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ADD                                                         M. Boucadair
Internet-Draft                                                    Orange
Intended status: Standards Track                                T. Reddy
Expires: October 4, 2020                                          McAfee
                                                                 D. Wing
                                                                  Citrix
                                                                 N. Cook
                                                            Open-Xchange
                                                           April 2, 2020


    DNS-over-HTTPS and DNS-over-TLS Server Discovery and Deployment
                    Considerations for Home Networks
                         draft-btw-add-home-05

Abstract

   This document discusses DoT/DoH deployment considerations for home
   networks.  It particularly sketches the required steps to use DoT/DoH
   capabilities provided by local networks.

   One of the goals of this document is to assess to what extent
   existing tools can be used to provide a DoT/DoH service.  As an
   outcome, new DHCP and Router Advertisement Options are specified in
   order to convey a DNS Authentication Domain Name.

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 October 4, 2020.

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 Deployment Scenarios . . . . . . . . . . . . . . . . .   5
     3.1.  Managed CPEs  . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Unmanaged CPEs  . . . . . . . . . . . . . . . . . . . . .   7
   4.  DNS Reference Identifier Option . . . . . . . . . . . . . . .   8
     4.1.  DHCPv6 DNS Reference Identifier Option  . . . . . . . . .   9
     4.2.  DHCP DNS Reference Identifier Option  . . . . . . . . . .  10
     4.3.  RA DNS Reference Identifier Option  . . . . . . . . . . .  12
   5.  DoH URI Templates . . . . . . . . . . . . . . . . . . . . . .  13
     5.1.  Define a Dedicated DHCP/RA Option . . . . . . . . . . . .  13
     5.2.  Retrieve the List Directly from the DoH Server  . . . . .  14
   6.  Locating DoH/DoT Servers  . . . . . . . . . . . . . . . . . .  14
     6.1.  DoT/DoH Auto-Upgrade  . . . . . . . . . . . . . . . . . .  16
     6.2.  Other Deployment Options  . . . . . . . . . . . . . . . .  17
   7.  Hosting DoH/DoT Forwarder in the CPE  . . . . . . . . . . . .  17
     7.1.  Managed CPEs  . . . . . . . . . . . . . . . . . . . . . .  18
       7.1.1.  ACME  . . . . . . . . . . . . . . . . . . . . . . . .  18
       7.1.2.  Redirection . . . . . . . . . . . . . . . . . . . . .  18
         7.1.2.1.  Server-Driven Redirection . . . . . . . . . . . .  18
         7.1.2.2.  Client-Initiated Redirection  . . . . . . . . . .  19
       7.1.3.  Auto-Upgrade based on Domains and their Sub-domains .  21
     7.2.  Unmanaged CPEs  . . . . . . . . . . . . . . . . . . . . .  22
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  23
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  24
     9.1.  DHCPv6 Option . . . . . . . . . . . . . . . . . . . . . .  24
     9.2.  DHCP Option . . . . . . . . . . . . . . . . . . . . . . .  25
     9.3.  RA Option . . . . . . . . . . . . . . . . . . . . . . . .  25
     9.4.  Service Name  . . . . . . . . . . . . . . . . . . . . . .  25
     9.5.  Encrypted DNS Types . . . . . . . . . . . . . . . . . . .  26
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  26
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  26
     11.2.  Informative References . . . . . . . . . . . . . . . . .  28
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  30




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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.

           (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




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   ISPs use DNS to provide additional services such as (but not limited
   to) malware filtering, parental control, or VoD (Video on Demand)
   optimization.  DNS is also a central component for mastering the
   quality of experience for current latency-sensitive services, but
   also emerging ones (such as those services that pertain to the Ultra
   Reliability and Low Latency Communications (uRLLC) or Enhanced Mobile
   Broadband (eMBB).

      For example, the latency targets set in the context of 5G are 1ms
      (uRLLC) and 4ms (eMBB).  An ISP will be able to address such
      demanding latency requirements assuming the corresponding services
      rely upon resources (network, compute, storage) that are located
      as close to the user as possible (e.g., by means of Edge Computing
      techniques and resources).  Such latency requirements are likely
      to be addressed by means of optimized designs (DNS, in
      particular), too.

   Relying upon local DNS resolvers will therefore contribute to meet
   the aforementioned service requirements.  The use of external
   resolvers is likely to induce an extra service delay which exceeds by
   far the service target.

