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Operations and Management Network Working Group DNS affiliation phishing takedown reputation TXT Security systems assess domain names one at a time. A new domain of an established organization therefore starts with no reputation, and a legitimate domain that resembles another organization's name can be reported as an impersonation. This document defines Domain Affiliation Confirmation (DAC), by which the operator of a parent domain confirms that a child domain is operated by it or under its authority. The child publishes a DNS record naming the parent and a random token; the parent confirms that token, in the DNS or over HTTPS, by naming the child. Verification takes at most three DNS queries. The parent publishes no list of its domains, and an affiliation is ended by deleting one record. The document also states what a confirmation means to relying parties such as takedown services and reputation systems. About This Document Discussion of this document takes place on the Domain Name System Operations (DNSOP) Working Group mailing list (dnsop@ietf.org). A reference implementation with test vectors accompanies this draft ().

Introduction

Problem Statement Mail filters, browser warning systems, secure web gateways, and brand-protection services evaluate domain names individually. Two consequences follow for organizations that operate more than one domain.

New domains start without reputation.

When an organization launches a product or service under a new domain name, that name is treated as unknown, and often as suspicious, because nothing tells the evaluating system that it is operated by the same organization as an established domain.

Legitimate look-alikes are reported as impersonations.

Brand-protection services search for newly registered names that resemble their clients' names. A legitimate domain of one organization can resemble the name of an unrelated organization. The service has no standard way to learn that the domain is operated by an established organization and may ask the domain's registrar or hosting provider to suspend it. Erroneous takedown requests can disable an entire legitimate domain .

Both problems would be reduced if an evaluating system could ask a precise question and get an answer from the party able to give it: "Does the operator of this established domain confirm that it operates this other domain?" No deployed mechanism answers it. Registration data is frequently redacted and was not designed to express relationships between domains, and certificates bind keys to names, not names to each other.

This Document This document defines Domain Affiliation Confirmation (DAC). The domain to be affiliated (the child) publishes a DNS record that names its parent domain and carries a random token. The parent confirms the affiliation by publishing, under a name derived from that token, a record that names the child. A verifier reads the child's record, reads the parent's confirmation, and checks that the confirmation names the child it started from. The document also states what a confirmation means (): the parent's operator indicates that it operates the child or authorizes its operation, and that reports of abuse concerning the child may be directed to it. A relationship that relying parties are not meant to act on would leave the problems above unsolved ().

Design Goals

1. Only the parent can create an affiliation. A claim the parent has not confirmed has no effect, and a confirmation for one child cannot be reused for another.
2. The parent publishes nothing from which its affiliated domains can be listed. Each child's claim is public, so affiliated domains can be found by examining domains individually, as with any public DNS data.
3. An affiliation is ended by deleting one record at the parent, without any change at the child. The change takes effect within cache and DNSSEC signature lifetimes ().
4. No new key material. The protocol relies on the parent's existing control of its DNS zone or, optionally, its web origin.
5. Bounded cost. Verification takes at most three TXT queries, plus one HTTPS request and its address resolution when the parent uses that channel, with no recursion and no searching.
6. Deployable with existing tools. Records are TXT records at underscored names .
7. The verification procedure does not consult a list of public suffixes. Every confirmation requires a record that only the parent's operator can publish.

Non-Goals DAC does not establish the identity, legitimacy, or licensing of the parent's operator. It conveys a statement by whoever operates the parent domain; whether that party deserves trust is for each relying party to decide (). DAC does not express relationships between independent organizations, such as partnerships or service providers (). DAC does not define where administrative boundaries in the DNS lie. Relying parties may still need such information to decide which names to evaluate.

Relationship to Other Work describes how a service verifies that a user controls a domain. DAC follows its practices for TXT records at underscored names and for tokens, but records a standing relationship between two domains rather than proving control of one domain to one service. lets a registry enforce that a group of names in that registry is held by one registrant. DAC operates across registries and is published by the domains' operators.

Requirements Language 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.

Terminology DNS terms are used as defined in . This document also uses the following terms.

Child domain:

The domain name whose affiliation is declared or verified.

Parent domain:

The domain name that the child domain names as its parent.

Parent operator:

The party that controls the content of the parent domain's DNS zone or, for confirmations over HTTPS, of the parent domain's HTTPS origin.

Claim:

The TXT record published by the child domain ().

Marker:

The TXT record by which a parent domain indicates that it participates and selects a confirmation channel ().

Confirmation:

The record by which the parent domain confirms one claim, published in the DNS () or over HTTPS ().

Token:

A random value, unique to one child domain, that links a claim to its confirmation ().

Verifier:

A party that carries out the procedure in .

Relying party:

A party that makes decisions using verification results, such as a takedown service, a mail filter, or a reputation system.

In examples, "bank.example" is the parent domain and "embank.example" is the child domain . The token used in examples, "mfrggzdfmztwq2lknnwg23tpobyxe43u", is illustrative and not random.

Protocol Overview An affiliation consists of three records. The claim is published by the child; the marker and the confirmation are published by the parent.

Records and Verification Steps " (claim) | | ._affiliation. | +-------------------------------+ | bank.example. | | TXT "v=DAC1; | | d=embank.example" | | (confirmation) | +--------------------------------+ Verifier: 1. TXT _affiliation.embank.example -> p=bank.example, t= 2. TXT _affiliation.bank.example -> marker, channel "dns" 3. TXT ._affiliation.bank.example -> d=embank.example 4. d= equals the child domain -> result "confirmed" ]]>

A claim copied to another domain fails because the confirmation names the original child. Confirmation names cannot be found without the corresponding claims, because they contain the token. The marker prevents a wildcard elsewhere in the parent's zone from answering for confirmation names () and tells the verifier which channel the parent uses.

