1Mail::SpamAssassin::PluUgsienr::CDoKnItMr(i3b)uted PerlMDaoiclu:m:eSnptaamtAisosnassin::Plugin::DKIM(3)
2
6 Mail::SpamAssassin::Plugin::DKIM - perform DKIM verification tests
7
9 loadplugin Mail::SpamAssassin::Plugin::DKIM [/path/to/DKIM.pm]
10 Taking into account signatures from any signing domains:
12 full DKIM_SIGNED eval:check_dkim_signed()
14 full DKIM_VALID eval:check_dkim_valid()
15 full DKIM_VALID_AU eval:check_dkim_valid_author_sig()
16 full DKIM_VALID_EF eval:check_dkim_valid_envelopefrom()
17 Taking into account signatures from specified signing domains only:
19 (quotes may be omitted on domain names consisting only of letters,
20 digits, dots, and minus characters)
21 full DKIM_SIGNED_MY1 eval:check_dkim_signed('dom1','dom2',...)
23 full DKIM_VALID_MY1 eval:check_dkim_valid('dom1','dom2',...)
24 full DKIM_VALID_AU_MY1 eval:check_dkim_valid_author_sig('d1','d2',...)
25 full __DKIM_DEPENDABLE eval:check_dkim_dependable()
27 Author Domain Signing Practices (ADSP) from any author domains:
29 header DKIM_ADSP_NXDOMAIN eval:check_dkim_adsp('N')
31 header DKIM_ADSP_ALL eval:check_dkim_adsp('A')
32 header DKIM_ADSP_DISCARD eval:check_dkim_adsp('D')
33 header DKIM_ADSP_CUSTOM_LOW eval:check_dkim_adsp('1')
34 header DKIM_ADSP_CUSTOM_MED eval:check_dkim_adsp('2')
35 header DKIM_ADSP_CUSTOM_HIGH eval:check_dkim_adsp('3')
36 Author Domain Signing Practices (ADSP) from specified author domains
38 only:
39 header DKIM_ADSP_MY1 eval:check_dkim_adsp('*','dom1','dom2',...)
41 describe DKIM_SIGNED Message has a DKIM or DK signature, not necessarily valid
43 describe DKIM_VALID Message has at least one valid DKIM or DK signature
44 describe DKIM_VALID_AU Message has a valid DKIM or DK signature from author's domain
45 describe DKIM_VALID_EF Message has a valid DKIM or DK signature from envelope-from domain
46 describe __DKIM_DEPENDABLE A validation failure not attributable to truncation
47 describe DKIM_ADSP_NXDOMAIN Domain not in DNS and no valid author domain signature
49 describe DKIM_ADSP_ALL Domain signs all mail, no valid author domain signature
50 describe DKIM_ADSP_DISCARD Domain signs all mail and suggests discarding mail with no valid author domain signature, no valid author domain signature
51 describe DKIM_ADSP_CUSTOM_LOW adsp_override is CUSTOM_LOW, no valid author domain signature
52 describe DKIM_ADSP_CUSTOM_MED adsp_override is CUSTOM_MED, no valid author domain signature
53 describe DKIM_ADSP_CUSTOM_HIGH adsp_override is CUSTOM_HIGH, no valid author domain signature
54 For compatibility with pre-3.3.0 versions, the following are synonyms:
56 OLD: eval:check_dkim_verified = NEW: eval:check_dkim_valid
58 OLD: eval:check_dkim_signall = NEW: eval:check_dkim_adsp('A')
59 OLD: eval:check_dkim_signsome = NEW: redundant, semantically always true
60 The __DKIM_DEPENDABLE eval rule deserves an explanation. The rule
62 yields true when signatures are supplied by a caller, OR ELSE when
63 signatures are obtained by this plugin AND either there are no
64 signatures OR a rule __TRUNCATED was false. In other words:
65 __DKIM_DEPENDABLE is true when failed signatures can not be attributed
66 to message truncation when feeding a message to SpamAssassin. It can
67 be consulted to prevent false positives on large but truncated messages
68 with poor man's implementation of ADSP by hand-crafted rules.
