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ADD                                                         M. Boucadair
Internet-Draft                                                    Orange
Intended status: Standards Track                                T. Reddy
Expires: July 26, 2021                                            McAfee
                                                                 D. Wing
                                                                  Citrix
                                                                 N. Cook
                                                            Open-Xchange
                                                               T. Jensen
                                                               Microsoft
                                                        January 22, 2021


   DHCP and Router Advertisement Options for Encrypted DNS Discovery
                         draft-btw-add-home-12

Abstract

   The document specifies new DHCP and IPv6 Router Advertisement options
   to discover encrypted DNS servers (e.g., DNS-over-HTTPS, DNS-over-
   TLS, DNS-over-QUIC).  Particularly, it allows to learn an
   authentication domain name together with a list of IP addresses and a
   port number to reach such encrypted DNS servers.  The discovery of
   DNS-over-HTTPS URI Templates is also discussed.

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 July 26, 2021.

Copyright Notice

   Copyright (c) 2021 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 . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Overview and Rationale  . . . . . . . . . . . . . . . . . . .   4
   4.  DHCPv6 Encrypted DNS Options  . . . . . . . . . . . . . . . .   6
     4.1.  Encrypted DNS ADN Option  . . . . . . . . . . . . . . . .   6
     4.2.  Encrypted DNS Address Option  . . . . . . . . . . . . . .   7
     4.3.  DHCPv6 Client Behavior  . . . . . . . . . . . . . . . . .   9
   5.  DHCPv4 Encrypted DNS Option . . . . . . . . . . . . . . . . .   9
     5.1.  Encrypted DNS Option  . . . . . . . . . . . . . . . . . .   9
     5.2.  DHCPv4 Client Behavior  . . . . . . . . . . . . . . . . .  11
   6.  IPv6 RA Encrypted DNS Options . . . . . . . . . . . . . . . .  11
     6.1.  Encrypted DNS ADN Option  . . . . . . . . . . . . . . . .  12
     6.2.  Encrypted DNS Address Option  . . . . . . . . . . . . . .  13
   7.  DoH URI Templates . . . . . . . . . . . . . . . . . . . . . .  14
   8.  Hosting Encrypted DNS Forwarder in Local Networks . . . . . .  16
     8.1.  Managed CPEs  . . . . . . . . . . . . . . . . . . . . . .  16
       8.1.1.  DNS Forwarders  . . . . . . . . . . . . . . . . . . .  16
       8.1.2.  ACME  . . . . . . . . . . . . . . . . . . . . . . . .  16
       8.1.3.  Auto-Upgrade Based on Domains and their Subdomains  .  16
     8.2.  Unmanaged CPEs  . . . . . . . . . . . . . . . . . . . . .  17
   9.  Legacy CPEs . . . . . . . . . . . . . . . . . . . . . . . . .  18
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  18
     10.1.  Spoofing Attacks . . . . . . . . . . . . . . . . . . . .  18
     10.2.  Deletion Attacks . . . . . . . . . . . . . . . . . . . .  19
     10.3.  Passive Attacks  . . . . . . . . . . . . . . . . . . . .  19
     10.4.  Wireless Security - Authentication Attacks . . . . . . .  20
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
     11.1.  Encrypted DNS Flag Bits  . . . . . . . . . . . . . . . .  20
     11.2.  DHCPv6 Options . . . . . . . . . . . . . . . . . . . . .  21
     11.3.  DHCPv4 Option  . . . . . . . . . . . . . . . . . . . . .  21
     11.4.  Neighbor Discovery Options . . . . . . . . . . . . . . .  21
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  22
   13. Contributing Authors  . . . . . . . . . . . . . . . . . . . .  22
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  22
     14.2.  Informative References . . . . . . . . . . . . . . . . .  23



