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ADD                                                         M. Boucadair
Internet-Draft                                                    Orange
Intended status: Standards Track                                T. Reddy
Expires: March 25, 2021                                           McAfee
                                                                 D. Wing
                                                                  Citrix
                                                                 N. Cook
                                                            Open-Xchange
                                                      September 21, 2020


DHCP and Router Advertisement Options for Encrypted DNS Discovery within
                             Home Networks
                         draft-btw-add-home-09

Abstract

   The document specifies new DHCP and Router Advertisement Options to
   discover encrypted DNS servers (e.g., DoH, DoT, DoQ).  Particularly,
   it allows to learn an Authentication Domain Name together with a list
   of IP addresses and optionally a port number to reach such encrypted
   DNS servers.

   This document focuses on encrypted DNS deployment within home
   networks.

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/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 25, 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   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 extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Sample Target Deployment Scenarios  . . . . . . . . . . . . .   5
     3.1.  Managed CPEs  . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Unmanaged CPEs  . . . . . . . . . . . . . . . . . . . . .   7
   4.  Encrypted DNS Discovery Options . . . . . . . . . . . . . . .   8
     4.1.  DHCPv6 Encrypted DNS Options  . . . . . . . . . . . . . .   9
     4.2.  DHCP Encrypted DNS Option . . . . . . . . . . . . . . . .  12
     4.3.  IPv6 RA Encrypted DNS Options . . . . . . . . . . . . . .  14
   5.  DoH URI Templates . . . . . . . . . . . . . . . . . . . . . .  17
   6.  Make Use of Discovered Encrypted DNS Server . . . . . . . . .  18
   7.  Hosting Encrypted DNS Forwarder in the CPE  . . . . . . . . .  19
     7.1.  Managed CPEs  . . . . . . . . . . . . . . . . . . . . . .  19
       7.1.1.  ACME  . . . . . . . . . . . . . . . . . . . . . . . .  19
       7.1.2.  Auto-Upgrade Based on Domains and their Subdomains  .  19
     7.2.  Unmanaged CPEs  . . . . . . . . . . . . . . . . . . . . .  20
   8.  Legacy CPEs . . . . . . . . . . . . . . . . . . . . . . . . .  21
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  22
     9.1.  Spoofing Attacks  . . . . . . . . . . . . . . . . . . . .  22
     9.2.  Deletion Attacks  . . . . . . . . . . . . . . . . . . . .  23
     9.3.  Passive Attacks . . . . . . . . . . . . . . . . . . . . .  23
     9.4.  Wireless Security - Authentication Attacks  . . . . . . .  23
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  23
     10.1.  DHCPv6 Options . . . . . . . . . . . . . . . . . . . . .  24
     10.2.  DHCP Option  . . . . . . . . . . . . . . . . . . . . . .  24
     10.3.  RA Options . . . . . . . . . . . . . . . . . . . . . . .  24
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  24
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  25
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  25
     12.2.  Informative References . . . . . . . . . . . . . . . . .  26
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29








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1.  Introduction

   Internet Service Providers (ISPs) traditionally provide DNS resolvers
   to their customers.  Typically, ISPs deploy the following mechanisms
   to advertise a list of DNS Recursive DNS server(s) to their
   customers:

   o  Protocol Configuration Options in cellular networks [TS.24008].

   o  DHCP [RFC2132] (Domain Name Server Option) or DHCPv6
      [RFC8415][RFC3646] (OPTION_DNS_SERVERS).

   o  IPv6 Router Advertisement [RFC4861][RFC8106] (Type 25 (Recursive
      DNS Server Option)).

   The communication between a customer's device (possibly via Customer
   Premises Equipment (CPE)) and an ISP-supplied DNS resolver takes
   place by using cleartext DNS messages (Do53)
   [I-D.ietf-dnsop-terminology-ter].  Some examples are depicted in
   Figure 1.  In the case of cellular networks, the cellular network
   will provide connectivity directly to a host (e.g., smartphone,
   tablet) or via a CPE.  Do53 mechanisms used within the Local Area
   Network (LAN) are similar in both fixed and cellular CPE-based
   broadband service offerings.



























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           (a) Fixed Networks
                      ,--,--,--.             ,--,--,--.
                   ,-'   +--+  `-.       ,-'   ISP    `-.
                  ( LAN  |H |    CPE----(                 )
                   `-.   +--+   ,-'       `-.          ,-'
                      `--'|-'--'             `--'--'--'
                          |                     |
                          |<=======Do53========>|

           (b) Cellular Networks
                           |<===========Do53=========>|
                      ,--,-|,--.                      |
                   ,-'   +--+   `-.               ,--,--,--.
                  ( LAN  |H |     CPE------------+          \
                   `-.   +--+   ,-'            ,'   ISP     `-.
                      `--'--'--'              (                )
                                         +-----+-.          ,-'
                         +--+            |        `--'--'--'
                         |H +------------+
                         +--+
           Legend:
            * H: refers to a host.

                    Figure 1: Sample Legacy Deployments

   This document focuses on the support of encrypted DNS such as DNS-
   over-HTTPS (DoH) [RFC8484], DNS-over-TLS (DoT) [RFC7858], or DNS-
   over-QUIC (DoQ) [I-D.ietf-dprive-dnsoquic] in local networks.  In
   particular, the document describes how a local encrypted DNS server
   can be discovered and used by connected hosts by means of DHCP,
   DHCPv6, and RA options (Section 4).  These options are designed to
   convey the following information: the DNS Authentication Domain Name
   (ADN) [RFC8310], a list of IP addresses, and optionally a port
   number.

   Some ISPs rely upon external resolvers (e.g., outsourced service or
   public resolvers); these ISPs provide their customers with the IP
   addresses of these resolvers.  These addresses are typically
   configured on CPEs using the same mechanisms listed above.  Likewise,
   users can modify the default DNS configuration of their CPEs (e.g.,
   supplied by their ISP) to configure their favorite DNS servers.  This
   document permits such deployments.

   Both managed and unmanaged CPEs are discussed in the document
   (Section 3).  Also, considerations related to hosting a DNS forwarder
   in the CPE are described (Section 7).





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   Hosts and/or CPEs may be connected to multiple networks; each
   providing their own DNS configuration using the discovery mechanisms
   specified in this document.  Nevertheless, it is out of the scope of
   this specification to discuss DNS selection of multi-interface
   devices.  The reader may refer to [RFC6731] for a discussion of
   issues and an example of DNS server selection for multi-interfaced
   devices.

