Network Working Group O. Kolkman
Internet-Draft NLNet
Intended status: BCP J. Peterson
Expires: September 15, 2011 NeuStar, Inc.
H. Tschofenig
Nokia Siemens Networks
B. Aboba
Microsoft Corporation
March 14, 2011
Architectural Considerations on Application Features in the DNS
draft-iab-dns-applications-01
Abstract
While the principal purpose of the Domain Name System (DNS) is to
translate Internet domain names to IP addresses, over time a number
of Internet applications have integrated supplemental features into
the DNS to support their operations. Many of these features assist
in locating the appropriate service in a domain, or in transforming
intermediary identifiers into names that the DNS can process.
Proposals to piggyback more sophisticated application behavior on top
of the DNS, however, have raised questions about the propriety of
instantiating some features in that way, especially those with
security sensitivities. This document explores the architectural
consequences of installing application features in the DNS, and
provides guidance for future work in this area.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 15, 2011.
Copyright Notice
Kolkman, et al. Expires September 15, 2011 [Page 1]
Internet-Draft Applications in DNS March 2011
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Kolkman, et al. Expires September 15, 2011 [Page 2]
Internet-Draft Applications in DNS March 2011
Table of Contents
1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Overview of DNS Application Usages . . . . . . . . . . . . . . 6
2.1. Locating Services in a Domain . . . . . . . . . . . . . . 6
2.2. NAPTR and DDDS . . . . . . . . . . . . . . . . . . . . . . 7
2.3. Arbitrary Data in the DNS . . . . . . . . . . . . . . . . 8
3. Challenges for the DNS . . . . . . . . . . . . . . . . . . . . 10
3.1. Compound Queries . . . . . . . . . . . . . . . . . . . . . 10
3.1.1. Responses Tailored to the Originator . . . . . . . . . 11
3.2. Metadata about Tree Structure . . . . . . . . . . . . . . 12
3.3. Using DNS as a Generic Database . . . . . . . . . . . . . 13
3.3.1. Large Data in the DNS . . . . . . . . . . . . . . . . 13
3.4. Administrative Structures Misaligned with the DNS . . . . 14
3.5. Domain Redirection . . . . . . . . . . . . . . . . . . . . 14
4. Principles and Guidance . . . . . . . . . . . . . . . . . . . 16
4.1. Private DNS Deployments . . . . . . . . . . . . . . . . . 17
5. Security Considerations . . . . . . . . . . . . . . . . . . . 19
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
8. Informative References . . . . . . . . . . . . . . . . . . . . 22
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24
Kolkman, et al. Expires September 15, 2011 [Page 3]
Internet-Draft Applications in DNS March 2011
1. Motivation
The Domain Name System (DNS) has long provided a general means of
translating easily-memorized domain names into numeric Internet
Protocol addresses, which made the Internet easier to use by
providing a valuable layer of indirection between well-known names
and lower layer protocol elements. [RFC0974], however, documented a
further use of the DNS: to manage application services operating in a
domain with the Mail Exchange (MX) resource record, which helped
email addressed to the domain to find an authoritative mail service
for the domain.
The seminal MX record served as a prototype for a long series of DNS
resource records that supported applications associated with a domain
name. The SRV resource record [RFC2052] provided a more general
mechanism for identifying services in a domain, complete with a
weighting system and selection among transports. The Naming
Authority Pointer (NAPTR, originally [RFC2168]) resource record,
especially in its reincarnation as the Dynamic Delegation Discovery
System (DDDS, [RFC3401]) framework, added a new wrinkle - a way of
casting any sort of string as a domain name, which might then be
"resolved" by the DNS to find NAPTR records. This enabled the
resolution of identifiers other than traditional domain names through
the DNS; the best-known example of this are telephone numbers, as
resolved by the DDDS application ENUM. Recent work such as
DomainKeys Identified Mail (DKIM, [RFC4871]) has enabled security
features of applications to be advertised through the DNS, via the
TXT resource record.
As the amount of application intelligence available to the DNS has
increased, however, some proposed extensions have become misaligned
with the foundational assumptions of the DNS. One such assumption is
that the resolution of domain names to IP addresses is public
information with no need for confidentiality - any security required
by an application or service is invoked after the DNS query, when the
resolved service has been contacted. Typically, the translation also
does not depend on the identity of the querier (although for load
balancing reasons or related optimizations, the DNS may return
different addresses in response to queries from different sources, or
even no response at all, which is discussed further in
Section 3.1.1). These assumptions permit the existence of a single
authoritative unique global root of the DNS (see [RFC2826], and also
underlie the scaling capabilities of the DNS, notably the ability of
intermediaries to cache responses. At the point where these
assumptions no longer apply to the data that an application requires,
one can reasonably question whether or not that application should
use the DNS to deliver that data.
