Network Working Group                                              Z. Hu
Internet-Draft                                                    L. Zhu
Intended status: Standards Track                            J. Heidemann
Expires: April 13, 2016                         USC/Information Sciences
                                                               A. Mankin
                                                              D. Wessels
                                                           Verisign Labs
                                                              P. Hoffman
                                                        October 11, 2015

        DNS over TLS: Initiation and Performance Considerations


   This document describes the use of TLS to provide privacy for DNS.
   Encryption provided by TLS eliminates opportunities for eavesdropping
   and on-path tampering with DNS queries in the network, such as
   discussed in RFC 7258.  In addition, this document specifies two
   usage profiles for DNS-over-TLS and provides advice on performance
   considerations to minimize overhead from using TCP and TLS with DNS.

   Note: this document was formerly named
   draft-ietf-dprive-start-tls-for-dns.  Its name has been changed to
   better describe the mechanism now used.  Please refer to working
   group archives under the former name for history and previous
   discussion.  [RFC Editor: please remove this paragraph prior to

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

   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 13, 2016.

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Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( 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
   2.  Reserved Words . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Establishing and Managing DNS-over-TLS Sessions  . . . . . . .  4
     3.1.  Session Initiation . . . . . . . . . . . . . . . . . . . .  4
     3.2.  TLS Handshake and Authentication . . . . . . . . . . . . .  4
     3.3.  Transmitting and Receiving Messages  . . . . . . . . . . .  5
     3.4.  Connection Reuse, Close and Reestablishment  . . . . . . .  5
   4.  Usage Profiles . . . . . . . . . . . . . . . . . . . . . . . .  6
     4.1.  Opportunistic Privacy Profile  . . . . . . . . . . . . . .  7
     4.2.  Pre-Deployed Profile . . . . . . . . . . . . . . . . . . .  7
   5.  Performance Considerations . . . . . . . . . . . . . . . . . .  8
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   7.  Design Evolution . . . . . . . . . . . . . . . . . . . . . . .  9
   8.  Implementation Status  . . . . . . . . . . . . . . . . . . . . 10
     8.1.  Unbound  . . . . . . . . . . . . . . . . . . . . . . . . . 10
     8.2.  ldns . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     8.3.  digit  . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     8.4.  getdns . . . . . . . . . . . . . . . . . . . . . . . . . . 11
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   10. Contributing Authors . . . . . . . . . . . . . . . . . . . . . 12
   11. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 12
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 13
     12.2. Informative References . . . . . . . . . . . . . . . . . . 14
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16

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1.  Introduction

   Today, nearly all DNS queries [RFC1034], [RFC1035] are sent
   unencrypted, which makes them vulnerable to eavesdropping by an
   attacker that has access to the network channel, reducing the privacy
   of the querier.  Recent news reports have elevated these concerns,
   and recent IETF work has specified privacy considerations for DNS

   Prior work has addressed some aspects of DNS security, but until
   recently there has been little work on privacy between a DNS client
   and server.  DNS Security Extensions (DNSSEC), [RFC4033] provide
   _response integrity_ by defining mechanisms to cryptographically sign
   zones, allowing end-users (or their first-hop resolver) to verify
   replies are correct.  By intention, DNSSEC does not protect request
   and response privacy.  Traditionally, either privacy was not
   considered a requirement for DNS traffic, or it was assumed that
   network traffic was sufficiently private, however these perceptions
   are evolving due to recent events [RFC7258].

   Other work that has offered the potential to encrypt between DNS
   clients and servers includes DNSCurve [dempsky-dnscurve],
   ConfidentialDNS [I-D.confidentialdns] and IPSECA [I-D.ipseca].  In
   addition to the present draft, the DPRIVE working group has recently
   adopted a DNS-over-DTLS [draft-ietf-dprive-dnsodtls] proposal.

   This document describes using DNS-over-TLS on a well-known port and
   also offers advice on performance considerations to minimize
   overheads from using TCP and TLS with DNS.

