Add DKIM documentation

diff --git a/doc/doc-docbook/spec.xfpt b/doc/doc-docbook/spec.xfpt
index 5e9f1ffbeaaf3308653138c1ff54039a6d3cc477..7add7e6041da3f201a088eccedd8516323a71040 100644 (file)
--- a/doc/doc-docbook/spec.xfpt
+++ b/doc/doc-docbook/spec.xfpt
@@ -1,4 +1,4 @@
-. $Cambridge: exim/doc/doc-docbook/spec.xfpt,v 1.53 2009/06/30 20:03:17 tom Exp $
+. $Cambridge: exim/doc/doc-docbook/spec.xfpt,v 1.54 2009/10/13 08:46:06 tom Exp $

 .
 . /////////////////////////////////////////////////////////////////////////////
 . This is the primary source of the Exim Manual. It is an xfpt document that is
 .
 . /////////////////////////////////////////////////////////////////////////////
 . This is the primary source of the Exim Manual. It is an xfpt document that is


@@ -34287,6 +34287,24 @@ ACL (acl_smtp_dkim), which can be called several times per message, with
 different signature context.
 .endlist
 
 different signature context.
 .endlist
 

+In typical Exim style, the verification implementation does not include any
+default "policy". Instead it enables you to build your own policy using
+Exim's standard controls.
+
+Please note that verification of DKIM signatures in incoming mail is turned
+on by default for logging purposes. For each signature in incoming email,
+exim will log a line displaying the most important signature details, and the
+signature status. Here is an example:
+.code
+2009-09-09 10:22:28 1MlIRf-0003LU-U3 DKIM: d=facebookmail.com s=q1-2009b c=relaxed/relaxed a=rsa-sha1 i=@facebookmail.com t=1252484542 [verification succeeded]
+.endd
+You might want to turn off DKIM verification processing entirely for internal
+or relay mail sources. To do that, set the &%dkim_disable_verify%& ACL
+control modifier. This should typically be done in the RCPT ACL, at points
+where you accept mail from relay sources (internal hosts or authenticated
+senders).
+
+

 .section "Signing outgoing messages" "SECID513"
 .cindex "DKIM" "signing"
 
 .section "Signing outgoing messages" "SECID513"
 .cindex "DKIM" "signing"
 


@@ -34296,19 +34314,19 @@ These options take (expandable) strings as arguments.
 .option dkim_domain smtp string&!! unset
 MANDATORY
 The domain you want to sign with. The result of this expanded
 .option dkim_domain smtp string&!! unset
 MANDATORY
 The domain you want to sign with. The result of this expanded

-option is put into the $dkim_domain expansion variable.
+option is put into the &%$dkim_domain%& expansion variable.

 
 .option dkim_selector smtp string&!! unset
 MANDATORY
 
 .option dkim_selector smtp string&!! unset
 MANDATORY

-This sets the key selector string. You can use the $dkim_domain expansion
+This sets the key selector string. You can use the &%$dkim_domain%& expansion

 variable to look up a matching selector. The result is put in the expansion
 variable to look up a matching selector. The result is put in the expansion

-variable $dkim_selector which should be used in the dkim_private_key option
-along with $dkim_domain.
+variable &%$dkim_selector%& which should be used in the &%dkim_private_key%&
+option along with &%$dkim_domain%&.

 
 .option dkim_private_key smtp string&!! unset
 MANDATORY
 
 .option dkim_private_key smtp string&!! unset
 MANDATORY

-This sets the private key to use. You can use the $dkim_domain and
-$dkim_selector expansion variables to determine the private key to use.
+This sets the private key to use. You can use the &%$dkim_domain%& and
+&%$dkim_selector%& expansion variables to determine the private key to use.

 The result can either
 .ilist
 be a valid RSA private key in ASCII armor, including line breaks.
 The result can either
 .ilist
 be a valid RSA private key in ASCII armor, including line breaks.


@@ -34317,7 +34335,8 @@ start with a slash, in which case it is treated as a file that contains
 the private key.
 .next
 be "0", "false" or the empty string, in which case the message will not
 the private key.
 .next
 be "0", "false" or the empty string, in which case the message will not

-be signed. This case will not result in an error, even if dkim_strict is set.
+be signed. This case will not result in an error, even if &%dkim_strict%&
+is set.

