While a feature is experimental, there will be a build-time
option whose name starts "EXPERIMENTAL_" that must be set in
order to include the feature. This file contains information
-about experimenatal features, all of which are unstable and
-liable to incompatibile change.
+about experimental features, all of which are unstable and
+liable to incompatible change.
-OCSP Stapling support
---------------------------------------------------------------
-
-X509 PKI certificates expire and can be revoked; to handle this, the
-clients need some way to determine if a particular certificate, from a
-particular Certificate Authority (CA), is still valid. There are three
-main ways to do so.
-
-The simplest way is to serve up a Certificate Revocation List (CRL) with
-an ordinary web-server, regenerating the CRL before it expires. The
-downside is that clients have to periodically re-download a potentially
-huge file from every certificate authority it knows of.
-
-The way with most moving parts at query time is Online Certificate
-Status Protocol (OCSP), where the client verifies the certificate
-against an OCSP server run by the CA. This lets the CA track all
-usage of the certs. This requires running software with access to the
-private key of the CA, to sign the responses to the OCSP queries. OCSP
-is based on HTTP and can be proxied accordingly.
-
-The only widespread OCSP server implementation (known to this writer)
-comes as part of OpenSSL and aborts on an invalid request, such as
-connecting to the port and then disconnecting. This requires
-re-entering the passphrase each time some random client does this.
-
-The third way is OCSP Stapling; in this, the server using a certificate
-issued by the CA periodically requests an OCSP proof of validity from
-the OCSP server, then serves it up inline as part of the TLS
-negotiation. This approach adds no extra round trips, does not let the
-CA track users, scales well with number of certs issued by the CA and is
-resilient to temporary OCSP server failures, as long as the server
-starts retrying to fetch an OCSP proof some time before its current
-proof expires. The downside is that it requires server support.
-
-If Exim is built with EXPERIMENTAL_OCSP and it was built with OpenSSL,
-then it gains one new option: "tls_ocsp_file".
-
-The file specified therein is expected to be in DER format, and contain
-an OCSP proof. Exim will serve it as part of the TLS handshake. This
-option will be re-expanded for SNI, if the tls_certificate option
-contains $tls_sni, as per other TLS options.
-
-Exim does not at this time implement any support for fetching a new OCSP
-proof. The burden is on the administrator to handle this, outside of
-Exim. The file specified should be replaced atomically, so that the
-contents are always valid. Exim will expand the "tls_ocsp_file" option
-on each connection, so a new file will be handled transparently on the
-next connection.
-
-Exim will check for a validity next update timestamp in the OCSP proof;
-if not present, or if the proof has expired, it will be ignored.
-
-At this point in time, we're gathering feedback on use, to determine if
-it's worth adding complexity to the Exim daemon to periodically re-fetch
-OCSP files and somehow handling multiple files.
-
-
-
-
-Brightmail AntiSpam (BMI) suppport
+Brightmail AntiSpam (BMI) support
--------------------------------------------------------------
Brightmail AntiSpam is a commercial package. Please see
1) Adding support for BMI at compile time
To compile with BMI support, you need to link Exim against
- the Brighmail client SDK, consisting of a library
+ the Brightmail client SDK, consisting of a library
(libbmiclient_single.so) and a header file (bmi_api.h).
You'll also need to explicitly set a flag in the Makefile to
include BMI support in the Exim binary. Both can be achieved
-Sender Policy Framework (SPF) support
---------------------------------------------------------------
-
-To learn more about SPF, visit http://www.openspf.org. This
-document does not explain the SPF fundamentals, you should
-read and understand the implications of deploying SPF on your
-system before doing so.
-
-SPF support is added via the libspf2 library. Visit
-
- http://www.libspf2.org/
-
-to obtain a copy, then compile and install it. By default,
-this will put headers in /usr/local/include and the static
-library in /usr/local/lib.
-
-To compile Exim with SPF support, set these additional flags in
-Local/Makefile:
-
-EXPERIMENTAL_SPF=yes
-CFLAGS=-DSPF -I/usr/local/include
-EXTRALIBS_EXIM=-L/usr/local/lib -lspf2
-
-This assumes that the libspf2 files are installed in
-their default locations.
