| Rfc | 6950 |
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| Title | Architectural Considerations on Application Features in the DNS |
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| Author | J.
Peterson, O. Kolkman, H. Tschofenig, B. Aboba |
|---|
| Date | October 2013 |
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| Format: | TXT, HTML |
|---|
| Status: | INFORMATIONAL |
|---|
|
Internet Architecture Board (IAB) J. Peterson
Request for Comments: 6950 NeuStar, Inc.
Category: Informational O. Kolkman
ISSN: 2070-1721 NLnet Labs
H. Tschofenig
Nokia Siemens Networks
B. Aboba
Skype
October 2013
Architectural Considerations on Application Features in the DNS
Abstract
A number of Internet applications rely on the Domain Name System
(DNS) to support their operations. Many applications use the DNS to
locate services for a domain; some, for example, transform
identifiers other than domain names into formats that the DNS can
process, and then fetch application data or service location data
from the DNS. Proposals incorporating sophisticated application
behavior using DNS as a substrate have raised questions about the
role of the DNS as an application platform. This document explores
the architectural consequences of using the DNS to implement certain
application features, and it provides guidance to future application
designers as to the limitations of the DNS as a substrate and the
situations in which alternative designs should be considered.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Architecture Board (IAB)
and represents information that the IAB has deemed valuable to
provide for permanent record. It represents the consensus of the
Internet Architecture Board (IAB). Documents approved for
publication by the IAB are not a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6950.
Copyright Notice
Copyright (c) 2013 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.
Table of Contents
1. Motivation ......................................................2
2. Overview of DNS Application Usages ..............................4
2.1. Locating Services in a Domain ..............................5
2.2. NAPTR and DDDS .............................................6
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 ...............12
3.2. Using DNS as a Generic Database ...........................14
3.2.1. Large Data in the DNS ..............................14
3.3. Administrative Structures Misaligned with the DNS .........16
3.3.1. Metadata about Tree Structure ......................18
3.4. Domain Redirection ........................................20
4. Private DNS and Split Horizon ..................................21
5. Principles and Guidance ........................................23
6. Security Considerations ........................................25
7. IAB Members at the Time of Approval ............................26
8. Acknowledgements ...............................................26
9. Informative References .........................................27
1. Motivation
The Domain Name System (DNS) has long provided a general means of
translating domain names into Internet Protocol addresses, which
makes the Internet easier to use by providing a valuable layer of
indirection between names and lower-layer protocol elements.
[RFC0974] documented a further use of the DNS: to locate an
application service operating in a domain, via the Mail Exchange (MX)
Resource Record; these records help email addressed to the domain to
find a mail service for the domain sanctioned by the zone
administrator.
The seminal MX record served as a prototype for other DNS resource
records that supported applications associated with a domain name.
The SRV Resource Record [RFC2052] provided a more general mechanism
for locating services in a domain, complete with a weighting system
and selection among transports. The Naming Authority Pointer (NAPTR)
Resource Record (originally described in [RFC2168]), especially as it
evolved into the more general Dynamic Delegation Discovery System
(DDDS) [RFC3401] framework, added a generic mechanism for storing
application data in the DNS. Primarily, this involved a client-side
algorithm for transforming a string into a domain name, which might
then be resolved by the DNS to find NAPTR records. This enabled the
resolution of identifiers that do not have traditional host
components through the DNS; the best-known examples of this are
telephone numbers, as resolved by the DDDS application ENUM. Recent
work, such as DomainKeys Identified Mail (DKIM) [RFC6376], has
enabled security features of applications to be advertised through
the DNS, via the TXT Resource Record.
The scope of application usage of the DNS has thus increased over
time. Applications in many environments require features such as
confidentiality, and as the contexts in which applications rely on
the DNS have increased, some application protocols have looked to
extend the DNS to include these sorts of capabilities. However, some
proposed usages of, and extensions to, the DNS have become misaligned
with both the DNS architecture and the DNS protocol. If we take the
example of confidentiality, we see that in the global public DNS, the
resolution of domain names to IP addresses is an exchange of public
information with no expectation of confidentiality. Thus, the
underlying query/response protocol has no encryption mechanism;
typically, any security required by an application or service is
invoked after the DNS query, when the resolved service has been
contacted. Only in private DNS environments (including split-horizon
DNS) where the identity of the querier is assured through some
external policy can the DNS maintain confidential records, by
providing distinct answers to the private and public users of the
DNS. In support of load-balancing or other optimizations, a DNS
server may return different addresses in response to queries from
different sources, or even no response at all; see Section 3.1.1 for
details.
This document provides guidance to application designers and
application protocol designers looking to use the DNS to support
features in their applications. It provides an overview of past
application usage of the DNS as well as a review of proposed new
usages. It identifies concerns and trade-offs and provides guidance
on the question, "Should I store this information in the DNS, or use
some other means?" when that question arises during protocol
development. These guidelines remind application protocol designers
of the strengths and weaknesses of the DNS in order to make it easier
for designers to decide what features the DNS should provide for
their application.
The guidance in this document complements the guidance on extending
the DNS given in [RFC5507]. Whereas [RFC5507] considers the
preferred ways to add new information to the underlying syntax of the
DNS (such as defining new resource records or adding prefixes or
suffixes to labels), the current document considers broader
implications of applications that rely on the DNS for the
implementation of certain features, be it through extending the DNS
or simply reusing existing protocol capabilities -- implications that
may concern the invocation of the resolver by applications; the
behavior of name servers, resolvers, or caches; extensions to the
underlying DNS protocol; the operational responsibilities of zone
administrators; security; or the overall architecture of names. When
existing DNS protocol fields are used in ways that their designers
did not intend to handle new applications, those applications may
demand further changes and extensions that are fundamentally at odds
with the strengths of the DNS.
2. Overview of DNS Application Usages
[RFC0882] identifies the original and fundamental connection between
the DNS and applications. It begins by describing how the
interdomain scope of applications creates "formidable problems when
we wish to create consistent methods for referencing particular
resources that are similar but scattered throughout the environment".
This motivated transitioning the "mapping between host names... and
ARPA Internet addresses" from a global table (the original "hosts"
file) to a "distributed database that performs the same function".
[RFC0882] also envisioned some ways to find the resources associated
with mailboxes in a domain: without these extensions, a user trying
to send mail to a foreign domain lacked a discovery mechanism to
locate the right host in the remote domain to which to connect.
While a special-purpose service discovery mechanism could be built
for each such application protocol that needed this functionality,
the universal support for the DNS encourages installing these
features into its public tree rather than inventing something new.
Thus, over time, several other applications leveraged DNS resource
records for locating services in a domain or for storing application
data associated with a domain in the DNS. This section gives
examples of various types of DNS usage by applications to date.
2.1. Locating Services in a Domain
The MX Resource Record provides the simplest example of an
application advertising its domain-level resources in the Domain Name
System. The MX Resource Record contains the domain name of a server
that receives mail on behalf of the administrative domain in
question; that domain name must itself be resolved to one or more
IP addresses through the DNS in order to reach the mail server.
While naming conventions for applications might serve a similar
purpose (a host might be named "mail.example.com", for example),
approaching service location through the creation of a new resource
record yields important benefits. For example, one can put multiple
MX records under the same name, in order to designate backup
resources or to load-balance across several such servers (see
[RFC1794]); these properties could not easily be captured by naming
conventions (see [RFC4367], though more recently DNS-based Service
Discovery (DNS-SD) [RFC6763] codifies service instance naming
conventions for use across applications to locate services in a
domain).
