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Versions: (draft-wing-dprive-dnsodtls) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 RFC 8094

DPRIVE                                                          T. Reddy
Internet-Draft                                                   D. Wing
Intended status: Standards Track                                P. Patil
Expires: April 20, 2016                                            Cisco
                                                        October 18, 2015


                         DNS over DTLS (DNSoD)
                     draft-ietf-dprive-dnsodtls-02

Abstract

   DNS queries and responses are visible to network elements on the path
   between the DNS client and its server.  These queries and responses
   can contain privacy-sensitive information which is valuable to
   protect.  An active attacker can send bogus responses causing
   misdirection of the subsequent connection.

   To counter passive listening and active attacks, this document
   proposes the use of Datagram Transport Layer Security (DTLS) for DNS,
   to protect against passive listeners and certain active attacks.  As
   DNS needs to remain fast, this proposal also discusses mechanisms to
   reduce DTLS round trips and reduce DTLS handshake size.  The proposed
   mechanism runs over port 853.

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 April 20, 2016.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.





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   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
   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.  Relationship to TCP Queries and to DNSSEC . . . . . . . . . .   3
   3.  Common problems with DNS Privacy  . . . . . . . . . . . . . .   3
     3.1.  Firewall Blocking Ports or DNS Privacy Protocol . . . . .   3
     3.2.  Authenticating the DNS Privacy Server . . . . . . . . . .   4
     3.3.  Downgrade attacks . . . . . . . . . . . . . . . . . . . .   5
   4.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Incremental Deployment  . . . . . . . . . . . . . . . . . . .   6
   6.  DTLS session initiation, Polling and Discovery  . . . . . . .   6
   7.  Performance Considerations  . . . . . . . . . . . . . . . . .   7
   8.  Established sessions  . . . . . . . . . . . . . . . . . . . .   8
   9.  Fragmentation and Reassembly  . . . . . . . . . . . . . . . .   9
     9.1.  Generating fragmented packets . . . . . . . . . . . . . .  10
     9.2.  Receiving fragmented packets  . . . . . . . . . . . . . .  11
     9.3.  The DNS-fragment Extension  . . . . . . . . . . . . . . .  12
   10. Anycast . . . . . . . . . . . . . . . . . . . . . . . . . . .  13
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  14
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     14.2.  Informative References . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   The Domain Name System is specified in [RFC1034] and [RFC1035].  DNS
   queries and responses are normally exchanged unencrypted and are thus
   vulnerable to eavesdropping.  Such eavesdropping can result in an
   undesired entity learning domains that a host wishes to access, thus
   resulting in privacy leakage.  DNS privacy problem is further
   discussed in [I-D.bortzmeyer-dnsop-dns-privacy].

   Active attackers have long been successful at injecting bogus
   responses, causing cache poisoning and causing misdirection of the
   subsequent connection (if attacking A or AAAA records).  A popular



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   mitigation against that attack is to use ephemeral and random source
   ports for DNS queries [RFC5452].

   This document defines DNS over DTLS (DNSoD, pronounced "dee-enn-sod")
   which provides confidential DNS communication for stub resolvers,
   recursive resolvers, iterative resolvers and authoritative servers.

   The motivations for proposing DNSoD are that

   o  TCP suffers from network head-of-line blocking, where the loss of
      a packet causes all other TCP segments to not be delivered to the
      application until the lost packet is re-transmitted.  DNSoD,
      because it uses UDP, does not suffer from network head-of-line
      blocking.

   o  DTLS session resumption consumes 1 round trip whereas TLS session
      resumption can start only after TCP handshake is complete.
      Although TCP Fast Open [RFC7413] can reduce that handshake, TCP
      Fast Open is not yet available in commercially-popular operating
      systems.

2.  Relationship to TCP Queries and to DNSSEC

   DNS queries can be sent over UDP or TCP.  The scope of this document,
   however, is only UDP.  DNS over TCP could be protected with TLS, as
   described in [I-D.ietf-dprive-dns-over-tls].  Alternatively, a shim
   protocol could be defined between DTLS and DNS, allowing large
   responses to be sent over DTLS itself, see Section 7.

   DNS Security Extensions (DNSSEC [RFC4033]) provides object integrity
   of DNS resource records, allowing end-users (or their resolver) to
   verify legitimacy of responses.  However, DNSSEC does not protect
   privacy of DNS requests or responses.  DNSoD works in conjunction
   with DNSSEC, but DNSoD does not replace the need or value of DNSSEC.