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   Appendix A.  Sample Target Deployment Scenarios . . . . . . . . .  26
     A.1.  Managed CPEs  . . . . . . . . . . . . . . . . . . . . . .  27
       A.1.1.  Direct DNS  . . . . . . . . . . . . . . . . . . . . .  28
       A.1.2.  Proxied DNS . . . . . . . . . . . . . . . . . . . . .  29
     A.2.  Unmanaged CPEs  . . . . . . . . . . . . . . . . . . . . .  30
       A.2.1.  ISP-facing Unmanaged CPEs . . . . . . . . . . . . . .  30
       A.2.2.  Internal Unmanaged CPEs . . . . . . . . . . . . . . .  30
   Appendix B.  Make Use of Discovered Encrypted DNS Servers . . . .  31
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  32

1.  Introduction

   This document focuses on the support of encrypted DNS such as DNS-
   over-HTTPS (DoH) [RFC8484], DNS-over-TLS (DoT) [RFC7858], or DNS-
   over-QUIC (DoQ) [I-D.ietf-dprive-dnsoquic] in local networks.

   In particular, the document specifies how a local encrypted DNS
   server can be discovered and used by connected hosts by means of DHCP
   [RFC2132], DHCPv6 [RFC8415], and IPv6 Router Advertisement (RA)
   [RFC4861] options.  These options are designed to convey the
   following information: the DNS Authentication Domain Name (ADN), a
   list of IP addresses, and optionally a port number.  The discovery of
   DoH URI Templates is discussed in Section 7.

   Sample target deployment scenarios are discussed in Appendix A; both
   managed and unmanaged Customer Premises Equipment (CPEs) are covered.
   It is out of the scope of this document to provide an exhaustive
   inventory of deployments where Encrypted DNS Options (Sections 4, 5,
   and 6) can be used.

   Considerations related to hosting a DNS forwarder in a local network
   are described in Section 8.

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].  The
   following additional terms are used:

   Do53:  refers to unencrypted DNS.

   Encrypted DNS:  refers to a scheme where DNS exchanges are
      transported over an encrypted channel.  Examples of encrypted DNS



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      are DNS-over-TLS (DoT) [RFC7858], DNS-over-HTTPS (DoH) [RFC8484],
      or DNS-over-QUIC (DoQ) [I-D.ietf-dprive-dnsoquic].

   Managed CPE:  refers to a CPE that is managed by an Internet Service
      Providers (ISP).

   Unmanaged CPE:  refers to a CPE that is not managed by an ISP.

   DHCP:  refers to both DHCPv4 and DHCPv6.

3.  Overview and Rationale

   This document describes how a DNS client can discover a local
   encrypted DNS server(s) using DHCP (Sections 4 and 5) and Neighbor
   Discovery protocol (Section 6).

   As reported in Section 1.7.2 of [RFC6125]:

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

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

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

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





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   A list of IP addresses to reach an encrypted DNS server can be
   returned in the option to accommodate current deployments relying
   upon primary and backup servers.  Whether one IP address or more are
   returned in an option is deployment specific.  For example, a router
   embedding a recursive server or forwarder has to include one single
   IP address pointing to one of its LAN-facing interfaces.  This
   address can be a private IPv4 address, a link-local address, a Unique
   Local IPv6 unicast Address (ULA), or a Global Unicast Address (GUA).

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

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

   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 dedicated management tools.  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.

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

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

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

   DHCP/RA options to discover encrypted DNS servers (including, DoH URI
   Templates should the WG pursue that approach pending feedback) takes
   precedence over DEER [I-D.pauly-add-deer] since DEER uses unencrypted
   DNS to an external DNS resolver, which is susceptible to both
   internal and external attacks whereas DHCP/RA is only vulnerable to
   internal attacks.




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4.  DHCPv6 Encrypted DNS Options

   This section defines two DHCPv6 options: DHCPv6 Encrypted DNS ADN
   option (Section 4.1) and DHCPv6 Encrypted DNS Address option
   (Section 4.2).

4.1.  Encrypted DNS ADN Option

   The DHCPv6 Encrypted DNS ADN option is used to configure an
   authentication domain name of the encrypted DNS server.  The format
   of this option is shown in Figure 1.

