package controlled locations (/usr/include and /usr/lib).
-2. Use the following global settings to configure DMARC:
+2. Use the following global options to configure DMARC:
Required:
dmarc_tld_file Defines the location of a text file of valid
the most current version can be downloaded
from a link at http://publicsuffix.org/list/.
See also util/renew-opendmarc-tlds.sh script.
+ The default for the option is currently
+ /etc/exim/opendmarc.tlds
Optional:
dmarc_history_file Defines the location of a file to log results
-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, within 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). [this can _almost_ be done in
- transport option expansions, but not quite: it requires tha DANE-present
- but STARTTLS-failing targets fallback to cleartext, which current DANE
- coding specifically blocks]
-
-Note that REQUIRETLS is only advertised once a TLS connection is achieved
-(in contrast to STARTTLS). If you want to check the advertising, do something
-like "swaks -s 127.0.0.1 -tls -q HELO".
-
-
-
-
Early pipelining support
------------------------
Ref: https://datatracker.ietf.org/doc/draft-harris-early-pipe/
+TLS Session Resumption
+----------------------
+TLS Session Resumption for TLS 1.2 and TLS 1.3 connections can be used (defined
+in RFC 5077 for 1.2). The support for this can be included by building with
+EXPERIMENTAL_TLS_RESUME defined. This requires GnuTLS 3.6.3 or OpenSSL 1.1.1
+(or later).
+
+Session resumption (this is the "stateless" variant) involves the server sending
+a "session ticket" to the client on one connection, which can be stored by the
+client and used for a later session. The ticket contains sufficient state for
+the server to reconstruct the TLS session, avoiding some expensive crypto
+calculation and one full packet roundtrip time.
+
+Operational cost/benefit:
+ The extra data being transmitted costs a minor amount, and the client has
+extra costs in storing and retrieving the data.
+
+In the Exim/Gnutls implementation the extra cost on an initial connection
+which is TLS1.2 over a loopback path is about 6ms on 2017-laptop class hardware.
+The saved cost on a subsequent connection is about 4ms; three or more
+connections become a net win. On longer network paths, two or more
+connections will have an average lower startup time thanks to the one
+saved packet roundtrip. TLS1.3 will save the crypto cpu costs but not any
+packet roundtrips.
+
+Security aspects:
+ The session ticket is encrypted, but is obviously an additional security
+vulnarability surface. An attacker able to decrypt it would have access
+all connections using the resumed session.
+The session ticket encryption key is not committed to storage by the server
+and is rotated regularly. Tickets have limited lifetime.
+
+There is a question-mark over the security of the Diffie-Helman parameters
+used for session negotiation. TBD. q-value; cf bug 1895
+
+Observability:
+ New log_selector "tls_resumption", appends an asterisk to the tls_cipher "X="
+element.
+
+Variables $tls_{in,out}_resumption have bit 0-4 indicating respectively
+support built, client requested ticket, client offered session,
+server issued ticket, resume used. A suitable decode list is provided
+in the builtin macro _RESUME_DECODE for ${listextract {}{}}.
+
+
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git://git.exim.org / exim.git / blobdiff