     [file version.texi not included]

This manual is for aegee-milter VERSION

Copyright © 2007-2019 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections being ``Free Software'' and ``Free Software Needs Free Documentation'', with the Front-Cover Texts being ``A GNU Manual,'' and with the Back-Cover Texts as in (a) below.

(a) The FSF's Back-Cover Text is: ``You are free to copy and modify this GNU Manual.  Buying copies from GNU Press supports the FSF in developing GNU and promoting software freedom.''

AEGEE-milter manual
*******************

This manual is for AEGEE-milter (version VERSION, updated UPDATED), a milter for sendmail and postfix for applying per recipient rules at SMTP level.

Introduction
************

Emails are exchanged between servers using the Simple Mail Transport Protocol.  With SMTP the client first communicates which are the recipents of a message.  The server tells for each recipient, whether it accepts emails for that destination.  Then the client sends the content of the message, and the server answers, whether the message is accepted for all previously accepted recipients or for none of them.

With this workflow it is not possible for a multi-recipient SMTP transaction to accept an email for some recipients and reject it for others.  Some users might want to reject whatever suspicious emails they receive, rather than storing them in their Junk folder.  Other users might want to have the suspicious mails in their Junk folders.  And a third group of users might want to deliver the message in their Junk folder, but draw the attention of the sender and, by rejecting the message at SMTP level, communicate to the sender that the message was delivered in the Junk folder.  When a mail is sent to more than one of the above groups, what can the MTA server do to satisfy all wishes?

SMTP makes such use cases hard to handle.  aegee-milter tries to help.

To the criticizers, stating that there is a prize for solving the shortcomings of SMTP, the answer is that one has to start somewhere, e.g. with email segmentation.

For understanding this manual the reader is expected to have general knowledge about the Simple Mail Transfer Protocol.  The manual assumes that the reader knows the concepts of SMTP transaction, distinction between SMTP client and SMTP server, reply code, enhanced status code.  The terms junk and spam are used interchangeably.

Objectives
**********

The objectives of aegee-milter are to:

  * save time of humans, spent processing data, at the cost of either delayed mail delivery, double delivery or rejection messages for partially delivered emails and partially undelivered emails.

  * solve the problem with legitimate emails being put in the Junk folder and are not read in time.

  * have a mail server with good IP reputation, which is achieved by reducing the amount of sent bounces.

  * use Sieve for deciding whether to accept or reject a message at SMTP level.

Sometimes mailing list subscribers post with email addresses, they are not subscribed to the list.  The mailing list manager can defer the decision on whether to accept the email to the mailing list owner, or bounce the message back to the sender, telling her that she cannot write to that address.  Bouncing messages has the risk of blacklisting the mail server in some backscatters DNSBLs.  Delegating the decision to the mailing list owner steals her time, forcing her to handle also spam messages, and does not scale.  aegee-milter offers to reject at SMTP level such messages and communicate in this way to the sender what is going on.  With this approach the time of the mailing list owner is saved, as the work is shifted to the sender.

Anti-spam software classifies sometimes legitimate emails as spam.  If this happens, the messages are dispositioned in the Junk folder and neither the sender nor the recipients are aware, that the mail is missing.  aegee-milter offers to reject the messages, telling the sender why the message was evaluated as spam, and possibly inserting in the rejection message how the recipient can be contacted (snail mail, phone, fax etc).  This saves the recipients the time usually spent digging in the Spam folder, and the concequences of reading a mail much later than expected.  (With aegee-milter mails are sometimes delivered delayed.  The assumption here is, that the disadvantages imposed by reading post-factum a message wrongly delivered in the spam folder are higher than the delay imposed by segmenting a message, or the risk of double delivery.) A further option is to deliver the message as Junk to the recipient, but use rejection at SMTP level, telling the sender that the mail was delivered as spam, why it was evaluated as spam, and what are the alternatives to contact the recipient.

Imperfect anti-spam software makes errors and senders can ask why their email was evaluated as spam.  When using anti-spam software from the public domain, the outcome of the anti-spam evaluation is not a secret.  Putting the output of the anti-spam software in the SMTP rejection reason, when the email is rejected due high spam probability, saves to the site administrator questions, why a specific email was evaluated as spam.

