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\input texinfo
@set{wmYear} 1997
@set{wmAuthor} Philip Hazel
@set{wmAuthor_email} <ph10@@cus.cam.ac.uk>
@set{COPYRIGHT1} Copyright @copyright{} @value{wmYear} University of Cambridge
@c %**start of header
@setfilename oview.info
@settitle Exim Overview
@paragraphindent 0
@c %**end of header

@titlepage
@title The Exim Mail Transfer Agent
@author @value{wmAuthor}
@page
@vskip 0pt plus 1filll
Permission is granted to make and distribute verbatim copies of this manual provided the
copyright notice and this permission notice are preserved on all copies.
@sp2
@value{COPYRIGHT1}@*
@end titlepage

@node       Top,       Exim Overview, (dir), (dir)
@top
Exim Mail Transfer Agent Overview@*
*********************************

The Exim Overview is converted mechanically into Texinfo format from its
original marked-up source. Some typographic representations are changed,
chapters and sections cannot be numbered, and Texinfo lacks the ability to mark
updated parts of the specification with change bars.

Because the chapters and sections are unnumbered, cross references are set to
their names. This makes the English a bit odd, with phrases like `see section
"Mail filtering"' but it seemed very cumbersome to change this to `see the
section entitled "Mail filtering"' each time.

@menu
* Exim Overview::
@end menu

@node Exim Overview, , Top, Top
@chapter Exim Overview
Date: 9 November 2000

Exim is a mail transfer agent (MTA) developed at the University of Cambridge
for use on Unix systems connected to the Internet. It is freely available under
the terms of the GNU General Public Licence. In overall style it is similar to
Smail 3, but its facilities are more extensive. It contains facilities for
verifying incoming sender and recipient addresses, for refusing mail from
specified hosts, networks, or senders, and for controlling mail relaying.

Exim is in production use at quite a few sites, some of which move hundreds of
thousands of messages per day. This document contains an overview description
of the way Exim works, with a certain amount of omission and simplification to
keep it fairly short. Please address any enquiries about Exim to Philip Hazel:
@example

Email:  <ph10@@cus.cam.ac.uk>
Phone:  +44 1223 334714
Fax:    +44 1223 334679
@sp 1
University of Cambridge
Computing Service
New Museums Site
Pembroke Street
Cambridge CB2 3QG
United Kingdom
@end example

This document is copyright (c) University of Cambridge 2000, but copying
permission is granted to all.
-------------------------------------------------------------------------@*
@sp 4
@center "If I have seen further it is by standing on the shoulders of giants."@*
							 (Isaac Newton)
@sp 4



@sp 2
@menu
* Background::
* Availability::
* Limitations::
* Main features::
* Performance::
* Interface::
* Method of operation::
* Mail filtering::
* Directors::
* Routers::
* Transports::
* Exim logs::
* Exim databases::
* SMTP batching::
* Retries::
* Header rewriting::
* Host verification::
* SMTP port reservation::
* Control of relaying::
* Sender verification::
* Sender lock out::
* Receiver verification::
* The `percent hack'::
* Security::
* The Exim Monitor::
@end menu

@node Background, Availability, Exim Overview, Exim Overview
@section Background

Exim owes a great deal to Smail 3 and its author, Ron Karr. Without the
experience of running and working on the Smail 3 code, I could never have
contemplated starting to write a new mailer. The general style of operation and
configuration are taken from Smail 3, though the actual code of Exim is
entirely new.

My intention was to write a mailer that had more functionality than Smail 3,
but which retained the simple lightweight approach, as this seemed to me to be
all that was needed for systems directly connected to the Internet, where most
messages are delivered almost immediately. On the central mail machines at
Cambridge University, `most' means around 98%.

Since the first versions of Exim went into service at the end of 1995, it has
continued to develop, and now has far more facilities than my original
conception.


@node Availability, Limitations, Background, Exim Overview
@section Availability

The current distribution of Exim is available from
@display

ftp://ftp.csx.cam.ac.uk/pub/software/email/exim/exim-n.nn.tar.gz
@end display

where n.nn is the version number. The distribution contains an @sc{ascii}
copy of the documentation; other formats are available from
@display

ftp://ftp.csx.cam.ac.uk/pub/software/email/exim/exim-html-n.nn.tar.gz
ftp://ftp.csx.cam.ac.uk/pub/software/email/exim/exim-pdf-n.nn.tar.gz
ftp://ftp.csx.cam.ac.uk/pub/software/email/exim/exim-postscript-n.nn.tar.gz
ftp://ftp.csx.cam.ac.uk/pub/software/email/exim/exim-texinfo-n.nn.tar.gz
@end display

The HTML documentation is also kept online via the Exim home page:
@display

http://www.exim.org
@end display

The following operating systems are currently supported: AIX, BSDI, DGUX,
FreeBSD, GNU/Hurd, GNU/Linux, HI-OSF (Hitachi), HP-UX, IRIX, MIPS RISCOS,
NetBSD, OpenBSD, QNX, SCO, SCO SVR4.2 (aka UNIX-SV), Solaris (aka SunOS5),
SunOS4, Tru64-Unix (aka DEC OSF1, aka Digital UNIX) Ultrix, and Unixware.


