dprive                                                      S. Dickinson
Internet-Draft                                                   Sinodun
Intended status: Standards Track                              D. Gillmor
Expires: December 12, 2016                                          ACLU
                                                                T. Reddy
                                                           June 10, 2016

         Authentication and (D)TLS Profile for DNS-over-(D)TLS


   This document describes how a DNS client can use a domain name to
   authenticate a DNS server that uses Transport Layer Security (TLS)
   and Datagram TLS (DTLS).  Additionally, it defines (D)TLS profiles
   for DNS clients and servers implementing DNS-over-TLS and DNS-over-

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 http://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 December 12, 2016.

Copyright Notice

   Copyright (c) 2016 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
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must

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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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Background  . . . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  Usage Profiles  . . . . . . . . . . . . . . . . . . . . .   6
       4.2.1.  DNS Resolution  . . . . . . . . . . . . . . . . . . .   8
     4.3.  Authentication  . . . . . . . . . . . . . . . . . . . . .   8
       4.3.1.  DNS-over-(D)TLS Bootstrapping Problems  . . . . . . .   8
       4.3.2.  Credential Verification . . . . . . . . . . . . . . .   8
       4.3.3.  Implementation guidance . . . . . . . . . . . . . . .   9
   5.  Authentication in Opportunistic DNS-over(D)TLS Privacy  . . .   9
   6.  Authentication in Strict DNS-over(D)TLS Privacy . . . . . . .   9
   7.  In Band Source of Domain Name: SRV Service Label  . . . . . .  10
   8.  Out of Band Sources of Domain Name  . . . . . . . . . . . . .  10
     8.1.  Full direct configuration . . . . . . . . . . . . . . . .  10
     8.2.  Direct configuration of name only . . . . . . . . . . . .  10
     8.3.  DHCP  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   9.  Credential Verification . . . . . . . . . . . . . . . . . . .  12
     9.1.  X.509 Certificate Based Authentication  . . . . . . . . .  12
     9.2.  DANE  . . . . . . . . . . . . . . . . . . . . . . . . . .  12
       9.2.1.  Direct DNS Lookup . . . . . . . . . . . . . . . . . .  13
       9.2.2.  TLS DNSSEC Chain extension  . . . . . . . . . . . . .  13
   10. Combined Credentials with SPKI Pinsets  . . . . . . . . . . .  13
   11. (D)TLS Protocol Profile . . . . . . . . . . . . . . . . . . .  14
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  15
     13.1.  Counter-measures to DNS Traffic Analysis . . . . . . . .  15
   14. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  15
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     15.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Appendix A.  Server capability probing and caching by DNS clients  18
   Appendix B.  Changes between revisions  . . . . . . . . . . . . .  19
     B.1.  -02 version . . . . . . . . . . . . . . . . . . . . . . .  19
     B.2.  -01 version . . . . . . . . . . . . . . . . . . . . . . .  19
     B.3.  draft-ietf-dprive-dtls-and-tls-profiles-00  . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

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1.  Introduction

   DNS Privacy issues are discussed in [RFC7626].  Two documents that
   provide DNS privacy between DNS clients and DNS servers are:

   o  Specification for DNS over Transport Layer Security (TLS)
      [RFC7858], referred to here as simply 'DNS-over-TLS'

   o  DNS-over-DTLS (DNSoD) [I-D.ietf-dprive-dnsodtls], referred to here
      simply as 'DNS-over-DTLS'

   Both documents are limited in scope to encrypting DNS messages
   between stub clients and recursive resolvers and the same scope is
   applied to this document (see Section 2 and Section 3).  The
   proposals here might be adapted or extended in future to be used for
   recursive clients and authoritative servers, but this application is
   out of scope for the DNS PRIVate Exchange (DPRIVE) Working Group per
   its current charter.

   This document defines two Usage Profiles (Strict and Opportunistic)
   for DTLS [RFC6347] and TLS [RFC5246] which define the security
   properties a user should expect when using that profile to connect to
   the available DNS servers.  In essence:

   o  the Strict Profile requires an encrypted connection and successful
      authentication of the DNS server which provides strong privacy
      guarantees (at the expense of providing no DNS service if this is
      not available).

   o  the Opportunistic Profile will attempt, but does not require,
      encryption and successful authentication; it therefore provides no
      privacy guarantees but offers maximum chance of DNS service.