Sieve is a standard language for filtering emails RFC 5228 (https://tools.ietf.org/html/rfc5228).  There is software that can upload and download Sieve scripts to a server using the ManageSieve protocol RFC 5804 (https://tools.ietf.org/html/rfc5804).  With Sieve, the user can inspect the headers a message has, in particular the amount of scores assigned by the anti-spam software, and based on them to decide to accept or reject the message.  When a message is rejected, the user can specify the text to be returned to the sender.  This text can include the telephone or fax number of the recipient and its snail mail address.  In this way, when the anti-spam software classifies a legitimate email as spam, the communication flow is not interrupted by silently delivering messages as Junk.

Circumventing the SMTP Limitations
**********************************

There are some approaches possible to circumvent the limitation of SMTP, that a multi-recipient message has to be either accepted or rejected for all recipients.  All approaches have drawbacks, but using none of them also has drawbacks.

One Recipient per Transaction. Defer for other Recipients
=========================================================

With this approach on each SMTP transaction the server accepts the email for the first recipient and defers it for all other recipients, that have different opinion on whether to accept the message from the first recipient.  Deferring is done with a "451 4.5.3 Use PRDR or retry immediately, possibly in the same TCP connection, after DATA End of data; retry=0" message.

The 451 reply code means, that there is a temporary error and the client shall retry later.

The 4.5.3 Enhanced Status Code means more recipients were specified for the message than could have been delivered by the protocol. This error should normally result in the segmentation of the message into two, the remainder of the recipients to be delivered on a subsequent delivery attempt. [RFC3463, Section 3.6].  RFC 5248 adds, that 4.5.3 can only be used with ESC 451.

Note, that RFC 821 defines 452 reply code, as "Insufficient storage", which RFC 5821 repeats, but adds, that 452 can also mean "Too many recipients" and that a conforming server should be able to accept at least 100 receipients per iteration before returning 452.

The ideal answer would be "452 4.5.3", stating twice the same thing, but this combination is not allowed ber RFC 5248.  If 452 is interpreted as "Insufficient storage now, after some time the server might have more place", then the client will impose some delay.  On the other side, with "451 4.5.3" there are no indications for the client to impose a particular delay on the retry.

When the SMTP client retries, the message has one recipient less than the initial message.  With this approach, a multi recipient message is split (segmented) into one message per recipient and on each iteration the message for the currently handled recipient can be accepted or rejected.

The advantage of this approach is, that the recipients preferences are matched exactly.  As a side effect, if a deferred message arrives the second time, the anti-spam software can evaluate it the second time as likely spam, while the earlier evaluation of the same message, could have classified the message as unlikely spam.

The disadvantage of this approach is, that it delays the delivery of the message for all recipients but the first one.  A message with many recipients can be delayed so much, that the SMTP client gives up retrying for some recipients.

The delayed delivery is not mitigated by inserting more than two IP addresses accepting SMTP connections.  In its default configuration, in order to preserve resources, Postfix does try to deliver emails to up to two different IP addresses, and after the second try falls asleep.  Courier MTA does try to deliver emails only up to one different IP address, before falling asleep.

The retry=0 hint means, that the client shall retry immediately.  This is specified in SMTP Service Extension for Greylisting Operations (https://tools.ietf.org/html/draft-santos-smtpgrey).  However, the specification does not say what to do with zero behind retry= and it is not known if there is some significant software implementing that internet draft.  Moreover, the hint is defined to be only useful with the GREYLIST service extension and aegee-milter does not take care to announce support for it.  Nevertheless adding retry=0 does not harm.

One Recipient per Transaction. Faked defer for other Recipients
===============================================================

Like One Recipient per Transaction. Defer for other Recipients, but delivers the messages promtly to the recipients, that want to accept the message, and remembers, the result of each action.  When the SMTP client retries, the next recipient is accepted in the RCPT TO: phase, together with all recipients that share the same opinion on accepting a message with that recipient.  At the end of DATA, the SMTP server communicates to the client the outcome of the iteration, but does not perform any delivery.  Remember, the action was performed on the first iteration.

On the first iteration, the SMTP server remembers the Message-Id, the recipients and the actions per recipient.

The advantage of this approach is, that compared to no faked deferring, it does not cause delivery delay to the recipients.

The disadvantages are, that the sender might get a notification, that the delivery of her message was delayed, while in fact the message was delivered.  Moreover, on retrying some SMTP clients change the Message-Id, so suppressing the action on a retry will not work and the receivers will get the same message more than once.

