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DPRIVE WG                                                       T. Reddy
Internet-Draft                                                    McAfee
Intended status: Standards Track                                 D. Wing
Expires: September 8, 2020                                        Citrix
                                                           M. Richardson
                                                Sandelman Software Works
                                                            M. Boucadair
                                                                  Orange
                                                           March 7, 2020


A Bootstrapping Procedure to Discover and Authenticate DNS-over-TLS and
                         DNS-over-HTTPS Servers
               draft-reddy-dprive-bootstrap-dns-server-08

Abstract

   This document specifies mechanisms to automatically bootstrap
   endpoints (e.g., hosts, Customer Equipment) to discover and
   authenticate DNS-over-TLS and DNS-over-HTTPS servers provided by a
   local network.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on September 8, 2020.

Copyright Notice

   Copyright (c) 2020 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
   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



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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Bootstrapping Endpoint Devices  . . . . . . . . . . . . . . .   6
   5.  Bootstrapping IoT Devices . . . . . . . . . . . . . . . . . .   8
   6.  DNS-over-(D)TLS and DNS-over-HTTPS Server Discovery Procedure   9
   7.  Connection Handshake and Service Invocation . . . . . . . . .  10
   8.  EST Service Discovery Procedure . . . . . . . . . . . . . . .  10
     8.1.  mDNS  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   9.  Network Reattachment  . . . . . . . . . . . . . . . . . . . .  11
   10. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  12
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  12
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  14
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     14.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   Traditionally a caching DNS server has been provided by local
   networks.  This provides benefits such as low latency to reach that
   DNS server (owing to its network proximity to the endpoint).
   However, if an endpoint is configured to use Internet-hosted or
   public DNS-over-TLS [RFC7858] or DNS-over-HTTPS [RFC8484] servers,
   any available local DNS server cannot serve DNS requests from local
   endpoints.  If public DNS servers are used instead of using local DNS
   servers, some operational problems can occur such as those listed
   below:

   o  "Split DNS" [RFC2775] to use the special internal-only domain
      names (e.g., "internal.example.com") in enterprise networks will
      not work, and ".local" and "home.arpa" names cannot be locally
      resolved in home networks.

   o  Content Delivery Networks (CDNs) that map traffic based on DNS may
      lose the ability to direct end-user traffic to a nearby service-
      specific cluster in cases where a DNS service is being used that
      is not affiliated with the local network and which does not send



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      "EDNS Client Subnet" (ECS) information [RFC7871] to the CDN's DNS
      authorities [CDN].

   If public DNS servers are used instead of using local DNS servers,
   the following discusses the impact on network-based security:

   o  Various network security services are provided by Enterprise
      networks to protect endpoints (e.g,. Hosts, IoT devices).
      Network-based security solutions such as Firewalls (FW) and
      Intrusion Prevention Systems (IPS) rely on network traffic
      inspection to implement perimeter-based security policies.  The
      network security services may for example prevent malware
      download, block known malicious URLs, enforce use of strong
      ciphers, stop data exfiltration, etc.  These network security
      services act on DNS requests originating from endpoints.

   o  However, if an endpoint is configured to use public DNS-over-TLS
      or DNS-over-HTTPS servers, network security services cannot act on
      DNS requests from these endpoints.

   o  In order to act on DNS requests from endpoints, network security
      services can block DNS-over-TLS traffic by dropping outgoing
      packets to destination port 853.  Identifying DNS-over-HTTPS
      traffic is far more challenging than DNS-over-TLS traffic.
      Network security services may try to identify the domains offering
      DNS-over-HTTPS servers, and DNS-over-HTTPS traffic can be blocked
      by dropping outgoing packets to these domains.  If an endpoint has
      enabled strict privacy profile (Section 5 of [RFC8310]), and the
      network security service blocks the traffic to the public DNS
      server, the DNS service won't be available to the endpoint and
      ultimately the endpoint cannot access Internet-reachable services.

   o  If an endpoint has enabled opportunistic privacy profile
      (Section 5 of [RFC8310]), and the network security service blocks
      traffic to the public DNS server, the endpoint will either
      fallback to an encrypted connection without authenticating the DNS
      server provided by the local network or fallback to clear text
      DNS, and cannot exchange encrypted DNS messages.