   This document focuses on the support of DNS-over-HTTPS (DoH)
   [RFC8484] or DNS-over-TLS (DoT) [RFC7858] in local networks.  In
   particular, the document describes how a local DoH/DoT server can be
   discovered and used by connected hosts.  This document specifies
   options that allow DNS clients to discover local DoT/DoH servers.
   Section 4 describes DHCP, DHCPv6, and RA options to convey the
   Authentication Domain Name (ADN, defined in [RFC8310]).

   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).

   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




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   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.

   'DoH/DoT' refers to DNS-over-HTTPS and/or DNS-over-TLS.

3.  Sample 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
   authentication domain name information (ADN) to managed CPEs if DoH/
   DoT is supported by a local network similar to what is depicted in
   Figure 2.

   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 by using a dedicated field in the discovery message:
   Encrypted DNS Type (Section 4).
















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           (a) Fixed Networks

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


           (b) Cellular Networks

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

                       Figure 2: DoH/DoT in the WAN

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

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

                     Figure 3: Direct DoH/DoT 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



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   the request to an upstream DoH/DoT/Do53 resolver.  Such deployment is
   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 DoH/DoT to its customers may enable DoH/DoT 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  )
                |     `-.          ,-'|      `-.          ,-'
                |        `--'--'--'   |         `--'--'--'
                |                     |             |
                |<======DoT/DoH======>|<==DoT/DoH==>|


                    Figure 4: Proxied DoH/DoT 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 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.  DoH/DoT sessions
   can be established by a host with the DoH/DoT servers of the ISP (see
   Figure 5).




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

   Figure 5: Direct DoH/DoT 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.  DoH/DoT 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
         |     `.         ,-'             `-.    -'         |
         |       `-'--'--'                   `--'           |
         |                                                  |
         |<======================DoT/DoH===================>|

     Figure 6: Direct DoH/DoT Sessions with a Third Party DNS Resolver

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

4.  DNS Reference Identifier Option

   This section describes how a DNS client can discover the ADN of local
   DoH/DoT 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 a DoH/DoT 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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   The DNS client establishes a DoH/DoT session with the discovered DNS
   IP address(es) (Section 6) and uses the mechanism discussed in
   Section 8 of [RFC8310] to authenticate the DNS server certificate
   using the authentication domain name conveyed in the DNS Reference
   Identifier.

   If the DNS Reference Identifier 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 DNS Reference Identifier Option

   The DHCPv6 DNS Reference Identifier option is used to configure an
   authentication domain name of the DoH/DoT server.  The format of this
   option is shown in Figure 7.

       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_DNS_RI          |         Option-length         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |F|Encr DNS Type|                                               |
      +-+-------------+                                               |
      |                                                               |
      ~                 Authentication Domain Name                    ~
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


             Figure 7: DHCPv6 DNS Reference Identifier Option

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

   o  Option-code: OPTION_V6_DNS_RI (TBA1, see Section 9.1)
   o  Option-length: Length of the enclosed data in octets.
   o  F bit: Forwarder bit.  If set, it instructs the DNS client to
      retrieve the forwarder identifier from the resolver identified by
      the Authentication Domain Name (Section 7).
   o  Encr DNS Type (Encrypted DNS Type): Indicates the type of the
      encrypted DNS server conveyed in this attribute.  The following
      values are defined:

         0: Any encrypted DNS
         1: DoT
         2: DoH
         See Section 9.5 for future assignment considerations.
   o  Authentication Domain Name: A fully qualified domain name of the
      DoH/DoT server.  This field is formatted as specified in
      Section 10 of [RFC8415].



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   An example of the Authentication Domain Name encoding is shown in
   Figure 8.  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 8: An example of the authentication-domain-name Encoding

   Multiple instances of OPTION_V6_DNS_RI 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 including
   OPTION_V6_DNS_RI 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 Type field to the requested
   encrypted DNS server.

   If the DHCPv6 client requested any encrypted DNS server, the DHCP
   client MUST be prepared to receive multiple DHCP OPTION_V6_DNS_RI
   options; each option is to be treated as a separate encrypted DNS
   server.

