Network Working Group                                              Z. Hu
Internet-Draft                                                    L. Zhu
Intended status: Standards Track                            J. Heidemann
Expires: August 16, 2015                        USC/Information Sciences
                                                               A. Mankin
                                                              D. Wessels
                                                           Verisign Labs
                                                              P. Hoffman
                                                          VPN Consortium
                                                       February 12, 2015

         TLS for DNS: Initiation and Performance Considerations


   This document offers an approach to initiating TLS for DNS: use of a
   dedicated DNS-over-TLS port, and fallback to a mechanism for
   upgrading a DNS-over-TCP connection over the standard port (TCP/53)
   to a DNS-over-TLS connection.  Encryption provided by TLS eliminates
   opportunities for eavesdropping on DNS queries in the network, such
   as discussed in RFC 7285.  In addition, this document discusses
   performance considerations to minimize overheads from using TCP and
   TLS with DNS, pertaining to both approaches.

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 August 16, 2015.

Copyright Notice

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

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( 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.

1.  Introduction

   Today, nearly all DNS queries ([RFC1034] and [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 ongoing efforts are beginning to identify privacy concerns about
   DNS ([I-D.ietf-dprive-problem-statement]).

   Prior work has addressed some aspects of DNS security, but there has
   been little work till recently 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.  DNSSEC by intention 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 [RFC7285].

   DNSCurve [draft-dempsky-dnscurve] defines a method to add
   confidentiality to the link between DNS clients and servers; however,
   it does so with a new cryptographic protocol and does not take
   advantage of an existing standard protocol such as TLS.
   ConfidentialDNS [draft-wijngaards-confidentialdns] and IPSECA
   [draft-osterweil-dane-ipsec] use opportunistic encryption to offer
   privacy for DNS queries and responses.  Finally, others have
   suggested DNS-over-TLS.  Unbound DNS software [unbound] includes a
   DNS-over-TLS implementation.  The present document goes beyond past
   DNS-over-TLS discussions by providing two modes of initiation for
   DNS-over-TLS, use of a well-known port, TBD, and use of a negotiation
   mechanism in an established connection.

   This document describes a protocol that is resilient in environments
   affected by differing middle box concerns.  The port-based initiation
   of TLS is very straightforward, but might be blocked by firewalls or
   be unwelcome to some DNS client or server implementations.  If port-

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   based initiation of TLS fails, there is an upgrade-based negotiation
   mechanism to enable DNS clients and servers to upgrade an existing
   DNS-over-TCP connection to a DNS-over-TLS connection, analogous to
   upgrade mechanisms in other uses of TLS, such as STARTTLS [RFC2595]
   used in SMTP [RFC3207], IMAP [RFC3501] and POP [RFC1939], to name
   just a few of many.  But like those, the upgrade-based approach has
   middle box considerations, particularly downgrade attacks, as
   discussed in Section 2.4.

   The protocol described here works for any DNS client to server
   communication using DNS-over-TCP.  In particular, the same protocol
   can be used for stub-recursive and recursive-authoritative
   communications.  We expect implementation initially between stubs and

   In specifying TLS negotiation for DNS, this document defines only the
   protocol extensions that are needed.  It does not describe how DNS
   clients might validate server certificates or specify trusted
   certificate authorities.  Solutions for certificate authentication
   are currently outside the scope of this document.

1.1.  Reserved Words

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

2.  Protocol Changes

   The only changes required for port-based DNS-over-TLS are those
   optimizing TCP and TLS performance discussed in the following.  The
   DNS protocol itself is unchanged.

   Clients and servers negotiate upgrade-based DNS-over-TLS by setting a
   bit in the Flags field of the EDNS0 [RFC6891] OPT meta-RR.  The "TLS
   OK" (TO) bit is defined as the second bit of the third and fourth
   bytes of the "extended RCODE and flags" portion of the EDNS0 OPT
   meta-RR, immediately adjacent to the "DNSSEC OK" (DO) bit [RFC4033]:

                     +0 (MSB)                +1 (LSB)
           0: |   EXTENDED-RCODE      |       VERSION         |
           2: |DO|TO|                  Z                      |

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2.1.  Use by DNS Clients

   DNS clients first try port-based DNS-over-TLS.  If that connection
   fails, they try upgrade-based DNS-over-TLS.

2.1.1.  Port-Based DNS-over-TLS for Clients

   DNS clients SHOULD first try using port-based DNS-over-TLS by
   establishing the TCP connection to the dedicated port TBD (number to
   be defined in Section 4).

