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Internet Architecture Board (IAB)                            J. PetersonRequest for Comments: 6950                                 NeuStar, Inc.Category: Informational                                       O. KolkmanISSN: 2070-1721                                               NLnet Labs                                                           H. Tschofenig                                                  Nokia Siemens Networks                                                                B. Aboba                                                                   Skype                                                            October 2013Architectural Considerations on Application Features in the DNSAbstract   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; some, for example, transform   identifiers other than domain names into formats that the DNS can   process, and then fetch application data or service location data   from the DNS.  Proposals incorporating sophisticated application   behavior using DNS as a substrate have raised questions about the   role of the DNS as an application platform.  This document explores   the architectural consequences of using the DNS to implement certain   application features, and it provides guidance to future application   designers as to the limitations of the DNS as a substrate and the   situations in which alternative designs should be considered.Status of This Memo   This document is not an Internet Standards Track specification; it is   published for informational purposes.   This document is a product of the Internet Architecture Board (IAB)   and represents information that the IAB has deemed valuable to   provide for permanent record.  It represents the consensus of the   Internet Architecture Board (IAB).  Documents approved for   publication by the IAB are not a candidate for any level of Internet   Standard; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6950.Peterson, et al.              Informational                     [Page 1]

RFC 6950               Application Features in DNS          October 2013Copyright Notice   Copyright (c) 2013 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.Table of Contents1. Motivation ......................................................22. Overview of DNS Application Usages ..............................42.1. Locating Services in a Domain ..............................52.2. NAPTR and DDDS .............................................62.3. Arbitrary Data in the DNS ..................................83. Challenges for the DNS .........................................103.1. Compound Queries ..........................................103.1.1. Responses Tailored to the Originator ...............123.2. Using DNS as a Generic Database ...........................143.2.1. Large Data in the DNS ..............................143.3. Administrative Structures Misaligned with the DNS .........163.3.1. Metadata about Tree Structure ......................183.4. Domain Redirection ........................................204. Private DNS and Split Horizon ..................................215. Principles and Guidance ........................................236. Security Considerations ........................................257. IAB Members at the Time of Approval ............................268. Acknowledgements ...............................................269. Informative References .........................................271.  Motivation   The Domain Name System (DNS) has long provided a general means of   translating domain names into Internet Protocol addresses, which   makes the Internet easier to use by providing a valuable layer of   indirection between names and lower-layer protocol elements.   [RFC0974] documented a further use of the DNS: to locate an   application service operating in a domain, via the Mail Exchange (MX)   Resource Record; these records help email addressed to the domain to   find a mail service for the domain sanctioned by the zone   administrator.Peterson, et al.              Informational                     [Page 2]

RFC 6950               Application Features in DNS          October 2013   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)   Resource Record (originally described in [RFC2168]), especially as it   evolved into the more general Dynamic Delegation Discovery System   (DDDS) [RFC3401] framework, added a generic mechanism for storing   application data in the DNS.  Primarily, this involved a client-side   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 examples of this are   telephone numbers, as resolved by the DDDS application ENUM.  Recent   work, such as DomainKeys Identified Mail (DKIM) [RFC6376], has   enabled security features of applications to be advertised through   the DNS, via the TXT Resource Record.   The scope of application usage of the DNS has thus increased over   time.  Applications in many environments require features such as   confidentiality, 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.  However, some   proposed usages of, and extensions to, the DNS have become misaligned   with both the DNS architecture and the DNS protocol.  If we take the   example of confidentiality, we see that in the global public DNS, the   resolution of domain names to IP addresses is an exchange of public   information with no expectation of confidentiality.  Thus, the   underlying query/response protocol has no 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 policy can the DNS maintain confidential records, by   providing distinct answers to the private and public users of the   DNS.  In support of load-balancing or other optimizations, a DNS   server may return different addresses in response to queries from   different sources, or even no response at all; seeSection 3.1.1 for   details.   This document provides guidance to application designers and   application protocol designers looking to use the DNS to support   features in their applications.  It provides an overview of past   application usage of the DNS as well as a review of proposed new   usages.  It identifies concerns and trade-offs and provides guidance   on 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 designersPeterson, et al.              Informational                     [Page 3]

RFC 6950               Application Features in DNS          October 2013   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 applications that rely on the DNS for the   implementation of certain features, 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   demand further changes and extensions that are fundamentally at odds   with the strengths of the DNS.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 transitioning the "mapping between host names... and   ARPA Internet addresses" from a global table (the original "hosts"   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 service discovery mechanism could be built   for each such application protocol that needed this functionality,   the universal support for the DNS encourages installing these   features into its public tree rather than inventing something new.   Thus, over time, several other applications leveraged DNS resource   records for locating services in a domain or for storing application   data associated with a domain in the DNS.  This section gives   examples of various types of DNS usage by applications to date.Peterson, et al.              Informational                     [Page 4]