A disadvantage compared to the not-faked deferral is that during a later retry, the anti spam software can consider a mail as spam, while it was not considered as spam at the first try.  To mitigate this disadvantage, the final mail store can perform anti-spam evaluation periodically after the delivery and mark messages as spam, even if they were not considered as spam at the time of delivery.  However, such delayed reevaluation can be performed only until the user checks its inbox or spam mailbox, as subsequent moving of emails between the two mailboxes will confuse the users.

Rejection message for Partial Delivery and Partial non-Delivery
===============================================================

An email for a recipient is either ordinary delivered, delivered as Junk or rejected.  A recipient can decide whether to communicate to the sender, that the message was just delivered, or delivered as Junk.  When a mail is directed to many recipients and the recipients have different opinion on whether the accept the message, the server can at the end of DATA return a message, to which recipients the message was ordinary delivered, to which recipients the message was rejected and to which recipients the mail was delivered as junk.  With the latter part, of course, taking the preference of the user to communicate to the sender, that the message was delivered as spam.

The advantage of this approach is, that the preferences of the users are enacted exactly and there are no delays.

A possible disadvantage is, that the rejection message contains, for the senders unusual, different statements about the disposition for each recipient.  The senders must be able to understand the language, in which the rejection message was written.

When an email from a mailing list manager is sent to many recipients and the rejection message spells that some recipients have and other have not received the message, the mailing list manager will interpret only the number codes and conclude, that the message was not delivered to any recipient.  On repeated non-delivery retries, the mailing list manager may remove all recipients from the mailing list.

Send Non Delivery Reports
=========================

It is possible, that the MTA accepts initially the email for all recipients and delivers later non-delivery reports from the recipients, not willing to accept the email.

The disadvantage is, that non-delivery reports (bounces) can blacklist the IP address of the server.  However, when aegee-milter is used with the MSA, the NDR are send only to local users and there are no bad side effects.

A combination of all Approaches
===============================

In theory, it is possible to combine different approaches for the same message: do some segmentation with deferred delays, some segmentation with faked delay, and some partial return replies.

aegee-milter does not offer such mixed mode of operation, as the author has no use-case for it.

Use PRDR
========

The Per Recipient Delivery Response SMTP Service Extension borrows a concept from LMTP (Local Mail Transfer Protocol).  LMTP is used to deliver mails on the receiving server to the mailboxes.  It assumes, that the server for the mailboxes cannot send bouces and, e.g. due quota limitations, does accept the message for some recipients of the mail transaction and reject it for others.  The communication, which recipients receive and reject the message is done after the message body is transmitted.  The specitication is in the doc/ directory.

The disadvantage is, that nearly no software implements PRDR.  When PRDR is available, aegee-milter does make use of it.

Do nothing
==========

Without doing nothing, the email workflow stays as you know it.  The disadvantage is, that the objectives aegee-milter has are not met.

Limitations of the different Modes
==================================

An MTA listens to port 25 and accepts connections from other MTAs.  The assumption is, that these MTAs do retry.

A program accepting emails from the user, MSA, listens to a different port.  The common submitting mail user agent does not automatically retry to resend a message on temporary failure, but requires interaction from the user in such cases.  When aegee-milter is applied in the MSA, then the “Rejection message for partial delivery and partial non-delivery” or "Send Non Delivery Reports" mode are the only one that can work, as they do not rely on retrying.  In the first mode, the errors are communitated during the SMTP dialog to the mail user agent, in the second mode, the errors are communicated as Non Delivery Report.

If a user installs a Sieve script, that is supposed to be executed during SMTP delivery from outside, it will be surprizing for the Sieve script owner to figure out, that the Sieve script was not called for mails delivered over the MSA.  To match the expectations of the users, aegee-milter shall be run also in the MSA.

Concepts -- Core -- Modules -- Lists
************************************

aegee-milter consists of "core", "modules" and "lists".

The modules say per recipient how to act.  They are queried at the RCPT TO:, end of headers and end of data whetherto accept the message for the recipient, or to reject it, and what message to use when rejecting the message.  Example for modules is mod_delay, which can throttle SMTP connections, or mod_sieve to execute Sieve scripts per recipient at SMTP Level.

The core glues aegee-milter to the MTA and asks each module, how to act.  Its role is to map the per recipient decisions of the modules, to the SMTP protocol, depending on the preferences of the site administrator.

