Network Working Group J. Klensin
Internet-Draft June 19, 2017
Intended status: Informational
Expires: December 21, 2017
DNS Privacy, Authorization, Special Uses, Encoding, Characters,
Matching, and Root Structure: Time for Another Look?
draft-klensin-dns-function-considerations-02
Abstract
The basic design of the Domain Name System was completed almost 30
years ago. The last half of that period has been characterized by
significant changes in requirements and expectations, some of which
either require changes to how the DNS is used or that can be
accommodated only poorly or not at all. This document asks the
question of whether it is time to either redesign and replace the DNS
to match contemporary requirements and expectations (rather than
continuing to try to design and implement incremental patches that
are not fully satisfactory) or to draw some clear lines about
functionality that is not really needed or that should be performed
somewhere else.
Author's Note
This draft is intended to draw a number of issues and references
together in one place and to start a discussion. It is obviously
incomplete, particularly with regard to the list of perceived issues
and deficiencies with that DNS. To avoid misunderstanding, I don't
completely believe some of the deficiencies listed below but am
merely providing information about claims of deficiencies. Input is
welcome, especially about what is missing (or plain wrong) and would
be greatly appreciated.
This document should be discussed on the IETF list or by private
conversation with the author.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 21, 2017.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Background and Hypothesis . . . . . . . . . . . . . . . . . . 4
3. Warts and Tensions With The Current DNS . . . . . . . . . . . 5
3.1. Multiple address types . . . . . . . . . . . . . . . . . 5
3.2. Matching Part I: Case Sensitivity in Labels and Other
Anomalies . . . . . . . . . . . . . . . . . . . . . . . . 5
3.3. Matching Part II: Non-ASCII ("internationalized") Domain
Name Labels . . . . . . . . . . . . . . . . . . . . . . . 6
3.4. Matching Part III: Label Synonyms, Equivalent Names, and
Variants . . . . . . . . . . . . . . . . . . . . . . . . 7
3.5. Query Privacy . . . . . . . . . . . . . . . . . . . . . . 8
3.6. Alternate Name Spaces for Public Use in the DNS
Framework: The CLASS Problem . . . . . . . . . . . . . . 9
3.7. Loose Synchronization . . . . . . . . . . . . . . . . . . 9
3.8. Private Name Spaces and Special Names . . . . . . . . . . 10
3.9. Alternate Query or Response Encodings . . . . . . . . . . 11
3.10. Distribution and Managment of Root Servers . . . . . . . 11
3.11. Identifiers Versus Brands and Other Convenience Names . . 11
3.12. A Single Hierarchy with a Centrally-controlled Root . . . 12
3.13. Newer Application Protocols and New Requirements . . . . 13
3.13.1. The Extensions . . . . . . . . . . . . . . . . . . . 13
3.13.2. Extensions and Deployment Pressures -- The TXT
RRTYPE . . . . . . . . . . . . . . . . . . . . . . . 14
3.13.3. Periods and Zone Cut Issues . . . . . . . . . . . . 15
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3.14. Scaling of Reputation and Other Ancillary Information . . 15
4. Searching and the DNS - An Historical Note . . . . . . . . . 16
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
7. Security Considerations . . . . . . . . . . . . . . . . . . . 17
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.1. Normative References . . . . . . . . . . . . . . . . . . 17
8.2. Informative References . . . . . . . . . . . . . . . . . 18
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 22
A.1. Changes from version -00 (2017-06-02) to -01 . . . . . . 22
A.2. Changes from version -01 (2017-06-06) to -02 . . . . . . 22
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 23
1. Introduction
This document explores contemporary expectations of the Internet's
domain system (DNS) and compares them to the assumptions and
properties of the DNS design. It is primarily intended to ask the
question of whether the differences are causing enough stresses on
the system, stresses that cannot be resolved satisfactorily by
further patching, that the Internet community should be considering
designing a new system, one that is better adapted to current needs
and expectations, and developing a deployment and transition strategy
for it. For those (perhaps the majority of us) for whom actually
replacing the DNS is too radical to be realistic, the document may be
useful in two other ways. It may provide a foundation for discussing
what functions the DNS should not be expected to support and how
those functions can be supported in other ways, perhaps via an
intermediate system that then calls on the DNS or by using some other
type of database technology for some set of functions while leaving
the basic DNS functions intact. Or it may provide a basis for
"better just get used to that and the way it works" discussions to
replace fantasies about what the DNS might do in some alternate
reality.
There is a key design or philosophical question associated with the
analysis in this document that the document does not address. It is
whether changes to perceived requirements to DNS functionality as
described here are, in most respects, evolutionary or whether many of
them are instances of trying to utilize the DNS for new requirements
because it exists and is already deployed independent of whether the
DNS is really appropriate or not. The latter might be an instance of
a problem often described in the IETF as "when all you have is a
hammer, everything looks like a nail".
While this document does not assume deep technical or operational
knowledge of the DNS, it does assume some knowledge and at least
general familiarity with the concepts of RFC 1034 [RFC1034] and RFC
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1035 [RFC1035] and the terminology discussed in RFC 7719 [RFC7719]
and elsewhere. Although some of the comments it contains might be
taken as hints or examples of different ways to think about the
design issues, it makes no attempt to explore, much less offer a
tutorial on, alternate naming systems or database technologies.
