Internet Key Exchange Protocol Version 2 (IKEv2) Configuration for Encrypted DNS
draft-ietf-ipsecme-add-ike-02

ipsecme                                                     M. Boucadair
Internet-Draft                                                    Orange
Intended status: Standards Track                                T. Reddy
Expires: 28 October 2022                                          Akamai
                                                                 D. Wing
                                                                  Citrix
                                                              V. Smyslov
                                                              ELVIS-PLUS
                                                           26 April 2022

   Internet Key Exchange Protocol Version 2 (IKEv2) Configuration for
                             Encrypted DNS
                     draft-ietf-ipsecme-add-ike-02

Abstract

   This document specifies new Internet Key Exchange Protocol Version 2
   (IKEv2) Configuration Payload Attribute Types for 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
   and may be updated, replaced, or obsoleted by other documents at any
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 28 October 2022.

Copyright Notice

   Copyright (c) 2022 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
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   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 . . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  ENCDNS_IP* Configuration Payload Attributes . . . . . . .   3
     3.2.  ENCDNS_DIGEST_INFO Configuration Payload Attribute  . . .   6
   4.  IKEv2 Protocol Exchange . . . . . . . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  Configuration Payload Attribute Types . . . . . . . . . .   9
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Appendix A.  Sample Deployment Scenarios  . . . . . . . . . . . .  12
     A.1.  Roaming Enterprise Users  . . . . . . . . . . . . . . . .  12
     A.2.  VPN Service Provider  . . . . . . . . . . . . . . . . . .  12
     A.3.  DNS Offload . . . . . . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   This document specifies encrypted DNS configuration for an Internet
   Key Exchange Protocol Version 2 (IKEv2) [RFC7296] initiator,
   particularly the Authentication Domain Name (ADN) of DNS servers that
   support encrypted DNS protocols such as DNS-over-HTTPS (DoH)
   [RFC8484], DNS-over-TLS (DoT) [RFC7858], or DNS-over-QUIC (DoQ)
   [I-D.ietf-dprive-dnsoquic].

   This document introduces new IKEv2 Configuration Payload Attribute
   Types (Section 3) for the support of encrypted DNS servers.  These
   attributes can be used to provision ADNs, a list of IP addresses, and
   a set of service parameters.

   Sample use cases are described in Appendix A.  The Configuration
   Payload Attribute Types defined in this document are not specific to
   these deployments, but can also be used in other deployment contexts.
   It is out of the scope of this document to provide a comprehensive
   list of deployment contexts.

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   The encrypted DNS server hosted by a 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
   server.  The encrypted DNS server can run on private IP addresses and
   its access can be restricted to clients connected to the VPN.

   Note that, for many years, typical designs have often considered that
   the DNS server was usually located inside the protected domain, but
   could be located outside of it.  With encrypted DNS, the latter
   option becomes plausible.

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 of the terms defined in [RFC8499].

   Also, this document uses of 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.

3.  IKEv2 Configuration Payload Attribute Types for Encrypted DNS

3.1.  ENCDNS_IP* Configuration Payload Attributes

   The ENCDNS_IP* IKEv2 Configuration Payload Attribute Types are used
   to configure encrypted DNS servers to an initiator.  All these
   attributes share the format that is shown in Figure 1.

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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             |
   +-+-----------------------------+---------------+---------------+
   |       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; is set to TBA1 or TBA2 values listed in Section 6.1.

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

      -  0 if the Configuration payload has types CFG_REQUEST (if no
         specific DNS server is requested) or CFG_ACK.

      -  (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 field is encoded
      following the rules specified in Section 2.4.1 of
      [I-D.ietf-dnsop-svcb-https].

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      AliasMode (Section 2.4.2 of [I-D.ietf-dnsop-svcb-https]) is not
      supported because such a mode will trigger additional Do53 queries
      while the data can be supplied directly in the IKE response.  As
      such, this field MUST NOT be set to 0.

   *  Num Addresses (1 octet) - Indicates the number of enclosed IPv4
      (for ENCDNS_IP4 attribute type) or IPv6 (for ENCDNS_IP6 attribute
      type) addresses.  It MUST NOT be set to 0 if the Configuration
      payload has types CFG_REPLY or CFG_SET.

   *  ADN Length (1 octet) - Indicates the length of the "Authentication
      Domain Name" field in octets.

