Email Feedback Reports for DKIM Signers
draft-brotman-dkim-fbl-02

Network Working Group                                         A. Brotman
Internet-Draft                                              Comcast, Inc
Intended status: Standards Track                           26 March 2024
Expires: 27 September 2024

                Email Feedback Reports for DKIM Signers
                       draft-brotman-dkim-fbl-02

Abstract

   Mechanism to discover a destination used to deliver user-supplied FBL
   reports to an original DKIM signer or other responsible parties.
   This allows the reporting entity to deliver reports for each party
   which has affixed a validating DKIM signature.  The discovery is made
   via DNS and the record is constructed using items within the DKIM
   signature in the message.

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
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 27 September 2024.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Discovery using DNS . . . . . . . . . . . . . . . . . . . . .   3
   3.  DNS Record Location . . . . . . . . . . . . . . . . . . . . .   3
   4.  DNS Record Format . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Report Contents . . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  arf . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     5.2.  xarf  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     5.3.  Aggregate FBL Reports . . . . . . . . . . . . . . . . . .   5
       5.3.1.  Report Format . . . . . . . . . . . . . . . . . . . .   6
   6.  Content Flag  . . . . . . . . . . . . . . . . . . . . . . . .   6
   7.  Delivery Methods  . . . . . . . . . . . . . . . . . . . . . .   6
     7.1.  mailto  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     7.2.  https . . . . . . . . . . . . . . . . . . . . . . . . . .   7
       7.2.1.  https Feedback-Type Header  . . . . . . . . . . . . .   7
   8.  Verifying External Destinations . . . . . . . . . . . . . . .   8
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
     9.1.  Feedback to Malicious Senders . . . . . . . . . . . . . .   8
     9.2.  Report Contents for ARF . . . . . . . . . . . . . . . . .   9
   10. Other Considerations  . . . . . . . . . . . . . . . . . . . .   9
     10.1.  Supplying FP Reports . . . . . . . . . . . . . . . . . .   9
     10.2.  Site Requirements  . . . . . . . . . . . . . . . . . . .   9
   11. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   9
   12. Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
   14. Normative References  . . . . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   Historically, Feedback Loops (FBL), typically comprised of False
   Positive (FP) and False Negative (FN) reports, have allowed users the
   ability to inform their Mailbox Provider (MBP) that they disagree
   with a message's placement in the Inbox or Spam folder.  In some
   situations, an MBP may then forward that feedback directly, or via an
   intermediary, to the original source system of that message.
   Traditionally, this source system identified via a registration
   system, typically tying a set of IPs or DKIM-based domains to a
   specific reporting location.

   By allowing reporters to discover the destination and reporting
   preferences on their own, this will reduce friction getting FBLs to
   the original DKIM signer(s).

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2.  Discovery using DNS

   There are alternative approaches for discovering the feedback
   information proposed.  This document describes a method for using DNS
   to discover a feedback address by utilizing the DKIM signature(s)
   within a message itself.

   The advantage of the DNS approach is that it can be changed after
   messages are delivered, allowing for old reports to be processed
   after migrating to a new report processing provider.  It also avoids
   common problems with modifying headers of messages that are already
   signed by another DKIM signature.

   Email service providers and intermediaries, which have a shared
   responsibility with an upstream sender, will commonly add their own
   DKIM signatures to the messages, thus resulting in the message having
   two signatures in different DKIM d= domains.  Dual-signed messages
   will result in feedback going to the location specified in the DNS
   for both domains.  Thus there is no reason to modify any message
   headers and potentially break the original DKIM signature.

3.  DNS Record Location

   The record will combine a label with the "d" value from the DKIM
   signature in the message being sent, optionally using a DNS wildcard
   (* character).  Such as the case where "d=example.org", the record
   would be located at:

   _feedback._domainkey.example.org

   or

   *._feedback._domainkey.example.org

   If the reporting destination needs to be different for individual
   DKIM selectors, each selector will need a DNS record with a value
   combined with a label with the "s=" value from the DKIM signature in
   the message being sent.  Such as the case where "d=example.org", and
   "s=contact", for example:

   contact._feedback._domainkey.example.org

   By including the selector, this allows a domain to be able to segment
   the feedback to various report processing providers, but a wildcard
   can no longer be used as a catch-all and an individual record must be
   created for each selector in use.  DKIM selectors are not supposed to
   be used for identification purposes, and they should change
   frequently to facilitate key rotation.

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   _The need for selector level feedback still needs to be assessed._

   All domain owners that want to ensure they receive all feedback
   should, at a minimum, publish a record at the following location as a
   catch-all:

   _feedback._domainkey.example.org

   The DNS entry will contain a TXT record described below.

