Internet DRAFT - draft-blank-ietf-bimi
draft-blank-ietf-bimi
Network S. Blank
Internet-Draft P. Goldstein
Intended status: Standards Track Valimail
Expires: 9 September 2021 T. Loder (ed)
Skye Logicworks LLC
T. Zink (ed)
M. Bradshaw (ed)
Fastmail
8 March 2021
Brand Indicators for Message Identification (BIMI)
draft-blank-ietf-bimi-02
Abstract
Brand Indicators for Message Identification (BIMI) permits Domain
Owners to coordinate with Mail User Agents (MUAs) to display brand-
specific Indicators next to properly authenticated messages. There
are two aspects of BIMI coordination: a scalable mechanism for Domain
Owners to publish their desired Indicators, and a mechanism for Mail
Transfer Agents (MTAs) to verify the authenticity of the Indicator.
This document specifies how Domain Owners communicate their desired
Indicators through the BIMI Assertion Record in DNS and how that
record is to be interpreted by MTAs and MUAs. MUAs and mail-
receiving organizations are free to define their own policies for
making use of BIMI data and for Indicator display as they see fit.
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 https://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 5 August 2021.
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Copyright Notice
Copyright (c) 2021 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 (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents 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 . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. High-Level Goals . . . . . . . . . . . . . . . . . . . . 7
2.2. Security . . . . . . . . . . . . . . . . . . . . . . . . 8
2.3. Out of Scope . . . . . . . . . . . . . . . . . . . . . . 8
3. Terminology and Definitions . . . . . . . . . . . . . . . . . 8
3.1. BIMI Assertion . . . . . . . . . . . . . . . . . . . . . 9
3.2. Indicator . . . . . . . . . . . . . . . . . . . . . . . . 9
3.3. Mark Verifying Authority (MVA) . . . . . . . . . . . . . 9
3.4. BIMI Evidence Document . . . . . . . . . . . . . . . . . 9
3.5. Verified Mark Certificate (VMC) . . . . . . . . . . . . . 9
3.6. Protocol Client . . . . . . . . . . . . . . . . . . . . . 10
3.7. Verifying Protocol Client . . . . . . . . . . . . . . . . 10
4. BIMI DNS Records . . . . . . . . . . . . . . . . . . . . . . 10
4.1. MUA Obligations . . . . . . . . . . . . . . . . . . . . . 11
4.2. Assertion Record Definition . . . . . . . . . . . . . . . 11
4.2.1. Declination to Publish . . . . . . . . . . . . . . . 13
4.2.2. Supported Image Formats for l= tag . . . . . . . . . 13
4.3. Selectors . . . . . . . . . . . . . . . . . . . . . . . . 13
5. BIMI Header Fields . . . . . . . . . . . . . . . . . . . . . 14
5.1. BIMI-Selector Header . . . . . . . . . . . . . . . . . . 14
5.2. BIMI-Location Header . . . . . . . . . . . . . . . . . . 15
5.3. BIMI-Indicator Header . . . . . . . . . . . . . . . . . . 16
5.4. Header Signing . . . . . . . . . . . . . . . . . . . . . 16
6. Domain Owner Actions . . . . . . . . . . . . . . . . . . . . 17
6.1. Determine and Publish Indicator(s) for Use . . . . . . . 17
6.2. Publish Assertion Records . . . . . . . . . . . . . . . . 17
6.3. Manage multiple uses of the same Indicator(s) within a
trust boundary . . . . . . . . . . . . . . . . . . . . . 17
6.4. Set the headers on outgoing email as appropriate . . . . 17
7. Receiver Actions . . . . . . . . . . . . . . . . . . . . . . 18
7.1. Authentication Requirements . . . . . . . . . . . . . . . 18
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7.2. Assertion Record Discovery . . . . . . . . . . . . . . . 19
7.3. Indicator Discovery. . . . . . . . . . . . . . . . . . . 20
7.4. Indicator Discovery With Evidence. . . . . . . . . . . . 20
7.5. Indicator Discovery Without Evidence. . . . . . . . . . . 21
7.6. Indicator Validation . . . . . . . . . . . . . . . . . . 21
7.7. Affix BIMI Status to Authentication Results Header
Field . . . . . . . . . . . . . . . . . . . . . . . . . 22
7.8. Handle Existing BIMI-Location and BIMI-Indicator
Headers . . . . . . . . . . . . . . . . . . . . . . . . 23
7.9. Construct BIMI-Location URI . . . . . . . . . . . . . . . 23
7.10. Construct BIMI-Indicator header . . . . . . . . . . . . . 23
8. Security Considerations . . . . . . . . . . . . . . . . . . . 24
8.1. Indirect Mail Flows . . . . . . . . . . . . . . . . . . . 24
8.2. Lookalike Domains and Copycat Indicators . . . . . . . . 24
8.3. Large files and buffer overflows . . . . . . . . . . . . 24
8.4. Slow DNS queries . . . . . . . . . . . . . . . . . . . . 24
8.5. Unaligned Indicators and asserting domains . . . . . . . 25
8.6. Unsigned BIMI-Selector Header . . . . . . . . . . . . . . 25
8.7. CGI scripts in Indicator payload . . . . . . . . . . . . 25
8.8. Metadata in Indicators . . . . . . . . . . . . . . . . . 25
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
9.1. Permanent Header Field Updates . . . . . . . . . . . . . 25
9.2. Registry for Supported BIMI Formats . . . . . . . . . . . 26
9.3. Other IANA needs . . . . . . . . . . . . . . . . . . . . 26
10. Normative References . . . . . . . . . . . . . . . . . . . . 26
11. Informative References . . . . . . . . . . . . . . . . . . . 27
Appendix A. Example Selector Discovery (INFORMATIVE) . . . . . . 28
A.1. No BIMI-Selector Header . . . . . . . . . . . . . . . . . 28
A.2. With BIMI-Selector Header . . . . . . . . . . . . . . . . 28
A.3. Without BIMI-Selector Header on a subdomain . . . . . . . 28
A.4. With BIMI-Selector Header on a subdomain . . . . . . . . 28
A.5. Invalid BIMI-Selector Header . . . . . . . . . . . . . . 29
Appendix B. Example Authentication-Results entry
(INFORMATIONAL) . . . . . . . . . . . . . . . . . . . . . 29
B.1. Successful BIMI lookup . . . . . . . . . . . . . . . . . 29
B.2. No BIMI record . . . . . . . . . . . . . . . . . . . . . 29
B.3. Declination to Publish . . . . . . . . . . . . . . . . . 29
B.4. Subdomain has no default record, but organizational domain
does . . . . . . . . . . . . . . . . . . . . . . . . . . 29
B.5. Subdomain and orgznizational domain have no record for
selector, but organization . . . . . . . . . . . . . . . 30
B.6. Subdomain has no record for selector, but organization
domain does . . . . . . . . . . . . . . . . . . . . . . . 30
Appendix C. Example BIMI Headers Construction (INFORMATIONAL) . 30
C.1. MTA Receives an email . . . . . . . . . . . . . . . . . . 30
C.2. MTA does its authentication checks . . . . . . . . . . . 30
C.3. MTA performs BIMI Assertion . . . . . . . . . . . . . . . 31
C.4. MTA appends to Authentication-Results . . . . . . . . . . 31
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C.5. MTA Constructs BIMI-Location and BIMI-Indicator
headers . . . . . . . . . . . . . . . . . . . . . . . . . 31
C.6. The MUA displays the Indicator . . . . . . . . . . . . . 32
Appendix D. Acknowledgements . . . . . . . . . . . . . . . . . . 32
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 32
1. Introduction
RFC EDITOR: PLEASE REMOVE THE FOLLOWING PARAGRAPH BEFORE PUBLISHING:
The source for this draft is maintained in GitHub at:
https://github.com/BLAHBLAHBLAH (https://github.com/BLAHBLAHBLAH)
This document defines Brand Indicators for Message Identification
(BIMI), which enables Domain Owners to coordinate with Mail Box
Providers (MBPs), Mail Transfer Agents (MTAs), and Mail User Agents
(MUAs) in the display of brand-specific Indicators next to properly
authenticated messages.
