MARF Working Group M. Kucherawy Internet-Draft Cloudmark Intended status: Standards Track January 23, 2012 Expires: July 26, 2012 Extensions to DKIM for Failure Reporting draft-ietf-marf-dkim-reporting-04 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/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on July 26, 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. Kucherawy Expires July 26, 2012 [Page 1] Internet-Draft DKIM Reporting Extensions January 2012 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1. Keywords . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.2. Imported Definitions . . . . . . . . . . . . . . . . . . . 4 3. Optional Reporting Address for DKIM . . . . . . . . . . . . . 5 4. Optional Reporting Address for DKIM-ADSP . . . . . . . . . . . 7 5. Requested Reports . . . . . . . . . . . . . . . . . . . . . . 9 5.1. Requested Reports for DKIM Failures . . . . . . . . . . . 9 5.2. Requested Reports for DKIM ADSP Failures . . . . . . . . . 9 6. Report Generation . . . . . . . . . . . . . . . . . . . . . . 10 6.1. Report Format . . . . . . . . . . . . . . . . . . . . . . 10 6.2. Envelope Sender Selection . . . . . . . . . . . . . . . . 10 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 7.1. DKIM Signature Tag Registration . . . . . . . . . . . . . 11 7.2. DKIM ADSP Tag Registration . . . . . . . . . . . . . . . . 11 8. Security Considerations . . . . . . . . . . . . . . . . . . . 12 8.1. Inherited Considerations . . . . . . . . . . . . . . . . . 12 8.2. Forgeries . . . . . . . . . . . . . . . . . . . . . . . . 12 8.3. Amplification Attacks . . . . . . . . . . . . . . . . . . 12 8.4. Automatic Generation . . . . . . . . . . . . . . . . . . . 12 8.5. Reporting Multiple Incidents . . . . . . . . . . . . . . . 13 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14 9.1. Normative References . . . . . . . . . . . . . . . . . . . 14 9.2. Informative References . . . . . . . . . . . . . . . . . . 14 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 15 Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 16 B.1. Example Use of DKIM Signature Extension Tags . . . . . . . 16 B.2. Example Use of DKIM ADSP Extension Tags . . . . . . . . . 16 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 18 Kucherawy Expires July 26, 2012 [Page 2] Internet-Draft DKIM Reporting Extensions January 2012 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 Mail Domain (ADMD; see [EMAIL-ARCH]). This document extends [DKIM] and [ADSP] to add an optional reporting address and some reporting parameters. Kucherawy Expires July 26, 2012 [Page 3] Internet-Draft DKIM Reporting Extensions January 2012 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]. The base64 encoding method is defined in [MIME]. Kucherawy Expires July 26, 2012 [Page 4] Internet-Draft DKIM Reporting Extensions January 2012 3. Optional Reporting Address for DKIM A domain name owner employing [DKIM] for e-mail 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 the following optional "tags" (as defined in [DKIM]) to the DKIM-Signature header fields, 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 bearing this signature fails 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 found in the "d=" tag of the signature whose validation failed. 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: sig-r-tag = %x72 *WSP "=" *WSP qp-section 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 5.1 for a list of valid tags. ABNF: sig-ro-type = ( "all" / "s" / "v" / "x" ) sig-ro-tag = %x72 %x6f *WSP "=" *WSP sig-ro-type *WSP 0* ( ":" *WSP sig-ro-type ) 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 signature authentication 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. Kucherawy Expires July 26, 2012 [Page 5] Internet-Draft DKIM Reporting Extensions January 2012 ABNF: sig-ri-tag = %x72 %x69 *WSP "=" *WSP 1*12DIGIT "/" 1*12DIGIT rs= Requested SMTP Error String (text; OPTIONAL; no default). The value is a dkim-quoted-printable string the signing domain requests be included in [SMTP] error strings if messages are rejected during a single SMTP session. ABNF: sig-rs-tag = %x72 %x73 *WSP "=" qp-section In the absence of an "r=" tag, all other tags listed above MUST be ignored. Kucherawy Expires July 26, 2012 [Page 6] Internet-Draft DKIM Reporting Extensions January 2012 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. This