Kolkman, et al. Expires September 15, 2011 [Page 4]
Internet-Draft Applications in DNS March 2011
Increasingly, however, the flexibility of the DDDS framework has
encouraged the repurposing of the DNS into a generic database. Since
the output of DDDS can be a Uniform Resource Indicator (URI
[RFC3986]), and URIs themselves are containers for basically
arbitrary data, one can query through the DDDS framework for an
arbitrary string (provided it can be formatted and contained within
the syntactical limits of a domain name) and receive as a response an
equally arbitrary chunk of data. The use of the DNS for generic
database lookups is especially attractive in environments that
already use the DDDS framework, where deployments would prefer to
reuse the existing query/response interface of the DNS rather than
installing a new and separate database capability.
The guidance in this document complements the guidance on extending
the DNS given in [RFC5507]. Whereas RFC5507 considers the preferred
ways to add new information to the underlying syntax of the DNS (such
as defining new resource records or adding prefixes or suffixes to
labels), the current document considers broader implications of
offloading application features to the DNS, be it through extending
the DNS or simply reusing existing protocol capabilities. It is the
features themselves, rather than any syntactical representation of
those features, that are considered here.
Kolkman, et al. Expires September 15, 2011 [Page 5]
Internet-Draft Applications in DNS March 2011
2. Overview of DNS Application Usages
While the original motivation for the Domain Name System was to
implement as an Internet service a means of masking lengthy numeric
addresses with strings that are easier to interpret and memorize, the
hierarchical system of domains rendered the DNS important for its
administrative properties as well as its mnemonics. Since the DNS
explained how to reach an administrative domain rather than simply a
host, it was natural to extend the DNS further to optimize for
reaching particular applications within a domain. Without these
extensions, a user trying to send mail to a foreign domain, for
example, lacked a discovery mechanism to locate the right host in the
remote domain to connect to for mail. While a special-purpose
discovery mechanism could be built by each such application protocol
that needed this functionality, the universality of the DNS invites
installing these features into its public tree.
2.1. Locating Services in a Domain
The MX resource record provides the simplest motivating example for
an application advertising its hosts in the Domain Name System. The
MX resource record contains the domain name of a server within the
administrative domain in question that receives mail; that domain
name must itself be resolved to an IP address through the DNS in
order to reach the mail server. While naming conventions for
applications might serve a similar purpose (a host might be named
"mail.example.com" for example), approaching service location through
the creation of a new resource record yields several important
benefits. Firstly, one can put multiple MX records in a zone, in
order to designate backup servers that can receive mail when the
primary server is offline. One can even load balance across several
such servers (see [RFC1794]. These properties could not easily be
captured by naming conventions (see [RFC4367]).
While the MX record represents a substantial improvement over naming
conventions as a means of service location, it remains specific to a
single application. Thus, the general approach of the MX record was
adapted to fit a broader class of application through the Service
(SRV) resource record (originally [RFC2052]). The SRV record allows
DNS resolvers to search for particular applications and underlying
transports (for example, HTTP running over TLS, see [RFC2818]) and to
learn the domain name and port where that service resides in a given
administrative domain. It also provides a weighting mechanism to
allow load balancing across several (presumably equivalent) instances
of a service in a domain.
The reliance of applications on the existence of MX and SRV records
has important implications for the way that applications manage
Kolkman, et al. Expires September 15, 2011 [Page 6]
Internet-Draft Applications in DNS March 2011
identifiers. Email identifiers of the form "user@domain" require the
presence of MX records to provide the convenience of simply
specifying that "domain" component rather than a "host.domain"
structure. While for applications like HTTP, naming conventions
continue to abound ("www.example.com"), the SRV algorithm queries for
an application-specific label combining the protocol and transport.
For a protocol like HTTP, the SRV label derives from the URL scheme
of the identifier invoked by the application. The application
identifier thus retained sole responsibility for carrying the desired
protocol and domain, but could offload to the DNS the location of the
host of that service within the domain, the port where the service
resided on that host, load balancing and fault tolerance, and related
application features. Ultimately, resolvers that acquire MX or SRV
records use them as intermediate transformations in order to arrive
at an eventual domain name that will resolve to the IP address of a
host to contact for the service.