3.  Common problems with DNS Privacy

   This section describes problems common to any DNS privacy solution.
   To achieve DNS privacy an encrypted and integrity-protected channel
   is needed between the client and server.  This channel can be
   blocked, and the client needs to react to such blockages.

3.1.  Firewall Blocking Ports or DNS Privacy Protocol

   When sending DNS over an encrypted channel, there are two choices:
   send the encrypted traffic over the DNS ports (UDP 53, TCP 53) or
   send the encrypted traffic over a different port.  The encrypted
   traffic is not normal DNS traffic, but rather is a cryptographic



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   handshake followed by encrypted payloads.  There can be firewalls,
   other security devices, or intercepting DNS proxies which block the
   non-DNS traffic or otherwise react negatively (e.g., quarantining the
   host for suspicious behavior).  Alternatively, if a different port is
   used for the encrypted traffic, a firewall or other security device
   might block that port or otherwise react negatively.

   There is no panacea, and only experiments on the Internet will
   uncover which technique or combination of techniques will work best.
   This document describes using DNSoD on a well-known port.

3.2.  Authenticating the DNS Privacy Server

   DNS privacy requires encrypting the query (and response) from passive
   attacks.  Such encryption typically provides integrity protection as
   a side-effect, which means on-path attackers cannot simply inject
   bogus DNS responses.  However, to provide stronger protection from
   active attackers pretending to be the server, the server itself needs
   to be authenticated.

   To authenticate the server providing DNS privacy, the DNS client
   needs to be configured with the names or IP addresses of those DNS
   privacy servers.  The server certificate MUST contain DNS-ID
   (subjectAltName) as described in Section 4.1 of [RFC6125].  DNS names
   and IP addresses can be contained in the subjectAltName entries.  The
   client MUST use the rules and guidelines given in section 6 of
   [RFC6125] to validate the DNS server identity.

   We imagine this could be implemented by adding the certificate name
   to the /etc/resolv.conf file, such as below:

     nameserver 8.8.8.8
     certificate google-public-dns.google.com
     nameserver 208.67.220.220
     certificate resolver.opendns.com

   For DNS privacy servers that don't have a certificate trust chain
   (e.g., because they are on a home network or a corporate network),
   the configured list of DNS privacy servers can contain the Subject
   Public Key Info (SPKI) fingerprint of the DNS privacy server (i.e., a
   simple whitelist of name and SPKI fingerprint).  The public key is
   used for the same reasons HTTP pinning [RFC7469] uses the public key.
   Raw public key-based authentication mechanism defined in [RFC7250]
   can be also used to authenticate the DNS server.







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   We imagine this could be implemented by adding the SPKI fingerprint
   to the /etc/resolv.conf file, such as below (line split for Internet
   Draft formatting):

     nameserver 192.168.1.1
     sha256 : "d6qzRu9zOECb90Uez27xWltNsj0e1Md7GkYYkVoZWmM="

   The only algorithm considered at this time is "sha256", i.e., the
   hash algorithm SHA256 [RFC6234]; additional algorithms may be allowed
   for use in this context in the future.  The quoted-string is a
   sequence of base 64 digits: the base64-encoded SPKI Fingerprint
   [RFC4648].

3.3.  Downgrade attacks

   Using DNS privacy with an authenticated server is most preferred, DNS
   privacy with an unauthenticated server is next preferred, and plain
   DNS is least preferred.  This section gives a non-normative
   discussion on common behaviors and choices.

   An implementation MAY attempt to obtain DNS privacy by contacting DNS
   servers on the local network (provided by DHCP) and on the Internet,
   and make those attempts in parallel to reduce user impact.  If DNS
   privacy cannot be successfully negotiated for whatever reason, the
   client can do three things:

   1.  refuse to send DNS queries on this network, which means the
       client cannot make effective use of this network, as modern
       networks require DNS; or,

   2.  use opportunistic security, as described in [RFC7435]. or,

   3.  send plain DNS queries on this network, which means no DNS
       privacy is provided.

   Heuristics can improve this situation, but only to a degree (e.g.,
   previous success of DNS privacy on this network may be reason to
   alert the user about failure to establish DNS privacy on this network
   now).  Still, the client (in cooperation with the end user) has to
   decide to use the network without the protection of DNS privacy.

4.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].




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5.  Incremental Deployment

   DNSoD can be deployed incrementally by the Internet Service Provider
   or as an Internet service.

   If the ISP's DNS resolver supports DNSoD, then DNS queries are
   protected from passive listening and from many active attacks along
   that path.

