1 From time to time, experimental features may be added to Exim.
2 While a feature is experimental, there will be a build-time
3 option whose name starts "EXPERIMENTAL_" that must be set in
4 order to include the feature. This file contains information
5 about experimental features, all of which are unstable and
6 liable to incompatible change.
9 Brightmail AntiSpam (BMI) support
10 --------------------------------------------------------------
12 Brightmail AntiSpam is a commercial package. Please see
13 http://www.brightmail.com for more information on
14 the product. For the sake of clarity, we'll refer to it as
18 0) BMI concept and implementation overview
20 In contrast to how spam-scanning with SpamAssassin is
21 implemented in exiscan-acl, BMI is more suited for per
22 -recipient scanning of messages. However, each messages is
23 scanned only once, but multiple "verdicts" for multiple
24 recipients can be returned from the BMI server. The exiscan
25 implementation passes the message to the BMI server just
26 before accepting it. It then adds the retrieved verdicts to
27 the messages header file in the spool. These verdicts can then
28 be queried in routers, where operation is per-recipient
29 instead of per-message. To use BMI, you need to take the
32 1) Compile Exim with BMI support
33 2) Set up main BMI options (top section of Exim config file)
34 3) Set up ACL control statement (ACL section of the config
36 4) Set up your routers to use BMI verdicts (routers section
38 5) (Optional) Set up per-recipient opt-in information.
40 These four steps are explained in more details below.
42 1) Adding support for BMI at compile time
44 To compile with BMI support, you need to link Exim against
45 the Brightmail client SDK, consisting of a library
46 (libbmiclient_single.so) and a header file (bmi_api.h).
47 You'll also need to explicitly set a flag in the Makefile to
48 include BMI support in the Exim binary. Both can be achieved
49 with these lines in Local/Makefile:
51 EXPERIMENTAL_BRIGHTMAIL=yes
52 CFLAGS=-I/path/to/the/dir/with/the/includefile
53 EXTRALIBS_EXIM=-L/path/to/the/dir/with/the/library -lbmiclient_single
55 If you use other CFLAGS or EXTRALIBS_EXIM settings then
56 merge the content of these lines with them.
58 Note for BMI6.x users: You'll also have to add -lxml2_single
59 to the EXTRALIBS_EXIM line. Users of 5.5x do not need to do
62 You should also include the location of
63 libbmiclient_single.so in your dynamic linker configuration
64 file (usually /etc/ld.so.conf) and run "ldconfig"
65 afterwards, or else the produced Exim binary will not be
66 able to find the library file.
69 2) Setting up BMI support in the Exim main configuration
71 To enable BMI support in the main Exim configuration, you
72 should set the path to the main BMI configuration file with
73 the "bmi_config_file" option, like this:
75 bmi_config_file = /opt/brightmail/etc/brightmail.cfg
77 This must go into section 1 of Exim's configuration file (You
78 can put it right on top). If you omit this option, it
79 defaults to /opt/brightmail/etc/brightmail.cfg.
81 Note for BMI6.x users: This file is in XML format in V6.xx
82 and its name is /opt/brightmail/etc/bmiconfig.xml. So BMI
83 6.x users MUST set the bmi_config_file option.
86 3) Set up ACL control statement
88 To optimize performance, it makes sense only to process
89 messages coming from remote, untrusted sources with the BMI
90 server. To set up a messages for processing by the BMI
91 server, you MUST set the "bmi_run" control statement in any
92 ACL for an incoming message. You will typically do this in
93 an "accept" block in the "acl_check_rcpt" ACL. You should
94 use the "accept" block(s) that accept messages from remote
95 servers for your own domain(s). Here is an example that uses
96 the "accept" blocks from Exim's default configuration file:
99 accept domains = +local_domains
104 accept domains = +relay_to_domains
109 If bmi_run is not set in any ACL during reception of the
110 message, it will NOT be passed to the BMI server.
