168 lines
7.7 KiB
PHP
168 lines
7.7 KiB
PHP
#
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# Exim ACL for greylisting. David Woodhouse <dwmw2@infradead.org>
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#
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# For full background on the logic behind greylisting and how this
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# ACL works, see https://github.com/Exim/exim/wiki/SimpleGreylisting
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#
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# UPDATING TO EXIM 4.94+
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# ======================
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#
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# Previous versions of this ACL specified the sqlite database filename
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# in the sqlite lookup strings directly, but since Exim 4.94 is it no
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# longer permitted to mix "tainted" text which comes from the message
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# itself, with the filename. Thus, you now have to set
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#
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# sqlite_dbfile = /var/spool/exim/db/greylist.db
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#
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# ... in the main configuration because it can't be specified within
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# the ACL in this file any more.
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# USING THIS ACL
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# ==============
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#
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# First set sqlite_dbfile in the main configuration file to point to
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# the greylist sqlite database, as described above.
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#
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# In your main ACLs, gather reason(s) for greylisting into a variable
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# named $acl_m_greylistreasons before invoking this ACL with
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# 'require acl = greylist_mail'. The reasons should be separate lines
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# of text, and will be reported in the SMTP rejection message as well
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# as the log message. Anything "suspicious" about the email can be
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# used as criteria here — being HTML, having even a few SpamAssassin
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# points, even lacking SPF authorisation (which is OK for greylisting
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# although you should never reject outright for an SPF "failure"
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# because of the flaws in SPF).
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#
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# Obviously you need to .include this file too in order to be able
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# to invoke this greylist_mail ACL.
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# HOW IT WORKS
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# ============
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#
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# When a suspicious mail is seen, we temporarily reject it and wait to see
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# if the sender tries again. Most spam robots won't bother. Real mail hosts
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# _will_ retry, and we'll accept it the second time. For hosts which are
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# observed to retry, we don't bother greylisting again in the future --
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# it's obviously pointless. We remember such hosts, or 'known resenders',
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# by a tuple of their IP address and the name they used in HELO.
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#
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# We also include the time of listing for 'known resenders', just in case
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# someone wants to expire them after a certain amount of time. So the
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# database table for these 'known resenders' looks like this:
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#
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# CREATE TABLE resenders (
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# host TEXT,
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# helo TEXT,
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# time INTEGER,
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# PRIMARY KEY (host, helo) );
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#
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# To remember mail we've rejected, we create an 'identity' from its sender
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# and recipient addresses and its Message-ID: header. We don't include the
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# sending IP address in the identity, because sometimes the second and
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# subsequent attempts may come from a different IP address to the original.
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#
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# We do record the original IP address and HELO name though, because if
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# the message _is_ retried from another machine, it's the _first_ one we
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# want to record as a 'known resender'; not just its backup path.
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#
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# Obviously we record the time too, so the main table of greylisted mail
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# looks like this:
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#
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# CREATE TABLE greylist (
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# id TEXT,
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# expire INTEGER,
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# host TEXT,
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# helo TEXT);
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#
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greylist_mail:
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# Firstly, accept if it was generated locally or by authenticated clients.
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accept hosts = :
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accept authenticated = *
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# Secondly, there's _absolutely_ no point in greylisting mail from
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# hosts which are known to resend their mail. Just accept it.
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accept condition = ${lookup sqlite {SELECT host from resenders \
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WHERE helo='${quote_sqlite:$sender_helo_name}' \
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AND host='$sender_host_address';} {1}}
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# Generate a hashed 'identity' for the mail, as described above.
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warn set acl_m_greyident = ${hash{20}{62}{$sender_address$recipients$h_message-id:}}
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# Attempt to look up this mail in the greylist database. If it's there,
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# remember the expiry time for it; we need to make sure they've waited
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# long enough.
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warn set acl_m_greyexpiry = ${lookup sqlite {SELECT expire FROM greylist \
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WHERE id='${quote_sqlite:$acl_m_greyident}';}{$value}}
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# If there's absolutely nothing suspicious about the email, accept it. BUT...
