ADD
Internet Engineering Task Force (IETF) T. Reddy
Internet-Draft Reddy.K
Request for Comments: 9704 Nokia
Intended status:
Category: Standards Track D. Wing
Expires: 22 December 2024
ISSN: 2070-1721 Citrix
K. Smith
Vodafone
B. Schwartz
Meta
20 June
December 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 an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list It represents the consensus of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid the IETF community. It has
received public review and has been approved for a maximum publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of six months RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be updated, replaced, or obsoleted by other documents obtained at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 22 December 2024.
https://www.rfc-editor.org/info/rfc9704.
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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 Preconfigured External Resolver . . . . . . . . 9
6.2. Using DNSSEC . . . . . . . . . . . . . . . . . . . . . . 10
7. Delegating DNSSEC across Across Split DNS Boundaries . . . . . . . . 10
8. Examples of Example Split-Horizon DNS Configuration . . . . . . . . . 12
8.1. Split-Horizon Entire Zone . . . . . . . . . . . . . . . . 13
8.1.1. Verification Using an External Resolver . . . . . . . 14
8.1.2.
8.2. Verification using 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. New DHCP Split DNS Authentication Algorithm . . . . . . . . 18
13.2. Provisioning Domains for Split DNS
13.2. New PvD Additional Information . 18 Type for Split DNS
13.3. New PvD Split DNS Claims Registry . . . . . . . . . . . 19
13.3.1. Guidelines for the Designated Experts . . . . . . . 20
13.4. DNS Underscore Name . . . . . . . . . . . . . . . . . . 20
14. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 21
15.1.
14.1. Normative References . . . . . . . . . . . . . . . . . . 21
15.2.
14.2. Informative References . . . . . . . . . . . . . . . . . 22
Acknowledgements
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") 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 using a local root [RFC8806], mDNS Multicast
DNS (mDNS) [RFC6762], and NXDOMAIN synthesis for .onion [RFC7686].
Networks usually offer clients a DNS resolver using means such as
(e.g.,
DHCP OFFER, offers or IPv6 Router Advertisement). 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 provides different answers for those domains -- depending on the source of the query, it is described as query -- provides
different answers for those domains, the network having 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 have are
given special treatment in this network (e.g., RDNSS Selection "Recursive DNS Server
(RDNSS) selection" [RFC6731],
"Access Network Domain Name" "access network domain name" [RFC5986],
and "Client FQDN"
[RFC4702][RFC4704] Fully Qualified Domain Name (FQDN)" [RFC4702] [RFC4704]
in DHCP, DHCP; "dnsZones" in Provisioning Domains
[RFC8801], (PvDs) [RFC8801]; and INTERNAL_DNS_DOMAIN
"INTERNAL_DNS_DOMAIN" [RFC8598] in IKEv2). Internet Key Exchange Protocol
Version 2 (IKEv2)). However, none of the local domain hint
mechanisms enables 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.
This specification relies on securely identified expects that local DNS servers, servers will be securely
identified and checks that each local domain hint will be checked 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] [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
"global DNS". The following additional terms are used throughout the
this document:
Encrypted DNS 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 Resolver: Refers to a DNS resolver that supports any
encrypted DNS scheme.
Split-Horizon DNS 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 global DNS. (See "Split the definition of "split DNS" in Section 6
of [RFC9499].)
Validated Split-Horizon A split horizon Split Horizon: Indicates that 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' "owner" and 'operator' "operator" are used
interchangeably and refer to the individual or entity responsible for
the management and maintenance of domains.
Lone lines in examples are wrapped using a single backslash ("\") per
[RFC8792].
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 Special-Use Domain Names registered with IANA [RFC6761],
most notably ".home.arpa", "home.arpa.", "resolver.arpa.", "ipv4only.arpa." "ipv4only.arpa.", and
".local",
"local.", are never unique to a specific DNS server's authority. All
special-use domain names
Special-Use Domain Names are outside the scope of this document and
MUST NOT be validated using the mechanism described in this document.
Use
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 split
DNS zone structure.
*
DNSSEC Compatibility: compatibility: The specification supports DNSSEC-based
[RFC9364] object
security for local zone contents. contents per [RFC9364].