      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    OPTION_V6_ENC_ADN          |         Option-length         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Enc DNS Flags |                                               |
     +---------------+                                               +
     |                                                               |
     ~                   authentication-domain-name                  ~
     |                                                               |
     +---------------------------------------------------------------+

                 Figure 1: DHCPv6 Encrypted DNS ADN Option

   The fields of the option shown in Figure 1 are as follows:

   Option-code:  OPTION_V6_ENC_ADN (TBA1, see Section 11.2)

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

   Enc DNS Flags (Encrypted DNS Flags):  Indicates the type(s) of the
      encrypted DNS server conveyed in this attribute.  The format of
      this 8-bit field is shown in Figure 2.

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

                    Figure 2: Encrypted DNS Flags Field

      T: If set, this bit indicates that the server supports DoT
         [RFC7858].

      H: If set, this bit indicates that the server supports DoH
         [RFC8484].





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      Q: If set, this bit indicates that the server supports DoQ
         [I-D.ietf-dprive-dnsoquic].

      U: Unassigned bits.  These bits MUST be unset by the sender.
         Associating a meaning with an unassigned bit can be done as per
         Section 11.1.

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

      To keep the packet small, if more than one encrypted DNS type
      (e.g., both DoH and DoT) are to be returned to a requesting client
      and the same ADN is used for these types, the corresponding bits
      must be set in the 'Encrypted DNS Types' field of the same option
      instance in a response.  For example, if the client requested DoH
      and DoT and the server supports both with the same ADN, then both
      T and H bits must be set.

   authentication-domain-name:  A fully qualified domain name of the
      encrypted DNS server.  This field is formatted as specified in
      Section 10 of [RFC8415].

      An example of the authentication-domain-name encoding is shown in
      Figure 3.  This example conveys the FQDN "doh1.example.com.", and
      the resulting Option-length field is 18.

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

    Figure 3: An Example of the DNS authentication-domain-name Encoding

4.2.  Encrypted DNS Address Option

   The DHCPv6 Encrypted DNS Address option is used to configure a list
   of IP addresses and a port number of the encrypted DNS server.  The
   format of this option is shown in Figure 4.










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      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    OPTION_V6_ENC_ADD          |         Option-length         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Enc DNS Flags |   Unassigned  |          Port Number          |
     +---------------+---------------+-------------------------------+
     |                                                               |
     |                         ipv6-address                          |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                         ipv6-address                          |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 4: DHCPv6 Encrypted DNS Address Option

   The fields of the option shown in Figure 4 are as follows:

   Option-code:  OPTION_V6_ENC_ADD (TBA2, see Section 11.2)

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

   Enc DNS Flags (Encrypted DNS Flags):  Indicates the type(s) of the
      encrypted DNS server conveyed in this attribute.  The format of
      this 8-bit field is shown in Figure 2.  In a request, these bits
      are set to indicate the requested encrypted DNS server type(s) by
      the client.  In a response, these bits are set as a function of
      the encrypted DNS supported by the server and the requested
      encrypted DNS server type(s).

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

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

   ipv6-address(es):  Indicates one or more IPv6 addresses to reach the
      encrypted DNS server.  An address can be link-local, ULA, or GUA.





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

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

4.3.  DHCPv6 Client Behavior

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

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

   The DHCPv6 client MUST silently discard multicast and host loopback
   addresses conveyed in OPTION_V6_ENC_ADD.

5.  DHCPv4 Encrypted DNS Option

5.1.  Encrypted DNS Option

   The DHCPv4 Encrypted DNS option is used to configure an
   authentication domain name, a list of IP addresses, and a port number
   of the encrypted DNS server.  The format of this option is
   illustrated in Figure 5.


















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                      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     |     TBA3      |     Length    |
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     | Enc DNS Flags | Num Addresses |
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     |           Port Number         |
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     |                               |
                     ~         IPv4 Address(es)      ~
                     |                               |
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     |                               |
                     ~  authentication-domain-name   ~
                     |                               |
                     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 5: DHCPv4 Encrypted DNS Option

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

   Code:  OPTION_V4_ENC_DNS (TBA3, see Section 11.3).

   Length:  Length of the enclosed data in octets.

   Enc DNS Flags (Encrypted DNS Flags):  Indicates the type(s) of the
      encrypted DNS server conveyed in this attribute.  The format of
      this field is shown in Figure 2.

