Network Working Group                                                IAB
Internet-Draft                                         P. Faltstrom, Ed.
Intended status: Standards Track Informational                           R. Austein, Ed.
Expires: August 21, 2008 January 15, 2009                                   P. Koch, Ed.
                                                       February 18,
                                                           July 14, 2008

                   Design Choices When Expanding DNS
                        draft-iab-dns-choices-05
                        draft-iab-dns-choices-06

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Copyright Notice

   Copyright (C) The IETF Trust (2008). January 15, 2009.

Abstract

   This note discusses how to extend the DNS with new data for a new
   application.  DNS extension discussions too often focus on reuse of
   the TXT Resource Record Type.  This document lists different
   mechanisms to extend the DNS, and concludes that the use of a new DNS
   Resource Record Type is the best solution.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Extension mechanisms . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Place selectors inside the RDATA of existing Resource
           Record Types . . . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Add a prefix to the owner name . . . . . . . . . . . . . .  5  6
     3.3.  Add a suffix to the owner name . . . . . . . . . . . . . .  6  7
     3.4.  Add a new Class  . . . . . . . . . . . . . . . . . . . . .  7
     3.5.  Add a new Resource Record Type . . . . . . . . . . . . . .  7  8
   4.  Zone boundaries are invisible to applications  . . . . . . . .  8  9
   5.  Why adding a new Resource Record Type is the preferred
       solution . . . . . . . . . . . . . . . . . . . . . . . . . . .  9 10
   6.  Conclusion and Recommendation  . . . . . . . . . . . . . . . . 12 13
   7.  Creating A New Resource Record Type  . . . . . . . . . . . . . 13 14
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13 14
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13 14
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 13 15
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14 16
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 14 16
     11.2. Informative References . . . . . . . . . . . . . . . . . . 14 16
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15 17
   Intellectual Property and Copyright Statements . . . . . . . . . . 16 18

1.  Introduction

   The DNS stores multiple categories of data.  The two most commonly
   used categories are infrastructure data for the DNS system itself (NS
   and SOA Resource Records) and data which have to do with mappings
   between domain names and IP addresses (A, AAAA and PTR Resource
   Records).  There are other categories as well, some of which are tied
   to specific applications like email (MX Resource Records), while
   others are generic Resource Record Types used to convey information
   for multiple protocols (SRV and NAPTR Resource Records).

   When storing data in the DNS for a new application, the data are
   usually tied goal must be
   to store data in such a "normal" domain name, so way that the application can query for the
   data it wants, while minimizing the impact on both existing
   applications.
   applications and the amount of extra data transfered to the client.
   This implies a number of design choices have to be made, where the
   most important is to ensure that an as precise selection of what data
   to return must be made already in the query.  A query that consists
   of the triple Owner, Resource Record Type and Resource Record Class.

   Historically, extending DNS to store application data tied to a
   domain name has been done in different ways at different times.  MX
   Resource Records were created as a new Resource Record Type
   specifically designed to support electronic mail.  SRV records are a
   generic type which use a prefixing scheme in combination with a base
   domain name.  Records associated with ENUM use a suffixing scheme.  NAPTR records add selection data inside the RDATA.  It
   is clear that the methods used to add new data types to the DNS have
   been inconsistent, and the purpose of this document is to attempt to
   clarify the implications of each of these methods, both for the
   applications that use them and for the rest of the DNS.

   This document talks extensively about use of DNS wildcards.  Many
   people might think use of wildcards is not something that happens
   today.  In reality though, wildcards are in use, especially for
   certain application-specific data such as MX Resource Records.
   Because of this, the choice has to be made with existence of
   wildcards in mind.

   Another overall issue that must be taken into account is what the new
   data in the DNS are to describe.  In some cases they might be
   completely new data.  In other cases they might be metadata tied to
   data that already exist in the DNS.  An example of new data is key
   information for SSH and data used for fighting spam authenticating sender of email
   messages (metadata tied to MX Resource Records).  If the new data are
   tied to data that already exist in the DNS, an analysis should be
   made as to whether having (for example) address records and SSH key
   information in different DNS zones is a problem, or if it is a bonus,
   and if it is, is a problem, whether the specification must require all of
   the related data to be in the same zone.  One specific difference
   between having the records in the same zone or not have to do with
   maintenance of the records.  If they are in the same zone, the same
   maintainer (from a DNS perspective) manages the two records.
   Specifically, they must be signed with the same DNSSEC keys if DNSSEC
   is in use.