Records

Common Syntax The three records share one tag-list syntax: a version tag followed by zero or more tags, separated by semicolons. The ABNF below uses case-sensitive strings ; WSP, ALPHA, and DIGIT are the core rules of . A TXT record is a DAC record if its text, after any leading spaces or tabs, begins with "v", optional whitespace, "=", optional whitespace, and "DAC1", followed by whitespace, ";", or the end of the text. Other TXT records are ignored, so DAC records can share a name with other TXT records. A DAC record is well-formed if it matches the "record" rule, contains no tag name more than once, and the value of every tag defined in this document matches the rule given for it, with an "x" value that is also a valid calendar date. A DAC record that is not well-formed is malformed.

• Tag names are case-sensitive. Verifiers MUST ignore tags they do not recognize. New tags are registered as described in ; adding a tag does not change the version.
• A TXT record consisting of several character-strings is interpreted as their concatenation without separators, as in . Publishers SHOULD keep each record within a single character-string of at most 255 octets.
• Domain names in records are in canonical form: A-labels in lowercase, without a trailing dot, at most 253 octets long, with a last label that is not entirely numeric. A label with hyphens in both the third and fourth positions MUST be a valid A-label. The "domain" rule expresses the character-level constraints; a name violating the others is malformed. Verifiers do not repair names.

Claim Record A child domain declares its parent with one TXT record at the name formed by prepending the label "_affiliation" to the child domain:

p (required):

The parent domain, matching the "p-value" rule.

t (required):

The token, matching the "t-value" rule ().

A child domain has at most one parent and MUST NOT publish more than one DAC record at its "_affiliation" name; a domain that is also a parent includes its marker tags in the same record (). The parent domain MUST NOT be the child domain itself. The claim name MAY be a CNAME, for example when the child's records are managed by the parent's operator.

Scope of an Affiliation A claim is published at, and an affiliation is verified for, one domain name. Organizations typically publish claims at their registrable domains, such as "embank.example", not at every host name. A relying party assessing a name below such a domain, for example "marketing.embank.example", determines the domain it regards as responsible for that name, as it already does when attributing reputation, and verifies that domain. Claims are not published at names like "marketing.embank.example" for this purpose. By default, a confirmation states nothing about names below the child domain, and relying parties apply their usual practice to them. A parent that operates, or authorizes the operation of, every name below the child states this by including the "s" tag with the value "subtree" in the confirmation (). The confirmation then also covers every name below the child. A parent MUST NOT include it when names below the child are operated by parties not acting under the parent's authority, such as customers of a hosting or platform service (). If a name below a child with a subtree confirmation publishes its own claim, that claim governs the name and the names below it.

Marker Record A parent domain that confirms affiliations publishes exactly one DAC record at the name formed by prepending "_affiliation" to the parent domain. A domain that is both a child and a parent publishes a single record there containing both the claim tags and the marker tags.

c (optional):

The confirmation channel. This document defines "dns", the default (), and "https" (). Other values are registered as described in . With "https", DNS confirmation records are not consulted.

r (optional):

A URI for reports of abuse concerning the parent's affiliated domains, with scheme "mailto" or "https". Semicolons and whitespace in the URI are percent-encoded. Verifiers ignore an "r" tag with any other scheme.

m (optional):

The mode: "enforce", the default, or "test". In "test" mode the parent is deploying the protocol and asks that its confirmations not be acted on (). Verifiers treat any value other than "enforce" as "test".

f (optional):

A URI to which verifiers may send reports of claims naming this parent that were not confirmed (). It is either "https://" followed by a host and an optional path, without userinfo, port, query, or fragment, or "mailto:" followed by exactly one address, without header fields. The host, or the domain of the address, is in canonical form () and is the parent domain or a name below it. Verifiers ignore an "f" tag that does not meet these conditions.

The marker MUST be a TXT record owned by the "_affiliation" name itself, not data reached through a CNAME or DNAME record. The marker makes "_affiliation" an existing node in the parent zone, so that a wildcard higher in the zone cannot answer for names below it (). The "_affiliation" name MAY be delegated as a separate zone, in which case that zone's operator publishes the marker and confirmations. A DNAME record at that name redirects the queries for confirmations to another zone and has the same effect as delegating them.

Confirmation Record (DNS Channel) For each confirmed child, a parent using the DNS channel publishes one TXT record at the name formed by prepending the child's token to the parent's "_affiliation" name:

d (required):

The child domain, matching the "d-value" rule.

s (optional):

The scope. The value "subtree" states that the confirmation also covers every name below the child domain (). Verifiers ignore any other value; other scopes would be defined with a new tag.

x (optional):

The last day of validity, as a full-date in UTC , matching the "x-value" rule. The confirmation applies until the end of that day, 23:59:59 UTC, and not afterwards. Without "x", the confirmation applies until it is removed.

An expiry date suits domains used for a limited time, such as campaign sites, and also limits the consequences of a confirmation that is not removed when its domain changes hands (), for example when it is set to the domain's registration expiry date and updated on renewal. A parent extends a confirmation by replacing the record with one carrying a later date. Registration expiry dates normally move forward when domains renew automatically, so a parent tying "x" to them updates the confirmation at each renewal. A confirmation name MUST NOT hold more than one DAC record. It MAY be a CNAME, for example to a name hosted by a DNS provider. The parent MUST NOT publish wildcard records at or below its "_affiliation" name.

Tokens A token links one claim to one confirmation. It makes confirmation names unpredictable, so that a parent's confirmations cannot be found by guessing names, whether by online queries or, for DNSSEC-signed zones using NSEC3, by offline guessing of hashed names. A token is not a secret: it is published in the claim and appears in queries. The protocol's resistance to forged affiliations does not depend on tokens being unknown ().

• A token MUST contain at least 128 bits, and SHOULD contain 160 bits, from a cryptographically secure random source . At 128 bits, guessing a confirmation name is infeasible and collisions between tokens are negligible.
• A token is encoded in base32 () with lowercase letters and without padding, giving 26 to 52 characters; a 160-bit token is 32 characters, well within the 63-octet limit on a DNS label . Case-sensitive encodings such as base64 cannot be used, because DNS name matching ignores case and some resolvers alter the case of query names.
• Tokens are opaque; verifiers do not decode them. A parent MUST use a different token for each child and SHOULD use a new token when re-establishing an affiliation that was previously ended.