RFC 6950               Application Features in DNS          October 20132.1.  Locating Services in a Domain   The MX Resource Record provides the simplest example of an   application advertising its domain-level 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], though more recently DNS-based Service   Discovery (DNS-SD) [RFC6763] codifies service instance naming   conventions for use across applications to locate services in a   domain).   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 applications through the Service   (SRV) Resource Record (originally described in [RFC2052]).  The SRV   record allows DNS resolvers to query for particular services and   underlying transports (for example, HTTP running over Transport Layer   Security (TLS) [RFC2818]) and to 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 the way that applications pass domain 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 than requiring an application to guess which   particular host handles mail on behalf of the domain.  While naming   conventions continue to abound ("www.example.com") for applications   like web browsing, SRV records allow applications to query for an   application-specific protocol and transport in the domain.  For the   Lightweight Directory Access Protocol (LDAP), the SRV service name   corresponds to the URL scheme of the identifier invoked by the   application (e.g., when "ldap://example.com" is the identifier, the   SRV query passed to the resolver is for "_ldap._tcp.example.com");   for other applications, the SRV service name that the application   passes to the resolver may be implicit in the identifier rather thanPeterson, et al.              Informational                     [Page 5]

RFC 6950               Application Features in DNS          October 2013   explicit.  In either case, the application delivers the service name   to the DNS to find the location of the host of that service for the   domain, the port where the service resides on that host, additional   locations or ports for load-balancing and fault tolerance, and   related application features.   Locating specific services for a domain was the first major function   for which applications started using the DNS beyond 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 applications that acquire MX (or SRV) records might need   to perform further queries or transformations in order to arrive at   an eventual domain name that will resolve to the IP addresses for the   service, [RFC1034] allowed 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.  At the same time, this is an example of an   extension of the DNS that cannot be universally relied on: many DNS   resolver implementations will ignore the addresses in the additional   section of the DNS answers because of the trustworthiness issues   described in [RFC2181].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 Uniform   Resource Identifier (URI) [RFC3986] framework, which incorporated   Uniform Resource 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 types of URIs into domain names.  This allows   identifiers with no recognizable domain component to be treated as   domain names for the purpose of name resolution.  Once these   transformations result in a domain name, applications can 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 describes, and then two fields that can contain   translations: a "replacement" field or a "regexp" (regular   expression) field, only one of which appears in a given NAPTR record   (see [RFC2168]).  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.  ThePeterson, et al.              Informational                     [Page 6]

RFC 6950               Application Features in DNS          October 2013   "regexp", on the other hand, allows regular expression   transformations on the original URI intended to turn it into an   identifier that the DNS can 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 domain 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 found in the in-addr domain [RFC0883], in which IPv4   addresses are encoded as domain names by applying a string   preparation algorithm that required reversing the octets and treating   each individual octet as a label in a domain name -- thus, for   example, the address 192.0.2.1 became 1.2.0.192.in-addr.arpa.  This   allowed resolvers to query the DNS to learn name(s) associated with   an IPv4 address.  The same mechanism has been applied to IPv6   addresses [RFC3596] and other sorts of identifiers that lack 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 described in [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 domain   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 the more mature form of the NAPTR standard, in the Dynamic   Delegation Discovery System (DDDS) [RFC3401] framework, the initial   transformation of an identifier (such as a telephone number) to a   domain name was called the "First Well Known Rule".  The address-   reversing mechanism, whereby a query name is formed by reversing an   IPv4 address and prepending it to the in-addr.arpa domain, is   generalized for the use of NAPTR: each application defines a "First   Well Known Rule" that translates a specific resource into a query   name.  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 regarding the   identifier that "The lexical structure of this string must imply a   unique delegation path", there is no requirement that the identifier   be hierarchical nor that the points of delegation in the domain namePeterson, et al.              Informational                     [Page 7]

RFC 6950               Application Features in DNS          October 2013   created by the "First Well Known Rule" correspond to any points of   administrative delegation inherent in the structure of the   identifier.   While this ability to look up names "which are not in domain name   syntax" does not change the underlying DNS protocol -- 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 (seeSection 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   URI encoding has ways of encapsulating basically arbitrary data: the   most extreme example is a data URL [RFC2397].  Thus, the returned   NAPTR record might be interpreted to produce output other than a   domain name that would subsequently be resolved to IP addresses and   contacted for a particular application -- it could give a literal   result that would be consumed by the application.  Originally, as   discussed in [RFC2168], the intended applicability of the regular   expression field in NAPTR was narrower: 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 to   "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 expression 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 that did not serve any of those aforementioned needs.   According to [RFC3402], the output of DDDS is wholly application-   specific: "the Application must define 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 and   receiving at the end of the process information about the   manufacturer.   Historically speaking, NAPTR did not 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 toPeterson, et al.              Informational                     [Page 8]