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] and
   [I-D.ietf-dnsop-terminology-ter].

   Do53 refers to unencrypted DNS.

   'Encrypted DNS' refers to a scheme where DNS exchanges are
   transported over an encrypted channel.  Examples of encrypted DNS are
   DNS-over-TLS (DoT) [RFC7858], DNS-over-HTTPS (DoH) [RFC8484], or DNS-
   over-QUIC (DoQ) [I-D.ietf-dprive-dnsoquic].

3.  Sample Target Deployment Scenarios

3.1.  Managed CPEs

   ISPs have developed an expertise in managing service-specific
   configuration information (e.g., CPE WAN Management Protocol
   [TR-069]).  For example, these tools may be used to provision the DNS
   server's ADN to managed CPEs if an encrypted DNS is supported by a
   local network similar to what is depicted in Figure 2.

   For example, DoH-capable (or DoT) clients establish the DoH (or DoT)
   session with the discovered DoH (or DoT) server.

   The DNS client discovers whether the DNS server in the local network
   supports DoH/DoT/DoQ by using a dedicated field in the discovery
   message: Encrypted DNS Types (Section 4).









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           (a) Fixed Networks

                      ,--,--,--.             ,--,--,--.
                   ,-'   +--+  `-.       ,-'   ISP    `-.
                  ( LAN  |H |    CPE----(    DNS Server  )
                   `-.   +--+   ,-'       `-.         ,-'
                      `--'|-'--'             `--'--'--'
                          |                     |
                          |<===Encrypted DNS===>|

           (b) Cellular Networks

                           |<=====Encrypted DNS======>|
                      ,--,-|,--.                      |
                   ,-'   +--+   `-.               ,--,--,--.
                  ( LAN  |H |     CPE------------+          \
                   `-.   +--+   ,-'            ,'   ISP     `-.
                      `--'--'--'              (    DNS Server  )
                                         +-----+-.          ,-'
                          +--+           |        `--'--'--'
                          |H +-----------+
                          +--+

                    Figure 2: Encrypted DNS in the WAN

   Figure 2 shows the scenario where the CPE relays the list of
   encrypted DNS servers it learns for the network by using mechanisms
   like DHCP or a specific Router Advertisement message.  In such
   context, direct encrypted DNS sessions will be established between a
   host serviced by a CPE and an ISP-supplied encrypted DNS server (see
   the example depicted in Figure 3 for a DoH/DoT-capable host).

                         ,--,--,--.             ,--,--,--.
                      ,-'          `-.       ,-'   ISP    `-.
              Host---(      LAN      CPE----(    DNS Server  )
                |     `-.          ,-'       `-.          ,-'
                |        `--'--'--'             `--'--'--'
                |                                   |
                |<=========Encrypted DNS===========>|

                  Figure 3: Direct Encrypted DNS Sessions

   Figure 4 shows a deployment where the CPE embeds a caching DNS
   forwarder.  The CPE advertises itself as the default DNS server to
   the hosts it serves.  The CPE relies upon DHCP or RA to advertise
   itself to internal hosts as the default DoT/DoH/Do53 server.  When
   receiving a DNS request it cannot handle locally, the CPE forwards
   the request to an upstream DoH/DoT/Do53 resolver.  Such deployment is



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   required for IPv4 service continuity purposes (e.g.,
   [I-D.ietf-v6ops-rfc7084-bis]) or for supporting advanced services
   within the home (e.g., malware filtering, parental control,
   Manufacturer Usage Description (MUD) [RFC8520] to only allow intended
   communications to and from an IoT device).  When the CPE behaves as a
   DNS forwarder, DNS communications can be decomposed into two legs:

   o  The leg between an internal host and the CPE.

   o  The leg between the CPE and an upstream DNS resolver.

   An ISP that offers encrypted DNS to its customers may enable
   encrypted DNS in both legs as shown in Figure 4.  Additional
   considerations related to this deployment are discussed in Section 7.

                         ,--,--,--.             ,--,--,--.
                      ,-'          `-.       ,-'   ISP    `-.
              Host---(      LAN      CPE----(    DNS Server  )
                |     `-.          ,-'|      `-.          ,-'
                |        `--'--'--'   |         `--'--'--'
                |                     |             |
                |<=====Encrypted=====>|<=Encrypted=>|
                          DNS                DNS

                 Figure 4: Proxied Encrypted DNS Sessions

3.2.  Unmanaged CPEs

   Customers may decide to deploy unmanaged CPEs (assuming the CPE is
   compliant with the network access technical specification that is
   usually published by ISPs).  Upon attachment to the network, an
   unmanaged CPE receives from the network its service configuration
   (including the DNS information) by means of, e.g., DHCP.  That DNS
   information is shared within the LAN following the same mechanisms as
   those discussed in Section 3.1.  A host can thus, for example,
   establish DoH/DoT session with a DoH/DoT server similar to what is
   depicted in Figure 3.

   Customers may also decide to deploy internal home routers (called
   hereafter, Internal CPEs) for a variety of reasons that are not
   detailed here.  Absent any explicit configuration on the internal CPE
   to override the DNS configuration it receives from the ISP-supplied
   CPE, an Internal CPE relays the DNS information it receives via DHCP/
   RA from the ISP-supplied CPE to connected hosts.  Encrypted DNS
   sessions can be established by a host with the DNS servers of the ISP
   (see Figure 5).





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                    ,--,--,--.                    ,--,--,--.
                 ,-'          Internal         ,-'    ISP   `-.
          Host--(    Network#A   CPE----CPE---(    DNS Server   )
           |     `-.          ,-'              `-.          ,-'
           |        `--'--'--'                    `--'--'--'
           |                                          |
           |<==============Encrypted DNS=============>|

     Figure 5: Direct Encrypted DNS Sessions with the ISP DNS Resolver
                              (Internal CPE)

   Similar to managed CPEs, a user may modify the default DNS
   configuration of an unmanaged CPE to use his/her favorite DNS servers
   instead.  Encrypted DNS sessions can be established directly between
   a host and a 3rd Party DNS server (see Figure 6).

