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
+Early pipelining support
+------------------------
+Ref: https://datatracker.ietf.org/doc/draft-harris-early-pipe/
-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.
+If compiled with EXPERIMENTAL_PIPE_CONNECT support is included for this feature.
+The server advertises the feature in its EHLO response, currently using the name
+"X_PIPE_CONNECT" (this will change, some time in the future).
+A client may cache this information, along with the rest of the EHLO response,
+and use it for later connections. Those later ones can send esmtp commands before
+a banner is received.
-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"
+Up to 1.5 roundtrip times can be taken out of cleartext connections, 2.5 on
+STARTTLS connections.
-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]
+In combination with the traditional PIPELINING feature the following example
+sequences are possible (among others):
-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".
+(client) (server)
+
+EHLO,MAIL,RCPT,DATA ->
+ <- banner,EHLO-resp,MAIL-ack,RCPT-ack,DATA-goahead
+message-data ->
+------
+
+EHLO,MAIL,RCPT,BDAT ->
+ <- banner,EHLO-resp,MAIL-ack,RCPT-ack
+message-data ->
+------
+
+EHLO,STARTTLS ->
+ <- banner,EHLO-resp,TLS-goahead
+TLS1.2-client-hello ->
+ <- TLS-server-hello,cert,hello-done
+client-Kex,change-cipher,finished ->
+ <- change-cipher,finished
+EHLO,MAIL,RCPT,DATA ->
+ <- EHLO-resp,MAIL-ack,RCPT-ack,DATA-goahead
+
+------
+(tls-on-connect)
+TLS1.2-client-hello ->
+ <- TLS-server-hello,cert,hello-done
+client-Kex,change-cipher,finished ->
+ <- change-cipher,finshed
+ <- banner
+EHLO,MAIL,RCPT,DATA ->
+ <- EHLO-resp,MAIL-ack,RCPT-ack,DATA-goahead
+
+Where the initial client packet is SMTP, it can combine with the TCP Fast Open
+feature and be sent in the TCP SYN.
+
+
+A main-section option "pipelining_connect_advertise_hosts" (default: *)
+and an smtp transport option "hosts_pipe_connect" (default: unset)
+control the feature.
+
+If the "pipelining" log_selector is enabled, the "L" field in server <=
+log lines has a period appended if the feature was advertised but not used;
+or has an asterisk appended if the feature was used. In client => lines
+the "L" field has an asterisk appended if the feature was used.
+
+The "retry_data_expire" option controls cache invalidation.
+Entries are also rewritten (or cleared) if the adverised features
+change.
+
+
+NOTE: since the EHLO command must be constructed before the connection is
+made it cannot depend on the interface IP address that will be used.
+Transport configurations should be checked for this. An example avoidance:
+
+ helo_data = ${if def:sending_ip_address \
+ {${lookup dnsdb{>! ptr=$sending_ip_address} \
+ {${sg{$value} {^([^!]*).*\$} {\$1}}} fail}} \
+ {$primary_hostname}}
+
+
+
+
+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 {}{}}.
--------------------------------------------------------------
git://git.exim.org / exim.git / blobdiff