BIMI is designed to be open and to work at Internet scale. BIMI is
intended to drive adoption of email authentication best practices by
leveraging existing DMARC [RFC7489] policies, the supporting
authentication methods DKIM [RFC6376] and SPF [RFC7208], and other
associated standards such as ARC [RFC8617].
The approach taken by BIMI is heavily influenced by the approach
taken in DKIM [RFC6376], in that BIMI:
* has no dependency on the deployment of any new Internet protocols
or services for indicator registration or revocation;
* makes no attempt to include encryption as part of the mechanism;
* is compatible with the existing email infrastructure and
transparent to the fullest extent possible;
* requires minimal new infrastructure;
* can be implemented independently of clients in order to reduce
deployment time;
* can be deployed incrementally; and
* allows delegation of indicator hosting to third parties.
To participate in BIMI, Domain Owners MUST have a strong [DMARC]
policy (quarantine or reject) on both the Organizational Domain, and
the RFC5322.From Domain of the message. Quarantine policies MUST NOT
have a pct less than pct=100.
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This document defines how Domain Owners specify their desired
indicators through the BIMI Assertion Record in DNS and how that
record is to be interpreted by MTAs and MUAs. This document does not
cover how domains or indicators are verified, how MUAs should display
the indicators, or how other protocols (i.e. IMAP, JMAP) can be
extended to work with BIMI. Other documents may cover these topics.
MUAs and Mail Box Providers (MBPs) are free to define their own
policies for making use of BIMI data and for indicator display as
they see fit.
2. Overview
The Sender Policy Framework (SPF [RFC7208]), DomainKeys Identified
Mail (DKIM [RFC6376]), "Domain-based Message Authentication,
Reporting, and Conformance" (DMARC [RFC7489]), and Authenticated
Received Chain (ARC [RFC8617]) provide mechanisms for domain-level
authentication of email messages. They enable cooperating email
senders and receivers to distinguish messages that are authorized to
use the domain name from those that are not. BIMI relies on these
authentication protocols, but is not a new authentication protocol
itself.
MUAs are increasingly incorporating graphical Indicators to indicate
the identity of the sender of a message. While a discussion of the
merits of doing this is beyond the scope of this document, at present
there are no open standards for publishing and aiding discovery of
preferred Indicators or for tying display of them to authentic
messages only.
Because of the desire to have brand-specific Indicators available,
some mail-receiving organizations have developed closed systems for
obtaining and displaying Brand Indicators for select domains. While
this has enabled these mail-receiving organizations to display brand
Indicators for a limited subset of messages, this closed approach has
a number of downsides:
1. It puts a significant burden on each mail-receiving organization,
because they must identify and manage a large database of Brand
Indicators.
2. Scalability is challenging for closed systems that attempt to
capture and maintain complete sets of data across the whole of
the Internet.
3. A lack of uniformity across different mail-receiving
organizations - each organization will have its own Indicator
set, which may or may not agree with those maintained by other
organizations for any given domain.
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4. Domain Owners have limited ability to influence the Brand
Indicator for the domain(s) they own, and any ability they do
have is likely to be dependent upon direct coordination with each
of many mail-receiving organizations.
5. Many Domain Owners have no ability to participate whatsoever as
they do not have the appropriate relationships to coordinate with
mail-receiving organizations.
6. MUAs that are not associated with a particular mail-receiving
organization are likely to be disadvantaged, because they are
unlikely to receive Indicators in a standardized manner or
optimized for their user interfaces.
This shows the need for a standardized mechanism by which Domain
Owners interested in ensuring that their Indicators are displayed
correctly and appropriately can publish and distribute Brand
Indicators for use by any participating MUA.
BIMI removes the substantial burden of curating and maintaining an
Indicator database from MUAs and MBPs, and allows each domain owner
to manage their own Indicators. As an additional benefit, mail-
originating organizations are incentivized to authenticate their
email as doing so will enable them to influence how email and
Indicators from the organization are displayed.
The structure of BIMI is as follows:
1. Domain Owners:
* Fully implement the DMARC [RFC7489] mechanism, to include:
- Creating and publishing in DNS [RFC1035] a DMARC [RFC7489]
policy record that meets the following criteria:
o The policy record MUST express either a Requested Mail
Receiver policy of "quarantine" with an effective
percentage of 100%, or a Requested Mail Receiver policy
of "reject" (with any percentage value).
o Is a subdomain policy is published it MUST NOT be "none"
o Be published for the Organizational Domain, and any
subdomains thereof
- Deploying authentication technologies to ensure Identifier
Alignment
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* Publish their preferred Brand Indicators via the DNS
[RFC1035].
2. Senders: Ensure mail is properly authenticated, and has a
sufficiently strict DMARC [RFC7489] policy.
3. MTAs and MBPs:
* Confirm authenticity of the message using DMARC [RFC7489] and
whatever other authentication mechanisms they wish to apply.
* Check for a corresponding BIMI record, obtaining references to
the indicator media and optional substantiation of indicator
ownership rights
* If both the message is authentic and the logo is deemed
acceptable, the receiver adds a header to the message which
can be used by the MUA to obtain the Domain Owner's preferred
brand indicator.
4. MUA: retrieves and displays the brand indicator as appropriate
based on its policy and user interface.