-
-You can now run SPF checks in incoming SMTP by using the "spf"
-ACL condition in either the MAIL, RCPT or DATA ACLs. When
-using it in the RCPT ACL, you can make the checks dependend on
-the RCPT address (or domain), so you can check SPF records
-only for certain target domains. This gives you the
-possibility to opt-out certain customers that do not want
-their mail to be subject to SPF checking.
-
-The spf condition takes a list of strings on its right-hand
-side. These strings describe the outcome of the SPF check for
-which the spf condition should succeed. Valid strings are:
-
- o pass The SPF check passed, the sending host
- is positively verified by SPF.
- o fail The SPF check failed, the sending host
- is NOT allowed to send mail for the domain
- in the envelope-from address.
- o softfail The SPF check failed, but the queried
- domain can't absolutely confirm that this
- is a forgery.
- o none The queried domain does not publish SPF
- records.
- o neutral The SPF check returned a "neutral" state.
- This means the queried domain has published
- a SPF record, but wants to allow outside
- servers to send mail under its domain as well.
- o err_perm This indicates a syntax error in the SPF
- record of the queried domain. This should be
- treated like "none".
- o err_temp This indicates a temporary error during all
- processing, including Exim's SPF processing.
- You may defer messages when this occurs.
-
-You can prefix each string with an exclamation mark to invert
-is meaning, for example "!fail" will match all results but
-"fail". The string list is evaluated left-to-right, in a
-short-circuit fashion. When a string matches the outcome of
-the SPF check, the condition succeeds. If none of the listed
-strings matches the outcome of the SPF check, the condition
-fails.
-
-Here is an example to fail forgery attempts from domains that
-publish SPF records:
-
-/* -----------------
-deny message = $sender_host_address is not allowed to send mail from ${if def:sender_address_domain {$sender_address_domain}{$sender_helo_name}}. \
- Please see http://www.openspf.org/Why?scope=${if def:sender_address_domain {mfrom}{helo}};identity=${if def:sender_address_domain {$sender_address}{$sender_helo_name}};ip=$sender_host_address
- spf = fail
---------------------- */
-
-You can also give special treatment to specific domains:
-
-/* -----------------
-deny message = AOL sender, but not from AOL-approved relay.
- sender_domains = aol.com
- spf = fail:neutral
---------------------- */
-
-Explanation: AOL publishes SPF records, but is liberal and
-still allows non-approved relays to send mail from aol.com.
-This will result in a "neutral" state, while mail from genuine
-AOL servers will result in "pass". The example above takes
-this into account and treats "neutral" like "fail", but only
-for aol.com. Please note that this violates the SPF draft.
-
-When the spf condition has run, it sets up several expansion
-variables.
-
- $spf_header_comment
- This contains a human-readable string describing the outcome
- of the SPF check. You can add it to a custom header or use
- it for logging purposes.
-
- $spf_received
- This contains a complete Received-SPF: header that can be
- added to the message. Please note that according to the SPF
- draft, this header must be added at the top of the header
- list. Please see section 10 on how you can do this.
-
- Note: in case of "Best-guess" (see below), the convention is
- to put this string in a header called X-SPF-Guess: instead.
-
- $spf_result
- This contains the outcome of the SPF check in string form,
- one of pass, fail, softfail, none, neutral, err_perm or
- err_temp.
-
- $spf_smtp_comment
- This contains a string that can be used in a SMTP response
- to the calling party. Useful for "fail".
-
-In addition to SPF, you can also perform checks for so-called
-"Best-guess". Strictly speaking, "Best-guess" is not standard
-SPF, but it is supported by the same framework that enables SPF
-capability. Refer to http://www.openspf.org/FAQ/Best_guess_record
-for a description of what it means.
-
-To access this feature, simply use the spf_guess condition in place
-of the spf one. For example:
-
-/* -----------------
-deny message = $sender_host_address doesn't look trustworthy to me
- spf_guess = fail
---------------------- */
-
-In case you decide to reject messages based on this check, you
-should note that although it uses the same framework, "Best-guess"
-is NOT SPF, and therefore you should not mention SPF at all in your
-reject message.
-
-When the spf_guess condition has run, it sets up the same expansion
-variables as when spf condition is run, described above.
-
-Additionally, since Best-guess is not standarized, you may redefine
-what "Best-guess" means to you by redefining spf_guess variable in
-global config. For example, the following:
-
-/* -----------------
-spf_guess = v=spf1 a/16 mx/16 ptr ?all
---------------------- */
-
-would relax host matching rules to a broader network range.