While the MX record represents a substantial improvement over naming
conventions as a means of service location, it remains specific to a
single application. Thus, the general approach of the MX record was
adapted to fit a broader class of applications through the Service
(SRV) Resource Record (originally described in [RFC2052]). The SRV
record allows DNS resolvers to query for particular services and
underlying transports (for example, HTTP running over Transport Layer
Security (TLS) [RFC2818]) and to learn a host name and port where
that service resides in a given domain. It also provides a weighting
mechanism to allow load-balancing across several instances of a
service.
The reliance of applications on the existence of MX and SRV records
has important implications for the way that applications manage
identifiers and the way that applications pass domain names to
resolvers. Email identifiers of the form "user@domain" rely on MX
records to provide the convenience of simply specifying a "domain"
component rather than requiring an application to guess which
particular host handles mail on behalf of the domain. While naming
conventions continue to abound ("www.example.com") for applications
like web browsing, SRV records allow applications to query for an
application-specific protocol and transport in the domain. For the
Lightweight Directory Access Protocol (LDAP), the SRV service name
corresponds to the URL scheme of the identifier invoked by the
application (e.g., when "ldap://example.com" is the identifier, the
SRV query passed to the resolver is for "_ldap._tcp.example.com");
for other applications, the SRV service name that the application
passes to the resolver may be implicit in the identifier rather than
explicit. In either case, the application delivers the service name
to the DNS to find the location of the host of that service for the
domain, the port where the service resides on that host, additional
locations or ports for load-balancing and fault tolerance, and
related application features.
Locating specific services for a domain was the first major function
for which applications started using the DNS beyond simple name
resolution. SRV broadened and generalized the precedent of MX to
make service location available to any application, rather than just
to mail. As applications that acquire MX (or SRV) records might need
to perform further queries or transformations in order to arrive at
an eventual domain name that will resolve to the IP addresses for the
service, [RFC1034] allowed that the Additional (data) section of DNS
responses may contain the corresponding address records for the names
of services designated by the MX record; this optimization, which
requires support in the authoritative server and the resolver, is an
initial example of how support for application features requires
changes to DNS operation. At the same time, this is an example of an
extension of the DNS that cannot be universally relied on: many DNS
resolver implementations will ignore the addresses in the additional
section of the DNS answers because of the trustworthiness issues
described in [RFC2181].
2.2. NAPTR and DDDS
The NAPTR Resource Record evolved to fulfill a need in the transition
from Uniform Resource Locators (URLs) to the more mature Uniform
Resource Identifier (URI) [RFC3986] framework, which incorporated
Uniform Resource Names (URNs). Unlike URLs, URNs typically do not
convey enough semantics internally to resolve them through the DNS,
and consequently a separate URI-transformation mechanism is required
to convert these types of URIs into domain names. This allows
identifiers with no recognizable domain component to be treated as
domain names for the purpose of name resolution. Once these
transformations result in a domain name, applications can retrieve
NAPTR records under that name in the DNS. NAPTR records contain a
far more rich and complex structure than MX or SRV Resource Records.
A NAPTR record contains two different weighting mechanisms ("order"
and "preference"), a "service" field to designate the application
that the NAPTR record describes, and then two fields that can contain
translations: a "replacement" field or a "regexp" (regular
expression) field, only one of which appears in a given NAPTR record
(see [RFC2168]). A "replacement", like NAPTR's ancestor the PTR
record, simply designates another domain name where one would look
for records associated with this service in the domain. The
"regexp", on the other hand, allows regular expression
transformations on the original URI intended to turn it into an
identifier that the DNS can resolve.
As the abstract of [RFC2915] says, "This allows the DNS to be used to
lookup services for a wide variety of resource names (including URIs)
which are not in domain name syntax". Any sort of hierarchical
identifier can potentially be encoded as a domain name, and thus
historically the DNS has often been used to resolve identifiers that
were never devised as a name for an Internet host. A prominent early
example is found in the in-addr domain [RFC0883], in which IPv4
addresses are encoded as domain names by applying a string
preparation algorithm that required reversing the octets and treating
each individual octet as a label in a domain name -- thus, for
example, the address 192.0.2.1 became 1.2.0.192.in-addr.arpa. This
allowed resolvers to query the DNS to learn name(s) associated with
an IPv4 address. The same mechanism has been applied to IPv6
addresses [RFC3596] and other sorts of identifiers that lack a domain
component. Eventually, this idea connected with activities to create
a system for resolving telephone numbers on the Internet, which
became known as ENUM (originally described in [RFC2916]). ENUM
borrowed from an earlier proposal, the "tpc.int" domain [RFC1530],
which provided a means for encoding telephone numbers as domain names
by applying a string preparation algorithm that required reversing
the digits and treating each individual digit as a label in a domain
name -- thus, for example, the number +15714345400 became
0.0.4.5.4.3.4.1.7.5.1.tpc.int. In the ENUM system, in place of
"tpc.int" the special domain "e164.arpa" was reserved for use.
In the more mature form of the NAPTR standard, in the Dynamic
Delegation Discovery System (DDDS) [RFC3401] framework, the initial
transformation of an identifier (such as a telephone number) to a
domain name was called the "First Well Known Rule". The address-
reversing mechanism, whereby a query name is formed by reversing an
IPv4 address and prepending it to the in-addr.arpa domain, is
generalized for the use of NAPTR: each application defines a "First
Well Known Rule" that translates a specific resource into a query
name. Its flexibility has inspired a number of proposals beyond ENUM
to encode and resolve unorthodox identifiers in the DNS. Provided
that the identifiers transformed by the "First Well Known Rule" have
some meaningful structure and are not overly lengthy, virtually
anything can serve as an input for the DDDS structure: for example,
civic addresses. Though [RFC3402] stipulates regarding the
identifier that "The lexical structure of this string must imply a
unique delegation path", there is no requirement that the identifier
be hierarchical nor that the points of delegation in the domain name
created by the "First Well Known Rule" correspond to any points of
administrative delegation inherent in the structure of the
identifier.
While this ability to look up names "which are not in domain name
syntax" does not change the underlying DNS protocol -- the names
generated by the DDDS algorithm are still just domain names -- it
does change the context in which applications pass name to resolvers
and can potentially require very different operational practices of
zone administrators (see Section 3.3). In terms of the results of a
DNS query, the presence of the "regexp" field of NAPTR records
enabled unprecedented flexibility in the types of identifiers that
applications could resolve with the DNS. Since the output of the
regular expression frequently took the form of a URI (in ENUM
resolution, for example, a telephone number might be converted into a
SIP URI [RFC3261]), anything that could be encoded as a URI might be
the result of resolving a NAPTR record -- which, as the next section
explores, essentially means arbitrary data.
2.3. Arbitrary Data in the DNS
URI encoding has ways of encapsulating basically arbitrary data: the
most extreme example is a data URL [RFC2397]. Thus, the returned
NAPTR record might be interpreted to produce output other than a
domain name that would subsequently be resolved to IP addresses and
contacted for a particular application -- it could give a literal
result that would be consumed by the application. Originally, as
discussed in [RFC2168], the intended applicability of the regular
expression field in NAPTR was narrower: the "regexp" field contained
a "substitution expression that is applied to the original URI in
order to construct the next domain name to lookup", in order to
"change the host that is contacted to resolve a URI" or as a way of
"changing the path or host once the URL has been assigned". The
regular expression tools available to NAPTR record authors, however,
grant much broader powers to alter the input string, and thus
applications began to rely on NAPTR to perform more radical
transformations that did not serve any of those aforementioned needs.
