mirror of https://github.com/tLDP/LDP
251 lines
8.3 KiB
Plaintext
251 lines
8.3 KiB
Plaintext
<!doctype linuxdoc system>
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<linuxdoc>
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<article>
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<title>sendmail address rewriting mini-HOWTO
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<author>Thomas Roessler, roessler@guug.de
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<date>v0.0, 6 May 1998
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<abstract> This document is a brief description of how to
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set up sendmail's configuration file for the home user's
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dial-up access. </abstract>
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<sect>Introduction
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<p>We assume that you have the kind of Internet access
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which seems to be most common at universities and online
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services nowadays: You dial into your provider's network
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using PPP over a serial connection. Your incoming mail is
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spooled at the provider's POP or IMAP server, while
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outgoing messages are to be sent via SMTP. You don't have
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a domain name of your own, so everything has to use
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<em/one/ address.
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<p>We assume that you have already installed a fairly
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recent version of Eric Allman's sendmail (version 8.8.8 is
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current at the time of this writing and should work fine).
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<p>This document is partially referring to specific
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properties of Debian GNU/Linux systems; users of different
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distributions will have to take some care.
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<p>Make sure you have the following information at hand:
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<itemize>
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<item>Your ISP's mail server
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<item>Your Internet mail address
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</itemize>
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<p>The configuration we are planning has two main goals:
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<enum>
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<item>Sending mail between various local users must be
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possible.
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<item>The outside world must see the local users'
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ISP mail addresses, not the local ones.
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</enum>
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<p>To achieve this, we will make use of sendmail's
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<tt/genericstable/ feature.
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<sect>File Roadmap
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<p>We will put all of sendmail's configuration files in a
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separate directory under <tt//etc/:
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<tt//etc/mail/. Usually, sendmail will expect
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these files to reside directly under <tt//etc/. To
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avoid problems, <tt//etc/sendmail.cf/ should be a
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symbolic link to <tt//etc/mail/sendmail.cf/.
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<p>The following files will populate <tt//etc/mail/:
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<itemize>=20
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<item><tt/aliases/ - contains additional local addresses
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<item><tt/genericsdomain/ - contains some information on your
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local host's configuration
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<item><tt/genericstable/ - contains the actual rewriting
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rules.
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<item><tt/sendmail.cf/ - sendmail's configuration file
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<item><tt/sendmail.mc/ - the source of <tt/sendmail.cf/.
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</itemize>
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Some of these files will be accompanied by <tt/.db/ files.
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They contain hashed databases for sendmail's direct use.
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<p>We assume that the <tt/cf/ part of sendmail's source
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tree resides under a directory named
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<tt//usr/lib/sendmail.cf/. This is the case
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on Debian GNU/Linux systems. Other distributions will put
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this stuff at different places. Please refer to your
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distribution's documentation for details.
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<sect>Configuring sendmail
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<sect1>The main configuration file
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<p>Sendmail uses a highly complex rule system for it's
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configuration. While you can do lots of neat tricks with
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this stuff, writing a <tt/sendmail.cf/ file from scratch
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is rather unusual and time-consuming. If you are
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interested in doing so, you should stop reading this
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document right now and instead read the "Bat Book" from
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O'Reilly.
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<p>Instead of hand-crafting these rules, we will rely on
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the <tt/m4/ macro processor to put together our
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configuration file from ready-made pieces which are
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distributed together with sendmail.
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<p>Let's look at the first lines of the <tt/sendmail.mc/ file:
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<code>
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include(/usr/lib/sendmail.cf/m4/cf.m4)
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VERSIONID(`sendmail.mc - roessler@guug.de')
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OSTYPE(debian)
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define(`ALIAS_FILE',`/etc/mail/aliases')
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</code>
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In the beginning, <tt/cf.m4/ is included. This m4 macro
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file contains lots of macro definitions for the rest of the
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file. Be sure that the path you give here is correct -
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the one we are representing in our example is typical for
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Debian GNU/Linux. The <tt/OSTYPE/ macro is used to give
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some useful defaults for certain configuration values. If
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you aren't using a Debian system, you should replace the
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word "debian" by "linux" here. <tt/ALIAS_FILE/ tells
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sendmail where to look for the list of aliases.
