X-Git-Url: https://git.exim.org/users/jgh/exim.git/blobdiff_plain/5ffbf3a3b9bbc8ebc7a0b29bf753febe283f5fa9..d6c829b9a350f61c98196768e3260beb3cbecbfe:/doc/doc-txt/experimental-spec.txt diff --git a/doc/doc-txt/experimental-spec.txt b/doc/doc-txt/experimental-spec.txt index 84fd54716..2b77d2647 100644 --- a/doc/doc-txt/experimental-spec.txt +++ b/doc/doc-txt/experimental-spec.txt @@ -390,237 +390,6 @@ 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: - -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 @@ -871,117 +640,61 @@ 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}} - - +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 --------------------------------------------------------------