-$Cambridge: exim/doc/doc-txt/experimental-spec.txt,v 1.8 2007/09/28 12:58:41 tom Exp $
-
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 experimenatal features, all of which are unstable and
-liable to incompatibile change.
-
-
-0. DKIM support
---------------------------------------------------------------
-
-DKIM support is implemented via libdkim. A compatible version
-is available here:
-
-http://duncanthrax.net/exim-experimental/libdkim-1.0.15-tk.tar.gz
-
-Build the lib according to the instructions in the enclosed
-INSTALL file.
-
-To build Exim with DKIM support, specify this in Local/Makefile:
-
-EXPERIMENTAL_DKIM=yes
-CFLAGS += -I/home/tom/libdkim/include
-LDFLAGS += -ldkim -lssl -lstdc++ -L/home/tom/libdkim/lib
-
-Remember to tweak the CFLAGS and LDFLAGS lines to match the
-location of the libdomainkeys includes and lib on your system.
-
-The current experimental implementation supports two independent
-functions:
-
-o Validate incoming DKIM-signed email.
-o Sign outgoing email with DKIM.
-
-The former is implemented in the ACLs for SMTP, the latter as
-an extension to the SMTP transport. That means both facilities
-are limited to SMTP I/O.
-
-
-1) Validate incoming email
-
-Incoming messages are fed to the DKIM validation process as they
-are received "on the wire". This happens synchronously to Exim's
-buffering of the message in the spool.
-
-You must set "control = dkim_verify" in one of the ACLs preceding
-DATA (you will typically use acl_smtp_rcpt), at a point where
-non-local, non-relay, non-submission mail is processed. If that
-control flag is not set, the message will NOT be verified.
-
-Example:
-
-warn log_message = Feeding message to DKIM validator.
- control = dk_verify
-
-You can then check for DKIM signatures in the ACL after data
-(acl_smtp_data), using the 'dkim' query-style lookup type. The
-query string should be a domain or DKIM identity:
-
-${lookup dkim{domain.example}}
-
-Such a lookup will yield one of the following strings:
-
-unverified: Exim did not (yet) verify the eventual DKIM
- signatures in this message. This may happen
- if a) You did not use control=dkim_verify
- or b) You are using the lookup before
- the DATA ACL.
-
-unsigned: The message does not have a signature from
- the specified domain.
-
-good: The message has a signature from the specified
- domain, and it verified successfully.
-
-bad: The message has a signature from the specified
- domain, but it did not verify.
-
-defer: A temporary DNS problem was encountered while
- trying to verify the signature.
-
-
-
-2) Sign outgoing email with DKIM
-
-Outgoing messages are signed just before Exim puts them "on
-the wire". The only thing that happens after DKIM signing is
-eventual TLS encryption.
-
-Signing is implemented by setting private options on the SMTP
-transport. These options take (expandable) strings as
-arguments.
-
- dkim_domain = <expanded string> [MANDATORY]
-
- The domain you want to sign with. Should optimally match
- the domain in the "From:" header of the message, but
- does not necessarily have to. The result of this expanded
- option is put into the $dkim_domain expansion variable.
-
- dkim_selector = <expanded string> [MANDATORY]
-
- This sets the key selector string. You can use the
- $dkim_domain expansion variable to look up a matching
- selector. The result is put in the expansion variable
- $dkim_selector which should be used in the dkim_private_key
- option along with $dkim_domain.
-
- dkim_private_key = <expanded string> [MANDATORY]
-
- This sets the private key to use. You can use the
- $dkim_domain and $dkim_selector expansion variables to
- determine the private key to use. The result can either
-
- o be a valid RSA private key in ASCII armor, including
- line breaks.
- o start with a slash, in which case it is treated as
- a file that contains the private key.
- o be "0", "false" or the empty string, in which case
- the message will not be signed. This case will not
- result in an error, even if dkim_strict is set.
-
- dkim_canon = <expanded string> [OPTIONAL]
-
- This option sets the canonicalization method used when
- signing a message. The DKIM RFC currently supports two
- methods: "simple" and "relaxed". The option defaults to
- "relaxed" when unset. Note: the current implementation
- only support using the same canonicalization method for
- both headers and body.
-
- dkim_strict = <expanded string> [OPTIONAL]
-
- This option defines how Exim behaves when signing a
- message that should be signed fails for some reason. When
- the expansion evaluates to either "1" or "true", Exim will
- defer. Otherwise Exim will send the message unsigned. You
- can use the $dkim_domain and $dkim_selector expansion
- variables here.
