Network Working Group S. Woolf
Internet-Draft Internet Systems Consortium, Inc.
Intended status: Informational X. Lee
Expires: September 15, 2011 J. Yao
CNNIC
March 14, 2011
Problem Statement: DNS Resolution of Aliased Names
draft-ietf-dnsext-aliasing-requirements-01.txt
Abstract
This document attempts to describe a set of issues that arises from
the desire to treat a set or group of names as "aliases" of each
other, "bundled," "variants," or "the same," which is problematic in
terms of corresponding behavior for DNS labels and FQDNs.
With the emergence of internationalized domain names, among other
potential use cases, two or more names that users will regard as
having identical meaning may sometimes require corresponding behavior
in the underlying infrastructure, possibly in the DNS itself. It's
not clear how to accommodate this required behavior of such names in
DNS resolution; in particular, it's not clear when they are best
accommodated in registry practices for generating names for lookup in
the DNS, existing DNS protocol elements and behavior, existing
application-layer mechanisms and practices, or some set of protocol
elements or behavior not yet defined. This document attempts to
describe some of these cases and the behavior of some of the possible
solutions discussed to date.
Status of this Memo
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 15, 2011.
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. What this document does . . . . . . . . . . . . . . . . . 5
1.2. What this document does not do . . . . . . . . . . . . . . 5
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 6
2.1. Registration of Domain Name Variants . . . . . . . . . . . 7
2.2. Identical DNS Resolution for Bundled DNS Names . . . . . . 8
2.3. Character Variants . . . . . . . . . . . . . . . . . . . . 9
2.3.1. An example: Simplified and Traditional Chinese . . . . 9
2.3.2. An example: Greek . . . . . . . . . . . . . . . . . . 9
2.3.3. An Example: Arabic . . . . . . . . . . . . . . . . . . 10
2.4. Use of Variants . . . . . . . . . . . . . . . . . . . . . 10
3. Operational Considerations . . . . . . . . . . . . . . . . . . 11
3.1. Zone Provisioning and Authority Servers . . . . . . . . . 11
3.1.1. Provisioning of 'aliases' in the registry . . . . . . 12
3.1.2. Impact of special mechanisms . . . . . . . . . . . . . 12
3.2. Recursive Resolvers . . . . . . . . . . . . . . . . . . . 12
3.3. Applications . . . . . . . . . . . . . . . . . . . . . . . 13
4. Proposed Requirements . . . . . . . . . . . . . . . . . . . . 14
5. Possible Solutions . . . . . . . . . . . . . . . . . . . . . . 15
5.1. Mapping or Redirection of Domain Names . . . . . . . . . . 16
5.1.1. Mapping itself (CNAME) . . . . . . . . . . . . . . . . 16
5.1.2. Mapping its descendants . . . . . . . . . . . . . . . 16
5.1.3. Mapping itself and its descendants . . . . . . . . . . 17
5.2. Zone Clone . . . . . . . . . . . . . . . . . . . . . . . . 17
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
7. Security Considerations . . . . . . . . . . . . . . . . . . . 18
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 19
9. Change History . . . . . . . . . . . . . . . . . . . . . . . . 19
9.1. draft-yao-dnsext-identical-resolution: Version 00 . . . . 19
9.2. draft-yao-dnsext-identical-resolution: Version 01 . . . . 19
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
10.1. Normative References . . . . . . . . . . . . . . . . . . . 19
10.2. Informative References . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21
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1. Introduction
As the Internet and the DNS have evolved beyond their original realms
of use, a set of needs and expectations has appeared about how DNS
labels behave that is informed significantly by common human
assumptions about how "names" or "words" work. One aspect of this is
the notion or expectation that multiple sets of names may be similar
to a human user, and expected to behave "the same" or as "aliases" of
one another, across multiple services and interactions. The DNS was
designed with the implicit expectation that names would be based on
ASCII characters, and the "similarity" or "sameness" property doesn't
seem to arise terribly often in the names people originally wanted to
use in the DNS; thus the requirements of identical resolution of
"aliased" or "bundled" names hasn't figured prominently as an
attribute that needed to be accommodated in the generation or lookup
of DNS names. However, with the standardization of internationalized
domain names protocols (ref: IDNA and IDNAbis), more and more
internationalized domain name labels [RFC3490] are appearing in DNS
zones. In some cases, these labels [RFC3743] are accompanied by the
expectation that they are "equivalent" or should behave "the same,"
often because these labels are derived from names or strings that
users consider "the same" in some languages. Accordingly, Internet
users hope for such labels to behave in DNS contexts as they expect
the corresponding human constructs to behave, regardless of the
specific service (smtp, http, etc.) involved..
