Network Working Group Z. Hu Internet-Draft L. Zhu Intended status: Standards Track J. Heidemann Expires: March 21, 2016 USC/Information Sciences Institute A. Mankin D. Wessels Verisign Labs P. Hoffman ICANN September 18, 2015 DNS over TLS: Initiation and Performance Considerations draft-ietf-dprive-dns-over-tls-00 Abstract This document describes the use of TLS to provide privacy for DNS. Encryption provided by TLS eliminates opportunities for eavesdropping on 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 publication] 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 March 21, 2016. Hu, et al. Expires March 21, 2016 [Page 1] Internet-Draft DNS over TLS September 2015 Copyright Notice Copyright (c) 2015 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 8.3. digit . . . . . . . . . . . . . . . . . . . . . . . . . . 11 8.4. getdns . . . . . . . . . . . . . . . . . . . . . . . . . . 11 9. Security Considerations . . . . . . . . . . . . . . . . . . . 11 10. Contributing Authors . . . . . . . . . . . . . . . . . . . . . 12 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12 12.1. Normative References . . . . . . . . . . . . . . . . . . . 12 12.2. Informative References . . . . . . . . . . . . . . . . . . 13 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16 Hu, et al. Expires March 21, 2016 [Page 2] Internet-Draft DNS over TLS September 2015 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 [RFC7626]. 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. Hu, et al. Expires March 21, 2016 [Page 3] Internet-Draft DNS over TLS September 2015 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", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 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 a designated port TBD identified in Section 6. DNS clients desiring privacy from DNS-over-TLS from a particular server SHOULD establish a TCP connection to port TBD 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 Hu, et al. Expires March 21, 2016 [Page 4] Internet-Draft DNS over TLS September 2015 [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 recursion. 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, 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 6.2.1.1). Since pipelined responses can arrive out-of-order, clients MUST match responses to outstanding queries using the ID field and port number. 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 "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. Proper management of established and idle connections is important to the healthy operation of a DNS server. An implementor of DNS-over- Hu, et al. Expires March 21, 2016 [Page 5] Internet-Draft DNS over TLS September 2015 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. Hu, et al. Expires March 21, 2016 [Page 6] Internet-Draft DNS over TLS September 2015 4.1. Opportunistic Privacy Profile For opportunistic privacy, analogous to SMTP opportunistic encryption [RFC7435] one does not require privacy, but one desires privacy when possible. 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 attackers. 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, for instance within the authenticated DHCP exchange [RFC3118]. Hu, et al. Expires March 21, 2016 [Page 7] Internet-Draft DNS over TLS September 2015 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 has been requested using the Hu, et al. Expires March 21, 2016 [Page 8] Internet-Draft DNS over TLS September 2015 Early Allocation process [RFC7120] for the well-known TCP port in this document. We further recommend that IANA reserve the same port number over UDP for the proposed DNS-over-DTLS protocol [draft-ietf-dprive-dnsodtls]. Service Name domain-s Transport Protocol(s) TCP/UDP Assignee IESG Contact TBD Description DNS query-response protocol run over TLS Reference This document 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. 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. Hu, et al. Expires March 21, 2016 [Page 9] Internet-Draft DNS over TLS September 2015 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 exist. 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, ssl-service-pem, ssl-upstream. See https://unbound.net/documentation/unbound.conf.html. 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:// Hu, et al. Expires March 21, 2016 [Page 10] Internet-Draft DNS over TLS September 2015 portal.sinodun.com/stash/projects/TDNS/repos/dns-over-tls_patches/ browse. 8.3. digit The digit DNS client from USC/ISI supports DNS-over-TLS. Source code available at http://www.isi.edu/ant/software/tdns/index.html. 8.4. getdns The getdns API implementation supports DNS-over-TLS. Source code available at https://getdnsapi.net. 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 (i.e., "pinning") 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 query. 2. Middleboxes [RFC3234] are be 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. 3. Any protocol interactions prior to the TLS handshake are performed in the clear and can be modified by a person-in-the- middle attacker. For this reason, clients MAY discard cached information about server capabilities advertised prior to the start of the TLS handshake. Hu, et al. Expires March 21, 2016 [Page 11] Internet-Draft DNS over TLS September 2015 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 UK Email: sara@sinodun.com URI: http://sinodun.com 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. 12. References 12.1. Normative References [I-D.ietf-dnsop-5966bis] Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and D. Wessels, "DNS Transport over TCP - Implementation Requirements", draft-ietf-dnsop-5966bis-02 (work in Hu, et al. Expires March 21, 2016 [Page 12] Internet-Draft DNS over TLS September 2015 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, May 2015, . 12.2. Informative References [I-D.confidentialdns] Wijngaards, W., "Confidential DNS", draft-wijngaards-dnsop-confidentialdns-03 (work in progress), March 2015, . [I-D.edns-tcp-keepalive] 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, . [I-D.edns0-padding] Mayrhofer, A., "The EDNS(0) Padding Option", draft-mayrhofer-edns0-padding-01 (work in progress), August 2015, . [I-D.ipseca] 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, . [RFC3118] Droms, R. and W. Arbaugh., Ed., "Authentication for DHCP Messages", RFC 3118, DOI 10.17487/RFC3118, June 2001, . [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, . [RFC5966] Bellis, R., "DNS Transport over TCP - Implementation Requirements", RFC 5966, DOI 10.17487/RFC5966, Hu, et al. Expires March 21, 2016 [Page 14] Internet-Draft DNS over TLS September 2015 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-dnscurve] Dempsky, M., "DNSCurve", draft-dempsky-dnscurve-01 (work in progress), August 2010, . [dnssec-trigger] NLnet Labs, "Dnssec-Trigger", May 2014, . [draft-ietf-dprive-dnsodtls] Reddy, T., Wing, D., and P. Patil, "DNS over DTLS (DNSoD)", draft-ietf-dprive-dnsodtls-01 (work in progress), June 2015, . [draft-ietf-tls-falsestart] Moeller, B. and A. Langley, "Transport Layer Security (TLS) False Start", draft-ietf-tls-falsestart-00 (work in progress), November 2014, . [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, . Authors' Addresses Zi Hu USC/Information Sciences Institute 4676 Admiralty Way, Suite 1133 Marina del Rey, CA 90292 USA Phone: +1 213 587-1057 Email: zihu@usc.edu Liang Zhu USC/Information Sciences Institute 4676 Admiralty Way, Suite 1133 Marina del Rey, CA 90292 USA Phone: +1 310 448-8323 Email: liangzhu@usc.edu John Heidemann USC/Information Sciences Institute 4676 Admiralty Way, Suite 1001 Marina del Rey, CA 90292 USA Phone: +1 310 822-1511 Email: johnh@isi.edu Allison Mankin Verisign Labs 12061 Bluemont Way Reston, VA 20190 Phone: +1 703 948-3200 Email: amankin@verisign.com Hu, et al. Expires March 21, 2016 [Page 16] Internet-Draft DNS over TLS September 2015 Duane Wessels Verisign Labs 12061 Bluemont Way Reston, VA 20190 Phone: +1 703 948-3200 Email: dwessels@verisign.com Paul Hoffman ICANN Email: paul.hoffman@icann.org Hu, et al. Expires March 21, 2016 [Page 17]