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Versions: 00 draft-ietf-dprive-dtls-and-tls-profiles

dprive                                                      S. Dickinson
Internet-Draft                                                   Sinodun
Intended status: Standards Track                              D. Gillmor
Expires: June 25, 2016                                              ACLU
                                                                T. Reddy
                                                                   Cisco
                                                       December 23, 2015


  Authentication and (D)TLS Profile for DNS-over-TLS and DNS-over-DTLS
               draft-dgr-dprive-dtls-and-tls-profiles-00

Abstract

   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-
   DTLS.

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 June 25, 2016.

Copyright Notice

   Copyright (c) 2015 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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Background  . . . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  Usage Profiles  . . . . . . . . . . . . . . . . . . . . .   5
     4.3.  Authentication  . . . . . . . . . . . . . . . . . . . . .   6
       4.3.1.  DNS-over-(D)TLS Bootstrapping Problems  . . . . . . .   6
       4.3.2.  Credential Verification . . . . . . . . . . . . . . .   7
       4.3.3.  Implementation guidance . . . . . . . . . . . . . . .   7
   5.  Authentication in Opportunistic DNS-over(D)TLS Privacy  . . .   7
   6.  Authentication in Strict DNS-over(D)TLS Privacy . . . . . . .   7
   7.  In Band Source of Domain Name: SRV Service Label  . . . . . .   8
   8.  Out of Band Sources of Domain Name  . . . . . . . . . . . . .   8
     8.1.  Full direct configuration . . . . . . . . . . . . . . . .   8
     8.2.  Direct configuration of name only . . . . . . . . . . . .   8
     8.3.  DHCP  . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   9.  Credential Verification . . . . . . . . . . . . . . . . . . .   9
     9.1.  X.509 Certificate Based Authentication  . . . . . . . . .   9
     9.2.  DANE  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
       9.2.1.  Direct DNS Lookup . . . . . . . . . . . . . . . . . .  10
       9.2.2.  TLS DNSSEC Chain extension  . . . . . . . . . . . . .  11
   10. Combined Credentials with SPKI Pinsets  . . . . . . . . . . .  11
   11. (D)TLS Protocol Profile . . . . . . . . . . . . . . . . . . .  12
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  13
     13.1.  Counter-measures to DNS Traffic Analysis . . . . . . . .  13
   14. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     15.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Appendix A.  Server capability probing and caching by DNS clients  16
   Appendix B.  Changes between revisions  . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   The DPRIVE working group has two active documents that provide DNS
   privacy between DNS clients and DNS servers (to address the concerns
   in [RFC7626]):

   o  DNS-over-TLS [I-D.ietf-dprive-dns-over-tls]



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   o  DNS-over-DTLS [I-D.ietf-dprive-dnsodtls]

   This document defines usage profiles and authentication mechanisms
   for DTLS [RFC6347] and TLS [RFC5246] that specify how a DNS client
   should authenticate a DNS server based on a domain name.  In
   particular, it describes:

   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.

   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 profiles defined here.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   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 [I-D.ietf-dprive-dns-over-tls] 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.



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   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 [I-D.ietf-dprive-dns-over-tls] and DNS-
      over-DTLS [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: [I-D.ietf-dprive-dns-over-tls] 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
      [I-D.ietf-dprive-dns-over-tls].  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.




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

4.2.  Usage Profiles

   A DNS client has a choice of privacy usage profiles available.  This
   choice is briefly discussed in both [I-D.ietf-dprive-dns-over-tls]
   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 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
      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."

      In the best case scenario (authenticated and encrypted connection)
      this is equivalent to Strict Privacy, in the worst case (clear
      text connection) this is equivalent to No Privacy.  Clients will
      try for the best case but are willing to fallback to intermediate
      cases and eventually the worst case scenario in order to obtain a
      response.  This provides an undetermined privacy guarantee to the
      user depending on what kind of connection is actually used.  This
      is discussed in section Section 5

   o  No Privacy: the DNS client does not require or attempt to use
      either encryption or authentication.  Queries are always sent in
      clear text.  This provides no privacy guarantees to the client.








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      +-----------------------+------------------+-----------------+
      |     Usage Profile     | Passive Attacker | Active Attacker |
      +-----------------------+------------------+-----------------+
      |       No Privacy      |        N         |        N        |
      | Opportunistic Privacy |        P         |      N (D)      |
      |     Strict Privacy    |        P         |        P        |
      +-----------------------+------------------+-----------------+

      P == protection; N == no protection; D == detection is possible

   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 which is preferable to No
   Privacy.  However since the different profiles require varying levels
   of configuration (or a trusted relationship with a provider) DNS
   clients will need to carefully select which profile to use based on
   their communication privacy needs.

   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.





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4.3.2.  Credential Verification

   The use of SPKI pinset verification is discussed in
   [I-D.ietf-dprive-dns-over-tls].

   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.