   This document does not talk about what one should store in the DNS.
   It also doesn't discuss whether DNS should be used for service
   discovery, or whether DNS should be used for storage of data specific
   for the service.  In general, DNS is a protocol that, apart from
   holding metadata that makes the DNS itself function (NS, SOA, DNSSEC
   Resource Record Types, etc), only holds references to service
   locations (SRV, NAPTR, A, AAAA Resource Record Types), but there are
   exceptions (such as MX Resource Records).

2.  Background

   See RFC 2929 [RFC2929] for a brief summary of DNS query structure.
   Readers interested in the full story should start with the base DNS
   specification in RFC 1035 [RFC1035], and continue with the various
   documents that update, clarify, and extend the base specification.

   When composing a DNS query, the parameters used by the protocol are a
   triple: a DNS name, a DNS class, and a DNS record Resource Record Type.
   Every Resource Record matching a particular name, type and class is
   said to belong to the same "RRSet", Resource Record Set (RRSet), and the whole
   RRSet is always returned to the client that queries for it.
   Splitting an RRSet is a protocol
   violation, violation (sending a partial RRSet,
   not truncating the DNS response), because it results can result in coherency
   problems with the DNS caching mechanism.  See RFC 2181 section 5
   [RFC2181] for more information.

   Some discussions around extensions to the DNS include arguments
   around MTU size.  Note that most discussions about DNS and MTU size
   are about the size of the whole DNS packet, not about the size of a
   single RRSet.

   Almost all DNS query traffic is carried over UDP, where a DNS message
   must fit within a single UDP packet.  DNS response messages are
   almost always larger than DNS query messages, so message size issues
   are almost always about responses, not queries.  The base DNS
   specification limits DNS messages over UDP to 512 octets; EDNS0
   [RFC2671] specifies a mechanism by which a client can signal its
   willingness to receive larger responses, but deployment of EDNS0 is
   not universal, in part because of firewalls that block fragmented UDP
   packets or EDNS0.  If a response message won't fit in a single
   packet, the name server returns a truncated response, at which point
   the client may retry using TCP.  DNS queries over TCP are not subject
   to this length limitation, but TCP imposes significantly higher per-
   query overhead on name servers than UDP.  It is also the case that
   the policies in deployed firewalls far too often is are such that it
   blocks DNS over TCP, so using TCP might not in reality be an option.
   There are also risks (although possibly small) that a change of
   routing while a TCP flow is open create creates problems when the DNS
   servers are deployed in an anycast environment.

3.  Extension mechanisms

   The DNS protocol is intended to be extensible to support new kinds of
   data.  This section examines the various ways in which this sort of
   extension can be accomplished.

3.1.  Place selectors inside the RDATA of existing Resource Record Types

   For a given query name, one might choose to have a single RRSet (all
   Resource Records sharing the same name, type, and class) shared by
   multiple applications, and have the different applications use
   selectors within the Resource Record data (RDATA) to determine which
   records are intended for which applications.  This sort of selector
   mechanism is usually referred to "subtyping", because it is in effect
   creating an additional type subsystem within a single DNS Resource
   Record Type.

   Examples of subtyping include NAPTR Resource Records (see [RFC3761]) [RFC3761] and
   the original DNSSEC KEY Resource Record Type ([RFC2535]) (before
   it [RFC2535] (which was
   later updated by RFC 3445 [RFC3445]).

   All DNS subtyping schemes share a common weakness: With subtyping
   schemes it is impossible for a client to query for just the data it
   wants.  Instead, the client must fetch the entire RRSet, then select
   the Resource Records in which it is interested.  Furthermore, since
   DNSSEC signatures operate on complete RRSets, the entire RRSet must
   be re-signed if any Resource Record in it changes.  As a result, each
   application that uses a subtyped Resource Record incurs higher
   overhead than any of the applications would have incurred had they
   not been using a subtyping scheme.  The fact the RRSet is always
   passed around as an indivisible unit increases the risk the RRSet
   will not fit in a UDP packet, which in turn increases the risk that
   the client will have to retry the query with TCP, which substantially
   increases the load on the name server.  More precisely: having one
   query fail over to TCP is not a big deal, but since the typical ratio
   of clients to servers in today's deployed DNS is very high, having a
   substantial number of DNS messages fail over to TCP may cause the
   queried name servers to be overloaded by TCP overhead.

