Establishing Local DNS Authority in Validated Split-Horizon Environments
draft-ietf-add-split-horizon-authority-14

ADD                                                             T. Reddy
Internet-Draft                                                     Nokia
Intended status: Standards Track                                 D. Wing
Expires: 22 December 2024                                         Citrix
                                                                K. Smith
                                                                Vodafone
                                                             B. Schwartz
                                                                    Meta
                                                            20 June 2024

Establishing Local DNS Authority in Validated Split-Horizon Environments
               draft-ietf-add-split-horizon-authority-14

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.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the Adaptive DNS Discovery
   Working Group mailing list (add@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/add/.

   Source for this draft and an issue tracker can be found at
   https://github.com/ietf-wg-add/draft-ietf-add-split-horizon-
   authority.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 22 December 2024.

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   Copyright (c) 2024 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
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Establishing Local DNS Authority  . . . . . . . . . . . . . .   6
     5.1.  Example . . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.2.  Conveying Authorization Claims  . . . . . . . . . . . . .   8
       5.2.1.  Using DHCP  . . . . . . . . . . . . . . . . . . . . .   8
       5.2.2.  Using Provisioning Domains  . . . . . . . . . . . . .   8
   6.  Validating Authority over Local Domain Hints  . . . . . . . .   9
     6.1.  Using a Pre-configured External Resolver  . . . . . . . .   9
     6.2.  Using DNSSEC  . . . . . . . . . . . . . . . . . . . . . .  10
   7.  Delegating DNSSEC across Split DNS Boundaries . . . . . . . .  10
   8.  Examples of Split-Horizon DNS Configuration . . . . . . . . .  12
     8.1.  Split-Horizon Entire Zone . . . . . . . . . . . . . . . .  13
       8.1.1.  Verification Using an External Resolver . . . . . . .  14
       8.1.2.  Verification using DNSSEC . . . . . . . . . . . . . .  15
   9.  Operational Efficiency in Split-Horizon Deployments . . . . .  16
   10. Validation with IKEv2 . . . . . . . . . . . . . . . . . . . .  17
   11. Authorization Claim Update  . . . . . . . . . . . . . . . . .  17
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  18
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
     13.1.  DHCP Split DNS Authentication Algorithm  . . . . . . . .  18
     13.2.  Provisioning Domains Split DNS Additional Information  .  18
     13.3.  New PvD Split DNS Claims Registry  . . . . . . . . . . .  19

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       13.3.1.  Guidelines for the Designated Experts  . . . . . . .  20
     13.4.  DNS Underscore Name  . . . . . . . . . . . . . . . . . .  20
   14. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  20
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  21
     15.2.  Informative References . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  25

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 a 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 Local Root [RFC8806], mDNS [RFC6762],
   and NXDOMAIN synthesis for .onion [RFC7686].

   Networks usually offer clients a DNS resolver using means such as
   (e.g., DHCP OFFER, IPv6 Router Advertisement).  Although this
   resolver is formally specified as a recursive resolver (e.g.,
   Section 5.1 of [RFC8106]), some networks provide a hybrid resolver
   instead.  If this resolver acts as an authoritative server for some
   names and provides different answers for those domains depending on
   the source of the query, it is described as the network having
   "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.

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   There are several existing mechanisms for a network to provide
   clients with "local domain hints", listing domain names that have
   special treatment in this network (e.g., RDNSS Selection [RFC6731],
   "Access Network Domain Name" [RFC5986], and "Client FQDN"
   [RFC4702][RFC4704] in DHCP, "dnsZones" in Provisioning Domains
   [RFC8801], and INTERNAL_DNS_DOMAIN [RFC8598] in IKEv2).  However,
   none of the local domain hint mechanisms enables 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.

   This specification relies on securely identified local DNS servers,
   and checks 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 the
   document:

   Encrypted DNS  A DNS protocol that provides an encrypted channel
      between a DNS client and server (e.g., DNS over TLS (DoT)
      [RFC7858], HTTPS (DoH) [RFC8484], 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 "Split DNS" in Section 6 of [RFC9499].)

   Validated Split-Horizon  A split horizon configuration for some name

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      is considered "validated" if the client has confirmed that a
      parent of that name has authorized this resolver to serve its own
      responses for that name.  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 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.

   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 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.

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

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., DNR),
   that identifying name MUST be the one 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.

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   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 5.1 and 5.2 of
   [I-D.ietf-dnsop-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 [RFC8792]

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

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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 $TBD1, "Split DNS Authentication".  In DHCPv4 [RFC2131], the
   long-options mechanism 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 defined in Section 5.

5.2.2.  Using Provisioning Domains

   When using Provisioning Domains [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 is represented by its "Mnemonic"
      string from the ZONEMD Hash Algorithms registry ([RFC8976],
      Section 5.2).

