-$Cambridge: exim/doc/doc-txt/experimental-spec.txt,v 1.2 2005/03/08 15:33:05 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.
-
+about experimental features, all of which are unstable and
+liable to incompatible change.
-1. Yahoo DomainKeys support
+PRDR support
--------------------------------------------------------------
-DomainKeys (DK) support is built into Exim using the
-"libdomainkeys" reference library implementation. It is
-available at
-
-http://domainkeys.sf.net
-
-You must build this library on your system and compile Exim
-against it. To build Exim with DK support, add these lines to
-your Local/Makefile:
-
-EXPERIMENTAL_DOMAINKEYS=yes
-CFLAGS += -I/home/tom/exim-cvs/extra/libdomainkeys
-LDFLAGS += -ldomainkeys -L/home/tom/exim-cvs/extra/libdomainkeys
-
-Remember to tweak the CFLAGS and LDFLAGS lines to match the
-location of the libdomainkeys includes and lib on your system.
+Per-Recipient Data Reponse is an SMTP extension proposed by Eric Hall
+in a (now-expired) IETF draft from 2007. It's not hit mainstream
+use, but has apparently been implemented in the META1 MTA.
-The current experimental implementation supports two
-independent functions:
+There is mention at http://mail.aegee.org/intern/sendmail.html
+of a patch to sendmail "to make it PRDR capable".
-o Validate incoming DK-signed email.
-o Sign outgoing email with DK.
+ ref: http://www.eric-a-hall.com/specs/draft-hall-prdr-00.txt
-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.
+If Exim is built with EXPERIMENTAL_PRDR there is a new config
+boolean "prdr_enable" which controls whether PRDR is advertised
+as part of an EHLO response, a new "acl_data_smtp_prdr" ACL
+(called for each recipient, after data arrives but before the
+data ACL), and a new smtp transport option "hosts_try_prdr".
+PRDR may be used to support per-user content filtering. Without it
+one must defer any recipient after the first that has a different
+content-filter configuration. With PRDR, the RCPT-time check
+for this can be disabled when the MAIL-time $smtp_command included
+"PRDR". Any required difference in behaviour of the main DATA-time
+ACL should however depend on the PRDR-time ACL having run, as Exim
+will avoid doing so in some situations (eg. single-recipient mails).
-1) Validate incoming email
-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.
-
-You must set "control = dk_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:
+OCSP Stapling support
+--------------------------------------------------------------
-warn log_message = Feeding message to DK validator.
- control = dk_verify
-
-You can check for the outcome of the DK check in the ACL after
-data (acl_smtp_data), using a number of ACL conditions and/or
-expansion variables.
-
-
-
-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.
- no key Public key missing in target domain DNS.
- bad format Public key available, but unuseable.
- non-participant Target domain states not to participate in DK.
- revoked The signing key has been revoked by the domain.
-
-
- 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".
-
-
- $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.
- no key Public key missing in target domain DNS.
- bad format Public key available, but unuseable.
- non-participant Target domain states not to participate in DK.
- revoked The signing key has been revoked by the domain.
-
-
- $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
-the wire". The only thing that happens after DK signing is
-eventual TLS encryption.
-
-Signing is implemented by setting private options on the SMTP
-transport. These options take (expandable) strings as
-arguments. The most important variable to use in these
-expansions is $dk_domain. It contains the domain that DK wants
-to sign for.
-
-
- 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_selector which should be used in the dk_private_key
- option along with $dk_domain.
-
-
- 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
- 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 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
- "simple" when unset.
-
-
- 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
- defer. Otherwise Exim will send the message unsigned. You
- can and should use the $dk_domain and $dk_selector
- expansion variables here.
-
-
- 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
+X.509 PKI certificates expire and can be revoked; to handle this, the
+clients need some way to determine if a particular certificate, from a
+particular Certificate Authority (CA), is still valid. There are three
+main ways to do so.
+
+The simplest way is to serve up a Certificate Revocation List (CRL) with
+an ordinary web-server, regenerating the CRL before it expires. The
+downside is that clients have to periodically re-download a potentially
+huge file from every certificate authority it knows of.
