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ADD                                                         M. Boucadair
Internet-Draft                                                    Orange
Intended status: Standards Track                                T. Reddy
Expires: September 7, 2020                                        McAfee
                                                                 D. Wing
                                                                  Citrix
                                                                 N. Cook
                                                            Open-Xchange
                                                           March 6, 2020


    DNS-over-HTTPS and DNS-over-TLS server Discovery and Deployment
              Considerations for Home and Mobile Networks
                         draft-btw-add-home-01

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 September 7, 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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Sample Deployment Scenarios . . . . . . . . . . . . . . . . .   4
   4.  DNS Reference Identifier Option . . . . . . . . . . . . . . .   6
     4.1.  DHCPv6 DNS Reference Identifier Option  . . . . . . . . .   7
     4.2.  DHCP DNS Reference Identifier Option  . . . . . . . . . .   8
     4.3.  RA DNS Reference Identifier Option  . . . . . . . . . . .   8
   5.  Locating DoH/DoT Servers  . . . . . . . . . . . . . . . . . .   9
   6.  DNS-over-TLS and DNS-over-HTTPS Server Discovery Procedure  .  11
   7.  Hosting DoH/DoT Forwarder in the CPE  . . . . . . . . . . . .  12
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
     9.1.  DHCPv6 Option . . . . . . . . . . . . . . . . . . . . . .  14
     9.2.  DHCP Option . . . . . . . . . . . . . . . . . . . . . . .  14
     9.3.  RA Option . . . . . . . . . . . . . . . . . . . . . . . .  14
     9.4.  Service Name  . . . . . . . . . . . . . . . . . . . . . .  15
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  15
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  15
     11.2.  Informative References . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

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)).





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   The communication between a customer's device (UE) (possibly via
   Customer Premise 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, connectivity can be
   provided to a UE or to a CPE.  Do53 mechanisms used within the LAN
   are similar in both fixed and cellular CPE-based broadband service
   offerings.

           (a) Fixed Networks

                      ,--,--,--.             ,--,--,--.
                   ,-'   +--+  `-.       ,-'   ISP    `-.
                  ( LAN  |UE|    CPE----(    DNS Server  )
                   `-.   +--+   ,-'       `-.          ,-'
                      `--'|-'--'             `--'--'--'
                          |                     |
                          |<=======Do53========>|


           (b) Cellular Networks

                           |<===========Do53=========>|
                      ,--,-|,--.                      |
                   ,-'   +--+   `-.               ,--,--,--.
                  ( LAN  |UE|     CPE------------+          \
                   `-.   +--+   ,-'            ,'   ISP     `-.
                      `--'--'--'              (    DNS Server  )
                                         +-----+-.          ,-'
                          +--+           |        `--'--'--'
                          |UE+-----------+
                          +--+

                    Figure 1: Sample Legacy Deployments

   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



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      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
   DHCP/RA options that allows DNS clients to discover local DoT/DoH
   servers.  Section 4 describes DHCPv4, DHCPv6 and RA options to convey
   the authentication domain name information (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.  This
   document permits such deployments.  It is up to an ISP to decide
   which list of DNS resolvers to advertise to its serviced 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].

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

3.  Sample Deployment Scenarios

   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
   authentication domain name information (ADN, defined in [RFC8310]) to
   managed CPEs if DoH/DoT is supported by a local network similar to
   what is depicted in Figure 2.





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   DNS clients may try to establish DoH/DoT sessions with discovered DNS
   servers to determine whether these servers support DoH and/or DoT
   (Section 5).  Alternatively, a DNS client may discover whether the
   DNS server in the local network supports DoH/DoT by using the
   mechanism discussed in Section 6.

           (a) Fixed Networks

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


           (b) Cellular Networks

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

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

                     Figure 3: Direct DoH/DoT Sessions



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   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
   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.

   Also, an ISP that wants to offer DoH/DoT to its customers may enable
   DoH/DoT in both legs as shown in Figure 4.  Additional considerations
   related to this approach are discussed in Section 7.