   Because of the size limitations, using a subtyping scheme to list a
   large number of services for a single domain name risks triggering
   truncation and fallback to TCP, which may in turn force the zone
   administrator to announce only a subset of available services.

3.2.  Add a prefix to the owner name

   By adding an application-specific prefix to a domain name, we get a
   different name/class/type triple, and therefore a different RRSet.
   One problem with adding prefixes has to do with wildcards, especially
   if one has records like

   *.example.com. IN MX 1 mail.example.com.

   and one wants records tied to those names.  Suppose one creates the
   prefix "_mail".  One would then have to say something like

   _mail.*.example.com. IN X-FOO A B C D

   but DNS wildcards only work with the "*" as the leftmost token in the
   domain name (see also RFC 4592 [RFC4592]).

   Even when a specific prefix is chosen, the data will still

   There have been proposals to be
   stored in some Resource Record Type.  This Resource Record Type can
   either be a record Type deal with the problem that has DNS wild-
   cards are always terminal records.  These proposals introduce an appropriate format
   additional set of trade-offs that would need to store the
   data or a new Resource be taken into account
   when assessing which extension mechanism to choose.  Aspects of extra
   response time needed to perform the extra queries, costs of pre-
   calculation of possible answers, or the costs induced to the system
   as a whole come to mind.  At the time of writing none of these
   proposals has been published as standards tracks RFCs.

   Even when a specific prefix is chosen, the data will still have to be
   stored in some Resource Record Type.  This implies Resource Record Type can
   either be an existing Resource Record Type that has an appropriate
   format to store the data or a new Resource Record Type.  One also
   might nee some other selection mechanism has to be applied as well, mechanism, such as ability to
   distinguish between the records in an RRSet given they have the same
   Resource Record Type.  Because of this, one needs to both register a
   unique prefix and define what Resource Record Type is to be used for
   this specific service.

   If the record has some relationship with another record in the zone,
   the fact that the two records can be in different zones might have
   implications on the trust the application has in the records.  For
   example:

   example.com.      IN MX    10 mail.example.com.
   _foo.example.com. IN X-BAR "metadata for the mail service"

   In this example, the two records might be in two different zones, and
   because of this might be administered by two different organisations,
   and signed by two different organizations entities when using DNSSEC.  Prefix has
   lately because of these two reasons been a very interesting solution
   for many protocol designers.  In some cases when using TXT records
   (add reference to DKIM), in other cases when adding new Resource
   Record Types (SRV).

3.3.  Add a suffix to the owner name

   Adding a suffix to a domain name changes the name/class/type triple,
   and therefore the RRSet.  In this case, since the query name can be
   set to exactly the data one wants the size of the RRSet is minimized.
   The problem with adding a suffix is that it creates a parallel tree
   within the IN class.  Further, there is no technical mechanism to
   ensure that the delegation for "example.com" and "example.com._bar"
   are made to the same organization.  Furthermore, data associated with
   a single entity will now be stored in two different zones, such as
   "example.com" and "example.com._bar", which, depending on who
   controls "_bar", can create new synchronization and update
   authorization issues.

   One way of solving the administrative issues is by using the DNAME
   Resource Record Type specified in RFC 2672 [RFC2672].

   Even when using a different name, the data will still have to be
   stored in some Resource Record Type.  This Resource Record Type can
   either be a "kitchen-sink record" or a new Resource Record Type. that has an appropriate format to
   store the data.  This implies that one might have to mix the prefix
   based selection mechanism with some other mechanism has to so that the right
   Resource Record can be applied as well,
   with implications detailed in other parts found out of this note. many in a potential larger RRSet.

   In RFC 2163 [RFC2163] an infix token is inserted directly below the
   TLD, but the result is equivalent to adding a suffix to the owner
   name (instead of creating a TLD one is creating a second level
   domain).