   Num Addresses:  Indicates the number of included IPv4 addresses.

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

   IPv4 Address(es):  Indicates one or more IPv4 addresses to reach the
      encrypted DNS server.  Both private and public IPv4 addresses can
      be included in this field.  The format of this field is shown in
      Figure 6.  This format assumes that an IPv4 address is encoded as
      a1.a2.a3.a4.










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

               Figure 6: Format of the IPv4 Addresses Field

   authentication-domain-name:  The domain name of the encrypted DNS
      server.  This field is formatted as specified in Section 10 of
      [RFC8415].  The format of this field is shown in Figure 7.  The
      values s1, s2, s3, etc. represent the domain name labels in the
      domain name encoding.

                   +-----+-----+-----+-----+-----+--
                   |  s1 |  s2 |  s3 |  s4 | s5  |  ...
                   +-----+-----+-----+-----+-----+--
                     authentication-domain-name

         Figure 7: Format of the Authentication Domain Name Field

   OPTION_V4_ENC_DNS is a concatenation-requiring option.  As such, the
   mechanism specified in [RFC3396] MUST be used if OPTION_V4_ENC_DNS
   exceeds the maximum DHCPv4 option size of 255 octets.

5.2.  DHCPv4 Client Behavior

   To discover an encrypted DNS server, the DHCPv4 client requests the
   Encrypted DNS server by including OPTION_V4_ENC_DNS in a Parameter
   Request List option [RFC2132].  The DHCPv4 client sets the Encrypted
   DNS Types field to the requested encrypted DNS server.

   If the DHCPv4 client requested more than one encrypted DNS server
   type, the DHCPv4 client MUST be prepared to receive multiple DHCP
   OPTION_V4_ENC_DNS options; each option is to be treated as a separate
   encrypted DNS server.

   The DHCPv4 client MUST silently discard multicast and host loopback
   addresses conveyed in OPTION_V4_ENC_DNS.

6.  IPv6 RA Encrypted DNS Options

   This section defines two Neighbor Discovery [RFC4861]: IPv6 Router
   Advertisement (RA) Encrypted DNS ADN option (Section 6.1) and IPv6 RA
   Encrypted DNS Address option (Section 6.2).  These options are useful
   in contexts similar to those discussed in Section 1.1 of [RFC8106].





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6.1.  Encrypted DNS ADN Option

   The IPv6 RA Encrypted DNS ADN option is used to configure an
   authentication domain name of the encrypted DNS server.  The format
   of this option is illustrated in Figure 8.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     TBA4      |     Length    | Enc DNS Flags |   Unassigned  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Lifetime                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                   authentication-domain-name                  ~
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 8: RA Encrypted DNS ADN Option

   The fields of the option shown in Figure 8 are as follows:

   Type:  8-bit identifier of the Encrypted DNS Option as assigned by
      IANA (TBA4, see Section 11.4).

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

   Enc DNS Flags (Encrypted DNS Flags):  Indicates the type(s) of the
      encrypted DNS server conveyed in this attribute.  The format of
      this field is shown in Figure 2.

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

   Lifetime:  32-bit unsigned integer.  The maximum time in seconds
      (relative to the time the packet is received) over which the
      discovered Authentication Domain Name is valid.

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

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

      A value of zero means that this Authentication Domain Name MUST no
      longer be used.




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   authentication Domain Name:  The domain name of the encrypted DNS
      server.  This field is formatted as specified in Section 10 of
      [RFC8415].

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

6.2.  Encrypted DNS Address Option

   The IPv6 RA Encrypted DNS Address option is used to configure a port
   number and a list of IPv6 addresses of the encrypted DNS server.  The
   format of this option is illustrated in Figure 9.  All of the
   addresses share the same Lifetime value.  Similar to [RFC8106], if it
   is desirable to have different Lifetime values per IP address,
   multiple Encrypted DNS Address options may be used.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     TBA5      |     Length    |         Unassigned            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                           Lifetime                            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Enc DNS Flags |  Unassigned   |           Port Number         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                         ipv6-address                          |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     |                         ipv6-address                          |
     |                                                               |
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                              ...                              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                 Figure 9: RA Encrypted DNS Address Option

   The fields of the RA Encrypted DNS Address option shown in Figure 9
   are as follows:

   Type:  8-bit identifier of the Encrypted DNS Address Option as
      assigned by IANA (TBA5, see Section 11.4).





