DPRIVE                                                          T. Reddy
Internet-Draft                                                   D. Wing
Intended status: Standards Track                                P. Patil
Expires: October 23, 2015                                          Cisco
                                                          April 21, 2015

                         DNS over DTLS (DNSoD)


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

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

Status of This Memo

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   This Internet-Draft will expire on October 23, 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
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Relationship to TCP Queries and to DNSSEC . . . . . . . . . .   3
   3.  Common problems with DNS Privacy  . . . . . . . . . . . . . .   3
     3.1.  Firewall Blocking Ports or DNS Privacy Protocol . . . . .   3
     3.2.  Authenticating the DNS Privacy Server . . . . . . . . . .   4
     3.3.  Downgrade attacks . . . . . . . . . . . . . . . . . . . .   4
   4.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Incremental Deployment  . . . . . . . . . . . . . . . . . . .   5
   6.  Demultiplexing, Polling, Port Usage, and Discovery  . . . . .   5
   7.  Performance Considerations  . . . . . . . . . . . . . . . . .   6
   8.  Established sessions  . . . . . . . . . . . . . . . . . . . .   7
   9.  DTLS Features and Cipher Suites . . . . . . . . . . . . . . .   8
   10. Anycast . . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   12. Security Considerations . . . . . . . . . . . . . . . . . . .   9
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     14.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     14.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

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

   Active attackers have long been successful at injecting bogus
   responses, causing cache poisoning and causing misdirection of the
   subsequent connection (if attacking A or AAAA records).  A popular
   mitigation against that attack is to use ephemeral and random source
   ports for DNS queries.

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

   The motivations for proposing DNSoD are that

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

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

2.  Relationship to TCP Queries and to DNSSEC

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

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

3.  Common problems with DNS Privacy

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

3.1.  Firewall Blocking Ports or DNS Privacy Protocol

   When sending DNS over an encrypted channel, there are two choices:
   send the encrypted traffic over the DNS ports (UDP 53, TCP 53) or
   send the encrypted traffic over a different port.  The encrypted
   traffic is not normal DNS traffic, but rather is a cryptographic
   handshake followed by encrypted payloads.  There can be firewalls,
   other security devices, or intercepting DNS proxies which block the
   non-DNS traffic or otherwise react negatively (e.g., quarantining the

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   host for suspicious behavior).  Alternatively, if a different port is
   used for the encrypted traffic, a firewall or other security device
   might block that port or otherwise react negatively.

   There is no panacea, and only experiments on the Internet will
   uncover which technique or combination of techniques will work best.
   The authors believe a combination of techniques will be necessary, as
   that has proven necessary with other protocols that desire to work on
   existing networks.

3.2.  Authenticating the DNS Privacy Server

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

   To authenticate the server providing DNS privacy, the DNS client
   needs to be configured with the names of those DNS privacy servers.
   When connecting a DNS privacy server, the server's IP address can be
   converted to its hostname by doing a DNS PTR lookup, verifying that
   the name matches the pre-configured list of DNS privacy servers, and
   finally validating its certificate trust chain or a local list of
   certificates.  For DNS privacy servers that don't have a certificate
   trust chain (e.g.,, because they are on a home network or a corporate
   network), the configured list of DNS privacy servers can contain the
   certificate fingerprint of the DNS privacy server (i.e., a simple
   whitelist of name and certificate fingerprint).

3.3.  Downgrade attacks

   Using DNS privacy with an authenticated server is most preferred, DNS
   privacy with an unauthenticated server is next preferred, and plain
   DNS is least preferred.  An implementation will attempt to obtain DNS
   privacy by contacting DNS servers on the local network (provided by
   DHCP) and on the Internet, and will make those attempts in parallel
   to reduce user impact.  If DNS privacy cannot be successfully
   negotiated for whatever reason, client can do three things:

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

   2.  use DNS privacy with an un-authorized server, which means an
       attacker could be spoofing the handshake with the DNS privacy
       server; or,

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

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

4.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in

5.  Incremental Deployment

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

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

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

6.  Demultiplexing, Polling, Port Usage, and Discovery

   [Note - This section requires further discussion]

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

   DNSoD can run over standard UDP port 53 as defined in [RFC1035].  A
   DNS client or server that does not implement this specification will
   not respond to the incoming DTLS packets because they don't parse as
   DNS packets (the DNS Opcode would be 15, which is undefined).  A DNS
   client or server that does implement this specification can

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   demultiplex DNS and DTLS packets by examining the third octet.  For
   TLS 1.2, which is what is defined by this specification, a DTLS
   packet will contain 253 in the third octet, whereas a DNS packet will
   never contain 253 in the third octet.

   There has been some concern with sending DNSoD traffic over the same
   port as normal, un-encrypted DNS traffic.  The intent of this section
   is to show that DNSoD could successfully be sent over port 53.
   Further analysis and testing on the Internet may be valuable to
   determine if multiplexing on port 53, using a separate port, or some
   fallback between a separate port and port 53 brings the most success.