Publishing an Affiliation

Creating an Affiliation

1. The parent operator publishes the marker, once, before its first affiliation.
2. The parent operator generates a token for the child and publishes the confirmation ( or ).
3. The child's operator publishes the claim. When parent and child are operated by the same organization, one tool performs all of these steps ().

Publishing the confirmation first lets the affiliation verify as soon as the claim is visible.

Ending an Affiliation The parent operator ends an affiliation by removing the confirmation; no change at the child is needed. A parent operator MUST remove the confirmation for a child domain that it no longer operates or authorizes, including when the child domain is transferred, sold, or allowed to expire, except as described in . A claim left in the child's zone, or copied there by a later registrant, has no effect once the confirmation is gone. A parent that withdraws from the protocol removes its marker, which ends all of its affiliations.

Moving a Child to Another Parent When a child domain moves from one parent to another, for example after an acquisition, the operators proceed in this order so that the child remains confirmed throughout:

1. The new parent publishes its marker, if it has none, and a confirmation for the child under a new token.
2. The child replaces its claim with one naming the new parent and the new token.
3. After verifying that the child's claim names the new parent, and after the TTL of the old claim has passed (86400 seconds if the old parent does not know it), the old parent removes its confirmation.

If the child domain leaves both organizations, for example because it is sold, the parent removes its confirmation no later than the transfer (), and the child's new operator replaces or removes the claim.

Organizations with Several Levels A parent domain can itself be the child domain of another affiliation, for example when a subsidiary's primary domain is affiliated with its group's primary domain. Each affiliation is confirmed separately and means only what states about that pair: a confirmation of the parent by its own parent says nothing about the parent's children. Verifiers MAY repeat the procedure with the parent domain as the child domain. A verifier that does so MUST NOT follow more than two levels beyond the first parent and MUST stop if a domain repeats.

Verification

Inputs and Outputs The input is a child domain in canonical form (). Verifiers that accept names containing non-ASCII characters convert them to A-labels as specified in ; the processing of with non-transitional processing is a common way of doing so. A name that cannot be converted yields "permerror". The output is a result () and, when available, the parent domain, the channel, whether the result is authenticated (), whether the confirmation covers names below the child domain (), the parent's mode and abuse contact, and the time for which the result may be reused ().

Procedure Verifiers MUST use a DNS interface that exposes the answer section of responses, so that CNAME and DNAME records in an answer can be detected. In the steps below, "no record" means that the query returned NXDOMAIN, or NOERROR with no DAC record; this includes a name that exists only as an empty non-terminal. NXDOMAIN and NOERROR without data are treated alike because servers differ in which they return. Other DNS outcomes are handled as described in .

1. Query TXT at "_affiliation." followed by the child domain.
   • No record: the result is "none".
   • More than one DAC record, or a malformed one: "permerror".
   • Neither a "p" nor a "t" tag: the record is only a marker, and the result is "none".
   • Only one of "p" and "t", or "p" equal to the child domain: "permerror".
2. Query TXT at "_affiliation." followed by the parent domain. Any "p" and "t" tags in the marker belong to the parent's own claim and are ignored here. Apply the following checks in order.
   1. The answer contains a CNAME or DNAME record: "permerror".
   2. No record: "not-confirmed".
   3. More than one DAC record, or a malformed one: "permerror".
   4. A "c" value not supported by the verifier: "not-confirmed".
3. If the channel is "dns", query TXT at the token, followed by "._affiliation.", followed by the parent domain.
   • No record: "not-confirmed".
   • More than one DAC record, or a malformed one: "permerror".
   If the channel is "https", obtain the confirmation as described in .
4. Examine the confirmation's "d" tag.
   • Missing: "permerror".
   • Not equal to the child domain: "not-confirmed".
   • Equal to the child domain, but the confirmation has an "x" tag and the time of verification is after the end of that day: "not-confirmed".
   • Otherwise: "confirmed". If the confirmation has "s=subtree", the verifier reports that the result covers names below the child domain.

Domain names are compared as octet strings in canonical form. The procedure does not query any other names, follow references found in record content, or consult a list of public suffixes.

Results

confirmed:

The parent confirms the child.

not-confirmed:

The child claims a parent, and the verifier did not obtain a confirmation. This covers a parent that does not participate, an ended affiliation, a copied or fabricated claim, and a channel the verifier does not support.

none:

The child makes no claim.

temperror:

A temporary failure prevented a result; the verifier can retry later.

permerror:

A record is malformed or the configuration is invalid; the result will not change until a record is corrected.

Only "confirmed" establishes an affiliation.

DNS Errors Any response code other than NOERROR and NXDOMAIN, a timeout, or a truncated response that cannot be retried over TCP yields "temperror". A response that fails DNSSEC validation ("bogus" ) also yields "temperror" and MUST NOT contribute to a "confirmed" result. Verifiers MUST support DNS over TCP.

Authenticated Results For the DNS channel, a result is authenticated if all DNS answers used were validated as secure , either by the verifier or by a validating resolver reached over a trusted path. For the HTTPS channel, the Web PKI authenticates the confirmation, but the decision to use that channel was taken from the marker. The result is authenticated only if the claim and the marker were validated as secure. Otherwise, a forged marker could select the HTTPS channel for a parent that does not use it (). Verifiers report whether a result is authenticated. The abuse contact is taken from the marker and is authenticated only when the marker is.

Caching A verifier MAY reuse a result for the smallest of: the TTLs of the DNS answers used, where the TTL of a negative answer is determined as in ; the freshness lifetime of the HTTPS response (); the time remaining until a confirmation's expiry; and 86400 seconds. After "temperror", verifiers retry with increasing intervals and SHOULD NOT retry more often than once a minute. Resolvers that serve stale data can extend the time for which an ended affiliation still verifies.

