



Internet Engineering Task Force (IETF)                        T. Reddy.K
Request for Comments: 9704                                         Nokia
Category: Standards Track                                        D. Wing
ISSN: 2070-1721                                                   Citrix
                                                                K. Smith
                                                                Vodafone
                                                             B. Schwartz
                                                                    Meta
                                                            January 2025


Establishing Local DNS Authority in Validated Split-Horizon Environments

Abstract

   When split-horizon DNS is deployed by a network, certain domain names
   can be resolved authoritatively by a network-provided DNS resolver.
   DNS clients that are not configured to use this resolver by default
   can use it for these specific domains only.  This specification
   defines a mechanism for domain owners to inform DNS clients about
   local resolvers that are authorized to answer authoritatively for
   certain subdomains.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9704.

Copyright Notice

   Copyright (c) 2025 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Terminology
   3.  Scope
   4.  Requirements
   5.  Establishing Local DNS Authority
     5.1.  Example
     5.2.  Conveying Authorization Claims
       5.2.1.  Using DHCP
       5.2.2.  Using Provisioning Domains
   6.  Validating Authority over Local Domain Hints
     6.1.  Using a Preconfigured External Resolver
     6.2.  Using DNSSEC
   7.  Delegating DNSSEC Across Split DNS Boundaries
   8.  Example Split-Horizon DNS Configuration
     8.1.  Verification Using an External Resolver
     8.2.  Verification Using DNSSEC
   9.  Operational Efficiency in Split-Horizon Deployments
   10. Validation with IKEv2
   11. Authorization Claim Update
   12. Security Considerations
   13. IANA Considerations
     13.1.  New DHCP Authentication Algorithm for Split DNS
     13.2.  New PvD Additional Information Type for Split DNS
     13.3.  New PvD Split DNS Claims Registry
       13.3.1.  Guidelines for the Designated Experts
     13.4.  DNS Underscore Name
   14. References
     14.1.  Normative References
     14.2.  Informative References
   Acknowledgements
   Authors' Addresses

1.  Introduction

   To resolve a DNS query, there are three main behaviors that an
   implementation can apply: (1) answer from a local database, (2) query
   the relevant authorities and their parents, or (3) ask a server to
   query those authorities and return the final answer.  Implementations
   that use these behaviors are called "authoritative nameservers",
   "full/recursive resolvers", and "forwarders" (or "stub resolvers"),
   respectively.  However, an implementation can also implement a
   mixture of these behaviors, depending on local policy, for each
   query.  Such an implementation is termed a "hybrid resolver".

   Most DNS resolvers are hybrids of some kind.  For example, stub
   resolvers support a local "hosts file" that preempts query
   forwarding, and most DNS forwarders and full resolvers can also serve
   responses from a local zone file.  Other standardized hybrid
   resolution behaviors include using a local root [RFC8806], Multicast
   DNS (mDNS) [RFC6762], and NXDOMAIN synthesis for .onion [RFC7686].

   Networks usually offer clients a DNS resolver using means such as
   DHCP offers or IPv6 Router Advertisements (RAs).  Although this
   resolver is formally specified as a recursive resolver (e.g., see
   Section 5.1 of [RFC8106]), some networks provide a hybrid resolver
   instead.  If this resolver acts as an authoritative server for some
   names and -- depending on the source of the query -- provides
   different answers for those domains, the network is said to be using
   "split-horizon DNS", because those names resolve in this way only
   from inside the network.

   DNS clients that use pure stub resolution, sending all queries to the
   network-provided resolver, will always receive the split-horizon
   results.  Conversely, clients that send all queries to a different
   resolver or implement pure full resolution locally will never receive
   them.  Clients that strictly implement either of these resolution
   behaviors are out of scope for this specification.  Instead, this
   specification enables hybrid clients to access split-horizon results
   from a network-provided hybrid resolver, while using a different
   resolution method for some or all other names.

   There are several existing mechanisms for a network to provide
   clients with "local domain hints", listing domain names that are
   given special treatment in this network (e.g., "Recursive DNS Server
   (RDNSS) selection" [RFC6731], "access network domain name" [RFC5986],
   and "Client Fully Qualified Domain Name (FQDN)" [RFC4702] [RFC4704]
   in DHCP; "dnsZones" in Provisioning Domains (PvDs) [RFC8801]; and
   "INTERNAL_DNS_DOMAIN" [RFC8598] in Internet Key Exchange Protocol
   Version 2 (IKEv2)).  However, none of the local domain hint
   mechanisms enable clients to determine whether this special treatment
   is authorized by the domain owner.  Instead, these specifications
   require clients to make their own determinations about whether to
   trust and rely on these hints.

   This document describes a mechanism between domain names, networks,
   and clients that allows the network to establish its authority over a
   domain to a client (Section 5).  Clients can use this protocol to
   confirm that a local domain hint was authorized by the domain owner
   (Section 6), which might influence its processing of that hint.  This
   process requires cooperation between the local DNS zone and the
   public zone.

   In this specification, network operators securely identify the local
   DNS servers, and clients check each local domain hint against a
   globally valid parent zone.

2.  Terminology

   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.

