TLS Client Authentication via DANE TLSA records
draft-ietf-dance-client-auth-16

Internet Engineering Task Force                                 S. Huque
Internet-Draft                                                Salesforce
Intended status: Standards Track                             V. Dukhovni
Expires: 6 May 2027                                  OpenSSL Corporation
                                                         2 November 2026

            TLS Client Authentication via DANE TLSA records
                    draft-ietf-dance-client-auth-16

Abstract

   The DANE TLSA protocol describes how to publish Transport Layer
   Security (TLS) server certificates or public keys in the DNS.  This
   document describes how to use the TLSA record to publish client
   certificates or public keys, and also the rules and considerations
   for using them with TLS.  In addition, it defines a new TLS
   extension, DANE Client Identity, to convey the client's domain name
   identity to the server.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 6 May 2027.

Copyright Notice

   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction and Motivation . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  Associating Client Identities in DNS TLSA Records . . . . . .   3
     2.1.  Format 1: Service Specific Client Identity  . . . . . . .   4
     2.2.  Format 2: IoT Device Identity . . . . . . . . . . . . . .   4
     2.3.  Format 3: Freeform Client Identity  . . . . . . . . . . .   5
   3.  Example TLSA Records for Clients  . . . . . . . . . . . . . .   5
     3.1.  Format 1: Service Specific Client Identity  . . . . . . .   5
     3.2.  Format 2: IoT Device Identity . . . . . . . . . . . . . .   6
     3.3.  Format 3: Freeform Client Identity  . . . . . . . . . . .   6
   4.  Authentication Model  . . . . . . . . . . . . . . . . . . . .   6
   5.  Client Identifiers in X.509 Certificates  . . . . . . . . . .   7
   6.  Signaling the Client's DANE Identity in TLS . . . . . . . . .   7
   7.  TLS DANE Client Identity Extension  . . . . . . . . . . . . .   8
   8.  Changes to Client and Server Behavior . . . . . . . . . . . .   8
     8.1.  Client Behavior . . . . . . . . . . . . . . . . . . . . .   9
     8.2.  Server Behavior . . . . . . . . . . . . . . . . . . . . .   9
   9.  Raw Public Keys . . . . . . . . . . . . . . . . . . . . . . .  11
   10. Operational Considerations  . . . . . . . . . . . . . . . . .  11
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  13
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
     13.1.  TLS ExtensionType Value  . . . . . . . . . . . . . . . .  15
     13.2.  Underscored and Globally Scoped DNS Node Names . . . . .  15
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  15
     14.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

1.  Introduction and Motivation

   The TLS [RFC9846] and DTLS [RFC9147] protocols optionally support the
   authentication of clients using X.509 certificates [RFC5280] or raw
   public keys [RFC7250].  TLS applications that perform DANE [RFC6698]
   [RFC7671] authentication of servers using TLSA records may also
   desire to authenticate clients using the same mechanism, especially
   if the client identity is in the form of or can be represented by a

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   DNS domain name.  Some design patterns from the Internet of Things
   (IoT) plan to make use of this form of authentication, where large
   networks of physical objects identified by DNS names may authenticate
   themselves using TLS to centralized device management and control
   platforms.  Other potential applications include authenticating the
   client side of SMTP transport security.

   In this document, the term TLS is used generically to describe both
   the TLS and DTLS (Datagram Transport Layer Security) protocols.  The
   protocol changes described can also be used with QUIC, since QUIC re-
   uses the TLS handshake [RFC9001].

   This specification requires TLS 1.3 [RFC9846] or DTLS 1.3 [RFC9147]
   (or later versions).  It relies on extensions carried within the
   encrypted CertificateRequest and Certificate handshake messages.
   Earlier versions of the protocols do not support such extensions,
   since their Certificate message carries no extension fields, and they
   send these messages unencrypted.  Carrying the client's DANE identity
   inside encrypted handshake messages protects it from disclosure on
   the network path; however, see Section 12 regarding its possible
   exposure through DNS queries.

