X-Git-Url: https://git.exim.org/users/heiko/exim.git/blobdiff_plain/de78e2d59e3924238648b8fb363a1412fa9e499d..3bb1e9f161fd22707bc7cfcd77772422ac0fdd8e:/doc/doc-txt/experimental-spec.txt diff --git a/doc/doc-txt/experimental-spec.txt b/doc/doc-txt/experimental-spec.txt index 4836a7d51..47cd93f3d 100644 --- a/doc/doc-txt/experimental-spec.txt +++ b/doc/doc-txt/experimental-spec.txt @@ -6,7 +6,7 @@ about experimental features, all of which are unstable and liable to incompatible change. -Brightmail AntiSpam (BMI) suppport +Brightmail AntiSpam (BMI) support -------------------------------------------------------------- Brightmail AntiSpam is a commercial package. Please see @@ -42,7 +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 @@ -292,190 +292,60 @@ These four steps are explained in more details below. -Sender Policy Framework (SPF) support +SRS (Sender Rewriting Scheme) Support (using libsrs_alt) -------------------------------------------------------------- +See also the main docs, for an alternative native support implementation. -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. - This should be treated like "none". - o permerror This indicates a syntax error in the SPF - record of the queried domain. You may deny - messages when this occurs. (Changed in 4.83) - o temperror This indicates a temporary error during all - processing, including Exim's SPF processing. - You may defer messages when this occurs. - (Changed in 4.83) - o err_temp Same as permerror, deprecated in 4.83, will be - removed in a future release. - o err_perm Same as temperror, deprecated in 4.83, will be - removed in a future release. - -You can prefix each string with an exclamation mark to invert -its 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, permerror or - temperror. - - $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. - - -A lookup expansion is also available. It takes an email -address as the key and an IP address as the database: +Exim can be built with SRS support using Miles Wilton's +libsrs_alt library. The current version of the supported +library is 0.5, there are reports of 1.0 working. - $lookup (username@domain} spf {ip.ip.ip.ip}} +In order to use SRS, you must get a copy of libsrs_alt from -The lookup will return the same result strings as they can appear in -$spf_result (pass,fail,softfail,neutral,none,err_perm,err_temp). -Currently, only IPv4 addresses are supported. +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 -SRS (Sender Rewriting Scheme) Support --------------------------------------------------------------- +EXPERIMENTAL_SRS_ALT=yes -Exiscan currently includes SRS support via Miles Wilton's -libsrs_alt library. The current version of the supported -library is 0.5. +in your Local/Makefile. -In order to use SRS, you must get a copy of libsrs_alt from +The built-in support, included by SUPPORT_SRS, +shuold *not* be enabled if you wish to use the libsrs_alt +version. -http://srs.mirtol.com/ +The following main-section options become available: + srs_config string + srs_hashlength int + srs_hashmin int + srs_maxage int + srs_secrets string + srs_usehash bool + srs_usetimestamp bool -Unpack the tarball, then refer to MTAs/README.EXIM -to proceed. You need to set +The redirect router gains these options (all of type string, unset by default): + srs + srs_alias + srs_condition + srs_dbinsert + srs_dbselect + +The following variables become available: + $srs_db_address + $srs_db_key + $srs_orig_recipient + $srs_orig_sender + $srs_recipient + $srs_status + +The predefined feature-macro _HAVE_SRS will be present. +Additional delivery log line elements, tagged with "SRS=" will show the srs sender. +For configuration information see https://github.com/Exim/exim/wiki/SRS . -EXPERIMENTAL_SRS=yes -in your Local/Makefile. DCC Support @@ -550,376 +420,7 @@ 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. - -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 above section on enabling the EXPERIMENTAL_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/. - -Optional: -dmarc_history_file Defines the location of a file to log results - of dmarc verification on inbound emails. The - contents are importable by the opendmarc tools - which will manage the data, send out DMARC - reports, and expire the data. Make sure the - directory of this file is writable by the user - exim runs as. - -dmarc_forensic_sender The email address to use when sending a - forensic report detailing alignment failures - if a sender domain's dmarc record specifies it - and you have configured Exim to send them. - Default: do-not-reply@$default_hostname - - -3. By default, the DMARC processing will run for any remote, -non-authenticated user. It makes sense to only verify DMARC -status of messages coming from remote, untrusted sources. You can -use standard conditions such as hosts, senders, etc, to decide that -DMARC verification should *not* be performed for them and disable -DMARC with a control setting: - - control = dmarc_disable_verify - -A DMARC record can also specify a "forensic address", which gives -exim an email address to submit reports about failed alignment. -Exim does not do this by default because in certain conditions it -results in unintended information leakage (what lists a user might -be subscribed to, etc). You must configure exim to submit forensic -reports to the owner of the domain. If the DMARC record contains a -forensic address and you specify the control statement below, then -exim will send these