It is perhaps worth noting that, while the perspective is different
and more than a dozen years have passed, many of the issues discussed
in this document were analyzed and described (most of them with more
extensive explanations) in a 2005 US National Research Council report
[NRC-Signposts].
Readers should note that several references are to obsolete
documents. That was done because they are intended to show the
documents and dates that introduced particular features or concepts.
When current versions are intended, they are referenced.
2. Background and Hypothesis
The domain name system (DNS) [RFC1034] was designed starting in the
early 1980s [RFC0799] [RFC0881] [RFC0882] with the main goal of
replacing the flat, centrally-administered, host table system
[RFC0810] [RFC0952] [RFC0953] with a hierarchical, administratively-
distributed, system. The DNS design included some features that were
judged to be unworkable and either replaced (e.g., the mail
destination (MD) and mail forwarder (MF) approach [RFC0882] that were
replaced by the MX approach [RFC0974]), abandoned (e.g., the
mechanism for using email local parts as labels described in RFC 1034
Section 3.3), or deprecated (e.g., the WKS RR TYPE [RFC1123]. Newer
ideas and requirements have identified a number of other features,
some of which were less developed than others. Of course the
original designers could not anticipate everything that has come to
be expected of the DNS in the last 30 years.
In recent years, demand for new and extended services and uses of the
DNS have, in turn, led to proposals for DNS extensions or changes of
various sorts. Some have been adopted, including a model for
negotiating extended functionality [RFC2671], others were found to be
impracticable, and still others continue to be under consideration.
A few features of the original DNS specification, such as the CLASS
property and label types, have also been suggested to be so badly
specified that they should be deprecated [Sullivan-Class].
Unlike earlier changes such as the IDNA mechanisms for better
incorporating non-ASCII labels without modifying the DNS structure
itself [RFC3490] [RFC5890], some recent proposals require or strongly
suggest changes to APIs, formats, or interfaces by programs that need
to retrieve information from the DNS or interpret that information.
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Requirements for such changes suggest that it may be time to stop
patching the DNS or trying to extend it in small increments, but to
consider development of a system that better meets today's needs and
a transition strategy to it.
The next section of this document discusses a number of issues with
the current DNS design that could appropriately be addressed by a
different and newer design model. In at least some cases, changing
the model and protocols could bring significant benefits to the
Internet and/or its administration.
This document is not a proposal for a new protocol. It is intended
to stimulate thought about how far we want to try to push the
existing DNS, to examine whether expectations of it are already
exceeding its plausible capabilities, and to start discussion of a
redesign or alternatives to one if the time for that discussion has
come.
3. Warts and Tensions With The Current DNS
As suggested above, there are many signs that the DNS is incapable of
meeting contemporary expectations of how it should work and
functionality it should support. Some of those expectations are
unrealistic under any imaginable circumstances; others are impossible
(or merely problematic) in the current DNS structure but could be
accommodated in a redesign. These are examples, rather than a
comprehensive list, and do not appear in any particular order.
3.1. Multiple address types
While returning both TYPE A (IPv4 address) and AAAA (IPv6 address)
records as additional information in response to any of several query
types (see RFC 3596 [RFC3596]) was a useful patch, it is easy to
imagine better choices. For example, except that it would have
required DNS modifications, we could have established a single
"address" query type (QTYPE) that could return whatever IPv4 and/or
IPv6 addresses were available, perhaps with preference information if
that were stored in the database, and without requiring the "ANY" be
used. Other solutions would have been plausible; that one is offered
only to combine an existence proof of at least one possibility and an
example of how the existing DNS design and implementations are
preventing us from thinking more broadly about possible solutions.
3.2. Matching Part I: Case Sensitivity in Labels and Other Anomalies
The DNS specifications assume that labels are octet strings and
octets with the high bit zero have seven-bit ASCII codes in the
remaining bits. They require that, when a domain name used in a
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query is matched to one stored in the database, those ASCII
characters be interpreted in a case-independent way, i.e., upper and
lower case letters are treated as equivalent (digits and symbols are
not affected). For non-ASCII octets, i.e., octets in labels with the
first bit turned on, there are no assumptions about the character
coding used, much less any rules about character case equivalence --
strings must be compared by matching bits in sequence. Even though
the current model for handling non-ASCII (i.e., "internationalized")
domain name labels (IDNs) [RFC5890] (and see Section 3.3 below)
encodes information so the DNS is not directly affected, the notion
that some characters in labels are handled in a case-insensitive way
and that others are case-sensitive (or that upper case must be
prohibited entirely as IDNA does) has caused a good deal of confusion
and resentment. Those concerns about inconsistent behavior and
mishandling (or suboptimal handling) of case relationships for some
languages have not been mitigated by repeated explanations that the
relationships between "decorated" lower-case characters and their
upper-case equivalent are often sensitive to language and locality
and therefore not deterministic with information available to DNS
servers.
3.3. Matching Part II: Non-ASCII ("internationalized") Domain Name
Labels
Quite independent of the case-sensitivity problem, one of the
fundamental properties of Unicode [Unicode] is that some abstract
characters can be represented in multiple ways, such as by a single,
precomposed, code point or by a base code point followed by one or
more code points that specify combining characters. While Unicode
Normalization can be used to eliminate many (but not all) of those
distinctions for comparison (matching) purposes, it is best applied
during matching rather than by changing one string into another. The
first version of IDNA ("IDNA2003") made the choice to change strings
during processing for either storage or retrieval [RFC3490]
[RFC3491]; the second ("IDNA2008") required that all strings be
normalized [RFC5891]. Neither is optimal, if only because
transforming the strings themselves implies that the input string in
an application may not be the same as the string used in processing
and perhaps later display.
