MARF Working Group M.S. Kucherawy
Internet-Draft Cloudmark
Intended status: Standards Track October 18, 2012
Expires: April 19, 2013

Extensions to DKIM for Failure Reporting
draft-ietf-marf-dkim-reporting-05

Abstract

This memo presents extensions to the DomainKeys Identified Mail (DKIM) specification to allow for detailed reporting of message authentication failures in an on-demand fashion.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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Copyright Notice

Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved.

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Table of Contents

1. Introduction

[DKIM] introduced a mechanism for message signing and authentication. It uses digital signing to associate a domain name with a message in a reliable (i.e. not forgeable) manner. The output is a verified domain name that can then be subjected to some sort of evaluation process (e.g., advertised sender policy, comparison to a known-good list, submission to a reputation service, etc.).

Deployers of message authentication technologies are increasingly seeking visibility into DKIM verification failures and conformance failures involving the published signing practices (e.g., [ADSP]) of an Administrative Management Domain (ADMD; see [EMAIL-ARCH]).

This document extends [DKIM] and [ADSP] to add an optional reporting address and some reporting parameters.

2. Definitions

2.1. Keywords

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 [KEYWORDS].

2.2. Imported Definitions

The ABNF token "qp-section" is imported from [MIME].

Numerous DKIM-specific terms used here are defined in [DKIM]. The definition of the ABNF token "domain-name" can also be found there.

3. Optional Reporting for DKIM

A domain name owner employing [DKIM] for email signing and authentication might want to know when signatures in use by specific keys are not successfully verifying. Currently there is no such mechanism defined.

This document adds optional "tags" (as defined in [DKIM]) to the DKIM-Signature header field and the DKIM key record in the DNS, using the formats defined in that specification.

3.1. Extension DKIM Signature Tag

The following tag is added to DKIM-Signature header fields when a Signer wishes to request that reports of failed verifications be generated by a Verifier:

rd=
Reporting Domain (plain-text; OPTIONAL; no default). The value MUST be a string containing a DNS domain name. This value will be used by the Verifier to confirm that failed verification reports are desired and collect report generation parameters. A complete description and illustration of how this is applied can be found in Section 3.3.

    sig-rd-tag = %x72.64 *WSP "=" *WSP domain-name
				

ABNF:

3.2. DKIM Reporting TXT Record

When a Signer wishes to advertise that it wants to receive failed verification reports, it also places in the DNS a TXT resource record (RR) whose content follows the same general syntax as DKIM key records, as defined in Section 3.6.1 of [DKIM], in that it is made up of a sequence of tag-value objects. In this case, the tags and values comprise the parameters to be used when generating the reports. The Verifier (if it supports this extension) will request the content of this record when it sees an "rd=" tag in a DKIM-Signature header field.

The reassembly rules of Section 3.6.2.2 of [DKIM] also apply here if the reporting TXT record consists of several string fragments.

Any tag found in the content of this record that is not registered with IANA as described in Section 7.3 MUST be ignored.

    rep-ra-tag = %x72.61 *WSP "=" *WSP qp-section
					
    rep-rp-tag = %x72.70 *WSP "=" *WSP 1*3DIGIT
					
    rep-rr-type = ( "all" / "s" / "v" / "x" )
    rep-rr-tag = %x72 %x6f *WSP "=" *WSP rep-rr-type
                 *WSP 0* ( ":" *WSP rep-rr-type )
					
    rep-rs-tag = %x72 %x73 *WSP "=" qp-section
					

The initial list of tags supported for the reporting TXT record is as follows:

ra=
Reporting Address (plain-text; REQUIRED). A dkim-quoted-printable string (see Section 2.11 of [DKIM]) containing the local-part of an email address to which a report SHOULD be sent when mail fails DKIM verification for one of the reasons enumerated below. The value MUST be interpreted as a local-part only. To construct the actual address to which the report is sent, the verifier simply appends to this value an "@" followed by the domain name found in the "rd=" tag of the DKIM-Signature header field. Therefore, an ADMD making use of this specification MUST ensure that an email address thus constructed can receive reports generated as described in Section 6. ABNF:
rp=
Requested Report Percentage (plain-text; OPTIONAL; default is "100"). The value is an integer value from 0 to 100 inclusive that indicates what percentage of incidents of signature authentication failures, selected at random, are to cause reports to be generated. The report generator SHOULD NOT issue reports for more than the requested percentage of incidents. Report generators MAY make use of the "Incidents:" field in [ARF] to indicate that there are more reportable incidents than there are reports. ABNF:
rr=
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 5.1 for a list of valid tags. ABNF:
rs=
Requested SMTP Error String (text; OPTIONAL; no default). The value is a dkim-quoted-printable string that the publishing ADMD requests be included in [SMTP] error strings if messages are rejected during the delivery SMTP session. ABNF:

In the absence of an "ra=" tag, all other tags listed above MUST be ignored. In other words, an implementation MUST NOT have a default value for the "ra=" tag.