   If the network security service fails to block DNS-over-TLS or DNS-
   over-HTTPS traffic, this can compromise the endpoint security; some
   of the potential security threats are listed below:

   o  The network security service cannot prevent an endpoint from
      accessing malicious domains.

   o  If the endpoint is an IoT device which is configured to use public
      DNS-over-TLS or DNS-over-HTTPS servers, and if a policy



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      enforcement point in the local network is programmed using, for
      example, a Manufacturer Usage Description (MUD) file [RFC8520] by
      a MUD manager to only allow intented communications to and from
      the IoT device, the policy enforcement point cannot enforce the
      network Access Control List (ACL) rules based on domain names
      (Section 8 of [RFC8520]).

   If the network security service successfully blocks DNS-over-TLS and
   DNS-over-HTTPS traffic, this can still compromise the endpoint
   security and privacy; some of the potential security threats are
   listed below:

   o  Pervasive monitoring of DNS traffic.

   o  An internal attacker can modify the DNS responses to re-direct the
      client to malicious servers.

   In addition, the local network's DNS server is advertised using DHCP/
   RA which is insecure and also provides no mechanism to securely
   authenticate the DNS server.  To overcome the above threats, this
   document specifies a mechanism to automatically bootstrap endpoints
   to discover and authenticate the DNS-over-TLS and DNS-over-HTTPS
   servers provided by their local network.  The overall procedure can
   be structured into the following steps:

   o  Bootstrapping (Section 4) is necessary only when connecting to a
      new network or when the network's DNS certificate has changed.
      Bootstrapping authenticates the Enrollment over Secure Transport
      (EST) [RFC7030] server to the endpoint.  After authenticating the
      EST server, DNS server certificate used by the local network is
      downloaded to the endpoint.  This DNS server certificate enables
      subsequent authenticated encrypted communication with the local
      DNS server (e.g., DNS-over-HTTPS) during in the connection phase.

   o  Discovery (Section 6) is performed by a previously bootstrapped
      endpoint whenever connecting to a network.  During discovery, the
      endpoint is instructed which privacy-enabling DNS protocol(s),
      port number(s), and IP addresses are supported on a local network.
      This effectively takes the place of DNS server IP address
      traditionally provided by IPv4 or IPv6 DHCP or by IPv6 Router
      Advertisement [RFC8106].

   o  Connection handshake and service invocation (Section 7): The DNS
      client initiates a TLS handshake with the DNS server learned in
      the discovery phase, and validates the DNS server's identity using
      the credentials obtained in the bootstrapping phase.





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   Note: The strict and opportunistic privacy profiles as defined in
   [RFC8310] only applies to DNS-over-TLS protocol, there has been no
   such distinction made for DNS-over-HTTPS protocol.

2.  Scope

   The problems discussed in Section 1 will be encountered in Enterprise
   networks.  Typically Enterprise networks do not assume that all
   devices in their network are managed by the IT team or Mobile Device
   Management (MDM) devices, especially in the quite common BYOD ("Bring
   Your Own Device") scenario.  The mechanisms specified in this
   document can be used by BYOD devices to discover and authenticate
   DNS-over-TLS and DNS-over-HTTPS servers provided by the Enterprise
   network.  This mechanism can also be used by IoT devices (managed by
   IT team) after onboarding to discover and authenticate DNS- over-TLS
   and DNS-over-HTTPS servers provided by the Enterprise network.

   WiFi as frequently deployed is vulnerable to various attacks
   ([Evil-Twin],[Krack] and [Dragonblood]).  Because of these attacks,
   only cryptographically authenticated communications are trusted on
   WiFi networks.  This means information provided by the network via
   DHCPv4, DHCPv6, or RA (e.g., NTP server, DNS server, default domain)
   are un-trusted because DHCP and RA are not authenticated.