@node Limitations, Main features, Availability, Exim Overview
@section Limitations

For the benefit of those reading this overview to see whether Exim is of
interest to them, its limitations are listed first.

@itemize @bullet

@item
Exim is written in ANSI C. This should not be much of a limitation these days.
However, to help with systems that lack a true ANSI C library, Exim avoids
making any use of the value returned by the @file{sprintf()} function, which is one
of the main incompatibilities. It has its own version of @file{strerror()} for use
with SunOS4 and any other system that lacks this function, and a macro can be
defined to turn @file{memmove()} into @file{bcopy()} if necessary.

@item
Exim is intended for use as an Internet mailer, and therefore handles addresses
in RFC 822 domain format only. It cannot handle `bang paths', though simple
two-component bang paths can be converted by a straightforward rewriting
configuration. This does not prevent Exim from being interfaced to UUCP,
provided domain addresses are used.

@item
Exim insists that every address it handles has a domain attached. For incoming
local messages, domainless addresses are automatically qualified with a
configured domain value. Configuration options specify from which remote
systems unqualified addresses are acceptable. They are qualified on receipt.

@item
The only external transport currently implemented is an SMTP transport over
a TCP/IP network (using sockets), suitable for machines on the Internet.
However, a pipe transport is available, and there are facilities for writing
messages to files in `batched SMTP' format; both of these can be used to send
messages to some other transport mechanism such as UUCP. Batched SMTP input is
also catered for.

@item
Exim is not designed for storing mail for dial-in hosts. When the volumes of
such mail are large, it is better to get the messages `delivered' into files
(that is, off Exim's queue) and subsequently passed on to the dial-in hosts by
other means.
@end itemize




@node Main features, Performance, Limitations, Exim Overview
@section Main features

Exim follows the same general approach of decentralized control that Smail 3
does. There is no central process doing overall management of mail delivery.
However, the independent delivery processes share data in the form of `hints',
which makes delivery more efficient in some cases. The hints are kept in a
number of DBM files. If any of these files are lost, the only effect is to
change the pattern of delivery attempts and retries.

Here is a summary of Exim's main features. More details are given in the
sections which follow.

@itemize @bullet

@item
Many configuration options can be given as expansion strings, which are
transformed in various ways when they are used. As these can include file
lookups, much of Exim's operation can be made table-driven if desired. For
example, it is possible to do local delivery on a machine on which the users do
not have accounts. The ultimate flexibility can be obtained (at a price) by
running a Perl interpreter while expanding a string.

@item
Regular expressions, compatible with Perl 5, are available in a number of
configuration parameters.

@item
Domain lists can include file lookups, making it possible to support a large
number of local domains, for example.

@item
Exim has flexible retry algorithms, applicable to mail routing as well as to
delivery.

@item
Exim contains header and envelope rewriting facilities.

@item
Unqualified addresses are accepted only from specified hosts or networks.

@item
Exim can perform multiple deliveries down the same SMTP channel after deliveries
to a host have been delayed.

@item
Exim can be configured to do local deliveries immediately but to leave remote
deliveries until the message is picked up by a queue-runner process. This
increases the likelihood of multiple messages being sent down a single SMTP
connection.

@item
When copies of a message have to be delivered to more than one remote host, up
to a configured maximum number of remote deliveries can be done in parallel.

@item
Exim has support for the SMTP AUTH extension for authenticating clients.

@item
Exim has support for encrypted connections using the SMTP STARTTLS extension.

@item
Exim supports optional checking of incoming return path (sender) and receiver
addresses as they are received by SMTP.

@item
SMTP calls from specific hosts and networks, optionally from specific idents,
can be locked out, and incoming SMTP messages from specific senders can also be
locked out. Blocked hosts can be identified explicitly, or via RBL lists, or
the SMTP AUTH and TLS mechanisms can be used.

@item
It is possible to control which hosts may use the Exim host as a relay for
onward transmission of mail. Again, the SMTP AUTH and TLS mechanisms can be
used in this connection.

@item
Messages on the queue can be `frozen' and `thawed' by the administrator.

@item
The maximum size of messages can be specified.

@item
Exim can handle a number of independent local domains on the same machine; each
domain can have its own alias files, etc. These are commonly called @dfn{virtual
domains}.

@item
Simple mailing lists can be handled directly by Exim itself (but for `serious'
mailing list operations, it is best to use it in conjunction with specialist
mailing list software).

@item
Exim stats a user's home directory before looking for a @file{.forward} file, in
order to detect the case of a missing NFS mount.

