Network Working Group S. Kitterman
Internet-Draft fTLD Registry Services
Intended status: Experimental July 27, 2019
Expires: January 28, 2020
DMARC (Domain-based Message Authentication, Reporting, and Conformance)
Extension For PSDs (Public Suffix Domains)
draft-ietf-dmarc-psd-05
Abstract
DMARC (Domain-based Message Authentication, Reporting, and
Conformance) is a scalable mechanism by which a mail-originating
organization can express domain-level policies and preferences for
message validation, disposition, and reporting, that a mail-receiving
organization can use to improve mail handling. DMARC policies can be
applied to individual domains or to all domains within an
organization. The design of DMARC precludes grouping policies for
domains based on policy published above the organizational level,
such as TLDs (Top Level Domains). Domains at this higher level of
the DNS tree (but not necessarily at the top of the DNS tree) can be
collectively referred to as Public Suffix Domains (PSDs). This
document describes an extension to DMARC to enable DMARC
functionality PSDs.
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
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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 January 28, 2020.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology and Definitions . . . . . . . . . . . . . . . . . 5
2.1. Conventions Used in This Document . . . . . . . . . . . . 5
2.2. Public Suffix Domain (PSD) . . . . . . . . . . . . . . . 5
2.3. Longest PSD . . . . . . . . . . . . . . . . . . . . . . . 5
2.4. Public Suffix Operator (PSO) . . . . . . . . . . . . . . 5
2.5. PSO Controlled Domain Names . . . . . . . . . . . . . . . 5
2.6. Non-existent Domains . . . . . . . . . . . . . . . . . . 5
3. PSD DMARC Updates to DMARC Requirements . . . . . . . . . . . 6
3.1. General Updates . . . . . . . . . . . . . . . . . . . . . 6
3.2. Section 6.1 DMARC Policy Record . . . . . . . . . . . . . 6
3.3. Section 6.3 General Record Format . . . . . . . . . . . . 6
3.4. Section 6.5. Domain Owner Actions . . . . . . . . . . . 7
3.5. Section 6.6.3. Policy Discovery . . . . . . . . . . . . 7
3.6. Section 7. DMARC Feedback . . . . . . . . . . . . . . . 7
4. Privacy Considerations . . . . . . . . . . . . . . . . . . . 7
4.1. Feedback leakage . . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6.1. Subdomain Policy Tag . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. The Experiment . . . . . . . . . . . . . . . . . . . 11
A.1. PSD DMARC Privacy Concern Mitigation . . . . . . . . . . 11
A.2. Non-Existent Subdomain Policy . . . . . . . . . . . . . . 12
Appendix B. DMARC PSD Registry Examples . . . . . . . . . . . . 12
B.1. DMARC PSD DNS Query Service . . . . . . . . . . . . . . . 12
B.2. DMARC Public Suffix Domain (PSD) Registry . . . . . . . . 12
Appendix C. Implementation . . . . . . . . . . . . . . . . . . . 13
C.1. Authheaders Module . . . . . . . . . . . . . . . . . . . 13
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 13
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
DMARC [RFC7489] provides a mechanism for publishing organizational
policy information to email receivers. DMARC allows policy to be
specified for both individual domains and for organizational domains
and their sub-domains within a single organization. For 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 such 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.
As an example, imagine a country code TLD (ccTLD) which has public
subdomains for government and commercial use (.gov.example and
.com.example). Within the .gov.example public suffix, use of DMARC
has been mandated and .gov.example has published its own DMARC
record:
"v=DMARC1; p=reject; rua=mailto:dmarc@dmarc.service.gov.example"
at
_dmarc.gov.example.
This DMARC record provides policy and a reporting destination for
mail sent from @gov.example. However, due to DMARC's current method
of discovering and applying policy at the organizational domain
level, the non-existent organizational domain of @tax.gov.example
does not and cannot fall under a DMARC policy.
This document provides a simple extension to DMARC [RFC7489] to allow
operators of Public Suffix Domains (PSDs) to express policy at the
level of the PSD that covers all organizational domains that do not
explicitly publish DMARC records, extends the DMARC policy query
functionality to detect and process such a policy, describes receiver
feedback for such policies, and provides controls to mitigate
potential privacy considerations associated with this extension.