document adds the following optional "tags" (as defined in [ADSP]) to the DKIM ADSP 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 [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: adsp-r-tag = %x72 *WSP "=" qp-section 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: adsp-ri-tag = %x72 %x69 *WSP "=" *WSP 1*12DIGIT "/" 1*12DIGIT Kucherawy Expires July 26, 2012 [Page 7] Internet-Draft DKIM Reporting Extensions January 2012 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 5.2 for a list of valid tags. ABNF: adsp-ro-type = ( "all" / "s" / "u" ) adsp-ro-tag = %x72 %x6f *WSP "=" *WSP adsp-ro-type *WSP 0* ( ":" *WSP adsp-ro-type ) 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: adsp-rs-tag = %x72 %x73 *WSP "=" qp-section In the absence of an "r=" tag, all other tags listed above MUST be ignored. Kucherawy Expires July 26, 2012 [Page 8] Internet-Draft DKIM Reporting Extensions January 2012 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. Kucherawy Expires July 26, 2012 [Page 9] Internet-Draft DKIM Reporting Extensions January 2012 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:<>". Kucherawy Expires July 26, 2012 [Page 10] Internet-Draft DKIM Reporting Extensions January 2012 7. IANA Considerations As required by [IANA-CONSIDERATIONS], this section contains registry information for the new [DKIM] key tags, and the new [ADSP] tags. 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) | current | | ro | (this document) | current | | rp | (this document) | current | | rs | (this document) | current | +------+-----------------+---------+ 7.2. DKIM ADSP Tag Registration IANA is requested to update the DKIM ADSP Tag Specification Registry to include the following new items: +------+-----------------+---------+ | TYPE | REFERENCE | STATUS | +------+-----------------+---------+ | r | (this document) | current | | ro | (this document) | current | | rp | (this document) | current | | rs | (this document) | current | +------+-----------------+---------+ Kucherawy Expires July 26, 2012 [Page 11] Internet-Draft DKIM Reporting Extensions January 2012 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. 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: a. A falsified authentication failure notification when the message was in fact delivered to the indicated recipient; b. 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. 8.3. 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. 8.4. 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 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. Kucherawy Expires July 26, 2012 [Page 12] Internet-Draft DKIM Reporting Extensions January 2012 In general ARF feedback loop terms, it is 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. 8.5. 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. Kucherawy Expires July 26, 2012 [Page 13] Internet-Draft DKIM Reporting Extensions January 2012 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., Ed., Hansen, T., Ed., and M. Kucherawy, Ed., "DomainKeys Identified Mail (DKIM) Signatures", RFC 6376, September 2011. [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. Kucherawy Expires July 26, 2012 [Page 14] Internet-Draft DKIM Reporting Extensions January 2012 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. Kucherawy Expires July 26, 2012 [Page 15] Internet-Draft DKIM Reporting Extensions January 2012 Appendix B. Examples This section contains examples of the use of each of the extensions defined by this memo. 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: o Reports about signature evaluation failures should be send to the address "dkim-errors" at the signer's domain; o Only reports about signature verification failures and expired signatures should be generated. 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: o The sending domain (i.e. the one that is advertising this policy) signs all mail it sends; o Reports about ADSP evaluation failures should be send to the address "dkim-adsp-errors" at the sender's domain; Kucherawy Expires July 26, 2012 [Page 16] Internet-Draft DKIM Reporting Extensions January 2012 o Only reports about unsigned messages should be generated. Kucherawy Expires July 26, 2012 [Page 17] Internet-Draft DKIM Reporting Extensions January 2012 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 Kucherawy Expires July 26, 2012 [Page 18]