   DNSoD can be offered as an Internet service, and a stub resolver or
   DNS resolver can be configured to point to that DNSoD server (rather
   than to the ISP-provided DNS server).

6.  DTLS session initiation, Polling and Discovery

   Many modern operating systems already detect if a web proxy is
   interfering with Internet communications, using proprietary
   mechanisms that are out of scope of this document.  After that
   mechanism has run (and detected Internet connectivity is working),
   the DNSoD procedure described in this document should commence.  This
   timing avoids delays in joining the network (and displaying an icon
   indicating successful Internet connection), at the risk that those
   initial DNS queries will be sent without protection afforded by
   DNSoD.

   DNSoD MUST run over standard UDP port 853 as defined in Section 11.
   A DNS server that supports DNSoD MUST listen for and accept DTLS
   packets on a designated port 853.

   The host should determine if the DNS server supports DNSoD by sending
   a DTLS ClientHello message.  A DNS server that does not support DNSoD
   will not respond to ClientHello messages sent by the client.  The
   client MUST use timer values defined in Section 4.2.4.1 of [RFC6347]
   for retransmission of ClientHello message and if no response is
   received from the DNS server.  After 15 seconds, it MUST cease
   attempts to re-transmit its ClientHello.  If the DNS client receives
   a hard ICMP error [RFC1122], it MUST immediately cease attempts to
   re-transmit its ClientHello.  Thereafter, the client MAY repeat that
   procedure in the event the DNS server has been upgraded to support
   DNSoD, but such probing SHOULD NOT be done more frequently than every
   24 hours and MUST NOT be done more frequently than every 15 minutes.
   This mechanism requires no additional signaling between the client
   and server.








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7.  Performance Considerations

   To reduce number of octets of the DTLS handshake, especially the size
   of the certificate in the ServerHello (which can be several
   kilobytes), DNS client and server can use raw public keys [RFC7250]
   or Cached Information Extension [I-D.ietf-tls-cached-info].  Cached
   Information Extension avoids transmitting the server's certificate
   and certificate chain if the client has cached that information from
   a previous TLS handshake.

   Multiple DNS queries can be sent over a single DTLS session and the
   DNSoD client need not wait for an outstanding reply before sending
   the next query.  The existing Query ID allows multiple requests and
   responses to be interleaved in whatever order they can be fulfilled
   by the DNS server.  This means DNSoD reduces the consumption of UDP
   port numbers, and because DTLS protects the communication between the
   DNS client and its server, the resolver SHOULD NOT use random
   ephemeral source ports (Section 9.2 of [RFC5452]) because such source
   port use would incur additional, unnecessary DTLS load on the DNSoD
   server.  When sending multiple queries over a single DTLS session,
   clients MUST take care to avoid Message ID collisions.  In other
   words, they MUST not re-use the DNS Message ID of an in-flight query.

   It is highly advantageous to avoid server-side DTLS state and reduce
   the number of new DTLS sessions on the server which can be done with
   [RFC5077].  This also eliminates a round-trip for subsequent DNSoD
   queries, because with [RFC5077] the DTLS session does not need to be
   re-established.

   Compared to normal DNS, DTLS adds at least 13 octets of header, plus
   cipher and authentication overhead to every query and every response.
   This reduces the size of the DNS payload that can be carried.
   Certain DNS responses are large (e.g., many AAAA records, TXT, SRV)
   and don't fit into a single UDP packet, causing a partial response
   with the truncation (TC) bit set.  The client is then expected to
   repeat the query over TCP, which causes additional name resolution
   delay.  We have considered two ideas, one that reduces the need to
   switch to TCP and another that eliminates the need to switch to TCP:

   o  To avoid IP fragmentation, DTLS handshake messages incorporate
      their own fragment offset and fragment length, but this is only
      for the handshake.  Payloads that cause the DTLS packet to exceed
      the path maximum MTU need their own fragmentation support
      Section 9.

   o  DNS client and server MUST support the EDNS0 option defined in
      [RFC6891] so that the DNS client can indicate to the DNS server




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      the maximum DNS response size it can handle without IP
      fragmentation.

   DNSoD puts an additional computational load on servers.  The largest
   gain for privacy is to protect the communication between the DNS
   client (the end user's machine) and its caching resolver.
   Implementing DNSoD on root servers is outside the scope of this
   document.

