Internet Key Exchange Protocol Version 2 (IKEv2) Configuration for Encrypted DNS
draft-ietf-ipsecme-add-ike-14

ipsecme                                                     M. Boucadair
Internet-Draft                                                    Orange
Intended status: Standards Track                                T. Reddy
Expires: 11 November 2023                                          Nokia
                                                                 D. Wing
                                                                  Citrix
                                                              V. Smyslov
                                                              ELVIS-PLUS
                                                             10 May 2023

   Internet Key Exchange Protocol Version 2 (IKEv2) Configuration for
                             Encrypted DNS
                     draft-ietf-ipsecme-add-ike-14

Abstract

   This document specifies new Internet Key Exchange Protocol Version 2
   (IKEv2) Configuration Payload Attribute Types to assign DNS resolvers
   that support encrypted DNS protocols, such as DNS-over-HTTPS (DoH),
   DNS-over-TLS (DoT), and DNS-over-QUIC (DoQ).

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 11 November 2023.

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

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   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  IKEv2 Configuration Payload Attribute Types for Encrypted
           DNS . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  ENCDNS_IP* Configuration Payload Attributes . . . . . . .   4
     3.2.  ENCDNS_DIGEST_INFO Configuration Payload Attribute  . . .   6
   4.  IKEv2 Protocol Exchange . . . . . . . . . . . . . . . . . . .   9
   5.  Subject Public Key Info (SPKI) Hash . . . . . . . . . . . . .  11
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  12
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     10.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Appendix A.  Configuration Payload Examples . . . . . . . . . . .  15
     A.1.  Configuration of Encrypted IPv6 DNS Resolvers without
           Suggested Values  . . . . . . . . . . . . . . . . . . . .  15
     A.2.  Configuration of Encrypted IPv6 DNS Resolvers with
           Suggested Values  . . . . . . . . . . . . . . . . . . . .  16
     A.3.  Split DNS . . . . . . . . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   This document specifies a mechanism to assign encrypted DNS
   configurations to an Internet Key Exchange Protocol Version 2 (IKEv2)
   [RFC7296] initiator.  Specifically, it assigns one or more
   Authentication Domain Names (ADNs) of DNS resolvers that support
   encrypted DNS protocols.  The specific protocols supported are
   described using the Service Parameters format defined in
   [I-D.ietf-dnsop-svcb-https]; supported protocols include DNS-over-
   HTTPS (DoH) [RFC8484], DNS-over-TLS (DoT) [RFC7858], and DNS-over-
   QUIC (DoQ) [RFC9250].

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   This document introduces three new IKEv2 Configuration Payload
   Attribute Types (Section 3) to add support for encrypted DNS
   resolvers.  The ENCDNS_IP4 and ENCDNS_IP6 attribute types
   (Section 3.1) are used to provision ADNs, a list of IP addresses, and
   a set of service parameters.  The ENCDNS_DIGEST_INFO attribute
   (Section 3.2) additionally allows a specific resolver certificate to
   be indicated by the IKEv2 responder.

   The encrypted DNS resolver hosted by a Virtual Private Network (VPN)
   provider can get a domain-validate certificate from a public
   Certificate Authority (CA).  The VPN client does not need to be
   provisioned with the root certificate of a private CA to authenticate
   the certificate of the encrypted DNS resolvers.  The encrypted DNS
   resolver can run on private IP addresses and its access can be
   restricted to clients connected to the VPN.

   For many years, typical designs have often considered that the DNS
   resolver was usually located inside the protected domain, but could
   be located outside of it.  With encrypted DNS, the latter option
   becomes plausible.  Note that existing VPN client implementations
   might not expect that the discovered DNS resolver IP addresses to be
   outside of the covered IP address ranges of the VPN tunnel.

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 uses the terms defined in [RFC8499].

   Also, this document uses the terms defined in [RFC7296].  In
   particular, readers should be familiar with "initiator" and
   "responder" terms used in that document.