@item
Exim contains an optional built-in mail filtering facility. This enables users
to set up their own mail filtering in a straightfoward manner without the need
to run an external program. There can also be a system filter file that applies
to all messages.

@item
There is support for multiple user mailboxes controlled by prefixes or suffixes
on the user name, either via the filter mechanism or through multiple
@file{.forward} files.

@item
Periodic warnings are automatically sent to messages' senders when delivery is
delayed -- the time between warnings is configurable.

@item
A queue run can be manually started to deliver just a particular portion of the
queue, or those messages with a recipient or sender whose address contains a
given string or matches a particular regular expression.

@item
Exim can be configured to run as root only when needed; in particular, it need
not run as root when receiving incoming messages or when sending out messages
over SMTP. It always uses @file{setuid} when doing local deliveries.

@item
I have tried to make the wording of delivery failure messages clearer and
simpler, for the benefit of those less-experienced people who are now using
email. However, the wording can be completely replaced by customized paragraphs
supplied in a file if necessary.

@item
Exim contains support for IPv6.

@item
The Exim Monitor is an optional extra; it displays information about Exim's
processing in an X window, and an administrator can perform a number of control
actions from the window interface. This does not, however, supplant
command-line access to all the control functions.
@end itemize



@node Performance, Interface, Main features, Exim Overview
@section Performance

Although I did not specifically set out to write a high-performance MTA, Exim
does seem to be fairly efficient. One site I heard of was a mailing list
exploder that sometimes handles over 100,000 deliveries a day on a big Linux
box, the record being 177,000 deliveries (791MB in total). Up to 13,000
deliveries in an hour have been reported. On larger systems, up to 800,000
messages a day have been reported.



@node Interface, Method of operation, Performance, Exim Overview
@section Interface

Like many MTAs, Exim has adopted the Sendmail interface so that it can be a
straight replacement for @file{usr/sbin/sendmail} or @file{/usr/lib/sendmail}. All the
relevant Sendmail options are implemented. There are also some additional
options that are compatible with Smail 3, and some further options that are new
to Exim.

The runtime configuration interface is a single file which is divided into a
number of sections. The entries in this file consist of keywords and values, in
the style of Smail 3 configuration files.

Control of messages on the queue can be done via certain privileged command
line options. There is also an optional monitor program called @file{eximon}, which
displays current information in an X window and contains interfaces to the
command line options.



@node Method of operation, Mail filtering, Interface, Exim Overview
@section Method of operation

When Exim receives a message, it writes two files in its spool directory. The
first contains the envelope information, the current status of the message,
and the headers, while the second contains the body of the message. The status
of the message includes a complete list of recipients and a list of those that
have already received the message. The header file gets updated during the
course of delivery if necessary.

A message remains in the spool directory until it is completely delivered to
its recipients or to an error address, or until it is deleted by an
administrator or by the user who originally created it. In cases when delivery
cannot proceed -- for example, when a message can neither be delivered to its
recipients nor returned to its sender, the message is marked `frozen' on the
spool, and no more deliveries are attempted. The administrator can thaw such
messages when the problem has been corrected, and can also freeze individual
messages by hand if necessary.

As delivery proceeds, Exim writes timestamped information about each address to
a per-message log file; this includes any delivery error messages. This log is
solely for the benefit of the administrator. All the information Exim itself
needs for delivery is kept in the header spool file. The message log file is
deleted with the spool files. If a message is delayed for more than a
configured time, a warning message is sent to the sender.

The main delivery processing elements of Exim are called directors,
routers, and transports. Code for a number of these is provided, and
compile-time options specify which ones are actually included in the binary.
Directors handle addresses that include one of the local domains, routers
handle remote addresses, and transports do actual deliveries.

When a message is to be delivered, the sequence of events is roughly as
follows:

@itemize @bullet

@item
If there is a system filter file, it is obeyed. This can check on the contents
of the message and its headers, and cause delivery to be abandoned or directed
to alternative or additional addresses, or it can freeze the message for human
attention. It can also add headers to the message, and set up `scores' which
can be used in local recipients' filter files.

@item
Each address is parsed and a check is made to see if it is local or not, by
comparing the domain with the list of local domains, which can be wildcarded,
or even held in a file if there are a large number of them.

@item
If an address is local, it is passed to each configured director in turn until
one is able to handle it. There are directors for handling aliases, for
handling @file{.forward} files, for checking on login names, and so on. If no
director can handle it, the address is failed. Directors can be targeted at
particular local domains, so several local domains can be processed
independently of each other.

@item
A director that accepts an address may set up a local or a remote transport for
it, or it may generate one or more new addresses (typically from alias
or forward files). New addresses are fed back into this process from the top,
but in order to avoid loops, a director will ignore any address which has an
identically-named ancestor that was processed by itself.

