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exim4/doc/experimental-spec.txt
Daniel Baumann 802ab461a9
Adding upstream version 4.98.2. 
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
2025-06-21 14:27:17 +02:00

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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.
Brightmail AntiSpam (BMI) support
--------------------------------------------------------------
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 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
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@$domain}lsearch{/etc/exim/bmi_optin_data}}
control = bmi_run
Of course, you can also use any other lookup method that
Exim supports, including LDAP, Postgres, MySQL, Oracle etc.,
as long as the result is a list of colon-separated opt-in
strings.
For a list of available opt-in strings, please contact your
Brightmail representative.
DCC Support
--------------------------------------------------------------
Distributed Checksum Clearinghouse; http://www.rhyolite.com/dcc/
*) Building exim
In order to build exim with DCC support add
EXPERIMENTAL_DCC=yes
to your Makefile. (Re-)build/install exim. exim -d should show
EXPERIMENTAL_DCC under "Support for".
*) Configuration
In the main section of exim.cf add at least
dccifd_address = /usr/local/dcc/var/dccifd
or
dccifd_address = <ip> <port>
In the DATA ACL you can use the new condition
dcc = *
After that "$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.
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 Experimental 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.
XCLIENT proxy support
---------------------------------------------------------------
Per https://www.postfix.org/XCLIENT_README.html
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)
If compiled with EXPERIMENTAL_XCLIENT=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).
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
--------------------------------------------------------------
End of file
--------------------------------------------------------------
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