HTTPS Channel A parent whose marker contains "c=https" publishes confirmations as HTTPS resources instead of DNS records, which suits operators that manage such data through a web application rather than their DNS provider. The claim and the marker remain DNS records; selecting this channel is done in the parent's DNS zone.

Resource The confirmation for a token is the resource identified by: /.well-known/affiliation/ ]]> A confirmed token is served with status code 200 and media type "text/plain". The content is one DAC confirmation record, with the same syntax as in the DNS, optionally followed by a single CRLF or LF: A token that is not confirmed is answered with 404 (Not Found) or 410 (Gone).

Fetching A verifier fetching a confirmation:

• MUST use the "https" scheme on port 443, with the parent domain as the host, and MUST validate the server's certificate for that name ;
• MUST NOT follow redirects;
• MUST NOT connect to addresses that the IANA special-purpose address registries mark as not globally reachable, and MUST connect to the address it checked;
• MUST NOT send cookies or other credentials; and
• MUST read no more than 1024 octets of content and SHOULD limit the request to 10 seconds.

Responses are handled as follows:

• 200 with media type "text/plain" (parameters are ignored) and content that is exactly one DAC record: the record is the confirmation, examined as in step 4 of . A malformed record, other content, another media type, or more than 1024 octets yields "permerror".
• 404 or 410: "not-confirmed".
• 5xx, a connection failure, a timeout, or a TLS failure: "temperror".
• Any other status code, including 3xx, or a parent domain that has no globally reachable address: "permerror".

The freshness lifetime of a response is zero if it carries Cache-Control no-store or no-cache, is otherwise taken from a max-age directive if present, and is otherwise 3600 seconds.

Meaning of a Confirmation This section states what a "confirmed" result means, so that relying parties can use it consistently. It does not prescribe relying-party policy.

What a Confirmation States A "confirmed" result means that, at the time of verification, the parent operator had published a statement that:

1. the child domain is operated by the parent operator or by a party acting under its authority; and
2. reports of abuse concerning the child domain may be directed to the parent operator, at the contact in the marker or by its usual means.

With "s=subtree", both statements also apply to every name below the child domain. The second element distinguishes an affiliation from a mere association: a parent that confirms a child is the expected recipient of complaints about it. The statement is made by whoever controls the parent domain's DNS zone, or its HTTPS origin when that channel is used.

What a Confirmation Does Not State A confirmation says nothing about the trustworthiness, identity, or licensing of the parent operator, or about the content served by the child. A confirmation from a parent domain registered yesterday is as valid as one from a long-established organization. A confirmation therefore conveys at most the standing that a relying party already attributes to the parent domain.

Considerations for Relying Parties

• Takedown decisions. When a domain resembling a protected name is confirmed as affiliated with a parent that the relying party already regards as legitimate, the resemblance is unlikely to be an impersonation of the parent's own name, and a conflict with another organization's name is a dispute between organizations. Relying parties can direct such reports to the parent's abuse contact or to the dispute processes for name conflicts. A confirmation alone does not justify delaying action on abuse that has been observed ().
• Reputation. A relying party may extend to a confirmed child some or all of the reputation it attributes to the parent. Attributing more standing to the child than to the parent is inconsistent with the meaning above, and taking abuse observed on a confirmed child into account when assessing the parent gives parent operators a reason to confirm only domains they control.
• Testing mode. Confirmations from a parent in "test" mode are intended for reporting to the parent, not for decisions.
• Other results. "none", "not-confirmed", "temperror", and "permerror" are not, by themselves, evidence that a domain is malicious. Most domains publish no claim. A claim that a well-known parent does not confirm may be an attempt to borrow its standing, but it may also result from misconfiguration or from records an attacker caused to appear in a victim's zone ().
• Assurance. Relying parties taking consequential actions can consider whether a result was authenticated () and, for unauthenticated results, the measures in .

Failure Reports A parent can ask verifiers to report claims that name it but are not confirmed, for example by a domain that copied a claim or fabricated one. Such reports let the parent learn about attempts to borrow its standing, and about confirmations that have expired or been removed by mistake. Sending reports is optional for verifiers.

When Reports Are Sent A verifier MAY send a report when it obtained the parent's marker, the marker carries an "f" tag, and the result is "not-confirmed" or "permerror" for a reason arising after step 2 of , that is, a missing, mismatched, expired, or malformed confirmation, or a confirmation that could not be obtained over HTTPS. Reports are not sent for "temperror" or for an unsupported channel. Reports are particularly useful while a parent is in "test" mode, and verifiers may send them then. A verifier MUST NOT send a report to an "f" URI that does not meet the conditions in ; in particular, reports go only to the parent domain or names below it, which prevents a marker from directing reports at third parties. A parent that wants reports processed by a provider points a name below its domain, or its mail routing, at that provider. Because an attacker can create any number of child domains, a verifier MUST send at most one message per report destination in 24 hours, combining reports into it, and SHOULD limit the number of reports per message, for example to 1000.

Report Format A report is a JSON array of objects, each with the following members:

version:

The string "DAC1".

observed:

The time of verification, as an RFC 3339 date-time in UTC.

child:

The child domain, in canonical form.

parent:

The parent domain, in canonical form.

result:

"not-confirmed" or "permerror".

reason:

One of the values in the "DAC Report Reasons" registry ().

reporter (optional):

A domain name identifying the organization sending the report.

Receivers ignore members and reason values they do not recognize.

Delivery For an "https" URI, the verifier sends the report in an HTTP POST request with media type "application/json", subject to the same restrictions as in , except that the content may be up to 65536 octets. For a "mailto" URI, the verifier sends an email message with the subject "DAC report for" followed by the parent domain, whose body, or one of whose body parts, is the report with media type "application/json". Verifiers SHOULD send report email from a domain that authenticates its mail, so that receivers can filter it. Failure to deliver a report is not retried more than once.