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


                    Figure 4: Proxied DoH/DoT Sessions

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 RA
   (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



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   authentication domain name to be presented as a reference identifier
   for DNS authentication purposes.

   The DNS client establishes a DoH/DoT session with the discovered DNS
   IP address(es) (Section 5) 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 5.

       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         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      |                 authentication-domain-name                    |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


             Figure 5: DHCPv6 DNS Reference Identifier Option

   The fields of the option shown in Figure 5 are as follows:

   o  Option-code: OPTION_V6_DNS_RI (TBA1, see Section 9.1)
   o  Option-length: Length of the authentication-domain-name field in
      octets.
   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].

   An example of the authentication-domain-name encoding is shown in
   Figure 6.  This example conveys the FQDN "doh1.example.com.".









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

      Figure 6: An example of the authentication-domain-name Encoding

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 7.

            Code  Length   Authentication domain name
           +-----+-----+-----+-----+-----+-----+-----+--
           |TBA2 |  n  |  s1 |  s2 |  s3 |  s4 | s5  |  ...
           +-----+-----+-----+-----+-----+-----+-----+--

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


             Figure 7: DHCPv4 DNS Reference Identifier Option

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

   o  Code: OPTION_V4_DNS_RI (TBA2, see Section 9.2).
   o  Length: Includes the length of the "authentication domain name"
      field in octets.
   o  Authentication domain name: The domain name of the DoH/DoT server.
      This field is formatted as specified in Section 10 of [RFC8415].

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 8.













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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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Type      |     Length    |           Reserved            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Lifetime                            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       :                  authentication-domain-name                   :
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 8: RA DNS Reference Identifier Option

   The fields of the option shown in Figure 8 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  Reserved: 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
      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].

5.  Locating DoH/DoT Servers

   A CPE or a host relies upon discovery mechanisms (such as PCO, DHCP,
   or RA) to retrieve DoH and DoT servers' reachability information.  In
   the various scenarios sketched in Section 3, Do53, DoH, and DoT may
   terminate on the same IP address (or distinct IP addresses as
   depicted in Figure 9).  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




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   learn the IP addresses of DoT/DoH servers while an authentication
   domain name is supplied by one of the options discussed in Section 4.

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


               Figure 9: Locating DoH/DoT/Do53 using RFC8106

   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 trusted resolver, the DNS
      client may auto-upgrade to use the pre-configured DoH/DoT server
      tied to the discovered DNS server IP address.  In such a case the
      DNS client may perform additional check out of band, such as
      confirming that the Do53 IP address and the DoH server are owned
      and operated by the same organisation.

   o  If the DNS reference identity (Section 4) is provided by means of
      DHCP/RA, 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.

   Additional options are discussed below:

   o  The Wi-Fi Alliance has released the Device Provisioning Protocol
      (DPP).  If DPP is used, the configurator can securely configure
      devices in the home network with the local DoT/DoH server using
      DPP.





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   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 (ADN) 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.

6.  DNS-over-TLS and DNS-over-HTTPS Server Discovery Procedure

   A DNS client discovers the DNS server in the local network supporting
   DNS-over-TLS and DNS-over-HTTPS protocols by using DNS-based Service
   Discovery (DNS-SD) [RFC6763].  DNS-SD provides generic solution for
   discovering services available in a local network.  DNS-SD defines a
   set of naming rules for certain DNS record types that they use for
   advertising and discovering services.  Section 4.1 of [RFC6763]
   specifies that a service instance name in DNS-SD has the following
   structure:

   <Instance> . <Service> . <Domain>

   The <Domain> portion specifies the authentication domain name (ADN).
   The <Service> portion of the DNS service instance name MUST be
   "_domain-s._tcp" or "_doh._tcp".  If no DNS-SD records can be
   retrieved, the discovery procedure fails for this authentication
   domain name.  However, before retrying a lookup that has failed, a
   DNS client MUST wait a time period that is appropriate for the
   encountered error (e.g., NXDOMAIN, timeout, etc.).  If no DNS-SD
   records can be retrieved, the DNS client can try connecting to the
   pre-configured public DNS servers (if any).

