sendmail itself beginning with the various tasks that
sendmail performs (such as mail routing, header rewriting, and so on) as well as its corresponding configuration files.
</P>
<A NAME="PAGENUM-99"><P>Page 99</P></A>
<CENTER>
<TABLE BGCOLOR="#FFFF99">
<TR><TD><B>
WARNING
</B></TD></TR>
<TR><TD>
<BLOCKQUOTE>
As with any large software package, sendmail has its share of bugs. Although the bugs
that cause sendmail to fail or crash the system have been almost completely eliminated,
security holes that provide root access are still found from time to time.
<BR>
Like any software that provides network connectivity, you
must keep track of security announcements from the
Computer Emergency Response Team (CERT,
www.cert.org) by either visiting its Web page, joining its mailing list, or reading its moderated
newsgroup comp.security.announce.
</BLOCKQUOTE></TD></TR>
</TABLE></CENTER>
<H4>
Internet Mail Protocols
</H4>
<P>To understand the different jobs that sendmail performs, you need to know a little about
Internet protocols. Protocols are simply agreed-upon standards that software and hardware
use to communicate.
</P>
<P>Protocols are usually layered, with higher levels using the lower ones as building blocks.
For example, the Internet Protocol (IP) sends packets of data back and forth without building
an end-to-end connection such as used by SMTP and other higher-level protocols. The
Transmission Control Protocol (TCP), which is built on top of IP, provides for
connection-oriented services such as those used by programs such as Telnet and the Simple Mail Transfer
Protocol (SMTP). Together, TCP/IP provides the basic network services for the Internet.
Higher-level protocols such as the File Transfer Protocol (FTP) and SMTP are built on top of TCP/IP.
The advantage of such layering is that programs which implement the SMTP or FTP protocols
don't have to know anything about transporting packets on the network and making connections
to other hosts. They can use the services provided by TCP/IP for that job.
</P>
<P>SMTP defines how programs exchange e-mail on the Internet. It doesn't matter whether
the program exchanging the e-mail is sendmail running on an HP workstation or an SMTP
client written for an Apple Macintosh. As long as both programs implement the SMTP
protocol correctly, they can exchange mail.
</P>
<P>The following example of the SMTP protocol in action might help demystify it a little.
The user betty at gonzo.gov is sending mail to joe at
whizzer.com:
</P>
<!-- CODE //-->
<PRE>
$ sendmail -v joe@whizzer.com < letter
joe@whizzer.com... Connecting to whizzer.com via tcp...
Trying 123.45.67.1... connected.
220-whizzer.com SMTP ready at Mon, 6 Jun 1997 18:56:22 -0500
220 ESMTP spoken here
>>> HELO gonzo.gov
250 whizzer.com Hello gonzo.gov [123.45.67.2], pleased to meet you
>>> MAIL From:<betty@gonzo.gov>
250 <betty@gonzo.gov>... Sender ok
>>> RCPT To:<joe@whizzer.com>
</PRE>
<!-- END CODE //-->
<A NAME="PAGENUM-100"><P>Page 100</P></A>
<!-- CODE //-->
<PRE>
250 <joe@whizzer.com>... Recipient ok
>>> DATA
354 Enter mail, end with "." on a line by itself
>>> .
250 SAA08680 Message accepted for delivery
>>> QUIT
221 whizzer.com closing connection
joe@whizzer.com... Sent
$
</PRE>
<!-- END CODE //-->
<P>The first line shows one way to invoke sendmail directly rather than let your favorite
Mail User Agent (MUA) such as Elm, Pine, or Mutt do it for you. The
-v option tells sendmail to be verbose and shows you the SMTP dialogue. The other lines show an SMTP client and
server carrying on a conversation. Lines prefaced with
>>> indicate the client (or sender) on
gonzo.gov, and the lines that immediately follow are the replies of the server (or receiver) on
whizzer.com. The first line beginning with 220 is the SMTP server announcing itself after the initial
connection, giving its hostname and the date and time, and the second line informs the client that
this server understands the Extended SMTP protocol (ESMTP) in case the client wants to use
it. Numbers such as 220 are reply codes that the SMTP client uses to communicate with the
SMTP server. The text following the reply codes is only for human consumption.
</P>
<P>Although this dialogue might still look a little mysterious, it will soon be very familiar if
you take the time to read RFC 821. Running
sendmail with its -v option also helps you understand how an SMTP dialogue works.
</P>
<H4><A NAME="ch07_ 4">
The Domain Name System and E-mail
</A></H4>
<P>Names like whizzer.com are convenient for humans, but computers insist on using numeric
IP addresses like 123.45.67.1. The Domain Name System (DNS) provides this hostname to
IP address translation and other important information.
</P>
<P>In the old days when most of us walked several miles to school through deep snow, there
were only a few thousand hosts on the Internet. All hosts were registered with the
Network Information Center (NIC), which distributed a host table listing the hostnames and IP addresses
of all the hosts on the Internet. Those simple times are gone forever. No one really knows
how many hosts are connected to the Internet now, but they number in the millions, and an
administrative entity like the NIC can't keep their names straight. Thus was born the DNS.
</P>
<P>The DNS distributes authority for naming and numbering hosts to autonomous
administrative domains. For example, a company called
whizzer.com could maintain all the information about the hosts in its own domain. When the host
a.whizzer.com wants to send mail or Telnet to the host
b.whizzer.com, it sends an inquiry over the network to the
whizzer.com nameserver, which might run on a host named
ns.whizzer.com. The ns.whizzer.com nameserver
would reply to a.whizzer.com with the IP address of
b.whizzer.com (and possibly other information), and the mail would be sent or the Telnet connection made. Because
ns.whizzer.com is authoritative for the
whizzer.com domain, it can answer any inquiries about
whizzer.com hosts regardless of where they originate; the authority for naming hosts in this domain has
been delegated.
</P>
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