113 4) Setting up routers to use BMI verdicts
115 When a message has been run through the BMI server, one or
116 more "verdicts" are present. Different recipients can have
117 different verdicts. Each recipient is treated individually
118 during routing, so you can query the verdicts by recipient
119 at that stage. From Exim's view, a verdict can have the
122 o deliver the message normally
123 o deliver the message to an alternate location
124 o do not deliver the message
126 To query the verdict for a recipient, the implementation
127 offers the following tools:
130 - Boolean router preconditions. These can be used in any
131 router. For a simple implementation of BMI, these may be
132 all that you need. The following preconditions are
135 o bmi_deliver_default
137 This precondition is TRUE if the verdict for the
138 recipient is to deliver the message normally. If the
139 message has not been processed by the BMI server, this
140 variable defaults to TRUE.
142 o bmi_deliver_alternate
144 This precondition is TRUE if the verdict for the
145 recipient is to deliver the message to an alternate
146 location. You can get the location string from the
147 $bmi_alt_location expansion variable if you need it. See
148 further below. If the message has not been processed by
149 the BMI server, this variable defaults to FALSE.
153 This precondition is TRUE if the verdict for the
154 recipient is NOT to deliver the message to the
155 recipient. You will typically use this precondition in a
156 top-level blackhole router, like this:
158 # don't deliver messages handled by the BMI server
164 This router should be on top of all others, so messages
165 that should not be delivered do not reach other routers
166 at all. If the message has not been processed by
167 the BMI server, this variable defaults to FALSE.
170 - A list router precondition to query if rules "fired" on
171 the message for the recipient. Its name is "bmi_rule". You
172 use it by passing it a colon-separated list of rule
173 numbers. You can use this condition to route messages that
174 matched specific rules. Here is an example:
176 # special router for BMI rule #5, #8 and #11
180 data = postmaster@mydomain.com
183 - Expansion variables. Several expansion variables are set
184 during routing. You can use them in custom router
185 conditions, for example. The following variables are
188 o $bmi_base64_verdict
190 This variable will contain the BASE64 encoded verdict
191 for the recipient being routed. You can use it to add a
192 header to messages for tracking purposes, for example:
197 headers_add = X-Brightmail-Verdict: $bmi_base64_verdict
198 transport = local_delivery
200 If there is no verdict available for the recipient being
201 routed, this variable contains the empty string.
203 o $bmi_base64_tracker_verdict
205 This variable will contain a BASE64 encoded subset of
206 the verdict information concerning the "rules" that
207 fired on the message. You can add this string to a
208 header, commonly named "X-Brightmail-Tracker". Example:
213 headers_add = X-Brightmail-Tracker: $bmi_base64_tracker_verdict
214 transport = local_delivery
216 If there is no verdict available for the recipient being
217 routed, this variable contains the empty string.
221 If the verdict is to redirect the message to an
222 alternate location, this variable will contain the
223 alternate location string returned by the BMI server. In
224 its default configuration, this is a header-like string
225 that can be added to the message with "headers_add". If
226 there is no verdict available for the recipient being
227 routed, or if the message is to be delivered normally,
228 this variable contains the empty string.
232 This is an additional integer variable that can be used
233 to query if the message should be delivered at all. You
234 should use router preconditions instead if possible.
236 $bmi_deliver is '0': the message should NOT be delivered.
237 $bmi_deliver is '1': the message should be delivered.
240 IMPORTANT NOTE: Verdict inheritance.
241 The message is passed to the BMI server during message
242 reception, using the target addresses from the RCPT TO:
243 commands in the SMTP transaction. If recipients get expanded
244 or re-written (for example by aliasing), the new address(es)
245 inherit the verdict from the original address. This means
246 that verdicts also apply to all "child" addresses generated
247 from top-level addresses that were sent to the BMI server.
250 5) Using per-recipient opt-in information (Optional)
252 The BMI server features multiple scanning "profiles" for
253 individual recipients. These are usually stored in a LDAP
254 server and are queried by the BMI server itself. However,
255 you can also pass opt-in data for each recipient from the
256 MTA to the BMI server. This is particularly useful if you
257 already look up recipient data in Exim anyway (which can
258 also be stored in a SQL database or other source). This
259 implementation enables you to pass opt-in data to the BMI
260 server in the RCPT ACL. This works by setting the
261 'bmi_optin' modifier in a block of that ACL. If should be
262 set to a list of comma-separated strings that identify the
263 features which the BMI server should use for that particular
264 recipient. Ideally, you would use the 'bmi_optin' modifier
265 in the same ACL block where you set the 'bmi_run' control
266 flag. Here is an example that will pull opt-in data for each
267 recipient from a flat file called
268 '/etc/exim/bmi_optin_data'.