3.3. DKIM Reporting Algorithm

The following algorithm, or one semantically equivalent to it, MUST be applied when an implementation is equipped to generate reports in compliance with this specification and its evaluation of the DKIM-Signature header field (see [DKIM]) on a message fails for some reason. Note that this processing is done as a reporting extension only; the outcome of the specified DKIM evaluation is otherwise unaffected.

  1. If the DKIM-Signature field did not contain an "rd=" tag, terminate.
  2. Issue a [DNS] TXT query to the name that results from appending the value of the "rd=" tag to the string "_report._domainkey". For example, if a DKIM-Signature header field contains "example.com", issue a DNS TXT query to "_report._domainkey.example.com".
  3. If the DNS query returns anything other than a success status code (0), also known as NOERROR, the implementation MAY log this locally; in either case, terminate.
  4. If the resultant TXT is in several string fragments, reassemble it as described in Section 3.6.2.2 of [DKIM].
  5. If the TXT content is syntactically invalid, the implementation MAY log this locally; in either case, terminate.
  6. If the reason for the signature evaluation failure does not match one of the report requests found in the "rr=" tag (or its default value), terminate.
  7. If a report percentage ("rp=") tag was present, select a random number between 0 and 99, inclusive; if the selected number is higher than the tag's value, terminate.
  8. Determine the reporting address by extracting the value of the "ra=" tag and appending to it "@" followed by the domain name found in the "rd=" tag of the DKIM-Signature header field.
  9. Construct and send a report in compliance with Section 6 of this memo that includes as its intended recipient the address constructed in the previous step.
  10. If the [SMTP] session during which the DKIM signautre was evaluated is still active and the SMTP server has not already given its response to the DATA command that relayed the message, and an "rs=" tag was present in the TXT record, the SMTP server SHOULD include the decoded string found in the "rs=" tag in its SMTP reply to the DATA command.

This algorithm has the following advantages over previous implementations:

  1. If the DKIM signature fails to verify, no additional DNS check is made to see if reporting is requested; the request is active in that it is included in the DKIM-Signature header field. (Previous implementations included the reporting address in the DKIM key record, which is not queried for certain failure cases. This meant, for full reporting, that the key record had to be retrieved even when it was not otherwise necessary.)
  2. The request is confirmed by the presence of a corresponding TXT record in the DNS, since the Signer thus provides the parameters required to construct and send the report. This means a malicious Signer cannot falsely assert that someone else wants failure reports. It can cause additional DNS traffic against the domain listed in the "rd=" signature tag, but negative caching of the requested DNS record will help to mitigate this issue.
  3. It is not possible for a Signer to direct reports to an email address outside of its own domain, preventing distributed email-based denial-of-service attacks.

4. Optional Reporting Address for DKIM-ADSP

There also exist cases in which a domain name owner employing [ADSP] for announcing signing practises with DKIM may want to know when messages are received without valid author domain signatures. Currently there is no such mechanism defined.

    adsp-ra-tag = %x72.61 *WSP "=" qp-section
				
    adsp-rp-tag = %x72.70 *WSP "=" *WSP 1*3DIGIT
				
    adsp-rr-type = ( "all" / "s" / "u" )
    adsp-rr-tag = %x72 %x6f *WSP "=" *WSP adsp-rr-type
                  *WSP 0* ( ":" *WSP adsp-rr-type )
				
    adsp-rs-tag = %x72 %x73 *WSP "=" qp-section
				

This document adds the following optional "tags" (as defined in [ADSP]) to the DKIM ADSP records, using the form defined in that specification:

ra=
Reporting Address (plain-text; OPTIONAL; no default). The value MUST be a dkim-quoted-printable string containing the local-part of an email address to which a report SHOULD be sent when mail claiming to be from this domain failed the verification algorithm described in [ADSP], in particular because a message arrived without a signature that validates, which contradicts what the ADSP record claims. The value MUST be interpreted as a local-part only. To construct the actual 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 ADSP, i.e., the one taken from the RFC5322.From domain of the message under evaluation. Therefore, a signer making use of this extension tag MUST ensure that an email address thus constructed can receive reports generated as described in Section 6. ABNF:
rp=
Requested Report Percentage (plain-text; OPTIONAL; default is "100"). The value is a single integer value from 0 to 100 inclusive that indicates what percentage of incidents of ADSP evaluation failures, selected at random, should cause reports to be generated. The report generator SHOULD NOT issue reports for more than the requested percentage of incidents. Report generators MAY make use of the "Incidents:" field in [ARF] to indicate that there are more reportable incidents than there are reports. ABNF:
rr=
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 5.2 for a list of valid tags. ABNF:
rs=
Requested SMTP Error String (plain-text; OPTIONAL; no default). The value is a string the signing domain requests be included in [SMTP] error strings when messages are rejected during a single SMTP session. ABNF:

In the absence of an "r=" tag, all other tags listed above MUST be ignored.