It would almost certainly be preferable, and more consistent with
Unicode recommendations, to use normalization (and perhaps other
techniques) at matching time rather than altering the strings at all,
even if there were still only a single matching algorithm, i.e.,
normalization were added to the existing ASCII-only case folding.
However, even Unicode's discussion of normalization [Unicode-UAX15]
indicates that there are special, language-dependent, cases (the most
commonly-cited example is the dotless "i" (U+0131)). Not only does
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the DNS lack any information about languages that could be used in a
mapping algorithm, but, as long as there is a requirement that there
be only one mapping algorithm for the entire system, that information
could not be used even if it were available. One could imagine a
successor system that would use information stored at nodes in the
hierarchy to specify different matching rules for subsidiary nodes
(or equivalent arrangements for non-hierarchical systems). It is not
clear whether that would be a good idea, but it certainly is not
possible with the DNS as we know it.
3.4. Matching Part III: Label Synonyms, Equivalent Names, and Variants
As the initial phases of work on IDNs started to conclude, it became
obvious that the nature and evolution of human language and writing
systems required treating some names as "the same as" others. The
first important example of this involved the relatively recent effort
to simplify the Chinese writing system, thereby creating a
distinction between "Simplified" and "Traditional" Chinese even
though the meaning of the characters remained the same in almost all
cases (in so-called ideographic character sets, characters have
meaning rather than representing sounds). A joint effort among the
relevant country code top level domain (TLD) registries and some
other interested parties produced a set of recommendations for
dealing with the issues with that script [RFC3743] and introduced the
concept of "variant" characters and domain names.
However, when names are seen as having meanings, rather than merely
being mnemonics, and especially when they represent brands or the
equivalent, or when spelling for a particular written language is not
completely standardized, there is an immediate demand to treat
different strings as exact equivalents. As a trivial English-
language example, it is widely understood that "colour" and "color"
represent the same word, so does that imply that, if they are used as
DNS labels in domain names all of whose other labels are identical,
should the two domain names be treated as identical? Examples for
other languages or writing systems, especially ones in which some or
all markings that distinguish characters by sound or that change the
pronunciation of words are optional, are often more numerous and more
problematic than national spelling differences in English, but they
are harder to explain to those unfamiliar with those other languages
or writing systems (and hard to illustrate in ASCII-only Internet-
Drafts and RFCs). Although approximations are possible, the DNS
cannot handle that requirement: not only do its aliasing mechanisms
(CNAME, DNAME, and various proposals for newer and different types of
aliasing [DNS-Aliases] [DNS-BNAME], not provide a strong enough
binding, but the ability to use those aliases from a subtree
controlled by one administrative entity to that of another one,
implies that there is little or no possibility of the owner (in
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either the DNS sense or the registrar-registrant one) of a particular
name to control the synonyms for it. Some of that issue can be dealt
with at the application level, e.g., by redirects in web protocols,
but taking that approach, which is the essential characteristic of
"if both names belong to the same owner, everything is ok"
approaches, results in names being handled in inconsistent ways in
different protocols.
A different way of looking at part of this issue (and, to some
degree, of the one discussed above in Section 3.3) is that these
perceived equivalences and desired transformations are context-
dependent, but the DNS resolution process is not [RFC6912].
Similar problems arise as people notice that some characters are
easily mistaken for others and that might be an opportunity for user
confusion and attacks. Commonly-cited examples include the Latin and
Cyrillic script "a" characters, which are identical [CACM-Homograph],
the characters in many scripts that look like open circles or
vertical or horizontal lines, and even the Latin script letter "l"
and the European digit "1", but examples abound in other scripts and
combinations of scripts as well. The most common proposed solution
within the DNS context has been to treat these cases, as well as
those involving orthographic variations, as "variants" and either ban
all but one (or a few) of the possible labels from the DNS (possibly
on a first come first served basis) or by ensuring that any
collection of such strings that are delegated as assigned to the same
ownership (see above). Neither solution is completely satisfactory:
if all but one string is excluded, users who guess at a different
form, perhaps in trying to transcribe characters from written or
printed form, don't find what they are looking for and, as pointed
out above, "same ownership" is sufficient only with carefully-
designed and administered applications protocol support and sometimes
not then.
Some of these issues are discussed at more length in an ICANN report
[ICANN-VIP].
3.5. Query Privacy
There has been growing concern in recent years that DNS queries occur
in clear text on the public Internet and that, if those queries can
be intercepted, they can expose a good deal of information about
interests and contacts that could compromise individual privacy.
While a number of proposals, including query name minimization
[RFC7816] and running DNS over an encrypted tunnel [RFC7858] have
been made to mitigate that problem, they all appear to share the
common properties of security patches rather than designed-in
security or privacy mechanisms. While experience may prove otherwise
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once (and if) they are widely deployed, it does not appear that any
of them are as satisfactory as a system with query privacy designed
in might be. More general tutorials on this issue have appeared
recently [Huston-DNSPrivacy].