   The users have to indicate to their system in some way that they
   desire bootstrapping to be performed only when connecting to a
   specific network (e.g., organization for which a user works or a user
   works temporarily within another corporation), similar to the way
   users disable VPN connection in specific network (e.g., Enterprise
   network) and enable VPN connection by default in other networks.  If
   the discovered DNS server meets the privacy preserving data policy
   requirements of the user, the user can select to use the discovered
   DNS-over-TLS and DNS-over-HTTPS servers.  In addition, if the
   discovered DNS-over-TLS and DNS-over-HTTPS servers is pre-configured
   in the OS or browser, user can inform the system to use the servers
   in untrusted networks (e.g. coffee shops, airports etc.).  It is
   strongly recommended to configure the DNS server to be used in
   untrusted networks provided the DNS server meets the privacy
   preserving data policy requirements of the user, offers malware
   filtering service and is pre-configured in the OS or browser.

3.  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.



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   This document uses the terms defined in [RFC8499].

4.  Bootstrapping Endpoint Devices

   The following steps detail the mechanism to automatically bootstrap
   an endpoint with the local network's DNS server certificate:

   1.  The endpoint authenticates to the local network and discovers the
       Enrollment over Secure Transport (EST) [RFC7030] server using the
       procedure discussed in Section 8.

   2.  The endpoint establishes provisional TLS connection with that EST
       server, i.e., the endpoint provisionally accepts the unverified
       TLS server certificate.  However, the endpoint MUST authenticate
       the EST server before it accepts the DNS server certificate.  The
       endpoint either uses password-based authenticated key exchange
       (PAKE) with TLS 1.3 [I-D.barnes-tls-pake] as an authentication
       method or uses the mutual authentication protocol for HTTP
       [RFC8120] to authenticate the discovered EST server.

       As a reminder, PAKE is an authentication method that allows the
       use of usernames and passwords over unencrypted channels without
       revealing the passwords to an eavesdropper.  Similarly, the
       mutual authentication for HTTP is based on PAKE and provides
       mutual authentication between an HTTP client and an HTTP server
       using username and password as credentials.  The cryptographic
       algorithms to use with the mutual authentication protocol for
       HTTP are defined in [RFC8121].

   3.  The endpoint needs to use PAKE scheme to perform authentication
       the first time it connects to an EST server.  If the EST server
       authentication is successful, the server's identity can be used
       to authenticate subsequent TLS connections to that EST server.
       The endpoint configures the reference identifier for the EST
       server using the DNS-ID identifier type in the EST server
       certificate.  On subsequent connections to the EST server, the
       endpoint MUST validate the EST server certificate using the
       Implict Trust Anchor database (i.e, the EST server certificate
       must pass PKIX certification path validation) and match the
       reference identifier against the EST server's identity according
       to the rules specified in Section 6.4 of [RFC6125].

   4.  The endpoint learns the End-Entity certificates [RFC8295] from
       the EST server.  The certificate provisioned to the DNS server in
       the local network will be treated as a End-Entity certificate.
       As a reminder, the End-Entity certificates must be validated by
       the endpoint using an authorized trust anchor (Section 3.2 of
       [RFC8295]).  The endpoint needs to identify the certificate



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       provisioned to the DNS server.  The SRV-ID identifier type
       [RFC6125] within subjectAltName entry MUST be used to identify
       the DNS server certificate.

       For example, DNS server certificate will include SRV-ID "_domain-
       s.example.net" along with DNS-ID "example.net".  The SRV service
       label "domain-s" is defined in Section 6 of [RFC7858].  As a
       reminder, the protocol component is not included in the SRV-ID
       [RFC4985].

   5.  The endpoint configures the authentication domain name (ADN)
       (defined in [RFC8310]) for the DNS server from the DNS-ID
       identifier type within subjectAltName entry in the DNS server
       certificate.  The DNS server certificate is associated with the
       ADN to be matched with the certificate given by the DNS server in
       TLS.  To some extent, this approach is similar to certificate
       usage PKIX-EE(1) defined in [RFC7671].