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
   [I-D.ietf-dprive-dns-over-tls] which MAY be used for DNS-over-(D)TLS.

   DNS clients issuing queries under an opportunistic profile which know
   of a domain name for a 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, and for debugging or
   diagnostic purposes if there are means to report the result of the
   authentication attempt.  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
   [I-D.ietf-dprive-dns-over-tls].

   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



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   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.

   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.

   It can then use opportunistic DNS connections to untrusted DNS
   servers (e.g. provided by the local DHCP service) to establish the IP



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   address of the intended privacy-enabling DNS server by doing a lookup
   of SRV records.  Such records MUST be validated using DNSSEC.

   Example: For a DNSSEC validating DNS client configured in this way to
   do strict DNS privacy to dns.example.net, it would opportunistically
   look up the SRV for _domain-s._tcp.dns.example.net and determine
   addresses (via opportunistic A and/or AAAA lookups) for the resulting
   SRV response(s).  The records obtained during this process would only
   be used if they were validated by the client using DNSSEC.

   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.

   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.

   Note that this requires the client to trust the DHCP server, and to
   have a secured/authenticated connection to it.  Therefore this
   mechanism may be limited to only certain environments.  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.]

   [QUESTION: The authors would like to solicit feedback on the use of
   DHCP to determine whether to purse a new DHCP option in a later
   version of this draft, or defer it.]

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



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   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
   server.

   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

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




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   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.shore-tls-dnssec-chain-extension].  If the DNS server supports
   this extension, it can provide the full chain to the client in the
   handshake.

   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
   credentials.

   [ 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 [I-D.ietf-dprive-dns-over-tls]
   MAY be used with DNS-over-(D)TLS.

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







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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
   later.

   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
      handshake

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

   Guidance specific to TLS or DTLS is provided in either
   [I-D.ietf-dprive-dnsodtls] or [I-D.ietf-dprive-dns-over-tls].



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12.  IANA Considerations

   This memo includes no request to IANA.

13.  Security Considerations

   Security considerations discussed in [RFC7525],
   [I-D.ietf-dprive-dnsodtls] and [I-D.ietf-dprive-dns-over-tls] 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
   traffic).

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

   o  EDNS(0) padding [I-D.ietf-dprive-edns0-padding], 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
      [I-D.ietf-dnsop-edns-client-subnet].  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 request.

14.  Acknowledgements

   Thanks to the authors of both [I-D.ietf-dprive-dnsodtls] and
   [I-D.ietf-dprive-dns-over-tls] 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 and Ray Bellis for review and discussion of the ideas
   presented here.







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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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc4985>.

   [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,
              <http://www.rfc-editor.org/info/rfc5246>.

   [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,
              <http://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, <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>.

15.2.  Informative References

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

   [I-D.ietf-dnsop-edns-client-subnet]
              Contavalli, C., Gaast, W., tale, t., and W. Kumari,
              "Client Subnet in DNS Queries", draft-ietf-dnsop-edns-
              client-subnet-06 (work in progress), December 2015.

   [I-D.ietf-dprive-dns-over-tls]
              Zi, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
              and P. Hoffman, "DNS over TLS: Initiation and Performance
              Considerations", draft-ietf-dprive-dns-over-tls-02 (work
              in progress), December 2015.

   [I-D.ietf-dprive-dnsodtls]
              Reddy, T., Wing, D., and P. Patil, "DNS over DTLS
              (DNSoD)", draft-ietf-dprive-dnsodtls-03 (work in
              progress), November 2015.

   [I-D.ietf-dprive-edns0-padding]
              Mayrhofer, A., "The EDNS(0) Padding Option", draft-ietf-
              dprive-edns0-padding-01 (work in progress), November 2015.

   [I-D.ietf-tls-cached-info]
              Santesson, S. and H. Tschofenig, "Transport Layer Security
              (TLS) Cached Information Extension", draft-ietf-tls-
              cached-info-21 (work in progress), December 2015.

   [I-D.ietf-tls-falsestart]
              Langley, A., Modadugu, N., and B. Moeller, "Transport
              Layer Security (TLS) False Start", draft-ietf-tls-
              falsestart-01 (work in progress), November 2015.






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   [I-D.shore-tls-dnssec-chain-extension]
              Shore, M., Barnes, R., Huque, S., and W. Toorop, "A DANE
              Record and DNSSEC Authentication Chain Extension for TLS",
              draft-shore-tls-dnssec-chain-extension-02 (work in
              progress), October 2015.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,
              <http://www.rfc-editor.org/info/rfc2131>.

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

   [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,
              <http://www.rfc-editor.org/info/rfc7626>.

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
   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.)




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Appendix B.  Changes between revisions

   [Note to RFC Editor: please remove this section prior to
   publication.]

Authors' Addresses

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

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


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

   Email: dkg@fifthhorseman.net


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

   Email: tireddy@cisco.com
















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