RFC 6950               Application Features in DNS          October 2013   retrieve", and 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; [RFC1035] also 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 requires no registration process.  [RFC1464]   experimented with a means of incorporating name/value pairs to the   TXT record structure, which allowed applications to distinguish   different chunks of data stored in a TXT record -- surely not just   "descriptive text" as the TXT originally specified.  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 others, 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 many applications.   Recently, DKIM [RFC6376] sidestepped the problem of TXT ambiguity by   storing keys under a specialized DNS naming structure that includes   the component "_domainkeys", which serves to restrict the scope of   that TXT solely to DKIM use.  Storing keys in the DNS became the   preferred solution for DKIM for several reasons: notably, because   email applications already queried the DNS in their ordinary   operations, 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 on the originating   side) 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 storing large data in the DNS, as   discussed inSection 3.2.1, as well as with the DKIM namespace   conventions that complicate the use of DNS wildcards (as discussed inSection 6.1.2 of [RFC6376] and in more general terms in [RFC5507]).   If prefixes are used to identify TXT records used by an application,   potentially the use of wildcards may furthermore cause leakages that   other applications will need to detect.Peterson, et al.              Informational                     [Page 9]

RFC 6950               Application Features in DNS          October 20133.  Challenges for the DNS   The methods discussed in the previous section for transforming   arbitrary identifiers into domain names and 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 a data URL, the DNS could effectively   implement the entire application feature set of any simple query-   response protocol.  Effectively, the DDDS framework considers the DNS   a generic database -- indeed, the DDDS framework was designed to work   with any sort of underlying database; as [RFC3403] says, 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 for   applications to 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.  The global public DNS was not designed to provide these   sorts of properties, and extending the DNS protocols to encompass   them could result in 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, or can only partly, 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, DNS RRsets are uniquely identified by domain name,   resource record type, and class.  DNS queries are based on this   3-tuple, and the replies are resource record sets that are to be   treated as atomic data elements (see [RFC2181]); to applications, the   behavior of the DNS has traditionally been that of an exact-match   query-response lookup mechanism.  Outside of the DNS space, however,   there are plenty of query-response applications that require aPeterson, et al.              Informational                    [Page 10]

RFC 6950               Application Features in DNS          October 2013   compound or relational search, one taking into account more than one   factor in formulating a response or one that uses no single factor as   a key to the database.  For example, in the 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 Public Switched Telephone Network (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 append additional data to ENUM queries to give the DNS   server sufficient information to return a suitable URI (seeSection 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.  For   example, 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 this particular 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 dubious, especially if more than one   additional parameter is required and the presence of parameters is   optional so that the application needs multiple queries to assess the   completeness of the information it needs to perform its function.   As an alternative, it has been proposed that we piggyback additional   query parameters as Extension Mechanisms for DNS (EDNS(0)) extensions   (see [RFC6891]).  This might be problematic for three reasons.   First, supporting EDNS(0) extensions requires significant changes to   name server behavior; these changes need to be supported by the   authoritative and recursive name servers on which the application   relies and might be very hard to realize on a global scale.  In   addition, the original stated applicability of the EDSN(0) mechanism,   as [RFC2671] states, was to "a particular transport level message and   not to any actual DNS data", and consequently the OPT ResourcePeterson, et al.              Informational                    [Page 11]

RFC 6950               Application Features in DNS          October 2013   Records it specifies are never to be forwarded.  The use of EDNS(0)   for compound queries, however, clearly is intended to discriminate   actual DNS data rather than to facilitate transport-layer handling.   Finally, [RFC6891] also specifies that "OPT RRs MUST NOT be cached,   forwarded, or stored" (see the next paragraph).  For these reasons,   this memo recommends against crafting compound DNS queries by using   EDNS(0).   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 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.  Therefore, were EDNS(0) to be employed   this way, its attributes would not be transitive, and if this were   not considered where intermediaries are employed, as is normally the   case in the global DNS, brokenness might occur.3.1.1.  Responses Tailored to the Originator   DNS responses tailored to the identity of their originator, where   some sort of administrative identity of the originator must be   conveyed to the DNS, constitute the most important subcase of these   compound queries.  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 breach 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 [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 technology such as the one   described by [EDNS-CLIENT-IP] would 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.  As a result, such technology cannot be rolled out in an   incremental, unilateral fashion but could only be successful when   implemented bilaterally (by authoritative server and recursive   resolver); this is a significant bar to deployment.Peterson, et al.              Informational                    [Page 12]