   This document makes use of the following terms:

   Do53:  refers to unencrypted DNS.

   Encrypted DNS:  refers to a scheme where DNS messages are sent over
      an encrypted channel.  Examples of encrypted DNS are DoT, DoH, and
      DoQ.

   ENCDNS_IP*:  refers to any IKEv2 Configuration Payload Attribute
      Types defined in Section 3.1.

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3.  IKEv2 Configuration Payload Attribute Types for Encrypted DNS

3.1.  ENCDNS_IP* Configuration Payload Attributes

   The ENCDNS_IP* IKEv2 Configuration Payload Attribute Types,
   ENCDNS_IP4 and ENCDNS_IP6, are used to configure encrypted DNS
   resolvers to an initiator.  Both attribute types share the format
   that is shown in Figure 1.  The information included in these
   attributes adheres to the recommendation in Section 3.1.9 of
   [I-D.ietf-add-dnr].

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-----------------------------+-------------------------------+
   |R|         Attribute Type      |            Length             |
   +-+-----------------------------+---------------+---------------+
   |       Service Priority        | Num Addresses |  ADN Length   |
   +-------------------------------+---------------+---------------+
   ~                         IP Addresses                          ~
   +---------------------------------------------------------------+
   ~                  Authentication Domain Name                   ~
   +---------------------------------------------------------------+
   ~                 Service Parameters (SvcParams)                ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                        Figure 1: Attributes Format

   The description of the fields of the attribute shown in Figure 1 is
   as follows:

   *  R (Reserved, 1 bit) - This bit MUST be set to zero and MUST be
      ignored on receipt (see Section 3.15.1 of [RFC7296] for details).

   *  Attribute Type (15 bits) - Identifier for Configuration Attribute
      Type.  This is set to TBA1 for ENCDNS_IP4 or TBA2 for ENCDNS_IP6,
      as registered in Section 8.

   *  Length (2 octets, unsigned integer) - Length of the enclosed data
      in octets.  In particular, this field is set to:

      -  0, if the Configuration payload has (i) type CFG_REQUEST and no
         specific DNS resolver is requested or (ii) type CFG_ACK.  If
         the 'Length' field is set to 0, then later fields shown in
         Figure 1 are not present.

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      -  (4 + 'Length of the ADN' + N * 4 + Length of SvcParams) for
         ENCDNS_IP4 attributes if the Configuration payload has types
         CFG_REQUEST or CFG_REPLY or CFG_SET; N being the number of
         included IPv4 addresses ('Num addresses').

      -  (4 + 'Length of the ADN' + N * 16 + Length of SvcParams) for
         ENCDNS_IP6 attributes if the Configuration payload has types
         CFG_REQUEST or CFG_REPLY or CFG_SET; N being the number of
         included IPv6 addresses ('Num addresses').

   *  Service Priority (2 octets) - The priority of this attribute
      compared to other ENCDNS_IP* instances.  This 16-bit unsigned
      integer is interpreted following the rules specified in
      Section 2.4.1 of [I-D.ietf-dnsop-svcb-https].  As AliasMode
      (Section 2.4.2 of [I-D.ietf-dnsop-svcb-https]) is not supported,
      this field MUST NOT be set to 0.  Note that AliasMode is not
      supported because such a mode will trigger additional Do53 queries
      while the data can be supplied directly in the IKE response.

   *  Num Addresses (1 octet) - Indicates the number of enclosed IPv4
      (for ENCDNS_IP4) or IPv6 (for ENCDNS_IP6) addresses.  This value
      MUST NOT be set to 0 if the Configuration payload is of type
      CFG_REPLY or CFG_SET.  This may be set to 0 in CFG_REQUEST to
      indicate that no IP address is encoded in the attribute.

   *  ADN Length (1 octet) - Indicates the length of the "Authentication
      Domain Name" field in octets.  When set to '0', this means that no
      ADN is enclosed in the attribute.