   This protocol is intended for deployments in which disclosure of the
   client's name to DNS resolvers and authoritative servers is
   acceptable, such as devices and services identified by machine names.
   It is generally unsuitable where the client's identity must be kept
   confidential, for example where client names identify individual
   people, unless the mitigations described in Section 12 are applied.

1.1.  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 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Associating Client Identities in DNS TLSA Records

   Different applications may have quite different conventions for
   naming clients via domain names.  This document thus does not
   prescribe a single format, but mentions a few that may have wide
   applicability.

   The name that the client conveys to the server (in the TLS DANE
   Client Identity extension, see Section 7) is the complete owner name
   of the client's TLSA record, that is, the exact DNS name the server
   queries.  Unlike server-side DANE [RFC6698], where the verifier

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   derives the TLSA owner name from transport coordinates (the
   _port._proto prefix), client naming conventions are application-
   specific; this document describes several below and does not
   prescribe one.  Because the TLS layer is general-purpose and has no
   knowledge of these conventions, the client conveys the full owner
   name and the server performs no construction of its own.

   Formats 1 and 2 below use underscore-prefixed labels.  Their global
   underscored node names [RFC8552], "_client" and "_device"
   respectively, are registered for the TLSA RR type (see Section 13).
   An application that defines its own naming convention employing
   underscore-prefixed labels needs to register the corresponding global
   underscored node name, as required by [RFC8552].

2.1.  Format 1: Service Specific Client Identity

   In this format, the owner name of the client TLSA record has the
   following structure:

   [_service]._client.[client-domain-name]

   The first label identifies the application service name.  It is
   followed by the fixed label "_client", and the remaining labels form
   the client domain name.

   Encoding the application service name into the owner name allows the
   same client domain name to have different authentication credentials
   for different application services.  There is no need to encode the
   transport label - the same name form is usable with both TLS and
   DTLS.

   The "_client" label is the global underscored node name of this
   format [RFC8552], and the _service label is scoped beneath it.  The
   _service label is expected to be a service name registered in the
   IANA Service Name Registry [SRVREG] prefixed with an underscore, but
   could also be a custom string defined by an application; such a
   string should be chosen so as not to coincide with a registered
   service name.  Labels beneath "_client" are not entered in the
   "Underscored and Globally Scoped DNS Node Names" registry.

   The RDATA or data field portion of the TLSA record is formed exactly
   as specified in [RFC6698] and [RFC7671], and carries the same
   meaning.

2.2.  Format 2: IoT Device Identity

   The Device Identity form of the TLSA record has the following
   structure:

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   [devicename]._device.[org-domain-name]

   The "_device" label interposed between the client device name labels
   and the organization domain labels allows management of all client
   identities to be delegated to a subzone or to another party.  The
   "_device" label is the global underscored node name of this format
   [RFC8552].

2.3.  Format 3: Freeform Client Identity

   In this format, the owner name of the client TLSA record is an
   ordinary DNS domain name, with no service label or other structural
   label interposed:

   [client-domain-name]

   This form imposes no structure beyond a domain name that identifies
   the client.  Because it need not contain any underscore-prefixed
   labels, it can be represented directly in the dNSName component of
   the Subject Alternative Name extension of an X.509 certificate.  It
   is therefore the most suitable form for deployments whose client
   certificates are issued by a public Certification Authority, or that
   otherwise rely on the certificate to carry the client's name, since
   public CAs and the dNSName type do not generally permit underscore-
   prefixed labels.

3.  Example TLSA Records for Clients

   The following examples are provided in the textual presentation
   format of the DNS TLSA record.

3.1.  Format 1: Service Specific Client Identity

   An example TLSA record for the client "device1.example.com." and the
   application service "smtp".  This record specifies the SHA-256 hash
   of the subject public key component of the end-entity certificate
   corresponding to the client.  The certificate usage for this record
   is 3 (DANE-EE) and thus is validated in accordance with Section 5.1
   of [RFC7671].

