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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 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
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
( 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:
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
You should also include the location of in your dynamic linker configuration
file (usually /etc/ 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
verify = recipient
control = bmi_run
accept domains = +relay_to_domains
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
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
driver = redirect
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
driver = redirect
bmi_rule = 5:8:11
data =
- Expansion variables. Several expansion variables are set
during routing. You can use them in custom router
conditions, for example. The following variables are
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:
driver = accept
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:
driver = accept
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
The example:
accept domains = +relay_to_domains
verify = recipient
bmi_optin = ${lookup{$local_part@$domain}lsearch{/etc/exim/bmi_optin_data}}
control = bmi_run
Of course, you can also use any other lookup method that
Exim supports, including LDAP, Postgres, MySQL, Oracle etc.,
as long as the result is a list of colon-separated opt-in
For a list of available opt-in strings, please contact your
Brightmail representative.
SRS (Sender Rewriting Scheme) Support
Exiscan currently includes SRS support via Miles Wilton's
libsrs_alt library. The current version of the supported
library is 0.5, there are reports of 1.0 working.
In order to use SRS, you must get a copy of libsrs_alt from
(not the original source, which has disappeared.)
Unpack the tarball, then refer to MTAs/README.EXIM
to proceed. You need to set
in your Local/Makefile.
DCC Support
Distributed Checksum Clearinghouse;
*) Building exim
In order to build exim with DCC support add
to your Makefile. (Re-)build/install exim. exim -d should show
EXPERIMENTAL_DCC under "Support for".
*) Configuration
In the main section of add at least
dccifd_address = /usr/local/dcc/var/dccifd
dccifd_address = <ip> <port>
In the DATA ACL you can use the new condition
dcc = *
After that "$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
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
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 = \
condition = ${if def:acl_m_dcc_override_client_ip}
log_message = dbg: acl_m_dcc_override_client_ip set to \
Then set something like
# cat /etc/mail/multipleip_sites
Use a reasonable IP. eg. one the sending cluster actually uses.
DMARC Support
DMARC combines feedback from SPF, DKIM, and header From: in order
to attempt to provide better indicators of the authenticity of an
email. This document does not explain the fundamentals, you
should read and understand how it works by visiting the website at
DMARC support is added via the libopendmarc library. Visit:
to obtain a copy, or find it in your favorite rpm package
repository. If building from source, this description assumes
that headers will be in /usr/local/include, and that the libraries
are in /usr/local/lib.
1. To compile Exim with DMARC support, you must first enable SPF.
Please read the Local/Makefile comments on enabling the SUPPORT_SPF
feature. You must also have DKIM support, so you cannot set the
DISABLE_DKIM feature. Once both of those conditions have been met
you can enable DMARC in Local/Makefile:
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:
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
See also util/ script.
dmarc_history_file Defines the location of a file to log results
of dmarc verification on inbound emails. The
contents are importable by the opendmarc tools
which will manage the data, send out DMARC
reports, and expire the data. Make sure the
directory of this file is writable by the user
exim runs as.
dmarc_forensic_sender Alternate email address to use when sending a
forensic report detailing alignment failures
if a sender domain's dmarc record specifies it
and you have configured Exim to send them.
If set, this is expanded and used for the
From: header line; the address is extracted
from it and used for the envelope from.
If not set, the From: header is expanded from
the dsn_from option, and <> is used for the
envelope from.
Default: unset.
3. By default, the DMARC processing will run for any remote,
non-authenticated user. It makes sense to only verify DMARC
status of messages coming from remote, untrusted sources. You can
use standard conditions such as hosts, senders, etc, to decide that
DMARC verification should *not* be performed for them and disable
DMARC with a control setting:
control = dmarc_disable_verify
A DMARC record can also specify a "forensic address", which gives
exim an email address to submit reports about failed alignment.
Exim does not do this by default because in certain conditions it
results in unintended information leakage (what lists a user might
be subscribed to, etc). You must configure exim to submit forensic
reports to the owner of the domain. If the DMARC record contains a
forensic address and you specify the control statement below, then
exim will send these forensic emails. It's also advised that you
configure a dmarc_forensic_sender because the default sender address
construction might be inadequate.
control = dmarc_enable_forensic
(AGAIN: You can choose not to send these forensic reports by simply
not putting the dmarc_enable_forensic control line at any point in
your exim config. If you don't tell it to send them, it will not
send them.)
There are no options to either control. Both must appear before
the DATA acl.
