X-Git-Url: https://git.exim.org/exim.git/blobdiff_plain/57233af5f91cdca9a0232a71fab2d12a538cb1a6..f41bc530a8b1a66e811e5c3b4da3df72ecba0e1d:/doc/doc-txt/experimental-spec.txt diff --git a/doc/doc-txt/experimental-spec.txt b/doc/doc-txt/experimental-spec.txt index 385f0526e..84fd54716 100644 --- a/doc/doc-txt/experimental-spec.txt +++ b/doc/doc-txt/experimental-spec.txt @@ -6,100 +6,7 @@ about experimental features, all of which are unstable and liable to incompatible change. -PRDR support --------------------------------------------------------------- - -Per-Recipient Data Reponse is an SMTP extension proposed by Eric Hall -in a (now-expired) IETF draft from 2007. It's not hit mainstream -use, but has apparently been implemented in the META1 MTA. - -There is mention at http://mail.aegee.org/intern/sendmail.html -of a patch to sendmail "to make it PRDR capable". - - ref: http://www.eric-a-hall.com/specs/draft-hall-prdr-00.txt - -If Exim is built with EXPERIMENTAL_PRDR there is a new config -boolean "prdr_enable" which controls whether PRDR is advertised -as part of an EHLO response, a new "acl_data_smtp_prdr" ACL -(called for each recipient, after data arrives but before the -data ACL), and a new smtp transport option "hosts_try_prdr". - -PRDR may be used to support per-user content filtering. Without it -one must defer any recipient after the first that has a different -content-filter configuration. With PRDR, the RCPT-time check -for this can be disabled when the MAIL-time $smtp_command included -"PRDR". Any required difference in behaviour of the main DATA-time -ACL should however depend on the PRDR-time ACL having run, as Exim -will avoid doing so in some situations (eg. single-recipient mails). - - - -OCSP Stapling support --------------------------------------------------------------- - -X.509 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 a new global 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 valid next update timestamp in the OCSP proof; -if not present, or if the proof has expired, it will be ignored. - -Also, given EXPERIMENTAL_OCSP and OpenSSL, the smtp transport gains -a "hosts_require_ocsp" option; a host-list for which an OCSP Stapling -is requested and required for the connection to proceed. The host(s) -should also be in "hosts_require_tls", and "tls_verify_certificates" -configured for the transport. - -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 @@ -135,7 +42,7 @@ 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 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 @@ -385,166 +292,18 @@ These four steps are explained in more details below. -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 dependent 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 standardized, 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 @@ -554,8 +313,10 @@ EXPERIMENTAL_SRS=yes in your Local/Makefile. + DCC Support -------------------------------------------------------------- +Distributed Checksum Clearinghouse; http://www.rhyolite.com/dcc/ *) Building exim @@ -625,7 +386,603 @@ Then set something like mout-xforward.gmx.net 82.165.159.12 mout.gmx.net 212.227.15.16 -Use a reasonable IP. eg. one the sending cluster acutally uses. +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:: 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/ none:fail:pass + + add_header = :at_start:${authresults {}} + + Note that it would be wise to strip incoming messages of A-R headers + that claim to be from our own . + +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 = : : [ : ] +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[=] Add an x= tag to the generated AMS header, with an expiry time. + If the value 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