   _smtp._client.device1.example.com. IN TLSA (
      3 1 1 d2abde240d7cd3ee6b4b28c54df034b9
            7983a1d16e8a410e4561cb106618e971 )

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3.2.  Format 2: IoT Device Identity

   An example TLSA record for the device named "sensor7" managed by the
   organization "example.com".  This record specifies the SHA-512 hash
   of the subject public key component of an EE certificate
   corresponding to the client.

   sensor7._device.example.com. IN TLSA (
      3 1 2 0f8b48ff5fd94117f21b6550aaee89c8
            d8adbc3f433c8e587a85a14e54667b25
            f4dcd8c4ae6162121ea9166984831b57
            b408534451fd1b9702f8de0532ecd03c )

   The example below shows a wildcard TLSA record mapped to a TLSA
   record with a DANE-TA usage mode.  This allows all client identifiers
   matching the wildcard to be authenticated by client certificates
   issued by an organization-managed Certification Authority.  This
   example presumes an organization-managed CA willing to issue the
   underscore-prefixed device name; see Section 10.

   *._device.example.com. IN TLSA (
      2 0 1 20efa254ecd5b646e701211095bc3fe4
            423e21941b0b29efb21da57ec944a9b5 )

3.3.  Format 3: Freeform Client Identity

   An example TLSA record for the client "device73.example.com", whose
   certificate is issued by a public Certification Authority.  This
   record specifies certificate usage 1 (PKIX-EE) and the SHA-256 hash
   of the subject public key component of the end-entity certificate
   corresponding to the client.

   device73.example.com. IN TLSA (
      1 1 1 d13576c4333389c6489df3566f29a449
            b413f3225ff87a3fb028535596a7af08 )

4.  Authentication Model

   The authentication model assumed in this document is the following:

   The client is assigned an identity corresponding to a DNS domain
   name.

   The client has a private and public key pair.  Where client
   certificates are being used, the client also has a certificate
   binding the name to its public key.  The certificate or public key
   has a corresponding TLSA record published in the DNS, which allows it
   to be authenticated directly via the DNS (using the DANE-TA or DANE-

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   EE certificate usage modes) or via a PKIX public CA system constraint
   if the client's certificate was issued by a public CA (using the
   PKIX-TA or PKIX-EE DANE usage modes).

5.  Client Identifiers in X.509 Certificates

   The client conveys its DNS name to the server using the TLS DANE
   Client Identity extension (see Section 7).

   With the DANE-EE(3) certificate usage, the presented certificate or
   public key is matched directly against the TLSA record, so the
   client's name need not be present in the certificate at all.

   With the DANE-TA(2), PKIX-TA(0), and PKIX-EE(1) certificate usages,
   authentication additionally requires the presented certificate to be
   matched to a reference identity, as specified in [RFC7671].  In these
   cases the client's name, as conveyed in the TLS DANE Client Identity
   extension, MUST be present as a dNSName entry in the certificate's
   Subject Alternative Name extension, and the server MUST verify that
   it matches; if it does not, the server MUST abort the connection with
   a handshake_failure alert.

   The choice of naming convention (Section 2) interacts with the
   certificate usage mode; see Section 10 for guidance.

6.  Signaling the Client's DANE Identity in TLS

   The client MUST explicitly signal that it has a DANE identity.  The
   most important reason is that the server needs an explicit indication
   from the client that it has a DANE record, so as to avoid unnecessary
   DNS queries in-band with the TLS handshake.

   The DANE Client Identity TLS extension is used for this purpose.  A
   conforming client always conveys its DANE client identity (i.e.,
   domain name) in this extension (see Section 7), so the server never
   needs to derive it from the certificate.  This is particularly
   important when using TLS raw public key authentication, where there
   is no certificate from which an identity could be extracted, and when
   a client certificate contains multiple identities, only a specific
   one of which has a DANE record.

   The format of this extension and the detailed client and server
   behavior are specified in Section 7.

