MARF Working Group S. Kitterman Internet-Draft Authentication Metrics Intended status: Standards Track June 27, 2011 Expires: December 29, 2011 SPF Authentication Failure Reporting using the Abuse Report Format draft-ietf-marf-spf-reporting-00 Abstract This memo presents extensions to the Abuse Reporting Format (ARF), and Sender Policy Framework (SPF) specifications 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 December 29, 2011. 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. Kitterman Expires December 29, 2011 [Page 1] Internet-Draft SPF Auth Failure Reporting June 2011 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.1. Keywords . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.2. Imported Definitions . . . . . . . . . . . . . . . . . . . 4 3. Optional Reporting Address for SPF . . . . . . . . . . . . . . 5 4. Requested Reports . . . . . . . . . . . . . . . . . . . . . . 7 4.1. Requested Reports for SPF Failures . . . . . . . . . . . . 7 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 5.1. SPF Modifier Registration . . . . . . . . . . . . . . . . 8 6. Security Considerations . . . . . . . . . . . . . . . . . . . 9 6.1. Inherited Considerations . . . . . . . . . . . . . . . . . 9 6.2. Reports From Unrelated Domains . . . . . . . . . . . . . . 9 6.3. Forgeries . . . . . . . . . . . . . . . . . . . . . . . . 9 6.4. Automatic Generation . . . . . . . . . . . . . . . . . . . 10 6.5. Envelope Sender Selection . . . . . . . . . . . . . . . . 10 6.6. Reporting Multiple Incidents . . . . . . . . . . . . . . . 10 7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12 7.1. Normative References . . . . . . . . . . . . . . . . . . . 12 7.2. Informative References . . . . . . . . . . . . . . . . . . 12 Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 14 Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 15 B.1. Minimal SPF DNS record change to add a reporting address . . . . . . . . . . . . . . . . . . . . . . . . . 15 B.2. SPF DNS record with reporting address, report interval, and requested report type . . . . . . . . . . . 15 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16 Kitterman Expires December 29, 2011 [Page 2] Internet-Draft SPF Auth Failure Reporting June 2011 1. Introduction [ARF] defines a message format for sending reports of abuse in the messaging infrastructure, with an eye toward automating both the generating and consumption of those reports. [SPF] is one mechanism for message sender authentication; it is "path-based" meaning it authenticates the route that a message took from origin to destination. As with other email authentication methods, like [DKIM], the output is a verified domain name that can then be subjected to some sort of evaluation process (e.g., 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, unauthorized path traversals (SPF failures), and conformance failures involving the published signing practices (e.g., [ADSP]) of an Administrative Mail Domain (ADMD; see [EMAIL-ARCH]). This document extends [SPF] to add an optional reporting address and an optional means of specifying a desired report format and other parameters. Extension of [ARF] to add features required for the reporting of these incidents is covered in [I-D.MARF-DKIM-REPORTING]. This document additionally creates a an IANA registry of [SPF] record modifiers to avoid modidier namespace collisions. Kitterman Expires December 29, 2011 [Page 3] Internet-Draft SPF Auth Failure Reporting June 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]. "local-part" is defined in [MAIL]. "addr-spec" is defined in [MAIL]. Kitterman Expires December 29, 2011 [Page 4] Internet-Draft SPF Auth Failure Reporting June 2011 3. Optional Reporting Address for SPF There exist cases in which a domain name owner employing [SPF] for announcing sending practises may want to know when messages are received via unauthorized routing. Currently there is no such mechanism defined. This document adds the following optional "modifier" (as defined in Section 4.6.1 of [SPF]) to SPF records, using the form defined in that specification: r= Reporting Address (plain-text; OPTIONAL; no default). MUST be a local-part or addr-spec (see Section 3.4.1 of [MAIL])\ containing an e-mail address to which a report SHOULD be sent when mail claiming to be from this domain (see Section 2.4 of [SPF] for a description of how domains are identified for SPF checks) has failed the evaluation algorithm described in [SPF], in particular because a message arrived via an unauthorized route . The format of this reply MUST be in the format specified by the "rf=" tag defined below. If only a local-part is provided, then 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 SPF record. r= modifiers in a record that was reached by following an include: mechanism MUST be ignored. ABNF: spf-report-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 report formats. See Section 5 for a description of the registry and [I-D.MARF-DKIM-REPORTING] 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. If no supported formats are requested, then no report is sent. ABNF: spf-rf-tag = %x72 %x66 *WSP "=" *WSP rep-format *WSP 0*( ":" *WSP rep-format ) Kitterman Expires December 29, 2011 [Page 5] Internet-Draft SPF Auth Failure Reporting June 2011 ri= Requested Report Interval (plain-text; OPTIONAL; default is "0"). The value is an unsigned 32-bit integer that specifies an interval, i.e. a number of reports, during which the report generator SHOULD NOT issue more than one report about a given type of incident should be generated. A value of "0" requests a report for every incident. 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: spf-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 4.1 for a list of valid tags. ABNF: spf-ro-type = ( "all" / "e" / "f" / "s" ) spf-ro-tag = %x72 %x6f *WSP "=" *WSP spf-ro-type *WSP 0* ( ":" *WSP spf-ro-type ) rs= Requested SMTP Error String (plain-text; OPTIONAL; no default). The value is a string the sending domain requests be included in [SMTP] error strings when messages are rejected. ABNF: spf-rs-tag = %x72 %x73 *WSP "=" qp-section Kitterman Expires December 29, 2011 [Page 6] Internet-Draft SPF Auth Failure Reporting June 2011 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 that do not match a requested report, and MUST ignore requests for reports not included in this these lists. 