This document also provides a new DMARC [RFC7489] tag to indicate
requested handling policy for non-existent subdommains. This is
provided specifically to support phased deployment of PSD DMARC, but
is expected to be useful more generally. Undesired rejection risks
for mail purporting to be from domains that do not exist are
substantially lower than for those that do, so the operational risk
of requesting harsh policy treatment (e.g. reject) is lower.
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As an additional benefit, the PSD DMARC extension clarifies existing
requirements. Based on the requirements of DMARC [RFC7489], DMARC
should function above the organizational level for exact domain
matches (i.e. if a DMARC record were published for 'example', then
mail from example@example should be subject to DMARC processing).
Testing had revealed that this is not consistently applied in
different implementations.
There are two types of Public Suffix Operators (PSOs) for which this
extension would be useful and appropriate:
o Branded PSDs (e.g., ".google"): These domains are effectively
Organizational Domains as discussed in DMARC [RFC7489]. They
control all subdomains of the tree. These are effectively private
domains, but listed in the Public Suffix List. They are treated
as Public for DMARC purposes. They require the same protections
as DMARC Organizational Domains, but are currently excluded.
o Multi-organization PSDs that require DMARC usage (e.g., ".bank"):
Because existing Organizational Domains using this PSD have their
own DMARC policy, the applicability of this extension is for non-
existent domains. The extension allows the brand protection
benefits of DMARC to extend to the entire PSD, including cousin
domains of registered organizations.
As an example, take the ".gov.example" described above and add that
for this entity emails about tax returns are sent from
tax.gov.example. It would not be surprising if fraudulent emails
were sent purporting to be from taxes.gov.example (taxes is a cousin
of tax - different enough to evade DMARC protection, but similar
enough to potentially confuse users). As defined in DMARC [RFC7489],
a DMARC record could be published for tax.gov.example, but there is
no general solution to publishing a DMARC policy to defend against
abuse of non-existent cousins such as taxes.gov.example. While an
explicit record could be published for this one domain, the universe
of possible cousins is such that this approach does not scale.
Due to the design of DMARC [RFC7489] and the nature of the Internet
email architecture [RFC5598], there are interoperability issues
associated with DMARC [RFC7489] deployment. These are discussed in
Interoperability Issues between DMARC and Indirect Email Flows
[RFC7960]. These issues are not typically applicable to PSDs, since
they (e.g., the ".gov.example" used above) do not typically send
mail.
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2. Terminology and Definitions
This section defines terms used in the rest of the document.
2.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2.2. Public Suffix Domain (PSD)
The global Internet Domain Name System (DNS) is documented in
numerous Requests for Comment (RFC). It defines a tree of names
starting with root, ".", immediately below which are Top Level Domain
names such as ".com" and ".us". They are not available for private
registration. In many cases the public portion of the DNS tree is
more than one level deep.
2.3. Longest PSD
Organizational Domain (DMARC [RFC7489] Section 3.2) with one left-
most label removed.
2.4. Public Suffix Operator (PSO)
A Public Suffix Operator manages operations within its PSD.
2.5. PSO Controlled Domain Names
PSO Controlled Domain Names are names in the DNS that are managed by
a PSO and are not available for use as Organizational Domains (the
term Organizational Domains is defined in DMARC [RFC7489]
Section 3.2). Depending on PSD policy, these will have one (e.g.,
".com") or more (e.g., ".co.uk") name components.
2.6. Non-existent Domains
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 a broader definition than that in NXDOMAIN [RFC8020].
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3. PSD DMARC Updates to DMARC Requirements
This document updates DMARC [RFC7489] as follows:
3.1. General Updates
References to "Domain Owners" also apply to PSOs.
3.2. Section 6.1 DMARC Policy Record
PSD DMARC records are published as a subdomain of the PSD. For the
PSD ".example", the PSO would post DMARC policy in a TXT record at
"_dmarc.example".
