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exim4/debian/README.Debian.xml
Daniel Baumann b6ead8c0a5
Adding debian version 4.98.2-1. 
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
2025-06-21 14:27:17 +02:00

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<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE article PUBLIC "-//OASIS//DTD DocBook XML V4.4//EN"
"http://www.docbook.org/xml/4.4/docbookx.dtd">
<article> <title>Exim 4 for Debian</title>
<section> <title>Introduction</title>
<para>
If you're reading this, you have found the README.Debian
file. This is good, thanks! Please continue reading this file in
its entirety. It is full of important information and has been
written with the questions in mind that keep popping up on the
mailing lists.
</para>
<section> <title>How to find your way around the Documentation</title>
<para>
Exim comes with very extensive documentation. Here is how to
find it.
<orderedlist>
<listitem>
<simpara>
A lot of information about Debian's Exim 4
packaging can be found in this document.
</simpara>
</listitem>
<listitem>
<simpara>
The packages contain a lot of Debian-specific man pages.
Use the <command>apropos exim</command> command to get a list.
</simpara>
</listitem>
<listitem>
<simpara>
Most files that control the default configuration are
documented in the exim4-config_files(5) man page, which
is symlinked to the file names. man &lt;filename&gt; should
lead you to the page.
</simpara>
</listitem>
<listitem>
<para>
The very extensive Upstream documentation is shipped
<orderedlist>
<listitem>
<simpara>
in text form
(<filename>/usr/share/doc/exim4-base/spec.txt.gz</filename>)
with the binary packages.
</simpara>
</listitem>
<listitem>
<simpara>
in HTML in the package
<filename>exim4-doc-html</filename>
</simpara>
</listitem>
<listitem>
<simpara>
as a Texinfo file in the package
<filename>exim4-doc-info</filename>
</simpara>
</listitem>
</orderedlist>
</para>
</listitem>
</orderedlist>
</para>
<para>
Please note that documentation found on the web or in other
parts of the Debian system (such as the Debian Reference)
might be outdated and thus give wrong advice. In doubt, the
documentation listed above should take precedence.
</para>
</section>
<section> <title>Getting Support</title>
<para>
For your questions and comments, there is a <ulink
url="http://lists.alioth.debian.org/mailman/listinfo/pkg-exim4-users">
Debian-specific mailing list</ulink>. Please ask Debian-specific
questions there, and only write to the upstream exim-users mailing
list if
you are sure that your question is not Debian-specific.
Debian-specific questions are more likely to find answers on
our pkg-exim4-users mailing list, while complex custom
configuration issues might be more easily solved on the
upstream exim-users mailing list because of the broader and
more experienced audience there. You can subscribe to
pkg-exim4-users <ulink
url="http://lists.alioth.debian.org/mailman/listinfo/pkg-exim4-users">
via the subscription web page;</ulink> you need to be
subscribed to post.
</para>
<para>
If you think that your question might be more easily answered
if one knows a bit about your configuration, you might want to
execute <command>reportbug --subject="none" --offline --quiet
--severity=wishlist --body="none" --output=exim4.reportbug
exim4-config</command> on the system in question, answer yes
to both "include [extended] configuration" questions and include
the contents of the exim4.reportbug file generated by this
command with your question. Please check whether the file
contains any confidential information before sending.
</para>
</section>
<section> <title>Packaging</title>
<para>
Similar to the Apache2 package, Exim 4 is an entirely
different package that does not currently offer a smooth
upgrade path from Debian's Exim 3 packages.
</para>
<para>
It is the first Exim package in Debian that can be configured
using debconf. However, the entire configuration framework is
extremely flexible, allowing you to get exactly the amount of
control you need for the job at hand.
</para>
<section> <title>Feature Sets in the daemon packages</title>
<para>
To use Exim 4, you need at least the following packages:
<variablelist>
<varlistentry>
<term>exim4-base</term>
<listitem>
<simpara>support files for all Exim MTA (v4) packages</simpara>
</listitem>
</varlistentry>
<varlistentry>
<term>exim4-config</term>
<listitem>
<simpara>configuration for the Exim MTA (v4)</simpara>
</listitem>
</varlistentry>
<varlistentry>
<term>exim4-daemon-light</term>
<listitem>
<simpara>lightweight exim MTA (v4) daemon</simpara>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
Just apting the metapackage <command>exim4</command> will pull
in the other packages per dependency. You'll get an exim daemon
with minimal feature set (no external lookups).
</para>
<para>
If you need more advanced features like LDAP, sqlite, PostgreSQL
and MySQL data lookups, SASL and SPA SMTP authentication, embedded
Perl interpreter, and exiscan-acl for integration of
virus-scanners and SpamAssassin, you can replace
<command>exim4-daemon-heavy</command> instead of
<command>exim4-daemon-light</command>. Additionally, the source
package offers infrastructure to build your own custom-tailored
exim4-daemon-custom which exactly fits your special local needs.
The infrastructure to do so is already in place, see
debian/rules for instructions.
</para>
</section>
<section> <title>How to build a custom daemon</title>
<para>
The process of building a custom daemon is partially
documented in the <filename>debian/rules</filename> file
in the source package. Patches for more documentation are welcome.
</para>
</section>
</section>
</section>
<section> <title>Configuration of Exim 4 in the Debian packages</title>
<para>
Generally, the Debian Exim 4 packages are configured through
debconf. You have been asked some questions on package installation,
and your initial Exim configuration has been created from your
answers. You can repeat the configuration process any time by invoking
<command>dpkg-reconfigure exim4-config</command>. If you are an
experienced Exim administrator and prefer to have your own,
hand-crafted, non-automatic Exim configuration, you will find
information about how to do so in
<xref linkend="completely-different-configuration"/>.
</para>
<para>
The debconf-driven configuration is mainly geared for a
one-domain shell account machine/workstation with local delivery
as suggested by the original upstream default configuration.
If you configure the packages to handle more than one local
domain, all local domains are treated identically. The domain
part is not used for routing and filtering decisions.
</para>
<para>
Despite the default configuration being extended somewhat from
the original upstream, chances are that you'll need to
manually change the Exim configuration with an editor if you intend to
do something that is not covered by the debconf-driven configuration.
It has never been the packages' intention to offer all possible
configuration methods through debconf. The configuration files are
there to be changed, feel free to do so if you see fit. The Debian
Exim 4 maintainers have tried to make the configuration as flexible as
possible so that manual intervention can be minimized.
</para>
<para>
If you need to make manual changes to the Exim configuration,
please be familiar with how Exim works. At minimum, have read this
README file and the manpages delivered with the Debian Exim 4
packages, and <filename>/usr/share/doc/exim4-base/spec.txt.gz</filename>
chapters <phrase>"How Exim receives and delivers mail"</phrase> and
<phrase>"The Exim run time configuration file"</phrase>.
<filename>spec.txt.gz</filename> is an excellent reference.
</para>
<para>
Please note that while most free-form fields in the
debconf-driven configuration have the entered string end up
verbatim in Exim's configuration file (and thus using more
advanced features like host, address and domain lists is possible
and will probably work), this is not officially supported.
Only plain lists are supported in the debconf dialogs. You may
use more advanced features, but they may stop working any time
during upgrades.
</para>
<section> <title>The Configuration System</title>
<section id="debconf-questions"> <title>The Debconf questions</title>
<para>
In this section, we try to document and explain the debconf
questions, which are themselves limited to a small screen of
information and might leave questions unanswered. Since you
can usually read this file only after having answered the
questions, the process can always be repeated by invoking
<command>dpkg-reconfigure exim4-config.</command>
<filename>/etc/exim4/update-exim4.conf.conf</filename>,
documented in the <command>update-exim4.conf</command>
manual page, is
a simple shell-script snippet used to store the answers
that you passed to debconf when initially configuring Exim.
You may also modify this file with an editor of your choice.
The package maintainer scripts can handle this and will
preserve your changes.
</para>
<section> <title>General type of mail configuration</title>
<para>
This is the main configuration question which will
control which of the remaining questions are
presented to you. It also controls things like daemon
invocation and delivery of outgoing mail.
</para>
<section> <title> internet site; mail is sent and
received directly using SMTP</title>
<para>
This option is suitable for a standalone system
with full internet connectivity.