69
71 This SpamAssassin plugin implements DKIM lookups as described by the
72 RFC 4871, as well as historical DomainKeys lookups, as described by RFC
73 4870, thanks to the support for both types of signatures by newer
74 versions of module Mail::DKIM.
75 It requires the "Mail::DKIM" CPAN module to operate. Many thanks to
77 Jason Long for that module.
78
80 The following tags are added to the set, available for use in reports,
81 header fields, other plugins, etc.:
82 _DKIMIDENTITY_
84 Agent or User Identifier (AUID) (the 'i' tag) from valid signatures;
85 _DKIMDOMAIN_
87 Signing Domain Identifier (SDID) (the 'd' tag) from valid signatures;
88 _DKIMSELECTOR_
90 DKIM selector (the 's' tag) from valid signatures;
91 Identities and domains from signatures which failed verification are
93 not included in these tags. Duplicates are eliminated (e.g. when there
94 are two or more valid signatures from the same signer, only one copy
95 makes it into a tag). Note that there may be more than one signature
96 in a message - currently they are provided as a space-separated list,
97 although this behaviour may change.
98
100 "Mail::DKIM" Mail::SpamAssassin::Plugin(3)
101 http://dkimproxy.sourceforge.net/
103 https://tools.ietf.org/rfc/rfc4871.txt
104 https://tools.ietf.org/rfc/rfc4870.txt
105 https://tools.ietf.org/rfc/rfc5617.txt
106 https://datatracker.ietf.org/group/dkim/about/
107
109 whitelist_from_dkim author@example.com [signing-domain]
110 Works similarly to whitelist_from, except that in addition to
111 matching an author address (From) to the pattern in the first
112 parameter, the message must also carry a valid Domain Keys
113 Identified Mail (DKIM) signature made by a signing domain (SDID,
114 i.e. the d= tag) that is acceptable to us.
115 Only one whitelist entry is allowed per line, as in
117 "whitelist_from_rcvd". Multiple "whitelist_from_dkim" lines are
118 allowed. File-glob style characters are allowed for the From
119 address (the first parameter), just like with
120 "whitelist_from_rcvd".
121 The second parameter (the signing-domain) does not accept full
123 file-glob style wildcards, although a simple '*.' (or just a '.')
124 prefix to a domain name is recognized and implies any subdomain of
125 the specified domain (but not the domain itself).
126 If no signing-domain parameter is specified, the only acceptable
128 signature will be an Author Domain Signature (sometimes called
129 first-party signature) which is a signature where the signing
130 domain (SDID) of a signature matches the domain of the author's
131 address (i.e. the address in a From header field).
132 Since this whitelist requires a DKIM check to be made, network
134 tests must be enabled.
135 Examples of whitelisting based on an author domain signature
137 (first-party):
138 whitelist_from_dkim joe@example.com
140 whitelist_from_dkim *@corp.example.com
141 whitelist_from_dkim *@*.example.com
142 Examples of whitelisting based on third-party signatures:
144 whitelist_from_dkim jane@example.net example.org
146 whitelist_from_dkim rick@info.example.net example.net
147 whitelist_from_dkim *@info.example.net example.net
148 whitelist_from_dkim *@* mail7.remailer.example.com
149 whitelist_from_dkim *@* *.remailer.example.com
150 def_whitelist_from_dkim author@example.com [signing-domain]
152 Same as "whitelist_from_dkim", but used for the default whitelist
153 entries in the SpamAssassin distribution. The whitelist score is
154 lower, because these are often targets for abuse of public mailers
155 which sign their mail.
156 unwhitelist_from_dkim author@example.com [signing-domain]
158 Removes an email address with its corresponding signing-domain
159 field from def_whitelist_from_dkim and whitelist_from_dkim tables,
160 if it exists. Parameters to unwhitelist_from_dkim must exactly
161 match the parameters of a corresponding whitelist_from_dkim or
162 def_whitelist_from_dkim config option which created the entry, for
163 it to be removed (a domain name is matched case-insensitively);
164 i.e. if a signing-domain parameter was specified in a whitelisting
165 command, it must also be specified in the unwhitelisting command.