The general issues of what "the same" means, or of defining
"variants" in human scripts as codified in Unicode (or anywhere else)
are well outside the scope of the DNS or the expertise of most of the
people who work on it. They are matters for philosophers and
linguists, and for applications developers, respectively. However,
to the extent that these issues can be specified as involving the
resolution of names in the DNS, it's reasonable to describe those
expectations and attempt to accommodate them.
There is some existing technology defined in the DNS for behavior
that can be described as one name behaving "the same" as another.
For a single node in the DNS tree, CNAME can be used to map one name
as an "alias" to another, "canonical" name. If there is a need to
map a subtree of the DNS-- a zone, or a domain and its subdomains--
to another domain, DNAME has been defined to allow this behavior.
However, there is no way currently defined to do both, as CNAME is
required to be the sole record at its node in the tree. Behavior
that combines the characteristics of CNAME and DNAME is not currently
defined in the DNS.
If existing protocol does not meet the zone administrator's need to
be able to treat one label, name, or zone as "the same" as another,
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there are also administrative mechanisms available for manipulating
databases underlying the generation and resolution of DNS names.
Registry operators have many mechanisms for working around DNS
protocol in order to get behavior they want for names in DNS zones,
and management of "aliases" is no exception. However, it is not
clear how much of the user and operator requirements for "aliases"
can be met by mechanisms for provisioning DNS zones, at acceptable
cost. Concerns have been raised about this approach particularly at
large scales and there is a need both to provision possibly
exponential numbers of domains and then to audit them for compliance
with parent registry policy.
1.1. What this document does
Attempts to think about "aliases" or similar concepts as applied to
the DNS have been difficult, both because use cases have been unclear
and because terminology for describing and distinguishing them has
not been readily available. This document attempts to provide both
brief descriptions of identified use cases, and a rough organization
for how to think about behavior in the DNS that might correspond to
the requirements derived from them as a way of evaluating proposed
solutions. This includes existing and additional possible solutions,
from the perspective of both DNS (authoritative server, resolver, and
client) and application needs.
As a departure point, we attempt to be rigorous about distinguishing
DNS "labels" from "words" (a human construct) and "strings" (which we
use here as machine-readable constructs that nonetheless may not
conform to DNS label constraints, such as IDNA U-labels). The
distinctions among what humans type or see, what applications use,
and what DNS stores and resolves are sometimes subtle but
particularly important.
A list of broad requirements is proposed for any DNS protocol changes
that might be undertaken.
We also review existing technologies (CNAME, DNAME) and proposed new
ones ("BNAME," "zone clones") against the proposed requirements.
1.2. What this document does not do
This document makes no attempt to solve or even describe
"translation" of one name into another in the DNS, which is likely to
be impossible. "Translation" in general, or even the particular
problem of determining when or why two DNS labels (or even FQDNs)
should be considered "the same", is simply not in scope for the DNS
protocol. We pre-suppose those decisions are made elsewhere and that
the DNS needs to deliver behavior in conformance with that external
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decision. In particular, we're talking about creating a property or
association among a set of DNS names as "sameness" or "alias-of", but
the correspondence between that set and any set of human- or
application-visible strings is created outside of the DNS database
and protocol. Language Variant Tables (RFC3743) serve as guides for
registration policy, but the associations they create are basically
expressed only as policy about what can be registered, not visible to
DNS resolvers, applications, or users.
Accordingly, this document makes no comment on policy regarding when
two names are "the same," what restrictions should be placed on their
generation or use outside those imposed by the DNS protocol, or the
ability of one approach over another to instantiate what a given user
regards as "the same" for a language, script, culture, community,
application, encoding, or purpose.
1.3. Terminology
All the basic terms used in this specification are defined in the
documents [RFC1034], [RFC1035], [RFC2672] and [RFC3490].
We also note that there is a wide variety of terminology in use to
describe the issues we attempt to treat in this document, and no
consensus on which apply under what conditions. Terms for "a set of
domain names that somehow need to be treated as similar" include
"bundle," "variants," or "clones". As uniformity of terms is one of
the goals of any work on this topic in the DNS, we try not to add to
the confusion in the problem statement but can't claim to have
finalized a recommendation in early versions of this document.
2. Problem Statement
From the point of view of the DNS, a number of attributes suggest
themselves as important dimensions for evaluating what "the same"
might mean.