[TBD: Potentially incorporate some discussion of Hammer's hostmeta or
the Webfingers approach as alternatives to using the DNS to identify
additional resources required for services]
2.2. NAPTR and DDDS
The NAPTR resource record evolved to fulfill a need in the transition
from Uniform Resource Locators (URLs) to the more mature URI
framework, which incorporated Uniform Resources Names (URNs). Unlike
URLs, URNs typically do not convey enough semantics internally to
resolve them through the DNS, and consequently a separate URI-
transformation mechanism is required to convert these types of URIs
into domain names. This allowed identifiers with no recognizable
domain component to treated as DNS names for the purpose of name
resolution. Once these transformations resulted in a domain name,
applications could retrieve NAPTR records from that zone in the DNS.
NAPTR records contain a far more rich and complex structure than MX
or SRV resource records. A NAPTR record contains two different
weighting mechanisms ("order" and "preference"), a "service" field to
designate the application that the NAPTR record described, and then
two fields that could contain translations: a "replacement" or
"regular expression" field, only one of which appeared in given NAPTR
record. A "replacement," like NAPTR's ancestor the PTR record,
simply designated another zone where one would look for records
associated with this service in the domain. The "regexp," on the
other hand, allowed sed-like transformations on the original URI
intended to transform it into an identifier that the DNS could
resolve.
The same mechanism could obviously be applied to other sorts of
identifiers that lacked a domain component, and thus this work
Kolkman, et al. Expires September 15, 2011 [Page 7]
Internet-Draft Applications in DNS March 2011
naturally combined with activities to create a system for resolving
telephone numbers on the Internet, which became known as ENUM
(originally [RFC2916]). ENUM borrowed from an earlier proposal, the
"tpc.int" domain ([RFC1530]), which provided a means for encoding
telephone numbers as domain names applying a string preparation
algorithm that required reversing the digits and treating each
individual digit as a zone of the DNS - thus, for example, the number
+15714345400 became 0.0.4.5.4.3.4.1.7.5.1.tpc.int. In the ENUM
system, in place of "tpc.int" the special domain "e164.arpa" was
reserved for use. In its more mature form in Dynamic Delegation and
Discovery Service (DDDS) ([RFC3401] passim) framework, this initial
transformation was called the "First Well Known Rule." Its
flexibility has inspired a number of proposals beyond ENUM to encode
and resolve unorthodox identifiers in the DNS. Provided that the
identifiers transformed by the First Well Known Rule have some
meaningful hierarchical structure and are not overly lengthy,
virtually anything can serve as an input for the DDDS structure: for
example, civic addresses (see draft-rosen-dns-sos).
The presence of the "regexp" field of NAPTR records enabled
unprecedented flexibility in the transformations that DNS resolution
could perform. Since the output of the regular expression frequently
took the form of a URI (in ENUM resolution, for example, a telephone
might be converted into a SIP URI), anything that could be encoded as
a URI might be the result of resolving a NAPTR record. Since URI
encoding has ways of carrying basically arbitrary data (see for
example, the base 64 encoded binary data in the data URL [RFC2397]),
resolving a NAPTR record might result in an output other than an
identifier which would subsequently be resolved to an IP address and
contacted for a particular application - it could give a literal
result consumed by the application. Thus, the DNS could effectively
implement the entire application feature set of any simple query-
response protocol. Effectively, the DDDS framework turned the DNS
into a generic database - indeed, the DNS serves as but one example
of a possible back-end for DDDS, and perhaps not the most suitable
one.
2.3. Arbitrary Data in the DNS
NAPTR did not pioneer the storage of arbitrary data in the DNS.
[RFC1464] defined the TXT record, a means to store arbitrary string
data in the DNS using a simple attribute name/value pair syntax. The
existence of TXT records has long provided new applications with a
rapid way of storing data associated with a domain name in the DNS,
as the attribute names require no registration process and can simply
be minted at will. Thus, an application that wants to store
additional data in the DNS can do so without registering a new
resource record type.
Kolkman, et al. Expires September 15, 2011 [Page 8]
Internet-Draft Applications in DNS March 2011
While lax policies surrounding the use of the TXT record has resulted
in a checkered past for standardizing application usage of TXT, it
has provided a technical solution for DKIM ([RFC4871]) to store
cryptographic keys for email in DNS. Storing keys in the DNS for
DKIM made sense for several reasons: notably, because the public keys
associated with email required wide public distribution, and because
email identifiers contain a domain component that applications can
easily use to consult the DNS. If the application had to negotiate
support for the DKIM mechanism with mail servers, it would give rise
to bid-down attacks that are not possible if the DNS delivers the
keys (provided that DNSSEC guarantees authenticity of zone files).
