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Network Working Group                                         O. Kolkman
Internet-Draft                                                     NLNet
Intended status: Informational                               J. Peterson
Expires: May 3, 2012                                       NeuStar, Inc.
                                                           H. Tschofenig
                                                  Nokia Siemens Networks
                                                                B. Aboba
                                                   Microsoft Corporation
                                                        October 31, 2011


    Architectural Considerations on Application Features in the DNS
                     draft-iab-dns-applications-03

Abstract

   A number of Internet applications rely on the Domain Name System
   (DNS) to support their operations.  Many applications use the DNS to
   locate services for a domain, for example; more recently, some
   applications have begun transforming identifiers other than domain
   names into formats that the DNS can process.  Proposals to
   incorporate more sophisticated application behavior into the DNS,
   however, have raised questions about the applicability and
   extensibility of the DNS.  This document explores the architectural
   consequences of installing certain application features in the DNS,
   and provides guidance to future application designers as to the sorts
   of ways that application can make use of the DNS.

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 May 3, 2012.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the



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   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
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   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.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.




























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Table of Contents

   1.  Motivation . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Overview of DNS Application Usages . . . . . . . . . . . . . .  6
     2.1.  Locating Services in a Domain  . . . . . . . . . . . . . .  6
     2.2.  NAPTR and DDDS . . . . . . . . . . . . . . . . . . . . . .  7
     2.3.  Arbitrary Data in the DNS  . . . . . . . . . . . . . . . .  9
   3.  Challenges for the DNS . . . . . . . . . . . . . . . . . . . . 11
     3.1.  Compound Queries . . . . . . . . . . . . . . . . . . . . . 11
       3.1.1.  Responses Tailored to the Originator . . . . . . . . . 13
     3.2.  Using DNS as a Generic Database  . . . . . . . . . . . . . 14
       3.2.1.  Large Data in the DNS  . . . . . . . . . . . . . . . . 14
     3.3.  Administrative Structures Misaligned with the DNS  . . . . 16
       3.3.1.  Metadata about Tree Structure  . . . . . . . . . . . . 17
     3.4.  Domain Redirection . . . . . . . . . . . . . . . . . . . . 19
   4.  Private DNS and Split Horizon  . . . . . . . . . . . . . . . . 21
   5.  Principles and Guidance  . . . . . . . . . . . . . . . . . . . 23
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 25
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 26
   8.  IAB Members  . . . . . . . . . . . . . . . . . . . . . . . . . 27
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 28
   10. Informative References . . . . . . . . . . . . . . . . . . . . 29
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 33




























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1.  Motivation

   The Domain Name System (DNS) has long provided a general means of
   translating easily-memorized domain names into numeric Internet
   Protocol addresses, which makes the Internet easier to use by
   providing a valuable layer of indirection between well-known names
   and lower-layer protocol elements.  [RFC0974] documented a further
   use of the DNS: to manage application services operating in a domain
   with the Mail Exchange (MX) resource record, which helped email
   addressed to the domain to find a mail service for the domain
   sanctioned by the zone administrator.

   The seminal MX record served as a prototype for other DNS resource
   records that supported applications associated with a domain name.
   The SRV resource record [RFC2052] provided a more general mechanism
   for locating services in a domain, complete with a weighting system
   and selection among transports.  The Naming Authority Pointer (NAPTR,
   originally [RFC2168]) resource record, especially as it evolved into
   the more general Dynamic Delegation Discovery System (DDDS, [RFC3401]
   passim) framework, added a new wrinkle - an algorithm for
   transforming a string into a domain name, which might then be
   resolved by the DNS to find NAPTR records.  This enabled the
   resolution of identifiers that do not have traditional host
   components through the DNS; the best-known example of this are
   telephone numbers, as resolved by the DDDS application ENUM.  Recent
   work such as Domain Keys Identified Mail (DKIM, [RFC4871]) has
   enabled security features of applications to be advertised through
   the DNS, via the TXT resource record.

   As the amount of application intelligence available to the DNS has
   increased, however, some proposed usages and extensions have become
   misaligned with both the architecture and underlying protocols of the
   DNS.  In the global public DNS, for example, the resolution of domain
   names to IP addresses is public information with no expectation of
   confidentiality, and thus the underlying query/response protocol has
   no authentication or encryption mechanism - typically, any security
   required by an application or service is invoked after the DNS query,
   when the resolved service has been contacted.  Only in private DNS
   environments (including split horizon DNS) where the identity of the
   querier is assured through some external means does the DNS maintain
   confidential records in this sense - although for load balancing
   reasons, localization or related optimizations, the public DNS may
   return different addresses in response to queries from different
   sources, or even no response at all, which is discussed further in
   Section 3.1.1.  Applications in many environments require these sorts
   of features from databases, and as the contexts in which applications
   rely on the DNS have increased, some application protocols have
   looked to extend the DNS to include these sorts of capabilities.



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   This document therefore provides guidance to designers of
   applications and application protocols on the use or extension of the
   DNS to provide features to applications.  It offers an overview of
   ways that applications have used the DNS in the past, as well as
   proposed ways that applications could use the DNS that might raise
   concerns.  It gives some reasons to decide the question, "Should I
   store this information in the DNS, or use some other means?" when
   that question arises during protocol development.  These guidelines
   remind application protocol designers of the strengths and weaknesses
   of the DNS in order to make it easier for designers to decide what
   features the DNS should provide for their application.

   The guidance in this document complements the guidance on extending
   the DNS given in [RFC5507].  Whereas [RFC5507] considers the
   preferred ways to add new information to the underlying syntax of the
   DNS (such as defining new resource records or adding prefixes or
   suffixes to labels), the current document considers broader
   implications of offloading application features to the DNS, be it
   through extending the DNS or simply reusing existing protocol
   capabilities - implications that may concern the invocation of the
   resolver by applications, the behavior of name servers, resolvers or
   caches, extensions to the underlying DNS protocol, the operational
   responsibilities of zone administrators, security, or the overall
   architecture of names.  When existing DNS protocol fields are used in
   ways that their designers did not intend to handle new applications,
   those applications may require further changes and extensions that
   are fundamentally at odds with the strengths of the DNS.
























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2.  Overview of DNS Application Usages

   [RFC0882] identifies the original and fundamental connection between
   the DNS and applications.  It begins by describing how the
   interdomain scope of applications creates "formidable problems when
   we wish to create consistent methods for referencing particular
   resources that are similar but scattered throughout the environment."
   This motivated "the need to have a mapping between host names... and
   ARPA Internet addresses" transition from a global table (the original
   "hosts.txt" file) to a "distributed database that performs the same
   function."  [RFC0882] also envisioned some ways to find the resources
   associated with mailboxes in a domain: without these extensions, a
   user trying to send mail to a foreign domain lacked a discovery
   mechanism to locate the right host in the remote domain to which to
   connect.

