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Versions: (draft-dgr-dprive-dtls-and-tls-profiles) 00 01 02 03 04 05 06 07 08 09 10 11

dprive                                                      S. Dickinson
Internet-Draft                                                   Sinodun
Intended status: Standards Track                              D. Gillmor
Expires: July 22, 2017                                              ACLU
                                                                T. Reddy
                                                                   Cisco
                                                        January 18, 2017


         Authentication and (D)TLS Profile for DNS-over-(D)TLS
               draft-ietf-dprive-dtls-and-tls-profiles-08

Abstract

   This document discusses Usage Profiles, based on one or more
   authentication mechanisms, which can be used for DNS over Transport
   Layer Security (TLS) or Datagram TLS (DTLS).  This document also
   specifies new authentication mechanisms - it describes several ways a
   DNS client can use an authentication domain name to authenticate a
   DNS server.  Additionally, it defines (D)TLS profiles for DNS clients
   and servers implementing DNS-over-(D)TLS.

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 July 22, 2017.

Copyright Notice

   Copyright (c) 2017 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



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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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .   6
   5.  Usage Profiles  . . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  DNS Resolution  . . . . . . . . . . . . . . . . . . . . .   9
   6.  Authentication in DNS-over(D)TLS  . . . . . . . . . . . . . .   9
     6.1.  DNS-over-(D)TLS Startup Configuration Problems  . . . . .   9
     6.2.  Credential Verification . . . . . . . . . . . . . . . . .  10
     6.3.  Summary of Authentication Mechanisms  . . . . . . . . . .  10
     6.4.  Combining Authentication Mechanisms . . . . . . . . . . .  13
     6.5.  Authentication in Opportunistic Privacy . . . . . . . . .  13
     6.6.  Authentication in Strict Privacy  . . . . . . . . . . . .  13
     6.7.  Implementation guidance . . . . . . . . . . . . . . . . .  14
   7.  Sources of Authentication Domain Names  . . . . . . . . . . .  14
     7.1.  In Band Sources: SRV Service Label  . . . . . . . . . . .  14
     7.2.  Out of Band Sources . . . . . . . . . . . . . . . . . . .  15
       7.2.1.  Full direct configuration . . . . . . . . . . . . . .  15
       7.2.2.  Direct configuration of name only . . . . . . . . . .  15
       7.2.3.  DHCP  . . . . . . . . . . . . . . . . . . . . . . . .  16
   8.  Authentication Domain Name based Credential Verification  . .  16
     8.1.  PKIX Certificate Based Authentication . . . . . . . . . .  16
     8.2.  DANE  . . . . . . . . . . . . . . . . . . . . . . . . . .  17
       8.2.1.  Direct DNS Lookup . . . . . . . . . . . . . . . . . .  18
       8.2.2.  TLS DNSSEC Chain extension  . . . . . . . . . . . . .  18
   9.  (D)TLS Protocol Profile . . . . . . . . . . . . . . . . . . .  18
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  19
     11.1.  Counter-measures to DNS Traffic Analysis . . . . . . . .  19
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  20
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  20
     13.2.  Informative References . . . . . . . . . . . . . . . . .  21
   Appendix A.  Server capability probing and caching by DNS clients  23
   Appendix B.  Changes between revisions  . . . . . . . . . . . . .  23
     B.1.  -08 version . . . . . . . . . . . . . . . . . . . . . . .  23
     B.2.  -07 version . . . . . . . . . . . . . . . . . . . . . . .  23
     B.3.  -06 version . . . . . . . . . . . . . . . . . . . . . . .  24
     B.4.  -05 version . . . . . . . . . . . . . . . . . . . . . . .  24
     B.5.  -04 version . . . . . . . . . . . . . . . . . . . . . . .  24



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     B.6.  -03 version . . . . . . . . . . . . . . . . . . . . . . .  25
     B.7.  -02 version . . . . . . . . . . . . . . . . . . . . . . .  25
     B.8.  -01 version . . . . . . . . . . . . . . . . . . . . . . .  25
     B.9.  draft-ietf-dprive-dtls-and-tls-profiles-00  . . . . . . .  25
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  26

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'.  Note that this document has the
      Intended status of Experimental.

   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 was
   out of scope for the Working Group charter at the time this document
   was finished.