   This document makes use of the terms defined in [RFC9499], e.g.,
   "global DNS".  The following additional terms are used throughout
   this document:

   Encrypted DNS:  A DNS protocol that provides an encrypted channel
      between a DNS client and server (e.g., DNS over TLS (DoT)
      [RFC7858], DNS (queries) over HTTPS (DoH) [RFC8484], DNS over QUIC
      (DoQ) [RFC9250]).

   Encrypted DNS Resolver:  Refers to a DNS resolver that supports any
      encrypted DNS scheme.

   Split-Horizon DNS:  The DNS service provided by a resolver that also
      acts as an authoritative server for some names, providing
      resolution results that are meaningfully different from those in
      the global DNS.  (See the definition of "split DNS" in Section 6
      of [RFC9499].)

   Validated Split Horizon:  A split-horizon configuration that is
      authorized by the parents of the affected names and confirmed by
      the client.  Such authorization generally extends to the entire
      subtree of names below the authorization point.

   In this document, the terms "owner" and "operator" are used
   interchangeably and refer to the individual or entity responsible for
   the management and maintenance of domains.

3.  Scope

   The protocol described in this document is designed to support the
   ability of a domain owner to create or authorize a split-horizon view
   of their domain.  The protocol does not support split-horizon views
   created by any other entity.  Thus, DNS filtering is not enabled by
   this protocol.

   The protocol is applicable to any type of network offering split-
   horizon DNS configuration.  The endpoint does not need any prior
   configuration to confirm that a local domain hint was indeed
   authorized by the domain.

   All of the Special-Use Domain Names registered with IANA [RFC6761],
   most notably "home.arpa.", "resolver.arpa.", "ipv4only.arpa.", and
   "local.", are never unique to a specific DNS server's authority.  All
   Special-Use Domain Names are outside the scope of this document and
   MUST NOT be validated using the mechanism described in this document.

   The use of this specification is limited to DNS servers that support
   authenticated encryption and split-horizon DNS names that are rooted
   in the global DNS.

4.  Requirements

   This solution seeks to fulfill the following requirements:

   No loss of security:  No unauthorized party can impersonate a zone
      unless they could already do so without the use of this
      specification.

   Least privilege:  Local resolvers do not hold any secrets that could
      weaken the security of the public zone if compromised.

   Local zone confidentiality:  The specification does not leak local
      network subdomains to anyone outside of the network.

   Flexibility:  The specification can represent and authorize a split
      DNS zone structure.

   DNSSEC compatibility:  The specification supports DNSSEC-based object
      security for local zone contents per [RFC9364].

5.  Establishing Local DNS Authority

   A participating network MUST offer one or more encrypted resolvers
   via DHCP and Router Advertisement options for the Discovery of
   Network-designated Resolvers (DNR) [RFC9463], Discovery of Designated
   Resolvers (DDR) [RFC9462], or an equivalent mechanism (see
   Section 10).

   To establish local authority, the network MUST convey one or more
   "authorization claims" to the client.  An authorization claim is an
   abstract structure comprising:

   *  An Authentication Domain Name (ADN) of a local encrypted resolver.

   *  The DNS name of the authorizing parent zone.

   *  A set of subdomains of this parent zone that are claimed by the
      named local resolver (potentially including the entire parent
      zone).  To claim the entire parent zone, the claimed subdomain
      will be represented as an asterisk symbol ("*").

   *  A ZONEMD Hash Algorithm (Section 5.3 of [RFC8976]).  For
      interoperability purposes, implementations MUST support the
      "mandatory to implement" hash algorithms defined in Section 2.2.3
      of [RFC8976].

   *  A high-entropy salt, up to 255 octets.

   If the local encrypted resolver is identified by name (e.g., using
   DNR), that identifying name MUST be the name used in any
   corresponding authorization claim.  Otherwise (e.g., DDR using IP
   addresses), the resolver MUST present a validatable certificate
   containing a subjectAltName that matches the authorization claim
   using the validation techniques for matching as described in
   [RFC9525].

   The network then provides each authorization claim to the parent zone
   operator.  If the contents are approved, the parent zone operator
   computes a "Verification Token" according to the following procedure:

   1.  Convert all subdomains into canonical form and sort them in
       canonical order (Section 6 of [RFC4034]).

   2.  Replace the suffix corresponding to the parent zone with a zero
       octet.

   3.  Let $X be the concatenation of the resulting pseudo-FQDNs.

   4.  Let len($SALT) be the number of octets of salt, as a single
       octet.

   5.  Let $TOKEN = hash(len($SALT) || $SALT || $X), where "||" denotes
       concatenation and hash is the ZONEMD Hash Algorithm.

   The zone operator then publishes a "Verification Record" with the
   following structure, following the best practices outlined in
   Sections 5.2 and 5.3 of [DOMAIN-VERIFICATION-TECHNIQUES]:

   *  Type = TXT

   *  Owner Name = Concatenation of the ADN, "_splitdns-challenge", and
      the parent zone name

   *  Contents = "key/value" pairs, e.g., "token=base64url($TOKEN)"
      (without padding)

   By publishing this record, the parent zone authorizes the local
   encrypted resolver to serve these subdomains authoritatively.