3.3. Section 6.3 General Record Format
A new tag is added after fo:
np: Requested Mail Receiver policy for non-existent subdomains
(plain-text; OPTIONAL). Indicates the policy to be enacted by the
Receiver at the request of the Domain Owner. It applies only to
non-existent subdomains of the domain queried and not to either
existing subdomains or the domain itself. Its syntax is identical
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)
or the policy specified by the "p" tag, if the 'sp' tag is not
present, MUST be applied for non-existent subdomains. Note that
"np" will be ignored for DMARC records published on subdomains of
Organizational Domains and PSDs due to the effect of the DMARC
policy discovery mechanism described in DMARC [RFC7489]
Section 6.6.3.
The following tag definitions from DMARC [RFC7489] are updated:
p: The sentence 'Policy applies to the domain queried and to
subdomains, unless subdomain policy is explicitly described using
the "sp" tag' is updated to read 'Policy applies to the domain
queried and to subdomains, unless subdomain policy is explicitly
described using the "sp" or "np" tags.'
sp: The sentence 'If absent, the policy specified by the "p" tag
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
applicable, the policy specified by the "p" tag MUST be applied
for subdomains.
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3.4. Section 6.5. Domain Owner Actions
In addition to the DMARC domain owner actions, PSOs that require use
of DMARC and participate in PSD DMARC ought to make that information
available to receivers. The mechanism for doing so is one of the
experimental elements of this document. See the experiment
description (Appendix A).
3.5. Section 6.6.3. Policy Discovery
A new step between step 3 and 4 is added:
3A. If the set is now empty and the longest PSD (Section 2.3) of the
Organizational Domain is one that the receiver has determined is
acceptable for PSD DMARC (discussed in the experiment description
(Appendix A)), the Mail Receiver MUST query the DNS for a DMARC
TXT record at the DNS domain matching the longest PSD
(Section 2.3) in place of the RFC5322.From domain in the message
(if different). A possibly empty set of records is returned.
As an example, for a message with the Organizational Domain of
"example.compute.cloudcompany.com.cctld", the query for PSD DMARC
would use "compute.cloudcompany.com.cctld" as the longest PSD
(Section 2.3). The receiver would check to see if that PSD is listed
in the DMARC PSD Registry, and if so, perform the policy lookup at
"_dmarc.compute.cloudcompany.com.cctld".
Note: Because the PSD policy query comes after the Organizational
Domain policy query, PSD policy is not used for Organizational
domains that have published a DMARC policy. Specifically, this is
not a mechanism to provide feedback addresses (RUA/RUF) when an
Organizational Domain has declined to do so.
3.6. Section 7. DMARC Feedback
Operational note for PSD DMARC: For PSOs, feedback for non-existent
domains is desirable and useful, just as it is for org-level DMARC
operators. See Section 4 of this document for discussion of Privacy
Considerations.
4. Privacy Considerations
These privacy considerations are developed based on the requirements
of [RFC6973]. The Privacy Considerations of [RFC7489] apply to this
document.
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4.1. Feedback leakage
Providing feedback reporting to PSOs can, in some cases, create
leakage of information outside of an organization to the PSO. This
leakage could potentially be utilized as part of a program of
pervasive surveillance (See [RFC7624]). There are roughly three
cases to consider:
o Single Organization PSDs (e.g., ".google"), RUA and RUF reports
based on PSD DMARC have the potential to contain information about
emails related to entities managed by the organization. Since
both the PSO and the Organizational Domain owners are common,
there is no additional privacy risk for either normal or non-
existent Domain reporting due to PSD DMARC.
o Multi-organization PSDs that require DMARC usage (e.g., ".bank"):
PSD DMARC based reports will only be generated for domains that do
not publish a DMARC policy at the organizational or host level.
For domains that do publish the required DMARC policy records, the
feedback reporting addresses (RUA and RUF) of the organization (or
hosts) will be used. The only direct feedback leakage risk for
these PSDs are for Organizational Domains that are out of
compliance with PSD policy. Data on non-existent cousin domains
would be sent to the PSO.
o Multi-organization PSDs (e.g., ".com") that do not mandate DMARC
usage: Privacy risks for Organizational Domains that have not
deployed DMARC within such PSDs are significant. For non-DMARC
Organizational Domains, all DMARC feedback will be directed to the
PSO. PSD DMARC is opt-out (by publishing a DMARC record at the
Organizational Domain level) vice opt-in, which would be the more
desirable characteristic. This means that any non-DMARC
organizational domain would have its feedback reports redirected
to the PSO. The content of such reports, particularly for
existing domains, is privacy sensitive.