The purpose of this structure is to reduce operational complexity at
each step. It is also to consolidate validation and Indicator
selection operations into the MTA, so that Domain Owners need only
publish a few simple records and MUAs only need simple display logic.
It is expected that MBPs implementing BIMI will do so in both their
MTAs and MUAs.
#Requirements {#requirements}
Specification of BIMI in this document is guided by the following
high-level goals, security dependencies, detailed requirements, and
items that are documented as out of scope.
An overview of the security challenges and design decisions is
documented at [BIMI-OVERVIEW].
2.1. High-Level Goals
BIMI has the following high-level goals:
* Allow Domain Owners to suggest appropriate Indicators for display
with authenticated messages originating from their domains.
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* Enable the authors of MUAs to display meaningful Indicators
associated with the Domain Owner to recipients of authenticated
email.
* Provide mechanisms to prevent attempts by malicious Domain Owners
to fraudulently represent messages from their domains as
originating with other entities.
* Work at Internet Scale.
* Encourage the adoption of Email Authentication Best Practices.
2.2. Security
Brand Indicators are a potential vector for abuse. BIMI creates a
relationship between sending organization and Mail Receiver so that
the receiver can display appropriately designated Indicators if the
sending domain is verified and has meaningful reputation with the
receiver. Without verification and reputation, there is no way to
prevent a bad actor exxample.com from using example.com's Brand
Indicators and behaving maliciously. This document does not cover
the different verification and reputation mechanisms available, but
BIMI relies upon them to be in deployed in order to control abuse.
2.3. Out of Scope
Several topics and issues are specifically out of scope for the
initial version of this work. These include the following:
* Publishing policy other than via the DNS.
* Specific requirements for Indicator display on MUAs.
* The explicit mechanisms used by Verifying Protocol Clients - this
will be deferred to a later document.
3. Terminology and Definitions
This section defines terms used in the rest of the document.
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 BCP 14
(https://tools.ietf.org/html/bcp14) [RFC2119] [RFC8174] when, and
only when, they appear in all capitals, as shown here.
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Readers are encouraged to be familiar with the contents of [RFC5598].
In particular, that document defines various roles in the messaging
infrastructure that can appear the same or separate in various
contexts. For example, a Domain Owner could, via the messaging
mechanisms on which BIMI is based, delegate reponsibility for
providing preferred brand indicators to a third party with another
role. This document does not address the distinctions among such
roles; the reader is encouraged to become familiar with that material
before continuing.
Syntax descriptions use Augmented BNF (ABNF) [RFC5234].
"Author Domain", "Domain Owner", "Organizational Domain", and "Mail
Receiver" are imported from DMARC [RFC7489] Section 3.
3.1. BIMI Assertion
The mechanism through which a Protocol Client verifies the BIMI
Assertion Record and constructs the URI(s) to the requested
Indicator(s) to be placed in the BIMI-Location header.
3.2. Indicator
The icon, logo, image, mark, or other graphical representation of the
brand. The Indicator is defined in a common image format with
restrictions detailed in the Assertion Record Definition (#assertion-
record-definition).
3.3. Mark Verifying Authority (MVA)
An entity or organization that can provide evidence of verification
of Indicators asserted by a Domain Owner to Verifying Protocol
Clients. The MVA may choose to uphold and confirm the meeting of
certain Indicator standards (ie. size, trademark, content, etc).
3.4. BIMI Evidence Document
A document published by a Mark Verifying Authority to assert evicence
of verification. These are defined in a separate document.
3.5. Verified Mark Certificate (VMC)
A certificate issued by a Certificate Authority in accordance with
the Verified Mark Certificate Guidelines. These guidelines are
defined in a separate document.
A Verified Mark Certificate is one example of a BIMI Evidence
Document.
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3.6. Protocol Client
An entity designed to obtain and correctly interpret the records
defined in this specification for the purpose of discovering and
fetching published Indicators.
3.7. Verifying Protocol Client
A Protocol Client that uses optional capabilities to obtain and
evaluate evidence concerning the Domain Owner's rights to use the
published Indicators.
4. BIMI DNS Records
Domain owners publish BIMI policies by adding BIMI Assertion Records
in the DNS as TXT records.
Published policies are interpreted and applied by Protocol Clients.
A Domain Owner signals intended BIMI participation for one or more of
its domains by publishing an Assertion Record in a subdomain under
it. In doing so, Domain Owners make specific requests of MUAs
regarding the preferred set of Indicators to be displayed with
messages that are confirmed to be authorized to appear from the
Domain Owner's domain.
The use of BIMI is opt-in. Receivers default to performing no BIMI-
specific message handling until they choose to do so, and then only
if a BIMI record for the sender's domain is found.
BIMI's use of the DNS is driven in part by BIMI's use of domain names
as the basis of sender identity and message authentication. Use of
the DNS as the policy publication service also has the benefit of
reusing an extremely well-established operations, administration, and
management infrastructure, rather than creating a new one.
BIMI's policy payload is intentionally only published via a DNS
record and not via one or more email headers. This serves three
purposes:
1. There is one and only one mechanism for both simple and complex
policies to be published.
2. Operational complexity is reduced. MTAs only need to check a
single record in a consistent manner to discover and enforce
policy.
3. Indicators SHOULD be verified and cached in advance, so that
malicious headers cannot be used as an attack vector.
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Per DNS [RFC1035], a TXT record can comprise several "character-
string" objects. BIMI TXT records with multiple strings must be
treated in an identical manner to SPF Section 3.3
(https://tools.ietf.org/html/rfc7208#section-3.3).
4.1. MUA Obligations
MUAs implementing the BIMI mechanism SHOULD make a best-effort
attempt to adhere to the Domain Owner's published BIMI policy.
However, MUAs have final control over the user interface published to
their end users, and MAY use alternate Indicators than those
specified in the BIMI assertion record or no Indicator at all.
4.2. Assertion Record Definition
All Domain Owner BIMI preferences are expressed in DNS TXT records
published in subdomains named "_bimi". Multiple sets of preferences
can be associated with a single RFC5322.From domain. To distinguish
between these different preferences, BIMI defines and uses
[selectors]{#selectors}. Senders declare which selector to use for a
given message by specifying the selector in an optional BIMI-Selector
header (#bimi-selector).
For example, the Domain Owner of "example.com" would post BIMI policy
in a TXT record at "default._bimi.example.com". Similarly, a Mail
Receiver wishing to query for BIMI policy regarding mail with an
RFC5322.From Author Domain of "example.com" and a selector "default"
(the default) would query the TXT record located at the subdomain of
"default._bimi.example.com". The DNS-based BIMI policy record is
referred to as the "BIMI Assertion Record" or "Assertion Record".