RFC 6950               Application Features in DNS          October 2013   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   selected, and potentially sensitive, 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 deployments (seeSection 4), which   typically rely on an underlying security layer, such as a physical   network, a clear perimeter demarcation at a network perimeter point   (with network-layer anti-spoofing countermeasures), 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 that should not be useful on   the public Internet anyway, in some cases this leakage would reveal   topology or other information that the name server administrator   hopes to keep private.  More recently, TSIG [RFC2845] has been   employed as a way of selecting among "views" in BIND; this provides a   stronger level of security than merely relying on the source IP   address, but typically many users share the same secret to access a   given view, and moreover TSIG does not provide confidentiality   properties to DNS messages -- without network-layer separation   between users of different views, eavesdroppers might capture the DNS   queries and responses.   The use of source IP addresses as a discriminator to select DNS   resource records, regardless of its lack of acceptance by the   standards community, has 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; for   example, [EDNS-OPT-CODE] provides a SIP URI in an EDNS(0) parameter.   Effectively, this 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 approximate a confidentiality mechanism per resource record,   where only a specific set of originators is 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.   However, without any underlying cryptographic security, this   mechanism must rely on external layers for security (such as VPNs)   rather than any direct assurance.  Again, caching, forwarding, 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 likely require significant rearchitecture   of the DNS.Peterson, et al.              Informational                    [Page 13]

RFC 6950               Application Features in DNS          October 20133.2.  Using DNS as a Generic Database   As previously noted, applications can use a method like the "First   Well Known Rule" of DDDS to transform an arbitrary string into a   domain name and then receive from the DNS arbitrary data stored in   TXT RRs, in the "regexp" of NAPTRs, or even in custom records.  Some   query-response applications, however, require queries and responses   that simply fall outside the syntactic capabilities of the DNS.  For   example, domain names themselves must consist of labels that do not   exceed 63 octets, while the total length of the encoded name may not   exceed 255 octets, and applications that use label characters outside   the traditional ASCII set may run into problems (however, see the   discussion in[RFC6055], Section 3 for definitive guidance on the use   of non-ASCII in the DNS).  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.  Some applications today use   quite large data URLs (containing a megabyte or more of data) as   workarounds in environments where only URIs can syntactically appear   (for example, in Apple iOS, to pass objects between applications).   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".  And while EDNS(0) allows for mechanisms   to negotiate DNS message sizes larger than the traditional 512   octets, there is a high risk that a long payload will cause UDP   fragmentation, in particular when the DNS message already carries   DNSSEC information.  If EDNS(0) is not available, or the negotiated   EDNS(0) packet size is too small to fit the data, or UDP fragments   are dropped, the DNS may (eventually) resort to using TCP.  While TCP   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 UDP-only DNS queries with a forged source address, and thePeterson, et al.              Informational                    [Page 14]

RFC 6950               Application Features in DNS          October 2013   victim receives the response.  The attacker relies on amplification,   where a small query generates a large response directed at the   victim.  Where the responder supports EDNS(0), 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 or "heavier" transport protocols such as TCP.  The   bandwidth that can be mustered in a reflective amplification attack   directed by a botnet reflecting off a recursive name server on a   high-bandwidth network is sobering.  For example, if the responding   resolver 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 200 Gbps of traffic on the victim, more than   sufficient to congest any site on today's Internet.   DNS reflection attacks typically utilize UDP queries; it is   prohibitively difficult to complete a TCP three-way handshake begun   from a forged source address for DNS reflection attacks.  Unless the   attacker uses EDNS(0) [RFC6891] 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 utilize   EDNS(0).  As the large disparity between the size of a query and size   of the response creates this amplification, techniques for mitigating   this disparity should be further studied, though this is beyond the   scope of this memo (for an analysis of the effects of limiting   EDNS(0) responses while still accommodating DNSSEC, see [Lindsay]).   For example, some implementations could limit EDNS(0) responses to a   specific ratio compared to the request size, where the precise ratio   can be configured on a per-deployment basis (taking into account   DNSSEC response sizes).  Without some means of mitigating the   potential for amplification, EDNS(0) could cause significant harm.   In summary, there are two operational forces that tend to drive the   practically available EDNS(0) sizes down: possible UDP fragmentation   and minimizing amplification in case of reflection attacks.  DNSSEC   data will use a significant fraction of the available space in a DNS   packet.  Therefore -- appreciating that given the current DNSSEC andPeterson, et al.              Informational                    [Page 15]