3.4.  Add a new Class

   DNS zones are class-specific in the sense that all the records in
   that zone share the same class as the zone's SOA record and the
   existence of a zone in one class does not guarantee the existence of
   the zone in any other class.  In practice, only the IN class has ever
   seen widespread deployment, and the administrative overhead of
   deploying an additional class would almost certainly be prohibitive.

   Nevertheless, one could in theory use the DNS class mechanism to
   distinguish between different kinds of data.  However, since the DNS
   delegation tree (represented by NS Resource Records) is itself tied
   to a specific class, attempting to resolve a query by crossing a
   class boundary may produce unexpected results because there is no
   guarantee that the name servers for the zone in the new class will be
   the same as the name servers in the IN class.  The MIT Hesiod system
   used a scheme like this for storing data in the HS class, but only on
   a very small scale (within a single institution), and with an
   administrative fiat requiring that the delegation trees for the IN
   and HS trees be identical.  The use of the HS class for such storage
   of non-sensitive data was over time replaced by use of LDAP.

   Even when using a different class, the data will still have to be
   stored in some Resource Record Type or another.  This Resource Record
   Type can either be a "kitchen-sink record" or a new Resource Record
   Type. that has an appropriate format to
   store the data.  This implies that one might have to mix the prefix
   based selection mechanism with some other mechanism has to so that the right
   Resource Record can be applied as
   well, with implications detailed in other parts found out of this note. many in a potential larger RRSet.

3.5.  Add a new Resource Record Type

   When adding a new Resource Record Type to the system, entities in
   four different roles have to be able to handle the new Type:

   1.  There must be a way to insert the new Resource Records into the
       zone of the Primary Master name server.  For some server
       implementations, the user interface only accepts record Resource Record
       Types which it understands (perhaps so that the implementation
       can attempt to validate the data).  Other implementations allow
       the zone administrator to enter an integer for the Resource
       Record Type code and the RDATA in Base64 or hexadecimal encoding
       (or even as raw data).  RFC 3597 [RFC3597] specifies a standard
       generic encoding for this purpose.
   2.  A slave authoritative name server must be able to do a zone
       transfer, receive the data from some other authoritative name
       server, and serve data from the zone even though the zone
       includes records of unknown Types.  Historically, some
       implementations have had problems parsing stored copies of the
       zone file after restarting, but those problems have not been seen
       for a few years.
   3.  A caching resolver (most commonly a recursive name server) will
       cache the records which are responses to queries.  As mentioned
       in RFC 3597 [RFC3597],there are various pitfalls where a
       recursive name server might end up having problems.
   4.  The application must be able to get the RRSet with a new Resource
       Record Type.  The application itself may understand the RDATA,
       but the resolver library might not.  Support for a generic
       interface for retrieving arbitrary DNS Resource Record Types has
       been a requirement since 1989 (see RFC 1123 [RFC1123] Section
       6.1.4.2).  Some stub resolver library implementations neglect to
       provide this functionality and cannot handle unknown Resource
       Record Types, but implementation of a new stub resolver library
       is not particularly difficult, and open source libraries that
       already provide this functionality are available.

   Historically adding a new Resource Record Type as been very
   problematic.  Review process has been cumbersome, DNS servers have
   not been able to handle new Resource Record Types, and firewalls has
   dropped queries or responses with for the firewall unknown Resource
   Record Types.  This is for example one of the reasons the ENUM
   standard reuse the NAPTR Resource Record.  A choice that today might
   have been wrong, and a new resource record type could have been a
   better choice.

   Today, there is a requirement that DNS software can handle unknown
   Resource Record Types, and investigations have shown that software
   that is deployed in general do support it.  Also the approval process
   for new Resource Record Types has been updated so it is more
   predictable on what effort is needed for various Resource Record
   Types.

4.  Zone boundaries are invisible to applications

   Regardless of the possible choices above we have seen a number of
   cases where the application made assumptions about the structure of
   the namespace and the location where specific information resides.
   We take a small sidestep to argue against such approaches.

   The DNS namespace is a hierarchy, technically speaking.  However,
   this only refers to the way names are built from multiple labels.
   DNS hierarchy neither follows nor implies administrative hierarchy.
   That said, it cannot be assumed that data attached to a node in the
   DNS tree is valid for the whole subtree.  Technically, there are zone
   boundaries partitioning the namespace and administrative boundaries
   (or policy boundaries) may even exist elsewhere.

