Extensions to DKIM for Failure Reporting
draft-ietf-marf-dkim-reporting-10

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.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/.

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This Internet-Draft will expire on August 14, 2012.

Copyright Notice

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

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

• 1. Introduction
• 2. Definitions
• 2.1. Keywords
• 2.2. Imported Definitions
• 2.3. Other Definitions
• 3. Optional Reporting for DKIM
• 3.1. Extension DKIM Signature Tag
• 3.2. DKIM Reporting TXT Record
• 3.3. DKIM Reporting Algorithm
• 4. Optional Reporting Address for DKIM-ADSP
• 5. Requested Reports
• 5.1. Requested Reports for DKIM Failures
• 5.2. Requested Reports for DKIM ADSP Failures
• 6. Report Generation
• 6.1. Report Format
• 6.2. Other Guidance
• 7. IANA Considerations
• 7.1. DKIM Signature Tag Registration
• 7.2. DKIM ADSP Tag Registration
• 7.3. DKIM Reporting Tag Registry
• 8. Security Considerations
• 8.1. Inherited Considerations
• 8.2. Deliberate Misuse
• 9. References
• 9.1. Normative References
• 9.2. Informative References
• Appendix A. Acknowledgements
• Appendix B. Examples
• Appendix B.1. Example Use of DKIM Signature Extension Tag
• Appendix B.2. Example DKIM Reporting TXT Record
• Appendix B.3. Example Use of DKIM ADSP Extension Tags
• Author's Address

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. Reports are generated using the format defined in [I-D.IETF-MARF-AUTHFAILURE-REPORT].

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.

2.3. Other Definitions

report generator:

A report generator is an entitiy that generates and sends reports. For the scope of this memo, the term refers to Verifiers, as defined in Section 2.2 of [DKIM], designed also to generate authentication failure reports according to this specification.

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:

r=

Reporting Requested (plain-text; OPTIONAL; no default). If present, this tag indicates that the Signer requests that Verifiers generate a report when verification of the DKIM signature fails. At present, the only legal value is the single character "y" (in either upper or lower case). A complete description and illustration of how this is applied can be found in Section 3.3.

    sig-r-tag = %x72 *WSP "=" *WSP "y"
				

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. A report generator will request the content of this record when it sees an "r=" 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" / "d" / "o" / "p"/ "s" / "v" / "x" )
    rep-rr-tag = %x72.72 *WSP "=" *WSP rep-rr-type
                 *WSP 0* ( ":" *WSP rep-rr-type )
					

    rep-rs-tag = %x72.73 *WSP "=" qp-section
					

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

ra=

Reporting Address (plain-text; OPTIONAL). 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 "d=" 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 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, the "rp=" and "rr=" tags MUST be ignored, and the report generator MUST NOT issue a report.

3.3. DKIM Reporting Algorithm

Report generators MUST apply the following algorithm, or one semantically equivalent to it, for each DKIM-Signature header field whose verification fails for some reason. Note that this processing is done as a reporting extension only; the outcome of the specified DKIM evaluation MUST be otherwise unaffected.

1. If the DKIM-Signature field did not contain a valid "r=" tag, terminate.
2. Issue a [DNS] TXT query to the name that results from appending the value of the "d=" tag in the DKIM-Signature field to the string "_report._domainkey.". For example, if the DKIM-Signature header field contains "d=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, or if multiple TXT records are returned, 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, 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. If no "ra=" tag was present, skip this step and the next one. Otherwise, determine the reporting address by extracting the value of the "ra=" tag and appending to it "@" followed by the domain name found in the "d=" 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 and cause unwanted mail to be generated. It can cause additional DNS traffic against the domain listed in the "d=" 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.

The above procedure does not permit the detection and reporting of messages including a fraudulent DKIM-Signature header field, where such signature did not include an "r=" tag. It might be useful to some Signers to receive such reports, but this use case is not supported. To support this, a Verifier would have to violate the first step above and continue even in the absence of an "r=" tag. Although this satisfies this reporting requirement (which is expected to be unusual), it also creates a possible denial-of-service attack as such Verifiers will always look for the reporting TXT record, so the generator of fraudulent messages could simply send a large volume of such messages to a number of destinations. Thus, the specified algorithm does not accommodate this case.

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" / "d" / "o" / "p" / "s" / "u" )
    adsp-rr-tag = %x72.72 *WSP "=" *WSP adsp-rr-type
                  *WSP 0* ( ":" *WSP adsp-rr-type )
				

    adsp-rs-tag = %x72.73 *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 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 "ra=" tag, the "rp=" and "rr=" tags MUST be ignored, and the report generator MUST NOT issue a report.

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 list.

5.1. Requested Reports for DKIM Failures

The following report requests are defined for DKIM keys:

all

All reports are requested.

d

Reports are requested for signature evaluation errors that resulted from DNS issues (e.g., key retrieval problems).

o

Reports are requested for any reason related to DKIM signature evaluation not covered by other report requests listed here.

p

Reports are requested for signatures that are rejected for local policy reasons at the Verifier that are related to DKIM signature evaluation.

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.

d

Reports are requested for messages that could not have [ADSP] evaluated due to DNS (policy retrieval) issues.

o

Reports are requested for any [ADSP]-related failure reason not covered by other report requests listed here.

p

Reports are requested for messages that are rejected for local policy reasons at the Verifier that are related to [ADSP].

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.IETF-MARF-AUTHFAILURE-REPORT].

6.2. Other Guidance

Additional guidance about the generation of these reports can be found in [I-D.IETF-MARF-AS], especially Section 9.

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 |
              +------+-----------------+--------+
              | r    | (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], [ADSP], [I-D.IETF-MARF-AS], and [I-D.IETF-MARF-AUTHFAILURE-REPORT]. Many security issues related to this draft are already covered in those documents.

8.2. Deliberate Misuse

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

9. References

9.1. Normative References

9.2. Informative References

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, John Levine, and Scott Kitterman.

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 Tag

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

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

Example 1: DKIM-Signature field using this extension

This example DKIM-Signature field contains the "r=" tag that indicates reports are requested on verification failure.

If this signature fails to verify, a TXT query will be sent to "_report._domainkey.example.com" to retrieve a reporting address and other report parameters.

Appendix B.2. Example DKIM Reporting TXT Record

An example DKIM Reporting TXT Record as defined by this memo:

    ra=dkim-errors; rp=100; rr=v:x
				

Example 2: Example DKIM Reporting TXT Record

This example, continuing from the previous one, shows a message that might be found at "_report._domainkey.example.com" in a TXT record. It makes the following requests:

• Reports about signature evaluation failures should be send to the address "dkim-errors" at the signer's domain;
• All (100%) incidents should be reported;
• Only reports about signature verification failures and expired signatures should be generated.

Appendix B.3. Example Use of DKIM ADSP Extension Tags

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

    dkim=all; ra=dkim-adsp-errors; rr=u
				

Example 3: DKIM ADSP record using these extensions

This example ADSP record makes the following assertions:

• The sending domain (i.e. the one that is advertising this policy) signs all mail it sends;
• Reports about ADSP evaluation failures should be send to the address "dkim-adsp-errors" at the Author's domain;
• Only reports about unsigned messages should be generated.

Author's Address