3.6. Alternate Name Spaces for Public Use in the DNS Framework: The
CLASS Problem
The DNS standards include specification of a CLASS value to "identify
a protocol family or instance of a protocol" RFC 1034, Section 3.6
and elsewhere [RFC1034]. While it was used effectively in the early
days of the DNS to manage different protocol families within the same
administrative environment, recent attempts to use it to either
partition the DNS namespace in other ways such as for non-ASCII names
(partially to address the issues in Section 3.2 Section 3.3) or to
use DNS mechanisms for entirely different namespaces have exposed
fundamental problems with the mechanism [Sullivan-Class], leading to
recommendations that it be dropped entirely.
Whether either the function CLASS was originally intended to provide
or the ones for which there have been attempts to use it more
recently are actually needed is a separate question; it is clear that
the current DNS technical and administrative model is unsuitable for
either function.
3.7. Loose Synchronization
The DNS model of master and slave servers, with the latter initiating
updates based on TTL values, together with more local caches, depends
heavily on an approach that has come to be called "loose
synchronization", i.e., that there can be no expectation that all of
the servers that might reasonably answer a query will have exactly
the same data unless those data have been unchanged for a rather long
period. Put differently, if some or all of the records associated
with a particular node in the DNS (informally, a fully-qualified
domain name (FQDN)) change, one cannot expect those changes to be
propagated immediately.
That model has worked rather well since the DNS was first deployed,
protecting the system from requirements, that are typical where
simultaneous update of multiple systems is needed, such as elaborate
locking, complex update mechanisms, or journaling. As has often been
pointed out with the Internet, implementation and operational
complexity are often the enemy of stability, security, and
robustness. Loose synchronization has helped keep the DNS as simple
and robust as possible.
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A number of recent ideas about using the DNS to store data that
change very rapidly and where the changes are important are, however,
largely incompatible with loose synchronization. Efforts to use very
short (or zero) refresh times (in SOA records for slave updates from
masters) and TTLs (for caches) to simulate nearly-simultaneous
updating may work up to a point but appear to impose very heavy loads
on servers and distribution mechanisms that were not designed to
accommodate that style of working. Similar observations can be made
about attempts to use the NOTIFY extension [RFC1996] or dynamic,
"server-push", updating rather than the traditional DNS mechanisms.
While the NOTIFY and push mechanisms normally provide refresh times
and update mechanisms faster than those specified in RFC 1034 and
1035, they imply that a "master" server must know the identities of
(and have good connectivity to all of) its slaves, defeating at least
some of the advantages associated with stealth slaves, particularly
those associated with reduction of query traffic across the Internet.
Those mechanisms do nothing for cache updates: unless servers keep
track of every query for names associated with a specific zone and
somehow notify the query source systems, the only alternative to
having information that might be obsolete stored in caches is to use
very short or zero TTLs so the cached data time out almost
immediately after being stored (or are not stored at all), requiring
a new query to an authoritative server each time a resolver makes a
query.
3.8. Private Name Spaces and Special Names
Almost since the DNS was first deployed, there have been situations
in which it is desirable to use DNS-like names, and often DNS
resolution mechanisms or modifications of them, with name spaces for
which globally-available and consistent resolution using the public
DNS is either unfeasible or undesirable (and for which the use of
CLASS is not an appropriate mechanism). The need to isolate names
and addresses on LANs from the public Internet, typically via "split
horizon" approaches, is one example of this requirement although
often not recognized as such. Another example that has generated a
good deal of controversy involves "special names" -- labels or
pseudo-labels, often in TLD positions, that signal that the full name
should not be subject to normal DNS resolution or other processing
[RFC6761] [DNSOP-Sutld].
Independent of troublesome policy questions about who should allocate
such names and the procedures to be used, they almost inherently
require either a syntax convention to identify them (there actually
was such a convention, but it was abandoned many years ago and there
is no plausible way to re-institute it) or tables of such names that
are known to, and kept updated on, every resolver on the Internet, at
least if spurious queries to the root servers are to be avoided.
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If the DNS were to be redesigned and replaced, we could recognize
this requirement as part of the design and handle it much better than
it is possible to handle it today.
3.9. Alternate Query or Response Encodings
The DNS specifies formats for queries and data responses, based on
the state of the art and best practices at the time it was designed.
Recent work has suggested that there would be significant advantages
to supporting at least a description of the DNS messages in one or
more alternate formats, such as JSON [Hoffman-DNS-JSON]
[Hoffman-SimpleDNS-JSON]. While that work has been carefully done to
avoid requiring changes to the DNS, much of the argument for having
such a JSON-based description format could easily be turned into an
argument that, if the DNS were being revised, that format might be
preferable as a more direct alternative to having DNS queries and
responses in the original form.
3.10. Distribution and Managment of Root Servers
The DNS model requires a collection of root servers that hold, at
minimum, information about top-level domains. Over the years, that
requirement has evolved from a technically fairly minor function,
normally carried out as a service to the broader Internet community
and its users and systems, to a subject that is intensely
controversial with regard to who should control those servers, how
they should be distributed and where they should be located. While a
number of mechanisms have been proposed and one (anycast [RFC7094])
is in very active use to mitigate some of the real and perceived
problems, it seems obvious that a DNS successor, designed for today's
perceived requirements, could handle these problems in a technically
more appropriate and less controversial way.