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

7.  Performance Considerations

   To reduce number of octets of the DTLS handshake, especially the size
   of the certificate in the ServerHello (which can be several
   kilobytes), we should consider using plain public keys
   [I-D.ietf-tls-oob-pubkey].  Considering that to authorize a certain
   DNS server the client already needs explicit configuration of the DNS
   servers it trusts, maybe the public key configuration problem is
   really no worse than the configuration problem of those whitelisted

   Multiple DNS queries can be sent over a single DNSoD security
   association.  The existing QueryID allows multiple requests and
   responses to be interleaved in whatever order they can be fulfilled
   by the DNS server.  This means DNSoD reduces the consumption of UDP
   port numbers, and because DTLS protects the communication between the
   DNS client and its server, the resolver SHOULD NOT use random
   ephemeral source ports (Section 9.2 of [RFC5452]) because such source
   port use would incur additional, unnecessary DTLS load on the DNSoD

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   It is highly advantageous to avoid server-side DTLS state and reduce
   the number of new DTLS security associations on the server which can
   be done with [RFC5077].  This also eliminates a round-trip for
   subsequent DNSoD queries, because with [RFC5077] the DTLS security
   association does not need to be re-established.  Note: with the shim
   (described below) perhaps we could send the query and the restore
   server-side state in the ClientHello packet.

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

   o  Path MTU can be determined using Packetization Layer Path MTU
      Discovery [RFC4821] using DTLS heartbeat.  [RFC4821] does not rely
      on ICMP or ICMPv6, and would not affect DNS state or
      responsiveness on the client or server.  However, it would be
      additional chattiness.

   o  To avoid IP fragmentation, DTLS handshake messages incorporate
      their own fragment offset and fragment length.  We might utilize a
      similar mechanism in a shim layer between DTLS and DNS, so that
      large DNS messages could be carried without causing IP

   DNSoD puts an additional computational load on servers.  The largest
   gain for privacy is to protect the communication between the DNS
   client (the end user's machine) and its caching resolver.  Because of
   the load imposed, and because of the infrequency of queries to root
   servers means the DTLS overhead is unlikely to be amoritized over the
   DNS queries sent over that DTLS connection, implementing DNSoD on
   root servers is NOT RECOMMENDED.

8.  Established sessions

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

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

      ClientHello             -------->

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

      ClientHello             -------->
      (contains cookie)
      (empty SessionTicket extension)
                                     (empty SessionTicket extension)
                              <--------      ServerHelloDone

      Finished                -------->
                              <--------             Finished

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

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

        Message Flow for Full Handshake Issuing New Session Ticket

9.  DTLS Features and Cipher Suites

   To improve interoperability, the set of DTLS features and cipher
   suites is restricted.  The DTLS implementation MUST disable
   compression.  DTLS compression can lead to the exposure of
   information that would not otherwise be revealed [RFC3749].  Generic
   compression is unnecessary since DNS provides compression features
   itself.  DNS over DTLS MUST only be used with cipher suites that have
   ephemeral key exchange, such as the ephemeral Diffie-Hellman (DHE)
   [RFC5246] or the elliptic curve variant (ECDHE) [RFC4492].  Ephemeral
   key exchange MUST have a minimum size of 2048 bits for DHE or
   security level of 128 bits for ECDHE.  Authenticated Encryption with

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   Additional Data (AEAD) modes, such as the Galois Counter Model (GCM)
   mode for AES [RFC5288] are acceptable.

10.  Anycast

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

11.  IANA Considerations

   If demultiplexing DTLS and DNS (using the third octet, Section 6) is
   useful, we should reserve DNS Opcode 15 to ensure DNS always has a 0
   bit where DTLS always has a 1 bit.

12.  Security Considerations

   Once a DNSoD client has established a security association with a
   particular DNS server, and outstanding normal DNS queries with that
   server (if any) have been received, the DNSoD client MUST ignore any
   subsequent normal DNS responses from that server, as all subsequent
   responses should be inside DNSoD.  This behavior mitigates all (?)
   attacks described in Measures for Making DNS More Resilient against
   Forged Answers [RFC5452].

   Security considerations discussed in DTLS [RFC6347] also apply to
   this document.

13.  Acknowledgements

   Thanks to Phil Hedrick for his review comments on TCP and to Josh
   Littlefield for pointing out DNSoD load on busy servers (most notably
   root servers).

14.  References

14.1.  Normative References

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

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   [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.

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

   [RFC4492]  Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
              Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
              for Transport Layer Security (TLS)", RFC 4492, May 2006.

   [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.

   [RFC5288]  Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
              Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
              August 2008.

   [RFC5452]  Hubert, A. and R. van Mook, "Measures for Making DNS More
              Resilient against Forged Answers", RFC 5452, January 2009.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, January 2012.

14.2.  Informative References

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

              Zi, Z., Zhu, L., Heidemann, J., Mankin, A., and D.
              Wessels, "Starting TLS over DNS", draft-hzhwm-start-tls-
              for-dns-01 (work in progress), July 2014.

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              Wouters, P., Tschofenig, H., Gilmore, J., Weiler, S., and
              T. Kivinen, "Using Raw Public Keys in Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", draft-ietf-tls-oob-pubkey-11 (work in progress),
              January 2014.

   [RFC3749]  Hollenbeck, S., "Transport Layer Security Protocol
              Compression Methods", RFC 3749, May 2004.

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, March 2007.

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

Authors' Addresses

   Tirumaleswar Reddy
   Cisco Systems, Inc.
   Cessna Business Park, Varthur Hobli
   Sarjapur Marathalli Outer Ring Road
   Bangalore, Karnataka  560103

   Email: tireddy@cisco.com

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

   Email: dwing@cisco.com

   Prashanth Patil
   Cisco Systems, Inc.

   Email: praspati@cisco.com

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