Security considerations: running local commands

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

From time to time, experimental features may be added to Exim.
While a feature  is experimental, there  will be a  build-time
option whose name starts  "EXPERIMENTAL_" that must be  set in
order to include the  feature. This file contains  information
about experimental  features, all  of which  are unstable and
liable to incompatible change.


PRDR support
--------------------------------------------------------------

Per-Recipient Data Reponse is an SMTP extension proposed by Eric Hall
in a (now-expired) IETF draft from 2007.  It's not hit mainstream
use, but has apparently been implemented in the META1 MTA.

There is mention at http://mail.aegee.org/intern/sendmail.html
of a patch to sendmail "to make it PRDR capable".

 ref: http://www.eric-a-hall.com/specs/draft-hall-prdr-00.txt

If Exim is built with EXPERIMENTAL_PRDR there is a new config
boolean "prdr_enable" which controls whether PRDR is advertised
as part of an EHLO response, a new "acl_data_smtp_prdr" ACL
(called for each recipient, after data arrives but before the
data ACL), and a new smtp transport option "hosts_try_prdr".

PRDR may be used to support per-user content filtering.  Without it
one must defer any recipient after the first that has a different
content-filter configuration.  With PRDR, the RCPT-time check
for this can be disabled when the MAIL-time $smtp_command included
"PRDR".  Any required difference in behaviour of the main DATA-time
ACL should however depend on the PRDR-time ACL having run, as Exim
will avoid doing so in some situations (eg.  single-recipient mails).



OCSP Stapling support
--------------------------------------------------------------

X.509 PKI certificates expire and can be revoked; to handle this, the
clients need some way to determine if a particular certificate, from a
particular Certificate Authority (CA), is still valid.  There are three
main ways to do so.

The simplest way is to serve up a Certificate Revocation List (CRL) with
an ordinary web-server, regenerating the CRL before it expires.  The
downside is that clients have to periodically re-download a potentially
huge file from every certificate authority it knows of.

The way with most moving parts at query time is Online Certificate
Status Protocol (OCSP), where the client verifies the certificate
against an OCSP server run by the CA.  This lets the CA track all
usage of the certs.  This requires running software with access to the
private key of the CA, to sign the responses to the OCSP queries.  OCSP
is based on HTTP and can be proxied accordingly.

The only widespread OCSP server implementation (known to this writer)
comes as part of OpenSSL and aborts on an invalid request, such as
connecting to the port and then disconnecting.  This requires
re-entering the passphrase each time some random client does this.

The third way is OCSP Stapling; in this, the server using a certificate
issued by the CA periodically requests an OCSP proof of validity from
the OCSP server, then serves it up inline as part of the TLS
negotiation.   This approach adds no extra round trips, does not let the
CA track users, scales well with number of certs issued by the CA and is
resilient to temporary OCSP server failures, as long as the server
starts retrying to fetch an OCSP proof some time before its current
proof expires.  The downside is that it requires server support.

If Exim is built with EXPERIMENTAL_OCSP and it was built with OpenSSL,
then it gains a new global option: "tls_ocsp_file".

The file specified therein is expected to be in DER format, and contain
an OCSP proof.  Exim will serve it as part of the TLS handshake.  This
option will be re-expanded for SNI, if the tls_certificate option
contains $tls_sni, as per other TLS options.

Exim does not at this time implement any support for fetching a new OCSP
proof.  The burden is on the administrator to handle this, outside of
Exim.  The file specified should be replaced atomically, so that the
contents are always valid.  Exim will expand the "tls_ocsp_file" option
on each connection, so a new file will be handled transparently on the
next connection.

Exim will check for a valid next update timestamp in the OCSP proof;
if not present, or if the proof has expired, it will be ignored.

Also, given EXPERIMENTAL_OCSP and OpenSSL, the smtp transport gains
a "hosts_require_ocsp" option; a host-list for which an OCSP Stapling
is requested and required for the connection to proceed.  The host(s)
should also be in "hosts_require_tls", and "tls_verify_certificates"
configured for the transport.

At this point in time, we're gathering feedback on use, to determine if
it's worth adding complexity to the Exim daemon to periodically re-fetch
OCSP files and somehow handling multiple files.