5. Requested Reports

This memo also includes, as the "ro" tags defined above, the means by which the signer can request reports for specific circumstances of interest. Verifiers MUST NOT generate reports for incidents that do not match a requested report, and MUST ignore requests for reports not included in this these lists.

5.1. 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.

5.2. Requested Reports for DKIM ADSP Failures

The following report requests are defined for ADSP records:

all
All reports are requested.
s
Reports are requested for messages that have a valid [DKIM] signature but do not match the published [ADSP] policy.
u
Reports are requested for messages that have no valid [DKIM] signature and do not match the published [ADSP] policy.

6. Report Generation

This section describes the process for generating and sending reports in accordance with the request of the signer and/or sender as described above.

6.1. Report Format

All reports generated as a result of requests contained in these extension parameters MUST be generated in compliance with [ARF] and its extension specific to this work, [I-D.MARF-AUTHFAILURE-REPORT].

6.2. Envelope Sender Selection

In the case of transmitted reports in the form of a new message (versus rejections during an [SMTP] session), it is necessary to construct the message so as to avoid amplification attacks, deliberate or otherwise. Thus, per Section 2 of [DSN], the envelope sender address of the report SHOULD be chosen to ensure that no delivery status reports will be issued in response to the report itself, and MUST be chosen so that these reports will not generate mail loops. Whenever an [SMTP] transaction is used to send a report, the MAIL FROM command MUST use a NULL return address, i.e. "MAIL FROM:<>".

7. IANA Considerations

As required by [IANA-CONSIDERATIONS], this section contains registry information for the new [DKIM] signature tags, and the new [ADSP] tags. It also creates a DKIM reporting tag registry.

7.1. DKIM Signature Tag Registration

IANA is requested to update the DKIM Signature Tag Specification Registry to include the following new items:

              +------+-----------------+--------+
              | TYPE | REFERENCE       | STATUS |
              +------+-----------------+--------+
              | rd   | (this document) | active |
              +------+-----------------+--------+
			

7.2. DKIM ADSP Tag Registration

IANA is requested to update the DKIM ADSP Specification Tag Registry to include the following new items:

              +------+-----------------+
              | TYPE | REFERENCE       |
              +------+-----------------+
              | ra   | (this document) |
              | rp   | (this document) |
              | rr   | (this document) |
              | rs   | (this document) |
              +------+-----------------+
			

7.3. DKIM Reporting Tag Registry

IANA is requested to create a sub-registry of the DKIM Parameters registry called "DKIM Reporting Tags". Additions to this registry follow the "Specification Required" rules, with the following columns required for all registrations:

Type:
The name of the tag being used in reporting records
Reference:
The document that specifies the tag being defined
Status:
The status of the tag's current use, either "active" indicating active use, or "historic" indicating discontinued or deprecated use

              +------+-----------------+--------+
              | TYPE | REFERENCE       | STATUS |
              +------+-----------------+--------+
              | ra   | (this document) | active |
              | rp   | (this document) | active |
              | rr   | (this document) | active |
              | rs   | (this document) | active |
              +------+-----------------+--------+
				

The initial registry entries are as follows:

8. Security Considerations

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

8.1. Inherited Considerations

Implementers are advised to consider the Security Considerations sections of [DKIM] and [ADSP].

8.2. Deliberate Misuse

Some threats caused by deliberate misuse of this mechanism are discussed in Section 3.3.

8.3. 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 reports of any kind should take appropriate precautions to minimize the potential damage from denial-of-service attacks.

Security threats related to forged reports include the sending of:

  1. A falsified authentication 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 these reports should themselves be signed with something like DKIM. On the other hand, if there's a problem with the DKIM infrastructure at the verifier, signing DKIM failure reports may produce reports that aren't trusted or even accepted by their intended recipients.

8.4. Amplification Attacks

Failure to compile with the normative statements in Section 6.2 can lead to amplification denial-of-service attacks. See that section for details.

8.5. Automatic Generation

The mechanisms described in this memo are primarily intended for use in generating reports to aid implementers of [DKIM] and [ADSP] and other related protocols in development and debugging. Therefore, they are not designed for prolonged forensic use. However, such uses are possible by ADMDs that want to keep a close watch for fraud or infrastructure problems.