   The false assumption has lead to an approach called "tree climbing",
   where a query that does not receive a positive response (either the
   requested RRSet was missing or the name did not exist) is retried by
   repeatedly stripping off the leftmost label (climbing towards the
   root) until the root domain is reached.  Sometimes these proposals
   try to avoid the query for the root or the TLD level, but still this
   approach has severe drawbacks:

   o  Technically, the DNS was built as a query - response tool without
      any search capability [RFC3467].  Adding the search mechanism
      imposes additional burden on the technical infrastructure, in the
      worst case on TLD and root name servers.
   o  For reasons similar to those outlined in RFC 1535, 1535 [RFC1535],
      querying for information in a domain outside the control of the
      intended entity may lead to incorrect results and may also put
      security at risk.  Finding the exact policy boundary is impossible
      without an explicit marker which does not exist at present.  At
      best, software can detect zone boundaries (e.g., by looking for
      SOA Resource Records), but some TLD registries register names
      starting at the second level (e.g., CO.UK), and there are various
      other "registry" types at second, third or other level domains
      that cannot be identified as such without policy knowledge
      external to the DNS.

   To restate, the zone boundary is purely a boundary that exists in the
   DNS for administrative purposes, and applications should be careful
   not to draw unwarranted conclusions from zone boundaries.  A
   different way of stating this is that the DNS does not support
   inheritance, e.g. a wildcard MX RRSet for a TLD will not be valid for
   any subdomain of that particular TLD.

5.  Why adding a new Resource Record Type is the preferred solution

   By now, the astute reader might be be wondering what conclusions to draw
   from all the issues presented so far.  We will now attempt to clear up
   the reader's confusion by following the thought processes of a
   typical application designer who wishes to store data in the DNS,
   showing how such a designer almost inevitably hits upon the idea of
   just using a TXT Resource Record, Records, why this is a bad thing, and why a
   new Resource Record Type should be allocated instead. instead, but also
   explain how to reuse an existing resource record, including TXT, can
   be made less harmful.

   The overall problem with most solutions has to do with two main
   issues:
   o  No semantics to prevent collision with other use
   o  Space considerations in the DNS message

   A typical application designer is not interested in the DNS for its
   own sake, but rather regards it as a distributed database in which
   application data can be stored.  As a result, the designer of a new
   application is usually looking for the easiest way to add whatever
   new data the application needs to the DNS in a way that naturally
   associates the data with a DNS name.

   As explained in Section 3.4, using the DNS class system as an
   extension mechanism is not really an option, and in fact most users
   of the system don't even realize that the mechanism exists.  As a
   practical matter, therefore any extension is likely to be within the
   IN class.

   Adding a new Resource Record Type is the technically correct answer
   from the DNS protocol standpoint (more on this below), but doing so
   requires some DNS expertise, due to the issues listed in Section 3.5.
   As a result,
   Consequently, this option is usually not considered.  Note that
   according to RFC 2929 [RFC2929], some Types require IETF Consensus,
   while others only require a specification.

   There is a drawback to defining new RR types that is worth
   mentioning.  The RRTYPE is a 16 bit value and hence a a limited
   resource.  In order to prevent herding the registry has a review
   based allocation policy [RFC2929], however this may not be sufficient
   if extension of the DNS by addition of new RR types takes up
   significantly and the registry starts nearing completion.  In that
   case the trade-offs with respect to choosing an extension mechanism
   may need to change.

   The application designer is thus left with the prospect of reusing
   some existing DNS Type within the IN class, but when the designer
   looks at the existing Types, almost all of them have well-defined
   semantics, none of which quite match the needs of the new
   application.  This has not completely prevented proposals from
   reusing existing Resource Record Types in ways incompatible with
   their defined semantics, but it does tend to steer application
   designers away from this approach.

   For example, Resource Record Type 40 was registered for the SINK
   record
   Resource Record Type.  This Resource Record Type was discussed in the
   DNSIND working group of the IETF, and it was decided at the 46th IETF
   to not move the I-D forward to become an RFC because of the risk of
   encouraging application designers to use the SINK Resource Record
   Type instead of registering a new Resource Record Type, which would
   result in infeasibly large SINK RRsets.

