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Internet Engineering Task Force (IETF)              Phillip Hallam-Baker
Internet-Draft                                         Comodo Group Inc.
Intended Status: Standards Track                             May 9, 2014
Expires: November 10, 2014

        DNS Privacy and Censorship: Use Cases and Requirements.


   This document describes use cases and requirements arising from
   privacy an free speech concerns in the DNS.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   Copyright (c) 2014 IETF Trust and the persons identified as the
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Table of Contents

   1.  Introduction.  . . . . . . . . . . . . . . . . . . . . . . . .  3
      1.1.  Related Work  . . . . . . . . . . . . . . . . . . . . . .  3
      1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .  3
      1.3.  Defined Terms . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Use Cases and Requirements . . . . . . . . . . . . . . . . . .  3
      2.1.  Core Use Cases  . . . . . . . . . . . . . . . . . . . . .  4
         2.1.1.  Client/Resolver Communications . . . . . . . . . . .  5
         2.1.2.  Resolver/Authoritative Communications  . . . . . . .  7
      2.2.  Constraints . . . . . . . . . . . . . . . . . . . . . . .  7
         2.2.1.  Legacy Deployment  . . . . . . . . . . . . . . . . .  7
         2.2.2.  Integrity Attacks  . . . . . . . . . . . . . . . . .  8
         2.2.3.  Limited message size . . . . . . . . . . . . . . . .  8
         2.2.4.  Confidentiality Requirements . . . . . . . . . . . .  8
         2.2.5.  Integrity Requirements . . . . . . . . . . . . . . .  9
         2.2.6.  Access Requirements  . . . . . . . . . . . . . . . .  9
   3.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   5.  Acnowledgementsts  . . . . . . . . . . . . . . . . . . . . . . 10
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
      6.1.  Normative References  . . . . . . . . . . . . . . . . . . 10
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10

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1. Introduction.

   Recent events have required urgent consideration of privacy concerns
   in Internet protocols. In particular the lack of confidentiality
   controls in the DNS [RFC1035] protocol is of considerable concern.

   This document illustrates the privacy and related concerns raised
   with a set of use cases which in turn give rise to a set of

1.1. Related Work

   DNS Security (DNSSEC), [RFC4033] is an existing standard that
   protects the integrity of data between a DNS authoritative server and
   the party making a request. DNSSEC does not provide confidentiality
   and only permits blocking of integrity checks to be detcted. Since
   the only action available to the client in this circumstance is to
   block access to the possibly compromized site or accept a downgrade
   attack, this is not a sufficient countermeasure against a denial of
   service attack.

   It is anticipated that the proposals developed to address the use
   cases and requirements specified in this document will compliment
   rather than replace the existing mechanisms provided in DNSSEC.

1.2. Terminology

   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [RFC2119]

1.3. Defined Terms

   [[These terms are deliberately left blank here or else we will spend
   time wordsmithing the defined term definitions rather than looking at
   the protocol.]

      Authoritative DNS Server

      Caching Recursive Resolver


      DNS Client

      Recursive Resolver

      Stub Resolver

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2. Use Cases and Requirements

   The immediate motivation for considering a DNS transport layer
   security protocol is the desire to improve the privacy of Internet
   communications by allowing encryption of DNS requests and responses.

   A closely related concern that is directly affected by

2.1. Core Use Cases

   The DNS is the Internet infrastructure that makes authoritative
   statements about DNS names. In particular the DNS is used to support
   discovery of Internet services by mapping DNS names onto IP

   In the conventional configuration, a client requiring information
   from the DNS does not access DNS authoritative servers directly and
   instead makes requests through a resolver. The resolver in turn
   determines which requests to make to answer the query and forwards
   the request to the authoritative server.

   +-------------+    +-------------+    +-------------+
   |   Client    |--->|  Resolver   |--->|Authoritative|
   +-------------+    +-------------+    +-------------+

   Due to the distributed and hierarchical nature of the DNS, answering
   a DNS query typically requires queries to multiple Authoritative
   servers. This process is known as Recursive Resolution of the DNS
   Query. In the typical configuration the Resolver is a 'Caching
   Recursive Resolver' capable of making Recursive Queries and caching
   the result to answer future queries. The client is typically a 'Stub
   Client' that is not capable of making recursive queries itself and
   must rely on a Recursive Resolver to do this for it.

   One of the major security weaknesses in the DNS infrastructure as
   currently deployed is that by default most Internet enabled devices
   accept DNS service from the servers offered to it by DHCP when a
   connection is established. Since the DNS is a naming service and thus
   a trusted service, DNS services SHOULD be trustworthy. The practice
   of relying on a the 'local' DNS resolver advertised in the DHCP
   connection is therefore highly unsatisfactory.