5.1.  Example

   Consider the following authorization claim:

   *  ADN = "resolver17.parent.example"

   *  Parent = "parent.example"

   *  Subdomains = "payroll.parent.example",
      "secret.project.parent.example"

   *  Hash Algorithm = SHA-384 [RFC6234]

   *  Salt = "example salt octets (should be random)"

   To approve this claim, the zone operator would publish the following
   record:

   NOTE: '\' line wrapping per RFC 8792

     resolver17.parent.example._splitdns-challenge.parent.example. \
     IN TXT "token=z1qyK7QWwQPkT-ZmVW-tAQbsNyYenTNBPp5ogYB8S1wesVCR\
     -KJDv2eFwfJcWQM"

5.2.  Conveying Authorization Claims

   The authorization claim is an abstract structure that must be encoded
   in some concrete syntax in order to convey it from the network to the
   client.  This section defines some encodings of the authorization
   claims.

5.2.1.  Using DHCP

   In DHCP, each authorization claim is encoded as a DHCP Authentication
   option ([RFC3118] and Section 21.11 of [RFC8415]), using the Protocol
   value 4, "Split-horizon DNS".  In DHCPv4 [RFC2131], the mechanism for
   splitting long options as described in Section 8 of [RFC3396] MUST be
   used if the Authentication option exceeds the maximum DHCPv4 option
   size of 255 octets.  The Algorithm field provides the ZONEMD Hash
   Algorithm, represented by its registered Value.  The Replay Detection
   Method value MUST be 0x00.  The Authentication Information MUST
   contain the following information, concatenated:

   1.  The ADN in canonical form.

   2.  The parent name in canonical form.

   3.  A one-octet "salt length" field.

   4.  The salt value.

   5.  The $X value as defined in Section 5.

5.2.2.  Using Provisioning Domains

   When using PvDs [RFC8801], the authorization claims are represented
   by the PvD Additional Information key "splitDnsClaims", whose value
   is a JSON array.  Each entry in the array MUST be a JSON object with
   the following structure:

   "resolver":  The ADN as a dot-separated name.

   "parent":  The parent zone name as a dot-separated name.

   "subdomains":  An array containing the claimed subdomains, as dot-
      separated names with the parent suffix already removed, in
      canonical order.  To claim the entire parent zone, the claimed
      subdomain will be represented as an asterisk symbol ("*").

   "algorithm":  The hash algorithm, represented by its "Mnemonic"
      string from the "ZONEMD Hash Algorithms" registry (Section 5.3 of
      [RFC8976]).

   "salt":  The salt, encoded in base64url [RFC4648].

   Future specifications aiming to define new keys will need to add them
   to the IANA registry defined in Section 13.3.  DNS client
   implementations will ignore any keys they don't recognize but may
   also report unknown keys.

6.  Validating Authority over Local Domain Hints

   To validate an authorization claim provided by the network, DNS
   clients MUST resolve the Verification Record for that name.  If the
   resolution produces an RRset containing the expected token for this
   claim, the client SHALL regard the named resolver as authoritative
   for the claimed subdomains.  Clients MUST ignore any unrecognized
   keys in the Verification Record.

   Each validation of authority applies only to a specific ADN.  If a
   network offers multiple encrypted resolvers, each claimed subdomain
   may be authorized for a distinct subset of the network-provided
   resolvers.

   A zone is termed a "Validated Split-Horizon zone" after successful
   validation using a "tamperproof" DNS resolution method, i.e., a
   method that is not subject to interference by the local network
   operator.  Two possible tamperproof resolution methods are presented
   below.

6.1.  Using a Preconfigured External Resolver

   This method applies only if the client is already configured with a
   default resolution strategy that sends queries to a resolver outside
   of the network over an encrypted transport.  That resolution strategy
   is considered tamperproof because any actor who could modify the
   response could already modify all of the user's other DNS responses.
   If the client cannot obtain a response from the external resolver
   within a reasonable timeframe, it MUST consider the verification
   process to have failed.

   To ensure that this assumption holds, clients MUST NOT relax the
   acceptance rules they would otherwise apply when using this resolver.
   For example, if the client would check the Authenticated Data (AD)
   bit or validate RRSIGs locally when using this resolver, it must also
   do so when resolving TXT records for this purpose.  The client MAY
   perform DNSSEC validation for the verification query even if it has
   disabled DNSSEC validation for other DNS queries.

6.2.  Using DNSSEC

   The client resolves the Verification Record using any resolution
   method of its choice (e.g., querying one of the network-provided
   resolvers, performing iterative resolution locally) and performs full
   DNSSEC validation locally [RFC6698].  The result is processed based
   on its DNSSEC validation state (Section 4.3 of [RFC4035]):

   *Secure*:  The response is used for validation.

   *Bogus* or *Indeterminate*:  The response is rejected, and validation
      is considered to have failed.

   *Insecure*:  The client SHOULD retry the validation process using a
      different method, such as the method described in Section 6.1, to
      ensure compatibility with unsigned names.  If the client chooses
      not to retry (e.g., no configured policy to validate the
      authorization claim using an external resolver), it MUST consider
      validation to have failed.