 .endlist
 
 .option dkim_canon smtp string&!! unset
 .endlist
 
 .option dkim_canon smtp string&!! unset


@@ -34325,22 +34344,180 @@ OPTIONAL
 This option sets the canonicalization method used when signing a message.
 The DKIM RFC currently supports two methods: "simple" and "relaxed".
 The option defaults to "relaxed" when unset. Note: the current implementation
 This option sets the canonicalization method used when signing a message.
 The DKIM RFC currently supports two methods: "simple" and "relaxed".
 The option defaults to "relaxed" when unset. Note: the current implementation

-only support using the same canonicalization method for both headers and body.
+only supports using the same canonicalization method for both headers and body.

 
 .option dkim_strict smtp string&!! unset
 OPTIONAL
 This  option  defines  how  Exim  behaves  when  signing a message that
 should be signed fails for some reason.  When the expansion evaluates to
 either "1" or "true", Exim will defer. Otherwise Exim will send the message
 
 .option dkim_strict smtp string&!! unset
 OPTIONAL
 This  option  defines  how  Exim  behaves  when  signing a message that
 should be signed fails for some reason.  When the expansion evaluates to
 either "1" or "true", Exim will defer. Otherwise Exim will send the message

-unsigned. You can use the $dkim_domain and $dkim_selector expansion
+unsigned. You can use the &%$dkim_domain%& and &%$dkim_selector%& expansion

 variables here.
 
 .option dkim_sign_headers smtp string&!! unset
 OPTIONAL
 When set, this option must expand to (or be specified as) a colon-separated
 variables here.
 
 .option dkim_sign_headers smtp string&!! unset
 OPTIONAL
 When set, this option must expand to (or be specified as) a colon-separated

-list of header names. These headers will be included in the message
-signature. When unspecified, the headers recommended in RFC4871 will be used.
+list of header names. Headers with these names will be included in the message
+signature. When unspecified, the header names recommended in RFC4871 will be
+used.
+
+
+.section "Verifying DKIM signatures in incoming mail" "SECID514"
+.cindex "DKIM" "verification"
+
+Verification of DKIM signatures in incoming email is implemented via the
+&%acl_smtp_dkim%& ACL. By default, this ACL is called once for each
+syntactically(!) correct signature in the incoming message.
+
+To evaluate the signature in the ACL a large number of expansion variables
+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
+more advanced policies. For that reason, the global option
+&%dkim_verify_signers%&, and a global expansion variable
+&%$dkim_signing_domains%& exist.

 
 