</para>
<itemizedlist>
<listitem>
<para>
The Exim SMTP daemon will accept messages
to local domains, and deliver them locally.
</para>
</listitem>
<listitem>
<para>
Outgoing mail will be delivered directly
to the mail exchange servers of the
recipient domain
</para>
</listitem>
</itemizedlist>
</section>
<section> <title> mail sent by smarthost; received via
SMTP or fetchmail</title>
<para>
This option is suitable for a standalone client system
which has restricted internet connectivity, for
example on a residential connection where an SMTP
smarthost is used. Some ISPs block outgoing SMTP
connections to combat the spam problem, thus
requiring the use of their smarthosts. It is
generally a good idea to use the ISPs smart host
if one is connected with a dynamic IP address
since quite a few sites do not accept mail
directly delivered from a dial-in pool.
</para>
<para>
fetchmail can be used to retrieve incoming mail
from the ISP's POP3 or IMAP mail server and
deliver it to Exim via SMTP.
</para>
<itemizedlist>
<listitem>
<para>
The Exim SMTP daemon will accept messages
to local domains, and deliver them locally.
</para>
</listitem>
<listitem>
<para>
Outgoing mail will always be delivered to
the smarthost configured in exim4.
</para>
</listitem>
</itemizedlist>
</section>
<section> <title>mail sent by smarthost; no local mail</title>
<para>
This option is suitable for a client system in a
computer pool which is not responsible for a local
e-mail domain. All locally generated e-mail is
sent to the smarthost without any local domains.
</para>
</section>
<section> <title>local delivery only; not on a network</title>
<para>
This option is suitable for a standalone system
with no networking at all. Only messages for configured
local domains are accepted and delivered locally;
messages for all other domains are rejected:
``Mailing to remote domains not supported''.
</para>
</section>
<section> <title>no configuration at this time</title>
<para>
This option disables most of Debian's automatisms
and leaves exim in an unconfigured state.
update-exim4.conf will still copy
<filename>/etc/exim4/exim4.conf.template</filename>
or concatenate the files from
<filename>/etc/exim4/conf.d,</filename> and will
not generate any configuration control macros.
Unless you manually edit the configuration source,
this will leave Exim with a syntactically invalid
configuration file, thus in a state where the
daemon won't even start.
</para>
<para>
Only choose this option if you know what you're
doing and are prepared to create your own Exim
configuration.
</para>
<para>
dpkg-conffile handling is still in place, and you
will be offered updates for configuration
snippets, as soon as they become available.
</para>
</section>
</section>
<section> <title>System mail name</title>
<para>
The "mail name" is the domain name used to "qualify"
mail addresses without a domain name.
</para>
<para>
This name will also be used by other programs. It
should be the single, full domain name (FQDN).
</para>
<para>
For example, if a mail address on the local host is
foo@example.org, then the correct value for this
option would be example.org.
</para>
<para>
Exim, as a rule, handles only fully qualified mail
addresses, that is, addresses with a local part, an @
sign and a domain. If confronted with an unqualified
address, that is, one without @ sign and without
domain, first thing exim does is qualify the address
by adding the @ sign and a domain.
</para>
<para>
This qualification happens for all addresses exim
encounters, be it sender, recipient or else.
</para>
<para>
The domain name used to qualify unqualified mail addresses
is called ``mail name'' on Debian systems and entered
in this debconf dialog. What you enter here will end
up in <filename>/etc/mailname,</filename> which is a
file that might be used by other programs as well.
</para>
<para>
In some configuration types, the package configuration
will offer you, at a later step, to hide this name
from outgoing messages by rewriting the headers.
</para>
</section>
<section> <title>IP addresses to listen on for incoming SMTP
connections</title>
<para>
Please enter a semicolon-separated list of IP addresses.
The Exim SMTP listener daemon will listen on all IP
addresses listed here.
</para>
<para>
An empty value will cause Exim to listen for connections
on all available network interfaces.
</para>
<para>
If this system does only receive e-mail directly from
local services (and not from other hosts),
it is suggested to prohibit external connections to the
local Exim daemon. Such services include e-mail
programs (MUSs) which talk to localhost only as well as
fetchmail. External connections are impossible when
127.0.0.1 is entered here, as this will disable listening
on public network interfaces.
</para>
<para>
Do not change this unless you know what you are doing.
Altering this value could post a security risk to your
system. For most users, the default value is sufficient.
</para>
</section>
<section> <title>Other destinations for which mail is accepted</title>
<para>
Please enter a semicolon-separated list of recipient
domains for which this machine should consider itself
the final destination. These domains are commonly
called 'local domains'. The local hostname and 'localhost'
are always added to the list given here.
</para>
<para>
By default all local domains will be treated
identically. If both a.example and b.example are
local domains, acc@a.example and acc@b.example will
be delivered to the same final destination. If
different domain names should be treated differently,
it is necessary to edit the config files afterwards.
</para>
<para>
The answer to this question ends up in the list of
domains that Exim will consider local domains. Mail
for recipients in one of these domains will be
subject to local alias expansion and then delivered
locally in the appropriate configuration types.
</para>
</section>
<section> <title>Domains to relay mail for</title>
<para>
Please enter a semicolon-separated list of recipient
domains for which this system will relay mail, for
example as a fallback MX or mail gateway. This means
that this system will accept mail for these domains
from anywhere on the Internet and deliver them
according to local delivery rules.
</para>
<para>
Do not mention local domains here. Wildcards may be used.
</para>
<para>
The answer to this question is a list of the domains
for which Exim will relay messages coming in from anywhere
on the Internet.
</para>
</section>
<section> <title>Machines to relay mail for</title>
<para>
Please enter a semicolon-separated list of IP address
ranges for which this system will unconditionally relay
mail, functioning as a smarthost.
</para>
<para>
You should use the standard address/prefix format
(e.g. 194.222.242.0/24 or 5f03:1200:836f::/48).
</para>
<para>
If this system should not be a smarthost for any
other host, leave this list blank.
</para>
<para>
Please note that systems not listed here can still use
SMTP AUTH to relay through this system. If this system
only has clients on dynamic IP addresses that use SMTP
AUTH, leave this list blank as well. Do
<emphasis>NOT</emphasis> list 0.0.0.0/0!
</para>
<para>
Warning: While it is possible to use
host<emphasis>names</emphasis> instead of IP addresses in this
list extra care needs to be taken in this case.
<emphasis>Unresolvable names in the host list will break
relaying.</emphasis> See
Exim specification chapter <phrase>"Domain, host, address, and
local part lists"</phrase>
and the exim4-config_files man page.
</para>
</section>
<section> <title>IP address or host name of the outgoing
smarthost</title>
<para>
Please enter the IP address or the host name of a mail
server that this system should use as outgoing
smarthost. If the smarthost only accepts your mail on
a port different from TCP/25, append two colons and
the port number (for example smarthost.example::587 or
192.168.254.254::2525). Colons in IPv6 addresses need
to be doubled.
</para>
<para>
If the smarthost requires authentication, please refer
to <xref linkend="smtp-auth"/> for notes about setting
up SMTP authentication.
</para>
<para>
Multiple smarthost entries are permitted, semicolon
separated. Each of the hosts is tried, in the order
specified (See Exim specification, chapter
<phrase>"The manualroute router"</phrase>, section
<phrase>"How the list of hosts is used"</phrase>.)
</para>
</section>
<section> <title>Hide local mail name in outgoing mail</title>
<para>
The headers of outgoing mail can be rewritten to make
it appear to have been generated on a different
system, replacing the local host name in From,
Reply-To, Sender and Return-Path.
</para>
</section>
<section> <title>Visible domain name for local users</title>
<para>
If you ask Exim to hide the local mail name in
outgoing mail, it will next ask you for the domain
name that should be visible for your local users.
These information is then used to establish the
appropriate rewriting rules.
</para>
</section>
<section> <title>Keep number of DNS queries minimal
(Dial-on-Demand)</title>
<para>
In normal mode of operation Exim does DNS lookups at
startup, and when receiving or delivering messages.
This is for logging purposes and allows keeping down
the number of hard-coded values in the configuration.
</para>
<para>
If this system does not have a DNS full service
resolver available at all times (for example if its
Internet access is a dial-up line using
dial-on-demand), this might have unwanted
consequences. For example, starting up Exim or
running the queue (even with no messages waiting)
might trigger a costly dial-up-event.
