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diff --git a/doc/experimental-spec.txt b/doc/experimental-spec.txt new file mode 100644 index 0000000..84fd547 --- /dev/null +++ b/doc/experimental-spec.txt @@ -0,0 +1,989 @@ +From time to time, experimental features may be added to Exim. +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 experimental features, all of which are unstable and +liable to incompatible change. + + +Brightmail AntiSpam (BMI) support +-------------------------------------------------------------- + +Brightmail AntiSpam is a commercial package. Please see +http://www.brightmail.com for more information on +the product. For the sake of clarity, we'll refer to it as +"BMI" from now on. + + +0) BMI concept and implementation overview + +In contrast to how spam-scanning with SpamAssassin is +implemented in exiscan-acl, BMI is more suited for per +-recipient scanning of messages. However, each messages is +scanned only once, but multiple "verdicts" for multiple +recipients can be returned from the BMI server. The exiscan +implementation passes the message to the BMI server just +before accepting it. It then adds the retrieved verdicts to +the messages header file in the spool. These verdicts can then +be queried in routers, where operation is per-recipient +instead of per-message. To use BMI, you need to take the +following steps: + + 1) Compile Exim with BMI support + 2) Set up main BMI options (top section of Exim config file) + 3) Set up ACL control statement (ACL section of the config + file) + 4) Set up your routers to use BMI verdicts (routers section + of the config file). + 5) (Optional) Set up per-recipient opt-in information. + +These four steps are explained in more details below. + +1) Adding support for BMI at compile time + + To compile with BMI support, you need to link Exim against + 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 + with these lines in Local/Makefile: + + EXPERIMENTAL_BRIGHTMAIL=yes + CFLAGS=-I/path/to/the/dir/with/the/includefile + EXTRALIBS_EXIM=-L/path/to/the/dir/with/the/library -lbmiclient_single + + If you use other CFLAGS or EXTRALIBS_EXIM settings then + merge the content of these lines with them. + + Note for BMI6.x users: You'll also have to add -lxml2_single + to the EXTRALIBS_EXIM line. Users of 5.5x do not need to do + this. + + You should also include the location of + libbmiclient_single.so in your dynamic linker configuration + file (usually /etc/ld.so.conf) and run "ldconfig" + afterwards, or else the produced Exim binary will not be + able to find the library file. + + +2) Setting up BMI support in the Exim main configuration + + To enable BMI support in the main Exim configuration, you + should set the path to the main BMI configuration file with + the "bmi_config_file" option, like this: + + bmi_config_file = /opt/brightmail/etc/brightmail.cfg + + This must go into section 1 of Exim's configuration file (You + can put it right on top). If you omit this option, it + defaults to /opt/brightmail/etc/brightmail.cfg. + + Note for BMI6.x users: This file is in XML format in V6.xx + and its name is /opt/brightmail/etc/bmiconfig.xml. So BMI + 6.x users MUST set the bmi_config_file option. + + +3) Set up ACL control statement + + To optimize performance, it makes sense only to process + messages coming from remote, untrusted sources with the BMI + server. To set up a messages for processing by the BMI + server, you MUST set the "bmi_run" control statement in any + ACL for an incoming message. You will typically do this in + an "accept" block in the "acl_check_rcpt" ACL. You should + use the "accept" block(s) that accept messages from remote + servers for your own domain(s). Here is an example that uses + the "accept" blocks from Exim's default configuration file: + + + accept domains = +local_domains + endpass + verify = recipient + control = bmi_run + + accept domains = +relay_to_domains + endpass + verify = recipient + control = bmi_run + + If bmi_run is not set in any ACL during reception of the + message, it will NOT be passed to the BMI server. + + +4) Setting up routers to use BMI verdicts + + When a message has been run through the BMI server, one or + more "verdicts" are present. Different recipients can have + different verdicts. Each recipient is treated individually + during routing, so you can query the verdicts by recipient + at that stage. From Exim's view, a verdict can have the + following outcomes: + + o deliver the message normally + o deliver the message to an alternate location + o do not deliver the message + + To query the verdict for a recipient, the implementation + offers the following tools: + + + - Boolean router preconditions. These can be used in any + router. For a simple implementation of BMI, these may be + all that you need. The following preconditions are + available: + + o bmi_deliver_default + + This precondition is TRUE if the verdict for the + recipient is to deliver the message normally. If the + message has not been processed by the BMI server, this + variable defaults to TRUE. + + o bmi_deliver_alternate + + This precondition is TRUE if the verdict for the + recipient is to deliver the message to an alternate + location. You can get the location string from the + $bmi_alt_location expansion variable if you need it. See + further below. If the message has not been processed by + the BMI server, this variable defaults to FALSE. + + o bmi_dont_deliver + + This precondition is TRUE if the verdict for the + recipient is NOT to deliver the message to the + recipient. You will typically use this precondition in a + top-level blackhole router, like this: + + # don't deliver messages handled by the BMI server + bmi_blackhole: + driver = redirect + bmi_dont_deliver + data = :blackhole: + + This router should be on top of all others, so messages + that should not be delivered do not reach other routers + at all. If the message has not been processed by + the BMI server, this variable defaults to FALSE. + + + - A list router precondition to query if rules "fired" on + the message for the recipient. Its name is "bmi_rule". You + use it by passing it a colon-separated list of rule + numbers. You can use this condition to route messages that + matched specific rules. Here is an example: + + # special router for BMI rule #5, #8 and #11 + bmi_rule_redirect: + driver = redirect + bmi_rule = 5:8:11 + data = postmaster@mydomain.com + + + - Expansion variables. Several expansion variables are set + during routing. You can use them in custom router + conditions, for example. The following variables are + available: + + o $bmi_base64_verdict + + This variable will contain the BASE64 encoded verdict + for the recipient being routed. You can use it to add a + header to messages for tracking purposes, for example: + + localuser: + driver = accept + check_local_user + headers_add = X-Brightmail-Verdict: $bmi_base64_verdict + transport = local_delivery + + If there is no verdict available for the recipient being + routed, this variable contains the empty string. + + o $bmi_base64_tracker_verdict + + This variable will contain a BASE64 encoded subset of + the verdict information concerning the "rules" that + fired on the message. You can add this string to a + header, commonly named "X-Brightmail-Tracker". Example: + + localuser: + driver = accept + check_local_user + headers_add = X-Brightmail-Tracker: $bmi_base64_tracker_verdict + transport = local_delivery + + If there is no verdict available for the recipient being + routed, this variable contains the empty string. + + o $bmi_alt_location + + If the verdict is to redirect the message to an + alternate location, this variable will contain the + alternate location string returned by the BMI server. In + its default configuration, this is a header-like string + that can be added to the message with "headers_add". If + there is no verdict available for the recipient being + routed, or if the message is to be delivered normally, + this variable contains the empty string. + + o $bmi_deliver + + This is an additional integer variable that can be used + to query if the message should be delivered at all. You + should use router preconditions instead if possible. + + $bmi_deliver is '0': the message should NOT be delivered. + $bmi_deliver is '1': the message should be delivered. + + + IMPORTANT NOTE: Verdict inheritance. + The message is passed to the BMI server during message + reception, using the target addresses from the RCPT TO: + commands in the SMTP transaction. If recipients get expanded + or re-written (for example by aliasing), the new address(es) + inherit the verdict from the original address. This means + that verdicts also apply to all "child" addresses generated + from top-level addresses that were sent to the BMI server. + + +5) Using per-recipient opt-in information (Optional) + + The BMI server features multiple scanning "profiles" for + individual recipients. These are usually stored in a LDAP + server and are queried by the BMI server itself. However, + you can also pass opt-in data for each recipient from the + MTA to the BMI server. This is particularly useful if you + already look up recipient data in Exim anyway (which can + also be stored in a SQL database or other source). This + implementation enables you to pass opt-in data to the BMI + server in the RCPT ACL. This works by setting the + 'bmi_optin' modifier in a block of that ACL. If should be + set to a list of comma-separated strings that identify the + features which the BMI server should use for that particular + recipient. Ideally, you would use the 'bmi_optin' modifier + in the same ACL block where you set the 'bmi_run' control + flag. Here is an example that will pull opt-in data for each + recipient from a flat file called + '/etc/exim/bmi_optin_data'. + + The file format: + + user1@mydomain.com: <OPTIN STRING1>:<OPTIN STRING2> + user2@thatdomain.com: <OPTIN STRING3> + + + The example: + + accept domains = +relay_to_domains + endpass + verify = recipient + bmi_optin = ${lookup{$local_part@$domain}lsearch{/etc/exim/bmi_optin_data}} + control = bmi_run + + Of course, you can also use any other lookup method that + Exim supports, including LDAP, Postgres, MySQL, Oracle etc., + as long as the result is a list of colon-separated opt-in + strings. + + For a list of available opt-in strings, please contact your + Brightmail representative. + + + + +SRS (Sender Rewriting Scheme) Support +-------------------------------------------------------------- + +Exiscan currently includes SRS support via Miles Wilton's +libsrs_alt library. The current version of the supported +library is 0.5, there are reports of 1.0 working. + +In order to use SRS, you must get a copy of libsrs_alt from + +https://opsec.eu/src/srs/ + +(not the original source, which has disappeared.) + +Unpack the tarball, then refer to MTAs/README.EXIM +to proceed. You need to set + +EXPERIMENTAL_SRS=yes + +in your Local/Makefile. + + + +DCC Support +-------------------------------------------------------------- +Distributed Checksum Clearinghouse; http://www.rhyolite.com/dcc/ + +*) Building exim + +In order to build exim with DCC support add + +EXPERIMENTAL_DCC=yes + +to your Makefile. (Re-)build/install exim. exim -d should show +EXPERIMENTAL_DCC under "Support for". + + +*) Configuration + +In the main section of exim.cf add at least + dccifd_address = /usr/local/dcc/var/dccifd +or + dccifd_address = <ip> <port> + +In the DATA ACL you can use the new condition + dcc = * + +After that "$dcc_header" contains the X-DCC-Header. + +Return values are: + fail for overall "R", "G" from dccifd + defer for overall "T" from dccifd + accept for overall "A", "S" from dccifd + +dcc = */defer_ok works as for spamd. + +The "$dcc_result" variable contains the overall result from DCC +answer. There will an X-DCC: header added to the mail. + +Usually you'll use + defer !dcc = * +to greylist with DCC. + +If you set, in the main section, + dcc_direct_add_header = true +then the dcc header will be added "in deep" and if the spool +file was already written it gets removed. This forces Exim to +write it again if needed. This helps to get the DCC Header +through to eg. SpamAssassin. + +If you want to pass even more headers in the middle of the +DATA stage you can set + $acl_m_dcc_add_header +to tell the DCC routines to add more information; eg, you might set +this to some results from ClamAV. Be careful. Header syntax is +not checked and is added "as is". + +In case you've troubles with sites sending the same queue items from several +hosts and fail to get through greylisting you can use +$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 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. + + 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, +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. +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 +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 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> [ : <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 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. + + + + +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/ + +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. + +Up to 1.5 roundtrip times can be taken out of cleartext connections, 2.5 on +STARTTLS connections. + +In combination with the traditional PIPELINING feature the following example +sequences are possible (among others): + +(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}} + + + + +-------------------------------------------------------------- +End of file +-------------------------------------------------------------- |