   Eliminating all of the above leaves the TXT Resource Record Type in
   the IN class.  The TXT RDATA format is free form text, and there are
   no existing semantics to get in the way.  Some attempts have been
   made, for example in draft-cheshire-dnsext-dns-sd
   [I-D.cheshire-dnsext-dns-sd], to specify a structured format for TXT
   Resource Record Types, but no such attempt has reached RFC status.
   Furthermore, the TXT Resource Record can obviously just be used as a
   bucket in which to carry around data to be used by some higher level
   parser, perhaps in some human readable programming or markup
   language.  Thus, for many applications, TXT Resource Records are the
   "obvious" choice.  Unfortunately, this conclusion, while
   understandable, is also wrong, for several reasons.

   The first reason why TXT Resource Records are not well suited to such
   use is precisely the lack of defined semantics that make them so
   attractive.  Arguably, the TXT Resource Record is misnamed, and
   should have been called the Local Container record, because the lack
   of defined semantics means that a TXT Resource Record means precisely
   what the data producer says it means.  This is fine, so long as TXT
   Resource Records are being used by human beings or by private
   agreement between data producer and data consumer.  However, it
   becomes a problem once one starts using them for standardized
   protocols in which there is no prior relationship between data
   producer and data consumer.  The reason for this is that that, if TXT
   records are used without one of the naming modifications discussed
   earlier (and in some cases even if one use such naming mechanisms),
   there is nothing to prevent collisions with some other incompatible
   use of TXT Resource Records.  This is even worse than the general
   subtyping problem described in Section 3.1, because TXT Resource
   Records don't even have a standardized selector field in which to
   store the subtype.  RFC 1464 [RFC1464] tried, but it was not a
   success.  At best a definition of a subtype is reduced to hoping that
   whatever scheme one has come up with will not accidently conflict
   with somebody else's subtyping scheme, and that it will not be
   possible to mis-parse one application's use of TXT Resource Records
   as data intended for a different application.  Any attempt to impose
   a standardized format within the TXT Resource Record format would be
   at least fifteen years too late even if it were put into effect
   immediately; at best, one can restrict the syntax that a particular
   application uses within a TXT Resource Record and accept the risk
   that unrelated TXT Resource Record uses will collide with it.

   Using one of the naming modifications discussed in Section 3.2 and
   Section 3.3 would address the subtyping problem, (and have been used
   in combinations with reuse of TXT record, such as for the dns/txt
   lookup mechanism in DKIM) but each of these approaches brings in new
   problems of its own.  The prefix approach (that for example SRV
   Resource Records use) does not work well with wildcards, which is a
   particular problem for mail-related applications, since MX Resource
   Records are probably the most common use of DNS wildcards.  The
   suffix approach doesn't have wildcard issues, but, as noted
   previously, it does have synchronization and update authorization
   issues, since it works by creating a second subtree in a different
   part of the global DNS name space.

   The next reason why TXT Resource Records are not well suited to
   protocol use has to do with the limited data space available in a DNS
   message.  As alluded to briefly in Section 3.1, typical DNS query
   traffic patterns involve a very large number of DNS clients sending
   queries to a relatively small number of DNS servers.  Normal path MTU
   discovery schemes do little good here because, from the server's
   perspective, there isn't enough repeat traffic from any one client
   for it to be worth retaining state.  UDP-based DNS is an idempotent
   query, whereas TCP-based DNS requires the server to keep state (in
   the form of TCP connection state, usually in the server's kernel) and
   roughly triples the traffic load.  Thus, 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.

   Subtyping schemes are therefore again problematic because they
   produce larger Resource RRSets than necessary, but verbose text
   encodings of data are also wasteful, since the data they hold can
   usually be represented more compactly in a Resource Record designed
   specifically to support the application's particular data needs.  If
   the data that need to be carried are so large that there is no way to
   make them fit comfortably into the DNS regardless of encoding, it is
   probably better to move the data somewhere else, and just use the DNS
   as a pointer to the data, as with NAPTR.

