Email Feedback Reports for DKIM Signers

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Table of Contents

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 could reduce friction getting FBLs to the original DKIM signer(s).¶

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.¶

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.¶

5. Report Contents

<feedback>
<report_metadata>
<date_range>
<begin>15141231</begin>
<end>15152525</end>
<date_range>
<report_id>20231212-8gKW3RA34VWa3ra</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.¶

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:[email protected]"¶

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

10. Other Considerations

11. Contributors

12. Notes

13. Appendex

14. References

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

Author's Address