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rfc:rfc8094

Internet Engineering Task Force (IETF) T. Reddy Request for Comments: 8094 Cisco Category: Experimental D. Wing ISSN: 2070-1721

                                                              P. Patil
                                                                 Cisco
                                                         February 2017
         DNS over Datagram Transport Layer Security (DTLS)

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

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for examination, experimental implementation, and
 evaluation.
 This document defines an Experimental Protocol for the Internet
 community.  This document is a product of the Internet Engineering
 Task Force (IETF).  It represents the consensus of the IETF
 community.  It has received public review and has been approved for
 publication by the Internet Engineering Steering Group (IESG).  Not
 all documents approved by the IESG are a candidate for any level of
 Internet Standard; see Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc8094.

Reddy, et al. Experimental [Page 1] RFC 8094 DNS over DTLS February 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
 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
    1.1. Relationship to TCP Queries and to DNSSEC ..................3
    1.2. Document Status ............................................4
 2. Terminology .....................................................4
 3. Establishing and Managing DNS over DTLS Sessions ................5
    3.1. Session Initiation .........................................5
    3.2. DTLS Handshake and Authentication ..........................5
    3.3. Established Sessions .......................................6
 4. Performance Considerations ......................................7
 5. Path MTU (PMTU) Issues ..........................................7
 6. Anycast .........................................................8
 7. Usage ...........................................................9
 8. IANA Considerations .............................................9
 9. Security Considerations .........................................9
 10. References ....................................................10
    10.1. Normative References .....................................10
    10.2. Informative References ...................................11
 Acknowledgements ..................................................13
 Authors' Addresses ................................................13

Reddy, et al. Experimental [Page 2] RFC 8094 DNS over DTLS February 2017

1. Introduction

 The Domain Name System is specified in [RFC1034] and [RFC1035].  DNS
 queries and responses are normally exchanged unencrypted; thus, they
 are vulnerable to eavesdropping.  Such eavesdropping can result in an
 undesired entity learning domain that a host wishes to access, thus
 resulting in privacy leakage.  The DNS privacy problem is further
 discussed in [RFC7626].
 This document defines DNS over DTLS, which provides confidential DNS
 communication between stub resolvers and recursive resolvers, stub
 resolvers and forwarders, and forwarders and recursive resolvers.
 DNS over DTLS 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.  DNS
 over DTLS might work equally between recursive clients and
 authoritative servers, but this application of the protocol is out of
 scope for the DNS PRIVate Exchange (DPRIVE) working group per its
 current charter.  This document does not change the format of DNS
 messages.
 The motivations for proposing DNS over DTLS are that:
 o  TCP suffers from network head-of-line blocking, where the loss of
    a packet causes all other TCP segments not to be delivered to the
    application until the lost packet is retransmitted.  DNS over
    DTLS, because it uses UDP, does not suffer from network head-of-
    line blocking.
 o  DTLS session resumption consumes one round trip, whereas TLS
    session resumption can start only after the TCP handshake is
    complete.  However, with TCP Fast Open [RFC7413], the
    implementation can achieve the same RTT efficiency as DTLS.
 Note: DNS over DTLS is an experimental update to DNS, and the
 experiment will be concluded when the specification is evaluated
 through implementations and interoperability testing.

1.1. 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 can be protected with TLS, as
 described in [RFC7858].  DNS over DTLS alone cannot provide privacy
 for DNS messages in all circumstances, specifically when the DTLS
 record size is larger than the path MTU.  In such situations, the DNS
 server will respond with a truncated response (see Section 5).

Reddy, et al. Experimental [Page 3] RFC 8094 DNS over DTLS February 2017

 Therefore, DNS clients and servers that implement DNS over DTLS MUST
 also implement DNS over TLS in order to provide privacy for clients
 that desire Strict Privacy as described in [DTLS].
 DNS Security Extensions (DNSSEC) [RFC4033] provide object integrity
 of DNS resource records, allowing end users (or their resolver) to
 verify the legitimacy of responses.  However, DNSSEC does not provide
 privacy for DNS requests or responses.  DNS over DTLS works in
 conjunction with DNSSEC, but DNS over DTLS does not replace the need
 or value of DNSSEC.