7.  Delegating DNSSEC Across Split DNS Boundaries

   When the local zone can be signed with globally trusted keys for the
   parent zone, support for DNSSEC can be accomplished by simply placing
   a zone cut at the parent zone and including a suitable DS record for
   the local resolver's DNSKEY.  Zones in this configuration appear the
   same to validating stubs whether or not they implement this
   specification.

   To enable DNSSEC validation of local DNS names without requiring the
   local resolver to hold DNSSEC private keys that are valid for the
   parent zone, parent zones MAY add a "ds=..." key to the Verification
   Record whose value is the RDATA of a single DS record, encoded in
   base64url.  This DS record authorizes a DNSKEY whose owner name is
   "resolver.arpa."

   To validate DNSSEC, the client first fetches and validates the
   Verification Record.  If it is valid and contains a "ds" key, the
   client MAY send a DNSKEY query for "resolver.arpa." to the local
   encrypted resolver.  At least one resulting DNSKEY Resource Record
   (RR) MUST match the DS RDATA from the "ds" key in the Verification
   Record.  All local resolution results for subdomains in this claim
   MUST offer RRSIGs that chain to a DNSKEY whose RDATA is identical to
   one of these approved DNSKEYs.

   The "ds" key MAY appear multiple times in a single Verification
   Record, in order to authorize multiple DNSKEYs for this local
   encrypted resolver.

   Note that when the local resolver does not have a globally trusted
   DNSKEY, any claimed subdomains MUST be marked as unsigned in the
   public DNS.  Otherwise, local resolution results would be rejected by
   validating stubs that do not implement this specification.

   NOTE: '\' line wrapping per RFC 8792

   ;; Parent zone.
   $ORIGIN parent.example.

   ; Parent zone's public Key Signing Key (KSK)
   ; and Zone Signing Key (ZSK).
   @ IN DNSKEY 257 3 5 ABCD...=
   @ IN DNSKEY 256 3 5 DCBA...=

   ; Verification Record containing DS RDATA for the local
   ; resolver's KSK.  This is an ordinary public TXT record,
   ; secured by RRSIGs from the public ZSK.
   resolver.example._splitdns-challenge IN TXT "token=abc...,ds=QWE..."

   ; NSEC record indicating that unsigned delegations are permitted at
   ; this subdomain.  This is required for compatibility with
   ; non-split-aware validating stub resolvers.  If the claimed label is
   ; confidential, the parent zone can conceal it using NSEC3 (with or
   ; without "opt-out").
   @ IN NSEC subdomain.parent.example. NS

   ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;

   ;; Local zone, claiming "subdomain.parent.example".

   ; The local resolver's KSK, validated by the Verification Record.
   ; It may not have a corresponding RRSIG.
   resolver.arpa. IN DNSKEY 257 3 5 ASDF...=

   ; Each claimed subdomain duplicates the local resolver's KSK at its
   ; zone apex and uses it to sign the ZSK.
   subdomain.parent.example.        IN DNSKEY 257 3 5 ASDF...=
   subdomain.parent.example.        IN DNSKEY 256 3 5 FDSA...=
   subdomain.parent.example         IN RRSIG DNSKEY 5 3 ...  \
           (KSK key tag) subdomain.parent.example. ...
   subdomain.parent.example.        IN AAAA 2001:db8::17
   subdomain.parent.example         IN RRSIG AAAA 5 3 ...    \
           (ZSK key tag) subdomain.parent.example. ...
   deeper.subdomain.parent.example. IN AAAA 2001:db8::18
   deeper.subdomain.parent.example  IN RRSIG AAAA 5 3 ...    \
           (ZSK key tag) subdomain.parent.example. ...

                     Figure 1: Example Use of "ds=..."

8.  Example Split-Horizon DNS Configuration

   Consider an organization that operates "example.com" and runs a
   different version of its global domain on its internal network.

   First, the host and network both need to support one of the discovery
   mechanisms described in Section 5.  Figure 2 shows discovery using
   information from the DNR and the PvD.

   Validation is then performed using either an external resolver
   (Section 8.1) or DNSSEC (Section 8.2).

   *Steps 1-2*:  The client determines the network's DNS server
      (dns.example.net) and PvD ID (pvd.example.com) using DNR and a
      PvD, along with one of the following: DNR Router Solicitation,
      DHCPv4, or DHCPv6.

   *Steps 3-5*:  The client connects to dns.example.net using an
      encrypted transport as indicated in DNR [RFC9463], authenticating
      the server to its name using TLS (Section 8 of [RFC8310]), and
      sends it a query for the address of pvd.example.com.

   *Steps 6-7*:  The client connects to the PvD server, validates its
      certificate, and retrieves the PvD Additional Information
      indicated by the associated PvD.  The JSON object contains:

      {
        "identifier": "pvd.example.com",
        "expires": "2025-05-23T06:00:00Z",
        "prefixes": ["2001:db8:1::/48", "2001:db8:4::/48"],
        "splitDnsClaims": [{
          "resolver": "dns.example.net",
          "parent": "example.com",
          "subdomains": ["*"],
          "algorithm": "SHA384",
          "salt": "abc...123"
        }]
      }

      The JSON keys "identifier", "expires", and "prefixes" are defined
      in [RFC8801].