One question is exactly what it is that's to be defined as "the
same"? Are the end results to be identical, and if so from what
perspective: that of the recursive resolver? The application? The
human consumer of content? Is it enough that lookups on the FQDN
portion of an email address result in the same A or AAAA records, or
does some intermediate mapping need to be maintained between MX
records in the resolution chain? What about the FQDN portion of a
URL handed back to an application, or in resolution processes that
include multiple lookups of records that may include FQDNs? Do there
need to be general rules specified for the handling of FQDNs in RDATA
of present and future RRtypes?
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Another question is the behavior of multiple names with respect to
one another: is it enough to define one as "canonical" or
"preferred," with the others considered as "variants" that are
transformed to the "preferred" form? Or is there a real need for
multiple names to be "equivalent", interchangeable, with none
considered "preferred" over the others? (We note here that no
requirement for complete interchangeability or identity has been
articulated, except anecdotally, and such equivalence would be
extremely difficult to define in the DNS.)
In addition, the tree structure of the DNS requires that we consider
the behavior of "identical" names across multiple zones in the
hierarchy. Are mappings to be maintained in names more than a level,
or two, deep? If so, with what characteristics, and what
characteristics are required for scalability?
A further question arises with respect to how applications should
interact with alias-specific DNS behavior. A basic requirement would
seem to be "First, do no harm," or in other words, any extensions to
DNS protocol in support of the desired "alias" behavior should not
interfere with applications that expect to do such interpretation on
their own. This concern is based in the expectation that DNS is
simple and predictable, operating strictly as infrastructure under
the process of creating "the user experience," not as part of it.
A key point in evaluating these questions is that DNS is a lookup-
based protocol. A DNS name either resolves, or not. There's no
search function in the DNS, no fuzzy match. It provides lookup on a
specified name. This is critically important because it means that
any work to define any kind of "sameness" that can't be expressed as
a lookup, such as selection among a set of candidate names for which
to return results, must be done elsewhere than in the DNS.
2.1. Registration of Domain Name Variants
To some degree, issues of "sameness" or creating an association among
a set of names have existed around the use of domain names from the
beginning. Points where the behavior of DNS labels have collided
with expectations around the behavior of words have included DNS
handling of case sensitivity, the kind of transformations a human
expects to "just work" around "try-this.example" vs.
"trythis.example", and continuing frustration that "confusingly
similar" names can be delegated to different parties by DNS
registries. However, the introduction of IDN has provided a forcing
function in that it has added visibility for a wider variety of
issues along these lines, and possibly the urgency of dealing with
them for large numbers of users.
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A need has been identified in connection with the introduction of IDN
for defining how "variants" might behave as DNS names. Specifically
defining "variant" is a matter for experts, but it's generally
conceded that recognition and careful management of cases where
multiple names are associated together as "variants" in the
expectation or preference of users are important; without such
management of grouped domain names, security risks may be increased
and the quality of user experience may be compromised. [RFC3743]
developed by JET (Joint Engineering Team) gives one possible solution
of how to manage registration of a domain name, intended to be
applied to the script and usage common across Chinese, Japanese, and
Korean users. [RFC3743] proposed an algorithm which will allocate a
group of names, consisting of a domain name and its variants, to the
same domain holder. It means that the domain holder will get control
of the domain name and its variants. [RFC4290] suggests the practice
in [RFC3743] to be used in registrations of internationalized domain
names. But [RFC3743] and [RFC4290] do not define how, exactly, these
bundles of names are to be treated by the registry or the DNS in
order to obtain the desired "identical" behavior. [RFC4690] said
that the "variant" model introduced in [RFC3743] and [RFC4290] can be
used by a registry to prevent the most negative consequences of
possible confusion, by ensuring either that both names are registered
to the same party in a given domain or that one of them is completely
prohibited. The principles described in [RFC3743], [RFC4290] and
[RFC4690] have been accepted by many registries. But the technical
details of how to guarantee that a bundle of domain names are
"identical" in the DNS remain unspecified.
2.2. Identical DNS Resolution for Bundled DNS Names
To some extent, the desired behavior can be described: "identical DNS
resolution" means that the process of resolving two domain names will
end with the same result, in most cases the same IP address. In the
history of DNS protocol development, there have been two attempts to
specify such "identical resolution" behavior:CNAME[RFC1034] which
maps or redirects itself, and DNAME[RFC2672] which maps or redirects
its descendants. In the case of bundles or groups of names, however,
some operators have asserted they need identical DNS resolution at
all levels' domain names, including the domain name itself and its
descendants. As alluded to above, registries are left with ad hoc
provisioning and database management mechanisms for managing variant
names, with some help from existing DNS protocol mechanisms for
mapping labels or FQDNs to each other. However, some are finding the
existing mechanisms to have unsatisfactory limitations; they are
seeking more guidance on the use of existing mechanisms, and perhaps
the addition of new ones in the DNS protocol.