+The global 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_signing_domains%& already contains a colon-
+separated list of signer domains for the message. When &%dkim_verify_signers%&
+is not specified in the main configuration, it defaults as:
+.code
+dkim_verify_signers = $dkim_signing_domains
+.endd
+This leads to the default behaviour of calling &%acl_smtp_dkim%& for each
+DKIM signature in the message. Current DKIM verifiers may want to explicitly
+call the ACL for known domains or identities. This would be achieved as follows:
+.code
+dkim_verify_signers = paypal.com:ebay.com:$dkim_signing_domains
+.endd
+This would result in &%acl_smtp_dkim%& always being called for "paypal.com"
+and "ebay.com", plus all domains that have signatures in the message. You can
+also be more creative in constructing your policy. Example:
+.code
+dkim_verify_signers = $sender_address_domain:$dkim_signing_domains
+.endd
+
+Inside the &%acl_smtp_dkim%&, the following expansion variables are
+available (from most to least important):
+
+.vlist
+.vitem &%$dkim_verify_status%&
+A string describing the general status of the signature. One of
+.ilist
+&%none%&: There is no signature in the message for the current domain or
+identity.
+.next
+&%invalid%&: The signature could not be verified due to a processing error.
+More detail is available in &%$dkim_verify_reason%&.
+.next
+&%fail%&: Verification of the signature failed.  More detail is
+available in &%$dkim_verify_reason%&.
+.next
+&%pass%&: The signature passed verification. It is valid.
+.endlist
+.vitem &%$dkim_verify_reason%&
+A string giving a litte bit more detail when &%$dkim_verify_status%& is either
+"fail" or "invalid". One of
+.ilist
+&%pubkey_unavailable%& (when &%$dkim_verify_status%&="invalid"): The public
+key for the domain could not be retrieved. This may be a temporary problem.
+.next
+&%pubkey_syntax%& (when &%$dkim_verify_status%&="invalid"): The public key
+record for the domain is syntactically invalid.
+.next
+&%bodyhash_mismatch%& (when &%$dkim_verify_status%&="fail"): The calculated
+body hash does not match the one specified in the signature header. This
+means that the message body was modified in transit.
+.next
+&%signature_incorrect%& (when &%$dkim_verify_status%&="fail"): The signature
+could not be verified. This may mean that headers were modified,
+re-written or otherwise changed in a way which is incompatible with
+DKIM verification. It may of course also mean that the signature is forged.
+.endlist
+.vitem &%$dkim_domain%&
+The signing domain. IMPORTANT: This variable is only populated if there is
+ab actual signature in the message. It does NOT neccessarily carry the
+domain that is currently being evaluated. Please use the &%dkim_signers%& ACL
+condition for that.
+.vitem &%$dkim_identity%&
+The signing identity. IMPORTANT: This variable is only populated if there is
+ab actual signature in the message. It does NOT neccessarily carry the
+identity that is currently being evaluated. Please use the &%dkim_signers%& ACL
+condition for that.
+.vitem &%$dkim_selector%&
+The key record selector string
+.vitem &%$dkim_algo%&
+The algorithm used. One of 'rsa-sha1' or 'rsa-sha256'.
+.vitem &%$dkim_canon_body%&
+The body canonicalization method. One of 'relaxed' or 'simple'.
+.vitem &%dkim_canon_headers%&
+The header canonicalization method. One of 'relaxed' or 'simple'.
+.vitem &%$dkim_copiedheaders%&
+A transcript of headers and their values which are included in the signature
+(copied from the 'z=' tag of the signature).
+.vitem &%$dkim_bodylength%&
+The number of signed body bytes. If zero ("0"), the body is unsigned. If no
+limit was set by the signer, "9999999999999" is returned. This makes sure
+that this variable always expands to an integer value.
+.vitem &%$dkim_created%&
+UNIX timestamp reflecting the date and time when the signature was created.
+When this was not specified by the signer, "0" is returned.
+.vitem &%$dkim_expires%&
+UNIX timestamp reflecting the date and time when the signer wants the
+signature to be treated as "expired". When this was not specified by the
+signer, "9999999999999" is returned. This makes it possible to do useful
+integer size comparisons against this value.
+.vitem &%$dkim_headernames%&
+A colon-separated list of names of headers included in the signature.
+.vitem &%$dkim_key_testing%&
+"1" if the key record has the "testing" flag set, "0" if not.
+.vitem &%$dkim_key_nosubdomaining%&
+"1" if the key record forbids subdomaining, "0" otherwise.
+.vitem &%$dkim_key_srvtype%&
+Service type (tag s=) from the key record. Defaults to "*" if not specified
+in the key record.
+.vitem &%$dkim_key_granularity%&
+Key granularity (tag g=) from the key record. Defaults to "*" if not specified
+in the key record.
+.vitem &%$dkim_key_notes%&
+Notes from the key record (tag n=)
+.endlist
+
+In addition, two ACL conditions are provided:
+
+.vlist
+.vitem &%dkim_signers%&
+ACL condition that checks a colon-separated list of domains or identities
+for a match against the domain or identity that the ACL is currently verifying.
+This is typically used to restrict an ACL verb to a group of domains or identities, like:
+
+.code
+# Warn when message apparently from GMail has no signature at all
+warn log_message = GMail sender without DKIM signature
+     sender_domains = gmail.com
+     dkim_signers = gmail.com
+     dkim_status = none
+.endd
+
+.vitem &%dkim_status%&
+ACL condition that checks a colon-separated list of possible DKIM verification
+results agains the actual result of verification. This is typically used
+to restrict an ACL verb to a list of verification outcomes, like:
+
+.code
+deny message = Message from Paypal with invalid or missing signature
+     sender_domains = paypal.com:paypal.de
+     dkim_signers = paypal.com:paypal.de
+     dkim_status = none:invalid:fail
+.endd
+
+The possible status keywords are: 'none','invalid','fail' and 'pass'. Please
+see the documentation of the &%$dkim_verify_status%& expansion variable above
+for more information of what they mean.
+.endlist

 
 
 . ////////////////////////////////////////////////////////////////////////////
 
 
 . ////////////////////////////////////////////////////////////////////////////