exim/exim-greylist.conf.inc

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# $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,
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# helo TEXT,
# time INTEGER,
# PRIMARY KEY (host, helo) );
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#
# 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,
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# expire INTEGER,
# host TEXT,
# helo TEXT);
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#
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. This is where the 5 minute
# timeout is set ($tod_epoch + 300), should you wish to change it.
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defer condition = ${if eq {$acl_m_greyexpiry}{} {1}}
log_message = Greylisted $h_message-id: for offences: ${sg {$acl_m_greylistreasons}{\n}{,}}\
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${lookup sqlite {GREYDB INSERT INTO greylist \
VALUES ( '$acl_m_greyident', \
'${eval10:$tod_epoch+300}', \
'$sender_host_address', \
'${quote_sqlite:$sender_helo_name}' );}}
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message = Your mail was considered suspicious for the following reason(s):\n$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}', \
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'$tod_epoch' ); }{1}{1}}