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
during domain parsing. Maintained by Mozilla,
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
directory of this file is writable by the user
exim runs as.
-dmarc_forensic_sender The email address to use when sending a
+dmarc_forensic_sender Alternate email address to use when sending a
forensic report detailing alignment failures
if a sender domain's dmarc record specifies it
and you have configured Exim to send them.
- Default: do-not-reply@$default_hostname
+
+ If set, this is expanded and used for the
+ From: header line; the address is extracted
+ from it and used for the envelope from.
+ If not set, the From: header is expanded from
+ the dsn_from option, and <> is used for the
+ envelope from.
+
+ Default: unset.
3. By default, the DMARC processing will run for any remote,
The spool files can then be processed by external processes and then
requeued into exim spool directories for final delivery.
+However, note carefully the warnings in the main documentation on
+qpool file formats.
The motivation/inspiration for the transport is to allow external
processes to access email queued by exim and have access to all the
The transport only takes one option:
* directory - This is used to specify the directory messages should be
-copied to
+copied to. Expanded.
The generic transport options (body_only, current_directory, disable_logging,
debug_print, delivery_date_add, envelope_to_add, event_action, group,
Note that it would be wise to strip incoming messages of A-R headers
that claim to be from our own <admd-identifier>.
-There are two new variables: $arc_state and $arc_state_reason.
+There are four new variables:
+
+ $arc_state One of pass, fail, none
+ $arc_state_reason (if fail, why)
+ $arc_domains colon-sep list of ARC chain domains, in chain order.
+ problematic elements may have empty list elements
+ $arc_oldest_pass lowest passing instance number of chain
+
+Example:
+ logwrite = oldest-p-ams: <${reduce {$lh_ARC-Authentication-Results:} \
+ {} \
+ {${if = {$arc_oldest_pass} \
+ {${extract {i}{${extract {1}{;}{$item}}}}} \
+ {$item} {$value}}} \
+ }>
Receive log lines for an ARC pass will be tagged "ARC".
Signing
--
-arc_sign = <admd-identifier> : <selector> : <privkey>
+arc_sign = <admd-identifier> : <selector> : <privkey> [ : <options> ]
An option on the smtp transport, which constructs and prepends to the message
an ARC set of headers. The textually-first Authentication-Results: header
is used as a basis (you must have added one on entry to the ADMD).
Expanded as a whole; if unset, empty or forced-failure then no signing is done.
-If it is set, all three elements must be non-empty.
-
+If it is set, all of the first three elements must be non-empty.
+
+The fourth element is optional, and if present consists of a comma-separated list
+of options. The options implemented are
+
+ timestamps Add a t= tag to the generated AMS and AS headers, with the
+ current time.
+ expire[=<val>] Add an x= tag to the generated AMS header, with an expiry time.
+ If the value <val> is an plain number it is used unchanged.
+ If it starts with a '+' then the following number is added
+ to the current time, as an offset in seconds.
+ If a value is not given it defaults to a one month offset.
+
+[As of writing, gmail insist that a t= tag on the AS is mandatory]
+
+Caveats:
+ * There must be an Authentication-Results header, presumably added by an ACL
+ while receiving the message, for the same ADMD, for arc_sign to succeed.
+ This requires careful coordination between inbound and outbound logic.
+
+ Only one A-R header is taken account of. This is a limitation versus
+ the ARC spec (which says that all A-R headers from within the ADMD must
+ be used).
+
+ * If passing a message to another system, such as a mailing-list manager
+ (MLM), between receipt and sending, be wary of manipulations to headers made
+ by the MLM.
+ + For instance, Mailman with REMOVE_DKIM_HEADERS==3 might improve
+ deliverability in a pre-ARC world, but that option also renames the
+ Authentication-Results header, which breaks signing.
+
+ * Even if you use multiple DKIM keys for different domains, the ARC concept
+ should try to stick to one ADMD, so pick a primary domain and use that for
+ AR headers and outbound signing.
+
+Signing is not compatible with cutthrough delivery; any (before expansion)
+value set for the option will result in cutthrough delivery not being
+used via the transport in question.
+
+
+
+
+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.
+
+ Since a new hints DB is used, the hints DB maintenance should be updated
+ to additionally handle "tls".
+
+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 (OpenSSL: 1hr, and one previous key is used for
+ overlap; GnuTLS 6hr but does not specify any overlap).
+ Tickets have limited lifetime (2hr, and new ones issued after 1hr under
+ OpenSSL. GnuTLS 2hr, appears to not do overlap).
+
+ 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 bits 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 {}{}}.
+
+Issues:
+ In a resumed session:
+ $tls_{in,out}_cipher will have values different to the original (under GnuTLS)
+ $tls_{in,out}_ocsp will be "not requested" or "no response", and
+ hosts_require_ocsp will fail
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git://git.exim.org / users / jgh / exim.git / blobdiff