The most commonly used e-mail program is sendmail, which is supplied with most Linux
versions. The sendmail system is extremely powerful and flexible, but it can at times be
annoying and difficult to configure and administer because of these very attributes.
Setting up sendmail and managing its use for most common e-mail tasks,however, is quite
easy, as this chapter will show you. If you are planning to to use sendmail as your mail
system, you will find that this chapter provides enough information for all but the most
complex networked system.
Before getting started, note that this chapter shows you how to set up more than
sendmail. Because sendmail is complex (the best reference manual to the mailer approaches
800 pages), it is often teamed with a utility called IDA, for a combined product often
known as sendmail+IDA. IDA makes sendmail much easier to use and is the most common method
of using sendmail with Linux. Indeed, with IDA in tow, sendmail becomes the easiest mail
transport package available for Linux.
If your system offers only a sendmail version prior to release 8, consider getting
sendmail+IDA from an FTP or BBS site. The convenience sendmail+IDA offers far outweighs
any hassles in obtaining the files. Some current Linux releases are offering sendmail
version 8, which is usually not supplied with IDA. Version 8 of sendmail is considerably
easier to set up than previous versions, and because this version is now supplied with
most Slackware Linux CD-ROMs (including the one supplied with this book), this chapter
also covers sendmail version 8 (without IDA). Check the FTP or BBS sites for more
information about sendmail+IDA for this (and later) releases.
The sendmail system by itself (without IDA) is configured primarily though a file
usually stored as /etc/sendmail.cf (although some systems place the file in
/usr/lib/sendmail.cf or other locations). The language used in the sendmail.cf file is
completely different than other configuration files and is very complex. To see for
yourself, examine the sendmail.cf file and try to make sense of it.
The sendmail.cf file handles the default actions of the sendmail system. Several other
files are involved in the configuration, too:
| decnetxtable | Converts generic addresses to DECnet addresses |
| genericfrom | Converts internal addresses into generic ones |
| mailertable | Defines any special treatment for remote hosts and domains |
| pathtable | Defines the UUCP paths to remote machines and domains |
| uucpxtable | Forces the delivery of UUCP mail from DNS addresses |
| uucprelays | Allows shortcuts to remote hosts |
| xaliases | Converts generic addresses to internal ones |
These tables are detailed later in this chapter. As mentioned, all the sendmail
configuration files are difficult to edit manually. Using sendmail+IDA makes configuration
much easier, as IDA handles configuration through table-driven options. Each table has a
much simpler syntax than the sendmail.cf file.
The sendmail+IDA system uses a preprocessor such as m4 or dbm to generate the proper
configuration files after you have specified values for many parameters. After using the
preprocessor, the system uses a Makefile to create the final configuration files.
When you use sendmail+IDA, the sendmail.cf file is not edited directly. Instead, a
configuration process generates the changes. The configuration routine is driven by a file
called sendmail.m4, which provides basic information about your system's name, the
pathnames used on your system, and the default mailer used. Although the sendmail.m4 file
can get pretty long, it needs only basic information for most Linux installations that use
UUCP or SMTP for mail transfers.
Many system administrators like to rename sendmail.m4 to match their system name (such as tpci.m4) in order to prevent the file from being overwritten by accident and to make it obvious which machine the file refers to. If you choose to rename your file, alter all references to sendmail.m4 in this chapter to reflect your new filename.
One of the most important sections of the sendmail.m4 file is the area that defines
directories. This area usually starts with a line defining LIBDIR:
dnl #define(LIBDIR, /usr/local/lib/mail)
The LIBDIR directory is where sendmail+IDA looks for configuration files and routing
tables. Usually this line is left alone, as the default path is the general location for
all Linux mail systems. If the path shown in the sendmail.m4 file is correct, don't modify
the file. This path is usually hardcoded into the sendmail binary, and doesn't need to be
overwritten by the sendmail.m4 file (or its generated sendmail.cf file). If you need to
change this path, you have to remove the dnl from the beginning of the line (which
essentially makes the line a comment), add the correct path, and then rebuild sendmail.cf.