According to [RFC3402], the output of DDDS is wholly application-
specific: "the Application must define what the expected output of
the Terminal Rule should be", and the example given in the document
is one of identifying automobile parts by inputting a part number and
receiving at the end of the process information about the
manufacturer.
Historically speaking, NAPTR did not pioneer the storage of arbitrary
data in the DNS. At the start, [RFC0882] observed that "it is
unlikely that all users of domain names will be able to agree on the
set of resources or resource information that names will be used to
retrieve", and consequently places little restriction on the
information that DNS records might carry: it might be "host
addresses, mailbox data, and other as yet undetermined information".
[RFC1035] defined the TXT record, a means to store arbitrary strings
in the DNS; [RFC1035] also specifically stipulates that a TXT
contains "descriptive text" and that "the semantics of the text
depends on the domain where it is found". The existence of TXT
records has long provided new applications with a rapid way of
storing data associated with a domain name in the DNS, as adding data
in this fashion requires no registration process. [RFC1464]
experimented with a means of incorporating name/value pairs to the
TXT record structure, which allowed applications to distinguish
different chunks of data stored in a TXT record -- surely not just
"descriptive text" as the TXT originally specified. In this fashion,
an application that wants to store additional data in the DNS can do
so without registering a new resource record type, though [RFC5507]
points out that it is "difficult to reliably distinguish one
application's record from others, and for its parser to avoid
problems when it encounters other TXT records".
While open policies surrounding the use of the TXT record have
resulted in a checkered past for standardizing application usage of
TXT, TXT has been used as a technical solution for many applications.
Recently, DKIM [RFC6376] sidestepped the problem of TXT ambiguity by
storing keys under a specialized DNS naming structure that includes
the component "_domainkeys", which serves to restrict the scope of
that TXT solely to DKIM use. Storing keys in the DNS became the
preferred solution for DKIM for several reasons: notably, because
email applications already queried the DNS in their ordinary
operations, because the public keys associated with email required
wide public distribution, and because email identifiers contain a
domain component that applications can easily use to consult the DNS.
If the application had to negotiate support for the DKIM mechanism
with mail servers, it would give rise to bid-down attacks (where
attackers misrepresent that DKIM is unsupported on the originating
side) that are not possible if the DNS delivers the keys (provided
that DNSSEC [RFC4033] guarantees authenticity of the data). However,
there are potential issues with storing large data in the DNS, as
discussed in Section 3.2.1, as well as with the DKIM namespace
conventions that complicate the use of DNS wildcards (as discussed in
Section 6.1.2 of [RFC6376] and in more general terms in [RFC5507]).
If prefixes are used to identify TXT records used by an application,
potentially the use of wildcards may furthermore cause leakages that
other applications will need to detect.
3. Challenges for the DNS
The methods discussed in the previous section for transforming
arbitrary identifiers into domain names and returning arbitrary data
in response to DNS queries both represent significant departures from
the basic function of translating host names to IP addresses, yet
neither fundamentally alters the underlying semantics of the DNS.
When we consider, however, that the URIs returned by DDDS might be
base-64-encoded binary data in a data URL, the DNS could effectively
implement the entire application feature set of any simple query-
response protocol. Effectively, the DDDS framework considers the DNS
a generic database -- indeed, the DDDS framework was designed to work
with any sort of underlying database; as [RFC3403] says, the DNS is
only one potential database for DDDS to use. Whether the DNS as an
underlying database can support the features that some applications
of DDDS require, however, is a more complicated question.
As the following subsections will show, the potential for
applications to rely on the DNS as a generic database gives rise to
additional requirements that one might expect to find in a database
access protocol: authentication of the source of queries for
comparison to access control lists, formulating complex relational
queries, and asking questions about the structure of the database
itself. The global public DNS was not designed to provide these
sorts of properties, and extending the DNS protocols to encompass
them could result in a fundamental alteration to its model.
Ultimately, this document concludes that efforts to retrofit these
capabilities into the DNS would be better invested in selecting, or
if necessary inventing, other Internet services with broader powers
than the DNS. If an application protocol designer wants these
properties from a database, in general this is a good indication that
the DNS cannot, or can only partly, meet the needs of the application
in question.
Since many of these new requirements have emerged from the ENUM
space, the following sections use ENUM as an illustrative example;
however, any application using the DNS as a feature-rich database
could easily end up with similar requirements.
3.1. Compound Queries
Traditionally, DNS RRsets are uniquely identified by domain name,
resource record type, and class. DNS queries are based on this
3-tuple, and the replies are resource record sets that are to be
treated as atomic data elements (see [RFC2181]); to applications, the
behavior of the DNS has traditionally been that of an exact-match
query-response lookup mechanism. Outside of the DNS space, however,
there are plenty of query-response applications that require a
compound or relational search, one taking into account more than one
factor in formulating a response or one that uses no single factor as
a key to the database. For example, in the telephony space,
telephone call routing often takes into account numerous factors
aside from the dialed number, including originating trunk groups,
interexchange carrier selection, number portability data, time of
day, and so on. All are considered simultaneously in generating a
route. While in its original conception ENUM hoped to circumvent the
traditional Public Switched Telephone Network (PSTN) and route
directly to Internet-enabled devices, the infrastructure ENUM effort
to support the migration of traditional carrier routing functions to
the Internet aspires to achieve feature parity with traditional
number routing. However, [RFC3402] explicitly states that "it is an
assumption of the DDDS that the lexical element used to make a
delegation decision is simple enough to be contained within the
Application Unique String itself. The DDDS does not solve the case
where a delegation decision is made using knowledge contained outside
the AUS and the Rule (time of day, financial transactions, rights
management, etc.)". Consequently, some consideration has been given
to ways to append additional data to ENUM queries to give the DNS
server sufficient information to return a suitable URI (see
Section 3.1.1).
From a sheer syntactical perspective, however, domain names do not
admit of this sort of rich structure. Several workarounds have
attempted to instantiate these sorts of features in DNS queries. For
example, the domain name itself could be compounded with the
additional parameters: one could take a name like
0.0.4.5.4.3.4.1.7.5.1.e164.arpa and append a trunk group identifier
to it, for example, of the form
tg011.0.0.4.5.4.3.4.1.7.5.1.e164.arpa. While in this particular case
a DNS server can adhere to its traditional behavior in locating
resource records, the syntactical viability of encoding additional
parameters in this fashion is dubious, especially if more than one
additional parameter is required and the presence of parameters is
optional so that the application needs multiple queries to assess the
completeness of the information it needs to perform its function.
As an alternative, it has been proposed that we piggyback additional
query parameters as Extension Mechanisms for DNS (EDNS(0)) extensions
(see [RFC6891]). This might be problematic for three reasons.
First, supporting EDNS(0) extensions requires significant changes to
name server behavior; these changes need to be supported by the
authoritative and recursive name servers on which the application
relies and might be very hard to realize on a global scale. In
addition, the original stated applicability of the EDSN(0) mechanism,
as [RFC2671] states, was to "a particular transport level message and
not to any actual DNS data", and consequently the OPT Resource
Records it specifies are never to be forwarded. The use of EDNS(0)
for compound queries, however, clearly is intended to discriminate
actual DNS data rather than to facilitate transport-layer handling.
