Provided by: spamassassin_3.4.4-1ubuntu1_all bug


       Mail::SpamAssassin::Plugin::DKIM - perform DKIM verification tests


        loadplugin Mail::SpamAssassin::Plugin::DKIM [/path/to/]

       Taking into account signatures from any signing domains:

        full   DKIM_SIGNED           eval:check_dkim_signed()
        full   DKIM_VALID            eval:check_dkim_valid()
        full   DKIM_VALID_AU         eval:check_dkim_valid_author_sig()
        full   DKIM_VALID_EF         eval:check_dkim_valid_envelopefrom()

       Taking into account signatures from specified signing domains only: (quotes may be omitted
       on domain names consisting only of letters, digits, dots, and minus characters)

        full   DKIM_SIGNED_MY1       eval:check_dkim_signed('dom1','dom2',...)
        full   DKIM_VALID_MY1        eval:check_dkim_valid('dom1','dom2',...)
        full   DKIM_VALID_AU_MY1     eval:check_dkim_valid_author_sig('d1','d2',...)

        full   __DKIM_DEPENDABLE     eval:check_dkim_dependable()

       Author Domain Signing Practices (ADSP) from any author domains:

        header DKIM_ADSP_NXDOMAIN    eval:check_dkim_adsp('N')
        header DKIM_ADSP_ALL         eval:check_dkim_adsp('A')
        header DKIM_ADSP_DISCARD     eval:check_dkim_adsp('D')
        header DKIM_ADSP_CUSTOM_LOW  eval:check_dkim_adsp('1')
        header DKIM_ADSP_CUSTOM_MED  eval:check_dkim_adsp('2')
        header DKIM_ADSP_CUSTOM_HIGH eval:check_dkim_adsp('3')

       Author Domain Signing Practices (ADSP) from specified author domains only:

        header DKIM_ADSP_MY1         eval:check_dkim_adsp('*','dom1','dom2',...)

        describe DKIM_SIGNED   Message has a DKIM or DK signature, not necessarily valid
        describe DKIM_VALID    Message has at least one valid DKIM or DK signature
        describe DKIM_VALID_AU Message has a valid DKIM or DK signature from author's domain
        describe DKIM_VALID_EF Message has a valid DKIM or DK signature from envelope-from domain
        describe __DKIM_DEPENDABLE     A validation failure not attributable to truncation

        describe DKIM_ADSP_NXDOMAIN    Domain not in DNS and no valid author domain signature
        describe DKIM_ADSP_ALL         Domain signs all mail, no valid author domain signature
        describe DKIM_ADSP_DISCARD     Domain signs all mail and suggests discarding mail with no valid author domain signature, no valid author domain signature
        describe DKIM_ADSP_CUSTOM_LOW  adsp_override is CUSTOM_LOW, no valid author domain signature
        describe DKIM_ADSP_CUSTOM_MED  adsp_override is CUSTOM_MED, no valid author domain signature
        describe DKIM_ADSP_CUSTOM_HIGH adsp_override is CUSTOM_HIGH, no valid author domain signature

       For compatibility with pre-3.3.0 versions, the following are synonyms:

        OLD: eval:check_dkim_verified = NEW: eval:check_dkim_valid
        OLD: eval:check_dkim_signall  = NEW: eval:check_dkim_adsp('A')
        OLD: eval:check_dkim_signsome = NEW: redundant, semantically always true

       The __DKIM_DEPENDABLE eval rule deserves an explanation. The rule yields true when
       signatures are supplied by a caller, OR ELSE when signatures are obtained by this plugin
       AND either there are no signatures OR a rule __TRUNCATED was false. In other words:
       __DKIM_DEPENDABLE is true when failed signatures can not be attributed to message
       truncation when feeding a message to SpamAssassin.  It can be consulted to prevent false
       positives on large but truncated messages with poor man's implementation of ADSP by hand-
       crafted rules.


       This SpamAssassin plugin implements DKIM lookups as described by the RFC 4871, as well as
       historical DomainKeys lookups, as described by RFC 4870, thanks to the support for both
       types of signatures by newer versions of module Mail::DKIM.

       It requires the "Mail::DKIM" CPAN module to operate. Many thanks to Jason Long for that


       The following tags are added to the set, available for use in reports, header fields,
       other plugins, etc.:

           Agent or User Identifier (AUID) (the 'i' tag) from valid signatures;

           Signing Domain Identifier (SDID) (the 'd' tag) from valid signatures;

           DKIM selector (the 's' tag) from valid signatures;

       Identities and domains from signatures which failed verification are not included in these
       tags. Duplicates are eliminated (e.g. when there are two or more valid signatures from the
       same signer, only one copy makes it into a tag).  Note that there may be more than one
       signature in a message - currently they are provided as a space-separated list, although
       this behaviour may change.