@item
If an address is not local, it is passed to each router in turn until one is
able to handle it. There are routers for looking up the domain in a local list,
for doing DNS or host file lookups, and for running external routing programs.
If no router can handle it, the address is failed.

@item
A router that accepts an address may set up a transport for it, or may
pass an altered address to subsequent routers, or it may discover that the
address is a local address after all. This typically happens when a partial
domain name is used and (for example) the DNS lookup is configured to try to
extend such names. In this case, the address is passed back to the directors.

@item
Routers normally set up remote transports for messages that are to be delivered
to other machines. However, a router can pass a message to a local transport,
and by this means messages for remote hosts can be routed to other transport
mechanisms such as UUCP via a file or a pipe.

@item
When all the directing and routing is done, addresses that have been
successfully handled are passed to their assigned transports. Local transports
normally handle only one address at a time, but remote ones can handle more
than one. Each local transport runs in a separate process under a
non-privileged uid.

@item
If there were any errors, a message is returned to an appropriate address (the
sender in the common case).

@item
If one or more addresses suffered a temporary failure, the message is left on
the queue, to be tried again later. Otherwise the spool files and message log
are deleted.
@end itemize



@node Mail filtering, Directors, Method of operation, Exim Overview
@section Mail filtering

Exim can be configured to allow users to set up filter files as an alternative
to the traditional @file{.forward} files. A filter file can test various
characteristics of a message, including the contents of the headers and the
start of the body, and direct delivery to specified addresses, files, or pipes
according to what it finds. The system-wide filter file uses the same control
syntax.


@node Directors, Routers, Mail filtering, Exim Overview
@section Directors

The existing directors are listed below. I use the RFC 822 term local part
to mean that portion of an address that comes before the @@ character.

@itemize @bullet

@item
@file{aliasfile}: This director handles local part expansion via a traditional alias
file. The name of the file is obtained by string expansion, and may therefore
depend on the local part or the domain. Generated pipe and file addresses can be
(independently) locked out.

The @file{aliasfile} director can also be used to test a list of local parts and
direct any messages for them to a specific transport. In this case the data
associated with the local part in the file is not used for address expansion,
but is available for other purposes. For example, files containing records of
the form
@display

  foo: uid=1234 gid=5678 mailbox=/home_1/foo/inbox
@end display

could be used on a system that did local deliveries without consulting its
@file{passwd} file. The @file{aliasfile} director could use the file to verify that the
local part was valid, and then the @file{appendfile} transport could use it to get a
uid, gid, and mailbox for the delivery.

The @file{aliasfile} director can also be configured to include the domain in the
key that is looked up, making it possible to hold aliases for several different
local domains in the same file. It is possible to specify a default address to
be used if nothing in the alias file matches the incoming address.

@item
@file{forwardfile}: This director handles local part expansion via a traditional
forward file or, if so configured, by a user's filter file. The name of the
file is obtained by string expansion, and may therefore depend on the
local part or the domain, though if it is not an absolute path it is
automatically assumed to be in the home directory of the user whose login name
is the local part. Mailing lists can be handled by file names of the form
@display

  /some/list/directory/$@{local_part@}
@end display

and it is possible to specify an error address for each list that depends on the
list name. Generated pipe and file addresses can be (independently) locked out.

@item
@file{localuser}: This director matches the local part of an address to a user of
the machine. It can also be configured to do a pattern match on the user's home
directory name. This makes it possible to partition the set of local users
according to their home directories.

@item
@file{smartuser}: This director matches any local part. It can be used to pass
messages for unknown users to a script that generates a helpful error message,
or it can be used to send such messages to another host, optionally changing
the envelope address in the process.
@end itemize

The configuration file determines which directors are actually used, and in
which order. It is possible to use the same director more than once, with
different options.

The addresses a director handles can be constrained in the following ways:

@itemize @bullet

@item
A specific set of local domains may be specified, in which case the director is
called only for addresses that contain one of those domains.

@item
A specific set of local parts may be specified, in which case the director is
called only for addresses that contain one of those local parts. This could be
used, for example, to handle `postmaster' independently of the particular local
domain.

@item
A specific set of sender addresses may be specified. This can be used to
implement closed mailing lists.

@item
A director may be configured to handle local parts that start with a certain
prefix and/or end with a certain suffix. For example, a director can be set up
to handle local parts of the form @file{xxxx-request} only.

@item
A flag controls whether a director is called when an address is being verified
on an incoming message (during the SMTP dialogue), as opposed to being directed
for delivery.
@end itemize

In addition, certain files can be required to exist or not exist for a given
director to be run.



@node Routers, Transports, Directors, Exim Overview
@section Routers

The existing routers are:

@itemize @bullet

@item
@file{domainlist:} This router searches a list of domains for the one it is trying
to route. The list may either be a string in the configuration file, possibly
including wild cards or regular expressions, or it may be in a file, or both
may be provided. In the case of a file, keys of the form @file{*.foo.bar.com} can
be used for simple wildcarding.