Finally, [RFC6891] also specifies that "OPT RRs MUST NOT be cached,
forwarded, or stored" (see the next paragraph). For these reasons,
this memo recommends against crafting compound DNS queries by using
EDNS(0).
The implications of these sorts of compound queries for recursion and
caching are potentially serious. The logic used by the authoritative
server to respond to a compound query may not be understood by any
recursive servers or caches; intermediaries that naively assume that
the response was selected based on the domain name, type, and class
alone might serve responses to queries in a different way than the
authoritative server intends. Therefore, were EDNS(0) to be employed
this way, its attributes would not be transitive, and if this were
not considered where intermediaries are employed, as is normally the
case in the global DNS, brokenness might occur.
3.1.1. Responses Tailored to the Originator
DNS responses tailored to the identity of their originator, where
some sort of administrative identity of the originator must be
conveyed to the DNS, constitute the most important subcase of these
compound queries. We must first distinguish this from cases where
the originating IP address or a similar indication is used to serve a
location-specific name. For those sorts of applications, which
generally lack security implications, relying on factors like the
source IP address introduces little harm; for example, when providing
a web portal customized to the region of the client, it would not be
a security breach if the client saw the localized portal of the wrong
country. Because recursive resolvers may obscure the origination
network of the DNS client, a recent proposal suggested introducing a
new DNS query parameter to be populated by DNS recursive resolvers in
order to preserve the originating IP address (see [EDNS-CLIENT-IP]).
However, aside from purely cosmetic uses, these approaches have known
limitations due to the prevalence of private IP addresses, VPNs, and
so on, which obscure the source IP address and instead supply the IP
address of an intermediary that may be very distant from the
originating endpoint. Implementing technology such as the one
described by [EDNS-CLIENT-IP] would require significant changes in
the operation of recursive resolvers and the authoritative servers
that would rely on the original source IP address to select resource
records, and moreover a fundamental change to caching behavior as
well. As a result, such technology cannot be rolled out in an
incremental, unilateral fashion but could only be successful when
implemented bilaterally (by authoritative server and recursive
resolver); this is a significant bar to deployment.
In other deployments in use today, including those based on the BIND
"views" feature, the source IP address is used to grant access to a
selected, and potentially sensitive, set of resource records. The
security implications of trusting the source IP address of a DNS
query have prevented most solutions along these lines from being
standardized (see [RFC6269]), though the practice remains widespread
in "split horizon" private DNS deployments (see Section 4), which
typically rely on an underlying security layer, such as a physical
network, a clear perimeter demarcation at a network perimeter point
(with network-layer anti-spoofing countermeasures), or an IPsec VPN,
to prevent spoofing of the source IP address. These deployments do
have a confidentiality requirement to prevent information intended
for a constrained audience (internal to an enterprise, for example)
from leaking to the public Internet -- while these internal network
resources may use private IP addresses that should not be useful on
the public Internet anyway, in some cases this leakage would reveal
topology or other information that the name server administrator
hopes to keep private. More recently, TSIG [RFC2845] has been
employed as a way of selecting among "views" in BIND; this provides a
stronger level of security than merely relying on the source IP
address, but typically many users share the same secret to access a
given view, and moreover TSIG does not provide confidentiality
properties to DNS messages -- without network-layer separation
between users of different views, eavesdroppers might capture the DNS
queries and responses.
The use of source IP addresses as a discriminator to select DNS
resource records, regardless of its lack of acceptance by the
standards community, has widespread acceptance in the field. Some
applications, however, go even further and propose extending the DNS
to add an application-layer identifier of the originator; for
example, [EDNS-OPT-CODE] provides a SIP URI in an EDNS(0) parameter.
Effectively, this conveyance of application-layer information about
the administrative identity of the originator through the DNS is a
weak authentication mechanism, on the basis of which the DNS server
makes an authorization decision before sharing resource records.
This can approximate a confidentiality mechanism per resource record,
where only a specific set of originators is permitted to see resource
records, or a case where a query for the same name by different
entities results in completely different resource record sets.
However, without any underlying cryptographic security, this
mechanism must rely on external layers for security (such as VPNs)
rather than any direct assurance. Again, caching, forwarding, and
recursion introduce significant challenges for applications that
attempt to offload this responsibility to the DNS. Achieving feature
parity with even the simplest authentication mechanisms available at
the application layer would likely require significant rearchitecture
of the DNS.
3.2. Using DNS as a Generic Database
As previously noted, applications can use a method like the "First
Well Known Rule" of DDDS to transform an arbitrary string into a
domain name and then receive from the DNS arbitrary data stored in
TXT RRs, in the "regexp" of NAPTRs, or even in custom records. Some
query-response applications, however, require queries and responses
that simply fall outside the syntactic capabilities of the DNS. For
example, domain names themselves must consist of labels that do not
exceed 63 octets, while the total length of the encoded name may not
exceed 255 octets, and applications that use label characters outside
the traditional ASCII set may run into problems (however, see the
discussion in [RFC6055], Section 3 for definitive guidance on the use
of non-ASCII in the DNS). The DNS therefore cannot be a completely
generic database. Similar concerns apply to the size of DNS
responses.
3.2.1. Large Data in the DNS
While the "data" URL specification [RFC2397] notes that it is "only
useful for short values", unfortunately it gives no particular
guidance about what "short" might mean. Some applications today use
quite large data URLs (containing a megabyte or more of data) as
workarounds in environments where only URIs can syntactically appear
(for example, in Apple iOS, to pass objects between applications).
The meaning of "short" in an application context is probably very
different from how we should understand it in a DNS message.
Referring to a typical public DNS deployment, [RFC5507] observes that
"there's a strong incentive to keep DNS messages short enough to fit
in a UDP datagram, preferably a UDP datagram short enough not to
require IP fragmentation". And while EDNS(0) allows for mechanisms
to negotiate DNS message sizes larger than the traditional 512
octets, there is a high risk that a long payload will cause UDP
fragmentation, in particular when the DNS message already carries
DNSSEC information. If EDNS(0) is not available, or the negotiated
EDNS(0) packet size is too small to fit the data, or UDP fragments
are dropped, the DNS may (eventually) resort to using TCP. While TCP
allows DNS responses to be quite long, this requires stateful
operation of servers, which can be costly in deployments where
servers have only fleeting connections to many clients. Ultimately,
there are forms of data that an application might store in the DNS
that exceed reasonable limits: in the ENUM context, for example,
something like storing base-64-encoded mp3 files of custom ringtones.
Designs relying on storage of large amounts of data within DNS RRs
furthermore need to minimize the potential damage achievable in a
reflection attack (see [RFC4732], Section 3), in which the attacker
sends UDP-only DNS queries with a forged source address, and the
victim receives the response. The attacker relies on amplification,
where a small query generates a large response directed at the
victim. Where the responder supports EDNS(0), an attacker may set
the requester maximum payload size to a larger value while querying
for a large resource record, such as a certificate [RFC4398]. Thus,
the combination of large data stored in DNS RRs and responders
supporting large payload sizes has the potential to increase the
potential damage achievable in a reflection attack.