4.2.  DHCP DNS Reference Identifier Option

   The DHCP DNS Reference Identifier option is used to configure an
   authentication domain name of the DoH/DoT server.  The format of this
   option is illustrated in Figure 9.





















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            0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |     TBA2      |     Length    |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
           |F|Encr DNS Type|               |
           +-+-+-+-+-+-+-+-+               |
           |                               |
           ~  Authentication Domain Name   ~
           |                               |
           +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

  with:

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

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


              Figure 9: DHCP DNS Reference Identifier Option

   The fields of the option shown in Figure 9 are as follows:

   o  Code: OPTION_V4_DNS_RI (TBA2, see Section 9.2).
   o  Length: Length of the enclosed data in octets.
   o  F bit: Forwarder bit.  If set, it instructs the DNS client to
      retrieve the forwarder identifier from the resolver identified by
      the Authentication Domain Name (Section 7).
   o  Encr DNS Type (Encrypted DNS Type): Indicates the type of the
      encrypted DNS server conveyed in this attribute.  The following
      values are defined:

         0: Any encrypted DNS
         1: DoT
         2: DoH
         See Section 9.5 for future assignment considerations.
   o  Authentication Domain Name: The domain name of the DoH/DoT server.
      This field is formatted as specified in Section 10 of [RFC8415].

   OPTION_V4_DNS_RI is a concatenation-requiring option.  As such, the
   mechanism specified in [RFC3396] MUST be used if OPTION_V4_DNS_RI
   exceeds the maximum DHCP option size of 255 octets.

   To discover an encrypted DNS server, the DHCP client requests the
   Encrypted DNS Reference Identifier by including OPTION_V4_DNS_RI in a



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   Parameter Request List option [RFC2132].  The DHCP client sets the
   Encrypted DNS Type field to the requested encrypted DNS server.

   If the DHCP client requested any encrypted DNS server, the DHCP
   client MUST be prepared to receive multiple DHCP OPTION_V4_DNS_RI
   options; each option is to be treated as a separate encrypted DNS
   server.

4.3.  RA DNS Reference Identifier Option

   The IPv6 Router Advertisement (RA) DNS Reference Identifier option is
   used to configure an authentication domain name of the DoH/DoT
   server.  The format of this option is illustrated in Figure 10.

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Type      |     Length    |F|Encr DNS Type|   Unassigned  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Lifetime                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       :                  Authentication Domain Name                   :
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 10: RA DNS Reference Identifier Option

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

   o  Type: 8-bit identifier of the DNS Reference Identifier Option as
      assigned by IANA (TBA3, see Section 9.3).
   o  Length: 8-bit unsigned integer.  The length of the option
      (including the Type and Length fields) is in units of 8 octets.
   o  F bit: Forwarder bit.  If set, it instructs the DNS client to
      retrieve the forwarder identifier from the resolver identified by
      the Authentication Domain Name (Section 7).
   o  Encr DNS Type (Encrypted DNS Type): Indicates the type of the
      encrypted DNS server conveyed in this attribute.  The following
      values are defined:

         0: Any encrypted DNS
         1: DoT
         2: DoH
         See Section 9.5 for future assignment considerations.
   o  Unassigned: This field is unused.  It MUST be initialized to zero
      by the sender and MUST be ignored by the receiver.




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   o  Lifetime: 32-bit unsigned integer.  The maximum time in seconds
      (relative to the time the packet is received) over which the
      authentication domain name MAY be used as a DNS Reference
      Identifier.

      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 the DNS Reference Identifier MUST no
      longer be used.
   o  Authentication Domain Name: The domain name of the DoH/DoT 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.

5.  DoH URI Templates

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

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

   o  DISCUSSION: More feedback is needed to assess whether URI RA/DHCP
      options have to be specified.

5.1.  Define a Dedicated DHCP/RA Option

   This solution assumes that DHCP servers and access routers maintain
   an updated list of the templates used by DoH resolvers.

   The following observations can be made:

   o  In order to avoid that stale DoH information is supplied to
      connected devices, each time the URI templates are updated at a
      DoH resolver (e.g., add a new DoH service, withdraw a DoH
      service), DHCP servers (or access routers for the RA case) have to
      be updated accordingly to reflect the DNS service change.