+
+The way with most moving parts at query time is Online Certificate
+Status Protocol (OCSP), where the client verifies the certificate
+against an OCSP server run by the CA. This lets the CA track all
+usage of the certs. This requires running software with access to the
+private key of the CA, to sign the responses to the OCSP queries. OCSP
+is based on HTTP and can be proxied accordingly.
+
+The only widespread OCSP server implementation (known to this writer)
+comes as part of OpenSSL and aborts on an invalid request, such as
+connecting to the port and then disconnecting. This requires
+re-entering the passphrase each time some random client does this.
+
+The third way is OCSP Stapling; in this, the server using a certificate
+issued by the CA periodically requests an OCSP proof of validity from
+the OCSP server, then serves it up inline as part of the TLS
+negotiation. This approach adds no extra round trips, does not let the
+CA track users, scales well with number of certs issued by the CA and is
+resilient to temporary OCSP server failures, as long as the server
+starts retrying to fetch an OCSP proof some time before its current
+proof expires. The downside is that it requires server support.
+
+If Exim is built with EXPERIMENTAL_OCSP and it was built with OpenSSL,
+then it gains a new global option: "tls_ocsp_file".
+
+The file specified therein is expected to be in DER format, and contain
+an OCSP proof. Exim will serve it as part of the TLS handshake. This
+option will be re-expanded for SNI, if the tls_certificate option
+contains $tls_sni, as per other TLS options.
+
+Exim does not at this time implement any support for fetching a new OCSP
+proof. The burden is on the administrator to handle this, outside of
+Exim. The file specified should be replaced atomically, so that the
+contents are always valid. Exim will expand the "tls_ocsp_file" option
+on each connection, so a new file will be handled transparently on the
+next connection.
+
+Exim will check for a valid next update timestamp in the OCSP proof;
+if not present, or if the proof has expired, it will be ignored.
+
+Also, given EXPERIMENTAL_OCSP and OpenSSL, the smtp transport gains
+a "hosts_require_ocsp" option; a host-list for which an OCSP Stapling
+is requested and required for the connection to proceed. The host(s)
+should also be in "hosts_require_tls", and "tls_verify_certificates"
+configured for the transport.
+
+For the client to be able to verify the stapled OCSP the server must
+also supply, in its stapled information, any intermediate
+certificates for the chain leading to the OCSP proof from the signer
+of the server certificate. There may be zero or one such. These
+intermediate certificates should be added to the server OCSP stapling
+file (named by tls_ocsp_file).
+
+At this point in time, we're gathering feedback on use, to determine if
+it's worth adding complexity to the Exim daemon to periodically re-fetch
+OCSP files and somehow handling multiple files.
+
+ A helper script "ocsp_fetch.pl" for fetching a proof from a CA
+ OCSP server is supplied. The server URL may be included in the
+ server certificate, if the CA is helpful.
+
+ One fail mode seen was the OCSP Signer cert expiring before the end
+ of vailidity of the OCSP proof. The checking done by Exim/OpenSSL
+ noted this as invalid overall, but the re-fetch script did not.
+
+
+
+
+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
+
+
+
+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
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
+using it in the RCPT ACL, you can make the checks dependent on
the RCPT address (or domain), so you can check SPF records
only for certain target domains. This gives you the
possibility to opt-out certain customers that do not want
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".
-
-
-4. SRS (Sender Rewriting Scheme) Support
+In addition to SPF, you can also perform checks for so-called
+"Best-guess". Strictly speaking, "Best-guess" is not standard
+SPF, but it is supported by the same framework that enables SPF
+capability. Refer to http://www.openspf.org/FAQ/Best_guess_record
+for a description of what it means.
+
+To access this feature, simply use the spf_guess condition in place
+of the spf one. For example:
+
+/* -----------------
+deny message = $sender_host_address doesn't look trustworthy to me
+ spf_guess = fail
+--------------------- */
+
+In case you decide to reject messages based on this check, you
+should note that although it uses the same framework, "Best-guess"
+is NOT SPF, and therefore you should not mention SPF at all in your
+reject message.