Since a reflection attack can be launched from any network that does
not implement source address validation, these attacks are difficult
to eliminate absent the ubiquitous deployment of source address
validation or "heavier" transport protocols such as TCP. The
bandwidth that can be mustered in a reflective amplification attack
directed by a botnet reflecting off a recursive name server on a
high-bandwidth network is sobering. For example, if the responding
resolver could be directed to generate a 10KB response in reply to a
50-octet query, then magnification of 200:1 would be attainable.
This would enable a botnet controlling 10000 hosts with 1 Mbps of
bandwidth to focus 200 Gbps of traffic on the victim, more than
sufficient to congest any site on today's Internet.
DNS reflection attacks typically utilize UDP queries; it is
prohibitively difficult to complete a TCP three-way handshake begun
from a forged source address for DNS reflection attacks. Unless the
attacker uses EDNS(0) [RFC6891] to enlarge the requester's maximum
payload size, a response can only reach 576 octets before the
truncate bit is set in the response. This limits the maximum
magnification achievable from a DNS query that does not utilize
EDNS(0). As the large disparity between the size of a query and size
of the response creates this amplification, techniques for mitigating
this disparity should be further studied, though this is beyond the
scope of this memo (for an analysis of the effects of limiting
EDNS(0) responses while still accommodating DNSSEC, see [Lindsay]).
For example, some implementations could limit EDNS(0) responses to a
specific ratio compared to the request size, where the precise ratio
can be configured on a per-deployment basis (taking into account
DNSSEC response sizes). Without some means of mitigating the
potential for amplification, EDNS(0) could cause significant harm.
In summary, there are two operational forces that tend to drive the
practically available EDNS(0) sizes down: possible UDP fragmentation
and minimizing amplification in case of reflection attacks. DNSSEC
data will use a significant fraction of the available space in a DNS
packet. Therefore -- appreciating that given the current DNSSEC and
EDNS(0) deployment experience, precise numbers are impossible to give
-- the generic payload available to other DNS data, given the premise
that TCP fallback is to be minimized, is likely to be closer to
several hundred octets than a few thousand octets.
3.3. Administrative Structures Misaligned with the DNS
While the DDDS framework enables any sort of alphanumeric data to
serve as a domain name through the application of the "First Well
Known Rule", the delegative structure of the resulting domain name
may not reflect any administrative division of responsibilities
inherent in the original data. While [RFC3402] requires only that
the "Application Unique String has some kind of regular, lexical
structure that the rules can be applied to", DDDS is first and
foremost a delegation system: its abstract stipulates that
"Well-formed transformation rules will reflect the delegation of
management of information associated with the string". Telephone
numbers in the United States, for example, are assigned and delegated
in a relatively complex manner. Historically, the first six digits
of a nationally specific number (called the "NPA/NXX") reflected a
point of administrative delegation from the number assignment agency
to a carrier; from these blocks of ten thousand numbers, the carrier
would in turn delegate individual assignments of the last four digits
(the "XXXX" portion) to particular customers. However, after the
rise of North American telephone number portability in the 1990s, the
first point of delegation went away: the delegation is effectively
from the number authority to the carrier for each complete ten-digit
number (NPA/NXX-XXXX). While technical implementation details differ
from nation to nation, number portability systems with similar
administrative delegations now exist worldwide.
To render these identifiers as domain names in accordance with the
DDDS Rule for ENUM yields a large flat administrative domain with no
points of administrative delegation from the country-code
administrator, e.g., 1.e164.arpa, down to the ultimate assignee of a
number. Under the assumption that one administrative domain is
maintained within one DNS zone containing potentially over five
billion names, scalability difficulties manifest in a number of
areas: the scalability that results from caching depends on these
points of delegation, so that cached results for intermediate servers
take the load off higher-tier servers. If there are no such
delegations, then as in the telephone number example the zone apex
server must bear the entire load for queries. Worse still, number
portability also introduces far more dynamism in number assignment,
where in some regions updated assignees for ported numbers must
propagate within fifteen minutes of a change in administration.
Jointly, these two problems make caching the zone extremely
problematic. Moreover, traditional tools for DNS replication, such
as the zone transfer protocols AXFR [RFC1034] and IXFR [RFC1995],
might not scale to accommodate zones with these dimensions and
properties.
In practice, the maximum sizes of telephone number administrative
domains are closer to 300M (the current amount of allocated telephone
numbers in the United States today -- still more than three times the
number of .com domain names), and one can alleviate some of the
scalability issues mentioned above by artificially dividing the
administrative domain into a hierarchy of DNS zones. Still, the fact
that traditional DNS management tools no longer apply to the
structures that an application tries to provision in the DNS is a
clue that the DNS might not be the right place for an application to
store its data.
While DDDS is the most obvious example of these concerns, the point
is more generic: for example, were address portability to be
implemented for IP addresses and their administration thus to become
non-hierarchical, the same concerns would apply to in-addr.arpa
names. The difficulty of mapping the DNS to administrative
structures can even occur with traditional domain names, where
applications expect clients to infer or locate zone cuts. In the web
context, for example, it can be difficult for applications to
determine whether two domains represent the same "site" when
comparing security credentials with URLs (see Section 3.4 below for
more on this). This has also caused known problems in determining
the scope of web cookies, in contexts where applications must infer
where administrative domains end in order to grant cookies that are
as narrowly scoped as possible.
In summary, the "First Well Known Rule" of DDDS provides a capability
that transforms arbitrary strings into domain names, but those names
play well with the DNS only when the input strings have an
administrative structure that maps to DNS delegations. In the first
place, delegation implies some sort of hierarchical structure. Any
mechanism to map a hierarchical identifier into a domain name should
be constructed such that the resulting domain name does match the
natural hierarchy of the original identifier. Although telephone
numbers, even in North America, have some hierarchical qualities
(like the geographical areas corresponding to their first three
digits), after the implementation of number portability these points
no longer mapped onto an administrative delegation. If the input
string to the DDDS does not have a hierarchical structure
representing administrative delegations that can map onto the DNS
distribution system, then that string probably is not a good
candidate for translating into a domain name.
3.3.1. Metadata about Tree Structure
There are also other ways in which the delegative structure of an
identifier may not map well onto the DNS. Traditionally, DNS
resolvers assume that when they receive a domain name from an
application the name is complete -- that it is not a fragment of a
domain name that a user is still in the middle of typing. For some
communications systems, however, this assumption does not apply.
ENUM use cases have surfaced a couple of optimization requirements to
reduce unnecessary calls and queries; proposed solutions include
metadata in the DNS that describes the contents and structure of ENUM
DNS trees to help applications handle incomplete queries or queries
for domains not in use.
In particular, the "send-n" proposal [ENUM-Send-N] hoped to reduce
the number of DNS queries sent in regions with variable-length
numbering plans. When a dialed number potentially has a variable
length, a telephone switch ordinarily cannot anticipate when a dialed
number is complete, as only the numbering plan administrator (who may
be a national regulator, or even in open number plans a private
branch exchange) knows how long a telephone number needs to be.
