Network Working Group | M.S. Kucherawy |
Internet Draft | Sendmail, Inc. |
<draft-kucherawy-dkim-reporting-01> | December 2007 |
Intended status: Standards Track | |
Expires: June 2008 |
Reporting of DKIM Verification Failures
draft-kucherawy-dkim-reporting-01
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This memo presents an extension to the DomainKeys Identified Mail (DKIM) specifications to allow public keys for verification to include a reporting address to be used to report message verification issues, and extends an Internet Message reporting format to be followed when generating such reports.
[1] introduced a standard for digital signing of messages for the purposes of sender authentication. There exist cases in which a domain name owner might want to receive reports from verifiers that determine DKIM-signed mail apparently from its domain is failing to verify according to [1] or is "Suspicious" according to [3].
This document extends [1] and [3] to add an optional reporting address to selector records, an optional means of specifying a desired report format, and furthermore extends [2] to add features required for DKIM reporting.
This memo presumes those specifications thus modified will issue as RFCs without these modifications. If the modifications are adopted prior to their publicatons, clearly those sections of this memo can be removed.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [5].
There exist cases in which a domain name owner employing [1] for e-mail signing and authentication may want to know when signatures in use by specific keys are not successfully verifying. Currently there is no such mechanism defined.
This document adds the following two optional "tags" (as defined in [1]) to the DKIM key records, using the form defined in that specification:
There also exist cases in which a domain name owner employing [3] for announcing signing practises with DKIM may want to know when messages are received unsigned or signed with third-party signatures contrary to the sender's published policy. Currently there is no such mechanism defined.
This document adds the following two optional "tags" (as defined in [3]) to the DKIM SSP records, using the form defined in that specification:
This memo also includes, as the "ro" tags defined above, the means by which the sender can request reports for specific circumstances of interest. Verifiers MUST NOT generate reports for incidents which do not match a requested report.
The following report requests are defined for DKIM keys:
The following report requests are defined for DKIM keys:
This section lists reporting formats supported by this DKIM reporting mechanism. Currently only one format is supported:
The current ARF format defined in [2] lacks some specific features required to do effective DKIM reporting. This section describes the extensions required to do so and thus required to conform to this specification.
A new feedback type of "dkim" is defined as an extension to Section 8.2 of [2]. See Section 7.3 for details.
The header names listed in that draft which may appear for this new feedback type include all shown in the draft except "Relevant-URI" and "Removal-Recipient" as they have no semantics relating to DKIM.
The following new ARF header names are defined as extensions to Section 8.1 of [2]:
The values which are base64 encodings may contain FWS for formatting purposes as per the usual header field wrapping defined in [6]. During decoding, any characters not in the base64 alphabet are ignored so that such line wrapping does not harm the value.
base64 is defined in [7].
The list of defined DKIM failure types, used in the "DKIM-Failure:" header (defined above), is as follows:
Supplementary data may be included in the form of [6]-compliant comments. For example, "Failure: ssp" could be augmented by a comment to indicate that the failed message was rejected because it was not signed when it should have been. See Appendix B for examples.
As required by [4], this section contains registry information for the new [1] key tag, the new [3] tag, and the extensions to [2].
IANA is requested to update the DKIM Key Tag Specification Registry to include the following new items:
+------+-----------------+ | TYPE | REFERENCE | +------+-----------------+ | r | (this document) | | rf | (this document) | | ro | (this document) | +------+-----------------+
IANA is requested to update the DKIM SSP Tag Specification Registry to include the following new items:
+------+-----------------+ | TYPE | REFERENCE | +------+-----------------+ | r | (this document) | | rf | (this document) | | ro | (this document) | +------+-----------------+
The following feedback type is added to the Feedback Report Feedback Type Registry:
Feedback Type: dkim Description: DKIM failure report Registration: (this document)
The following headers are added to the Feedback Report Header Names Registry:
Field Name: DKIM-Canonicalized-Body Description: Canonicalized body, per DKIM Multiple Appearances: No Related "Feedback-Type": dkim
Field Name: DKIM-Canonicalized-Headers Description: Canonicalized headers, per DKIM Multiple Appearances: No Related "Feedback-Type": dkim
Field Name: DKIM-Failure Description: Type of DKIM failure Multiple Appearances: No Related "Feedback-Type": dkim
Field Name: DKIM-Identity Description: Identity from DKIM signature Multiple Appearances: No Related "Feedback-Type": dkim
Field Name: DKIM-Selector Description: Selector from DKIM signature Multiple Appearances: No Related "Feedback-Type": dkim
Security issues with respect to these DKIM reports are similar to those found in [8].
These reports may be forged as easily as ordinary Internet electronic mail. User agents and automatic mail handling facilities (such as mail distribution list exploders) that wish to make automatic use of DSNs should take appropriate precautions to minimize the potential damage from denial-of-service attacks.
Security threats related to forged DSNs include the sending of:
Perhaps the simplest means of mitigating this threat is to assert that DKIM reports should themselves be signed. This is under consideration.
Automatic generation of these reports by verifying agents can cause a denial-of-service attack when a large volume of e-mail is sent that causes DKIM verification failures for whatever reason.
It is unclear what a good solution for this issue is. Limiting the rate of generation of these messages may be apropos but threatens to inhibit the distribution of important and possibly time-sensitive information.
[1] | Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, J., and M. Thomas, “ DomainKeys Identified Mail (DKIM) Signatures ”, RFC 4817, May 2007. |
[2] | Shafranovich, Y., Levine, J., and P. Hoffman, “ An Extensible Format for Email Feedback Reports ”, I-D DRAFT-SHAFRANOVICH-FEEDBACK-REPORT-03, June 2007. |
[3] | Allman, E., Delany, M., and J. Fenton, “ DKIM Sender Signing Practises ”, I-D DRAFT-IETF-DKIM-SSP-01. |
[4] | Alvestrand, H. and T. Narten, “ Guidelines for Writing an IANA Considerations Section in RFCs ”, RFC 2434, October 1998. |
[5] | Bradner, S., “ Key words for use in RFCs to Indicate Requirement Levels ”, RFC 2119, March 1997. |
[6] | Resnick, P., “ Internet Message Format ”, RFC 2822, April 2001. |
[7] | Freed, N. and N. Borenstein, “ Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies ”, RFC 2045, November 1996. |
[8] | Moore, K. and G. Vaudreuil, “ An Extensible Message Format for Delivery Status Notifications ”, RFC 1894, January 1996. |
The author wishes to acknowledge the following for their review and constructive criticism of this proposal: (add names here)
This section contains examples of the use of each of the extensions defined by this memo.
Things still to be done:
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