   *  IP Address(es) (variable) - One or more IPv4 or IPv6 addresses to
      be used to reach the encrypted DNS server that is identified by
      the name in the Authentication Domain Name.

   *  Authentication Domain Name (variable) - A fully qualified domain
      name of the encrypted DNS server following the syntax defined in
      [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].  The following service
      parameters are RECOMMENDED to be supported by an implementation:

      alpn:  Used to indicate the set of supported protocols
         (Section 7.1 of [I-D.ietf-dnsop-svcb-https]).

      port:  Used to indicate the target port number for the encrypted
         DNS connection (Section 7.2 of [I-D.ietf-dnsop-svcb-https]).

      ech:  Used to enable Encrypted ClientHello (ECH) (Section 7.3 of
         [I-D.ietf-dnsop-svcb-https]).

      dohpath:  Used to supply a relative DoH URI Template (Section 5.1
         of [I-D.ietf-add-svcb-dns]).

      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, 443 for DoH, and 853 for DoQ.

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

3.2.  ENCDNS_DIGEST_INFO Configuration Payload Attribute

   The format of ENCDNS_DIGEST_INFO configuration payload attribute is
   shown in Figure 2.

                        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             |
   +-+-----------------------------+---------------+---------------+
   |                    RESERVED                   |  ADN Length   |
   +-----------------------------------------------+---------------+
   ~                  Authentication Domain Name                   ~
   +---------------------------------------------------------------+
   ~                  Hash Algorithm Identifiers                   ~
   +---------------------------------------------------------------+
   ~                     Certificate Digest                        ~
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 2: ENCDNS_DIGEST_INFO Attribute Format

   *  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 6.1.

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

   *  RESERVED (3 octets) - These bits are reserved for future use.
      These bits MUST be set to zero by the sender and MUST be ignored
      by the receiver.

   *  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 server 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.

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   *  Hash Algorithm Identifiers (variable) - In a request, this field
      specifies a list of 16-bit hash algorithm identifiers that are
      supported by the encrypted DNS client.  In a response, this field
      specifies the 16-bit hash algorithm identifier selected by the
      server to generate the digest of its certificate.

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

      There is no padding between the hash algorithm identifiers.

      Note that SHA2-256 is mandatory to implement.

   *  Certificate Digest (variable) - MUST only be present in a
      response.  This field includes the digest of the encrypted DNS
      server certificate using the algorithm identified in the 'Hash
      Algorithm Identifiers' field.

4.  IKEv2 Protocol Exchange

   This section describes how an initiator can be configured with an
   encrypted DNS server (e.g., DoH, DoT) using IKEv2.

   Initiators indicate the support of an encrypted DNS in the
   CFG_REQUEST payloads by including one or two ENCDNS_IP* attributes,
   while responders supply the encrypted DNS configuration in the
   CFG_REPLY payloads.  Concretely:

      If the initiator supports encrypted DNS, it includes one or two
      ENCDNS_IP* attributes in the CFG_REQUEST.  For each IP address
      family the initiator MUST include exactly one attribute with the
      Length field set to 0 if no specific DNS server is requested.  The
      initiator MAY include the ENCDNS_DIGEST_INFO attribute with a list
      of hash algorithms that are supported by the encrypted DNS client.

      For each ENCDNS_IP* attribute from the CFG_REQUEST, if the
      responder supports the corresponding address family, and absent
      any policy, 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 addresses MUST include at
      least one IP address.  The responder may ignore suggested values
      (if any).  Multiple instances of the same ENCDNS_IP* attribute MAY
      be returned if distinct ADNs or service parameters are to be
      returned by the responder.  The same or distinct IP addresses can
      be returned in such instances.  These instances SHOULD be
      processed following their service priority (i.e., smaller service
      priority indicates a higher preference).

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      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 server 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 servers.

   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
   server certificate using the authentication domain name conveyed in
   ENCDNS_IP*.

   If the CFG_REPLY includes an ENCDNS_DIGEST_INFO attribute, the DNS
   client has to create a digest of the DNS server 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 server certificate is successfully validated.  If so,
   the client continues with the TLS connection as normal.  Otherwise,
   the client MUST treat the server certificate validation failure as a
   non-recoverable error.  This approach is similar to certificate usage
   PKIX-EE(1) defined in [RFC7671].

   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 servers.