8.  Established sessions

   In DTLS, all data is protected using the same record encoding and
   mechanisms.  When the mechanism described in this document is in
   effect, DNS messages are encrypted using the standard DTLS record
   encoding.  When a user of DTLS wishes to send an DNS message, it
   delivers it to the DTLS implementation as an ordinary application
   data write (e.g., SSL_write()).  A single DTLS session can be used to
   receive multiple DNS requests and generate DNS multiple responses.

   DNSoD client and server can use DTLS heartbeat [RFC6520] to verify
   that the peer still has DTLS state.  DTLS session is terminated by
   the receipt of an authenticated message that closes the connection
   (e.g., a DTLS fatal alert).




























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      Client                                          Server
      ------                                          ------

      ClientHello             -------->


                              <-------    HelloVerifyRequest
                                            (contains cookie)

      ClientHello             -------->
      (contains cookie)
      (empty SessionTicket extension)
                                                 ServerHello
                                     (empty SessionTicket extension)
                                                Certificate*
                                          ServerKeyExchange*
                                         CertificateRequest*
                              <--------      ServerHelloDone

      Certificate*
      ClientKeyExchange
      CertificateVerify*
      [ChangeCipherSpec]
      Finished                -------->
                                            NewSessionTicket
                                          [ChangeCipherSpec]
                              <--------             Finished


      DNS Request             --------->

                              <---------  DNS Response


        Message Flow for Full Handshake Issuing New Session Ticket

9.  Fragmentation and Reassembly

   This section describes an optional procedure the client and server
   can negotiate to send large DNS responses without IP fragmentation or
   reassembly.

   Large DNS responses cannot exceed the DNS maximum payload size (512)
   unless a larger size is negotiated with EDNS0.  Even using EDNS0,
   requesting responses larger the path MTU causes IP fragmentation.  If
   the response exceeds that size it is truncated and the TC bit set,
   forcing a DNS client that wants the entire response to establish a
   TCP connection and send the query again over TCP.  This slows down



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   DNS lookups, and is even more troublesome if a TLS session also needs
   to be established.

   To avoid these problems with DNS over DTLS, the DNS client and the
   DNS server can indicate support for a new application-layer
   fragmentation and reassembly mechanism, by using the new DTLS
   extension "DNS-fragment" in the DTLS ClientHello, and indicate how
   many fragments the client is willing to receive.  If the server
   supports this extension, it includes "DNS-fragment" in its DTLS
   ServerHello and indicates how many fragments it is willing to send in
   a response.  The EDNS0 value controls the size of the responses,
   including the size of fragmented responses.  If both the DTLS client
   and DTLS server indicate support DNS-fragment, and the DNS server's
   response exceeds the EDNS0-indicated size, the DNS server fragments
   the response into packets that are no larger than the EDNS0-indicated
   size, and sends them all to the DNS client.  Logically, the layering
   of the fragmentation is like this,

                              |     DNS       |
                              | fragmentation |
                              |     DTLS      |
                              |     UDP       |
                              |      IP       |

9.1.  Generating fragmented packets

   The response is formed, and separate packets are sent with their own
   fragmentation header, as follows:

                                             1  1  1  1  1  1
               0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |               DNS Query ID                    |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |QR| 1| M| M| Fragment-count  | Fragment-ID     |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
             |     Master Fragment Sequence Number (MFSN)    |
             +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

   The above fragment header appears at the beginning of all fragments.
   The fields are defined as follows:

      DNS Query ID: Is the same as the ID value of the DNS response;
      this means the first fragment has the DNS Query ID value appear
      twice in the packet, and means subsequent fragments will contain
      their associated DNS Query ID in the fragmentation header.





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      QR: This is the QR field in a normal DNS packet, and is always
      set, because fragmentation/reassembly is only defined in this
      document for responses.

      OP: This bit is always set.  This bit corresponds to first bit of
      the Opcode field of a normal DNS packet; that Opcode field for a
      normal DNS packet must be all 0.  By setting this bit to 1, this
      fragmentation header is distinguished from a normal DNS packet.

      M: The next two "M" bits must be 0, for future use.

      Fragment-count: For each query ID that generates a fragmented
      response, this field is set to the number of fragments that will
      be sent (that is, the highest Fragment-ID minus 1).

      Fragment-ID: Starts at 0 for the first fragmented segment and is
      incremented for each fragment.

      Master Fragment Sequence Number: starts at 0 and is incremented
      for the first packet of each fragmented response.

   The length of each fragment is calculated from the UDP packet size
   minus the DTLS overhead.