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   o  This dependency may be affordable if the ISP providing the
      connectivity service is also the one operating the DoH resolver.

   o  Nevertheless, if the DNS service is provided by a distinct entity
      than the ISP, an out-of-band mechanism is required to synchronize
      the list of DoH services that are active on a DoH resolver vs. the
      list maintained locally by the ISP.

   o  Also, it is not clear how enclosed URI templates will be validated
      by DHCP clients given that future specifications may allow for
      other variables in the URI.

   o  Including a large list of templates may cause the size of an RA to
      exceed the link MTU.  In such case, multiple RAs must be used.

   RA/DHCP has the following advantages:

   o  Notify clients whenever there is a change in the DoH service
      configuration.

   o  The DoH client can immediately use available DoH services.

   o  It is convenient if very few (stable) URIs are in use.

   o  It allows for customized configuration within the home network.
      For example, a child host can be provided by the CPE with a DoH
      server that supports parental filtering, while other hosts can be
      provisioned with a DoH server that does not enable such
      capabilities.

5.2.  Retrieve the List Directly from the DoH Server

   Upon discovery of a DoH resolver (Section 4), the DoH client contacts
   that DoH resolver to retrieve the list of supported DoH services
   (e.g., use of a well-known URI).  That information is cached by the
   DoH client for a given period (e.g., 24h).  DoH clients re-iterates
   that request regularly (e.g., 24h) to retrieve an updated list of
   supported DoH services.  Note that a "push" mode can be considered
   using the mechanism defined in [I-D.ietf-dnssd-push].

   This approach allows to avoid adherence of DoH servers with DHCP
   servers (or access routers) for (de)activating new DoH services.

6.  Locating DoH/DoT Servers

   A CPE or a host relies upon discovery mechanisms (such as PCO, DHCP,
   or RA) to retrieve DoH/DoT servers' reachability information.  In the
   various scenarios sketched in Section 3, Do53, DoH, and DoT may



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   terminate on the same IP address (or distinct IP addresses as
   depicted in Figure 12).  Terminating Do53/DoH/DoT on the same or
   distinct IP addresses is deployment-specific.

   From an IP reachability standpoint, DoH/DoT servers SHOULD be located
   by their address literals rather than their names.  This avoids
   adding a dependency on another server to resolve the DoH/DoT name.
   Concretely, if Do53/DoH/DoT terminate on same IP addresses, existing
   discovery mechanisms [RFC2132][RFC3646][RFC8106] can be leveraged to
   learn the IP addresses of DoT/DoH servers while an authentication
   domain name is supplied by one of the options discussed in Section 4.
   An example is depicted in Figure 11.

                    Legacy Do53
                      client
                          |<===DHCP===>|
                          |    {@1}    |       |
                          |            |       |
                          |======Do53 Query===>|
                          |            |       |   --,--,-
                         ,+-,--,--.    |       |,/  ISP   \.
                      ,-'          `-. |     ,-'            `-.
            DoH/DoT --(      LAN      CPE----( S (@1)         )
       capable client  `-.          ,-'|      `-.           ,-'
                |        `--'--'--'    |       | `--'--'--'
                |<========DHCP========>|       |
                |       {RI, @1}       |       |
                |                              |
                |<=========DoT/DoH============>|

   Legend:
     * S: DNS server
     * {@1}: IP address of S; returned in a DHCP
       Domain Name Server option
     * RI: DNS Reference Identifier

            Figure 11: Locating DoH/DoT/Do53 (Same DNS Server)














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                    Legacy Do53
                      client
                          |<===RA======|
                          | {RI,@1,@2} |             |
                          |            |             |
                          |========Do53 Query=======>|
                          |            |           --,--,-
                         ,+-,--,--.    |        ,/  S1 (@1)\.
                      ,-'          `-. |     ,-'    ISP     `-.
            DoH/DoT --(      LAN      CPE----(                 )
       capable client  `-.          ,-'|      `-.   S2 (@2)  ,-'
                |        `--'--'--'    |         `--'--'--'
                |<=========RA==========|             |
                |      {RI,@1,@2}      |             |
                |                                    |
                |<===============DoT/DoH============>|

   Legend:
     * S1: Do53 server
     * S2: DoH/DoT server
     * @1: IP address of S1
     * @1: IP address of S2
     * RI: DNS Reference Identifier

            Figure 12: Locating DoH/DoT/Do53 (Distinct Servers)

   The following sub-sections discusses the conditions under which
   discovered DoT/DoH server can be used.