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<p>The following lines tell sendmail to use the
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<tt/genericstable/ feature, and where to find the
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configuration files needed to use it: <code>
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FEATURE(masquerade_envelope) FEATURE(genericstable, `hash
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-o /etc/mail/genericstable')
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GENERICS_DOMAIN_FILE(`/etc/mail/genericsdomain') </code>
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The <tt/masquerade_envelope/ feature tells sendmail to
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apply header rewriting to the <em/envelope/ sender of a
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message. This is the mail address to which external mail
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delivery subsystems will direct their delivery failure
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reports and warning messages. The <tt/generics*/ files
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will be explained below.
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<p>Now, we have to define a so-called smart host, that is, a
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machine which will handle outgoing mail for your system.
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Note that this machine may be different from your ISP's
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POP and IMAP servers. If in doubt, contact the hotline.
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The code in the master configuration file:
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<code>
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define(`SMART_HOST',`mail-out.your.provider')
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</code>
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Please replace <em/mail-out.your.provider/ by the fully
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qualified hostname of your internet service provider.
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<p>The final two lines include the "mailer" definitions
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which are needed by sendmail to find out how to handle
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various types of mail:
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<code>
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MAILER(local)
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MAILER(smtp)
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</code>
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<p>To generate the <tt/sendmail.cf/ file from this
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<tt/sendmail.mc/, type the following commands (as root):
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<code>
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# m4 sendmail.mc > _sendmail.cf
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# mv -f _sendmail.cf sendmail.cf
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</code>
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Note the technique of writing <tt/m4/'s output to a
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temporary file which is thereafter moved to the proper
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place. This helps us to prevent sendmail from reading
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partially written configuration files.
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<sect1>Address rewriting
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<p>First, we have to tell sendmail what addresses are to
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be considered local (and thus should be subjected to the
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rewriting). This is quite simple: Just put the fully
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qualified host name of your machine into the file
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<tt//etc/mail/genericsdomain/. To get your
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host's fully qualified name, type the following command:
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<code> $ hostname -f </code>
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<p>Now, let's come to the rewriting table proper:
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<tt//etc/mail/genericstable/. This file
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consists of two white-space separated columns. The first
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column contains the local address, the second column
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contains the e-mail address which should be used instead.
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The file may look like this:
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<code>
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harry harryx@your.isp
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maude maudey@her.isp
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root fredx@your.isp
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news fredx@your.isp
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</code>
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Note that there should be one entry for <em/each/ account
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on the local machine, so that automatically generated mail
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which leaks out of the local system carries correct header
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information.
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<p>For performance reasons, sendmail won't use this text
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file directly, but rely on a "hashed" version instead. To
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generate it, type the following command:
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<code>
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# makemap -r hash genericstable.db < genericstable
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</code>
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<p>Note that the rewriting rules from the
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<tt/genericstable/ will <em/not/ apply to local mail or to
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messages you receive from outside - the mapping is only
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used if a message leaves your local system for your ISP's
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smart host.
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<sect1>Aliases
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<p>The aliases file contains additional local names which
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are only valid for local messages. This is useful for
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administrative accounts like <tt/root/ which receive
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automatically generated messages from your system.
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<p>A reasonable start for
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<tt//etc/mail/aliases/ could look like the
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following file:
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<code>
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root: fred
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news: root
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postmaster: root
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mail: root
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www: root
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nobody: /dev/null
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MAILER-DAEMON: nobody
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</code>
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This example will forward local mail for the <tt/root/,
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<tt/news/, <tt/postmaster/, <tt/mail/, and <tt/www/ users
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to <tt/fred/, while messages for <tt/nobody/ and
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<tt/MAILER-DAEMON/ will be redirected to
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<tt//dev/null/.
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<p>Just like the <tt/genericstable/, <tt/aliases/ may
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contain <em/lots/ of entries. Thus, it would once again
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be inefficient for sendmail to use the text file we just
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described. The same mechanism as with <tt/genericstable/
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is used for <tt/aliases/: A hashed database is generated.
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Instead of using <tt/makemap/ directly, you can type in
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the command <tt/newaliases/ this time. It will
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automatically take care of all what's needed.
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<sect>Further reading
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<p>The sendmail source distribution includes quite a bit
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of documentation. Read it, especially the file
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<tt/cf/README/.
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<p>If you are interested to dive deeper into sendmail's
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configuration options, you want to get the "Bat Book" from
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O'Reilly: Bryan Costales, Eric Allman, and Neil Rickert:
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"sendmail". O'Reilly, 1993.
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</article></linuxdoc>
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