                 ,--,--,--.                  ,--,
               ,'         Internal        ,-'    '-     3rd Party
        Host--(  Network#A  CPE----CPE---(   ISP   )--- DNS Server
         |     `.         ,-'             `-.    -'         |
         |       `-'--'--'                   `--'           |
         |                                                  |
         |<=================Encrypted DNS==================>|

      Figure 6: Direct Encrypted DNS Sessions with a Third Party DNS
                                 Resolver

   Section 7.2 discusses considerations related to hosting a forwarder
   in the Internal CPE.

4.  Encrypted DNS Discovery Options

   This section describes how a DNS client can discover a local
   encrypted DNS server(s) using DHCP (Sections 4.1 and 4.2) and
   Neighbor Discovery protocol (Section 4.3).

   As reported in Section 1.7.2 of [RFC6125]:

      "few certification authorities issue server certificates based on
      IP addresses, but preliminary evidence indicates that such
      certificates are a very small percentage (less than 1%) of issued
      certificates".

   In order to allow for PKIX-based authentication between a DNS client
   and an encrypted DNS server while accommodating the current best
   practices for issuing certificates, this document allows for
   configuring an authentication domain name to be presented as a
   reference identifier for DNS authentication purposes.



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   To avoid adding a dependency on another server to resolve the ADN,
   the options return a list of IP addresses to locate the encrypted DNS
   server.  In the various scenarios sketched in Section 3, encrypted
   DNS servers may terminate on the same IP address or distinct IP
   addresses.  Terminating encrypted DNS servers on the same or distinct
   IP addresses is deployment specific.  It is RECOMMENDED to return
   both the ADN and a list of IP addresses to a requesting host.

   Note that in order to optimize the size of discovery messages when
   all servers terminate on the same IP address, a host may rely upon
   the discovery mechanisms specified in [RFC2132][RFC3646][RFC8106] to
   retrieve a list of IP addresses to reach their DNS servers.
   Nevertheless, this approach requires a client that supports more than
   one encrypted DNS to probe that list of IP addresses.  To avoid such
   probing, the options defined in the following subsections associate
   an IP address with an encrypted DNS type.  No probing is required in
   such design.

   A list of IP addresses to reach an encrypted DNS server can be
   returned in the option to accommodate current deployments relying
   upon primary and backup servers.  Whether one IP address or more are
   returned in an option is deployment specific.  For example, a home
   router embedding a forwarder has to include one single IP address
   pointing to one of its LAN-facing interfaces.

   If more than one IP address are to be returned in an Encrypted DNS
   server option, these addresses are ordered in the preference for use
   by the client.

   Because DoT and DoQ may make use of customized port numbers instead
   of default ones, the Encrypted DNS server options are designed to
   return alternate port numbers.

   The DNS client establishes an encrypted DNS session with the
   discovered DNS IP address(es) and port number, and uses the mechanism
   discussed in Section 8 of [RFC8310] to authenticate the DNS server
   certificate using the authentication domain name conveyed in the
   encrypted DNS options.

   If the encrypted DNS is discovered by a host using both RA and DHCP,
   the rules discussed in Section 5.3.1 of [RFC8106] MUST be followed.

4.1.  DHCPv6 Encrypted DNS Options

   The DHCPv6 Encrypted DNS ADN option is used to configure an
   authentication domain name of the encrypted DNS server.  The format
   of this option is shown in Figure 7.




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       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    OPTION_V6_ENC_ADN          |         Option-length         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Enc DNS Type |                 Unassigned                    |
      +---------------+-----------------------------------------------+
      |                                                               |
      ~                  DNS Authentication Domain Name               ~
      |                                                               |
      +---------------------------------------------------------------+

                 Figure 7: DHCPv6 Encrypted DNS ADN Option

   The fields of the option shown in Figure 7 are as follows:

   o  Option-code: OPTION_V6_ENC_ADN (TBA1, see Section 10.1)
   o  Option-length: Length of the enclosed data in octets.
   o  Encr DNS Types (Encrypted DNS Types): Indicates the type(s) of the
      encrypted DNS server conveyed in this attribute.  The format of
      this 8-bit field is shown in Figure 8.

                             +-+-+-+-+-+-+-+-+
                             |U|U|U|U|U|Q|H|T|
                             +-+-+-+-+-+-+-+-+

                       Figure 8: Encrypted DNS Types

         T: If set, this bit indicates that the server supports DoT
         [RFC7858].
         H: If set, this bit indicates that the server supports DoH
         [RFC8484].
         Q: If set, this bit indicates that the server supports DoQ
         [I-D.ietf-dprive-dnsoquic].
         U: Unassigned bits.  These bits MUST be unset by the sender.
         Associating a meaning with an unassigned bit can be done via
         Standards Action [RFC8126].

      In a request, these bits are assigned to indicate the requested
      encrypted DNS server type(s) by the client.  In a response, these
      bits are set as a function of the encrypted DNS supported by the
      server and the requested encrypted DNS server type(s).

      To keep the packet small, if more than one encrypted DNS type
      (e.g., both DoH and DoT) are to be returned to a requesting client
      and the same ADN is used for these types, the corresponding bits
      must be set in the 'Encrypted DNS Types' field of the same option
      instance in a response.  For example, if the client requested DoH




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      and DoT and the server supports both with the same ADN, then both
      T and H bits must be set.
   o  Authentication Domain Name: A fully qualified domain name of the
      encrypted DNS server.  This field is formatted as specified in
      Section 10 of [RFC8415].

   An example of the Authentication Domain Name encoding is shown in
   Figure 9.  This example conveys the FQDN "doh1.example.com.".

        +------+------+------+------+------+------+------+------+------+
        | 0x04 |   d  |   o  |   h  |  1   | 0x07 |   e  |   x  |   a  |
        +------+------+------+------+------+------+------+------+------+
        |   m  |   p  |   l  |   e  | 0x03 |   c  |   o  |   m  | 0x00 |
        +------+------+------+------+------+------+------+------+------+

      Figure 9: An example of the authentication-domain-name Encoding

   The DHCPv6 Encrypted DNS Address option is used to configure a list
   of IP addresses and a port number of the encrypted DNS server.  The
   format of this option is shown in Figure 10.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    OPTION_V6_ENC_ADD          |         Option-length         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Enc DNS Type |   Unassigned  |          Port Number          |
      +---------------+---------------+-------------------------------+
      |                                                               |
      |                         IPv6 Address                          |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      |                         IPv6 Address                          |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                              ...                              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 10: DHCPv6 Encrypted DNS Address Option

   The fields of the option shown in Figure 10 are as follows:

   o  Option-code: OPTION_V6_ENC_ADD (TBA2, see Section 10.1)

   o  Option-length: Length of the enclosed data in octets.