</para>
<para>
This option should be selected if this system is
using Dial-on-Demand. If it has always-on Internet
access, this option should be disabled.
</para>
</section>
<section><title>Delivery method for local mail</title>
<para>
Exim is able to store locally delivered mail in
different formats. The most commonly used ones are
mbox and Maildir. mbox uses a single file for the
complete mail folder stored in /var/mail/. With
Maildir format every single message is stored in a
separate file in ~/Maildir/.
</para>
<para>
Please note that most mail tools in Debian expect the
local delivery method to be mbox in their default.
</para>
</section>
<section> <title>Split configuration into small files</title>
<para>
Our packages offer two (actually three, see
<xref linkend="completely-different-configuration"/>)
possibilities:
</para>
<orderedlist>
<listitem>
<simpara>
Generate Exim's configuration from
<filename>/etc/exim4/exim4.conf.template,</filename>
which is basically a normal Exim run-time
configuration file which will be supplemented
with some macros generated from Debconf in a
post-processing step before it is passed to exim.
</simpara>
</listitem>
<listitem>
<simpara>
Generate Exim's configuration from the
multiple files in
<filename>/etc/exim4/conf.d/</filename>. The
directories in
<filename>/etc/exim4/conf.d/</filename>
correspond to the sections of the Exim
run-time configuration file, so you should
easily find your way around there.
</simpara>
</listitem>
</orderedlist>
<para>
Splitting the configuration across multiple files
means that you have the actual configuration file
automatically generated from the files below
<filename>/etc/exim4/conf.d/</filename> by invoking
<command>update-exim4.conf</command>. Each section
of Exim's configuration has its own subdirectory and
the files in there are supposed to be read in
alphanumeric order.
<filename>router/00_exim4-config_header</filename>
is followed by
<filename>router/100_exim4-config_domain_literal</filename>,
...
</para>
<para>
If you chose unsplit configuration,
<command>update-exim4.conf</command> builds the
configuration from
<filename>/etc/exim4/exim4.conf.template</filename>,
which is basically the files from
<filename>/etc/exim4/conf.d/</filename> concatenated
together at package build time, and thus guarantees
consistency on the target system.
</para>
<para>
In both cases, <command>update-exim4.conf</command>
generates exim configuration macros from the debconf
configuration values and puts them into
the actual configuration file, which is then used by
the Exim daemon. See the
<command>update-exim4.conf</command> manual
page for more in-depth information about this
mechanism.
</para>
<para>
Benefits of the split configuration approach:
<itemizedlist>
<listitem>
<simpara>
it means less work for you when upgrading.
If we shipped one big file and modified
for example the Maildir transport in a new
version you won't have to do manual
conffile merging unless you had changed
exactly <emphasis>this</emphasis>
transport.
</simpara>
</listitem>
<listitem>
<simpara>
It allows other packages (e.g. sa-exim) to
modify Exim's configuration by dropping
files into
<filename>/etc/exim4/conf.d</filename>.
This needs, however quite exact syncing
between the exim4 packages and the other,
cooperating package.
</simpara>
</listitem>
</itemizedlist>
</para>
<para>
Drawbacks of the split configuration approach:
<itemizedlist>
<listitem>
<simpara>
It is more fragile. If files from
different sources (package, manually
changed, or other package) get out of
sync, it is possible for Exim to break
until you manually correct this. This can
for example happen if we decide to add a
new option to the Debian setup of a later
version, and you have already set this
option in a local file.
</simpara>
</listitem>
</itemizedlist>
</para>
<para>
Benefits of the unsplit configuration approach:
<itemizedlist>
<listitem>
<simpara>
People familiar with configuring Exim may
find this approach easier to understand as
<filename>exim4.conf.template</filename>
basically is a complete Exim configuration
file which will only undergo some basic
string replacement before is it passed to
exim.
</simpara>
</listitem>
<listitem>
<simpara>
Split-config's fragility mentioned
above does not occur.
</simpara>
</listitem>
</itemizedlist>
</para>
<para>
Drawbacks of the unsplit configuration approach:
<itemizedlist>
<listitem>
<simpara>
Will require manual intervention in case of an
upgrade.
</simpara>
</listitem>
</itemizedlist>
</para>
<para>
If in doubt go for the unsplit config, because it is
easier to roll back to Debian's default configuration
in one step. If you intend to do many changes to the
Debian setup, you might want to use the split config
at the price of having to more closely examine the
config file after an update.
</para>
<para>
We'd appreciate a patch that uses ucf and the
3-way-merge mechanism offered by that package. It
might be the best way to handle the big configuration
file.
</para>
<para>
If you are using unsplit configuration, have local
changes to <filename>/etc/exim4/conf.d/</filename>
(either made by yourself or by other packages dropping
their own routers or transports in) and want to
re-generate
<filename>/etc/exim4/exim4.conf.template</filename> to
activate these changes, you can do so by using
<command>update-exim4.conf.template</command>.
</para>
</section>
</section>
<section> <title>Access Control in the default configuration</title>
<para>
The Debian exim 4 packages come with a default configuration
that allows flexible access control and blacklisting of
sites and hosts. The acls involved can be found in
/etc/exim4/conf.d/acl, or in /etc/exim4/exim4.conf.template,
depending on which configuration scheme you use. Most
rejections of messages due to this mechanism happen at RCPT
time. Local configuration of the mechanisms happens through
data files in /etc/exim4 or via Exim macros that you can set
in /etc/exim4/conf.d/main, so there is normally no need to
change the files in the acl subdirectory in a split-config
setup. If you use the non-split config, you need to edit
/etc/exim4/exim4.conf.template, which, as a big
dpkg-conffile, won't give you any advantage of the .ifdef
scheme.
</para>
<para>
The data files are documented in the exim4-config_files man
page.
</para>
<para>
The access lists delivered with the exim4 packages also
contain quite a few configuration options that are too
restrictive to be active by default on a real-life site.
These are masked by .ifdef statements, can be activated by
setting the appropriate macros, and are documented in the
ACL files itself.
</para>
</section>
<section id='macros'> <title>Using Exim Macros to control the
configuration</title>
<para>
Our configuration can be controlled in a limited way by
setting macros. That way, you can switch on and off certain
parts of the default configuration and/or override values set
in Debconf without having to touch the dpkg-conffiles. While
touching dpkg-conffiles itself is explicitly allowed and wanted,
it can be quite a nuisance to be asked on package upgrade
whether one wants to use the locally changed file or the
file changed by the package maintainer.
</para>
<para>
Whenever you see an <command>.ifdef</command> or
<command>.ifndef</command> clause in the configuration file,
you can control the appropriate clause by setting the macro in
a local configuration file. <command>.ifndef</command> checks
whether a specific macro is set (to a nonempty value), the
actual value does not matter. (Both
<quote>EXIM4_EXAMPLE = true</quote> and
<quote>EXIM4_EXAMPLE = false</quote> pass this test.)
For split configuration, you can
drop the local configuration file anywhere in
<filename>/etc/exim4/conf.d/main</filename>. Just make sure it
gets read before the macro is first used.
<filename>000_localmacros</filename> is a possible name,
guaranteeing first order. For a non-split configuration,
<filename>/etc/exim4/exim4.conf.localmacros</filename> gets
read before
<filename>/etc/exim4/exim4.conf.template</filename>. To
actually set the macro <varname>EXIM4_EXAMPLE</varname> to the
value "this is a sample", write the following line
</para>
<para>
EXIM4_EXAMPLE = this is a sample
</para>
<para>
into the appropriate file. For more detailed discussion of the
general macro mechanism, see the Exim specification, chapter
<phrase>"The Exim run time configuration file"</phrase>, for
details how macro expansion works.
</para>
</section>
<section> <title>How does this work?</title>
<para>
The script <command>update-exim4.conf</command> parses the
<filename>/etc/exim4/update-exim4.conf.conf</filename> file
and provides the configuration for the exim daemon.
</para>
<para>
Depending on the value of
<varname>dc_use_split_config</varname>, it either
<itemizedlist>
<listitem>
<simpara>
takes all the files below
<filename>/etc/exim4/conf.d/</filename> and
concatenates them together or
</simpara>
</listitem>
<listitem>
<simpara>
uses <filename>exim4.conf.template</filename> as
input.
