draft-ietf-dmarc-psd-09.txt   draft-ietf-dmarc-psd-10.txt 
Network Working Group S. Kitterman Network Working Group S. Kitterman
Internet-Draft fTLD Registry Services Internet-Draft fTLD Registry Services
Intended status: Experimental September 22, 2020 Intended status: Experimental January 23, 2021
Expires: March 26, 2021 Expires: July 27, 2021
DMARC (Domain-based Message Authentication, Reporting, and Conformance) Experimental DMARC Extension For Public Suffix Domains
Extension For PSDs (Public Suffix Domains) draft-ietf-dmarc-psd-10
draft-ietf-dmarc-psd-09
Abstract Abstract
DMARC (Domain-based Message Authentication, Reporting, and DMARC (Domain-based Message Authentication, Reporting, and
Conformance) is a scalable mechanism by which a mail-originating Conformance) is a scalable mechanism by which a mail-originating
organization can express domain-level policies and preferences for organization can express domain-level policies and preferences for
message validation, disposition, and reporting, that a mail-receiving message validation, disposition, and reporting, that a mail-receiving
organization can use to improve mail handling. The design of DMARC organization can use to improve mail handling. The design of DMARC
presumes that domain names represent either nodes in the tree below presumes that domain names represent either nodes in the tree below
which registrations occur, or nodes where registrations have which registrations occur, or nodes where registrations have
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 26, 2021. This Internet-Draft will expire on July 27, 2021.
Copyright Notice Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Example . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Discussion . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology and Definitions . . . . . . . . . . . . . . . . . 5 2. Terminology and Definitions . . . . . . . . . . . . . . . . . 5
2.1. Conventions Used in This Document . . . . . . . . . . . . 5 2.1. Conventions Used in This Document . . . . . . . . . . . . 5
2.2. Public Suffix Domain (PSD) . . . . . . . . . . . . . . . 5 2.2. Public Suffix Domain (PSD) . . . . . . . . . . . . . . . 5
2.3. Organizational Domain . . . . . . . . . . . . . . . . . . 5 2.3. Organizational Domain . . . . . . . . . . . . . . . . . . 6
2.4. Longest PSD . . . . . . . . . . . . . . . . . . . . . . . 5 2.4. Longest PSD . . . . . . . . . . . . . . . . . . . . . . . 6
2.5. Public Suffix Operator (PSO) . . . . . . . . . . . . . . 6 2.5. Public Suffix Operator (PSO) . . . . . . . . . . . . . . 6
2.6. PSO Controlled Domain Names . . . . . . . . . . . . . . . 6 2.6. PSO Controlled Domain Names . . . . . . . . . . . . . . . 6
2.7. Non-existent Domains . . . . . . . . . . . . . . . . . . 6 2.7. Non-existent Domains . . . . . . . . . . . . . . . . . . 6
3. PSD DMARC Updates to DMARC Requirements . . . . . . . . . . . 6 3. PSD DMARC Updates to DMARC Requirements . . . . . . . . . . . 6
3.1. General Updates . . . . . . . . . . . . . . . . . . . . . 6 3.1. General Updates . . . . . . . . . . . . . . . . . . . . . 7
3.2. Changes in Section 6.3 "General Record Format" . . . . . 6 3.2. Changes in Section 6.3 "General Record Format" . . . . . 7
3.3. Changes in Section 6.5 "Domain Owner Actions" . . . . . . 7 3.3. Changes in Section 6.5 "Domain Owner Actions" . . . . . . 7
3.4. Changes in Section 6.6.1 "Extract Author Domain" . . . . 7 3.4. Changes in Section 6.6.1 "Extract Author Domain" . . . . 7
3.5. Changes in Section 6.6.3 "Policy Discovery" . . . . . . . 7 3.5. Changes in Section 6.6.3 "Policy Discovery" . . . . . . . 8
3.6. Changes in Section 7 "DMARC Feedback" . . . . . . . . . . 8 3.6. Changes in Section 7 "DMARC Feedback" . . . . . . . . . . 8
4. Privacy Considerations . . . . . . . . . . . . . . . . . . . 8 4. Privacy Considerations . . . . . . . . . . . . . . . . . . . 8
4.1. Feedback leakage . . . . . . . . . . . . . . . . . . . . 8 4.1. Feedback leakage . . . . . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9 5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6.1. Subdomain Policy Tag . . . . . . . . . . . . . . . . . . 10 6.1. Subdomain Policy Tag . . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 7. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . 10 7.1. Normative References . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . 10 7.2. Informative References . . . . . . . . . . . . . . . . . 11
