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Tutorial Article: Linux Internet Server: Setting Up a Mail Hub Part 3

By Paul Dunne

Read on to learn how to use Linux to serve the mail transport needs of your organization.

Questions regarding this article should be directed to the author at paul@tiny1.demon.co.uk

This article builds on my two earlier articles on installing Linux and configuring Linux for Internet access, listed here:

Introduction

In this article, we will cover setting up a Linux machine to act as a mail hub for the local network, allowing workstations to send and receive Internet mail without themselves being directly connected to the outside world. You should already know how to install Linux and connect a Linux machine to the Internet: previous articles in this series have examined these issues.

Getting and Compiling Sendmail

Although Sendmail comes ready built with almost any Linux distribution you care to name, there are advantages to knowing how to ``roll your own.'' A widely-used, important program such as Sendmail inevitably attracts a lot of attention from crackers. Updates to the program incorporating fixes for the latest security holes are regularly being made available. These updates are available as patches against the source code, so you need the source if you want to keep up. To provide the optimum degree of security for your site, down-load the Sendmail source from a reputab le site, then configure, compile and install Sendmail yourself,

There is a Sendmail Web page (<URL:http://www.sendmail.org/>), and many sites mirror the source code from there.

Extract the source into a directory -- /usr/src/sendmail-x.x.xx -- and change into the /src directory therein. In there is a makesendmail shell script that will do all the work for you. It is easiest to do the configuration in the source directory, before actually installing the binary and configuration files.

Configuring the Mail Hub

Introduction

Tackling the Sendmail configuration process from scratch is tough, no two ways about it. The main configuration file, sendmail.cf , is designed to be easy for the Sendmail program to read and parse; therefore, it appears somewhat inscrutable to humans. Fortunately, it is a task that rarely has to be done from the beginning.

The Sendmail distribution includes several sample sendmail.cf files, one or other of which can be adapted to most configurations with a few changes; and any modern Linux distribution will also include these in its Sendmail installation.

Version 8 Sendmail has introduced an important simplification into the configuration process, by shifting the user intervention from direct editing of the Sendmail configuration file to making changes to files of m4(1) macros, which is easier and more intuitive. I consider use of the m4 macros in this section, whereas the section on configuring a Sendmail client that connects to our mail hub will deal with the /etc/sendmail.cf file directly, as that process is sufficiently simple to be easily accomplished without the aid of the m4 macros.

Generating a Sendmail configuration

The Sendmail configuration apparatus is in the cf/ subdirectory. T he structure of this subdirectory is as follows:

[ Editor's note: Thus, the first subdirectory listed below is cf/m4 , the second cf/cf , and so forth.]

m4/
This contains support routines, which should not be changed.
cf/
The configuration files themselves. They have ``.mc'' suffixes, and must be run through the Unix m4 program to become usable. The resulting output should have a ``.cf'' suffix.
ostype/
Definitions describing a particular operating system type. These should always be referenced using the OSTYPE macro in the ``.mc'' file.
domain/
Definitions describing a particular domain, referenced using the DOMAIN macro in the ``.mc'' file.
mailer/
Descriptions of mailers, referenced using the MAILER macro in the ``.mc'' file.
sh/
Shell files used when building the ``.cf'' file from the ``.mc'' file in the cf/ subdirectory.
feature/
These hold special features that you might want to include. They should be referenced using the FEATURE macro.
hack/
Local hacks from Berkeley (the home of Sendmail), of no more than voyeuristic interest, if that.
siteconfig/
Site configuration -- for instance, tables of locally connected UUCP sites.

A root ``.mc'' file for the Mail-hub

The base ``.mc'' file forms the starting point for m4 directives that invoke other macro files. All of the definitions in our base ``.mc'' file in turn reference other ``.mc files. Thus, order is important: follow that given here.

VERSIONID(`@(#)unixworld-online.mc  8.5 (Virtual Reality, Maan) 19/8/97')
OSTYPE(linux)
FEATURE(nouucp)
MAILER(local)
MAILER(smtp)

In what follows, I shall simply explicate what I have done here. For the full in formation, see the README file in the cf/ subdirectory. There is also a good explanation in a previous UnixWorld Online Sendmail tutorial .