4.1. Requested Reports for SPF Failures The following report requests are defined for SPF keys: all All reports are requested. e Reports are requested for messages that produced an SPF result of "TempError" or "PermError. f Reports are requested for messages that produced an SPF result of "Fail". s Reports are requested for messages that produced an SPF result of "SoftFail". Kitterman Expires December 29, 2011 [Page 7] Internet-Draft SPF Auth Failure Reporting June 2011 5. IANA Considerations As required by [IANA-CONSIDERATIONS], this section contains registry information for the new [SPF] tags. 5.1. SPF Modifier Registration IANA is requested to create the Sender Policy Framework Modifier Registry, to include a list of all registered SPF modifier names and their defining documents. New registrations or updates MUST be published in accordance with the "Specification Required" guidelines as described in [IANA-CONSIDERATIONS]. New registrations and updates MUST contain the following information: 1. Name of the modifier being registered or updated 2. The document in which the specification of the modifier is published 3. New or updated status, which MUST be one of: current: The field is in current use deprecated: The field is in current use but its use is discouraged historic: The field is no longer in current use An update may make a notation on an existing registration indicating that a registered field is historic or deprecated if appropriate. +------------+-----------------+---------+ | MODIFIER | REFERENCE | STATUS | +------------+-----------------+---------+ | exp | RFC4408 | current | | redirect | RFC4408 | current | | r | (this document) | current | | rf | (this document) | current | | ri | (this document) | current | | ro | (this document) | current | | rs | (this document) | current | +------------+-----------------+---------+ Kitterman Expires December 29, 2011 [Page 8] Internet-Draft SPF Auth Failure Reporting June 2011 6. Security Considerations Security issues with respect to these reports are similar to those found in [DSN]. 6.1. Inherited Considerations Implementors are advised to consider the Security Considerations sections of [SPF] and [ARF]. 6.2. Reports From Unrelated Domains SPF records can be used by other domains via include mechanisms and redirect modifiers. If reporting addresses included in these records are specified with a full addr-spec then reports for other, potentially unrelated, domains may be reported to this address. In theory, malicious senders might use this as a path for generating large numbers of feedback reports. To mitigate this issue, specify reporting addresses with a local-part so that reports will be directed to the original domain from which the message causing the feedback report was sent. 6.3. 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 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 authentication information, such as result, domain, etc. Perhaps the simplest means of mitigating this threat is to assert that these reports should themselves be signed with something like DKIM or sent from sources authorized by SPF. 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. Kitterman Expires December 29, 2011 [Page 9] Internet-Draft SPF Auth Failure Reporting June 2011 6.4. 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 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 discovered data in the DNS. 6.5. 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:<>". The HELO/EHLO command SHOULD pass [SPF] HELO checks. 6.6. 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. Kitterman Expires December 29, 2011 [Page 10] Internet-Draft SPF Auth Failure Reporting June 2011 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. Kitterman Expires December 29, 2011 [Page 11] Internet-Draft SPF Auth Failure Reporting June 2011 7. References 7.1. Normative References [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. [I-D.MARF-DKIM-REPORTING] Kucherawy, M., "Extensions to DKIM for Failure Reporting", May 2011. [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. [MAIL] Resnick, P., "Internet Message Format", RFC 5322, October 2008. [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. [SPF] Wong, M. and W. Schlitt, "Sender Policy Framework (SPF) for Authorizing Use of Domains in E-Mail, Version 1", RFC 4408, April 2006. 7.2. Informative References [ADSP] Allman, E., Delany, M., Fenton, J., and J. Levine, "DKIM Sender Signing Practises", RFC 5617, August 2009. [DSN] Moore, K. and G. Vaudreuil, "An Extensible Message Format Kitterman Expires December 29, 2011 [Page 12] Internet-Draft SPF Auth Failure Reporting June 2011 for Delivery Status Notifications", RFC 3464, January 2003. Kitterman Expires December 29, 2011 [Page 13] Internet-Draft SPF Auth Failure Reporting June 2011 Appendix A. Acknowledgements The author wishes to acknowledge the following for their review and constructive criticism of this proposal: Murray Kucherawy and Tim Draegen. Kitterman Expires December 29, 2011 [Page 14] Internet-Draft SPF Auth Failure Reporting June 2011 Appendix B. Examples B.1. Minimal SPF DNS record change to add a reporting address v=spf1 mx:example.org r=postmaster -all B.2. SPF DNS record with reporting address, report interval, and requested report type v=spf1 mx:example.org r=postmaster@example.net rf=arf ri=10 ro=e -all Kitterman Expires December 29, 2011 [Page 15] Internet-Draft SPF Auth Failure Reporting June 2011 Author's Address Scott Kitterman Authentication Metrics 3611 Scheel Dr Ellicott City, MD 21042 US Phone: +1 301 325 5475 Email: skitterman@authmetrics.com Kitterman Expires December 29, 2011 [Page 16]