Appendix A. PSD DMARC Privacy Concern Mitigation Experiment . . 11 Appendix A. PSD DMARC Privacy Concern Mitigation Experiment . . 12
Appendix B. DMARC PSD Registry Examples . . . . . . . . . . . . 12 Appendix B. DMARC PSD Registry Examples . . . . . . . . . . . . 12
B.1. DMARC PSD DNS Query Service . . . . . . . . . . . . . . . 12 B.1. DMARC PSD DNS Query Service . . . . . . . . . . . . . . . 12
B.2. DMARC Public Suffix Domain (PSD) Registry . . . . . . . . 12 B.2. DMARC Public Suffix Domain (PSD) Registry . . . . . . . . 12
B.3. DMARC PSD PSL Extension . . . . . . . . . . . . . . . . . 13 B.3. DMARC PSD PSL Extension . . . . . . . . . . . . . . . . . 13
Appendix C. Implementations . . . . . . . . . . . . . . . . . . 13 Appendix C. Implementations . . . . . . . . . . . . . . . . . . 13
C.1. Authheaders Module . . . . . . . . . . . . . . . . . . . 13 C.1. Authheaders Module . . . . . . . . . . . . . . . . . . . 13
C.2. Zdkimfilter Module . . . . . . . . . . . . . . . . . . . 13 C.2. Zdkimfilter Module . . . . . . . . . . . . . . . . . . . 14
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 13 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction 1. Introduction
DMARC [RFC7489] provides a mechanism for publishing organizational DMARC [RFC7489] provides a mechanism for publishing organizational
policy information to email receivers. DMARC allows policy to be policy information to email receivers. DMARC allows policy to be
specified for both individual domains and for organizational domains specified for both individual domains and for organizational domains
and their sub-domains within a single organization. DMARC leverages and their sub-domains within a single organization.
public suffix lists to determine which domains are organizational
domains. It presumes that public suffix list listed domains are not To determine the organizational domain for a message under
organizational domains and not subject to DMARC processing; domains evaluation, and thus where to look for a policy statement, DMARC
are either organizational domains, sub-domains of organizational makes use of a Public Suffix List. The process for doing this can be
domains, or listed on a public suffix list. For domains listed in a found in Section 3.2 of the DMARC specification.
public suffix list, i.e. TLDs and domains that exist between TLDs and
organization level domains, policy can only be published for the DMARC as specified presumes that domain names present in a PSL are
exact domain. No method is available for these domains to express not organizational domains and thus not subject to DMARC processing;
policy or receive feedback reporting for sub-domains. This missing domains are either organizational domains, sub-domains of
method allows for the abuse of non-existent organizational-level organizational domains, or listed on a PSL. For domains listed in a
domains and prevents identification of domain abuse in email. PSL, i.e., TLDs and domains that exist between TLDs and organization
level domains, policy can only be published for the exact domain. No
method is available for these domains to express policy or receive
feedback reporting for sub-domains. This missing method allows for
the abuse of non-existent organizational-level domains and prevents
identification of domain abuse in email.
This document specifies experimental updates to the DMARC and PSL
algorithm cited above, in an attempt to mitigate this abuse.
1.1. Example
As an example, imagine a country code TLD (ccTLD) which has public As an example, imagine a country code TLD (ccTLD) which has public
subdomains for government and commercial use (.gov.example and subdomains for government and commercial use (".gov.example" and
.com.example). Suppose there exists a registered domain ".com.example"). A PSL whose maintainer is aware of this country's
"tax.gov.example" that is responsible for taxation in this imagined domain structurewould include entries for both of these in the PSL,
country. However, by exploiting the typically unauthenticated nature indicating that they are PSDs below which registrations can occur.
of email, there are regular malicious campaigns to impersonate this Suppose further that there exists a domain "tax.gov.example",
registered within ".gov.example", that is responsible for taxation in
this imagined country.