1.2. Document Status

 This document is an Experimental RFC.  One key aspect to judge
 whether the approach is usable on a large scale is by observing the
 uptake, usability, and operational behavior of the protocol in large-
 scale, real-life deployments.
 This DTLS solution was considered by the DPRIVE working group as an
 option to use in case the TLS-based approach specified in [RFC7858]
 turns out to have some issues when deployed.  At the time of writing,
 it is expected that [RFC7858] is what will be deployed, and so this
 specification is mainly intended as a backup.
 The following guidelines should be considered when performance
 benchmarking DNS over DTLS:
 1.  DNS over DTLS can recover from packet loss and reordering, and
     does not suffer from network head-of-line blocking.  DNS over
     DTLS performance, in comparison with DNS over TLS, may be better
     in lossy networks.
 2.  The number of round trips to send the first DNS query over DNS
     over DTLS is less than the number of round trips to send the
     first DNS query over TLS.  Even if TCP Fast Open is used, it only
     works for subsequent TCP connections between the DNS client and
     server (Section 3 in [RFC7413]).
 3.  If the DTLS 1.3 protocol [DTLS13] is used for DNS over DTLS, it
     provides critical latency improvements for connection
     establishment over DTLS 1.2.

2. 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] .

Reddy, et al. Experimental [Page 4] RFC 8094 DNS over DTLS February 2017

3. Establishing and Managing DNS over DTLS Sessions

3.1. Session Initiation

 By default, DNS over DTLS MUST run over standard UDP port 853 as
 defined in Section 8, unless the DNS server has mutual agreement with
 its clients to use a port other than 853 for DNS over DTLS.  In order
 to use a port other than 853, both clients and servers would need a
 configuration option in their software.
 The DNS client should determine if the DNS server supports DNS over
 DTLS by sending a DTLS ClientHello message to port 853 on the server,
 unless it has mutual agreement with its server to use a port other
 than port 853 for DNS over DTLS.  Such another port MUST NOT be port
 53 but MAY be from the "first-come, first-served" port range (User
 Ports [RFC6335], range 1024-49151).  This recommendation against the
 use of port 53 for DNS over DTLS is to avoid complications in
 selecting use or non-use of DTLS and to reduce risk of downgrade
 attacks.
 A DNS server that does not support DNS over DTLS will not respond to
 ClientHello messages sent by the client.  If no response is received
 from that server, and the client has no better round-trip estimate,
 the client SHOULD retransmit the DTLS ClientHello according to
 Section 4.2.4.1 of [RFC6347].  After 15 seconds, it SHOULD cease
 attempts to retransmit its ClientHello.  The client MAY repeat that
 procedure to discover if DNS over DTLS service becomes available from
 the DNS server, 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.
 DNS clients and servers MUST NOT use port 853 to transport cleartext
 DNS messages.  DNS clients MUST NOT send and DNS servers MUST NOT
 respond to cleartext DNS messages on any port used for DNS over DTLS
 (including, for example, after a failed DTLS handshake).  There are
 significant security issues in mixing protected and unprotected data;
 therefore, UDP connections on a port designated by a given server for
 DNS over DTLS are reserved purely for encrypted communications.

3.2. DTLS Handshake and Authentication

 The DNS client initiates the DTLS handshake as described in
 [RFC6347], following the best practices specified in [RFC7525].
 After DTLS negotiation completes, if the DTLS handshake succeeds
 according to [RFC6347], the connection will be encrypted and would
 then be protected from eavesdropping.

Reddy, et al. Experimental [Page 5] RFC 8094 DNS over DTLS February 2017

 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, DNS client MUST use the authentication mechanisms discussed
 in [DTLS].  This document does not propose new ideas for
 authentication.