4. You can now run DMARC checks in incoming SMTP by using the
"dmarc_status" ACL condition in the DATA ACL. You are required to
call the spf condition first in the ACLs, then the "dmarc_status"
condition. Putting this condition in the ACLs is required in order
for a DMARC check to actually occur. All of the variables are set
up before the DATA ACL, but there is no actual DMARC check that
occurs until a "dmarc_status" condition is encountered in the ACLs.
The dmarc_status condition takes a list of strings on its
right-hand side. These strings describe recommended action based
on the DMARC check. To understand what the policy recommendations
mean, refer to the DMARC website above. Valid strings are:
o accept The DMARC check passed and the library recommends
accepting the email.
o reject The DMARC check failed and the library recommends
rejecting the email.
o quarantine The DMARC check failed and the library recommends
keeping it for further inspection.
o none The DMARC check passed and the library recommends
no specific action, neutral.
o norecord No policy section in the DMARC record for this
sender domain.
o nofrom Unable to determine the domain of the sender.
o temperror Library error or dns error.
o off The DMARC check was disabled for this email.
You can prefix each string with an exclamation mark to invert its
meaning, for example "!accept" will match all results but
"accept". The string list is evaluated left-to-right in a
short-circuit fashion. When a string matches the outcome of the
DMARC check, the condition succeeds. If none of the listed
strings matches the outcome of the DMARC check, the condition
Of course, you can also use any other lookup method that Exim
supports, including LDAP, Postgres, MySQL, etc, as long as the
result is a list of colon-separated strings.
Performing the check sets up information used by the
${authresults } expansion item.
Several expansion variables are set before the DATA ACL is
processed, and you can use them in this ACL. The following
expansion variables are available:
o $dmarc_status
This is a one word status indicating what the DMARC library
thinks of the email. It is a combination of the results of
DMARC record lookup and the SPF/DKIM/DMARC processing results
(if a DMARC record was found). The actual policy declared
in the DMARC record is in a separate expansion variable.
o $dmarc_status_text
This is a slightly longer, human readable status.
o $dmarc_used_domain
This is the domain which DMARC used to look up the DMARC
policy record.
o $dmarc_domain_policy
This is the policy declared in the DMARC record. Valid values
are "none", "reject" and "quarantine". It is blank when there
is any error, including no DMARC record.
A now-redundant variable $dmarc_ar_header has now been withdrawn.
Use the ${authresults } expansion instead.
5. How to enable DMARC advanced operation:
By default, Exim's DMARC configuration is intended to be
non-intrusive and conservative. To facilitate this, Exim will not
create any type of logging files without explicit configuration by
you, the admin. Nor will Exim send out any emails/reports about
DMARC issues without explicit configuration by you, the admin (other
than typical bounce messages that may come about due to ACL
processing or failure delivery issues).
In order to log statistics suitable to be imported by the opendmarc
tools, you need to:
a. Configure the global setting dmarc_history_file.
b. Configure cron jobs to call the appropriate opendmarc history
import scripts and truncating the dmarc_history_file.
In order to send forensic reports, you need to:
a. Configure the global setting dmarc_forensic_sender.
b. Configure, somewhere before the DATA ACL, the control option to
enable sending DMARC forensic reports.
6. Example usage:
warn domains = +local_domains
hosts = +local_hosts
control = dmarc_disable_verify
warn !domains = +screwed_up_dmarc_records
control = dmarc_enable_forensic
warn condition = (lookup if destined to mailing list)
set acl_m_mailing_list = 1
warn dmarc_status = accept : none : off
!authenticated = *
log_message = DMARC DEBUG: $dmarc_status $dmarc_used_domain
warn dmarc_status = !accept
!authenticated = *
log_message = DMARC DEBUG: '$dmarc_status' for $dmarc_used_domain
warn dmarc_status = quarantine
!authenticated = *
set $acl_m_quarantine = 1
# Do something in a transport with this flag variable
deny condition = ${if eq{$dmarc_domain_policy}{reject}}
condition = ${if eq{$acl_m_mailing_list}{1}}
message = Messages from $dmarc_used_domain break mailing lists
deny dmarc_status = reject
!authenticated = *
message = Message from $dmarc_used_domain failed sender's DMARC policy, REJECT
warn add_header = :at_start:${authresults {$primary_hostname}}
DSN extra information
If compiled with EXPERIMENTAL_DSN_INFO extra information will be added
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.
Remote-MTA: X-ip; []:587
Several addresses may correspond to the (already available)
dns name for the remote MTA.
The remote MTA connect-time greeting.
X-Remote-MTA-smtp-greeting: X-str; 220 ESMTP Exim x.yz Tue, 2 Mar 1999 09:44:33 +0000
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.