The first line is for housekeeping, and puts the version line into the output file so you can keep track of changes.

OSTYPE

The first macro defines our operating system. You must define an operating system environment, or the configuration file build will not work. For us, the OS is Linux, so we use the file in the ostype directory named linux.mc . This contains such things as default file locations and other OS-specific material. You shouldn't need to change it.

FEATURE
nouucp
The only feature we use is nouucp, which says, don't do anything special with UUCP addresses at all.
nullclient
This feature could be used to do what we will do later on by hand, that is, generate a stripped-do wn configuration file that does noting but forward all mail to a central hub via a local SMTP-based network. The argument is the name of that hub.
MAILER

The MAILER macros use macro files to specify rules to handle one or more mailers. Here, we invoke definitions for a local mailer and an SMTP mailer. As a general rule, put the MAILER definitions last in your .mc file, and always put MAILER(smtp) before MAILER(uucp) -- several features and definitions will modify the definition of mailers, and the SMTP mailer modifies the UUCP mailer.

local
The local and prog mailers. You will almost always need these; the only exception is if you relay ALL your mail to another site. This mailer is included automatically.
smtp
The Simple Mail Transport Protocol mailer. This does not hide hosts behind a gateway or another other such hack; it assumes a world where everyone is running a name server. This file actually defines four mailers: smtp for regular (old-style) SMTP to other servers, esmtp for extended SMTP to other servers, smtp8 to do SMTP to other servers without convertin g 8-bit data to MIME (essentially, you are saying here that you know the other end is 8-bit clean even if it doesn't say so), and relay for transmission to our RELAY_HOST, LUSER_RELAY, or MAILER_HUB.

The sendmail.cf File

The final Sendmail configuration file is produced by invoking m4 with the .mc file specified above as its argument. The command line looks like this:


m4 ../m4/cf.m4 config.mc > config.cf

Where cf.m4 -- in ../m4 relative to the current directory -- is a general set of macro definitions that is always required, config.mc is the macro file we've developed above, and config.cf the output, the Sendmail configur ation file, which will be installed as /etc/sendmail.cf .

A Sendmail Installation

After you complete the configuration, as root type ``pmake install'' to install the new Sendmail program. (Note: that's pmake, to use the 4.4BSD make, not the GNU make which Linux uses). Of course, you should back up your old program first in case you need to ``back off'' and return to it. The files installed include:

/usr/sbin/sendmail
This is the actual Sendmail program binary. There may be a symbolic link in Sendmail's historic location, /usr/lib , pointing here, but /usr/sbin/ is now the actual directory location.
/etc/sendmail.cf
This is the configuration file that we generated.
/usr/bin/newaliases
This is another symbolic link to /usr/sbin/sendmail . When invoked by this name, Sendmail will rebuild the aliases database.
/var/spool/mqueue
This is the directory, where incoming and outgoing mail is kept awaiting delivery. It should have mode 700, to prevent inquisitive users from peeking at other users' mail.
/etc/aliases
This is the systemwide aliases file.
/usr/lib/sendmail.hf
This is the help file for Sendmail.
/etc/sendmail.st
This optional file can be used by Sendmail to record statistics.
/usr/bin/mailq
This is also a symbolic link to /usr/sbin/sendmail . When invoked under this name, Sendmail prints the contents of the mail queue.

Operations

Starting Sendmail on boot-up

You will most likely want to have the Sendmail daemon started every time the machine boots up. In Slackware, this is done by adding a line to the appropriate ``rc'' file in the /etc/rc.d directory. The Slackware insta ll procedure puts this in /etc/rc.d/rc.M . The code should look like this:


if [ -x /usr/sbin/sendmail ]
    echo "sendmail "
    /usr/sbin/sendmail -bd -q1h
fi

This checks to see if the file is executable, then tells the system console that it's about to invoke Sendmail, invokes Sendmail in daemon mode ( -bd ), and sets it to process the mail queue every hour ( -q1h ).