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accept condition = ${if eq {$acl_m_greylistreasons}{} {1}}
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condition = ${if eq {$acl_m_greyexpiry}{} {1}}
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# ..if this same mail was greylisted before (perhaps because it came from a
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# host which *was* suspicious), then we still want to mark that original host
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# as a "known resender". If we don't, then hosts which attempt to deliver from
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# a dodgy Legacy IP address but then fall back to using IPv6 after greylisting
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# will *never* see their Legacy IP address added to the 'known resenders' list.
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accept condition = ${if eq {$acl_m_greylistreasons}{} {1}}
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acl = write_known_resenders
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# If the mail isn't already the database -- i.e. if the $acl_m_greyexpiry
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# variable we just looked up is empty -- then try to add it now. This is
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# where the 5 minute timeout is set ($tod_epoch + 300), should you wish
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# to change it.
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warn condition = ${if eq {$acl_m_greyexpiry}{} {1}}
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set acl_m_dontcare = ${lookup sqlite {INSERT INTO greylist \
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VALUES ( '$acl_m_greyident', \
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'${eval10:$tod_epoch+300}', \
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'$sender_host_address', \
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'${quote_sqlite:$sender_helo_name}' );}}
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# Be paranoid, and check if the insertion succeeded (by doing another lookup).
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# Otherwise, if there's a database error we might end up deferring for ever.
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defer condition = ${if eq {$acl_m_greyexpiry}{} {1}}
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condition = ${lookup sqlite {SELECT expire FROM greylist \
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WHERE id='${quote_sqlite:$acl_m_greyident}';} {1}}
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message = Your mail was considered suspicious for the following reason(s):\n$acl_m_greylistreasons \
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The mail has been greylisted for 5 minutes, after which it should be accepted. \
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We apologise for the inconvenience. Your mail system should keep the mail on \
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its queue and retry. When that happens, your system will be added to the list \
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genuine mail systems, and mail from it should not be greylisted any more. \
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In the event of problems, please contact postmaster@$qualify_domain
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log_message = Greylisted <$h_message-id:> from <$sender_address> for offences: ${sg {$acl_m_greylistreasons}{\n}{,}}
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# Handle the error case (which should never happen, but would be bad if it did).
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# First by whining about it in the logs, so the admin can deal with it...
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warn condition = ${if eq {$acl_m_greyexpiry}{} {1}}
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log_message = Greylist insertion failed. Bypassing greylist.
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# ... and then by just accepting the message.
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accept condition = ${if eq {$acl_m_greyexpiry}{} {1}}
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# OK, we've dealt with the "new" messages. Now we deal with messages which
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# _were_ already in the database...
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# If the message was already listed but its time hasn't yet expired, keep rejecting it
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defer condition = ${if > {$acl_m_greyexpiry}{$tod_epoch}}
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message = Your mail was previously greylisted and the time has not yet expired.\n\
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You should wait another ${eval10:$acl_m_greyexpiry-$tod_epoch} seconds.\n\
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Reason(s) for greylisting: \n$acl_m_greylistreasons
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accept acl = write_known_resenders
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write_known_resenders:
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# The message was listed but it's been more than five minutes. Accept it now and whitelist
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# the _original_ sending host by its { IP, HELO } so that we don't delay its mail again.
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warn set acl_m_orighost = ${lookup sqlite {SELECT host FROM greylist \
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WHERE id='${quote_sqlite:$acl_m_greyident}';}{$value}}
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set acl_m_orighelo = ${lookup sqlite {SELECT helo FROM greylist \
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WHERE id='${quote_sqlite:$acl_m_greyident}';}{$value}}
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set acl_m_dontcare = ${lookup sqlite {INSERT INTO resenders \
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VALUES ( '$acl_m_orighost', \
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'${quote_sqlite:$acl_m_orighelo}', \
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'$tod_epoch' ); }}
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logwrite = Added host $acl_m_orighost with HELO '$acl_m_orighelo' to known resenders
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accept
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