272 user1@mydomain.com: <OPTIN STRING1>:<OPTIN STRING2>
273 user2@thatdomain.com: <OPTIN STRING3>
278 accept domains = +relay_to_domains
281 bmi_optin = ${lookup{$local_part@$domain}lsearch{/etc/exim/bmi_optin_data}}
284 Of course, you can also use any other lookup method that
285 Exim supports, including LDAP, Postgres, MySQL, Oracle etc.,
286 as long as the result is a list of colon-separated opt-in
289 For a list of available opt-in strings, please contact your
290 Brightmail representative.
295 SRS (Sender Rewriting Scheme) Support
296 --------------------------------------------------------------
298 Exiscan currently includes SRS support via Miles Wilton's
299 libsrs_alt library. The current version of the supported
300 library is 0.5, there are reports of 1.0 working.
302 In order to use SRS, you must get a copy of libsrs_alt from
304 https://opsec.eu/src/srs/
306 (not the original source, which has disappeared.)
308 Unpack the tarball, then refer to MTAs/README.EXIM
309 to proceed. You need to set
313 in your Local/Makefile.
318 --------------------------------------------------------------
319 Distributed Checksum Clearinghouse; http://www.rhyolite.com/dcc/
323 In order to build exim with DCC support add
327 to your Makefile. (Re-)build/install exim. exim -d should show
328 EXPERIMENTAL_DCC under "Support for".
333 In the main section of exim.cf add at least
334 dccifd_address = /usr/local/dcc/var/dccifd
336 dccifd_address = <ip> <port>
338 In the DATA ACL you can use the new condition
341 After that "$dcc_header" contains the X-DCC-Header.
344 fail for overall "R", "G" from dccifd
345 defer for overall "T" from dccifd
346 accept for overall "A", "S" from dccifd
348 dcc = */defer_ok works as for spamd.
350 The "$dcc_result" variable contains the overall result from DCC
351 answer. There will an X-DCC: header added to the mail.
355 to greylist with DCC.
357 If you set, in the main section,
358 dcc_direct_add_header = true
359 then the dcc header will be added "in deep" and if the spool
360 file was already written it gets removed. This forces Exim to
361 write it again if needed. This helps to get the DCC Header
362 through to eg. SpamAssassin.
364 If you want to pass even more headers in the middle of the
365 DATA stage you can set
366 $acl_m_dcc_add_header
367 to tell the DCC routines to add more information; eg, you might set
368 this to some results from ClamAV. Be careful. Header syntax is
369 not checked and is added "as is".
371 In case you've troubles with sites sending the same queue items from several
372 hosts and fail to get through greylisting you can use
373 $acl_m_dcc_override_client_ip
375 Setting $acl_m_dcc_override_client_ip to an IP address overrides the default
376 of $sender_host_address. eg. use the following ACL in DATA stage:
378 warn set acl_m_dcc_override_client_ip = \
379 ${lookup{$sender_helo_name}nwildlsearch{/etc/mail/multipleip_sites}{$value}{}}
380 condition = ${if def:acl_m_dcc_override_client_ip}
381 log_message = dbg: acl_m_dcc_override_client_ip set to \
382 $acl_m_dcc_override_client_ip
384 Then set something like
385 # cat /etc/mail/multipleip_sites
386 mout-xforward.gmx.net 82.165.159.12
387 mout.gmx.net 212.227.15.16
389 Use a reasonable IP. eg. one the sending cluster actually uses.
394 --------------------------------------------------------------
396 DMARC combines feedback from SPF, DKIM, and header From: in order
397 to attempt to provide better indicators of the authenticity of an
398 email. This document does not explain the fundamentals, you
399 should read and understand how it works by visiting the website at
400 http://www.dmarc.org/.
402 DMARC support is added via the libopendmarc library. Visit:
404 http://sourceforge.net/projects/opendmarc/
406 to obtain a copy, or find it in your favorite rpm package
407 repository. If building from source, this description assumes
408 that headers will be in /usr/local/include, and that the libraries
409 are in /usr/local/lib.
411 1. To compile Exim with DMARC support, you must first enable SPF.
