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Versions: (draft-jdfalk-marf-as) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 RFC 6650

MARF Working Group                                               J. Falk
Internet-Draft                                               Return Path
Updates: 5965 (if approved)                            M. Kucherawy, Ed.
Intended status: Standards Track                               Cloudmark
Expires: October 27, 2012                                 April 25, 2012


 Creation and Use of Email Feedback Reports: An Applicability Statement
                  for the Abuse Reporting Format (ARF)
                         draft-ietf-marf-as-16

Abstract

   RFC 5965 defines an extensible, machine-readable format intended for
   mail operators to report feedback about received email to other
   parties.  This Applicability Statement describes common methods for
   utilizing this format for reporting both abuse and authentication
   failure events.  Mailbox Providers of any size, mail sending
   entities, and end users can use these methods as a basis to create
   procedures that best suit them.  Some related optional mechanisms are
   also discussed.

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 October 27, 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



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   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 . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Discussion . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Definitions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Solicited and Unsolicited Reports  . . . . . . . . . . . . . .  4
   4.  Generating And Handling Solicited Abuse Reports  . . . . . . .  4
     4.1.  General Considerations for Feedback Providers  . . . . . .  5
     4.2.  Where To Send Reports  . . . . . . . . . . . . . . . . . .  5
     4.3.  What To Put In Reports . . . . . . . . . . . . . . . . . .  5
     4.4.  General Considerations for Feedback Consumers  . . . . . .  5
     4.5.  What To Expect . . . . . . . . . . . . . . . . . . . . . .  6
     4.6.  What To Do With Reports  . . . . . . . . . . . . . . . . .  6
   5.  Generating and Handling Unsolicited Abuse Reports  . . . . . .  6
     5.1.  General Considerations . . . . . . . . . . . . . . . . . .  6
     5.2.  When To Generate Reports . . . . . . . . . . . . . . . . .  7
     5.3.  Where To Send Reports  . . . . . . . . . . . . . . . . . .  7
     5.4.  What To Put In Reports . . . . . . . . . . . . . . . . . .  8
     5.5.  What To Do With Reports  . . . . . . . . . . . . . . . . .  9
   6.  Generating Automatic Authentication Failure Reports  . . . . . 10
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
     8.1.  In Other Documents . . . . . . . . . . . . . . . . . . . . 11
     8.2.  Forgeries  . . . . . . . . . . . . . . . . . . . . . . . . 11
     8.3.  Amplification Attacks  . . . . . . . . . . . . . . . . . . 11
     8.4.  Automatic Generation . . . . . . . . . . . . . . . . . . . 12
     8.5.  Reporting Multiple Incidents . . . . . . . . . . . . . . . 12
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 13
     10.2. Informative References . . . . . . . . . . . . . . . . . . 14
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15












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1.  Introduction

   The Abuse Reporting Format (ARF) was initially developed for two very
   specific use cases.  Initially, it was intended to be used for
   reporting feedback between large email operators, or from large email
   operators to end user network access operators, any of whom could be
   presumed to have automated abuse-handling systems.  Secondarily, it
   is used by those same large mail operators to send those same reports
   to other entities, including those involved in sending bulk email for
   commercial purposes.  In either case, the reports would be triggered
   by direct end user action such as clicking on a "report spam" button
   in their email client.

   Though other uses for the ARF format defined in [RFC5965] have been
   discussed (and may be documented similarly in the future), abuse
   remains the primary application, with a small amount of adoption of
   extensions that enable authentication failure reporting.

   This Applicability Statement provides direction for using the Abuse
   Reporting Format (ARF) in both contexts.  It also includes some
   statements about the use of ARF in conjunction with other email
   technologies.

   The purpose for reporting abusive messages is to stop recurrences.
   The methods described in this document focus on automating abuse
   reporting as much as practical, so as to minimize the work of a
   site's abuse team.  There are further reasons why abuse feedback
   generation is worthwhile, such as instruction of mail filters or
   reputation trackers, or to initiate investigations of particularly
   egregious abuses.  These other applications are not discussed in this
   memo.

   Further introduction to this topic may be found in [RFC6449], which
   is effectively an Applicability Statement written outside of the IETF
   and thus never achieved IETF consensus.  Much of the content for that
   document was input to this one.

   At the time of publication of this document, five feedback types are
   registered.  This document only discusses two of them ("abuse" and
   "auth-failure") as they are seeing sufficient use in practice that
   applicability statements can be made about them.  The others, i.e.,
   "fraud" [RFC5965] and "not-spam" [RFC6430], are either too new or too
   seldomly used to be included here.