RFC 6950               Application Features in DNS          October 2013   EDNS(0) deployment experience, precise numbers are impossible to give   -- the generic payload available to other DNS data, given the premise   that TCP fallback is to be minimized, is likely to be closer to   several hundred octets than a few thousand octets.3.3.  Administrative Structures Misaligned with the DNS   While the DDDS framework enables any sort of alphanumeric data to   serve as a domain name through the application of the "First Well   Known Rule", the delegative structure of the resulting domain name   may not reflect any administrative division of responsibilities   inherent in the original data.  While [RFC3402] requires only that   the "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 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 administrative domain with no   points of administrative delegation from the country-code   administrator, e.g., 1.e164.arpa, down to the ultimate assignee of a   number.  Under the assumption that one administrative domain is   maintained within one DNS zone containing potentially over five   billion names, scalability difficulties manifest in a number of   areas: the scalability that results from caching depends on these   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 assignees for ported numbers must   propagate within fifteen minutes of a change in administration.   Jointly, these two problems make caching the zone extremely   problematic.  Moreover, traditional tools for DNS replication, suchPeterson, et al.              Informational                    [Page 16]

RFC 6950               Application Features in DNS          October 2013   as the zone transfer protocols AXFR [RFC1034] and IXFR [RFC1995],   might not scale to accommodate zones with these dimensions and   properties.   In practice, the maximum sizes of telephone number administrative   domains are closer to 300M (the current amount of allocated telephone   numbers in the United States today -- still more than three times the   number of .com domain names), and one can alleviate some of the   scalability issues mentioned above by artificially dividing the   administrative domain into a hierarchy of DNS zones.  Still, the fact   that traditional DNS management tools no longer apply to the   structures that an application tries to provision in the DNS is a   clue that the DNS might not be the right place for an application to   store its data.   While DDDS is the most obvious example of these concerns, the point   is more generic: for example, were address portability to be   implemented for IP addresses and their administration thus to become   non-hierarchical, the same concerns would apply to in-addr.arpa   names.  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 domains represent the same "site" when   comparing security credentials with URLs (seeSection 3.4 below for   more on this).  This has also caused known problems in determining   the scope of web cookies, in contexts where applications must infer   where administrative domains end in order to grant cookies that are   as narrowly scoped as possible.   In summary, the "First Well Known Rule" of DDDS provides a capability   that transforms arbitrary strings into domain names, but those names   play well with the DNS only when the input strings have an   administrative structure that maps to DNS delegations.  In the first   place, delegation implies some sort of hierarchical structure.  Any   mechanism to map a hierarchical identifier into a domain name should   be constructed such that the resulting domain 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, then that string probably is not a good   candidate for translating into a domain name.Peterson, et al.              Informational                    [Page 17]

RFC 6950               Application Features in DNS          October 20133.3.1.  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 the name is complete -- that it is not a fragment of a   domain 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; proposed solutions include   metadata in the DNS that describes the contents and structure of ENUM   DNS trees to help applications handle incomplete queries or queries   for domains not in use.   In particular, the "send-n" proposal [ENUM-Send-N] hoped 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 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 that 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 this mechanism simply reduces the time the system   will wait before sending an ENUM query after collecting the final   dialed digit.  A tangentially related proposal, [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   records available in the DNS, whether or not the domain had been   allocated by a registry to a registrant (which is a different   condition than a name merely being unresolvable, per the semantics of   NXDOMAIN).Peterson, et al.              Informational                    [Page 18]

RFC 6950               Application Features in DNS          October 2013   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 ends has some features in common with the   traditional parent zone use of the NS record type, except that   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 one administrative domain would need to   remain synchronized with the state of the resources it predicts in   another administrative domain in the DNS namespace, in a case like   overlap dialing where the carrier delegates to a zone controlled by   an enterprise.  When dealing with external resources associated with   other namespaces, like number assignments in the PSTN or the   databases of a registry operator, other synchronization requirements   arise; maintaining that synchronization requires that the DNS have   "semi-real time" updates that may conflict with scale and caching   mechanisms of the DNS.   Placing metadata in the DNS 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 e164.arpa   root 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 "infrastructure" ENUM models (seeSection 4).   Ultimately, these metadata proposals share some qualities with DNS   redirection services offered by ISPs (for example, [DNS-REDIRECT])   that "help" web users who try to browse to sites that do not exist.   Similarly, metadata proposals like [ENUM-UNUSED] create DNS records   for unallocated zones that redirect to a service provider's web page.   However, in the [DNS-REDIRECT] cases, at least the existence or   non-existence of a domain name is a fact about the Internet   namespace, rather than about an external namespace like the telephony   E.164 namespace (which must be synchronized with the DNS tree in the   metadata proposals).  In send-n, different leaf zones that administer   telephone numbers of different lengths can only provision their hints   at their own apex, which provides an imperfect optimization: they   cannot install it themselves in a parent, both because they lack the   authority and because different zones would want to provision   contradictory data.  The later the hint appears in the tree, however,   the less optimization will result.  An application protocol designer   managing identifiers whose administrative model does not map well   onto the DNS namespace and delegations structure would be better   served to implement such features outside the DNS.Peterson, et al.              Informational                    [Page 19]

