-$Cambridge: exim/doc/doc-txt/experimental-spec.txt,v 1.2 2005/03/08 15:33:05 tom Exp $
+$Cambridge: exim/doc/doc-txt/experimental-spec.txt,v 1.11 2008/02/12 12:52:51 nm4 Exp $
From time to time, experimental features may be added to Exim.
While a feature is experimental, there will be a build-time
liable to incompatibile change.
+0. DKIM support
+--------------------------------------------------------------
+
+DKIM support is implemented via libdkim. A compatible version
+is available here:
+
+http://duncanthrax.net/exim-experimental/libdkim-1.0.15-tk.tar.gz
+
+Build the lib according to the instructions in the enclosed
+INSTALL file.
+
+To build Exim with DKIM support, specify this in Local/Makefile:
+
+EXPERIMENTAL_DKIM=yes
+CFLAGS += -I/home/tom/libdkim/include
+LDFLAGS += -ldkim -lssl -lstdc++ -L/home/tom/libdkim/lib
+
+Remember to tweak the CFLAGS and LDFLAGS lines to match the
+location of the libdomainkeys includes and lib on your system.
+
+The current experimental implementation supports two independent
+functions:
+
+o Validate incoming DKIM-signed email.
+o Sign outgoing email with DKIM.
+
+The former is implemented in the ACLs for SMTP, the latter as
+an extension to the SMTP transport. That means both facilities
+are limited to SMTP I/O.
+
+
+1) Validate incoming email
+
+Incoming messages are fed to the DKIM validation process as they
+are received "on the wire". This happens synchronously to Exim's
+buffering of the message in the spool.
+
+You must set "control = dkim_verify" in one of the ACLs preceding
+DATA (you will typically use acl_smtp_rcpt), at a point where
+non-local, non-relay, non-submission mail is processed. If that
+control flag is not set, the message will NOT be verified.
+
+Example:
+
+warn log_message = Feeding message to DKIM validator.
+ control = dk_verify
+
+You can then check for DKIM signatures in the ACL after data
+(acl_smtp_data), using the 'dkim' query-style lookup type. The
+query string should be a domain or DKIM identity:
+
+${lookup dkim{domain.example}}
+
+Such a lookup will yield one of the following strings:
+
+unverified: Exim did not (yet) verify the eventual DKIM
+ signatures in this message. This may happen
+ if a) You did not use control=dkim_verify
+ or b) You are using the lookup before
+ the DATA ACL.
+
+unsigned: The message does not have a signature from
+ the specified domain.
+
+good: The message has a signature from the specified
+ domain, and it verified successfully.
+
+bad: The message has a signature from the specified
+ domain, but it did not verify.
+
+defer: A temporary DNS problem was encountered while
+ trying to verify the signature.
+
+
+
+2) Sign outgoing email with DKIM
+
+Outgoing messages are signed just before Exim puts them "on
+the wire". The only thing that happens after DKIM signing is
+eventual TLS encryption.
+
+Signing is implemented by setting private options on the SMTP
+transport. These options take (expandable) strings as
+arguments.
+
+ dkim_domain = <expanded string> [MANDATORY]
+
+ The domain you want to sign with. Should optimally match
+ the domain in the "From:" header of the message, but
+ does not necessarily have to. The result of this expanded
+ option is put into the $dkim_domain expansion variable.
+
+ dkim_selector = <expanded string> [MANDATORY]
+
+ This sets the key selector string. You can use the
+ $dkim_domain expansion variable to look up a matching
+ selector. The result is put in the expansion variable
+ $dkim_selector which should be used in the dkim_private_key
+ option along with $dkim_domain.
+
+ dkim_private_key = <expanded string> [MANDATORY]
+
+ This sets the private key to use. You can use the
+ $dkim_domain and $dkim_selector expansion variables to
+ determine the private key to use. The result can either
+
+ o be a valid RSA private key in ASCII armor, including
+ line breaks.
+ o start with a slash, in which case it is treated as
+ a file that contains the private key.