   While a special-purpose discovery mechanism could be built by each
   such application protocol that needed this functionality, the
   universality of the DNS invites installing these features into its
   public tree.  Over time, several other applications leveraged
   resource records for locating services in a domain or for storing
   application data associated with a domain in the DNS.  This section
   gives an overview of such application usage to date.

2.1.  Locating Services in a Domain

   The MX resource record provides the simplest motivating example for
   an application advertising its resources in the Domain Name System.
   The MX resource record contains the domain name of a server that
   receives mail on behalf of the administrative domain in question;
   that domain name must itself be resolved to one or more IP addresses
   through the DNS in order to reach the mail server.  While naming
   conventions for applications might serve a similar purpose (a host
   might be named "mail.example.com" for example), approaching service
   location through the creation of a new resource record yields
   important benefits.  For example, one can put multiple MX records
   under the same name, in order to designate backup resources or to
   load balance across several such servers (see [RFC1794]); these
   properties could not easily be captured by naming conventions (see
   [RFC4367]).

   While the MX record represents a substantial improvement over naming
   conventions as a means of service location, it remains specific to a
   single application.  Thus, the general approach of the MX record was
   adapted to fit a broader class of application through the Service
   (SRV) resource record (originally [RFC2052]).  The SRV record allows
   DNS resolvers to query for particular services and underlying
   transports (for example, HTTP running over TLS, see [RFC2818]) and to



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   learn a host name and port where that service resides in a given
   domain.  It also provides a weighting mechanism to allow load
   balancing across several instances of a service.

   The reliance of applications on the existence of MX and SRV records
   has important implications for the way that applications manage
   identifiers, and that way that applications pass DNS names to
   resolvers.  Email identifiers of the form "user@domain" rely on MX
   records to provide the convenience of simply specifying a "domain"
   component rather requiring to guess which particular host handles
   mail on behalf of the domain.  While for applications like web
   browsing, naming conventions continue to abound ("www.example.com"),
   SRV records allow applications to query for an application-specific
   protocol and transport.  For HTTP, the SRV service name corresponds
   to the URL scheme of the identifier invoked by the application (e.g.,
   when "http://example.com" is the identifier, the SRV query is for
   "_http._tcp.example.com"); for other applications, the SRV service
   that the application passes to the resolver may be implicit in the
   identifier.  The identifier used at the application layer thus
   designates the desired protocol and domain, but the application
   offloads to the DNS finding the location of the host of that service
   for the domain, the port where the service resided on that host, load
   balancing and fault tolerance, and related application features.
   Ultimately, resolvers that acquire MX or SRV records may use them as
   intermediate transformations in order to arrive at an eventual domain
   name that will resolve to the IP addresses to contact for the
   service.

   Locating specific services for a domain is thus the first major
   function that applications offloaded to the DNS in addition to simple
   name resolution.  SRV broadened and generalized the precedent of MX
   to make service location available to any application, rather than
   just to mail.  As the domain name of the located service might
   require a second DNS query to resolve, [RFC1034] shows that the
   Additional Data section of DNS responses may contain the
   corresponding address records for the names of services designated by
   the MX record; this optimization, which requires support in the
   authoritative server and the resolver, is an initial example of how
   support for application features requires changes to DNS operation.

2.2.  NAPTR and DDDS

   The NAPTR resource record evolved to fulfill a need in the transition
   from Uniform Resource Locators (URLs) to the more mature URI
   [RFC3986] framework, which incorporated Uniform Resources Names
   (URNs).  Unlike URLs, URNs typically do not convey enough semantics
   internally to resolve them through the DNS, and consequently a
   separate URI-transformation mechanism is required to convert these



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   types of URIs into domain names.  This allowed identifiers with no
   recognizable domain component to be treated as DNS names for the
   purpose of name resolution.  Once these transformations resulted in a
   domain name, applications could retrieve NAPTR records under that
   name in the DNS.  NAPTR records contain a far more rich and complex
   structure than MX or SRV resource records.  A NAPTR record contains
   two different weighting mechanisms ("order" and "preference"), a
   "service" field to designate the application that the NAPTR record
   described, and then two fields that could contain translations: a
   "replacement" or "regular expression" field, only one of which
   appeared in given NAPTR record.  A "replacement," like NAPTR's
   ancestor the PTR record, simply designates another domain name where
   one would look for records associated with this service in the
   domain.  The "regexp," on the other hand, allows regular expression
   transformations on the original URI intended to transform it into an
   identifier that the DNS could resolve.

   As the Abstract of [RFC2915] says, "This allows the DNS to be used to
   lookup services for a wide variety of resource names (including URIs)
   which are not in domain name syntax."  Any sort of hierarchical
   identifier can potentially be encoded as a DNS name, and thus
   historically the DNS has often been used to resolve identifiers that
   were never devised as a name for an Internet host.  A prominent early
   example is the in-addr domain [RFC0882], which transposes an IPv4
   address into a domain name in order to query the DNS for name(s)
   associated with the address.  Similar mechanisms could obviously be
   applied to other sorts of identifiers that lacked a domain component.
   Eventually, this idea connected with activities to create a system
   for resolving telephone numbers on the Internet, which became known
   as ENUM (originally [RFC2916]).  ENUM borrowed from an earlier
   proposal, the "tpc.int" domain [RFC1530], which provided a means for
   encoding telephone numbers as domain names by applying a string
   preparation algorithm that required reversing the digits and treating
   each individual digit as a label in a DNS name - thus, for example,
   the number +15714345400 became 0.0.4.5.4.3.4.1.7.5.1.tpc.int.

   In the ENUM system, in place of "tpc.int" the special domain
   "e164.arpa" was reserved for use.  In its more mature form in the
   Dynamic Delegation and Discovery Service (DDDS) ([RFC3401] passim)
   framework, this initial transformation of the telephone number to a
   domain name was called the "First Well Known Rule."  Its flexibility
   has inspired a number of proposals beyond ENUM to encode and resolve
   unorthodox identifiers in the DNS.  Provided that the identifiers
   transformed by the First Well Known Rule have some meaningful
   structure and are not overly lengthy, virtually anything can serve as
   an input for the DDDS structure: for example, civic addresses.
   Though [RFC3402] stipulates of the identifier that "the lexical
   structure of this string must imply a unique delegation path," there



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   is no requirement that the identifier be hierarchical, nor that the
   points of delegation in the domain name created by the First Well
   Known Rule correspond to any points of administrative delegation
   implied by the structure of the identifier.

