Email Feedback Reports for DKIM Signers

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. This document is meant to create a method by which a MBP can discover how to report feedback in the form of an FBL. By utilizing this new method, and allowing reporters to discover the destination and reporting preferences on their own, this could reduce friction getting FBLs to the original DKIM signer(s). This is not meant to document how a MBP can provide feedback about DKIM-related issues.

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.

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). Using this abbreviated DKIM-Signature as an example: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=example.org; s=selector1; t=1767558375; bh=8Fgi2BuhJ2eE9KTZlip4By07JDsm5vSu2HdVM=; h=From:Reply-To:To:Subject:Date; b=6DZuSCPnTrFCeEtRSOu8RRuuiV9vw...== Such as the case where "d=example.org", the record would be located at: _feedback._domainkey.example.org or *._feedback._domainkey.example.org Either of the records above would result in all reports for all selectors going to the declared "ra" address. This is the default action, unless a selector-based record is created (described below). 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=selector1", for example: selector1._feedback._domainkey.example.org By including the selector, this allows a domain to be able to segment the feedback to various report processing providers. It is possible to have a wildcard record, as well as a specific record. Doing so may be ideal as it allows for there to be a fallback if a selector is overlooked during configuration. The need for selector level feedback still needs to be assessed. A record will be valid only for the exact "d=" domain that is in the DNS label, and will not apply to sub-domains. All domain owners that want to ensure they receive all feedback for a given domain 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.

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 . 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 receipt transfer methods supported by the DKIM signer. This MAY coexist with the "rfr" attribute. rfr: An optional field to refer the report generator(s) to another DNS entry. This option MAY coexist with the "ra" tag. (NOTE: there should probably be a limit on how many rfr will be followed) 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 "n". See notes about content below. 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.

DKIM Requirements If a sender utilizes the h or hp attributes in their DNS record, those fields MUST be covered by the DKIM signature that is requesting the report. If the header is not signed by the proper requester (or not valid), the receiver SHOULD refuse to generate any reports for those related messages.

Content flag It should be noted that not all MBPs will honor the 'c' flag, and may only send reports without full content. Entities requesting reports SHOULD be able to handle reports with and without content.

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.example.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=SendingIdentifier" _feedback._domainkey.example.org TXT
"v=DKIMRFBLv1;ra=mailto:fbl@example.org;hp=Campaign-Id;c=n" _feedback.domainkey.sender.com TXT
"v=DKIMRFBLv1;ra=mailto:fbl@other.com;h=SendingIdentifier;hp=Campaign-Id;c=y"

Report Contents The reports are meant to be mimialistic, and include the minimum information necessary for the signing entity to determine how to identify the source of the message. Additionally, the contents of the report itself MAY be base64-encoded. NOTE: Originally, there was a section here to be ARF/XARF. That has been removed.

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.

Delivery Methods Reports MUST be sent to the address(es) specified by the "ra" tag. If the declaration has a "rfr" tag, those recipient address(es) MUST also receive the reports.

mailto Refer to

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 specified by the rest of the declaration. NOTE: Consider if HTTPS should be supported, based on historical usage patterns for other similar mechanisms The report generator SHOULD follow redirects. The HTTPS method MUST be POST. HTTPS 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 Header in Email DKIM-FBL: <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

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 section 7.3 or the associated IANA declarations. If this header is absent, the Feedback-Type MUST be considered "abuse".

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 'ra' tag if the 'ra' tag is 'mailto'. Domain alignment applies to the domain defined in the URI of the header referenced by the 'ra' 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.

Using the domain of the destination
Prepend "_report._feedback"
Prepend the values from d= and s= from the original signature.
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"

Security Considerations

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.

Report Contents for ARF Noting in 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 , may not need to include that information, regardless of the flag settings. Not all report generators will honor the c flag, may redact content based on local policies. A report receiver should be prepared to receive reports with and without content.

Other Considerations

Supplying FP Reports It is at the discretion of the report generator as to whether they supply False Positive reports, or aggregate information, to the report requester.

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.

Unaligned Domains A report generator may decide that they would only like to provide reports to the aligned signer in a message. That is their discretion.

Contributors

Notes

Appendix

Samples

Sample message DKIM-Signature: d=example.com;s=Selector1;h=From:To:Subject:Message-Id:Campaign-Id:Date From: "Sender" marketing@example.com To: "Customer" recipient@example.net Subject: SubjectHere Message-Id: awav4w4vaw.aw4473737bab.AWAe@sender Campaign-Id: 20240314a_Sender FBL-Message-Id: fgjm7Bbbse56b.Sender.recipient.example.net Date: March 24th, 2024 12:34.000UTC Click here for stuff <EOM>

Sample DNS and Reports

Content-requested DNS: v=DKIMRFBLv1;ra=mailto:fbl@example.com;c=y

No Content Requested DNS: v=DKIMRFBLv1;ra=mailto:fbl@example.com;c=n;h=Campaign-Id

No Content, Summary only DNS: v=DKIMRFBLv1;ra=mailto:fbl@example.com;c=n;hp=FBL-Message-Id Nothing should be delivered, as the FBL-Message-Id is not signed

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