Auths: fix possible OOB write in external authenticator. Bug 2999

[exim.git] / doc / doc-txt / experimental-spec.txt

diff --git a/doc/doc-txt/experimental-spec.txt b/doc/doc-txt/experimental-spec.txt
index cff937f7fb0815d69bd43c5bf0a1a3c8d999b70e..f61db629ea444bb9a35856068eba08c0ed7fc781 100644 (file)
--- a/doc/doc-txt/experimental-spec.txt
+++ b/doc/doc-txt/experimental-spec.txt
@@ -1,414 +1,12 @@
-$Cambridge: exim/doc/doc-txt/experimental-spec.txt,v 1.9 2007/10/04 13:21:06 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
 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.
-
-  dkim_sign_headers = <expanded string> [OPTIONAL]
-
-    When set, this option must expand to (or be specified as)
-    a colon-separated list of header names. These headers will
-    be included in the message signature. When unspecified,
-    the recommended headers will be used. Currently, these
-    are:
-
-    from:sender:reply-to:subject:date:
-    message-id:to:cc:mime-version:content-type:
-    content-transfer-encoding:content-id:
-    content-description:resent-date:resent-from:
-    resent-sender:resent-to:resent-cc:resent-message-id:
-    in-reply-to:references:
-    list-id:list-help:list-unsubscribe:
-    list-subscribe:list-post:list-owner:list-archive
-
-
-
-
-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.
-
-
-  $dk_result
-
-    Contains a  human-readable result  of the  DK check,  more
-    verbose than $dk_status. Useful for logging purposes.
-
-
+about experimental  features, all  of which  are unstable and
+liable to incompatible change.

 
 

-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
 --------------------------------------------------------------
 
 Brightmail  AntiSpam  is  a  commercial  package.  Please  see


@@ -444,7 +42,7 @@ 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
 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
   (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


@@ -694,139 +292,408 @@ These four steps are explained in more details below.
 
 
 
 
 
 

-3. Sender Policy Framework (SPF) support
+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.
+
+
+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.

 
 

-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
+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>.  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 = <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.
+
+
+
+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.
+

 
 

-Exiscan  currently  includes SRS  support  via Miles  Wilton's
-libsrs_alt library. The current version of the supported
-library is 0.5.

 
 

-In order to  use SRS, you  must get a  copy of libsrs_alt from
+XCLIENT proxy support
+---------------------------------------------------------------
+Per https://www.postfix.org/XCLIENT_README.html

 
 

-http://srs.mirtol.com/
+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)

 
 

-Unpack the tarball, then refer to MTAs/README.EXIM
-to proceed. You need to set
+If compiled with EXPERIMENTAL_XCLIENT=yes :-

 
 

-EXPERIMENTAL_SRS=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).

 
 

-in your Local/Makefile.
+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

 
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