   While this ability to look up names "which are not in the domain name
   syntax" does not change the underlying DNS protocols - the names
   generated by the DDDS algorithm are still just domain names - it does
   change the context in which applications pass name to resolvers, and
   can potentially require very different operational practices of zone
   administrators(see Section 3.3).  In terms of the results of a DNS
   query, the presence of the "regexp" field of NAPTR records enabled
   unprecedented flexibility in the types of identifiers that
   applications could resolve with the DNS.  Since the output of the
   regular expression frequently took the form of a URI (in ENUM
   resolution, for example, a telephone number might be converted into a
   SIP URI [RFC3261]), anything that could be encoded as a URI might be
   the result of resolving a NAPTR record - which, as the next section
   explores, essentially means arbitrary data.

2.3.  Arbitrary Data in the DNS

   Since URI encoding has ways of carrying basically arbitrary data,
   resolving a NAPTR record might result in an output other than an
   identifier which would subsequently be resolved to IP addresses and
   contacted for a particular application - it could give a literal
   result consumed by the application.  Originally, in [RFC2168], the
   intended applicability of the regular expression field in NAPTR was
   much more narrow: the regexp field contained a "substitution
   expression that is applied to the original URI in order to construct
   the next domain name to lookup," in order "change the host that is
   contacted to resolve a URI" or as a way of "changing the path or host
   once the URL has been assigned."  The regular express tools available
   to NAPTR record authors, however, grant much broader powers to alter
   the input string, and thus applications began to rely on NAPTR to
   perform more radical transformations.  By [RFC3402], the output of
   DDDS is wholly application-specific: "the Application must determine
   what the expected output of the Terminal Rule should be," and the
   example given in the document is one of identifying automobile parts
   by inputting a part number is receiving at the end of the process
   receiving information about the manufacturer (though this example
   reflects that the DNS is only one database to which the DDDS
   framework might apply).

   NAPTR did not however pioneer the storage of arbitrary data in the
   DNS.  At the start, [RFC0882] observed that "it is unlikely that all
   users of domain names will be able to agree on the set of resources
   or resource information that names will be used to retrieve," and



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   consequently places little restriction on the information that DNS
   records might carry: it might be "host addresses, mailbox data, and
   other as yet undetermined information."  [RFC1035] defined the TXT
   record, a means to store arbitrary strings in the DNS; [RFC1034]
   specifically stipulates that a TXT contains "descriptive text" and
   that "the semantics of the text depends on the domain where it is
   found."  The existence of TXT records has long provided new
   applications with a rapid way of storing data associated with a
   domain name in the DNS, as adding data in this fashion require no
   registration process.  [RFC1464] added a means of incorporating name/
   value pairs to the TXT record structure, which allowed applications
   to differentiate different chunks of data stored in a TXT record.  In
   this fashion, an application that wants to store additional data in
   the DNS can do so without registering a new resource record type -
   though [RFC5507] points out that it is "difficult to reliably
   distinguish one application's record from otters, and for its parser
   to avoid problems when it encounters other TXT records."

   While open policies surrounding the use of the TXT record have
   resulted in a checkered past for standardizing application usage of
   TXT, TXT has been used as a technical solution for DKIM [RFC4871] to
   store necessary information about the security of email in DNS,
   though within a narrow naming convention (records stored under
   "_domainkey" subdomains).  Storing keys in the DNS became the
   preferred solution for DKIM for several reasons: notably, because the
   public keys associated with email required wide public distribution,
   and because email identifiers contain a domain component that
   applications can easily use to consult the DNS.  If the application
   had to negotiate support for the DKIM mechanism with mail servers, it
   would give rise to bid-down attacks (where attackers misrepresent
   that DKIM is unsupported in the domain) that are not possible if the
   DNS delivers the keys (provided that DNSSEC [RFC4033] guarantees
   authenticity of the data).  However, there are potential issues with
   story large data in the DNS, as discussed in Section 3.2.1.

















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3.  Challenges for the DNS

   The methods discussed in the previous section for transforming
   arbitrary identifiers into domain names, and for returning arbitrary
   data in response to DNS queries, both represent significant
   departures from the basic function of translating host names to IP
   addresses, yet neither fundamentally alters the underlying semantics
   of the DNS.  When we consider, however, that the URIs returned by
   DDDS might be base 64 encoded binary data in the data URL [RFC2397],
   the DNS could effectively implement the entire application feature
   set of any simple query-response protocol.  Effectively, the DDDS
   framework overlays the DNS with a generic database - indeed, the DDDS
   framework was designed to work with any sort of underlying database;
   as [RFC3403] shows, the DNS is only one potential database for DDDS
   to use.  Whether the DNS as an underlying database can support the
   features that some applications of DDDS require, however, is a more
   complicated question.

   As the following subsections will show, the potential that
   applications might rely on the DNS as a generic database gives rise
   to additional requirements that one might expect to find in a
   database access protocol: authentication of the source of queries for
   comparison to access control lists, formulating complex relational
   queries, and asking questions about the structure of the database
   itself.  DNS was not designed to provide these sorts of properties,
   and extending the DNS to encompass them would represent a fundamental
   alteration to its model.  Ultimately, this document concludes that
   efforts to retrofit these capabilities into the DNS would be better
   invested in selecting, or if necessary inventing, other Internet
   services with broader powers than the DNS.  If an application
   protocol designer wants these properties from a database, in general
   this is a good indication that the DNS cannot meet the needs of the
   application in question.

   Since many of these new requirements have emerged from the ENUM
   space, the following sections use ENUM as an illustrative example;
   however, any application using the DNS as a feature-rich database
   could easily end up with similar requirements.

3.1.  Compound Queries

   Traditionally, the DNS requires resolvers to supply no information
   other than the domain name, the type and the class of records sought
   in order to receive a reply from an authoritative server.  Outside of
   the DNS space, however, there are plenty of query-response
   applications that require a compound or relational search, which
   takes into account more than one factor in formulating a response or
   uses no single factor as a key to the database.  For example, in the



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   telephony space, telephone call routing often takes into account
   numerous factors aside from the dialed number, including originating
   trunk groups, interexchange carrier selection, number portability
   data, time of day, and so on.  All are considered simultaneously in
   generating a route.  While in its original conception, ENUM hoped to
   circumvent the traditional PSTN and route directly to Internet-
   enabled devices, the infrastructure ENUM effort to support the
   migration of traditional carrier routing functions to the Internet
   aspires to achieve feature parity with traditional number routing.
   However, [RFC3402] explicitly states that "it is an assumption of the
   DDDS that the lexical element used to make a delegation decision is
   simple enough to be contained within the Application Unique String
   itself.  The DDDS does not solve the case where a delegation decision
   is made using knowledge contained outside the AUS and the Rule (time
   of day, financial transactions, rights management, etc.)."
   Consequently, some consideration has been given to ways to add
   additional data to ENUM queries to give the DNS server sufficient
   information to return a suitable URI (see Section 3.1.1).