Kolkman, et al. Expires September 15, 2011 [Page 9]
Internet-Draft Applications in DNS March 2011
3. Challenges for the DNS
These methods for transforming arbitrary identifiers into domain
names, and for returning arbitrary data in response to DNS queries,
both represent significant extensions from the original concept of
the DNS, yet neither fundamentally alters the underlying model of the
DNS. The promise that applications might rely on the DNS as a
generic database, however, invariably gives rise to additional
requirements that one might expect to find in a database access
protocol: authentication of the source of queries for comparison to
access control lists, formulating complex relational queries, and
asking questions about the structure of the database itself. DNS was
not designed to provide these sorts of properties, and extending the
DNS to encompass them would represent a fundamental alteration to its
model. If an application desires these properties from a database,
in general this is a good indication that the DNS cannot meet the
needs of the application in question.
Since many of these new requirements have emerged from the ENUM
space, the following sections use ENUM as an illustrative example;
however, any application using the DNS as a feature-rich database
could easily end up with similar requirements.
3.1. Compound Queries
Traditionally, the DNS requires resolvers to supply no information
other than the domain name (along with the type and class of records
sought) in order to receive a reply from an authoritative server.
Outside of the DNS space, however, there are plenty of query-response
applications that require a compound or relational search, which
takes into account more than one factor in formulating a response or
uses no single factor as a key to the database. For example, in the
telephony space, telephone call routing often takes into account
numerous factors aside from the dialed number, including originating
trunk groups, interexchange carrier selection, number portability
data, time of day, and so on. All are considered simultaneously in
generating a route. While in its original conception, ENUM hoped to
circumvent the traditional PSTN and route directly to Internet-
enabled devices, the infrastructure ENUM effort to support the
migration of traditional carrier routing functions to the Internet
aspires to achieve feature parity with traditional number routing.
Consequently, some consideration has been given to ways to add
additional data to ENUM queries to give the DNS server sufficient
information to return a suitable URI.
Several workaround have attempted to instantiate these sorts of
features in the DNS. Most commonly, proposals piggyback additional
query parameters as eDNS0 extensions (see [RFC2671]). Alternatively,
Kolkman, et al. Expires September 15, 2011 [Page 10]
Internet-Draft Applications in DNS March 2011
the domain name itself can be compounded with the additional
parameters: one could take a name like
0.0.4.5.4.3.4.1.7.5.1.e164.arpa and append a trunk group identifier
to it, for example, of the form
tg011.0.0.4.5.4.3.4.1.7.5.1.e164.arpa. While in the latter case, a
DNS server can adhere to its traditional behavior in locating
resource records, the syntactical viability of encoding additional
parameters in this fashion is very dubious, especially if more than
one additional parameter is required and the presence of parameters
is optional. The former eDNS0 case requires significant changes to
DNS server behavior. Moreover, the implications of these sorts of
compound queries for recursion and caching are potentially serious.
3.1.1. Responses Tailored to the Originator
The most important subcase of the compound queries are DNS responses
tailored to the identity of their originator, where some sort of
administrative identity of the originator must be conveyed to the
DNS. We must first distinguish this from cases where the originating
IP address is used to serve a location-specific name. For those
sorts of applications, which general lack security implications (for
example, providing a web portal customized to the region of the
client), relying on the source IP address introduces little harm.
Because recurse resolvers may obscure the origination network of the
DNS client, a recent proposal suggested introducing a new DNS query
parameter to be populated by DNS recursive resolvers in order to
preserve the originating IP address (see
draft-vandergaast-edns-client-ip). However, aside from purely
cosmetic uses, these approaches have many known limitations due to
the prevalence of VPNs, onion routing systems, and so on. Some
transitional deployments today, however, use the source IP address is
often mapped to the administrative identity of the originator. The
security implications of trusting the source IP address of a DNS
query have prevented most solutions along these lines from being
standardized (see draft-ietf-intarea-shared-addressing-issues),
though the practice remains widespread in private DNS deployment (see
Section 4.1). Some applications go even further and propose
extending the DNS to add an application-layer identifier of the
originator rather than an IP address; for example,
draft-kaplan-enum-source-uri provides a SIP URI in an eDNS0 parameter
(though without any specific provision for cryptographically
verifying the claimed identity). Effectively, the conveyance of this
information about the administrative identity of the originator is a
weak authentication mechanism, on the basis of which the DNS server
makes an authorization decision before sharing resource records.