RFC 6950               Application Features in DNS          October 20133.4.  Domain Redirection   Most Internet application services provide a redirection feature --   when one attempts to contact a service, the service may refer the   person to a different service instance, potentially in another   domain, that is for whatever reason better suited to service a   request.  In HTTP and SIP, for example, this feature is implemented   by the 300 class responses containing one or more URIs, which 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 protocols supporting redirection, such as HTTP and SIP, have   available security mechanisms, including TLS, that can use   certificates to attest that a 300 response came from the domain that   the originator initially hoped to contact.   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;   this requires a virtual hosting service to possess a certificate   corresponding to the hosted domain.  (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]).  This restriction rules out many styles   of hosting deployments common in the web world today, however.   [HARD-PROBLEM] explores this problem space.  [RFC6125] proposes a   solution for all applications that use TLS, which relies on new   application-specific identifiers in certificates, as does [RFC4985]);   note, however, that support for such certificates would requirePeterson, et al.              Informational                    [Page 20]

RFC 6950               Application Features in DNS          October 2013   changes to existing certificate authority practices as well as   application behavior.  With DNSSEC in place, DNS-based Authentication   of Named Entities (DANE) [RFC6394] offers another way to bind   certificates to particular applications and services.   All of these application-layer measures attempt to mirror the   delegation of administrative authority in the DNS, when the DNS   itself serves as the ultimate authority on how domains are delegated.   (Note: changing the technical delegation structure by changing NS   records in the DNS is not the same as administrative delegation,   e.g., when a domain changes ownership.)  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 an administrative 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.   This inevitably gives rise to various design trade-offs involving   performance, load, and related factors, but in these application   environments, the security properties typically take priority.4.  Private DNS and Split Horizon   The classic view of the uniqueness of domain names in the DNS 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.   [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; while some deployments   differentiate internal queries from public queries by the source IP   address, the concerns inSection 3.1.1 relating to trusting source IP   addresses apply to such deployments.  When the internal address space   range is private [RFC1918], this makes it both easier for the server   to discriminate public from private and harder for public entities to   impersonate nodes in the private network.  Networks that are madePeterson, et al.              Informational                    [Page 21]

RFC 6950               Application Features in DNS          October 2013   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 distinct networks.   The use cases that motivate split-horizon DNS typically involve   restricting access to some network services -- intranet resources   such as internal web sites, 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 such as the   "First Well Known Rule" of DDDS into "domain name syntax".  Telephone   calls, for example, are traditionally routed through highly mediated   networks, in which 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 responses tailored to   the originator, and for confidentiality, is easily motivated when the   data returned by the DNS is no longer "describing protocol addresses,   network resources and services" [RFC2826] but instead is arbitrary   data.  Although, for example, ENUM was originally intended for   deployment in the global public root of the DNS (under e164.arpa),   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 more 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 privatePeterson, et al.              Informational                    [Page 22]

RFC 6950               Application Features in DNS          October 2013   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 private   environments, experience teaches us that these implementations will   begin to interact with the public DNS and that the private practices   will leak.   While such leakage is not a problem when using the mechanisms   described in Sections3.1,3.2, and3.5 (with private RR types) of   [RFC5507], other extension mechanisms might cause confusion or harm   if leaked.  The use of a dedicated suffix (Section 3.3 of [RFC5507])   in a private environment might cause confusion if leaked to the   public Internet, for example.   That this kind of leakage of protocol elements from private   deployments to public deployments does happen has been demonstrated,   for example, with SIP: SIP implemented a category of supposedly   private extensions ( the "P-" headers) that saw widespread success   and use outside of the constrained environments for which they were   specifically designed.  There is no reason to think that   implementations with similar "private" extensions to the DNS   protocols would not similarly end up in use in public environments.5.  Principles and Guidance   The success of the global public DNS relies on the fact that it is a   distributed database, one that has the property that it is loosely   coherent and offers lookup instead of search functionality.  Loose   coherency means that answers to queries are coherent within the   bounds of data replication between authoritative servers (as   controlled by the administrator of the zone) and caching behavior by   recursive name servers.  Today, this term increasingly must also   include load-balancing or related features that rely on the source IP   address of the resolver.  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.Peterson, et al.              Informational                    [Page 23]

