NAME¶

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

SYNOPSIS¶

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

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 ARC signatures (Authenticated Received Chain, RFC 8617) from any signing domains:

 full   ARC_SIGNED            eval:check_arc_signed()
 full   ARC_VALID             eval:check_arc_valid()

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.

DESCRIPTION¶

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 module.

TAGS¶

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

  _DKIMIDENTITY_
    Agent or User Identifier (AUID) (the 'i' tag) from valid signatures;
  _DKIMDOMAIN_
    Signing Domain Identifier (SDID) (the 'd' tag) from valid signatures;
  _DKIMSELECTOR_
    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.

SEE ALSO¶

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

  http://dkimproxy.sourceforge.net/
  https://tools.ietf.org/rfc/rfc4871.txt
  https://tools.ietf.org/rfc/rfc4870.txt
  https://tools.ietf.org/rfc/rfc5617.txt
  https://datatracker.ietf.org/group/dkim/about/

USER SETTINGS¶

welcomelist_from_dkim author@example.com [signing-domain]

Previously whitelist_from_dkim which will work interchangeably until 4.1.

Works similarly to welcomelist_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 welcomelist entry is allowed per line, as in "welcomelist_from_rcvd". Multiple "welcomelist_from_dkim" lines are allowed. File-glob style characters are allowed for the From address (the first parameter), just like with "welcomelist_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 itself).

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 welcomelist requires a DKIM check to be made, network tests must be enabled.

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

  welcomelist_from_dkim joe@example.com
  welcomelist_from_dkim *@corp.example.com
  welcomelist_from_dkim *@*.example.com
    

Examples of welcomelisting based on third-party signatures:

  welcomelist_from_dkim jane@example.net      example.org
  welcomelist_from_dkim rick@info.example.net example.net
  welcomelist_from_dkim *@info.example.net    example.net
  welcomelist_from_dkim *@*                   mail7.remailer.example.com
  welcomelist_from_dkim *@*                   *.remailer.example.com
    

def_welcomelist_from_dkim author@example.com [signing-domain]

Previously def_whitelist_from_dkim which will work interchangeably until 4.1.

Same as "welcomelist_from_dkim", but used for the default welcomelist entries in the SpamAssassin distribution. The welcomelist score is lower, because these are often targets for abuse of public mailers which sign their mail.

unwelcomelist_from_dkim author@example.com [signing-domain]

Previously unwhitelist_from_dkim which will work interchangeably until 4.1.

Removes an email address with its corresponding signing-domain field from def_welcomelist_from_dkim and welcomelist_from_dkim tables, if it exists. Parameters to unwelcomelist_from_dkim must exactly match the parameters of a corresponding welcomelist_from_dkim or def_welcomelist_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 welcomelisting command, it must also be specified in the unwelcomelisting 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 subdomain).

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

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_ALL, DKIM_ADSP_DISCARD, DKIM_ADSP_CUSTOM_LOW, DKIM_ADSP_CUSTOM_MED, 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.

Example:

  adsp_override *.mydomain.example.com   discardable
  adsp_override *.neversends.example.com discardable
  adsp_override ebay.com
  adsp_override *.ebay.com
  adsp_override ebay.co.uk
  adsp_override *.ebay.co.uk
  adsp_override paypal.com
  adsp_override *.paypal.com
  adsp_override amazon.com
  adsp_override ealerts.bankofamerica.com
  adsp_override americangreetings.com
  adsp_override egreetings.com
  adsp_override bluemountain.com
  adsp_override hallmark.com   all
  adsp_override *.hallmark.com all
  adsp_override youtube.com    custom_high
  adsp_override google.com     custom_low
  adsp_override gmail.com      custom_low
  adsp_override googlemail.com custom_low
  adsp_override yahoo.com      custom_low
  adsp_override yahoo.com.au   custom_low
  adsp_override yahoo.se       custom_low
  adsp_override junkmailerkbw0rr.com nxdomain
  adsp_override junkmailerd2hlsg.com 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 welcomelisting. 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. This option is evaluated on ARC signatures checks as well.

ADMINISTRATOR SETTINGS¶

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).