DKIM: preferences for verify algorithms

[exim.git] / doc / doc-docbook / spec.xfpt

diff --git a/doc/doc-docbook/spec.xfpt b/doc/doc-docbook/spec.xfpt
index aa39965052faf3ebbf1917dc56bc940c427884f0..8bba6feb21d04a198040005b8a3c4a1e9e06ba5d 100644 (file)
--- a/doc/doc-docbook/spec.xfpt
+++ b/doc/doc-docbook/spec.xfpt
@@ -14347,7 +14347,9 @@ listed in more than one group.
 See also the &'Policy controls'& section above.
 
 .table2
 See also the &'Policy controls'& section above.
 
 .table2

-.row &%dkim_verify_signers%&         "DKIM domain for which DKIM ACL is run"
+.row &%dkim_verify_hashes%&          "DKIM hash methods accepted for signatures"
+.row &%dkim_verify_keytypes%&        "DKIM key types accepted for signatures"
+.row &%dkim_verify_signers%&         "DKIM domains for which DKIM ACL is run"

 .row &%host_lookup%&                 "host name looked up for these hosts"
 .row &%host_lookup_order%&           "order of DNS and local name lookups"
 .row &%recipient_unqualified_hosts%& "may send unqualified recipients"
 .row &%host_lookup%&                 "host name looked up for these hosts"
 .row &%host_lookup_order%&           "order of DNS and local name lookups"
 .row &%recipient_unqualified_hosts%& "may send unqualified recipients"


@@ -15092,6 +15094,27 @@ etc. are ignored. If IP literals are enabled, the &(ipliteral)& router declines
 to handle IPv6 literal addresses.
 
 
 to handle IPv6 literal addresses.
 
 

+.new
+.option dkim_verify_hashes main "string list" "sha256 : sha512 : sha1"
+.cindex DKIM "selecting signature algorithms"
+This option gives a list of hash types which are acceptable in signatures,
+and an order of processing.
+Signatures with algorithms not in the list will be ignored.
+
+Note that the presence of sha1 violates RFC 8301.
+Signatures using the rsa-sha1 are however (as of writing) still common.
+The default inclusion of sha1 may be dropped in a future release.
+
+.option dkim_verify_keytypes main "string list" "ed25519 : rsa"
+This option gives a list of key types which are acceptable in signatures,
+and an order of processing.
+Signatures with algorithms not in the list will be ignored.
+
+.option dkim_verify_minimal main boolean false
+If set to true, verification of signatures will terminate after the
+first success.
+.wen
+

 .option dkim_verify_signers main "domain list&!!" $dkim_signers
 .cindex DKIM "controlling calls to the ACL"
 This option gives a list of DKIM domains for which the DKIM ACL is run.
 .option dkim_verify_signers main "domain list&!!" $dkim_signers
 .cindex DKIM "controlling calls to the ACL"
 This option gives a list of DKIM domains for which the DKIM ACL is run.


@@ -39913,15 +39936,28 @@ RFC 6376 lists these tags as RECOMMENDED.
 
 Verification of DKIM signatures in SMTP incoming email is done for all
 messages for which an ACL control &%dkim_disable_verify%& has not been set.
 
 Verification of DKIM signatures in SMTP incoming email is done for all
 messages for which an ACL control &%dkim_disable_verify%& has not been set.

+.new
+.cindex DKIM "selecting signature algorithms"
+Individual classes of signature algorithm can be ignored by changing
+the main options &%dkim_verify_hashes%& or &%dkim_verify_keytypes%&.
+The &%dkim_verify_minimal%& option can be set to cease verification
+processing for a message once the first passing signature is found.
+.wen
+

 .cindex authentication "expansion item"
 Performing verification sets up information used by the
 &$authresults$& expansion item.
 
 .cindex authentication "expansion item"
 Performing verification sets up information used by the
 &$authresults$& expansion item.
 

-The results of that verification are then made available to the
+.new
+For most purposes the default option settings suffice and the remainder
+of this section can be ignored.
+.wen
+
+The results of verification are made available to the

 &%acl_smtp_dkim%& ACL, which can examine and modify them.
 &%acl_smtp_dkim%& ACL, which can examine and modify them.

-By default, this ACL is called once for each
-syntactically(!) correct signature in the incoming message.

 A missing ACL definition defaults to accept.
 A missing ACL definition defaults to accept.

+By default, the ACL is called once for each
+syntactically(!) correct signature in the incoming message.

 If any ACL call does not accept, the message is not accepted.
 If a cutthrough delivery was in progress for the message, that is
 summarily dropped (having wasted the transmission effort).
 If any ACL call does not accept, the message is not accepted.
 If a cutthrough delivery was in progress for the message, that is
 summarily dropped (having wasted the transmission effort).


@@ -39932,11 +39968,11 @@ containing the signature status and its details are set up during the
 runtime of the ACL.
 
 Calling the ACL only for existing signatures is not sufficient to build
 runtime of the ACL.
 
 Calling the ACL only for existing signatures is not sufficient to build

-more advanced policies. For that reason, the global option
-&%dkim_verify_signers%&, and a global expansion variable
+more advanced policies. For that reason, the main option
+&%dkim_verify_signers%&, and an expansion variable

 &%$dkim_signers%& exist.
 
 &%$dkim_signers%& exist.
 

-The global option &%dkim_verify_signers%& can be set to a colon-separated
+The main option &%dkim_verify_signers%& can be set to a colon-separated

 list of DKIM domains or identities for which the ACL &%acl_smtp_dkim%& is
 called. It is expanded when the message has been received. At this point,
 the expansion variable &%$dkim_signers%& already contains a colon-separated
 list of DKIM domains or identities for which the ACL &%acl_smtp_dkim%& is
 called. It is expanded when the message has been received. At this point,
 the expansion variable &%$dkim_signers%& already contains a colon-separated


@@ -39974,7 +40010,7 @@ If multiple signatures match a domain (or identity), the ACL is called once
 for each matching signature.
 
 
 for each matching signature.
 
 

-Inside the &%acl_smtp_dkim%&, the following expansion variables are
+Inside the DKIM ACL, the following expansion variables are

 available (from most to least important):
 
 
 available (from most to least important):
 
 


@@ -40068,8 +40104,12 @@ DKIM signatures identified as having been signed with historic
 algorithms (currently, rsa-sha1) have permanently failed evaluation
 .endd
 
 algorithms (currently, rsa-sha1) have permanently failed evaluation
 .endd
 

-To enforce this you must have a DKIM ACL which checks this variable
-and overwrites the &$dkim_verify_status$& variable as discussed above.
+To enforce this you must either have a DKIM ACL which checks this variable
+and overwrites the &$dkim_verify_status$& variable as discussed above,
+.new
+or have set the main option &%dkim_verify_hashes%& to exclude
+processing of such signatures.
+.wen

 
 .vitem &%$dkim_canon_body%&
 The body canonicalization method. One of 'relaxed' or 'simple'.
 
 .vitem &%$dkim_canon_body%&
 The body canonicalization method. One of 'relaxed' or 'simple'.