3.3. 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 DTLS user wishes to send a DNS message, the data is
 delivered to the DTLS implementation as an ordinary application data
 write (e.g., SSL_write()).  A single DTLS session can be used to send
 multiple DNS requests and receive multiple DNS responses.
 To mitigate the risk of unintentional server overload, DNS over DTLS
 clients MUST take care to minimize the number of concurrent DTLS
 sessions made to any individual server.  For any given client/server
 interaction, it is RECOMMENDED that there be no more than one DTLS
 session.  Similarly, servers MAY impose limits on the number of
 concurrent DTLS sessions being handled for any particular client IP
 address or subnet.  These limits SHOULD be much looser than the
 client guidelines above because the server does not know, for
 example, if a client IP address belongs to a single client, is
 representing multiple resolvers on a single machine, or is
 representing multiple clients behind a device performing Network
 Address Translation (NAT).
 In between normal DNS traffic, while the communication to the DNS
 server is quiescent, the DNS client MAY want to probe the server
 using DTLS heartbeat [RFC6520] to ensure it has maintained
 cryptographic state.  Such probes can also keep alive firewall or NAT
 bindings.  This probing reduces the frequency of needing a new
 handshake when a DNS query needs to be resolved, improving the user
 experience at the cost of bandwidth and processing time.
 A DTLS session is terminated by the receipt of an authenticated
 message that closes the connection (e.g., a DTLS fatal alert).  If
 the server has lost state, a DTLS handshake needs to be initiated
 with the server.  For the server, to mitigate the risk of
 unintentional server overload, it is RECOMMENDED that the default DNS
 over DTLS server application-level idle time be set to several
 seconds and not set to less than a second, but no particular value is

Reddy, et al. Experimental [Page 6] RFC 8094 DNS over DTLS February 2017

 specified.  When no DNS queries have been received from the client
 after idle timeout, the server MUST send a DTLS fatal alert and then
 destroy its DTLS state.  The DTLS fatal alert packet indicates the
 server has destroyed its state, signaling to the client that if it
 wants to send a new DTLS message, it will need to re-establish
 cryptographic context with the server (via full DTLS handshake or
 DTLS session resumption).  In practice, the idle period can vary
 dynamically, and servers MAY allow idle connections to remain open
 for longer periods as resources permit.

4. Performance Considerations

 The DTLS protocol profile for DNS over DTLS is discussed in
 Section 11 of [DTLS].  To reduce the number of octets of the DTLS
 handshake, especially the size of the certificate in the ServerHello
 (which can be several kilobytes), DNS clients and servers can use raw
 public keys [RFC7250] or Cached Information Extension [RFC7924].
 Cached Information Extension avoids transmitting the server's
 certificate and certificate chain if the client has cached that
 information from a previous TLS handshake.  TLS False Start [RFC7918]
 can reduce round trips by allowing the TLS second flight of messages
 (ChangeCipherSpec) to also contain the (encrypted) DNS query.
 It is highly advantageous to avoid server-side DTLS state and reduce
 the number of new DTLS sessions on the server that can be done with
 TLS Session Resumption without server state [RFC5077].  This also
 eliminates a round trip for subsequent DNS over DTLS queries, because
 with [RFC5077] the DTLS session does not need to be re-established.
 Since responses within a DTLS session can arrive out of order,
 clients MUST match responses to outstanding queries on the same DTLS
 connection using the DNS Message ID.  If the response contains a
 question section, the client MUST match the QNAME, QCLASS, and QTYPE
 fields.  Failure by clients to properly match responses to
 outstanding queries can have serious consequences for
 interoperability (Section 7 of [RFC7766]).

5. Path MTU (PMTU) Issues

 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.  The
 DNS client and server MUST support the Extension Mechanisms for DNS
 (EDNS0) option defined in [RFC6891] so that the DNS client can
 indicate to the DNS server the maximum DNS response size it can
 reassemble and deliver in the DNS client's network stack.  If the DNS
 client does set the EDNS0 option defined in [RFC6891], then the
 maximum DNS response size of 512 bytes plus DTLS overhead will be