   Additionally, a number of authentication mechanisms are defined that
   specify how a DNS client should authenticate a DNS server based on a
   domain name.  In particular, the following is described:

   o  How a DNS client can obtain a domain name for a DNS server to use
      for (D)TLS authentication.

   o  What are the acceptable credentials a DNS server can present to
      prove its identity for (D)TLS authentication based on a given
      domain name.

   o  How a DNS client can verify that any given credential matches the
      domain name obtained for a DNS server.

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   It should be noted that [RFC7858] includes a description of a
   specific case of a Strict Usage Profile using a single authentication
   mechanism (SPKI pinning).  This draft generalises the picture by
   separating the Usage Profile, which is based purely on the security
   properties it offers the user, from the specific mechanism that is
   used for authentication.  Therefore the "Out-of-band Key-pinned
   Privacy Profile" described in the DNS-over-TLS draft would qualify as
   a "Strict Usage Profile" that used SPKI pinning for authentication.

   This document also defines a (D)TLS protocol profile for use with
   DNS.  This profile defines the configuration options and protocol
   extensions required of both parties to optimize connection
   establishment and session resumption for transporting DNS, and to
   support the authentication mechanisms defined here.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   Several terms are used specifically in the context of this draft:

   o  DNS client: a DNS stub resolver or forwarder/proxy.  In the case
      of a forwarder, the term "DNS client" is used to discuss the side
      that sends queries.

   o  DNS server: a DNS recursive resolver or forwarder/proxy.  In the
      case of a forwarder, the term "DNS server" is used to discuss the
      side that responds to queries.

   o  Privacy-enabling DNS server: A DNS server that:

      *  MUST implement DNS-over-TLS [RFC7858] and MAY implement DNS-
         over-DTLS [I-D.ietf-dprive-dnsodtls].

      *  Can offer at least one of the credentials described in
         Section 9.

      *  Implements the (D)TLS profile described in Section 11.

   o  (D)TLS: For brevity this term is used for statements that apply to
      both Transport Layer Security [RFC5246] and Datagram Transport
      Layer Security [RFC6347].  Specific terms will be used for any
      statement that applies to either protocol alone.

   o  DNS-over-(D)TLS: For brevity this term is used for statements that
      apply to both DNS-over-TLS [RFC7858] and DNS-over-DTLS

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      [I-D.ietf-dprive-dnsodtls].  Specific terms will be used for any
      statement that applies to either protocol alone.

   o  Credential: Information available for a DNS server which proves
      its identity for authentication purposes.  Credentials discussed
      here include:

      *  X.509 certificate

      *  DNSSEC validated chain to a TLSA record

      but may also include SPKI pinsets.

   o  SPKI Pinsets: [RFC7858] describes the use of cryptographic digests
      to "pin" public key information in a manner similar to HPKP
      [RFC7469].  An SPKI pinset is a collection of these pins that
      constrains a DNS server.

   o  Reference Identifier: a Reference Identifier as described in
      [RFC6125], constructed by the DNS client when performing TLS
      authentication of a DNS server.

3.  Scope

   This document is limited to domain-name-based authentication of DNS
   servers by DNS clients (as defined in the terminology section), and
   the (D)TLS profiles needed to support this.  As such, the following
   things are out of scope:

   o  Authentication of authoritative servers by recursive resolvers.

   o  Authentication of DNS clients by DNS servers.

   o  SPKI-pinset-based authentication.  This is defined in [RFC7858].
      However, Section 10 does describe how to combine that approach
      with the domain name based mechanism described here.

   o  Any server identifier other than domain names, including IP
      address, organizational name, country of origin, etc.