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   o  Encr DNS Types (Encrypted DNS Types): Indicates the type(s) of the
      encrypted DNS server conveyed in this attribute.  The format of
      this 8-bit field is shown in Figure 8.  In a request, these bits
      are set to indicate the requested encrypted DNS server type(s) by
      the client.  In a response, these bits are set as a function of
      the encrypted DNS supported by the server and the requested
      encrypted DNS server type(s).

   o  Unassigned: These bits MUST be unset by the sender.  Associating a
      meaning with an unassigned bit can be done via Standards Action
      [RFC8126].

   o  Port Number: If not null, Indicates the port number to be used for
      the encrypted DNS.  If this field is to zero, this indicates that
      default port numbers should be used.  As a reminder, the default
      port number is 853 for DoT and 443 for DoH.

   o  IPv6 Address(es): Indicates one or more IPv6 addresses to reach
      the encrypted DNS server.

   Multiple instances of OPTION_V6_ENC_ADN (or OPTION_V6_ENC_ADD) may be
   returned to a DHCPv6 client; each pointing to a distinct encrypted
   DNS server type.

   To discover an encrypted DNS server, the DHCPv6 client includes
   OPTION_V6_ENC_ADN and OPTION_V6_ENC_ADD in an Option Request Option
   (ORO), as in Sections 18.2.1, 18.2.2, 18.2.4, 18.2.5, 18.2.6, and
   21.7 of [RFC8415].  The DHCPv6 client sets the Encrypted DNS Types
   field to the requested encrypted DNS server type(s).

   If the DHCPv6 client requested more than one encrypted DNS server
   type, the DHCP client MUST be prepared to receive multiple
   OPTION_V6_ENC_ADN (or OPTION_V6_ENC_ADD) options; each option is to
   be treated as a separate encrypted DNS server.

   If more than one encrypted DNS server types is supported on the same
   IP address and default port numbers are used, one instance of
   OPTION_V6_ENC_ADD option with the appropriate bits set in "Encr DNS
   Types" field should be returned by the DHCP server.

4.2.  DHCP Encrypted DNS Option

   The DHCP Encrypted DNS option is used to configure an authentication
   domain name, a list of IP addresses, and a port number of the
   encrypted DNS server.  The format of this option is illustrated in
   Figure 11.





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             0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |     TBA3      |     Length    |
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            | Encr DNS Types| Num Addresses |
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |           Port Number         |
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |                               |
            ~         IPv4 Address(es)      ~
            |                               |
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
            |                               |
            ~  Authentication Domain Name   ~
            |                               |
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                   Figure 11: DHCP Encrypted DNS Option

   The fields of the option shown in Figure 11 are as follows:

   o  Code: OPTION_V4_ENC_DNS (TBA3, see Section 10.2).
   o  Length: Length of the enclosed data in octets.
   o  Encr DNS Types (Encrypted DNS Types): Indicates the type(s) of the
      encrypted DNS server conveyed in this attribute.  The format of
      this field is shown in Figure 8.
   o  Num Addresses: Indicates the number of included IPv4 addresses.
   o  Port Number: If not null, it indicates the port number to be used
      for the encrypted DNS.  A null value indicates that default port
      numbers are used.  As a reminder, the default port number is 853
      for DoT and 443 for DoH.
   o  IPv4 Addresses: Indicates one or more IPv4 addresses to reach the
      encrypted DNS server.  The format of this field is shown in
      Figure 12.

           0     8     16    24    32    40    48
           +-----+-----+-----+-----+-----+-----+--
           |  a1 |  a2 |  a3 |  a4 |  a1 |  a2 | ...
           +-----+-----+-----+-----+-----+-----+--
            IPv4 Address 1          IPv4 Address 2 ...

     This format assumes that an IPv4 address is encoded as a1.a2.a3.a4.

               Figure 12: Format of the IPv4 Addresses Field
   o  Authentication Domain Name: The domain name of the DoH/DoT server.
      This field is formatted as specified in Section 10 of [RFC8415].
      The format of this field is shown in Figure 13.



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           +-----+-----+-----+-----+-----+--
           |  s1 |  s2 |  s3 |  s4 | s5  |  ...
           +-----+-----+-----+-----+-----+--
            Authentication Domain Name

     The values s1, s2, s3, etc. represent the domain name labels in the
     domain name encoding.

         Figure 13: Format of the Authentication Domain Name Field

   OPTION_V4_ENC_DNS is a concatenation-requiring option.  As such, the
   mechanism specified in [RFC3396] MUST be used if OPTION_V4_ENC_DNS
   exceeds the maximum DHCP option size of 255 octets.

   To discover an encrypted DNS server, the DHCP client requests the
   Encrypted DNS server by including OPTION_V4_ENC_DNS in a Parameter
   Request List option [RFC2132].  The DHCP client sets the Encrypted
   DNS Types field to the requested encrypted DNS server.

   If the DHCP client requested more than one encrypted DNS server type,
   the DHCP client MUST be prepared to receive multiple DHCP
   OPTION_V4_ENC_DNS options; each option is to be treated as a separate
   encrypted DNS server.

4.3.  IPv6 RA Encrypted DNS Options

   This section defines two Neighbor Discovery [RFC4861]: IPv6 Router
   Advertisement (RA) Encrypted DNS ADN option and IPv6 RA Encrypted DNS
   Address option.

   The IPv6 RA Encrypted DNS ADN option is used to configure an
   authentication domain name of the encrypted DNS server.  The format
   of this option is illustrated in Figure 14.

        0                   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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     TBA3      |     Length    | Encr DNS Types|   Unassigned  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Lifetime                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       :                   Authentication Domain Name                  :
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                  Figure 14: RA Encrypted DNS ADN Option



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   The fields of the option shown in Figure 14 are as follows:

   o  Type: 8-bit identifier of the Encrypted DNS Option as assigned by
      IANA (TBA3, see Section 10.3).
   o  Length: 8-bit unsigned integer.  The length of the option
      (including the Type and Length fields) is in units of 8 octets.
   o  Encr DNS Types (Encrypted DNS Types): Indicates the type(s) of the
      encrypted DNS server conveyed in this attribute.  The format of
      this field is shown in Figure 8.
   o  Unassigned: This field is unused.  It MUST be initialized to zero
      by the sender and MUST be ignored by the receiver.
   o  Lifetime: 32-bit unsigned integer.  The maximum time in seconds
      (relative to the time the packet is received) over which the
      discovered Authentication Domain Name is valid.