This can parlay into a selective confidentiality mechanism, where
only a specific set of originators are permitted to see resource
records, or a case where a query for the same name by different
Kolkman, et al. Expires September 15, 2011 [Page 11]
Internet-Draft Applications in DNS March 2011
entities results in completely different resource record sets. The
DNS, however, substantially lacks the protocol semantics to manage
access control list for data, and again, caching and recursion
introduce significant challenges for applications that attempt to
offload this responsibility to the DNS. Achieving feature parity
with even the simplest authentication mechanisms available at the
application layer would like require significant rearchitecture of
the DNS.
3.2. Metadata about Tree Structure
ENUM use cases have also surfaced a couple of optimization
requirements to reduce unnecessary calls and queries by including
metadata that describes the contents and structure of ENUM DNS trees.
In particular, the "send-n" proposal (draft-bellis-enum-send-n) hopes
to reduce the number of DNS queries sent in cases where a telephone
system is collecting dialed digits in a region that supports
"overlap" dialing, a practice which compensates for variable-length
numbering plans. When the dialed number potentially has a variable
length, a telephone switch ordinarily cannot anticipate when a dialed
number is complete, as only the terminating customer premise
equipment (typically a private branch exchange) knows how long a
telephone number needs to be. The "send-n" proposal offloads to the
DNS the responsibility for informing the telephone switch the minimum
number of digits that must be collected by placing in zones
corresponding to incomplete telephone numbers some resource records
which state how many more digits are required - effectively how many
steps down the DNS tree one must take before querying the DNS again.
With this information, the application is not required to query the
DNS every time a new digit is dialed, but can wait to collect
sufficient digits to receive a response. As an optimization, this
practice thus saves the resources of the DNS server - though it does
not result in faster call set-up, as the call cannot complete until
all digits are collected. A tangentially related proposal,
draft-ietf-enum-void, similarly places resource records in the DNS
that tell the application that it need not attempt to reach a number
on the PSTN, as the number is unassigned.
Both proposals optimize application behavior by placing metadata in
the DNS that predicts the success of future queries or application
invocations. These predictions require that the metadata remain
synchronized with the state of the resources it predicts.
Maintaining that synchronization, however, requires that the DNS have
semi-real time updates that may conflict with scale and caching
mechanisms. It may also raise questions about the authority and
delegation model, and whether the entities that control the zones
where changes occur have the authority to populate the zones where
synchronization must be maintained; in send-n, different leaf zones
Kolkman, et al. Expires September 15, 2011 [Page 12]
Internet-Draft Applications in DNS March 2011
might want to populate different information in a common parent. It
is ultimately unclear why this data is better maintained by the DNS
than in an unrelated application protocol, nor why applications
should wait until they are collecting digits to learn this
information.
3.3. Using DNS as a Generic Database
As previously noted, the use of the First Well Known Rule of DDDS
combined with data URLs effectively allows the DNS to answer queries
for arbitrary strings and to return arbitrary data as value. Some
query-response applications, however, require queries and responses
that simply fall outside the syntactic capabilities of the DNS.
Domain names themselves must conform with certain syntactic
constraints: they must consist of labels that do not exceed 63
characters while the total length of the encoded name may not exceed
255 octets, they must obey fairly strict encoding rules, and so on.
3.3.1. Large Data in the DNS
While the data URL specification (RFC2397) notes that it is "only
useful for short values," many applications today use quite large
data URLs as workarounds in environments where only URIs can
syntactically appear. While the use of TCP and eDNS0 allows DNS
responses to be quite long, nonetheless there are forms of data that
an application might store in the DNS that exceed reasonable limits:
in the ENUM context, for example, something like storing base 64
encoded mp3 files of custom ringtones.
Designs relying on storage of large amounts of data within DNS RRs
futhermore need to minimize the potential damage achievable in a
reflection attack, in which the attacker sends DNS queries with a
forged source address, and the victim receives the response. By
generating a large response to a small query, the attacker can
magnify the bandwidth directed at the victim.
Since it is difficult to complete a TCP three-way handshake begun
from a forged source address, DNS reflection attacks utilize UDP
queries. Unless the attacker utilizes EDNS0 [RFC2671] to enlarge the
requester's maximum payload size, a response can only reach 576
octets before the truncate bit is set in the response. This limits
the maximum magnification achievable from a DNS query that does not
utilize EDNS0.