Handling Reports Reports are unauthenticated assertions by whoever sent them. A parent treats a report as a prompt to verify the named child itself, not as evidence, and rate-limits what it accepts. Reports are not a substitute for the monitoring described in .

Operational Considerations

Automation and Inventory Unless an expiry date is used, the records contain no dates, so creating an affiliation is a one-time operation with nothing to renew. An organization with many domains maintains the records with a tool that generates tokens and writes records through its DNS providers' interfaces. Affiliations go stale when domains change hands. The most reliable control is to derive confirmations from the organization's registrar accounts or domain inventory, so that a confirmation is removed when its domain leaves the inventory. Registrars and DNS providers can offer this as a service.

TTLs, Signatures, and Revocation Time The time for an ended affiliation to stop verifying is normally bounded by the TTL of the confirmation (or the freshness lifetime of the HTTPS confirmation) and by the 86400-second limit in ; serve-stale resolvers and replay can extend it (, ). A TTL of 3600 seconds is a reasonable default. For signed zones, the validity period of signatures over confirmation records also bounds how long a deleted confirmation can be replayed (); parents wanting prompt revocation keep it short for names below "_affiliation". The negative-caching TTL of the parent zone affects how soon a new confirmation becomes visible.

Staged Deployment A parent can publish its marker with "m=test", create confirmations, and check results before changing the marker to "enforce". Relying parties can likewise verify in a reporting-only mode before acting on results.

Choosing and Changing Channels The DNS channel needs only DNS records and benefits from DNSSEC. The HTTPS channel suits parents that prefer to manage confirmations in a web application; it requires that the parent domain itself serve HTTPS on port 443 without redirecting, which many organizations' apex domains do not do by default. A parent that changes channel publishes confirmations in the new channel before changing the marker and removes the old ones after the marker's TTL has passed. Confirmations may be hosted by a DNS provider through CNAME records or by delegating the "_affiliation" name; the provider can then create and remove confirmations ().

Load and Scale Verification costs at most three TXT queries. Markers are shared by all children of a parent and are cached accordingly. Most domains publish no claim, and for them the cost is one query. A parent using the HTTPS channel receives a request for each verification of one of its children that is not answered from a cache; such parents SHOULD serve confirmations from infrastructure able to handle that load and SHOULD set a Cache-Control max-age.

DNS Configuration

• Parents SHOULD sign their zones with DNSSEC so that verifiers can obtain authenticated results. A signed parent zone SHOULD use NSEC3 with the parameters in , compact denial of existence , or minimally covering NSEC records , and SHOULD NOT use NSEC records that allow the zone to be walked ().
• Parents SHOULD restrict zone transfers to their own secondary servers .
• Parents and children MUST NOT publish wildcard records at or below their "_affiliation" names, and SHOULD NOT publish wildcard CNAME records that cover those names.

Monitoring Parent operators can check newly observed domains, for example from Certificate Transparency logs, for claims naming their domain. A claim carrying a token the parent did not issue indicates a domain attempting to borrow its standing; a claim for a domain no longer in the parent's inventory indicates a confirmation that should already have been removed.

Security Considerations

Threat Model The attacker wants a domain it controls to verify as affiliated with a parent it does not control, to prevent a legitimate affiliation from verifying, to use an affiliation to delay action against abuse, or to learn a parent's affiliated domains. The attacker can register domains, publish records in zones it controls, place content on web hosting it has access to, observe public DNS data, and in some cases forge or replay DNS responses on the path to a verifier. Compromise of the parent's DNS zone or web origin is discussed in .

Fabricated Claims Anyone can publish a claim naming any parent. The claim is confirmed only if a record exists under the parent's "_affiliation" node, or at the parent's HTTPS origin when the parent has selected that channel, that names the claiming domain. Absent forged responses (), only the parent operator can create such a record.

Copied Claims and Token Disclosure Claims and tokens are public. An attacker that copies a legitimate claim to its own domain obtains a confirmation naming the legitimate child, and the result is "not-confirmed". The binding between token and child is in the parent's confirmation, which the attacker cannot alter.

Stale Confirmations If a parent fails to remove the confirmation of a child domain that has changed hands, the new registrant of the child inherits the affiliation. This is the most likely failure in practice. The requirement in , inventory-driven automation (), and monitoring () address it. Relying parties with access to registration data can treat a confirmation with caution when the child's registration changed after the confirmation was first observed. Likewise, if a parent domain expires and is registered by someone else, the new registrant can confirm arbitrary domains. Relying parties that attribute standing to a parent based on its history can reassess a parent whose registration has changed rather than carrying standing across the change.

Forged DNS Responses An attacker able to forge DNS responses to a verifier, for example through cache poisoning or by hijacking routes to authoritative servers, can produce any result for unsigned zones. For the DNS channel, the forgery has to cover the parent's records; forging the claim alone cannot produce "confirmed". Forging the marker can switch the channel in either direction. In particular, an attacker that can forge the marker of an unsigned parent zone and can place a file at a path under the parent's HTTPS origin, for example through shared hosting or an upload feature, can obtain "confirmed" for any domain. Such a result is not authenticated (), which is why the Web PKI alone does not make an HTTPS result authenticated. Results authenticated with DNSSEC are not affected by forged responses. Relying parties taking consequential actions on unauthenticated results can perform the queries from several network locations and require the answers to agree, as certification authorities are required to do for domain validation .

Replay of Deleted Records An attacker on the path to a verifier can replay a confirmation that the parent has deleted. For unsigned zones, any response can be forged anyway. For signed zones, a replayed RRset validates until its signatures expire, which can be weeks. Parents that want deletion to take effect promptly against such attackers use short signature validity periods for names below "_affiliation" (). A confirmation with an expiry date cannot be replayed to effect beyond that date.