</simpara>
</listitem>
</itemizedlist>
The debconf-managed information from
<filename>/etc/exim4/update-exim4.conf.conf</filename> is
merged into the generated configuration file by generating a
number of Exim configuration macros.
</para>
<para>
<varname>DCsmarthost</varname>, for example, is set to the
value of <varname>$dc_smarthost</varname>
in <filename>/etc/exim4/update-exim4.conf.conf</filename>
which holds the answer to "Which machine will act as the
smarthost and handle outgoing mail?"
</para>
<para>
The result of these operations is saved as
<filename>/var/lib/exim4/config.autogenerated</filename>,
which is <emphasis>not</emphasis> a dpkg-conffile! Manual
changes to this file will be overwritten by
<command>update-exim4.conf</command>.
</para>
<para>
Please consult <command>update-exim4.conf</command> manpage
for more detailed information.
</para>
<para>
<command>update-exim4.conf</command> is invoked by the init
script prior to any operation that may invoke an exim process,
and gives an error message if the generated config file is
syntactically invalid. If you want to activate your changes to
files in conf.d/ just execute <command>invoke-rc.d exim4 restart</command>.
</para>
</section>
<section id="howto-change-config"><title>How do I do minor tweaks to the configuration?</title>
<para>
Some times, you want to do minor adjustments to the Exim
configuration to make Exim behave exactly like you want it
to behave. There are the following possibilities to modify
Exim's behavior.
</para>
<section><title>Adjustments supported by the debconf configuration</title>
<para>
If you want to modify parameters that are supported by the
debconf configuration, things are easy. Just invoke
<command>dpkg-reconfigure exim4-config</command> or hand-edit
<filename>/etc/exim4/update-exim4.conf.conf</filename> to your
liking and restart Exim.
</para>
<para>
You can find explanation of the debconf questions in <xref
linkend="debconf-questions"/>.
Additionally,
<filename>/etc/exim4/update-exim4.conf.conf</filename>
is documented in the <command>update-exim4.conf</command>
man page.
</para>
</section>
<section><title>Adjustments controlled by macros in the Debian Exim configuration</title>
<para>
Some aspects of the Debian Exim configuration can be
controlled by Exim macros. To find out about these, you
need basic understanding of Exim configuration. Just look
in our Exim configuration and see which macro needs to be
set to a different value to alter Exim's behavior.
</para>
<para>
<xref linkend="macros"/> gives a closer explanation about
how to do this.
</para>
</section>
<section><title>Making direct changes to the Debian Exim configuration</title>
<para>
You can, of course, make direct change to the
configuration. All configuration files in /etc/exim4 are
dpkg-conffiles, and you can thus edit them any time. Your
changes will be preserved through updates. You need to
know about how to configure Exim to be successful.
</para>
<para>
If you use unsplit configuration, edit
<filename>/etc/exim4/exim4.conf.template</filename>. If you use
split configuration, edit the Exim configuration snippets in
<filename>/etc/exim4/conf.d</filename>.
</para>
<para>
More information about how the Exim configuration is built
can be found in this document and in the
<command>update-exim4.conf</command> manual page.
</para>
</section>
</section>
<section id="completely-different-configuration"> <title>Using a completely different configuration scheme</title>
<para>
If you are an experienced Exim administrator, you might feel
working with our pre-fabricated configuration
cumbersome and complex. You might feel right if you need to
make more complex changes and do not need to receive updates
from us. This section is going to tell about how to use
your own configuration.
</para>
<para>
But, you might profit from keeping the Debian magic. Most
files that come with Debian exim4 are conffiles. Debian is
going to care about your changes and keeps them around.
Additionally, a lot of configuration options can be
overridden with a macro, which does not require you to
actually change our configuration file. A lot of people are
using our configuration scheme, and maybe it is going to
save you a lot of time if you decide to spend some time
familiarizing yourself with our scheme.
</para>
<section> <title>Override exim4-config configuration magic</title>
<para>
If you are only running a small number of systems and
want to completely disable Debian's magic, just take
your monolithic configuration file and install it as
<filename>/etc/exim4/exim4.conf</filename>. Exim will
use that file verbatim. To have something to start,
you can either take
<filename>/etc/exim4/exim4.conf.template</filename>,
run <command>update-exim4.conf --keepcomments --output
/etc/exim4/exim4.conf</command>, or use upstream's
default configuration file that is installed as
<filename>/usr/share/doc/exim4-base/examples/example.conf.gz</filename>.
You are going to lose all magic you get from packaging
though, so you need to be familiar with Exim to build
an actually working config.
</para>
<para>
<filename>/var/lib/exim4/config.autogenerated</filename>,
the file generated by
<command>update-exim4.conf</command>, is ignored as soon
as <filename>/etc/exim4/exim4.conf</filename> is found.
You should not edit
<filename>/etc/exim4/exim4.conf</filename> directly when
Exim is running, because the forked processes Exim starts
for SMTP receiving or queue running would use the new
configuration file, while the original main exim-daemon
would still use the old configuration file.
</para>
<para>
Some third-party HOWTOs that reference Debian and
claim to make things easy suggest dumping a
pre-fabricated, static config file to
<filename>/etc/exim4/exim4.conf</filename>. This is
considered bad advice by the Debian maintainers since
you are going to disable all updates and service magic
that Debian might deliver in the future this way. If
you do not know exactly what you're doing here, this
is a bad choice. We try to comment on external HOWTOs
found on the web in the <ulink
url="http://wiki.debian.org/PkgExim4UserFAQ">Debian
Exim4 User FAQ</ulink> to help you find out which
advice to follow.
</para>
</section>
<section> <title>Replacing exim4-config with your own exim4 configuration package.</title>
<para>
We split off Exim's configuration system (debconf,
<command>update-exim4.conf</command>, and the files in
<filename>/etc/exim4/conf.d)</filename> to a separate
package, exim4-config. If you want to, you can replace
exim4-config by something entirely different. The other
packages don't care. Your package needs to:
<itemizedlist>
<listitem>
<simpara>
Provides: exim4-config-2, Conflicts:
exim4-config-2,exim4-config
</simpara>
</listitem>
<listitem>
<simpara>
drop the Exim 4 configuration either into
<filename>/var/lib/exim4/config.autogenerated</filename>
or into <filename>/etc/exim4/exim4.conf</filename>.
</simpara>
</listitem>
</itemizedlist>
Your package must provide an executable <command>update-exim4.conf</command>
that must be in root's path (<filename>/usr/sbin</filename> recommended). The init
script will invoke that executable prior to invoking the
actual exim daemon. If you do not need that script, have it exit 0.
</para>
<para>
If you want to create your own configuration packages, there is a
number of helpers available.
<itemizedlist>
<listitem>
<simpara>
The Exim 4 Debian svn repository holds sources for a
exim4-config-simple package which contains a simple, not
debconf-driven configuration scheme as an example which can
be used as a template for a classical, exim4.conf based
configuration scheme.
</simpara>
</listitem>
<listitem>
<simpara>
The Exim 4 Debian svn repository holds sources for a
exim4-config-medium package which contains the conf.d
driven configuration of the main package with the
debconf interaction removed. This can be used to create
your own non-debconf configuration package that uses the
conf.d mechanism.
</simpara>
</listitem>
<listitem>
<simpara>
Finally, you can invoke the script
<filename>debian/config-custom/create-custom-config-package</filename>
which will create a new source package
"exim4-config-custom" with the debconf-driven config
scheme of exim4-config for your local modification.
</simpara>
</listitem>
</itemizedlist>
Please note that exim4-config-simple and
exim4-config-medium are only targeted to be used as a
template. The configurations contained are not
suitable for productive use. Of course, the Debian
maintainers appreciate any patches you might find
suitable. The scripts in exim4-config-simple and
exim4-config-medium may not work at all in your
environment. Unfortunately, they have not been
updated in a long time as well. We are willing to
accept patches.
</para>
<para>
See the development web page for links to the subversion
repository.
</para>
<para>
Exchanging the entire exim4-config package with
something custom comes particularly handy for sites
that have more than a few machines that are
similarly configured, but do not want to use the
original exim4-config package. Build your own
exim4-config-custom or exim4-config-foo, and simply
apt that package to the machines that need to have
that configuration. Future updates can then be
handled via the dpkg-conffile mechanism, properly
detecting local modifications.