   *  IP Address(es) (variable) - Includes one or more IP addresses that
      can be used to reach the encrypted DNS resolver identified by the
      Authentication Domain Name.  For ENCDNS_IP4 this field contains
      one or more 4-octet IPv4 addresses, and for ENCDNS_IP6 this field
      contains one or more 16-octet IPv6 addresses.

   *  Authentication Domain Name (variable) - A fully qualified domain
      name of the encrypted DNS resolver, in DNS presentation format and
      using an Internationalized Domain Names for Applications (IDNA)
      A-label [RFC5890].  The name MUST NOT contain any terminators
      (e.g., NULL, CR).

      An example of a valid ADN for DoH server is "doh1.example.com".

   *  Service Parameters (SvcParams) (variable) - Specifies a set of
      service parameters that are encoded following the rules in
      Section 2.1 of [I-D.ietf-dnsop-svcb-https].  Section 3.1.5 of
      [I-D.ietf-add-dnr] lists a set of service parameters that are
      recommended to be supported by implementations.

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      The service parameters MUST NOT include "ipv4hint" or "ipv6hint"
      SvcParams as they are superseded by the included IP addresses.

      If no "port" service parameter is included, this indicates that
      default port numbers should be used.  As a reminder, the default
      port number is 853 for DoT (Section 6 of [RFC7858]), 443 for DoH
      (Section 8.1 of [RFC8484]), and 853 for DoQ (Section 8 of
      [RFC9250]).

      The service parameters apply to all IP addresses in the ENCDNS_IP*
      Configuration Payload Attribute.

3.2.  ENCDNS_DIGEST_INFO Configuration Payload Attribute

   The ENCDNS_DIGEST_INFO configuration payload attribute (Figure 2)
   allows IKEv2 responders to specify a certificate digest that
   initiators can use when validating TLS connections to encrypted
   resolvers.  This attribute can also be sent by the initiator to
   request specific hash algorithms for such digests.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-----------------------------+-------------------------------+
   |R|         Attribute Type      |            Length             |
   +-+-------------+---------------+-------------------------------+
   | Num Hash Algs |  ADN Length   |                               |
   +---------------+---------------+                               +
   ~                Authentication Domain Name                     ~
   +-------------------------------+-------------------------------+
   ~                List of Hash Algorithm Identifiers             ~
   +---------------------------------------------------------------+
   ~                       Certificate Digest                      ~
   +---------------------------------------------------------------+

               Figure 2: ENCDNS_DIGEST_INFO Attribute Format

   Some of the fields shown in Figure 2 can be omitted as further
   detailed below.

   The format of ENCDNS_DIGEST_INFO attribute if the Configuration
   payload has type CFG_REQUEST is shown in Figure 3.

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                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-----------------------------+-------------------------------+
   |R|         Attribute Type      |            Length             |
   +-+-------------+---------------+-------------------------------+
   | Num Hash Algs |  ADN Length   |                               |
   +---------------+---------------+                               +
   ~                List of Hash Algorithm Identifiers             ~
   +---------------------------------------------------------------+

        Figure 3: ENCDNS_DIGEST_INFO Attribute Format in CFG_REQUEST

   The description of the fields of the attribute shown in Figure 3 is
   as follows:

   *  R (Reserved, 1 bit) - This bit MUST be set to zero and MUST be
      ignored on receipt (see Section 3.15.1 of [RFC7296] for details).

   *  Attribute Type (15 bits) - Identifier for Configuration Attribute
      Type; is set to TBA3 value listed in Section 8.

   *  Length (2 octets, unsigned integer) - Length of the enclosed data
      in octets.  This field MUST be set to "2 + (2 * 'number of
      included hash algorithm identifiers')".

   *  Num Hash Algs (1 octet) - Indicates the number of included 'Hash
      Algorithm Identifiers'.  This field MUST be set to "(Length -
      2)/2".