Wildcards, Aliases, and Delegation If a parent zone contained a wildcard such as "*.bank.example" and no "_affiliation" node existed, queries for confirmation names would be answered from the wildcard . If that wildcard were a CNAME into a zone operated by a third party, such as a hosting platform, a tenant of that platform might be able to publish a TXT record there confirming a domain of its choosing. The marker prevents this: since its owner name exists, a wildcard above it cannot match names below it, and verifiers reject a marker reached through a CNAME or DNAME record. An empty non-terminal at "_affiliation" also blocks the wildcard but yields "not-confirmed" because there is no marker. Confirmation names may be CNAMEs, and the "_affiliation" name may be delegated or carry a DNAME record. The operator of the target or delegated zone can then create and remove confirmations. Parents delegate in this way only to parties they would trust to publish confirmations on their behalf.

Compromise of the Parent An attacker controlling the parent's DNS zone can confirm any domain; it can also redirect the parent domain itself, so DAC does not add to the consequences of that compromise. An attacker controlling only the parent's web origin can confirm domains through the HTTPS channel only if the parent's marker selects that channel or the marker is forged (). Verifiers report the channel used, and relying parties concerned about web compromise can give less weight to confirmations obtained over HTTPS. A parent can detect confirmations it did not create by comparing them with its inventory, and recovers by deleting them.

Use of Affiliations to Deflect Takedowns An attacker that controls a parent domain to which relying parties attribute standing, for example an expired domain re-registered for its history or a compromised DNS account, can confirm look-alike domains used for phishing. Takedown requests may then be directed to an abuse contact that the attacker controls. Relying parties should not let a confirmation alone delay action against abuse they have observed, and can reassess parents whose registration has changed () or whose set of confirmed domains grows unexpectedly.

Inducing Adverse Results An attacker able to place a TXT record at "_affiliation" under a victim's domain, for example through a dangling or wildcard CNAME in the victim's zone pointing to a platform the attacker uses, controls the victim's claim. It can make the victim appear to claim a parent that does not confirm it, which is one reason why results other than "confirmed" are not by themselves evidence of malice (). It can also name a parent the attacker operates and confirm the claim there, making the victim appear affiliated with the attacker and directing abuse reports about the victim to the attacker's contact. Children therefore MUST NOT publish wildcards covering their "_affiliation" name () and SHOULD make their claim name a CNAME only to names they or their parent operator control.

Resource Consumption Verification is bounded at three TXT queries and one HTTPS request with its address resolution, with no recursion and no references in record content, avoiding the lookup-limit problems experienced with ; following several levels () is bounded at nine queries. An attacker can publish claims with random tokens naming a participating parent, so that each verification causes a query for a new name at the parent's authoritative servers, or a request to its HTTPS origin. Aggressive use of DNSSEC-validated caches can absorb such queries only where denial of existence covers ranges of names; the configurations recommended in to prevent zone walking, and unsigned zones, give up that protection, so parents using them depend on verifiers limiting their query rates. Verifiers that accept names to verify from untrusted sources SHOULD rate-limit the queries and requests they send for each parent domain.

Subtree Confirmations A subtree confirmation extends the parent's statement to names that the parent does not list individually. If anyone other than the parent's organization can create names below the child, for example because the child offers subdomains to customers or has a delegation or wildcard record pointing to a third party, those parties' names would appear affiliated with the parent. A subtree confirmation is also not evidence about the content of any particular name below the child; abuse observed there is handled as in . This is why parents include "s=subtree" only for children whose entire namespace they control (). A dangling record below the child, such as a CNAME to a deprovisioned cloud resource, has the same effect if taken over, so parents using subtree confirmations keep the child's zone free of such records.

Failure Reports An attacker can publish many claims naming a parent so that verifiers send reports to it; the per-pair limit in and parent-side rate limiting bound the effect. Restricting report destinations to the parent's own names prevents markers from being used to direct reports at third parties. A forged marker can direct reports from a verifier to a name chosen by the attacker only within the same limits as other forged responses (). Reports are never evidence that a domain is malicious ().

Expiry Dates Expiry dates are evaluated against the verifier's clock. An expiry date limits how long a forgotten confirmation remains effective but does not replace removing confirmations ().

Confusable Names An attacker can create its own parent and children with names resembling a legitimate organization's. Their affiliations verify, but convey only the standing of the attacker's parent (). User interfaces that display affiliations SHOULD display names in a form that makes confusable characters evident .

Privacy Considerations A claim publicly discloses that the child declares an affiliation with the parent; this disclosure is the purpose of the claim. The parent publishes no list of its domains: a query at the parent reveals nothing about which children exist, and confirmations can be located only from the corresponding claims. A third party can still find affiliated domains by examining domains one by one, for example those listed in Certificate Transparency logs or zone files, so organizations should assume that every domain publishing a claim is discoverable. Domains that publish no claim, such as names held defensively or not yet launched, are not revealed. This property depends on confirmation names not being enumerable. Zone transfers and DNSSEC denial of existence using NSEC records can reveal them; NSEC3 hashes of names containing random tokens of 128 bits or more cannot feasibly be reversed. gives recommendations. Queries reveal to resolver operators, and to passive DNS collections, which domains a verifier is evaluating. QNAME minimisation reduces exposure of full names to servers higher in the DNS tree. Failure reports disclose to the parent which of the verifier's evaluations involved claims naming the parent. Verifiers that do not wish to disclose this do not send reports, and the reports contain no information about why an evaluation took place. The abuse contact in the marker is published, and parents SHOULD use role addresses. Publishers SHOULD NOT include personal data in records.

IANA Considerations

Underscored and Globally Scoped DNS Node Name IANA is requested to add the following entry to the "Underscored and Globally Scoped DNS Node Names" registry : Underscored Node Name

RR Type	_NODE NAME	Reference
TXT	_affiliation	RFC XXXX

Well-Known URI IANA is requested to register the following in the "Well-Known URIs" registry :

URI Suffix:

affiliation

Change Controller:

IETF

Specification Document:

RFC XXXX,

Status:

permanent

Related Information:

None

Domain Affiliation Confirmation Registry Group IANA is requested to create a registry group titled "Domain Affiliation Confirmation (DAC)" containing the three registries below. Registrations in all three are made under the Specification Required policy .