-
-
-
-
-
-1. Yahoo DomainKeys support
---------------------------------------------------------------
-
-DomainKeys (DK) support is built into Exim using the
-"libdomainkeys" reference library implementation. It is
-available at
-
-http://domainkeys.sf.net
-
-You must build this library on your system and compile Exim
-against it. To build Exim with DK support, add these lines to
-your Local/Makefile:
-
-EXPERIMENTAL_DOMAINKEYS=yes
-CFLAGS += -I/home/tom/exim-cvs/extra/libdomainkeys
-LDFLAGS += -ldomainkeys -L/home/tom/exim-cvs/extra/libdomainkeys
-
-Remember to tweak the CFLAGS and LDFLAGS lines to match the
-location of the libdomainkeys includes and lib on your system.
-
-The current experimental implementation supports two
-independent functions:
-
-o Validate incoming DK-signed email.
-o Sign outgoing email with DK.
-
-The former is implemented in the ACLs for SMTP, the latter as
-an extension to the SMTP transport. That means both facilities
-are limited to SMTP I/O.
-
-
-
-1) Validate incoming email
-
-Incoming messages are fed to the DK validation process as they
-are received "on the wire". This happens synchronously to
-Exim's buffering of the message in the spool.
-
-You must set "control = dk_verify" in one of the ACLs
-preceding DATA (you will typically use acl_smtp_rcpt), at a
-point where non-local, non-relay, non-submission mail is
-processed. If that control flag is not set, the message will
-NOT be verified.
-
-Example:
-
-warn log_message = Feeding message to DK validator.
- control = dk_verify
-
-You can check for the outcome of the DK check in the ACL after
-data (acl_smtp_data), using a number of ACL conditions and/or
-expansion variables.
-
-
-
-1.1.) DK ACL conditions
-
- dk_sender_domains = <domain list>
-
- This condition takes a domainlist as argument and
- succeeds if the domain that DK has been verifying for is
- found in the list.
-
-
- dk_senders = <address list>
-
- This condition takes an addresslist as argument and
- succeeds if the address that DK has been verifying for
- is found in the list.
-
-
- dk_sender_local_parts = <local part list>
-
- This condition takes a local_part list as argument
- and succeeds if the domain that DK has been
- verifying for is found in the list.
-
-
- dk_status = <colon separated list of keywords>
-
- This condition takes a list of keywords as argument, and
- succeeds if one of the listed keywords matches the outcome
- of the DK check. The available keywords are:
-
- good DK check succeeded, mail is verified.
- bad DK check failed.
- no signature Mail is not signed with DK.
- no key Public key missing in target domain DNS.
- bad format Public key available, but unuseable.
- non-participant Target domain states not to participate in DK.
- revoked The signing key has been revoked by the domain.
-
-
- dk_policy = <colon separated list of keywords>
-
- This condition takes a list of keywords as argument, and
- succeeds if one of the listed keywords matches the policy
- announced by the target domain. The available keywords
- are:
-
- signsall The target domain signs all outgoing email.
- testing The target domain is currently testing DK.
-
-
- dk_domain_source = <colon separated list of keywords>
-
- This condition takes a list of keywords as argument, and
- succeeds if one of the listed keywords matches the
- location where DK found the sender domain it verified for.
- The available keywords are:
-
- from The domain came from the "From:" header.
- sender The domain came from the "Sender:" header.
- none DK was unable to find the responsible domain.
-
-
-
-1.2.) DK verification expansion variables
-
- $dk_sender_domain
-
- Contains the domain that DK has verified for.
-
-
- $dk_sender
-
- Contains the address that DK has verified for.
-
-
- $dk_sender_local_part
-
- Contains the local part that DK has verified for.
-
-
- $dk_sender_source
-
- Contains the "source" of the above three variables, one of
-
- "from" The address came from the "From:" header.
- "sender" The address came from the "Sender:" header.
-
- When DK was unable to find a valid address, this variable
- is "0".
-
-
- $dk_signsall
-
- Is "1" if the target domain signs all outgoing email,
- "0" otherwise.
-
-
- $dk_testing
-
- Is "1" if the target domain is testing DK, "0" otherwise.
-
-
- $dk_is_signed
-
- Is "1" if the message is signed, "0" otherwise.
-
-
- $dk_status
-
- Contains the outcome of the DK check as a string, commonly
- used to add a "DomainKey-Status:" header to messages. Will
- contain one of:
-
- good DK check succeeded, mail is verified.
- bad DK check failed.
- no signature Mail is not signed with DK.
- no key Public key missing in target domain DNS.
- bad format Public key available, but unuseable.
- non-participant Target domain states not to participate in DK.
- revoked The signing key has been revoked by the domain.
+about experimental features, all of which are unstable and
+liable to incompatible change.
- $dk_result
-
- Contains a human-readable result of the DK check, more
- verbose than $dk_status. Useful for logging purposes.
