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OPENDKIM README
===============
$Id: README,v 1.6 2009/08/11 16:35:22 cm-msk Exp $
This directory has the latest open source DKIM filter software from The
OpenDKIM Project.
Report any bugs to opendkim-users@lists.opendkim.org.
There is a web site at http://www.opendkim.org that is home for the latest
updates.
+--------------+
| INTRODUCTION |
+--------------+
The OpenDKIM Project is a community effort to develop and maintain an open
source library for producing DKIM-aware applications, and a "milter"-based
filter for providing DKIM service.
The DKIM sender authentication system was originally created by the E-mail
Signing Technology Group (ESTG) and is now a proposed standard of the
IETF (RFC4871). DKIM is an amalgamation of the DomainKeys (DK) proposal by
Yahoo!, Inc. and the Internet Identified Mail (IIM) proposal by Cisco.
"milter" is a portmanteau of "mail filter" and refers to a protocol and API
for communicating mail traffic information between MTAs and mail filtering
plug-in applications. It was originally invented at Sendmail, Inc. but
has also been adapted to other MTAs.
The OpenDKIM project started as a code fork of version 2.8.3 of the open source
"dkim-milter" package developed and maintained by Sendmail, Inc. The license
used by The OpenDKIM project is found in the LICENSE file. Portions of this
project are also covered by the Sendmail Open Source License, available in
this distribution in the file "LICENSE.Sendmail". See the copyright notice(s)
in each source file to determine whether or not it is covered by both licenses.
This package consists of a library that implements the DKIM service and a
milter-based filter application that can plug in to any milter-aware MTA to
provide that service to sufficiently recent sendmail, Postfix or other MTAs
that support the milter protocol.
An optional asynchronous resolver library is also provided to work around
limitations of the basic BIND resolver which comes installed on most
systems.
+--------------+
| DEPENDENCIES |
+--------------+
To compile and operate, this package requires the following:
o OpenSSL (http://www.openssl.org, or ask your software vendor for a package)
(any version will get you started, however v0.9.8 is required if you want
to be able to sign or verify messages using the SHA256 message digest
algorithm)
o sendmail v8.13.0 (or later), or Postfix 2.3, (or later) and libmilter.
o Access to a working nameserver (required only for verification).
+-----------------------+
| RELATED DOCUMENTATION |
+-----------------------+
The man page for opendkim (the actual filter program) is present in the
opendkim directory of this source distribution. There is additional
information in the INSTALL and FEATURES files, and in the README file in the
opendkim directory. Changes are documented in the RELEASE_NOTES file.
HTML-style documentation for libopendkim is available in libopendkim/docs in
this source distribution.
General information about DKIM can be found at http://www.dkim.org
Mailing lists discussing and supporting the DKIM software found in this
package are maintained via a list server at opendkim.org. Visit
http://www.opendkim.org to subscribe or browse archives. The available
lists are:
opendkim-announce (moderated) Release announcements.
opendkim-users General OpenDKIM user questions and answers.
opendkim-dev Chatter among OpenDKIM developers.
opendkim-cvs Automated source code change announcements.
Bug tracking is done via the trackers on SourceForge at
http://sourceforge.net/projects/opendkim. You can enter new bug
reports there, but please check first for older bugs already open,
or even already closed, before opening a new issue.
+---------+
| WARNING |
+---------+
Since OpenDKIM uses cryptography, the following information from OpenSSL
applies to this package as well.
PLEASE REMEMBER THAT EXPORT/IMPORT AND/OR USE OF STRONG CRYPTOGRAPHY
SOFTWARE, PROVIDING CRYPTOGRAPHY HOOKS OR EVEN JUST COMMUNICATING
TECHNICAL DETAILS ABOUT CRYPTOGRAPHY SOFTWARE IS ILLEGAL IN SOME
PARTS OF THE WORLD. SO, WHEN YOU IMPORT THIS PACKAGE TO YOUR
COUNTRY, RE-DISTRIBUTE IT FROM THERE OR EVEN JUST EMAIL TECHNICAL
SUGGESTIONS OR EVEN SOURCE PATCHES TO THE AUTHOR OR OTHER PEOPLE
YOU ARE STRONGLY ADVISED TO PAY CLOSE ATTENTION TO ANY EXPORT/IMPORT
AND/OR USE LAWS WHICH APPLY TO YOU. THE AUTHORS ARE NOT LIABLE FOR
ANY VIOLATIONS YOU MAKE HERE. SO BE CAREFUL, IT IS YOUR RESPONSIBILITY.