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2.3. Character Variants
Many defined scripts as used in many different languages have
"character variants" included. There is no uniform definition of
variants, and in fact their characteristics differ widely, but it's
possible to define some. For example, the definition of variant
characters in the JET Guidelines [RFC3743], intended for use with the
CJK language/script communities, is roughly this: One conceptual
character can be identified with several different code points in
character sets for computer use. In UNICODE definitions of some
scripts, including Han (chinese), some characters can be identified
as "compatibility variants" of another character, which usually
implies that the first can be remapped to the second without the loss
of any meaning. In this document, variant characters are two or more
characters that may be similar in appearance or identical in meaning
(similarity in appearance is not required by the definition but often
occurs).
With the introduction of IDNs in the DNS, perhaps most prominently in
the root zone, decisions about how to deal with IDN variants is a
significant challenge ahead of us. We describe here a couple of
examples, Chinese and Greek; comparable situations exist in Arabic,
Cyrillic, and others.
2.3.1. An example: Simplified and Traditional Chinese
For example, the IDN TLD "China"(U+4E2D U+56FD) and its variant
(U+4E2D U+570B) are in the root today. The first one (U+4E2D U+56FD)
can be considered the "original" IDN TLD and the second one (U+4E2D
U+570B) can be considered the IDN TLD "variant". Ideally, it should
be possible to treat the original IDN TLD and its IDN TLD variant as
"identical" for purposes of DNS resolution, in a way similar to the
case mapping most DNS users take for granted. However, this analogy
is a bit perilous, and turns out to be hard to use as a guide to what
behavior is actually desirable, not least because it's not fully
consistent even within the DNS.
At this writing, four Han script IDN TLDs are in the root, including
two pairs comprising a Traditional Chinese name and its Simplified
counterpart. These operators will, in an ideal world, be able to
share some operational experience around implementation of registry
policy regarding managing multiple DNS trees as "the same"
2.3.2. An example: Greek
In Greek, almost every word has the "tonos" accent sign, but where it
is placed (on which character) can vary. Further, some words end in
a final sigma, which is represented differently to sigma appearing
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elsewhere in the word. If a registry wishes to be able to enforce
the association among all of the domain names that correspond to a
"word" in Greek, with all its possible Unicode strings, some
mechanism must be used to enumerate the "variant" names and tie them
together. This makes sense from the human factors perspective, as
depending on how the user types something, results may include a
different domain to what was expected, although the user may have the
firm belief that "the same word" was input in multiple cases.
As an example, the domain names "xn--0xadhj4a.gr" and "xn--
0xaafjl.gr" appear to a native speaker/reader of Greek to represent
"the same word," in a sense very much like the case insensitivity
that native users of Latin script take for granted in the DNS.
2.3.3. An Example: Arabic
[STW: [to be added]
2.4. Use of Variants
It's reasonable to pose the question at this point, without
necessarily being able to answer it yet, of what is the ideal or
intended impact of solving the issue identified so far for registries
on applications and end users. Ultimately, simplifying the
provisioning side may result in the same semantics as we have today
for zones maintained in parallel but for less work. However, we
later assert a proposed requirement that synthesizing the same record
as a query would have obtained from an enumerated parallel tree isn't
enough-- that the property of association or "sameness" we're
creating with specific mechanisms needs to be useful in some specific
way to the consumer of the data.
The trigger for raising the questions discussed here is based in user
expectations that one name, in certain circumstances to be determined
and somehow encoded by humans, can be treated as interchangeable with
another with regards to a particular context or activity, with
resolution of domain names as part of the context or activity.
However, it's useful to note here that satisfying that set of user
expectations may or may not reasonably be done in the DNS, wholly or
in part.
There are two arguments for placing functionality that links one name
to another as "the same" in the DNS. Their validity is not yet
determined, but they amount to:
1. The expectation that two or more names be "the same" is often
expressed as a desire to register the associated names as a
"bundle" or otherwise link them as domain names. This is because
the domain name in a URL or email address is often presented to
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the user as semantically meaningful, based on strings used to
derive DNS labels-- proper names, "words," etc. This brings such
concerns to the attention of providers of domain names, and
suggests at least exploring how to answer the question near where
it's asked-- i.e. the registry.
2. The desire for names on the Internet to act like words is often
service-independent; users want to be able to use identical
strings in the course of invoking multiple services that seem to
be related, such as going to a webpage and then sending email to
an address in "the same" domain (probably an FQDN). It's been
noted that people are very comfortable with a certain amount of
fuzziness about "alternative spellings" and assorted other
variations within the notion of "sameness", but they nonetheless
often want such an association to exist. In cases where a set of
variant strings is parseable in the application, has
corresponding A-labels that can be looked up in the DNS, etc. but
only a subset can be typed on the user's input device or rendered
on the user's screen, the association may be necessary to the
successful completion of the activity the user is attempting.