   It is RECOMMENDED that a value not exceeding 10 is used by the DNS
   client and DNS server during their DNS-fragment negotiation
   [RFC6928].  If the server needs to generate more fragments than were
   negotiated, MUST set the TC bit and SHOULD send the number of
   fragments negotiated.

   The fragments MUST be at least 64 bytes (minimum Ethernet MTU) minus
   DTLS, UDP, and IP overhead.  The fragments need not all be the same
   size.  The DNS client indicates the maximum DNS size using EDNS0,
   which constrains the size of the response packet on the wire.  When
   generating fragmented packets, the DNS server MUST NOT generate
   fragments that exceed the maximum DNS size.

9.2.  Receiving fragmented packets

   Upon receipt of a DTLS packet, DTLS processing is performed and the
   Opcode field is examined to determine if reassembly is required
   before processing as a DNS packet, as depicted below:









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               +--------+
               | 00000 -+------------> DNS processing
      Opcode = |        |                  ^
               |        |                  |
               | 1xxxx -+--->reassembly----+
               +--------+

   If reassembly needs to be performed, the packets are matched
   according to their DNS query ID value (at the top of the fragment
   header), their Master Fragment Sequence Number, and ordered by their
   Fragment-ID.  Once all the fragments have been received (that is, all
   fragments from 0 through the Fragment-ID matching the frag-count
   minus 1), the fragment headers are removed and the DNS payload is
   handed to the DNS layer.  Due to network loss or packet corruption,
   some fragments might not be received, which will cause the DNS layer
   to perform a normal re-transmission of the DNS query, with the same
   query ID.  The re-transmitted answer, which will be fragmented
   identically to the original answer (assuming that resource record did
   not change between the two answers), will have a different Master
   Fragment Sequence Number.

      Design Note: Tha MFSN protects against corruption caused by DNS
      resource record changing between the initial query and its re-
      transmitted query.

   After a time out, incomplete fragments are discarded by the receiver.

   If the Fragment-ID is 0 and the DNS Query ID value in the fragment
   header does not match the ID value in the DNS header, a DTLS Alert is
   generated and an error is logged.

9.3.  The DNS-fragment Extension

   A new extension type ("DNS_fragment(TBA)") is defined and MUST be
   included by the client in its "ClientHello" message if it wants to
   use fragmentation, and MUST be included in the ServerHello if the
   server agrees to use fragmentation.

   enum { DNS-fragment(TBA), (65535) } ExtensionType;

   The "extension_data" field of the "DNS-fragment" extension MUST
   contain a "MaxNumOfFragments" value, which is the maximum number of
   fragments the client wants to receive (indicated in the ClientHello),
   and the maximum number of fragments the server will send (indicated
   in the ServerHello).

   uint8 MaxNoOfFragments;




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   The value indicated in the ServerHello MUST be less than or equal to
   the value indicated in the ClientHello, and if not the client MUST
   terminate the DTLS association with an Alert, and MAY establish a new
   DTLS association without the dns_fragment extension.

10.  Anycast

   DNS servers are often configured with anycast addresses.  While the
   network is stable, packets transmitted from a particular source to an
   anycast address will reach the same server that has the cryptographic
   context from the DNS over DTLS handshake.  But when the network
   configuration changes,a DNS over DTLS packet can be received by a
   server that does not have the necessary cryptographic context.  To
   encourage the client to initiate a new DTLS handshake, DNS servers
   SHOULD generate a DTLS Alert message in response to receiving a DTLS
   packet for which the server does not have any cryptographic context.

11.  IANA Considerations

   This document adds a new extension for DTLS, in accordance with
   [RFC5246]:

   enum { DNS-Fragment(TBA), (65535) } ExtensionType;

   [[ NOTE: This value needs to be assigned by IANA ]]

   This extension MUST only be used with DTLS.

   IANA is requested to add the following value to the "Service Name and
   Transport Protocol Port Number Registry" registry in the System
   Range.  The registry for that range requires IETF Review or IESG
   Approval [RFC6335] and such a review has been requested using the
   Early Allocation process [RFC7120] for the well-known UDP port in
   this document.

       Service Name            domain-s
       Transport Protocol(s)   UDP/TCP
       Port                    853
       Assignee                IESG
       Contact                 dwing@cisco.com
       Description             DNS query-response protocol runs over
                               DTLS and TLS
       Reference               This document








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12.  Security Considerations

   The interaction between a DNS client and DNS server requires Datagram
   Transport Layer Security (DTLS) with a ciphersuite offering
   confidentiality protection and guidance given in [RFC7525] must be
   followed to avoid attacks on DTLS.  Once a DNSoD client has
   established a security association with a particular DNS server, and
   outstanding normal DNS queries with that server (if any) have been
   received, the DNSoD client MUST ignore any subsequent normal DNS
   responses from that server, as all subsequent responses should be
   encrypted.  This behavior mitigates all possible attacks described in
   Measures for Making DNS More Resilient against Forged Answers
   [RFC5452].