forensic emails. It's also advised that you -configure a dmarc_forensic_sender because the default sender address -construction might be inadequate. - - control = dmarc_enable_forensic - -(AGAIN: You can choose not to send these forensic reports by simply -not putting the dmarc_enable_forensic control line at any point in -your exim config. If you don't tell it to send them, it will not -send them.) - -There are no options to either control. Both must appear before -the DATA acl. - - -4. You can now run DMARC checks in incoming SMTP by using the -"dmarc_status" ACL condition in the DATA ACL. You are required to -call the spf condition first in the ACLs, then the "dmarc_status" -condition. Putting this condition in the ACLs is required in order -for a DMARC check to actually occur. All of the variables are set -up before the DATA ACL, but there is no actual DMARC check that -occurs until a "dmarc_status" condition is encountered in the ACLs. - -The dmarc_status condition takes a list of strings on its -right-hand side. These strings describe recommended action based -on the DMARC check. To understand what the policy recommendations -mean, refer to the DMARC website above. Valid strings are: - - o accept The DMARC check passed and the library recommends - accepting the email. - o reject The DMARC check failed and the library recommends - rejecting the email. - o quarantine The DMARC check failed and the library recommends - keeping it for further inspection. - o none The DMARC check passed and the library recommends - no specific action, neutral. - o norecord No policy section in the DMARC record for this - sender domain. - o nofrom Unable to determine the domain of the sender. - o temperror Library error or dns error. - o off The DMARC check was disabled for this email. - -You can prefix each string with an exclamation mark to invert its -meaning, for example "!accept" will match all results but -"accept". The string list is evaluated left-to-right in a -short-circuit fashion. When a string matches the outcome of the -DMARC check, the condition succeeds. If none of the listed -strings matches the outcome of the DMARC check, the condition -fails. - -Of course, you can also use any other lookup method that Exim -supports, including LDAP, Postgres, MySQL, etc, as long as the -result is a list of colon-separated strings. - -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. - - o $dmarc_ar_header - This is the entire Authentication-Results header which you can - add using an add_header modifier. - - -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 - add_header = $dmarc_ar_header - - warn dmarc_status = !accept - !authenticated = * - log_message = DMARC DEBUG: '$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 - - - -DANE ------------------------------------------------------------- -DNS-based Authentication of Named Entities, as applied -to SMTP over TLS, provides assurance to a client that -it is actually talking to the server it wants to rather -than some attacker operating a Man In The Middle (MITM) -operation. The latter can terminate the TLS connection -you make, and make another one to the server (so both -you and the server still think you have an encrypted -connection) and, if one of the "well known" set of -Certificate Authorities has been suborned - something -which *has* been seen already (2014), a verifiable -certificate (if you're using normal root CAs, eg. the -Mozilla set, as your trust anchors). - -What DANE does is replace the CAs with the DNS as the -trust anchor. The assurance is limited to a) the possibility -that the DNS has been suborned, b) mistakes made by the -admins of the target server. The attack surface presented -by (a) is thought to be smaller than that of the set -of root CAs. - -It also allows the server to declare (implicitly) that -connections to it should use TLS. An MITM could simply -fail to pass on a server's STARTTLS. - -DANE scales better than having to maintain (and -side-channel communicate) copies of server certificates -for every possible target server. It also scales -(slightly) better than having to maintain on an SMTP -client a copy of the standard CAs bundle. It also -means not having to pay a CA for certificates. - -DANE requires a server operator to do three things: -1) run DNSSEC. This provides assurance to clients -that DNS lookups they do for the server have not -been tampered with. The domain MX record applying -to this server, its A record, its TLSA record and -any associated CNAME records must all be covered by -DNSSEC. -2) add TLSA DNS records. These say what the server -certificate for a TLS connection should be. -3) offer a server certificate, or certificate chain, -in TLS connections which is traceable to the one -defined by (one of?) the TSLA records - -There are no changes to Exim specific to server-side -operation of DANE. - -The TLSA record for the server may have "certificate -usage" of DANE-TA(2) or DANE-EE(3). The latter specifies -the End Entity directly, i.e. the certificate involved -is that of the server (and should be the sole one transmitted -during the TLS handshake); this is appropriate for a -single system, using a self-signed certificate. - DANE-TA usage is effectively declaring a specific CA -to be used; this might be a private CA or a public, -well-known one. A private CA at simplest is just -a self-signed certificate which is used to sign -cerver certificates, but running one securely does -require careful arrangement. If a private