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

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

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

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

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

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

   Enc DNS Flags (Encrypted DNS Flags):  Indicates the type(s) of the
      encrypted DNS server conveyed in this attribute.  The format of
      this field is shown in Figure 2.

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

   ipv6-address(es):  One or more IPv6 addresses of the encrypted DNS
      server.  An address can be link-local, ULA, or GUA.

7.  DoH URI Templates

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

   Upon discovery of a DoH resolver (Sections 4, 5, and 6), the DoH
   client may contact that DoH resolver to retrieve the list of
   supported DoH services using DEER [I-D.pauly-add-deer].  This will
   allow the client to discover the resolver's supported DoH templates
   or DoH resolvers that the discovered resolver designates using DNS
   SVCB queries [I-D.schwartz-svcb-dns].  The designated DoH resolvers
   and DoH resolver discovered using DHCP/RA may be hosted on the same
   or distinct IP addresses.



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

   o  ADN: doh.example.com

   o  Locator: 2001:db8:1::1

   The client will use DEER [I-D.pauly-add-deer] to discover the DoH
   templates supported by the DNS server at the Locator (2001:db8:1::1).
   In addition to the checks included in DEER, clients should verify the
   ADN (doh.example.com) is valid for the certificate provided by the
   DoH resolver.  However, the IP address of the DEER-discovered
   resolver may differ from the Locator field value.  This will allow
   the ISP to offer different DoH services to the endpoints attached to
   local networks.

   Alternatively, dedicated DHCP/RA options may be defined to convey an
   URI template in order to avoid additional network traffic to
   bootstrap DoH configuration.  An example of the format of such an
   option is depicted 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   OPTION_V6_DOH_TEMPLATE      |         Option-length         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     ~                         uri-template-data                     ~
     |                             . . .                             |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

            Figure 10: Example of a DHCPv6 URI Template Option

      Note: More feedback from the WG is needed to decide which
      approach(es) to follow.

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






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8.  Hosting Encrypted DNS Forwarder in Local Networks

   This section discusses some deployment considerations (not
   recommendations) to host an encrypted DNS forwarder within a local
   network.

8.1.  Managed CPEs

   The section discusses mechanisms that can be used to host an
   encrypted DNS forwarder in a managed CPE (Appendix A.1).

8.1.1.  DNS Forwarders

   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.

8.1.2.  ACME

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

8.1.3.  Auto-Upgrade Based on Domains and their Subdomains

   If the ADN conveyed in DHCP/RA (Sections 4, 5, and 6) is
   preconfigured in popular OSes or browsers as a verified resolver and
   the auto-upgrade (Appendix B) is allowed for both the preconfigured
   ADN and its sub-domains, the encrypted DNS client will learn the
   local encrypted DNS forwarder using DHCP/RA and auto-upgrade because
   the preconfigured ADN would match the subjectAltName value in the
   server certificate.  For example, if the preconfigured ADN is
   "*.example.com" and the discovered encrypted DNS forwarder is
   "cpe1.example.com", auto-upgrade will take place.

   In this case, the CPE can communicate the ADN of the local DoH
   forwarder (Section 8.1.2) to internal hosts using DHCP/RA (Sections
   4, 5, and 6).

   Let's suppose that "*.example.net" is preconfigured as a verified
   resolved in the browser or OS.  If the encrypted DNS client discovers
   a local forwarder "cpe1-internal.example.net", the encrypted DNS
   client will auto-upgrade because the preconfigured ADN would match



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   subjectAltName value "cpe1-internal.example.net" of type dNSName.  As
   shown in Figure 11, the auto-upgrade to a rogue server advertising
   "rs.example.org" will fail because it does not match "*.example.net".