The local mailer used by sendmail is defined in the line that contains the variable
LOCAL_MAILER_DEF:
define(LOCAL_MAILER_DEF, mailers.linux)dnl
This line is necessary because sendmail doesn't handle mail delivery. Another program
takes care of this step instead. By default, the value used for the local mailer (which is
almost always deliver) is contained in a file called mailers.linux. This file is
referenced in the LOCAL_MAILER_DEF entry in the sendmail.m4 file, which means you need to
check the mailers.linux file in the same subdirectory (usually
/usr/local/lib/mail/mailers.linux) to ensure the deliver program (or whatever delivery
agent you use) is properly entered. A typical mailers.linux file looks like the following:
# mailers.linux Mlocal, P=/usr/bin/deliver, F=SlsmFDMP, S=10, R=25/10, A=deliver $u Mprog, P=/bin/sh, F=lsDFMeuP, S=10, A=sh -c $u
The deliver mail delivery agent is also specified in the file Sendmail.mc, which is
used to build sendmail.cf. If the name of your delivery agent is not deliver, check the
Sendmail.mc file to make sure your mail delivery agent is properly specified. (If you are
using deliver, don't worry about this file.) The Sendmail.mc file is important and must be
read in when sendmail.m4 is processed. There is usually a line in sendmail.m4 that makes
sure this action occurs. The line, which usually occurs at the top of the sendmail.m4
file, looks like the following:
include(Sendmail.mc)dnl
You may need to specify some entries in the PSEUDODOMAINS variable. This variable is
used to handle systems that can't expand into domain names properly, usually UUCP
networks. The entries in the PSEUDODOMAINS field tells sendmail+IDA not to use DNS for
these networks (which would always fail). Typically, the PSEUDODOMAINS variable is set to
the following values:
define(PSEUDODOMAINS, BITNET UUCP)dnl
You can use the PSEUDONYMS variable to hide your machine names from the outside world.
For example, a mail recipient on another network sees only the address tpci.com regardless
of whether mail was sent from merlin.tpci.com or chatton.tpci.com. When you use the
PSEUDONYMS variable, sendmail accepts mail from all machines identified in the PSEUDONYMS
field. The PSEUDONYMS field is usually used as shown in the following line:
define(PSEUDONYMS, tpci.com)dnl
This entry lets any machine with the network type tpci.com send mail through sendmail.
To define the name of your local machine, you use the DEFAULT_HOST variable. This
variable is usually defined as the same name as your mail server (or your basic machine's
name if you are not on a network). For example, you can use the following entry to set the
default mail server's name:
define(DEFAULT_HOST, merlin.tpci.com)dnl
If you do not set a valid name for the DEFAULT_HOST variable, no mail will be returned
properly to your system.
If your system is not a mail gateway to the Internet (or other networks that are
accessible from your LAN), you can set your Linux system to send mail on to another
machine for processing by setting the RELAY_HOST and RELAY_MAILER variables in
sendmail.c4. These variables set the name of the mail server that all mail should be
passed on to. For example, to set your local system to route all outbound mail to a
machine called wizard, you set the following two lines as shown:
define(RELAY_HOST, wizard)dnl
define(RELAY_MAILER, UUCP=A)dnl
The RELAY_MAILER line specifies the mailer to use to send messages on to the
RELAY_HOST.
If you are working with a UUCP-based mail system, you should modify a few more entries
in the sendmail.m4 file. These modifications are necessary because a UUCP mail system is
often addressed differently than a DNS-based system. The UUCP-specific entries in the
sendmail.m4 file usually look like the following:
define(UUCPNAME, tpci)dnl
define(UUCPNODES, |uuname|sort|uniq)dnl
define(BANGIMPLIESUUCP)dnl
define(BANGONLYUUCP)dnl
The first line defining UUCPNAMES specifies the name of the local system in UUCP terms.
This name can be different than the name used in DNS, although usually these names will be
similar. The UUCPNODES variable defines the commands that are used to produce a list of
hostnames for systems you connect to via UUCP.
The BANGIMPLIESUUCP variable tells sendmail to assume that any address containing an
exclamation mark (called a bang) is UUCP style. The BANGONLYUUCP variable does the same
thing. Both variables are usually set to on (not commented out) because few DNS users use
the bang addressing method.
Several lines in the sendmail.m4 file define configuration tables. For the most part,
these configuration tables are under the directory defined by LIBDIR. This section of the
sendmail.m4 file has several lines that look like the following:
define(ALIASES, LIBDIR/aliases)dnl
define(DOMAINTABLE, LIBDIR/domaintable)dnl
There are about seven configuration file definitions in total. You can change any of
these values if you want, but be sure to move the files themselves to the specified
location. On the whole, it is best to leave the files in their default locations.