However, where the responder supports EDNS0, an attacker may set the
requester maximum payload size to a larger value while querying for a
large RR, such as a certificate [RFC4398]. Thus the combination of
large data stored in DNS RRs and responders supporting large
Kolkman, et al. Expires September 15, 2011 [Page 13]
Internet-Draft Applications in DNS March 2011
requester payload sizes has the potential to increase the potential
damage achievable in a reflection attack. Since a reflection attack
can be launched from any network that does not implement source
address validation, these attacks are difficult to eliminate absent
the ubiquitous deployment of source address validation. Since
reflection attacks are most damaging when launched from high
bandwidth networks, the implementation of source address validation
on these networks is particularly important.
The bandwidth that can be mustered in a reflection attack directed by
a botnet controlling broadband hosts is sobering. For example, if a
responder could be directed to generate a 10KB response in reply to a
50 octet query, then magnification of 200:1 would be attainable.
This would enable a botnet controlling 10000 hosts with 1 Mbps of
bandwidth to focus 2000 Gbps of traffic on the victim, more than
sufficient to congest any site on the Internet.
3.4. Administrative Structures Misaligned with the DNS
While the DDDS framework enables any sort of alphanumeric data to
serve as a DNS name through the application of the First Well Known
Rule, the delegative structure of the resulting DNS name may not
reflect the division of responsibilities for the resources that the
alphanumeric data indicates. Telephone numbers in the United States,
for example, are assigned and delegated in a relatively complex
manner: the first three digits of a nationally specific number are an
"area code" which is understood as an indivisible component of the
number, yet for the purpose of the DNS, those three digits are ranked
hierarchically.
The difficulty of mapping the DNS to administrative structures can
even occur with traditional domain names, where applications expect
clients to infer or locate zone cuts.
3.5. Domain Redirection
Most Internet application services provide a redirection feature -
when you attempt to contact a service, the service may refer you to a
different service instance, potentially in another domain, that is
for whatever reason better suited to address a request. In HTTP and
SIP, for example, this feature is implemented by the 300 class
responses containing one or more better URIs that may indicate that a
resource has moved temporarily or permanently to another service.
Several tools in the DNS. including the SRV record, can provide a
similar feature at a DNS level, and consequently some applications as
an optimization offload the responsibility for redirection to the
DNS; NAPTR can also provide this capability on a per-application
basis, and numerous DNS resource records can provide redirection on a
Kolkman, et al. Expires September 15, 2011 [Page 14]
Internet-Draft Applications in DNS March 2011
per-domain basis. This can prevent the unnecessary expenditure of
application resources on a function that could be performed as a
component of a DNS lookup that is already a prerequisite for
contacting the service. Consequently, in some deployment
architectures this DNS-layer redirection is used for virtual hosting
services.
Implementing domain redirection in the DNS, however, has important
consequences for application security. Un the absence of universal
DNSSEC, applications must blindly trust the DNS in order to believe
that their request has not been hijacked and redirected to a
potentially malicious domain, unless some subsequent application
mechanism can provide the necessary assurance. By way of contrast,
for application-layer redirections protocols like HTTP and SIP have
widely deployed security mechanisms such as TLS that can use
certificates to vouch that a 300 response came from the domain that
the originator initially hoped to contact.
A number of applications have attempted to provide an after-the-fact
security mechanism that verifies the authority of a DNS delegation in
the absence of DNSSEC. The specification for deferencing SIP URIs
([RFC3263], reaffirmed in [RFC5922]) requires that during TLS
establishment, the site eventually reached by a SIP request present a
certificate corresponding to the original URI expected by the user
(in other words, if example.com redirects to example.net in the DNS,
this mechanism expects that example.net will supply a certificate for
example.com in TLS), which requires a virtual hosting service to
possess a certificate corresponding to the hosted domain. This
restriction rules out many styles of hosting deployments common the
web world today, however. [I-D.barnes-hard-problem] explores this
problem space, and [I-D.saintandre-tls-server-id-check] proposes a
solution for all applications that use TLS. Potentially, new types
of certificates (similar to [RFC4985] might bridge this gap, but
support for those certificates would require changes to existing
certificate authority practices as well as application behavior.
All of these application-layer measures attempt to mirror the
delegation of authority in the DNS, when the itself DNS serves as the
ultimate authority on how domains are delegated. The difficulty of
synchronizing a static instrument like a certificate with a
delegation in the DNS, however, exposes the fundamentally problematic
nature of this endeavor. In environments where DNSSEC is not
available, the problems with securing DNS-layer redirections would be
avoided by performing redirections in the application-layer.