   +---------+         +--------------------+  +------------+ +--------+
   | Client  |         | Network's          |  | Network    | | Router |
   |         |         | Encrypted Resolver |  | PvD Server | |        |
   +---------+         +--------------------+  +------------+ +--------+
      |                                     |         |            |
      | Router Solicitation or              |         |            |
      | DHCPv4/DHCPv6 (1)                   |         |            |
      |----------------------------------------------------------->|
      |                                     |         |            |
      |  Response with DNR ADN &            |         |            |
      |  PvD FQDN (2)                       |         |            |
      |<-----------------------------------------------------------|
      | ----------------------------\       |         |            |
      |-| now knows DNR ADN &       |       |         |            |
      | | PvD FQDN                  |       |         |            |
      | |---------------------------/       |         |            |
      |                                     |         |            |
      | TLS connection to dns.example.net (3)         |            |
      |------------------------------------>|         |            |
      | ---------------------------\        |         |            |
      |-| validate TLS certificate |        |         |            |
      | |--------------------------/        |         |            |
      |                                     |         |            |
      | resolve pvd.example.com (4)         |         |            |
      |------------------------------------>|         |            |
      |                                     |         |            |
      |            A or AAAA records (5)    |         |            |
      |<------------------------------------|         |            |
      |                                     |         |            |
      | https://pvd.example.com/.well-known/pvd (6)   |            |
      |---------------------------------------------->|            |
      |                                     |         |            |
      |  200 OK (JSON Additional Information) (7)     |            |
      |<----------------------------------------------|            |
      | ----------------------------------\ |         |            |
      |-| {..., "splitDnsClaims": [...] } | |         |            |
      | |---------------------------------/ |         |            |

       Figure 2: An Example of Learning Local Claims of DNS Authority

8.1.  Verification Using an External Resolver

   Figure 3 shows the steps performed to verify the local claims of DNS
   authority using an external resolver.

   *Steps 1-2*:  The client uses an encrypted DNS connection to an
      external resolver to issue TXT queries for the Verification
      Records.  The TXT lookup returns a token that matches the claim.

   *Step 3*:  The client has validated that example.com has authorized
      dns.example.net to serve example.com.  When the client connects
      using an encrypted transport as indicated in DNR [RFC9463], it
      will authenticate the server to its name using TLS (Section 8 of
      [RFC8310]) and send queries to resolve any names that fall within
      the claimed zones.

   +---------+                  +--------------------+  +----------+
   | Client  |                  | Network's          |  | External |
   |         |                  | Encrypted Resolver |  | Resolver |
   +---------+                  +--------------------+  +----------+
        |                                          |         |
        | TLS connection                           |         |
        |--------------------------------------------------->|
        | ---------------------------\             |         |
        |-| validate TLS certificate |             |         |
        | |--------------------------|             |         |
        |                                          |         |
        | TXT? dns.example.net.\                   |         |
        |   _splitdns-challenge.example.com (1)    |         |
        |--------------------------------------------------->|
        |                                          |         |
        |  TXT "token=ABC..." (2)                  |         |
        |<---------------------------------------------------|
        | --------------------------------\        |         |
        |-| dns.example.net is authorized |        |         |
        | ----------------------\---------|        |         |
        |-| finished validation |                  |         |
        | |---------------------|                  |         |
        |                                          |         |
        |  use dns.example.net when                |         |
        |  resolving example.com (3)               |         |
        |----------------------------------------->|         |
        |                                          |         |

           Figure 3: Verifying Claims Using an External Resolver

8.2.  Verification Using DNSSEC

   Figure 4 shows the steps performed to verify the local claims of DNS
   authority using DNSSEC.

   *Steps 1-2*:  The DNSSEC-validating client queries the network's
      encrypted resolver to issue TXT queries for the Verification
      Records.  The TXT lookup will return a signed response containing
      the expected token.  The client then performs full DNSSEC
      validation locally.

   *Step 3*:  If the DNSSEC validation is successful and the token
      matches, then this authorization claim is validated.  Once the
      client connects using an encrypted transport as indicated in DNR
      [RFC9463], it will authenticate the server to its name using TLS
      (Section 8 of [RFC8310]) and send queries to resolve any names
      that fall within the claimed zones.

   +---------+                                    +--------------------+
   | Client  |                                    | Network's          |
   |         |                                    | Encrypted Resolver |
   +---------+                                    +--------------------+
     |                                                               |
     | DNSSEC OK (DO), TXT? dns.example.net.\                        |
     |   _splitdns-challenge.example.com (1)                         |
     |-------------------------------------------------------------->|
     |                                                               |
     | TXT token=DEF..., Signed Answer (RRSIG) (2)                   |
     |<--------------------------------------------------------------|
     | -------------------------------------\                        |
     |-| DNSKEY+TXT matches RRSIG, use TXT  |                        |
     | |------------------------------------|                        |
     | --------------------------------\                             |
     |-| dns.example.net is authorized |                             |
     | |-------------------------------|                             |
     | ----------------------\                                       |
     |-| finished validation |                                       |
     | |---------------------|                                       |
     |                                                               |
     | use encrypted network-designated resolver for example.com (3) |
     |-------------------------------------------------------------->|
     |                                                               |

           Figure 4: An Example of Verifying Claims Using DNSSEC

9.  Operational Efficiency in Split-Horizon Deployments

   In many split-horizon deployments, all non-public domain names are
   placed in a separate child zone (e.g., internal.example.com).  In
   this configuration, the message flow is similar to the flow described
   in Section 8.1, except that queries for hosts not within the
   subdomain (e.g., www.example.com) are sent to the external resolver
   rather than the resolver for internal.example.com.