4.  Discussion

4.1.  Background

   To protect against passive attacks DNS privacy requires encrypting
   the query (and response).  Such encryption typically provides
   integrity protection as a side-effect, which means on-path attackers
   cannot simply inject bogus DNS responses.  For DNS privacy to also

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   provide protection against active attackers pretending to be the
   server, the client must authenticate the server.

   This draft discusses Usage Profiles, which provide differing levels
   of privacy guarantees to DNS clients, based on the requirements for
   authentication and encryption, regardless of the context (for
   example, which network the client is connected to).  A Usage Profile
   is a distinct concept to a usage policy or usage model, which might
   dictate which Profile should be used in a particular context
   (enterprise vs coffee shop), with a particular set of DNS Servers or
   with reference to other external factors.  A description of the
   variety of usage policies is out of scope of this document, but may
   be the subject of a future I-D.

4.2.  Usage Profiles

   A DNS client has a choice of privacy usage profiles available.  This
   choice is briefly discussed in both [RFC7858] and
   [I-D.ietf-dprive-dnsodtls].  In summary, the usage profiles are:

   o  Strict Privacy: the DNS client requires both an encrypted and
      authenticated connection to a privacy-enabling DNS Server.  A hard
      failure occurs if this is not available.  This requires the client
      to securely obtain information it can use to authenticate the
      server.  This profile can include some initial meta queries
      (performed using Opportunistic Privacy) to securely obtain the IP
      address and authentication information for the privacy-enabling
      DNS server to which the DNS client will subsequently connect.  The
      rationale for this is that requiring Strict Privacy for such meta
      queries would introduce significant deployment obstacles.  This
      profile provides strong privacy guarantees to the client.  This is
      discussed in detail in Section 6.

   o  Opportunistic Privacy: the DNS client uses Opportunistic Security
      as described in [RFC7435]

         "... the use of cleartext as the baseline communication
         security policy, with encryption and authentication negotiated
         and applied to the communication when available."

      The use of Opportunistic Privacy is intended to support
      incremental deployment of security capabilities with a view to
      widespread adoption of Strict Privacy.  It should be employed when
      the DNS client might otherwise settle for cleartext; it provides
      the maximum protection available.  As described in [RFC7435] it
      might result in

      *  an encrypted and authenticated connection

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      *  an encrypted connection

      *  a clear text connection

      *  hard failure

      depending on the fallback logic of the client, the available
      authentication information and the capabilities of the DNS Server.
      In the first three cases the DNS client is willing to continue
      with a connection to the DNS Server and perform resolution of

   To compare the two Usage profiles the table below shows successful
   Strict Privacy along side the 3 possible successful outcomes of
   Opportunistic Privacy.  In the best case scenario for Opportunistic
   (authenticated and encrypted connection) it is equivalent to Strict
   Privacy.  In the worst case scenario it is equivalent to clear text.
   Clients using Opportunistic Privacy SHOULD try for the best case but
   MAY fallback to intermediate cases and eventually the worst case
   scenario in order to obtain a response.  It therefore provides no
   privacy guarantee to the user and varying protection depending on
   what kind of connection is actually used.  Note that there is no
   requirement in Opportunistic to notify the user what type of
   connection is actually used, the detection described below is only
   possible if such connection information is available.  This is
   discussed in Section 5.

    | Usage Profile | Connection | Passive Attacker | Active Attacker |
    |     Strict    |    A, E    |        P         |        P        |
    | Opportunistic |    A, E    |        P         |        P        |
    | Opportunistic |     E      |        P         |      N (D)      |
    | Opportunistic |            |      N (D)       |      N (D)      |

   P == protection; N == no protection; D == detection is possible; A ==
            Authenticated Connection; E == Encrypted Connection

   Table 1: DNS Privacy Protection by Usage Profile and type of attacker

   Since Strict Privacy provides the strongest privacy guarantees it is
   preferable to Opportunistic Privacy.

   However since the two profiles require varying levels of
   configuration (or a trusted relationship with a provider) and DNS
   server capabilities, DNS clients will need to carefully select which
   profile to use based on their communication privacy needs.  For the

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   case where a client has a trusted relationship with a provider it is
   expected that the provider will provide either a domain name or SPKI
   pinset via a secure out-of-band mechanism and therefore Strict
   Privacy should be used.