   Figure 1 illustrates a sequence diagram for bootstrapping an endpoint
   with the local network's DNS server certificate.































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 +----------+                                     +--------+  +--------+
 | Endpoint |                                     |  EST   |  |  DNS   |
 |          |                                     | Server |  | Server |
 +----------+                                     +--------+  +--------+
         | DNS-SD query to discover the EST server      |          |
         |-------------------------------------------------------->|
         |                                              |          |
         | optional: mDNS query to                      |          |
         |  discover the EST server                     |          |
         |--------------------------------------------->|          |
         |                                              |          |
         | Establish provisional TLS connection         |          |
         |<-------------------------------------------->|          |
         |                                              |          |
         | PAKE scheme to authenticate the EST server   |          |
         |<-------------------------------------------->|          |
         |                                              |          |
 [Generate reference identifier for the EST server      |          |
  to compare with the EST server certificate            |          |
  in subsequent TLS connections]                        |          |
         |                                              |          |
         |      Get EE certificates                     |          |
         |--------------------------------------------->|          |
         |                                              |          |
 [Identify the DNS server certificate in EE             |          |
  certificates to match with the certificate            |          |
  by the DNS server in TLS handshake]                |          |
                                                        |          |
 [Configure ADN and associate DNS server certificate]   |          |
         |                                              |          |

                 Figure 1: Bootstrapping Endpoint Devices

5.  Bootstrapping IoT Devices

   The following steps explain the mechanism to automatically bootstrap
   IoT devices with local network's CA certificates and DNS server
   certificate:

   o  Bootstrapping Remote Secure Key Infrastructures (BRSKI) discussed
      in [I-D.ietf-anima-bootstrapping-keyinfra] provides a solution for
      secure automated bootstrap of devices.  BRSKI specifies means to
      provision credentials on devices to be used to operationally
      access networks.  In addition, BRSKI provides an automated
      mechanism for the bootstrap distribution of CA certificates from
      the EST server.  The IoT device can use BRSKI to automatically
      bootstrap the IoT device using the IoT manufacturer provisioned
      X.509 certificate, in combination with a registrar provided by the



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      local network and IoT device manufacturer's authorizing service
      (MASA):

      1.  The IoT device authenticates to the local network using the
          IoT manufacturer provisioned X.509 certificate.  The IoT
          device can request and get a voucher from the MASA service via
          the registrar.  The voucher is signed by the MASA service and
          includes the local network's CA public key.

      2.  The IoT device validates the signed voucher using the
          manufacturer installed trust anchor associated with the MASA,
          stores the CA's public key and validates the provisional TLS
          connection to the registrar.

      3.  The IoT device requests the full EST distribution of current
          CA certificates (Section 5.9.1 in
          [I-D.ietf-anima-bootstrapping-keyinfra]) from the registrar
          operating as a BRSKI-EST server.  The IoT devices stores the
          CA certificates as Explicit Trust Anchor database entries.
          The IoT device uses the Explicit Trust Anchor database to
          validate the DNS server certificate.

      4.  The IoT device learns the End-Entity certificates from the
          BRSKI-EST server.  The certificate provisioned to the DNS
          server in the local network will be treated as an End-Entity
          certificate.  The IoT device needs to identify the certificate
          provisioned to the DNS server.  The SRV-ID identifier type
          within subjectAltName entry MUST be used to identify the DNS
          server certificate.

      5.  The endpoint configures the ADN for the DNS server from the
          DNS-ID identifier type within subjectAltName entry in the DNS
          server certificate.  The DNS server certificate is associated
          with the ADN to be matched with the certificate given by the
          DNS server in TLS.

