$Cambridge: exim/doc/doc-txt/experimental-spec.txt,v 1.12 2009/06/11 14:07:57 tom Exp $
From time to time, experimental features may be added to Exim.
While a feature is experimental, there will be a build-time
option whose name starts "EXPERIMENTAL_" that must be set in
order to include the feature. This file contains information
about experimenatal features, all of which are unstable and
liable to incompatibile change.
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 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 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 standarized, 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.
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End of file
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