4.2.1.  DNS Resolution

   A DNS client SHOULD select a particular usage profile when resolving
   a query.  A DNS client MUST NOT fallback from Strict Privacy to
   Opportunistic Privacy during the resolution process as this could
   invalidate the protection offered against active attackers.

4.3.  Authentication

   This document describes authentication mechanisms that can be used in
   either Strict or Opportunistic Privacy for DNS-over-(D)TLS.

4.3.1.  DNS-over-(D)TLS Bootstrapping Problems

   Many (D)TLS clients use PKIX authentication [RFC6125] based on a
   domain name for the server they are contacting.  These clients
   typically first look up the server's network address in the DNS
   before making this connection.  A DNS client therefore has a
   bootstrap problem.  DNS clients typically know only the IP address of
   a DNS server.

   As such, before connecting to a DNS server, a DNS client needs to
   learn the domain name it should associate with the IP address of a
   DNS server for authentication purposes.  Sources of domains names are
   discussed in Section 7 and Section 8.

   One advantage of this domain name based approach is that it
   encourages association of stable, human recognisable identifiers with
   secure DNS service providers.

4.3.2.  Credential Verification

   The use of SPKI pinset verification is discussed in [RFC7858].

   In terms of domain name based verification, once a domain name is
   known for a DNS server a choice of mechanisms can be used for
   authentication.  Section 9 discusses these mechanisms in detail,
   namely X.509 certificate based authentication and DANE.

   Note that the use of DANE adds requirements on the ability of the
   client to get validated DNSSEC results.  This is discussed in more
   detail in Section 9.2.

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4.3.3.  Implementation guidance

   Section 11 describes the (D)TLS profile for DNS-over(D)TLS.
   Additional considerations relating to general implementation
   guidelines are discussed in both Section 13 and in Appendix A.

5.  Authentication in Opportunistic DNS-over(D)TLS Privacy

   An Opportunistic Security [RFC7435] profile is described in [RFC7858]
   which MAY be used for DNS-over-(D)TLS.

   DNS clients issuing queries under an opportunistic profile which know
   of a domain name or SPKI pinset for a given privacy-enabling DNS
   server MAY choose to try to authenticate the server using the
   mechanisms described here.  This is useful for detecting (but not
   preventing) active attack, since the fact that authentication
   information is available indicates that the server in question is a
   privacy-enabling DNS server to which it should be possible to
   establish an authenticated, encrypted connection.  In this case,
   whilst a client cannot know the reason for an authentication failure,
   from a privacy standpoint the client should consider an active attack
   in progress and proceed under that assumption.  Attempting
   authentication is also useful for debugging or diagnostic purposes if
   there are means to report the result.  This information can provide a
   basis for a DNS client to switch to (preferred) Strict Privacy where
   it is viable.

6.  Authentication in Strict DNS-over(D)TLS Privacy

   To authenticate a privacy-enabling DNS server, a DNS client needs to
   know the domain name for each server it is willing to contact.  This
   is necessary to protect against active attacks on DNS privacy.

   A DNS client requiring Strict Privacy MUST either use one of the
   sources listed in Section 8 to obtain a domain name for the server it
   contacts, or use an SPKI pinset as described in [RFC7858].

   A DNS client requiring Strict Privacy MUST only attempt to connect to
   DNS servers for which either a domain name or a SPKI pinset is known
   (or both).  The client MUST use the available verification mechanisms
   described in Section 9 to authenticate the server, and MUST abort
   connections to a server when no verification mechanism succeeds.

   With Strict Privacy, the DNS client MUST NOT commence sending DNS
   queries until at least one of the privacy-enabling DNS servers
   becomes available.

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   A privacy-enabling DNS server may be temporarily unavailable when
   configuring a network.  For example, for clients on networks that
   require registration through web-based login (a.k.a. "captive
   portals"), such registration may rely on DNS interception and
   spoofing.  Techniques such as those used by DNSSEC-trigger
   [dnssec-trigger] MAY be used during network configuration, with the
   intent to transition to the designated privacy-enabling DNS servers
   after captive portal registration.  The system MUST alert by some
   means that the DNS is not private during such bootstrap.