Kolkman, et al. Expires September 15, 2011 [Page 15]
Internet-Draft Applications in DNS March 2011
4. Principles and Guidance
The success of the DNS relies on the fact that it is a distributed
database, one that has the property that it is loosely coherent and
that it offers lookup instead of search functionality. Loose
coherency means that answers to queries are coherent within the
bounds of data replication between authoritative servers and caching
behavior by recursive name servers.
It is likely that the DNS provides a good match whenever applications
needs are aligned with the following properties:
Data can be stored in such a way that a single query name, class
and type provide a direct answer
Data is indexed by keys that are semantically and syntactically
uncomplicated
Answers only depend on the question (name, type and class), not on
the identity of the entity doing the query
Data stored in the DNS is resilient to data propagation and
caching behavior
Whenever one of the four properties above does not apply to ones data
one should seriously consider whether the DNS is the best place to
store actual data. On the other hand, good indicators that the DNS
is not the appropriate tool for solving problems is when you have to
worry about:
Trying to establish domain boundaries within the tree - the
delegation point in the DNS is something that applications should
in general not be aware off
Working from application identifiers that cannot be resolved by
the DNS without excessively complex transformations
The sensitivity of the data provided by the DNS, especially
confidentiality
Highly volatile data that synchronizes with the state of
applications external to the DNS
Answers dependent on the application-layer identity or location of
the source of the query
There are many useful application features that can safely be
offloaded to the DNS: aside from locating services in a domain, the
Kolkman, et al. Expires September 15, 2011 [Page 16]
Internet-Draft Applications in DNS March 2011
DNS clearly can assist in the resolution of identifiers without a
domain component (including URNs), and moreover it can host some
static application data, like the cryptographic keys used by DKIM for
email, which are well suited to storage in the DNS. However, the
prospects for offloading application features like authentication of
query originators, structuring compound questions and implementing
metadata about the tree structure are more remote. While clearly
DNS-layer redirection is a widely deployed alternative to
application-layer redirection, many applications that choose to
offload this have struggled to meet the resulting security
challenges.
In cases where applications require these sorts of features, they are
simply better instantiated by independent application-layer protocols
than the DNS. The query-response semantics of the DNS are easily
replicated by HTTP, for example, and the objects which HTTP can carry
both in queries and responses can easily contain the necessary
structure to manage compound queries. Similarly, HTTP has numerous
ways to provide the necessary authentication, authorization and
confidentiality properties that some features require.
Where the administrative delegations of the DNS form a necessary
component in the instantiation of an application feature, there are
various ways that the DNS can bootstrap access to an independent
application-layer protocol better suited to field the queries in
question. For example, since NAPTR records can contain URIs, those
URI can point to an external query-response service such as HTTP,
with a NAPTR service field that signal to applications that questions
of interest can be answered at that service.
4.1. Private DNS Deployments
Today, many deployments that want to install these rich application
features in DNS do so in private environments rather than in the
public DNS tree. There are two motivations for this: in the first
place, proprietary non-standard parameters can easily be integrated
into DNS queries or responses; secondly, confidentiality and custom
responses can be provided by deploying, respectively, underlying VPNs
to shield the private tree from public queries, and effectively
different virtual DNS trees for each administrative entity that might
launch a query (so-called "split horizon" DNS being one such
example). In these constrained environments, caching and recursive
resolvers can be managed or eliminated in order to prevent any
unexpected intermediary behavior.
While these deployments address the requirements of applications that
rely on them, by definition these techniques will not form the basis
of a standard solution. Moreover, as implementations come to support
Kolkman, et al. Expires September 15, 2011 [Page 17]
Internet-Draft Applications in DNS March 2011
these proprietary parameters, it seems almost certain that these
private techniques will begin to leak into the public DNS.
Therefore, keeping these features within higher-layer applications
rather than offloading them to the DNS is preferred.
Kolkman, et al. Expires September 15, 2011 [Page 18]
Internet-Draft Applications in DNS March 2011
5. Security Considerations
Many of the concerns about offloading application features to the DNS
revolve around security. Section 3.5 discusses a security problem
concerning redirection that has surfaced in a number of protocols
(see [I-D.barnes-hard-problem]). The perceived need to authenticate
the source of DNS queries (see Section 3.1.1 and authorize access to
particular resource records also illustrates the fundamental security
principles that arise from offloading certain application features to
the DNS.