   Initiation of DNS-over-TLS is very straightforward.  By establishing
   a connection over a well-known port, clients and servers expect and
   agree to negotiate a TLS session to secure the channel.  Deployment
   will be gradual.  Not all servers will support DNS-over-TLS and the
   well-known port might be blocked by some firewalls.  Clients will be
   expected to keep track of servers that support TLS and those that
   don't.  Clients and servers will adhere to the TLS implementation
   recommendations and security considerations of [RFC7525].

   The protocol described here works for any DNS client to server
   communication using DNS-over-TCP.  That is, it may be used for
   queries and responses between stub clients and recursive servers as
   well as between recursive clients and authoritative servers.

   This document describes two profiles in Section 4 providing different
   levels of assurance of privacy: an opportunistic privacy profile and
   a pre-deployed profile.

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   An earlier version of this document described a technique for
   upgrading a DNS-over-TCP connection to a DNS-over-TLS session with,
   essentially, "STARTTLS for DNS".  To simplify the protocol, this
   document now only uses a well-known port to specify TLS use, omitting
   the upgrade approach.  The upgrade approach no longer appears in this
   document, which now focuses exclusively on the use of a well-known
   port for DNS-over-TLS.

2.  Reserved Words

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Establishing and Managing DNS-over-TLS Sessions

3.1.  Session Initiation

   A DNS server that supports DNS-over-TLS SHOULD listen for and accept
   TCP connections on port 853.

   DNS clients desiring privacy from DNS-over-TLS from a particular
   server SHOULD establish a TCP connection to port 853 on the server.
   Upon successful establishment of the TCP connection, client and
   server SHOULD immediately initiate a TLS handshake using the
   procedure described in [RFC5246].

   DNS clients SHOULD remember server IP addresses that don't support
   DNS-over-TLS, including timeouts, connection refusals, and TLS
   handshake failures, and not request DNS-over-TLS from them for a
   reasonable period (such as one hour per server).  DNS clients
   following a pre-deployed privacy profile MAY be more aggressive about
   retrying DNS-over-TLS connection failures.

3.2.  TLS Handshake and Authentication

   Once the DNS client succeeds in connecting via TCP on the well-known
   port for DNS-over-TLS, it proceeds with the TLS handshake [RFC5246],
   following the best practices specified in [RFC7525]).

   The client will then authenticate the certificate, if required.  DNS-
   over-TLS does not propose new ideas for certificate authentication.
   Depending on the privacy profile in use Section 4, the DNS client may
   choose not to require authentication of the certificate, or it may
   make use of a certificate that is part of the Certificate Authority
   infrastructure [RFC5280] authenticated in the manner of HTTP/TLS

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   [RFC2818].  DANE [RFC6698] provides mechanisms to root certificate
   trust with DNSSEC.  The DNS queries in DANE authentication of the
   certificate for DNS-over-TLS MAY be in the clear to avoid trust

   After TLS negotiation completes, the connection will be encrypted and
   is now protected from eavesdropping.  At this point, normal DNS
   queries SHOULD take place.

3.3.  Transmitting and Receiving Messages

   All messages (requests and responses) in the established TLS session
   MUST use the two-octet length field described in Section 4.2.2 of
   [RFC1035].  For reasons of efficiency, DNS clients and servers SHOULD
   transmit the two-octet length field, and the message described by
   that length field, to the TCP layer at the same time (e.g., in a
   single "write" system call) to make it more likely that all the data
   will be transmitted in a single TCP segment
   ([I-D.ietf-dnsop-5966bis], Section 8).

   In order to minimize latency, clients SHOULD pipeline multiple
   queries over a TLS session.  When a DNS client sends multiple queries
   to a server, it should not wait for an outstanding reply before
   sending the next query ([I-D.ietf-dnsop-5966bis], Section

   Since pipelined responses can arrive out-of-order, clients MUST match
   responses to outstanding queries using the ID field, query name,
   type, and class.  Failure by clients to properly match responses to
   outstanding queries can have serious consequences for
   interoperability ([I-D.ietf-dnsop-5966bis], Section 7).

