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* SHOULD YOU USE THIS PACKAGE? *

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Since version 4.50, Exim has the content-scanning extension formerly

known as "exiscan" built-in. It has a number of advantages and

disadvantages compared to SA-Exim.

Advantages of built-in content-scanning interface:

 * One less configuration file to edit.

 * Spam control policy integrates better with Exim's ACL system.

 * It's possible to tell SA which user to scan for (the -u parameter of

   spamc). SA-Exim can't do that (yet).

 * Finer control over the mail header is possible, but not in a clean

   way (it involves putting all header fields you might possibly want

   to add in the report template, and using rather complicated

   expansion expressions to extract the wanted ones from

   $spam_report). At any rate, you can choose a prefix different from

   "X-Spam-".

Advantages of SA-Exim:

 * It is possible to use the report_safe feature, which turns mail

   deemed to be spam into a message/rfc822 attachment of a report

   message. (Note however that if you do, then any X-SA-* fields added

   to help the greylisting module can't be removed.)

 * All the add_header and rewrite_header options in

   /etc/spamassassin/local.cf will be obeyed. In other words,

   everything will be *almost* as if you filtered the mail through

   spamassassin on the command line.

 * So-called teergrubing ("tarpitting") is possible in a way that

   isn't possible with exiscan (I'm not in any way saying that it

   works as a counterattack against spammers).

 * You can simply add the sa-exim package to a standard exim4

   installation and it should, in principle, instantly work (except

   you have to uncomment one line in sa-exim.conf).

Both alternatives enable you to defer, greylist, reject, and blackhole

mail, optionally saving copies, at configurable score levels.

*****************

* CONFIGURATION *

*****************

This version of the sa-exim package defaults to placing a configuration

sniplet in /etc/exim4/conf.d/. Depending on what you have answered to the

DebConf questions while configuring Exim4, the module will be loaded

automatically, or human intervention is required.

To find out what configuration file Exim4 is using, issue:

  $ exim4 -bV | tail -1

  Configuration file is /path/to/configfile

If /path/to/configfile shows:

  - /etc/exim4/exim4.conf

    You are using the hand-crafted configuration file.

	See the 'HAND-CRAFTED' section below.

  - /var/lib/exim4/config.autogenerated

    You are using the debianized configuration scheme - with either

    'split' or 'unsplit' configuration file.

	See the 'DEBIANIZED' section below.

HAND-CRAFTED

------------

Use 'grep "local_scan_path" /etc/exim4/exim4.conf" to see if the sa-exim

line is included in the configuration. If grep returns something, check

if it matches the following line. If grep returns nothing, you have to

manually add the following line to the exim4.conf file and restart exim4.

    local_scan_path = /usr/lib/exim4/local_scan/sa-exim.so

Change or add the line above and manually restart exim4 by issuing

'invoke-rc.d exim4 reload' or '/etc/init.d/exim4 reload' as root.

DEBIANIZED

----------

Use 'grep "local_scan_path" /var/lib/exim4/config.autogenerated' to

see if the sa-exim line is included in the configuration. If grep

returns something, you're set and already using the sa-exim module. If

grep returns nothing, we need to figure out a few things:

Issue:

	$ grep "use_split_config" /etc/exim4/update-exim4.conf.conf

      dc_use_split_config='true'

If your result shows 'false' where mine shows 'true', then you're

using the unsplit configuration, generated from

/etc/exim4/exim4.conf.template.  If you haven't customized that file

you could edit /etc/exim4/update-exim4.conf.conf by hand, change the

'false' to 'true' and issue 'update-exim4.conf' as root. Then, check

again if the sa-exim module line is included. It should. If it still

isn't: mail me. If it is, restart exim4 by issuing 'invoke-rc.d exim4

restart' or '/etc/init.d/exim4 restart' as root. If you *have*

customized /etc/exim4/exim4.conf.template, then you'd better stick

with the unsplit configuration scheme and add the local_scan_path

setting by hand, like with the hand-crafted configuration file.

