Discovery of Encrypted DNS Resolvers: Deployment Considerations
draft-boucadair-add-deployment-considerations-00

Document Type Active Internet-Draft (individual)
Authors Mohamed Boucadair  , Tirumaleswar Reddy.K  , Dan Wing  , Neil Cook  , Tommy Jensen 
Last updated 2021-05-17
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ADD                                                    M. Boucadair, Ed.
Internet-Draft                                                    Orange
Intended status: Informational                             T. Reddy, Ed.
Expires: November 18, 2021                                        McAfee
                                                                 D. Wing
                                                                  Citrix
                                                                 N. Cook
                                                            Open-Xchange
                                                               T. Jensen
                                                               Microsoft
                                                            May 17, 2021

    Discovery of Encrypted DNS Resolvers: Deployment Considerations
            draft-boucadair-add-deployment-considerations-00

Abstract

   The document discusses some deployment considerations of the various
   options to discover encrypted DNS servers (e.g., DNS-over-HTTPS, DNS-
   over-TLS, DNS-over-QUIC).

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 18, 2021.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents

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   carefully, as they describe your rights and restrictions with respect
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   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 . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Sample Target Deployment Scenarios  . . . . . . . . . . . . .   3
     3.1.  Managed CPEs  . . . . . . . . . . . . . . . . . . . . . .   5
       3.1.1.  Direct DNS  . . . . . . . . . . . . . . . . . . . . .   5
       3.1.2.  Proxied DNS . . . . . . . . . . . . . . . . . . . . .   6
     3.2.  Unmanaged CPEs  . . . . . . . . . . . . . . . . . . . . .   7
       3.2.1.  ISP-facing Unmanaged CPEs . . . . . . . . . . . . . .   7
       3.2.2.  Internal Unmanaged CPEs . . . . . . . . . . . . . . .   7
   4.  Hosting Encrypted DNS Forwarder in Local Networks . . . . . .   8
     4.1.  Managed CPEs  . . . . . . . . . . . . . . . . . . . . . .   8
       4.1.1.  DNS Forwarders  . . . . . . . . . . . . . . . . . . .   8
       4.1.2.  ACME  . . . . . . . . . . . . . . . . . . . . . . . .   9
     4.2.  Unmanaged CPEs  . . . . . . . . . . . . . . . . . . . . .   9
   5.  Legacy CPEs . . . . . . . . . . . . . . . . . . . . . . . . .  10
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   [I-D.ietf-add-dnr] specifies how a local encrypted DNS server can be
   discovered 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 a
   set of service parameters.

   This document discusses deployment considerations for the discovery
   of encrypted DNS servers 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.

   Sample target deployment scenarios are discussed in Section 3; both
   managed and unmanaged Customer Premises Equipment (CPEs) are covered.

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   It is out of the scope of this document to provide an exhaustive
   inventory of deployments where Encrypted DNS options can be used.

   Considerations related to hosting a DNS forwarder in a local network
   are described in Section 4.

2.  Terminology

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

   Encrypted DNS options:  refers to the options defined in
      [I-D.ietf-add-dnr].

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

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

   DHCP:  refers to both DHCPv4 and DHCPv6.

3.  Sample Target Deployment Scenarios

   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)).

   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 1.  In the case of cellular networks, the cellular
   network will provide connectivity directly to a host (e.g.,
   smartphone, tablet) or via a CPE.  Do53 mechanisms used within the

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   Local Area Network (LAN) are similar in both fixed and cellular CPE-
   based broadband service offerings.

   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.

             (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 1: Sample Legacy Deployments

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3.1.  Managed CPEs

   This section focuses on CPEs that are managed by ISPs.

3.1.1.  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 2.

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

   The DNS client discovers whether the DNS server in the local network
   supports DoH/DoT/DoQ by using the service paramters (ALPN).

             (a) Fixed Networks

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

             (b) Cellular Networks

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

                    Figure 2: Encrypted DNS in the WAN

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

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

                  Figure 3: Direct Encrypted DNS Sessions

3.1.2.  Proxied DNS

   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., 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 4.  Additional
   considerations related to this deployment are discussed in Section 4.

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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 4: Proxied Encrypted DNS Sessions

3.2.  Unmanaged CPEs

3.2.1.  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 Section 3.1.  A host can thus establish DoH/DoT
   session with a DoH/DoT server similar to what is depicted in Figure 3
   or Figure 4.

3.2.2.  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 5).

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

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

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

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

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

   Section 4.2 discusses considerations related to hosting a forwarder
   in the Internal CPE.

4.  Hosting Encrypted DNS Forwarder in Local Networks

   This section discusses some deployment considerations to host an
   encrypted DNS forwarder within a local network.

4.1.  Managed CPEs

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

4.1.1.  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.

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4.1.2.  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.

4.2.  Unmanaged CPEs

   The approach specified in Section 4.1 does not apply for hosting a
   DNS forwarder in an unmanaged CPE.

   The unmanaged CPE 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
      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.

   Figure 7 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.

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              ,--,--,--.                         ,--,
            ,'         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 7: Example of an Internal CPE Hosting a Forwarder

5.  Legacy CPEs

   Hosts serviced by legacy CPEs that can't be upgraded to support the
   options defined in Sections 4, 5, and 6 of [I-D.ietf-add-dnr] 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 discovery using the resolver IP address
   as defined in Section 4 of [I-D.ietf-add-ddr] to discover the
   designated resolvers.

   The guidance in Sections 4.1 and 4.2 of [I-D.ietf-add-ddr] related to
   the designated resolver verification has to be followed in such case.

6.  Security Considerations

   DNR-related security considerations are discussed in Section 7 of
   [I-D.ietf-add-dnr].

7.  IANA Considerations

   This document does not require any IANA action.

8.  Acknowledgements

   This text was initially part of [I-D.ietf-add-dnr].

9.  References

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9.1.  Normative References

   [I-D.ietf-add-dnr]
              Boucadair, M., Reddy, T., Wing, D., Cook, N., and T.
              Jensen, "DHCP and Router Advertisement Options for the
              Discovery of Network-designated Resolvers (DNR)", draft-
              ietf-add-dnr-00 (work in progress), February 2021.

9.2.  Informative References

   [I-D.ietf-add-ddr]
              Pauly, T., Kinnear, E., Wood, C. A., McManus, P., and T.
              Jensen, "Discovery of Designated Resolvers", draft-ietf-
              add-ddr-00 (work in progress), February 2021.

   [I-D.ietf-dprive-dnsoquic]
              Huitema, C., Mankin, A., and S. Dickinson, "Specification
              of DNS over Dedicated QUIC Connections", draft-ietf-
              dprive-dnsoquic-02 (work in progress), February 2021.

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

   [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>.

   [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>.

   [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>.

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

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

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   [RFC8415]  Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
              Richardson, M., Jiang, S., Lemon, T., and T. Winters,
              "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
              RFC 8415, DOI 10.17487/RFC8415, November 2018,
              <https://www.rfc-editor.org/info/rfc8415>.

   [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>.

Authors' Addresses

   Mohamed Boucadair (editor)
   Orange
   Rennes  35000
   France

   Email: mohamed.boucadair@orange.com

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   Tirumaleswar Reddy (editor)
   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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