sendmail address rewriting mini-HOWTO
 Thomas Roessler, [email protected]
 v0.0, 6 May 1998

 This document is a brief description of how to set up sendmail's con
 figuration file for the home user's dial-up access.

 1.  Introduction

 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:


   Your ISP's mail server

   Your Internet mail address

 The configuration we are planning has two main goals:


 1. Sending mail between various local users must be possible.

 2. The outside world must see the local users' ISP mail addresses, not
    the local ones.


 To achieve this, we will make use of sendmail's genericstable feature.


 2.  File Roadmap

 We will put all of sendmail's configuration files in a separate
 directory under /etc: /etc/mail.  Usually, sendmail will expect these
 files to reside directly under /etc.  To avoid problems,
 /etc/sendmail.cf should be a symbolic link to /etc/mail/sendmail.cf.


 The following files will populate /etc/mail:

   =20

   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 sendmail.cf.

 Some of these files will be accompanied by .db files.  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.


 3.  Configuring sendmail

 3.1.  The main configuration file

 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 O'Reilly.


 Instead of hand-crafting these rules, we will rely on the 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 sendmail.mc file:


 ______________________________________________________________________
 include(/usr/lib/sendmail.cf/m4/cf.m4)
 VERSIONID(`sendmail.mc - [email protected]')
 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.  ALIAS_FILE tells 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')
 ______________________________________________________________________

 The masquerade_envelope feature tells sendmail to apply header rewrit
 ing 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:

 ______________________________________________________________________
 define(`SMART_HOST',`mail-out.your.provider')
 ______________________________________________________________________


 Please replace mail-out.your.provider by the fully qualified hostname
 of your internet service provider.


 The final two lines include the "mailer" definitions which are needed
 by sendmail to find out how to handle various types of mail:

 ______________________________________________________________________
 MAILER(local)
 MAILER(smtp)
 ______________________________________________________________________

 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
 ______________________________________________________________________


 Note the technique of writing m4's output to a temporary file which is
 thereafter moved to the proper place.  This helps us to prevent send
 mail from reading partially written configuration files.


 3.2.  Address rewriting

 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   [email protected]
 maude   [email protected]
 root    [email protected]
 news    [email protected]
 ______________________________________________________________________

 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 smart
 host.


 3.3.  Aliases

 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 following
 file:


 ______________________________________________________________________
 root: fred
 news: root
 postmaster: root
 mail: root
 www: root

 nobody: /dev/null
 MAILER-DAEMON: nobody
 ______________________________________________________________________

 This example will forward local mail for the root, news, postmaster,
 mail, and www users to fred, while messages for nobody and MAILER-
 DAEMON will be redirected to /dev/null.


 Just like the genericstable, aliases may 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 genericstable is
 used for aliases: A hashed database is generated.  Instead of using
 makemap directly, you can type in the command newaliases this time.
 It will automatically take care of all what's needed.


 4.  Further reading

 The sendmail source distribution includes quite a bit of
 documentation.  Read it, especially the file cf/README.


 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.

You are viewing proxied material from ftp.icm.edu.pl. The copyright of proxied material belongs to its original authors. Any comments or complaints in relation to proxied material should be directed to the original authors of the content concerned. Please see the disclaimer for more details.