RFC 9091 | PSD DMARC | July 2021 |
Kitterman & Wicinski | Experimental | [Page] |
Domain-based Message Authentication, Reporting, and Conformance (DMARC), defined in RFC 7489, permits a domain-controlling organization to express domain-level policies and preferences for message validation, disposition, and reporting, which a mail-receiving organization can use to improve mail handling.¶
DMARC distinguishes the portion of a name that is a Public Suffix Domain (PSD), below which Organizational Domain names are created. The basic DMARC capability allows Organizational Domains to specify policies that apply to their subdomains, but it does not give that capability to PSDs. This document describes an extension to DMARC to fully enable DMARC functionality for PSDs.¶
Some implementations of DMARC consider a PSD to be ineligible for DMARC enforcement. This specification addresses that case.¶
This document is not an Internet Standards Track specification; it is published for examination, experimental implementation, and evaluation.¶
This document defines an Experimental Protocol for the Internet community. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are candidates for any level of Internet Standard; see Section 2 of RFC 7841.¶
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc9091.¶
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.¶
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 subdomains within a single organization.¶
To determine the Organizational Domain for a message under evaluation, and thus where to look for a policy statement, DMARC makes use of a public suffix list. The process for doing this can be found in Section 3.2 of the DMARC specification [RFC7489]. Currently, the most common public suffix list being used is the one maintained by the Mozilla Foundation and made public at <https://publicsuffix.org>.¶
In the basic DMARC model, Public Suffix Domains (PSDs) are not Organizational Domains and are thus not subject to DMARC processing. In DMARC, domains fall into one of three categories: Organizational Domains, subdomains of Organizational Domains, or PSDs. A PSD can only publish DMARC policy for itself and not for any subdomains under it. In some cases, this limitation allows for the abuse of non-existent organizational-level domains and hampers identification of domain abuse in email.¶
This document specifies experimental updates to the DMARC specification [RFC7489] in an attempt to mitigate this abuse.¶
As an example, imagine a Top-Level Domain (TLD), ".example", that has public subdomains for government and commercial use (".gov.example" and ".com.example"). The maintainer of a list of such a PSD structure would include entries for both of these subdomains, thereby indicating that they are PSDs, below which Organizational Domains can be registered. Suppose further that there exists a legitimate domain called "tax.gov.example", registered within ".gov.example".¶
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 "t4x.gov.example". Such domains are not registered.¶
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;" "rua=mailto:dmc@dmarc.svc.gov.example" )¶
This DMARC record provides policy and a reporting destination for mail sent from @gov.example. Similarly, "tax.gov.example" will have a DMARC record that specifies policy for mail sent from addresses @tax.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 @t4x.gov.example does not and cannot fall under a DMARC policy.¶
Defensively registering all variants of "tax" is not a scalable strategy. The intent of this specification, therefore, is to enhance the DMARC discovery method by enabling an agent receiving such a message to be able to determine that a relevant policy is present at "gov.example", which is precluded by the current DMARC specification.¶
This document provides a simple extension to [RFC7489] to allow operators of Public Suffix Domains (PSDs) to:¶
This document also provides a new DMARC tag to indicate requested handling policy for non-existent subdomains. 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.¶
As an additional benefit, the PSD DMARC extension clarifies existing requirements. Based on the requirements of [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 has 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:¶
Due to the design of DMARC and the nature of the Internet email architecture [RFC5598], there are interoperability issues associated with DMARC deployment. These are discussed in "Interoperability Issues between Domain-based Message Authentication, Reporting, and Conformance (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.¶
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", "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.¶
The global Internet Domain Name System (DNS) is documented in numerous RFCs. It defines a tree of names starting with root, ".", immediately below which are Top-Level Domain names such as ".com" and ".us". The domain name structure consists of a tree of names, each of which is made of a sequence of words ("labels") separated by period characters. The root of the tree is simply called ".". The Internet community at large, through processes and policies external to this work, selects points in this tree at which to register domain names "owned" by independent organizations. Real-world examples are ".com", ".org", ".us", and ".gov.uk". Names at which such registrations occur are called "Public Suffix Domains (PSDs)", and a registration consists of a label selected by the registrant to which a desirable PSD is appended. For example, "ietf.org" is a registered domain name, and ".org" is its PSD.¶
The term "Organizational Domain" is defined in Section 3.2 of [RFC7489].¶
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.¶
A Public Suffix Operator is an organization that manages operations within a PSD, particularly the DNS records published for names at and under that domain name.¶
PSO-Controlled Domain Names are names in the DNS that are managed by a PSO and are not available for use as Organizational Domains. PSO-Controlled Domain Names may have one (e.g., ".com") or more (e.g., ".co.uk") name components, depending on PSD policy.¶
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 [RFC8020].¶
To participate in this experiment, implementations should interpret [RFC7489] as described in the following subsections.¶
References to "Domain Owners" also apply to PSOs.¶
The following paragraph is added to this section. 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 absent) 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 Section 6.6.3 of [RFC7489].¶
The following tag definitions from DMARC are updated:¶
The ABNF [RFC5234] for DMARC is updated to include a new definition, "dmarc-nprequest":¶
dmarc-nprequest = "np" *WSP "=" *WSP ( "none" / "quarantine" / "reject" )¶
The "dmarc-record" definition is also updated to include the following:¶
dmarc-record = dmarc-version dmarc-sep [dmarc-request] [dmarc-sep dmarc-srequest] [dmarc-sep dmarc-auri] [dmarc-sep dmarc-furi] [dmarc-sep dmarc-adkim] [dmarc-sep dmarc-aspf] [dmarc-sep dmarc-ainterval] [dmarc-sep dmarc-fo] [dmarc-sep dmarc-rfmt] [dmarc-sep dmarc-percent] [dmarc-sep] [dmarc-sep dmarc-nprequest] ; components other than dmarc-version and ; dmarc-request may appear in any order¶
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. This document is an experimental mechanism for doing so; see the description in Appendix B.¶
Experience with DMARC has shown that some implementations short-circuit messages, bypassing DMARC policy application, when the domain name extracted by the receiver (from the RFC5322.From domain) is on the public suffix list used by the receiver. This negates the capability being created by this specification. Therefore, the following paragraph is appended to Section 6.6.1 of the DMARC specification [RFC7489]:¶
Note that domain names that appear on a public suffix list are not exempt from DMARC policy application and reporting.¶
A new step is added between steps 3 and 4:¶
- 3A.