Brightmail AntiSpam (BMI) suppport
--------------------------------------------------------------

Brightmail  AntiSpam  is  a  commercial  package.  Please  see
http://www.brightmail.com    for    more    information     on
the product. For  the sake of  clarity, we'll refer  to it  as
"BMI" from now on.


0) BMI concept and implementation overview

In  contrast  to   how  spam-scanning  with   SpamAssassin  is
implemented  in  exiscan-acl,  BMI  is  more  suited  for  per
-recipient  scanning of  messages. However,  each messages  is
scanned  only  once,  but  multiple  "verdicts"  for  multiple
recipients can be  returned from the  BMI server. The  exiscan
implementation  passes  the  message to  the  BMI  server just
before accepting it.  It then adds  the retrieved verdicts  to
the messages header file in the spool. These verdicts can then
be  queried  in  routers,  where  operation  is  per-recipient
instead  of per-message.  To use  BMI, you  need to  take the
following steps:

  1) Compile Exim with BMI support
  2) Set up main BMI options (top section of Exim config file)
  3) Set up ACL control statement (ACL section of the config
     file)
  4) Set up your routers to use BMI verdicts (routers section
     of the config file).
  5) (Optional) Set up per-recipient opt-in information.

These four steps are explained in more details below.

1) Adding support for BMI at compile time

  To compile with BMI support,  you need to link Exim  against
  the   Brighmail  client   SDK,  consisting   of  a   library
  (libbmiclient_single.so)  and  a  header  file  (bmi_api.h).
  You'll also need to explicitly set a flag in the Makefile to
  include BMI support in the Exim binary. Both can be achieved
  with  these lines in Local/Makefile:

  EXPERIMENTAL_BRIGHTMAIL=yes
  CFLAGS=-I/path/to/the/dir/with/the/includefile
  EXTRALIBS_EXIM=-L/path/to/the/dir/with/the/library -lbmiclient_single

  If  you use  other CFLAGS  or EXTRALIBS_EXIM  settings then
  merge the content of these lines with them.

  Note for BMI6.x users: You'll also have to add -lxml2_single
  to the EXTRALIBS_EXIM line. Users of 5.5x do not need to  do
  this.

  You    should     also    include     the    location     of
  libbmiclient_single.so in your dynamic linker  configuration
  file   (usually   /etc/ld.so.conf)   and   run    "ldconfig"
  afterwards, or  else the  produced Exim  binary will  not be
  able to find the library file.


2) Setting up BMI support in the Exim main configuration

  To enable BMI  support in the  main Exim configuration,  you
  should set the path to the main BMI configuration file  with
  the "bmi_config_file" option, like this:

  bmi_config_file = /opt/brightmail/etc/brightmail.cfg

  This must go into section 1 of Exim's configuration file (You
  can  put it  right on  top). If  you omit  this option,  it
  defaults to /opt/brightmail/etc/brightmail.cfg.

  Note for BMI6.x users: This  file is in XML format  in V6.xx
  and its  name is  /opt/brightmail/etc/bmiconfig.xml. So  BMI
  6.x users MUST set the bmi_config_file option.


3) Set up ACL control statement

  To  optimize performance,  it makes  sense only  to process
  messages coming from remote, untrusted sources with the  BMI
  server.  To set  up a  messages for  processing by  the BMI
  server, you MUST set the "bmi_run" control statement in  any
  ACL for an incoming message.  You will typically do this  in
  an "accept"  block in  the "acl_check_rcpt"  ACL. You should
  use the "accept" block(s)  that accept messages from  remote
  servers for your own domain(s). Here is an example that uses
  the "accept" blocks from Exim's default configuration file:


  accept  domains       = +local_domains
          endpass
          verify        = recipient
          control       = bmi_run

  accept  domains       = +relay_to_domains
          endpass
          verify        = recipient
          control       = bmi_run

  If bmi_run  is not  set in  any ACL  during reception of the
  message, it will NOT be passed to the BMI server.