Consequently, a switch trying to resolve such a number through a
system like ENUM might send a query for a telephone number that has
only partially been dialed in order to test its completeness. The
send-n proposal installs in the DNS a hint informing the telephone
switch of the minimum number of digits that must be collected by
placing in zones corresponding to incomplete telephone numbers some
resource records that state how many more digits are required --
effectively how many steps down the DNS tree one must take before
querying the DNS again. Unsurprisingly, those boundaries reflect
points of administrative delegation, where the parent in a number
plan yields authority to a child. With this information, the
application is not required to query the DNS every time a new digit
is dialed but can wait to collect sufficient digits to receive a
response. As an optimization, this practice thus saves the resources
of the DNS server, though the call cannot complete until all digits
are collected, so this mechanism simply reduces the time the system
will wait before sending an ENUM query after collecting the final
dialed digit. A tangentially related proposal, [ENUM-UNUSED],
similarly places resource records in the DNS that tell the
application that it need not attempt to reach a number on the
telephone network, as the number is unassigned -- a comparable
general DNS mechanism would identify, for a domain name with no
records available in the DNS, whether or not the domain had been
allocated by a registry to a registrant (which is a different
condition than a name merely being unresolvable, per the semantics of
NXDOMAIN).
Both proposals optimize application behavior by placing metadata in
the DNS that predicts the success of future queries or application
invocation by identifying points of administrative delegation or
assignment in the tree. In some respects, marking a point in the
tree where a zone begins or ends has some features in common with the
traditional parent zone use of the NS record type, except that
instead of pointing to a child zone these metadata proposals point to
distant grandchildren. While this does not change resolver behavior
as such (instead, it changes the way that applications invoke the
resolver), it does have implications for the practices for zone
administrators. Metadata in one administrative domain would need to
remain synchronized with the state of the resources it predicts in
another administrative domain in the DNS namespace, in a case like
overlap dialing where the carrier delegates to a zone controlled by
an enterprise. When dealing with external resources associated with
other namespaces, like number assignments in the PSTN or the
databases of a registry operator, other synchronization requirements
arise; maintaining that synchronization requires that the DNS have
"semi-real time" updates that may conflict with scale and caching
mechanisms of the DNS.
Placing metadata in the DNS may also raise questions about the
authority and delegation model. Who gets to supply records for
unassigned names? While in the original but little-used e164.arpa
root of ENUM this would almost certainly be a numbering plan
administrator, it is far less clear who that would be in the more
common and successful "infrastructure" ENUM models (see Section 4).
Ultimately, these metadata proposals share some qualities with DNS
redirection services offered by ISPs (for example, [DNS-REDIRECT])
that "help" web users who try to browse to sites that do not exist.
Similarly, metadata proposals like [ENUM-UNUSED] create DNS records
for unallocated zones that redirect to a service provider's web page.
However, in the [DNS-REDIRECT] cases, at least the existence or
non-existence of a domain name is a fact about the Internet
namespace, rather than about an external namespace like the telephony
E.164 namespace (which must be synchronized with the DNS tree in the
metadata proposals). In send-n, different leaf zones that administer
telephone numbers of different lengths can only provision their hints
at their own apex, which provides an imperfect optimization: they
cannot install it themselves in a parent, both because they lack the
authority and because different zones would want to provision
contradictory data. The later the hint appears in the tree, however,
the less optimization will result. An application protocol designer
managing identifiers whose administrative model does not map well
onto the DNS namespace and delegations structure would be better
served to implement such features outside the DNS.
3.4. Domain Redirection
Most Internet application services provide a redirection feature --
when one attempts to contact a service, the service may refer the
person to a different service instance, potentially in another
domain, that is for whatever reason better suited to service a
request. In HTTP and SIP, for example, this feature is implemented
by the 300 class responses containing one or more URIs, which may
indicate that a resource has moved temporarily or permanently to
another service. Several tools in the DNS, including the SRV record,
can provide a similar feature at a DNS level, and consequently some
applications as an optimization offload the responsibility for
redirection to the DNS; NAPTR can also provide this capability on a
per-application basis, and numerous DNS resource records can provide
redirection on a per-domain basis. This can prevent the unnecessary
expenditure of application resources on a function that could be
performed as a component of a DNS lookup that is already a
prerequisite for contacting the service. Consequently, in some
deployment architectures this DNS-layer redirection is used for
virtual hosting services.
Implementing domain redirection in the DNS, however, has important
consequences for application security. In the absence of universal
DNSSEC, applications must blindly trust that their request has not
been hijacked at the DNS layer and redirected to a potentially
malicious domain, unless some subsequent application mechanism can
provide the necessary assurance. By way of contrast, application-
layer protocols supporting redirection, such as HTTP and SIP, have
available security mechanisms, including TLS, that can use
certificates to attest that a 300 response came from the domain that
the originator initially hoped to contact.
A number of applications have attempted to provide an after-the-fact
security mechanism that verifies the authority of a DNS delegation in
the absence of DNSSEC. The specification for dereferencing SIP URIs
([RFC3263], reaffirmed in [RFC5922]), requires that during TLS
establishment the site eventually reached by a SIP request present a
certificate corresponding to the original URI expected by the user;
this requires a virtual hosting service to possess a certificate
corresponding to the hosted domain. (In other words, if example.com
redirects to example.net in the DNS, this mechanism expects that
example.net will supply a certificate for example.com in TLS, per the
HTTP precedent in [RFC2818]). This restriction rules out many styles
of hosting deployments common in the web world today, however.
[HARD-PROBLEM] explores this problem space. [RFC6125] proposes a
solution for all applications that use TLS, which relies on new
application-specific identifiers in certificates, as does [RFC4985]);
note, however, that support for such certificates would require
changes to existing certificate authority practices as well as
application behavior. With DNSSEC in place, DNS-based Authentication
of Named Entities (DANE) [RFC6394] offers another way to bind
certificates to particular applications and services.
All of these application-layer measures attempt to mirror the
delegation of administrative authority in the DNS, when the DNS
itself serves as the ultimate authority on how domains are delegated.
(Note: changing the technical delegation structure by changing NS
records in the DNS is not the same as administrative delegation,
e.g., when a domain changes ownership.) Synchronizing a static
instrument like a certificate with a delegation in the DNS, however,
is problematic because delegations are not static: revoking and
reissuing a certificate every time an administrative delegation
changes is cumbersome operationally. In environments where DNSSEC is
not available, the problems with securing DNS-layer redirections
would be avoided by performing redirections in the application layer.
This inevitably gives rise to various design trade-offs involving
performance, load, and related factors, but in these application
environments, the security properties typically take priority.
4. Private DNS and Split Horizon
The classic view of the uniqueness of domain names in the DNS is
given in [RFC2826]:
DNS names are designed to be globally unique, that is, for any one
DNS name at any one time there must be a single set of DNS records
uniquely describing protocol addresses, network resources and
services associated with that DNS name. All of the applications
deployed on the Internet which use the DNS assume this, and
Internet users expect such behavior from DNS names.
[RFC2826] "does not preclude private networks from operating their
own private name spaces" but notes that if private networks "wish to
make use of names uniquely defined for the global Internet, they have
to fetch that information from the global DNS naming hierarchy".
There are various DNS deployments outside of the global public DNS,
including "split horizon" deployments and DNS usages on private (or
virtual private) networks. In a split horizon, an authoritative
server gives different responses to queries from the public Internet
than they do to internal resolvers; while some deployments
differentiate internal queries from public queries by the source IP
address, the concerns in Section 3.1.1 relating to trusting source IP
addresses apply to such deployments. When the internal address space
range is private [RFC1918], this makes it both easier for the server
to discriminate public from private and harder for public entities to
impersonate nodes in the private network. Networks that are made
private physically, or logically by cryptographic tunnels, make these
private deployments more secure. The most complex deployments along
these lines use multiple virtual private networks to serve different
answers for the same name to many distinct networks.