The Mail Queue

The mail queue lives in the /var/spool/mqueue directory, by default. All mail messages are held as two files, one named df XXXnnnnn , the other qf XXXnnnnn , where XXX is a three-letter sequence, nnnnn a five-digit sequence, both used simply to give every message a unique identifier. The ``qf'' or queue control file contains the e-mail message header and various processing information, whereas the ``df'' or data file contains the body of the e-mail message. There are other files, but they are transient and usually of interest only to Sendmail.

Logging

Sendmail uses the Unix syslogd(8) facility. Usually, this is set up to log all Sendmail transactions to /var/log/maillog , by default.

Security

Sendmail has a reputation as a security nightmare, but this is largely undeserved, particularly with Version 8, which has solved a lot of the problems that plagued previous versions. Actually, the degree of Sendmail security is largely the responsibility of the system administrator (you!). Some specific points to watch for:

  • Make sure the aliases file isn't writable except by trusted system personnel. This includes both the text and database version.
  • Make sure that other files that Sendmail reads, such as the mailertable, are only writable by trusted system personnel.
  • The queue directory sh ould definitely not be world writable. In fact, opinions vary on the correct permissions for the mail queue. One school of thought holds that 700 is the safest way; the other that 711 is permissible, allowing the queue to be searched by a Sendmail process that has relinquised its root privileges. Use 700 to be on the safe side; it will always be possible to relax this slightly should it cause problems.

Clients

Providing e-mail service to other machines in the network can be done in two ways. The first is to use SMTP to act as a ``mail hub'' that sends and receives internal network (and optionally local) mail on behalf of the other machines. Secondly, a POP service can be set up, where local users use client software on their computers to collect their mail via the POP3 protocol, and send mail via SMTP to the server.

The /etc/aliases File

Users on the local network must be identifiable by the Sendmail process running on the server machine. In the case of POP mailboxes, this is done by creating a normal user account. In the case of Linux clients collecting mail through Sendmail themselves, this is done by adding the appropriate alias to /etc/aliases . For example, on my local network, any mail arriving at my mail server (tiny1.demon.co.uk) destined for user bob is sent on to bob@donner.example.com on my internal network by the following line in /etc/aliases :

bob:    bob@donner.example.com 

The simplest way to make sure that mail comes back to the right place is to set the Reply-To header in all outgoing mail to point to the account on the mail-hub system, not the originating machine. This can be done in the options settings of your POP3 mailer, or will be handled for you by Sendmail on a Linux client.

It's recommended to have the mail-hub use the Sendmail ``masquerade'' feature, so that the headers of mail messages originating from your priv ate network are rewritten to look as though they came from the hub.

First, we turn on masquerading:

MASQUERADE_AS(mailhub.example.com)

where mailhub.example.com is the name by which your mail-hub is known on the Internet.

Normally, the only addresses that are masqueraded are those that come from this host. In our situation, that doesn't do a lot of good, as it's mail from other local hosts we want altered by the mail-hub to make it look as though the mail-hub sent it. We do this with the line:

MASQUERADE_DOMAIN(otherhost.example.com)

which has the effect that any mail from otherhost.example.com will, when relayed, be rewritten to have the MASQUERADE_AS address. This can be a space-separated list of names, or you can keep the list in a file, in which case the line to use in the ``.mc'' file is:

MASQUERADE_DOMAIN_FILE(filename)

Other Linux boxes

There are two ways o f handling the configuration for a Sendmail client that merely routes all outbound mail to a mail-hub. One can use the m4 configuration process, as described above for the mail-hub, and I'll run through that here.

The ``.mc'' file is simple, just:

divert(0)dnl
VERSIONID(`@(#)dumbclient.mc 28/10/97')
OSTYPE(linux)
FEATURE(nullclient, mailhost)

Where mailhost is the fully qualified domain name of the mail-hub to which all mail is to be forwarded.

This case is simple enough that the /etc/sendmail.cf file is not so arcane as it can be, so it is worth taking the opportunity to examine it in detail.