6.1.  DoT/DoH Auto-Upgrade

   Additional considerations are discussed below for the use of DoH and
   DoT servers provided by local networks:

   o  If the DNS server's IP address discovered by using DHCP/RA is pre-
      configured 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 pre-configured DoH/DoT 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 DoH server are owned and operated
      by the same organisation.

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





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      In such case, the DNS client matches the domain name in the DNS
      Reference Identifier DHCP/RA option with the 'DNS-ID' identifier
      type within subjectAltName entry in the server certificate
      conveyed in the TLS handshake.

   Such an auto-upgrade mechanism would be compatible with the
   Redirection method of Section 7.1.2, for managed CPEs hosting a DoT/
   DoH forwarder.

6.2.  Other Deployment Options

   Some deployment options to securely configure hosts are discussed
   below.  These options are provided for the sake of completeness.

   o  If Device Provisioning Protocol (DPP) [DPP] is used, the
      configurator can securely configure devices in the home network
      with the local DoT/DoH server using DPP.  If the DoT/DoH servers
      use raw public keys [RFC7250], the Subject Public Key Info (SPKI)
      pin set [RFC7250] of raw public keys may be encoded in a QR code.
      The configurator (e.g., mobile device) can scan the QR code and
      provision SPKI pin set in OS/Browser.  The configurator can in-
      turn securely configure devices (e.g., thermostat) in the home
      network with the SPKI pin set using DPP.

   o  If a CPE is co-located with security services within the home
      network, the CPE can use WPA-PSK but with unique pre-shared keys
      for different endpoints to deal with security issues.  In such
      networks, [I-D.reddy-dprive-bootstrap-dns-server] may be used to
      securely bootstrap endpoint devices with the authentication domain
      name and DNS server certificate of the local network's DoH/DoT
      server.

      The OS would not know if the WPA pre-shared-key is the same for
      all clients or a unique pre-shared key is assigned to the host.
      Hence, the user has to indicate to the system that a unique pre-
      shared key is assigned to trigger the bootstrapping procedure.

      If the device joins a home network using a single shared password
      among all the attached devices, a compromised device can host a
      fake access point, and the device cannot be securely bootstrapped
      with the home network's DoH/DoT server.

7.  Hosting DoH/DoT Forwarder in the CPE








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7.1.  Managed CPEs

   The following mechanisms can be used to host a DoH/DoT 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 DoH/DoT 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.

   If the CPE announces itself as a DNS forwarder, the F-bit MUST NOT be
   set.

7.1.2.  Redirection

7.1.2.1.  Server-Driven Redirection

   The mechanism specified in this section is specific to DoH.

   An ISP-managed CPE can be configured with the ISP's DoH resolver IP
   addresses and ADN, which it will communicate to internal hosts using
   DHCP/RA (Section 4).  Upon joining the network, a DoH client follows
   the procedure specified in Section 6.1 to upgrade to DoH.

   Once the DoH session is established, the ISP DoH server uses HTTP
   redirection (Section 6.4.4 in [RFC7231] and [RFC7538]) to redirect
   the DNS client to the DoH forwarder hosted on the CPE (e.g.,
   cpe1-internal.example.net).  The DNS client either uses Do53 to
   resolve the domain name in the redirected URI and eventually
   establishes DoH session with the DoH forwarder in the CPE reachable
   on the LAN interface.  A simplified example is illustrated in
   Figure 13.

   PKIX authentication [RFC6125] based upon the domain name in the
   redirected URI will detect rogue DNS servers.






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   A DNS client that successfully connects to a redirected DoH server
   may choose to locally cache the server host IP addresses in order to
   not have to repeat the Do53 query.

                                                   --,--,-
                         ,+-,--,--.             ,/  ISP   \.
                      ,-'          `-.       ,-'            `-.
            DoH/DoT --(      LAN      CPE----( S (@1)          )
       capable client  `-.          ,-'|      `-.           ,-'
                |        `--'--'--'    |       | `--'--'--'
                |<========DHCP========>|       |
                |       {RI, @1}       |       |
                |                              |
                |<============DoH=============>|
                |<--308 Permanent Redirect-----|
                |                              |
                |--------Do53 Query----------->|
                |<--------CPE's LAN @----------|
                |                      |
                |<========DoH=========>|
                |                      |
   Legend:
     * S: DoH/DoT server
     * @1: IP address of S

       Figure 13: A Simplified Example of Server-Driven Redirection

      Notes:

      *  As an optimization, the redirect response can be modified to
         include a list of IPv4/IPv6 addresses in addition to the CPE's
         name.

