Establishing Local DNS Authority in Validated Split-Horizon Environments

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, mDNS, and NXDOMAIN synthesis for .onion. Networks usually offers clients a DNS resolver using means such as (e.g., DHCP OFFER, IPv6 Router Advertisement). Although this server is formally specified as a recursive resolver (e.g., ), 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. 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, "Access Network Domain Name", and "Client FQDN" in DHCP, "dnsZones" in Provisioning Domains , and INTERNAL_DNS_DOMAIN in 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 (). Clients can use this protocol to confirm that a local domain hint was authorized by the domain owner (), 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.

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 when, and only when, they appear in all capitals, as shown here. This document makes use of the terms defined in , 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) , HTTPS (DoH) , QUIC (DoQ) ).
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 .)
Validated Split-Horizon: A split horizon configuration for some name 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.

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 , 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.

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.
DNSSEC Compatibility: The specification supports DNSSEC-based object security for local zone contents.

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) , Discovery of Designated Resolvers (DDR) , or an equivalent mechanism (see ). 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 (). For interoperability purposes implementations MUST support the "mandatory to implement" hash algorithms defined in .
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 . 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:

Convert all subdomains into canonical form and sort them in canonical order ().
Replace the suffix corresponding to the parent zone with a zero octet.
Let $X be the concatenation of the resulting pseudo-FQDNs.
Let len($SALT) be the number of octets of salt, as a single octet.
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 advices such as in Sections 5.1 and 5.2 of :

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.

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

To approve this claim, the zone operator would publish the following record: NOTE: '\' line wrapping per resolver17.parent.example._splitdns-challenge.parent.example. \ IN TXT "token=z1qyK7QWwQPkT-ZmVW-tAQbsNyYenTNBPp5ogYB8S1wesVCR\ -KJDv2eFwfJcWQM"

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.

Using DHCP In DHCP, each Authorization Claim is encoded as a DHCP Authentication Option ( and ), using the Protocol value $TBD1, "Split DNS Authentication". In DHCPv4 , the long-options mechanism described in 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:

The ADN in canonical form.
The parent name in canonical form.
A one-octet "salt length" field.
The salt value.
The $X value defined in .

Using Provisioning Domains When using Provisioning Domains, 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 ().
"salt": The salt, encoded in base64url.

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

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.

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. 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.

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 . The result is processed based on its DNSSEC validation state ():

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 , to ensure compatibility with unsigned names.

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. 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.

Example use of "ds=..." ;; 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. ...

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).

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 . shows discovery using DNR and PvD. Validation is then perfomed using either an external resolver or DNSSEC.

Steps 1-2: The client determines the network's DNS server (dns.example.net) and Provisioning Domain (pvd.example.com) using DNR and PvD, 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, authenticating the server to its name using TLS (), 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 .

An Example of Learning Local Claims of DNS Authority | | | | | | 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": [...] } | | | | | |---------------------------------/ | | | ]]>

Verification Using an External Resolver 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, it will authenticate the server to its name using TLS (), and send queries to resolve any names that fall within the claimed zones.

Verifying claims using an external resolver | | ---------------------------\ | | |-| 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) | | |----------------------------------------->| | | | | ]]>

Verification using DNSSEC 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.
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, it will authenticate the server to its name using TLS (), and send queries to resolve any names that fall within the claimed zones.

An Example of Verifying Claims using DNSSEC | | | | 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) | |-------------------------------------------------------------->| | | ]]>

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 , 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 , 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 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".

Validation with IKEv2 When the VPN 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 . 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.

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:

DHCP reconfiguration can be initiated by the DHCP server to prompt 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.
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 ().
The old verification record needs to be maintained until the DHCP lease time or PvD Additional Information expiry.

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 ) is achieved by providing implementations with flexibility to choose hashing algorithms from the ZONEMD Schemes registry (). The entropy of salt depends on a high-quality pseudo-random number generator. For further discussion on random number generation, see . The salt MUST be regenerated whenever the authorization claim is updated.

IANA Considerations

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)

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:

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)

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 , are listed below and will be added to the IANA registry:

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" registry will be administered by IANA through Expert Review . Experts are requested to ensure that defined keys do not overlap in names or semantics.

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.

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)

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.