6.  DNS-over-(D)TLS and DNS-over-HTTPS Server Discovery Procedure

   A DNS client discovers the DNS server in the local network supports
   DNS-over-TLS and DNS-over-HTTPS protocols by using the mechanism
   discussed in Section 6 of [I-D.btw-add-home].  If the endpoint has
   enabled strict privacy profile and access to the pre-configured
   public DNS servers is blocked, the DNS service won't be available to
   the endpoint and ultimately the endpoint cannot access Internet-
   reachable services.  If the endpoint has enabled opportunistic
   privacy profile and access to the pre-configured public DNS servers
   is blocked, the endpoint will either fallback to an encrypted




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   connection without authenticating the DNS server provided by the
   local network or fallback to clear text DNS.

7.  Connection Handshake and Service Invocation

   The DNS client initiates TLS handshake with the DNS server, the DNS
   server presents its certificate in ServerHello message, and the DNS
   client MUST match the DNS server certificate downloaded in Step 4 in
   Section 4 or Section 5 with the certificate provided by the DNS
   server in TLS handshake.  If the match is successful, the DNS client
   MUST validate the server certificate using the Implicit Trust Anchor
   database (i.e., the DNS server certificate must pass PKIX
   certification path validation).

   If the match is successful and server certificate is successfully
   validated, the client continues with the connection as normal.
   Otherwise, the client MUST treat the server certificate validation
   failure as a non-recoverable error.  If the DNS client cannot reach
   or establish an authenticated and encrypted connection with the
   privacy-enabling DNS server provided by the local network, the DNS
   client can fallback to the privacy-enabling public DNS server.

8.  EST Service Discovery Procedure

   A EST client discovers the EST server in the local network by using
   DNS-based Service Discovery (DNS-SD) [RFC6763] or Multicast DNS
   (mDNS) [RFC6762].  The <Domain> portion specifies the DNS sub-domain
   where the service instance is registered.  It may be "local.",
   indicating the mDNS local domain, or it may be a conventional domain
   name such as "example.com.".  The <Service> portion of the EST
   service instance name MUST be "_est._tcp".

8.1.  mDNS

   A EST client application can proactively discover an EST server being
   advertised in the site by multicasting a PTR query to the following:

   o  "_est._tcp.local"

   A EST server can send out gratuitous multicast DNS answer packets
   whenever it starts up, wakes from sleep, or detects a change in EST
   server configuration.  EST client application can receive these
   gratuitous packets and cache information contained in them.








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9.  Network Reattachment

   On subsequent attachments to the network, the endpoint discovers the
   privacy-enabling DNS server using the authentication domain name
   (configured in Step 5 of Section 4 or Section 5), initiates TLS
   handshake with the DNS server and follows the mechanism discussed in
   Section 7 to validate the DNS server certificate.

   If the DNS server certificate is invalid (e.g., revoked or expired)
   or the procedure to discover the privacy-enabling DNS server fails
   (e.g.  the domain name of the privacy-enabling DNS server has changed
   because the Enterprise network has switched to a public privacy-
   enabling DNS server capable of blocking access to malicious domains),
   the endpoint discovers and initiates TLS handshake with the EST
   server, and uses the validation techniques described in [RFC6125] to
   compare the reference identifier (created in Step 2 of Section 4 in
   this document) to the EST server certificate and verifies the entire
   certification path as per [RFC5280].  The endpoint then gets the DNS
   server certificate from the EST server.  If the DNS-ID identifier
   type within subjectAltName entry in the DNS server certificate does
   not match the configured ADN, the ADN is replaced with the DNS-ID
   identifier type.  The DNS server certificate associated with the ADN
   is replaced with the one provided by the EST server.  If the ADN has
   changed, the endpoint discovers the privacy-enabling DNS server,
   initiates TLS handshake with the DNS server and follows the mechanism
   discussed in Section 7 to validate the DNS server certificate.

   Figure 2 illustrates a sequence diagram for re-configuring an
   endpoint with ADN and local network's DNS server certificate on
   subsequent attachments to the network.





