The lists are data sources or sinks.  The core and the modules can read or write data to a list, by knowing its name.  E.g. to log data, core send data to a list called “log“.  Modules rely on the presence of lists with specific name.  That is, if a module is programmed to put or get data in a list with particular name and this list is not configured to be loaded, then aegee-milter crashes at the moment, when data for that list is inserted or queried.

More on the core, modules and lists is expained in the Developer's documentation.  As user, the understanding of these concepts is necessary for doing the right configuration.

The order in which modules are executed is significants.  The per module documentation explains the rationale of the built-in order.  aegee-milter's is built by default with an module execution order, that makes sense.

Installation
************

aegee-milter is distributed by its author in source code.  To install the software the manual describes the specifics on compiling aegee-milter.  The reader is supposed to have general knowledge on compiling software with autoconf.

The dependencies are both build-time and run-time dependencies: at build time, the corresponding headers must be present.

Dependencies

  * (required) libmilter to connect aegee-milter with the MTA (sendmail or postfix).  libmilter is part of sendmail.

  * (required) Glib (https://developer.gnome.org/glib/stable/) for reading the configuration file

  * (optional) liblistserv (https://mail.aegee.org/software/liblistserv/), memcached (http://memcached.org/) and libmemcached (https://libmemcached.org/libMemcached.html) to remove subscribers from listserv mailing lists on received ARF report, needed by the ARF module

  * (optional) Cyrus Imap (https://www.cyrusimap.org/) to execute Sieve scripts and for the ARF module.  Only version 3.0 is tested: 3.1 was never tested and 2.5 has worked good at some moment in the past without known problems.

Patches in contrib/

  * Sendmail does some validations on the milter protocol, which validations do not permit to execute PRDR.  A patch lifts these limitations.

  * Cyrus Imap does not install some headers files in $(prefix)/include, necessary to compile aegee-milter: lib/libconfig.h, lib/prot.h, lib/ptrarray.h, lib/util.h, lib/byteorder64.h, imap/conversations.h, imap/mailbox.h, imap/message.h, imap/message_guid.h, imap/quota.h, imap/sequence.h, sieve/grammar.h, sieve/varlist.h.

  * Move struct conversations_open *open_conversations from conversation.h to conversations.c.  This patch is included in Cyrus Imap 3.0.12.

  * RFC 5429 Sieve Email Filtering: Reject and Extended Reject Extensions (https://tools.ietf.org/html/rfc5429) does not recommend to permit using ereject or reject with the actions "keep", "fileinto" and "redirect".

Configuration
*************

On start aegee-milter reads its configuration file.  The configuration file is called aegee-milter.ini .  The file must be located either under $sysconfdir during installation, or passed to aegee-milter with the -c parameter.

The format of the file is INI-style.  It is parsed according to glib's rules (https://developer.gnome.org/glib/stable/glib-Key-value-file-parser.html).  The section names and the keys within a section are case-sensitive.

A module or a list is loaded at execution time, if there is a section in the configuration file with the name of the module or the list.  Sections may be emtpy.  For configuring each module or list there is a separate section.  Sections refering non-existent modules or lists are ignored.

The core needs two sections, called General and Milter.

An example for a INI file

     [General]
     bounce-mode=delayed
     pidfile=/var/run/aegee-milter.pid
     expire=1
     sendmail=/usr/local/sbin/sendmail -C /etc/mail/sendmail/sendmail.cf

     [Milter]
     socket=local:/var/run/aegee-milter.sock
     timeout =3600

     [mod_arf]
     [mod_sieve_global]
     [mod_sieve]
     [mod_via]
     [mod_relayed]
     [mod_rewrite_from]
     [mod_equal]
     [mod_delay]
     ehlo=1
     data=1
     mail=1
     rcpt=1
     [list_timsieved_scripts]
     [list_memcached]
     [list_log]
     file=/var/log/aegee-milter.log
     [list_listserv]
     #email=xxx
     #password=yyyy
     #host=localhost

The details of the list_ and mod_ sections are explained in the documentation of the respective list/module sections below.

Connecting aegee-milter with the MTA
====================================

Milters can be configured either with sendmail or with postfix.

aegee-milter needs the macros j at the connect stage and the i macro on envfrom.