RFC 6950               Application Features in DNS          October 2013   It is likely that the DNS provides a good match whenever the needs of   applications are aligned with the following properties:   o  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   o  Data stored in the DNS is indexed by keys that do not violate the      syntax or semantics of domain names   o  Applications invoke the DNS to resolve complete names, not      fragments   o  Answers do not depend on an application-layer identity of the      entity doing the query   o  Ultimately, applications invoke the DNS to assist in communicating      with a service whose name is resolved through the DNS   Whenever one of the five 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, if one has to worry about   the following items, then these items are good indicators that the   DNS is not the appropriate tool for solving problems:   o  Populating metadata about domain boundaries within the tree -- the      points of administrative delegation in the DNS are something that      applications are not in general aware of   o  Domain names derived from identifiers that do not share a semantic      or administrative model compatible with the DNS   o  Selective disclosure of data stored in and provided by the DNS   o  DNS responses not fitting into UDP packets, unless EDNS(0) is      available, and only then with the caveats discussed inSection 3.2.1   In cases where applications require these sorts of features, they are   likely better instantiated by independent application-layer protocols   than the DNS.  For example, the objects that HTTP can carry in both   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 asPeterson, et al.              Informational                    [Page 24]

RFC 6950               Application Features in DNS          October 2013   the redirection properties discussed inSection 3.4.  These   differences highlight the fact that the DNS and HTTP offer very   different services and have different applicabilities; while both are   query-response protocols, HTTP should not be doing the job of DNS,   and DNS should not be doing the job of HTTP.  Similarly, DNS should   not be doing the job of Diameter, LDAP, or other application-layer   protocols.  The overhead of using any application-layer protocol may   not be appropriate for all environments, of course, but even in   environments where a more lightweight protocol is appropriate, DNS is   usually 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 [URI-RR] Resource Records   can contain URIs, those URIs can in turn point to an external query-   response service such as an HTTP service where more syntactically and   semantically rich queries and answers might be exchanged.  Any   protocol designer considering where to implement features must   perform their own gap analysis and determine whether or not   implementing some features is worth the potential cost in increased   network state, latency, and so on, but implementing some features   simply requires heavier structures than others.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 (seeSection 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.  AsSection 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 (regarding the use of EDNS(0) and   TCP) the presence of large sets of records (e.g., ANY queries) is not   recommended.Section 3.4 discusses a security problem concerning   redirection that has surfaced in a number of protocols (see   [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, nor a confidential mechanism.   Some implementations may support authenticating users through TSIG,   provided that the security association with a compliant server has   been pre-established, though authentication is typically not neededPeterson, et al.              Informational                    [Page 25]

RFC 6950               Application Features in DNS          October 2013   for queries in the global public DNS.  The existing feature set of   DNSSEC is, however, sufficient for providing security for most of the   ways that applications traditionally have used the DNS.  The   deployment of DNSSEC ([RFC4033] and related specifications) is   heartily encouraged.  Nothing in this document is intended to   discourage the implementation, deployment, or use of Secure DNS   Dynamic Updates [RFC3007], though this document does recommend that   large data in the DNS be treated in accordance with the guidance inSection 3.2.1.7.  IAB Members at the Time of Approval   Internet Architecture Board Members at the time this document was   approved were:   Bernard Aboba   Jari Arkko   Marc Blanchet   Ross Callon   Alissa Cooper   Spencer Dawkins   Joel Halpern   Russ Housley   David Kessens   Danny McPherson   Jon Peterson   Dave Thaler   Hannes Tschofenig8.  Acknowledgements   The IAB appreciates the comments and often spirited disagreements of   Eric Osterweil, John Levine, Stephane Bortzmeyer, Ed Lewis, Dave   Crocker, Ray Bellis, Lawrence Conroy, Ran Atkinson, Patrik Faltstrom,   and Eliot Lear.Peterson, et al.              Informational                    [Page 26]