3.11. Identifiers Versus Brands and Other Convenience Names
A key design element of the original network object naming systems
for the ARPANET, largely inherited by the DNS, was that the names,
while expected to be mnemonic, were identifiers and their being
highly distinguishable and not prone to ambiguity was important.
That led to very restrictive rules about what could appear in a name.
In the case of the host table, the restrictions that came to the DNS
(largely via SMTP) as the "preferred syntax" [RFC 1034 Section 3.5]
or what we now often call the letter-digit-hyphen (LDH) rule.
Similar rules to make identifiers easier to use, less prone to
ambiguity, or less likely to interfere with syntax in more formal
languages occur frequently. For example, almost every programming
language has restrictions on what can appear in an identifier and
Unicode provides general recommendations about identifier composition
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[Unicode-USA31]. Both are quite restrictive as compared to the
number of characters and total number of strings that can be written
using that character coding system.
In the last decade or two, another perspective has emerged, largely
without being explicitly understood or acknowledged. In it, the DNS
is really (and primarily) a system for expressing thoughts and
concepts. Those include free expression of ideas in as close to
natural language as possible as well as representation of product
names and brands. That view requires letter-like characters that
might not be reasonable in identifiers along with a variety of
symbols and punctuation and might require indicators of preferred
type styles to provide information in a form that exactly matches
personal or legal preferences. That perspective would argue for
standardizing word and sentence separators, removing the 63 octet per
label limit and probably the limit of 255 octets on the total length
of a domain name, and maybe even eliminating the hierarchy or
allowing separators for labels in presentation form (now fixed at "."
for the DNS) to be different according to context. At least it
suggests that the original design was defective in not prioritizing
those uses over support for unique and unambiguous identifiers.
So we have two, or, depending on how one counts, three very different
use cases. The historical one is support for unique identifiers.
The other is expression of ideas and, if one considers it separate,
presentation of brand and product names. Because they inherently
involve different constraints, priorities, and success criteria,
these perspectives are, at best, only loosely compatible.
We cannot simultaneously optimize both the identifier perspective and
either or both of the others in the same system. At best, there are
some complex trade-offs involved. Even then, it is not clear that
the same DNS (or other system) can accommodate all of them. Until we
come to terms with that, the differences manifest themselves with
friction among communities, most often with tension between "we want
to do (or use or sell) these types of labels" and "not good for the
operational Internet or the DNS".
3.12. A Single Hierarchy with a Centrally-controlled Root
A good many Internet policy discussions in the last two decades have
revolved around such questions of how many top level domains there
should be and what they should be, who should control them and how,
how (or if) their individual operations and policy decisions should
be accountable to others, and what processes should be used (and by
what entities or organizational structures) to make those decisions.
Several people have pointed out that, if we were designing a next-
generation DNS using today's technology, it should be possible to
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remove the technical requirement for a central authority over the
root (some people have suggested that blockchain approaches would be
helpful for this purpose; others believe they just would not scale
adequately, at least at acceptable cost). Whether that would be
desirable or not is fairly obviously a question of perspective and
priorities.
3.13. Newer Application Protocols and New Requirements
New work done in other areas has led to demands for new DNS features,
many of them involving data values that require recursively
referencing the DNS. Early record types that did that were
accompanied by restrictions that reduced the risk of looping
references or other difficulties. For example, while the MX RRTYPE
has a fully-qualified domain name as its data, SMTP imposes "primary
name" restrictions that prevent the name used from being, e.g., a
CNAME. While loops with CNAMEs are possible, RFC 1034, Section 3.6,
includes a discussion about ways to avoid problems and how they
should be handled. Some newer protocols and conventions can cause
more stress. There are separate issues with additions and with how
the DNS has been extended to try to deal with them.
3.13.1. The Extensions
Some examples of DNS extensions for new protocol demands that
illustrate, or have led to, increased stress include:
NAPTR Requires far more complex data in the DNS for ENUM (e.g., VoIP
and SIP) support, including URI information and hence recursive or
repeated lookups, than any of the RRTYPEs originally supported.
The RRSET associate with these records can become quite large
becaues the separator between the various records is part of the
RDATA, and not the {owner, class, type} triple (a problem slightly
related to the problem with overloading of TXT RTYPE discussed in
Section 3.13.2. This problem, and similar ones for some of the
cases below. may suggest that any future design is in need of a
different TYPE model such as systematic arrangements for subtypes
or some explicit hierarchy in the TYPEs.
URI Has a URI as its data, typically also requiring recursive or
repeated lookups.
Service location (SRV) and credential information (including SPF)
Require structured data and, especially for the later,
significantly more data, than most original RRTYPEs.
URI/URL The early design decision for the World Wide Web that its
mechanism for identifying digital web content (now known as
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Uniform Resource Identifiers [RFC3986]) did so by using domain
names and hence the network location of the information or other
material. That, in turn, has required systems intended to improve
web performance by locating and retrieving a "nearest copy" rather
than the single copy designated by the URL to intercept DNS
queries and respond with values that are not precisely those
stored for the designated domain name in the DNS or to otherwise
access information in a way not supported by the DNS itself.
In addition to the stresses these new functions cause, incremental
deployment of systems that utilize them means that some functions
will work in some environments and not others. This has been
especially problematic with complex, multi-record, functions like
DNSSEC that provide or require special validation mechanisms such as
DNNSEC.