DAC Tags Each entry contains a tag name matching the "tag-name" rule, the record in which it appears (claim, marker, or confirmation; a tag may appear in more than one), a short description, and a reference. The designated expert checks that the tag is clearly defined and that verifiers that ignore it remain interoperable. The initial contents are: Initial DAC Tags

Tag	Record	Description	Reference
v	claim, marker, confirmation	version	RFC XXXX
p	claim	parent domain	RFC XXXX
t	claim	token	RFC XXXX
c	marker	confirmation channel	RFC XXXX
r	marker	abuse contact URI	RFC XXXX
m	marker	mode	RFC XXXX
d	confirmation	child domain	RFC XXXX
s	confirmation	scope	RFC XXXX
x	confirmation	expiry date	RFC XXXX
f	marker	failure report URI	RFC XXXX

DAC Confirmation Channels Each entry contains a channel name consisting of lowercase letters, digits, and hyphens, a short description, and a reference. The designated expert checks that the specification states how a confirmation is obtained, which outcomes map to which results, and the security properties of the channel. The initial contents are: Initial DAC Confirmation Channels

Channel	Description	Reference
dns	TXT record under the parent's "_affiliation" name	RFC XXXX
https	Resource under "/.well-known/affiliation/"	RFC XXXX

DAC Report Reasons Each entry contains a reason value consisting of lowercase letters, digits, and hyphens, a short description, and a reference. The designated expert checks that the reason describes a distinct outcome of the procedure in or of a channel. The initial contents are: Initial DAC Report Reasons

Reason	Description	Reference
unconfirmed-token	No confirmation exists for the claim's token	RFC XXXX
different-child	The confirmation names another domain	RFC XXXX
expired	The confirmation's expiry date has passed	RFC XXXX
malformed-confirmation	The confirmation is malformed, not unique, or cannot be obtained	RFC XXXX

Implementation Status This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in . The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist. According to , "this will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. It is up to the individual working groups to use this information as they see fit".

Organization:

The author.

Implementation:

"dac", a library and command-line tool in Go: .

Description:

A verifier implementing and with its own DNS client, failure reporting (), a publisher that maintains an inventory of affiliations and generates claims, markers and confirmations, a server for the HTTPS channel, checks of published records, and a test DNS server. All test vectors of are automated tests, and every record example in this document is checked against the ABNF in .

Level of maturity:

Prototype; not yet deployed.

Coverage:

All of the protocol.

Licensing:

BSD 3-Clause.

Contact:

The author.

References Normative References Informative References IAB CA/Browser Forum Mozilla Foundation Video Games Chronicle

Design Rationale

Why TXT Records discourages overloading TXT records, mainly because unrelated uses collide at the same owner name. DAC uses a dedicated underscored name and a version tag, which avoid that collision. A dedicated resource record type would remove the need for version-tag parsing, and authoritative servers handle unknown types . TXT was chosen because DNS provisioning interfaces used by domain operators support it universally, and because experience with the dedicated SPF type , which removed, showed the deployment cost of requiring one. The data model is simple enough that a dedicated type could be added later without changing it.

Why Not a List of Domains If the parent published a list of its domains, a single query at the parent would disclose the organization's entire portfolio at once, including domains not otherwise easy to find, which is a ready-made target list for attackers and can reveal products before launch. Every new domain would require editing a shared record, and the list would be bounded by record size or moved to a separate document fetched over HTTPS, adding a dependency. Hashing list entries does not help, because domain names are guessable. Per-child confirmations reveal nothing at the parent and are each located only from a claim.

Why Not Signatures A parent could publish a public key and sign each child's name, with the signature published by the child, so that the parent's zone would not change per child. Signatures remain valid after a child changes hands, however, so they need expiry dates, periodic re-signing of every child, revocation lists for urgent cases, and procedures for key rotation and compromise. DAC instead relies on the parent's existing control of its DNS zone or web origin, at the cost of one record per child at the parent.

Why the Procedure Does Not Use the Public Suffix List Mechanisms that infer relationships between names depend on the Public Suffix List , a community-maintained list without a standard update model; replaced its use in DMARC for this reason. DAC infers nothing: each confirmation requires a record that only the parent's operator can publish. A claim naming a public suffix as its parent is confirmed only if that suffix's operator confirms it. Relying parties may still use boundary information to decide which names to evaluate and how to attribute standing.

Why Affiliation Is Not Transitive Across Organizations A mechanism in which organizations vouch for one another, with vouching that propagates, lets an attacker gain standing by compromising or paying any participant, dilutes the meaning of each step, and makes revocation depend on parties who may not know about the chain. DAC expresses only that one operator operates or authorizes a domain. The protocol cannot tell whether two levels belong to the same organization, which is why a confirmation says nothing about the confirmed domain's own children ().

Why One Parent per Child In designs where every member of a group lists, and must be confirmed by, every other member, each new domain requires changes at every member, and a verifier must compute the group as the closure of pairwise links, at a cost that grows with the size of the group and has to be capped by arbitrary limits. DAC relates each child to exactly one parent, so adding a domain changes two records and verification has a fixed cost. Organizations with several levels are handled one affiliation at a time ().

Why Authentication Is Reported Rather Than Required Requiring DNSSEC or an HTTPS origin on both sides of every relationship would exclude most domains, including names held defensively that serve no content, and would make the mechanism unusable for most of the organizations it is meant to help. DAC instead reports whether each result is authenticated (), so that relying parties can require authentication where their decisions warrant it, and states precisely when an HTTPS confirmation counts as authenticated.

Why No Aggregators DAC needs no index, registry of relationships, or submission service. Every verifier obtains results directly from the domains concerned, so there is no additional party to operate, trust, or keep current, and no endpoint that can be used to make a service send traffic to arbitrary domains. Aggregators can be built on DAC, but nothing depends on them.