      The value of Lifetime SHOULD by default be at least 3 *
      MaxRtrAdvInterval, where MaxRtrAdvInterval is the maximum RA
      interval as defined in [RFC4861].

      A value of all one bits (0xffffffff) represents infinity.

      A value of zero means that this Authentication Domain Name MUST no
      longer be used.
   o  Authentication Domain Name: The domain name of the encrypted DNS
      server.  This field is formatted as specified in Section 10 of
      [RFC8415].

      This field MUST be padded with zeros so that its size is a
      multiple of 8 octets.

   The IPv6 RA Encrypted DNS Address option is used to configure a port
   number and a list of IPv6 addresses of the encrypted DNS server.  The
   format of this option is illustrated in Figure 15.  All of the
   addresses share the same Lifetime value.  Simialr to [RFC8106], if it
   is desirable to have different Lifetime values per IP address,
   multiple Encrypted DNS Address options may be used.















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        0                   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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     TBA5      |     Length    |         Unassigned            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Lifetime                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Encr DNS Types|  Unassigned   |           Port Number         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       :                         IPv6 Address(es)                      :
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                Figure 15: RA Encrypted DNS Address Option

   The fields of the RA Encrypted DNS Address option shown in Figure 15
   are as follows:

   o  Type: 8-bit identifier of the Encrypted DNS Address Option as
      assigned by IANA (TBA5, see Section 10.3).

   o  Length: 8-bit unsigned integer.  The length of the option
      (including the Type and Length fields) is in units of 8 octets.

   o  Unassigned: This field is unused.  It MUST be initialized to zero
      by the sender and MUST be ignored by the receiver.

   o  Lifetime: 32-bit unsigned integer.  The maximum time in seconds
      (relative to the time the packet is received) over which the
      discovered encrypted DNS IPv6 addresses are valid.

      The value of Lifetime SHOULD by default be at least 3 *
      MaxRtrAdvInterval, where MaxRtrAdvInterval is the maximum RA
      interval as defined in [RFC4861].

      A value of all one bits (0xffffffff) represents infinity.

      A value of zero means that these IPv6 addresses MUST no longer be
      used.

   o  Port Number: If not null, it indicates the port number to be used
      for the encrypted DNS.  A null value indicates that default port
      numbers must be used.  As a reminder, the default port number is
      853 for DoT and 443 for DoH.





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   o  Encr DNS Types (Encrypted DNS Types): Indicates the type(s) of the
      encrypted DNS server conveyed in this attribute.  The format of
      this field is shown in Figure 8.

   o  IPv6 Address(es) : One or more IPv6 addresses of the encrypted DNS
      server.  Link-local addresses MAY be used.

5.  DoH URI Templates

   DoH servers may support more than one URI Template [RFC8484].  Also,
   if the resolver hosts several DoH services (e.g., no-filtering,
   blocking adult content, blocking malware), these services can be
   discovered as templates.  The following discusses a mechanism for a
   DoH client to retrieve the list of supported templates by a DoH
   server.

   Upon discovery of a DoH resolver (Section 4), the DoH client may
   contact that DoH server to retrieve the list of supported DoH
   services using the well-known URI.  DoH clients re-iterates that
   request regularly to retrieve an updated list of supported DoH
   services.

   Let's suppose that a host has discovered an encrypted DNS server that
   is DoH-capable.  The host has also discovered the following
   information:

   o  ADN: doh.example.com

   o  Locator: 2001:db8:1::1

   The client requests "https://doh.example.com/.well-known/resinfo" to
   retrieve the list of the URI Templates associated with this
   discovered server.

   Alternatively, dedicated DHCP/RA options may be defined to convey an
   URI template.  An example of such option is depicted in Figure 16.















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       0                   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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   OPTION_V6_DOH_TEMPLATE      |         Option-length         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      ~                         uri-template-data                     ~
      |                             . . .                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      Each instance of the uri-template-data is formatted as follows:

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+-+-+-+-+-+
      |   uri-template-len            |          URI Template         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...-+-+-+-+-+-+-+

            Figure 16: Example of a DHCPv6 URI Template Option

      Note: More feedback form the WG is needed to decide which approach
      to follow.

   How a DoH client makes use of the configured DoH services is out of
   scope of this document.

6.  Make Use of Discovered Encrypted DNS Server

   Even if the use of a discovered encrypted DNS server is beyond the
   discovery process and falls under encrypted server selection, the
   following discusses typical conditions under which discovered
   encrypted DNS server can be used.

   o  If the DNS server's IP address discovered by using DHCP/RA is
      preconfigured in the OS or Browser as a verified resolver (e.g.,
      part of an auto-upgrade program such as [Auto-upgrade]), the DNS
      client auto-upgrades to use the preconfigured encrypted DNS server
      tied to the discovered DNS server IP address.  In such a case the
      DNS client will perform additional checks out of band, such as
      confirming that the Do53 IP address and the encrypted DNS server
      are owned and operated by the same organisation.

   o  Similarly, if the ADN conveyed in DHCP/RA (Section 4) is
      preconfigured in the OS or browser as a verified resolver, the DNS
      client auto-upgrades to establish an encrypted a DoH/DoT/DoQ
      session with the ADN.

      In such case, the DNS client matches the domain name in the
      Encrypted DNS DHCP/RA option with the 'DNS-ID' identifier type




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      within subjectAltName entry in the server certificate conveyed in
      the TLS handshake.

7.  Hosting Encrypted DNS Forwarder in the CPE

7.1.  Managed CPEs

   The following mechanisms can be used to host an encrypted DNS
   forwarder in a managed CPE (Section 3.1).

7.1.1.  ACME

   The ISP can assign a unique FQDN (e.g., cpe1.example.com) and a
   domain-validated public certificate to the encrypted DNS forwarder
   hosted on the CPE.  Automatic Certificate Management Environment
   (ACME) [RFC8555] can be used by the ISP to automate certificate
   management functions such as domain validation procedure, certificate
   issuance and certificate revocation.