3.13.2. Extensions and Deployment Pressures -- The TXT RRTYPE
Unfortunately (but unsurprisingly) and despite IETF efforts to make
things easier [RFC6895], DNS support libraries have often been slow
to add full support for new RRTYPEs, impeding deployment of
applications that depend on them. Both to get faster deployment and,
at least until recently, to avoid burdensome IETF approval
procedures, many application designers have chosen to push protocol-
critical information into records with TXT RRTYPE, a record type that
was originally intended to include only information equivalent to
comments.
This causes two problems. First, TXT records used this way tend to
get long and complex, which is a problem in itself if one is trying
to minimize TCP connections. Second, applications that are
attempting to obtain data cannot merely ask for the relevant QTYPE,
they must obtain all of the records with QTYPE TXT and parse them to
determine which ones are of interest. That would be easier if there
was some standard for how to do that parsing but, at least in part
because the clear preference in the DNS design is for distinct
RRTYPEs for different kinds of information, there is no such
standard.
On the other hand, this issue is somewhat different from most of the
others described in this document because (as the IETF has
recommended several times) the problem is easily solved within the
current DNS design by allocating and supporting new RRTYPEs when
needed rather than using TXT as a workaround (that does not mean that
other solutions are impossible, either with the current DNS or some
other design). The problem then lies in the implementations and/or
mechanisms that deter or impede rapid deployment of support for new
RRTYPEs.
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3.13.3. Periods and Zone Cut Issues
One of the DNS characteristics that is poorly understood by non-
experts is that the period (".", U+002E) character can be used in
three different ways:
o As a label separator in the presentation form that also designates
a "zone break" (delegation boundary). For example,
foo.bar.example.com indicates the owner, "foo", of records in the
"bar.example.com" zone.
o As a label separator in the presentation form that does not
designate a zone break. For example, foo.bar.example.com
indicates the owner, "foo.bar", of records in the "example.com"
zone.
o As a character within a label, including as a substitute for an
at-sign ("@") when an email address appears in an SOA record or in
a label that denotes such an address (see Section 2).
In general, these cases cannot be distinguished by looking at them.
The third is problematic for non-DNS reasons, e.g.,
"john.doe.example.net" is ambiguous as to whether it should be
interpreted as a simple FQDN, as a notation for john.doe@example.net,
for john@doe.example.net, and so on.
The distinction between the first two cases was probably not
important as the DNS was originally intended to be used. However, as
soon as RRTYPEs (other than NS records that define the zone cut) are
used that are sensitive to the boundaries between zones, the
distinctions become important to people other than the relevant zone
administrators. DNSSEC involves one such set of relationships. It
increases the importance of questions about what should go in a
parent zone and what should go in child zones and how much difference
it makes if NS records in a parent zone for a child zone are
consistent with the records and data in the child zone. This also
causes application issues, may raise questions about relationships
between registrars and one or more registries or, if they are
separate, DNS operators.
3.14. Scaling of Reputation and Other Ancillary Information
The original design for DNS administration, reflected in RFC 1591
[RFC1591] and elsewhere, assumed that all domains would exhibit a
very high level of responsibility toward and for the community and
that level of responsibility would be enforced if necessary. More
recent decisions have taken things in the direction of "registrant
beware" and even "user and applications beware" even some recent
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protocols at least partially reflect the original model (see, e.g.,
IDNA [RFC5890] and the discussion in a recent Internet-Draft
[Klensin-5891bis]. One possible approach to the problems, especially
security problems, that are enabled by the new environment is to
establish reputation systems associated with clearly-defined
administrative boundaries and with warnings to users.
The IETF DBOUND WG [IETF-DBOUND] addressed ways to establish and
document boundaries more precise than simple dependencies on TLDs but
it was not successful in producing a standard.
A TLD reputation-based approach was adopted by some web browsers
after IDNs and a growing number of gTLDs were introduced; that
approach was based on a simple list and does not scale to the current
size of the DNS or even the DNS root.
4. Searching and the DNS - An Historical Note
Some of the issues identified above might reasonably be addressed,
not by changing the DNS itself but by changing our model of what it
is about and how it is used. Specifically, one key assumption when
the DNS (and the host table system before it) was designed was that
it was a naming system for network resources, not, e.g., digital
content. As such, exact matching was important, it was reasonable to
have labels treated as mnemonics that did not necessarily have
linguistic or semantic meaning except to those using them, and so on.
A return to that model, presumably by having user-facing applications
call on an intermediate layer to disambiguate user-friendly names and
map them to DNS names (network object locators) would significantly
reduce stress on the DNS and would also allow dealing with types of
matching and similar or synonymous strings that cannot be handled
algorithmically no matter how much DNS matching rules were altered.
In the early part of the last decade, the IETF explored that approach
a little bit in the context of IDNs and what were then called
"Internet keywords" [DNS-search]. It may be time to look at that
approach again and more deeply in the context of more recent
developments.
It is worth noting that, while that "search" approach, or some other
approach that abstracted and separated several of the issues
identified in Section 3 from the DNS protocol and database
themselves, it does not address all of them. At least some elements
of several of those issues, such as the synchronization ones
described in Section 3.7, are inherent in the DNS design and, if we
are not going to replace the DNS, we had best get used to them.