3.4.  Connection Reuse, Close and Reestablishment

   For DNS clients that use library functions such as "getaddrinfo()"
   and "gethostbyname()", current implementations are known to open and
   close TCP connections each DNS call.  To avoid excess TCP
   connections, each with a single query, clients SHOULD reuse a single
   TCP connection to the recursive resolver.  Alternatively they may
   prefer to use UDP to a DNS-over-TLS enabled caching resolver on the
   same machine that then uses a system-wide TCP connection to the
   recursive resolver.

   In order to amortize TCP and TLS connection setup costs, clients and
   servers SHOULD NOT immediately close a connection after each
   response.  Instead, clients and servers SHOULD reuse existing
   connections for subsequent queries as long as they have sufficient
   resources.  In some cases, this means that clients and servers may
   need to keep idle connections open for some amount of time.

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   Proper management of established and idle connections is important to
   the healthy operation of a DNS server.  An implementor of DNS-over-
   TLS SHOULD follow best practices for DNS-over-TCP, as described in
   [I-D.ietf-dnsop-5966bis].  Failure to do so may lead to resource
   exhaustion and denial-of-service.

   Whereas client and server implementations from the [RFC1035] era are
   known to have poor TCP connection management, this document
   stipulates that successful negotiation of TLS indicates the
   willingness of both parties to keep idle DNS connections open,
   independent of timeouts or other recommendations for DNS-over-TCP
   without TLS.  In other words, software implementing this protocol is
   assumed to support idle, persistent connections and be prepared to
   manage multiple, potentially long-lived TCP connections.

   This document does not make specific recommendations for timeout
   values on idle connections.  Clients and servers should reuse and/or
   close connections depending on the level of available resources.
   Timeouts may be longer during periods of low activity and shorter
   during periods of high activity.  Current work in this area may also
   assist DNS-over-TLS clients and servers select useful timeout values
   [I-D.edns-tcp-keepalive] [tdns].

   Clients and servers that keep idle connections open MUST be robust to
   termination of idle connection by either party.  As with current DNS-
   over-TCP, DNS servers MAY close the connection at any time (perhaps
   due to resource constraints).  As with current DNS-over-TCP, clients
   MUST handle abrupt closes and be prepared to reestablish connections
   and/or retry queries.

   When reestablishing a DNS-over-TCP connection that was terminated, as
   discussed in [I-D.ietf-dnsop-5966bis], TCP Fast Open [RFC7413] is of
   benefit.  DNS servers SHOULD enable fast TLS session resumption
   [RFC5077] and this SHOULD be used when reestablishing connections.

   When closing a connection, DNS servers SHOULD use the TLS close-
   notify request to shift TCP TIME-WAIT state to the clients.
   Additional requirements and guidance for optimizing DNS-over-TCP are
   provided by [RFC5966], [I-D.ietf-dnsop-5966bis].

4.  Usage Profiles

   This protocol provides flexibility to accommodate several different
   use cases.  Two usage profiles are defined here to identify specific
   design points in performance and privacy.  Other profiles are
   possible but are outside the scope of this document.

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4.1.  Opportunistic Privacy Profile

   For opportunistic privacy, analogous to SMTP opportunistic encryption
   [RFC7435] one does not require privacy, but one desires privacy when

   With opportunistic privacy, a client might learn of a TLS-enabled
   recursive DNS resolver from an untrusted source (such as DHCP while
   roaming), it might or might not validate the TLS certificate.  These
   choices maximize availability and performance, but they leave the
   client vulnerable to on-path attacks that remove privacy.

   Opportunistic privacy can be used by any current client, but it only
   provides guaranteed privacy when there are no on-path active

4.2.  Pre-Deployed Profile

   For pre-deployed privacy, the DNS client has one or more trusted
   recursive DNS providers.  This profile provides strong privacy
   guarantees to the user.

   With pre-deployed privacy, a client retains a copy of the TLS
   certificate (and/or other authentication credentials as appropriate)
   and IP address of each provider.  The client will only use DNS
   servers for which this information has been pre-configured.  The
   possession of a trusted, pre-deployed TLS certificate allows the
   client to detect person-in-the-middle and downgrade attacks.

   With pre-deployed privacy, the DNS client MUST signal to the user
   when none of the designated DNS servers are available, and MUST NOT
   provide DNS service until at least one of the designated DNS servers
   becomes available.