If the domain is found, its entry can either specify a single replacement
domain name that is passed on to subsequent routers, or it can specify a list
of domain names that are looked up by this router. The lookup can be done by
the @file{gethostbyname} function, or by DNS lookup, and in the latter case it is
configurable whether MX or A records or both are used. As well as providing
explicit routing for certain domains, the domainlist router can be used to set
up gateways for partial domains (for example, for @file{*.uucp}) and it can also be used as
a `smarthost' router by using the all-inclusive wild card.

@item
@file{lookuphost:} This router looks up domain names either by calling the
@file{gethostbyname} function, or by using the DNS. In the latter case, it can be
configured to use the DNS resolver options for qualifying single-component
names and for searching parent domains. It is also possible to specify explicit
text strings for widening domains that are not found initially. It is possible
to insist on the presence of MX records for certain sets of domains. A
configuration option controls whether the message's headers are rewritten when
a domain name is changed.

@item
@file{queryprogram:} This router passes the address to a command that runs in a
separate process under an unprivileged uid and gid. The command returns a line
of text specifying whether it matched the domain or not. If it did match, it
may specify a transport name, or it may specify that the transport specified
for the router is used. The command may also send back a new domain name to
replace the current one, and specify a method of looking this name up
(@file{gethostbyname}, DNS, or pass to next router).
@end itemize

The configuration file determines which routers are actually used, and in
which order. It is possible to use the same router more than once, with
different options.

Like directors, routers can be constrained to handle only certain domains or
certain local parts (though I haven't seen a good use for that yet), or
messages from certain senders. If a router times out, either the delivery can
be deferred, or the address can be passed on to the next router.

A flag controls whether a router is called when an address is being verified on
an incoming message, as opposed to being routed for delivery.


@node Transports, Exim logs, Routers, Exim Overview
@section Transports

Local and remote transports are handled differently. A local transport is
always run in a separate process with an appropriate real uid and gid. Their
values can be specified in the transport's configuration, or passed over from
the director that handled the address. Remote transports run under Exim's own
uid. The existing transports are:

@itemize @bullet

@item
@file{appendfile:} This local transport appends the message to a file whose name is
specified as a string containing variable expansions. The current local part
can be inserted via the expansion mechanism, and file names such as
@display

/home/$@{local_part@}/inbox
/var/mail/$@{local_part@}
@end display

are typical examples. However, it is possible to look up each individual user's
inbox name in a file, should that be required.

Exclusive access to the file is ensured by using the traditional mailbox
locking strategy of creating a lock file. The lock creation process uses a
`hitching post' algorithm (similar to that used by Pine) which is robust
when the mailbox file is NFS-mounted. The file is also locked using the
@file{lockf} function, but either one of the locking mechanisms can be turned off
if necessary.

The @file{appendfile} transport can also be configured to deliver each message into
a separate file in a given directory, optionally in `maildir' format. In this
case, no locking is needed.

Options on this transport allow for the insertion of a prefix line (for
example, `From xxx...') and a suffix line, special processing of message lines
starting with `From', and the addition of @file{Return-path}, @file{Delivery-date}, and
@file{Envelope-to} headers. If the mailbox file is not a regular file, or does not
have the correct owner, group, or permissions, no delivery takes place; the
address is deferred and the postmaster is informed, except that, if the file's
permissions are greater than those required, Exim reduces the permissions
and carries on. There are additional checks to reduce the possibility of
security exposures caused by race conditions.

@item
@file{pipe}: This local transport passes the message via a pipe to a specified
command (program or script) which is run in a separate process under a given
uid and gid. Various parameters of the message are passed as environment
variables, and there are the same options as for @file{appendfile} for controlling
the form of the message.

The returned status of the command may be used to determine success or failure,
or it can be ignored. A configuration option specifies whether any standard
output generated by the command is to be returned to the sender (either always,
or only if the command fails). If this is set unconditionally and output is
actually generated, the delivery is deemed to have failed, whatever the
returned status of the command. The maximum amount of output generated by the
command can be controlled, and a timeout may be set for it.

@item
@file{smtp}: This remote transport delivers a message using SMTP over TCP/IP. All
addresses in the message that route to the same set of hosts, and have the same
error address (return path), are normally sent in a single transaction. An
explicit list of hosts can be set for the transport, or a host list may be
attached to an address by one of the routers. If all the hosts are temporarily
unable to accept the message, it is delivered to one of a list of fallback
hosts, if configured.
@end itemize



@node Exim logs, Exim databases, Transports, Exim Overview
@section Exim logs

Exim writes to three different log files:

@itemize @bullet

@item
The main log records the arrival of each message and the result of each
delivery attempt in a single line in each case. The format is as compact as
possible, in an attempt to keep down the size of log files. A number of other
events are also recorded in the main log.