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 +----------+                                     +--------+  +--------+
 | Endpoint |                                     |  EST   |  |  DNS   |
 |          |                                     | Server |  | Server |
 +----------+                                     +--------+  +--------+
         | DNS-SD query to discover the EST server      |          |
         |-------------------------------------------------------->|
         |                                              |          |
         | optional: mDNS query to                      |          |
         | discover the EST server                      |          |
         |--------------------------------------------->|          |
         |                                              |          |
         | Establish TLS connection                     |          |
         | and validate EST server certificate          |          |
         |<-------------------------------------------->|          |
         |                                              |          |
         |      Get EE certificates                     |          |
         |<-------------------------------------------->|          |
         |                                              |          |
 [Identify the DNS server certificate in EE             |          |
  certificates to match with the certificate            |          |
  by the DNS server in TLS handshake]                |          |
                                                        |          |
 [Re-configure ADN and associate DNS server certificate]|          |
         |                                              |          |


   Figure 2: Bootstrapping Endpoint Devices on subsequent attachments to
                                the network

10.  Privacy Considerations

   [RFC7626] discusses DNS privacy considerations in both "on the wire"
   (Section 2.4 of [RFC7626]) and "in the server" (Section 2.5 of
   [RFC7626] contexts.  The mechanism defined in
   [I-D.reddy-dprive-dprive-privacy-policy] can be used by the DNS
   server to communicate its privacy statement URL and filtering policy
   to a DNS client.  This communication is cryptographically signed to
   attest to its authenticity.  By evaluating the DNS privacy statement,
   filtering policy and the signatory, the user can choose to use the
   discovered DNS server if it meets privacy preserving data policy and
   filtering requirements of the user.

11.  Security Considerations

   The bootstrapping procedure to obtain the certificate of the local
   networks DNS server uses a client identity and password to
   authenticate the EST server using PAKE schemes.  Security




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   considerations such as those discussed in [I-D.barnes-tls-pake] or
   [RFC8120] and [RFC8121] need to be taken into consideration.

   Users cannot be expected to enable or disable the bootstrapping or
   the discovery procedure as they switch networks.  Thus, it is
   RECOMMENDED that users indicate to their system in some way that they
   desire bootstrapping to be performed when connecting to a specific
   network, similar to the way users disable VPN connection in specific
   network (e.g., Enterprise network) and enable VPN connection by
   default in other networks.

   If an endpoint has enabled strict privacy profile, and the network
   security service blocks the traffic to the privacy-enabling public
   DNS server, a hard failure occurs and the user is notified.  The user
   has a choice to switch to another network or if the user trusts the
   network, the user can enable strict privacy profile with the DNS-
   over-TLS or DNS-over-HTTPS server discovered in the network instead
   of downgrading to opportunistic privacy profile.

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

   If the browser or OS is pre-configured with a list of DNS servers
   where some perform malware filtering and others do not, an attacker
   can prevent contacting the preferred filtering DNS servers causing a
   downgrade attack to a non-filtering DNS server, which the attacker
   can leverage to deliver malware.  To prevent such an attack, it is
   RECOMMENDED if any pre-configured DNS servers perform malware
   filtering that all pre-configured DNS servers perform malware
   filtering.

   Related to the downgrade attack described in the previous paragraph,
   if the browser or OS is pre-configured to use a DNS server that
   filters malware, it MUST NOT use locally-learned DNS servers (e.g.,
   learned via DHCP) unless they also perform malware filtering and also
   conform to the user's privacy policy.

   Security considerations in [I-D.ietf-anima-bootstrapping-keyinfra]
   need to be taken into consideration for IoT devices.



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12.  IANA Considerations

   IANA is requested to allocate the SRV service name of "est".

13.  Acknowledgments

   Thanks to Joe Hildebrand, Harsha Joshi, Shashank Jain, Patrick
   McManus, Bob Harold, Livingood Jason, Winfield Alister, Eliot Lear,
   Stephane Bortzmeyer, Ted Lemon and Sara Dickinson for the discussion
   and comments.

14.  References

14.1.  Normative References

   [I-D.btw-add-home]
              Boucadair, M., Reddy.K, T., Wing, D., and N. Cook, "DNS-
              over-HTTPS and DNS-over-TLS server Discovery and
              Deployment Considerations for Home and Mobile Networks",
              draft-btw-add-home-01 (work in progress), March 2020.