         [RFC7838] allows an origin's resource to be authoritatively
         available at a separate network location, nevertheless it
         requires the alternative service to present a certificate for
         the origin's host name.  This is not acceptable for the case
         discussed in this section.

      *  Clients not caching the redirect information may experience an
         extra delay for each request.

7.1.2.2.  Client-Initiated Redirection

   This section assumes that the procedure discussed in Section 7.1.2.1
   is disabled.





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   An ISP-managed CPE can be configured with the ISP's DoH/DoT resolver
   IP addresses and ADN, which it will communicate to internal hosts
   using DHCP/RA (Section 4).  In addition, the CPE sets the F-bit to
   inform the client that a forwarder is available locally.

   Upon joining the LAN, a DoH/DoT client follows the procedure
   specified in Section 6.1 to upgrade to DoH/DoT.

   Once the DoH/DoT session is established, and given that F-bit is set,
   the DoH/DoT client uses SRV resource record lookup for discovering
   the DoH/DoT forwarder available in the local network to redirect the
   DNS client to the DoH/DoT forwarder hosted on the CPE (e.g.,
   cpe1-internal.example.net).  A simplified example is illustrated in
   Figure 14.

   _domain-s._tcp.example.net  SRV 0 0 853 cpe1-internal.example.net
   cpe1-internal.example.net  AAAA 2001:db8:8:4::2
   cpe1-internal.example.net  A 198.51.100.2

                      Figure 14: DoT Redirect Example

   The <Service> portion of the DNS service instance name MUST be
   "_domain-s._tcp" (Section 6 of [RFC7858]) or "_doh._tcp"
   (Section 9.4).

   The SRV record can only convey a sub-domain of the parent domain
   otherwise the SRV record MUST be rejected by the DoH/DoT client.

   A DNS client that successfully connects to a redirected DoH/DoT
   server may choose to locally cache the server IP addresses in order
   to avoid repeating the SRV lookup.




















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                                                   --,--,-
                         ,+-,--,--.             ,/  ISP   \.
                      ,-'          `-.       ,-'            `-.
            DoH/DoT --(      LAN      CPE----( S (@1)          )
       capable client  `-.          ,-'|      `-.           ,-'
                |        `--'--'--'    |       | `--'--'--'
                |<========DHCP========>|       |
                |   {ADN, F=1, @1}     |       |
                |                              |
                | /==========================\ |
                || DoH: SRV Lookup (ADN)      ||
                || cpe1-internal.example.net  ||
                || AAAA ==> 2001:db8:8:4::2   ||
                || A    ==> 198.51.100.2      ||
                | \==========================/ |
                |
                |
                |<========DoH=========>|
                |                      |
   Legend:
     * S: DoH/DoT server
     * @1: IP address of S
     * F: Forwarding bit

      Figure 15: A Simplified Example of Client-Initiated Redirection

7.1.3.  Auto-Upgrade based on Domains and their Sub-domains

   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 6.1) is allowed for both the pre-configured ADN and its sub-
   domains, the redirection mechanisms discussed in Section 7.1.2 can be
   avoided.  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 DoH/DoT client discovers a
   local forwarder "cpe1-internal.example.net", the DoH/DoT client will
   auto-upgrade because the pre-configured ADN would match
   subjectAltName value "cpe1-internal.example.net" of type dNSName.  As
   shown in Figure 16, 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, F=0, @rs}
                |              |                  --,--,-
                |        ,+-,--+--.             ,/  ISP   \.
                |     ,-'          `-.       ,-'            `-.
            DoH/DoT --(      LAN      CPE----( S (@1)          )
       capable client  `-.          ,-'|      `-.           ,-'
                |        `--'--'--'    |         `--'--'--'
                |<========DHCP========>|
                |{ADN=                 |
                |  cpe1-internal.example.net, F=0, @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 16: 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 a DoH/DoT forwarder on the unmanaged CPE.  This assumes the
   following:

   o  The DoH/DoT server certificate is managed by the entity in-charge
      of hosting the DoT/DoH forwarder.