   Stub resolvers do not change their recursive resolvers often.  A
   slight delay in failing to establish a port-based DNS-over-TLS
   connection is probably minor relative to the benefit of encrypted DNS
   queries and responses.  The stub resolver should give a reasonable
   amount of time for the recursive resolver to start the TLS setup,
   such as a few seconds.

   It SHOULD be an implementation and/or local determination as to
   whether to attempt TLS via the dedicated port first and then fall
   back to STARTTLS use, or to choose some other order of attempts and

2.1.2.  Sending Queries for Upgrade-Based DNS-over-TLS

   Setting the TO bit in queries sent using UDP transport has no
   protocol meaning.  However, the client MAY set the TO bit when using
   UDP transport.  The server MUST ignore the TO bit when receiving UDP

   DISCUSSION: community advice is sought on this.  The advantage of
   allowing a client to send UDP on TO is that servers can collect
   information on deployment (as happened with the DO bit).  The
   disadvantage is that a meaningless bit (TO over UDP) might cause
   confusion, and some middleboxes might not pass a UDP query with the
   TO bit set.

   DNS clients set the TO bit in the initial query sent to a server
   using TCP transport to signal their desire that the TCP connection be
   upgraded to TLS.  DNS clients SHOULD NOT set the TO bit on queries
   when using TCP or TLS transport because doing so has no meaning in
   this protocol.

   Since the motivation for upgrade-based DNS-over-TLS is to preserve
   privacy, DNS clients SHOULD use an initial (unprotected) query that
   reveals no private information in the initial TO=1 query to a server.
   To provide a standard "dummy" query, it is RECOMMENDED to send the
   initial query with RD=0, QNAME="STARTTLS", QCLASS=CH, and QTYPE=TXT

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   ("STARTTLS/CH/TXT") analogous to administrative queries already in
   widespread use [RFC4892].

   After sending the initial TO=1 query using TCP transport, DNS clients
   MUST wait for the initial response before sending any subsequent
   queries over the same TCP connection.

2.1.3.  Receiving Responses for Upgrade-Based DNS-over-TLS

   A DNS client that receives a response using UDP transport that has
   the TO bit set handles that response as usual.  It MAY record the
   server's support for DNS-over-TLS and use that information as part of
   its server selection algorithm in the case where multiple servers are
   available to service a particular query.

   A DNS client that has sent the TO bit using TCP transport and
   receives a response to its initial query that has the TO bit set MUST
   immediately initiate a TLS handshake using the procedure described in
   [RFC5246].  (Note that this document does not yet deal with what
   happens when the TLS handshake does not succeed.)

   DISCUSSION: are there any cases in which a DNS client that sent TO on
   DNS-over-TCP and receives TO in the initial response from the server
   would not initiate the TLS handshake?  Is there any reason for this
   to be SHOULD rather than MUST?

   A DNS client that receives a response to its initial query using TCP
   transport that has the TO bit clear MUST not initiate a TLS handshake
   and SHOULD utilize the existing TCP connection for subsequent
   queries.  DNS clients SHOULD remember server IP addresses that don't
   support upgrade-based DNS-over-TLS, including TLS handshake failures,
   and not request DNS-over-TLS from them for reasonable period (such as
   one hour per server).

2.2.  Use by DNS Servers

   A DNS server that supports DNS-over-TLS SHOULD support port-based
   DNS-over-TLS, and SHOULD support upgrade-based DNS-over TLS.

2.2.1.  Receiving Queries for Upgrade-Based DNS-over-TLS

   A DNS server receiving a query over UDP with the TO bit ignores that
   bit.  A DNS server receiving a query over an existing TLS connection
   with the TO bit ignores that bit.

   A DNS server receiving an initial query over TCP that has the TO bit
   set MAY inform the client it is willing to establish a TLS session,
   as described in the next section.

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   A DNS server receiving subsequent queries over TCP MUST ignore the TO
   bit.  (A client wishing to start TLS after the initial query MUST
   open a new TCP connection to do so.)

2.2.2.  Sending Responses

   A DNS server sending a response over UDP to a query that had an OPT
   meta-RR SHOULD set the TO bit to indicate its general support for
   DNS-over-TLS, as long as it is willing and able to support a TLS
   connection with the particular client.

   A DNS server receiving an initial query over TCP that has the TO bit
   set MAY set the TO bit in its response.  The server MUST then proceed
   with the TLS handshake protocol.

   A DNS server receiving a "dummy" STARTTLS/CH/TXT query over TCP MUST
   respond with RCODE=0 and a TXT RR in the Answer section.  Contents of
   the TXT RR are strictly informative (for humans) and MUST NOT be
   interpreted by the client software.  Recommended TXT RDATA values are
   "STARTTLS" or "NO_TLS".