@item
The reject log records information from messages that are rejected because
their return paths are invalid (a configurable option), or the sender or
sending host appears in a blocking list. The headers are written to this log if
they have been read, following a copy of the one-line message that is also
written to the main log.

@item
The panic log is written to when Exim suffers a disaster and has to bomb out.
@end itemize

A utility script for renaming and compressing the main and reject logs each
night is provided. There are also scripts for extracting statistics from log
files and for searching log files for message entries that match a given
pattern. For example, one can pull out all entries relating to messages for a
given local part.

It is possible to configure Exim to write its logging data to syslog instead
of, or as well as, to local files.



@node Exim databases, SMTP batching, Exim logs, Exim Overview
@section Exim databases

Exim maintains a number of databases in DBM files to help it perform efficient
mail delivery. In effect, the files contain hints, and if they are lost it is
not a disaster -- Exim's performance just suffers a bit. The databases are:

@itemize @bullet

@item
@file{retry}: This contains information about each failing remote host and temporary
failing local delivery -- when the first failure was detected, when the
delivery (or directing or routing) was last tried, and when it should next be
tried. More details about retry algorithms are given below.

@item
@file{wait-smtp}: This contains information about messages that are waiting for
particular hosts after an SMTP delivery failure (see the next section).

@item
@file{reject}: This contains information about SMTP message rejections (see below).

@item
@file{serialize-<xxx>}: Exim can be configured to serialize connections to certain
hosts and some kinds of queue run, in other words, do only one of them at a
time. The data for implementing this is kept in database files whose names
begin with @file{serialize-}.
@end itemize

There is a utility program that lists the contents of one of these databases,
and another that allows manual modifications to be applied in some cases.
Database records are timestamped, and there is a utility that removes records
that are older than a given period, and also cleans up @file{wait-smtp} records
containing references to messages that no longer exist. Running this daily or
weekly should be sufficient to keep the files reasonably tidy.

Exim can use any one of the following DBM libraries: @file{ndbm}, @file{gdbm}, @file{DB 1.85},
or @file{DB 2.x}.





@node SMTP batching, Retries, Exim databases, Exim Overview
@section SMTP batching

When an SMTP delivery attempt fails, causing the message to be deferred till
later, Exim updates a DBM database that contains records keyed by host name
plus IP address. Each record holds a list of messages that are waiting for that
host and address.

When an SMTP delivery succeeds, Exim consults the database to see if there are
any other messages waiting for the same host and address. If it finds any, it
creates a new Exim process and passes it the open SMTP channel and a message
identification. The new process then delivers the waiting message down the
existing channel and may in turn cause the creation of yet another process. Any
other waiting addresses in the message are skipped. The maximum number of
messages sent down one connection is configurable.

This scheme achieves some SMTP efficiency when a number of messages have been
queued up for a given host, without the overhead of a heavyweight queueing
apparatus.



@node Retries, Header rewriting, SMTP batching, Exim Overview
@section Retries

When a message cannot immediately be directed, routed, or delivered, it
remains on the queue and another delivery attempt occurs at a later time. While
failures to deliver to remote hosts are the most common cause of this, it is
also possible for a message to be deferred as a result of temporary local
delivery failure, or following directing or routing. A local delivery can fail
if the user is over quota, while directing can be delayed if a user's home
directory is not available (for example, a missing NFS mount), and therefore the
existence of a @file{.forward} file cannot be tested. Routing can be delayed by DNS
timeouts.

Exim can be given a set of rules which specify how often to retry deferred
addresses, and when to give up. These rules apply to directing and routing as
well as to transporting, and are keyed by (wildcarded) domain name or, for
local users, by local part and domain name, either of which can be wildcarded.

Each rule is actually a sequential list of subrules, which are applied
successively as time passes. At present there are two kinds of subrule: fixed
interval, and geometrically increasing interval. For example, it is possible to
specify a rule such as `retry every 15 minutes for 2 hours; then increase the
interval between retries by a factor of 1.5 each time until 8 hours have
passed; then retry every 8 hours until 4 days have passed; then give up'. The
times are measured from when the address first failed, so, for example, if a
host has been down for two days, new messages will immediately go on to the
8-hour retry schedule.

Exim does not have an elaborate series of alarm clocks to cause retries to
happen exactly on schedule. A queue-runner process is started periodically,
to attempt delivery, one by one, of messages containing addresses that have
passed their next retry time. If such an address fails again, a new retry time
is computed, and so subsequent messages queued for the same address get skipped.
The queue is not processed sequentially, but in a `random' order, to prevent
one rogue message that causes a problem blocking other messages to the same
destination for ever.

When the maximum time for retrying has passed, pending addresses are failed.
However, a next try time is still computed from the final subrule. Until that
time is reached, any new messages for the address are immediately failed. When
the next try time is passed, one further delivery attempt is made; if this
fails, a new next try time is computed, and so on.