   This document specifies 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.  Section 5 presents a generalised
   discussion of Usage Profiles by separating the Usage Profile, which
   is based purely on the security properties it offers the user, from
   the specific mechanism(s) that are used for authentication.  The
   Profiles described are:

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

   The above Usage Profiles attempt authentication of the server using
   at least one authentication mechanism.  Section 6.4 discusses how to
   combine authentication mechanisms to determine the overall
   authentication result.  Depending on that overall authentication



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   result (and whether encryption is available) the Usage Profile will
   determine if the connection should proceed, fallback or fail.

   One authentication mechanism is already described in [RFC7858].  That
   document specifies an SPKI based authentication mechanism for DNS-
   over-TLS in the context of a specific case of a Strict Usage Profile
   using that single authentication mechanism.  Therefore the "Out-of-
   band Key-pinned Privacy Profile" described in [RFC7858] would qualify
   as a "Strict Usage Profile" that used SPKI pinning for
   authentication.

   This document extends the use of SPKI pinset based authentication so
   that it is considered a general authentication mechanism that can be
   used with either DNS-over-(D)TLS Usage Profile.  That is, the SPKI
   pinset mechanism described in [RFC7858] MAY be used with DNS-
   over-(D)TLS.

   This document also describes a number of additional authentication
   mechanisms all of which specify how a DNS client should authenticate
   a DNS server based on an 'authentication domain name'.  In
   particular, the following is described:

   o  How a DNS client can obtain the combination of an authentication
      domain name and IP address for a DNS server.  See Section 7.

   o  What are the acceptable credentials a DNS server can present to
      prove its identity for (D)TLS authentication based on a given
      authentication domain name.  See Section 8.

   o  How a DNS client can verify that any given credential matches the
      authentication domain name obtained for a DNS server.  See
      Section 8.

   In Section 9 this document 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 all currently specified authentication mechanisms.

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:





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   o  DNS client: a DNS stub resolver or forwarder.  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.  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 8.

      *  Implements the (D)TLS profile described in Section 9.

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

   o  Authentication domain name: A domain name that can be used to
      authenticate a DNS Privacy enabling server.  Sources of
      authentication domain names are discussed in Section 7.

   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  Authentication information: Information a DNS client may use as
      the basis of an authentication mechanism.  In this context that
      can be either a:

      *  a SPKI pinset or

      *  an authentication domain name

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



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

      *  PKIX certificate

      *  DNSSEC validated chain to a TLSA record

      but may also include SPKI pinsets.

3.  Scope

   This document is limited to describing

   o  Usage Profiles based on general authentication mechanisms

   o  The details of domain-name-based authentication of DNS servers by
      DNS clients (as defined in the terminology section)

   o  The (D)TLS profiles needed to support authentication in DNS-
      over-(D)TLS.

   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  The details of how to perform SPKI-pinset-based authentication.
      This is defined in [RFC7858].

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

4.  Discussion

   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.

   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



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   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 future work.

5.  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].  These 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 authentication 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 Profile is discussed in detail in
      Section 6.6.

   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

      *  an encrypted connection

      *  a clear text connection

      depending on the fallback logic of the client, the available
      authentication information and the capabilities of the DNS Server.



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      In all these cases the DNS client is willing to continue with a
      connection to the DNS Server and perform resolution of queries.

   To compare the two Usage profiles the table below shows successful
   Strict Privacy along side the 3 possible outcomes of Opportunistic
   Privacy.  In the best case scenario for Opportunistic Privacy (an
   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 the intermediate case 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 Security to notify
   the user what type of connection is actually used, the 'detection'
   described below is only possible if such connection information is
   available.  However, if it is available and the user is informed that
   an unencrypted connection was used to connect to a server then the
   user should assume (detect) that the connection is subject to both
   active and passive attack since the DNS queries are sent in clear
   text.  This might be particularly useful if a new connection to a
   certain server is unencrypted when all previous connections were
   encrypted.  Similarly if the user is informed that an encrypted but
   unauthenticated connection was used then user can detect that the
   connection may be subject to active attack.  This is discussed in
   Section 6.5.

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

   Strict Privacy provides the strongest privacy guarantees and
   therefore SHOULD always be implemented in DNS clients along with
   Opportunistic Privacy.