- If the set is now empty and the longest PSD ([RFC9091], Section 2.4) of the Organizational Domain is one that the receiver has determined is acceptable for PSD DMARC (based on the data in one of the DMARC PSD Registry Examples described in Appendix B of [RFC9091]), the Mail Receiver MUST query the DNS for a DMARC TXT record at the DNS domain matching the longest PSD 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.example", the query for PSD DMARC would use "compute.cloudcompany.com.example" as the longest PSD. 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.example".¶
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.¶
The following paragraph is added to this section:¶
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 [RFC9091] for discussion of privacy considerations for PSD DMARC.¶
These privacy considerations are developed based on the requirements of [RFC6973]. Additionally, the privacy considerations of [RFC7489] apply to the mechanisms described by this document. To participate in this experiment, implementations should be aware of the privacy considerations described in this section. If this experiment is successful, this section should be incorporated into the "Privacy Considerations" section as "Feedback Leakage".¶
Providing feedback reporting to PSOs can, in some cases, cause information to leak out 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:¶
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 MUST be 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 participate in DMARC, any feedback reporting related to multi-organizational PSDs MUST be limited to non-existent domains except in cases where the reporter knows that PSO requires use of DMARC (by checking the DMARC PSD Registry).¶
This document does not change the security considerations of [RFC7489] and [RFC7960].¶
The risks of the issues identified in Section 12.3 of [RFC7489] ("DNS Security") are amplified by PSD DMARC. In particular, consequences of DNS cache poisoning (or name chaining) are increased because a successful attack would potentially have a much wider scope (see [RFC3833] for details).¶
The risks of the issues identified in Section 12.5 of [RFC7489] ("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.¶
IANA has added a new tag to the "DMARC Tag Registry" in the "Domain-based Message Authentication, Reporting, and Conformance (DMARC) Parameters" registry. The "Status" column is defined in Section 11.4 of [RFC7489].¶
The new entry is as follows:¶
Tag Name | Reference | Status | Description |
---|---|---|---|
np | RFC 9091 | current | Requested handling policy for non-existent subdomains |
The experiment being performed has three different questions that are looking to be addressed in this document.¶
To facilitate experimentation around mitigation of data leakage, samples of the DNS-based and IANA-like registries are available at [PSD-DMARC].¶
A sample stand-alone DNS query service is available at [PSD-DMARC]. It was developed based on the contents suggested for an IANA registry in an earlier draft version of this document. Usage of the service is described at [PSD-DMARC].¶
[PSD-DMARC] 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 PSDs that is suitable for use in build updates for PSD DMARC-capable software.¶
PSDs that are deploying DMARC and are participating in PSD DMARC must register their public suffix domain in this new registry. The requirement has to be documented in a manner that satisfies the terms of Expert Review, per [RFC8126]. 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 authoritative registry can be found here: <https://psddmarc.org>¶
[PSD-DMARC] provides a file formatted like the Public Suffix List (PSL) in order to facilitate identification of PSD DMARC participants. Contents are functionally identical to the IANA-like registry but presented in a different format.¶
When using this approach, the input domain of the extension lookup is supposed to be the output domain of the regular PSL lookup, i.e., the Organizational Domain. This alternative data approach is potentially useful since DMARC implementations already need to be able to parse the data format, so it should be easier to implement.¶
There are two known implementations of PSD DMARC available for testing.¶
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 [PSD-DMARC].¶
The zdkimfilter module is a separately available add-on to Courier-MTA.¶
Mostly used for DomainKeys Identified Mail (DKIM) signing, it can be configured to also verify, apply DMARC policies, and send Aggregate Reports. For PSD DMARC, it uses the PSL extension list approach, which is available from [PSD-DMARC].¶
Thanks to the following individuals for their contributions (both public and private) to improving this document: 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.¶
A special mention to Dave Crocker for coming up with the name.¶