RFC 6950               Application Features in DNS          October 20139.  Informative References   [DNS-REDIRECT]              Creighton, T., Griffiths, C., Livingood, J., and R. Weber,              "DNS Redirect Use by Service Providers", Work in Progress,              October 2010.   [EDNS-CLIENT-IP]              Contavalli, C., van der Gaast, W., Leach, S., and D.              Rodden, "Client IP information in DNS requests", Work in              Progress, May 2010.   [EDNS-OPT-CODE]              Kaplan, H., Walter, R., Gorman, P., and M. Maharishi,              "EDNS Option Code for SIP and PSTN Source Reference Info",              Work in Progress, October 2011.   [ENUM-Send-N]              Bellis, R., "IANA Registrations for the 'Send-N'              Enumservice", Work in Progress, June 2008.   [ENUM-UNUSED]              Stastny, R., Conroy, L., and J. Reid, "IANA Registration              for Enumservice UNUSED", Work in Progress, March 2008.   [HARD-PROBLEM]              Barnes, R. and P. Saint-Andre, "High Assurance              Re-Direction (HARD) Problem Statement", Work in Progress,              July 2010.   [Lindsay]  Lindsay, G., "DNSSEC and DNS Amplification Attacks",              April 2012.   [RFC0882]  Mockapetris, P., "Domain names: Concepts and facilities",RFC 882, November 1983.   [RFC0883]  Mockapetris, P., "Domain names: Implementation              specification",RFC 883, November 1983.   [RFC0974]  Partridge, C., "Mail routing and the domain system",              STD 10,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.Peterson, et al.              Informational                    [Page 27]

RFC 6950               Application Features in DNS          October 2013   [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, B., Karrenberg, D., de 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.   [RFC2168]  Daniel, R. and M. Mealling, "Resolution of Uniform              Resource Identifiers using the Domain Name System",RFC 2168, June 1997.   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS              Specification",RFC 2181, July 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.   [RFC2826]  Internet Architecture Board, "IAB Technical Comment on the              Unique DNS Root",RFC 2826, May 2000.   [RFC2845]  Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.              Wellington, "Secret Key Transaction Authentication for DNS              (TSIG)",RFC 2845, 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.Peterson, et al.              Informational                    [Page 28]

RFC 6950               Application Features in DNS          October 2013   [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.   [RFC3596]  Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,              "DNS Extensions to Support IP Version 6",RFC 3596,              October 2003.   [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.   [RFC4367]  Rosenberg, J., Ed., and IAB, "What's in a Name: False              Assumptions about DNS Names",RFC 4367, February 2006.   [RFC4398]  Josefsson, S., "Storing Certificates in the Domain Name              System (DNS)",RFC 4398, March 2006.   [RFC4732]  Handley, M., Ed., Rescorla, E., Ed., and IAB, "Internet              Denial-of-Service Considerations",RFC 4732,              December 2006.   [RFC4985]  Santesson, S., "Internet X.509 Public Key Infrastructure              Subject Alternative Name for Expression of Service Name",RFC 4985, August 2007.Peterson, et al.              Informational                    [Page 29]

RFC 6950               Application Features in DNS          October 2013   [RFC5486]  Malas, D., Ed., and D. Meyer, Ed., "Session Peering for              Multimedia Interconnect (SPEERMINT) Terminology",RFC 5486, March 2009.   [RFC5507]  IAB, Faltstrom, P., Ed., Austein, R., Ed., and P. Koch,              Ed., "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.   [RFC6055]  Thaler, D., Klensin, J., and S. Cheshire, "IAB Thoughts on              Encodings for Internationalized Domain Names",RFC 6055,              February 2011.   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and              Verification of Domain-Based Application Service Identity              within Internet Public Key Infrastructure Using X.509              (PKIX) Certificates in the Context of Transport Layer              Security (TLS)",RFC 6125, March 2011.   [RFC6269]  Ford, M., Ed., Boucadair, M., Durand, A., Levis, P., and              P. Roberts, "Issues with IP Address Sharing",RFC 6269,              June 2011.   [RFC6376]  Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,              "DomainKeys Identified Mail (DKIM) Signatures",RFC 6376,              September 2011.   [RFC6394]  Barnes, R., "Use Cases and Requirements for DNS-Based              Authentication of Named Entities (DANE)",RFC 6394,              October 2011.   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service              Discovery",RFC 6763, February 2013.   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms              for DNS (EDNS(0))", STD 75,RFC 6891, April 2013.   [URI-RR]   Faltstrom, P. and O. Kolkman, "The Uniform Resource              Identifier (URI) DNS Resource Record", Work in Progress,              July 2013.Peterson, et al.              Informational                    [Page 30]

RFC 6950               Application Features in DNS          October 2013Authors' Addresses   Jon Peterson   NeuStar, Inc.   EMail: jon.peterson@neustar.biz   Olaf Kolkman   NLnet Labs   EMail: olaf@nlnetlabs.nl   Hannes Tschofenig   Nokia Siemens Networks   EMail: Hannes.Tschofenig@gmx.net   Bernard Aboba   Skype   EMail: Bernard_aboba@hotmail.comPeterson, et al.              Informational                    [Page 31]