PSOs will receive feedback on non-existent domains, which may be
similar to existing Organizational Domains. Feedback related to such
cousin domains have a small risk of carrying information related to
an actual Organizational Domain. To minimize this potential concern,
PSD DMARC feedback is best limited to Aggregate Reports. Feedback
Reports carry more detailed information and present a greater risk.
Due to the inherent Privacy and Security risks associated with PSD
DMARC for Organizational Domains in multi-organization PSDs that do
not particpate in DMARC, any Feedback Reporting related to multi-
organizational PSDs ought to be limited to non-existent domains
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except in cases where the reporter knows that PSO requires use of
DMARC.
5. Security Considerations
This document does not change the Security Considerations of
[RFC7489] and [RFC7960].
The risks of the issues identified in [RFC7489], Section 12.3, DNS
Security, are amplified by PSD DMARC. In particular, DNS cache
poisoning (or Name Chaining), see [RFC3833] for details, consequences
are increased because a sucessful attack would potentially have a
much wider scope.
The risks of the issues identified in [RFC7489], Section 12.5,
External Reporting Addresses, are amplified by PSD DMARC. By design,
PSD DMARC causes unrequested reporting of feedback to entities
external to the Organizational Domain. This is discussed in more
detail in Section 4.
6. IANA Considerations
This section describes actions requested to be completed by IANA.
6.1. Subdomain Policy Tag
IANA is requested to add a new tag to DMARC Tag Registry in the
Domain-based Message Authentication, Reporting, and Conformance
(DMARC) Parameters Registry.
The entry is as follows:
+----------+-----------+---------+-------------------------------+
| Tag Name | Reference | Status | Description |
+----------+-----------+---------+-------------------------------+
| np | this | current | Requested handling policy for |
| | document | | non-existent subdomains |
+----------+-----------+---------+-------------------------------+
7. References
7.1. Normative References
[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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[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>.
[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>.
7.2. Informative References
[psddmarc.org]
multiple, "PSD DMARC Web Site", April 2019,
<https://psddmarc.org/>.
[PSL] multiple, "Public Suffix List", April 2019,
<https://publicsuffix.org/>.
[RFC3833] Atkins, D. and R. Austein, "Threat Analysis of the Domain
Name System (DNS)", RFC 3833, DOI 10.17487/RFC3833, August
2004, <https://www.rfc-editor.org/info/rfc3833>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<https://www.rfc-editor.org/info/rfc5226>.
[RFC5598] Crocker, D., "Internet Mail Architecture", RFC 5598,
DOI 10.17487/RFC5598, July 2009,
<https://www.rfc-editor.org/info/rfc5598>.
[RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
Morris, J., Hansen, M., and R. Smith, "Privacy
Considerations for Internet Protocols", RFC 6973,
DOI 10.17487/RFC6973, July 2013,
<https://www.rfc-editor.org/info/rfc6973>.
[RFC7624] Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
Trammell, B., Huitema, C., and D. Borkmann,
"Confidentiality in the Face of Pervasive Surveillance: A
Threat Model and Problem Statement", RFC 7624,
DOI 10.17487/RFC7624, August 2015,
<https://www.rfc-editor.org/info/rfc7624>.
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[RFC7960] Martin, F., Ed., Lear, E., Ed., Draegen. Ed., T., Zwicky,
E., Ed., and K. Andersen, Ed., "Interoperability Issues
between Domain-based Message Authentication, Reporting,
and Conformance (DMARC) and Indirect Email Flows",
RFC 7960, DOI 10.17487/RFC7960, September 2016,
<https://www.rfc-editor.org/info/rfc7960>.
[RFC8020] Bortzmeyer, S. and S. Huque, "NXDOMAIN: There Really Is
Nothing Underneath", RFC 8020, DOI 10.17487/RFC8020,
November 2016, <https://www.rfc-editor.org/info/rfc8020>.
Appendix A. The Experiment
There are two experimental questions addressed in this document: one
regarding mitigation of PSD related privacy concerns and the other on
the utility of specifying separate DMARC policies for non-existent
sub-domains.