The use cases that motivate split-horizon DNS typically involve
restricting access to some network services -- intranet resources
such as internal web sites, development servers, or directories, for
example -- while preserving the ease of use offered by domain names
for internal users. While for many of these resources the split
horizon would not return answers to public resolvers for those
internal resources (those records are kept confidential from the
public), in some cases the same name (e.g., "mail.example.com") might
resolve to one host internally but another externally. The
requirements for multiple-VPN private deployments, however, are
different: in this case the authoritative server gives different, and
confidential, answers to a set of resolvers querying for the same
name. While these sorts of use cases rarely arise for traditional
domain names, where, as [RFC2826] says, users and applications expect
a unique resolution for a name, they can easily arise when other
sorts of identifiers have been translated by a mechanism such as the
"First Well Known Rule" of DDDS into "domain name syntax". Telephone
calls, for example, are traditionally routed through highly mediated
networks, in which an attempt to find a route for a call often
requires finding an appropriate intermediary based on the source
network and location rather than finding an endpoint (see the
distinction between the Look-Up Function and Location Routing
Function in [RFC5486]). Moreover, the need for responses tailored to
the originator, and for confidentiality, is easily motivated when the
data returned by the DNS is no longer "describing protocol addresses,
network resources and services" [RFC2826] but instead is arbitrary
data. Although, for example, ENUM was originally intended for
deployment in the global public root of the DNS (under e164.arpa),
the requirements of maintaining telephone identifiers in the DNS
quickly steered operators into private deployments.
In private environments, it is also easier to deploy any necessary
extensions than it is in the public DNS: in the first place,
proprietary non-standard extensions and parameters can more easily be
integrated into DNS queries or responses, as the implementations of
resolvers and servers can likely be controlled; secondly,
confidentiality and custom responses can be provided by deploying,
respectively, underlying physical or virtual private networks to
shield the private tree from public queries, and effectively
different virtual DNS trees for each administrative entity that might
launch a query; thirdly, in these constrained environments, caching
and recursive resolvers can be managed or eliminated in order to
prevent any unexpected intermediary behavior. While these private
deployments serve an important role in the marketplace, there are
risks in using techniques intended only for deployment in private and
constrained environments as the basis of a standard solution. When
proprietary parameters or extensions are deployed in private
environments, experience teaches us that these implementations will
begin to interact with the public DNS and that the private practices
will leak.
While such leakage is not a problem when using the mechanisms
described in Sections 3.1, 3.2, and 3.5 (with private RR types) of
[RFC5507], other extension mechanisms might cause confusion or harm
if leaked. The use of a dedicated suffix (Section 3.3 of [RFC5507])
in a private environment might cause confusion if leaked to the
public Internet, for example.
That this kind of leakage of protocol elements from private
deployments to public deployments does happen has been demonstrated,
for example, with SIP: SIP implemented a category of supposedly
private extensions ( the "P-" headers) that saw widespread success
and use outside of the constrained environments for which they were
specifically designed. There is no reason to think that
implementations with similar "private" extensions to the DNS
protocols would not similarly end up in use in public environments.
5. Principles and Guidance
The success of the global public DNS relies on the fact that it is a
distributed database, one that has the property that it is loosely
coherent and offers lookup instead of search functionality. Loose
coherency means that answers to queries are coherent within the
bounds of data replication between authoritative servers (as
controlled by the administrator of the zone) and caching behavior by
recursive name servers. Today, this term increasingly must also
include load-balancing or related features that rely on the source IP
address of the resolver. It is critical that application designers
who intend to use the DNS to support their applications consider the
implications that their uses have for the behavior of resolvers;
intermediaries, including caches and recursive resolvers; and
authoritative servers.
It is likely that the DNS provides a good match whenever the needs of
applications are aligned with the following properties:
o Data stored in the DNS can be propagated and cached using
conventional DNS mechanisms, without intermediaries needing to
understand exceptional logic (considerations beyond the name,
type, and class of the query) used by the authoritative server to
formulate responses
o Data stored in the DNS is indexed by keys that do not violate the
syntax or semantics of domain names
o Applications invoke the DNS to resolve complete names, not
fragments
o Answers do not depend on an application-layer identity of the
entity doing the query
o Ultimately, applications invoke the DNS to assist in communicating
with a service whose name is resolved through the DNS
Whenever one of the five properties above does not apply to one's
data, one should seriously consider whether the DNS is the best place
to store actual data. On the other hand, if one has to worry about
the following items, then these items are good indicators that the
DNS is not the appropriate tool for solving problems:
o Populating metadata about domain boundaries within the tree -- the
points of administrative delegation in the DNS are something that
applications are not in general aware of
o Domain names derived from identifiers that do not share a semantic
or administrative model compatible with the DNS
o Selective disclosure of data stored in and provided by the DNS
o DNS responses not fitting into UDP packets, unless EDNS(0) is
available, and only then with the caveats discussed in
Section 3.2.1
In cases where applications require these sorts of features, they are
likely better instantiated by independent application-layer protocols
than the DNS. For example, the objects that HTTP can carry in both
queries and responses can easily contain the necessary structure to
manage compound queries. Many applications already use HTTP because
of widespread support for it in middleboxes. Similarly, HTTP has
numerous ways to provide the necessary authentication, authorization,
and confidentiality properties that some features require, as well as
the redirection properties discussed in Section 3.4. These
differences highlight the fact that the DNS and HTTP offer very
different services and have different applicabilities; while both are
query-response protocols, HTTP should not be doing the job of DNS,
and DNS should not be doing the job of HTTP. Similarly, DNS should
not be doing the job of Diameter, LDAP, or other application-layer
protocols. The overhead of using any application-layer protocol may
not be appropriate for all environments, of course, but even in
environments where a more lightweight protocol is appropriate, DNS is
usually not the only alternative.
Where the administrative delegations of the DNS form a necessary
component in the instantiation of an application feature, there are
various ways that the DNS can bootstrap access to an independent
application-layer protocol better suited to field the queries in
question. For example, since NAPTR or URI [URI-RR] Resource Records
can contain URIs, those URIs can in turn point to an external query-
response service such as an HTTP service where more syntactically and
semantically rich queries and answers might be exchanged. Any
protocol designer considering where to implement features must
perform their own gap analysis and determine whether or not
implementing some features is worth the potential cost in increased
network state, latency, and so on, but implementing some features
simply requires heavier structures than others.
6. Security Considerations
Many of the concerns about how applications use the DNS discussed in
this document involve security. The perceived need to authenticate
the source of DNS queries (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. As Section 3.2.1 observes, large data in the DNS can
provide a means of generating reflection attacks, and without the
remedies discussed in that section (regarding the use of EDNS(0) and
TCP) the presence of large sets of records (e.g., ANY queries) is not
recommended. Section 3.4 discusses a security problem concerning
redirection that has surfaced in a number of protocols (see
[HARD-PROBLEM]).
While DNSSEC, were it deployed universally, can play an important
part in securing application redirection in the DNS, DNSSEC does not
provide a means for a resolver to authenticate itself to a server,
nor a framework for servers to return selective answers based on the
authenticated identity of resolvers, nor a confidential mechanism.