+
+When the spf_guess condition has run, it sets up the same expansion
+variables as when spf condition is run, described above.
+
+Additionally, since Best-guess is not standardized, you may redefine
+what "Best-guess" means to you by redefining spf_guess variable in
+global config. For example, the following:
+
+/* -----------------
+spf_guess = v=spf1 a/16 mx/16 ptr ?all
+--------------------- */
+
+would relax host matching rules to a broader network range.
+
+
+SRS (Sender Rewriting Scheme) Support
--------------------------------------------------------------
Exiscan currently includes SRS support via Miles Wilton's
-libsrs_alt library. The current version of the supported
+libsrs_alt library. The current version of the supported
library is 0.5.
In order to use SRS, you must get a copy of libsrs_alt from
in your Local/Makefile.
+DCC Support
+--------------------------------------------------------------
+
+*) Building exim
+
+In order to build exim with DCC support add
+
+EXPERIMENTAL_DCC=yes
+
+to your Makefile. (Re-)build/install exim. exim -d should show
+EXPERIMENTAL_DCC under "Support for".
+
+
+*) Configuration
+
+In the main section of exim.cf add at least
+ dccifd_address = /usr/local/dcc/var/dccifd
+or
+ dccifd_address = <ip> <port>
+
+In the DATA ACL you can use the new condition
+ dcc = *
+
+After that "$dcc_header" contains the X-DCC-Header.
+
+Return values are:
+ fail for overall "R", "G" from dccifd
+ defer for overall "T" from dccifd
+ accept for overall "A", "S" from dccifd
+
+dcc = */defer_ok works as for spamd.
+
+The "$dcc_result" variable contains the overall result from DCC
+answer. There will an X-DCC: header added to the mail.
+
+Usually you'll use
+ defer !dcc = *
+to greylist with DCC.
+
+If you set, in the main section,
+ dcc_direct_add_header = true
+then the dcc header will be added "in deep" and if the spool
+file was already written it gets removed. This forces Exim to
+write it again if needed. This helps to get the DCC Header
+through to eg. SpamAssassin.
+
+If you want to pass even more headers in the middle of the
+DATA stage you can set
+ $acl_m_dcc_add_header
+to tell the DCC routines to add more information; eg, you might set
+this to some results from ClamAV. Be careful. Header syntax is
+not checked and is added "as is".
+
+In case you've troubles with sites sending the same queue items from several
+hosts and fail to get through greylisting you can use
+$acl_m_dcc_override_client_ip
+
+Setting $acl_m_dcc_override_client_ip to an IP address overrides the default
+of $sender_host_address. eg. use the following ACL in DATA stage:
+
+ warn set acl_m_dcc_override_client_ip = \
+ ${lookup{$sender_helo_name}nwildlsearch{/etc/mail/multipleip_sites}{$value}{}}
+ condition = ${if def:acl_m_dcc_override_client_ip}
+ log_message = dbg: acl_m_dcc_override_client_ip set to \
+ $acl_m_dcc_override_client_ip
+
+Then set something like
+# cat /etc/mail/multipleip_sites
+mout-xforward.gmx.net 82.165.159.12
+mout.gmx.net 212.227.15.16
+
+Use a reasonable IP. eg. one the sending cluster acutally uses.
+
+DMARC Support
+--------------------------------------------------------------
+
+DMARC combines feedback from SPF, DKIM, and header From: in order
+to attempt to provide better indicators of the authenticity of an
+email. This document does not explain the fundamentals, you
+should read and understand how it works by visiting the website at
+http://www.dmarc.org/.
+
+DMARC support is added via the libopendmarc library. Visit:
+
+ http://sourceforge.net/projects/opendmarc/
+
+to obtain a copy, or find it in your favorite rpm package
+repository. If building from source, this description assumes
+that headers will be in /usr/local/include, and that the libraries
+are in /usr/local/lib.
+
+1. To compile Exim with DMARC support, you must first enable SPF.