4) Setting up routers to use BMI verdicts

  When a message has been  run through the BMI server,  one or
  more "verdicts" are  present. Different recipients  can have
  different verdicts. Each  recipient is treated  individually
  during routing, so you  can query the verdicts  by recipient
  at  that stage.  From Exim's  view, a  verdict can  have the
  following outcomes:

  o deliver the message normally
  o deliver the message to an alternate location
  o do not deliver the message

  To query  the verdict  for a  recipient, the  implementation
  offers the following tools:


  - Boolean router  preconditions. These  can be  used in  any
    router. For a simple  implementation of BMI, these  may be
    all  that  you  need.  The  following  preconditions   are
    available:

    o bmi_deliver_default

      This  precondition  is  TRUE  if  the  verdict  for  the
      recipient is  to deliver  the message  normally. If  the
      message has not been  processed by the BMI  server, this
      variable defaults to TRUE.

    o bmi_deliver_alternate

      This  precondition  is  TRUE  if  the  verdict  for  the
      recipient  is to  deliver the  message to  an alternate
      location.  You  can  get the  location  string  from the
      $bmi_alt_location expansion variable if you need it. See
      further below. If the message has not been processed  by
      the BMI server, this variable defaults to FALSE.

    o bmi_dont_deliver

      This  precondition  is  TRUE  if  the  verdict  for  the
      recipient  is  NOT  to   deliver  the  message  to   the
      recipient. You will typically use this precondition in a
      top-level blackhole router, like this:

        # don't deliver messages handled by the BMI server
        bmi_blackhole:
          driver = redirect
          bmi_dont_deliver
          data = :blackhole:

      This router should be on top of all others, so  messages
      that should not be delivered do not reach other  routers
      at all. If   the  message  has  not  been  processed  by
      the  BMI server, this variable defaults to FALSE.


  - A list router  precondition to query  if rules "fired"  on
    the message for the recipient. Its name is "bmi_rule". You
    use  it  by  passing it  a  colon-separated  list of  rule
    numbers. You can use this condition to route messages that
    matched specific rules. Here is an example:

      # special router for BMI rule #5, #8 and #11
      bmi_rule_redirect:
        driver = redirect
        bmi_rule = 5:8:11
        data = postmaster@mydomain.com


  - Expansion variables. Several  expansion variables are  set
    during  routing.  You  can  use  them  in  custom   router
    conditions,  for  example.  The  following  variables  are
    available:

    o $bmi_base64_verdict

      This variable  will contain  the BASE64  encoded verdict
      for the recipient being routed. You can use it to add  a
      header to messages for tracking purposes, for example:

      localuser:
        driver = accept
        check_local_user
        headers_add = X-Brightmail-Verdict: $bmi_base64_verdict
        transport = local_delivery

      If there is no verdict available for the recipient being
      routed, this variable contains the empty string.

    o $bmi_base64_tracker_verdict

      This variable  will contain  a BASE64  encoded subset of
      the  verdict  information  concerning  the  "rules" that
      fired  on the  message. You  can add  this string  to a
      header, commonly named "X-Brightmail-Tracker". Example:

      localuser:
        driver = accept
        check_local_user
        headers_add = X-Brightmail-Tracker: $bmi_base64_tracker_verdict
        transport = local_delivery

      If there is no verdict available for the recipient being
      routed, this variable contains the empty string.

    o $bmi_alt_location

      If  the  verdict  is  to  redirect  the  message  to  an
      alternate  location,  this  variable  will  contain  the
      alternate location string returned by the BMI server. In
      its default configuration, this is a header-like  string
      that can be added to the message with "headers_add".  If
      there is  no verdict  available for  the recipient being
      routed, or if the  message is to be  delivered normally,
      this variable contains the empty string.

    o $bmi_deliver

      This is an additional integer variable that can be  used
      to query if the message should be delivered at all.  You
      should use router preconditions instead if possible.

      $bmi_deliver is '0': the message should NOT be delivered.
      $bmi_deliver is '1': the message should be delivered.


  IMPORTANT NOTE: Verdict inheritance.
  The  message  is passed  to  the BMI  server  during message
  reception,  using the  target addresses  from the  RCPT TO:
  commands in the SMTP transaction. If recipients get expanded
  or re-written (for example by aliasing), the new address(es)
  inherit the  verdict from  the original  address. This means
  that verdicts also apply to all "child" addresses  generated
  from top-level addresses that were sent to the BMI server.