BIMI Assertion Records follow the extensible "tag-value" syntax for
DNS-based key records as defined in DKIM [RFC6376].
Assertion Records are defined precisely. Mail receivers MUST NOT
attempt to fix syntactical or capitalization errors. If a required
tag is missing, or its value not well-formed, it is an error.
This section creates a registry for known BIMI tags and registers the
initial set defined in this document. Only tags defined in this
document or in later extensions, and thus added to the registry, are
to be processed; unknown tags MUST be ignored.
The following tags are introduced as the initial valid BIMI tags:
v= Version (plain-text; REQUIRED). Identifies the record retrieved
as a BIMI record. It MUST have the value of "BIMI1" for
implementations compliant with this version of BIMI. The value of
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this tag MUST match precisely; if it does not match or it is absent,
the entire retrieved record MUST be ignored. It MUST be the first
tag in the list.
ABNF:
bimi-version = %x76 *WSP "=" *WSP %x42.49.4d.49 1DIGIT
a= Authority Evidence Location (plain-text; URI; OPTIONAL). If
present, this tag MUST have an empty value or its value MUST be a
single URI. An empty value for the tag is interpreted to mean the
Domain Owner does not wish to publish or does not have authority
evidence to disclose. The URI, if present, MUST contain a fully
qualified domain name (FQDN) and MUST specify HTTPS as the URI scheme
("https"). The URI SHOULD specify the location of a publicly
retrievable BIMI Evidence Document. The format for evidence
documents is defined in a separate document.
If the a= tag is not present, it is assumed to have an empty value.
ABNF:
bimi-evidence-location = %x61 *WSP "=" bimi-uri
bimi-uri = \[FWS\] URI \[FWS\]
; "URI" is imported from [URI]
; HTTPS only
; commas within a URI (ASCII ; 0x2C) MUST be encoded
l= location (URI; REQUIRED). The value of this tag is either empty
indicating declination to publish, or a single URI representing the
location of a Brand Indicator file. The only supported transport is
HTTPS.
ABNF:
bimi-location = %x6c *WSP "=" bimi-uri
Therefore, the formal definition of the BIMI Assertion Record, using
ABNF [RFC5234], is as follows:
bimi-sep = *WSP %x3b *WSP
bimi-record = bimi-version (bimi-sep bimi-location) (bimi-sep bimi-evidence-location) \[bimi-sep\]
; components other than bimi-version may appear in any order
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4.2.1. Declination to Publish
If both the "l=" and "a=" tags are empty, it is an explicit refusal
to participate in BIMI. This is distinct from not publishing a BIMI
record. For example, an empty BIMI record enables a Domain Owner to
decline BIMI participation for a subdomain when its organizational
domain has default Indicators available. Furthermore, messages sent
using a selector that has declined to publish will not show an
Indicator while messages with other selectors would display normally.
An explicit declination to publish looks like:
v=BIMI1; l=; a=;
4.2.2. Supported Image Formats for l= tag
Any format in the BIMI-formats IANA registry are acceptable targets
for the l= tag. If an l= tag URI ends with any other image format
suffix, or if the document retrievable from the location(s) in the l=
tag are of any other format, the evaluation of the record MUST be
treated as a permanent error.
As of the publishing of this document, only SVG and SVGZ, as defined
in RFC6170 section 5.2 (https://tools.ietf.org/html/rfc6170#section-
5.2) is acceptable in the l= tag. Further restrictions apply to the
SVG; these are documented elsewhere.
4.3. Selectors
To support publishing and display of more than one distinct Brand
Indicator per domain, the brand Indicator namespace is subdivided for
publishing of multiple Assertion Records using "selectors".
Selectors allow the Domain Owner to choose the brand Indicator, for
example, by type of recipient, by message source, or by other
considerations like seasonal branding. BIMI selectors are modeled
after DKIM selectors (https://tools.ietf.org/html/rfc6376#section-
3.1).
The selector "default" is the default Assertion Record. Domain
Owners can specify which other selector to use on a per-message basis
by utilizing the BIMI-Selector Header (#bimi-selector).
Periods are allowed in selectors and are component separators. When
BIMI Assertion Records are retrieved from the DNS, periods in
selectors define DNS label boundaries in a manner similar to the
conventional use in domain names. In a DNS implementation, this can
be used to allow delegation of a portion of the selector namespace.
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ABNF:
selector = sub-domain *( "." sub-domain )
; from [SMTP] Domain,
; excluding address-literal
The number of selectors for each domain is determined by the Domain
Owner. Many Domain Owners will be satisfied with just one selector,
whereas organizations with more complex branding requirements can
choose to manage disparate selectors. BIMI sets no maximum limit on
the number of selectors.
5. BIMI Header Fields
Once BIMI policies are published in DNS via Assertion Records, Domain
Owners can provide additional guidance to Mail Receivers, and Mail
Receivers to their MUAs through the use of BIMI header fields.
BIMI header fields are case insensitive. If a required tag is
missing, it is an error.
5.1. BIMI-Selector Header
BIMI DNS records are placed in <selector>._bimi.<domain>, and by
default they are placed in default._bimi.<domain>. That is, for
example.com, the default Assertion Record is located in the DNS at
default._bimi.example.com. However, a Domain Owner may override the
use of the default selector and specify the use of an alternative
using the RFC5322-compliant header 'BIMI-Selector'. The BIMI-
Selector header consists of key value pairs:
v= Version (plain-text; REQUIRED). The version of BIMI. It MUST
have the value of "BIMI1" for implementations compliant with this
version of BIMI. The value of this tag MUST match precisely; if it
does not or it is absent, the entire retrieved record MUST be
ignored. It MUST be the first tag in the list.
ABNF:
bimi-header-version = "v" *WSP "=" *WSP "BIMI" 1DIGIT
s= Selector (plain-text; REQUIRED). The location of the BIMI DNS
record, when combined with the RFC5322.From domain.
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ABNF:
bimi-selector = "s" *WSP "=" *WSP selector
And the formal definition of the BIMI Selector Header, using ABNF, is
as follows:
bimi-selector-header = bimi-header-version bimi-sep bimi-selector \[bimi-sep\]
5.2. BIMI-Location Header
BIMI-Location is the header a Mail Receiver inserts that tells the
MUA where to get the BIMI Indicator from.
The syntax of the header is as follows:
v= Version (plain-text; REQUIRED). The version of BIMI. It MUST
have the value of "BIMI1" for implementations compliant with this
version of BIMI. The value of this tag MUST match precisely; if it
does not or it is absent, he entire header MUST be ignored. It MUST
be the first tag in the list.