</para>
<para>
In the future, it might be possible that Debian will
contain multiple flavours of Exim4 configuration.
However, these packages would have to be maintained
by someone else because the exim4 package
maintainers think that the scheme delivered with
exim4-config is the least of all evils and would
rather not spend the time to maintain multiple configuration
schemes while only actually using one. It would be
nice to have a configuration scheme using a
monolithic config file, managed by ucf in
three-way-merge mode. If anybody feels ready to
maintain it, please go ahead.
</para>
</section>
</section>
</section>
<section id="TLS"> <title>Using TLS</title>
<para>
Both exim4-daemon-heavy and exim4-daemon-light support TLS/SSL
using the GnuTLS library.
</para>
<section> <title>Exim 4 as TLS/SSL client</title>
<para>
Exim will use TLS
via STARTTLS <emphasis>automatically</emphasis> as client if
the server Exim connects to offers it.
</para>
<para>
This means that you will not need any special configuration if
you want to use opportunistic TLS for outgoing mail. However,
to enforce TLS and successful certificate verification, a few
things need to be configured.
</para>
<para>
To enforce TLS and prevent fallback to unencrypted
connections, ensure that hosts_require_tls = * is in effect on
the respective transport. For the remote_smtp_smarthost
transport, this setting can be controlled via the
REMOTE_SMTP_SMARTHOST_HOSTS_REQUIRE_TLS macro.
</para>
<para>
The certificate presented by the remote host is checked
against the system CA certificate store
(<filename>/etc/ssl/certs/</filename>) and the verification
result is logged (CV=...).
For the remote_smtp_smarthost transport successful
certificate verification against the system trust store is
enforced by default on encrypted connections.
(<quote>REMOTE_SMTP_SMARTHOST_TLS_VERIFY_HOSTS = *</quote>
is set by default). Set this macro to an empty value to
disable this. To check against a certificate not present in
the system trust store point
REMOTE_SMTP_SMARTHOST_TLS_VERIFY_CERTIFICATES (which sets
tls_verify_certificates) to a file containing this (set of)
trusted certificates.
</para>
<para>
Successful certificate verification is
<emphasis>not enforced</emphasis>
by default for other transports.
</para>
<para>
Another possibility would be to use DANE for certificate
verification. This requires support on the server side and
a resolver with DNSSEC support on the client side.
</para>
<para>
If your
server setup mandates the use of client certificates, you
need to amend your remote_smtp and/or remote_smtp_smarthost
transports with a tls_certificate option. This is not
commonly needed.
</para>
<para id="tls_client_certicate">
To make exim send a TLS certificate to the remote host set
REMOTE_SMTP_TLS_CERTIFICATE/REMOTE_SMTP_PRIVATEKEY or for
the remote_smtp_smarthost transport
REMOTE_SMTP_SMARTHOST_TLS_CERTIFICATE/REMOTE_SMTP_SMARTHOST_PRIVATEKEY
respectively.
</para>
<para>
To use TLS on connect set <quote>protocol = smtps</quote> on
the respective transport. (For the remote_smtp_smarthost
transport the macro REMOTE_SMTP_SMARTHOST_PROTOCOL can be used.
</para>
</section>
<section> <title>TLS support for Exim as server</title>
<para>
Incoming TLS is enabled by default. It can be disabled by
setting MAIN_TLS_ADVERTISE_HOSTS to an empty value.
</para>
<para>
The MAIN_TLS_ENABLE macro allows setting/finetuning exim's
TLS options. If it is not set upstream defaults will be
used. Exim will then use on-connect autogenerated self-signed
certificates. This is not optimal both for performance
reasons and because these certificates cannot be verified by
connecting clients/servers.
</para>
<para>
Each time an on-demand cert is created exim will log a info message
in the mainlog that looks like this:
<blockquote>
Warning: No server certificate defined; will use a selfsigned one.
Suggested action: either install a certificate or change
tls_advertise_hosts option
</blockquote>
This informative message can be ignored.
</para>
<para>
To avoid the (small) performance issue and the log message one can
locally create certificates. The exim-gencert script (which requires
openssl) can be helpful for this purpose. It is shipped in
<filename>/usr/share/doc/exim4-base/examples/</filename> and
takes care of proper access privileges on the private key
file when installing key/certificate in
<filename>/etc/exim4/</filename>.
</para>
<para>
One can also get a certificate from a CA and install the key in
<filename>/etc/exim4/exim.key</filename> and the certificate
in <filename>/etc/exim4/exim.crt</filename>.
</para>
<para>
To enable use of the installed certificates set the macro
MAIN_TLS_ENABLE in a local configuration file as described in
<xref linkend="macros"/>.
</para>
<para>
Exim will advertise STARTTLS when
connected to on the normal SMTP ports. Alternatively/additionally
TLS on connect on Port 465 can be enabled by including
<code>-oX 465:25 -oP /run/exim4/exim.pid</code>
in EXIMSERVICE/EXIMDAEMONOPTS in
<filename>/etc/default/exim4</filename> and
"tls_on_connect_ports=465" in the main configuration section.
</para>
<para>
The -oP is needed because Exim does not write an implicit pid
file if -oX is given. Without pid file, service file/init-script
and cronjob will malfunction.
</para>
<para>
It might be appropriate to add "+tls_cipher" to
any log_selector statement you might already have, or to add a
log_selector statement setting these two options in a local
configuration file. (For Debian's configuration simply define
the MAIN_LOG_SELECTOR macro.)
This option makes Exim log what cipher
your Exim and the peer's mailer have negotiated to use to
encrypt the transaction.
</para>
<para>
Exim can be configured to ask a client for a certificate and to
try to verify it. Debian's exim configuration used to enable
this by default, but stopped doing so since it caused TLS errors
with a couple of popular clients (Outlook, Incredimail, etc.).
To enable this again set the macro MAIN_TLS_TRY_VERIFY_HOSTS to
the lists hosts whose certificates you want to check. (Use * to
try checking all hosts. The value of the macro is used to
populate exim's main option tls_try_verify_hosts.) You should
also point MAIN_TLS_VERIFY_CERTIFICATES to a file containing the
accepted certificates, since its default setting
(/etc/ssl/certs/ca-certificates.crt) can contain a large list of
certificates which causes the interoperabilty problems with
Outlook et.al. noted above.
</para>
<para>
The server certificate is only used for incoming connections,
please consult <xref linkend="tls_client_certicate"/> for the
corresponding outgoing conncection options.
</para>
</section>
<section> <title>Troubleshooting</title>
<para>
If Exim complains in an SMTP session that TLS is unavailable,
the Exim mainlog or paniclog frequently has exact information
about what might be wrong. Fo example, you might see
</para>
<para>
2003-01-27 19:06:45 TLS error on connection from localhost [127.0.0.1]
(cert/key setup): Error while reading file)
</para>
<para>
showing that there has been an error while accessing the
certificate or the private key file.
</para>
<para>
Insuffient entropy available is a frequent cause of TLS
failures in Exim context. If Exim logs "not enough random bytes
available", or simply hangs silently when an encrypted
connection should be established, then Exim was
unable to read enough random data from
<filename>/dev/random</filename> to do whatever cryptographic
operation is requested. Please check that your
<filename>/dev/random</filename> device is setup properly.
</para>
<para>
You might also find "TLS error on connection to [...]
(gnutls_handshake): The Diffie-Hellman prime sent by the server is
not acceptable (not long enough)." given as reason. Exim by default
requires a DH prime length of 1024 bits. This requirement can be
downgraded by setting the tls_dh_min_bits option on the SMTP
transport. The setting is accessible in the Debian configuration by
setting the macro TLS_DH_MIN_BITS. (e.g. "TLS_DH_MIN_BITS = 768").
</para>
</section>
</section>
<section id="smtp-auth"> <title>SMTP-AUTH</title>
<para>
Exim can do SMTP AUTH both as a client and as a server.
</para>
<para>
AUTH PLAIN and AUTH LOGIN are disabled for connections which are
not protected by SSL/TLS per default. These authentication
methods use cleartext passwords, and allowing the
transmission of cleartext passwords on unencrypted connections
is a security risk. Therefore, the default configuration configures
Exim not to use and/or allow AUTH PLAIN and AUTH LOGIN over
unencrypted connections.