166 Useful for removing undesired default entries from a distributed
168 configuration by a local or site-specific configuration or by
169 "user_prefs".
170 adsp_override domain [signing-practices]
172 Currently few domains publish their signing practices (RFC 5617 -
173 ADSP), partly because the ADSP rfc is rather new, partly because
174 they think hardly any recipient bothers to check it, and partly for
175 fear that some recipients might lose mail due to problems in their
176 signature validation procedures or mail mangling by mailers beyond
177 their control.
178 Nevertheless, recipients could benefit by knowing signing practices
180 of a sending (author's) domain, for example to recognize forged
181 mail claiming to be from certain domains which are popular targets
182 for phishing, like financial institutions. Unfortunately, as
183 signing practices are seldom published or are weak, it is hardly
184 justifiable to look them up in DNS.
185 To overcome this chicken-or-the-egg problem, the "adsp_override"
187 mechanism allows recipients using SpamAssassin to override
188 published or defaulted ADSP for certain domains. This makes it
189 possible to manually specify a stronger (or weaker) signing
190 practices than a signing domain is willing to publish (explicitly
191 or by default), and also save on a DNS lookup.
192 Note that ADSP (published or overridden) is only consulted for
194 messages which do not contain a valid DKIM signature from the
195 author's domain.
196 According to RFC 5617, signing practices can be one of the
198 following: "unknown", "all" and "discardable".
199 "unknown": The domain might sign some or all email - messages from
201 the domain may or may not have an Author Domain Signature. This is
202 a default if a domain exists in DNS but no ADSP record is found.
203 "all": All mail from the domain is signed with an Author Domain
205 Signature.
206 "discardable": All mail from the domain is signed with an Author
208 Domain Signature. Furthermore, if a message arrives without a
209 valid Author Domain Signature, the domain encourages the
210 recipient(s) to discard it.
211 ADSP lookup can also determine that a domain is "out of scope",
213 i.e., the domain does not exist (NXDOMAIN) in the DNS.
214 To override domain's signing practices in a SpamAssassin
216 configuration file, specify an "adsp_override" directive for each
217 sending domain to be overridden.
218 Its first argument is a domain name. Author's domain is matched
220 against it, matching is case insensitive. This is not a regular
221 expression or a file-glob style wildcard, but limited wildcarding
222 is still available: if this argument starts by a "*." (or is a sole
223 "*"), author's domain matches if it is a subdomain (to one or more
224 levels) of the argument. Otherwise (with no leading asterisk) the
225 match must be exact (not a subdomain).
226 An optional second parameter is one of the following keywords
228 (case-insensitive): "nxdomain", "unknown", "all", "discardable",
229 "custom_low", "custom_med", "custom_high".
230 Absence of this second parameter implies "discardable". If a domain
232 is not listed by a "adsp_override" directive nor does it explicitly
233 publish any ADSP record, then "unknown" is implied for valid
234 domains, and "nxdomain" for domains not existing in DNS. (Note:
235 domain validity is only checked with versions of Mail::DKIM 0.37 or
236 later (actually since 0.36_5), the "nxdomain" would never turn up
237 with older versions).
238 The strong setting "discardable" is useful for domains which are
240 known to always sign their mail and to always send it directly to
241 recipients (not to mailing lists), and are frequent targets of
242 fishing attempts, such as financial institutions. The "discardable"
243 is also appropriate for domains which are known never to send any
244 mail.
245 When a message does not contain a valid signature by the author's
247 domain (the domain in a From header field), the signing practices
248 pertaining to author's domain determine which of the following
249 rules fire and contributes its score: DKIM_ADSP_NXDOMAIN,
250 DKIM_ADSP_ALL, DKIM_ADSP_DISCARD, DKIM_ADSP_CUSTOM_LOW,
251 DKIM_ADSP_CUSTOM_MED, DKIM_ADSP_CUSTOM_HIGH. Not more than one of
252 these rules can fire for messages that have one author (but see
253 below). The last three can only result from a 'signing-practices'
254 as given in a "adsp_override" directive (not from a DNS lookup),
255 and can serve as a convenient means of providing a different score
256 if scores assigned to DKIM_ADSP_ALL or DKIM_ADSP_DISCARD are not
257 considered suitable for some domains.