Why the Meaning of a Confirmation Is Defined A mechanism that declares a relationship between domains, but whose specification forbids relying parties to draw conclusions from it, gives those parties nothing to act on, and the problems in remain. DAC addresses a specific operational problem with intended consumers, takedown services and reputation systems, and states what a confirmation means (), including where reports of abuse are to be sent. It also bounds that meaning: a confirmation conveys no more standing than the parent already has (). The author invites operators of such services to comment on whether the stated meaning meets their needs.

Complete Example The operator of bank.example launches embank.example and uses the DNS channel. A verifier evaluating embank.example:

1. queries TXT at _affiliation.embank.example and obtains parent "bank.example" and the token;
2. queries TXT at _affiliation.bank.example and obtains the marker: channel "dns", mode "enforce", contact "mailto:abuse@bank.example";
3. queries TXT at mfrggzdfmztwq2lknnwg23tpobyxe43u._affiliation.bank.example and obtains "d=embank.example", which equals the child domain.

The result is "confirmed", reusable for 3600 seconds, and authenticated if all three answers validate as secure. A mail filter assessing "statements.embank.example" verifies "embank.example", the domain it already treats as responsible for that name. Because the confirmation above has no "s" tag, the result states nothing about "statements.embank.example" itself. If bank.example operates every name under embank.example, it publishes the confirmation as "v=DAC1; d=embank.example; s=subtree", and the result covers "statements.embank.example" too. An attacker registers embank-login.example and publishes a copy of the claim. Step 3 returns "d=embank.example", which differs from "embank-login.example", so the result is "not-confirmed". When the operator of bank.example sells embank.example, it deletes the confirmation; from then on, embank.example yields "not-confirmed" whatever its new operator publishes.

Test Vectors Each vector describes a change to the data of and the expected result for the child domain "embank.example" (in vector 6, "embank-login.example"). T is the example token. The reference implementation () runs these vectors as tests. Test Vectors

#	Change from the example	Result
1	None	confirmed
2	No claim at the child	none
3	Child has only "v=spf1 -all"	none
4	Child has "v=DAC10; p=bank.example; t=T"	none
5	No marker at the parent	not-confirmed
6	Child is embank-login.example with a copy of the claim	not-confirmed
7	Claim carries a token the parent never issued	not-confirmed
8	Confirmation deleted	not-confirmed
9	Marker name is an empty non-terminal (NOERROR, no data)	not-confirmed
10	Marker answer contains a CNAME	permerror
11	Marker answer contains a CNAME and no TXT data	permerror
12	Claim answer contains a CNAME	confirmed
13	Confirmation answer contains a CNAME	confirmed
14	Two DAC records at the claim name	permerror
15	Two DAC records at the confirmation name	permerror
16	Confirmation repeats the "d" tag	permerror
17	Confirmation has no "d" tag	permerror
18	Token in uppercase	permerror
19	Token of 8 characters	permerror
20	Token of 26 characters, confirmed under that token	confirmed
21	Claim has "p=Bank.Example."	permerror
22	Claim has an additional tag "z=future"	confirmed
23	Claim repeats the "p" tag	permerror
24	Claim names embank.example as its parent	permerror
25	Parent is the single-label name "bank"	confirmed
26	Claim has "p=192.0.2.1"	permerror
27	Marker has "m=test"	confirmed, mode test
28	Marker has "m=other"	confirmed, mode test
29	Confirmation has "s=subtree"	confirmed, covers names below
30	Confirmation has "s=other"	confirmed, child only
31	Confirmation has "x=2099-12-31"	confirmed
32	Confirmation has "x=2020-01-01"	not-confirmed (report reason "expired")
33	Confirmation has "x=2027-02-30"	permerror
34	Confirmation has an "x" value equal to the current UTC date	confirmed
35	Confirmation has "s=subtree; x=2099-12-31"	confirmed, covers names below
36	Marker has "f=mailto:dac-reports@bank.example"; claim carries a token the parent never issued	not-confirmed; report with reason "unconfirmed-token" to that address
37	Marker has "f=mailto:reports@other.example"; claim carries a token the parent never issued	not-confirmed; no report
38	Marker has "f=mailto:r@bank.example?cc=x@other.example"	no reports (tag ignored)
39	Marker has "f=https://user@bank.example/r" or "f=https://bank.example:8443/r"	no reports (tag ignored)
40	Marker has "f=https://reports.bank.example/dac"; "c=https" and the resource returns 404	not-confirmed; report reason "unconfirmed-token"
41	Confirmation repeats the "d" tag, marker has a valid "f"	permerror; report reason "malformed-confirmation"
42	Child record has only marker tags ("v=DAC1; r=mailto:abuse@embank.example")	none
43	Child record has claim and marker tags; its own children verify against it	confirmed (both)
44	Claim has "p=bank.example" but no "t"	permerror
45	Marker has "r=ftp://x.example"	confirmed, no contact
46	Marker has "c=future" or "c=HTTPS"	not-confirmed
47	SERVFAIL, timeout, or DNSSEC bogus at any of the three names	temperror
48	Confirmation name returns NOERROR without data	not-confirmed
49	"c=https"; 200, "text/plain", "v=DAC1; d=embank.example"	confirmed
50	"c=https"; 200, "text/plain; charset=utf-8", "v=DAC1; d=embank.example"	confirmed
51	"c=https"; 200, "text/plain", "Cache-Control: no-store", "v=DAC1; d=embank.example"	confirmed, not reusable
52	"c=https"; 404	not-confirmed
53	"c=https"; 301	permerror
54	"c=https"; 200 with "text/html"	permerror
55	"c=https"; content longer than 1024 octets	permerror
56	"c=https"; content is two records	permerror
57	"c=https"; malformed record	permerror
58	"c=https"; 503 or connection failure	temperror
59	"c=https"; parent has no globally reachable address	permerror
60	"c=https"; no resource, but a DNS confirmation exists	not-confirmed

Acknowledgments This document builds on the practices collected in and on operational experience with SPF, DMARC, MTA-STS, and ACME.