MARF Working Group M. Kucherawy Internet-Draft Cloudmark Intended status: Standards Track September 10, 2011 Expires: March 13, 2012 Extensions to DKIM for Failure Reporting draft-ietf-marf-dkim-reporting-03 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 March 13, 2012. Copyright Notice Copyright (c) 2011 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 March 13, 2012 [Page 1] Internet-Draft DKIM Reporting Extensions September 2011 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. Reporting Formats . . . . . . . . . . . . . . . . . . . . . . 10 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 7.1. DKIM Key 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. Automatic Generation . . . . . . . . . . . . . . . . . . . 12 8.4. Envelope Sender Selection . . . . . . . . . . . . . . . . 13 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 Key Extension Tags . . . . . . . . . . 16 B.2. Example Use of DKIM ADSP Extension Tags . . . . . . . . . 16 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 18 Kucherawy Expires March 13, 2012 [Page 2] Internet-Draft DKIM Reporting Extensions September 2011 1. Introduction [DKIM] introduced a mechanism for message sender 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 sender 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 an optional means of specifying a desired report format and other parameters. Similar documents extend [ARF] and SPF (RFC4408) to improve the reporting capabilities of those standards as well, and are intended to be deployed together. The full suite of documents is: o RFCxxxx: Abuse Report Format (ARF) Extensions for Authentication Failure Reporting o RFCxxxx+1: DKIM and ADSP Extensions for Authentication Failure Reporting (this memo) o RFCxxxx+2: SPF Extensions for Authentication Failure Reporting Kucherawy Expires March 13, 2012 [Page 3] Internet-Draft DKIM Reporting Extensions September 2011 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 March 13, 2012 [Page 4] Internet-Draft DKIM Reporting Extensions September 2011 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 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 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. 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. ABNF: key-r-tag = %x72 *WSP "=" *WSP qp-section 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 that is taken from the registered list of report formats. See Section 7 for a description of the registry and Section 6 for a description of recognized formats. The verifier generating reports MUST generate a report using the first token in the list that represents a report format it is capable of generating. ABNF: rep-format = ( "arf" / "smtp" ) key-rf-tag = %x72 %x66 *WSP "=" *WSP rep-format *WSP 0*( ":" *WSP rep-format ) ri= Requested Report Interval (plain-text; OPTIONAL; default is "0"). The value is an unsigned 32-bit integer that specifies an interval during which the report generator SHOULD NOT issue more than one report about a given incident type. A value of "0" Kucherawy Expires March 13, 2012 [Page 5] Internet-Draft DKIM Reporting Extensions September 2011 requests a report for every incident; a value of "1" requests a report for every other incident; etc. Where the requested interval is not zero, the agent generating a report SHOULD include an "Incidents:" field in the generated report so the receiving agent has some indication of how many reports were suppressed. ABNF: key-ri-tag = %x72 %x69 *WSP "=" *WSP 1*DIGIT 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: key-ro-type = ( "all" / "s" / "v" / "x" ) key-ro-tag = %x72 %x6f *WSP "=" *WSP key-ro-type *WSP 0* ( ":" *WSP key-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. ABNF: key-rs-tag = %x72 %x73 *WSP "=" qp-section In the absence of an "r=" tag, all other tags listed above MUST be ignored. Kucherawy Expires March 13, 2012 [Page 6] Internet-Draft DKIM Reporting Extensions September 2011 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 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 format of this reply MUST be in the format specified by the "rf=" tag, defined 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 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. ABNF: adsp-r-tag = %x72 *WSP "=" qp-section 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 that is taken from the registered list of DKIM report formats. See Section 7 for a description of the registry and Section 6 for a description of recognized formats. The verifier generating reports MUST generate a report using the first token in the list that represents a report format it is capable of generating. ABNF: adsp-rf-tag = %x72 %x66 *WSP "=" *WSP rep-format *WSP 0*( ":" *WSP rep-format ) ri= Requested Report Interval (plain-text; OPTIONAL; default is "0"). The value is an unsigned 32-bit integer that specifies an interval during which the report generator SHOULD NOT issue more than one report about a given type of incident should be Kucherawy Expires March 13, 2012 [Page 7] Internet-Draft DKIM Reporting Extensions September 2011 generated. A value of "0" requests a report for every incident; a value of "1" requests a report for every other incident; etc. Where the requested interval is not zero, the agent generating a report SHOULD include an "Incidents:" field in the generated report so the receiving agent has some indication of how many reports were suppressed. ABNF: adsp-ri-tag = %x72 %x69 *WSP "=" *WSP 1*DIGIT 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. 