7.  In Band Source of Domain Name: SRV Service Label

   This specification adds a SRV service label "domain-s" for privacy-
   enabling DNS servers.

   Example service records (for TLS and DTLS respectively):

      _domain-s._tcp.dns.example.com.  SRV 0 1 853 dns1.example.com.
      _domain-s._tcp.dns.example.com.  SRV 0 1 853 dns2.example.com.

      _domain-s._udp.dns.example.com.  SRV 0 1 853 dns3.example.com.

8.  Out of Band Sources of Domain Name

8.1.  Full direct configuration

   DNS clients may be directly and securely provisioned with the domain
   name of each privacy-enabling DNS server.  For example, using a
   client specific configuration file or API.

   In this case, direct configuration for a DNS client would consist of
   both an IP address and a domain name for each DNS server.

8.2.  Direct configuration of name only

   A DNS client may be configured directly and securely with only the
   domain name of its privacy-enabling DNS server.  For example, using a
   client specific configuration file or API.

   A DNS client might learn of a default recursive DNS resolver from an
   untrusted source (such as DHCP's DNS server option [RFC3646]).  It
   can then use opportunistic DNS connections to untrusted recursive DNS
   resolver to establish the IP address of the intended privacy-enabling
   DNS server by doing a lookup of SRV records.  Such records MUST be
   validated using DNSSEC.  Private DNS resolution can now be done by
   the DNS client against the configured privacy-enabling DNS server.


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   o  A DNSSEC validating DNS client is configured with the domain name
      dns.example.net for a privacy-enabling DNS server

   o  Using Opportunistic Privacy to a default DNS resolver (acquired,
      for example, using DHCP) the client performs look ups for

      *  SRV record for _domain-s._tcp.dns.example.net to obtain the
         server host name

      *  A and/or AAAA lookups to obtain IP address for the server host

   o  Client validates all the records obtained in the previous step
      using DNSSEC.

   o  If the records successfully validate the client proceeds to
      connect to the privacy-enabling DNS server using Strict Privacy.

   A DNS client so configured that successfully connects to a privacy-
   enabling DNS server MAY choose to locally cache the looked up
   addresses in order to not have to repeat the opportunistic lookup.

8.3.  DHCP

   Some clients may have an established trust relationship with a known
   DHCP [RFC2131] server for discovering their network configuration.
   In the typical case, such a DHCP server provides a list of IP
   addresses for DNS servers (see section 3.8 of [RFC2132]), but does
   not provide a domain name for the DNS server itself.

   In the future, a DHCP server might use a DHCP extension to provide a
   list of domain names for the offered DNS servers, which correspond to
   IP addresses listed.

   Use of such a mechanism with any DHCP server when using an
   Opportunistic profile is reasonable, given the security expectation
   of that profile.  However when using a Strict profile the DHCP
   servers used as sources of domain names MUST be considered secure and
   trustworthy.  This document does not attempt to describe secured and
   trusted relationships to DHCP servers.

   [NOTE: It is noted (at the time of writing) that whilst some
   implementation work is in progress to secure IPv6 connections for
   DHCP, IPv4 connections have received little to no implementation
   attention in this area.]

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9.  Credential Verification

9.1.  X.509 Certificate Based Authentication

   When a DNS client configured with a domain name connects to its
   configured DNS server over (D)TLS, the server may present it with an
   X.509 certificate.  In order to ensure proper authentication, DNS
   clients MUST verify the entire certification path per [RFC5280].  The
   DNS client additionally uses [RFC6125] validation techniques to
   compare the domain name to the certificate provided.