X-Remote-MTA-helo-response: X-str; Hello localhost []
Only the first line of a multiline response is recorded.
This string sometimes presents the remote MTA's view of
the peer IP connecting to it.
The reporting MTA detailed diagnostic.
X-Exim-Diagnostic: X-str; SMTP error from remote mail server after RCPT TO:<d3@myhost.test.ex>: 550 hard error
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
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 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
to your Local/Makefile. (Re-)build/install exim. exim -d should show
Experimental_LMDB in the line "Support for:".
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.
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
Sample configuration:
driver = accept
transport = scan
driver = queuefile
directory = /var/spool/baruwa-scanner/input
In order to build exim with Queuefile transport support add or uncomment
to your Local/Makefile. (Re-)build/install exim. exim -d should show
Experimental_QUEUEFILE in the line "Support for:".
ARC support
Note that this is not an RFC yet, so may change.
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 the set of
mail-handling systems that the mail transits. 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.
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
$arc_oldest_pass lowest passing instance number of chain
logwrite = oldest-p-ams: <${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".
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]
* 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.
If compiled with EXPERIMENTAL_REQUIRETLS support is included for this
feature, where a REQUIRETLS option is added to the MAIL command.
The client may not retry in clear if the MAIL+REQUIRETLS fails (or was never
offered), and the server accepts an obligation that any onward transmission
by SMTP of the messages accepted will also use REQUIRETLS - or generate a
fail DSN.
The Exim implementation includes
- a main-part option tls_advertise_requiretls; host list, default "*"
- an observability variable $requiretls returning yes/no
- an ACL "control = requiretls" modifier for setting the requirement
- Log lines and Received: headers capitalise the S in the protocol
element: "P=esmtpS"
Differences from spec:
- we support upgrading the requirement for REQUIRETLS, including adding
it from cold, within an MTA. The spec only define the sourcing MUA
as being able to source the requirement, and makes no mention of upgrade.
- No support is coded for the RequireTLS header (which can be used
to annul DANE and/or STS policiy). [this can _almost_ be done in
transport option expansions, but not quite: it requires tha DANE-present
but STARTTLS-failing targets fallback to cleartext, which current DANE
coding specifically blocks]
Note that REQUIRETLS is only advertised once a TLS connection is achieved
(in contrast to STARTTLS). If you want to check the advertising, do something
like "swaks -s -tls -q HELO".
Early pipelining support
If compiled with EXPERIMENTAL_PIPE_CONNECT support is included for this feature.
The server advertises the feature in its EHLO response, currently using the name
"X_PIPE_CONNECT" (this will change, some time in the future).
A client may cache this information, along with the rest of the EHLO response,
and use it for later connections. Those later ones can send esmtp commands before
a banner is received.
Up to 1.5 roundtrip times can be taken out of cleartext connections, 2.5 on
STARTTLS connections.
In combination with the traditional PIPELINING feature the following example
sequences are possible (among others):
(client) (server)
<- banner,EHLO-resp,MAIL-ack,RCPT-ack,DATA-goahead
message-data ->
<- banner,EHLO-resp,MAIL-ack,RCPT-ack
message-data ->
<- banner,EHLO-resp,TLS-goahead
TLS1.2-client-hello ->
<- TLS-server-hello,cert,hello-done
client-Kex,change-cipher,finished ->
<- change-cipher,finshed
<- EHLO-resp,MAIL-ack,RCPT-ack,DATA-goahead
TLS1.2-client-hello ->
<- TLS-server-hello,cert,hello-done
client-Kex,change-cipher,finished ->
<- change-cipher,finshed
<- banner
<- EHLO-resp,MAIL-ack,RCPT-ack,DATA-goahead
Where the initial client packet is SMTP, it can combine with the TCP Fast Open
feature and be sent in the TCP SYN.
A main-section option "pipelining_connect_advertise_hosts" (default: *)
and an smtp transport option "hosts_pipe_connect" (default: unset)
control the feature.
If the "pipelining" log_selector is enabled, the "L" field in server <=
log lines has a period appended if the feature was advertised but not used;
or has an asterisk appended if the feature was used. In client => lines
the "L" field has an asterisk appended if the feature was used.
The "retry_data_expire" option controls cache invalidation.
Entries are also rewritten (or cleared) if the adverised features
NOTE: since the EHLO command must be constructed before the connection is
made it cannot depend on the interface IP address that will be used.
Transport configurations should be checked for this. An example avoidance:
helo_data = ${if def:sending_ip_address \
{${lookup dnsdb{>! ptr=$sending_ip_address} \
{${sg{$value} {^([^!]*).*\$} {\$1}}} fail}} \
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