1.1.  Discussion

   [RFC Editor: please remove this section prior to publication.]




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   This document is being discussed within the IETF MARF Working Group,
   on the marf@ietf.org mailing list.


2.  Definitions

   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 [RFC2119], and are
   intended to replace the Requirement Levels described in Section 3.3
   of [RFC2026].

   Some of the terminology used in this document is taken from
   [RFC5598].

   "Mailbox Provider" refers to an organization that accepts, stores,
   and offers access to [RFC5322] messages ("email messages") for end
   users.  Such an organization has typically implemented SMTP
   ([RFC5321]), and might provide access to messages through IMAP
   ([RFC3501]), POP ([RFC1939]), a proprietary interface designed for
   HTTP ([RFC2616]), or a proprietary protocol.


3.  Solicited and Unsolicited Reports

   The original application of [RFC5965], and still by far the most
   common, is when two mail systems make a private agreement to exchange
   abuse reports, usually reports due to recipients manually reporting
   messages as spam.  We refer to these as solicited reports.

   Other uses for ARF involve such reports sent between parties that
   don't know each other.  These unsolicited reports are sent without
   prior arrangement between the parties as to the context and meaning
   of the reports, so the constraints on how these unsolicited reports
   need to be structured such that the reports generated are likely to
   be useful to the recipient, to what address(es) they can usefully be
   sent, what issues the can be used to report, and how they can be
   handled by the receiver of the report are very different.

   The two cases are covered separately in following sections.


4.  Generating And Handling Solicited Abuse Reports

   [The numbered items in these subsections are not intended to be in a
   paricular sequence.  The numbers are here during document development
   to make it easier to identify the items for discussion, and will be
   removed before publication.]



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4.1.  General Considerations for Feedback Providers

   1.  A Mailbox Provider receives reports of abusive or unwanted mail
       from its users, most often by providing a "report spam" button
       (or similar nomenclature) in the MUA (Mail User Agent).  The
       method of transferring this message and any associated metadata
       from the MUA to the Mailbox Provider's ARF processing system is
       not defined by any standards document, but is discussed further
       in Section 3.2 of [RFC6449].  Policy concerns related to the
       collection of this data are discussed in Section 3.4 of
       [RFC6449].
   2.  To implement the recommendations of this memo, the reports are
       formatted per [RFC5965], and transmitted as an email message
       ([RFC5322]), typically using SMTP ([RFC5321]).
   3.  Ongoing maintenance of an ARF processing system is discussed in
       Section 3.6 of [RFC6449].

4.2.  Where To Send Reports

   1.  The Mailbox Provider SHOULD NOT send reports to addresses that
       have not explicitly requested them.  A valid deviation might be
       the result of local policy instructions.  The process whereby
       such parties may request the reports is discussed in Section 3.5
       of [RFC6449].

4.3.  What To Put In Reports

   1.  The reports SHOULD use "Feedback-Type: abuse", for its type.
       Although a Mailbox Provider generating the reports can use other
       types appropriate to the nature of the abuse being reported, the
       operator receiving the reports might not treat different feedback
       types differently.
   2.  The following fields are optional in [RFC5965], but SHOULD be
       used in this context when their corresponding values are
       available: Original-Mail-From, Arrival-Date, Source-IP, Original-
       Rcpt-To.  Other optional fields can be included, as the
       implementer feels is appropriate.
   3.  User-identifiable data MAY be obscured as described in [RFC6590].

4.4.  General Considerations for Feedback Consumers

   1.  ARF report streams are established proactively between Feedback
       Providers and Feedback Consumers.  Recommendations for preparing
       to make that request are discussed in Section 4.1 of [RFC6449].
   2.  Operators MUST be able to accept ARF [RFC5965] reports as email
       messages [RFC5322] over SMTP [RFC5321].  These and other types of
       email messages that can be received are discussed in Section 4.2
       of [RFC6449].



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   3.  Recipients of feedback reports that are part of formal feedback
       arrangements have to be capable of handling large volumes of
       reports.  This could require automation of report processing.
       Discussion of this can be found in Section 4.4 of [RFC6449].