412 Please read the Local/Makefile comments on enabling the SUPPORT_SPF
413 feature. You must also have DKIM support, so you cannot set the
414 DISABLE_DKIM feature. Once both of those conditions have been met
415 you can enable DMARC in Local/Makefile:
417 EXPERIMENTAL_DMARC=yes
418 LDFLAGS += -lopendmarc
419 # CFLAGS += -I/usr/local/include
420 # LDFLAGS += -L/usr/local/lib
422 The first line sets the feature to include the correct code, and
423 the second line says to link the libopendmarc libraries into the
424 exim binary. The commented out lines should be uncommented if you
425 built opendmarc from source and installed in the default location.
426 Adjust the paths if you installed them elsewhere, but you do not
427 need to uncomment them if an rpm (or you) installed them in the
428 package controlled locations (/usr/include and /usr/lib).
431 2. Use the following global options to configure DMARC:
434 dmarc_tld_file Defines the location of a text file of valid
435 top level domains the opendmarc library uses
436 during domain parsing. Maintained by Mozilla,
437 the most current version can be downloaded
438 from a link at http://publicsuffix.org/list/.
439 See also util/renew-opendmarc-tlds.sh script.
440 The default for the option is currently
441 /etc/exim/opendmarc.tlds
444 dmarc_history_file Defines the location of a file to log results
445 of dmarc verification on inbound emails. The
446 contents are importable by the opendmarc tools
447 which will manage the data, send out DMARC
448 reports, and expire the data. Make sure the
449 directory of this file is writable by the user
452 dmarc_forensic_sender Alternate email address to use when sending a
453 forensic report detailing alignment failures
454 if a sender domain's dmarc record specifies it
455 and you have configured Exim to send them.
457 If set, this is expanded and used for the
458 From: header line; the address is extracted
459 from it and used for the envelope from.
460 If not set, the From: header is expanded from
461 the dsn_from option, and <> is used for the
467 3. By default, the DMARC processing will run for any remote,
468 non-authenticated user. It makes sense to only verify DMARC
469 status of messages coming from remote, untrusted sources. You can
470 use standard conditions such as hosts, senders, etc, to decide that
471 DMARC verification should *not* be performed for them and disable
472 DMARC with a control setting:
474 control = dmarc_disable_verify
476 A DMARC record can also specify a "forensic address", which gives
477 exim an email address to submit reports about failed alignment.
478 Exim does not do this by default because in certain conditions it
479 results in unintended information leakage (what lists a user might
480 be subscribed to, etc). You must configure exim to submit forensic
481 reports to the owner of the domain. If the DMARC record contains a
482 forensic address and you specify the control statement below, then
483 exim will send these forensic emails. It's also advised that you
484 configure a dmarc_forensic_sender because the default sender address
485 construction might be inadequate.
487 control = dmarc_enable_forensic
489 (AGAIN: You can choose not to send these forensic reports by simply
490 not putting the dmarc_enable_forensic control line at any point in
491 your exim config. If you don't tell it to send them, it will not
494 There are no options to either control. Both must appear before
498 4. You can now run DMARC checks in incoming SMTP by using the
499 "dmarc_status" ACL condition in the DATA ACL. You are required to
500 call the spf condition first in the ACLs, then the "dmarc_status"
501 condition. Putting this condition in the ACLs is required in order
502 for a DMARC check to actually occur. All of the variables are set
503 up before the DATA ACL, but there is no actual DMARC check that
504 occurs until a "dmarc_status" condition is encountered in the ACLs.
506 The dmarc_status condition takes a list of strings on its
507 right-hand side. These strings describe recommended action based
508 on the DMARC check. To understand what the policy recommendations
509 mean, refer to the DMARC website above. Valid strings are:
511 o accept The DMARC check passed and the library recommends
513 o reject The DMARC check failed and the library recommends
515 o quarantine The DMARC check failed and the library recommends
516 keeping it for further inspection.
517 o none The DMARC check passed and the library recommends
518 no specific action, neutral.
519 o norecord No policy section in the DMARC record for this
521 o nofrom Unable to determine the domain of the sender.
522 o temperror Library error or dns error.
523 o off The DMARC check was disabled for this email.
