-$Cambridge: exim/doc/doc-txt/experimental-spec.txt,v 1.3 2005/06/27 18:32:20 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
liable to incompatibile change.
-
-1. Yahoo DomainKeys support
+OCSP Stapling 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.
-
-The current experimental implementation supports two
-independent functions:
-
-o Validate incoming DK-signed email.
-o Sign outgoing email with DK.
-
-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 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:
-
-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.
+X509 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.
- $dk_sender_local_part
+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.
- Contains the local part that DK has verified for.
+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.
- $dk_sender_source
+If Exim is built with EXPERIMENTAL_OCSP and it was built with OpenSSL,
+then it gains one new option: "tls_ocsp_file".
- Contains the "source" of the above three variables, one of
+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.
- "from" The address came from the "From:" header.
- "sender" The address came from the "Sender:" header.
+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.
- When DK was unable to find a valid address, this variable
- is "0".
+Exim will check for a validity next update timestamp in the OCSP proof;
+if not present, or if the proof has expired, it will be ignored.
+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.
- $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
+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
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
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.
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
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
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
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.
-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.
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
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
--------------------- */
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
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
+SRS (Sender Rewriting Scheme) Support
--------------------------------------------------------------
Exiscan currently includes SRS support via Miles Wilton's
git://git.exim.org / exim.git / blobdiff