</para>
<para>
It is thus recommended to set up Exim to use TLS to encrypt
the connections. Please refer to <xref linkend="TLS"/> for
documentation about this. Note that most Microsoft clients
need special handling for TLS.
</para>
<section> <title>Using Exim as SMTP-AUTH client</title>
<para>
If you want to set up Exim as SMTP AUTH client for delivery
to your internet access provider's smarthost put the name of
the server, your login and password in
<filename>/etc/exim4/passwd.client</filename>. See the man
page for exim4-config_files(5) for more information about the
required format.
</para>
<para>
If you need to enable AUTH PLAIN or AUTH LOGIN for unencrypted
connections because your service provider does support neither
TLS encryption nor the CRAM MD5 authentication method, you can
do so by setting the AUTH_CLIENT_ALLOW_NOTLS_PASSWORDS macro.
Please refer to <xref linkend="macros"/> for an explanation of
how best to do this.
</para>
<para>
<filename>/etc/exim4/passwd.client</filename> needs to be
readable for the exim user (user Debian-exim, group
Debian-exim). It is suggested that you keep the default
permissions root:Debian-exim 0640.
</para>
</section>
<section> <title>Using Exim as SMTP-AUTH server</title>
<para>
The configuration files include many, verbosely commented,
examples for server-side smtp-authentication which just need
to be uncommented.
</para>
<para>
If you need to enable AUTH PLAIN or AUTH LOGIN for unencrypted
connections because your clients neither support TLS encryption
nor the CRAM MD5 authentication method, you can do so by setting
the AUTH_SERVER_ALLOW_NOTLS_PASSWORDS macro. Please refer to
<xref linkend="macros"/> for an explanation of how best to
do this.
</para>
<para>
If you want to authenticate against system passwords (e.g.
<filename>/etc/shadow</filename>) the easiest way is to use
saslauthd in the Debian package sasl2-bin. You have to add the
exim-user (currently Debian-exim) to the sasl group, to give
exim permission to use the saslauthd service.
</para>
<para>
The Debian exim4 maintainers consider using system login
passwords a bad idea for the following reasons:
<itemizedlist>
<listitem>
<simpara>
A compromised password will give access to a system account.
</simpara>
</listitem>
<listitem>
<simpara>
E-Mail passwords could accidentally be transmitted unencrypted.
</simpara>
</listitem>
<listitem>
<simpara>
E-Mail passwords are likely to be stored with the
client software, which greatly increases the chance of a
compromise.
</simpara>
</listitem>
</itemizedlist>
</para>
</section>
</section>
<section> <title>How the Exim daemon is started</title>
<para>
The Debian Exim 4 packages' init script is located in
<filename>/etc/init.d/exim4</filename>. Apart from the
functions that are required by Debian policy and the LSB, it
supports the commands <command>what</command>, which executes
<command>exiwhat</command> to show what your Exim processes
are doing, and <command>force_stop</command> which
unconditionally kills all Exim processes.
</para>
<para>
The init script can be configured to start listening and/or
queue running daemons. This configuration can be found in
<filename>/etc/default/exim4</filename>. This file is
extensively documented.
</para>
</section>
<section> <title>Miscellaneous packaging issues</title>
<section> <title>The daily cron job</title>
<para>
Exim4's daily cron job
(<filename>/etc/cron.daily/exim4-base</filename>)
does basic housekeeping tasks:
<itemizedlist>
<listitem>
<simpara>
It reads <filename>/etc/default/exim4</filename>, so you
can use this file to change any of the variables used in
the cron job.
</simpara>
</listitem>
<listitem>
<simpara>
It is a no-op if no Exim4 binary is found.
</simpara>
</listitem>
<listitem>
<simpara>
If <command>$E4BCD_DAILY_REPORT_TO</command> is set
to a non-empty string, the output of eximstats is
mailed to the address given in that variable. The
default is empty, so no reports are sent. Options
for eximstats can be given in
<command>$E4BCD_DAILY_REPORT_OPTIONS</command>.
</simpara>
</listitem>
<listitem>
<simpara>
A non-empty paniclog is a nearly sure sign of bad
things going on. Thus, the cron job will send out
warning messages to the syslog and root if it finds
the panic log non-empty.
Please note that the paniclog is not rotated daily,
so existing issues will be reported daily until
either the paniclog is rotated due to its sheer
size, or you manually move it away, for example by
calling <command>logrotate -f
/etc/logrotate.d/exim4-paniclog</command> from a shell.
</simpara>
<simpara>
Just in case your system logs transient error
situations to the panic log as well (see, for
example,
<ulink url="https://bugs.exim.org/show_bug.cgi?id=92">Exim Bug 92</ulink>),
you can configure
<command>$E4BCD_PANICLOG_NOISE</command> to a
regular expression. If the paniclog contains only
lines that match that regular expression, no warning
messages are generated.
</simpara>
<simpara>
If you want to disable paniclog monitoring
completely, set <command>$E4BCD_WATCH_PANICLOG</command>
to no. <command>E4BCD_WATCH_PANICLOG=once</command> will
rotate a non-empty paniclog automatically after sending out
the warning e-mail.
</simpara>
<simpara>
The <command>E4BCD_PANICLOG_LINES</command> setting can be
used to limit the number of lines of paniclog quoted in
warning email. It is set to 10 by default.
</simpara>
</listitem>
<listitem>
<simpara>
It tidies up the retry and hints databases.
</simpara>
</listitem>
</itemizedlist>
</para>
</section>
</section>
<section> <title>Using Exim with inetd/xinetd</title>
<para>
Exim4 is run as a separate daemon instead of inetd/xinetd for
two reasons:
<variablelist>
<varlistentry>
<term>Ease of maintenance:</term>
<listitem>
<simpara>
update-inetd is difficult to impossible to handle
correctly (Just check the archived bug reports of Exim.)
and update-inetd seems to be unmaintained for a long
time, nobody dares to touch it. To quote Mark Baker, the
maintainer of Exim (v3): "I really wish I had never used
inetd in the first place, but simply set up exim to run
as a daemon, but it's too late to change that now."
</simpara>
</listitem>
</varlistentry>
<varlistentry>
<term>Extended features</term>
<listitem>
<simpara>
Running from <command>inetd</command> interferes with
Exim's resource controls (e.g it disables
smtp_accept_max_per_host and smtp_accept_max).
</simpara>
</listitem>
</varlistentry>
</variablelist>
</para>
<para>
If you introduce bugs on your systems by running from (x)inetd
you are on your own! If you want to run exim from
<command>xinetd</command>, follow these steps:
<orderedlist>
<listitem>
<simpara>
Disable Exim 4's listening daemon by standard means
for the local init system (e.g. mask the service
with systemd) and take care of running the queue (e.g. with
a cronjob).
</simpara>
</listitem>
<listitem>
<para>
Create <filename>/etc/xinetd.d/exim4</filename>
<programlisting>
service smtp
{
disable = no
flags = NAMEINARGS
socket_type = stream
protocol = tcp
wait = no
user = Debian-exim
group = Debian-exim
server = /usr/sbin/exim4
server_args = exim4 -bs
}
</programlisting>
</para>
</listitem>
<listitem>
<simpara>Run <command>invoke-rc.d exim4 restart; invoke-rc.d
(x)inetd restart</command></simpara>
</listitem>
</orderedlist>
</para>
<para>If you want to use plain inetd,
<orderedlist>
<listitem>
<simpara>
Disable Exim 4's listening daemon by standard means
for the local init system (e.g. mask the service
with systemd) and take care of running the queue (e.g. with
a cronjob).
</simpara>
</listitem>
<listitem>
<simpara>
Insert following line into
<filename>/etc/inetd.conf</filename>:<programlisting>
smtp stream tcp nowait Debian-exim /usr/sbin/exim4 exim4 -bs
</programlisting>
</simpara>
</listitem>
</orderedlist>
</para>
</section>
<section> <title>Handling incoming mail for local accounts with low UID</title>
<para>
Since system accounts (mail, uucp, lp etc) are usually aliased
to root, and root's mailbox is usually read by a human, these
account names have started to be a common target for spammers.
The Debian Exim 4 packages have a mechanism to deal with this
situation. However, since this derives rather far from normal
behavior, it is disabled by default.