   The designated DNS provider may be temporarily unavailable when
   configuring a network.  For example, for clients on networks that
   require authentication through web-based login, such authentication
   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 DNS
   provider after authentication.  The user MUST be alerted that the DNS
   is not private during such bootstrap.

   Methods for pre-deployment of the designated DNS provider are outside
   the scope of this document.  In corporate settings, such information
   may be provided at system installation.

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5.  Performance Considerations

   DNS-over-TLS incurs additional latency at session startup.  It also
   requires additional state (memory) and increased processing (CPU).

   1.  Latency: Compared to UDP, DNS-over-TCP requires an additional
       round-trip-time (RTT) of latency to establish a TCP connection.
       TCP Fast Open [RFC7413] can eliminate that RTT when information
       exists from prior connections.  The TLS handshake adds another
       two RTTs of latency.  Clients and servers should support
       connection keepalive (reuse) and out-of-order processing to
       amortize connection setup costs.  Fast TLS connection resumption
       [RFC5077] further reduces the setup delay and avoids the DNS
       server keeping per-client session state.  TLS False Start
       [draft-ietf-tls-falsestart] can also lead to a latency reduction
       in certain situations.

   2.  State: The use of connection-oriented TCP requires keeping
       additional state at the server in both the kernel and
       application.  The state requirements are of particular concern on
       servers with many clients, although memory-optimized TLS can add
       only modest state over TCP.  Smaller timeout values will reduce
       the number of concurrent connections, and servers can
       preemptively close connections when resource limits are exceeded.

   3.  Processing: Use of TLS encryption algorithms results in slightly
       higher CPU usage.  Servers can choose to refuse new DNS-over-TLS
       clients if processing limits are exceeded.

   4.  Number of connections: To minimize state on DNS servers and
       connection startup time, clients SHOULD minimize creation of new
       TCP connections.  Use of a local DNS request aggregator (a
       particular type of forwarder) allows a single active DNS-over-TLS
       connection from any given client computer to its server.
       Additional guidance can be found in [I-D.ietf-dnsop-5966bis].

   A full performance evaluation is outside the scope of this
   specification.  A more detailed analysis of the performance
   implications of DNS-over-TLS (and DNS-over-TCP) is discussed in
   [tdns] and [I-D.ietf-dnsop-5966bis].

6.  IANA Considerations

   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 was requested using the Early

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   Allocation process [RFC7120] for the well-known TCP port in this

   We further recommend that IANA reserve the same port number over UDP
   for the proposed DNS-over-DTLS protocol [draft-ietf-dprive-dnsodtls].

   IANA responded to the early allocation request with the following
   TEMPORARY assignment:

    Service Name            domain-s
    Port Number             853
    Transport Protocol(s)   TCP/UDP
    Assignee                IETF DPRIVE Chairs
    Contact                 Paul Hoffman
    Description             DNS query-response protocol run over TLS/DTLS
    Reference               This document

   The TEMPORARY assignment expires 2016-10-08.  IANA is requested to
   make the assigmnent permanent upon publication of this document as an

7.  Design Evolution

   [Note to RFC Editor: please do not remove this section prior to
   publication as it may be useful to future Foo-over-TLS efforts]

   Earlier versions of this document proposed an upgrade-based approach
   to establishing a TLS session.  The client would signal its interest
   in TLS by setting a "TLS OK" bit in the EDNS0 flags field.  A server
   would signal its acceptance by responding with the TLS OK bit set.

   Since we assume the client doesn't want to reveal (leak) any
   information prior to securing the channel, we proposed the use of a
   "dummy query" that clients could send for this purpose.  The proposed
   query name was STARTTLS, query type TXT, and query class CH.

   The TLS OK signaling approach has both advantages and disadvantages.
   One important advantage is that clients and servers could negotiate
   TLS.  If the server is too busy, or doesn't want to provide TLS
   service to a particular client, it can respond negatively to the TLS
   probe.  An ancillary benefit is that servers could collect
   information on adoption of DNS-over-TLS (via the TLS OK bit in
   queries) before implementation and deployment.  Another anticipated
   advantage is the expectation that DNS-over-TLS would work over port
   53.  That is, no need to "waste" another port and deploy new firewall
   rules on middleboxes.