525 You can prefix each string with an exclamation mark to invert its
526 meaning, for example "!accept" will match all results but
527 "accept". The string list is evaluated left-to-right in a
528 short-circuit fashion. When a string matches the outcome of the
529 DMARC check, the condition succeeds. If none of the listed
530 strings matches the outcome of the DMARC check, the condition
533 Of course, you can also use any other lookup method that Exim
534 supports, including LDAP, Postgres, MySQL, etc, as long as the
535 result is a list of colon-separated strings.
537 Performing the check sets up information used by the
538 ${authresults } expansion item.
540 Several expansion variables are set before the DATA ACL is
541 processed, and you can use them in this ACL. The following
542 expansion variables are available:
545 This is a one word status indicating what the DMARC library
546 thinks of the email. It is a combination of the results of
547 DMARC record lookup and the SPF/DKIM/DMARC processing results
548 (if a DMARC record was found). The actual policy declared
549 in the DMARC record is in a separate expansion variable.
552 This is a slightly longer, human readable status.
555 This is the domain which DMARC used to look up the DMARC
558 o $dmarc_domain_policy
559 This is the policy declared in the DMARC record. Valid values
560 are "none", "reject" and "quarantine". It is blank when there
561 is any error, including no DMARC record.
563 A now-redundant variable $dmarc_ar_header has now been withdrawn.
564 Use the ${authresults } expansion instead.
567 5. How to enable DMARC advanced operation:
568 By default, Exim's DMARC configuration is intended to be
569 non-intrusive and conservative. To facilitate this, Exim will not
570 create any type of logging files without explicit configuration by
571 you, the admin. Nor will Exim send out any emails/reports about
572 DMARC issues without explicit configuration by you, the admin (other
573 than typical bounce messages that may come about due to ACL
574 processing or failure delivery issues).
576 In order to log statistics suitable to be imported by the opendmarc
578 a. Configure the global setting dmarc_history_file.
579 b. Configure cron jobs to call the appropriate opendmarc history
580 import scripts and truncating the dmarc_history_file.
582 In order to send forensic reports, you need to:
583 a. Configure the global setting dmarc_forensic_sender.
584 b. Configure, somewhere before the DATA ACL, the control option to
585 enable sending DMARC forensic reports.
590 warn domains = +local_domains
592 control = dmarc_disable_verify
594 warn !domains = +screwed_up_dmarc_records
595 control = dmarc_enable_forensic
597 warn condition = (lookup if destined to mailing list)
598 set acl_m_mailing_list = 1
601 warn dmarc_status = accept : none : off
603 log_message = DMARC DEBUG: $dmarc_status $dmarc_used_domain
605 warn dmarc_status = !accept
607 log_message = DMARC DEBUG: '$dmarc_status' for $dmarc_used_domain
609 warn dmarc_status = quarantine
611 set $acl_m_quarantine = 1
612 # Do something in a transport with this flag variable
614 deny condition = ${if eq{$dmarc_domain_policy}{reject}}
615 condition = ${if eq{$acl_m_mailing_list}{1}}
616 message = Messages from $dmarc_used_domain break mailing lists
618 deny dmarc_status = reject
620 message = Message from $dmarc_used_domain failed sender's DMARC policy, REJECT
622 warn add_header = :at_start:${authresults {$primary_hostname}}
626 DSN extra information
627 ---------------------
628 If compiled with EXPERIMENTAL_DSN_INFO extra information will be added
629 to DSN fail messages ("bounces"), when available. The intent is to aid
630 tracing of specific failing messages, when presented with a "bounce"
631 complaint and needing to search logs.
634 The remote MTA IP address, with port number if nonstandard.
636 Remote-MTA: X-ip; [127.0.0.1]:587
638 Several addresses may correspond to the (already available)
639 dns name for the remote MTA.
641 The remote MTA connect-time greeting.
643 X-Remote-MTA-smtp-greeting: X-str; 220 the.local.host.name ESMTP Exim x.yz Tue, 2 Mar 1999 09:44:33 +0000
645 This string sometimes presents the remote MTA's idea of its
646 own name, and sometimes identifies the MTA software.
648 The remote MTA response to HELO or EHLO.
650 X-Remote-MTA-helo-response: X-str; 250-the.local.host.name Hello localhost [127.0.0.1]
652 Only the first line of a multiline response is recorded.
654 This string sometimes presents the remote MTA's view of
655 the peer IP connecting to it.