Reddy, et al. Experimental [Page 7] RFC 8094 DNS over DTLS February 2017

 well within the Path MTU.  If the Path MTU is not known, an IP MTU of
 1280 bytes SHOULD be assumed.  The client sets its EDNS0 value as if
 DTLS is not being used.  The DNS server MUST ensure that the DNS
 response size does not exceed the Path MTU, i.e., each DTLS record
 MUST fit within a single datagram, as required by [RFC6347].  The DNS
 server must consider the amount of record expansion expected by the
 DTLS processing when calculating the size of DNS response that fits
 within the path MTU.  The Path MTU MUST be greater than or equal to
 the DNS response size + DTLS overhead of 13 octets + padding size
 ([RFC7830]) + authentication overhead of the negotiated DTLS cipher
 suite + block padding (Section 4.1.1.1 of [RFC6347]).  If the DNS
 server's response were to exceed that calculated value, the server
 MUST send a response that does fit within that value and sets the TC
 (truncated) bit.  Upon receiving a response with the TC bit set and
 wanting to receive the entire response, the client behavior is
 governed by the current Usage Profile [DTLS].  For Strict Privacy,
 the client MUST only send a new DNS request for the same resource
 record over an encrypted transport (e.g., DNS over TLS [RFC7858]).
 Clients using Opportunistic Privacy SHOULD try for the best case (an
 encrypted and authenticated transport) but MAY fall back to
 intermediate cases and eventually the worst case scenario (clear
 text) in order to obtain a response.

6. 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 or routing changes, a DNS over DTLS packet can be
 received by a server that does not have the necessary cryptographic
 context.  Clients using DNS over DTLS need to always be prepared to
 re-initiate the DTLS handshake, and in the worst case this could even
 happen immediately after re-initiating a new handshake.  To encourage
 the client to initiate a new DTLS handshake, DNS servers SHOULD
 generate a DTLS fatal alert message in response to receiving a DTLS
 packet for which the server does not have any cryptographic context.
 Upon receipt of an unauthenticated DTLS fatal alert, the DTLS client
 validates the fatal alert is within the replay window
 (Section 4.1.2.6 of [RFC6347]).  It is difficult for the DTLS client
 to validate that the DTLS fatal alert was generated by the DTLS
 server in response to a request or was generated by an on- or off-
 path attacker.  Thus, upon receipt of an in-window DTLS fatal alert,
 the client SHOULD continue retransmitting the DTLS packet (in the
 event the fatal alert was spoofed), and at the same time it SHOULD
 initiate DTLS session resumption.  When the DTLS client receives an
 authenticated DNS response from one of those DTLS sessions, the other
 DTLS session should be terminated.

Reddy, et al. Experimental [Page 8] RFC 8094 DNS over DTLS February 2017

7. Usage

 Two Usage Profiles, Strict and Opportunistic, are explained in
 [DTLS].  The order of preference for DNS usage is as follows:
 1.  Encrypted DNS messages with an authenticated server
 2.  Encrypted DNS messages with an unauthenticated server
 3.  Plaintext DNS messages

8. IANA Considerations

 This specification uses port 853 already allocated in the IANA port
 number registry as defined in Section 6 of [RFC7858].

9. Security Considerations

 The interaction between a DNS client and a DNS server requires
 Datagram Transport Layer Security (DTLS) with a ciphersuite offering
 confidentiality protection.  The guidance given in [RFC7525] MUST be
 followed to avoid attacks on DTLS.  The DNS client SHOULD use the TLS
 Certificate Status Request extension (Section 8 of [RFC6066]),
 commonly called "OCSP stapling" to check the revocation status of the
 public key certificate of the DNS server.  OCSP stapling, unlike OCSP
 [RFC6960], does not suffer from scale and privacy issues.  DNS
 clients keeping track of servers known to support DTLS enables
 clients to detect downgrade attacks.  To interfere with DNS over
 DTLS, an on- or off-path attacker might send an ICMP message towards
 the DTLS client or DTLS server.  As these ICMP messages cannot be
 authenticated, all ICMP errors should be treated as soft errors
 [RFC1122].  If the DNS query was sent over DTLS, then the
 corresponding DNS response MUST only be accepted if it is received
 over the same DTLS connection.  This behavior mitigates all possible
 attacks described in Measures for Making DNS More Resilient against
 Forged Answers [RFC5452].  The security considerations in [RFC6347]
 and [DTLS] are to be taken into account.
 A malicious client might attempt to perform a high number of DTLS
 handshakes with a server.  As the clients are not uniquely identified
 by the protocol and can be obfuscated with IPv4 address sharing and
 with IPv6 temporary addresses, a server needs to mitigate the impact
 of such an attack.  Such mitigation might involve rate limiting
 handshakes from a certain subnet or more advanced DoS/DDoS techniques
 beyond the scope of this document.

