Support REQUIRETLS

[exim.git] / doc / doc-txt / experimental-spec.txt

diff --git a/doc/doc-txt/experimental-spec.txt b/doc/doc-txt/experimental-spec.txt
index aa93e07bf7da4933dea9d8bb5f13d7e199049924..43f14237bc502097d9dae4ab7ae7ddf6e3494c8d 100644 (file)
--- a/doc/doc-txt/experimental-spec.txt
+++ b/doc/doc-txt/experimental-spec.txt
@@ -860,6 +860,37 @@ used via the transport in question.
 
 
 
+
+REQUIRETLS support
+------------------
+Ref: https://tools.ietf.org/html/draft-ietf-uta-smtp-require-tls-03
+
+If compiled with EXPERIMENTAL_REQUIRETLS support is included for this
+feature, where a REQUIRETLS option is added to the MAIL command.
+The client may not retry in clear if the MAIL+REQUIRETLS fails (or was never
+offered), and the server accepts an obligation that any onward transmission
+by SMTP of the messages accepted will also use REQUIRETLS - or generate a
+fail DSN.
+
+The Exim implementation includes
+- a main-part option tls_advertise_requiretls; host list, default "*"
+- an observability variable $requiretls returning yes/no
+- an ACL "control = requiretls" modifier for setting the requirement
+- Log lines and Received: headers capitalise the S in the protocol
+  element: "P=esmtpS"
+
+Differences from spec:
+- we support upgrading the requirement for REQUIRETLS, including adding
+  it from cold, withing an MTA.  The spec only define the sourcing MUA
+  as being able to source the requirement, and makes no mention of upgrade.
+- No support is coded for the RequireTLS header (which can be used
+  to annul DANE and/or STS policiy). [can this be done in ACL?]
+
+Note that REQUIRETLS is only advertised once a TLS connection is acheived
+(in contrast to STARTTLS).  If you want to check the advertising, do something
+like "swaks -s 127.0.0.1 -tls -q HELO".
+
+
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