Network Working GroupM.S. Kucherawy
Internet DraftSendmail, 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

Status of this Memo

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Copyright Notice

Copyright © The IETF Trust (2007). All Rights Reserved.

Abstract

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. Introduction

[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.

1.1 Definitions

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].


2. Optional Key Reporting Address for DKIM

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:

r=
Reporting Address (plain-text; OPTIONAL; no default). The value MUST be a dkim-quoted-printable string containing the local-part of an e-mail address to which a report SHOULD be sent when mail signed with this key fails verification because either (a) the signature verification itself failed, or (b) the body hash test failed. The format of this reply is selected by the value of the "rf=" tag, defined below. To generate a complete address to which the report is sent, the verifier simply appends to this value an "@" followed by the domain found in the "d=" tag of the signature whose validation failed.
rf=
Reporting Format (plain-text; OPTIONAL; default is "arf"). The value MUST be a colon-separated list of tokens representing desired reporting formats in order of preference. Each element of the list MUST be a token which is taken from the registered list of DKIM report formats. See Section 7 for a description of the registry and Section 5 for a description of recognized formats. The verifier generating reports MUST generate a report using the first token in the list which represents a report format it is capable of generating.
ro=
Requested Reports (plain-text; OPTIONAL; default is "all"). The value MUST be a colon-separated list of tokens representing those conditions under which a report is desired. See Section 4.1 for a list of valid tags.

3. Optional Key Reporting Address for DKIM-SSP

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:

r=
Reporting Address (plain-text; OPTIONAL; no default). The value MUST be a dkim-quoted-printable string containing the local-part of an e-mail address to which a report SHOULD be sent when mail claiming to be from this domain failed the verification algorithm described in [3], in particular because a message arrived unsigned or signed by a third-party signature in violation of the sender's published policy. The format of this reply MUST be in the format specified by the "rf=" tag defined below. To generate a complete address to which the report is sent, the verifier simply appends to this value an "@" followed by the domain whose policy was queried in order to evaluate the sender's SSP.
rf=
Reporting Format (plain-text; OPTIONAL; default is "arf"). The value MUST be a colon-separated list of tokens representing desired reporting formats in decreasing order of preference. Each element of the list MUST be a token which is taken from the registered list of DKIM report formats. See Section 7 for a description of the registry and Section 5 for a description of recognized formats. The verifier generating reports MUST generate a report using the first token in the list which represents a report format it is capable of generating.
ro=
Requested Reports (plain-text; OPTIONAL; default is "all"). The value MUST be a colon-separated list of tokens representing those conditions under which a report is desired. See Section 4.2 for a list of valid tags.

4. Requested Reports

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.

4.1 Requested Reports for DKIM Failures

The following report requests are defined for DKIM keys:

all
All reports are requested.
s
Reports are requested for signature or key syntax errors.
v
Reports are requested for signature verification failures or body hash mismatches.
x
Reports are requested for signatures rejected by the verifier because the expiration time has passed.

4.2 Requested Reports for DKIM SSP Failures

The following report requests are defined for DKIM keys:

all
All reports are requested.
s
Reports are requested for messages for which the SSP algorithm determines the message is both signed and Suspicious.
u
Reports are requested for messages for which the SSP algorithm determines the message is both unsigned and Suspicious.

5. Reporting Formats

This section lists reporting formats supported by this DKIM reporting mechanism. Currently only one format is supported:

arf:
Abuse Reporting Format, as defined in [2], and as extended in Section 6.

6. Extension ARF Fields for DKIM Reporting

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.

6.1 New ARF Feedback Type

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.

6.2 New ARF Header Names

The following new ARF header names are defined as extensions to Section 8.1 of [2]:

DKIM-Canonicalized-Body:
A base64 encoding of the canonicalized body of the message as generated by the verifier. This header and value MUST be present for reports using feedback type "dkim" when reporting a "bodyhash" failure.
DKIM-Canonicalized-Headers:
A base64 encoding of the canonicalized header of the message as generated by the verifier. This header and value MUST be present for reports using feedback type "dkim".
DKIM-Failure:
Indicates the type of DKIM failure that is being reported. The list of valid values is enumerated below. This header and value MUST be present for reports using feedback type "dkim".
DKIM-Identity:
The identity of the signature which failed verification, taken from the "i=" tag of the signature. This header and value MUST be present for reports using feedback type "dkim" when reporting anything other than an "ssp" failure.
DKIM-Selector:
The selector of the signature which failed verification, taken from the "s=" tag of the signature. This header and value MUST be present for reports using feedback type "dkim" when reporting anything other than an "ssp" failure.

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].

6.3 DKIM Failure Types

The list of defined DKIM failure types, used in the "DKIM-Failure:" header (defined above), is as follows:

bodyhash:
The body hash in the signature and the body hash computed by the verifier did not match.
granularity:
The key referenced by the signature on the message was not authorized for use by the sender.
revoked:
The key referenced by the signature on the message has been revoked.
signature:
The signature on the message did not successfully verify against the header hash and public key.
ssp:
The sender's published signing practises determined the message is suspicious.

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.


7. IANA Considerations

As required by [4], this section contains registry information for the new [1] key tag, the new [3] tag, and the extensions to [2].

7.1 DKIM Key Tag Registration

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) |
                        +------+-----------------+
			

7.2 DKIM SSP Tag Registration

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) |
                        +------+-----------------+
			

7.3 Updates to ARF Feedback Types

The following feedback type is added to the Feedback Report Feedback Type Registry:

    Feedback Type: dkim
    Description: DKIM failure report
    Registration: (this document)
			

7.4 Updates to ARF Header Names

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
			

8. Security Considerations

Security issues with respect to these DKIM reports are similar to those found in [8].

8.1 Forgeries

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:

  1. A falsified DKIM failure notification when the message was in fact delivered to the indicated recipient;
  2. Falsified signature information, such as selector, domain, etc.

Perhaps the simplest means of mitigating this threat is to assert that DKIM reports should themselves be signed. This is under consideration.

8.2 Automatic Generation

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.


9. References

9.1 Normative References

[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.

9.2 Informative References

[8]Moore, K. and G. Vaudreuil, “ An Extensible Message Format for Delivery Status Notifications ”, RFC 1894, January 1996.

Author's Address

Murray S. KucherawySendmail, Inc.6475 Christie Ave., Suite 350Emeryville, CA 94608USPhone: +1 510 594 5400EMail:

A. Acknowledgements

The author wishes to acknowledge the following for their review and constructive criticism of this proposal: (add names here)


B. Examples

This section contains examples of the use of each of the extensions defined by this memo.

B.1 Example Use of DKIM Key Extension Tags

B.2 Example Use of DKIM SSP Extension Tags

B.3 Example Use of ARF Extension Headers


C. To-Do List

Things still to be done:


D. Public Discussion

[REMOVE BEFORE PUBLICATION]

Public discussion of this proposed specification is handled via the mail-vet-discuss@mipassoc.org mailing list. The list is open. Access to subscription forms and to list archives can be found at http://mipassoc.org/mailman/listinfo/mail-vet-discuss.

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