4.5.  What To Expect

   1.  The list of vaild Feedback-Types is defined in [RFC5965], which
       created an IANA registry for valid values to allow for
       extensions.  However, an automated report processing system MUST
       NOT reject (in the SMTP sense) a report based solely on an
       unknown Feedback-Type, to allow for handling of new types that
       are not yet supported.  The automated system can simply set
       reports of unknown types aside for manual handling.  However,
       Mailbox Providers might only make use of the "abuse" Feedback-
       Type.  Therefore, report receivers might be required to do
       additional analysis to separate different types of abuse reports
       after receipt if they do not have prior specific knowledge of the
       sender of the report.
   2.  Reports receivers MUST accept reports that have obscured their
       user-identifiable data as described in [RFC6590].  That document
       also discusses the handling of such reports.  This technique is
       also discussed in Section 4.4 of [RFC6449].

4.6.  What To Do With Reports

   1.  Section 4.3 of [RFC6449] discusses actions that mail operators
       might take upon receiving a report (or multiple reports).


5.  Generating and Handling Unsolicited Abuse Reports

   [The numbered items in these subsections are not intended to be in a
   paricular sequence.  The numbers are here during document development
   to make it easier to identify the items for discussion, and will be
   removed before publication.]

5.1.  General Considerations

   1.  It is essential for report recipients to be capable of throttling
       reports being sent to avoid damage to their own installations.
       Therefore, Feedback Providers MUST provide a way for report
       recipients to request that no further reports be sent.
       Unfortunately, no standardized mechanism for such requests exists
       to date, and all existing mechanisms for meeting this requirement
       are out-of-band.





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   2.  Message authentication is generally a good idea, but it is
       especially important to encourage credibility of and thus
       response to unsolicited reports.  Therefore, as with any other
       message, Feedback Providers sending unsolicited reports SHOULD
       send reports that they believe will pass Sender Policy Framework
       ([RFC4408]) and/or DomainKeys Identified Mail ([RFC6376]) checks.

5.2.  When To Generate Reports

   1.  Handling of unsolicited reports has a significant cost to the
       report receiver.  Senders of unsolicited reports, especially
       those sending large volumes of them automatically SHOULD NOT send
       reports that cannot be used as a basis for action by the
       recipient, whether this is due to the report being sent about an
       incident that is not abuse-related, the report being sent to an
       email address that won't result in action, or the content or
       format of the report being hard for the recipient to read or use.
   2.  Feedback Providers SHOULD NOT report all mail sent from a
       particular sender merely because some of it is determined to be
       abusive.
   3.  Mechanical reports of mail that "looks like" spam, based solely
       on the results of inline content analysis tools, SHOULD NOT be
       sent since, because of their subjective nature, they are unlikely
       to provide a basis for the recipient to take action.  Complaints
       generated by end users about mail that is determined by them to
       be abusive, or mail delivered to "spam trap" or "honeypot"
       addresses, are far more likely to be accurate and MAY be sent.
   4.  If a Feedback Provider applies the Sender Policy Framework
       [RFC4408] to arriving messages, a report SHOULD NOT be generated
       to the RFC5321.MailFrom domain if the SPF evaluation produced a
       "Fail", "SoftFail", "TempError" or "PermError" report, as no
       reliable assertion or assumption can be made that use of the
       domain was authorized.  A valid exception would be specific
       knowledge that the SPF result is not definitive for that domain
       under those circumstances (for example, a message that is also
       signed using DomainKeys Identified Mail (DKIM, [RFC6376]) by the
       same domain, and that signature validates).

5.3.  Where To Send Reports

   1.  Rather than generating feedback reports themselves, MUAs SHOULD
       make abuse reports back to their mailbox providers so that they
       can generate and send ARF messages on behalf of end users (see
       Section 3.2 of [RFC6449]).  This allows centralized processing
       and tracking of reports, and provides training input to filtering
       systems.  There is, however, no standard mechanism for this
       signaling between MUAs and mailbox providers to trigger abuse
       reports.



