Internet DRAFT - draft-brotman-dkim-fbl
draft-brotman-dkim-fbl
Network Working Group A. Brotman
Internet-Draft Comcast, Inc
Intended status: Standards Track 23 October 2023
Expires: 25 April 2024
Email Feedback Reports for DKIM Signers
draft-brotman-dkim-fbl-01
Abstract
Mechanism to discover a destination used to deliver user-supplied FBL
reports to an original DKIM signer or other responsible parties.
This should allow the reporting entity to deliver reports via email
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
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This Internet-Draft will expire on 25 April 2024.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Discovery using DNS . . . . . . . . . . . . . . . . . . . . . 2
3. DNS Record Location . . . . . . . . . . . . . . . . . . . . . 3
4. DNS Record Format . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Samples . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Report Contents . . . . . . . . . . . . . . . . . . . . . . . 5
6. Verifying External Destinations . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . 5
7.1. Feedback to Malicious Senders . . . . . . . . . . . . . . 6
7.2. Report Contents for ARF . . . . . . . . . . . . . . . . . 6
8. Other Considerations . . . . . . . . . . . . . . . . . . . . 6
8.1. Supplying FP Reports . . . . . . . . . . . . . . . . . . 6
8.2. Site Requirements . . . . . . . . . . . . . . . . . . . . 6
8.3. Report Delivery . . . . . . . . . . . . . . . . . . . . . 6
9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 6
10. Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
12. Normative References . . . . . . . . . . . . . . . . . . . . 6
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
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, a MBP may then forward that complaint directly, or via an
intermediary, to the original source system of that message.
Additionally, these complaints reach the source system via a
registration system, typically tying a set of IPs or DKIM-based
domains to a specific reporting address.
By allowing reporters to discover the destination on their own, this
should make getting FBLs to the original DKIM signer(s) easier.
2. Discovery using DNS
There are alternative approaches for discovering the feedback
information proposed. Particularly, by adding a new header to every
outbound message to define the feedback address.
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 receiving endpoint. It also avoids
common problems with modifying headers of messages that are already
signed by another DKIM signature.
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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 breaking 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":
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 mimimum, 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.
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4. DNS Record Format
The DNS record should 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 email 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 (",").
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.
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 to the report. This field is OPTIONAL, and may
contain only one attribute.
f: Format requested by report receiver. Options are "arf" and
"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.
4.1. Samples
_feedback._domainkey.example.org TXT
"v=DKIMRFBLv1;ra=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=fbl@example.org;rfr=_feedback._domainkey.example.org
"
*._feedback._domainkey.example.org TXT
"v=DKIMRFBLv1;ra=other_fbl@example.org"
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5. Report Contents
When the report format is specified as "arf", the report contents
should adhere to [RFC5965].
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)
6. Verifying External Destinations
In order to limit the possibility of misdirected reports, if the
receiving entity domain does not match the d= of the DKIM signature,
there must be a DNS record to verify the external destination.
Consider the record:
foo._feedback._domainkey.example.org TXT
"v=DKIMRFBLv1 ; ra=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"
7. Security Considerations
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7.1. 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.
7.2. 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.
8. Other Considerations
8.1. Supplying FP Reports
It is at the discretion of the report generator as to whether they
supply False Positive reports to the report requester.
8.2. 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.
8.3. Report Delivery
In this document, only delivery via SMTP is specified. However, a
separate document could be created to allow for feedback via HTTPS,
UDP, or something yet to be defined.
9. Contributors
10. Notes
11. References
12. Normative References
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[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
Email: alex_brotman@comcast.com
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