We assume that you have the kind of Internet access which seems to be most common at universities and online services nowadays: You dial into your provider's network using PPP over a serial connection. Your incoming mail is spooled at the provider's POP or IMAP server, while outgoing messages are to be sent via SMTP. You don't have a domain name of your own, so everything has to use one address.
We assume that you have already installed a fairly recent version of Eric Allman's sendmail (version 8.8.8 is current at the time of this writing and should work fine).
This document is partially referring to specific properties of Debian GNU/Linux systems; users of different distributions will have to take some care.
Make sure you have the following information at hand:
The configuration we are planning has two main goals:
To achieve this, we will make use of sendmail's
We will put all of sendmail's configuration files in a
separate directory under
/etc/mail. Usually, sendmail will expect
these files to reside directly under
/etc/sendmail.cf should be a
symbolic link to
The following files will populate
aliases- contains additional local addresses
genericsdomain- contains some information on your local host's configuration
genericstable- contains the actual rewriting rules.
sendmail.cf- sendmail's configuration file
sendmail.mc- the source of
Some of these files will be accompanied by
They contain hashed databases for sendmail's direct use.
We assume that the
cf part of sendmail's source
tree resides under a directory named
/usr/lib/sendmail.cf. This is the case
on Debian GNU/Linux systems. Other distributions will put
this stuff at different places. Please refer to your
distribution's documentation for details.
Sendmail uses a highly complex rule system for it's
configuration. While you can do lots of neat tricks with
this stuff, writing a
sendmail.cf file from scratch
is rather unusual and time-consuming. If you are
interested in doing so, you should stop reading this
document right now and instead read the "Bat Book" from
Instead of hand-crafting these rules, we will rely on
m4 macro processor to put together our
configuration file from ready-made pieces which are
distributed together with sendmail.
Let's look at the first lines of the
include(/usr/lib/sendmail.cf/m4/cf.m4) VERSIONID(`sendmail.mc - email@example.com') OSTYPE(debian) define(`ALIAS_FILE',`/etc/mail/aliases')
In the beginning,
cf.m4 is included. This m4 macro
file contains lots of macro definitions for the rest of the
file. Be sure that the path you give here is correct -
the one we are representing in our example is typical for
Debian GNU/Linux. The
OSTYPE macro is used to give
some useful defaults for certain configuration values. If
you aren't using a Debian system, you should replace the
word "debian" by "linux" here.
sendmail where to look for the list of aliases.
The following lines tell sendmail to use the
genericstable feature, and where to find the
configuration files needed to use it:
FEATURE(masquerade_envelope) FEATURE(genericstable, `hash -o /etc/mail/genericstable') GENERICS_DOMAIN_FILE(`/etc/mail/genericsdomain')
masquerade_envelopefeature tells sendmail to apply header rewriting to the envelope sender of a message. This is the mail address to which external mail delivery subsystems will direct their delivery failure reports and warning messages. The
generics*files will be explained below.
Now, we have to define a so-called smart host, that is, a machine which will handle outgoing mail for your system. Note that this machine may be different from your ISP's POP and IMAP servers. If in doubt, contact the hotline. The code in the master configuration file:
The final two lines include the "mailer" definitions which are needed by sendmail to find out how to handle various types of mail:
To generate the
sendmail.cf file from this
sendmail.mc, type the following commands (as root):
# m4 sendmail.mc > _sendmail.cf # mv -f _sendmail.cf sendmail.cf
m4's output to a temporary file which is thereafter moved to the proper place. This helps us to prevent sendmail from reading partially written configuration files.
First, we have to tell sendmail what addresses are to
be considered local (and thus should be subjected to the
rewriting). This is quite simple: Just put the fully
qualified host name of your machine into the file
/etc/mail/genericsdomain. To get your
host's fully qualified name, type the following command:
$ hostname -f
Now, let's come to the rewriting table proper:
/etc/mail/genericstable. This file
consists of two white-space separated columns. The first
column contains the local address, the second column
contains the e-mail address which should be used instead.
The file may look like this:
harry firstname.lastname@example.org maude email@example.com root firstname.lastname@example.org news email@example.com
Note that there should be one entry for each account on the local machine, so that automatically generated mail which leaks out of the local system carries correct header information.
For performance reasons, sendmail won't use this text file directly, but rely on a "hashed" version instead. To generate it, type the following command:
# makemap -r hash genericstable.db < genericstable
Note that the rewriting rules from the
genericstable will not apply to local mail or to
messages you receive from outside - the mapping is only
used if a message leaves your local system for your ISP's
The aliases file contains additional local names which
are only valid for local messages. This is useful for
administrative accounts like
root which receive
automatically generated messages from your system.
A reasonable start for
/etc/mail/aliases could look like the
root: fred news: root postmaster: root mail: root www: root nobody: /dev/null MAILER-DAEMON: nobody
This example will forward local mail for the
fred, while messages for
MAILER-DAEMON will be redirected to
Just like the
contain lots of entries. Thus, it would once again
be inefficient for sendmail to use the text file we just
described. The same mechanism as with
is used for
aliases: A hashed database is generated.
Instead of using
makemap directly, you can type in
newaliases this time. It will
automatically take care of all what's needed.
The sendmail source distribution includes quite a bit
of documentation. Read it, especially the file
If you are interested to dive deeper into sendmail's configuration options, you want to get the "Bat Book" from O'Reilly: Bryan Costales, Eric Allman, and Neil Rickert: "sendmail". O'Reilly, 1993.