Reddy, et al. Experimental [Page 9] RFC 8094 DNS over DTLS February 2017

10. References

10.1. 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>.
 [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>.
 [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>.
 [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>.

Reddy, et al. Experimental [Page 10] RFC 8094 DNS over DTLS February 2017

 [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
            "Recommendations for Secure Use of Transport Layer
            Security (TLS) and Datagram Transport Layer Security
            (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
            2015, <http://www.rfc-editor.org/info/rfc7525>.
 [RFC7830]  Mayrhofer, A., "The EDNS(0) Padding Option", RFC 7830,
            DOI 10.17487/RFC7830, May 2016,
            <http://www.rfc-editor.org/info/rfc7830>.

10.2. Informative References

 [DTLS]     Dickinson, S., Gillmor, D., and T. Reddy, "Authentication
            and (D)TLS Profile for DNS-over-(D)TLS", Work in
            Progress, draft-ietf-dprive-dtls-and-tls-profiles-08,
            January 2017.
 [DTLS13]   Rescorla, E. and H. Tschofenig, "The Datagram Transport
            Layer Security (DTLS) Protocol Version 1.3", Work in
            Progress, draft-rescorla-tls-dtls13-00, October 2016.
 [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>.
 [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
            Extensions: Extension Definitions", RFC 6066,
            DOI 10.17487/RFC6066, January 2011,
            <http://www.rfc-editor.org/info/rfc6066>.
 [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>.
 [RFC6960]  Santesson, S., Myers, M., Ankney, R., Malpani, A.,
            Galperin, S., and C. Adams, "X.509 Internet Public Key
            Infrastructure Online Certificate Status Protocol - OCSP",
            RFC 6960, DOI 10.17487/RFC6960, June 2013,
            <http://www.rfc-editor.org/info/rfc6960>.

Reddy, et al. Experimental [Page 11] RFC 8094 DNS over DTLS February 2017

 [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>.
 [RFC7626]  Bortzmeyer, S., "DNS Privacy Considerations", RFC 7626,
            DOI 10.17487/RFC7626, August 2015,
            <http://www.rfc-editor.org/info/rfc7626>.
 [RFC7766]  Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and
            D. Wessels, "DNS Transport over TCP - Implementation
            Requirements", RFC 7766, DOI 10.17487/RFC7766, March 2016,
            <http://www.rfc-editor.org/info/rfc7766>.
 [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>.
 [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>.

Reddy, et al. Experimental [Page 12] RFC 8094 DNS over DTLS February 2017

Acknowledgements

 Thanks to Phil Hedrick for his review comments on TCP and to Josh
 Littlefield for pointing out DNS over DTLS load on busy servers (most
 notably root servers).  The authors would like to thank Simon
 Josefsson, Daniel Kahn Gillmor, Bob Harold, Ilari Liusvaara, Sara
 Dickinson, Christian Huitema, Stephane Bortzmeyer, Alexander
 Mayrhofer, Allison Mankin, Jouni Korhonen, Stephen Farrell, Mirja
 Kuehlewind, Benoit Claise, and Geoff Huston for discussions and
 comments on the design of DNS over DTLS.  The authors would like to
 give special thanks to Sara Dickinson for her help.

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
 Dan Wing
 Email: dwing-ietf@fuggles.com
 Prashanth Patil
 Cisco Systems, Inc.
 Email: praspati@cisco.com

Reddy, et al. Experimental [Page 13]

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