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7.  TLS DANE Client Identity Extension

   The DANE Client Identity extension type, "dane_clientid", will have a
   value assigned and registered in the IANA "TLS ExtensionType Values"
   registry.  Its extension_data field always has the following format:

   opaque ClientName<0..2^8-1>;

   The ClientName field contains the full owner name of the client's
   TLSA record in the common display format described in Section 2 of
   [RFC9499], without the trailing dot.  This is the exact DNS name the
   server queries; the server performs no further construction of it
   (see Section 2).  The lower bound length value of 0 octets indicates
   that no client name is present (this form is used by the TLS server,
   in its CertificateRequest message, to advertise that it supports the
   DANE client authentication protocol).

   Each label of the name MUST consist only of ASCII letters, digits,
   hyphens, and underscores, so that the name never requires the escape
   sequences of the [RFC1035] presentation format.  The name MUST be a
   valid domain name, and is therefore at most 255 octets in wire format
   [RFC1035].  Given the character restriction above, it is then at most
   253 octets long in this form, and so always fits within the
   ClientName field.  Internationalized labels MUST be expressed in
   their A-label form [RFC5890], which is the form present in the DNS
   and, per Section 7.2 of [RFC5280] as updated by [RFC9549], the form
   required in a certificate's dNSName; U-labels MUST NOT be used.

   A TLS server implementing this specification MUST send the
   "dane_clientid" extension, with a zero-length ClientName, in its
   CertificateRequest message, to indicate that it understands the
   extension and is capable of performing DANE client authentication.
   The server supplies no name of its own.

   A TLS client implementing this specification and intending to use
   DANE client authentication with the TLS server MUST send an extension
   of type "dane_clientid", with a nonzero-length ClientName populated
   with the full owner name of the client's corresponding DNS TLSA
   record (see Section 2), in its Certificate message.  Per
   Section 4.5.1 of [RFC9846], the client may only send the extension if
   the server's CertificateRequest message included it; a server
   receiving it otherwise aborts the handshake with an
   "unsupported_extension" alert (Section 4.3 of [RFC9846]).

8.  Changes to Client and Server Behavior

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8.1.  Client Behavior

   DANE client authentication requires that a DNSSEC [RFC9364] signed
   TLSA record, corresponding to the client's X.509 certificate or
   public key, be published at the owner name the client conveys in its
   ClientName (see Section 2).

   A client can only use this protocol with a server that supports it
   and advertises that support by including a DANE Client Identity
   extension (with a zero-length ClientName) in its CertificateRequest
   message.  When the client receives such a CertificateRequest, and
   intends to authenticate using DANE, it presents its certificate or
   public key and MUST send the DANE Client Identity extension in its
   Certificate message, carrying a nonzero-length ClientName populated
   with the full owner name of its TLSA record, as specified in
   Section 7.

8.2.  Server Behavior

   A server that requires DANE client authentication MUST NOT negotiate
   a protocol version below TLS 1.3 (or DTLS 1.3) for connections where
   such authentication is mandatory (see Section 1).

   A TLS Server implementing this specification performs the following
   steps:

   1.  Request a client certificate in the TLS handshake's
       CertificateRequest message, that includes a DANE Client Identity
       extension with a zero-length ClientName.

   2.  Receive the client's Certificate message, together with any DANE
       Client Identity extension it carries.

   3.  If the client sent a nonzero-length ClientName in the DANE Client
       Identity extension of its Certificate message, extract the
       client's domain name from it.  If the client sent the extension
       with a zero-length ClientName, this is a protocol violation (a
       conforming client sends a nonzero-length name, per Section 7);
       the server MUST abort the connection with a handshake_failure
       alert.  If the client did not send the extension at all, it has
       not requested DANE client authentication, and this specification
       does not apply to the connection.  A server that supports DANE
       client authentication alongside other modes (for example,
       traditional TLS client authentication, or accepting
       unauthenticated clients) then handles the connection according to
       its general client-authentication policy; a server that requires
       DANE client authentication MUST abort the connection with a
       handshake_failure alert.