The ABNF for bimi-header-version is imported exactly from the
[BIMI Selector Header](#bimi-selector).
l: location of the BIMI Indicator (URI; OPTIONAL if a bimi-evidence-
location-header-uri is specified, otherwise REQUIRED.). Inserted by
the MTA after performing the required checks and obtaining the
applicable domain's published Assertion Record. The value of this
tag is a URI representing the location of the Brand Indicator file.
HTTPS is the only supported transport.
ABNF:
bimi-location-header-uri = "l" *WSP "=" bimi-uri
a: location of the BIMI Evidence Document (URI; REQUIRED if the BIMI
Evidence Document was verified). Inserted by the MTA after
performing the required checks and obtaining the applicable domain's
published Assertion Record. The value of this tag is a URI
representing the location of the BIMI Evidence Document. HTTPS is
the only supported transport.
ABNF:
bimi-evidenced-location-header-uri = "a" *WSP "=" bimi-uri
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And the formal definition of the BIMI Location Header, using ABNF, is
as follows:
bimi-location-header-location-only = bimi-location-header-uri
bimi-location-header-evidence-only = bimi-evidence-location-header-uri
bimi-location-header-both = bimi-location-header-uri bimi-evidence-location-header-uri
bimi-location-options = bimi-location-header-location-only / bimi-location-header-evidence-only / bimi-location-header-both
bimi-location-header = bimi-header-version bimi-sep bimi-location-options \[bimi-sep\]
5.3. BIMI-Indicator Header
BIMI-Indicator is the header a Mail Receiver inserts to pass a
verified Indicator to the MUA.
The header contains the SVG of the Indicator encoded as base64, and
is inserted by the MTA after performing the required checks and
obtaining the applicable domain's published Assertion Record. The
contents of this tag MUST match the SVG Indicator content retrieved
from the URI specified in the BIMI-Location header. If he Indicator
was supplied as a gzipped SVGZ file then the MTA MUST uncompress the
file before base64 encoding.
base64string = ALPHADIGITPS *([FWS] ALPHADIGITPS)
[ [FWS] "=" [ [FWS] "=" ] ]
And the formal definition of the BIMI Indicator Header, using ABNF,
is as follows:
bimi-indicator-header = bimi-sep base64string \[bimi-sep\]
5.4. Header Signing
If present, the BIMI-Selector header SHOULD be included in the DMARC-
aligned DKIM signature used to confirm authenticity of the message.
If it is not included in the DMARC-compliant DKIM signature, the
header SHOULD be ignored.
Receivers MAY choose to apply additional methods to validate the
BIMI-Selector header, for example by evaluating a trusted [ARC]
chain. In this case the Receiver MAY choose to treat the message as
if the BIMI-Selector header was signed.
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The BIMI-Location and BIMI-Indicator headers MUST NOT be DKIM signed.
This header is untrusted by definition, and is only for use between
an MTA and its MUAs, after DKIM has been validated by the MTA.
Therefore, signing this header is meaningless, and any messages with
it signed are either coming from malicious or misconfigured third
parties.
6. Domain Owner Actions
This section includes a walk through of the actions a Domain Owner
takes when setting up Assertion Records and sending email messages.
6.1. Determine and Publish Indicator(s) for Use
Domain Owners should consider which Indicator file formats to choose
when setting up their BIMI Assertion Records. For a Sender, BIMI
provides control over which Indicators are eligible and can be chosen
for display, but not the ultimate manner in which the MUA will
display the Indicator.
6.2. Publish Assertion Records
For each set of Indicators and domains, publish the appropriate
Assertion Record as either "default" or a named selector as a DNS TXT
record within the appropriate "_bimi" namespace.
6.3. Manage multiple uses of the same Indicator(s) within a trust
boundary
For Domain Owners with multiple domains that wish to share the same
set of Indicators within a trust boundary and only manage those
Indicators from a single DNS location, it is RECOMMENDED to use DNS
CNAMEs.
Using a CNAME here is functionally similar to the SPF redirect
modifier. Since BIMI does not require l= tags to be aligned to the
Author Domain, CNAMEs present a cleaner solution than extending the
protocol.
6.4. Set the headers on outgoing email as appropriate
Once a default Assertion Record has been published for an Author
Domain, all emails from this domain should display the appropriate
Indicator in participating MUAs.
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If a non-default Indicator is desired, the BIMI-Selector header
should be set appropriately. If for some reason this selector cannot
be accessed by the Protocol Client, the fallback is the default
Assertion Record on the Organization domain.
The BIMI-Location header MUST NOT be set by email senders, and
Protocol Clients MUST ignore it.
7. Receiver Actions
This section includes a walk through of the actions a Protocol Client
takes when evaluating an email message for BIMI Assertion.
7.1. Authentication Requirements
Before applying BIMI processing for a message, a receiver MUST verify
that the message passed the following BIMI authentication
requirements:
1. If more than 1 RFC5322.From header is present in the message, or
any RFC5322.From header contains more than 1 email address then
BIMI processing MUST NOT be performed for this message.
2. Start with the DNS domain found in the RFC5322.From header in the
message. Define this DNS domain as the Author Domain.
3. Find the Organizational Domain for the Author Domain. Define
this DNS domain as the Author Organizational Domain. If the
Author Domain is an Organizational Domain then this will be
identical to the Author Domain.
4. Evaluate the DMARC [RFC7489] result for the Author Domain.
Define the result as the BIMI DMARC Result.
5. If the BIMI DMARC result is not 'pass', then the receiver MAY
choose to apply additional authentication methods, for example by
evaluating a trusted ARC [RFC8617] chain, a list of trusted
forwarders, or by applying a local policy. In this case the
Receiver MAY choose to treat the message as if the BIMI DMARC
Result was 'pass'.
6. If the DMARC [RFC7489] result for the Author Domain is not
'pass', and the message could not be authenticated by any
additional authentication method, then BIMI processing MUST NOT
be performed for this message.
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7. If the DMARC [RFC7489] policy for the Author Domain or Author
Organizational Domain is p=none then BIMI processing MUST NOT be
performed for this message.
8. If the DMARC [RFC7489] record for the Author Domain or Author
Organizational Domain includes a subdomain policy, and that
subdomain policy is sp=none then BIMI processing MUST NOT be
performed for this message.
9. If the DMARC [RFC7489] policy for the Author Domain or Author
Organizational Domain is p=quarantine, and the DMARC [RFC7489]
record defines a percentage tag, then that tag MUST be pct=100,
otherwise BIMI processing MUST NOT be performed for this message.
7.2. Assertion Record Discovery
Through the BIMI Assertion Record (#assertion-record-definition),
Domain Owners use DNS TXT records to advertise their preferences.