+Please read the above section on enabling the EXPERIMENTAL_SPF
+feature. You must also have DKIM support, so you cannot set the
+DISABLE_DKIM feature. Once both of those conditions have been met
+you can enable DMARC in Local/Makefile:
+
+EXPERIMENTAL_DMARC=yes
+LDFLAGS += -lopendmarc
+# CFLAGS += -I/usr/local/include
+# LDFLAGS += -L/usr/local/lib
+
+The first line sets the feature to include the correct code, and
+the second line says to link the libopendmarc libraries into the
+exim binary. The commented out lines should be uncommented if you
+built opendmarc from source and installed in the default location.
+Adjust the paths if you installed them elsewhere, but you do not
+need to uncomment them if an rpm (or you) installed them in the
+package controlled locations (/usr/include and /usr/lib).
+
+
+2. Use the following global settings to configure DMARC:
+
+Required:
+dmarc_tld_file Defines the location of a text file of valid
+ top level domains the opendmarc library uses
+ during domain parsing. Maintained by Mozilla,
+ the most current version can be downloaded
+ from a link at http://publicsuffix.org/list/.
+
+Optional:
+dmarc_history_file Defines the location of a file to log results
+ of dmarc verification on inbound emails. The
+ contents are importable by the opendmarc tools
+ which will manage the data, send out DMARC
+ reports, and expire the data. Make sure the
+ directory of this file is writable by the user
+ exim runs as.
+
+dmarc_forensic_sender The email address to use when sending a
+ forensic report detailing alignment failures
+ if a sender domain's dmarc record specifies it
+ and you have configured Exim to send them.
+ Default: do-not-reply@$default_hostname
+
+
+3. By default, the DMARC processing will run for any remote,
+non-authenticated user. It makes sense to only verify DMARC
+status of messages coming from remote, untrusted sources. You can
+use standard conditions such as hosts, senders, etc, to decide that
+DMARC verification should *not* be performed for them and disable
+DMARC with a control setting:
+
+ control = dmarc_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_forensic_enable
+
+(AGAIN: You can choose not to send these forensic reports by simply
+not putting the dmarc_forensic_enable control line at any point in
+your exim config. If you don't tell it to send them, it will not
+send them.)
+
+There are no options to either control. Both must appear before
+the DATA acl.
+
+
+4. You can now run DMARC checks in incoming SMTP by using the
+"dmarc_status" ACL condition in the DATA ACL. You are required to
+call the spf condition first in the ACLs, then the "dmarc_status"
+condition. Putting this condition in the ACLs is required in order
+for a DMARC check to actually occur. All of the variables are set
+up before the DATA ACL, but there is no actual DMARC check that
+occurs until a "dmarc_status" condition is encountered in the ACLs.
+
+The dmarc_status condition takes a list of strings on its
+right-hand side. These strings describe recommended action based
+on the DMARC check. To understand what the policy recommendations
+mean, refer to the DMARC website above. Valid strings are:
+
+ o accept The DMARC check passed and the library recommends
+ accepting the email.
+ o reject The DMARC check failed and the library recommends
+ rejecting the email.
+ o quarantine The DMARC check failed and the library recommends
+ keeping it for further inspection.
+ o 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
+fails.
+
+Of course, you can also use any other lookup method that Exim
+supports, including LDAP, Postgres, MySQL, etc, as long as the
+result is a list of colon-separated strings;
+
+Several expansion variables are set before the DATA ACL is
+processed, and you can use them in this ACL. The following
+expansion variables are available:
+
+ o $dmarc_status
+ This is a one word status indicating what the DMARC library
+ thinks of the email.
+
+ o $dmarc_status_text
+ This is a slightly longer, human readable status.
+
+ o $dmarc_used_domain
+ This is the domain which DMARC used to look up the DMARC
+ policy record.
+
+ o $dmarc_ar_header
+ This is the entire Authentication-Results header which you can
+ add using an add_header modifier.
+
+
+5. How to enable DMARC advanced operation:
+By default, Exim's DMARC configuration is intended to be
+non-intrusive and conservative. To facilitate this, Exim will not
+create any type of logging files without explicit configuration by
+you, the admin. Nor will Exim send out any emails/reports about
+DMARC issues without explicit configuration by you, the admin (other
+than typical bounce messages that may come about due to ACL
+processing or failure delivery issues).