4.  DNS Record Format

   The DNS record MUST contain the information necessary for a report
   generator to send the feedback to the proper location.

   v: A string identifying the record.  The value must be "DKIMRFBLv1"

   ra: An address destination for reports.  The address should match the
   format defined in [RFC5321].  If there is a "rfr" entry, the "ra" may
   be omitted.  If there is more than one target address, the entries
   must be separated by a comma (",").  The destination MUST use a
   classification of "mailto" or "https", indicating the transfer
   methods supported by the DKIM signer.

   rfr: An optional field to refer the report generator(s) to another
   DNS entry.

   c: Content flag.  If set to 'n', the reporting entity SHOULD remove
   all content beyond the headers of the original message that is being
   reported.  The default is "y".

   h: The header by which the signer can identify the recipient, sender,
   and campaign.  If a report generator is trying to create a
   minimalistic report, this would be the minimum amount of information
   to properly act on the report.  This field is OPTIONAL, and MUST
   contain only one attribute.

   hp: The header by which the signer can only identify the campaign.
   If present, the report generator may use the hp header instead of the
   h header if the recipient needs to remain private and there is no
   expectation of future sending to the recipient to be suppressed.
   This field is OPTIONAL, and MUST contain only one attribute.

   f: Format requested by report receiver.  Options are "arf" or "xarf".
   Default is "arf", and multiple values may be separated by a comma
   (,).  If a report sender is unable to generate a report in a
   requested format, they SHOULD NOT send a report.

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4.1.  Examples

   _feedback._domainkey.example.org TXT
   "v=DKIMRFBLv1;ra=mailto://reporting@feedback.example.org"

   contact._feedback._domainkey.example.org TXT
   "v=DKIMRFBLv1;rfr=_feedback._domainkey.example.org"

   contact._feedback._domainkey.example.org TXT
   "v=DKIMRFBLv1;ra=mailto://fbl@example.org;rfr=_feedback._domainkey.ex
   ample.org"

   *._feedback._domainkey.example.org TXT
   "v=DKIMRFBLv1;ra=mailto://other_fbl@example.org"

   _feedback._domainkey.example.org TXT
   "v=DKIMRFBLv1;c=n;ra=https://ra.example.org/
   reports;h=SendingIdentifer" (https://ra.example.org/
   reports;h=SendingIdentifer")

5.  Report Contents

5.1.  arf

   When the report format is specified as "arf", the report contents
   should adhere to [RFC5965]

5.2.  xarf

   When the report format is chosen as "xarf" [XARF], the report
   generator should reference the materials below as to the format.
   XARF follows a JSON format and the format may change over time to
   match that specification.

   The current format can be referenced:

   https://github.com/abusix/xarf/blob/master/schemas/3/spam.schema.json
   (https://github.com/abusix/xarf/blob/master/schemas/3/
   spam.schema.json)

5.3.  Aggregate FBL Reports

   A reporting entity may desire to send only aggregate data for a given
   time period, and that report may not contain any content of the
   original messages.  When this is the case, the reporter should
   utilize the hp field in the DNS declaration to be used as the value
   by which the sender will be able to recognize the message stream.

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   The hp field MUST be signed by the corresponding DKIM signature, and
   that signature must validate.  As a message may be signed by multiple
   signatures, it's possible that there could be multiple headers match
   an hp definition.

5.3.1.  Report Format

   NOTE: This could be created as an XARF, TBD

   <feedback>
   <report_metadata>
   <date_range>
   <begin>15141231</begin>
   <end>15152525</end>
   <date_range>
   <report_id>20231212-favw3ra34vwa3ra</report_id>
   <selector>dkim_sel</selector>
   <domain>dkim_domain</domain>
   <feedback_header>HeaderName</feedback_header>
   <header_contents>123:c21vRA3-V3A</header_contents>
   </report_metadata>
   <record>
   <row>
   <source_ip>1.2.3.4</source_ip>
   <fn_complaints>3</fn_complaints>
   </row>
   </record>
   </feedback>

6.  Content Flag

   Some DKIM signers may prefer that they only receive headers from a
   reporter.  The reporter SHOULD attempt to adhere to those wishes of
   the signer.  In a situation where c=n and h has a value, the report
   generator would send a report with only that single header. if the
   'hp' tag has a value then the report generator MAY use that value
   instead of the 'h' tag if the recipient's privacy needs to be
   preserved at the expense of future sending possibly not being
   suppressed to that address.

7.  Delivery Methods

   Reports MUST be sent to the address specified by the "ra" tag.

7.1.  mailto

   Refer to [RFC5965]

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7.2.  https

   A DKIM signer may specify that they wish to receive reports via
   HTTPS.  When doing so, the reporter should continue to use the format
   specifed by the rest of the declaration.

   The report generator SHOULD follow redirects.