   [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>.

   [RFC4985]  Santesson, S., "Internet X.509 Public Key Infrastructure
              Subject Alternative Name for Expression of Service Name",
              RFC 4985, DOI 10.17487/RFC4985, August 2007,
              <https://www.rfc-editor.org/info/rfc4985>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/info/rfc5280>.

   [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>.

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              DOI 10.17487/RFC6762, February 2013,
              <https://www.rfc-editor.org/info/rfc6762>.




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

   [RFC7030]  Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
              "Enrollment over Secure Transport", RFC 7030,
              DOI 10.17487/RFC7030, October 2013,
              <https://www.rfc-editor.org/info/rfc7030>.

   [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>.

   [RFC8121]  Oiwa, Y., Watanabe, H., Takagi, H., Maeda, K., Hayashi,
              T., and Y. Ioku, "Mutual Authentication Protocol for HTTP:
              Cryptographic Algorithms Based on the Key Agreement
              Mechanism 3 (KAM3)", RFC 8121, DOI 10.17487/RFC8121, April
              2017, <https://www.rfc-editor.org/info/rfc8121>.

   [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>.

   [RFC8295]  Turner, S., "EST (Enrollment over Secure Transport)
              Extensions", RFC 8295, DOI 10.17487/RFC8295, January 2018,
              <https://www.rfc-editor.org/info/rfc8295>.

   [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>.

14.2.  Informative References

   [CDN]      "End-User Mapping: Next Generation Request Routing for
              Content Delivery", 2015,
              <https://conferences.sigcomm.org/sigcomm/2015/pdf/papers/
              p167.pdf>.

   [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.barnes-tls-pake]
              Barnes, R. and O. Friel, "Usage of PAKE with TLS 1.3",
              draft-barnes-tls-pake-04 (work in progress), July 2018.

   [I-D.ietf-anima-bootstrapping-keyinfra]
              Pritikin, M., Richardson, M., Eckert, T., Behringer, M.,
              and K. Watsen, "Bootstrapping Remote Secure Key
              Infrastructures (BRSKI)", draft-ietf-anima-bootstrapping-
              keyinfra-37 (work in progress), February 2020.

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

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

   [RFC2775]  Carpenter, B., "Internet Transparency", RFC 2775,
              DOI 10.17487/RFC2775, February 2000,
              <https://www.rfc-editor.org/info/rfc2775>.

   [RFC7626]  Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626,
              DOI 10.17487/RFC7626, August 2015,
              <https://www.rfc-editor.org/info/rfc7626>.

   [RFC7671]  Dukhovni, V. and W. Hardaker, "The DNS-Based
              Authentication of Named Entities (DANE) Protocol: Updates
              and Operational Guidance", RFC 7671, DOI 10.17487/RFC7671,
              October 2015, <https://www.rfc-editor.org/info/rfc7671>.

   [RFC7871]  Contavalli, C., van der Gaast, W., Lawrence, D., and W.
              Kumari, "Client Subnet in DNS Queries", RFC 7871,
              DOI 10.17487/RFC7871, May 2016,
              <https://www.rfc-editor.org/info/rfc7871>.

   [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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   [RFC8120]  Oiwa, Y., Watanabe, H., Takagi, H., Maeda, K., Hayashi,
              T., and Y. Ioku, "Mutual Authentication Protocol for
              HTTP", RFC 8120, DOI 10.17487/RFC8120, April 2017,
              <https://www.rfc-editor.org/info/rfc8120>.

   [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>.

   [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>.

Authors' Addresses

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

   Email: kondtir@gmail.com


   Dan Wing
   Citrix Systems, Inc.
   USA

   Email: dwing-ietf@fuggles.com


   Michael C. Richardson
   Sandelman Software Works
   USA

   Email: mcr+ietf@sandelman.ca


   Mohamed Boucadair
   Orange
   Rennes  35000
   France

   Email: mohamed.boucadair@orange.com





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