However, by exploiting the typically unauthenticated nature of email,
there are regular malicious campaigns to impersonate this
organization that use similar-looking ("cousin") domains such as organization that use similar-looking ("cousin") domains such as
"t4x.gov.example". These domains are not registered. Within the "t4x.gov.example". Such domains are not registered.
".gov.example" public suffix, use of DMARC has been mandated, so
"gov.example" publishes the following DMARC DNS record: Within the ".gov.example" public suffix, use of DMARC has been
mandated, so "gov.example" publishes the following DMARC DNS record:
_dmarc.gov.example. IN TXT ( "v=DMARC1; p=reject; " _dmarc.gov.example. IN TXT ( "v=DMARC1; p=reject; "
"rua=mailto:dmc@dmarc.svc.gov.example" ) "rua=mailto:dmc@dmarc.svc.gov.example" )
This DMARC record provides policy and a reporting destination for This DMARC record provides policy and a reporting destination for
mail sent from @gov.example. However, due to DMARC's current method mail sent from @gov.example. However, due to DMARC's current method
of discovering and applying policy at the organizational domain of discovering and applying policy at the organizational domain
level, the non-existent organizational domain of @t4x.gov.example level, the non-existent organizational domain of @t4x.gov.example
does not and cannot fall under a DMARC policy. does not and cannot fall under a DMARC policy.
Defensively registering all variants of "tax" is obviously not a Defensively registering all variants of "tax" is obviously not a
scalable strategy. The intent of this specification, therefore, is scalable strategy. The intent of this specification, therefore, is
to enhance the DMARC algorithm by enabling an agent receiving such a to enhance the DMARC algorithm by enabling an agent receiving such a
message to be able to determine that a relevant policy is present at message to be able to determine that a relevant policy is present at
"gov.example", which is precluded by the current DMARC algorithm. "gov.example", which is precluded by the current DMARC algorithm.
1.2. Discussion
This document provides a simple extension to DMARC [RFC7489] to allow This document provides a simple extension to DMARC [RFC7489] to allow
operators of Public Suffix Domains (PSDs) to: operators of Public Suffix Domains (PSDs) to:
o Express policy at the level of the PSD that covers all o Express policy at the level of the PSD that covers all
organizational domains that do not explicitly publish DMARC organizational domains that do not explicitly publish DMARC
records records
o Extends the DMARC policy query functionality to detect and process o Extends the DMARC policy query functionality to detect and process
such a policy such a policy
o Describes receiver feedback for such policies o Describes receiver feedback for such policies
o Provides controls to mitigate potential privacy considerations o Provides controls to mitigate potential privacy considerations
associated with this extension associated with this extension
This document also provides a new DMARC [RFC7489] tag to indicate This document also provides a new DMARC tag to indicate requested
requested handling policy for non-existent subdommains. This is handling policy for non-existent subdommains. This is provided
provided specifically to support phased deployment of PSD DMARC, but specifically to support phased deployment of PSD DMARC, but is
is expected to be useful more generally. Undesired rejection risks expected to be useful more generally. Undesired rejection risks for
for mail purporting to be from domains that do not exist are mail purporting to be from domains that do not exist are
substantially lower than for those that do, so the operational risk substantially lower than for those that do, so the operational risk
of requesting harsh policy treatment (e.g. reject) is lower. of requesting harsh policy treatment (e.g. reject) is lower.
As an additional benefit, the PSD DMARC extension clarifies existing As an additional benefit, the PSD DMARC extension clarifies existing
requirements. Based on the requirements of DMARC [RFC7489], DMARC requirements. Based on the requirements of DMARC [RFC7489], DMARC
should function above the organizational level for exact domain should function above the organizational level for exact domain
matches (i.e. if a DMARC record were published for 'example', then matches (i.e. if a DMARC record were published for 'example', then
mail from example@example should be subject to DMARC processing). mail from example@example should be subject to DMARC processing).
Testing had revealed that this is not consistently applied in Testing had revealed that this is not consistently applied in
different implementations. different implementations.
