#

# Exim ACL for greylisting. David Woodhouse <dwmw2@infradead.org>

#

# For full background on the logic behind greylisting and how this

# ACL works, see https://github.com/Exim/exim/wiki/SimpleGreylisting

#

# UPDATING TO EXIM 4.94+

# ======================

#

# Previous versions of this ACL specified the sqlite database filename 

# in the sqlite lookup strings directly, but since Exim 4.94 is it no 

# longer permitted to mix "tainted" text which comes from the message 

# itself, with the filename. Thus, you now have to set

#

# sqlite_dbfile = /var/spool/exim/db/greylist.db

#

# ... in the main configuration because it can't be specified within

# the ACL in this file any more.

# USING THIS ACL

# ==============

#

# First set sqlite_dbfile in the main configuration file to point to

# the greylist sqlite database, as described above.

#

# In your main ACLs, gather reason(s) for greylisting into a variable 

# named $acl_m_greylistreasons before invoking this ACL with

# 'require acl = greylist_mail'. The reasons should be separate lines

# of text, and will be reported in the SMTP rejection message as well

# as the log message. Anything "suspicious" about the email can be

# used as criteria here — being HTML, having even a few SpamAssassin

# points, even lacking SPF authorisation (which is OK for greylisting

# although you should never reject outright for an SPF "failure"

# because of the flaws in SPF).

#

# Obviously you need to .include this file too in order to be able

# to invoke this greylist_mail ACL.

# HOW IT WORKS

# ============

#

# When a suspicious mail is seen, we temporarily reject it and wait to see

# if the sender tries again. Most spam robots won't bother. Real mail hosts

# _will_ retry, and we'll accept it the second time. For hosts which are

# observed to retry, we don't bother greylisting again in the future --

# it's obviously pointless. We remember such hosts, or 'known resenders',

# by a tuple of their IP address and the name they used in HELO.

#

# We also include the time of listing for 'known resenders', just in case

# someone wants to expire them after a certain amount of time. So the 

# database table for these 'known resenders' looks like this:

#

# CREATE TABLE resenders (

#        host            TEXT,

#        helo            TEXT,

#        time            INTEGER,

#    PRIMARY KEY (host, helo) );

#

# To remember mail we've rejected, we create an 'identity' from its sender

# and recipient addresses and its Message-ID: header. We don't include the

# sending IP address in the identity, because sometimes the second and 

# subsequent attempts may come from a different IP address to the original.

#

# We do record the original IP address and HELO name though, because if

# the message _is_ retried from another machine, it's the _first_ one we

# want to record as a 'known resender'; not just its backup path.

#

# Obviously we record the time too, so the main table of greylisted mail

# looks like this:

#

# CREATE TABLE greylist (

#        id              TEXT,

#        expire          INTEGER,

#        host            TEXT,

#        helo            TEXT);

#

greylist_mail:

  # Firstly,  accept if it was generated locally or by authenticated clients.

  accept hosts = :

  accept authenticated = *

  # Secondly, there's _absolutely_ no point in greylisting mail from

  # hosts which are known to resend their mail. Just accept it.

  accept condition = ${lookup sqlite {SELECT host from resenders \

			       WHERE helo='${quote_sqlite:$sender_helo_name}' \

			       AND host='$sender_host_address';} {1}}

  # Generate a hashed 'identity' for the mail, as described above.

  warn set acl_m_greyident = ${hash{20}{62}{$sender_address$recipients$h_message-id:}}