The increasing number of small computers on the Internet has caused there to be
a lot of messages addressed to hosts that are never going to listen. The retry
logic described above should reduce the amount of wasted time spent on trying
to deliver such messages. However, some administrators are unhappy about this
rather draconian approach, which can cause an address to be failed without any
deliveries being attempted. Exim can alternatively be configured always to try
at least once those hosts whose last failure was before the arrival of the
message. This option increases the number of attempts to deliver to dead hosts.

Retry rules can be predicated on particular errors as well as on domain names,
and for domains that are looked up in the DNS, further discrimination on
whether MX records were used or not is also possible. Thus it is possible to
treat `connection refused' and `connection timed out' differently, or to
distinguish between `connection refused and there was only an A record' and
`connection refused from a host pointed to by an MX record'.

When a local delivery fails because a user is over quota, the retry rule can be
predicated on the length of time since the mailbox was last read. For example,
if the mailbox has been recently read, the delivery can be retried for a while;
otherwise it can be failed quickly.



@node Header rewriting, Host verification, Retries, Exim Overview
@section Header rewriting

There are those who argue that header rewriting is a totally Bad Thing; there
are others who swear they cannot live without it. Exim provides the facility --
you do not have to use it!

Exim can be configured to rewrite the address portions of headers when a
message is received. From release 3.20, Exim can also be configured to rewrite
addresses in header lines at transport time. Rewriting rules can be targeted at
individual headers and the envelope fields; it is possible, for example, just
to rewrite the `From' header and no others.

Rewriting rules are keyed by local part and domain, either of which can be
wildcarded, and the replacement text is a general expansion string which can
contain file lookups. This makes it possible to replace login names by
`friendly' names in outgoing addresses via a DBM lookup, for example. The other
most common rewriting requirement of replacing @file{*.foo.bar} with @file{foo.bar} is
also easily handled.

Headers are also automatically rewritten by Exim in two cases:

@itemize @bullet

@item
If a locally-generated message contains addresses without domains, a
configured qualifying domain is added to each of them. It is also possible to
specify which remote systems are permitted to send messages containing
unqualified addresses. These too get qualified on reception.

@item
Routing of a domain may reveal that it was only a partial domain, in which case
the headers are rewritten to contain the full domain. For example, as a result
of routing, an address such as @file{xxx@@foo} may turn into @file{xxx@@foo.bar.ac.uk}.
@end itemize


In addition to the rewriting rules, Exim can be configured to add or delete
specific headers at transport time. The configuration can either be on the
transport, in which case it applies to all copies of the message sent by that
transport, or on a director or router, in which case it applies only to
copies of the message that are associated with the addresses that the router or
director handled.


@node Host verification, SMTP port reservation, Header rewriting, Exim Overview
@section Host verification

Exim can be configured to accept incoming SMTP calls from certain hosts only,
or it can be configured to reject calls from certain hosts. In both cases, the
test may include an RFC 1413 identification check. A system that gets all its
mail via a central hub might want to lock out the rest of the world, while
a number of systems under one management might want to exchange mail only via
the standard mailer, and hence reject mail from all but certain specified ids
within the group.

Hosts for rejection can be identified explicitly, or via the RBL DNS databases.
Alternatively, the SMTP AUTH extension mechanism may be used for authenticating
hosts.

When a host fails an acceptance test, Exim can either give an error code
immediately on connection, or allow the connection to proceed and then give
error codes to all the message's recipients. The latter approach is useful when
using the mechanism to reject unsolicited junk mail and mail bombs, because it
normally prevents the sender from trying again with the same message.



@node SMTP port reservation, Control of relaying, Host verification, Exim Overview
@section SMTP port reservation

The maximum number of simultaneous incoming SMTP calls can be set, and in
addition, a number of them can be reserved for particular hosts or particular
IP networks. It is also possible to specify a system load value above which
only calls from the reserved hosts are accepted.


@node Control of relaying, Sender verification, SMTP port reservation, Exim Overview
@section Control of relaying

A host is said to act as a relay if it accepts an incoming message from an
external host and delivers it to an external host. Unscrupulous persons have
been known to use unsuspecting hosts as relays in an attempt to disguise the
origin of messages. An Exim host can be configured to accept mail from any host
for onward transmission to a limited set of domains only, and to accept mail
only from a specified list of hosts or networks for onward transmission to any
domain. Such hosts may also be identified by means of the SMTP AUTH mechanism.
It is also possible to enable relaying to any domain whose MX records point to
the local host, without having to list the domains explicitly.


@node Sender verification, Sender lock out, Control of relaying, Exim Overview
@section Sender verification

The return path of a message (also known as the `envelope sender') is used when
Exim has to return an error message. If this is a bad address, the error
message cannot be delivered, and the postmaster has to sort things out.

Sender verification (a configurable option that applies to SMTP input) is
intended to pass this work to a foreign postmaster, by refusing to accept the
message in the first place. There is an exception list which can specify
certain hosts (with optional RFC 1413 identifications) that are allowed to
bypass the check.