The file decnetxtable is used to translate domain names into DECnet style names. This
file is a holdover from earlier versions of sendmail and will probably never be necessary
for Linux users (unless your Linux machine is on a DECnet system).
The domaintable file forces sendmail to perform specific instructions after using DNS.
The file, which is almost never used on Linux systems, allows you to provide expansion of
short-form names. Suppose you often send mail to the host reallylongname.reallybignet.com,
but you don't want to type that entry each time. You could place the following entry in
the domaintable file
reallylongname.reallybignet.com big.com
so that whenever you send mail to bill@big.com sendmail expands the address to
bill@reallylongname.reallybignet.com. You also can use the domaintable file to correct
common typographic mistakes. For example, if many users accidently send mail to abcdef.com
instead of abcdfe.com, you could add a line to the domaintable file that corrects the
domain name:
abcdfe.com abcdef.com
The format of the domaintable file is always the correct domain name followed by the
incorrect (or shortened) domain name.
The genericfrom table hides local usernames and machine addresses by converting local
usernames to a generic ID that has no obvious connection to the username. Linux systems
seldom use this table because the general convention is to use real names on e-mail and
similar data. The companion file, xaliases, performs the generic to real conversion when
mail comes back from the outside world.
The mailertable table defines any special handling for hosts or domains. Most often,
mailertable specifies how certain domains or hosts are accessed and which protocol to use
for these domains and hosts. You don't have to modify this file if your system only uses
UUCP, but you should verify its contents if you use SMTP or DNS.
The mailertable file is read from the first line down, and sendmail processes mail
based on each line in the file. For this reason, place the most specific rules at the top
of the file, followed by more general rules. Rules give the method of connection first and
then list the remote system or domain:
mailer delimiter relayname remote
In this syntax, mailer is the transport to use, delimiter is a special character,
relayname is the name of the system to pass the mail to, and remote is the remote host or
domain name. The mailer can be one of the following values:
| TCP-A | TCP with Internet-style addresses |
| TCP-U | TCP with UUCP-style addresses |
| UUCP-A | UUCP with Internet-style addresses |
The delimiter has a special meaning and must be one of the following characters:
| ! | Strips the hostname from the address before forwarding |
| , | Doesn't modify the address at all |
| : | Removes the hostname only if intermediate hosts are specified |
You can build the mailertable rules quite easily when you are forwarding mail to a
remote mail server. For example, to force sendmail to use UUCP through a remote mail
server called wizard to connect to the remote system roy.sailing.org, add a rule like the
following to the mailertable file:
UUCP-A,wizard roy.sailing.org
On a more general level, a rule like this one
TCP-A,wizard chatton.com
forwards any mail destined for the remote network chatton.com to the local mail server
wizard via TCP.
The pathtable table defines explicit routing to remote hosts and networks. The format
of each line in the pathtable file uses a syntax similar to a UUCP path alias, with
entries appearing alphabetically in the file. The pathtable file is rarely used because
most Linux systems can handle the routing without explicit instructions.
The uucprelays file short-circuits the UUCP path to a remote site when a better path
exists. For example, if your users often use the path wizard!bignet!merlin!tpci and you
create a direct link to tpci, you could use the uucprelays file to redirect the mail.
The uucpxtable file is used when a UUCP style address has to be used for mail delivery.
The file provides the instructions for converting a DNS format address to a UUCP format
address. If you are using a mail server other than your current machine or want to use
UUCP to connect to specific machines because of reliability factors, this table is
necessary.
This file contains entries that lists the UUCP style name followed by the domain name,
as follows:
chatton chatton.com
This entry tells sendmail that any mail for chatton.com should be rerouted via UUCP to
chatton (UUCP style addressing). This entry forces mail addresses to yvonne@chatton.com to
be rewritten as chatton!yvonne, an address that UUCP can handle.
Now that you have configured the sendmail.m4 file and its dependent files, you can use
the m4 processor to generate the sendmail.cf file. When the sendmail.m4 file is ready to
be processed, issue the command
make sendmail.cf
or substitute your site name if you renamed your sendmail.m4 file to reflect your site
name. (In other words, if you created a file called tpci.m4, specify tpci.cf in the
command).