This argues in turn for some mechanism that is not dependent on
the specific service, protocol, or application involved, since
leaving it up to service-specific mechanisms, or conventions in
the use of DNS records or other mechanisms not really intended
for the purpose, leads to confusion and inconsistency.
3. Operational Considerations
Any change to a mature infrastructure protocol such as DNS needs to
be informed by consideration of the tradeoffs among providing the
associated service, using the service, and possibly conflicting means
of offering comparable functionality. In the case of DNS, this
requires that we look at provisioning (populating zones and the
mechanics of authority servers responding to queries), service by
intermediate and client resolvers, and related capabilities provided
with existing facilities in the DNS and in applications.
3.1. Zone Provisioning and Authority Servers
The initial motivation for discussion of support for aliases in the
DNS was provided by operators of top-level domain (TLD) registries.
The problem facing them lies in scaling the provision of "bundles" of
names that users expect to be treated "the same", as in the examples
previously described.
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3.1.1. Provisioning of 'aliases' in the registry
The most obvious way to provision multiple names as "the same" is to
delegate each separately, and then maintain the contents of the
delegated zones together, from the same backend database or by some
similar mechanism. This has the advantage of requiring no new
technology; it can be done, and is done today, entirely with
provisioning logic and registry policy.
However, it doubles the work and the number of records required. If
provisioning isn't done carefully, errors can arise, leaving
inconsistencies. And provisioning multiple trees does nothing to
link the resulting names directly; there is no property of
"association" or isomorphism created in the DNS that corresponds to
user or application expectations for "sameness". There is no way to
tell, from resolving a name in one tree, that it's part of a set or
bundle of related names.
Separate provisioning also poses a limitation for some registry
operators in that there is no way to verify that the trees are being
maintained in parallel without exhaustively walking the zones, which
may be large or nested to multiple levels. In the case, for example,
of a zone A.B.C.example.com, in which each of A,B,C are derived from
strings with a single character variant each, eight zones must be
maintained in parallel and possibly available for audit by the
authority over example.com, depending on its delegation policy.
3.1.2. Impact of special mechanisms
Once we begin to consider mechanisms for maintaining parallel zones
or "aliases", we need to look at how the "alias" or association
property is created and where the burden of maintaining it lies. In
the case of proposed mechanisms, we attempt to describe them below.
Existing mechanisms besides the simple, straightforward provisioning
of zones that are identical except for the ownernames of
corresponding records include wildcards, CNAME, and DNAME. See
below, but here we note that they require special processing by the
authority server in order to synthesize responses that are supposed
to be the equivalent of simply providing the parallel zones by one-
to-one enumeration.
3.2. Recursive Resolvers
Another area where it's necessary to review requirements and impacts
of changes to the DNS is in resolver expectations and behavior, given
that recursive resolvers do most of the work of getting the data out
of DNS that provisioning activities put into it.
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In practice, much of the work of special processing falls to
resolvers. In particular, any scheme that can result in multiple
queries and some need to chain the answers or disambiguate multiple
answers is going to make more work for resolvers, and is going to
need to specify that work in careful detail. Any ambiguity or lack
of precision in specifying the use of "aliases" will propagate back
to applications, and quite possibly leave applications writers and
users worse off than they were without DNS mechanisms intended to
"help".
Ideally any new RRtypes defined to support "aliases" would be
provisioned on the authority server and require no special
processing, which would make them transparent to intermediate
resolvers. However, depending on how much such RRs and their
processing need to be visible to the application to be effective,
this may not be possible.
3.3. Applications
The most complex part of the analysis of costs and benefits of
defining new technology for support of "aliases" by DNS is in
determining what applications would do with such new mechanisms and
how it would help to have them. In particular, it is critically
important not to simply provide additional complexity, even in the
name of making provisioning on the server side easier, unless there's
some clear benefit to it for the ultimate client of the DNS as well--
the user who is trying to "do something" on the Internet.
Such a clear benefit could come from the ability, alluded to above,
to provide a facility that was anchored in the DNS and so did not
have to be re-invented anew for each application or protocol that
wished to have user-transparent access to the ability to reduce
"aliases" to a canonical domain name without necessarily being aware,
a priori, that the name was part of a set that could be deemed "the
same".
An example used more than once in discussion is provided by SSL, as a
protocol that uses domain names without necessarily using the DNS
protocol per se. SSL certificates are tied to one domain name. It
would be helpful to applications to have a non-protocol-specific way
to identify securely cases where multiple domain names can be
canonicalized to the domain name used for an SSL certificate.