   *  ADN Length (1 octet) - MUST be set to 0.

   *  List of Hash Algorithm Identifiers (variable) - Specifies a list
      of 16-bit hash algorithm identifiers that are supported by the
      encrypted DNS client.  This list may be controlled by a local
      policy.

      The values of this field are taken from the Hash Algorithm
      Identifiers of IANA's "Internet Key Exchange Version 2 (IKEv2)
      Parameters" registry [IANA-IKE-HASH].

      There is no padding between the hash algorithm identifiers.

      Note that SHA2-256 is mandatory to implement (see Section 5).

   The format of ENCDNS_DIGEST_INFO attribute if the Configuration
   payload has types CFG_REPLY or CFG_SET is shown in Figure 4.

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                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-----------------------------+-------------------------------+
   |R|         Attribute Type      |            Length             |
   +-+-----------------------------+---------------+---------------+
   | Num Hash Algs |  ADN Length   |                               |
   +---------------+---------------+                               +
   ~                Authentication Domain Name                     ~
   +-------------------------------+-------------------------------+
   | Hash Algorithm Identifier     |                               ~
   +-------------------------------+                               +
   ~                     Certificate Digest                        ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Figure 4: ENCDNS_DIGEST_INFO Attribute Format in CFG_REPLY or CFG_SET

   The description of the fields of the attribute shown in Figure 3 is
   as follows:

   *  R (Reserved, 1 bit) - This bit MUST be set to zero and MUST be
      ignored on receipt (see Section 3.15.1 of [RFC7296] for details).

   *  Attribute Type (15 bits) - Identifier for Configuration Attribute
      Type; is set to TBA3 value listed in Section 8.

   *  Length (2 octets, unsigned integer) - Length of the data in
      octets.

   *  Num Hash Algs (1 octet) - MUST be set to 1.

   *  ADN Length (1 octet) - Indicates the length of the "Authentication
      Domain Name" field in octets.  When set to '0', this means that
      the digest applies on the ADN conveyed in the ENCDNS_IP*
      Configuration Payload Attribute(s).

   *  Authentication Domain Name (variable) - A fully qualified domain
      name of the encrypted DNS resolver following the syntax defined in
      [RFC5890].  The name MUST NOT contain any terminators (e.g., NULL,
      CR).  A name is included only when multiple ADNs are included in
      the ENCDNS_IP* Configuration Payload Attributes.

   *  Hash Algorithm Identifier (2 octets) - Specifies the 16-bit hash
      algorithm identifier selected by the DNS resolver to generate the
      digest of its certificate.

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   *  Certificate Digest (variable) - This field includes the Subject
      Public Key Info (SPKI) hash (Section 5) of the encrypted DNS
      resolver certificate using the algorithm identified in the 'Hash
      Algorithm Identifier' field.  The length of this field is "Length
      - 4 - 'ADN Length'".

   The ENCDNS_DIGEST_INFO attribute may be present in the Configuration
   payload of CFG_ACK.  In such a case, the ENCDNS_DIGEST_INFO MUST be
   returned with zero-length data.

   As discussed in Section 3.15.1 of [RFC7296], there are no defined
   uses for the CFG_SET/CFG_ACK exchange.  The use of the
   ENCDNS_DIGEST_INFO attribute for these messages is provided for
   completeness.

4.  IKEv2 Protocol Exchange

   This section describes how the attributes defined in Section 3 are
   used to configure an IKEv2 initiator with one or more encrypted DNS
   resolvers.  As a reminder, badly formatted attributes or unacceptable
   fields are handled as per Section 2.21 of [RFC7296].