-
-
SRS (Sender Rewriting Scheme) Support
--------------------------------------------------------------
Exiscan currently includes SRS support via Miles Wilton's
libsrs_alt library. The current version of the supported
-library is 0.5.
+library is 0.5, there are reports of 1.0 working.
In order to use SRS, you must get a copy of libsrs_alt from
-http://srs.mirtol.com/
+https://opsec.eu/src/srs/
+
+(not the original source, which has disappeared.)
Unpack the tarball, then refer to MTAs/README.EXIM
to proceed. You need to set
in your Local/Makefile.
+
DCC Support
--------------------------------------------------------------
+Distributed Checksum Clearinghouse; http://www.rhyolite.com/dcc/
*) Building exim
After that "$dcc_header" contains the X-DCC-Header.
-Returnvalues are:
+Return values are:
fail for overall "R", "G" from dccifd
defer for overall "T" from dccifd
accept for overall "A", "S" from dccifd
If you want to pass even more headers in the middle of the
DATA stage you can set
$acl_m_dcc_add_header
-to tell the DCC routines add more information; eg, you might set
+to tell the DCC routines to add more information; eg, you might set
this to some results from ClamAV. Be careful. Header syntax is
not checked and is added "as is".
+In case you've troubles with sites sending the same queue items from several
+hosts and fail to get through greylisting you can use
+$acl_m_dcc_override_client_ip
+
+Setting $acl_m_dcc_override_client_ip to an IP address overrides the default
+of $sender_host_address. eg. use the following ACL in DATA stage:
+
+ warn set acl_m_dcc_override_client_ip = \
+ ${lookup{$sender_helo_name}nwildlsearch{/etc/mail/multipleip_sites}{$value}{}}
+ condition = ${if def:acl_m_dcc_override_client_ip}
+ log_message = dbg: acl_m_dcc_override_client_ip set to \
+ $acl_m_dcc_override_client_ip
+
+Then set something like
+# cat /etc/mail/multipleip_sites
+mout-xforward.gmx.net 82.165.159.12
+mout.gmx.net 212.227.15.16
+
+Use a reasonable IP. eg. one the sending cluster actually uses.
+
+
+
+DMARC Support
+--------------------------------------------------------------
+
+DMARC combines feedback from SPF, DKIM, and header From: in order
+to attempt to provide better indicators of the authenticity of an
+email. This document does not explain the fundamentals, you
+should read and understand how it works by visiting the website at
+http://www.dmarc.org/.
+
+DMARC support is added via the libopendmarc library. Visit:
+
+ http://sourceforge.net/projects/opendmarc/
+
+to obtain a copy, or find it in your favorite rpm package
+repository. If building from source, this description assumes
+that headers will be in /usr/local/include, and that the libraries
+are in /usr/local/lib.
+
+1. To compile Exim with DMARC support, you must first enable SPF.
+Please read the Local/Makefile comments on enabling the SUPPORT_SPF
+feature. You must also have DKIM support, so you cannot set the
+DISABLE_DKIM feature. Once both of those conditions have been met
+you can enable DMARC in Local/Makefile:
+
+EXPERIMENTAL_DMARC=yes
+LDFLAGS += -lopendmarc
+# CFLAGS += -I/usr/local/include
+# LDFLAGS += -L/usr/local/lib
+
+The first line sets the feature to include the correct code, and
+the second line says to link the libopendmarc libraries into the
+exim binary. The commented out lines should be uncommented if you
+built opendmarc from source and installed in the default location.
+Adjust the paths if you installed them elsewhere, but you do not
+need to uncomment them if an rpm (or you) installed them in the
+package controlled locations (/usr/include and /usr/lib).
+
+
+2. Use the following global settings to configure DMARC:
+
+Required:
+dmarc_tld_file Defines the location of a text file of valid
+ top level domains the opendmarc library uses
+ 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.
+
+Optional:
+dmarc_history_file Defines the location of a file to log results
+ of dmarc verification on inbound emails. The
+ contents are importable by the opendmarc tools
+ which will manage the data, send out DMARC
+ reports, and expire the data. Make sure the
+ directory of this file is writable by the user
+ exim runs as.