Automatic generation of these reports by verifying agents can cause a denial-of-service attack when a large volume of email is sent that causes authentication failures for whatever reason.

Limiting the rate of generation of these messages may be appropriate but threatens to inhibit the distribution of important and possibly time-sensitive information.

In general ARF feedback loop terms, it is often suggested that report generators only create these (or any) ARF reports after an out-of-band arrangement has been made between two parties. This mechanism then becomes a way to adjust parameters of an authorized abuse report feedback loop that is configured and activated by private agreement rather than starting to send them automatically based solely on data found in the DKIM signatures, which could have been fraudulently inserted.

8.6. Reporting Multiple Incidents

If it is known that a particular host generates abuse reports upon certain incidents, an attacker could forge a high volume of messages that will trigger such a report. The recipient of the report could then be innundated with reports. This could easily be extended to a distributed denial-of-service attack by finding a number of report-generating servers.

The incident count referenced in [ARF] provides a limited form of mitigation. The host generating reports may elect to send reports only periodically, with each report representing a number of identical or near-identical incidents. One might even do something inverse-exponentially, sending reports for each of the first ten incidents, then every tenth incident up to 100, then every 100th incident up to 1000, etc. until some period of relative quiet after which the limitation resets.

The use of this for "near-identical" incidents in particular causes a degradation in reporting quality, however. If for example a large number of pieces of spam arrive from one attacker, a reporting agent may decide only to send a report about a fraction of those messages. While this averts a flood of reports to a system administrator, the precise details of each incident are similarly not sent.

9. References

9.1. Normative References

[ADSP] Allman, E., Delany, M., Fenton, J. and J. Levine, "DKIM Sender Signing Practises ", RFC 5617, August 2009.
[ARF] Shafranovich, Y., Levine, J. and M. Kucherawy, "An Extensible Format for Email Feedback Reports ", RFC 5965, August 2010.
[DKIM] Crocker, D., Hansen, T. and M. Kucherawy, "DomainKeys Identified Mail (DKIM) Signatures ", RFC 6376, September 2011.
[DNS] Mockapetris, P., "Domain names - implementation and specification ", STD 13, RFC 1035, November 1987.
[EMAIL-ARCH] Crocker, D, "Internet Mail Architecture", RFC 5598, October 2008.
[I-D.MARF-AUTHFAILURE-REPORT] Fontana, H., "Authentication Failure Reporting using the Abuse Report Format", I-D draft-ietf-marf-authfailure-report, January 2012.
[IANA-CONSIDERATIONS] Alvestrand, H. and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs ", RFC 5226, May 2008.
[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels ", RFC 2119, March 1997.
[MIME] Freed, N. and N. Borenstein, "Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies ", RFC 2045, November 1996.
[SMTP] Klensin, J., "Simple Mail Transfer Protocol ", RFC 5321, October 2008.

9.2. Informative References

[DSN] Moore, K. and G. Vaudreuil, "An Extensible Message Format for Delivery Status Notifications ", RFC 3464, January 2003.

Appendix A. Acknowledgements

The authors wish to acknowledge the following for their review and constructive criticism of this proposal: Steve Atkins, Monica Chew, Dave Crocker, Tim Draegen, Frank Ellermann, JD Falk, and John Levine.

Appendix B. Examples

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

Appendix B.1. Example Use of DKIM Signature Extension Tags

A DKIM-Signature field including use of the extensions defined by this memo:

    DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple;
            d=example.com; s=jan2012;
            r=dkim-errors; ro=v:x;
            h=from:to:subject:date:message-id;
            bh=YJAYwiNdc3wMh6TD8FjVhtmxaHYHo7Z/06kHQYvQ4tQ=;
            b=jHF3tpgqr6nH/icHKIqFK2IJPtCLF0CRJaz2Hj1Y8yNwTJ
              IMYIZtLccho3ymGF2GYqvTl2nP/cn4dH+55rH5pqkWNnuJ
              R9z54CFcanoKKcl9wOZzK9i5KxM0DTzfs0r8
				

Example 1: DKIM-Signature field using these extensions

This example DKIM-Signature field contains the following data in addition to the basic DKIM signature data:

Appendix B.2. Example Use of DKIM ADSP Extension Tags

A DKIM ADSP record including use of the extensions defined by this memo:

    dkim=all; r=dkim-adsp-errors; ro=u
				

Example 2: DKIM ADSP record using these extensions

This example ADSP record makes the following assertions:

Author's Address

Murray S. Kucherawy Cloudmark 128 King St., 2nd Floor San Francisco, CA 94107 US Phone: +1 415 946 3800 EMail: msk@cloudmark.com