       "Mail::DKIM" Mail::SpamAssassin::Plugin(3)


       whitelist_from_dkim [signing-domain]
           Works similarly to whitelist_from, except that in addition to matching an author
           address (From) to the pattern in the first parameter, the message must also carry a
           valid Domain Keys Identified Mail (DKIM) signature made by a signing domain (SDID,
           i.e. the d= tag) that is acceptable to us.

           Only one whitelist entry is allowed per line, as in "whitelist_from_rcvd".  Multiple
           "whitelist_from_dkim" lines are allowed. File-glob style characters are allowed for
           the From address (the first parameter), just like with "whitelist_from_rcvd".

           The second parameter (the signing-domain) does not accept full file-glob style
           wildcards, although a simple '*.' (or just a '.') prefix to a domain name is
           recognized and implies any subdomain of the specified domain (but not the domain

           If no signing-domain parameter is specified, the only acceptable signature will be an
           Author Domain Signature (sometimes called first-party signature) which is a signature
           where the signing domain (SDID) of a signature matches the domain of the author's
           address (i.e. the address in a From header field).

           Since this whitelist requires a DKIM check to be made, network tests must be enabled.

           Examples of whitelisting based on an author domain signature (first-party):

             whitelist_from_dkim *
             whitelist_from_dkim *@*

           Examples of whitelisting based on third-party signatures:

             whitelist_from_dkim *
             whitelist_from_dkim *@*         
             whitelist_from_dkim *@*                   *

       def_whitelist_from_dkim [signing-domain]
           Same as "whitelist_from_dkim", but used for the default whitelist entries in the
           SpamAssassin distribution.  The whitelist score is lower, because these are often
           targets for abuse of public mailers which sign their mail.

       unwhitelist_from_dkim [signing-domain]
           Removes an email address with its corresponding signing-domain field from
           def_whitelist_from_dkim and whitelist_from_dkim tables, if it exists.  Parameters to
           unwhitelist_from_dkim must exactly match the parameters of a corresponding
           whitelist_from_dkim or def_whitelist_from_dkim config option which created the entry,
           for it to be removed (a domain name is matched case-insensitively);  i.e. if a
           signing-domain parameter was specified in a whitelisting command, it must also be
           specified in the unwhitelisting command.

           Useful for removing undesired default entries from a distributed configuration by a
           local or site-specific configuration or by "user_prefs".

       adsp_override domain [signing-practices]
           Currently few domains publish their signing practices (RFC 5617 - ADSP), partly
           because the ADSP rfc is rather new, partly because they think hardly any recipient
           bothers to check it, and partly for fear that some recipients might lose mail due to
           problems in their signature validation procedures or mail mangling by mailers beyond
           their control.

           Nevertheless, recipients could benefit by knowing signing practices of a sending
           (author's) domain, for example to recognize forged mail claiming to be from certain
           domains which are popular targets for phishing, like financial institutions.
           Unfortunately, as signing practices are seldom published or are weak, it is hardly
           justifiable to look them up in DNS.

           To overcome this chicken-or-the-egg problem, the "adsp_override" mechanism allows
           recipients using SpamAssassin to override published or defaulted ADSP for certain
           domains. This makes it possible to manually specify a stronger (or weaker) signing
           practices than a signing domain is willing to publish (explicitly or by default), and
           also save on a DNS lookup.

           Note that ADSP (published or overridden) is only consulted for messages which do not
           contain a valid DKIM signature from the author's domain.

           According to RFC 5617, signing practices can be one of the following: "unknown", "all"
           and "discardable".

           "unknown": The domain might sign some or all email - messages from the domain may or
           may not have an Author Domain Signature. This is a default if a domain exists in DNS
           but no ADSP record is found.

           "all": All mail from the domain is signed with an Author Domain Signature.

           "discardable": All mail from the domain is signed with an Author Domain Signature.
           Furthermore, if a message arrives without a valid Author Domain Signature, the domain
           encourages the recipient(s) to discard it.

           ADSP lookup can also determine that a domain is "out of scope", i.e., the domain does
           not exist (NXDOMAIN) in the DNS.

           To override domain's signing practices in a SpamAssassin configuration file, specify
           an "adsp_override" directive for each sending domain to be overridden.