   A DNS client that implements DNS-over-(D)TLS SHOULD NOT default to
   the use of clear text (no privacy).



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   The choice between the two profiles depends on a number of factors
   including which is more important to the particular client:

   o  DNS service at the cost of no privacy guarantee (Opportunistic) or

   o  guaranteed privacy at the potential cost of no DNS service
      (Strict).

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

   A DNS server that implements DNS-over-TLS SHOULD provide at least one
   credential in order that those DNS clients that wish to do so are
   able to use Strict Privacy (see Section 2).

5.1.  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.

6.  Authentication in DNS-over(D)TLS

   This section describes authentication mechanisms and how they can be
   used in either Strict or Opportunistic Privacy for DNS-over-(D)TLS.

6.1.  DNS-over-(D)TLS Startup Configuration Problems

   Many (D)TLS clients use PKIX authentication [RFC6125] based on an
   authentication 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.  Such a DNS client therefore has a
   bootstrap problem.  DNS clients typically know only the IP address of
   a DNS server.

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

   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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6.2.  Credential Verification

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

   In terms of domain name based verification, once an authentication
   domain name is known for a DNS server a choice of authentication
   mechanisms can be used for credential verification.  Section 8
   discusses these mechanisms in detail, namely PKIX 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 8.2.

6.3.  Summary of Authentication Mechanisms

   This section provides an overview of the various authentication
   mechanisms.  The table below indicates how the DNS client obtains
   information to use for authentication for each option; either
   statically via direct configuration or dynamically.  Of course, the
   Opportunistic Usage Profile does not require authentication and so a
   client using that profile may choose to connect to a Privacy Enabling
   DNS server on the basis of just an IP address.




























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   +---+------------+-------------+------------------------------------+
   | # | Static     | Dynamically | Short name: Description            |
   |   | Config     | Obtained    |                                    |
   +---+------------+-------------+------------------------------------+
   | 1 | SPKI + IP  |             | SPKI: SPKI pinset(s) and IP        |
   |   |            |             | address obtained out of band       |
   |   |            |             | [RFC7858]                          |
   |   |            |             |                                    |
   | 2 | ADN + IP   |             | ADN: ADN and IP address obtained   |
   |   |            |             | out of band (see Section 7.2.1)    |
   |   |            |             |                                    |
   | 3 | ADN        | IP          | ADN only: DNSSEC validated         |
   |   |            |             | Opportunistic lookups to a NP DNS  |
   |   |            |             | server for SRV, A/AAAA (see        |
   |   |            |             | Section 7.2.2)                     |
   |   |            |             |                                    |
   | 4 |            | ADN + IP    | DHCP: DHCP configuration only (see |
   |   |            |             | Section 7.2.3)                     |
   |   |            |             |                                    |
   | 5 | [ADN + IP] | [ADN + IP]  | DANE: DNSSEC chain obtained via    |
   |   |            | TLSA record | Opportunistic lookups to NP DNS    |
   |   |            |             | server (see Section 8.2.1)         |
   |   |            |             |                                    |
   | 6 | [ADN + IP] | [ADN + IP]  | TLS extension: DNSSEC chain        |
   |   |            | TLSA record | provided by PE DNS server in TLS   |
   |   |            |             | DNSSEC chain extension (see        |
   |   |            |             | Section 8.2.2)                     |
   +---+------------+-------------+------------------------------------+

      SPKI == SPKI pinset(s), IP == IP Address, ADN == Authentication
    Domain Name, NP == Network provided, PE == Privacy-enabling, [ ] ==
           Data may be obtained either statically or dynamically

              Table 2: Overview of Authentication Mechanisms

   The following summary attempts to present some key attributes of each
   of the mechanisms (using the 'Short name' from Table 2), indicating
   attractive attributes with a '+' and undesirable attributes with a
   '-'.