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   2.  Feedback Providers SHOULD NOT send reports to recipients that are
       uninvolved or only peripherally involved.  For example, they
       SHOULD NOT send reports to the operator of every Autonomous
       System in the path between the apparent originating system and
       the operator generating the report.  Instead, they need to send
       reports to recipients that are both responsible for the messages
       and are able to do something about them.
   3.  Deciding where to send an unsolicited report will typically rely
       on heuristics.  Abuse addresses in WHOIS ([RFC3912]) records of
       the IP address relaying the subject message and/or of the domain
       name found in the results of a PTR ("reverse lookup") query on
       that address are likely reasonable candidates, as is the
       abuse@domain role address (see [RFC2142]) of related domains.
       Unsolicited reports SHOULD NOT be sent to email addresses that
       are not clearly intended to handle abuse reports.  Legitimate
       candidates include those found in WHOIS records or on a web site
       that either are explicitly described as an abuse contact, or are
       of the form "abuse@domain".
   4.  Where an abusive message is authenticated using a domain-level
       authentication technology such as DKIM ([RFC6376]) or SPF
       ([RFC4408]), the domain that has been verified by the
       authentication mechanism is often a reasonable candidate for
       receiving feedback about the message.  For DKIM, though, while
       the authenticated domain has some responsibility for the mail
       sent, it can be a poor contact point for abuse issues (for
       example, it could represent the message's author but not its
       sender, it could identify the bad actor responsible for the
       message, or it could refer to a domain that cannot receive mail
       at all).
   5.  Often, unsolicited reports will have no meaning if sent to abuse
       reporting addresses belonging to the abusive parties themselves.
       In fact, it is possible that such reports might reveal
       information about complainants.  Reports SHOULD NOT be sent to
       such addresses if they can be identified beforehand, except where
       the abusive party is known to be responsive to such reports.

5.4.  What To Put In Reports

   1.  Reports SHOULD use "Feedback-Type: abuse", but can use other
       types as appropriate.  However, the Mailbox Provider generating
       the reports cannot assume that the operator receiving the reports
       will treat different Feedback-Types differently.
   2.  Reports SHOULD include the following optional fields whenever
       their corresponding values are available and applicable to the
       report: Original-Mail-From, Arrival-Date, Source-IP, Original-
       Rcpt-To.  Other optional fields can be included, as the
       implementer feels is appropriate.




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   3.  Experience suggests use of ARF is advisable in most contexts.
       Automated recipient systems can handle abuse reports sent in ARF
       format at least as well as any other format such as plain text,
       with or without a copy of the message attached.  That holds even
       for systems that did not request ARF format reports, assuming
       such reports are generated considering the possibility of
       recipients that don't use automated ARF parsing.  Anyone sending
       unsolicited reports in ARF format can legitimately presume that
       some recipients will only be able to access the human readable
       (first, text/plain) part of it, and SHOULD include all
       information needed also in this part.  Further, they SHOULD
       ensure that the report is readable when viewed as plain text, to
       give low-end ticketing systems as much assistance as possible.
       In extreme cases, failure to take these steps may result in the
       report being discarded or ignored.

5.5.  What To Do With Reports

   1.  Receivers of unsolicited reports can take advantage of the
       standardized parts of the ARF format to automate processing.
       Independent of the sender of the report, they can improve
       processing by separating valid from invalid reports by, for
       example, looking for references to IP address ranges, domains,
       and mailboxes for which the recipient organization is responsible
       in the copy of the reported message, and by correlating multiple
       reports of similar messages to identify bulk email senders.
   2.  Per Section 4.4 of [RFC6449], a network service provider MAY use
       ARF data for automated forwarding of feedback messages to the
       originating customer.
   3.  Published abuse mailbox addresses SHOULD NOT reject non-ARF
       messages based solely on the format, as generation of ARF
       messages can occasionally be unavailable or not applicable.
       Deviation from this requirement could be done due to local policy
       decisions regarding other message criteria.
   4.  Although [RFC6449] suggests that replying to feedback is not
       useful, in the case of receipt of ARF reports where no feedback
       arrangement has been established, a non-automated reply might be
       desirable to indicate what action resulted from the complaint,
       heading off more severe filtering by the Feedback Provider.  In
       addition, using an address that cannot receive replies precludes
       any requests for additional information, and increases the
       likelihood that further reports will be discarded or blocked.
       Thus, a Feedback Provider sending unsolicited reports SHOULD NOT
       generate reports for which a reply cannot be received.  Where an
       unsolicited report results in the establishment of contact with a
       responsible and responsive party, this can be saved for future
       complaint handling and possible establishment of a formal
       (solicited) feedback arrangement.  See Section 3.5 of [RFC6449]



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       for a discussion of establishment of feedback arrangements.


6.  Generating Automatic Authentication Failure Reports

   [These numbered items are not intended to be in a paricular sequence.
   The numbers are here during document development to make it easier to
   identify the items for discussion, and will be removed before
   publication.]

   There are some cases where report generation is caused by automation
   rather than user request.  A specific example of this is reporting,
   using the ARF format (or extensions to it), of messages that fail
   particular message authentication checks.  Examples of this include
   [I-D.IETF-MARF-DKIM-REPORTING] and [I-D.IETF-MARF-SPF-REPORTING].
   The considerations presented below apply in those cases.