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5. Acknowledgements
Many of the concerns and ideas described in this document reflect
conversations over a period of many years, some rooted in DNS
"keyword" and "search" discussions that paralleled the development of
Internationalized Domain Names (IDNs). Conversations with, or
writings of, Rob Austein, Christine Borgman, Carolina Carvalho, Vint
Cerf, Lyman Chapin, Patrik Faltstrom, Geoff Huston, Xiaodong Lee,
Karen Liu, Yaqub Mueller, Andrew Sullivan, Paul Twomey, Suzanne
Woolf, Jiankang Yao, other participants in the circa 2003 "DNS
Search" effort and in the ICANN SSAC Working Party on IDNs, and some
others whose names were sadly forgotten were particularly important
to either the content of this document or the motivation for writing
it even though they may not agree with the conclusions I have reached
and bear no responsibility for them.
Many of the subsections of Section 3 were extracted from comments
first made in conjunctions with recent email discussions. Comments
from Suzanne Woolf about an early draft were particularly important
as was material developed with suggestions from Patrik Faltstrom,
especially Section 3.13. Feedback and suggestions from several of
the above and from Stephane Bortzmeyer, Tony Finch, and George
Sadowsky were extremely helpful for improving the clarity and
accuracy of parts of the document, especially so for a broader
audience.
6. IANA Considerations
[[CREF1: RFC Editor: Please remove this section before publication.]]
This memo includes no requests to or actions for IANA.
7. Security Considerations
From both security and privacy perspectives, a replacement for the
DNS would not have to go very far to be a significant improvement.
8. References
8.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<http://www.rfc-editor.org/info/rfc1034>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>.
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8.2. Informative References
[CACM-Homograph]
Gabrilovich, E. and A. Gontmakher, "The Homograph Attack",
Communications of the ACM 45(2):128, February 2002,
<http://www.cs.technion.ac.il/~gabr/papers/
homograph_full.pdf>.
[DNS-Aliases]
Woolf, S., Lee, X., and J. Yao, "Problem Statement: DNS
Resolution of Aliased Names", March 2011,
<https://datatracker.ietf.org/doc/draft-ietf-dnsext-
aliasing-requirements/>.
[DNS-BNAME]
Yao, J., Lee, X., and P. Vixie, "Bundled DNS Name
Redirection", May 2016, <https://datatracker.ietf.org/doc/
draft-yao-dnsext-bname/>.
[DNS-search]
IETF, "Internet Resource Name Search Service", 2003,
<https://datatracker.ietf.org/wg/irnss/about/>.
While it met several times informally and as one or more
BOFs, this effort never really got off the ground. That
was due in part to the IETF decision to go forward with
the IDNA approach and in part by signs that the "keyword"
efforts were beginning to fall apart.
[DNSOP-Sutld]
Lemon, T., Droms, R., and W. Kumari, "Special-Use Domain
Names Problem Statement", June 2017,
<https://datatracker.ietf.org/doc/draft-ietf-dnsop-sutld-
ps>.
[Hoffman-DNS-JSON]
Hoffman, P., "Representing DNS Messages in JSON", May
2017, <https://datatracker.ietf.org/doc/draft-hoffman-dns-
in-json/>.
[Hoffman-SimpleDNS-JSON]
Hoffman, P., "Simple DNS Queries and Responses in JSON",
June 2017, <https://datatracker.ietf.org/doc/draft-
hoffman-simplednsjsonn/>.
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[Huston-DNSPrivacy]
Huston, G. and J. Silva Dama, "DNS Privacy", Internet
Protocol Journal Vol 20, No 1, March 2017,
<http://ipj.dreamhosters.com/wp-
content/uploads/issues/2017/ipj20-1.pdf>.
[ICANN-VIP]
ICANN, "IDN Variant Issues Project: Final Integrated
Issues Report Published and Proposed Project Plan for Next
Steps is Now Open for Public Comment", February 2012,
<https://www.icann.org/news/announcement-2012-02-20-en>.
[IETF-DBOUND]
IETF, "Domain Boundaries (dbound)", 2017,
<https://datatracker.ietf.org/wg/dbound/about/>.
[Klensin-5891bis]
Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Registry Restrictions and
Recommendations", March 2017,
<https://datatracker.ietf.org/doc/draft-klensin-idna-
rfc5891bis/>.
[NRC-Signposts]
National Research Council, "Signposts in Cyberspace: The
Domain Name System and Internet Navigation"", 2005,
<https://www.nap.edu/catalog/11258/signposts-in-
cyberspace-the-domain-name-system-and-internet-
navigation>.
[RFC0799] Mills, D., "Internet name domains", RFC 799,
DOI 10.17487/RFC0799, September 1981,
<http://www.rfc-editor.org/info/rfc799>.
[RFC0810] Feinler, E., Harrenstien, K., Su, Z., and V. White, "DoD
Internet host table specification", RFC 810,
DOI 10.17487/RFC0810, March 1982,
<http://www.rfc-editor.org/info/rfc810>.
[RFC0881] Postel, J., "Domain names plan and schedule", RFC 881,
DOI 10.17487/RFC0881, November 1983,
<http://www.rfc-editor.org/info/rfc881>.
[RFC0882] Mockapetris, P., "Domain names: Concepts and facilities",
RFC 882, DOI 10.17487/RFC0882, November 1983,
<http://www.rfc-editor.org/info/rfc882>.
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[RFC0952] Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet
host table specification", RFC 952, DOI 10.17487/RFC0952,
October 1985, <http://www.rfc-editor.org/info/rfc952>.