   Initiators first indicate support for encrypted DNS by including
   ENCDNS_IP* attributes in their CFG_REQUEST payloads.  Responders
   supply encrypted DNS configuration by including ENCDNS_IP* attributes
   in their CFG_REPLY payloads.  Concretely:

      If the initiator supports encrypted DNS, it includes either or
      both of the ENCDNS_IP4 and ENCDNS_IP6 attributes in its
      CFG_REQUEST.  If the initiator does not want to request specific
      DNS resolvers, it sets the Length field to 0 for the attribute.
      For a given attribute type, the initiator MAY send either an empty
      attribute or a list of distinct suggested resolvers.  The
      initiator MAY also include the ENCDNS_DIGEST_INFO attribute with a
      list of hash algorithms that are supported by the encrypted DNS
      client.

      If the request includes multiple bitwise identical attributes,
      only the first occurrence is processed, and the rest SHOULD be
      ignored by the responder.  The responder MAY discard the full
      request if the count of repeated attributes exceeds an
      (implementation specific) threshold.

      For each ENCDNS_IP* attribute from the CFG_REQUEST, if the
      responder supports the corresponding address family, and absent
      any policy restrictions, the responder sends back ENCDNS_IP*
      attribute(s) in the CFG_REPLY with an appropriate list of IP
      addresses, service parameters, and an ADN.  The list of IP

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      addresses MUST include at least one IP address.  The service
      parameters SHOULD include at least the "alpn" service parameter.
      The "alpn" service parameter may not be required in contexts such
      as a variant of DNS over CoAP where the messages are encrypted
      using Object Security for Constrained RESTful Environments
      (OSCORE) [RFC8613].

      The responder MAY ignore suggested values from the initiator (if
      any).  Multiple instances of the same ENCDNS_IP* attribute MAY be
      returned if distinct ADNs or service parameters need to be
      assigned to the initiator.  In such instances, the different
      attributes can have matching or distinct IP addresses.  These
      instances MUST be presented to a local DNS client following their
      service priority (i.e., smaller service priority values indicates
      a higher preference).

      In addition, the responder MAY return the ENCDNS_DIGEST_INFO
      attribute to convey a digest of the certificate of the encrypted
      DNS and the identifier of the hash algorithm that is used to
      generate the digest.

      If the CFG_REQUEST includes an ENCDNS_IP* attribute but the
      CFG_REPLY does not include an ENCDNS_IP* matching the requested
      address family, this is an indication that requested address
      family is not supported by the responder or the responder is not
      configured to provide corresponding resolver addresses.

      If the initiator receives both ENCDNS_IP* and INTERNAL_IP6_DNS (or
      INTERNAL_IP4_DNS) attributes, it is RECOMMENDED that the initiator
      uses the encrypted DNS resolvers.

   The DNS client establishes an encrypted DNS session (e.g., DoT, DoH,
   DoQ) with the address(es) conveyed in ENCDNS_IP* and uses the
   mechanism discussed in Section 8 of [RFC8310] to authenticate the DNS
   resolver certificate using the authentication domain name conveyed in
   ENCDNS_IP*.

   If the CFG_REPLY includes an ENCDNS_DIGEST_INFO attribute, the client
   has to create an SPKI hash (Section 5) of the DNS resolver
   certificate received in the TLS handshake using the negotiated hash
   algorithm in the ENCDNS_DIGEST_INFO attribute.  If the computed
   digest for an ADN matches the one sent in the ENCDNS_DIGEST_INFO
   attribute, the encrypted DNS resolver certificate is successfully
   validated.  If so, the client continues with the TLS connection as
   normal.  Otherwise, the client MUST treat the resolver certificate
   validation failure as a non-recoverable error.  This approach is
   similar to certificate usage PKIX-EE(1) with selector SPKI(1) defined
   in [RFC7671] but without PKIX validation.

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   If the IPsec connection is a split-tunnel configuration and the
   initiator negotiated INTERNAL_DNS_DOMAIN as per [RFC8598], the DNS
   client resolves the internal names using ENCDNS_IP* DNS resolvers.

