X-Git-Url: https://git.exim.org/users/jgh/exim.git/blobdiff_plain/4398228719841b08524520b8c07faeb3bee89705..e5903596a0f6b078f5854ca591963e21c7f7328a:/doc/doc-txt/experimental-spec.txt?ds=sidebyside diff --git a/doc/doc-txt/experimental-spec.txt b/doc/doc-txt/experimental-spec.txt index b981f3b7a..301152f1a 100644 --- a/doc/doc-txt/experimental-spec.txt +++ b/doc/doc-txt/experimental-spec.txt @@ -428,16 +428,17 @@ 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: +2. Use the following global options to configure DMARC: -Optional: +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/. - If unset, "/etc/exim/opendmarc.tlds" (hardcoded) - is used. + See also util/renew-opendmarc-tlds.sh script. + The default for the option is currently + /etc/exim/opendmarc.tlds Optional: dmarc_history_file Defines the location of a file to log results @@ -448,11 +449,19 @@ dmarc_history_file Defines the location of a file to log results directory of this file is writable by the user exim runs as. -dmarc_forensic_sender The email address to use when sending a +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. - Default: do-not-reply@$default_hostname + + If set, this is expanded and used for the + From: header line; the address is extracted + from it and used for the envelope from. + If not set, the From: header is expanded from + the dsn_from option, and <> is used for the + envelope from. + + Default: unset. 3. By default, the DMARC processing will run for any remote, @@ -525,6 +534,9 @@ Of course, you can also use any other lookup method that Exim supports, including LDAP, Postgres, MySQL, etc, as long as the result is a list of colon-separated strings. +Performing the check sets up information used by the +${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: @@ -548,9 +560,8 @@ expansion variables are available: 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. +A now-redundant variable $dmarc_ar_header has now been withdrawn. +Use the ${authresults } expansion instead. 5. How to enable DMARC advanced operation: @@ -590,7 +601,6 @@ b. Configure, somewhere before the DATA ACL, the control option to 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 = * @@ -609,6 +619,8 @@ b. Configure, somewhere before the DATA ACL, the control option to !authenticated = * message = Message from $dmarc_used_domain failed sender's DMARC policy, REJECT + warn add_header = :at_start:${authresults {$primary_hostname}} + DSN extra information @@ -707,6 +719,8 @@ 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 +737,7 @@ 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, @@ -791,7 +805,21 @@ standard header. Note that it would be wise to strip incoming messages of A-R headers that claim to be from our own . -There are two new variables: $arc_state and $arc_state_reason. +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". @@ -803,7 +831,7 @@ 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 three elements must be non-empty. +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 @@ -822,12 +850,18 @@ 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. @@ -838,6 +872,142 @@ used via the transport in question. + +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}} + + + + +TLS Session Resumption +---------------------- +TLS Session Resumption for TLS 1.2 and TLS 1.3 connections can be used (defined +in RFC 5077 for 1.2). The support for this can be included by building with +EXPERIMENTAL_TLS_RESUME defined. This requires GnuTLS 3.6.3 or OpenSSL 1.1.1 +(or later). + +Session resumption (this is the "stateless" variant) involves the server sending +a "session ticket" to the client on one connection, which can be stored by the +client and used for a later session. The ticket contains sufficient state for +the server to reconstruct the TLS session, avoiding some expensive crypto +calculation and one full packet roundtrip time. + +Operational cost/benefit: + The extra data being transmitted costs a minor amount, and the client has + extra costs in storing and retrieving the data. + + In the Exim/Gnutls implementation the extra cost on an initial connection + which is TLS1.2 over a loopback path is about 6ms on 2017-laptop class hardware. + The saved cost on a subsequent connection is about 4ms; three or more + connections become a net win. On longer network paths, two or more + connections will have an average lower startup time thanks to the one + saved packet roundtrip. TLS1.3 will save the crypto cpu costs but not any + packet roundtrips. + + Since a new hints DB is used, the hints DB maintenance should be updated + to additionally handle "tls". + +Security aspects: + The session ticket is encrypted, but is obviously an additional security + vulnarability surface. An attacker able to decrypt it would have access + all connections using the resumed session. + The session ticket encryption key is not committed to storage by the server + and is rotated regularly (OpenSSL: 1hr, and one previous key is used for + overlap; GnuTLS 6hr but does not specify any overlap). + Tickets have limited lifetime (2hr, and new ones issued after 1hr under + OpenSSL. GnuTLS 2hr, appears to not do overlap). + + There is a question-mark over the security of the Diffie-Helman parameters + used for session negotiation. TBD. q-value; cf bug 1895 + +Observability: + New log_selector "tls_resumption", appends an asterisk to the tls_cipher "X=" + element. + + Variables $tls_{in,out}_resumption have bits 0-4 indicating respectively + support built, client requested ticket, client offered session, + server issued ticket, resume used. A suitable decode list is provided + in the builtin macro _RESUME_DECODE for ${listextract {}{}}. + +Issues: + In a resumed session: + $tls_{in,out}_cipher will have values different to the original (under GnuTLS) + $tls_{in,out}_ocsp will be "not requested" or "no response", and + hosts_require_ocsp will fail + + -------------------------------------------------------------- End of file --------------------------------------------------------------