exim/exim-greylist.conf.inc

# $Id: acl-greylist-sqlite,v 1.1 2006/06/13 13:56:54 dwmw2 Exp $

GREYDB=/var/spool/exim/db/greylist.db

# ACL for greylisting. Place reason(s) for greylisting into a variable named
# $acl_m_greylistreasons before invoking 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.
#
# 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 PRIMARY KEY,
#        helo            TEXT,
#        time            INTEGER
# );
#
# 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 PRIMARY KEY,
#        expire          INTEGER,
#        host            TEXT,
#        helo            TEXT
# );
#

greylist_mail:
  # First, accept if it there's absolutely nothing suspicious about it...
  accept condition = ${if eq{$acl_m_greylistreasons}{} {1}}
  # ... or 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 hosts = sqlite;GREYDB SELECT host from resenders \
			       WHERE helo='${quote_sqlite:$sender_helo_name}' \
			       AND host='$sender_host_address';

  # Generate the mail identity (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 = ${lookup sqlite {GREYDB SELECT expire FROM greylist \
				WHERE id='${quote_sqlite:$acl_m_greyident}';}{$value}}

  # If the mail isn't already the database, defer it with an appropriate 
  # message, and add it. Do the addition to the greylist database as a 
  # hackish side-effect of the log-message, appending 'success' or 'failure'
  # to the log-message depending on whether the SQL worked (which it always
  # should). This is where the 5 minute timeout is set ($tod_epoch + 300)
  # should you wish to change it.
  defer condition = ${if eq {$acl_m_greyexpiry}{} {1}}
	log_message = Greylisted $h_message-id: for offences: ${sg {$acl_m_greylistreasons}{\n}{,}}:\
		${lookup sqlite {GREYDB INSERT INTO greylist \
					VALUES ( '$acl_m_greyident', \
						 '${eval10:$tod_epoch+300}', \
						 '$sender_host_address', \
						 '${quote_sqlite:$sender_helo_name}' );}\
			{success}{failure}}
        message = Your mail was considered suspicious for the following reason(s):\n$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

  # 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

  # The message was listed but it's been more than five minutes. Accept it now and whitelist
  # the sending host by its { IP, HELO } so that we don't delay its mail again. The addition
  # to the database is again done as a hackish side-effect; this time a side-effect of
  # evaluating a condition which comes out as '1' whether the database bit succeeds or not.
  accept condition = ${lookup sqlite {GREYDB INSERT INTO resenders \
			VALUES ( '$sender_host_address', \
				 '${quote_sqlite:$sender_helo_name}', \
				 '$tod_epoch' ); }{1}{1}}