A certain amount of SPAM mail contains invalid return paths. Apart from this,
there are two main causes: misconfigured mailers (gateways in particular), and
users fooling around with mail. Sender verification catches both of them. It
operates by passing the sender address through the directors and routers in
verification mode; if this fails, the message is not accepted.

The first thing foreign postmasters ask when they learn about an apparently
legitimate rejected message is `What were the headers?'. For this reason, and
also to collect evidence in cases of mail forgery, Exim does not initially
reject a message after the MAIL FROM command in the SMTP session. It reads
the message, so as to be able to write the headers to the rejection log, and
then gives a hard error response to the sending host.

Unfortunately, several mailers believe that any error response after the data
for a message has been sent indicates a temporary error. Consequently, such
mailers will continue to try to send a message that has been rejected as
described above. To prevent this, whenever a message is rejected, Exim records
the time, bad address, and host in a DBM database. If the same host sends the
same bad address within 24 hours, it is rejected immediately at the MAIL
FROM command.

Sadly, even this doesn't stop some mailers from repeatedly trying to send the
message. As a last resort, if the same host sends the same bad address for a
third time in 24 hours, the MAIL FROM command is accepted, but all
subsequent RCPT TO commands are rejected. If this does not stop a remote
mailer then it is badly broken.

If the attempt to verify the sender address cannot be completed (typically
because of a DNS timeout) Exim returns a temporary error code after the
MAIL FROM command, which should cause the remote mailer to try again
later. However, it is possible to configure Exim to accept the message in these
circumstances.

Many messages with bad return paths in fact contain perfectly valid `From' or
`Reply-to' headers. For administrators that want a quieter life, there is a
configuration option which causes Exim to check these headers if the return
path is bad, and if a good address is found, to use it to replace the return
path. The old value is retained in an header whose name begins with `X-'.



@node Sender lock out, Receiver verification, Sender verification, Exim Overview
@section Sender lock out

More and more unsolicited junk mail is being seen on the Internet. It is
sometimes useful to be able to reject messages (from any host) with particular
sender addresses in the envelope. Exim can be configured to reject messages
whose sender addresses match certain patterns, either by failing the MAIL
FROM command, or (because some mailers take no notice of that) by failing all
RCPT TO commands.



@node Receiver verification, The `percent hack', Sender lock out, Exim Overview
@section Receiver verification

Exim can be configured so that it checks the addresses given in incoming SMTP
RCPT TO commands as they are received. A failing address can be
immediately rejected, or it can be logged and accepted. If verification cannot
be completed (typically because of a DNS timeout) either a temporary error code
can be given, or the address can be logged and accepted.


@node The `percent hack', Security, Receiver verification, Exim Overview
@section The `percent hack'

The so-called `percent hack' is the feature of mailers whereby a local part
containing a percent sign gets interpreted as an entire new address, with the
percent replaced by @@. This is used for explicit mail routing and sometimes
for testing. In Exim, it is possible to configure which local domains, if any,
allow the `percent hack', though this is not recommended. Such usage, if
configured, is, however, subject to the relay controls.



@node Security, The Exim Monitor, The `percent hack', Exim Overview
@section Security

Exim is written as a single binary that has to run @file{setuid} to root. I did
start off trying to write it as a number of different modules, but soon came to
the conclusion that, for this design of mailer, it was not worth it, because
the functions don't decompose cleanly. For example, if you want to verify
addresses while receiving mail you need all the directing and routing apparatus
to be available.

Exim runs each local delivery in a separate process which is @file{setuid} to the
relevant local user. In addition, it can be configured to run under a given
non-root uid (and gid) for much of the rest of the time, and this is the
recommended practice. In particular, it need not be root while sending or
receiving SMTP mail.

Exim checks the permissions and owners of files to which messages are to be
appended, and refuses to proceed with the delivery if things are not right.

Delivery of messages to pipes or files is supported only as a result of
expanding an address via an alias or a forward file, provided this is permitted
by the configuration. Externally generated local addresses cannot specify files
or pipes -- no special action is taken for addresses starting with the file or
pipe characters, so they will usually fail.

Use of the VRFY, EXPN, and ETRN functions in SMTP connections is
controlled by configuration options. The DEBUG function is not supported
at all.


@node The Exim Monitor, , Security, Exim Overview
@section The Exim Monitor

A program for monitoring Exim and displaying information in an X window is
provided. This can be configured to show stripcharts of incoming and outgoing
mail in various categories. It also shows a `tail' of the main log file, and
information about messages on the queue.

There is a menu of operations that can be performed by suitably
privileged users. Messages can be frozen, thawed, deleted, caused to be
delivered, modified, or returned to their senders from this interface. However,
all these actions can also be performed from the command line interface as
well.

@sp 1
@center  *  *  *
@contents
@bye