2.3.  Established Sessions

   After TLS negotiation completes, the connection will be encrypted and
   is now protected from eavesdropping and normal DNS queries SHOULD
   take place, following DNS-over-TCP framing ([RFC1035], section

   Both clients and servers SHOULD follow existing DNS-over-TCP timeout
   rules, which are often implementation- and situation-dependent.  In
   the absence of any other advice, the RECOMMENDED timeout values are
   30 seconds for recursive name servers, 60 seconds for clients of
   recursive name servers, 10 seconds for authoritative name servers,
   and 20 seconds for clients of authoritative name servers.  Current
   work in this area may assist DNS-over-TLS clients and servers select
   useful timeout values [draft-wouters-edns-tcp-keepalive] [tdns].

   As with current DNS-over-TCP, DNS servers MAY close the connection at
   any time (e.g., 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.  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].  As
   discussed in [I-D.ietf-dnsop-5966bis], TCP Fast Open [RFC7413] is of

   DNS servers SHOULD enable fast TLS session resumption [RFC5077] to

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   avoid keeping per-client session state.

2.4.  Downgrade Attacks and Middleboxes

   Middleboxes [RFC3234] may be present in some networks and have been
   known to interfere with normal DNS resolution and create problems for
   DNS-over-TLS.  Remarkably, downgrade attacks can affect plaintext
   protocols that utilize "STARTTLS" signaling in a similar way.  A DNS
   client attempting upgrade-based DNS-over-TLS through a middlebox, or
   in the presence of a downgrade attack, could have one of the
   following outcomes.  (These outcomes are similar to those discussed
   in prior RFCs, such as [RFC3207].)

   o  The DNS client sends a TO=1 query and receives a TO=0 response.
      In this case there is no upgrade to TLS and DNS resolution occurs
      normally, without encryption.

   o  The DNS client sends a TO=1 query and receives a TO=1 response,
      but the middlebox does not understand the TLS negotiation.
      Middleboxes SHOULD clear TO in replies if they are not prepared to
      pass through TLS negotiation.  Clients SHOULD retry DNS without TO
      set if negotiation fails, and then retry with TLS after a
      reasonable period (see Section 2.1.3).

   o  The DNS client sends a TO=1 query but receives no response at all.
      The middlebox might be silently dropping the query due to the
      presence of the TO bit, when it should, in fact, ignore and pass
      through unknown flag bits [RFC6891].  The client SHOULD fall back
      to normal (unencrypted) DNS for a reasonable period (as discussed
      in Section 2.1.3).

   In general, clients that attempt TLS and fail can either fall back on
   unencrypted DNS, or wait and retry later, depending on their privacy

3.  Performance Considerations

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

   1.  Latency: Compared to UDP, DNS-over-TCP requires an additional
       round-trip-time (RTT) of latency to establish the connection.
       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.  Moreover,
       TLS connection resumption can further reduce the setup delay.

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   2.  State: The use of connection-oriented TCP requires keeping
       additional state in both kernels and applications.  TLS has
       marginal increases in state over TCP alone.  The state
       requirements are of particular concerns on servers with many
       clients.  Smaller timeout values will reduce the number of
       concurrent connections, and servers can preemptively close
       connections when resources limits are exceeded.

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

   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 a
   technical report [tdns].

4.  IANA Considerations

   This document defines a new bit ("TO") in the Flags field of the
   EDNS0 OPT meta-RR.  At the time of approval of this draft in the
   standards track, as per the IANA Considerations of RFC 6891, IANA is
   requested to reserve the second leftmost bit of the flags as the TO
   bit, immediately adjacent to the DNSSEC DO bit, as shown in
   Section 2.

   IANA is requested add the following value to the "Service Name and
   Transport Protocol Port Number Registry" registry.  That registry is
   populated by expert review [RFC6335], and such a review will be
   requested if this document progresses.  It would be desirable to be
   assigned port 54 upon completion of review.

       Service Name            DNS-over-TLS
       Transport Protocol(s)   TCP
       Assignee                IESG
       Contact                 TBD
       Description             DNS query-response protocol run over TLS
       Reference               This document

5.  Security Considerations

   The goal of this proposal is to address the security risks that arise
   because DNS queries may be eavesdropped upon, as described above.
   There are a number of residual risks that may impact this goal.

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   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; we refer to the TLS RFCs for discussion
       of these security issues.

   2.  Any protocol interactions prior to the TLS handshake are
       performed in the clear and can be modified by a man-in-the-middle
       attacker.  For this reason, clients MAY discard cached
       information about server capabilities advertised prior to the
       start of the TLS handshake.