6.  Conclusion and Recommendation

   Given the problems detailed in Section 5, it is worth reexamining the
   oft-jumped-to conclusion that specifying a new Resource Record Type
   is hard.  Historically, this was indeed the case, but recent surveys
   suggest that support for unknown Resource Record Types [RFC3597] is
   now widespread, and because of that the DNS infrastructure can handle
   new resource record types.  The lack of support for unknown Types is
   mostly an issue for relatively old provision software and
   applications that would probably need to be upgraded in any case as
   part of supporting a new application. feature (that require the new Resource
   Record Type).  One should also remember that deployed DNS software
   today should support DNSSEC, and software recent enough to do so will
   likely support both unknown Resource Record Types [RFC3597] and EDNS0
   [RFC2671].

   Of all the issues detailed in Section 3.5, provisioning the data is
   in some respects the most difficult.  The problems can be divided in
   two, the ability to manage the zone on the master server, and the
   ability for secondary servers to do zone transfers (AXFR or IXFR)
   with the new data.  Investigations show that the problem here is less
   difficult for the authoritative name servers themselves than the
   front-end systems used to enter (and perhaps validate) the data.
   Hand editing does not work well for maintenance of large zones, so
   some sort of tool is necessary, and the tool may not be tightly
   coupled to the name server implementation itself.  Note, however,
   that this provisioning problem exists to some degree with any new
   form of data to be stored in the DNS, regardless of data format,
   Resource Record type, or naming scheme.  Including the type (even if TXT Resource Record Type. Types are in use),
   or naming scheme.  Adapting front-end systems to support a new
   Resource Record type Type may be a bit more difficult than reusing an
   existing type, but this appears to be a minor difference in degree
   rather than a difference in kind.

   Given the various issues described in this note, we believe that:
   o  there is no magic solution which allows a completely painless
      addition of new data to the DNS, but
   o  on the whole, the best solution is still to use the DNS Resource
      Record Type mechanism designed for precisely this purpose, and
   o  of all the alternate solutions, the "obvious" approach of using
      TXT Resource Records is almost certainly the worst.
   This especially for the two reasons outlined above (lack of semantics
   and its implications, and size leading to the need to use TCP).

7.  Creating A New Resource Record Type

   The process for creating a new Resource Record Type is specified in
   draft-ietf-dnsext-2929bis [I-D.ietf-dnsext-2929bis].

8.  IANA Considerations

   This document does not require any IANA actions.

9.  Security Considerations

   DNS RRSets can be signed using DNSSEC.  DNSSEC is almost certainly
   necessary for any application mechanism that stores authorization
   data in the DNS.  DNSSEC signatures significantly increase the size
   of the messages transported, and because of this, the DNS message
   size issues discussed in Section 3.1 and Section 5 are more serious
   than they might at first appear.

   Adding new Resource Record Types (as discussed in Section 3.5) can
   create two different kinds of problems.  In DNS software and in
   applications.  In the DNS software, it might conceivably trigger bugs
   and other bad behavior in software that is not compliant with RFC
   3597 [RFC3597], but most such DNS software is old enough and insecure
   enough that it should be updated for other reasons in any case.  In
   applications and provisioning software, the changes for the new
   features that need the new data in DNS can be updated to understand
   the structure of the new data format (regardless of whether a new
   Resource Record Type is used or some other mechanism is chosen.
   Basic API support for retrieving arbitrary Resource Record Types has
   been a requirement since 1989 (see [RFC1123]). 1989[RFC1123].

   Any new protocol that proposes to use the DNS to store data used to
   make authorization decisions would be well advised not only to use
   DNSSEC but also to encourage upgrades to DNS server software recent
   enough not to be riddled with well-known exploitable bugs.  Because
   of this, support for new Resource Record Types will not be as hard as
   people might think at first.

10.  Acknowledgements

   This document has been created during a number of years, with input
   from many people.  The question on how to expand and use the DNS is
   sensitive, and a document like this can not please everyone.  The
   goal is instead to describe the architecture and tradeoffs, and make
   some recommendations about best practices.