CA is used -then either all clients must be primed with it, or -(probably simpler) the server TLS handshake must transmit -the entire certificate chain from CA to server-certificate. -If a public CA is used then all clients must be primed with it -(losing one advantage of DANE) - but the attack surface is -reduced from all public CAs to that single CA. -DANE-TA is commonly used for several services and/or -servers, each having a TLSA query-domain CNAME record, -all of which point to a single TLSA record. - -The TLSA record should have a Selector field of SPKI(1) -and a Matching Type field of SHA2-512(2). - -At the time of writing, https://www.huque.com/bin/gen_tlsa -is useful for quickly generating TLSA records; and commands like - - openssl x509 -in -pubkey -noout /dev/null \ - | openssl sha512 \ - | awk '{print $2}' - -are workable for 4th-field hashes. - -For use with the DANE-TA model, server certificates -must have a correct name (SubjectName or SubjectAltName). - -The use of OCSP-stapling should be considered, allowing -for fast revocation of certificates (which would otherwise -be limited by the DNS TTL on the TLSA records). However, -this is likely to only be usable with DANE-TA. NOTE: the -default of requesting OCSP for all hosts is modified iff -DANE is in use, to: - - hosts_request_ocsp = ${if or { {= {0}{$tls_out_tlsa_usage}} \ - {= {4}{$tls_out_tlsa_usage}} } \ - {*}{}} - -The (new) variable $tls_out_tlsa_usage is a bitfield with -numbered bits set for TLSA record usage codes. -The zero above means DANE was not in use, -the four means that only DANE-TA usage TLSA records were -found. If the definition of hosts_request_ocsp includes the -string "tls_out_tlsa_usage", they are re-expanded in time to -control the OCSP request. - -This modification of hosts_request_ocsp is only done if -it has the default value of "*". Admins who change it, and -those who use hosts_require_ocsp, should consider the interaction -with DANE in their OCSP settings. - - -For client-side DANE there are two new smtp transport options, -hosts_try_dane and hosts_require_dane. They do the obvious thing. -[ should they be domain-based rather than host-based? ] - -DANE will only be usable if the target host has DNSSEC-secured -MX, A and TLSA records. - -A TLSA lookup will be done if either of the above options match -and the host-lookup succeded using dnssec. -If a TLSA lookup is done and succeeds, a DANE-verified TLS connection -will be required for the host. - -(TODO: specify when fallback happens vs. when the host is not used) - -If DANE is requested and useable (see above) the following transport -options are ignored: - hosts_require_tls - tls_verify_hosts - tls_try_verify_hosts - tls_verify_certificates - tls_crl - tls_verify_cert_hostnames - -If DANE is not usable, whether requested or not, and CA-anchored -verification evaluation is wanted, the above variables should be set -appropriately. - -Currently dnssec_request_domains must be active (need to think about that) -and dnssec_require_domains is ignored. - -If verification was successful using DANE then the "CV" item -in the delivery log line will show as "CV=dane". - -There is a new variable $tls_out_dane which will have "yes" if -verification succeeded using DANE and "no" otherwise (only useful -in combination with EXPERIMENTAL_EVENT), and a new variable -$tls_out_tlsa_usage (detailed above). +Use a reasonable IP. eg. one the sending cluster actually uses. @@ -958,14 +459,226 @@ 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 somtimes give extra information over the + 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. +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. + +[RFC 8617 was published 2019/06. Draft 11 was 2018/01. A review of the +changes has not yet been done] + +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 a set of +mail-handling systems that mail transits that are all under the control +of one organisation. 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. + + +Building with ARC Support +-- +Enable using EXPERIMENTAL_ARC=yes in your Local/Makefile. +You must also have DKIM present (not disabled), and you very likely +want to have SPF enabled. + + +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. + + + +Dovecot authenticator via inet socket +-------------------------------------------------------------- +If Dovecot is configured similar to :- + +service auth { +... +#SASL + inet_listener { + name = exim + port = 12345 + } +... +} + +then an Exim authenticator can be configured :- + + dovecot-plain: + driver = dovecot + public_name = PLAIN + server_socket = dovecot_server_name 12345 + server_tls = true + server_set_id = $auth1 + +If the server_socket does not start with a / it is taken as a hostname (or IP); +and a whitespace-separated port number must be given. + + + + +Logging protocol unusual states +--------------------------------------------------------------- +An extra log_selector, "protocol_detail" has been added in the default build. +The name may change in future, hence the Experimenal status. + +Currrently the only effect is to enable logging, under TLS, +of a TCP RST received directly after a QUIT (in server mode). +Outlook is consistently doing this; not waiting for the SMTP response +to its QUIT, not properly closing the TLS session and not properly closing +the TCP connection. Previously this resulted is an error from SSL_write +being logged. -------------------------------------------------------------- End of file