   From a sheer syntactical perspective, however, domain names do not
   admit of this sort of rich structure.  Several workarounds have
   attempted to instantiate these sorts of features in DNS queries.
   Most commonly, proposals piggyback additional query parameters as
   EDNS0 extensions (see [RFC2671]); alternatively, the domain name
   itself could be compounded with the additional parameters: one could
   take a name like 0.0.4.5.4.3.4.1.7.5.1.e164.arpa and append a trunk
   group identifier to it, for example, of the form
   tg011.0.0.4.5.4.3.4.1.7.5.1.e164.arpa.  While in the latter case, a
   DNS server can adhere to its traditional behavior in locating
   resource records, the syntactical viability of encoding additional
   parameters in this fashion is very dubious, especially if more than
   one additional parameter is required and the presence of parameters
   is optional.  The former EDNS0 case requires significant changes to
   name server behavior.  The intended applicability of the EDNS0
   mechanism, as the [RFC2761] states, is to "a particular transport
   level message and not to any actual DNS data," and consequently the
   OPT Resource Records it specifies are never to be forwarded; the use
   of EDNS0 for compound queries, however, clearly is intended to
   discriminate actual DNS data rather than to facilitate transport-
   layer handling.  [RFC2761] also specifies that only one OPT pseudo-
   Resource Record can appear in a single request, so the use of this
   mechanism can only compound a query by a single field.  For these
   reasons, this draft recommends against the use of eDNS0 as a
   mechanism for crafting compound DNS queries.

   Moreover, the implications of these sorts of compound queries for
   recursion and caching are potentially serious.  The logic used by the
   authoritative server to respond to a compound query may not be



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   understood by any recursive servers or caches; intermediaries that
   naively assume that the response was selected based on the domain
   name, type and class alone might serve responses to queries in a
   different way than the authoritative server intends.  Even
   comparatively unambiguous expressions of server preference like the
   time-to-live parameter are not always honored by caches.

3.1.1.  Responses Tailored to the Originator

   The most important subcase of the compound queries are DNS responses
   tailored to the identity of their originator, where some sort of
   administrative identity of the originator must be conveyed to the
   DNS.  We must first distinguish this from cases where the originating
   IP address or a similar indication is used to serve a location-
   specific name.  For those sorts of applications, which generally lack
   security implications, relying on factors like the source IP address
   introduces little harm for example, when providing a web portal
   customized to the region of the client, it would not be a security
   breech if the client saw the localized portal of the wrong country.
   Because recursive resolvers may obscure the origination network of
   the DNS client, a recent proposal suggested introducing a new DNS
   query parameter to be populated by DNS recursive resolvers in order
   to preserve the originating IP address (see
   [I-D.vandergaast-edns-client-ip]).  However, aside from purely
   cosmetic uses, these approaches have known limitations due to the
   prevalence of private IP addresses, VPNs and so on which obscure the
   source IP address and instead supply the IP address of an
   intermediary that may be very distant from the originating endpoint.
   Implementing vandergaast does require significant changes in the
   operation of recursive resolvers and the authoritative servers that
   would rely on the original source IP address to select Resource
   Records, and moreover a fundamental change to caching behavior as
   well.

   In other deployments in use today, including those based on the BIND
   "views" feature, the source IP address is used to grant access to a
   private set of resource records.  The security implications of
   trusting the source IP address of a DNS query have prevented most
   solutions along these lines from being standardized (see [RFC6269]),
   though the practice remains widespread in "split horizon" private DNS
   deployment (see Section 4) which typically rely on an underlying
   security layer, such as a physical network or an IPsec VPN, to
   prevent spoofing of the source IP address.  These deployments do have
   a confidentiality requirement to prevent information intended for a
   constrained audience (internal to an enterprise, for example) from
   leaking to the public Internet -- while these internal network
   resources may use private IP addresses which would not be useful on
   the public Internet anyway, in some cases this leakage would reveal



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   topology or other information that the name server provisioner hopes
   to keep private.

   These first two cases, regardless of their acceptance by the
   standards community, have widespread acceptance in the field.  Some
   applications, however, go even further and propose extending the DNS
   to add an application-layer identifier of the originator rather than
   an IP address; for example,
   [I-D.kaplan-dnsext-enum-sip-source-ref-opt-code] provides a SIP URI
   in an eDNS0 parameter (though without any specific provision for
   cryptographically verifying the claimed identity).  Effectively, the
   conveyance of application-layer information about the administrative
   identity of the originator through the DNS is a weak authentication
   mechanism, on the basis of which the DNS server makes an
   authorization decision before sharing resource records.  This can
   parlay into a per-Resource Record confidentiality mechanism, where
   only a specific set of originators are permitted to see resource
   records, or a case where a query for the same name by different
   entities results in completely different resource record sets.  The
   DNS, however, substantially lacks the protocol semantics to manage
   access control lists for data, and again, caching and recursion
   introduce significant challenges for applications that attempt to
   offload this responsibility to the DNS.  Achieving feature parity
   with even the simplest authentication mechanisms available at the
   application layer would like require significant rearchitecture of
   the DNS.

3.2.  Using DNS as a Generic Database

   As previously noted, the use of the First Well Known Rule of DDDS
   combined with data URLs (or potentially TXT records) effectively
   allows the DNS to answer queries for arbitrary strings and to return
   arbitrary data as a value.  Some query-response applications,
   however, require queries and responses that simply fall outside the
   syntactic capabilities of the DNS.  Domain names themselves must
   conform to certain syntactic constraints: they must consist of labels
   that do not exceed 63 characters while the total length of the
   encoded name may not exceed 255 octets, they must obey fairly strict
   encoding rules, and so on.  The DNS therefore cannot be a completely
   generic database.  Similar concerns apply to the size of DNS
   responses.

3.2.1.  Large Data in the DNS

   While the data URL specification [RFC2397] notes that it is "only
   useful for short values," unfortunately it gives no particular
   guidance about what "short" might mean.  Many applications today use
   quite large data URLs (containing a megabyte or more of data) as



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   workarounds in environments where only URIs can syntactically appear.
   The meaning of "short" in an application context is probably very
   different from how we should understand it in a DNS message.
   Referring to a typical public DNS deployment, [RFC5507] observes that
   "there's a strong incentive to keep DNS messages short enough to fit
   in a UDP datagram, preferably a UDP datagram short enough not to
   require IP fragmentation," which entails a maximum size of 512
   octets.  While the use of TCP and eDNS0 allows DNS responses to be
   quite long, this requires stateful operation of servers which can be
   costly in deployments where servers have only fleeting connections to
   many clients.  Ultimately, there are forms of data that an
   application might store in the DNS that exceed reasonable limits: in
   the ENUM context, for example, something like storing base 64 encoded
   mp3 files of custom ringtones.