If you use OpenSSL then make sure you read their README file which
contains information about patents etc.
+---------------------+
| DIRECTORY STRUCTURE |
+---------------------+
libar An optional thread-safe asynchronous resolver library.
libopendkim A library that implements the proposed DKIM standard.
libopendkim/docs HTML documentation describing the API provided by libopendkim.
opendkim A milter-based filter application which uses libopenkim (and
optionally libar) to provide DKIM service via an MTA using
the milter protocol.
+----------------+
| RUNTIME ISSUES |
+----------------+
WARNING: sendmail symbol 'X' not available
The filter attempted to get some information from the MTA which the MTA
did not provide.
At various points in the interaction between the MTA and the filter, certain
macros containing information about the job in progress or the connection
being handled are passed from the MTA to the filter. The names of the macros
the MTA should pass to the filter are defined by the "Milter.macros"
settings in sendmail.cf, e.g. "Milter.macros.connect",
"Milter.macros.envfrom", etc. This message indicates that the filter needed
the contents of macro X, but that macro was not passed down from the MTA.
Typically the values needed by this filter are passed from the MTA if the
sendmail.cf was generated by the usual m4 method. If you do not have
those options defined in your sendmail.cf, make sure your M4 configuration
files are current and rebuild your sendmail.cf to get appropriate lines
added to your sendmail.cf, and then restart sendmail.
MTA timeouts
By default, the MTA is configured to wait up to ten seconds for a response
from a filter before giving up. When querying remote nameservers
for key and policy data, the DKIM filter may not get a response from the
resolver within that time frame, and thus this MTA timeout will occur.
This can cause messages to be rejected, temp-failed or delivered without
verification, depending on the failure mode selected for the filter.
When using the standard resolver library provided with your system, the
DNS timeout cannot be adjusted. If you encounter this problem, you must
increase the time the MTA waits for replies. See the documentation in
the sendmail open source distribution (libmilter/README in particular)
for instructions on changing these timeouts.
When using the provided asynchronous resolver library, you can use the
"-T" command line option to change the timeout so that it is shorter than
the MTA timeout.
d2i_PUBKEY_bio() failed
After retrieving and decoding a public key to perform a message verification,
the OpenSSL library attempted to make use of that key but failed. The
known possible causes are:
(a) Memory exhaustion
(b) Key corruption
If you're set to temp-fail messages in these cases, the remote end will
probably retry the message. If the same message fails again later,
the key is probably corrupted or otherwise invalid.
You can try compiling with _FFR_LOG_SSL_ERRORS to have the filter extract
OpenSSL library errors and include them in the system log when such
conditions occur for further diagnostics. In one observed case, a corrupt
key also caused the following to be logged:
error:0D0680A8:asn1 encoding routines:ASN1_CHECK_TLEN:wrong tag;
error:0D07803A:asn1 encoding routines:ASN1_ITEM_EX_D2I:nested asn1 error
+-----------------------------------------+
| Sendmail MASQUERADING AND GENERICSTABLE |
+-----------------------------------------+
There are two features of the sendmail MTA which, if activated, can interfere
with successful use of the DKIM service. The two features are MASQUERADE_AS
and FEATURE(genericstable). See cf/README in the open source sendmail
source code distribution for more information.
Due to the way the milter protocol is incorporated into the MTA, opendkim
sees the headers before they are modified as required by those two features.
This means the DKIM signature is generated based on the headers originally
injected by the mail client and not on the headers which are actually sent
out by the MTA. As a result, the verifying agent at the receiver's side
will be unable to verify the signature as the signed data and the received
data don't match.
The suggested solutions to this problem are:
(1) Send mail with the headers already written as needed, obviating the
need for these features (or just turn them off).
(2) Have two MTAs set up, either on separate boxes or on the same box.
The first MTA should do all of the rewriting (i.e. use these two
features) and the second one should use opendkim to add the signature
and do no rewriting at all.
(3) Have multiple DaemonPortOptions lines in your configuration file. The
first daemon port (port 25) does the header rewriting and then routes
the message to the second port; the latter does no rewriting but does the
signing and then sends the message on its way.