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   *  "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.  DNS client
   implementations will ignore any keys they don't recognize but may
   also report about 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 Pre-configured 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 a 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 timeout period, 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.  Alternatively, a
   client might perform DNSSEC validation for the verification query
   even if it has disabled DNSSEC validation for other DNS queries.

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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 ([RFC4035], Section 4.3):

      *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 one 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 simply by 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, base64url-
   encoded.  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 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.

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   The "ds" key MAY appear multiple times in a single Verification
   Record, in order to authorize multiple DNSKEYs for this local
   encrypted resolver.  If the "ds" key is not present in a valid
   Verification Record, the client MUST disable DNSSEC validation when
   resolving the claimed subdomains via this local encrypted resolver.

   Note that in this configuration, any claimed subdomains MUST be
   marked as unsigned in the public DNS.  Otherwise, resolution results
   would be rejected by validating stubs that do not implement this
   specification.

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   ;; Parent zone.
   $ORIGIN parent.example.

   ; Parent zone's public KSK and 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.  Examples of Split-Horizon DNS Configuration

   Two examples are shown below.  The first example shows a company with
   an internal-only DNS server that claims the entire zone for that
   company (e.g., *.example.com).  In the second example, the internal
   servers resolves only a subdomain of the company's zone (e.g.,
   *.internal.example.com).

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8.1.  Split-Horizon Entire Zone

   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
   DNR and PvD.

   Validation is then perfomed using either an external resolver
   (Section 8.1.1) or DNSSEC (Section 8.1.2).

      *Steps 1-2*: The client determines the network's DNS server
      (dns.example.net) and Provisioning Domain (pvd.example.com) using
      DNR [RFC9463] and PvD [RFC8801], using one of DNR Router
      Solicitation, DHCPv4, or DHCPv6.

      *Step 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 ([RFC8310], Section 8), 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 provisioning domain JSON
      information indicated by the associated PvD.  The PvD 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].

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   +---------+         +--------------------+  +------------+ +--------+
   | 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

8.1.1.  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.

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      *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 ([RFC8310],
      Section 8), 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

8.1.2.  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
      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.

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      *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
      ([RFC8310], Section 8), 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 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 in Section 8.1, the internal DNS server will need a certificate
   signed by a Certification Authority (CA) trusted by the client.

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   Although placing internal domains inside a child domain is
   unnecessary to prevent leakage, such placement reduces the frequency
   of changes to the Verification Record, this document recommends 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, not
   detailed here.

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 clients for dynamically requesting 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.

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   2.  The sequence number in the RA PvD option will be incremented,
       requiring clients to fetch PvD additional information from the
       HTTPS server due to the updated sequence number in the new RA
       ([RFC8801], Section 4.1).

   3.  The old verification record needs to be maintained until the DHCP
       lease time or PvD Additional Information expiry.

12.  Security Considerations

   The Authentication Domain Names 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 flexibility to choose hashing algorithms from
   the ZONEMD Schemes registry ([RFC8976], Section 5.2).

   The entropy of salt depends on a high-quality pseudo-random 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.  DHCP Split DNS Authentication Algorithm

   IANA is requested to add the following entry to the "Protocol Name
   Space Values" registry on the "Dynamic Host Configuration Protocol
   (DHCP) Authentication Option Name Spaces" page:

   *  Value: $TBD1

   *  Description: Split-horizon DNS

   *  Reference: (This Document)

13.2.  Provisioning Domains Split DNS Additional Information

   IANA is requested to add the following entry to the "Additional
   Information PvD Keys" registry under the "Provisioning Domains
   (PvDs)" registry group:

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   *  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: (This document)

13.3.  New PvD Split DNS Claims Registry

   IANA is requested to create a new registry "PvD Split DNS Claims"
   Registry, within the "Provisioning Domains (PvDs)" registry page.
   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 will be
   added to the IANA registry:

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

                      Figure 5: Split DNS Claims

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   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" 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 is requested to add the following entry to the "Underscored and
   Globally Scoped DNS Node Names" registry under the "Domain Name
   System (DNS) Parameters" registry group:

   *  RR Type: TXT

   *  _NODE NAME: _splitdns-challenge

   *  Reference: (This document)

14.  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.

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   Thanks to Mohamed Boucadair for the Shepherd review.

15.  References

15.1.  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>.

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   [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>.

15.2.  Informative References

   [I-D.ietf-dnsop-domain-verification-techniques]
              Sahib, S. K., Huque, S., Wouters, P., and E. Nygren,
              "Domain Control Validation using DNS", Work in Progress,
              Internet-Draft, draft-ietf-dnsop-domain-verification-
              techniques-04, 3 March 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-dnsop-
              domain-verification-techniques-04>.

   [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>.

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   [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>.

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   [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>.

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Authors' Addresses

   Tirumaleswar Reddy
   Nokia
   India
   Email: kondtir@gmail.com

   Dan Wing
   Citrix Systems, Inc.
   4988 Great America Pkwy
   Santa Clara, CA 95054
   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

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