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

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

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





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   Figure 17 illustrates an example of an unmanaged CPE hosting a
   forwarder which connects to a 3rd party DoH/DoT 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}      |                                   |
      |<=====DoT/DoH=====>|<=============DoT/DoH=============>|

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

         Figure 17: Example of an Internal CPE Hosting a Forwarder

8.  Security Considerations

   An attacker can get a domain name, domain-validated public
   certificate from a CA, host a DoT/DoH server and claim the best DNS
   privacy preservation policy.  Also, an attacker within the home
   network can use the public IP address, get an 'IP address'-validated
   public certificate from a CA, host a DoT/DoH server and claim the
   best DNS privacy preservation policy.

   Because DHCP/RA messages are not encrypted or protected against
   modification in any way, their content can be spoofed or modified by
   compromised devices within the home network.  An attacker can spoof
   the DHCP/RA response to provide the attacker's DoT/DoH server.  Note
   that such an attacker can launch other attacks as discussed in
   Section 22 of [RFC8415].  Furthermore, if the browser or the OS is
   pre-configured with a list of DNS servers and some of which perform
   malware filtering while others do not, an attacker can prevent
   contacting the preferred filtering DNS servers causing a downgrade
   attack to a non-filtering DNS server, which the attacker can leverage
   to deliver malware.

   The use of DoH/DoT also depends on the user's policies.  For example,
   the user may indicate his/her consent to use (or not) the locally-
   discovered DoH/DoT server or request to review human-readable privacy
   policy information of a selected DNS server and to assess whether
   that DNS server performs DNS-based content filtering (e.g.,



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   [I-D.reddy-dprive-dprive-privacy-policy]).  The DNS client is assumed
   to adhere to these policies.  This document does not make any
   assumption about the structure of such policies nor mandates specific
   requirements.  Such policies and their handling is out of scope.

   DoH/DoT servers discovered using insecure discovery mechanisms like
   DHCP/RA are used by a DNS client if the insecurely discovered DoH/DoT
   server is pre-configured in the OS or the browser.  Section 6.1
   identifies a set of deployment options under which DHCP/RA RI options
   can be used.

   If the insecurely discovered DoH/DoT server is not pre-configured in
   the OS or browser, the client may validate the signatory (e.g.,
   cryptographically attested by the ISP).  However, as discussed above,
   the use of policies to select servers is out of scope of this
   document.

   DoT/DoH 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] based upon
   the authentication domain name in the Reference Identifier 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).

   The resolution of redirected domain using Do53 (Section 7.1.2.1) is a
   minimal leakage (see Section 6.3 of [RFC8310]).  To protect against
   DNS-vectored attacks, DNSSEC can be used to ensure the validity of
   the received DNS records received.  Impersonation of a DoH/DoT
   forwarder is prevented by validating the certificate presented by the
   DoH/DoT forwarder.

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

9.  IANA Considerations

9.1.  DHCPv6 Option

   IANA is requested to assign the following new DHCPv6 Option Code in
   the registry maintained in: https://www.iana.org/assignments/dhcpv6-
   parameters/dhcpv6-parameters.xhtml#dhcpv6-parameters-2.








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   +-------+------------------+---------+-------------+----------------+
   | Value | Description      | Client  | Singleton   | Reference      |
   |       |                  | ORO     | Option      |                |
   +-------+------------------+---------+-------------+----------------+
   | TBA1  | OPTION_V6_DNS_RI | Yes     | Yes         | [ThisDocument] |
   +-------+------------------+---------+-------------+----------------+

9.2.  DHCP Option

   IANA is requested to assign the following new DHCP Option Code in the
   registry maintained in: https://www.iana.org/assignments/bootp-dhcp-
   parameters/bootp-dhcp-parameters.xhtml#options.

   +------+------------------+-------+----------------+----------------+
   | Tag  | Name             | Data  | Meaning        | Reference      |
   |      |                  | Length|                |                |
   +------+------------------+-------+----------------+----------------+
   | TBA2 | OPTION_V4_DNS_RI | N     | DoT/DoH server | [ThisDocument] |
   |      |                  |       | authentication |                |
   |      |                  |       | domain name    |                |
   +------+------------------+-------+----------------+----------------+

9.3.  RA Option

   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
   http://www.iana.org/assignments/icmpv6-parameters/
   icmpv6-parameters.xhtml#icmpv6-parameters-5.