258 RFC 5322 permits a message to have more than one author - multiple
260 addresses may be listed in a single From header field. RFC 5617
261 defines that a message with multiple authors has multiple signing
262 domain signing practices, but does not prescribe how these should
263 be combined. In presence of multiple signing practices, more than
264 one of the DKIM_ADSP_* rules may fire.
265 As a precaution against firing DKIM_ADSP_* rules when there is a
267 known local reason for a signature verification failure, the
268 domain's ADSP is considered 'unknown' when DNS lookups are disabled
269 or a DNS lookup encountered a temporary problem on fetching a
270 public key from the author's domain. Similarly, ADSP is considered
271 'unknown' when this plugin did its own signature verification
272 (signatures were not passed to SA by a caller) and a metarule
273 __TRUNCATED was triggered, indicating the caller intentionally
274 passed a truncated message to SpamAssassin, which was a likely
275 reason for a signature verification failure.
276 Example:
278 adsp_override *.mydomain.example.com discardable
280 adsp_override *.neversends.example.com discardable
281 adsp_override ebay.com
283 adsp_override *.ebay.com
284 adsp_override ebay.co.uk
285 adsp_override *.ebay.co.uk
286 adsp_override paypal.com
287 adsp_override *.paypal.com
288 adsp_override amazon.com
289 adsp_override ealerts.bankofamerica.com
290 adsp_override americangreetings.com
291 adsp_override egreetings.com
292 adsp_override bluemountain.com
293 adsp_override hallmark.com all
294 adsp_override *.hallmark.com all
295 adsp_override youtube.com custom_high
296 adsp_override google.com custom_low
297 adsp_override gmail.com custom_low
298 adsp_override googlemail.com custom_low
299 adsp_override yahoo.com custom_low
300 adsp_override yahoo.com.au custom_low
301 adsp_override yahoo.se custom_low
302 adsp_override junkmailerkbw0rr.com nxdomain
304 adsp_override junkmailerd2hlsg.com nxdomain
305 # effectively disables ADSP network DNS lookups for all other domains:
307 adsp_override * unknown
308 score DKIM_ADSP_ALL 2.5
310 score DKIM_ADSP_DISCARD 25
311 score DKIM_ADSP_NXDOMAIN 3
312 score DKIM_ADSP_CUSTOM_LOW 1
314 score DKIM_ADSP_CUSTOM_MED 3.5
315 score DKIM_ADSP_CUSTOM_HIGH 8
316 dkim_minimum_key_bits n (default: 1024)
318 The smallest size of a signing key (in bits) for a valid signature
319 to be considered for whitelisting. Additionally, the eval function
320 check_dkim_valid() will return false on short keys when called with
321 explicitly listed domains, and the eval function
322 check_dkim_valid_author_sig() will return false on short keys
323 (regardless of its arguments). Setting the option to 0 disables a
324 key size check.
325 Note that the option has no effect when the eval function
327 check_dkim_valid() is called with no arguments (like in a rule
328 DKIM_VALID). A mere presence of some valid signature on a message
329 has no reputational value (without being associated with a
330 particular domain), regardless of its key size - anyone can prepend
331 its own signature on a copy of some third party mail and re-send
332 it, which makes it no more trustworthy than without such signature.
333 This is also a reason for a rule DKIM_VALID to have a near-zero
334 score, i.e. a rule hit is only informational.
335
337 dkim_timeout n (default: 5)
338 How many seconds to wait for a DKIM query to complete, before
339 scanning continues without the DKIM result. A numeric value is
340 optionally suffixed by a time unit (s, m, h, d, w, indicating
341 seconds (default), minutes, hours, days, weeks).
342perl v5.36.0 2022-07-23Mail::SpamAssassin::Plugin::DKIM(3)