   The applicability statement for this use case is somewhat smaller as
   many of the issues associated with abuse reports are not relevant to
   reports about authentication failures.

   1.  Automatic feedback generators MUST select actual message
       recipients based on data provided by willing report receivers.
       In particular, recipients MUST NOT be selected using heuristics.
   2.  If the message under evaluation by the Verifier is an ARF
       ([RFC5965]) message, a report MUST NOT be automatically
       generated.
   3.  The message for a new report sent via SMTP MUST be constructed so
       as to avoid amplification attacks, deliberate or otherwise.  The
       envelope sender address of the report MUST be chosen so that
       these reports will not generate mail loops.  Similar to Section 2
       of [RFC3464], the envelope sender address of the report MUST be
       chosen to ensure that no feedback reports will be issued in
       response to the report itself.  Therefore, when an SMTP
       transaction is used to send a report, the MAIL FROM command
       SHOULD use the NULL reverse-path, i.e., "MAIL FROM:<>".  An
       exception to this would be the use of a reverse-path selected
       such that SPF checks on the report will pass; in such cases, the
       operator will need to make provisions to avoid the amplification
       attack or mail loop via other means.
   4.  Reports SHOULD use "Feedback-Type: auth-failure", but MAY use
       other types as appropriate.  However, the Mailbox Provider
       generating the reports cannot assume that the operator receiving
       the reports will treat different Feedback-Types differently.
   5.  These reports SHOULD include the following optional fields,
       although they are optional in [RFC5965], whenever their
       corresponding values are available: Original-Mail-From, Arrival-
       Date, Source-IP, Original-Rcpt-To.  Other optional fields can be



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       included, as the implementer feels is appropriate.


7.  IANA Considerations

   [RFC Editor: please remove this section prior to publication.]

   This document has no IANA actions.


8.  Security Considerations

8.1.  In Other Documents

   Implementers are strongly urged to review, at a minimum, the Security
   Considerations sections of [RFC5965] and [RFC6449].

8.2.  Forgeries

   Feedback Providers that relay user complaints directly, rather than
   by reference to a stored message (e.g., IMAP or POP), could be duped
   into sending a complaint about a message that the complaining user
   never actually received, as an attack on the purported originator of
   the falsified message.  Feedback Providers need to be resilient to
   such attack methods.

   Also, 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.

   Perhaps the simplest means of mitigating this threat is to assert
   that these reports should themselves be signed with something like
   DKIM and/or authorized by something like SPF.  Note, however, that if
   there is a problem with the email infrastructure at either end, DKIM
   and/or SPF may result in reports that aren't trusted or even accepted
   by their intended recipients, so it is important to make sure those
   components are properly configured.  Use of both technologies in
   tandem can resolve this concern to a degree since they generally have
   disjoint failure modes.

8.3.  Amplification Attacks

   Failure to comply with the recommendations regarding selection of the
   envelope sender can lead to amplification denial-of-service attacks.
   This is discussed in Section 6 as well as in [RFC3464].



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8.4.  Automatic Generation

   ARF ([RFC5965]) reports have historically been generated individually
   as a result of some kind of human request, such as someone clicking a
   "Report Abuse" button in a mail reader.  In contrast, the mechanisms
   described in some extension documents (i.e.,
   [I-D.IETF-MARF-DKIM-REPORTING] and [I-D.IETF-MARF-SPF-REPORTING]) are
   focused around automated reporting.  This obviously implies the
   potential for much larger volumes or higher frequency of messages,
   and thus greater mail system load (both for Feedback Providers and
   report receivers).

   Those mechanisms are primarily intended for use in generating reports
   to aid implementers of DKIM ([RFC6376]), ADSP ([RFC5617]), and SPF
   ([RFC4408]), and other related protocols during development and
   debugging.  They are not generally intended for prolonged forensic
   use, specifically because of these load concerns.  However, extended
   use is possible by ADMDs that want to keep a close watch for fraud or
   infrastructure problems.  It is important to consider the impact of
   doing so on both Feedback Providers and the requesting ADMDs.

   A sender requesting these reports can cause its mail servers to be
   overwhelmed if it sends out signed messages whose signatures fail to
   verify for some reason, provoking a large number of reports from
   Feedback Providers.  Similarly, a Feedback Provider could be
   overwhelmed by a large volume of messages requesting reports whose
   signatures fail to validate, as those now need to send reports back
   to the signer.