      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 to include
      INTERNAL_DNS_DOMAIN even in the absence of INTERNAL_IP6_DNS (or
      INTERNAL_IP4_DNS) attributes.

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5.  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 servers even in case of split-VPN configuration minimizes 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.

   The initiator may trust the encrypted DNS servers supplied by means
   of IKEv2 from a trusted responder more than the locally provided DNS
   servers, especially in the case of connecting to unknown or untrusted
   networks (e.g., coffee shops or hotel networks).

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

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

6.  IANA Considerations

6.1.  Configuration Payload Attribute Types

   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].

                                  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_ENCDNS_DIGEST_INFO YES   0 or more  RFC XXXX

7.  Acknowledgements

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

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   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.

8.  References

8.1.  Normative References

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

   [I-D.ietf-add-svcb-dns]
              Schwartz, B., "Service Binding Mapping for DNS Servers",
              Work in Progress, Internet-Draft, draft-ietf-add-svcb-dns-
              03, 22 April 2022, <https://www.ietf.org/archive/id/draft-
              ietf-add-svcb-dns-03.txt>.

   [I-D.ietf-dnsop-svcb-https]
              Schwartz, B., 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-08, 12 October 2021,
              <https://www.ietf.org/archive/id/draft-ietf-dnsop-svcb-
              https-08.txt>.

   [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>.

   [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>.

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   [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>.

8.2.  Informative References

   [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, 13 July 2020,
              <https://www.ietf.org/archive/id/draft-arkko-farrell-arch-
              model-t-04.txt>.

   [I-D.ietf-dprive-dnsoquic]
              Huitema, C., Dickinson, S., and A. Mankin, "DNS over
              Dedicated QUIC Connections", Work in Progress, Internet-
              Draft, draft-ietf-dprive-dnsoquic-12, 20 April 2022,
              <https://www.ietf.org/archive/id/draft-ietf-dprive-
              dnsoquic-12.txt>.

   [IANA-IKE] "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>.

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   [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>.

Appendix A.  Sample Deployment Scenarios

A.1.  Roaming Enterprise Users

   In this Enterprise scenario (Section 1.1.3 of [RFC7296]), a roaming
   user connects to the Enterprise network through an IPsec tunnel.  The
   split-tunnel Virtual Private Network (VPN) configuration allows the
   endpoint to access hosts that resides in the Enterprise network
   [RFC8598] using that tunnel; other traffic not destined to the
   Enterprise does not traverse the tunnel.  In contrast, a non-split-
   tunnel VPN configuration causes all traffic to traverse the tunnel
   into the enterprise.

   For both split- and non-split-tunnel configurations, the use of
   encrypted DNS instead of Do53 provides privacy and integrity
   protection along the entire path (rather than just to the VPN
   termination device) and can communicate the encrypted DNS server
   policies.

   For split-tunnel VPN configurations, the endpoint uses the
   Enterprise-provided encrypted DNS server to resolve internal-only
   domain names.

   For non-split-tunnel VPN configurations, the endpoint uses the
   Enterprise-provided encrypted DNS server to resolve both internal and
   external domain names.

   Enterprise networks are susceptible to internal and external attacks.
   To minimize that risk all enterprise traffic is encrypted
   (Section 2.1 of [I-D.arkko-farrell-arch-model-t]).

A.2.  VPN Service Provider

   Legacy VPN service providers usually preserve end-users' data
   confidentiality by sending all communication traffic through an
   encrypted tunnel.  A VPN service provider can also provide guarantees
   about the security of the VPN network by filtering malware and
   phishing domains.

   Browsers and OSes support DoH/DoT; VPN providers may no longer expect
   DNS clients to fallback to Do53 just because it is a closed network.

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   The encrypted DNS server hosted by the VPN service provider can be
   securely discovered by the endpoint using the IKEv2 Configuration
   Payload Attribute Type.

A.3.  DNS Offload

   VPN service providers typically allow split-tunnel VPN configuration
   in which users can choose applications that can be excluded from the
   tunnel.  For example, users may exclude applications that restrict
   VPN access.

   The encrypted DNS server hosted by the VPN service provider can be
   securely discovered by the endpoint using the IKEv2 Configuration
   Payload Attribute Type.

Authors' Addresses

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

   Tirumaleswar Reddy
   Akamai
   Embassy Golf Link Business Park
   Bangalore 560071
   Karnataka
   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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