+
+dmarc_forensic_sender The 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
+
+
+3. By default, the DMARC processing will run for any remote,
+non-authenticated user. It makes sense to only verify DMARC
+status of messages coming from remote, untrusted sources. You can
+use standard conditions such as hosts, senders, etc, to decide that
+DMARC verification should *not* be performed for them and disable
+DMARC with a control setting:
+
+ control = dmarc_disable_verify
+
+A DMARC record can also specify a "forensic address", which gives
+exim an email address to submit reports about failed alignment.
+Exim does not do this by default because in certain conditions it
+results in unintended information leakage (what lists a user might
+be subscribed to, etc). You must configure exim to submit forensic
+reports to the owner of the domain. If the DMARC record contains a
+forensic address and you specify the control statement below, then
+exim will send these forensic emails. It's also advised that you
+configure a dmarc_forensic_sender because the default sender address
+construction might be inadequate.
+
+ control = dmarc_enable_forensic
+
+(AGAIN: You can choose not to send these forensic reports by simply
+not putting the dmarc_enable_forensic control line at any point in
+your exim config. If you don't tell it to send them, it will not
+send them.)
+
+There are no options to either control. Both must appear before
+the DATA acl.
+
+
+4. You can now run DMARC checks in incoming SMTP by using the
+"dmarc_status" ACL condition in the DATA ACL. You are required to
+call the spf condition first in the ACLs, then the "dmarc_status"
+condition. Putting this condition in the ACLs is required in order
+for a DMARC check to actually occur. All of the variables are set
+up before the DATA ACL, but there is no actual DMARC check that
+occurs until a "dmarc_status" condition is encountered in the ACLs.
+
+The dmarc_status condition takes a list of strings on its
+right-hand side. These strings describe recommended action based
+on the DMARC check. To understand what the policy recommendations
+mean, refer to the DMARC website above. Valid strings are:
+
+ o accept The DMARC check passed and the library recommends
+ accepting the email.
+ o reject The DMARC check failed and the library recommends
+ rejecting the email.
+ o quarantine The DMARC check failed and the library recommends
+ keeping it for further inspection.
+ o none The DMARC check passed and the library recommends
+ no specific action, neutral.
+ o norecord No policy section in the DMARC record for this
+ sender domain.
+ o nofrom Unable to determine the domain of the sender.
+ o temperror Library error or dns error.
+ o off The DMARC check was disabled for this email.
+
+You can prefix each string with an exclamation mark to invert its
+meaning, for example "!accept" will match all results but
+"accept". The string list is evaluated left-to-right in a
+short-circuit fashion. When a string matches the outcome of the
+DMARC check, the condition succeeds. If none of the listed
+strings matches the outcome of the DMARC check, the condition
+fails.
+
+Of course, you can also use any other lookup method that Exim
+supports, including LDAP, Postgres, MySQL, etc, as long as the
+result is a list of colon-separated strings.
+
+Performing the check sets up information used by the
+${authresults } expansion item.
+
+Several expansion variables are set before the DATA ACL is
+processed, and you can use them in this ACL. The following
+expansion variables are available:
+
+ o $dmarc_status
+ This is a one word status indicating what the DMARC library
+ thinks of the email. It is a combination of the results of
+ DMARC record lookup and the SPF/DKIM/DMARC processing results
+ (if a DMARC record was found). The actual policy declared
+ in the DMARC record is in a separate expansion variable.
+
+ o $dmarc_status_text
+ This is a slightly longer, human readable status.
+
+ o $dmarc_used_domain
+ This is the domain which DMARC used to look up the DMARC
+ policy record.
+
+ o $dmarc_domain_policy
+ This is the policy declared in the DMARC record. Valid values
+ are "none", "reject" and "quarantine". It is blank when there
+ is any error, including no DMARC record.
+
+A now-redundant variable $dmarc_ar_header has now been withdrawn.
+Use the ${authresults } expansion instead.
+
+
+5. How to enable DMARC advanced operation:
+By default, Exim's DMARC configuration is intended to be
+non-intrusive and conservative. To facilitate this, Exim will not
+create any type of logging files without explicit configuration by
+you, the admin. Nor will Exim send out any emails/reports about
+DMARC issues without explicit configuration by you, the admin (other
+than typical bounce messages that may come about due to ACL
+processing or failure delivery issues).