-
-
-
-2) Sign outgoing email with DK
-
-Outgoing messages are signed just before Exim puts them "on
-the wire". The only thing that happens after DK signing is
-eventual TLS encryption.
-
-Signing is implemented by setting private options on the SMTP
-transport. These options take (expandable) strings as
-arguments. The most important variable to use in these
-expansions is $dk_domain. It contains the domain that DK wants
-to sign for.
-
-
- dk_selector = <expanded string> [MANDATORY]
-
- This sets the key selector string. You can use the
- $dk_domain expansion variable to look up a matching
- selector. The result is put in the expansion variable
- $dk_selector which should be used in the dk_private_key
- option along with $dk_domain.
-
-
- dk_private_key = <expanded string> [MANDATORY]
-
- This sets the private key to use. You SHOULD use the
- $dk_domain and $dk_selector expansion variables to
- determine the private key to use. The result can either
-
- o be a valid RSA private key in ASCII armor, including
- line breaks.
- o start with a slash, in which case it is treated as
- a file that contains the private key.
- o be "0", "false" or the empty string, in which case
- the message will not be signed. This case will not
- result in an error, even if dk_strict is set.
-
-
- dk_canon = <expanded string> [OPTIONAL]
-
- This option sets the canonicalization method used when
- signing a message. The DK draft currently supports two
- methods: "simple" and "nofws". The option defaults to
- "simple" when unset.
-
-
- dk_strict = <expanded string> [OPTIONAL]
-
- This option defines how Exim behaves when signing a
- message that should be signed fails for some reason. When
- the expansion evaluates to either "1" or "true", Exim will
- defer. Otherwise Exim will send the message unsigned. You
- can and should use the $dk_domain and $dk_selector
- expansion variables here.
-
-
- dk_domain = <expanded string> [NOT RECOMMENDED]
-
- This option overrides DKs autodetection of the signing
- domain. You should only use this option if you know what
- you are doing. The result of the string expansion is also
- put in $dk_domain.
-
-
-
-
-2. Brightmail AntiSpam (BMI) suppport
+Brightmail AntiSpam (BMI) support
--------------------------------------------------------------
Brightmail AntiSpam is a commercial package. Please see
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
-3. Sender Policy Framework (SPF) support
---------------------------------------------------------------
-
-To learn more about SPF, visit http://spf.pobox.com. 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 dependend 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.
- o err_perm This indicates a syntax error in the SPF
- record of the queried domain. This should be
- treated like "none".
- o err_temp This indicates a temporary error during all
- processing, including Exim's SPF processing.
- You may defer messages when this occurs.
-
-You can prefix each string with an exclamation mark to invert
-is 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 a simple example to fail forgery attempts from domains
-that publish SPF records:
-
-/* -----------------
-deny message = $sender_host_address is not allowed to send mail from $sender_address_domain
- 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.
-
- $spf_result
- This contains the outcome of the SPF check in string form,
- one of pass, fail, softfail, none, neutral, err_perm or
- err_temp.
-
- $spf_smtp_comment
- This contains a string that can be used in a SMTP response
- to the calling party. Useful for "fail".
-
-
-
-4. SRS (Sender Rewriting Scheme) Support
+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.
+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
-http://srs.mirtol.com/
+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
in your Local/Makefile.
+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
+
+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
+ $srs_status
+
+The predefined feature-macro _HAVE_SRS will be present.
+Additional delivery log line elements, tagged with "SRS=" will show the srs sender.
+For configuration information see https://github.com/Exim/exim/wiki/SRS .
+
+
+
+
+DCC Support
+--------------------------------------------------------------
+Distributed Checksum Clearinghouse; http://www.rhyolite.com/dcc/
+
+*) Building exim
+
+In order to build exim with DCC support add
+
+EXPERIMENTAL_DCC=yes
+
+to your Makefile. (Re-)build/install exim. exim -d should show
+EXPERIMENTAL_DCC under "Support for".
+
+
+*) Configuration
+
+In the main section of exim.cf add at least
+ dccifd_address = /usr/local/dcc/var/dccifd
+or
+ dccifd_address = <ip> <port>
+
+In the DATA ACL you can use the new condition
+ dcc = *
+
+After that "$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:<d3@myhost.test.ex>: 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.
+
+
+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.
+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/<acceptable_list> none:fail:pass
+
+ add_header = :at_start:${authresults {<admd-identifier>}}
+
+ Note that it would be wise to strip incoming messages of A-R headers
+ that claim to be from our own <admd-identifier>.
+
+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 = <admd-identifier> : <selector> : <privkey> [ : <options> ]
+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[=<val>] Add an x= tag to the generated AMS header, with an expiry time.
+ If the value <val> 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.
+
+
+
+
+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
git://git.exim.org / exim.git / blobdiff