   In real world circumstances this configuration is further complicated
   by firewalls, NAT devices and other middleboxes. Many of which filter
   or in some cases modify DNS protocol packets whether or not they are
   addressed to that device.

   For the purposes of considering the privacy of the DNS protocol,
   there are two important protocol interactions to consider:

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      *  Communications between a Client and a Resolver

      *  Communications between a Resolver and an Authoritative Server

   The DNS protocol supports both modes of interaction without special
   provision for either case. From a security point of view, the two
   interactions have different characteristics and give rise to
   different use cases.

2.1.1. Client/Resolver Communications

   Communications between the client and the resolver reveal a lot of
   privacy sensitive information about the user. A DNS query for the
   address of a controversial Web Site indicates with high probability
   that a user is viewing material from the site.

   In the typical configuration a DNS client makes use of the DNS
   resolution server(s) advertised by DHCP when a network configuration
   is established or server(s) that are configured manually by an

   In either case the relationship between the client and the resolver
   is at minimum persistent for the length of time the network
   association is active. In the case that the DNS service is selected
   and confinugred manually, the security relationship might last for
   years or the entire life of the device. Local Resolver

   For the sake of completeness, we state the case in which a client
   obtains DNS service from a local DNS server advertised at the time
   the network connection is established as a use case. Note however
   that the privacy concerns that can be protected in such circumstances
   are necessarily limited as the user has no idea where the service is
   being provided from.

   [U-LOCAL]: User connects to untrusted local network and wishes to use
   the locally provided DNS service.

   While a user may not intend to use the local DNS service, there are
   many real world network configurations that attempt to impose this on
   the user for a variety of reasons. In particular hotels and other
   providers of local wireless Internet often make use of a 'captive DNS
   resolver' to direct users to a portal for a variety of business
   purposes that include limiting use of the wireless network to
   particular parties.

   While it is clearly impossible to provide robust privacy protections
   to users who accept core network functions from random untrustworthy
   sources, the ability to establish network connections in such
   circumstances is essential.

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   A public resolver allows users to avoid the numerous security
   vulnerabilities inherent in the local resolver model. Instead of
   taking trusted services from random, anonymous providers, the user
   selects a particular DNS resolution provider to be used regardless of
   which network is in use.

   Many Public DNS resolution services are for-profit commercial
   ventures. The business models supporting such services include
   advertising and data-mining the DNS log file data.

   [[U-PUBLIC] The user takes DNS resolution service from a selected
   provider offering a public DNS resolution service. Selected Subscriber Resolver

   In an alternative business model the DNS resolution service is
   visible to the public Internet but only answers requests from paying
   subscribers. While such a service might not be considered
   sufficiently attractive for it to be offered as a stand-alone
   service, an ISP or security provider might offer a privacy enhanced
   DNS as part of a more general offering.

   [[U-SUBSCRIBER] The user takes DNS resolution service from a selected
   provider offering the service on a subscription model of some form. Selected Private Resolver

   Most medium to large enterprises run their own DNS services as part
   of their trusted network infrastructure.

   Although the DNS is conceptually a single uniform namespace, many
   Internet sites regard the DNS names of their internal network
   machines to be secret. Protecting the secrecy of such names being one
   of the principle attractions of a DNS privacy protocol to such
   enterprises. this leads to the widespread use of 'split-horizon' DNS
   in which different views of the DNS namespace are visible depending
   on whether a machine is inside or outside the enterprise.

   [U-PRIVATE] A device takes DNS resolution service from a private
   service restricted to authorized use. Hybrid Resolver

   To reduce equipment costs and in response to employee demand, many
   enterprises now support a Bring Your Own Device (BYOD) model in which
   a device that is the property of the owner. Such a device requires
   access to a private DNS service to access enterprise resources within
   a hidden split-horizon DNS. But the owner might not wish their

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   private use of the device to be visible to their employer.

   [U-HYBRID] A user makes use of different DNS resolution services for
   different portions of the DNS namespace.

2.1.2. Resolver/Authoritative Communications

   Communications between a Resolver and an Authoritative Server can
   also leak privacy sensitive data. Such leakage is mitigated at
   resolvers with a large number of users and a high traffic load.

   Unlike clients which typically direct DNS requests to a single
   resolver or a small number of resolvers, resolvers typically interact
   with a large number of authoritative servers. Some of which service a
   large number of DNS domains and others service are restricted to a
   publishing data for a specific enterprise.

   Although these use cases are not distinguished in the DNS protocol,
   the privacy implications and protocol constraints of interactions
   with the two types of server are very different. Any interaction
   between a resolver and an authoritative server that responds to
   requests for a single domain with a single host effectively discloses
   the nature of the request regardless of whether encryption is used.
   At the other extreme, traffic analysis of interactions with
   authoritative services serving a large number of domains revealls
   much less.