   The managed CPE should support a configuration parameter to instruct
   the CPE whether it has to relay the encrypted DNS server received
   from the ISP's network or has to announce itself as a forwarder
   within the local network.  The default behavior of the CPE is to
   supply the encrypted DNS server received from the ISP's network.

7.1.2.  Auto-Upgrade Based on Domains and their Subdomains

   If the ADN conveyed in DHCP/RA (Section 4) is pre-configured in
   popular OSes or browsers as a verified resolver and the auto-upgrade
   (Section 6) is allowed for both the pre-configured ADN and its sub-
   domains, the encrypted DNS client will learn the local encrypted DNS
   forwarder using DHCP/RA and auto-upgrade because the left-most label
   of the pre-configured ADN would match the subjectAltName value in the
   server certificate.  Concretely, the CPE can communicate the ADN of
   the local DoH forwarder (Section 7.1.1) to internal hosts using DHCP/
   RA (Section 4).

   Let's suppose that "example.net" is pre-configured as a verified
   resolved in the browser or OS.  If the encrypted DNS client discovers
   a local forwarder "cpe1-internal.example.net", the encrypted DNS
   client will auto-upgrade because the pre-configured ADN would match
   subjectAltName value "cpe1-internal.example.net" of type dNSName.  As
   shown in Figure 17, the auto-upgrade to a rogue server advertising
   "rs.example.org" will fail.







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                            Rogue Server
                |              |
                X<==DHCP=======|
                | {ADN=        |
                |  rs.example.org, @rs}
                |              |                  --,--,-
                |        ,+-,--+--.             ,/  ISP   \.
                |     ,-'          `-.       ,-'            `-.
            DoH/DoT --(      LAN      CPE----( S (@1)          )
       capable client  `-.          ,-'|      `-.           ,-'
                |        `--'--'--'    |         `--'--'--'
                |<========DHCP========>|
                |{ADN=                 |
                |  cpe1-internal.example.net, @i}
                |
                |<========DoH=========>|
                |                      |
   Legend:
     * S: DoH/DoT server
     * @1: IP address of S
     * @i: internal IP address of the CPE
     * @rs: IP address of a rogue server

   Figure 17: A Simplified Example of Auto-upgrade based on Sub-domains

7.2.  Unmanaged CPEs

   The approach specified in Section 7.1 does not apply for hosting a
   DNS forwarder in an unmanaged CPE.

   The unmanaged CPE administrator (referred to as administrator) can
   host an encrypted DNS forwarder on the unmanaged CPE.  This assumes
   the following:

   o  The encrypted DNS server certificate is managed by the entity in-
      charge of hosting the encrypted DNS forwarder.

      Alternatively, a security service provider can assign a unique
      FQDN to the CPE.  The encrypted DNS forwarder will act like a
      private encrypted DNS server only be accessible from within the
      home network.

   o  The encrypted DNS forwarder will either be configured to use the
      ISP's or a 3rd party encrypted DNS server.

   o  The unmanaged CPE will advertise the encrypted DNS forwarder ADN
      using DHCP/RA to internal hosts.




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   Figure 18 illustrates an example of an unmanaged CPE hosting a
   forwarder which connects to a 3rd party encrypted DNS server.  In
   this example, the DNS information received from the managed CPE (and
   therefore from the ISP) is ignored by the Internal CPE hosting the
   forwarder.

              ,--,--,--.                         ,--,
            ,'         Internal   Managed     ,-'    '-     3rd Party
     Host--(  Network#A  CPE--------CPE------(   ISP   )--- DNS Server
      |     `.         ,-'|          |        `-.    -'       |
      |       `-'--'--'   |          |<==DHCP==>|`--'         |
      |                   |<==DHCP==>|          |             |
      |<======DHCP=======>|          |                        |
      |     {RI, @i}      |                                   |
      |<==Encrypted DNS==>|<==========Encrypted DNS==========>|

     Legend:
       * @i: IP address of the DNS forwarder hosted in the Internal
             CPE.

         Figure 18: Example of an Internal CPE Hosting a Forwarder

8.  Legacy CPEs

   Hosts serviced by legacy CPEs that can't be upgraded to support the
   options defined in Section 4 won't be able to learn the encrypted DNS
   server hosted by the ISP, in particular.  If the ADN is not
   discovered using DHCP/RA, such hosts will have to fallback to use the
   special-use domain name defined in [I-D.pp-add-resinfo] to discover
   the encrypted DNS server and to retrieve the list of supported DoH
   services using the RESINFO RRtype [I-D.pp-add-resinfo].

   If the Do53 and Encrypted DNS servers are hosted on different public
   IP addresses, the DNS server certificate can include the IP address
   of the Do53 server in the subjectAltName extension.  After validating
   the server certificate, the client can retrieve the IP address in the
   certificate to identify that both Do53 and Encrypted DNS servers are
   operated by the same entity.

   If the Do53 and Encrypted DNS server are hosted on a private IP
   address, they should use the same private IP address to prove to the
   client they are operated by the same entity.

   The DHCP/RA option to discover ADN takes precedence over special-use
   domain name since the special-use domain name is susceptible to both
   internal and external attacks whereas DHCP/RA is only vulnerable to
   internal attacks.




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9.  Security Considerations

9.1.  Spoofing Attacks

   Because DHCP/RA messages are not encrypted or protected against
   modification within the LAN, their content can be spoofed or modified
   by active attackers (e.g., compromised devices within the home
   network).  An active attacker (Section 3.3 of [RFC3552]) can spoof
   the DHCP/RA response to provide the attacker's Encrypted DNS server.
   Note that such an attacker can launch other attacks as discussed in
   Section 22 of [RFC8415].  The attacker can get a domain name, domain-
   validated public certificate from a CA, host an Encrypted DNS server
   and claim the best DNS privacy preservation policy.  Also, an
   attacker can use a public IP address, get an 'IP address'-validated
   public certificate from a CA, host an Encryptd DNS server and claim
   the best DNS privacy preservation policy.

   Attacks of spoofed or modified DHCP responses and RA messages by
   attackers within the home network may be mitigated by making use of
   the following mechanisms:

   o  DHCPv6-Shield described in [RFC7610], the CPEs discards DHCP
      response messages received from any local endpoint.

   o  RA-Guard described in [RFC7113], the CPE discards RAs messages
      received from any local endpoint.

   o  Source Address Validation Improvement (SAVI) solution for DHCP
      described in [RFC7513], the CPE filters packets with forged source
      IP addresses.