+ o be "0", "false" or the empty string, in which case
+ the message will not be signed. This case will not
+ result in an error, even if dkim_strict is set.
+
+ dkim_canon = <expanded string> [OPTIONAL]
+
+ This option sets the canonicalization method used when
+ signing a message. The DKIM RFC currently supports two
+ methods: "simple" and "relaxed". The option defaults to
+ "relaxed" when unset. Note: the current implementation
+ only support using the same canonicalization method for
+ both headers and body.
+
+ dkim_strict = <expanded string> [OPTIONAL]
+
+ This option defines how Exim behaves when signing a
+ message that should be signed fails for some reason. When
+ the expansion evaluates to either "1" or "true", Exim will
+ defer. Otherwise Exim will send the message unsigned. You
+ can use the $dkim_domain and $dkim_selector expansion
+ variables here.
+
+ dkim_sign_headers = <expanded string> [OPTIONAL]
+
+ When set, this option must expand to (or be specified as)
+ a colon-separated list of header names. These headers will
+ be included in the message signature. When unspecified,
+ the recommended headers will be used. Currently, these
+ are:
+
+ from:sender:reply-to:subject:date:
+ message-id:to:cc:mime-version:content-type:
+ content-transfer-encoding:content-id:
+ content-description:resent-date:resent-from:
+ resent-sender:resent-to:resent-cc:resent-message-id:
+ in-reply-to:references:
+ list-id:list-help:list-unsubscribe:
+ list-subscribe:list-post:list-owner:list-archive
+
+
+
1. Yahoo DomainKeys support
--------------------------------------------------------------
Incoming messages are fed to the DK validation process as they
are received "on the wire". This happens synchronously to
-Exim's buffering of the message in the spool.
+Exim's buffering of the message in the spool.
You must set "control = dk_verify" in one of the ACLs
preceding DATA (you will typically use acl_smtp_rcpt), at a
1.1.) DK ACL conditions
dk_sender_domains = <domain list>
-
+
This condition takes a domainlist as argument and
succeeds if the domain that DK has been verifying for is
found in the list.
dk_senders = <address list>
-
+
This condition takes an addresslist as argument and
succeeds if the address that DK has been verifying for
is found in the list.
dk_sender_local_parts = <local part list>
-
+
This condition takes a local_part list as argument
and succeeds if the domain that DK has been
verifying for is found in the list.
dk_status = <colon separated list of keywords>
-
+
This condition takes a list of keywords as argument, and
succeeds if one of the listed keywords matches the outcome
of the DK check. The available keywords are:
-
+
good DK check succeeded, mail is verified.
bad DK check failed.
no signature Mail is not signed with DK.
dk_policy = <colon separated list of keywords>
-
+
This condition takes a list of keywords as argument, and
succeeds if one of the listed keywords matches the policy
announced by the target domain. The available keywords
are:
-
+
signsall The target domain signs all outgoing email.
testing The target domain is currently testing DK.
dk_domain_source = <colon separated list of keywords>
-
+
This condition takes a list of keywords as argument, and
succeeds if one of the listed keywords matches the
location where DK found the sender domain it verified for.
The available keywords are:
-
+
from The domain came from the "From:" header.
sender The domain came from the "Sender:" header.
none DK was unable to find the responsible domain.
1.2.) DK verification expansion variables
$dk_sender_domain
-
+
Contains the domain that DK has verified for.
-
-
+
+
$dk_sender
-
+
Contains the address that DK has verified for.
-
-
+
+
$dk_sender_local_part
-
+
Contains the local part that DK has verified for.
-
-
+
+
$dk_sender_source
-
+
Contains the "source" of the above three variables, one of
-
+
"from" The address came from the "From:" header.
"sender" The address came from the "Sender:" header.
-
+
When DK was unable to find a valid address, this variable
- is "0".
+ is "0".
$dk_signsall
-
+
Is "1" if the target domain signs all outgoing email,
"0" otherwise.
-
-
+
+
$dk_testing
-
+
Is "1" if the target domain is testing DK, "0" otherwise.