   1.  SPKI

          + Minimal leakage (Note that the ADN is always leaked in the
          SNI field in the Client Hello in TLS when communicating with a
          privacy enabling DNS server)

          - Overhead of on-going key management required




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   2.  ADN

          + Minimal leakage

          + One-off direct configuration only

   3.  ADN only

          + Minimal one-off direct configuration, only a human
          recognisable domain name needed

          - SRV, A/AAAA meta queries leaked to network provided DNS
          server

   4.  DHCP

          + No static config

          - Requires a non-standard or future DHCP option in order to
          provide the ADN

          - Requires secure and trustworthy connection to DHCP server

   5.  DANE

          The ADN and/or IP may be obtained statically or dynamically
          and the relevant attributes of that method apply

          + DANE options (e.g. matching on entire certificate)

          - Requires a DNSSEC validating stub implementation (deployment
          of which is limited at the time of writing)

          - DNSSEC chain meta queries leaked to network provided DNS
          server

   6.  TLS extension

          The ADN and/or IP may be obtained statically or dynamically
          and the relevant attributes of that method apply

          + Reduced latency compared with 'DANE'

          + No network provided DNS server required if ADN and IP
          statically configured

          + DANE options (e.g. matching on entire certificate)




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          - Requires a DNSSEC validating stub implementation

6.4.  Combining Authentication Mechanisms

   This draft does not make explicit recommendations about how an 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 an authentication 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 an authentication domain
   name and a SPKI pinset for a DNS server SHOULD match on both a valid
   credential for the authentication domain name and a valid SPKI pinset
   (if both are available) when connecting to that DNS server.  The
   overall authentication result should only be considered successful if
   both authentication mechanisms are successful.

6.5.  Authentication in Opportunistic 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
   authentication information 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.

6.6.  Authentication in Strict Privacy

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





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   A DNS client requiring Strict Privacy MUST either use one of the
   sources listed in Section 7.2 to obtain an authentication 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 at least one piece of authentication
   information is known.  The client MUST use the available verification
   mechanisms described in Section 8 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.

6.7.  Implementation guidance

   Section 9 describes the (D)TLS profile for DNS-over(D)TLS.
   Additional considerations relating to general implementation
   guidelines are discussed in both Section 11 and in Appendix A.

7.  Sources of Authentication Domain Names

7.1.  In Band Sources: 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.






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7.2.  Out of Band Sources

7.2.1.  Full direct configuration

   DNS clients may be directly and securely provisioned with the
   authentication 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.

7.2.2.  Direct configuration of name only

   A DNS client may be configured directly and securely with only the
   authentication 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.

   Example:

   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
         name

   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.



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7.2.3.  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 authentication 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 authentication domain names MUST be
   considered secure and trustworthy.  This document does not attempt to
   describe secured and trusted relationships to DHCP servers.

   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.

8.  Authentication Domain Name based Credential Verification

8.1.  PKIX Certificate Based Authentication

   When a DNS client configured with an authentication domain name
   connects to its configured DNS server over (D)TLS, the server may
   present it with an PKIX 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 authentication domain name: A DNS-ID and an SRV-ID
   [RFC4985].  The DNS-ID is simply the authentication domain name
   itself.  The SRV-ID uses a "_domain-s." prefix.  So if the configured
   authentication 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 PKIX
   certificate's subjectAltName extension as described in section 6 of



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   [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.

8.2.  DANE

   DANE [RFC6698] provides mechanisms to root certificate and raw public
   key trust with DNSSEC.  However this requires the DNS client to have
   an authentication domain name for the DNS Privacy Server which must
   be obtained via a trusted source.

   This section assumes a solid understanding of both DANE [RFC6698] and
   DANE Operations [RFC7671].  A few pertinent issues covered in these
   documents are outlined here as useful pointers, but familiarity with
   both these documents in their entirety is expected.

   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."

   It is noted that [RFC7671] covers the following topics:

   o  Section 4.1: Opportunistic Security and PKIX Usages and
      Section 14: Security Considerations, which both discuss the use of
      PKIX-TA(0) and PKIX-EE(1) for OS.

   o  Section 5: Certificate-Usage-Specific DANE Updates and Guidelines.
      Specifically Section 5.1 which outlines the combination of
      Certificate Usage DANE-EE(3) and Selector Usage SPKI(1) with Raw
      Public Keys [RFC7250].  Section 5.1 also discusses the security
      implications of this mode, for example, it discusses key lifetimes
      and specifies that validity period enforcement is based solely on
      the TLSA RRset properties for this case.

   o  Section 13: Operational Considerations, which discusses TLSA TTLs
      and signature validity periods.

   The specific DANE record for a DNS Privacy Server would take the
   form:

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




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

8.2.1.  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].

8.2.2.  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
   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.