      Note: [RFC8598] requires INTERNAL_IP6_DNS (or INTERNAL_IP4_DNS)
      attribute to be mandatory present when INTERNAL_DNS_DOMAIN is
      included.  This specification relaxes that constraint in the
      presence of ENCDNS_IP* attributes.  That is, if ENCDNS_IP*
      attributes are supplied, it is allowed for responders to include
      INTERNAL_DNS_DOMAIN even in the absence of INTERNAL_IP6_DNS (or
      INTERNAL_IP4_DNS) attributes.

5.  Subject Public Key Info (SPKI) Hash

   The SPKI hash of the encrypted DNS resolver certificate is the output
   of a cryptographic hash algorithm whose input is the DER-encoded
   ASN.1 representation of the SPKI.

   Implementations MUST support SHA2-256 [RFC6234].

6.  Security Considerations

   This document adheres to the security considerations defined in
   [RFC7296].  In particular, this document does not alter the trust on
   the DNS configuration provided by a responder.

   Networks are susceptible to internal attacks as discussed in
   Section 3.2 of [I-D.arkko-farrell-arch-model-t].  Hosting encrypted
   DNS resolvers even in case of split-VPN configuration can minimize
   the attack vector (e.g., a compromised network device cannot monitor/
   modify DNS traffic).  This specification describes a mechanism to
   restrict access to the DNS messages to only the parties that need to
   know.

   If the IKEv2 responder has used NULL Authentication method [RFC7619]
   to authenticate itself, the initiator MUST NOT use returned
   ENCDNS_IP* resolvers configuration unless it is pre-configured, e.g.,
   in the operating system or the application.

   This specification does not extend the scope of accepting DNSSEC
   trust anchors beyond the usage guidelines defined in Section 6 of
   [RFC8598].

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7.  Privacy Considerations

   As discussed in [RFC9076], the use of encrypted DNS does not reduce
   the data available in the DNS resolver.  For example, the reader may
   refer to Section 8 of [RFC8484] or Section 7 of [RFC9250] for a
   discussion on specific privacy considerations to encrypted DNS.

8.  IANA Considerations

   This document requests IANA to assign the following new IKEv2
   Configuration Payload Attribute Types from the "IKEv2 Configuration
   Payload Attribute Types" namespace available at [IANA-IKE-CFG].

                                  Multi-
   Value  Attribute Type          Valued  Length     Reference
   ------ ------------------       -----  ---------  ---------
    TBA1  ENCDNS_IP4                YES   0 or more  RFC XXXX
    TBA2  ENCDNS_IP6                YES   0 or more  RFC XXXX
    TBA3  ENCDNS_DIGEST_INFO        YES   0 or more  RFC XXXX

9.  Acknowledgements

   Many thanks to Yoav Nir, Christian Jacquenet, Paul Wouters, and Tommy
   Pauly for the review and comments.

   Yoav and Paul suggested the use of one single attribute carrying both
   the name and an IP address instead of depending on the existing
   INTERNAL_IP6_DNS and INTERNAL_IP4_DNS attributes.

   Thanks to Tero Kivinen for the Shepherd review and Roman Danyliw for
   the AD review.

   Thanks to Stewart Bryant for the gen-art review, Dhruv Dhody for the
   ops-dir review, and Patrick Mevzek for the dns-dir review.

   Thanks to Paul Wouters, Zaheduzzaman Sarker, Eric Vyncke, and Robert
   Wilton for the comments during the IESG review.

10.  References

10.1.  Normative References

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   [I-D.ietf-dnsop-svcb-https]
              Schwartz, B. M., Bishop, M., and E. Nygren, "Service
              binding and parameter specification via the DNS (DNS SVCB
              and HTTPS RRs)", Work in Progress, Internet-Draft, draft-
              ietf-dnsop-svcb-https-12, 11 March 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-dnsop-
              svcb-https-12>.

   [IANA-IKE-HASH]
              "IKEv2 Hash Algorithms",
              <https://www.iana.org/assignments/ikev2-parameters/ikev2-
              parameters.xhtml#hash-algorithms>.