657 The reporting MTA detailed diagnostic.
659 X-Exim-Diagnostic: X-str; SMTP error from remote mail server after RCPT TO:<d3@myhost.test.ex>: 550 hard error
661 This string sometimes give extra information over the
662 existing (already available) Diagnostic-Code field.
665 Note that non-RFC-documented field names and data types are used.
670 LMDB is an ultra-fast, ultra-compact, crash-proof key-value embedded data store.
671 It is modeled loosely on the BerkeleyDB API. You should read about the feature
672 set as well as operation modes at https://symas.com/products/lightning-memory-mapped-database/
674 LMDB single key lookup support is provided by linking to the LMDB C library.
675 The current implementation does not support writing to the LMDB database.
677 Visit https://github.com/LMDB/lmdb to download the library or find it in your
678 operating systems package repository.
680 If building from source, this description assumes that headers will be in
681 /usr/local/include, and that the libraries are in /usr/local/lib.
683 1. In order to build exim with LMDB lookup support add or uncomment
685 EXPERIMENTAL_LMDB=yes
687 to your Local/Makefile. (Re-)build/install exim. exim -d should show
688 Experimental_LMDB in the line "Support for:".
690 EXPERIMENTAL_LMDB=yes
692 # CFLAGS += -I/usr/local/include
693 # LDFLAGS += -L/usr/local/lib
695 The first line sets the feature to include the correct code, and
696 the second line says to link the LMDB libraries into the
697 exim binary. The commented out lines should be uncommented if you
698 built LMDB from source and installed in the default location.
699 Adjust the paths if you installed them elsewhere, but you do not
700 need to uncomment them if an rpm (or you) installed them in the
701 package controlled locations (/usr/include and /usr/lib).
703 2. Create your LMDB files, you can use the mdb_load utility which is
704 part of the LMDB distribution our your favourite language bindings.
706 3. Add the single key lookups to your exim.conf file, example lookups
709 ${lookup{$sender_address_domain}lmdb{/var/lib/baruwa/data/db/relaydomains.mdb}{$value}}
710 ${lookup{$sender_address_domain}lmdb{/var/lib/baruwa/data/db/relaydomains.mdb}{$value}fail}
711 ${lookup{$sender_address_domain}lmdb{/var/lib/baruwa/data/db/relaydomains.mdb}}
716 Queuefile is a pseudo transport which does not perform final delivery.
717 It simply copies the exim spool files out of the spool directory into
718 an external directory retaining the exim spool format.
720 The spool files can then be processed by external processes and then
721 requeued into exim spool directories for final delivery.
722 However, note carefully the warnings in the main documentation on
725 The motivation/inspiration for the transport is to allow external
726 processes to access email queued by exim and have access to all the
727 information which would not be available if the messages were delivered
728 to the process in the standard email formats.
730 The mailscanner package is one of the processes that can take advantage
731 of this transport to filter email.
733 The transport can be used in the same way as the other existing transports,
734 i.e by configuring a router to route mail to a transport configured with
735 the queuefile driver.
737 The transport only takes one option:
739 * directory - This is used to specify the directory messages should be
742 The generic transport options (body_only, current_directory, disable_logging,
743 debug_print, delivery_date_add, envelope_to_add, event_action, group,
744 headers_add, headers_only, headers_remove, headers_rewrite, home_directory,
745 initgroups, max_parallel, message_size_limit, rcpt_include_affixes,
746 retry_use_local_part, return_path, return_path_add, shadow_condition,
747 shadow_transport, transport_filter, transport_filter_timeout, user) are
750 Sample configuration:
762 directory = /var/spool/baruwa-scanner/input
765 In order to build exim with Queuefile transport support add or uncomment
767 EXPERIMENTAL_QUEUEFILE=yes
769 to your Local/Makefile. (Re-)build/install exim. exim -d should show
770 Experimental_QUEUEFILE in the line "Support for:".
775 Specification: https://tools.ietf.org/html/draft-ietf-dmarc-arc-protocol-11
776 Note that this is not an RFC yet, so may change.
778 ARC is intended to support the utility of SPF and DKIM in the presence of
779 intermediaries in the transmission path - forwarders and mailinglists -
780 by establishing a cryptographically-signed chain in headers.
782 Normally one would only bother doing ARC-signing when functioning as
783 an intermediary. One might do verify for local destinations.