   As specified in Section 8.1, the internal DNS server will need a
   certificate signed by a Certification Authority (CA) trusted by the
   client.

   Although placing internal domains inside a child domain is not
   necessary to prevent leakage, such placement reduces the frequency of
   changes to the Verification Record.  This document recommends that
   the internal domains be kept in a child zone of the local domain
   hints advertised by the network.  For example, if the PvD "dnsZones"
   entry is "internal.example.com" and the network-provided DNS resolver
   is "ns1.internal.example.com", the network operator can structure the
   internal domain names as "private1.internal.example.com",
   "private2.internal.example.com", etc.  The network-designated
   resolver will be used to resolve the subdomains of the local domain
   hint "*.internal.example.com".

10.  Validation with IKEv2

   When the endpoint is using a VPN tunnel and the tunnel is IPsec, the
   encrypted DNS resolver hosted by the VPN service provider can be
   securely discovered by the endpoint using the ENCDNS_IP* IKEv2
   Configuration Payload Attribute Types defined in [RFC9464].  The VPN
   client can use the mechanism defined in Section 6 to validate that
   the discovered encrypted DNS resolver is authorized to answer for the
   claimed subdomains.

   Other VPN tunnel types have similar configuration capabilities.  Note
   that those capabilities are not discussed in this document.

11.  Authorization Claim Update

   A Verification Record is only valid until it expires.  Expiry occurs
   when the Time To Live (TTL) or DNSSEC signature validity period ends.
   Shortly before Verification Record expiry, clients MUST fetch the
   Verification Records again and repeat the verification procedure.
   This ensures the availability of updated and valid Verification
   Records.

   A new Verification Record must be added to the RRset before the
   corresponding authorization claim is updated.  After the claim is
   updated, the following procedures can be used:

   1.  DHCP reconfiguration can be initiated by a DHCP server that has
       previously communicated with a DHCP client and negotiated for the
       DHCP client to listen for Reconfigure messages, to prompt the
       DHCP client to dynamically request the updated authorization
       claim.  This process avoids the need for the client to wait for
       its current lease to complete and request a new one, enabling the
       lease renewal to be driven by the DHCP server.

   2.  The sequence number in the RA PvD Option can be incremented,
       requiring clients to fetch PvD Additional Information from the
       HTTPS server due to the updated sequence number in the new RA
       (Section 4.1 of [RFC8801]).

   3.  The old Verification Record needs to be maintained until the DHCP
       lease or PvD Additional Information expires.

12.  Security Considerations

   The ADNs of authorized local encrypted resolvers are revealed in the
   owner names of Verification Records.  This makes it easier for domain
   owners to understand which resolvers they are currently authorizing
   to implement split DNS.  However, this could create a confidentiality
   issue if the local encrypted resolver's name contains sensitive
   information or is part of a secret subdomain.  To mitigate the impact
   of such leakage, local resolvers should be given names that do not
   reveal any sensitive information.

   The security properties of hashing algorithms are not fixed.
   Algorithm agility (see [RFC7696]) is achieved by providing
   implementations with the flexibility to choose hashing algorithms
   from the "ZONEMD Hash Algorithms" registry (Section 5.3 of
   [RFC8976]).

   The entropy of a salt depends on a high-quality pseudorandom number
   generator.  For further discussion on random number generation, see
   [RFC4086].  The salt MUST be regenerated whenever the authorization
   claim is updated.

13.  IANA Considerations

13.1.  New DHCP Authentication Algorithm for Split DNS

   IANA has added the following entry to the "Protocol Name Space
   Values" registry in the "Dynamic Host Configuration Protocol (DHCP)
   Authentication Option Name Spaces" registry group:

   Value:  4

   Description:  Split-horizon DNS

   Reference:  RFC 9704

13.2.  New PvD Additional Information Type for Split DNS

   IANA has added the following entry to the "Additional Information PvD
   Keys" registry in the "Provisioning Domains (PvDs)" registry group:

   JSON key:  splitDnsClaims

   Description:  Verifiable locally served domains

   Type:  Array of Objects

   Example:
      [{
        "resolver": "dns.example.net",
        "parent": "example.com",
        "subdomains": ["sub"],
        "algorithm": "SHA384",
        "salt": "abc...123"
      }]

   Reference:  RFC 9704

13.3.  New PvD Split DNS Claims Registry

   IANA has created a new registry called "PvD Split DNS Claims" within
   the "Provisioning Domains (PvDs)" registry group.  This new registry
   reserves JSON keys for use in sub-dictionaries under the
   splitDnsClaims JSON key.  The initial contents of this registry, as
   discussed in Section 5.2.2, are listed below and have been added to
   the registry:

    +==========+================+=======+===================+=========+
    |JSON key  | Description    |Type   | Example           |Reference|
    +==========+================+=======+===================+=========+
    |resolver  | The            |String | "dns.example.net" |RFC 9704 |
    |          | Authentication |       |                   |         |
    |          | Domain Name    |       |                   |         |
    +----------+----------------+-------+-------------------+---------+
    |parent    | The parent     |String | "example.com"     |RFC 9704 |
    |          | zone name      |       |                   |         |
    +----------+----------------+-------+-------------------+---------+
    |subdomains| An array       |Array  | ["sub"]           |RFC 9704 |
    |          | containing the |of     |                   |         |
    |          | claimed        |Strings|                   |         |
    |          | subdomains     |       |                   |         |
    +----------+----------------+-------+-------------------+---------+
    |algorithm | The hash       |String | "SHA384"          |RFC 9704 |
    |          | algorithm      |       |                   |         |
    +----------+----------------+-------+-------------------+---------+
    |salt      | The salt       |String | "abc...123"       |RFC 9704 |
    |          | (base64url)    |       |                   |         |
    +----------+----------------+-------+-------------------+---------+

                         Table 1: Split DNS Claims

   The keys defined in this document are mandatory.  Any new assignments
   of keys will be considered as optional for the purpose of the
   mechanism described in this document.

   New assignments in the "PvD Split DNS Claims" registry will be
   administered by IANA through Expert Review [RFC8126].  Experts are
   requested to ensure that defined keys do not overlap in names or
   semantics.

13.3.1.  Guidelines for the Designated Experts

   It is suggested that multiple designated experts be appointed for
   registry change requests.

   Criteria that should be applied by the designated experts include
   determining whether the proposed registration duplicates existing
   entries and whether the registration description is clear and fits
   the purpose of this registry.

   Registration requests are evaluated within a three-week review period
   on the advice of one or more designated experts.  Within the review
   period, the designated experts will either approve or deny the
   registration request, communicating this decision to IANA.  Denials
   should include an explanation and, if applicable, suggestions as to
   how to make the request successful.

13.4.  DNS Underscore Name

   IANA has added the following entry to the "Underscored and Globally
   Scoped DNS Node Names" registry in the "Domain Name System (DNS)
   Parameters" registry group:

   RR Type:  TXT

   _NODE NAME:  _splitdns-challenge

   Reference:  RFC 9704

14.  References

14.1.  Normative References

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

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,
              <https://www.rfc-editor.org/info/rfc2131>.

   [RFC3118]  Droms, R., Ed. and W. Arbaugh, Ed., "Authentication for
              DHCP Messages", RFC 3118, DOI 10.17487/RFC3118, June 2001,
              <https://www.rfc-editor.org/info/rfc3118>.

   [RFC3396]  Lemon, T. and S. Cheshire, "Encoding Long Options in the
              Dynamic Host Configuration Protocol (DHCPv4)", RFC 3396,
              DOI 10.17487/RFC3396, November 2002,
              <https://www.rfc-editor.org/info/rfc3396>.

   [RFC4034]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Resource Records for the DNS Security Extensions",
              RFC 4034, DOI 10.17487/RFC4034, March 2005,
              <https://www.rfc-editor.org/info/rfc4034>.

   [RFC4035]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "Protocol Modifications for the DNS Security
              Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
              <https://www.rfc-editor.org/info/rfc4035>.

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <https://www.rfc-editor.org/info/rfc4086>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/rfc4648>.

   [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>.

   [RFC6761]  Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
              RFC 6761, DOI 10.17487/RFC6761, February 2013,
              <https://www.rfc-editor.org/info/rfc6761>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [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>.

   [RFC8415]  Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
              Richardson, M., Jiang, S., Lemon, T., and T. Winters,
              "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
              RFC 8415, DOI 10.17487/RFC8415, November 2018,
              <https://www.rfc-editor.org/info/rfc8415>.

   [RFC8801]  Pfister, P., Vyncke, É., Pauly, T., Schinazi, D., and W.
              Shao, "Discovering Provisioning Domain Names and Data",
              RFC 8801, DOI 10.17487/RFC8801, July 2020,
              <https://www.rfc-editor.org/info/rfc8801>.

   [RFC8976]  Wessels, D., Barber, P., Weinberg, M., Kumari, W., and W.
              Hardaker, "Message Digest for DNS Zones", RFC 8976,
              DOI 10.17487/RFC8976, February 2021,
              <https://www.rfc-editor.org/info/rfc8976>.

   [RFC9525]  Saint-Andre, P. and R. Salz, "Service Identity in TLS",
              RFC 9525, DOI 10.17487/RFC9525, November 2023,
              <https://www.rfc-editor.org/info/rfc9525>.

14.2.  Informative References

   [DOMAIN-VERIFICATION-TECHNIQUES]
              Sahib, S., Huque, S., Wouters, P., and E. Nygren, "Domain
              Control Validation using DNS", Work in Progress, Internet-
              Draft, draft-ietf-dnsop-domain-verification-techniques-06,
              21 October 2024, <https://datatracker.ietf.org/doc/html/
              draft-ietf-dnsop-domain-verification-techniques-06>.

