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lib/Mail/SpamAssassin/Plugin/DKIM.pm  view on Meta::CPAN

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 C<welcomelist_from_rcvd>.
Multiple C<welcomelist_from_dkim> lines are allowed. File-glob style characters
are allowed for the From address (the first parameter), just like with
C<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

=item def_welcomelist_from_dkim author@example.com [signing-domain]

Previously def_whitelist_from_dkim which will work interchangeably until 4.1.

Same as C<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.

=item 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 C<user_prefs>.

=item 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 C<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:
C<unknown>, C<all> and C<discardable>.

C<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.

C<all>: All mail from the domain is signed with an Author Domain Signature.

C<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 C<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): C<nxdomain>, C<unknown>, C<all>, C<discardable>,
C<custom_low>, C<custom_med>, C<custom_high>.

Absence of this second parameter implies C<discardable>. If a domain is not
listed by a C<adsp_override> directive nor does it explicitly publish any
ADSP record, then C<unknown> is implied for valid domains, and C<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 C<nxdomain>
would never turn up with older versions).