   People that have helped include: Dean Andersson, Loa Andersson, Mark
   Andrews, John Angelmo, Roy Badami, Dan Bernstein, Alex Bligh,
   Nathaniel Borenstein, Stephane Bortzmeyer, Brian Carpenter, Leslie
   Daigle, Elwyn Davies, Mark Delany, Richard Draves, Martin Duerst,
   Donald Eastlake, Robert Elz, Jim Fenton, Tony Finch, Jim Gilroy,
   Olafur Gudmundsson, Eric Hall, Philip Hallam-Baker, Ted Hardie, Bob
   Hinden, Paul Hoffman, Geoff Houston, Christian Huitema, Johan Ihren,
   John Klensin, Olaf Kolkman, Ben Laurie, William Leibzon, John Levine,
   Edward Lewis, David MacQuigg, Allison Manking, Bill Manning, Danny
   McPherson, David Meyer, Pekka Nikander, Mans Nilsson, Masataka Ohta,
   Douglas Otis, Michael Patton, Jonathan Rosenberg, Anders Rundgren,
   Miriam Sapiro, Pekka Savola, Chip Sharp, James Snell, Dave Thaler,
   Michael Thomas, Paul Vixie, Sam Weiler, Florian Weimer, Bert Wijnen,
   and Dan Wing.

   Members of the IAB when this document was made available were: Loa
   Andersson, Leslie Daigle, Elwyn Davies, Kevin Fall, Gonzalo Camarillo, Stuart Cheshire Russ Housley, Olaf
   Kolkman, Gregory Lebovitz, Barry Leiba, Kurtis Lindqvist, Andrew
   Malis, Danny McPherson, David Oran, Eric Rescorla, Dave Thaler, and Lixia Zhang.

11.  References

11.1.  Normative References

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

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

   [RFC2535]  Eastlake 3rd,  Eastlake, D., "Domain Name System Security Extensions",
              RFC 2535, March 1999.

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

   [RFC3597]  Gustafsson, A., "Handling of Unknown DNS Resource Record
              (RR) Types", RFC 3597, September 2003.

11.2.  Informative References

   [I-D.cheshire-dnsext-dns-sd]
              Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", draft-ietf-dnsext-2929bis-06 (work in
              progress), August 2006.

   [I-D.ietf-dnsext-2929bis]
              Eastlake 3rd, D., "Domain Name System (DNS) IANA
              Considerations",
              draft-ietf-dnsext-2929bis-06 draft-cheshire-dnsext-dns-sd-03 (work in
              progress), August 2007.

   [RFC1123]  Braden, R., "Requirements for Internet Hosts - Application
              and Support", RFC 1123, STD 3, RFC 1123, October 1989.

   [RFC1535]  Gavron, E., "A Security Problem and Proposed Correction
              With Widely Deployed DNS Software", RFC 1535,
              October 1993.

   [RFC2163]  Allocchio, C., "Using the Internet DNS to Distribute MIXER
              Conformant Global Address Mapping (MCGAM)", RFC 2163,
              January 1998.

   [RFC2181]  Elz, R. and R. Bush, "Clarifications to the DNS
              Specification", RFC 2181, July 1997.

   [RFC2672]  Crawford, M., "Non-Terminal DNS Name Redirection",
              RFC 2672, August 1999.

   [RFC2929]  Eastlake 3rd,  Eastlake, D., Brunner-Williams, E., and B. Manning,
              "Domain Name System (DNS) IANA Considerations", RFC 2929, BCP 42,
              RFC 2929, September 2000.

   [RFC3445]  Massey, D. and S. Rose, "Limiting the Scope of the KEY
              Resource Record (RR)", RFC 3445, December 2002.

   [RFC3467]  Klensin, J., "Role of the Domain Name System (DNS)",
              RFC 3467, February 2003.

   [RFC3761]  Faltstrom, P. and M. Mealling, "The E.164 to Uniform
              Resource Identifiers (URI) Dynamic Delegation Discovery
              System (DDDS) Application (ENUM)", RFC 3761, April 2004.

   [RFC4592]  Lewis, E., "The Role of Wildcards in the Domain Name
              System", RFC 4592, July 2006.

Authors' Addresses

   Internet Architecture Board

   Email: iab@iab.org

   Patrik Faltstrom (editor)

   Email: paf@cisco.com

   Rob Austein (editor)

   Email: sra@isc.org

   Peter Koch (editor)

   Email: pk@denic.de

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