Preference discovery is accomplished via a method similar to the
method used for DMARC [RFC7489] records. This method, and the
important differences between BIMI and DMARC [RFC7489] mechanisms,
are discussed below.
Assertion Record Discovery MUST NOT be attempted if the message
authentication fails per Receiver policy.
To balance the conflicting requirements of supporting wildcarding,
allowing subdomain policy overrides, and limiting DNS query load,
Protocol Clients MUST employ the following lookup scheme for the
appropriate BIMI record for the message:
1. Start with the DNS domain found in the RFC5322.From header in the
message. Define this DNS domain as the Author Domain.
2. If the message for which the Indicator is being determined
specifies a selector value in the BIMI Selector Header (#bimi-
selector), use this value for the selector. Otherwise the value
'default' MUST be used for the selector.
3. Clients MUST query the DNS for a BIMI TXT record at the DNS
domain constructed by concatenating the selector, the string
'_bimi', and the Author Domain. A possibly empty set of records
is returned.
4. Records that do not start with a "v=" tag that identifies the
current version of BIMI MUST be discarded.
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5. If the set is now empty, the Client MUST query the DNS for a BIMI
TXT record at the DNS domain constructed by concatenating the
selector, the string '_bimi', and the Organizational Domain (as
defined in DMARC [RFC7489]) corresponding to the Author Domain.
A custom selector that does not exist falls back to
<selector>._bimi.<organizationalDomain>. A possibly empty set of
records is returned.
6. Records that do not start with a "v=" tag that identifies the
current version of BIMI MUST be discarded.
7. If the remaining set contains multiple records or no records,
Assertion Record Discovery terminates and BIMI processing MUST
NOT be performed for this message.
8. If the remaining set contains only a single record, this record
is used for BIMI Assertion.
7.3. Indicator Discovery.
1. If the retrieved Assertion Record does not include a valid bimi-
location in the l= tag, then Indicator Discovery has failed, and
the Indicator MUST NOT be displayed. The bimi-location entry
MUST be a URI with a HTTPS transport.
2. If the retrieved Assertion Record includes a bimi-evidence-
location entry in the a= tag, and the receiver supports BIMI
Evidence Document validation, then proceed to the Indicator
Discovery With Evidence (#indicator-discovery-with-evidence)
step.
3. If the receiver does not support BIMI Evidence Document
validation, or the retrieved Assertion Record does not include a
bimi-evidence-location entry, then proceed to the Indicator
Discovery Without Evidence (#indicator-discovery-without-
evidence) step.
7.4. Indicator Discovery With Evidence.
Individual types of BIMI Evidence Document MAY specify extra
discovery and validation steps. These will be defined in separate
documents.
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7.5. Indicator Discovery Without Evidence.
If an Assertion Record is found, and it has empty bimi-location and
bimi-evidence-location then this is a Declination to Publish record.
BIMI processing MUST not occur on this message and the MTA SHOULD
reflect this in the Authentication-Results header by adding a
bimi=declined entry.
If an Assertion Record is found, and has an empty or missing bimi-
evidence-location entry then no evidence has is presented, and the
Indicator MUST be retrieved from the URI specified in the bimi-
location entry using the following algorithm:
1. Retrieve the SVG Indicator from the URI specified in the l= tag.
This MUST be a URI with a HTTPS transport.
2. If the TLS server identity certificate presented during the TLS
session setup does not chain-up to a root certificate the Client
trusts then Indicator validation has failed and the Indicator
MUST NOT be displayed.
3. Proceed to the Indicator Validation (#indicator-validation) step.
7.6. Indicator Validation
1. Check the file size of the retrieved Indicator against
recommended maximum sizes as defined in this document, and in the
BIMI SVG document. A receiver MAY choose to implement their own
file size restrictions. If the Indicator is larger than the
maximum size the the receiver MAY choose not to display the
Indicator. A receiver MAY choose to implement the size limit as
a retrieval limit rather than retrieving the entire document and
then checking the size.
2. If the SVG Indicator is missing, or is not a valid SVG or SVGZ
document then validation has failed and the Indicator MUST NOT be
displayed.
3. Check the retrieved Indicator against the SVG validation steps
specified in this document, and in the BIMI SVG document.
4. If Indicator verification has passed, and the Indicator is from a
trusted source, then the Indicator MAY be displayed per receiver
policy.
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7.7. Affix BIMI Status to Authentication Results Header Field
Upon completion of Assertion Record Discovery, Indicator Discovery,
and Indicator Validation, an MTA SHOULD affix the result in the
Authentication-Results header using the following syntax, with the
following key=value pairs:
bimi: Result of the bimi lookup (plain-text; REQUIRED). Range of
values are 'pass' (BIMI successfully validated), 'none' (no BIMI
record present), 'fail' (syntax error in the BIMI record, failure in
Discovery or Validation steps, or some other error), 'temperror' (DNS
lookup problem), 'declined' (The domain owner published an explicit
declination record), or 'skipped' (BIMI check was not performed,
possibly because the message did not comply with the minimum
requirements such as passing DMARC, or the MTA does not trust the
sending domain). The MTA MAY put comments in parentheses after bimi
result, e.g., "bimi=fail (Invalid SVG)", "bimi=skipped (sender not
trusted)" or "bimi=skipped (message failed DMARC)".
header.d: Domain of the BIMI Assertion Record which was evaluated
(plain-text; REQUIRED if bimi=pass). For example, this will be the
organizational domain if the BIMI lookup used the fallback record,
otherwise it will be the RFC5322.From domain.
header.selector: Selector of the BIMI Assertion Record which was
evaluated (plain-text; REQUIRED if bimi=pass). For example, if a
BIMI-Selector Header was present and used to discover a BIMI
Assertion Record then this will be the Selector used, otherwise this
will be 'default'.
policy.authority: Authority verification status of the Brand
Identifier (plain-text; REQUIRED if the BIMI Evidence Document was
checked). If the Authority Evidence presented in the BIMI Assertion
Record was checked and found to be valid then this MUST be set to
pass. If the validation failed then this MUST be set to fail. If no
Authority Evidence was presented, or the MTA did not check the
Authority Evidence then this SHOULD be set to none.
policy.authority-uri: The URI of the BIMI Evidence Document checked,
as found in the a= tag of the BIMI Assertion Record (plain-text;
OPTIONAL).
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7.8. Handle Existing BIMI-Location and BIMI-Indicator Headers
Regardless of success of the BIMI lookup, if a BIMI-Location or a
BIMI-Indicator header is already present in a message it MUST be
either removed or renamed. This is because the MTA performing BIMI-
related processing immediately prior to a Mail Delivery Agent (or
within the same administrative realm) is the only entity allowed to
specify the BIMI-Location or BIMI-Indicator headers (e.g. not the
sending MTA, and not an intermediate MTA). Allowing one or more
existing headers through to a MUA is a security risk.