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   However, at the same time, there was uncertainty whether or not
   middleboxes would pass the TLS OK bit, given that the EDNS0 flags
   field has been unchanged for many years.  Another disadvantage is
   that the TLS OK bit may make downgrade attacks easy and
   indistinguishable from broken middleboxes.  From a performance
   standpoint, the upgrade-based approach had the disadvantage of
   requiring 1xRTT additional latency for the dummy query.

   Following this proposal, DNS-over-DTLS was proposed separately.  DNS-
   over-DTLS claimed it could work over port 53, but only because a non-
   DTLS server interprets a DNS-over-DTLS query as a response.  That is,
   the non-DTLS server observes the QR flag set to 1.  While this
   technically works, it seems unfortunate and perhaps even undesirable.

   DNS over both TLS and DTLS can benefit from a single well-known port
   and avoid extra latency and mis-interpreted queries as responses.

8.  Implementation Status

   [Note to RFC Editor: please remove this section and reference to RFC
   6982 prior to publication.]

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in RFC 6982.
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may

   According to RFC 6982, "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

8.1.  Unbound

   The Unbound recursive name server software added support for DNS-
   over-TLS in version 1.4.14.  The unbound.conf configuration file has
   the following configuration directives: ssl-port, ssl-service-key,

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   ssl-service-pem, ssl-upstream.  See

8.2.  ldns

   Sinodun Internet Technologies has implemented DNS-over-TLS in the
   ldns library from NLnetLabs.  This also gives DNS-over-TLS support to
   the drill DNS client program.  Patches available at https://

8.3.  digit

   The digit DNS client from USC/ISI supports DNS-over-TLS.  Source code
   available at

8.4.  getdns

   The getdns API implementation supports DNS-over-TLS.  Source code
   available at

9.  Security Considerations

   Use of DNS-over-TLS is designed to address the privacy risks that
   arise out of the ability to eavesdrop on DNS messages.  It does not
   address other security issues in DNS, and there are a number of
   residual risks that may affect its success at protecting privacy:

   1.  There are known attacks on TLS, such as person-in-the-middle and
       protocol downgrade.  These are general attacks on TLS and not
       specific to DNS-over-TLS; please refer to the TLS RFCs for
       discussion of these security issues.  Clients and servers MUST
       adhere to the TLS implementation recommendations and security
       considerations of [RFC7525].  DNS clients keeping track of
       servers known to support TLS enables clients to detect downgrade
       attacks.  For servers with no connection history and no apparent
       support for TLS, clients depending on their Privacy Profile and
       privacy requirements may choose to (a) try another server when
       available, (b) continue without TLS, or (c) refuse to forward the

   2.  Middleboxes [RFC3234] are present in some networks and have been
       known to interfere with normal DNS resolution.  Use of a
       designated port for DNS-over-TLS should avoid such interference.
       In general, clients that attempt TLS and fail can either fall
       back on unencrypted DNS, or wait and retry later, depending on
       their Privacy Profile and privacy requirements.

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   3.  Any DNS protocol interactions prior to the TLS handshake that are
       performed in the clear can be modified by a person-in-the-middle
       attacker.  For example, unencrypted queries and responses might
       take place over port 53 between a client and server prior to TLS.
       For this reason, clients MAY discard cached information about
       server capabilities advertised prior to the start of the TLS

   4.  This document does not itself specify ideas to resist known
       traffic analysis or side channel leaks.  Even with encrypted
       messages, a well-positioned party may be able to glean certain
       details from an analysis of message timings and sizes.  Clients
       and servers may consider the use of a padding method to address
       privacy leakage due to message sizes [I-D.edns0-padding]

10.  Contributing Authors

   The below individuals contributed significantly to the draft.  The
   RFC Editor prefers a maximum of 5 names on the front page, and so we
   have listed additional authors in this section.

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

11.  Acknowledgments

   The authors would like to thank Stephane Bortzmeyer, John Dickinson,
   Daniel Kahn Gillmor, Brian Haberman, Kim-Minh Kaplan, Bill Manning,
   George Michaelson, Eric Osterweil, and Glen Wiley for reviewing this
   Internet-draft.  They also thank Nikita Somaiya for early work on
   this idea.