   3.  As with other uses of STARTTLS-upgrade to TLS, the mechanism
       specified here is susceptible to downgrade attacks, where a
       person-in-the-middle prevents a successful TLS upgrade.  Keeping
       track of servers known to support TLS (i.e., "pinning") enables
       clients to detect downgrade attacks.  For servers with no
       connection history, clients may choose to refuse non-TLS DNS, or
       they may continue without TLS, depending on their privacy

   4.  This document does not propose new ideas for certificate
       authentication for TLS in the context of DNS.  Several external
       methods are possible, although each has weaknesses.  The current
       Certificate Authority infrastructure [RFC5280] is used by HTTP/
       TLS [RFC2818].  With many trusted CAs, this approach has
       recognized weaknesses [CA_Compromise].  Some work is underway to
       partially address these concerns (for example, with certificate
       pinning [certificate_pinning], but more work is needed.  DANE
       [RFC6698] provides mechanisms to root certificate trust with
       DNSSEC.  That use here must be carefully evaluated to address
       potential issues in trust recursion.  For stub-to-recursive
       resolver use, certificate authentication is sometimes either easy
       or nearly impossible.  If the recursive resolver is manually
       configured, its certificate can be authenticated when it is
       configured.  If the recursive resolver is automatically
       configured (such as with DHCP [RFC2131]), it could use DHCP
       authentication mechanisms [RFC3118]).

   Ongoing discussion and development of opportunistic TLS (connections
   without CA validation, [RFC7435]) may be relevant to DNS-over-TLS.

6.  Acknowledgments

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

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   Work by Zi Hu, Liang Zhu, and John Heidemann in this paper 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.

7.  References

7.1.  Normative References

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

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

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

   [RFC5077]  Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
              "Transport Layer Security (TLS) Session Resumption without
              Server-Side State", RFC 5077, January 2008.

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

   [RFC5966]  Bellis, R., "DNS Transport over TCP - Implementation
              Requirements", RFC 5966, August 2010.

   [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, August 2011.

   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891, April 2013.

7.2.  Informative References

              Infosec Island Admin, "CA Compromise", January 2012, <http

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              Dickinson, J., Bellis, R., Mankin, A., and D. Wessels,
              "DNS Transport over TCP - Implementation Requirements",
              draft-ietf-dnsop-5966bis-00 (work in progress),
              December 2014.

              Bortzmeyer, S., "DNS privacy considerations",
              draft-ietf-dprive-problem-statement-00 (work in progress),
              October 2014.

   [RFC1939]  Myers, J. and M. Rose, "Post Office Protocol - Version 3",
              STD 53, RFC 1939, May 1996.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, March 1997.

   [RFC2595]  Newman, C., "Using TLS with IMAP, POP3 and ACAP",
              RFC 2595, June 1999.

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

   [RFC3118]  Droms, R. and W. Arbaugh, "Authentication for DHCP
              Messages", RFC 3118, June 2001.

   [RFC3207]  Hoffman, P., "SMTP Service Extension for Secure SMTP over
              Transport Layer Security", RFC 3207, February 2002.

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

              4rev1", RFC 3501, March 2003.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, March 2005.

   [RFC4892]  Woolf, S. and D. Conrad, "Requirements for a Mechanism
              Identifying a Name Server Instance", RFC 4892, June 2007.

   [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, May 2008.

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)

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              Protocol: TLSA", RFC 6698, August 2012.

   [RFC7285]  Alimi, R., Penno, R., Yang, Y., Kiesel, S., Previdi, S.,
              Roome, W., Shalunov, S., and R. Woundy, "Application-Layer
              Traffic Optimization (ALTO) Protocol", RFC 7285,
              September 2014.

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

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

              OWASP, "Certificate and Public Key Pinning", 2014, <https:

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

              Osterweil, E., Wiley, G., Mitchell, D., and A. Newton,
              "Opportunistic Encryption with DANE Semantics and IPsec:
              IPSECA", draft-osterweil-dane-ipsec-00 (work in progress),
              February 2014,

              Wijngaards, W., "Confidential DNS",
              draft-wijngaards-dnsop-confidentialdns-00 (work in
              progress), November 2013, <

              Wouters, P. and J. Abley, "The edns-tcp-keepalive EDNS0
              Option", draft-wouters-edns-tcp-keepalive-00 (work in
              progress), October 2013, <

   [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,

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   [unbound]  NLnet Labs, Verisign labs, "Unbound", December 2013,

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
   VPN Consortium


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