-$Cambridge: exim/doc/doc-txt/experimental-spec.txt,v 1.7 2007/07/27 13:56:24 magnus 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
--------------------------------------------------------------
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.
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
--------------------- */
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
git://git.exim.org / exim.git / blobdiff