If the original email message had a DKIM signature, it has already
been evaluated. Removing the BIMI-Location header at this point
should not invalidate the signature since it should not be included
within it per this spec.
7.9. Construct BIMI-Location URI
This header MUST NOT be added if Discovery or Validation steps
failed.
The URI used to retrieve the validated SVG Indicator. If the
receiver extracted the Indicator from the BIMI Evidence Document then
this SHOULD be the bimi-evidence-location added with a a= tag,
otherwise it SHOULD be the bimi-location added with a l= tag. If
both a= and l= tags are included then the MTA MUST perform checks to
ensure that the SVG Indicator referenced by the bimi-location is
identical to the SVG Indicator extracted from the BIMI Evidence
Document.
7.10. Construct BIMI-Indicator header
This header MUST NOT be added if Discovery or Validation steps
failed.
Encode the SVG Indicator retrieved and validated during the Indicator
Discovery and Indicator Validation steps as base64 encoded data. If
the Indicator was compressed with gzip when retrieved then the data
MUST be uncompressed before being base64 encoded.
The MTA MUST fold the header to be within the line length limits of
SMTP [RFC5321].
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8. Security Considerations
The consistent use of Brand Indicators is valuable for Domain Owners,
Mail Receivers, and End Users. However, the routine display of brand
Indicators represents an attractive target for abuse, especially for
determined malicious actors. Great care is warranted. The
discussion following as an incomplete list of considerations.
8.1. Indirect Mail Flows
If a mail store ingests a message from another mail store through
some other means, the message may or may not have BIMI headers added
already. If the receiving store trusts the other mail store, it may
simply use existing headers. Or, it may re-evaluate BIMI policy and
requirements, and create or replace the BIMI-Location header.
8.2. Lookalike Domains and Copycat Indicators
Publishing BIMI records is not sufficient for an MTA to signal to the
MUA to load the BIMI Indicator. For example, the Domain Owner may
also need to have a sufficiently strong reputation with the MTA. The
receiver may use a manually maintained list of large brands, it may
import a list from a third party of acceptable domains, or it may
apply its own reputation heuristics before deciding whether or not to
load the BIMI Indicator. BIMI does not specify what MTAs may bring
to bear as additional factors.
8.3. Large files and buffer overflows
The MTA or MUA should perform some basic analysis and avoid loading
Indicators that are too large or too small. The Receiver may choose
to maintain a manual list and do the inspection of its list offline
so it doesn't have to do it at time-of-scan.
8.4. Slow DNS queries
All email Receivers already have to query for DNS records, and all of
them have built-in timeouts when performing DNS queries.
Furthermore, the use of caching when loading Indicators can help cut
down on load time. Virtually all email clients have some sort of
image-downloading built-in and make decisions when to load or not
load Indicators.
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8.5. Unaligned Indicators and asserting domains
There is no guarantee that a group responsible for managing Brand
Indicators will have access to put these Indicators directly in any
specific location of a domain, and requiring that Indicators live on
the asserted domain is too high a bar. Additionally, letting a brand
have Indicator locations outside its domain may be desirable so that
someone sending legitimate authenticated email on the Domain Owner's
behalf can manage and set selectors as an authorized third party
without requiring access to the Domain Owner's DNS or web services.
8.6. Unsigned BIMI-Selector Header
If a Domain Owner relies on SPF but not DKIM for email
authentication, then adding a requirement of DKIM may create too high
of a bar for that sender. On the other hand, Receivers doing BIMI
assertion may factor in the lack of DKIM signing when deciding
whether to add a BIMI-Location header.
8.7. CGI scripts in Indicator payload
MTAs and MVAs should aggressively police Indicators to ensure they
are the Indicators they claim to be, are within appropriate size
limits, and pass other sanity checks. Additionally, MTAs might cache
good Indicators and serve them directly to their MUAs, which would in
practice bypass any malicious dynamic payload set to trigger against
an end user but not an MTA.
8.8. Metadata in Indicators
Domain Owners should be careful to strip any metadata out of
published Indicators that they don't want to expose or which might
bloat file size. MTAs and MVAs might wish to inspect and remove such
data from Indicators before exposing them to end users.
9. IANA Considerations
IANA will need to reserve three new entries for the "Permanent
Message Header Field Names" registry and create a registry for
support file formats for BIMI.
9.1. Permanent Header Field Updates
Header field name: BIMI-Selector
Applicable protocol: mail
Status: standard
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Author/Change controller: IETF
Specification document: This one
Header field name: BIMI-Location
Applicable protocol: mail
Status: standard
Author/Change controller: IETF
Specification document: This one
Header field name: BIMI-Indicator
Applicable protocol: mail
Status: standard
Author/Change controller: IETF
Specification document: This one
9.2. Registry for Supported BIMI Formats
Names of support file types supported by BIMI must be registered by
IANA.
New entries are assigned only for values that have been documented in
a published RFC that has had IETF Review, per [IANA-CONSIDERATIONS].
Each method must register a name, the file extension, the
specification that defines it, and a description.
9.3. Other IANA needs
10. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
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[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008,
<https://www.rfc-editor.org/info/rfc5321>.
[RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598,
DOI 10.17487/RFC5598, July 2009,
<https://www.rfc-editor.org/info/rfc5598>.
[RFC6376] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
"DomainKeys Identified Mail (DKIM) Signatures", STD 76,
RFC 6376, DOI 10.17487/RFC6376, September 2011,
<https://www.rfc-editor.org/info/rfc6376>.
[RFC7208] Kitterman, S., "Sender Policy Framework (SPF) for
Authorizing Use of Domains in Email, Version 1", RFC 7208,
DOI 10.17487/RFC7208, April 2014,
<https://www.rfc-editor.org/info/rfc7208>.
[RFC7489] Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
Message Authentication, Reporting, and Conformance
(DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
<https://www.rfc-editor.org/info/rfc7489>.
[RFC8617] Andersen, K., Long, B., Ed., Blank, S., Ed., and M.
Kucherawy, Ed., "The Authenticated Received Chain (ARC)
Protocol", RFC 8617, DOI 10.17487/RFC8617, July 2019,
<https://www.rfc-editor.org/info/rfc8617>.
11. Informative References
[BIMI-OVERVIEW]
"An Overview of the Design of BIMI",
<http://tools.ietf.org/html/draft-bkl-bimi-overview-
00.html>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
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Appendix A. Example Selector Discovery (INFORMATIVE)
This section shows several examples of how a receiving MTA should
determine which Assertion Record to use depending on the BIMI-
Selector header.
A.1. No BIMI-Selector Header
The domain example.com does not send with a BIMI-Selector header.
