X-Git-Url: https://git.exim.org/exim.git/blobdiff_plain/617d39327e65b7fccc41a12b4a5e2940d6327c9f..ddd83c1e419a8939d6655fc061de16b271446162:/doc/doc-txt/experimental-spec.txt diff --git a/doc/doc-txt/experimental-spec.txt b/doc/doc-txt/experimental-spec.txt index 4ed6f2518..f61db629e 100644 --- a/doc/doc-txt/experimental-spec.txt +++ b/doc/doc-txt/experimental-spec.txt @@ -292,28 +292,6 @@ These four steps are explained in more details below. -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/ @@ -390,227 +368,6 @@ 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: - -Optional: -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/. - If unset, "/etc/exim/opendmarc.tlds" (hardcoded) - is used. - -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 $dmarc_used_domain failed sender's DMARC policy, REJECT - - - DSN extra information --------------------- If compiled with EXPERIMENTAL_DSN_INFO extra information will be added @@ -653,52 +410,6 @@ Rationale: Note that non-RFC-documented field names and data types are used. -LMDB Lookup support -------------------- -LMDB is an ultra-fast, ultra-compact, crash-proof key-value embedded data store. -It is modeled loosely on the BerkeleyDB API. You should read about the feature -set as well as operation modes at https://symas.com/products/lightning-memory-mapped-database/ - -LMDB single key lookup support is provided by linking to the LMDB C library. -The current implementation does not support writing to the LMDB database. - -Visit https://github.com/LMDB/lmdb to download the library or find it in your -operating systems package repository. - -If building from source, this description assumes that headers will be in -/usr/local/include, and that the libraries are in /usr/local/lib. - -1. In order to build exim with LMDB lookup support add or uncomment - -EXPERIMENTAL_LMDB=yes - -to your Local/Makefile. (Re-)build/install exim. exim -d should show -Experimental_LMDB in the line "Support for:". - -EXPERIMENTAL_LMDB=yes -LDFLAGS += -llmdb -# CFLAGS += -I/usr/local/include -# LDFLAGS += -L/usr/local/lib - -The first line sets the feature to include the correct code, and -the second line says to link the LMDB libraries into the -exim binary. The commented out lines should be uncommented if you -built LMDB from source and installed in the default location. -Adjust the paths if you installed them elsewhere, but you do not -need to uncomment them if an rpm (or you) installed them in the -package controlled locations (/usr/include and /usr/lib). - -2. Create your LMDB files, you can use the mdb_load utility which is -part of the LMDB distribution our your favourite language bindings. - -3. Add the single key lookups to your exim.conf file, example lookups -are below. - -${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. @@ -707,6 +418,8 @@ 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 @@ -723,7 +436,7 @@ the queuefile driver. The transport only takes one option: * directory - This is used to specify the directory messages should be -copied to +copied to. Expanded. The generic transport options (body_only, current_directory, disable_logging, debug_print, delivery_date_add, envelope_to_add, event_action, group, @@ -761,6 +474,9 @@ 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. @@ -769,10 +485,18 @@ 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. +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 @@ -789,19 +513,187 @@ standard header. 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 . + that claim to be from our own . Eg: + + remove_header = \N^(?i)Authentication-Results\s*::\s*example.org;\N + +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 = : : +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. + + + +Limits ESMTP extension +--------------------------------------------------------------- +Per https://datatracker.ietf.org/doc/html/draft-freed-smtp-limits-01 +(as of 2023/08/04, version -05 has been published. It does not seems +to be substantively different.) + +If compiled with EXPERIMENTAL_ESMTP_LIMITS=yes :- + +As a server, Exim will advertise, in the EHLO response, the limit for RCPT +commands set by the recipients_max main-section config option (if it is set), +and the limit for MAIL commands set by the smtp_accept_max_per_connection +option. + +Note that as of writing, smtp_accept_max_per_connection is expanded but +recipients_max is not. + +A new main-section option "limits_advertise_hosts" controls whether +the limits are advertised; the default for the option is "*". + +As a client, Exim will: + + - note an advertised MAILMAX; the lower of the value given and the + value from the transport connection_max_messages option is used. + + - note an advertised RCPTMAX; the lower of the + value given and the value from the transport max_rcpt option is used. + Parallisation of transactions is not done if due to a RCPTMAX, unlike + max_rcpt. + + - note an advertised RCPTDOMAINMAX, and behave as if the transport + multi_domains option was set to false. The value advertised is ignored. + +Values advertised are only noted for TLS connections and ones for which +the server does not advertise TLS support. + + + +XCLIENT proxy support +--------------------------------------------------------------- +Per https://www.postfix.org/XCLIENT_README.html + +XCLIENT is an ESMTP extension supporting an inbound proxy. +The only client immplementation known is in Nginx +(https://nginx.org/en/docs/mail/ngx_mail_proxy_module.html) + +If compiled with EXPERIMENTAL_XCLIENT=yes :- + +As a server, Exim will advertise XCLIENT support (conditional on a new option +"hosts_xclient") and service XCLIENT commands with parameters + ADDR + NAME + PORT + LOGIN + DESTADDR + DESTPORT +A fresh HELO/EHLO is required after a succesful XCLIENT, and the usual +values are derived from that (making the HELO and PROTO paramemters redundant). +An XCLIENT command must give both ADDR and PORT parameters if no previous +XCLIENT has succeeded in the SMTP session. +After a success: + $proxy_session variable becomes "yes" + $proxy_local_address, $proxy_local_port have the proxy "inside" values + $proxy_external_address, $proxy_external_port have the proxy "outside" values + $sender_host_address, $sender_host_port have the remot client values -------------------------------------------------------------- End of file