              Encrypted DNS                              CPE
              capable client                             (@i)
                    |                                     |
                    |<=================DHCP===============|
                    | {ADN=cpe1-internal.example.net, @i} |
                    |                                     |
                    |                   Rogue Server      |
                    |                       (@rs)         |
                    |                         |           |
                    X<===========DHCP=========|           |
                    |{ADN=rs.example.org, @rs}|           |
                    |                         |           |
                    |                                     |
                    |<=================DoH===============>|
                    |                                     |

              Legend:
                * @i: internal IP address of the CPE
                * @rs: IP address of a rogue server

    Figure 11: A Simplified Example of Auto-upgrade based on Subdomains

8.2.  Unmanaged CPEs

   The approach specified in Section 8.1 does not apply for hosting a
   DNS forwarder in an unmanaged CPE.

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

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

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

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

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



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   Figure 12 illustrates an example of an unmanaged CPE hosting a
   forwarder which connects to a 3rd party encrypted DNS server.  In
   this example, the DNS information received from the managed CPE (and
   therefore from the ISP) is ignored by the Internal CPE hosting the
   forwarder.

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

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

         Figure 12: Example of an Internal CPE Hosting a Forwarder

9.  Legacy CPEs

   Hosts serviced by legacy CPEs that can't be upgraded to support the
   options defined in Sections 4, 5, and 6 won't be able to learn the
   encrypted DNS server hosted by the ISP, in particular.  If the ADN is
   not discovered using DHCP/RA, such hosts will have to fallback to use
   DEER as defined in [I-D.pauly-add-deer] to discover the encrypted DNS
   server and to retrieve the list of supported DoH services using the
   SVCB RRtype [I-D.schwartz-svcb-dns] without verifying the hostname of
   discovered templates with the ADN.  Other guidance in DEER relating
   to resolver verification must be followed in this case.  This will
   prevent an unencrypted resolver on a local address from referring to
   an encrypted resolver at a different address without an out-of-band
   configuration in the client beyond the scope of this document or
   DEER.

10.  Security Considerations

10.1.  Spoofing Attacks

   DHCP/RA messages are not encrypted or protected against modification
   within the LAN.  Unless mitigated (described below), the content of
   DHCP and RA messages can be spoofed or modified by active attackers,
   such as compromised devices within the local network.  An active
   attacker (Section 3.3 of [RFC3552]) can spoof the DHCP/RA response to
   provide the attacker's Encrypted DNS server.  Note that such an



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   attacker can launch other attacks as discussed in Section 22 of
   [RFC8415].  The attacker can get a domain name with a domain-
   validated public certificate from a CA and host an Encrypted DNS
   server.  Also, an attacker can use a public IP address and get an 'IP
   address'-validated public certificate from a CA to host an Encrypted
   DNS server.

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

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

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

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

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

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

10.2.  Deletion Attacks

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

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

10.3.  Passive Attacks

   A passive attacker (Section 3.2 of [RFC3552]) can identify a host is
   using DHCP/RA to discover an encrypted DNS server and can infer that




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   host is capable of using DoH/DoT/DoQ to encrypt DNS messages.
   However, a passive attacker cannot spoof or modify DHCP/RA messages.

10.4.  Wireless Security - Authentication Attacks

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

   If the pre-shared-key is the same for all clients that connect to the
   same WLAN, the shared key will be available to all nodes, including
   attackers, so it is possible to mount an active on-path attack.  Man-
   in-the-middle attacks are possible within local networks because such
   WLAN authentication lacks peer entity authentication.

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

11.  IANA Considerations

11.1.  Encrypted DNS Flag Bits

                                             1 2 3 4 5 6 7 8
                                            +-+-+-+-+-+-+-+-+
   Encrypted DNS Types is a set of 8 flags: |U|U|U|U|U|Q|H|T|
                                            +-+-+-+-+-+-+-+-+

   where flag bits in positions 1-5 are for future assignment as
   additional flag bits.