Aditionally, as of the writing of this document operational and
policy constraints prevent this experiment from being deployed
globally. If the experiment shows that PSD DMARC solves a real
problem and can be used at a large scale, the results could prove to
be useful in removing constraints outside of the IETF that would
permit broader deployment".
A.1. PSD DMARC Privacy Concern Mitigation
To mitigate the privacy concerns associated with Multi-organization
PSDs that do not mandate DMARC usage, see Section 4.1, a mechanism to
indicate which PSDs do not present this privacy risk is appropriate.
There are multiple approaches that are possible.
The experiment is to evaluate different possible approaches. The
experiment will be complete when there is rough consensus on a
technical approach that is demonstrated to be operationally usable
and effective at mitigating the privacy concern.
The mechanism needs the following attributes:
o Be reliably, publicly accessible
o Be under configuration control based on a public set of criteria
o List PSDs that either mandate DMARC for their registrants or for
which all lower level domains are controlled by the PSO and that
the relevant PSO has indicated a desire for the PSD to participate
in PSD DMARC
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o Have a small operational footprint (e.g. provide a documented,
lightweight mechanism for developers and operators to retrieve the
list of PSD DMARC participants)
o Not allow PSO to add PSDs to the PSD DMARC participants list
without third party review
As of this writing, three approaches have been proposed. None of
them are ideal:
o An extension to the Public Suffix List at [PSL]
o A dedicated registry queried via DNS - an example of such a
service is described in Appendix B.1 below
o An IANA registry
A.2. Non-Existent Subdomain Policy
PSOs that plan to implement PSD DMARC have indicated that the ability
to describe distinct policies for existing and non- existing sub-
domains would facilitate PSD DMARC deployment. There are also
suggestions that it would be more generally useful for DMARC.
During the period of the experiment, uptake of the new 'np' tag will
be evaluated to support assessment of the utility of including 'np'
in a future, non-experimental update.
Appendix B. DMARC PSD Registry Examples
To facilitate experimentation around data leakage mitigation, samples
of the DNS based and IANA like registries are available at
[psddmarc.org].
B.1. DMARC PSD DNS Query Service
A sample stand-alone DNS query service is available at
[psddmarc.org]. It was developed based on the contents suggested for
an IANA registry in an earlier revision of this draft. Usage of the
service is described on the web site.
B.2. DMARC Public Suffix Domain (PSD) Registry
[psddmarc.org] provides an IANA like DMARC Public Suffix Domain (PSD)
Registry as a stand-alone DNS query service. It follows the contents
and structure described below. There is a Comma Separated Value
(CSV) version of the listed PSD domains which is suitable for use in
build updates for PSD DMARC capable software.
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Names of PSDs participating in PSD DMARC must be registered this new
registry. New entries are assigned only for PSDs that require use of
DMARC. The requirement has to be documented in a manner that
satisfies the terms of Expert Review,per [RFC5226]. The Designated
Expert needs to confirm that provided documentation adequately
describes PSD policy to require domain owners to use DMARC or that
all domain owners are part of a single organization with the PSO.
The initial set of entries in this registry is as follows:
+-------------+---------------+
| PSD | Status |
+-------------+---------------+
| .bank | current |
+-------------+---------------+
| .insurance | current |
+-------------+---------------+
| .gov.uk | current |
+-------------+---------------+
Appendix C. Implementation
There is one known implementation of PSD DMARC available for testing.
C.1. Authheaders Module
The authheaders Python module and command line tool is available for
download or installation from Pypi (Python Packaging Index).
It supports both use of the DNS based query service and download of
the CSV registry file from [psddmarc.org].
Acknowledgements
Thanks to the following individuals for their contributions (both
public and private) to improving this document. Special shout out to
Dave Crocker for naming the beast.
Kurt Andersen, Seth Blank, Dave Crocker, Heather Diaz, Tim Draegen,
Zeke Hendrickson, Andrew Kennedy, John Levine, Dr Ian Levy, Craig
Schwartz, Alessandro Vesely, and Tim Wicinski
Author's Address
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Scott Kitterman
fTLD Registry Services
600 13th Street, NW, Suite 400
Washington, DC 20005
United States of America
Phone: +1 301 325-5475
Email: scott@kitterman.com
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