The configuration of a milter with the MTA is for aegee-milter not different, than for any other milter.  See bullet Configure Your MTA for an example.  Consult the manual of the MTA.

Order of Milters
----------------

It makes sense to first check the incoming emails for viruses.   If emails containing viruses are reject, processing of a message can stop here.

The spam evaluation shall happen before aegee-mitler is run.  The idea is that the anti-spam software inserts in headers the result of the spam-evaluation, the Sieve module of aegee-milter evaluates the inserted headers and based on them takes decisions.

It is recommened to run aegee-milter after the DMARC evaluation was performed.  aegee-milter can redirect messages and applying the DMARC policy (reject) after a message failing DMARC was redirected makes things more complicated.

Configuration: Section General
==============================

This section specifies global options.  The section can be skipped.

  * pidfile If present, aegee-milter writes in this file its Process ID.

  * sendmail How to invoke sendmail.  The default is /usr/bin/sendmail.

  * expire (Currently unused). How often the lists shall expunge outdated information.

Configuration: Section Milter
=============================

This section specifies the milter interface.

  * socket The socket where aegee-milter shall connect with the MTA.

  * timeout (optional) Sets the number of seconds aegee-milter will wait for an MTA communication (read or write) before timing out. The default timeout is 7210 seconds.

List Log
========

The list is loaded, when the configuration file contains section [list_log].  The section may contain the key "file". If "file" is present, on loading list_log, the file is opened and a marker is inserted.  On proper shut down, another marker is inserted.  The modules send data to list_log and the latter fills the data to the file.

On the roadmap is to add ability to list_log to add data to syslog and journald.

List Memcached
==============

The list is loaded, when the configuration file contains section [list_memcached].

With params the parameters passed to initialize libmemcached.  The deault is --SERVER=localhost --HASH-WITH-NAMESPACE --NAMESPACE=aegee-milter --TCP-NODELAY --TCP-KEEPALIVE --BINARY-PROTOCOL.

List Timsieved Scripts
======================

The list is loaded, when the configuration file contains section [list_timsieved_scripts].

For a domain, user, and script name, this list determines the location on the file system, where the Sieve script is located.

The only option in the section is imapd.conf.  If present, it specifies from which file to load the default Cyrus Imap configuration.  When absent, the usual Cyrus Imap mechanisms are used to find the configuration file.

list_timsieved_scripts reads from imapd.conf the variables sievedir, virtdomains defaultdomain, and fulldirhash.  How the paths are calculated is best seen by looking at the source code.  As a rule of thumb, if timsieved uploads a sieve script for a user and timsieved and aegee-milter share the same imapd.conf file, then aegee-milter will find the file uploaded by aegee-milter.

list_timsieved_scripts looks for scripts per recipient based on the RCPT TO: parameter.

For addresses without domain, the defaultdomain applies.

In a mail system there are different forms of equivalence between two addresses.  The MTA has an alias system, and virtual domains, that are used to determine if two addresses deliver emails to the same destination.  The authentication system can consider also two distinct user names as equivalent.

aegee-milter knows nothing about the aliasas in the MTA or the authentication system.  If two domains are supposed to be handled as equivalent by aegee-milter, then either there must be hard or symbolic links on the file system, so that aegee-milter can find the right file on the file system.  Likewise, if a person has two addresses and the same Sieve filtering rules shall be applied to emails to any of the addresses, the file system must have on the right, distinct place the Sieve scripts for both addresses.  Think of two uploads over timsieved for the one and for the other address.

Besides personal scripts, there are also global scripts per domain.  If a global script is searched for a particular domain and is missing, the global scripts of the default domain are not retried.

A final hack: in terms of ManageSieve there is a default script per user.  timsieved can be used change the default (active) script.  These scripts can be executed by two different systems: by aegee-milter at SMTP time, which has the capability to reject messages at SMTP level, but cannot call fileinto, and by Cyrus Imap as MDA which cannot perform reject at SMTP level, but can do fileinto.  To permit of both systems to coexists, when list_timsieved_scripts loads the default script for a recipient, it first check if a file default.siv.bc exists.  That is, if for the user over ManageSieve a file called default.siv was uploaded.  If this file exists, then aegee-milter uses it in place of the default script.  Otherwise list_timsieved_milter delivers the name of the default (active) Sieve script.

Module ARF
==========

The module is loaded, when the configuration file contains section [mod_arf].