   Encrypted DNS sessions with rogue servers spoofing the IP address of
   a DNS server will fail because the DNS client will fail to
   authenticate that rogue server based upon PKIX authentication
   [RFC6125], particularly the authentication domain name in the
   Encrypted DNS Option.  DNS clients that ignore authentication
   failures and accept spoofed certificates will be subject to attacks
   (e.g., redirect to malicious servers, intercept sensitive data).

   Encrypted DNS connections received from outside the home network MUST
   be discarded by the encrypted DNS forwarder in the CPE.  This
   behavior adheres to REQ#8 in [RFC6092]; it MUST apply for both IPv4
   and IPv6.








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9.2.  Deletion Attacks

   If the DHCP responses or RAs are dropped by the attacker, the client
   can fallback to use a pre-configured encrypted DNS server.  However,
   the use of policies to select servers is out of scope of this
   document.

   Note that deletion attack is not specific to DHCP/RA.

9.3.  Passive Attacks

   A passive attacker (Section 3.2 of [RFC3552]) can identify a host is
   using DHCP/RA to discover an encrypted DNS server and can infer that
   host is capable of using DoH/DoT/DoQ to encrypt DNS messages.
   However, a passive attacker cannot spoof or modify DHCP/RA messages.

9.4.  Wireless Security - Authentication Attacks

   Wireless LAN (WLAN) as frequently deployed in home networks is
   vulnerable to various attacks (e.g., [Evil-Twin], [Krack],
   [Dragonblood]).  Because of these attacks, only cryptographically
   authenticated communications are trusted on WLANs.  This means
   information provided by such networks via DHCP, DHCPv6, or RA (e.g.,
   NTP server, DNS server, default domain) are untrusted because DHCP
   and RA are not authenticated.

   With the current deployments (2020), the pre-shared-key is the same
   for all clients that connect to the same WLAN.  This results in the
   key being shared to attackers resulting in security breach.  Man-in-
   the-middle attacks are possible within home networks because WLAN
   authentication lacks peer entity authentication.

   This leads to the need for provisioning unique credentials for
   different clients.  Endpoints can be provisioned with unique
   credentials (username and password, typically) provided by the home
   network administraor to mutually authenticate to the home WLAN Access
   Point (e.g., 802.1x Wireless User Authentication on OpenWRT [dot1x],
   EAP-pwd [RFC8146]).  Not all of endpoint devices (e.g., IoT devices)
   support 802.1x supplicant and need an alternate mechanism to connect
   to the home network.  To address this limitation, unique pre-shared
   keys can be created for each such device and WPA-PSK is used (e.g.,
   [PSK]).

10.  IANA Considerations







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10.1.  DHCPv6 Options

   IANA is requested to assign the following new DHCPv6 Option Code in
   the registry maintained in [DHCPV6].

   +-------+-------------------+---------+------------+----------------+
   | Value | Description       | Client  | Singleton  | Reference      |
   |       |                   | ORO     | Option     |                |
   +-------+-------------------+---------+------------+----------------+
   | TBA1  | OPTION_V6_ENC_ADN | Yes     | No         | [ThisDocument] |
   | TBA2  | OPTION_V6_ENC_ADD | Yes     | No         | [ThisDocument] |
   +-------+-------------------+---------+------------+----------------+

10.2.  DHCP Option

   IANA is requested to assign the following new DHCP Option Code in the
   registry maintained in [BOOTP].

   +------+------------------+-------+----------------+----------------+
   | Tag  | Name             | Data  | Meaning        | Reference      |
   |      |                  | Length|                |                |
   +------+------------------+-------+----------------+----------------+
   | TBA3 | OPTION_V4_ENC_DNS| N     | Encrypted DNS  | [ThisDocument] |
   |      |                  |       | Server         |                |
   +------+------------------+-------+----------------+----------------+

10.3.  RA Options

   IANA is requested to assign the following new IPv6 Neighbor Discovery
   Option type in the "IPv6 Neighbor Discovery Option Formats" sub-
   registry under the "Internet Control Message Protocol version 6
   (ICMPv6) Parameters" registry maintained in [ND].

       +------+----------------------------------+----------------+
       | Type | Description                      | Reference      |
       +------+----------------------------------+----------------+
       | TBA4 | DNS Encrypted DNS ADN Option     | [ThisDocument] |
       | TBA5 | DNS Encrypted DNS Address Option | [ThisDocument] |
       +------+----------------------------------+----------------+

11.  Acknowledgements

   Many thanks to Christian Jacquenet for the review.

   Thanks to Tommy Jensen, Stephen Farrell, Martin Thomson, Vittorio
   Bertola, Stephane Bortzmeyer, Ben Schwartz, and Iain Sharp for the
   comments.




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   Thanks to Mark Nottingham for the feedback on HTTP redirection.

   The use of DHCP to retrieve an authentication domain name was
   discussed in Section 7.3.1 of [RFC8310] and
   [I-D.pusateri-dhc-dns-driu].

12.  References

12.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
              Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,
              <https://www.rfc-editor.org/info/rfc2132>.

   [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>.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <https://www.rfc-editor.org/info/rfc4861>.

   [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>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [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>.




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   [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>.

12.2.  Informative References

   [Auto-upgrade]
              The Unicode Consortium, "DoH providers: criteria, process
              for Chrome", <docs.google.com/document/
              d/128i2YTV2C7T6Gr3I-81zlQ-_Lprnsp24qzy_20Z1Psw/edit>.

   [BOOTP]    , <https://www.iana.org/assignments/bootp-dhcp-parameters/
              bootp-dhcp-parameters.xhtml#options>.

   [DHCPV6]   , <https://www.iana.org/assignments/dhcpv6-parameters/
              dhcpv6-parameters.xhtml#dhcpv6-parameters-2>.

   [dot1x]    Cisco, "Basic 802.1x Wireless User Authentication",
              <https://openwrt.org/docs/guide-user/network/wifi/
              wireless.security.8021x>.

   [DPP]      The Wi-Fi Alliance, "Device Provisioning Protocol
              Specification", <https://www.wi-fi.org/file/device-
              provisioning-protocol-specification>.

   [Dragonblood]
              The Unicode Consortium, "Dragonblood: Analyzing the
              Dragonfly Handshake of WPA3 and EAP-pwd",
              <https://papers.mathyvanhoef.com/dragonblood.pdf>.

