### -*-m4-*-
###
### Spam filtering for distorted.org.uk Exim configuration
###
### (c) 2012 Mark Wooding
###

###----- Licensing notice ---------------------------------------------------
###
### This program is free software; you can redistribute it and/or modify
### it under the terms of the GNU General Public License as published by
### the Free Software Foundation; either version 2 of the License, or
### (at your option) any later version.
###
### This program is distributed in the hope that it will be useful,
### but WITHOUT ANY WARRANTY; without even the implied warranty of
### MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
### GNU General Public License for more details.
###
### You should have received a copy of the GNU General Public License
### along with this program; if not, write to the Free Software Foundation,
### Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.

DIVERT(null)
###--------------------------------------------------------------------------
### Spam filtering.

## The Exim documentation tells lies.
##
## : *${run{*<_command_>* *<_args_>*}{*<_string1_>*}{*<_string2_>*}}*
## :     The command and its arguments are first expanded separately, [...]
##
## They aren't.  The whole command-and-args are expanded together, and then
## split at unquoted spaces.  This unpleasant hack sorts out the mess.
m4_define(<:SHQUOTE:>, <:"${rxquote:$1}":>)

## Utilities for collecting spam limits.
m4_define(<:SPAMLIMIT_CHECK:>,
	<:${if match{$1}{\N^-?[0-9]+$\N} {spam_limit=$1} {}}:>)

m4_define(<:SPAMLIMIT_ROUTER:>,
<:$1:
	driver = redirect
	data = :unknown:
	verify_only = true
	condition = ${if !eq{$acl_c_mode}{submission}}
	condition = ${extract{spam_limit}{$address_data}{false}{true}}:>)

m4_define(<:SPAMLIMIT_SET:>,
	<:address_data = \
		${if def:address_data {$address_data}{}} \
		m4_ifelse(<:$2:>, <::>, <::>, <:$2 \
		:>)$1:>)

m4_define(<:SPAMLIMIT_LOOKUP:>,
	<:condition = ${if exists{$1}}
	SPAMLIMIT_SET(<:${lookup {$2@$3/$4} nwildlsearch {$1} \
			       {SPAMLIMIT_CHECK(<:$value:>)}}:>, <:$5:>):>)

m4_define(<:SPAMLIMIT_USERV:>,
	<:SPAMLIMIT_SET(<:${run {/usr/bin/timeout 5s \
					/usr/bin/userv CONF_userv_opts \
					SHQUOTE($1) exim-spam-limit \
					SHQUOTE($4) \
					SHQUOTE($2) SHQUOTE(@$3)} \
				{SPAMLIMIT_CHECK(<:$value:>)}}:>, <:$5:>):>)

m4_define(<:GET_ADDRDATA:>,
	<:extract{<:$1:>}{${if def:address_data{$address_data}{}}}:>)

SECTION(global, policy)m4_dnl
spamd_address = CONF_spamd_address CONF_spamd_port

SECTION(acl, rcpt-hooks)m4_dnl
	## Do per-recipient spam-filter processing.
	require	 acl = rcpt_spam

SECTION(acl, misc)m4_dnl
skip_spam_check:

	## If the client is trusted, or this is a new submission, don't
	## bother with any of this.  We will have verified the sender
	## fairly aggressively before granting this level of trust.
	accept	 hosts = +trusted
	accept	 condition = ${if eq{$acl_c_mode}{submission}}

	## If all domains have disabled spam checking then don't check.
	accept	!condition = $acl_c_spam_check_domain

	## Otherwise we should check.
	deny

rcpt_spam:

	## If this is a virtual domain, and it says `spam-check=no', then we
	## shouldn't check spam.  But we can't check domains at DATA time, so
	## instead we must track whether all recipients have disabled
	## checking.
	warn	!domains = ${if exists{CONF_sysconf_dir/domains.conf} \
			 {partial0-lsearch; CONF_sysconf_dir/domains.conf} \
			 {}}
		 set acl_c_spam_check_domain = true
	warn	!condition = $acl_c_spam_check_domain
		 condition = DOMKV(spam-check, {${expand:$value}}{true})
		 set acl_c_spam_check_domain = true

	## See if we should do this check.
	accept	 acl = skip_spam_check

	## Always accept mail to `postmaster'.  Currently this is not
	## negotiable; maybe a tweak can be added to `domains.conf' if
	## necessary.
	accept	 local_parts = postmaster