9.  (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:




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   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 [RFC7918] which reduces round-trips by allowing
      the TLS second flight of messages (ChangeCipherSpec) to also
      contain the (encrypted) DNS query

   o  Cached Information Extension [RFC7924] which avoids transmitting
      the server's certificate and certificate chain if the client has
      cached that information from a previous TLS handshake

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

10.  IANA Considerations

   This memo includes no request to IANA.

11.  Security Considerations

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

11.1.  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 [RFC7830], which allows encrypted queries and
      responses to hide their size.

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




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   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
      request.

12.  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, Christian
   Huitema and John Levine for review and discussion of the ideas
   presented here.

13.  References

13.1.  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>.



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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, <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>.

   [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>.

   [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, <http://www.rfc-editor.org/info/rfc7671>.

   [RFC7830]  Mayrhofer, A., "The EDNS(0) Padding Option", RFC 7830,
              DOI 10.17487/RFC7830, May 2016,
              <http://www.rfc-editor.org/info/rfc7830>.

   [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>.

13.2.  Informative References

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






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   [dnssec-trigger]
              NLnetLabs, "Dnssec-Trigger", May 2014,
              <https://www.nlnetlabs.nl/projects/dnssec-trigger/>.

   [I-D.ietf-dprive-dnsodtls]
              Reddy, T., Wing, D., and P. Patil, "Specification for DNS
              over Datagram Transport Layer Security (DTLS)", draft-
              ietf-dprive-dnsodtls-15 (work in progress), December 2016.

   [I-D.ietf-tls-dnssec-chain-extension]
              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-02 (work in
              progress), January 2017.

   [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>.

   [RFC3646]  Droms, R., Ed., "DNS Configuration options for Dynamic
              Host Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
              DOI 10.17487/RFC3646, December 2003,
              <http://www.rfc-editor.org/info/rfc3646>.

   [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>.

   [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,
              <http://www.rfc-editor.org/info/rfc7871>.







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   [RFC7918]  Langley, A., Modadugu, N., and B. Moeller, "Transport
              Layer Security (TLS) False Start", RFC 7918,
              DOI 10.17487/RFC7918, August 2016,
              <http://www.rfc-editor.org/info/rfc7918>.

   [RFC7924]  Santesson, S. and H. Tschofenig, "Transport Layer Security
              (TLS) Cached Information Extension", RFC 7924,
              DOI 10.17487/RFC7924, July 2016,
              <http://www.rfc-editor.org/info/rfc7924>.

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.)

Appendix B.  Changes between revisions

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

B.1.  -08 version

   Removed hard failure as an option for Opportunistic Usage profile.

   Added a new section comparing the Authentication Mechanisms

B.2.  -07 version

   Re-work of the Abstract and Introduction to better describe the
   contents in this version.




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   Terminology: New definition of 'authentication information'.

   Scope: Changes to the Scope section.

   Moved discussion of combining authentication mechanism earlier.

   Changes to the section headings and groupings to make the
   presentation more logical.

B.3.  -06 version

   Introduction: Re-word discussion of Working group charter.

   Introduction: Re-word first and third bullet point about 'obtaining'
   a domain name and IP address.

   Introduction: Update reference to DNS-over-TLS draft.

   Terminology: Change forwarder/proxy to just forwarder

   Terminology: Add definition of 'Authentication domain name' and use
   this throughout

   Section 4.2: Remove parenthesis in the table.

   Section 4.2: Change the text after the table as agreed with Paul
   Hoffman.

   Section 4.3.1: Change title and remove brackets around last
   statement.

   Section 11: Split second paragraph.

B.4.  -05 version

   Add more details on detecting passive attacks to section 4.2

   Changed X.509 to PKIX throughout

   Change comment about future I-D on usage policies.

B.5.  -04 version

   Introduction: Add comment that DNS-over-DTLS draft is Experiments

   Update 2 I-D references to RFCs.





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B.6.  -03 version

   Section 9: Update DANE section with better references to RFC7671 and
   RFC7250

B.7.  -02 version

   Introduction: Added paragraph on the background and scope of the
   document.

   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
   table.

   Section 8.3: Clarified statement about use of DHCP in Opportunistic
   profile

   Title abbreviated.

B.8.  -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
   relationships.

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

   Section 8.2: Clarify description and example.

B.9.  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
   fixed.






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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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