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   4.  If the server applies name-based authorization rules (see below),
       it SHOULD first check the client's domain name against them, and
       reject a name that is not permitted without performing the
       lookup.  Otherwise, look up the TLSA record set in the DNS, using
       the client's domain name obtained in the previous step directly
       as the DNS query name.  The response MUST be cryptographically
       validated using DNSSEC.  The server could perform DNSSEC
       validation itself, authenticating the full chain back to a
       configured trust anchor (normally the DNS root).  Alternatively,
       it could also be configured to trust responses obtained via a
       validating resolver to which it has a secure connection, by
       requiring the Authenticated Data (AD) bit to be set in the
       responses.  If any result other than a DNSSEC validated TLSA
       record set is returned, the server MUST either abort the
       connection with a handshake_failure TLS alert, or, if server
       policy allows, treat the client as unauthenticated.  Such other
       results include DNSSEC validation failures, unauthenticated
       answers (because of an unsigned DNS zone or insecure delegation
       along the path to the TLSA owner name), NXDOMAIN, or NODATA
       responses.

   5.  Extract the RDATA of the TLSA records and match them to the
       presented client certificate according to the rules specified in
       the DANE TLS protocol [RFC6698] [RFC7671].  If successfully
       matched, the client is authenticated and the TLS session
       proceeds.  If unsuccessful, the server MUST either abort the
       connection with a handshake_failure TLS alert, or treat the
       client as unauthenticated, if TLS server policy allows.

   6.  If there are multiple records in the TLSA record set, then the
       client is authenticated as long as at least one of the TLSA
       records matches, subject to [RFC7671] digest agility, which
       SHOULD be implemented.

   When the client's name is expected to appear in the certificate, and
   the certificate contains multiple dNSName identities, the ClientName
   sent in the TLS DANE Client Identity extension unambiguously
   indicates which one is the client's name.  If the name in the TLS
   DANE Client Identity extension does not match one of the dNSNames in
   the certificate, then the server MUST abort the connection with a
   handshake_failure TLS alert.

   The client identity authenticated by this protocol is the name
   carried in the ClientName field.  The TLS implementation needs to
   make this name available to the application.  Other names in the
   client's certificate are not authenticated by this protocol; in
   particular, with the DANE-EE usage, names in the certificate are not
   checked at all (see Section 5.1 of [RFC7671]).

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   Servers may have their own allowlisting and authorization rules for
   which client names they accept.  For example a TLS server may be
   configured to only allow TLS sessions from clients with client names
   within a specific domain or set of domains.  If such rules are not
   met, the TLS server MUST either abort the connection with a
   handshake_failure TLS alert, or treat the client as unauthenticated,
   if TLS server policy allows.

   Because the ClientName is supplied by a not yet authenticated peer,
   the lookup in step 4 can be directed at any name, including names in
   zones whose authoritative servers respond slowly or not at all.  This
   matters most for servers that accept arbitrary client names, where no
   allowlist applies.  Servers should bound the time spent on the
   lookup, treating a timeout as a failure under step 4, and should rely
   on caching of TLSA responses, per their TTLs, to avoid repeated
   queries.

9.  Raw Public Keys

   When using raw public keys in TLS [RFC7250], there is no certificate
   to carry the client's name, so the DANE Client Identity extension
   (which a conforming client always sends; see Section 7) is the only
   means by which the client's identity is conveyed.  The associated
   DANE TLSA records employ only certificate usage 3 (DANE-EE) and a
   selector value of 1 (SPKI), as described in Section 5.1 of [RFC7671].
   The client's public key is bound to its name solely by these DNSSEC-
   validated TLSA records: the server matches the SubjectPublicKeyInfo
   in the client's Certificate message against them, either in full
   (matching type 0) or by its SHA-256 or SHA-512 digest (matching types
   1 and 2), and the client's CertificateVerify message proves
   possession of the corresponding private key.

10.  Operational Considerations

   The naming conventions described in Section 2 interact with the DANE
   certificate usage mode a deployment chooses, because of the way a
   client's name may need to appear in its certificate.

