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Open source ARC implementation
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OPENARC README ============== This directory has the latest open source ARC filter software from The Trusted Domain Project. There is a web site at http://www.trusteddomain.org that is home for the latest updates. +--------------+ | INTRODUCTION | +--------------+ The Trusted Domain Project (TDP) is a non-profit corporation dedicated to research and development of technologies that promote trust in the use of domain names in on the Internet. OpenARC is an initiative of TDP, and is primarily community effort to develop and maintain an open source library for producing ARC-aware applications, and a "milter"-based filter for providing ARC service. ARC is a technology proposal, put forward by a consortium of organizations, intended to allow a chain of message handlers (typically email operators) to confirm handling by trusted upstream handlers in an attempt to confirm the valid use of certain identifiers in the message. ARC is still experimental, and its specification may change. This package is intended for use by operators willing to take part in the experiment and provide their feedback to the development team. "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. A substantial amount of the code here is based on code developed as part of The OpenDKIM Project, also a TDP activity, which 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 ARC 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. +--------------+ | 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 or later is required if you want to be able to sign or verify messages using the SHA256 message digest algorithm which is generally required for current DKIM applications. o sendmail v8.13.0 (or later), or Postfix 2.3, (or later) and libmilter. (These are only required if you are building the filter.) o Access to a working nameserver (required only for signature verification). o On Linux systems, either libbsd (BSD compatibility library) or some other library that provides strlcat() and strlcpy(). o If you are interested in tinkering with the build and packaging structure, you may need to upgrade to these versions of GNU's "autotools" components: autoconf (GNU Autoconf) 2.61 automake (GNU automake) 1.7 (or 1.9 to avoid warnings) ltmain.sh (GNU libtool) 2.2.6 (or 1.5.26 after make maintainer-clean) +-----------------------+ | RELATED DOCUMENTATION | +-----------------------+ The man page for openarc (the actual filter program) is present in the openarc directory of this source distribution. There is additional information in the INSTALL and FEATURES files, and in the README file in the openarc directory. Changes are documented in the RELEASE_NOTES file. HTML-style documentation for libarc is available in libarc/docs in this source distribution. Mailing lists discussing and supporting the ARC software found in this package are maintained via a list server at trusteddomain.org. Visit http://www.trusteddomain.org to subscribe or browse archives. The available lists are: openarc-announce (moderated) Release announcements. openarc-users General OpenARC user questions and answers. openarc-dev Chatter among OpenARC developers. Bug tracking is done via the issue trackers on GitHub, at https://github.com/mskucherawy/OpenARC/issues. 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 OpenARC 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 | +---------------------+ contrib A collection of user contributed scripts that may be useful. docs A collection of RFCs and drafts related to opendkim. libopenarc A library that implements the proposed ARC service. libopenarc/docs HTML documentation describing the API provided by libopenarc. openarc A milter-based filter application which uses libopenarc to provide ARC service via an MTA using the milter protocol. +----------------+ | RUNTIME ISSUES | +----------------+ WARNING: symbol 'X' not available The filter attempted to get some information from the MTA that 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. In the case of sendmail, 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 ARC 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. 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. Other OpenARC issues: Report any bugs to the email address email@example.com or to the GitHub issue trackers accessible at: https://github.com/mskucherawy/OpenARC/issues +-----------------------------+ | Sendmail REWRITING FEATURES | +-----------------------------+ 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, openarc sees the headers before they are modified as required by those two features. This means any 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. There is also a feature of Sendmail that will cause it to alter addresses after signing but before they are transmitted. The feature, which is on by default, passes addresses in header fields to the resolver functions to ensure they are canonical. This can result in the replacement of those strings in the sent message with their canonical forms only after the message is signed, which will invalidate the signatures. To suppress this feature, add the following line to your sendmail.mc and submit.mc configuration files, re-generate your configuration and restart the filter: FEATURE(`nocanonify') -- Copyright (c) 2010-2012, 2014, 2016, 2017, The Trusted Domain Project. All rights reserved.