   [Evil-Twin]
              The Unicode Consortium, "Evil twin (wireless networks)",
              <https://en.wikipedia.org/wiki/
              Evil_twin_(wireless_networks)>.

   [I-D.box-add-requirements]
              Box, C., Pauly, T., Wood, C., and T. Reddy.K,
              "Requirements for Adaptive DNS Discovery", draft-box-add-
              requirements-00 (work in progress), September 2020.

   [I-D.ietf-dnsop-terminology-ter]
              Hoffman, P., "Terminology for DNS Transports and
              Location", draft-ietf-dnsop-terminology-ter-02 (work in
              progress), August 2020.





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   [I-D.ietf-dprive-dnsoquic]
              Huitema, C., Mankin, A., and S. Dickinson, "Specification
              of DNS over Dedicated QUIC Connections", draft-ietf-
              dprive-dnsoquic-00 (work in progress), April 2020.

   [I-D.ietf-v6ops-rfc7084-bis]
              Palet, J., "Basic Requirements for IPv6 Customer Edge
              Routers", draft-ietf-v6ops-rfc7084-bis-04 (work in
              progress), June 2017.

   [I-D.pp-add-resinfo]
              Sood, P. and P. Hoffman, "DNS Resolver Information Self-
              publication", draft-pp-add-resinfo-02 (work in progress),
              June 2020.

   [I-D.pusateri-dhc-dns-driu]
              Pusateri, T. and W. Toorop, "DHCPv6 Options for private
              DNS Discovery", draft-pusateri-dhc-dns-driu-00 (work in
              progress), July 2018.

   [Krack]    The Unicode Consortium, "Key Reinstallation Attacks",
              2017, <https://www.krackattacks.com/>.

   [ND]       , <http://www.iana.org/assignments/icmpv6-parameters/
              icmpv6-parameters.xhtml#icmpv6-parameters-5>.

   [PSK]      Cisco, "Identity PSK Feature Deployment Guide",
              <https://www.cisco.com/c/en/us/td/docs/wireless/
              controller/technotes/8-5/
              b_Identity_PSK_Feature_Deployment_Guide.html>.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003,
              <https://www.rfc-editor.org/info/rfc3552>.

   [RFC3646]  Droms, R., Ed., "DNS Configuration options for Dynamic
              Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
              DOI 10.17487/RFC3646, December 2003,
              <https://www.rfc-editor.org/info/rfc3646>.

   [RFC6092]  Woodyatt, J., Ed., "Recommended Simple Security
              Capabilities in Customer Premises Equipment (CPE) for
              Providing Residential IPv6 Internet Service", RFC 6092,
              DOI 10.17487/RFC6092, January 2011,
              <https://www.rfc-editor.org/info/rfc6092>.





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   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
              2011, <https://www.rfc-editor.org/info/rfc6125>.

   [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>.

   [RFC7113]  Gont, F., "Implementation Advice for IPv6 Router
              Advertisement Guard (RA-Guard)", RFC 7113,
              DOI 10.17487/RFC7113, February 2014,
              <https://www.rfc-editor.org/info/rfc7113>.

   [RFC7250]  Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
              Weiler, S., and T. Kivinen, "Using Raw Public Keys in
              Transport Layer Security (TLS) and Datagram Transport
              Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
              June 2014, <https://www.rfc-editor.org/info/rfc7250>.

   [RFC7513]  Bi, J., Wu, J., Yao, G., and F. Baker, "Source Address
              Validation Improvement (SAVI) Solution for DHCP",
              RFC 7513, DOI 10.17487/RFC7513, May 2015,
              <https://www.rfc-editor.org/info/rfc7513>.

   [RFC7610]  Gont, F., Liu, W., and G. Van de Velde, "DHCPv6-Shield:
              Protecting against Rogue DHCPv6 Servers", BCP 199,
              RFC 7610, DOI 10.17487/RFC7610, August 2015,
              <https://www.rfc-editor.org/info/rfc7610>.

   [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>.

   [RFC7969]  Lemon, T. and T. Mrugalski, "Customizing DHCP
              Configuration on the Basis of Network Topology", RFC 7969,
              DOI 10.17487/RFC7969, October 2016,
              <https://www.rfc-editor.org/info/rfc7969>.

   [RFC8146]  Harkins, D., "Adding Support for Salted Password Databases
              to EAP-pwd", RFC 8146, DOI 10.17487/RFC8146, April 2017,
              <https://www.rfc-editor.org/info/rfc8146>.





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   [RFC8305]  Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
              Better Connectivity Using Concurrency", RFC 8305,
              DOI 10.17487/RFC8305, December 2017,
              <https://www.rfc-editor.org/info/rfc8305>.

   [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>.

   [RFC8489]  Petit-Huguenin, M., Salgueiro, G., Rosenberg, J., Wing,
              D., Mahy, R., and P. Matthews, "Session Traversal
              Utilities for NAT (STUN)", RFC 8489, DOI 10.17487/RFC8489,
              February 2020, <https://www.rfc-editor.org/info/rfc8489>.

   [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>.

   [RFC8520]  Lear, E., Droms, R., and D. Romascanu, "Manufacturer Usage
              Description Specification", RFC 8520,
              DOI 10.17487/RFC8520, March 2019,
              <https://www.rfc-editor.org/info/rfc8520>.

   [RFC8555]  Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
              <https://www.rfc-editor.org/info/rfc8555>.

   [TR-069]   The Broadband Forum, "CPE WAN Management Protocol",
              December 2018, <https://www.broadband-
              forum.org/technical/download/TR-069.pdf>.

   [TS.24008]
              3GPP, "Mobile radio interface Layer 3 specification; Core
              network protocols; Stage 3 (Release 16)", December 2019,
              <http://www.3gpp.org/DynaReport/24008.htm>.

Authors' Addresses

   Mohamed Boucadair
   Orange
   Rennes  35000
   France

   Email: mohamed.boucadair@orange.com






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   Tirumaleswar Reddy
   McAfee, Inc.
   Embassy Golf Link Business Park
   Bangalore, Karnataka  560071
   India

   Email: TirumaleswarReddy_Konda@McAfee.com


   Dan Wing
   Citrix Systems, Inc.
   USA

   Email: dwing-ietf@fuggles.com


   Neil Cook
   Open-Xchange
   UK

   Email: neil.cook@noware.co.uk






























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