***************

* GREYLISTING *

***************

Greylisting is implemented as a SpamAssassin module. To enable it you

need to add the following five lines to your SpamAssassin

configuration:

loadplugin Greylisting /usr/share/perl5/Mail/SpamAssassin/Plugin/Greylisting.pm

header GREYLIST_ISWHITE eval:greylisting("( 'dir' => '/var/spool/sa-exim/tuplets'; 'method' => 'dir'; 'greylistsecs' => '1800'; 'dontgreylistthreshold' => 11; 'connectiphdr' => 'X-SA-Exim-Connect-IP'; 'envfromhdr' => 'X-SA-Exim-Mail-From'; 'rcpttohdr' => 'X-SA-Exim-Rcpt-To'; 'greylistnullfrom' => 1; 'greylistfourthbyte' => 0 )")

describe GREYLIST_ISWHITE The incoming server has been whitelisted for this recipient and sender

score GREYLIST_ISWHITE  -1.5

priority GREYLIST_ISWHITE 99999

(It is a long-standing bug that the module is installed in the wrong

directory, which is why the full path has to be specified on the

loadplugin line, but fixing it is probably not worth the disruption of

existing installations.)

If two messages from the same /24 IPv4 network or /64 IPv6 network (or

individual IP address, depending on greylistfourthbyte), with the same

sender, with the same list of recipient, and with a score below

dontgreylistthreshold are seen at least greylistsecs apart, the

triplet will be whitelisted and the GREYLIST_ISWHITE rule will be

considered to match thenceforth. That will signal to the local_scan

library to raise SAtempreject to let the message through, in addition

to the negative spam score it carries.

Notice that messages can be permanently rejected (score above

SApermreject) and still get a triplet whitelisted if the score is

below dontgreylistthreshold. If dontgreylistthreshold or SAtempreject

+ SAgreylistraisetempreject are less than SApermreject, some mail may

be temporarily rejected indefinitely.

See README.Greylisting for more details.

***********************

* SPAMD CONFIGURATION *

***********************

By default, spamd runs as root and assumes the identity of the user it

is told it is scanning mail on behalf of by whoever connects to it

(see README.spamd.gz in the spamassassin package for a discussion on

security). When SA-Exim runs spamc, this user will normally be

Debian-exim. You can set the SAspamcUser option in sa-exim.conf to

override this, but since a mail can have multiple recipients and is

only scanned once, per-user setups are problematic. Also, the

greylisting module won't work unless all users can write to the

tuplets directory.

Thus, when using SpamAssassin together with SA-Exim you may want to

run spamd under a specific system account by modifying the OPTIONS

variable in /etc/default/spamassassin to include a --username option.

However, if you ONLY use SpamAssassin with SA-Exim this is in practice

not strictly necessary.

You should NOT run spamd as the "nobody" user and/or the "nogroup"

group if you configure SpamAssassin to use sa-exim's greylisting

module, the bayesian classifier, or any helper module that needs to

write files, because nobody/nogroup should be completely unprivileged

and thus not own any files. Instead you should create a dedicated

account to run spamd under. You can then adjust the ownership of

/var/spool/sa-exim/tuplets and the username in

/etc/cron.d/greylistclean accordingly.

***********************************

* PROBLEMS WITH BAYES AUTO-EXPIRY *

***********************************

When scanning mail during the SMTP dialogue there is somewhat limited

time before the remote host gives up, even if they should wait for at

least ten minutes. To avoid Exim returning a temporary error status,

or the remote host giving up prematurely and in some cases for good,

SA-Exim overrides Exim's timeout handler and accepts the message if

SpamAssassin takes too long, by default 240 seconds.

Using SpamAssassin's Bayesian learning module means that it will

automatically expire old tokens when its database has grown too large.

That can take several minutes. If it takes too long, SA-Exim will

abort it, meaning that SpamAssassin will run auto-expiry again next

time, and be aborted, and so on...

If this happens, you have a few remedies:

1) Set SAtimeout to a higher value in /etc/exim4/sa-exim.conf.

2) Run sa-learn --force-expire periodically. How you run it depends on

   how you've configured SpamAssassin. Running it as Debian-exim may

   be sufficient.

2 a) In addition, you can add

   bayes_auto_expire 0

   to /etc/spamassassin/local.cf. This may not be a good idea if

   SpamAssassin, for whatever reason, is also used as a more

   traditional filter from e.g. .procmailrc, as all users will need to

   run sa-learn --force-expire then.

2 b) If you get a lot of mail, consider adding

   bayes_learn_to_journal 1

   to local.cf. See the Mail::SpamAssassin::Conf(3) manual page for

   more information.

**********************************

* NOTICE ABOUT SPAMC CONFIG FILE *

**********************************

Recent versions of spamc can read command-line parameters and switches

from a configuration file called /etc/spamassassin/spamc.conf. If that

file specifies conflicting options, it will prevent SA-Exim from

working. For now, you'll have to make sure that it doesn't.

 -- Magnus Holmgren <holmgren@debian.org>, Fri, 22 Jul 2016 09:58:32 +0200