   Designs relying on storage of large amounts of data within DNS RRs
   furthermore need to minimize the potential damage achievable in a
   reflection attack (see [RFC4732], Section 3), in which the attacker
   sends DNS queries with a forged source address, and the victim
   receives the response.  By generating a large response to a small
   query, the attacker can magnify the bandwidth directed at the victim.
   Where the responder supports EDNS0, an attacker may set the requester
   maximum payload size to a larger value while querying for a large
   Resource Record, such as a certificate [RFC4398].  Thus the
   combination of large data stored in DNS RRs and responders supporting
   large payload sizes has the potential to increase the potential
   damage achievable in a reflection attack.  Since a reflection attack
   can be launched from any network that does not implement source
   address validation, these attacks are difficult to eliminate absent
   the ubiquitous deployment of source address validation.  Since
   reflection attacks are most damaging when launched from high
   bandwidth networks, the implementation of source address validation
   on these networks is particularly important.

   The bandwidth that can be mustered in a reflection attack directed by
   a botnet controlling broadband hosts is sobering.  For example, if a
   responder could be directed to generate a 10KB response in reply to a
   50 octet query, then magnification of 200:1 would be attainable.
   This would enable a botnet controlling 10000 hosts with 1 Mbps of
   bandwidth to focus 2000 Gbps of traffic on the victim, more than
   sufficient to congest any site on the Internet.

   Since it is difficult to complete a TCP three-way handshake begun
   from a forged source address, DNS reflection attacks utilize UDP
   queries.  Unless the attacker uses EDNS0 [RFC2671] to enlarge the
   requester's maximum payload size, a response can only reach 576
   octets before the truncate bit is set in the response.  This limits
   the maximum magnification achievable from a DNS query that does not



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   utilize EDNS0.  As the large disparity between the size of a query
   and size of the response creates this amplification, implementations
   should limit EDNS0 responses to a specific ratio compared to the
   request size.  The precise ratio can be configured on a per-
   deployment basis, but without this limitation, EDNS0 can cause
   significant harm.

3.3.  Administrative Structures Misaligned with the DNS

   While the DDDS framework enables any sort of alphanumeric data to
   serve as a DNS name through the application of the First Well Known
   Rule, the delegative structure of the resulting DNS name may not
   reflect the division of responsibilities for the resources that the
   alphanumeric data indicates.  While [RFC3402] requires only that
   "Application Unique String has some kind of regular, lexical
   structure that the rules can be applied to," DDDS is first and
   foremost a delegation system: its abstract stipulates that "Well-
   formed transformation rules will reflect the delegation of management
   of information associated with the string."  Telephone numbers in the
   United States, for example, are assigned and delegated in a
   relatively complex manner.  Historically, the first six digits of a
   nationally specific number (called the "NPA/NXX") reflected a point
   of administrative delegation from the number assignment agency to a
   carrier; from these blocks of ten thousand numbers, the carrier would
   in turn delegate individual assignments of the last four digits (the
   "XXXX" portion) to particular customers.  However, after the rise of
   North American telephone number portability in the 1990s, the first
   point of delegation went away: the delegation is effectively from the
   number authority to the carrier for each the complete ten-digit
   number (NPA/NXX-XXXX).  While technical implementation details differ
   from nation to nation, number portability systems with similar
   administrative delegations now exist worldwide.

   To render these identifiers as domain names in accordance with the
   DDDS Rule for ENUM yields a large flat zone with no points of
   administrative delegation (or zone cuts) from the country-code
   administrator, e.g. 1.e164.arpa, down to the ultimately assignee of a
   number.  The scalability difficulties of maintaining a single DNS
   zone with potentially over five billion names in it manifest in a
   number of areas (in practice, there are closer to 300M allocated
   telephone numbers in the United States today, but that is still more
   than three times the number of .com domain names).  The scalability
   that results from caching depends on points of delegation so that
   cached results for intermediate servers take the load off higher tier
   servers.  If there are no such delegations, then as in the telephone
   number example the zone apex server must bear the entire load for
   queries.  Worse still, number portability also introduces far more
   dynamism in number assignment, where in some regions updated



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   assignees for ported numbers must propagate within fifteen minutes of
   a change in administration.  Jointly, these two problems cacheing the
   zone extremely problematic.  Moreover, traditional tools for DNS
   replication, such as the zone transfer protocol AXFR [RFC1034] and
   IXFR [RFC1995], are not designed to accommodate zones with these
   dimensions and properties.  When traditional DNS management tools no
   longer apply to the structures that an application tries to provision
   in the DNS, this is another clue that the DNS might not be the right
   place for an application to store its data.

   DDDS provides a capability that transforms arbitrary strings into
   domain names, but those names play well with the DNS only when the
   input string have an administrative structure that maps on DNS
   delegations.  In the first place, delegation implies some sort of
   hierarchical structure.  Any mechanism to map a hierarchical
   identifier into a DNS name should be constructed such that the
   resulting DNS name does match the natural hierarchy of the original
   identifier.  Although telephone numbers, even in North America, have
   some hierarchical qualities (like the geographical areas
   corresponding to their first three digits), after the implementation
   of number portability these points no longer mapped onto an
   administrative delegation.  If the input string to the DDDS does not
   have a hierarchical structure representing administrative delegations
   that can map onto the DNS distribution system, that string probably
   is not a good candidate for translating into a domain name.

   The difficulty of mapping the DNS to administrative structures can
   even occur with traditional domain names, where applications expect
   clients to infer or locate zone cuts.  In the web context, for
   example, it can be difficult for applications to determine whether
   two domain names represent the same "site" when

3.3.1.  Metadata about Tree Structure

   There are also other ways in which the delegative structure of an
   identifier may not map well onto the DNS.  Traditionally, DNS
   resolvers assume that when they receive a domain name from an
   application, that the name is complete - that it is not a fragment of
   a DNS name that a user is still in the middle of typing.  For some
   communications systems, however, this assumption does not apply.
   ENUM use cases have surfaced a couple of optimization requirements to
   reduce unnecessary calls and queries by including metadata in the DNS
   that describes the contents and structure of ENUM DNS trees to help
   applications to handle incomplete queries or queries for domains not
   in use.  In particular, the "send-n" proposal
   [I-D.bellis-enum-send-n] hopes to reduce the number of DNS queries
   sent in regions with variable-length numbering plans.  When a dialed
   number potentially has a variable length, a telephone switch



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   ordinarily cannot anticipate when a dialed number is complete, as
   only the numbering plan administrator (who may be a national
   regulator, or even in open number plans a private branch exchange)
   knows how long a telephone number needs to be.  Consequently, a
   switch trying to resolve such a number through a system like ENUM
   might send a query for a telephone number that has only partially
   been dialed in order to test its completeness.  The "send-n" proposal
   installs in the DNS a hint informing the telephone switch of the
   minimum number of digits that must be collected by placing in zones
   corresponding to incomplete telephone numbers some Resource Records
   which state how many more digits are required - effectively how many
   steps down the DNS tree one must take before querying the DNS again.
   Unsurprisingly, those boundaries reflect points of administrative
   delegation, where the parent in a number plan yields authority to a
   child.  With this information, the application is not required to
   query the DNS every time a new digit is dialed, but can wait to
   collect sufficient digits to receive a response.  As an optimization,
   this practice thus saves the resources of the DNS server - though the
   call cannot complete until all digits are collected, so at best this
   mechanism only reduces the time the system will wait before sending
   an ENUM query after collecting the final dialed digit.  A
   tangentially related proposal, [I-D.ietf-enum-unused], similarly
   places resource records in the DNS that tell the application that it
   need not attempt to reach a number on the telephone network, as the
   number is unassigned - a comparable general DNS mechanism would
   identify, for a domain name with no heather or not the domain had
   been allocated to an owner.