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






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      +--------------+-------+---------------------+----------------+
      | Bit Position | Label | Description         |   Reference    |
      +--------------+-------+---------------------+----------------+
      |      1       |   U   | Unassigned          |                |
      |      2       |   U   | Unassigned          |                |
      |      3       |   U   | Unassigned          |                |
      |      4       |   U   | Unassigned          |                |
      |      5       |   U   | Unassigned          |                |
      |      6       |   Q   | DNS-over-QUIC (DoQ) | [ThisDocument] |
      |      7       |   H   | DNS-over-HTTP (DoH) | [ThisDocument] |
      |      8       |   T   | DNS-over-TLS (DoT)  | [ThisDocument] |
      +--------------+-------+---------------------+----------------+

   New flag bits are assigned via Standards Action [RFC8126].

11.2.  DHCPv6 Options

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

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

11.3.  DHCPv4 Option

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

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

11.4.  Neighbor Discovery Options

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




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

12.  Acknowledgements

   Many thanks to Christian Jacquenet and Michael Richardson for the
   review.

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

   Thanks to Mark Nottingham for the feedback on HTTP redirection.

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

13.  Contributing Authors

      Nicolai Leymann
      Deutsche Telekom
      Germany

      Email: n.leymann@telekom.de

14.  References

14.1.  Normative References

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

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

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





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

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

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

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

   [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>.

14.2.  Informative References

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

   [BOOTP]    "BOOTP Vendor Extensions and DHCP Options",
              <https://www.iana.org/assignments/bootp-dhcp-parameters/
              bootp-dhcp-parameters.xhtml#options>.

   [DHCPV6]   "DHCPv6 Option Codes", <https://www.iana.org/assignments/
              dhcpv6-parameters/dhcpv6-parameters.xhtml#dhcpv6-
              parameters-2>.

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

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



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

   [I-D.ietf-dprive-dnsoquic]
              Huitema, C., Mankin, A., and S. Dickinson, "Specification
              of DNS over Dedicated QUIC Connections", draft-ietf-
              dprive-dnsoquic-01 (work in progress), October 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.pauly-add-deer]
              Pauly, T., Kinnear, E., Wood, C., McManus, P., and T.
              Jensen, "Discovery of Equivalent Encrypted Resolvers",
              draft-pauly-add-deer-00 (work in progress), November 2020.

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

   [I-D.schwartz-svcb-dns]
              Schwartz, B., "Service Binding Mapping for DNS Servers",
              draft-schwartz-svcb-dns-01 (work in progress), August
              2020.

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

   [ND]       "IPv6 Neighbor Discovery Option Formats",
              <http://www.iana.org/assignments/icmpv6-parameters/
              icmpv6-parameters.xhtml#icmpv6-parameters-5>.

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

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





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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>.

   [RFC6731]  Savolainen, T., Kato, J., and T. Lemon, "Improved
              Recursive DNS Server Selection for Multi-Interfaced
              Nodes", RFC 6731, DOI 10.17487/RFC6731, December 2012,
              <https://www.rfc-editor.org/info/rfc6731>.

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

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

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

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

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





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   [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>.

   [RFC8484]  Hoffman, P. and P. McManus, "DNS Queries over HTTPS
              (DoH)", RFC 8484, DOI 10.17487/RFC8484, October 2018,
              <https://www.rfc-editor.org/info/rfc8484>.

   [RFC8499]  Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
              Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
              January 2019, <https://www.rfc-editor.org/info/rfc8499>.

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

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

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

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

Appendix A.  Sample Target Deployment Scenarios

   Internet Service Providers (ISPs) traditionally provide DNS resolvers
   to their customers.  To that aim, 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  DHCPv4 [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 (possibly via Customer
   Premises Equipment (CPE)) and an ISP-supplied DNS resolver takes
   place by using cleartext DNS messages (Do53).  Some examples are
   depicted in Figure 13.  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
                                              ,--,--,--.
                 +-+      LAN     +---+    ,-'           `-.
                 |H+--------------+CPE+---+      ISP        )
                 +-+              +---+    `-.          ,-'
                  |                           `--'--'--'
                  |                               |
                  |<=============Do53============>|
                  |                               |

             (b) Cellular Networks

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

             Legend:
              * H: refers to a host.