Once the file has been processed, copy it to the /etc directory (which is where the
file normally resides) and start up sendmail with the command
/usr/lib/sendmail -bd -q1h
or reboot your machine (as sendmail usually starts from the rc startup files).
The latest version of sendmail supplied for most Linux systems is version 8. If you
didn't see versions 6 or 7, don't worry; they didn't exist. The sendmail system jumped
from release 5.X to 8. As of the date of publication, sendmail8+IDA wasn't in general
distribution, but it should be available by the time you read this. There are several
releases of sendmail version 8, just to make life more complicated. They are all pretty
much the same as far as installation and configuration are concerned.
Most Linux CD-ROMs have the source code for sendmail version 8 already compiled into a
binary. If you do not have the binary, change to the source directory and use the make
command. The Slackware CD-ROM includes the precompiled binaries.
Each release of sendmail version 8 has an installation guide included. On the Slackware CD-ROM release accompanying this book, the installation guide is in the directory /usr/doc/sendmail/op as the file op.me. To format and display this file properly on the screen, use the following command:
nroff -me op.me | more
If you want to send the output to the printer instead, replace the more command with lp, or save the output to a file and print that file directly.
For the most part, sendmail version 8 is similar in configuration details to the other
releases of sendmail. One change is the inclusion of four different UUCP mail routines.
Choose one of the four UUCP versions based on the following features:
Whichever version of UUCP you choose, copy or link it to the normal UUCP binary on your
Linux system.
As mentioned earlier in this chapter, the sendmail configuration files are daunting to
say the least. To make matters easier for administrators, a number of templates are
supplied with version 8 that cover most of the common installation configurations. You
have to copy one of these templates over the sendmail configuration file and change a few
parameters relating to your specific system, but you can leave the majority of the file.
Using these templates greatly shortens and simplifies the installation process. (If none
of these templates suits your installation, you must manually adjust the configuration
file. This process is beyond the scope of this book, so check the supplied documentation
for more information.)
The sendmail system uses a number of directories to hold the files it needs, including
the configuration file templates. Most of these files are processed by the macro language
processor m4. The directories and their contents are as follows:
| cf | This directory provides descriptions of hosts used by sendmail. The raw data files end in .mc (for master configuration) and are used as input; the output produces a file ending with .cf. |
| hack | This directory is a list of hacks used by the m4 macros to provide fixes or extra specifications. These should be avoided as much as possible. The information they supply is properly provided in the main configuration files. |
| m4 | This directory contains files with information applicable to all configuration files (site-independent information). |
| mailer | This directory contains files defining the mailers that are used by m4. The files generally provide information for local, UUCP, USENET, fax software, and similar types of mailers. |
| ostype | This directory contains files describing the operating system and its environment. |
| sh | This directory contains shell files used by m4. |
| siteconfig | This directory contains local site information. |
A number of files are important to the sendmail installation and configuration process.
These files, their ownership and permission settings (where appropriate), and their
purposes are as follows:
In order to start sendmail automatically when your Linux system boots, modify the
/etc/rc files to include these lines (if they don't already exist):
if [ -f /usr/sbin/sendmail -a -f /etc/sendmail.cf ]; then (cd /var/spool/mqueue; rm -f [lnx]f*) /usr/sbin/sendmail -bd -q30m & echo -n ' sendmail' >/dev/console fi
The directory paths should reflect the paths used on your system, of course. The rm
command used in this script removes any lock files that are left over from the system
after the last shutdown.
The Slackware Linux distribution that accompanies this book, and most other releases of
Linux that provide sendmail version 8, have a few templates provided that set most of the
needed parameters. The templates on the Linux CD-ROM accompanying this book are as
follows:
If you choose to use one of the template files, examine it carefully to make sure the
directories and system names are correct for your system. Make any changes directly into
the file. To install one of these configuration files (or any other that you may find that
suits your needs), issue the command
m4 config_file.mc > /etc/sendmail.cf
where config_file.mc is the configuration file you want to install. This command
executes the macro language processor and copies the resulting file over the standard
sendmail configuration file.
This chapter examined the configuration process for sendmail+IDA, the most common form of sendmail used with Linux. If you plan on using smail instead, read Chapter 36.