+
+In order to log statistics suitable to be imported by the opendmarc
+tools, you need to:
+a. Configure the global setting dmarc_history_file.
+b. Configure cron jobs to call the appropriate opendmarc history
+ import scripts and truncating the dmarc_history_file.
+
+In order to send forensic reports, you need to:
+a. Configure the global setting dmarc_forensic_sender.
+b. Configure, somewhere before the DATA ACL, the control option to
+ enable sending DMARC forensic reports.
+
+
+6. Example usage:
+(RCPT ACL)
+ warn domains = +local_domains
+ hosts = +local_hosts
+ control = dmarc_disable_verify
+
+ warn !domains = +screwed_up_dmarc_records
+ control = dmarc_enable_forensic
+
+ warn condition = (lookup if destined to mailing list)
+ set acl_m_mailing_list = 1
+
+(DATA ACL)
+ warn dmarc_status = accept : none : off
+ !authenticated = *
+ log_message = DMARC DEBUG: $dmarc_status $dmarc_used_domain
+
+ warn dmarc_status = !accept
+ !authenticated = *
+ log_message = DMARC DEBUG: '$dmarc_status' for $dmarc_used_domain
+
+ warn dmarc_status = quarantine
+ !authenticated = *
+ set $acl_m_quarantine = 1
+ # Do something in a transport with this flag variable
+
+ deny condition = ${if eq{$dmarc_domain_policy}{reject}}
+ condition = ${if eq{$acl_m_mailing_list}{1}}
+ message = Messages from $dmarc_used_domain break mailing lists
+
+ deny dmarc_status = reject
+ !authenticated = *
+ message = Message from $dmarc_used_domain failed sender's DMARC policy, REJECT
+
+ warn add_header = :at_start:${authresults {$primary_hostname}}
+
+
+
+DSN extra information
+---------------------
+If compiled with EXPERIMENTAL_DSN_INFO extra information will be added
+to DSN fail messages ("bounces"), when available. The intent is to aid
+tracing of specific failing messages, when presented with a "bounce"
+complaint and needing to search logs.
+
+
+The remote MTA IP address, with port number if nonstandard.
+Example:
+ Remote-MTA: X-ip; [127.0.0.1]:587
+Rationale:
+ Several addresses may correspond to the (already available)
+ dns name for the remote MTA.
+
+The remote MTA connect-time greeting.
+Example:
+ X-Remote-MTA-smtp-greeting: X-str; 220 the.local.host.name ESMTP Exim x.yz Tue, 2 Mar 1999 09:44:33 +0000
+Rationale:
+ This string sometimes presents the remote MTA's idea of its
+ own name, and sometimes identifies the MTA software.
+
+The remote MTA response to HELO or EHLO.
+Example:
+ X-Remote-MTA-helo-response: X-str; 250-the.local.host.name Hello localhost [127.0.0.1]
+Limitations:
+ Only the first line of a multiline response is recorded.
+Rationale:
+ This string sometimes presents the remote MTA's view of
+ the peer IP connecting to it.
+
+The reporting MTA detailed diagnostic.
+Example:
+ X-Exim-Diagnostic: X-str; SMTP error from remote mail server after RCPT TO:<d3@myhost.test.ex>: 550 hard error
+Rationale:
+ This string sometimes give extra information over the
+ existing (already available) Diagnostic-Code field.
+
+
+Note that non-RFC-documented field names and data types are used.
+
+
+LMDB Lookup support
+-------------------
+LMDB is an ultra-fast, ultra-compact, crash-proof key-value embedded data store.
+It is modeled loosely on the BerkeleyDB API. You should read about the feature
+set as well as operation modes at https://symas.com/products/lightning-memory-mapped-database/
+
+LMDB single key lookup support is provided by linking to the LMDB C library.
+The current implementation does not support writing to the LMDB database.
+
+Visit https://github.com/LMDB/lmdb to download the library or find it in your
+operating systems package repository.
+
+If building from source, this description assumes that headers will be in
+/usr/local/include, and that the libraries are in /usr/local/lib.
+
+1. In order to build exim with LMDB lookup support add or uncomment
+
+EXPERIMENTAL_LMDB=yes
+
+to your Local/Makefile. (Re-)build/install exim. exim -d should show
+Experimental_LMDB in the line "Support for:".