   The HTTP method MUST be POST.

   HTTP GET requests to the URL MUST provide easy to follow instructions
   for users to report complaints.

   The report generator SHOULD NOT remove parameters from the URL before
   submitting the report unless the 'hp' tag is specified.  If the 'hp'
   tag is specified then the parameters can be removed if the report
   generator needs to preserve the privacy of the recipient at the
   expense of the report not causing suppressed sending to that
   recipient in the future.

   DNS record

   v=DKIMRFBLv1;c=n;ra=https://ra.example.org/dkim-
   fbl?track=xzy;h=Message-Id;hp=Feedback-Id (https://ra.example.org/
   dkim-fbl?track=xzy;h=Message-Id;hp=Feedback-Id)

   Header in Email

   DKIM-FBL: https://ra.example.com/reports (https://ra.example.com/
   reports) Message-Id: opaque@example.com Feedback-Id: opaque

   Resulting POST request

   POST /dkim-fbl?optional=opaquePart HTTP/1.1 Host: ra.example.com
   Content-Type: application/x-www-form-urlencoded Feedback-Type: abuse
   Content-Length: 26

   ... NEED examples of each: arf and xarf to provide the 'h' or 'hp'

7.2.1.  https Feedback-Type Header

   A reporter MAY include a HTTP header that denotes which report type
   is being delivered.  If used, the header MUST be titled "Feedback-
   Type", and adhere to the definition referenced in [RFC5965] section
   7.3 or the associated IANA declarations.  If this header is absent,
   the Feedback-Type MUST be considered "abuse".

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8.  Verifying External Destinations

   In order to limit the possibility of misdirected reports, if the
   receiving entity domain does not align to the d= of the DKIM
   signature, there must be a DNS record to verify the external
   destination.

   Domain alignment is determined by the logic defined by [DMARCbis].
   Domain alignment applies to domain of the email address in the 'rua'
   tag if the 'f' tag is 'arf' or 'xarf'.  Domain alignment applies to
   the domain defined in the URI of the header referenced by the 'rua'
   tag if the 'f' tag is 'https'

   Consider the record:

   foo._feedback._domainkey.example.org TXT
   "v=DKIMRFBLv1;ra=mailto://reporting@othersite.com"

   In order for "othersite.com" to receive reports for this DKIM
   signature, a record must exist at specified location, and contain a
   specified value.

   1.  Using the domain of the destination
   2.  Prepend "_report._feedback"
   3.  Prepend the values from d= and s= from the original signature.
   4.  Ensure the value is set to "v=DKIMRFBLv1"

   foo.example.org._report._feedback.othersite.com TXT "v=DKIMRFBLv1"

   If the feedback receiver is comfortable with receiving feedback for
   all selectors within a domain, then they may omit the s= value from
   the DNS record location.  The record would be named:

   example.org._report._feedback.othersite.com TXT "v=DKIMRFBLv1"

9.  Security Considerations

9.1.  Feedback to Malicious Senders

   There is some concern that a MBP may provide some advantage or useful
   information to a malicious entity by providing them with FBL data.
   Each MBP should use their own judgement when deciding where to send
   reports.  It is possible that an attacker could use this information
   to attempt to bypass anti-spam filters, or to validate a recipient at
   a given site.

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9.2.  Report Contents for ARF

   Noting in [RFC5965] section 2.g, there should be enough information
   for most senders to process a complaint without the content of the
   message.  While the c flag allows the report receiver to state that
   they do not wish to receive content, the report generator, as per
   [RFC5965] does not need to include that information, regardless of
   the flag settings.

10.  Other Considerations

10.1.  Supplying FP Reports

   It is at the discretion of the report generator as to whether they
   supply False Positive reports to the report requester.

10.2.  Site Requirements

   A report generator may place some requirements on the sender in order
   to be eligible to receive reports.  This could include something such
   as a DMARC policy requirements, TLS usage, or some level of
   reputation.

11.  Contributors

12.  Notes

13.  References

   [DMARCbis] https://datatracker.ietf.org/doc/draft-ietf-dmarc-
   dmarcbis/ (https://datatracker.ietf.org/doc/draft-ietf-dmarc-
   dmarcbis/)

14.  Normative References

   [RFC5321]  Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
              DOI 10.17487/RFC5321, October 2008,
              <https://www.rfc-editor.org/info/rfc5321>.

   [RFC5965]  Shafranovich, Y., Levine, J., and M. Kucherawy, "An
              Extensible Format for Email Feedback Reports", RFC 5965,
              DOI 10.17487/RFC5965, August 2010,
              <https://www.rfc-editor.org/info/rfc5965>.

Author's Address

   Alex Brotman
   Comcast, Inc

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   Email: alex_brotman@comcast.com

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