This modules parses emails received at XXX and sent from the Yahoo, Microsoft or Seznam.cz feedback loop service.  It extracts from them the email addresses, which complained about received spam, the email addresse, which was used to send the message.  If the latter is a mailing list, mod_arf checks if the complaining address is subscribed to the mailing list.  If it is, it removes the address from the mailing list, and sends notifications to both the removed address, and the list-owners.  If the mailing list is a sub-list of another list, the email address is removed also from the super list and the notification is sent to the list owners of the super list, too.

If the complaining address is not subscribed to the mailing list or the sending address is not a mailing list, then an email notification is sent to the local administrators.

Once a combination sender-address/complaining address is handled, mod_arf inserts a record via list_memcached and whenever the same combination appears within a month, no more messages are sent.  This is useful, if for the same mailing list a recipient marks more than one messages in a row as spam.  Then the FBL service sends several notifications and only the first of it is relevant.

Note, mod_arf works only with local listserv mailing lists and the sent messages are customized for AEGEE.  Before using this module, the messages in the source code need to be customized.

Module block_sender
===================

The module is loaded, when the configuration file contains section [mod_block_sender].

Module delay
============

The module is loaded, when the configuration file contains section [mod_delay].

The more time spammers sent in waiting, the less amount of spam they send.  mod_delay ensures that the SMPT server spends a minimum amount of time in each phase during the delivery.

  * ehlo the minimum amount of seconds that the client will wait for an answer of EHLO

  * mail the minimum amount of seconds that the client will wait for an answer of MAIL FROM:

  * rcpt the minimum amount of seconds that the client will wait for an answer of RCPT TO:

  * data the minimum amount of seconds that the client will wait for an answer of end-of-data-fullstop.

If another module needs more time for a phase, than mandated by mod_delay, then mod_delay does not impose additional delays.

Module equal
============

The module is loaded, when the configuration file contains section [mod_equal].

This module makes sure, that the envelope sender and the first recipient of the mail envelope are distinct addresses.  There is some amout of spam, where the envolope sender and recipient coincide and for some sites, there are no use cases, where the sender can coincide with the first recipient.

Module relayed
==============

The module is loaded, when the configuration file contains section [mod_relayed].

For mails that are not rejected, a line with the sender and recipient is inserted in list_log.  With sendmail this information is hard to extract from the log files.

Module sieve
============

The module is loaded, when the configuration file contains section [mod_sieve].

It executes a Sieve script on every incoming mail per recipient.

The only actions executed are redirect, reject (rejects at SMTP level), explicit keep and discard.  Actions fileinto and vacation are ignored.  Cyrus Imap 3.1 has action ereject, but aegee-milter works with 3.0 and 3.0 has no ereject.

Calling the Sieve extensions from RFC 5490 is undefined behaviour.

Long rejection lines are split (folded) by aegee-milter when sent over SMTP, so the Sieve script can contain very long text after the reject action.

The executed script is the default script for the recipient.  If there is no such script, then the default global script for the domain is executed.  If there is no such script, mod_sieve does nothing.

Module sieve_global
===================

The module is loaded, when the configuration file contains section [mod_sieve_global].

It executes the same Sieve script for every incoming email.

The remarks for see Module sieve apply also here.

The executed script is called "global.siv" and must be present in the global scripts for the domain of the recipient.  If the script is missing, sieve_global does nothing.

Module via
==========

The module is loaded, when the configuration file contains section [mod_via].

Its purpose is to log the second last mailhost from the Received: header via list_log.

Invoking aegee-milter
*********************

On start aegee-milter looks for its configuration file aegee-milter.ini in the $sysconfdir, set at configure time.  To change the directory, where the configuration file is looked for, or the configuration file, use the -c command line parameter.

By default, aegee-milter forks in the background.  To run it in the foreground, use -f.

To print the version of aegee-milter or a short usage information, use -v and -h respectively.

The following invocation looks for aegee-milter.ini in /etc/mails/am and starts aegee-milter as daemon process.

     aegee-milter -c /etc/mails/am/

The following invocation loads the configuration file /etc/mails/am.ini and keeps the process in the foreground:

     aegee-milter -f -c /etc/mails/am.ini

Bugs and Community
******************

Interested parties, willing to report bugs, send patches or propose feature requests, are invited to join the milter-l@lists.aegee.org mailing list by visiting https://lists.aegee.org/join/milter-l.  Leaving the mailing list can be triggered from the same URL.  Writing to the mailing list is permitted only with addresses that are subscribed: One of Sender:, From:, Resent-Sender:, or Resent-From: headers must match, otherwise the posting is rejected.