+
+EXPERIMENTAL_LMDB=yes
+LDFLAGS += -llmdb
+# CFLAGS += -I/usr/local/include
+# LDFLAGS += -L/usr/local/lib
+
+The first line sets the feature to include the correct code, and
+the second line says to link the LMDB libraries into the
+exim binary. The commented out lines should be uncommented if you
+built LMDB from source and installed in the default location.
+Adjust the paths if you installed them elsewhere, but you do not
+need to uncomment them if an rpm (or you) installed them in the
+package controlled locations (/usr/include and /usr/lib).
+
+2. Create your LMDB files, you can use the mdb_load utility which is
+part of the LMDB distribution our your favourite language bindings.
+
+3. Add the single key lookups to your exim.conf file, example lookups
+are below.
+
+${lookup{$sender_address_domain}lmdb{/var/lib/baruwa/data/db/relaydomains.mdb}{$value}}
+${lookup{$sender_address_domain}lmdb{/var/lib/baruwa/data/db/relaydomains.mdb}{$value}fail}
+${lookup{$sender_address_domain}lmdb{/var/lib/baruwa/data/db/relaydomains.mdb}}
+
+
+Queuefile transport
+-------------------
+Queuefile is a pseudo transport which does not perform final delivery.
+It simply copies the exim spool files out of the spool directory into
+an external directory retaining the exim spool format.
+
+The spool files can then be processed by external processes and then
+requeued into exim spool directories for final delivery.
+
+The motivation/inspiration for the transport is to allow external
+processes to access email queued by exim and have access to all the
+information which would not be available if the messages were delivered
+to the process in the standard email formats.
+
+The mailscanner package is one of the processes that can take advantage
+of this transport to filter email.
+
+The transport can be used in the same way as the other existing transports,
+i.e by configuring a router to route mail to a transport configured with
+the queuefile driver.
+
+The transport only takes one option:
+
+* directory - This is used to specify the directory messages should be
+copied to. Expanded.
+
+The generic transport options (body_only, current_directory, disable_logging,
+debug_print, delivery_date_add, envelope_to_add, event_action, group,
+headers_add, headers_only, headers_remove, headers_rewrite, home_directory,
+initgroups, max_parallel, message_size_limit, rcpt_include_affixes,
+retry_use_local_part, return_path, return_path_add, shadow_condition,
+shadow_transport, transport_filter, transport_filter_timeout, user) are
+ignored.
+
+Sample configuration:
+
+(Router)
+
+scan:
+ driver = accept
+ transport = scan
+
+(Transport)
+
+scan:
+ driver = queuefile
+ directory = /var/spool/baruwa-scanner/input
+
+
+In order to build exim with Queuefile transport support add or uncomment
+
+EXPERIMENTAL_QUEUEFILE=yes
+
+to your Local/Makefile. (Re-)build/install exim. exim -d should show
+Experimental_QUEUEFILE in the line "Support for:".
+
+
+ARC support
+-----------
+Specification: https://tools.ietf.org/html/draft-ietf-dmarc-arc-protocol-11
+Note that this is not an RFC yet, so may change.
+
+ARC is intended to support the utility of SPF and DKIM in the presence of
+intermediaries in the transmission path - forwarders and mailinglists -
+by establishing a cryptographically-signed chain in headers.
+
+Normally one would only bother doing ARC-signing when functioning as
+an intermediary. One might do verify for local destinations.
+
+ARC uses the notion of a "ADministrative Management Domain" (ADMD).
+Described in RFC 5598 (section 2.3), this is essentially the set of
+mail-handling systems that the mail transits. A label should be chosen to
+identify the ADMD. Messages should be ARC-verified on entry to the ADMD,
+and ARC-signed on exit from it.
+
+
+Verification
+--
+An ACL condition is provided to perform the "verifier actions" detailed
+in section 6 of the above specification. It may be called from the DATA ACL
+and succeeds if the result matches any of a given list.
+It also records the highest ARC instance number (the chain size)
+and verification result for later use in creating an Authentication-Results:
+standard header.
+
+ verify = arc/<acceptable_list> none:fail:pass
+
+ add_header = :at_start:${authresults {<admd-identifier>}}
+
+ 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 three new variables: $arc_state, $arc_state_reason, $arc_domains:
+
+ $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> [ : <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.
+
+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.
+ * 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.
+
+
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