Internet DRAFT - draft-ietf-add-split-horizon-authority
draft-ietf-add-split-horizon-authority
ADD T. Reddy
Internet-Draft Nokia
Intended status: Standards Track D. Wing
Expires: 8 June 2024 Citrix
K. Smith
Vodafone
B. Schwartz
Meta
6 December 2023
Establishing Local DNS Authority in Validated Split-Horizon Environments
draft-ietf-add-split-horizon-authority-07
Abstract
When split-horizon DNS is deployed by a network, certain domains can
be resolved authoritatively by the network-provided DNS resolver.
DNS clients that are not configured to use this resolver can use it,
but only to resolve these domains. This specification defines a
mechanism for domain owners to inform 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
and may be updated, replaced, or obsoleted by other documents at any
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This Internet-Draft will expire on 8 June 2024.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
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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 . . . . . . . . . . . . . . 5
5.1. Example . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.2. Conveying Authorization Claims . . . . . . . . . . . . . 7
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 . . . . . . . . . . . . . . . . . . . . . . 9
7. Delegating DNSSEC across Split DNS Boundaries . . . . . . . . 10
8. Examples of Split-Horizon DNS Configuration . . . . . . . . . 11
8.1. Split-Horizon Entire Zone . . . . . . . . . . . . . . . . 11
8.1.1. Verification using an external resolver . . . . . . . 13
8.1.2. Verification using DNSSEC . . . . . . . . . . . . . . 14
8.2. Internal-only Subdomains . . . . . . . . . . . . . . . . 15
9. Validation with IKEv2 . . . . . . . . . . . . . . . . . . . . 16
10. Authorization Claim Update . . . . . . . . . . . . . . . . . 16
11. Security Considerations . . . . . . . . . . . . . . . . . . . 17
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
12.1. DHCP Split DNS Authentication Algorithm . . . . . . . . 17
12.2. Provisioning Domains Split DNS Additional Information . 17
12.3. DNS Underscore Name . . . . . . . . . . . . . . . . . . 18
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13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 18
14. References . . . . . . . . . . . . . . . . . . . . . . . . . 18
14.1. Normative References . . . . . . . . . . . . . . . . . . 18
14.2. Informative References . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
1. Introduction
To resolve a DNS query, there are three essential 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. We term such an implementation a "hybrid resolver".
Most DNS resolvers are hybrids of some kind. For example, stub
resolvers frequently 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].
In many network environments, the network offers clients a DNS server
(e.g. DHCP OFFER, IPv6 Router Advertisement). Although this server
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, we say that the network has "split-horizon DNS", because those
names resolve in this way only from inside the network.
Network 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 [RFC6731],
"Access Network Domain Name" [RFC5986], and "Client FQDN"
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[RFC4702][RFC4704] in DHCP, "dnsZones" in Provisioning Domains
[RFC8801], and INTERNAL_DNS_DOMAIN [RFC8598] 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 specification describes a protocol between domains, 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
(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 [RFC8499], 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)).
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 [RFC8499].)
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.
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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 [IANA-SUDN],
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
typical Split DNS zone structure.
* DNSSEC Compatibility: The specification supports DNSSEC-based
object security for local zone contents.
5. Establishing Local DNS Authority
To establish its authority over some DNS zone, a participating
network MUST offer one or more encrypted resolvers via DNR
[I-D.ietf-add-dnr], DDR [I-D.ietf-add-ddr], or an equivalent
mechanism (see Section 9).
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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.
To establish its authority, the network MUST provide 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
byte.
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 || $X). Where "||" denotes
concatenation.
The zone operator then publishes a "Verification Record" with the
following structure, following the advice in Sections 5.1 and 5.2 of
[I-D.ietf-dnsop-domain-verification-techniques]:
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* Type = TXT.
* Owner Name = Concatenation of the ADN, "_splitdns-challenge", and
the parent zone name.
* Contents = "token=base64url($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
* Salt = "example salt bytes (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 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.
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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, 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
(RDM) 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).
* "salt": The salt, encoded in base64url.
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6. Validating Authority over Local Domain Hints
To validate an Authorization Claim provided by the network,
participating 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
Authentication Domain Name. 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.
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 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), 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.
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*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.
7. Delegating DNSSEC across Split DNS Boundaries
We wish 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. To support this configuration, 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 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.
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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..."
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; Local zone, claiming "subdomain.parent.example".
; The local resolver's KSK, validated by the Verification Record.
resolver.arpa. IN DNSKEY 257 3 5 ASDF...=
; Each claimed subdomain has its own ZSK, which is signed by the
; KSK and is used to sign records at that subdomain and below.
subdomain.parent.example. IN DNSKEY 256 3 5 FDSA...=
subdomain.parent.example. IN AAAA 2001:db8::17
deeper.subdomain.parent.example. IN AAAA 2001:db8::18
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).