</para>
<para>
To enable it, set the macro FIRST_USER_ACCOUNT_UID to a numeric,
non-zero value. Incoming mail for local users that have a UID
lower than FIRST_USER_ACCOUNT_UID is rejected with the message "no
mail to system accounts". Incoming mail for local users that
have a UID greater or equal FIRST_USER_ACCOUNT_UID are processed as
usual. Therefore, the default value of 0 ensures that the
mechanism is disabled. On Debian systems, setting
FIRST_USER_ACCOUNT_UID to 500 or 1000 (depending on your local policy)
will disable incoming mail for system accounts.
</para>
<para>
Just in case that you need exceptions to the rule,
<filename>/etc/exim4/lowuid-aliases</filename> is an alias
file that is only honored for local accounts with UID lower
than FIRST_USER_ACCOUNT_UID. If you define an alias for such an
account here, incoming mail is processed according to the
alias. If you alias the account to itself, messages are
delivered to the account itself, which is an exception to the
rule that messages for low-UID accounts are rejected. The
format of <filename>/etc/exim4/lowuid-aliases</filename> is
just another alias file.
</para>
</section>
<section> <title>How to bypass local routing specialities</title>
<para>
Sometimes, it might be desirable to be able to bypass local
routing specialities like the alias file or a user-forward
file. This is possible in the Debian Exim4 packages by
prefixing the account name with "real-". For a local account
name "foo", "real-foo@hostname.example" will result in direct
delivery to foo's local Mailbox.
</para>
<para>
This feature is by default only available for locally
generated messages. If you want it to be accessible for
messages delivered from remote as well, set the Exim macro
COND_LOCAL_SUBMITTER to true. If you do not want this at all,
set the macro to false. Please note that the userforward
router uses this feature to get error messages delivered, i.e.
notifying the user of a syntax error in her
<filename>.forward</filename> file.
</para>
</section>
<section> <title>Using more complex deliveries from alias files</title>
<para>
Delivery to arbitrary files, directory or to pipes in the
<filename>/etc/aliases</filename> file is disabled by default
in the Debian Exim 4 packages. The delivery process including the
program being piped to would run as the exim admin-user
Debian-exim, which might open up security holes.
</para>
<para>
Invoking pipes from <filename>/etc/aliases</filename> file is
widely considered obsolete and deprecated. The Debian Exim
package maintainers would like to suggest using a dedicated
router/transport pair to invoke local processes for mail
processing. For example, the Debian mailman package contains a
<filename>/usr/share/doc/mailman/README.Exim4.Debian</filename> file
that gives a good example how to implement this. Using a
dedicated router/transport pair have the following advantages:
<itemizedlist>
<listitem>
<para>
The router/transport pair can be put in place by another
package, giving a well-defined transaction point between
Exim 4 and $PACKAGE.
</para>
</listitem>
<listitem>
<para>
Not allowing pipe deliveries from alias files makes it
harder to accidentally run programs with wrong
privileges.
</para>
</listitem>
<listitem>
<para>
It is possible to run different pipe processes under
different accounts.
</para>
</listitem>
<listitem>
<para>
Even if only invoking a single local program, it is easier
to do with your dedicated router/transport since you won't
need to change this file, making automatic updates of this
file possible for future versions of the Exim 4 packages. If
you do local changes here, dpkg conffile handling will
bother you on future updates.
</para>
</listitem>
</itemizedlist>
If you insist on using <filename>/etc/aliases</filename> in
the traditional way, you will need to activate the
respective functions by setting the transport options on the
system_aliases router appropriately. Macros are defined to make
this easier. See
<filename>/etc/exim4/conf.d/router/400_exim4-config_system_aliases</filename>
for information about which macros are available. You might
find the address_file, address_pipe and/or address_directory
transports that are used for the userforward router helpful in
writing your own transports for use in the system_aliases router.
</para>
<para>
If any of your aliases expand to pipes or files or directories
you should set up a user and a group for these deliveries to run
under. You can do this by setting the "user" and if a different
one than the user's primary group is wanted also
a "group" option.
Alternatively, you can specify "user" and/or "group" on the
transports that are used.
</para>
</section>
<section> <title>Putting Exim 4 and UUCP together</title>
<para>
UUCP is a traditional way to execute remote jobs (e.g. spool
mails), and as a lot of old things there are much more than one
way to do it. However, today, the ways to handle it have boiled
down to more or less two different ways.
</para>
<para>
Our recommendation is to use bsmtp/rsmtp wherever possible,
because it supports all kinds of mail addresses (also the empty
ones in bounces), and is also better from the security point of
view.
</para>
<section> <title>Sending mail via UUCP</title>
<section> <title>rmail with full addresses</title>
<para>
rmail is the oldest way to transfer mail to a remote system.
However, today it is normally required to use addresses with
full domains for that (Well, they look like any normal address
for you, and we do not tell about the other way to not confuse
you ;). If you want this, you can use this transport:
</para>
<programlisting>
rmail:
debug_print = "T: rmail for $pipe_addresses"
driver=pipe
command = uux - -r -a$sender_address -gC $domain_data!rmail $pipe_addresses
return_fail_output
user=uucp
batch_max = 20
</programlisting>
<para>
However, all recipients are handled via the command line, so
you are discouraged to use it.
</para>
</section>
<section> <title>bsmtp/rsmtp</title>
<para>
This is a more efficient way to transfer mails. It works
like sending SMTP via a pipe, but instead of waiting for an
answer, the SMTP is just batched; from this is also the name
batched SMTP or short bsmtp.
</para>
<para>
Furthermore, this way won't fail on addresses like "
"@do.main. If you want this, please use this, if the remote
site uses rsmtp (e.g. is Exim 4):
</para>
<programlisting>
rsmtp:
debug_print = "T: rsmtp for $pipe_addresses"
driver=pipe
command = /usr/bin/uux - -r -a$sender_address -gC $domain_data!rsmtp
use_bsmtp
return_fail_output
user=uucp
batch_max = 100
</programlisting>
<para>
and this if it wants bsmtp as the command:
</para>
<programlisting>
bsmtp:
debug_print = "T: bsmtp for $pipe_addresses"
driver=pipe
command = /usr/bin/uux - -r -a$sender_address -gC $domain_data!bsmtp
use_bsmtp
return_fail_output
user=uucp
batch_max = 100
</programlisting>
<para>
Of course, these examples can be extended for e.g.
compression (but you can also use ssh for compression, if
you want).
</para>
</section>
<section> <title>The router</title>
<para>
You need a router to tell Exim 4 which mails to forward to
UUCP. You can use this one; please adopt the last line. Of
course, it is also possible to send mail via more than one way.
</para>
<programlisting>
uucp_router:
debug_print = "R: uucp_router for $local_part@$domain"
driver=accept
require_files = +/usr/bin/uux
domains = wildlsearch;/etc/exim4/uucp
transport = rsmtp
</programlisting>
<para>
The file <filename>/etc/exim4/uucp</filename> looks like:
</para>
<programlisting>
*.do.main uucp.name.of.remote.side
</programlisting>
</section>
<section> <title>Speaking UUCP with the smarthost</title>
<para>
If you have a leaf system (i.e. all your mail not for your
local system goes to a single remote system), you can just
forward all non-local mail to the remote UUCP system. In
this case, you can replace "domains = ..." with "domains = !
+local_domains", but then you need also to replace
$domain_data in the transport by the UUCP-name of your
smarthost. The file <filename>/etc/exim4/uucp</filename> is
not needed in this case.
</para>
</section>
</section>
<section> <title>Receiving mail via UUCP</title>
<section> <title>Allow UUCP to use any envelope address</title>
<para>
Depending how much you trust your local users, you might use
trusted_users and add uucp to it or use
local_sender_retain=true and local_from_check=false.
</para>
</section>
<section> <title>If you get batched smtp</title>
<para>
Allow uucp to execute rsmtp via
<programlisting>
commands rmail rnews rsmtp
</programlisting>
in your <filename>/etc/uucp/sys</filename>, and ask the
sending site to use rsmtp (and not bsmtp) as the batched
command.
</para>
</section>
</section>
</section>
<section> <title>Notes on running SpamAssassin at SMTP time</title>
<para>
Exim can run
<ulink url="https://spamassassin.apache.org/">
SpamAssassin</ulink> while receiving a message by SMTP which
allows one to avoid acceptance of spam messages. The Debian
configuration contains some example code for running SpamAssassin,
but like all filtering this needs to be handled carefully.