   A DNS client constructs two Reference Identifiers for the server
   based on the domain name: A DNS-ID and an SRV-ID [RFC4985].  The DNS-
   ID is simply the domain name itself.  The SRV-ID uses a "_domain-s."
   prefix.  So if the configured domain name is "dns.example.com", then
   the two Reference Identifiers are:

      DNS-ID: dns.example.com

      SRV-ID: _domain-s.dns.example.com

   If either of the Reference Identifiers are found in the X.509
   certificate's subjectAltName extension as described in section 6 of
   [RFC6125], the DNS client should accept the certificate for the

   A compliant DNS client MUST only inspect the certificate's
   subjectAltName extension for these Reference Identifiers.  In
   particular, it MUST NOT inspect the Subject field itself.

9.2.  DANE

   DANE [RFC6698] provides mechanisms to root certificate and raw public
   keys trust with DNSSEC.  However this requires a domain name which
   must be obtained via a trusted source.

   It is noted that [RFC6698] says

      "Clients that validate the DNSSEC signatures themselves MUST use
      standard DNSSEC validation procedures.  Clients that rely on
      another entity to perform the DNSSEC signature validation MUST use
      a secure mechanism between themselves and the validator."

   The specific DANE record would take the form:

      _853._tcp.[server-domain-name] for TLS

      _853._udp.[server-domain-name] for DTLS

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9.2.1.  Direct DNS Lookup

   The DNS client MAY choose to perform the DNS lookups to retrieve the
   required DANE records itself.  The DNS queries for such DANE records
   MAY use opportunistic encryption or be in the clear to avoid trust
   recursion.  The records MUST be validated using DNSSEC as described
   above in [RFC6698].

9.2.2.  TLS DNSSEC Chain extension

   The DNS client MAY offer the TLS extension described in
   [I-D.ietf-tls-dnssec-chain-extension].  If the DNS server supports
   this extension, it can provide the full chain to the client in the

   If the DNS client offers the TLS DNSSEC Chain extension, it MUST be
   capable of validating the full DNSSEC authentication chain down to
   the leaf.  If the supplied DNSSEC chain does not validate, the client
   MUST ignore the DNSSEC chain and validate only via other supplied

   [ TODO: specify guidance for DANE parameters to be used here.  For
   example, a suggestion to use Certificate Usage of 3 (EE-DANE)
   (section 2.1.1 of [RFC6698]) and a Selector of 1 (SPKI) (section
   2.1.2) would completely remove all X.509 and certificate authorities
   from the verification path and allows for private certification ]

   [ TODO: discuss combination of DNSSEC Chain Extension with cert
   validation.  Note that the combination depends on the Certificate
   Usage value of the TLSA response. ]

10.  Combined Credentials with SPKI Pinsets

   The SPKI pinset profile described in [RFC7858] MAY be used with DNS-

   This draft does not make explicit recommendations about how a SPKI
   pinset based authentication mechanism should be combined with a
   domain based mechanism from an operator perspective.  However it can
   be envisaged that a DNS server operator may wish to make both an SPKI
   pinset and a domain name available to allow clients to choose which
   mechanism to use.  Therefore, the following is guidance on how
   clients ought to behave if they choose to configure both, as is
   possible in HPKP [RFC7469].

   A DNS client that is configured with both a domain name and a SPKI
   pinset for a DNS sever SHOULD match on both a valid credential for

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   the domain name and a valid SPKI pinset when connecting to that DNS

11.  (D)TLS Protocol Profile

   This section defines the (D)TLS protocol profile of DNS-over-(D)TLS.

   There are known attacks on (D)TLS, such as machine-in-the-middle and
   protocol downgrade.  These are general attacks on (D)TLS and not
   specific to DNS-over-TLS; please refer to the (D)TLS RFCs for
   discussion of these security issues.  Clients and servers MUST adhere
   to the (D)TLS implementation recommendations and security
   considerations of [RFC7525] except with respect to (D)TLS version.
   Since encryption of DNS using (D)TLS is virtually a green-field
   deployment DNS clients and server MUST implement only (D)TLS 1.2 or

   Implementations MUST NOT offer or provide TLS compression, since
   compression can leak significant amounts of information, especially
   to a network observer capable of forcing the user to do an arbitrary
   DNS lookup in the style of the CRIME attacks [CRIME].