   [U-A-BULK] Interaction between a resolver and an authoritative server
   supporting a large number of domains.

   [U-A-TAIL] Interaction between a resolver and an authoritative server
   supporting a small number of domains such that the interaction is
   effectively disclosure of the nature of the communication.

2.2. Constraints

   Any proposal to address the use cases must operate within the
   constraints set by existing DNS infrastructure and administration

2.2.1. Legacy Deployment

   The DNS protocol specification was first published in 1987 and has
   evolved significantly over time. While the vast majority of deployed
   DNS servers support modern features such as EDN(0) and DNSSEC, many
   do not. Likewise, most DNS clients and servers accept messages longer
   than the 500 byte minimum implementation requirement.

   Regretably, while most DNS clients and servers are capable of
   supporting features introduced since [RFC1035], many middle-box
   devices including firewalls and residential network gateway devices

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   do not.

2.2.2. Integrity Attacks

   One of the core security vulnerabilities of the original DNS protocol
   is that responses are only weakly bound to requests, thus enabling an
   attack known as 'DNS-Spoofing'.

   While DNSSEC is intended to provide a long term solution to the
   problem of DNS spoofing, deployment of DNSSEC is currently the rare
   exception rather than the rule.

2.2.3. Limited message size

   One of the chief performance limitations of the DNS as currently
   deployed is that most DNS servers will only accept a single request
   per DNS message. Th despite support for multiple queries in a single
   request in the DNS protocol,

2.2.4. Confidentiality Requirements

         Prevent or mitigate disclosure  of request and response data
         against a passive attacker.

         Prevent or mitigate disclosure  of request and response data
         against an active attacker after first contact.

         Prevent or mitigate disclosure of request and response data
         against an active attacker on every contact.

         Prevent or mitigate transaction linking that would disclose
         communications made in different network contexts as
         originating from the same soure..

         Prevent or mitigate disclosure from the pattern of

         Protect the client against profiling by a resolver.

         Protect the confidentiality of messages against profiling by
         authoritative servers.

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   Any form of disclosure could potentially be damaging. It is not the
   potential for harm but rather the likelihood of harm and the
   difficulty of mounting an attack that distinguishes these
   requirements in practice.

   As with any security concern, confidentiality is a property of a
   system and not a particular component in that system. In particular
   any protocol that employs end-to-end IP transport (i.e. not via TOR)
   will leak indentifiable information about the client in the source IP
   address of a request. While such disclosure will inevitably limit the
   degree to which a technology that is built on end-to-end IP can
   protect privacy, this does not mean that the requirements such as [R-
   C-LINK] should be abandoned: A careless proposal could make matters
   much worse.

   For example consider the case in which a protocol proposal uses a
   static identifier that is never changed to specify an encryption key
   to a resolver. For a static machine with a fixed IP address, such an
   identifier would leak no more information than the IP address. But
   used on a mobile device, the static identifier would allow a passive
   observer to determine that a collection of communications to the
   resolver from different IP addresses all came from the same machine.
   Since the IP address effectively discloses location, such entries
   would essentially disclose the travelling history of the device and
   thus allow the user's travel pattern to be inferred.

2.2.5. Integrity Requirements

         Prevent spoofing of DNS responses by active attack that is only
         possible within a narrow window of opportunity. For example
         during a one-time key exchange.

         Prevent spoofing of DNS responses by active attack on any query

2.2.6. Access Requirements

         Support anonymous access to a DNS resolution service

         Support authentication of the client requesting access to a DNS
         resolution service

         Prevent Message amplification attack

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         Prevent Denial of Service attack on the service

   Note that [[R-CANON] and [[RCAUTH] are mutually exclusive. While it
   is desirable for a solution to be capable of supporting both it is
   not possible for a request to be anonymous and authenticated at the
   same time by definition. The access requirement  [[RCAUTH] is also
   distinct from the spoofing countermeasure requirements [R-PSPOOF] and
   [R-ASPOOF]. The access requirement [[RCAUTH] requires that the
   service identify the source of a request. The anti-spoofing
   requirements require that responses be authenticated against the
   requests made.

3. Security Considerations

   The use cases set out above give rise to the following requirements.

   The term 'requirement' is used to refer to protocol features that
   might be considered desirable without taking a position as to whether
   they are necessary or desirable in practice. A proposal that is
   simpler or more performant may be considered to be superior to one
   that satisfies every requirement.

4. IANA Considerations


5. Acnowledgementsts

6. References

6.1. Normative References

   [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.,Rose, S.,
              "DNS Security Introduction and Requirements", RFC 4033,
              March 2005.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, 1 November 1987.

Author's Address

   Phillip Hallam-Baker
   Comodo Group Inc.


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