Currently HTTP has such an ability, but it's considered awkward to
use and does not help writers or users of other application
protocols.
An important characteristic of such a solution for applications,
however, is that the writer and user be able to tell when such a
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mechanism was invoked in the DNS, to avoid interference among
multiple possible ways to find "aliases" and compare them. This in
turn implies a fair amount of complexity to be inflicted not only on
DNS protocol but also on API/library writers seeking to use such new
facilities, particularly given the caution above that DNS is a lookup
protocol and must be given precise sequences of bits to look up.
Another characteristic of an "aliases" mechanism of interest to
application writers is the difficulty, and therefore the likely speed
and breadth of deployment, of such a DNS-based mechanism for
canonicalizing aliased names. DNS is notorious, as an aging
infrastructure protocol, for the long tail of deployment of
significant protocol features. Again, a feature can be designed to
be fairly easy to deploy, but without an incentive such as faster
application development or more secure applications, it still may not
see wide uptake even after it's present in current code bases.
An additional assumption often made needs to be examined here, as
well: that applications, and applications writers, have
straightforward, well-defined ways of interacting with the DNS, into
which new functionality can obviously and straightforwardly be added.
This can be true, as in the case of a query for a specific RRtype at
an unambiguously determined name by an application designed to find
and take advantage of the data represented in records of that RRtype.
However, it does not have to be, and often isn't: there's no standard
DNS API, and no standard abstraction for applications to interact
with the DNS. There is confusion about how to use DNS effectively,
and over the actual behavior of existing "alias" mechanisms such as
CNAME. Adding new technology is likely to be accompanied by
challenges not only in getting it deployed to the installed base, but
in getting its uses clearly documented for applications writers.
4. Proposed Requirements
These observations and examples, along with general discussion to
date, lead to the following tentative set of actual requirements.
1. Any mechanism proposed in the DNS to support "aliases" or
multiple names as "the same" MUST be workable for DNSSEC- signed
zones.
2. Any mechanism proposed in the DNS to support "aliases" or
multiple names as "the same" MUST be "backwards compatible," in
that it MUST NOT change the established behavior of existing
RRtypes and query processing.
3. Any mechanism proposed SHOULD NOT require more overhead of
registries, authoritative servers, or clients than existing
mechanisms for approximating the desired behavior, such as
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provisioning of multiple parallel trees or CNAME processing. If
a new solution is more work than existing mechanisms, imperfect
as they may be, it's not clear where the incentives would lie to
deploy it. This is particularly a concern for implementors and
application developers.
4. Any mechanism proposed MAY require new RRtypes and special
processing for them.
5. Any mechanism proposed MUST NOT only reduce costs of generating
and providing authoritative service for DNS zones. It would be
too easy to reduce costs on the authority server provider while
adding costs elsewhere, particularly in terms of complexity.
Given the central importance of DNS service to Internet
operations, any change undertaken to lower the cost to providers
may be useful, but should not simply shift costs to DNS users,
whether applications or end users.
5. Possible Solutions
Currently, there are several possible mechanisms to support identical
DNS resolution of "bundled" or "variant" names as "aliases" in the
DNS. Existing mechanisms in the DNS include CNAME and DNAME. In
addition, as described briefly above, registry operators have a great
many techniques for applying policy to what names can be registered,
and provisioning technology to how they are instantiated in the DNS,
in support of keeping "variant" names behaving similarly to each
other, or in preventing the use of such variants as might be
considered confusing or dangerous.
In addition, there are new proposals for DNS protocol to support
"aliases" in the DNS as part of the desired behavior of "variant"
names: Names direction[BNAME], and "Zone clone".
All of the solutions have their advantages and disadvantages. In
particular, there are a couple of limitations they all share. Every
mechanism existing or proposed to support "aliases" in the DNS
requires that one name be designated as the "canonical" name
("preferred" in the terminology of the JET variant mechanism) and any
others bundled with it are to be considered "variants" or "aliases".
The only known way to enforce a symmetrical or equivalent association
is via careful registry provisioning within and across domains. In
addition, the different "alias" mechanisms differ in subtle ways that
have to be carefully reviewed against the desired behavior of the DNS
in support of different types of "variants".
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5.1. Mapping or Redirection of Domain Names
5.1.1. Mapping itself (CNAME)
It was recognized as part of the original specification of the DNS
that a host can have many names; in fact this expectation predates
the DNS, referring to the earlier specification of host names. In
the simplest case for "aliases", Internet users need these multiple
names to be resolved to the same IP address by a DNS server. The
CNAME record [RFC1034], where "CNAME" is an abbreviation for
"Canonical Name", is a way to designate aliases of the "real" or
canonical name of a host. In some cases, CNAME can be used to
produce the necessary association a bundle of variant domain names.