785 ARC uses the notion of a "ADministrative Management Domain" (ADMD).
786 Described in RFC 5598 (section 2.3), this is essentially the set of
787 mail-handling systems that the mail transits. A label should be chosen to
788 identify the ADMD. Messages should be ARC-verified on entry to the ADMD,
789 and ARC-signed on exit from it.
794 An ACL condition is provided to perform the "verifier actions" detailed
795 in section 6 of the above specification. It may be called from the DATA ACL
796 and succeeds if the result matches any of a given list.
797 It also records the highest ARC instance number (the chain size)
798 and verification result for later use in creating an Authentication-Results:
801 verify = arc/<acceptable_list> none:fail:pass
803 add_header = :at_start:${authresults {<admd-identifier>}}
805 Note that it would be wise to strip incoming messages of A-R headers
806 that claim to be from our own <admd-identifier>.
808 There are four new variables:
810 $arc_state One of pass, fail, none
811 $arc_state_reason (if fail, why)
812 $arc_domains colon-sep list of ARC chain domains, in chain order.
813 problematic elements may have empty list elements
814 $arc_oldest_pass lowest passing instance number of chain
817 logwrite = oldest-p-ams: <${reduce {$lh_ARC-Authentication-Results:} \
819 {${if = {$arc_oldest_pass} \
820 {${extract {i}{${extract {1}{;}{$item}}}}} \
824 Receive log lines for an ARC pass will be tagged "ARC".
829 arc_sign = <admd-identifier> : <selector> : <privkey> [ : <options> ]
830 An option on the smtp transport, which constructs and prepends to the message
831 an ARC set of headers. The textually-first Authentication-Results: header
832 is used as a basis (you must have added one on entry to the ADMD).
833 Expanded as a whole; if unset, empty or forced-failure then no signing is done.
834 If it is set, all of the first three elements must be non-empty.
836 The fourth element is optional, and if present consists of a comma-separated list
837 of options. The options implemented are
839 timestamps Add a t= tag to the generated AMS and AS headers, with the
841 expire[=<val>] Add an x= tag to the generated AMS header, with an expiry time.
842 If the value <val> is an plain number it is used unchanged.
843 If it starts with a '+' then the following number is added
844 to the current time, as an offset in seconds.
845 If a value is not given it defaults to a one month offset.
847 [As of writing, gmail insist that a t= tag on the AS is mandatory]
850 * There must be an Authentication-Results header, presumably added by an ACL
851 while receiving the message, for the same ADMD, for arc_sign to succeed.
852 This requires careful coordination between inbound and outbound logic.
854 Only one A-R header is taken account of. This is a limitation versus
855 the ARC spec (which says that all A-R headers from within the ADMD must
858 * If passing a message to another system, such as a mailing-list manager
859 (MLM), between receipt and sending, be wary of manipulations to headers made
861 + For instance, Mailman with REMOVE_DKIM_HEADERS==3 might improve
862 deliverability in a pre-ARC world, but that option also renames the
863 Authentication-Results header, which breaks signing.
865 * Even if you use multiple DKIM keys for different domains, the ARC concept
866 should try to stick to one ADMD, so pick a primary domain and use that for
867 AR headers and outbound signing.
869 Signing is not compatible with cutthrough delivery; any (before expansion)
870 value set for the option will result in cutthrough delivery not being
871 used via the transport in question.
876 Early pipelining support
877 ------------------------
878 Ref: https://datatracker.ietf.org/doc/draft-harris-early-pipe/
880 If compiled with EXPERIMENTAL_PIPE_CONNECT support is included for this feature.
881 The server advertises the feature in its EHLO response, currently using the name
882 "X_PIPE_CONNECT" (this will change, some time in the future).
883 A client may cache this information, along with the rest of the EHLO response,
884 and use it for later connections. Those later ones can send esmtp commands before
885 a banner is received.