+
+In order to log statistics suitable to be imported by the opendmarc
+tools, you need to:
+a. Configure the global setting dmarc_history_file.
+b. Configure cron jobs to call the appropriate opendmarc history
+ import scripts and truncating the dmarc_history_file.
+
+In order to send forensic reports, you need to:
+a. Configure the global setting dmarc_forensic_sender.
+b. Configure, somewhere before the DATA ACL, the control option to
+ enable sending DMARC forensic reports.
+
+
+6. Example usage:
+(RCPT ACL)
+ warn domains = +local_domains
+ hosts = +local_hosts
+ control = dmarc_disable_verify
+
+ warn !domains = +screwed_up_dmarc_records
+ control = dmarc_enable_forensic
+
+(DATA ACL)
+ warn dmarc_status = accept : none : off
+ !authenticated = *
+ log_message = DMARC DEBUG: $dmarc_status $dmarc_used_domain
+ add_header = $dmarc_ar_header
+
+ warn dmarc_status = !accept
+ !authenticated = *
+ log_message = DMARC DEBUG: '$dmarc_status' for $dmarc_used_domain
+
+ warn dmarc_status = quarantine
+ !authenticated = *
+ set $acl_m_quarantine = 1
+ # Do something in a transport with this flag variable
+
+ deny dmarc_status = reject
+ !authenticated = *
+ message = Message from $domain_used_domain failed sender's DMARC policy, REJECT
+
+
+
+Transport post-delivery actions
+--------------------------------------------------------------
+
+An arbitrary per-transport string can be expanded on successful delivery,
+and (for SMTP transports) a second string on deferrals caused by a host error.
+This feature may be used, for example, to write exim internal log information
+(not available otherwise) into a database.
+
+In order to use the feature, you must set
+
+EXPERIMENTAL_TPDA=yes
+
+in your Local/Makefile
+
+and define the expandable strings in the runtime config file, to
+be executed at end of delivery.
+
+Additionally, there are 6 more variables, available at end of
+delivery:
+
+tpda_delivery_ip IP of host, which has accepted delivery
+tpda_delivery_port Port of remote host which has accepted delivery
+tpda_delivery_fqdn FQDN of host, which has accepted delivery
+tpda_delivery_local_part local part of address being delivered
+tpda_delivery_domain domain part of address being delivered
+tpda_delivery_confirmation SMTP confirmation message
+
+In case of a deferral caused by a host-error:
+tpda_defer_errno Error number
+tpda_defer_errstr Error string possibly containing more details
+
+The $router_name and $transport_name variables are also usable.
+
+
+To take action after successful deliveries, set the following option
+on any transport of interest.
+
+tpda_delivery_action
+
+An example might look like:
+
+tpda_delivery_action = \
+${lookup pgsql {SELECT * FROM record_Delivery( \
+ '${quote_pgsql:$sender_address_domain}',\
+ '${quote_pgsql:${lc:$sender_address_local_part}}', \
+ '${quote_pgsql:$tpda_delivery_domain}', \
+ '${quote_pgsql:${lc:$tpda_delivery_local_part}}', \
+ '${quote_pgsql:$tpda_delivery_ip}', \
+ '${quote_pgsql:${lc:$tpda_delivery_fqdn}}', \
+ '${quote_pgsql:$message_exim_id}')}}
+
+The string is expanded after the delivery completes and any
+side-effects will happen. The result is then discarded.
+Note that for complex operations an ACL expansion can be used.
+
+
+In order to log host deferrals, add the following option to an SMTP
+transport:
+
+tpda_host_defer_action
+
+This is a private option of the SMTP transport. It is intended to
+log failures of remote hosts. It is executed only when exim has
+attempted to deliver a message to a remote host and failed due to
+an error which doesn't seem to be related to the individual
+message, sender, or recipient address.
+See section 47.2 of the exim documentation for more details on how
+this is determined.