   If DoH is supported by the DNS server, the DNS client may request the
   URI resource record type [RFC7553] using the domain name discovered
   using DNS Reference Identifier DHCP/RA option (Section 4) to use the
   HTTPS URI scheme (Section 3 of [RFC8484]).






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7.  Hosting DoH/DoT Forwarder in the CPE

   The following mechanisms can be used to host a DoH/DoT forwarder in
   the CPE:

   o  If a CPE is co-located with security services (e.g., malware
      filtering, parental control, MUD), 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 to
      automate certificate management functions such as domain
      validation procedure, certificate issuance and certificate
      revocation.

   o  Alternatively, the security service provider can assign a unique
      FQDN to the managed CPE.  The DoT/DoH forwarder will act like a
      public DoT/DoH server but will only be accessible from within the
      home network.  DNS queries received outside the home network must
      be discarded by the DoH/DoT forwarder.  This behavior adheres to
      REQ#8 in [RFC6092], and must apply for both IPv4 and IPv6.

   o  If the ISP DoH resolver is pre-configured as a trusted resolver in
      browsers, the CPE is managed by the ISP, and the ISP has assigned
      a domain-validated public certificate to the DoH forwarder hosted
      on the CPE, the ISP can configure the CPE to convey the ISP DoH/
      DoT resolver IP addresses and the ISP DoH/DoT ADN in DHCP/RA to
      internal hosts (Section 4).  If the ISP DNS server IP address is
      pre-configured in the browser as a trusted resolver, the DNS
      client auto-upgrades to use the DoH/DoT server tied with the
      discovered DNS server IP address.

      If the ADN in DHCP/RA is pre-configured in the OS or browser as a
      trusted resolver, the client auto-upgrades to establish DoH
      session with the ADN.

      Once the DoH session is established, the ISP DoH/DoT server uses
      HTTP redirection (Section 6.4.4 in [RFC7231]) to redirect the DNS
      client to the DoH forwarder hosted on the CPE.  The DNS client
      uses Do53 to resolve the domain name in the redirected URI and
      eventually establishes DoH session with the DoH forwarder on the
      CPE.

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



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   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 primary attacks against the methods described in Section 6 are
   the ones that would lead to impersonation of a DNS server and
   spoofing the DNS response to indicate that the DNS server does not
   support DoH or DoT.  To protect against DNS-vectored attacks, secured
   DNS (DNSSEC) can be used to ensure the validity of the received DNS
   records received.  Impersonation of a DoH/DoT server is prevented by
   validating the certificate presented by the DoH/DoT server.  If DHCP/
   RA conveys an ADN, but the DNS-SD lookup indicates that the DNS
   server does not support DoH/DoT, the DNS client can detect the DNS
   response is spoofed.

   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.  The DNS client must adhere to these
   policies.

   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.

   If the insecurely discovered DoH/DoT server is not pre-configured in
   the OS or browser, its policy information must be cryptographically
   attested by the ISP (e.g., [I-D.reddy-dprive-dprive-privacy-policy]);
   user consent is required to use the locally-discovered DoH/DoT
   server.

   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).



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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.

   +-------+------------------+---------+-------------+----------------+
   | 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] |
        +------+---------------------------------+----------------+







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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.

        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]

10.  Acknowledgements

   Many thanks to Christian Jacquenet for the review.

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>.

   [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>.

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
              <https://www.rfc-editor.org/info/rfc6763>.

   [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>.







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   [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>.

   [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>.

11.2.  Informative References

   [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-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-(D)TLS and DNS-over-HTTPS Servers", draft-reddy-
              dprive-bootstrap-dns-server-07 (work in progress),
              February 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.



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   [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>.

   [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>.

   [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>.

   [RFC7553]  Faltstrom, P. and O. Kolkman, "The Uniform Resource
              Identifier (URI) DNS Resource Record", RFC 7553,
              DOI 10.17487/RFC7553, June 2015,
              <https://www.rfc-editor.org/info/rfc7553>.

   [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", March
              2018, <https://www.broadband-forum.org/technical/download/
              TR-069.pdf>.






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   [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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