But the CNAME only maps itself, not its descendants; in fact it is
defined to not have descendants, as it is the only name at a node in
the DNS tree and can't exist at the same name as delegation. In the
case of IDN variants, however, it is often desirable that the name
map both itself and its descendants.
In terms, however, of deployment and availability, it's useful to
note that CNAME is already part of the installed base of DNS
authority servers and intermediate mode resolvers. Using it for this
purpose requires description of how to do it and how it behaves, but
that already is available. There are no issues of uptake or
backwards compatibility or new code or new documentation.
5.1.2. Mapping its descendants
In order to maintain the address-to-name mappings in a context of
network renumbering, a DNAME record or Delegation Name record defined
by [RFC2672] was invented to create an alias for all its subdomains.
In contrast, the CNAME record creates an alias only of a single name
(and not of its subdomains). As with the CNAME record, the DNS
lookup will continue by retrying the lookup with the new name. If a
DNS resolver sends a query without EDNS[EDNS0], or with EDNS version
0, then a name server synthesizes a CNAME record to simulate the
semantics of the DNAME record. A DNAME record is very much like the
CNAME record, but while the CNAME record only applies for one name,
with a DNAME record one can create aliases for all the records for
its subdomain.
DNAME is can be considered slightly less widely deployed than CNAME
for the EDNS0 compatibility reason described above, but it's been
defined in the DNS for quite some time, and includes a backwards
compatibility mechanism in the CNAME synthesis just described, so use
of DNAME does not rely on deployment of resolver code capable of
special processing for DNAME; it relies entirely on authority server
capability.
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5.1.3. Mapping itself and its descendants
Bundling of "variant" strings or names as domain names, possibly
along with other use cases not yet identified, require the ability to
map a whole tree of the domain space to another domain. The current
DNS protocols do not support this function. A new DNS resource
record [BNAME] has been proposed to deal with this problem.
The advantage of BNAME is that it would enable a class of "aliasing"
behavior that some operators find desirable, particularly in
preference to some of the provisioning overhead they describe having
to deploy to support potentially large numbers of "bundles" of
variants at multiple levels of the DNS tree. The disadvantage is
that it may not provide the behavior people really want while
requiring the time and resources to code and deploy any new DNS
facility.
Alternatively, a proposal has been made that would leave CNAME as
already specified, but eliminating the constraint that a CNAME must
be alone at a node in the DNS tree. This would avoid any coding and
deployment overhead associated with new RRtypes, while obtaining the
desired behavior. Concerns expressed about it, however, include the
possible (but not yet specified) effort required for backwards
compatibility to avoid harm to implementations that expect, and use,
the old behavior.
Both of these mechanisms would require both authority server and
resolver changes to enable the new capability.
5.2. Zone Clone
The proposal of "zone clone" or "dns shadow", is an alternative
solution for a higher level of support than the DNS currently
provides for "alias" behavior across zones. In this scheme, a new
RRtype, SHADOW, is specified; it can exist at a zone apex and can be
used to define "clones" or "shadows" of the zone content so that
records in the zone are reachable via lookups from multiple
delegations. This mechanism varies fundamentally from CNAME/DNAME/
BNAME in that it creates a local copy on each cooperating
authoritative server that has the original zone, reachable by the
names specified in the SHADOW RR. Its scope, then, is the zone as
maintained by an authoritative server rather than a single RRset
(even one corresponding to a delegation).
This scheme has the advantage that it allows a SHADOW zone to be used
in all the same contexts as the canonical or underlying zone,
including contexts where a CNAME or DNAME (or, presumably, a BNAME)
cannot appear, such as in the RDATA of certain RRtypes. Of the
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proposed DNS protocol mechanisms, it probably comes closest to the
behavior some have requested as "equivalence," where none of the
bundled or SHADOW names is canonical or preferred over the others.
It does implicate an unknown level of effort to implement and
support.
6. IANA Considerations
There are no obvious IANA considerations in this memo; we reiterate
that the determination of which names are to be considered "the same"
is explicitly out of scope.
7. Security Considerations
[STW: Looking for examples for this section.]
Unsolved issues that will have to be considered in the definition of
what "the same" means for the DNS include the implications for
DNSSEC, and whether "identical" resolution includes DNSSEC validation
in the expected "identical" behavior.