           Its first argument is a domain name. Author's domain is matched against it, matching
           is case insensitive. This is not a regular expression or a file-glob style wildcard,
           but limited wildcarding is still available: if this argument starts by a "*." (or is a
           sole "*"), author's domain matches if it is a subdomain (to one or more levels) of the
           argument. Otherwise (with no leading asterisk) the match must be exact (not a

           An optional second parameter is one of the following keywords (case-insensitive):
           "nxdomain", "unknown", "all", "discardable", "custom_low", "custom_med",

           Absence of this second parameter implies "discardable". If a domain is not listed by a
           "adsp_override" directive nor does it explicitly publish any ADSP record, then
           "unknown" is implied for valid domains, and "nxdomain" for domains not existing in
           DNS. (Note: domain validity is only checked with versions of Mail::DKIM 0.37 or later
           (actually since 0.36_5), the "nxdomain" would never turn up with older versions).

           The strong setting "discardable" is useful for domains which are known to always sign
           their mail and to always send it directly to recipients (not to mailing lists), and
           are frequent targets of fishing attempts, such as financial institutions. The
           "discardable" is also appropriate for domains which are known never to send any mail.

           When a message does not contain a valid signature by the author's domain (the domain
           in a From header field), the signing practices pertaining to author's domain determine
           which of the following rules fire and contributes its score: DKIM_ADSP_NXDOMAIN,
           DKIM_ADSP_CUSTOM_HIGH. Not more than one of these rules can fire for messages that
           have one author (but see below). The last three can only result from a
           'signing-practices' as given in a "adsp_override" directive (not from a DNS lookup),
           and can serve as a convenient means of providing a different score if scores assigned
           to DKIM_ADSP_ALL or DKIM_ADSP_DISCARD are not considered suitable for some domains.

           RFC 5322 permits a message to have more than one author - multiple addresses may be
           listed in a single From header field.  RFC 5617 defines that a message with multiple
           authors has multiple signing domain signing practices, but does not prescribe how
           these should be combined. In presence of multiple signing practices, more than one of
           the DKIM_ADSP_* rules may fire.

           As a precaution against firing DKIM_ADSP_* rules when there is a known local reason
           for a signature verification failure, the domain's ADSP is considered 'unknown' when
           DNS lookups are disabled or a DNS lookup encountered a temporary problem on fetching a
           public key from the author's domain. Similarly, ADSP is considered 'unknown' when this
           plugin did its own signature verification (signatures were not passed to SA by a
           caller) and a metarule __TRUNCATED was triggered, indicating the caller intentionally
           passed a truncated message to SpamAssassin, which was a likely reason for a signature
           verification failure.


             adsp_override *   discardable
             adsp_override * discardable

             adsp_override *
             adsp_override *
             adsp_override *
             adsp_override   all
             adsp_override * all
             adsp_override    custom_high
             adsp_override     custom_low
             adsp_override      custom_low
             adsp_override custom_low
             adsp_override      custom_low
             adsp_override   custom_low
             adsp_override       custom_low

             adsp_override nxdomain
             adsp_override nxdomain

             # effectively disables ADSP network DNS lookups for all other domains:
             adsp_override *              unknown

             score DKIM_ADSP_ALL          2.5
             score DKIM_ADSP_DISCARD     25
             score DKIM_ADSP_NXDOMAIN     3

             score DKIM_ADSP_CUSTOM_LOW   1
             score DKIM_ADSP_CUSTOM_MED   3.5
             score DKIM_ADSP_CUSTOM_HIGH  8

       dkim_minimum_key_bits n             (default: 1024)
           The smallest size of a signing key (in bits) for a valid signature to be considered
           for whitelisting. Additionally, the eval function check_dkim_valid() will return false
           on short keys when called with explicitly listed domains, and the eval function
           check_dkim_valid_author_sig() will return false on short keys (regardless of its
           arguments). Setting the option to 0 disables a key size check.

           Note that the option has no effect when the eval function check_dkim_valid() is called
           with no arguments (like in a rule DKIM_VALID). A mere presence of some valid signature
           on a message has no reputational value (without being associated with a particular
           domain), regardless of its key size - anyone can prepend its own signature on a copy
           of some third party mail and re-send it, which makes it no more trustworthy than
           without such signature. This is also a reason for a rule DKIM_VALID to have a near-
           zero score, i.e. a rule hit is only informational.


       dkim_timeout n             (default: 5)
           How many seconds to wait for a DKIM query to complete, before scanning continues
           without the DKIM result. A numeric value is optionally suffixed by a time unit (s, m,
           h, d, w, indicating seconds (default), minutes, hours, days, weeks).