   [RFC4702]  Stapp, M., Volz, B., and Y. Rekhter, "The Dynamic Host
              Configuration Protocol (DHCP) Client Fully Qualified
              Domain Name (FQDN) Option", RFC 4702,
              DOI 10.17487/RFC4702, October 2006,
              <https://www.rfc-editor.org/info/rfc4702>.

   [RFC4704]  Volz, B., "The Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6) Client Fully Qualified Domain Name (FQDN)
              Option", RFC 4704, DOI 10.17487/RFC4704, October 2006,
              <https://www.rfc-editor.org/info/rfc4704>.

   [RFC5986]  Thomson, M. and J. Winterbottom, "Discovering the Local
              Location Information Server (LIS)", RFC 5986,
              DOI 10.17487/RFC5986, September 2010,
              <https://www.rfc-editor.org/info/rfc5986>.

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,
              <https://www.rfc-editor.org/info/rfc6234>.

   [RFC6731]  Savolainen, T., Kato, J., and T. Lemon, "Improved
              Recursive DNS Server Selection for Multi-Interfaced
              Nodes", RFC 6731, DOI 10.17487/RFC6731, December 2012,
              <https://www.rfc-editor.org/info/rfc6731>.

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              DOI 10.17487/RFC6762, February 2013,
              <https://www.rfc-editor.org/info/rfc6762>.

   [RFC7686]  Appelbaum, J. and A. Muffett, "The ".onion" Special-Use
              Domain Name", RFC 7686, DOI 10.17487/RFC7686, October
              2015, <https://www.rfc-editor.org/info/rfc7686>.

   [RFC7696]  Housley, R., "Guidelines for Cryptographic Algorithm
              Agility and Selecting Mandatory-to-Implement Algorithms",
              BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,
              <https://www.rfc-editor.org/info/rfc7696>.

   [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>.

   [RFC8106]  Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
              "IPv6 Router Advertisement Options for DNS Configuration",
              RFC 8106, DOI 10.17487/RFC8106, March 2017,
              <https://www.rfc-editor.org/info/rfc8106>.

   [RFC8310]  Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles
              for DNS over TLS and DNS over DTLS", RFC 8310,
              DOI 10.17487/RFC8310, March 2018,
              <https://www.rfc-editor.org/info/rfc8310>.

   [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>.

   [RFC8598]  Pauly, T. and P. Wouters, "Split DNS Configuration for the
              Internet Key Exchange Protocol Version 2 (IKEv2)",
              RFC 8598, DOI 10.17487/RFC8598, May 2019,
              <https://www.rfc-editor.org/info/rfc8598>.

   [RFC8792]  Watsen, K., Auerswald, E., Farrel, A., and Q. Wu,
              "Handling Long Lines in Content of Internet-Drafts and
              RFCs", RFC 8792, DOI 10.17487/RFC8792, June 2020,
              <https://www.rfc-editor.org/info/rfc8792>.

   [RFC8806]  Kumari, W. and P. Hoffman, "Running a Root Server Local to
              a Resolver", RFC 8806, DOI 10.17487/RFC8806, June 2020,
              <https://www.rfc-editor.org/info/rfc8806>.

   [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>.

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

   [RFC9462]  Pauly, T., Kinnear, E., Wood, C. A., McManus, P., and T.
              Jensen, "Discovery of Designated Resolvers", RFC 9462,
              DOI 10.17487/RFC9462, November 2023,
              <https://www.rfc-editor.org/info/rfc9462>.

   [RFC9463]  Boucadair, M., Ed., Reddy.K, T., Ed., Wing, D., Cook, N.,
              and T. Jensen, "DHCP and Router Advertisement Options for
              the Discovery of Network-designated Resolvers (DNR)",
              RFC 9463, DOI 10.17487/RFC9463, November 2023,
              <https://www.rfc-editor.org/info/rfc9463>.

   [RFC9464]  Boucadair, M., Reddy.K, T., Wing, D., and V. Smyslov,
              "Internet Key Exchange Protocol Version 2 (IKEv2)
              Configuration for Encrypted DNS", RFC 9464,
              DOI 10.17487/RFC9464, November 2023,
              <https://www.rfc-editor.org/info/rfc9464>.

   [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>.

Acknowledgements

   Thanks to Mohamed Boucadair, Jim Reid, Tommy Pauly, Paul Vixie,
   Michael Richardson, Bernie Volz, Éric Vyncke, and Vinny Parla for the
   discussion and comments.

   Thanks to Tianran Zhou for the opsdir review, Anthony Somerset for
   the dnsdir review, Watson Ladd for the secdir review, Bob Halley for
   the intdir review, and Mallory Knodel for the genart review.

   Thanks to Mohamed Boucadair for the Shepherd review.

Authors' Addresses

   Tirumaleswar Reddy.K
   Nokia
   India
   Email: kondtir@gmail.com


   Dan Wing
   Citrix Systems, Inc.
   United States of America
   Email: danwing@gmail.com


   Kevin Smith
   Vodafone Group
   One Kingdom Street
   London
   United Kingdom
   Email: kevin.smith@vodafone.com


   Benjamin Schwartz
   Meta Platforms, Inc.
   Email: ietf@bemasc.net