While DNSSEC, were it deployed universally, can play an important
part in securing application redirection in the DNS, DNSSEC does not
provide a means for a resolver to authenticate itself to a server,
nor a framework for servers to return selective answers based on the
authenticated identity of resolvers.
Kolkman, et al. Expires September 15, 2011 [Page 19]
Internet-Draft Applications in DNS March 2011
6. IANA Considerations
This document contains no considerations for the IANA.
Kolkman, et al. Expires September 15, 2011 [Page 20]
Internet-Draft Applications in DNS March 2011
7. Acknowledgements
The IAB would like to thank Ed Lewis, Ray Bellis, Lawrence Conroy,
Ran Atkinson, Patrik Faltstrom and Eliot Lear for their
contributions.
Kolkman, et al. Expires September 15, 2011 [Page 21]
Internet-Draft Applications in DNS March 2011
8. Informative References
[I-D.barnes-hard-problem]
Barnes, R. and P. Saint-Andre, "High Assurance Re-
Direction (HARD) Problem Statement",
draft-barnes-hard-problem-00 (work in progress),
July 2010.
[I-D.saintandre-tls-server-id-check]
Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
in Certificates Used with Transport Layer Security",
draft-saintandre-tls-server-id-check-09 (work in
progress), August 2010.
[RFC0974] Partridge, C., "Mail routing and the domain system",
RFC 974, January 1986.
[RFC1464] Rosenbaum, R., "Using the Domain Name System To Store
Arbitrary String Attributes", RFC 1464, May 1993.
[RFC1530] Malamud, C. and M. Rose, "Principles of Operation for the
TPC.INT Subdomain: General Principles and Policy",
RFC 1530, October 1993.
[RFC1794] Brisco, T., "DNS Support for Load Balancing", RFC 1794,
April 1995.
[RFC2052] Gulbrandsen, A. and P. Vixie, "A DNS RR for specifying the
location of services (DNS SRV)", RFC 2052, October 1996.
[RFC2168] Daniel, R. and M. Mealling, "Resolution of Uniform
Resource Identifiers using the Domain Name System",
RFC 2168, June 1997.
[RFC2397] Masinter, L., "The "data" URL scheme", RFC 2397,
August 1998.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, August 1999.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC2826] Internet Architecture Board, "IAB Technical Comment on the
Unique DNS Root", RFC 2826, May 2000.
[RFC2916] Faltstrom, P., "E.164 number and DNS", RFC 2916,
September 2000.
Kolkman, et al. Expires September 15, 2011 [Page 22]
Internet-Draft Applications in DNS March 2011
[RFC3263] Rosenberg, J. and H. Schulzrinne, "Session Initiation
Protocol (SIP): Locating SIP Servers", RFC 3263,
June 2002.
[RFC3401] Mealling, M., "Dynamic Delegation Discovery System (DDDS)
Part One: The Comprehensive DDDS", RFC 3401, October 2002.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC4366] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 4366, April 2006.
[RFC4367] Rosenberg, J. and IAB, "What's in a Name: False
Assumptions about DNS Names", RFC 4367, February 2006.
[RFC4871] Allman, E., Callas, J., Delany, M., Libbey, M., Fenton,
J., and M. Thomas, "DomainKeys Identified Mail (DKIM)
Signatures", RFC 4871, May 2007.
[RFC4985] Santesson, S., "Internet X.509 Public Key Infrastructure
Subject Alternative Name for Expression of Service Name",
RFC 4985, August 2007.
[RFC5507] IAB, Faltstrom, P., Austein, R., and P. Koch, "Design
Choices When Expanding the DNS", RFC 5507, April 2009.
[RFC5922] Gurbani, V., Lawrence, S., and A. Jeffrey, "Domain
Certificates in the Session Initiation Protocol (SIP)",
RFC 5922, June 2010.
Kolkman, et al. Expires September 15, 2011 [Page 23]
Internet-Draft Applications in DNS March 2011
Authors' Addresses
Olaf Kolkman
NLNet
Email: olaf@nlnetlabs.nl
Jon Peterson
NeuStar, Inc.
Email: jon.peterson@neustar.biz
Hannes Tschofenig
Nokia Siemens Networks
Email: Hannes.Tschofenig@gmx.net
Bernard Aboba
Microsoft Corporation
Email: bernarda@microsoft.com
Kolkman, et al. Expires September 15, 2011 [Page 24]