+
+Example:
+
+tpda_host_defer_action = \
+${lookup mysql {insert into delivlog set \
+ msgid = '${quote_mysql:$message_exim_id}', \
+ senderlp = '${quote_mysql:${lc:$sender_address_local_part}}', \
+ senderdom = '${quote_mysql:$sender_address_domain}', \
+ delivlp = '${quote_mysql:${lc:$tpda_delivery_local_part}}', \
+ delivdom = '${quote_mysql:$tpda_delivery_domain}', \
+ delivip = '${quote_mysql:$tpda_delivery_ip}', \
+ delivport = '${quote_mysql:$tpda_delivery_port}', \
+ delivfqdn = '${quote_mysql:$tpda_delivery_fqdn}', \
+ deliverrno = '${quote_mysql:$tpda_defer_errno}', \
+ deliverrstr = '${quote_mysql:$tpda_defer_errstr}' \
+ }}
+
+
+Redis Lookup
+--------------------------------------------------------------
+
+Redis is open source advanced key-value data store. This document
+does not explain the fundamentals, you should read and understand how
+it works by visiting the website at http://www.redis.io/.
+
+Redis lookup support is added via the hiredis library. Visit:
+
+ https://github.com/redis/hiredis
+
+to obtain a copy, 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 Redis lookup support add
+
+EXPERIMENTAL_REDIS=yes
+
+to your Local/Makefile. (Re-)build/install exim. exim -d should show
+Experimental_Redis in the line "Support for:".
+
+EXPERIMENTAL_REDIS=yes
+LDFLAGS += -lhiredis
+# 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 hiredis libraries into the
+exim binary. The commented out lines should be uncommented if you
+built hiredis 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 Redis lookup support:
+
+Required:
+redis_servers This option provides a list of Redis servers
+ and associated connection data, to be used in
+ conjunction with redis lookups. The option is
+ only available if Exim is configured with Redis
+ support.
+
+For example:
+
+redis_servers = 127.0.0.1/10/ - using database 10 with no password
+redis_servers = 127.0.0.1//password - to make use of the default database of 0 with a password
+redis_servers = 127.0.0.1// - for default database of 0 with no password
+
+3. Once you have the Redis servers defined you can then make use of the
+experimental Redis lookup by specifying ${lookup redis{}} in a lookup query.
+
+4. Example usage:
+
+(Host List)
+hostlist relay_from_ips = <\n ${lookup redis{SMEMBERS relay_from_ips}}
+
+Where relay_from_ips is a Redis set which contains entries such as "192.168.0.0/24" "10.0.0.0/8" and so on.
+The result set is returned as
+192.168.0.0/24
+10.0.0.0/8
+..
+.
+
+(Domain list)
+domainlist virtual_domains = ${lookup redis {HGET $domain domain}}
+
+Where $domain is a hash which includes the key 'domain' and the value '$domain'.
+
+(Adding or updating an existing key)
+set acl_c_spammer = ${if eq{${lookup redis{SPAMMER_SET}}}{OK}}
+
+Where SPAMMER_SET is a macro and it is defined as
+
+"SET SPAMMER <some_value>"
+
+(Getting a value from Redis)
+
+set acl_c_spam_host = ${lookup redis{GET...}}
+
+
+Proxy Protocol Support
+--------------------------------------------------------------
+
+Exim now has Experimental "Proxy Protocol" support. It was built on
+specifications from:
+http://haproxy.1wt.eu/download/1.5/doc/proxy-protocol.txt
+
+The purpose of this function is so that an application load balancer,
+such as HAProxy, can sit in front of several Exim servers and Exim
+will log the IP that is connecting to the proxy server instead of
+the IP of the proxy server when it connects to Exim. It resets the
+$sender_address_host and $sender_address_port to the IP:port of the
+connection to the proxy. It also re-queries the DNS information for
+this new IP address so that the original sender's hostname and IP
+get logged in the Exim logfile. There is no logging if a host passes or
+fails Proxy Protocol negotiation, but it can easily be determined and
+recorded in an ACL (example is below).