There are two types of Public Suffix Operators (PSOs) for which this There are two types of Public Suffix Operators (PSOs) for which this
extension would be useful and appropriate: extension would be useful and appropriate:
o Branded PSDs (e.g., ".google"): These domains are effectively o Branded PSDs (e.g., ".google"): These domains are effectively
Organizational Domains as discussed in DMARC [RFC7489]. They Organizational Domains as discussed in DMARC [RFC7489]. They
control all subdomains of the tree. These are effectively private control all subdomains of the tree. These are effectively private
domains, but listed in the Public Suffix List. They are treated domains, but listed in the Public Suffix List. They are treated
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as DMARC Organizational Domains, but are currently unable to as DMARC Organizational Domains, but are currently unable to
benefit from DMARC. benefit from DMARC.
o Multi-organization PSDs that require DMARC usage (e.g., ".bank"): o Multi-organization PSDs that require DMARC usage (e.g., ".bank"):
Because existing Organizational Domains using this PSD have their Because existing Organizational Domains using this PSD have their
own DMARC policy, the applicability of this extension is for non- own DMARC policy, the applicability of this extension is for non-
existent domains. The extension allows the brand protection existent domains. The extension allows the brand protection
benefits of DMARC to extend to the entire PSD, including cousin benefits of DMARC to extend to the entire PSD, including cousin
domains of registered organizations. domains of registered organizations.
Due to the design of DMARC [RFC7489] and the nature of the Internet Due to the design of DMARC and the nature of the Internet email
email architecture [RFC5598], there are interoperability issues architecture [RFC5598], there are interoperability issues associated
associated with DMARC [RFC7489] deployment. These are discussed in with DMARC [RFC7489] deployment. These are discussed in
Interoperability Issues between DMARC and Indirect Email Flows Interoperability Issues between DMARC and Indirect Email Flows
[RFC7960]. These issues are not typically applicable to PSDs, since [RFC7960]. These issues are not typically applicable to PSDs, since
they (e.g., the ".gov.example" used above) do not typically send they (e.g., the ".gov.example" used above) do not typically send
mail. mail.
2. Terminology and Definitions 2. Terminology and Definitions
This section defines terms used in the rest of the document. This section defines terms used in the rest of the document.
2.1. Conventions Used in This Document 2.1. Conventions Used in This Document
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PSD. PSD.
2.3. Organizational Domain 2.3. Organizational Domain
The term Organizational Domains is defined in DMARC [RFC7489] The term Organizational Domains is defined in DMARC [RFC7489]
Section 3.2. Section 3.2.
2.4. Longest PSD 2.4. Longest PSD
The longest PSD is the Organizational Domain with one label removed. The longest PSD is the Organizational Domain with one label removed.
It names the immediate parent node of the Organizational Domain in
the DNS namespace tree.
2.5. Public Suffix Operator (PSO) 2.5. Public Suffix Operator (PSO)
A Public Suffix Operator is an organization which manages operations A Public Suffix Operator is an organization which manages operations
within a PSD, particularly the DNS records published for names at and within a PSD, particularly the DNS records published for names at and
under that domain name. under that domain name.
2.6. PSO Controlled Domain Names 2.6. PSO Controlled Domain Names
PSO Controlled Domain Names are names in the DNS that are managed by PSO Controlled Domain Names are names in the DNS that are managed by
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".co.uk") name components, depending on PSD policy. ".co.uk") name components, depending on PSD policy.
2.7. Non-existent Domains 2.7. Non-existent Domains
For DMARC purposes, a non-existent domain is a domain for which there For DMARC purposes, a non-existent domain is a domain for which there
is an NXDOMAIN or NODATA response for A, AAAA, and MX records. This is an NXDOMAIN or NODATA response for A, AAAA, and MX records. This
is a broader definition than that in NXDOMAIN [RFC8020]. is a broader definition than that in NXDOMAIN [RFC8020].
3. PSD DMARC Updates to DMARC Requirements 3. PSD DMARC Updates to DMARC Requirements
This document updates DMARC [RFC7489] as follows: This document updates DMARC as follows:
3.1. General Updates 3.1. General Updates
References to "Domain Owners" also apply to PSOs. References to "Domain Owners" also apply to PSOs.