   Work by Zi Hu, Liang Zhu, and John Heidemann on this document is
   partially sponsored by the U.S. Dept. of Homeland Security (DHS)
   Science and Technology Directorate, HSARPA, Cyber Security Division,
   BAA 11-01-RIKA and Air Force Research Laboratory, Information
   Directorate under agreement number FA8750-12-2-0344, and contract
   number D08PC75599.

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12.  References

12.1.  Normative References

              Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and
              D. Wessels, "DNS Transport over TCP - Implementation
              Requirements", draft-ietf-dnsop-5966bis-02 (work in
              progress), July 2015.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <>.

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

   [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, <>.

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

   [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,

   [RFC7120]  Cotton, M., "Early IANA Allocation of Standards Track Code
              Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120,
              January 2014, <>.

   [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,

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              May 2015, <>.

12.2.  Informative References

              Wijngaards, W., "Confidential DNS",
              draft-wijngaards-dnsop-confidentialdns-03 (work in
              progress), March 2015, <

              Wouters, P., Abley, J., Dickinson, S., and R. Bellis, "The
              edns-tcp-keepalive EDNS0 Option",
              draft-ietf-dnsop-edns-tcp-keepalive-02 (work in progress),
              July 2015, <

              Mayrhofer, A., "The EDNS(0) Padding Option",
              draft-mayrhofer-edns0-padding-01 (work in progress),
              August 2015, <

              Osterweil, E., Wiley, G., Okubo, T., Lavu, R., and A.
              Mohaisen, "Opportunistic Encryption with DANE Semantics
              and IPsec: IPSECA", draft-osterweil-dane-ipsec-03 (work in
              progress), July 2015,

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, DOI 10.17487/
              RFC2818, May 2000,

   [RFC3234]  Carpenter, B. and S. Brim, "Middleboxes: Taxonomy and
              Issues", RFC 3234, DOI 10.17487/RFC3234, February 2002,

   [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,

   [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,

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   [RFC5966]  Bellis, R., "DNS Transport over TCP - Implementation
              Requirements", RFC 5966, DOI 10.17487/RFC5966,
              August 2010, <>.

   [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, <>.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258,
              May 2014, <>.

   [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,

   [RFC7435]  Dukhovni, V., "Opportunistic Security: Some Protection
              Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
              December 2014, <>.

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

              Dempsky, M., "DNSCurve", draft-dempsky-dnscurve-01 (work
              in progress), August 2010,

              NLnet Labs, "Dnssec-Trigger", May 2014,

              Reddy, T., Wing, D., and P. Patil, "DNS over DTLS
              (DNSoD)", draft-ietf-dprive-dnsodtls-01 (work in
              progress), June 2015, <

              Moeller, B. and A. Langley, "Transport Layer Security
              (TLS) False Start", draft-ietf-tls-falsestart-00 (work in
              progress), November 2014,

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   [tdns]     Zhu, L., Hu, Z., Heidemann, J., Wessels, D., Mankin, A.,
              and N. Somaiya, "T-DNS: Connection-Oriented DNS to Improve
              Privacy and Security", Technical report ISI-TR-688,
              February 2014, <Technical report, ISI-TR-688,

Authors' Addresses

   Zi Hu
   USC/Information Sciences Institute
   4676 Admiralty Way, Suite 1133
   Marina del Rey, CA  90292

   Phone: +1 213 587-1057

   Liang Zhu
   USC/Information Sciences Institute
   4676 Admiralty Way, Suite 1133
   Marina del Rey, CA  90292

   Phone: +1 310 448-8323

   John Heidemann
   USC/Information Sciences Institute
   4676 Admiralty Way, Suite 1001
   Marina del Rey, CA  90292

   Phone: +1 310 822-1511

   Allison Mankin
   Verisign Labs
   12061 Bluemont Way
   Reston, VA  20190

   Phone: +1 703 948-3200

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   Duane Wessels
   Verisign Labs
   12061 Bluemont Way
   Reston, VA  20190

   Phone: +1 703 948-3200

   Paul Hoffman


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