Please use common sense when writing to the mailing list on non-technical matters and avoid making statements, which you would not make talking directly to a person.

Interested parties, for which subscribing to a mailing list is inconvenient, can write to milter@aegee.org.  The author of aegee-milter prefers, that interested parties use the mailing list.

Contributions, meaning patches sent in any form, will be handled in reasonable time.  Patches, for which there are no cmpeling reasons to reject them, will be accepted.  This means, in the event that the author has no time to review a patch and there are no objections over the mailing list three months after proposing a patch, that the patch will be accepted.

Changes that have no other purpose, except to silence warnings from the compiler, linker or code analyzer will be accepted, unless they slow down the code.  The rationale is, that if such changes are not accepted, in the future somebody will propose them again and handling similar proposals several times is avoidable work.

The language is C++14.  Lifting to C++17 will reduce the amount of systems, where the software can be compiled.

Shorter code is better, but rewriting the whole program to be a single line of code, leads nowhere.

There is no stict code-style.

Text is in British English.

There is no plan for the event that the author is not capable to respond, but ideas for such a plan are welcomed.

These guidelines might change in the future.

Roadmap
*******

Priorities are, in this order:

  * Fixing problem in features that are claimed to be implemented.

  * Updating the documentation

  * Implementing Reject for all recipients, spelling in the reject reason the applied action per recipient.

  * Implementing PRDR rejection mode.  This is updating sendmail or postfix to be able to offer PRDR, when acting as server, and aegee-milter to be able to provide “Per recipient delivery response” over the milter interface.

  * Implementing “faked delay” rejection mode.  That is, from the perspective of the sender performing segmenting,

Development
***********

Developing the Core
===================

When a message is sent to many recipient and is handled by many modules, aegee-milter creates internally something like a table.  Each column represents a recipient and each row is a module.  When a module provides the same results for two recipients, then that cell in the table is extended and the two modules share the cell.

For example for a message with three recipients, that is handled by the modules delay and sieve, the table looks like:

  rec1   rec2   rec3
mod_sieve   A   B   A
mod_delay   D   D   D

The same latter in two cells means that the cell is shared.

Here, mod_sieve think that the result of the evaluation for rec1 and rec3 will be the same, therefore the cell in row mod_sieve in columns rec1 and rec3 is shared.  mod_delay has for all recipients the same handling, therefore all cells in the last row a shared.

aegee-milter executes the code for each cell only once.  In the above example mod_sieve will be executed for rec1 and rec3.  For rec2 it will not be executed, because the result of executing it will be the same, as for rec1.  mod_delay will be executed only once for this message and all recipients, since the result for executing the module for each recipient is the same.

The core exports a variable GKeyFile *inifile, which lists and modules can import with extern "C" GKeyFile *inifile.  It points to the open INI configuration file.  Lists and modules shall check the value only in the sections with their own name.  Currently, if there is an unrecognized option in a section, the corresponding list/module throws an exception and aegee-milter terminates.  However, if there is an unrecognized section name, the section is ignored.

The core stores per SMTP transaction data in a variable of type Privdata.  This variable is passed to any call of the modules and is not passed to call to methods from the lists.  It turned out in practice, that lists do not need the per SMTP transaction data, in order to serve their purpose.

The build System
================

aegee-milter is build with autoconf, automake and libtool.  There is a single Makefile, except for libtool.

Lists and modules are libtool dlpreopened (libtool) dlpreopened objects.  This means, that each list and module is first linked for itself and then the linked list or module is linked in the executable of aegee-milter.  At startup aegee-milter iterates over its ELF symbols, stripping them to the first dot.

If a section in the INI file exists with that name, and the preloaded name starts with 'm', aegee-milter tries to load it as module.

If a section in the INI file exists with that name, and the preloaded name starts with 'l', aegee-milter tries to load it as list.

At make install the libtool dlpreloaded objects are installed.  This is a known deficiency, since the installed objects are not needed at runtime, with an unknown solution.