[RFC0953] Harrenstien, K., Stahl, M., and E. Feinler, "Hostname
Server", RFC 953, DOI 10.17487/RFC0953, October 1985,
<http://www.rfc-editor.org/info/rfc953>.
[RFC0974] Partridge, C., "Mail routing and the domain system",
STD 10, RFC 974, DOI 10.17487/RFC0974, January 1986,
<http://www.rfc-editor.org/info/rfc974>.
[RFC1123] Braden, R., Ed., "Requirements for Internet Hosts -
Application and Support", STD 3, RFC 1123,
DOI 10.17487/RFC1123, October 1989,
<http://www.rfc-editor.org/info/rfc1123>.
[RFC1591] Postel, J., "Domain Name System Structure and Delegation",
RFC 1591, DOI 10.17487/RFC1591, March 1994,
<http://www.rfc-editor.org/info/rfc1591>.
[RFC1996] Vixie, P., "A Mechanism for Prompt Notification of Zone
Changes (DNS NOTIFY)", RFC 1996, DOI 10.17487/RFC1996,
August 1996, <http://www.rfc-editor.org/info/rfc1996>.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, DOI 10.17487/RFC2671, August 1999,
<http://www.rfc-editor.org/info/rfc2671>.
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, DOI 10.17487/RFC3490, March 2003,
<http://www.rfc-editor.org/info/rfc3490>.
[RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep
Profile for Internationalized Domain Names (IDN)",
RFC 3491, DOI 10.17487/RFC3491, March 2003,
<http://www.rfc-editor.org/info/rfc3491>.
[RFC3596] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
"DNS Extensions to Support IP Version 6", RFC 3596,
DOI 10.17487/RFC3596, October 2003,
<http://www.rfc-editor.org/info/rfc3596>.
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[RFC3743] Konishi, K., Huang, K., Qian, H., and Y. Ko, "Joint
Engineering Team (JET) Guidelines for Internationalized
Domain Names (IDN) Registration and Administration for
Chinese, Japanese, and Korean", RFC 3743,
DOI 10.17487/RFC3743, April 2004,
<http://www.rfc-editor.org/info/rfc3743>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<http://www.rfc-editor.org/info/rfc3986>.
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, DOI 10.17487/RFC5890, August 2010,
<http://www.rfc-editor.org/info/rfc5890>.
[RFC5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891,
DOI 10.17487/RFC5891, August 2010,
<http://www.rfc-editor.org/info/rfc5891>.
[RFC6761] Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
RFC 6761, DOI 10.17487/RFC6761, February 2013,
<http://www.rfc-editor.org/info/rfc6761>.
[RFC6895] Eastlake 3rd, D., "Domain Name System (DNS) IANA
Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895,
April 2013, <http://www.rfc-editor.org/info/rfc6895>.
[RFC6912] Sullivan, A., Thaler, D., Klensin, J., and O. Kolkman,
"Principles for Unicode Code Point Inclusion in Labels in
the DNS", RFC 6912, DOI 10.17487/RFC6912, April 2013,
<http://www.rfc-editor.org/info/rfc6912>.
[RFC7094] McPherson, D., Oran, D., Thaler, D., and E. Osterweil,
"Architectural Considerations of IP Anycast", RFC 7094,
DOI 10.17487/RFC7094, January 2014,
<http://www.rfc-editor.org/info/rfc7094>.
[RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", RFC 7719, DOI 10.17487/RFC7719, December
2015, <http://www.rfc-editor.org/info/rfc7719>.
[RFC7816] Bortzmeyer, S., "DNS Query Name Minimisation to Improve
Privacy", RFC 7816, DOI 10.17487/RFC7816, March 2016,
<http://www.rfc-editor.org/info/rfc7816>.
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[RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "Specification for DNS over Transport
Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
2016, <http://www.rfc-editor.org/info/rfc7858>.
[Sullivan-Class]
Sullivan, A., "The DNS Is Not Classy: DNS Classes
Considered Useless", July 2016,
<https://datatracker.ietf.org/doc/draft-sullivan-dns-
class-useless/>.
[Unicode] The Unicode Consortium, "The Unicode Standard, Version
9.0.0,", ISBN 978-1-936213-13-9, 2016,
<http://www.unicode.org/versions/Unicode9.0.0/>.
[Unicode-UAX15]
Davis, M. and K. Whistler, "Unicode Normalization Forms",
February 2016, <http://unicode.org/reports/tr15/>.
[Unicode-USA31]
Davis, M., "Unicode Identifier and Pattern Syntax", May
2016, <http://unicode.org/reports/tr31/>.
Appendix A. Change Log
RFC Editor: Please remove this appendix before publication.
A.1. Changes from version -00 (2017-06-02) to -01
o Many editorial corrections
o Addition of new (some replacing prior placeholder) sections,
especially to the list of issues with the current DNS design and
notably including Section 3.13.
A.2. Changes from version -01 (2017-06-06) to -02
o Improved the discussion ins several sections, including a somewhat
muddled description in Section 3.7
o Revised the Introduction to make the context for this document
somewhat more clear.
o Added several more references even though still not nearly enough
to make this document a comprehensive bibliography (which is not
intended).
o Many editorial corrections and a few added references.
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Author's Address
John C Klensin
1770 Massachusetts Ave, Ste 322
Cambridge, MA 02140
USA
Phone: +1 617 245 1457
Email: john-ietf@jck.com
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