</para>
<para>
SpamAssassin's default report should not be used in a add_header
statement since it contains empty lines. (This triggers e.g.
Amavis' warning "BAD HEADER SECTION, Improper folded header field
made up entirely of whitespace".) This is a safe, terse alternative:
<programlisting>
clear_report_template
report (_SCORE_ / _REQD_ requ) _TESTSSCORES(,)_ autolearn=_AUTOLEARN_
</programlisting>
</para>
<para>
Rejecting spam messages: Do not reject spam-messages received on
(non-spam) mailing lists, this can/will cause auto-unsubscription.
This also applies to messages received via forwarding services
(e.g. @debian.org addresses). If theses messages are rejected the
forwarding services will need to send a bounce address to the
spammer and will probably disable the forwarding if it happens all
the time. You will need to have some kind of whitelist to exclude
these hosts.
</para>
<para>
Security considerations: By default <command>spamd</command>
runs as root and changes uid/gid to the requested user to run
SpamAssassin. The example uses SpamAssassin default non-privileged
user (nobody) which prevents use of Bayesian filtering since this
requires persistent storage. You might want to setup a dedicated
user for exim spam scanning and use that one, either for a separate
SpamAssassin user profile or to run SpamAssassin as non-privileged
user.
</para>
</section>
</section>
<section> <title>Updating from Exim 3</title>
<para>
If you use <command>exim4-config</command> from Debian, you will
get the debconf based configuration scheme that is intended to
cover the majority of cases.
</para>
<para>
If <command>exim4-config</command> is installed while an Exim 3
package is present on the system,
<command>exim4-config</command> tries to parse the Exim 3 config
file to determine the answers that were given to
<command>eximconfig</command> on Exim 3 installation. These
answers are then taken as default values for the debconf based
configuration process. Be warned! <command>eximconfig</command>
from the Exim 3 packages does not record the explicit answers
given on Exim 3 configuration. So we have to guess the answers
from the Exim 3 configuration file
<filename>/etc/exim/exim.conf</filename>, which is bound to fail
if the config file has been modified after using
<command>eximconfig</command>.
</para>
<para>
This is the reason why we refrained from doing a "silent update", but
only use the guessed answers to get reasonable defaults for our
debconf based configuration process.
</para>
<para>
Please note that we do not use the
<command>exim_convert4r4</command> script, but try to configure
the Exim 4 package in the same way Exim 3 was. This will
hopefully aid future updates.
</para>
<para>
If you have used a customized Exim 3 configuration, you can of
course use <command>exim_convert4r4</command>, and install the
resulting file as <filename>/etc/exim4/exim4.conf</filename>
after careful inspection. Exim 4 will then use that file and
ignore the file that it generated from the debconf
configuration. To aid future updates, we do, however, encourage
you not to use the
<filename>exim_convert4r4-generated</filename> file verbatim but
instead drop appropriate configuration snippets in their
appropriate place in <filename>/etc/exim4/conf.d</filename>.
</para>
</section>
<section> <title>Misc Notes</title>
<section> <title>PAM</title>
<para>
On Debian systems the PAM modules run as the same user
as the calling program, so they cannot do anything you
could not do yourself, and in particular cannot access
<filename>/etc/shadow</filename> unless the user is in group
shadow. - If you want to use
<filename>/etc/shadow</filename> for Exim's SMTP AUTH you
will need to run exim as group shadow. Only
exim4-daemon-heavy is linked against libpam. We suggest using
saslauthd instead.
</para>
</section>
<section> <title>Account name restrictions</title>
<para>
In the default configuration, Exim cannot locally deliver
mail to accounts which have capitals in their name. This is
caused by the fact that Exim converts the local part of incoming
mail to lower case before the comparison done by the
check_local_user directive in routers is done.
</para>
<para>
The router option caseful_local_part can be used to control
this, and we decided not to set this option in the Debian
configuration since it would be a rather big change to Exim's
default behavior.
</para>
</section>
<section> <title>No deliveries to root!</title>
<para>
No Exim 4 version released with any Debian OS can run
deliveries as root. If you don't redirect mail for root via
<filename>/etc/aliases</filename> to a nonprivileged
account, the mail will be delivered to
<filename>/var/mail/mail</filename> with permissions 0600 and
owner mail:mail.
</para>
<para>
This redirection is done by the mail4root router which
is last in the list and will thus catch mail for root that has not
been taken care of earlier.
</para>
</section>
<section> <title>Debugging maintainer and init scripts</title>
<para>
Most of the scripts that come with this Debian package do a
<command>set -x</command> if invoked with the environment
variable EX4DEBUG defined and non-zero. This is particularly
handy if you need to debug the maintainer scripts that are
invoked during package installation. Since dpkg redirects
stdout of maintainer scripts, calling dpkg with EX4DEBUG
set might yield interesting results. If in doubt, invoke
the maintainer scripts with EX4DEBUG set manually directly
from the command line.
</para>
</section>
<section> <title>SELinux</title>
<para>
There is no SELinux policy for Exim4 available so far.
Until this is resolved, users should use postfix or
sendmail if they intend to run SELinux.
</para>
<para>
The Debian Exim4 maintainers would appreciate if
somebody could write an SELinux policy. We will gladly
use them in the Debian packages as long as there is
somebody available to test, debug and support.
</para>
</section>
<section> <title>misc</title>
<itemizedlist>
<listitem>
<simpara>
<command>convert4r4</command> is installed as
<filename>/usr/sbin/exim_convert4r4.</filename>
</simpara>
</listitem>
<listitem>
<simpara>
The charset for $header_foo expansions defaults to
UTF-8 instead of ISO-8859-1.
</simpara>
</listitem>
<listitem>
<simpara>
<ulink url="http://marc.merlins.org/linux/exim/">
Marc Merlin's Exim 4 Page</ulink> has a lot of ACL
examples.
</simpara>
</listitem>
<listitem>
<simpara>
For an example of Exim usage in a
<emphasis>large</emphasis> installation, see
Tony Finch's
<ulink
url="http://www-uxsup.csx.cam.ac.uk/~fanf2/hermes/doc/talks/2005-02-eximconf/">
paper</ulink>
about the Exim installation at University of Cambridge:
</simpara>
</listitem>
</itemizedlist>
</section>
</section>
<section> <title>Debian modifications to the Exim source</title>
<itemizedlist>
<listitem>
<simpara>
Install the exim binary as /usr/sbin/exim4 instead of
/usr/sbin/exim-&lt;version&gt; with a symlink /usr/sbin/exim. Also
adapt the documentation.
</simpara>
</listitem>
<listitem>
<simpara>
Make the build reproducible. Pull date/time from debian/changelog
and use it as build time instead of using __DATE__.
</simpara>
</listitem>
<listitem>
<simpara>
Documentation updates
</simpara>
<itemizedlist>
<listitem>
<simpara>
Mention how to install the Debian packaged perl-modules needed
for eximstats' graphs.
</simpara>
</listitem>
<listitem>
<simpara>
Add a warning about convert4r4.
</simpara>
</listitem>
<listitem>
<simpara>
Point to the <ulink
url="mailto:pkg-exim4-users@lists.alioth.debian.org">
Debian-specific mailing list</ulink> instead of
the <ulink url="mailto:exim-users@exim.org">official
exim-users list</ulink>.
</simpara>
</listitem>
</itemizedlist>
</listitem>
<listitem>
<simpara>
<ulink
url="http://marc.merlins.org/linux/exim/files/sa-exim-current/">localscan_dlopen.patch</ulink>:
This patch makes it possible to use and switch between
different local_scan
functions without recompiling Exim. Use
local_scan_path = /path/to/sharedobject to utilize
local_scan() in <filename>/path/to/sharedobject</filename>.
</simpara>
</listitem>
</itemizedlist>
</section>
<section> <title>Credits</title>
<para><variablelist>
<varlistentry>
<term><ulink url="mailto:aba@not.so.argh.org">Andreas
Barth</ulink></term>
<listitem>
<simpara>UUCP documentation</simpara>
</listitem>
</varlistentry>
<varlistentry>
<term>Dan Weber, Ryen Underwood</term>
<listitem>
<simpara>inetd/xinetd documentation</simpara>
</listitem>
</varlistentry>
</variablelist>
</para>
</section>
</article>
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