   Section 4.2.1.6 of [RFC5280] requires that a dNSName in the Subject
   Alternative Name extension be in the "preferred name syntax", which
   permits only letters, digits, and hyphens within a label.
   Underscore-prefixed labels, such as the _service and _client labels
   of the service-specific format or the _device label of the device-
   identity format (Section 2), therefore cannot appear in a conformant
   dNSName.  This restriction applies to any dNSName, independent of the
   DANE certificate usage mode.

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   With the DANE-EE(3) usage the client's name need not appear in the
   certificate (Section 5), and with raw public keys there is no
   certificate at all (Section 9).  Deployments using these can use any
   of the naming formats in Section 2, including those with underscore-
   prefixed labels, since the name is conveyed only in the TLS DANE
   Client Identity extension and matched directly against the TLSA
   record.

   With the DANE-TA(2), PKIX-TA(0), and PKIX-EE(1) usages the client's
   name must appear as a dNSName in the certificate and be matched
   against it (Section 5), and must therefore be in the preferred name
   syntax.

   Whether an underscore-bearing name can be used in these latter cases
   depends in practice on which certification authority issues the
   client certificate.  The preferred name syntax restriction is not
   enforced by many TLS and X.509 toolkits, which will create, encode,
   and verify certificates containing underscore-prefixed dNSNames.  An
   organization operating its own certification authority can therefore
   issue such certificates for use with the DANE-TA usage, as in the
   wildcard example of Section 3.2.  Commercial certification
   authorities, however, enforce the restriction and will not issue
   certificates containing underscore-prefixed dNSNames; deployments
   relying on them, typically with the PKIX-TA and PKIX-EE usages,
   cannot use the underscore-bearing formats and should adopt a naming
   convention that avoids such labels, for example the freeform identity
   of Section 2.3.

   It is expected that most deployments of this protocol will use the
   DANE certificate usages (DANE-EE(3) or DANE-TA(2)), often with an
   organization-managed CA.  As described above, the naming conventions
   of Section 2, including the underscore-bearing formats, work in
   practice with these usages.  The considerations concerning the
   preferred name syntax apply primarily to the comparatively less
   common deployments that rely on the PKIX-TA and PKIX-EE usages with
   certificates from a commercial certification authority.

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   The TLSA publisher requirements and operational considerations in
   Sections 8 and 13 of [RFC7671] apply to client TLSA records, with the
   roles of client and server reversed.  In particular, when a client's
   key or certificate is rotated, TLSA records matching the new
   credential should be published, and the TTL of the existing records
   allowed to elapse, before the client starts using it.  With the DANE-
   EE(3) usage, certificate expiration is ignored (Section 5.1 of
   [RFC7671]), so removing or replacing the client's TLSA records is the
   means of revoking its credential.  Servers may continue to accept a
   revoked credential until previously cached records expire, and an
   attacker able to replay them can extend this until their DNSSEC
   signatures expire.  The TTL and signature validity period of client
   TLSA records should be chosen with this in mind.

   Client authentication depends on the server obtaining DNSSEC-
   validated responses, and so on the availability of its validating
   resolver and of correctly signed client zones.  An outage of either
   causes all affected clients to fail authentication, or, where server
   policy allows, to be treated as unauthenticated (Section 8.2).
   Operators should provision redundant validating resolvers and monitor
   the DNSSEC signatures of client zones.

   Although the server sends only a generic alert on authentication
   failure (Section 12), it should log the specific cause (e.g., DNSSEC
   validation failure, an insecure response, NXDOMAIN or NODATA, lookup
   timeout, TLSA mismatch, or rejection by local policy) along with the
   ClientName, so that operators can diagnose failures.  Such logs
   contain client identities and should be protected accordingly.

11.  Acknowledgements

   For detailed reviews and helpful comments, the authors would like to
   thank Wes Hardaker, Joey Salazar, Eric Rescorla, Paul Wouters, Ash
   Wilson, Robert Moskowitz, Bill Woodcock, Olle Johansson, Michael
   Richardson, Sandoche Balakrichenan, Rick van Rein, Mike Ounsworth,
   Deb Cooley, Rich Salz, Kaveh Ranjbar, Matt Brown, and other members
   of the IETF DANCE and TLS working groups.