   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
   Feedback Providers only create these (or any) ARF reports after an
   out-of-band arrangement has been made between two parties.  These
   extension mechanisms then become ways 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 messages, which may
   have unintended consequences.

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



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   distributed denial-of-service attack by finding a number of report-
   generating servers.

   The incident count referenced in ARF ([RFC5965]) provides a limited
   form of mitigation.  The host generating reports can elect to send
   reports only periodically, with each report representing a number of
   identical or nearly-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 "nearly-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
   could 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.

   Other rate limiting provisions might be considered, including
   detection of a temporary failure response from the report destination
   and thus halting report generation to that destination for some
   period, or simply imposing or negotiating a hard limit on the number
   of reports to be sent to a particular receiver in a given time frame.


9.  Acknowledgements

   The author and editor wish to thank Steve Atkins, John Levine, Shmuel
   Metz, S. Moonesamy, and Alessandro Vesely for their contributions to
   this memo.

   All of the Best Practices referenced by this document are found in
   [RFC6449], written within the Collaboration Committee of the
   Messaging Anti-Abuse Working Group (MAAWG).

   Finally, the original author wishes to thank the doctors and staff at
   the University of Texas MD Anderson Cancer Center for doing what they
   do.


10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.



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   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              October 2008.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              October 2008.

   [RFC5598]  Crocker, D., "Internet Mail Architecture", RFC 5598,
              July 2009.

   [RFC5965]  Shafranovich, Y., Levine, J., and M. Kucherawy, "An
              Extensible Format for Email Feedback Reports", RFC 5965,
              August 2010.

10.2.  Informative References

   [I-D.IETF-MARF-DKIM-REPORTING]
              Kucherawy, M., "Extensions to DKIM for Failure Reporting",
              draft-ietf-marf-dkim-reporting (work in progress),
              January 2012.

   [I-D.IETF-MARF-SPF-REPORTING]
              Kitterman, S., "SPF Authentication Failure Reporting using
              the Abuse Report Format", draft-ietf-marf-spf-reporting
              (work in progress), January 2012.

   [RFC1939]  Myers, J. and M. Rose, "Post Office Protocol - Version 3",
              STD 53, RFC 1939, May 1996.

   [RFC2026]  Bradner, S., "The Internet Standards Process -- Revision
              3", BCP 9, RFC 2026, October 1996.

   [RFC2142]  Crocker, D., "MAILBOX NAMES FOR COMMON SERVICES, ROLES AND
              FUNCTIONS", RFC 2142, May 1997.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC3464]  Moore, K. and G. Vaudreuil, "An Extensible Message Format
              for Delivery Status Notifications", RFC 3464,
              January 2003.

   [RFC3501]  Crispin, M., "INTERNET MESSAGE ACCESS PROTOCOL - VERSION
              4rev1", RFC 3501, March 2003.

   [RFC3912]  Daigle, L., "WHOIS Protocol Specification", RFC 3912,
              September 2004.




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   [RFC4408]  Wong, M. and W. Schlitt, "Sender Policy Framework (SPF)
              for Authorizing Use of Domains in E-Mail, Version 1",
              RFC 4408, April 2006.

   [RFC5617]  Allman, E., Fenton, J., Delany, M., and J. Levine,
              "DomainKeys Identified Mail (DKIM) Author Domain Signing
              Practices (ADSP)", RFC 5617, August 2009.

   [RFC6376]  Crocker, D., Hansen, T., and M. Kucherawy, "DomainKeys
              Identified Mail (DKIM) Signatures", RFC 6376,
              September 2011.

   [RFC6430]  Li, K. and B. Leiba, "Email Feedback Report Type Value:
              not-spam", RFC 6430, November 2011.

   [RFC6449]  Falk, J., "Complaint Feedback Loop Operational
              Recommendations", RFC 6449, November 2011.

   [RFC6590]  Falk, J. and M. Kucherawy, "Redaction of Potentially
              Sensitive Data from Mail Abuse Reports", RFC 6590,
              April 2012.


Authors' Addresses

   J.D. Falk
   Return Path
   100 Mathilda Street, Suite 100
   Sunnyvale, CA  94089
   USA

   URI:   http://www.returnpath.net/


   M. Kucherawy (editor)
   Cloudmark
   128 King St., 2nd Floor
   San Francisco, CA  94107
   US

   Email: msk@cloudmark.com










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