5) Using per-recipient opt-in information (Optional)

  The  BMI server  features multiple  scanning "profiles"  for
  individual recipients.  These are  usually stored  in a LDAP
  server and are  queried by the  BMI server itself.  However,
  you can also  pass opt-in data  for each recipient  from the
  MTA to the  BMI server. This  is particularly useful  if you
  already look  up recipient  data in  Exim anyway  (which can
  also be  stored in  a SQL  database or  other source).  This
  implementation enables you  to pass opt-in  data to the  BMI
  server  in  the  RCPT   ACL.  This  works  by   setting  the
  'bmi_optin' modifier in  a block of  that ACL. If  should be
  set to a list  of comma-separated strings that  identify the
  features which the BMI server should use for that particular
  recipient. Ideally, you  would use the  'bmi_optin' modifier
  in the same  ACL block where  you set the  'bmi_run' control
  flag. Here is an example that will pull opt-in data for each
  recipient      from       a      flat       file      called
  '/etc/exim/bmi_optin_data'.

  The file format:

    user1@mydomain.com: <OPTIN STRING1>:<OPTIN STRING2>
    user2@thatdomain.com: <OPTIN STRING3>


  The example:

    accept  domains       = +relay_to_domains
            endpass
            verify        = recipient
            bmi_optin     = ${lookup{$local_part@$domain}lsearch{/etc/exim/bmi_optin_data}}
            control       = bmi_run

  Of course,  you can  also use  any other  lookup method that
  Exim supports, including LDAP, Postgres, MySQL, Oracle etc.,
  as long as  the result is  a list of  colon-separated opt-in
  strings.

  For a list of available opt-in strings, please contact  your
  Brightmail representative.




Sender Policy Framework (SPF) support
--------------------------------------------------------------

To learn  more  about  SPF, visit   http://www.openspf.org. 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 dependent 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 an example to fail forgery attempts from domains that
publish SPF records:

/* -----------------
deny message = $sender_host_address is not allowed to send mail from ${if def:sender_address_domain {$sender_address_domain}{$sender_helo_name}}.  \
              Please see http://www.openspf.org/Why?scope=${if def:sender_address_domain {mfrom}{helo}};identity=${if def:sender_address_domain {$sender_address}{$sender_helo_name}};ip=$sender_host_address
     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.

  Note: in case of "Best-guess" (see below), the convention is
  to put this string in a header called X-SPF-Guess: instead.

  $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".

In addition to SPF, you can also perform checks for so-called
"Best-guess".  Strictly speaking, "Best-guess" is not standard
SPF, but it is supported by the same framework that enables SPF
capability.  Refer to http://www.openspf.org/FAQ/Best_guess_record
for a description of what it means.

To access this feature, simply use the spf_guess condition in place
of the spf one.  For example:

/* -----------------
deny message = $sender_host_address doesn't look trustworthy to me
     spf_guess = fail
--------------------- */

In case you decide to reject messages based on this check, you
should note that although it uses the same framework, "Best-guess"
is NOT SPF, and therefore you should not mention SPF at all in your
reject message.

When the spf_guess condition has run, it sets up the same expansion
variables as when spf condition is run, described above.

Additionally, since Best-guess is not standardized, you may redefine
what "Best-guess" means to you by redefining spf_guess variable in
global config.  For example, the following:

/* -----------------
spf_guess = v=spf1 a/16 mx/16 ptr ?all
--------------------- */

would relax host matching rules to a broader network range.


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.

In order to  use SRS, you  must get a  copy of libsrs_alt from

http://srs.mirtol.com/

Unpack the tarball, then refer to MTAs/README.EXIM
to proceed. You need to set

EXPERIMENTAL_SRS=yes

in your Local/Makefile.


DCC Support
--------------------------------------------------------------

*) 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 acutally 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 above section on enabling the EXPERIMENTAL_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/.

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 The 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


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_verify_disable

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_forensic_enable

(AGAIN: You can choose not to send these forensic reports by simply
not putting the dmarc_forensic_enable 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 norecord    No policy section in the DMARC record for this
                sender domain.
  o nofrom      Unable to determine the domain of the sender.
  o none        There is no DMARC record for this sender domain.
  o error       Library error or dns error.

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;

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.

  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_ar_header
    This is the entire Authentication-Results header which you can
    add using an add_header modifier.


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_verify_disable

  warn    !domains       = +screwed_up_dmarc_records
          control        = dmarc_enable_forensic

(DATA ACL)
  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 = *
          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    dmarc_status   = reject
          !authenticated = *
          message        = Message from $domain_used_domain failed sender's DMARC policy, REJECT




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