                   Figure 13: Sample Legacy Deployments

A.1.  Managed CPEs

   This section focuses on CPEs that are managed by ISPs.






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A.1.1.  Direct DNS

   ISPs have developed an expertise in managing service-specific
   configuration information (e.g., CPE WAN Management Protocol
   [TR-069]).  For example, these tools may be used to provision the DNS
   server's ADN to managed CPEs if an encrypted DNS is supported by a
   local network similar to what is depicted in Figure 14.

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

   The DNS client discovers whether the DNS server in the local network
   supports DoH/DoT/DoQ by using a dedicated field in the discovery
   message: Encrypted DNS Types (Sections 4, 5, and 6) .

             (a) Fixed Networks

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

             (b) Cellular Networks

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

                    Figure 14: Encrypted DNS in the WAN

   Figure 14 shows the scenario where the CPE relays the list of
   encrypted DNS servers it learns for the network by using mechanisms



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   like DHCP or a specific Router Advertisement message.  In such
   context, direct encrypted DNS sessions will be established between a
   host serviced by a CPE and an ISP-supplied encrypted DNS server (see
   the example depicted in Figure 15 for a DoH/DoT-capable host).

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

                 Figure 15: Direct Encrypted DNS Sessions

A.1.2.  Proxied DNS

   Figure 16 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., Section 5.4.1 of
   [I-D.ietf-v6ops-rfc7084-bis]) or for supporting advanced services
   within a local network (e.g., malware filtering, parental control,
   Manufacturer Usage Description (MUD) [RFC8520] to only allow intended
   communications to and from an IoT device).  When the CPE behaves as a
   DNS forwarder, DNS communications can be decomposed into two legs:

   o  The leg between an internal host and the CPE.

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

   An ISP that offers encrypted DNS to its customers may enable
   encrypted DNS in one or both legs as shown in Figure 16.  Additional
   considerations related to this deployment are discussed in Section 8.














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

   (b)
                         ,--,--,--.             ,--,--,--.
             Legacy   ,-'          `-.       ,-'   ISP    `-.
              Host---(      LAN      CPE----(    DNS Server  )
                |     `-.          ,-'|      `-.          ,-'
                |        `--'--'--'   |         `--'--'--'
                |                     |             |
                |<=======Do53========>|<=Encrypted=>|
                |                     |     DNS     |

                 Figure 16: Proxied Encrypted DNS Sessions

A.2.  Unmanaged CPEs

A.2.1.  ISP-facing 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 Appendix A.1.  A host can thus establish DoH/DoT
   session with a DoH/DoT server similar to what is depicted in
   Figure 15 or Figure 16.

A.2.2.  Internal Unmanaged CPEs

   Customers may also decide to deploy internal routers (called
   hereafter, Internal CPEs) for a variety of reasons that are not
   detailed here.  Absent any explicit configuration on the internal CPE
   to override the DNS configuration it receives from the ISP-supplied
   CPE, an Internal CPE relays the DNS information it receives via DHCP/
   RA from the ISP-supplied CPE to connected hosts.  Encrypted DNS
   sessions can be established by a host with the DNS servers of the ISP
   (see Figure 17).





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

    Figure 17: Direct Encrypted DNS Sessions with the ISP DNS Resolver
                              (Internal CPE)

   Similar to managed CPEs, a user may modify the default DNS
   configuration of an unmanaged CPE to use his/her favorite DNS servers
   instead.  Encrypted DNS sessions can be established directly between
   a host and a 3rd Party DNS server (see Figure 18).

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

      Figure 18: Direct Encrypted DNS Sessions with a Third Party DNS
                                 Resolver

   Section 8.2 discusses considerations related to hosting a forwarder
   in the Internal CPE.

Appendix B.  Make Use of Discovered Encrypted DNS Servers

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

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

   o  Similarly, if the ADN conveyed in DHCP/RA (Sections 4, 5, and 6)
      is preconfigured in the OS or browser as a verified resolver, the



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      DNS client auto-upgrades to establish an encrypted a DoH/DoT/DoQ
      session with the ADN.

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

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


   Tommy Jensen
   Microsoft
   USA

   Email: tojens@microsoft.com





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