5. Establishing Local DNS Authority
A participating network MUST offer one or more encrypted resolvers
via DHCP and Router Advertisement Options 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"
"authorization claims" to the client. An "Authorization Claim" 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 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 name used in any corresponding
Authorization Claim.
authorization claim. Otherwise (e.g., DDR using IP addresses), the
resolver MUST present a validatable certificate containing a
subjectAltName that matches the Authorization Claim authorization claim using the
validation techniques for matching as described in [RFC9525].
The network then provides each Authorization Claim 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 $X), where "||" denotes
concatenation and hash is the ZONEMD Hash Algorithm.
The zone operator then publishes a "Verification Record" with the
following structure, following the best practices outlined in
Sections 5.1 and 5.2 and 5.3 of
[I-D.ietf-dnsop-domain-verification-techniques]: [DOMAIN-VERIFICATION-TECHNIQUES]:
* Type = TXT. TXT
* Owner Name = Concatenation of the ADN, "_splitdns-challenge", and
the parent zone name. name
* Contents = "key/value" pairs, e.g., "token=base64url($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"
5.2. Conveying Authorization Claims
The Authorization Claim 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. authorization
claims.
5.2.1. Using DHCP
In DHCP, each Authorization Claim authorization claim is encoded as a DHCP Authentication
Option
option ([RFC3118] and Section 21.11 of [RFC8415]), using the Protocol
value $TBD1, "Split DNS Authentication". 4, "Split-horizon DNS". In DHCPv4 [RFC2131], the
long-options mechanism for
splitting long options as described in Section 8 of [RFC3396] MUST be
used if the authentication 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 Provisioning Domains PvDs [RFC8801], the Authorization Claims authorization claims are represented
by the PvD Additional Information key "splitDnsClaims", whose value
is a JSON Array. 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 algorithm, represented by its "Mnemonic"
string from the ZONEMD "ZONEMD Hash Algorithms Algorithms" registry ([RFC8976],
Section 5.2).
* (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. 13.3. 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 authorization claim provided by the network, DNS
clients MUST resolve the Verification Record for that name. If the
resolution produces an RRSet RRset containing the expected token for this
Claim,
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 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 a an encrypted transport. That resolution strategy
is considered "tamperproof" 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.
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), locally) and performs full
DNSSEC validation locally [RFC6698]. The result is processed based
on its DNSSEC validation state ([RFC4035], Section 4.3): (Section 4.3 of [RFC4035]):
*Secure*: The response is used for validation.
*Bogus* or *Indeterminate*: The response is rejected rejected, and validation
is considered to have failed.
*Insecure*: The client SHOULD retry the validation process using a
different method, such as the one 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 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 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, base64url-
encoded. encoded in
base64url. This DS record authorizes a DNSKEY whose Owner Name 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 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. 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.
;; Parent zone.
$ORIGIN parent.example.
; Parent zone's public KSK Key Signing Key (KSK)
; and ZSK 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
; 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 Use of "ds=..."
8. Examples of Example 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).
8.1. Split-Horizon Entire Zone
Consider an organization that operates "example.com", "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. PvD information.
Validation is then perfomed performed using either an external resolver
(Section 8.1.1) 8.1) or DNSSEC (Section 8.1.2). 8.2).
*Steps 1-2*: The client determines the network's DNS server
(dns.example.net) and Provisioning Domain PvD information (pvd.example.com) using DNR
[RFC9463] and PvD PvDs [RFC8801], using one of the following: DNR
Router Solicitation, DHCPv4, or DHCPv6.
*Step
*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 ([RFC8310], Section 8), (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 provisioning domain PvD 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].
+---------+ +--------------------+ +------------+ +--------+
| 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.
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 ([RFC8310],
Section 8), (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 Claims Using an external resolver
8.1.2. External Resolver
8.2. Verification using 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 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 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),
(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 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
unnecessary to prevent leakage, such placement reduces the frequency
of changes to the Verification Record, this 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*_* 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, capabilities. Note
that those capabilities are not
detailed here. discussed in this document.
11. Authorization Claim Update
A verification record 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 Verification Record expiry, clients MUST fetch the
verification records
Verification Records again and repeat the verification procedure.
This ensures the availability of updated and valid verification
records. Verification
Records.