Some implementations may support authenticating users through TSIG,
provided that the security association with a compliant server has
been pre-established, though authentication is typically not needed
for queries in the global public DNS. The existing feature set of
DNSSEC is, however, sufficient for providing security for most of the
ways that applications traditionally have used the DNS. The
deployment of DNSSEC ([RFC4033] and related specifications) is
heartily encouraged. Nothing in this document is intended to
discourage the implementation, deployment, or use of Secure DNS
Dynamic Updates [RFC3007], though this document does recommend that
large data in the DNS be treated in accordance with the guidance in
Section 3.2.1.
7. IAB Members at the Time of Approval
Internet Architecture Board Members at the time this document was
approved were:
Bernard Aboba
Jari Arkko
Marc Blanchet
Ross Callon
Alissa Cooper
Spencer Dawkins
Joel Halpern
Russ Housley
David Kessens
Danny McPherson
Jon Peterson
Dave Thaler
Hannes Tschofenig
8. Acknowledgements
The IAB appreciates the comments and often spirited disagreements of
Eric Osterweil, John Levine, Stephane Bortzmeyer, Ed Lewis, Dave
Crocker, Ray Bellis, Lawrence Conroy, Ran Atkinson, Patrik Faltstrom,
and Eliot Lear.
9. Informative References
[DNS-REDIRECT]
Creighton, T., Griffiths, C., Livingood, J., and R. Weber,
"DNS Redirect Use by Service Providers", Work in Progress,
October 2010.
[EDNS-CLIENT-IP]
Contavalli, C., van der Gaast, W., Leach, S., and D.
Rodden, "Client IP information in DNS requests", Work in
Progress, May 2010.
[EDNS-OPT-CODE]
Kaplan, H., Walter, R., Gorman, P., and M. Maharishi,
"EDNS Option Code for SIP and PSTN Source Reference Info",
Work in Progress, October 2011.
[ENUM-Send-N]
Bellis, R., "IANA Registrations for the 'Send-N'
Enumservice", Work in Progress, June 2008.
[ENUM-UNUSED]
Stastny, R., Conroy, L., and J. Reid, "IANA Registration
for Enumservice UNUSED", Work in Progress, March 2008.
[HARD-PROBLEM]
Barnes, R. and P. Saint-Andre, "High Assurance
Re-Direction (HARD) Problem Statement", Work in Progress,
July 2010.
[Lindsay] Lindsay, G., "DNSSEC and DNS Amplification Attacks",
April 2012.
[RFC0882] Mockapetris, P., "Domain names: Concepts and facilities",
RFC 882, November 1983.
[RFC0883] Mockapetris, P., "Domain names: Implementation
specification", RFC 883, November 1983.
[RFC0974] Partridge, C., "Mail routing and the domain system",
STD 10, RFC 974, January 1986.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC1464] Rosenbaum, R., "Using the Domain Name System To Store
Arbitrary String Attributes", RFC 1464, May 1993.
[RFC1530] Malamud, C. and M. Rose, "Principles of Operation for the
TPC.INT Subdomain: General Principles and Policy",
RFC 1530, October 1993.
[RFC1794] Brisco, T., "DNS Support for Load Balancing", RFC 1794,
April 1995.
[RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G.,
and E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC1995] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
August 1996.
[RFC2052] Gulbrandsen, A. and P. Vixie, "A DNS RR for specifying the
location of services (DNS SRV)", RFC 2052, October 1996.
[RFC2168] Daniel, R. and M. Mealling, "Resolution of Uniform
Resource Identifiers using the Domain Name System",
RFC 2168, June 1997.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC2397] Masinter, L., "The "data" URL scheme", RFC 2397,
August 1998.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, August 1999.
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC2826] Internet Architecture Board, "IAB Technical Comment on the
Unique DNS Root", RFC 2826, May 2000.
[RFC2845] Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B.
Wellington, "Secret Key Transaction Authentication for DNS
(TSIG)", RFC 2845, May 2000.
[RFC2915] Mealling, M. and R. Daniel, "The Naming Authority Pointer
(NAPTR) DNS Resource Record", RFC 2915, September 2000.
[RFC2916] Faltstrom, P., "E.164 number and DNS", RFC 2916,
September 2000.
[RFC3007] Wellington, B., "Secure Domain Name System (DNS) Dynamic
Update", RFC 3007, November 2000.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3263] Rosenberg, J. and H. Schulzrinne, "Session Initiation
Protocol (SIP): Locating SIP Servers", RFC 3263,
June 2002.
[RFC3401] Mealling, M., "Dynamic Delegation Discovery System (DDDS)
Part One: The Comprehensive DDDS", RFC 3401, October 2002.
[RFC3402] Mealling, M., "Dynamic Delegation Discovery System (DDDS)
Part Two: The Algorithm", RFC 3402, October 2002.
[RFC3403] Mealling, M., "Dynamic Delegation Discovery System (DDDS)
Part Three: The Domain Name System (DNS) Database",
RFC 3403, October 2002.
[RFC3596] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
"DNS Extensions to Support IP Version 6", RFC 3596,
October 2003.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
[RFC4367] Rosenberg, J., Ed., and IAB, "What's in a Name: False
Assumptions about DNS Names", RFC 4367, February 2006.
[RFC4398] Josefsson, S., "Storing Certificates in the Domain Name
System (DNS)", RFC 4398, March 2006.
[RFC4732] Handley, M., Ed., Rescorla, E., Ed., and IAB, "Internet
Denial-of-Service Considerations", RFC 4732,
December 2006.
[RFC4985] Santesson, S., "Internet X.509 Public Key Infrastructure
Subject Alternative Name for Expression of Service Name",
RFC 4985, August 2007.
[RFC5486] Malas, D., Ed., and D. Meyer, Ed., "Session Peering for
Multimedia Interconnect (SPEERMINT) Terminology",
RFC 5486, March 2009.
[RFC5507] IAB, Faltstrom, P., Ed., Austein, R., Ed., and P. Koch,
Ed., "Design Choices When Expanding the DNS", RFC 5507,
April 2009.
[RFC5922] Gurbani, V., Lawrence, S., and A. Jeffrey, "Domain
Certificates in the Session Initiation Protocol (SIP)",
RFC 5922, June 2010.
[RFC6055] Thaler, D., Klensin, J., and S. Cheshire, "IAB Thoughts on
Encodings for Internationalized Domain Names", RFC 6055,
February 2011.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, March 2011.
[RFC6269] Ford, M., Ed., Boucadair, M., Durand, A., Levis, P., and
P. Roberts, "Issues with IP Address Sharing", RFC 6269,
June 2011.
[RFC6376] Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
"DomainKeys Identified Mail (DKIM) Signatures", RFC 6376,
September 2011.
[RFC6394] Barnes, R., "Use Cases and Requirements for DNS-Based
Authentication of Named Entities (DANE)", RFC 6394,
October 2011.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, February 2013.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891, April 2013.
[URI-RR] Faltstrom, P. and O. Kolkman, "The Uniform Resource
Identifier (URI) DNS Resource Record", Work in Progress,
July 2013.
Authors' Addresses
Jon Peterson
NeuStar, Inc.
EMail: jon.peterson@neustar.biz
Olaf Kolkman
NLnet Labs
EMail: olaf@nlnetlabs.nl
Hannes Tschofenig
Nokia Siemens Networks
EMail: Hannes.Tschofenig@gmx.net
Bernard Aboba
Skype
EMail: Bernard_aboba@hotmail.com