        +------+---------------------------------+----------------+
        | Type | Description                     | Reference      |
        +------+---------------------------------+----------------+
        | TBA3 | DNS Reference Identifier Option | [ThisDocument] |
        +------+---------------------------------+----------------+

9.4.  Service Name

   IANA is requested to allocate the following service name from the
   registry available at: https://www.iana.org/assignments/service-
   names-port-numbers/service-names-port-numbers.xhtml.









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        Service Name:            doh
        Port Number:             N/A
        Transport Protocol(s):   TCP
        Description:             DNS-over-HTTPS
        Assignee:                IESG <iesg@ietf.org>
        Contact:                 IETF Chair <chair@ietf.org>
        Reference:               [ThisDocument]

9.5.  Encrypted DNS Types

   This document requests IANA to create a new registry called
   "Encrypted DNS Types".  The initial values of the registery is as
   follows:

               +-------+----------------------+-----------+
               | Value | Description          | Reference |
               +-------+----------------------+-----------+
               | 0     | Any                  | RFC XXXX  |
               | 1     | DNS-over-TLS (DoT)   | RFC XXXX  |
               | 2     | DNS-over-HTTPs (DoH) | RFC XXXX  |
               +-------+----------------------+-----------+

   New values are assigned on a First Come, First Served (FCFS) basis
   (Section 4.4 of [RFC8126]).

10.  Acknowledgements

   Many thanks to Christian Jacquenet for the review.

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

   Thanks to Mark Nottingham for the feedback on HTTP redirection.

11.  References

11.1.  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>.





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   [RFC3396]  Lemon, T. and S. Cheshire, "Encoding Long Options in the
              Dynamic Host Configuration Protocol (DHCPv4)", RFC 3396,
              DOI 10.17487/RFC3396, November 2002,
              <https://www.rfc-editor.org/info/rfc3396>.

   [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>.

   [RFC7538]  Reschke, J., "The Hypertext Transfer Protocol Status Code
              308 (Permanent Redirect)", RFC 7538, DOI 10.17487/RFC7538,
              April 2015, <https://www.rfc-editor.org/info/rfc7538>.

   [RFC7858]  Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "Specification for DNS over Transport
              Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
              2016, <https://www.rfc-editor.org/info/rfc7858>.

   [RFC8106]  Jeong, J., Park, S., Beloeil, L., and S. Madanapalli,
              "IPv6 Router Advertisement Options for DNS Configuration",
              RFC 8106, DOI 10.17487/RFC8106, March 2017,
              <https://www.rfc-editor.org/info/rfc8106>.

   [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>.

   [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>.

   [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>.




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11.2.  Informative References

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

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

   [I-D.ietf-dnsop-terminology-ter]
              Hoffman, P., "Terminology for DNS Transports and
              Location", draft-ietf-dnsop-terminology-ter-01 (work in
              progress), February 2020.

   [I-D.ietf-dnssd-push]
              Pusateri, T. and S. Cheshire, "DNS Push Notifications",
              draft-ietf-dnssd-push-25 (work in progress), October 2019.

   [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.reddy-dprive-bootstrap-dns-server]
              Reddy.K, T., Wing, D., Richardson, M., and M. Boucadair,
              "A Bootstrapping Procedure to Discover and Authenticate
              DNS-over-TLS and DNS-over-HTTPS Servers", draft-reddy-
              dprive-bootstrap-dns-server-08 (work in progress), March
              2020.

   [I-D.reddy-dprive-dprive-privacy-policy]
              Reddy.K, T., Wing, D., Richardson, M., and M. Boucadair,
              "DNS Server Privacy Statement and Filtering Policy with
              Assertion Token", draft-reddy-dprive-dprive-privacy-
              policy-03 (work in progress), March 2020.

   [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>.

   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
              DOI 10.17487/RFC7231, June 2014,
              <https://www.rfc-editor.org/info/rfc7231>.

   [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>.

   [RFC7838]  Nottingham, M., McManus, P., and J. Reschke, "HTTP
              Alternative Services", RFC 7838, DOI 10.17487/RFC7838,
              April 2016, <https://www.rfc-editor.org/info/rfc7838>.

   [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>.

   [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>.






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   [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


   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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