+
+1. To compile Exim with Proxy Protocol support, put this in
+Local/Makefile:
+
+EXPERIMENTAL_PROXY=yes
+
+2. Global configuration settings:
+
+proxy_required_hosts = HOSTLIST
+
+The proxy_required_hosts option will require any IP in that hostlist
+to use Proxy Protocol. The specification of Proxy Protocol is very
+strict, and if proxy negotiation fails, Exim will not allow any SMTP
+command other than QUIT. (See end of this section for an example.)
+The option is expanded when used, so it can be a hostlist as well as
+string of IP addresses. Since it is expanded, specifying an alternate
+separator is supported for ease of use with IPv6 addresses.
+
+To log the IP of the proxy in the incoming logline, add:
+ log_selector = +proxy
+
+A default incoming logline (wrapped for appearance) will look like this:
+
+ 2013-11-04 09:25:06 1VdNti-0001OY-1V <= me@example.net
+ H=mail.example.net [1.2.3.4] P=esmtp S=433
+
+With the log selector enabled, an email that was proxied through a
+Proxy Protocol server at 192.168.1.2 will look like this:
+
+ 2013-11-04 09:25:06 1VdNti-0001OY-1V <= me@example.net
+ H=mail.example.net [1.2.3.4] P=esmtp PRX=192.168.1.2 S=433
+
+3. In the ACL's the following expansion variables are available.
+
+proxy_host The src IP of the proxy server making the connection
+proxy_port The src port the proxy server is using
+proxy_session Boolean, yes/no, the connected host is required to use
+ Proxy Protocol.
+
+There is no expansion for a failed proxy session, however you can detect
+it by checking if $proxy_session is true but $proxy_host is empty. As
+an example, in my connect ACL, I have:
+
+ warn condition = ${if and{ {bool{$proxy_session}} \
+ {eq{$proxy_host}{}} } }
+ log_message = Failed required proxy protocol negotiation \
+ from $sender_host_name [$sender_host_address]
+
+ warn condition = ${if and{ {bool{$proxy_session}} \
+ {!eq{$proxy_host}{}} } }
+ # But don't log health probes from the proxy itself
+ condition = ${if eq{$proxy_host}{$sender_host_address} \
+ {false}{true}}
+ log_message = Successfully proxied from $sender_host_name \
+ [$sender_host_address] through proxy protocol \
+ host $proxy_host
+
+4. Runtime issues to be aware of:
+ - Since the real connections are all coming from your proxy, and the
+ per host connection tracking is done before Proxy Protocol is
+ evaluated, smtp_accept_max_per_host must be set high enough to
+ handle all of the parallel volume you expect per inbound proxy.
+ - The proxy has 3 seconds (hard-coded in the source code) to send the
+ required Proxy Protocol header after it connects. If it does not,
+ the response to any commands will be:
+ "503 Command refused, required Proxy negotiation failed"
+ - If the incoming connection is configured in Exim to be a Proxy
+ Protocol host, but the proxy is not sending the header, the banner
+ does not get sent until the timeout occurs. If the sending host
+ sent any input (before the banner), this causes a standard Exim
+ synchronization error (i.e. trying to pipeline before PIPELINING
+ was advertised).
+ - This is not advised, but is mentioned for completeness if you have
+ a specific internal configuration that you want this: If the Exim
+ server only has an internal IP address and no other machines in your
+ organization will connect to it to try to send email, you may
+ simply set the hostlist to "*", however, this will prevent local
+ mail programs from working because that would require mail from
+ localhost to use Proxy Protocol. Again, not advised!
+
+5. Example of a refused connection because the Proxy Protocol header was
+not sent from a host configured to use Proxy Protocol. In the example,
+the 3 second timeout occurred (when a Proxy Protocol banner should have
+been sent), the banner was displayed to the user, but all commands are
+rejected except for QUIT:
+
+# nc mail.example.net 25
+220-mail.example.net, ESMTP Exim 4.82+proxy, Mon, 04 Nov 2013 10:45:59
+220 -0800 RFC's enforced
+EHLO localhost
+503 Command refused, required Proxy negotiation failed
+QUIT
+221 mail.example.net closing connection
+
+
+
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