   Both proposals optimize application behavior by placing metadata in
   the DNS that predicts the success of future queries or application
   invocation by identifying points of administrative delegation or
   assignment in the tree.  In some respects, marking a point in the
   tree where a zone begins or end has some features in common with the
   traditional parent zone use of the NS record type, except instead of
   pointing to a child zone, these metadata proposals point to distant
   grandchildren.  While this does not change resolver behavior as such
   (instead, it changes the way that applications invoke the resolver),
   it does have implications for the practices for zone administrators.
   Metadata in the authoritative server remain synchronized with the
   state of the resources it predicts.  Maintaining that
   synchronization, however, requires that the DNS have semi-real time
   updates that may conflict with scale and caching mechanisms.

   It may also raise questions about the authority and delegation model.
   Who gets to supply records for unassigned names?  While in the
   original but little-used "golden" root model of ENUM, this would
   almost certainly be a numbering plan administrator, it is far less
   clear who that would be in the more common and successful



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   "infrastructure" ENUM models (see Section 4).  Ultimately, these
   metadata proposals share some qualities with DNS redirection services
   offered by ISPs (for example, [I-D.livingood-dns-redirect]) which
   "help" web users who try to browser to sites that do not exist -
   though in those cases, at least the existence or non-existence of a
   domain name is a fact about the Internet, rather than about an
   external network like the telephone system which must be synchronized
   with the DNS tree in the metadata proposals.  In send-n, different
   leaf zones, who administer telephone numbers of different lengths,
   can only provision their hints at their own apex, which provides an
   imperfect optimization: they cannot install it in a parent, both
   because they lack the authority and because different zones would
   want to provision contradictory data.  An application protocol
   designer who wants to manage identifiers whose administrative model
   does not map well onto the DNS would be better served to implement
   such features outside the DNS.

3.4.  Domain Redirection

   Most Internet application services provide a redirection feature -
   when you attempt to contact a service, the service may refer you to a
   different service instance, potentially in another domain, that is
   for whatever reason better suited to address a request.  In HTTP and
   SIP, for example, this feature is implemented by the 300 class
   responses containing one or more better URIs that may indicate that a
   resource has moved temporarily or permanently to another service.
   Several tools in the DNS, including the SRV record, can provide a
   similar feature at a DNS level, and consequently some applications as
   an optimization offload the responsibility for redirection to the
   DNS; NAPTR can also provide this capability on a per-application
   basis, and numerous DNS resource records can provide redirection on a
   per-domain basis.  This can prevent the unnecessary expenditure of
   application resources on a function that could be performed as a
   component of a DNS lookup that is already a prerequisite for
   contacting the service.  Consequently, in some deployment
   architectures this DNS-layer redirection is used for virtual hosting
   services.

   Implementing domain redirection in the DNS, however, has important
   consequences for application security.  In the absence of universal
   DNSSEC, applications must blindly trust that their request has not
   been hijacked at the DNS layer and redirected to a potentially
   malicious domain, unless some subsequent application mechanism can
   provide the necessary assurance.  By way of contrast, application-
   layer redirections protocols like HTTP and SIP have widely deployed
   security mechanisms such as TLS that can use certificates to vouch
   that a 300 response came from the domain that the originator
   initially hoped to contact.



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   A number of applications have attempted to provide an after-the-fact
   security mechanism that verifies the authority of a DNS delegation in
   the absence of DNSSEC.  The specification for dereferencing SIP URIs
   ([RFC3263], reaffirmed in [RFC5922]), requires that during TLS
   establishment, the site eventually reached by a SIP request present a
   certificate corresponding to the original URI expected by the user
   (in other words, if example.com redirects to example.net in the DNS,
   this mechanism expects that example.net will supply a certificate for
   example.com in TLS, per the HTTP precedent in [RFC2818]), which
   requires a virtual hosting service to possess a certificate
   corresponding to the hosted domain.  This restriction rules out many
   styles of hosting deployments common in the web world today, however.
   [I-D.barnes-hard-problem] explores this problem space, and
   [I-D.saintandre-tls-server-id-check] proposes a solution for all
   applications that use TLS.  Potentially, new types of certificates
   (similar to [RFC4985]) might bridge this gap, but support for those
   certificates would require changes to existing certificate authority
   practices as well as application behavior.

   All of these application-layer measures attempt to mirror the
   delegation of authority in the DNS, when the DNS itself serves as the
   ultimate authority on how domains are delegated.  Synchronizing a
   static instrument like a certificate with a delegation in the DNS,
   however, is problematic because delegations are not static: revoking
   and reissuing a certificate every time a delegation changes is
   cumbersome operationally.  In environments where DNSSEC is not
   available, the problems with securing DNS-layer redirections would be
   avoided by performing redirections in the application-layer.























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4.  Private DNS and Split Horizon

   The classic view of the uniqueness of DNS names is given in
   [RFC2826]:

   DNS names are designed to be globally unique, that is, for any
   one DNS name at any one time there must be a single set of DNS
   records uniquely describing protocol addresses, network resources and
   services associated with that DNS name.  All of the applications
   deployed on the Internet which use the DNS assume this, and Internet
   users expect such behavior from DNS names.

   However, [RFC2826] "does not preclude private networks from operating
   their own private name spaces," but notes that if private networks
   "wish to make use of names uniquely defined for the global Internet,
   they have to fetch that information from the global DNS naming
   hierarchy."  There are various DNS deployments outside of the global
   public DNS, including "split horizon" deployments and DNS usages on
   private (or virtual private) networks.  In a split horizon, an
   authoritative server gives different responses to queries from the
   public Internet than they do to internal resolvers; as they typically
   differentiate internal from public queries by the source IP address,
   the concerns in Section 3.1.1 apply to such deployments.  When the
   internal address space range is private [RFC1918], this both makes it
   easier for the server to discriminate public from private and harder
   for public entities to impersonate nodes in the internal network.
   Networks that are made private physically, or logically by
   cryptographic tunnels, make these private deployments more secure.
   The most complex deployments along these lines use multiple virtual
   private networks to serve different answers for the same name to many
   networks.