   Implementations compliant with this profile MUST implement all of the
   following items:

   o  TLS session resumption without server-side state [RFC5077] which
      eliminates the need for the server to retain cryptographic state
      for longer than necessary.

   o  Raw public keys [RFC7250] which reduce the size of the
      ServerHello, and can be used by servers that cannot obtain
      certificates (e.g., DNS servers on private networks).

   Implementations compliant with this profile SHOULD implement all of
   the following items:

   o  TLS False Start [I-D.ietf-tls-falsestart] which reduces round-
      trips by allowing the TLS second flight of messages
      (ChangeCipherSpec) to also contain the (encrypted) DNS query

   o  Cached Information Extension [I-D.ietf-tls-cached-info] which
      avoids transmitting the server's certificate and certificate chain
      if the client has cached that information from a previous TLS

   [NOTE: The references to (works in progress) should be upgraded to
   MUST's if those references become RFC's prior to publication of this

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   Guidance specific to TLS is provided in [RFC7858] and that specific
   to DTLS it is provided in[I-D.ietf-dprive-dnsodtls].

12.  IANA Considerations

   This memo includes no request to IANA.

13.  Security Considerations

   Security considerations discussed in [RFC7525],
   [I-D.ietf-dprive-dnsodtls] and [RFC7858] apply to this document.

13.1.  Counter-measures to DNS Traffic Analysis

   This section makes suggestions for measures that can reduce the
   ability of attackers to infer information pertaining to encrypted
   client queries by other means (e.g. via an analysis of encrypted
   traffic size, or via monitoring of resolver to authoritative

   DNS-over-(D)TLS clients and servers SHOULD consider implementing the
   following relevant DNS extensions

   o  EDNS(0) padding [RFC7830], which allows encrypted queries and
      responses to hide their size.

   DNS-over-(D)TLS clients SHOULD consider implementing the following
   relevant DNS extensions

   o  Privacy Election using Client Subnet in DNS Queries [RFC7871].  If
      a DNS client does not include an EDNS0 Client Subnet Option with a
      SOURCE PREFIX-LENGTH set to 0 in a query, the DNS server may
      potentially leak client address information to the upstream
      authoritative DNS servers.  A DNS client ought to be able to
      inform the DNS Resolver that it does not want any address
      information leaked, and the DNS Resolver should honor that

14.  Acknowledgements

   Thanks to the authors of both [I-D.ietf-dprive-dnsodtls] and
   [RFC7858] for laying the ground work that this draft builds on and
   for reviewing the contents.  The authors would also like to thank
   John Dickinson, Shumon Huque, Melinda Shore, Gowri Visweswaran, Ray
   Bellis, Stephane Bortzmeyer, Jinmei Tatuya, Paul Hoffman and
   Christian Huitema for review and discussion of the ideas presented

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15.  References

15.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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,

   [RFC5077]  Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
              "Transport Layer Security (TLS) Session Resumption without
              Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
              January 2008, <http://www.rfc-editor.org/info/rfc5077>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

   [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,

   [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, <http://www.rfc-editor.org/info/rfc6125>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <http://www.rfc-editor.org/info/rfc6347>.

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
              2012, <http://www.rfc-editor.org/info/rfc6698>.

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   [RFC7250]  Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
              Weiler, S., and T. Kivinen, "Using Raw Public Keys in
              Transport Layer Security (TLS) and Datagram Transport
              Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
              June 2014, <http://www.rfc-editor.org/info/rfc7250>.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <http://www.rfc-editor.org/info/rfc7525>.

   [RFC7830]  Mayrhofer, A., "The EDNS(0) Padding Option", RFC 7830,
              DOI 10.17487/RFC7830, May 2016,

   [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, <http://www.rfc-editor.org/info/rfc7858>.

15.2.  Informative References

   [CRIME]    Rizzo, J. and T. Duong, "The CRIME Attack", 2012.

              NLnetLabs, "Dnssec-Trigger", May 2014,

              Reddy, T., Wing, D., and P. Patil, "DNS over DTLS
              (DNSoD)", draft-ietf-dprive-dnsodtls-06 (work in
              progress), April 2016.