Another area of possible peril includes SSL certificates, "Host"
headers as seen by web servers, and other security-relevant data
often associated with domain names. It will have to be considered
whether, and how, the "sameness" property maps into the expected
behavior of security-related protocols that use domain names,
particularly given that it's unlikely that all operators will ever
use the same set of constructs (whether in the DNS or elsewhere) to
signal whether different "names" are "the same" for purposes of the
function of a particular application or protocol.
In addition, there is a large cluster of security risks at the user
and application levels that motivate significant portions of the
interest in what it means to treat a set of names as "aliases" of
each other. One set of issues is around the expectation that two
strings are seen as "different" by the user in some obvious way (such
as visually) but need to be treated as "the same". The potential for
user confusion and subversion is not hard to imagine in cases where
two visually distinct strings are nonetheless likely to be expected
by the user to behave "the same" in some functional way. This is the
case we have attempted to address here.
There is a separate but complementary set of issues that arise around
cases where strings that look "the same" should nonetheless be
treated as different-- the so-called "confusing visual similarity"
problem. The easy example is substituting the Unicode codepoint for
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a character in one script, or a string of them, for the Unicode
codepoints for similar-looking characters in an altogether different
script. This has a different set of potential risks to users, and
has not been discussed here. It's often closely related to the
"alias" issue we have attempted to deal with, however, which poses
risks of its own to analysis of the either subject.
8. Acknowledgements
Most of the ideas here and much of the text is taken from discussions
on the DNSEXT and DNSOP WG mailing lists. Particular help is
acknowledged from the authors of the proposed solutions drafts, and
from the many contributors to the IDNAbis work and its underpinnings.
Special thanks at the intersection of DNS and IDNAbis is owed to
Patrik Faltstrom, Cary Karp, John Klensin, Vaggelis Segredakis, and
Andrew Sullivan for their patient explanations.
9. Change History
[[anchor28: RFC Editor: Please remove this section.]]
9.1. draft-yao-dnsext-identical-resolution: Version 00
o Domain Name Identical Resolution Problem Statement (initial
attempt)
9.2. draft-yao-dnsext-identical-resolution: Version 01
o Expanded introduction
o Added Greek example
o Added some detail to descriptions of proposed solutions
10. References
10.1. Normative References
[ASCII] American National Standards Institute (formerly United
States of America Standards Institute), "USA Code for
Information Interchange", ANSI X3.4-1968, 1968.
[EDNS0] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, August 1999.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
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[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2136] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, April 1997.
[RFC2672] Crawford, M., "Non-Terminal DNS Name Redirection",
RFC 2672, August 1999.
[RFC3490] Faltstrom, P., Hoffman, P., and A. Costello,
"Internationalizing Domain Names in Applications (IDNA)",
RFC 3490, March 2003.
[RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record
(RR) Types", RFC 3597, September 2003.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 3629, November 2003.
[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, April 2004.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005.
[RFC4290] Klensin, J., "Suggested Practices for Registration of
Internationalized Domain Names (IDN)", RFC 4290,
December 2005.
[RFC4690] Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and
Recommendations for Internationalized Domain Names
(IDNs)", RFC 4690, September 2006.
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10.2. Informative References
[BNAME] Yao, J., Lee, X., and P. Vixie, "Bundle DNS Name
Redirection", draft-yao-dnsext-bname-01.txt (work in
progress), 12 2009.
[CNAME-DNAME]
Sury, O., "CNAME+DNAME Name Redirection",
draft-sury-dnsext-cname-dname-00.txt (work in progress),
4 2010.
[IDN-TLD-Variants]
Yao, J. and X. Lee, "IDN TLD Variants Implementation
Guideline", draft-yao-dnsop-idntld-implementation-01.txt
(work in progress), 11 2009.
[RFC2672bis]
Rose, S. and W. Wijngaards, "Update to DNAME Redirection
in the DNS", Internet-Draft ietf-dnsext-rfc2672bis-dname-
17.txt, 6 2009.
[SHADOW] Vixie, P., "Use of DNS to Carry Configuration Metadata
Concerning Automatic Replication of Zones",
draft-vixie-dnsext-dnsshadow-00.txt (work in progress),
2 2010.
Authors' Addresses
Suzanne Woolf
Internet Systems Consortium, Inc.
950 Charter St.
Redwood City, CA 94063
Phone: +1 650 423 1333
Email: woolf@isc.org
Xiaodong LEE
CNNIC
No.4 South 4th Street, Zhongguancun
Beijing
Phone: +86 10 58813020
Email: lee@cnnic.cn
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Jiankang YAO
CNNIC
No.4 South 4th Street, Zhongguancun
Beijing
Phone: +86 10 58813007
Email: yaojk@cnnic.cn
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