   The use cases that motivate split horizon DNS typically involve
   restricting access to some network services - intranet resources like
   internal web site, development servers or directories, for example -
   while preserving the ease of use offered by domain names for internal
   users.  While for many of these resources, the split horizon would
   not return answers to public resolvers for those internal resources
   (those records are kept confidential from the public), in some cases,
   the same name (e.g., "mail.example.com") might resolve to one host
   internally but another externally.  The requirements for multiple-VPN
   private deployments, however, are different: in this case the
   authoritative server gives different, and confidential, answers to a
   set of resolvers querying for the same name.  While these sorts of
   use cases rarely arise for traditional domain names, where, as
   [RFC2826] says, users and applications expect a unique resolution for
   a name, they can easily arise when other sorts of identifiers have
   been translated by a mechanism like DDDS into "domain name syntax."



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   Telephone calls, for example, are traditionally routed through highly
   mediated networks, and an attempt to find a route for a call often
   requires finding an appropriate intermediary based on the source
   network and location rather than finding an endpoint (see the
   distinction between the Look-Up Function and Location Routing
   Function in [RFC5486]).  Moreover, the need for selective responses
   and confidentially is easily motivated when the data returned by the
   DNS is no longer "describing protocol addresses, network resources
   and services," but instead arbitrary data.  Although ENUM was
   originally intended for deployment in the global public root of the
   DNS (under e164.arpa), for example, the requirements of maintaining
   telephone identifiers in the DNS quickly steered operators into
   private deployments.

   In private environments, it is also easier to deploy any necessary
   extensions than it is in the public DNS: in the first place,
   proprietary non-standard extensions and parameters can easily be
   integrated into DNS queries or responses as the implementations of
   resolvers and servers can likely be controlled; secondly,
   confidentiality and custom responses can be provided by deploying,
   respectively, underlying physical or virtual private networks to
   shield the private tree from public queries, and effectively
   different virtual DNS trees for each administrative entity that might
   launch a query; thirdly, in these constrained environments, caching
   and recursive resolvers can be managed or eliminated in order to
   prevent any unexpected intermediary behavior.  While these private
   deployments serve an important role in the marketplace, there are
   risks in using techniques intended only for deployment in private and
   constrained environments as the basis of a standard solution.  When
   proprietary parameters or extensions are deployed in these private
   environments, experience teaches us that these implementations will
   begin to interact with the public DNS, and that the private practices
   will leak.  The history of SIP, for example, was full of supposedly
   private extensions (notably the "P-" headers) which saw widespread
   success and use outside of the constrained environments for which
   they were specifically designed.  Therefore, keeping these features
   within higher-layer applications rather than offloading them to the
   DNS is preferred.













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5.  Principles and Guidance

   The success of the DNS relies on the fact that it is a distributed
   database, one that has the property that it is loosely coherent and
   that it offers lookup instead of search functionality.  Loose
   coherency means that answers to queries are coherent within the
   bounds of data replication between authoritative servers and caching
   behavior by recursive name servers.  It is critical that application
   designers who intend to use the DNS to support their applications
   consider the implications that their uses have for the behavior of
   resolvers, intermediaries including caches and recursive resolvers,
   and authoritative servers.

   It is likely that the DNS provides a good match whenever applications
   needs are aligned with the following properties:

      Data stored in the DNS can be propagated and cached using
      conventional DNS mechanisms, without intermediaries needing to
      understand exceptional logic (considerations beyond the name, type
      and class of the query) used by the authoritative server to
      formulate responses

      Data stored in the DNS is indexed by keys that do not violate the
      syntax or semantics of domain names

      Applications invoke the DNS to resolve complete names, not
      fragments

      Answers do not depend on an application-layer identity of the
      entity doing the query

      Ultimately, applications invoke the DNS to assist in communicating
      with the domain of the name they are resolving

   Whenever one of the four properties above does not apply to one's
   data, one should seriously consider whether the DNS is the best place
   to store actual data.  On the other hand, good indicators that the
   DNS is not the appropriate tool for solving problems is when you have
   to worry about:

      Populating metadata about domain boundaries within the tree - the
      points of administrative delegation in the DNS are something that
      applications should in general not be aware off

      Domain names derived from identifiers that do not share a
      semantics or administrative model compatible with the DNS





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      Selective confidentiality of data stored in and provided by the
      DNS

      DNS responses not fitting into UDP packets, unless EDNS0 is
      available, and only then with the caveats discussed above

   In cases where applications require these sorts of features, they are
   simply better instantiated by independent application-layer protocols
   than the DNS.  The query-response semantics of the DNS are similar to
   HTTP, for example, and the objects which HTTP can carry both in
   queries and responses can easily contain the necessary structure to
   manage compound queries.  Many applications already use HTTP because
   of widespread support for it in middleboxes.  Similarly, HTTP has
   numerous ways to provide the necessary authentication, authorization
   and confidentiality properties that some features require, as well as
   the redirection properties discussed in Section 3.4.  The overhead of
   using HTTP may not be appropriate for all environments, but even in
   environments where a more lightweight protocol is appropriate, DNS is
   not the only alternative.

   Where the administrative delegations of the DNS form a necessary
   component in the instantiation of an application feature, there are
   various ways that the DNS can bootstrap access to an independent
   application-layer protocol better suited to field the queries in
   question.  For example, since NAPTR or URI resource
   [I-D.faltstrom-uri] records can contain URIs, those URIs can in turn
   point to an external query-response service such as HTTP where more
   syntactically and semantically rich queries and answers might be
   exchanged.






















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6.  Security Considerations

   Many of the concerns about how applications use the DNS discussed in
   this document involve security.  The perceived need to authenticate
   the source of DNS queries (see Section 3.1.1) and authorize access to
   particular resource records also illustrates the fundamental security
   principles that arise from offloading certain application features to
   the DNS.  As Section 3.2.1 observes, large data in the DNS can
   provide a means of generating reflection attacks, and without the
   remedies discussed in that section (especially the use of EDNS0 and
   TCP) the presence of large records is not recommended.  Section 3.4
   discusses a security problem concerning redirection that has surfaced
   in a number of protocols (see [I-D.barnes-hard-problem]).

   While DNSSEC, were it deployed universally, can play an important
   part in securing application redirection in the DNS, DNSSEC does not
   provide a means for a resolver to authenticate itself to a server,
   nor a framework for servers to return selective answers based on the
   authenticated identity of resolvers.  The existing feature set of
   DNSSEC is however sufficient for providing security for most of the
   ways that applications traditionally have used the DNS.  Nothing in
   this document is intended to discourage either the implementation,
   deployment, or use of Secure DNS Dynamic Updates ([RFC2136],
   [RFC3007]) to the DNS system, DNS Security ([RFC4033] and related
   specifications), or the use of Secure DNS Dynamic Updates in
   combination with DNS Security.

