12.  Security Considerations

   This document extends the use of DANE TLSA records [RFC6698]
   [RFC7671] to TLS client authentication.  Placing client identities in
   the DNS may pose privacy issues for certain applications, depending
   on the nature of the clients, the structure and content of the client
   names, and the mechanisms by which they are queried.  In particular,
   client names that correspond to human persons may pose a graver
   privacy risk than machine identities.  Applications employing this
   protocol should carefully assess those potential issues, as described

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   below.

   In TLS 1.3, the client identity is encrypted in the Certificate
   message, and thus normally protected from disclosure on the wire.
   DANE authentication however relies on the peer (the TLS server in
   this case) subsequently looking up the client's DANE record in the
   DNS.  Unless encrypted DNS transport is used (e.g., DNS over TLS
   [RFC7858], DNS over HTTPS [RFC8484], or DNS over QUIC [RFC9250]),
   this can expose the client name in the clear, and thus deployers of
   DANE client authentication should carefully evaluate any resulting
   privacy risks.  To mitigate this risk, deployments should ensure
   encrypted DNS is used to the extent possible.  This could involve
   turning on encrypted DNS between the TLS server and its local
   resolver, enabling it at the authoritative servers hosting the DANE
   client records, and using data minimization techniques like query
   name minimization [RFC9156] to avoid exposing the most specific parts
   of the client DNS names to layers that (largely) have no encrypted
   DNS today (e.g., the DNS root, and many DNS Top Level Domains).

   A possible way to avoid the TLS server looking up the client's DANE
   record in the DNS, is to have the client instead perform the DNS
   lookups for its own DANE record set, and assemble it and the
   associated full DNSSEC authentication chain in a new Certificate
   Extension that is sent to the TLS server.  A specification to do this
   in the other direction (from the server to the client) already
   exists: "TLS DNSSEC Chain Extension" [RFC9102].  However, there are
   several challenges with this approach: It is quite a heavyweight
   operation that some constrained clients may have challenges with (for
   example LoRaWAN clients).  In order to construct the DANE
   authentication chain, the client would need to perform DNS queries
   which would still leak its identity to the local network environment
   without encrypted DNS.  Lastly, there may be client-side network
   impediments to making this work, e.g., middleboxes that prevent
   DNSSEC-enabled queries from succeeding - one of the original
   motivations for RFC 9102 in the first place.

   The service specific client identity form lends itself to a structure
   that might make it easy for the same client to have multiple
   identities corresponding to different applications using the same
   public key (e.g., by using wildcards and DANE-EE mode), which could
   make this protocol susceptible to cross-protocol attacks where
   traffic is redirected from one service to another.  Deployers of this
   protocol should avoid this by not sharing per client credentials
   across distinct applications.

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   This mechanism cannot operate over TLS 1.2 or earlier (see
   Section 1).  A server that negotiated an earlier version would not
   receive the client's DANE identity, and authentication could be
   silently bypassed.  Section 8.2 therefore requires TLS 1.3 or DTLS
   1.3 where DANE client authentication is mandatory.

   This specification uses a single, generic TLS alert
   (handshake_failure) for the various client authentication failure
   conditions it describes, to avoid leaking unnecessary information to
   potential adversaries.

13.  IANA Considerations

13.1.  TLS ExtensionType Value

   IANA is requested to create the following entry in the "TLS
   ExtensionType Values" registry:

   Extension Name "dane_clientid" with value TBD, "TLS 1.3" column
   values set to "CR, CT", "DTLS-Only" column set to "N", "Recommended"
   column set to "N", and "Comment" column set to "For TLS 1.3 or
   later".

   The 'N' designation in the "Recommended" column is because this
   extension has very specific use cases.