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).
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*Steps 1-2*: The client determines the network's DNS server
(dns.example.net) and Provisioning Domain (pvd.example.com) using
DNR [I-D.ietf-add-dnr] 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 [I-D.ietf-add-dnr],
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 hostnames & | | |
| PvD FQDN (2) | | |
|<-----------------------------------------------------------|
| ----------------------------\ | | |
|-| now knows DNR hostnames & | | | |
| | 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: Learning Local Claims of DNS Authority
8.1.1. Verification using an external resolver
The figure below 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
[I-D.ietf-add-dnr], 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
The figure below 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*: The DNSSEC validation is successful and the token
matches, so this Authorization Claim is validated. When the
client connects using an encrypted transport as indicated in DNR
[I-D.ietf-add-dnr], 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: Verifying claims using DNSSEC
8.2. Internal-only Subdomains
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 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”.
9. 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 [I-D.ietf-ipsecme-add-ike]. 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.
10. 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.
When the verification record expires, clients MUST fetch the
verification records again and repeat the verification procedure.
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 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.
2. The sequence number in the RA (Router Advertisement) PvD (Prefix
Delegation) 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).
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3. The old verification record needs to be maintained until the DHCP
lease time or PvD Additional Information expiry.
11. 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, but it could
create a confidentiality problem if the local encrypted resolver's
name is inside a secret subdomain. To avoid leakage, local resolvers
should be given a name that does not reveal any sensitive information
(perhaps in addition to the more sensitive name).
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).
12. IANA Considerations
12.1. 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 DNS
* Reference: (This Document)
12.2. Provisioning Domains Split DNS Additional Information
IANA is requested to add the following entry to the "Additional
Information PvD Keys" registry on the "Provisioning Domains (PvDs)"
page:
* JSON key: "splitDnsClaims"
* Description: "Verifiable locally served domains"
* Type: Array of Objects
* Example:
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[{
"resolver": "dns.example.net",
"parent": "example.com",
"subdomains": ["sub"],
"algorithm": "SHA384",
"salt": "abc...123"
}]
* Reference: (This document)
12.3. DNS Underscore Name
IANA is requested to add the following entry to the "Underscored and
Globally Scoped DNS Node Names" registry on the "Domain Name System
(DNS) Parameters" page:
* RR Type: TXT
* _NODE NAME: _splitdns-challenge
* Reference: (This document)
13. Acknowledgements
Thanks to Mohamed Boucadair, Jim Reid, Tommy Pauly, Paul Vixie,
Michael Richardson, Bernie Volz and Vinny Parla for the discussion
and comments.
Thanks to Tianran Zhou for the opsdir review, Watson Ladd for the
secdir review and Bob Halley for the intdir review.
14. References
14.1. Normative References
[IANA-SUDN]
IANA, "Special-Use Domain Names",
<https://www.iana.org/assignments/special-use-domain-
names/special-use-domain-names.xhtml>.
[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>.
[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>.
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[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>.
[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>.
[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013,
<https://www.rfc-editor.org/info/rfc6762>.
[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>.
14.2. Informative References
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[I-D.ietf-add-ddr]
Pauly, T., Kinnear, E., Wood, C. A., McManus, P., and T.
Jensen, "Discovery of Designated Resolvers", Work in
Progress, Internet-Draft, draft-ietf-add-ddr-10, 5 August
2022, <https://datatracker.ietf.org/doc/html/draft-ietf-
add-ddr-10>.
[I-D.ietf-add-dnr]
Boucadair, M., Reddy.K, T., Wing, D., Cook, N., and T.
Jensen, "DHCP and Router Advertisement Options for the
Discovery of Network-designated Resolvers (DNR)", Work in
Progress, Internet-Draft, draft-ietf-add-dnr-16, 27 April
2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
add-dnr-16>.
[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-03, 17 October 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-dnsop-
domain-verification-techniques-03>.
[I-D.ietf-ipsecme-add-ike]
Boucadair, M., Reddy.K, T., Wing, D., and V. Smyslov,
"Internet Key Exchange Protocol Version 2 (IKEv2)
Configuration for Encrypted DNS", Work in Progress,
Internet-Draft, draft-ietf-ipsecme-add-ike-14, 10 May
2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
ipsecme-add-ike-14>.
[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>.
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[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>.
[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>.
[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>.
[RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
January 2019, <https://www.rfc-editor.org/info/rfc8499>.
[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>.
[RFC9162] Laurie, B., Messeri, E., and R. Stradling, "Certificate
Transparency Version 2.0", RFC 9162, DOI 10.17487/RFC9162,
December 2021, <https://www.rfc-editor.org/info/rfc9162>.
Authors' Addresses
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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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