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 March 13, 2012 [Page 8] Internet-Draft DKIM Reporting Extensions September 2011 5. 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 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 but do not match the published [ADSP] policy. Kucherawy Expires March 13, 2012 [Page 9] Internet-Draft DKIM Reporting Extensions September 2011 6. Reporting Formats This section lists reporting formats supported by this reporting mechanism. Initially, two formats are supported: arf: Abuse Reporting Format, as defined in [ARF] and extended in RFCxxxx. smtp: An [SMTP] error with a string descriptive of the problem that caused the sender authentication to fail. This explicitly requests evaluation of sender authentication concurrent with the SMTP session, and rejection (if appropriate) whenever possible rather than acceptance of the message and later generation of a feedback report of some kind (e.g. "arf", above) when verification fails. The presence of an "rs" tag (see Section 3 and Section 4) further requests a specific substring be included in the reply to ease automatic handling of such errors by sending or relaying MTAs. Kucherawy Expires March 13, 2012 [Page 10] Internet-Draft DKIM Reporting Extensions September 2011 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 Key Tag Registration IANA is requested to update the DKIM Key Tag Specification Registry to include the following new items: +------+-----------------+---------+ | TYPE | REFERENCE | STATUS | +------+-----------------+---------+ | r | (this document) | current | | rf | (this document) | current | | ri | (this document) | current | | ro | (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 | | rf | (this document) | current | | ri | (this document) | current | | ro | (this document) | current | | rs | (this document) | current | +------+-----------------+---------+ Kucherawy Expires March 13, 2012 [Page 11] Internet-Draft DKIM Reporting Extensions September 2011 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 DSNs of any kind should take appropriate precautions to minimize the potential damage from denial-of-service attacks. Security threats related to forged DSNs 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. 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 sender 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 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 Kucherawy Expires March 13, 2012 [Page 12] Internet-Draft DKIM Reporting Extensions September 2011 discovered data in the DNS. 8.4. Envelope Sender Selection In the case of transmitted reports in the form of a new message, 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:<>". 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 March 13, 2012 [Page 13] Internet-Draft DKIM Reporting Extensions September 2011 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] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, J., and M. Thomas, "DomainKeys Identified Mail (DKIM) Signatures", RFC 4871, May 2007. [EMAIL-ARCH] Crocker, D., "Internet Mail Architecture", RFC 5598, October 2008. [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 March 13, 2012 [Page 14] Internet-Draft DKIM Reporting Extensions September 2011 Appendix A. Acknowledgements The authors wish to acknowledge the following for their review and constructive criticism of this proposal: Monica Chew, Dave Crocker, Tim Draegen and JD Falk. Kucherawy Expires March 13, 2012 [Page 15] Internet-Draft DKIM Reporting Extensions September 2011 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 Key Extension Tags A DKIM key record including use of the extensions defined by this memo: v=DKIM1; k=rsa; t=y; r=dkim-errors; rf=arf; ro=v:x; p=MIGfMA0GCS qGSIb3DQEBAQUAA4GNADCBiQKBgQDh2vbhJTijCs2qbyJcwRCa8WqDTxI+PisFJo faPtoDJy0Qn41uNayCajfKADVcLqc87sXQS6GxfchPfzx7Vh9crYdxRbN/o/URCu ZsKmym1i1IPTwRLcXSnuKS0XDs1eRW2WQHGYlXksUDqSHWOS3ZO1W5t/FLcZHpIl l/80xs4QIDAQAB Example 1: DKIM key record using these extensions This example DKIM key record contains the following data in addition to the basic DKIM key data: o Reports about signature evaluation failures should be send to the address "dkim-errors" at the sender's domain; o The sender's domain requests reports in the "arf" format; 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; rf=arf; 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; o The sender's domain requests reports in the "arf" format; Kucherawy Expires March 13, 2012 [Page 16] Internet-Draft DKIM Reporting Extensions September 2011 o Only reports about unsigned messages should be generated. Kucherawy Expires March 13, 2012 [Page 17] Internet-Draft DKIM Reporting Extensions September 2011 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 March 13, 2012 [Page 18]