13.2.  Underscored and Globally Scoped DNS Node Names

   IANA is requested to add the following entries to the "Underscored
   and Globally Scoped DNS Node Names" registry [RFC8552]:

   +=========+============+============================+
   | RR Type | _NODE NAME | Reference                  |
   +=========+============+============================+
   | TLSA    | _client    | This document, Section 2.1 |
   +---------+------------+----------------------------+
   | TLSA    | _device    | This document, Section 2.2 |
   +---------+------------+----------------------------+

                          Table 1

14.  References

14.1.  Normative References

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, DOI 10.17487/RFC5890, August 2010,
              <https://www.rfc-editor.org/info/rfc5890>.

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
              2012, <https://www.rfc-editor.org/info/rfc6698>.

   [RFC7250]  Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
              Weiler, S., and T. Kivinen, "Using Raw Public Keys in
              Transport Layer Security (TLS) and Datagram Transport
              Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
              June 2014, <https://www.rfc-editor.org/info/rfc7250>.

   [RFC7671]  Dukhovni, V. and W. Hardaker, "The DNS-Based
              Authentication of Named Entities (DANE) Protocol: Updates
              and Operational Guidance", RFC 7671, DOI 10.17487/RFC7671,
              October 2015, <https://www.rfc-editor.org/info/rfc7671>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8552]  Crocker, D., "Scoped Interpretation of DNS Resource
              Records through "Underscored" Naming of Attribute Leaves",
              BCP 222, RFC 8552, DOI 10.17487/RFC8552, March 2019,
              <https://www.rfc-editor.org/info/rfc8552>.

   [RFC9147]  Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
              <https://www.rfc-editor.org/info/rfc9147>.

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   [RFC9364]  Hoffman, P., "DNS Security Extensions (DNSSEC)", BCP 237,
              RFC 9364, DOI 10.17487/RFC9364, February 2023,
              <https://www.rfc-editor.org/info/rfc9364>.

   [RFC9499]  Hoffman, P. and K. Fujiwara, "DNS Terminology", BCP 219,
              RFC 9499, DOI 10.17487/RFC9499, March 2024,
              <https://www.rfc-editor.org/info/rfc9499>.

   [RFC9549]  Housley, R., "Internationalization Updates to RFC 5280",
              RFC 9549, DOI 10.17487/RFC9549, March 2024,
              <https://www.rfc-editor.org/info/rfc9549>.

   [RFC9846]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 9846, DOI 10.17487/RFC9846, July 2026,
              <https://www.rfc-editor.org/info/rfc9846>.

14.2.  Informative References

   [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "Specification for DNS over Transport
              Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
              2016, <https://www.rfc-editor.org/info/rfc7858>.

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
              <https://www.rfc-editor.org/info/rfc8484>.

   [RFC9001]  Thomson, M., Ed. and S. Turner, Ed., "Using TLS to Secure
              QUIC", RFC 9001, DOI 10.17487/RFC9001, May 2021,
              <https://www.rfc-editor.org/info/rfc9001>.

   [RFC9102]  Dukhovni, V., Huque, S., Toorop, W., Wouters, P., and M.
              Shore, "TLS DNSSEC Chain Extension", RFC 9102,
              DOI 10.17487/RFC9102, August 2021,
              <https://www.rfc-editor.org/info/rfc9102>.

   [RFC9156]  Bortzmeyer, S., Dolmans, R., and P. Hoffman, "DNS Query
              Name Minimisation to Improve Privacy", RFC 9156,
              DOI 10.17487/RFC9156, November 2021,
              <https://www.rfc-editor.org/info/rfc9156>.

   [RFC9250]  Huitema, C., Dickinson, S., and A. Mankin, "DNS over
              Dedicated QUIC Connections", RFC 9250,
              DOI 10.17487/RFC9250, May 2022,
              <https://www.rfc-editor.org/info/rfc9250>.

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   [SRVREG]   IANA, "Service Name and Transport Protocol Port Number
              Registry", <https://www.iana.org/assignments/service-
              names-port-numbers/service-names-port-numbers.txt>.

Authors' Addresses

   Shumon Huque
   Salesforce
   Email: shuque@gmail.com

   Viktor Dukhovni
   OpenSSL Corporation
   Email: ietf-dane@dukhovni.org

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