	## Collect the user's spam threshold from the `address_data'
	## variable, where it was left by the `fetch_spam_limit' router
	## during recipient verification.  (This just saves duplicating this
	## enormous expression.)
	warn	 set acl_m_this_spam_limit = \
			${sg {${GET_ADDRDATA(spam_limit){$value}{nil}}} \
			     {^(|.*\\D.*)\$}{CONF_spam_max}}

	warn	 condition = ${GET_ADDRDATA(user){true}{false}}
		 set acl_m_spam_users = \
			${if def:acl_m_spam_users {$acl_m_spam_users::}{}}\
			${GET_ADDRDATA(user) \
				{$value=${sg{$local_part@$domain}\
					    {([!:])}{!\$1}}} \
				fail}

	## If there's a spam limit already established, and it's different
	## from this user's limit, then the sender will have to try this user
	## again later.
	defer	!hosts = +trusted
		 message = "You'd better try this one later"
		 condition = ${if def:acl_m_spam_limit {true}{false}}
		 condition = ${if ={$acl_m_spam_limit} \
				   {$acl_m_this_spam_limit} \
				  {false}{true}}

	## There's no limit set yet, or the user's limit is the same as the
	## existing one, or the client's local and we're not checking for
	## spam anyway.  Whichever way, it's safe to set it now.
	warn	 set acl_m_spam_limit = $acl_m_this_spam_limit

	## All done.
	accept

SECTION(acl, data-hooks)m4_dnl
	## Do spam checking.
	require	 acl = data_spam

SECTION(acl, misc)m4_dnl
data_spam:

	## See if we should do this check.
	accept	 acl = skip_spam_check

	## Check header validity.
	require	 verify = header_syntax

	## Check the message for spam, comparing to the configured limit.
	warn	 spam = exim:true

	## Format some reporting stuff.
	warn

		 ## Convert the limit (currently 10x fixed point) into a
		 ## decimal for presentation.
		 set acl_m_spam_limit_presentation = \
			${sg{$acl_m_spam_limit}{\N(\d)$\N}{.\$1}}

		 ## Convert the report into something less obnoxious.  Plain
		 ## old SpamAssassin has an `X-Spam-Status' header which
		 ## lists the matched rules and provides some other basic
		 ## information.  Try to extract something similar from the
		 ## report.
		 ##
		 ## This is rather fiddly.

		 ## Firstly, escape angle brackets, because we'll be using
		 ## them for our own purposes.
		 set acl_m_spam_tests = ${sg{$spam_report}{([!<>])}{!\$1}}

		 ## Trim off the blurb paragraph and the preview.  The rest
		 ## should be fairly well behaved.  Wrap double angle-
		 ## brackets around the remainder; these can't appear in the
		 ## body because we escaped them all earlier.
		 set acl_m_spam_tests = \
			${sg{$acl_m_spam_tests} \
			    {\N^(?s).*\n Content analysis details:(.*)$\N} \
			    {<<\$1>>}}

		 ## Extract the information about the matching rules and
		 ## their scores.  Leave `<<...>>' around everything else.
		 set acl_m_spam_tests = \
			${sg{$acl_m_spam_tests} \
			    {\N(?s)\n\s*(-?[\d.]+)\s+([-\w]+)\s\N} \
			    {>>\$2:\$1,<<}}

		 ## Strip everything still in `<<...>>' pairs, including any
		 ## escaped characters inside.
		 set acl_m_spam_tests = \
			${sg{$acl_m_spam_tests}{\N(?s)<<([^!>]+|!.)*>>\N}{}}

		 ## Trim off a trailing comma.
		 set acl_m_spam_tests = ${sg{$acl_m_spam_tests}{,\s*\$}{}}

		 ## Undo the escaping.
		 set acl_m_spam_tests = ${sg{$acl_m_spam_tests}{!(.)}{\$1}}

	## If we've decided to reject, then leave a dropping in the log file
	## so that users can analyse rejections for incoming messages, and
	## tell the sender to get knotted.
	deny	 message = Tinned meat product detected ($spam_score)
		 log_message = Spam rejection \
			score=$spam_score \
			limit=$acl_m_spam_limit_presentation \
			tests=$acl_m_spam_tests \
			users=$acl_m_spam_users
		 condition = ${if >{$spam_score_int}{$acl_m_spam_limit} \
				  {true}{false}}

	## Insert headers from the spam check now that we've decided to
	## accept the message.
	warn
		 ADD_HEADER(<:X-CONF_header_token-SpamAssassin-Score: \
			$spam_score/$acl_m_spam_limit_presentation \
			($spam_bar):>)
		 ADD_HEADER(<:X-CONF_header_token-SpamAssassin-Status: \
			score=$spam_score, \
			limit=$acl_m_spam_limit_presentation, \n\t\
			tests=$acl_m_spam_tests:>)

	## We're good.
	accept

DIVERT(null)
###----- That's all, folks --------------------------------------------------