-
-
+
+
$dk_is_signed
-
+
Is "1" if the message is signed, "0" otherwise.
-
-
+
+
$dk_status
-
+
Contains the outcome of the DK check as a string, commonly
used to add a "DomainKey-Status:" header to messages. Will
contain one of:
-
+
good DK check succeeded, mail is verified.
bad DK check failed.
no signature Mail is not signed with DK.
$dk_result
-
+
Contains a human-readable result of the DK check, more
verbose than $dk_status. Useful for logging purposes.
-
+
2) Sign outgoing email with DK
-Outgoing messages are signed just before exim puts them "on
+Outgoing messages are signed just before Exim puts them "on
the wire". The only thing that happens after DK signing is
eventual TLS encryption.
dk_selector = <expanded string> [MANDATORY]
-
+
This sets the key selector string. You can use the
$dk_domain expansion variable to look up a matching
selector. The result is put in the expansion variable
dk_private_key = <expanded string> [MANDATORY]
-
+
This sets the private key to use. You SHOULD use the
$dk_domain and $dk_selector expansion variables to
determine the private key to use. The result can either
-
+
o be a valid RSA private key in ASCII armor, including
line breaks.
o start with a slash, in which case it is treated as
o be "0", "false" or the empty string, in which case
the message will not be signed. This case will not
result in an error, even if dk_strict is set.
-
+
dk_canon = <expanded string> [OPTIONAL]
-
+
This option sets the canonicalization method used when
signing a message. The DK draft currently supports two
methods: "simple" and "nofws". The option defaults to
dk_strict = <expanded string> [OPTIONAL]
-
+
This option defines how Exim behaves when signing a
message that should be signed fails for some reason. When
the expansion evaluates to either "1" or "true", Exim will
dk_domain = <expanded string> [NOT RECOMMENDED]
-
+
This option overrides DKs autodetection of the signing
domain. You should only use this option if you know what
you are doing. The result of the string expansion is also
put in $dk_domain.
-
-
-2. Brighmail AntiSpam (BMI) suppport
+
+
+2. Brightmail AntiSpam (BMI) suppport
--------------------------------------------------------------
Brightmail AntiSpam is a commercial package. Please see
following steps:
1) Compile Exim with BMI support
- 2) Set up main BMI options (top section of exim config file)
+ 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.
+These four steps are explained in more details below.
1) Adding support for BMI at compile time
with these lines in Local/Makefile:
EXPERIMENTAL_BRIGHTMAIL=yes
- CFLAGS=-DBRIGHTMAIL -I/path/to/the/dir/with/the/includefile
+ 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"
able to find the library file.
-2) Setting up BMI support in the exim main configuration
+2) Setting up BMI support in the Exim main configuration
- To enable BMI support in the main exim configuration, you
+ 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 exims configuration file (You
+
+ 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
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 exims default configuration file:
-
+ the "accept" blocks from Exim's default configuration file:
+
accept domains = +local_domains
endpass
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.
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 Exims view, a verdict can have the
+ 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
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:
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
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
+ 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
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
-
+ control = bmi_run
+
Of course, you can also use any other lookup method that
- exim supports, including LDAP, Postgres, MySQL, Oracle etc.,
+ 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.
-
-
-
+
+
+
3. Sender Policy Framework (SPF) support
--------------------------------------------------------------
-To learn more about SPF, visit http://spf.pobox.com. This
+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
+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
+To compile Exim with SPF support, set these additional flags in
Local/Makefile:
EXPERIMENTAL_SPF=yes
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.
+ 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
strings matches the outcome of the SPF check, the condition
fails.
-Here is a simple example to fail forgery attempts from domains
-that publish SPF records:
+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 $sender_address_domain
+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
--------------------- */
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 SPF-Received: header that can be
+ 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.
+
4. SRS (Sender Rewriting Scheme) Support
--------------------------------------------------------------
Exiscan currently includes SRS support via Miles Wilton's
-libsrs_alt library. The current version of the supported
+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
git://git.exim.org / users / heiko / exim.git / blobdiff