887 Up to 1.5 roundtrip times can be taken out of cleartext connections, 2.5 on
888 STARTTLS connections.
890 In combination with the traditional PIPELINING feature the following example
891 sequences are possible (among others):
895 EHLO,MAIL,RCPT,DATA ->
896 <- banner,EHLO-resp,MAIL-ack,RCPT-ack,DATA-goahead
900 EHLO,MAIL,RCPT,BDAT ->
901 <- banner,EHLO-resp,MAIL-ack,RCPT-ack
906 <- banner,EHLO-resp,TLS-goahead
907 TLS1.2-client-hello ->
908 <- TLS-server-hello,cert,hello-done
909 client-Kex,change-cipher,finished ->
910 <- change-cipher,finished
911 EHLO,MAIL,RCPT,DATA ->
912 <- EHLO-resp,MAIL-ack,RCPT-ack,DATA-goahead
916 TLS1.2-client-hello ->
917 <- TLS-server-hello,cert,hello-done
918 client-Kex,change-cipher,finished ->
919 <- change-cipher,finshed
921 EHLO,MAIL,RCPT,DATA ->
922 <- EHLO-resp,MAIL-ack,RCPT-ack,DATA-goahead
924 Where the initial client packet is SMTP, it can combine with the TCP Fast Open
925 feature and be sent in the TCP SYN.
928 A main-section option "pipelining_connect_advertise_hosts" (default: *)
929 and an smtp transport option "hosts_pipe_connect" (default: unset)
932 If the "pipelining" log_selector is enabled, the "L" field in server <=
933 log lines has a period appended if the feature was advertised but not used;
934 or has an asterisk appended if the feature was used. In client => lines
935 the "L" field has an asterisk appended if the feature was used.
937 The "retry_data_expire" option controls cache invalidation.
938 Entries are also rewritten (or cleared) if the adverised features
942 NOTE: since the EHLO command must be constructed before the connection is
943 made it cannot depend on the interface IP address that will be used.
944 Transport configurations should be checked for this. An example avoidance:
946 helo_data = ${if def:sending_ip_address \
947 {${lookup dnsdb{>! ptr=$sending_ip_address} \
948 {${sg{$value} {^([^!]*).*\$} {\$1}}} fail}} \
954 TLS Session Resumption
955 ----------------------
956 TLS Session Resumption for TLS 1.2 and TLS 1.3 connections can be used (defined
957 in RFC 5077 for 1.2). The support for this can be included by building with
958 EXPERIMENTAL_TLS_RESUME defined. This requires GnuTLS 3.6.3 or OpenSSL 1.1.1
961 Session resumption (this is the "stateless" variant) involves the server sending
962 a "session ticket" to the client on one connection, which can be stored by the
963 client and used for a later session. The ticket contains sufficient state for
964 the server to reconstruct the TLS session, avoiding some expensive crypto
965 calculation and one full packet roundtrip time.
967 Operational cost/benefit:
968 The extra data being transmitted costs a minor amount, and the client has
969 extra costs in storing and retrieving the data.
971 In the Exim/Gnutls implementation the extra cost on an initial connection
972 which is TLS1.2 over a loopback path is about 6ms on 2017-laptop class hardware.
973 The saved cost on a subsequent connection is about 4ms; three or more
974 connections become a net win. On longer network paths, two or more
975 connections will have an average lower startup time thanks to the one
976 saved packet roundtrip. TLS1.3 will save the crypto cpu costs but not any
979 Since a new hints DB is used, the hints DB maintenance should be updated
980 to additionally handle "tls".
983 The session ticket is encrypted, but is obviously an additional security
984 vulnarability surface. An attacker able to decrypt it would have access
985 all connections using the resumed session.
986 The session ticket encryption key is not committed to storage by the server
987 and is rotated regularly. Tickets have limited lifetime.
989 There is a question-mark over the security of the Diffie-Helman parameters
990 used for session negotiation. TBD. q-value; cf bug 1895
993 New log_selector "tls_resumption", appends an asterisk to the tls_cipher "X="
996 Variables $tls_{in,out}_resumption have bit 0-4 indicating respectively
997 support built, client requested ticket, client offered session,
998 server issued ticket, resume used. A suitable decode list is provided
999 in the builtin macro _RESUME_DECODE for ${listextract {}{}}.
1002 In a resumed session:
1003 $tls_{in,out}_certificate_verified will be unset (undler OpenSSL)
1004 verify = certificate will be false (undler OpenSSL)
1005 $tls_{in,out}_cipher will have values different to the original (under GnuTLS)
1006 $tls_{in,out}_ocsp will be "not requested" or "no response"
1009 --------------------------------------------------------------
1011 --------------------------------------------------------------