   The DNS Fragment extension does not impact security of DTLS session
   establishment or application data exchange.  DNS Fragment provides
   fragmentation and reassembly of the encrypted DNS payload.

13.  Acknowledgements

   Thanks to Phil Hedrick for his review comments on TCP and to Josh
   Littlefield for pointing out DNSoD load on busy servers (most notably
   root servers).  The authors would like to thank Simon Josefsson,
   Daniel Kahn Gillmor, Bob Harold, Ilari Liusvaara and Sara Dickinson
   for discussions and comments on the design of DNSoD.

14.  References

14.1.  Normative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <http://www.rfc-editor.org/info/rfc1034>.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <http://www.rfc-editor.org/info/rfc1035>.

   [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>.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,
              <http://www.rfc-editor.org/info/rfc4033>.




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   [RFC4492]  Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
              Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
              for Transport Layer Security (TLS)", RFC 4492,
              DOI 10.17487/RFC4492, May 2006,
              <http://www.rfc-editor.org/info/rfc4492>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <http://www.rfc-editor.org/info/rfc4648>.

   [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>.

   [RFC5288]  Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
              Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
              DOI 10.17487/RFC5288, August 2008,
              <http://www.rfc-editor.org/info/rfc5288>.

   [RFC5452]  Hubert, A. and R. van Mook, "Measures for Making DNS More
              Resilient against Forged Answers", RFC 5452,
              DOI 10.17487/RFC5452, January 2009,
              <http://www.rfc-editor.org/info/rfc5452>.

   [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>.

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,
              <http://www.rfc-editor.org/info/rfc6234>.

   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165,
              RFC 6335, DOI 10.17487/RFC6335, August 2011,
              <http://www.rfc-editor.org/info/rfc6335>.



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   [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>.

   [RFC6520]  Seggelmann, R., Tuexen, M., and M. Williams, "Transport
              Layer Security (TLS) and Datagram Transport Layer Security
              (DTLS) Heartbeat Extension", RFC 6520,
              DOI 10.17487/RFC6520, February 2012,
              <http://www.rfc-editor.org/info/rfc6520>.

   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891,
              DOI 10.17487/RFC6891, April 2013,
              <http://www.rfc-editor.org/info/rfc6891>.

   [RFC7120]  Cotton, M., "Early IANA Allocation of Standards Track Code
              Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January
              2014, <http://www.rfc-editor.org/info/rfc7120>.

   [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>.

   [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>.

14.2.  Informative References

   [I-D.bortzmeyer-dnsop-dns-privacy]
              Bortzmeyer, S., "DNS privacy considerations", draft-
              bortzmeyer-dnsop-dns-privacy-02 (work in progress), April
              2014.

   [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-01 (work
              in progress), October 2015.






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   [I-D.ietf-tls-cached-info]
              Santesson, S. and H. Tschofenig, "Transport Layer Security
              (TLS) Cached Information Extension", draft-ietf-tls-
              cached-info-19 (work in progress), March 2015.

   [RFC1122]  Braden, R., Ed., "Requirements for Internet Hosts -
              Communication Layers", STD 3, RFC 1122,
              DOI 10.17487/RFC1122, October 1989,
              <http://www.rfc-editor.org/info/rfc1122>.

   [RFC3749]  Hollenbeck, S., "Transport Layer Security Protocol
              Compression Methods", RFC 3749, DOI 10.17487/RFC3749, May
              2004, <http://www.rfc-editor.org/info/rfc3749>.

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
              <http://www.rfc-editor.org/info/rfc4821>.

   [RFC6928]  Chu, J., Dukkipati, N., Cheng, Y., and M. Mathis,
              "Increasing TCP's Initial Window", RFC 6928,
              DOI 10.17487/RFC6928, April 2013,
              <http://www.rfc-editor.org/info/rfc6928>.

   [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>.

   [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
              <http://www.rfc-editor.org/info/rfc7413>.

Authors' Addresses

   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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   Dan Wing
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, California  95134
   USA

   Email: dwing@cisco.com


   Prashanth Patil
   Cisco Systems, Inc.
   Bangalore
   India

   Email: praspati@cisco.com




































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