There isn't the space for a full run-down on the syntax of the file. For the full story, consult the irreplaceable Sendmail book, published by O'Reilly & Associates , available at a discount from Amazon books .

The file is divided into sections, not for Sendmail, but to make it easier for humans to maintain (and explain).

Sendmail ``commands'' are usually one-letter in length, and must be at the beginning of a line. Generally, there is no whitespace between a command letter and its arguments.

The first part, Macros, shows variables (known as ``macros'' in Sendmail parlance) defined by use of the ``D'' (Define Macro), command. All the macros defined here are explained by comments on the line preceding them -- a wise practice that should not be confined to example files!

### Defined Macros (1)
# The name of the mail hub
DRwotan.example.com
# The hub as it is known to the outside world
DHtiny1.demon.co.uk
# The local official domain name
Dj$w
# Our domain name
DDexample.com
# Identity of the error message sender
DnMailer-Daemon
# Look of the Unix From line
DlFrom $g  $d
# The characters that separate address components
Do.:%@!^=/[]
# Default form for the sender's a
ddress
Dq<$g>

The second section, Classes, is for a special type of variable, a class, which can hold multiple values. The command letter here is ``C''. The class we define in this example is w , which holds a list of alternative host names for the machine (that is, other than the fully-qualified domain name, or FQDN).

### Defined Classes (2)
# All possible names for local machine
Cw localhost donner

The third section specifies Sendmail options. These can be provided on the command line used to invoke Sendmail, but because they are so numerous, it makes more sense to specify them in the configuration file.

# default delivery mode (in background)
Odbackground
# temporary file permissions---0600 for secure mail
OF0600
# default UID & GID
Ou1
Og1
# level at which to syslog errors
OL9
# Wait for SMTP replies.
Or1h
# default messages to old style
OoTrue
# Replace unquoted spaces with a dot
OB.

The fourth part specifies the headers that must appear in every mail message. These are the headers that Sendmail will add if the mail user agent (MUA) has not already done so.

### Header Declarations (4)
HFrom: $q
HReceived: by $j id $i; $b
H?x?Full-Name: $?x$x$.
H?D?Date: $a
H?M?Message-Id: <$t.$i@$j>

The fifth section is a set of priority settings. Sendmail will process the mail in its queue in order of decreasing priority, beginning with ``special-delivery''. The level of priority is set by the MUA with the Precedence: header line.

### Priorities (5)
Pspecial-delivery=100
Pfirst-class=0
Plist=-30
Pbulk=-60
Pjunk=-100

The sixth section defines a set of mailers that Sendmail will use to actually deliver mail. Sendmail is a mail transport agent , it doesn't deliver the message itself. These lines all begin with a capital ``M''. The ``local'' and ``prog'' mailers are mandatory. The real work here is done by the special mailer [IPC], which invokes internal Sendmail routines rather than an exter nal mailer program, to send all mail to a ``smart host'' using SMTP.

### Mailer Delivery Agent Definitions (6)
# Mailer to forward all mail to the hub machine
Mhub,	P=[IPC],	S=10,	R=0,	F=xmDFMuCX,	A=IPC $h
# Sendmail requires these, but we won't use them
Mlocal,	P=/bin/mail,	S=0,	R=0,	F=lsDFMShP,	A=deliver $u
Mprog,	P=/bin/sh,	S=0,	R=0,	F=lsDFMeu,	A=sh -c $u

The seventh and last section is the heart of Sendmail, the rule sets. These define the re-writing of addresses. The basic idea here is that there are two sides, a left-hand side (LHS) and a right-hand side (RHS). The LHS represents a pattern to match against input, and the RHS is the transformation to effect upon the input if a match is made. LHS and RHS are separated by tabs; comments are in the third column. You can learn more about these rules from this tutorial article .

