Experimental Domain-Based Message Authentication, Reporting, and Conformance (DMARC) Extension for Public Suffix Domains fTLD Registry Services
Suite 400 600 13th Street, NW Washington DC 20005 United States of America +1 301 325-5475 scott@kitterman.com
Elkins 26241 United States of America WV tjw.ietf@gmail.com
DMARC email authentication TLD 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.
Introduction DMARC 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 the DMARC specification. Currently, the most common public suffix list being used is the one maintained by the Mozilla Foundation and made public at . 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 in an attempt to mitigate this abuse.
Example 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: 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.
Discussion This document provides a simple extension to 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
  • Extend the DMARC policy query functionality to detect and process such a policy
  • Describe receiver feedback for such policies
  • Provide controls to mitigate potential privacy considerations associated with this extension
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 , 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:
Branded PSDs (e.g., ".google"):
These domains are effectively Organizational Domains as discussed in . They control all subdomains of the tree. These are effectively private domains but listed in the current public suffix list. They are treated as public for DMARC purposes. They require the same protections as DMARC Organizational Domains but are currently unable to benefit from DMARC.
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.
Due to the design of DMARC and the nature of the Internet email architecture, 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" . These issues are not typically applicable to PSDs since they (e.g., the ".gov.example" used above) do not typically send mail.
Terminology and Definitions This section defines terms used in the rest of the document.
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 when, and only when, they appear in all capitals, as shown here.
Public Suffix Domain (PSD) 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.
Organizational Domain The term "Organizational Domain" is defined in .
Longest PSD 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.
Public Suffix Operator (PSO) 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 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.
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 .
PSD DMARC Updates to DMARC Requirements To participate in this experiment, implementations should interpret as described in the following subsections.
General Updates References to "Domain Owners" also apply to PSOs.
Changes in Section 6.3 ("General Record Format") 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 .
The following tag definitions from DMARC 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.'
Changes in Section 6.4 ("Formal Definition") The ABNF for DMARC is updated to include a new definition, "dmarc-nprequest": The "dmarc-record" definition is also updated to include the following:
Changes in 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. This document is an experimental mechanism for doing so; see the description in .
Changes in Section 6.6.1 ("Extract Author Domain") 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 the DMARC specification:
Note that domain names that appear on a public suffix list are not exempt from DMARC policy application and reporting.
Changes in Section 6.6.3 ("Policy Discovery") A new step is added between steps 3 and 4:
3A.
If the set is now empty and the longest PSD ([RFC9091], ) 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 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.
Changes in Section 7 ("DMARC Feedback") 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 of [RFC9091] for discussion of privacy considerations for PSD DMARC.
Privacy Considerations These privacy considerations are developed based on the requirements of . Additionally, the privacy considerations of 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 ). There are roughly three cases to consider:
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.
Multi-organization PSDs that require DMARC usage (e.g., ".bank"):
Reports based on PSD DMARC 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 risk of feedback leakage 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.
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) instead of 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 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).
Security Considerations This document does not change the security considerations of and . The risks of the issues identified in ("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 for details). The risks of the issues identified in ("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 .
IANA Considerations 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 . The new entry is as follows:
Tag Name Reference Status Description
np RFC 9091 current Requested handling policy for non-existent subdomains
References Normative References Informative References Public Suffix Domain DMARC
PSD DMARC Privacy Concern Mitigation Experiment The experiment being performed has three different questions that are looking to be addressed in this document.
  • modifies policy discovery to add an additional DNS lookup. To determine if this lookup is useful, PSDs will add additional DMARC records in place and will analyze the DMARC reports. Success will be determined if a consensus of PSDs that publish DMARC records are able to collect useful data.
  • adds the "np" tag for non-existent subdomains (DNS NXDOMAIN). PSOs wishing to test this will add this flag to their DMARC record and will analyze DMARC reports for deployment. Success will be determined if organizations find explicitly blocking non-existent subdomains desirable and that doing so provides added value.
  • discusses three cases where providing feedback could cause information to leak out of an organization. This experiment will analyze the Feedback Reports generated for each case to determine if there is information leakage.
DMARC PSD Registry Examples To facilitate experimentation around mitigation of data leakage, samples of the DNS-based and IANA-like registries are available at .
DMARC PSD DNS Query Service A sample stand-alone DNS query service is available at . 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 .
DMARC PSD Registry 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 . 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:
DMARC PSD PSL Extension 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.
Implementations There are two known implementations of PSD DMARC available for testing.
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 .
Zdkimfilter Module 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 .
Acknowledgements Thanks to the following individuals for their contributions (both public and private) to improving this document: , , , , , , , , , , , and . A special mention to for coming up with the name.