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ADD WG                                                          T. Reddy
Internet-Draft                                                    McAfee
Intended status: Standards Track                                 D. Wing
Expires: November 6, 2020                                         Citrix
                                                           M. Richardson
                                                Sandelman Software Works
                                                            M. Boucadair
                                                                  Orange
                                                             May 5, 2020


A Bootstrapping Procedure to Discover and Authenticate DNS-over-TLS and
            DNS-over-HTTPS Servers for IoT and BYOD Devices
                 draft-reddy-add-iot-byod-bootstrap-00

Abstract

   This document specifies mechanisms to bootstrap endpoints (e.g.,
   hosts, IoT devices) to discover and authenticate DNS-over-TLS and
   DNS-over-HTTPS servers provided by a local network for IoT/BYOD
   devices in Enterprise networks.

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 November 6, 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Bootstrapping Endpoint Devices  . . . . . . . . . . . . . . .   5
   5.  Bootstrapping IoT Devices . . . . . . . . . . . . . . . . . .   8
   6.  Connection Handshake and Service Invocation . . . . . . . . .   9
   7.  EST Service Discovery Procedure . . . . . . . . . . . . . . .  10
   8.  Network Reattachment  . . . . . . . . . . . . . . . . . . . .  10
   9.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  12
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  12
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  13
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     13.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

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 (DoT) [RFC7858] or DNS-over-HTTPS (DoH) [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
      "EDNS Client Subnet" (ECS) information [RFC7871] to the CDN's DNS
      authorities [CDN].



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   If public DNS servers are used instead of local DNS servers, the
   following discusses the impacts 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.  However,
      if an endpoint is configured to use public DoH/DoT 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 DoT traffic by dropping outgoing packets to
      destination port 853.  Identifying DoH traffic is far more
      challenging than DoT traffic.  Network security services may try
      to identify the domains offering DoH servers, and DoH 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 DoH/DoT 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
      DoH/DoT servers, and if a policy 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




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      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 DoT and DoH
   traffic, this can still compromise the endpoint security and privacy;
   some of the potential security threats are listed below:

   o  Networks are susceptible to internal attacks as discussed in
      Section 3.2 of [I-D.arkko-farrell-arch-model-t].  An internal
      attacker can modify the DNS responses to re-direct the client to
      malicious servers.

   o  Pervasive monitoring of DNS traffic.

   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 bootstrap endpoints to discover and
   authenticate the DoT and DoH 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 procedure 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., DoH) during in the
      connection phase.

   o  Connection handshake and service invocation (Section 6): 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.

   Note:   The strict and opportunistic privacy profiles as defined in
      [RFC8310] only applies to DoT protocol, there has been no such
      distinction made for DoH 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 DoT



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   and DoH 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 DoT and DoH servers provided by the
   Enterprise network.

   WLAN as frequently deployed is vulnerable to various attacks
   ([Evil-Twin],[Krack] and [Dragonblood]).  Because of these attacks,
   only cryptographically authenticated communications are trusted on
   WLAN networks.  This means information provided by the network via
   DHCP, DHCPv6, or RA (e.g., NTP server, DNS server, default domain)
   are untrusted because DHCP and RA are not authenticated.
   [I-D.btw-add-home] discusses DoH/DoT server discovery using DHCP/RA
   but requires the DoH/DoT server to be pre-configured in the endpoint
   (OS or Browser) or the DNS client must be able cryptographically
   identify it is connecting to a DoT/DoH server hosted by a specific
   organization (e.g., ISP or Enterprise) (see
   [I-D.reddy-add-server-policy-selection]) to prevent the client from
   connecting to a attackers server.

   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
   DoT and DoH servers.

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.

   This document makes use of the terms defined in [RFC8499] and
   [I-D.ietf-dnsop-terminology-ter].

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

4.  Bootstrapping Endpoint Devices

   If an endpoint uses the credentials (username and password) provided
   by the IT admin to mutually authenticate to the Enterprise WLAN
   Access Point, the following steps can be used to securely bootstrap
   the endpoint with the authentication domain name (ADN, defined in



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   [RFC8310] and DNS server certificate of the local network's DoH/DoT
   server:

   1.  The endpoint authenticates to the local network and discovers the
       Enrollment over Secure Transport (EST) [RFC7030] server using the
       procedure discussed in Section 7.

   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].

       Note that the Crypto Forum Research Group (cfrg) is discussing
       selection of one or more PAKE to recommend to the wider IETF
       community.  This step will be further updated to reflect the
       outcome of the discussion.


   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 [RFC6125]) 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



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       the endpoint using an authorized trust anchor (Section 3.2 of
       [RFC8295]).  The endpoint needs to identify the certificate
       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] for DoT
       protocol.  The SRV service label "doh" is defined in Section 10.4
       of [I-D.btw-add-home] for DoH protocol.

   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 ADN and 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 bootstrap IoT devices
   supporting Bootstrapping Remote Secure Key Infrastructures (BRSKI)
   discussed in [I-D.ietf-anima-bootstrapping-keyinfra] with local
   network's CA certificates, ADN 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 bootstrap the IoT
      device using the IoT manufacturer provisioned X.509 certificate,



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      in combination with a registrar provided by the 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 (see Step 4 in Section 4).

      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.  Connection Handshake and Service Invocation

   The DNS client resolves the ADN using the mechanism discussed in
   Section 7.2 of [RFC8310].  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 an authorized trust anchor.





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   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.

   Note: This section will be further updated to reflect the outcome of
   the discussion in [I-D.btw-add-home] for a DoH client to retrieve the
   list of supported URI templates by a DoH server (Section 3 of
   [RFC8484]).

7.  EST Service Discovery Procedure

   An 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".

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

      "_est._tcp.local"

   An 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.

8.  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 6 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



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   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 6 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.

 +----------+                                     +--------+  +--------+
 | 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







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9.  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-add-server-policy-selection] 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 client can use the discovered
   DNS server if it meets privacy preserving data policy and filtering
   requirements of the user.

10.  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
   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 DoH/DoT
   server discovered in the network instead of downgrading to
   opportunistic privacy profile.

   The primary attacks against the methods described in Section 7 are
   the ones that would lead to impersonation of a EST server and
   spoofing the DNS response to indicate that the network does not
   support any privacy-enabling protocols or point to a malicious DoH/
   DoT server.  To protect against DNS-vectored attacks, secured DNS
   (DNSSEC) can be used to ensure the validity of the DNS records
   received.  Impersonation of the EST server is prevented by
   authenticating the EST server using the PAKE scheme.  The PAKE scheme
   is only used once to configure the reference identifier of the EST
   server and the server certificate is validated for subsequent TLS
   connections to the EST server.



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

11.  IANA Considerations

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

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

12.  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.

13.  References

13.1.  Normative References

   [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-41 (work in progress), April 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>.

   [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>.







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   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
              2011, <https://www.rfc-editor.org/info/rfc6125>.

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

   [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>.





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13.2.  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>.

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

   [I-D.arkko-farrell-arch-model-t]
              Arkko, J. and S. Farrell, "Challenges and Changes in the
              Internet Threat Model", draft-arkko-farrell-arch-model-
              t-03 (work in progress), March 2020.

   [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.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 Networks", draft-btw-
              add-home-05 (work in progress), April 2020.

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

   [I-D.reddy-add-server-policy-selection]
              Reddy.K, T., Wing, D., Richardson, M., and M. Boucadair,
              "DNS Server Selection: DNS Server Information with
              Assertion Token", draft-reddy-add-server-policy-
              selection-01 (work in progress), April 2020.

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






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   [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>.

   [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









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   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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