   [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>.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, DOI 10.17487/RFC5890, August 2010,
              <https://www.rfc-editor.org/info/rfc5890>.

   [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>.

   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
              2014, <https://www.rfc-editor.org/info/rfc7296>.

   [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>.

   [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>.

   [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>.

10.2.  Informative References

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   [I-D.arkko-farrell-arch-model-t]
              Arkko, J. and S. Farrell, "Challenges and Changes in the
              Internet Threat Model", Work in Progress, Internet-Draft,
              draft-arkko-farrell-arch-model-t-04, 14 July 2020,
              <https://datatracker.ietf.org/api/v1/doc/document/draft-
              arkko-farrell-arch-model-t/>.

   [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>.

   [IANA-IKE-CFG]
              "IKEv2 Configuration Payload Attribute Types",
              <https://www.iana.org/assignments/ikev2-parameters/
              ikev2-parameters.xhtml#ikev2-parameters-21>.

   [RFC7619]  Smyslov, V. and P. Wouters, "The NULL Authentication
              Method in the Internet Key Exchange Protocol Version 2
              (IKEv2)", RFC 7619, DOI 10.17487/RFC7619, August 2015,
              <https://www.rfc-editor.org/info/rfc7619>.

   [RFC7671]  Dukhovni, V. and W. Hardaker, "The DNS-Based
              Authentication of Named Entities (DANE) Protocol: Updates
              and Operational Guidance", RFC 7671, DOI 10.17487/RFC7671,
              October 2015, <https://www.rfc-editor.org/info/rfc7671>.

   [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>.

   [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>.

   [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>.

   [RFC8613]  Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
              <https://www.rfc-editor.org/info/rfc8613>.

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   [RFC9076]  Wicinski, T., Ed., "DNS Privacy Considerations", RFC 9076,
              DOI 10.17487/RFC9076, July 2021,
              <https://www.rfc-editor.org/info/rfc9076>.

   [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>.

Appendix A.  Configuration Payload Examples

A.1.  Configuration of Encrypted IPv6 DNS Resolvers without Suggested
      Values

   Figure 5 depicts an example of a CFG_REQUEST to request the
   configuration of IPv6 DNS resolvers without providing any suggested
   values.  In this example, the initiator uses the ENCDNS_DIGEST_INFO
   attribute to indicate that the encrypted DNS client supports SHA2-256
   (2), SHA2-384 (3), and SHA2-512 (4) hash algorithms for certificate
   digests.  The label of these algorithms is taken from
   [IANA-IKE-HASH].  The use of INTERNAL_IP6_ADDRESS is explained in
   [RFC7296]; it is thus not reiterated here.

   CP(CFG_REQUEST) =
     INTERNAL_IP6_ADDRESS()
     INTERNAL_IP6_DNS()
     ENCDNS_IP6()
     ENCDNS_DIGEST_INFO(0, (SHA2-256, SHA2-384, SHA2-512))

                      Figure 5: Example of CFG_REQUEST

   Figure 6 depicts an example of a CFG_REPLY that can be sent by a
   responder as a response the above CFG_REQUEST.  This response
   indicates the following information to identify the encrypted DNS
   resolver:

   *  Its Service Priority is 1

   *  Its single (1) IPv6 address (2001:db8:99:88:77:66:55:44)

   *  Its authentication domain name (doh.example.com).  This ADN has a
      length of 15.

   *  Its supported HTTP version (h2)

   *  The relative form of the URI Template (/dns-query{?dns})

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   *  The SPKI hash of the resolver's certificate using SHA2-256
      (8b6e7a5971cc6bb0b4db5a71...)