### The Rules Sets (7)
S0 select delivery agent
R@$+	$#error $: Missing user name
R$+	$#
hub $@$R $:$1	forward to hub
<p>
S3 preprocessing for all rule sets
R$*<>$*		$n	handle <> error addresses
R$*<$*<$*>$*>$*	$2<$3>$4	de-nest brackets
R$*<$*>$*	$2	basic RFC822 parsing

S10 rewrite the sender for the hub
R$-		$@$1@$H	user -> user@hub
R$-@$w		$@$1@$H	user@local -> user@hub
R$-@$=w		$@$1@$H	user@othernames -> user@hub
R$-@$=w.$D	$@$1@$H	user@domain -> user@hub

S1 dummy ruleset 1 (unused)

For more information on the Sendmail configuration files and their customization see our Sendmail tutorial article .

Windows

Collecting e-mail from the mail-hub from a Windows box is a simple matter of using the POP3 protocol to collect the mail from a POP3 server, which grabs mail from the standard Unix mail spool, /var/spool/mail . A POP service should be installed as part of your standard Linux set-up. If its not, then use pkgtool or whatever to install it from the CD.

The entry in /etc/services should be as follows:

pop-2		109/tcp				# PostOffice V.2
pop-3		110/tcp				# PostOffice V.3

The service will be started automatically by inetd when a service request is received. This behavior is set up by the following lines in /etc/inetd.conf :

pop-2	stream	tcp	nowait	root	/sbin/tcpd	/usr/sbin/in.pop3d
pop-3	stream	tcp	nowait	root	/sbin/tcpd	/usr/sbin/in.pop3d

To use this approach, each mail account must be a ``real'' Unix account on the mail hub, not just an entry in /etc/aliases . It's best to make the account unusable for log in by entering a * in the password field of /etc/passwd and specifying a bogus shell -- like /bin/false -- in the last field of the password file entry.

I use Eudora Lite, which is a freeware, stripped-down version of Eudora Pro. It is a fine e-mail client in its own ri ght, available from Qualcomm . Setting it up is simply a matter of pointing it at the mail hub, telling it the POP user name and password. One glitch I found was that not all options are saved to the EUDORA.INI file. Specifically, I had to set ``UseWinSock=1'' and ``UseDialup=0'' by editing the ``ini'' file, as changing these options from the menu had no effect.

Conclusion

Thus, we now have a working mail hub, providing Internet e-mail services to the local network.

Addendum: Editing /etc/sendmail.cf by hand

Let's say you have an existing Sendmail installation, one that ``came free'' when you installed Linux. You don't want to go to the bother of fussing about with getting the Sendmail source, figuring out m4, and the rest. Well, while I can't hope to cover all the details of the Sendmail configuration file syntax here, I can tell you the minimum changes yo u need to make to transform a generic sendmail.cf into one you can use. Because of the complexity of this file, I will list here only the things one has to change.

In the section above on configuring a Linux client for the local network , I have gone into more detail, which should suffice to give you a general understanding of the file syntax. Here, I will presume that there is already a suitable sendmail.cf on the machine (provided either by the Linux distribution, or from the Sendmail sources).

The ``w'' macro contains any other names that this host is known by, besides the FQDN. So, if the machine is known to your ISP as example1.com, say, but to your local network as example2.com, you need to put example2.com here. You might as well put ``localhost'' here, too.

Cwlocalhost example2.com

The ``S'' macro can contain the name of a smart relay host, to which all non-local mail is forwarded without further ado. Some sites can deliver mail to the local network, but cannot look up hosts on the Internet with DNS. To ensure delivery of all mail, such sites need to forward all non-local mail to a smart (or well-connected) host. For example, a customers of Demon, a UK ISP, would use post.demon.co.uk here.

DSpost.demon.co.uk

If you are on the end of a SLIP or PPP link, and you use a dial-on-demand system (controlled by diald(8) , for example), you won't want Sendmail to try to deliver mail straight-away, because that would keep the link going up and down all the time, with a phenomenal inflationary effect on your phone bill. To make Sendmail only attempt to deliver mail when actually processing the queue, set the Delivery Mode option to `deferred,'' as in:

O DeliveryMode=deferred

Those are all the changes you should need to make. The other parameters changes how Sendmail behaves, but should wo rk on your site without modification.

Author Biography

Paul Dunne is a writer and consultant who specializes in Linux. He has a home page that describes in more detail what he does.

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