Developing Modules
==================

If aegee-milter considers a symbol to be loaded as module, it extracts and executes a function called 'init' from the dlpreopened object.  If this function exists, and returns a SoModule object, then the module is loaded and added to AegeeMilter::so_modules.

Modules are executed for each recipient in the order they are loaded by aegee-milter and the order of loading depends on the order modules are appended to aegee-milter during linking.

Each module derives from the SoModule class.

aegee-milter offers a module to store data per SMTP session, per transaction and per recipient.

When a method from a module is called, the first passed argument is of type Privdata.

To compare, whether for two recipients a module will have the same results, on the SMTP transaction (accept/reject/reject stating the mesage was quarantiened), the core calls for the second recipient on the Equal method with two recipients as parameter.  If it returns true, then the core executes that module only once, for the first recipient, and the result of the execution is applied to both recipients.

Note, that if Equals() returns false, the implications are not only increased run-time needs.  Depending on the mode of aegee-milter, this can segment the message, in one way or another imposing delivery delays.

aegee-milter calls the Run method of the modules at these stages:

  * after RCPT:

  * after end of headers

  * after end of data

To see at which stage a module is called, it can execute

Developing Lists
================

If aegee-milter considers a symbol to be loaded as list, it extracts and executes a function called 'init' from the dlpreopened object.  If this function exists, and returns a SoList object, then the Tables() method of that object is called.  A single SoList object can provide several tables.  If it returns a non-empty std::vector<std::string>, then the std::strings are added to AegeeMilter::so_tables and linked to the libtool dlpreopened object.  Otherwise the list is unloaded.

The order in which lists are loaded is insignificant.

Each list derives from the SoList class.

The method Query gets as parameters: the table name, from which to obtain information, the user (recipient), for which to obtain information and what information is asked for that user.

The method Insert inserts for a table, user and key a value.

The method Remove is used to remove from a table and user a specific key.  Currently no list implements this method and no module calls it.

The method Expire is meant to expunge outdated data from a table and to be triggered regularly by aegee-milter.  Currently no list implements it and the default implementation does nothing.  The idea is to insert in a table information when vacation/out-of-office messages are sent, and to delete the information.

How the parameters are exactly interpretet is left to the modules and lists implementations and the agreements between them.  For instance, list_log::Insert uses the user and key to compose an appropriate log message and list_log::Query is not called.  list_timsieved_scripts::Query encodes in the user either which user and domain are meant, or if the global scripts for particular domain are requested.  The key is used to request either the default script, or a particular script name.  list_timsieved_scripts::Insert is not used by any module and is not implemented.

History
*******

aegee-milter was developed for a mail server that served a lot of aliases redirecting to different email providers, and mailing lists with mailboxes on different email providers.  The challenge was to keep the IP reputation good.  This means not to forward received spam, recognized as such by the anti-spam software and not to send bounces.  Bounces were sent from the mailing list manager e.g. when a sender uses an address for posting which cannot post to the mailing list.

The further, some mailboxes received more spam than the others, so their spam filter had to be set more aggressively.  Thus one mail sent simultaneosly to an address with more aggressive spam filter and with less aggressive spam filter had to be rejected for the first recient and accepted for the second.

When the author of aegee-milter took over a mail server and installed anti-spam software, he modified the mailing lists to bounce emails evaluated as spam.

The mail server

GNU GENERAL PUBLIC LICENSE
**************************

Version 3, 29 June 2007

     Copyright © 2007 Free Software Foundation, Inc. http://fsf.org/

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     If the Program specifies that a proxy can decide which future versions of the GNU General Public License can be used, that proxy's public statement of acceptance of a version permanently authorizes you to choose that version for the Program.

     Later license versions may give you additional or different permissions.  However, no additional obligations are imposed on any author or copyright holder as a result of your choosing to follow a later version.

 15. Disclaimer of Warranty.

     THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW.  EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM ``AS IS'' WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.  THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU.  SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION.

 16. Limitation of Liability.

     IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR CONVEYS THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.

 17. Interpretation of Sections 15 and 16.

     If the disclaimer of warranty and limitation of liability provided above cannot be given local legal effect according to their terms, reviewing courts shall apply local law that most closely approximates an absolute waiver of all civil liability in connection with the Program, unless a warranty or assumption of liability accompanies a copy of the Program in return for a fee.

END OF TERMS AND CONDITIONS
===========================

GNU Free Documentation License
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     [file fdl.texi not included]