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: : user2@thatdomain.com: The example: accept domains = +relay_to_domains endpass verify = recipient bmi_optin = ${lookup{$local_part@$domain}lsearch{/etc/exim/bmi_optin_data}} control = bmi_run Of course, you can also use any other lookup method that Exim supports, including LDAP, Postgres, MySQL, Oracle etc., as long as the result is a list of colon-separated opt-in strings. For a list of available opt-in strings, please contact your Brightmail representative. DCC Support -------------------------------------------------------------- Distributed Checksum Clearinghouse; http://www.rhyolite.com/dcc/ *) Building exim In order to build exim with DCC support add EXPERIMENTAL_DCC=yes to your Makefile. (Re-)build/install exim. exim -d should show EXPERIMENTAL_DCC under "Support for". *) Configuration In the main section of exim.cf add at least dccifd_address = /usr/local/dcc/var/dccifd or dccifd_address = In the DATA ACL you can use the new condition dcc = * After that "$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. 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. 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 . 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 = : : [ : ] 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 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 --------------------------------------------------------------