A new verification record Verification Record must be added to the RRset before the
corresponding Authorization Claim 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 client to dynamically requesting request the updated Authorization
Claim. 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 will be incremented,
requiring clients to fetch PvD additional information Additional Information from the
HTTPS server due to the updated sequence number in the new RA
([RFC8801], Section 4.1).
(Section 4.1 of [RFC8801]).
3. The old verification record Verification Record needs to be maintained until the DHCP
lease time or PvD Additional Information expiry. expires.
12. Security Considerations
The Authentication Domain Names ADNs of authorized local encrypted resolvers are revealed in the Owner Names
owner names of Verification Records. This makes it easier for domain
owners to understand which resolvers they are currently authorizing
to implement Split 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 agility (see [RFC7696]) is achieved by providing
implementations with the flexibility to choose hashing algorithms
from the ZONEMD Schemes "ZONEMD Hash Algorithms" registry ([RFC8976], Section 5.2). (Section 5.3 of
[RFC8976]).
The entropy of a salt depends on a high-quality pseudo-random 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 Split DNS Authentication Algorithm for Split DNS
IANA is requested to add has added the following entry to the "Protocol Name Space
Values" registry on in the "Dynamic Host Configuration Protocol (DHCP)
Authentication Option Name Spaces" page:
* registry group:
Value: $TBD1
* 4
Description: Split-horizon DNS
*
Reference: (This Document) RFC 9704
13.2. Provisioning Domains Split DNS New PvD Additional Information Type for Split DNS
IANA is requested to add has added the following entry to the "Additional Information PvD
Keys" registry under in the "Provisioning Domains (PvDs)" registry group:
*
JSON key: "splitDnsClaims"
* splitDnsClaims
Description: "Verifiable Verifiable locally served domains"
* domains
Type: Array of Objects
*
Example:
[{
"resolver": "dns.example.net",
"parent": "example.com",
"subdomains": ["sub"],
"algorithm": "SHA384",
"salt": "abc...123"
}]
*
Reference: (This document) RFC 9704
13.3. New PvD Split DNS Claims Registry
IANA is requested to create has created a new registry called "PvD Split DNS Claims"
Registry, within
the "Provisioning Domains (PvDs)" registry page. 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 will be have been added to
the IANA registry:
+------------+-----------------------+---------+-----------------+-----------+
| JSON
+==========+================+=======+===================+=========+
|JSON key | Description | Type |Type | Example | Reference |
+------------+-----------------------+---------+-----------------+-----------+
| resolver |Reference|
+==========+================+=======+===================+=========+
|resolver | The Authentication |String | String |"dns.example.net"| [RFCXXXX] "dns.example.net" |RFC 9704 |
| | Domain Name | Authentication | | | |
| | Domain Name | | | | parent
+----------+----------------+-------+-------------------+---------+
|parent | The parent zone name | String |String | "example.com" | [RFCXXXX] |
| |RFC 9704 |
| | zone name | | | subdomains |
+----------+----------------+-------+-------------------+---------+
|subdomains| An array containing |Array | Array of| ["sub"] |RFC 9704 |
| | | containing the claimed subdomains| Strings |of | | | [RFCXXXX]
| | claimed |Strings| | |
| | subdomains | | | algorithm |
+----------+----------------+-------+-------------------+---------+
|algorithm | The hash algorithm | String |String | "SHA384" | [RFCXXXX] | |RFC 9704 |
| | algorithm | | | | salt
+----------+----------------+-------+-------------------+---------+
|salt | The salt (base64url) | String |String | "abc...123" | [RFCXXXX] |RFC 9704 |
| | (base64url) | | | |
+------------+-----------------------+---------+-----------------+-----------+
Figure 5:
+----------+----------------+-------+-------------------+---------+
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" 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 is requested to add has added the following entry to the "Underscored and Globally
Scoped DNS Node Names" registry under in the "Domain Name System (DNS)
Parameters" registry group:
*
RR Type: TXT
*
_NODE NAME: _splitdns-challenge
*
Reference: (This document) RFC 9704
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.
Thanks to Mohamed Boucadair for the Shepherd review.
15. References
15.1.
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>.
15.2.
14.2. Informative References
[I-D.ietf-dnsop-domain-verification-techniques]
[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
techniques-06, 21 October 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-dnsop-
domain-verification-techniques-04>.
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 Reddy.K
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