  # Attempt to look up this mail in the greylist database. If it's there,

  # remember the expiry time for it; we need to make sure they've waited

  # long enough.

  warn set acl_m_greyexpiry = ${lookup sqlite {SELECT expire FROM greylist \

				WHERE id='${quote_sqlite:$acl_m_greyident}';}{$value}}

  # If there's absolutely nothing suspicious about the email, accept it. BUT...

  accept condition = ${if eq {$acl_m_greylistreasons}{} {1}}

         condition = ${if eq {$acl_m_greyexpiry}{} {1}}

  # ..if this same mail was greylisted before (perhaps because it came from a

  # host which *was* suspicious), then we still want to mark that original host

  # as a "known resender". If we don't, then hosts which attempt to deliver from

  # a dodgy Legacy IP address but then fall back to using IPv6 after greylisting

  # will *never* see their Legacy IP address added to the 'known resenders' list.

  accept condition = ${if eq {$acl_m_greylistreasons}{} {1}}

         acl = write_known_resenders

  # If the mail isn't already the database -- i.e. if the $acl_m_greyexpiry

  # variable we just looked up is empty -- then try to add it now. This is 

  # where the 5 minute timeout is set ($tod_epoch + 300), should you wish

  # to change it.

  warn  condition = ${if eq {$acl_m_greyexpiry}{} {1}}

	set acl_m_dontcare = ${lookup sqlite {INSERT INTO greylist \

					VALUES ( '$acl_m_greyident', \

						 '${eval10:$tod_epoch+300}', \

						 '$sender_host_address', \

						 '${quote_sqlite:$sender_helo_name}' );}}

  # Be paranoid, and check if the insertion succeeded (by doing another lookup).

  # Otherwise, if there's a database error we might end up deferring for ever.

  defer condition = ${if eq {$acl_m_greyexpiry}{} {1}}

        condition = ${lookup sqlite {SELECT expire FROM greylist \

				WHERE id='${quote_sqlite:$acl_m_greyident}';} {1}}

        message = Your mail was considered suspicious for the following reason(s):\n$acl_m_greylistreasons \

		  The mail has been greylisted for 5 minutes, after which it should be accepted. \

		  We apologise for the inconvenience. Your mail system should keep the mail on \

		  its queue and retry. When that happens, your system will be added to the list \

		  genuine mail systems, and mail from it should not be greylisted any more. \

		  In the event of problems, please contact postmaster@$qualify_domain

	log_message = Greylisted <$h_message-id:> from <$sender_address> for offences: ${sg {$acl_m_greylistreasons}{\n}{,}}

  # Handle the error case (which should never happen, but would be bad if it did).

  # First by whining about it in the logs, so the admin can deal with it...

  warn   condition = ${if eq {$acl_m_greyexpiry}{} {1}}

         log_message = Greylist insertion failed. Bypassing greylist.

  # ... and then by just accepting the message.

  accept condition = ${if eq {$acl_m_greyexpiry}{} {1}}

  # OK, we've dealt with the "new" messages. Now we deal with messages which

  # _were_ already in the database...

  # If the message was already listed but its time hasn't yet expired, keep rejecting it

  defer condition = ${if > {$acl_m_greyexpiry}{$tod_epoch}}

	message = Your mail was previously greylisted and the time has not yet expired.\n\

		  You should wait another ${eval10:$acl_m_greyexpiry-$tod_epoch} seconds.\n\

		  Reason(s) for greylisting: \n$acl_m_greylistreasons

  accept acl = write_known_resenders

write_known_resenders:

  # The message was listed but it's been more than five minutes. Accept it now and whitelist

  # the _original_ sending host by its { IP, HELO } so that we don't delay its mail again.

  warn set acl_m_orighost = ${lookup sqlite {SELECT host FROM greylist \

				WHERE id='${quote_sqlite:$acl_m_greyident}';}{$value}}

       set acl_m_orighelo = ${lookup sqlite {SELECT helo FROM greylist \

				WHERE id='${quote_sqlite:$acl_m_greyident}';}{$value}}

       set acl_m_dontcare = ${lookup sqlite {INSERT INTO resenders \

				VALUES ( '$acl_m_orighost', \

					 '${quote_sqlite:$acl_m_orighelo}', \

					 '$tod_epoch' ); }}

       logwrite = Added host $acl_m_orighost with HELO '$acl_m_orighelo' to known resenders

  accept