&*Note*&: Under current versions of OpenSSL, when a list of more than one
file is used, the &$tls_in_ourcert$& veriable is unreliable.
+
+&*Note*&: OCSP stapling is not usable when a list of more than one file is used.
.wen
If the option contains &$tls_out_sni$& and Exim is built against OpenSSL, then
Usable for GnuTLS 3.4.4 or 3.3.17 or OpenSSL 1.1.0 (or later).
+.new
+&*Note*&: There is currently no support for multiple OCSP proofs to match the
+multiple certificates facility.
+.wen
+
.option tls_on_connect_ports main "string list" unset
.cindex SSMTP
If the &%tls_verify_certificates%& option is set on the &(smtp)& transport, it
specifies a collection of expected server certificates.
-These may be the system default set (depending on library version),
-a file or,
-depending on library version, a directory,
-must name a file or,
-for OpenSSL only (not GnuTLS), a directory.
+These may be
+the system default set (depending on library version),
+a file,
+or (depending on library version) a directory.
The client verifies the server's certificate
against this collection, taking into account any revoked certificates that are
in the list defined by &%tls_crl%&.
default "policy". Instead it enables you to build your own policy using
Exim's standard controls.
+.new
Please note that verification of DKIM signatures in incoming mail is turned
-on by default for logging purposes. For each signature in incoming email,
+on by default for logging (in the <= line) purposes.
+
+Additional log detail can be enabled using the &%dkim_verbose%& log_selector.
+When set, 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 (with line-breaks added for clarity):
.code
c=relaxed/relaxed a=rsa-sha1
i=@facebookmail.com t=1252484542 [verification succeeded]
.endd
+.wen
+
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
.section "Signing outgoing messages" "SECDKIMSIGN"
.cindex "DKIM" "signing"
+.new
+For signing to be usable you must have published a DKIM record in DNS.
+Note that RFC 8301 says:
+.code
+rsa-sha1 MUST NOT be used for signing or verifying.
+
+Signers MUST use RSA keys of at least 1024 bits for all keys.
+Signers SHOULD use RSA keys of at least 2048 bits.
+.endd
+.wen
+.wen
+
Signing is enabled by setting private options on the SMTP transport.
These options take (expandable) strings as arguments.
Each element in turn is put into the &%$dkim_domain%& expansion variable
while expanding the remaining signing options.
.wen
-If it is empty after expansion, DKIM signing is not done.
+If it is empty after expansion, DKIM signing is not done,
+and no error will result even if &%dkim_strict%& is set.
.option dkim_selector smtp string list&!! unset
This sets the key selector string.
Each element in turn is put in the expansion
variable &%$dkim_selector%& which may be used in the &%dkim_private_key%&
option along with &%$dkim_domain%&.
-If the option is empty after expansion, DKIM signing is not done for this domain.
.wen
+If the option is empty after expansion, DKIM signing is not done for this domain,
+and no error will result even if &%dkim_strict%& is set.
.option dkim_private_key smtp string&!! unset
This sets the private key to use.
be signed. This case will not result in an error, even if &%dkim_strict%&
is set.
.endlist
-If the option is empty after expansion, DKIM signing is not done.
.new
+Note that RFC 8301 says:
+.code
+Signers MUST use RSA keys of at least 1024 bits for all keys.
+Signers SHOULD use RSA keys of at least 2048 bits.
+.endd
+.wen
+
.option dkim_hash smtp string&!! sha256
Can be set alternatively to &"sha1"& to use an alternate hash
-method. Note that sha1 is now condidered insecure, and deprecated.
+method.
+
+.new
+Note that RFC 8301 says:
+.code
+rsa-sha1 MUST NOT be used for signing or verifying.
+.endd
+.wen
.option dkim_identity smtp string&!! unset
If set after expansion, the value is used to set an "i=" tag in
.endlist
.new
-This variable can be overwritten using an ACL 'set' modifier.
+This variable can be overwritten, with any value, using an ACL 'set' modifier.
.wen
.vitem &%$dkim_domain%&
.vitem &%$dkim_algo%&
The algorithm used. One of 'rsa-sha1' or 'rsa-sha256'.
+.new
+Note that RFC 8301 says:
+.code
+rsa-sha1 MUST NOT be used for signing or verifying.
+
+DKIM signatures identified as having been signed with historic
+algorithms (currently, rsa-sha1) have permanently failed evaluation
+.endd
+
+To enforce this you must have a DKIM ACL which checks this variable
+and overwrites the &$dkim_verify_status$& variable as discussed above.
+.wen
+
.vitem &%$dkim_canon_body%&
The body canonicalization method. One of 'relaxed' or 'simple'.
.vitem &%$dkim_key_length%&
Number of bits in the key.
+
+.new
+Note that RFC 8301 says:
+.code
+Verifiers MUST NOT consider signatures using RSA keys of
+less than 1024 bits as valid signatures.
+.endd
+
+To enforce this you must have a DKIM ACL which checks this variable
+and overwrites the &$dkim_verify_status$& variable as discussed above.
+.wen
+
.endlist
In addition, two ACL conditions are provided:
git://git.exim.org / exim.git / blobdiff