3.2. Changes in Section 6.3 "General Record Format" 3.2. Changes in Section 6.3 "General Record Format"
A new tag is added after "fo": A new tag is added after "fo":
np: Requested Mail Receiver policy for non-existent subdomains np: Requested Mail Receiver policy for non-existent subdomains
(plain-text; OPTIONAL). Indicates the policy to be enacted by the (plain-text; OPTIONAL). Indicates the policy to be enacted by the
Receiver at the request of the Domain Owner. It applies only to Receiver at the request of the Domain Owner. It applies only to
non-existent subdomains of the domain queried and not to either non-existent subdomains of the domain queried and not to either
existing subdomains or the domain itself. Its syntax is identical existing subdomains or the domain itself. Its syntax is identical
to that of the "p" tag defined below. If the 'np' tag is absent, to that of the "p" tag defined below. If the 'np' tag is absent,
the policy specified by the "sp" tag (if the 'sp' tag is present) the policy specified by the "sp" tag (if the 'sp' tag is present)
or the policy specified by the "p" tag, if the 'sp' tag is not or the policy specified by the "p" tag, if the 'sp' tag is not
present, MUST be applied for non-existent subdomains. Note that present, MUST be applied for non-existent subdomains. Note that
"np" will be ignored for DMARC records published on subdomains of "np" will be ignored for DMARC records published on subdomains of
Organizational Domains and PSDs due to the effect of the DMARC Organizational Domains and PSDs due to the effect of the DMARC
policy discovery mechanism described in DMARC [RFC7489] policy discovery mechanism described in DMARC Section 6.6.3.
Section 6.6.3.
The following tag definitions from DMARC [RFC7489] are updated: The following tag definitions from DMARC are updated:
p: The sentence 'Policy applies to the domain queried and to p: The sentence 'Policy applies to the domain queried and to
subdomains, unless subdomain policy is explicitly described using subdomains, unless subdomain policy is explicitly described using
the "sp" tag' is updated to read 'Policy applies to the domain the "sp" tag' is updated to read 'Policy applies to the domain
queried and to subdomains, unless subdomain policy is explicitly queried and to subdomains, unless subdomain policy is explicitly
described using the "sp" or "np" tags.' described using the "sp" or "np" tags.'
sp: The sentence 'If absent, the policy specified by the "p" tag sp: The sentence 'If absent, the policy specified by the "p" tag
MUST be applied for subdomains' is updated to read 'If both the MUST be applied for subdomains' is updated to read 'If both the
'sp' tag is absent and the 'np' tag is either absent or not 'sp' tag is absent and the 'np' tag is either absent or not
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for doing so. See the [this document] experiment description for doing so. See the [this document] experiment description
(Appendix A). (Appendix A).
3.4. Changes in Section 6.6.1 "Extract Author Domain" 3.4. Changes in Section 6.6.1 "Extract Author Domain"
Experience with DMARC has shown that some implementations short- Experience with DMARC has shown that some implementations short-
circuit messages, bypassing DMARC policy application, when the domain circuit messages, bypassing DMARC policy application, when the domain
name extracted by the receiver (from the RFC5322.From) is on the name extracted by the receiver (from the RFC5322.From) is on the
public suffix list used by the receiver. This negates the capability public suffix list used by the receiver. This negates the capability
being created by this specification. Therefore, the following being created by this specification. Therefore, the following
paragraph is appended to Section 6.6.1 of DMARC [RFC7489]: paragraph is appended to Section 6.6.1 of DMARC:
Note that domain names that appear on a public suffix list are not Note that domain names that appear on a public suffix list are not
exempt from DMARC policy application and reporting. exempt from DMARC policy application and reporting.
3.5. Changes in Section 6.6.3 "Policy Discovery" 3.5. Changes in Section 6.6.3 "Policy Discovery"
A new step between step 3 and 4 is added: A new step between step 3 and 4 is added:
3A. If the set is now empty and the longest PSD (Section 2.4) of the 3A. If the set is now empty and the longest PSD (Section 2.4) of the
Organizational Domain is one that the receiver has determined is Organizational Domain is one that the receiver has determined is
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