   CP(CFG_REPLY) =
     INTERNAL_IP6_ADDRESS(2001:db8:0:1:2:3:4:5/64)
     ENCDNS_IP6(1, 1, 15,
                   (2001:db8:99:88:77:66:55:44),
                   "doh.example.com",
                   (alpn=h2 dohpath=/dns-query{?dns}))
     ENCDNS_DIGEST_INFO(0, SHA2-256,
                           8b6e7a5971cc6bb0b4db5a71...)

                       Figure 6: Example of CFG_REPLY

   In the example depicted in Figure 6, no ADN is included in the
   ENCDNS_DIGEST_INFO attribute because only one ADN is provided in the
   ENCDNS_IP6 attribute.  There is no ambiguity to identify the
   encrypted resolver associated with the supplied digest.

A.2.  Configuration of Encrypted IPv6 DNS Resolvers with Suggested
      Values

   An initiator may provide suggested values in the CFG_REQUEST when
   requesting an encrypted DNS resolver.  For example, the initiator
   may:

   *  Indicate a preferred resolver that is identified by an IPv6
      address (see Figure 7).

      CP(CFG_REQUEST) =
        INTERNAL_IP6_ADDRESS()
        INTERNAL_IP6_DNS()
        ENCDNS_IP6(1, 1, 0,
                      (2001:db8:99:88:77:66:55:44))

          Figure 7: Example of CFG_REQUEST with a Preferred Resolver
                         Identified by Its IP Address

   *  Indicate a preferred resolver that is identified by an ADN (see
      Figure 8).

      CP(CFG_REQUEST) =
        INTERNAL_IP6_ADDRESS()
        INTERNAL_IP6_DNS()
        ENCDNS_IP6(1, 0, 15, "doh.example.com")

          Figure 8: Example of CFG_REQUEST with a Preferred Resolver
                            Identified by Its ADN

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   *  Indicate a preferred transport protocol (DoT, in the example
      depicted in Figure 9)

      CP(CFG_REQUEST) =
        INTERNAL_IP6_ADDRESS()
        INTERNAL_IP6_DNS()
        ENCDNS_IP6(1, 0, 0, (alpn=dot))

         Figure 9: Example of CFG_REQUEST with a Preferred Transport
                                   Protocol

   *  or any combination thereof.

A.3.  Split DNS

   An initiator may also indicate that it supports Split DNS by
   including the INTERNAL_DNS_DOMAIN attribute in a CFG_REQUEST as shown
   in Figure 10.  In this example, the initiator does not indicate any
   preference for the requested encrypted DNS server nor which DNS
   queries will be forwarded through the IPsec tunnel.

   CP(CFG_REQUEST) =
     INTERNAL_IP6_ADDRESS()
     INTERNAL_IP6_DNS()
     ENCDNS_IP6()
     INTERNAL_DNS_DOMAIN()

        Figure 10: Example of CFG_REQUEST with Support of Split DNS

   Figure 11 shows an example of a reply of the responder.  Absent any
   prohibited local policy, the initiator uses the encrypted DNS server
   (doh.example.com) for any subsequent DNS queries for "example.com"
   and its subdomains.

   CP(CFG_REPLY) =
     INTERNAL_IP6_ADDRESS(2001:db8:0:1:2:3:4:5/64)
     ENCDNS_IP6(1, 1, 15,
                   (2001:db8:99:88:77:66:55:44),
                   "doh.example.com",
                   (alpn=h2 dohpath=/dns-query{?dns}))
     INTERNAL_DNS_DOMAIN(example.com)

          Figure 11: Example of CFG_REPLY with INTERNAL_DNS_DOMAIN

Authors' Addresses

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   Mohamed Boucadair
   Orange
   35000 Rennes
   France
   Email: mohamed.boucadair@orange.com

   Tirumaleswar Reddy
   Nokia
   India
   Email: kondtir@gmail.com

   Dan Wing
   Citrix Systems, Inc.
   United States of America
   Email: dwing-ietf@fuggles.com

   Valery Smyslov
   ELVIS-PLUS
   Russian Federation
   Email: svan@elvis.ru

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