1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
|
#
# 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 = ${lookup sqlite {INSERT INTO greylist \
VALUES ( '${quote_sqlite:$acl_m_greyident}', \
'${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
|