From: sender@example.com
The MTA would lookup default._bimi.example.com for the BIMI DNS
record.
A.2. With BIMI-Selector Header
The domain example.com sends with a BIMI-Selector header:
From: sender@example.com
BIMI-Selector: v=BIMI1; s=selector;
The MTA would lookup selector._bimi.example.com.
A.3. Without BIMI-Selector Header on a subdomain
The domain foo.example.com sends without a BIMI-Selector header:
From: sender@foo.example.com
The MTA would lookup default._bimi.foo.example.com for the BIMI DNS
record. If it did not exist, it would lookup
default._bimi.example.com.
A.4. With BIMI-Selector Header on a subdomain
The domain foo.example.com sends without a BIMI-Selector header:
From: sender@foo.example.com
BIMI-Selector: v=BIMI1; s=myselector;
The MTA would lookup myselector._bimi.foo.example.com for the BIMI
DNS record. If it did not exist, it would fall back to the lookup
myselector._bimi.example.com.
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A.5. Invalid BIMI-Selector Header
The domain example.com sends with a BIMI-Selector header, but does
not include the required field 'v=':
From: sender@example.com
BIMI-Selector: s=myselector;
The MTA would ignore this header, and lookup
default._bimi.example.com.
Appendix B. Example Authentication-Results entry (INFORMATIONAL)
This section shows example Authentication-Results stamps based on
different BIMI lookup statuses.
B.1. Successful BIMI lookup
From: sender@example.com
BIMI-Selector: v=BIMI1; s=myselector;
Authentication-Results: bimi=pass header.d=example.com header.selector=myselector;
B.2. No BIMI record
From: sender@sub.example.com
Authentication-Results: bimi=none;
In this example, sub.example.com does not have a BIMI record at
default._bimi.sub.example.com, nor does default._bimi.example.com
B.3. Declination to Publish
From: sender@example.com
Authentication-Results: bimi=declined;
In this example the record found at default._bimi.example.com was
"v=BIMI1; l=; a=;", indicating a Declination to Publish a BIMI
Assertion Record, and so indicating that BIMI processing should not
occur on this message.
B.4. Subdomain has no default record, but organizational domain does
From: sender@sub.example.com
Authentication-Results: bimi=pass header.d=example.com header.selector=default;
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B.5. Subdomain and orgznizational domain have no record for selector,
but organization
domain has a default
From: sender@sub.example.com
BIMI-Selector: v=BIMI1; s=myselector;
Authentication-Results: bimi=none;
In this example, the sender specified a DNS record at
myselector._bimi.sub.example.com but it did not exist. The fallback
is to use myselector._bimi.example.com, which also does not exist.
The assertion record does exist for the default selector at the
organizational domain default._bimi.example.com, however this is not
used as the sender specified a selector of myselector.
B.6. Subdomain has no record for selector, but organization domain does
From: sender@sub.example.com
BIMI-Selector: v=BIMI1; s=myselector;
Authentication-Results: bimi=pass header.d=example.com header.selector=myselector;
In this example, the sender specified a DNS record at
myselector._bimi.sub.example.com but it did not exist. The fallback
is to use myselector._bimi.example.com.
Appendix C. Example BIMI Headers Construction (INFORMATIONAL)
This section shows how an example MTA might evaluate an incoming
email for BIMI participation, and how it could share that
determination with its MUA(s).
C.1. MTA Receives an email
The MTA receives the following DKIM signed message:
DKIM-Signature: v=1; s=myExample; d=example.com; h=From;BIMI-Selector;Date;bh=...;b=...
From: sender@example.com
BIMI-Selector: v=BIMI1; s=brand;
BIMI-Location: image.example.com/bimi/logo/example-bimi.svg
Subject: Hi, this is a message from the good folks at Example Learning
C.2. MTA does its authentication checks
The receiving MTA receives the message and performs an SPF
verification (which fails), a DKIM verification (which passes), and a
DMARC verification (which passes). The domain is verified and has
good reputation. The Receiver proceeds to perform a BIMI lookup.
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C.3. MTA performs BIMI Assertion
The MTA sees that the message has a BIMI-Selector header, and it is
covered by the DKIM-Signature, and the DKIM-Signature that passed
DKIM is the one that covers the BIMI-Selector header. The MTA sees
the header validates and contains 'v=BIMI1', and 's=brand'. It
performs a DNS query for brand._bimi.example.com and retrieves:
brand._bimi.example.com IN TXT "v=BIMI1; l=https://image.example.com/bimi/logo/"
The MTA verifies the syntax of the BIMI DNS record, and it, too
passes.
The MTA knows it has previously retrieved the Indicator referenced by
the BIMI DNS record, and had already successfully checked this
Indicator against the published SVG profile. The MTA retrieves the
Indicator from the cache.
C.4. MTA appends to Authentication-Results
The MTA computes and affixes the results of the BIMI to the
Authentication-Results header:
Authentication-Results: spf=fail smtp.mailfrom=example.com;
dkim=pass (signature was verified) header.d=example.com;
dmarc=pass header.from=example.com;
bimi=pass header.d=example.com header.selector=brand;
C.5. MTA Constructs BIMI-Location and BIMI-Indicator headers
The MTA base64 encodes the retrieved Indicator and constructs a new
BIMI-Indicator header.
The MTA constructs a BIMI-Location header with a version tag, and an
l tag indicating the URL from which the Indicator was retrieved.
Finally, the MTA removes any existing BIMI-Location and BIMI-
Indicator headers, and stamps the new ones:
BIMI-Location: v=BIMI1; l=https://image.example.com/bimi/logo/
BIMI-Indicator: PD94bW...8L3N2Zz4K
That the original sender signed a BIMI-Location header against this
spec is irrelevant. It was used for DKIM validation and then thrown
out by the MTA.
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C.6. The MUA displays the Indicator
The mail is opened from the mail store in an MUA. The MUA performs
locally defined checks to make sure that it can trust the BIMI-
Indicator header. Finally, the MUA extracts the Indicator from the
BIMI-Indicator header and displays it to the user.
Appendix D. Acknowledgements
Many people have contributed to the development of BIMI. Along with
thanks to members of the current AuthIndicators Working Group, the
editors wish to acknowledge the efforts of Sri Somanchi, Don
Cardinal, Steve Jones, and John Levine.
Authors' Addresses
Seth Blank
Valimail
Email: seth@valimail.com
Peter Goldstein
Valimail
Email: peter@valimail.com
Thede Loder
Skye Logicworks LLC
Email: thede@skyelogicworks.com
Terry Zink
Email: tzink@terryzink.com
Marc Bradshaw
Fastmail
Email: marc@fastmailteam.com
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