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7.  IANA Considerations

   This document contains no considerations for the IANA.
















































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8.  IAB Members

   Internet Architecture Board Members at the time this document was
   published were:

   [TO BE INSERTED]













































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9.  Acknowledgements

   The IAB appreciates the comments and often spirited disagreements of
   Ed Lewis, Dave Crocker, Ray Bellis, Lawrence Conroy, Ran Atkinson,
   Patrik Faltstrom and Eliot Lear.














































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10.  Informative References

   [I-D.barnes-hard-problem]
              Barnes, R. and P. Saint-Andre, "High Assurance Re-
              Direction (HARD) Problem Statement",
              draft-barnes-hard-problem-00 (work in progress),
              July 2010.

   [I-D.bellis-enum-send-n]
              Bellis, R., "IANA Registrations for the 'Send-N'
              Enumservice", draft-bellis-enum-send-n-02 (work in
              progress), June 2008.

   [I-D.faltstrom-uri]
              Faltstrom, P. and O. Kolkman, "The Uniform Resource
              Identifier (URI) DNS Resource Record",
              draft-faltstrom-uri-06 (work in progress), October 2010.

   [I-D.ietf-enum-unused]
              Stastny, R., Conroy, L., and J. Reid, "IANA Registration
              for Enumservice UNUSED", draft-ietf-enum-unused-04 (work
              in progress), March 2008.

   [I-D.kaplan-dnsext-enum-sip-source-ref-opt-code]
              Kaplan, H., Walter, R., Gorman, P., and M. Maharishi,
              "EDNS Option Code for SIP and PSTN Source Reference Info",
              draft-kaplan-dnsext-enum-sip-source-ref-opt-code-03 (work
              in progress), October 2011.

   [I-D.livingood-dns-redirect]
              Creighton, T., Griffiths, C., Livingood, J., and R. Weber,
              "DNS Redirect Use by Service Providers",
              draft-livingood-dns-redirect-03 (work in progress),
              October 2010.

   [I-D.saintandre-tls-server-id-check]
              Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              in Certificates Used with Transport Layer Security",
              draft-saintandre-tls-server-id-check-09 (work in
              progress), August 2010.

   [I-D.vandergaast-edns-client-ip]
              Contavalli, C., Gaast, W., Leach, S., and D. Rodden,
              "Client IP information in DNS requests",
              draft-vandergaast-edns-client-ip-01 (work in progress),
              May 2010.




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   [RFC0882]  Mockapetris, P., "Domain names: Concepts and facilities",
              RFC 882, November 1983.

   [RFC0974]  Partridge, C., "Mail routing and the domain system",
              RFC 974, January 1986.

   [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.

   [RFC1464]  Rosenbaum, R., "Using the Domain Name System To Store
              Arbitrary String Attributes", RFC 1464, May 1993.

   [RFC1530]  Malamud, C. and M. Rose, "Principles of Operation for the
              TPC.INT Subdomain: General Principles and Policy",
              RFC 1530, October 1993.

   [RFC1794]  Brisco, T., "DNS Support for Load Balancing", RFC 1794,
              April 1995.

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

   [RFC1995]  Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
              August 1996.

   [RFC2052]  Gulbrandsen, A. and P. Vixie, "A DNS RR for specifying the
              location of services (DNS SRV)", RFC 2052, October 1996.

   [RFC2136]  Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
              "Dynamic Updates in the Domain Name System (DNS UPDATE)",
              RFC 2136, April 1997.

   [RFC2168]  Daniel, R. and M. Mealling, "Resolution of Uniform
              Resource Identifiers using the Domain Name System",
              RFC 2168, June 1997.

   [RFC2397]  Masinter, L., "The "data" URL scheme", RFC 2397,
              August 1998.

   [RFC2671]  Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
              RFC 2671, August 1999.

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




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   [RFC2826]  Internet Architecture Board, "IAB Technical Comment on the
              Unique DNS Root", RFC 2826, May 2000.

   [RFC2915]  Mealling, M. and R. Daniel, "The Naming Authority Pointer
              (NAPTR) DNS Resource Record", RFC 2915, September 2000.

   [RFC2916]  Faltstrom, P., "E.164 number and DNS", RFC 2916,
              September 2000.

   [RFC3007]  Wellington, B., "Secure Domain Name System (DNS) Dynamic
              Update", RFC 3007, November 2000.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC3263]  Rosenberg, J. and H. Schulzrinne, "Session Initiation
              Protocol (SIP): Locating SIP Servers", RFC 3263,
              June 2002.

   [RFC3401]  Mealling, M., "Dynamic Delegation Discovery System (DDDS)
              Part One: The Comprehensive DDDS", RFC 3401, October 2002.

   [RFC3402]  Mealling, M., "Dynamic Delegation Discovery System (DDDS)
              Part Two: The Algorithm", RFC 3402, October 2002.

   [RFC3403]  Mealling, M., "Dynamic Delegation Discovery System (DDDS)
              Part Three: The Domain Name System (DNS) Database",
              RFC 3403, October 2002.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

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

   [RFC4366]  Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
              and T. Wright, "Transport Layer Security (TLS)
              Extensions", RFC 4366, April 2006.

   [RFC4367]  Rosenberg, J. and IAB, "What's in a Name: False
              Assumptions about DNS Names", RFC 4367, February 2006.

   [RFC4732]  Handley, M., Rescorla, E., and IAB, "Internet Denial-of-
              Service Considerations", RFC 4732, December 2006.



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   [RFC4871]  Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
              J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
              Signatures", RFC 4871, May 2007.

   [RFC4985]  Santesson, S., "Internet X.509 Public Key Infrastructure
              Subject Alternative Name for Expression of Service Name",
              RFC 4985, August 2007.

   [RFC5486]  Malas, D. and D. Meyer, "Session Peering for Multimedia
              Interconnect (SPEERMINT) Terminology", RFC 5486,
              March 2009.

   [RFC5507]  IAB, Faltstrom, P., Austein, R., and P. Koch, "Design
              Choices When Expanding the DNS", RFC 5507, April 2009.

   [RFC5922]  Gurbani, V., Lawrence, S., and A. Jeffrey, "Domain
              Certificates in the Session Initiation Protocol (SIP)",
              RFC 5922, June 2010.

   [RFC6269]  Ford, M., Boucadair, M., Durand, A., Levis, P., and P.
              Roberts, "Issues with IP Address Sharing", RFC 6269,
              June 2011.





























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Authors' Addresses

   Olaf Kolkman
   NLNet

   Email: olaf@nlnetlabs.nl


   Jon Peterson
   NeuStar, Inc.

   Email: jon.peterson@neustar.biz


   Hannes Tschofenig
   Nokia Siemens Networks

   Email: Hannes.Tschofenig@gmx.net


   Bernard Aboba
   Microsoft Corporation

   Email: bernarda@microsoft.com



























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