              Santesson, S. and H. Tschofenig, "Transport Layer Security
              (TLS) Cached Information Extension", draft-ietf-tls-
              cached-info-23 (work in progress), May 2016.

              Shore, M., Barnes, R., Huque, S., and W. Toorop, "A DANE
              Record and DNSSEC Authentication Chain Extension for TLS",
              draft-ietf-tls-dnssec-chain-extension-00 (work in
              progress), June 2016.

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              Langley, A., Modadugu, N., and B. Moeller, "Transport
              Layer Security (TLS) False Start", draft-ietf-tls-
              falsestart-02 (work in progress), May 2016.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,

   [RFC2132]  Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor
              Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997,

   [RFC3646]  Droms, R., Ed., "DNS Configuration options for Dynamic
              Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
              DOI 10.17487/RFC3646, December 2003,

   [RFC7435]  Dukhovni, V., "Opportunistic Security: Some Protection
              Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
              December 2014, <http://www.rfc-editor.org/info/rfc7435>.

   [RFC7469]  Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
              Extension for HTTP", RFC 7469, DOI 10.17487/RFC7469, April
              2015, <http://www.rfc-editor.org/info/rfc7469>.

   [RFC7626]  Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626,
              DOI 10.17487/RFC7626, August 2015,

   [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,

Appendix A.  Server capability probing and caching by DNS clients

   This section presents a non-normative discussion of how DNS clients
   might probe for and cache privacy capabilities of DNS servers.

   Deployment of both DNS-over-TLS and DNS-over-DTLS will be gradual.
   Not all servers will support one or both of these protocols and the
   well-known port might be blocked by some middleboxes.  Clients will
   be expected to keep track of servers that support DNS-over-TLS and/or
   DNS-over-DTLS, and those that have been previously authenticated.

   If no server capability information is available then (unless
   otherwise specified by the configuration of the DNS client) DNS

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   clients that implement both TLS and DTLS should try to authenticate
   using both protocols before failing or falling back to a lower
   security.  DNS clients using opportunistic security should try all
   available servers (possibly in parallel) in order to obtain an
   authenticated encrypted connection before falling back to a lower
   security.  (RATIONALE: This approach can increase latency while
   discovering server capabilities but maximizes the chance of sending
   the query over an authenticated encrypted connection.)

Appendix B.  Changes between revisions

   [Note to RFC Editor: please remove this section prior to

B.1.  -02 version

   Introduction: Added paragraph on the background and scope of the

   Introduction and Discussion: Added more information on what a Usage
   profiles is (and is not) the the two presented here.

   Introduction: Added paragraph to make a comparison with the Strict
   profile in RFC7858 clearer.

   Section 4.2: Re-worked the description of Opportunistic and the

   Section 8.3: Clarified statement about use of DHCP in Opportunistic

   Title abbreviated.

B.2.  -01 version

   Section 4.2: Make clear that the Strict Privacy Profile can include
   meta queries performed using Opportunistic Privacy.

   Section 4.2, Table 1: Update to clarify that Opportunistic Privacy
   does not guarantee protection against passive attack.

   Section 4.2: Add sentence discussing client/provider trusted

   Section 5: Add more discussion of detection of active attacks when
   using Opportunistic Privacy.

   Section 8.2: Clarify description and example.

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B.3.  draft-ietf-dprive-dtls-and-tls-profiles-00

   Re-submission of draft-dgr-dprive-dtls-and-tls-profiles with name
   change to draft-ietf-dprive-dtls-and-tls-profiles.  Also minor nits

Authors' Addresses

   Sara Dickinson
   Sinodun Internet Technologies
   Magdalen Centre
   Oxford Science Park
   Oxford  OX4 4GA

   Email: sara@sinodun.com
   URI:   http://sinodun.com

   Daniel Kahn Gillmor
   125 Broad Street, 18th Floor
   New York  NY 10004

   Email: dkg@fifthhorseman.net

   Tirumaleswar Reddy
   Cisco Systems, Inc.
   Cessna Business Park, Varthur Hobli
   Sarjapur Marathalli Outer Ring Road
   Bangalore, Karnataka  560103

   Email: tireddy@cisco.com

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