Network Working Group IAB
Internet-Draft P. Faltstrom, Ed.
Intended status: Informational R. Austein, Ed.
Expires: January 15, 2009 P. Koch, Ed.
July 14, 2008
Design Choices When Expanding DNS
draft-iab-dns-choices-06
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Abstract
This note discusses how to extend the DNS with new data for a new
application. DNS extension discussions too often focus on reuse of
the TXT Resource Record Type. This document lists different
mechanisms to extend the DNS, and concludes that the use of a new DNS
Resource Record Type is the best solution.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Background . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Extension mechanisms . . . . . . . . . . . . . . . . . . . . . 5
3.1. Place selectors inside the RDATA of existing Resource
Record Types . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Add a prefix to the owner name . . . . . . . . . . . . . . 6
3.3. Add a suffix to the owner name . . . . . . . . . . . . . . 7
3.4. Add a new Class . . . . . . . . . . . . . . . . . . . . . 7
3.5. Add a new Resource Record Type . . . . . . . . . . . . . . 8
4. Zone boundaries are invisible to applications . . . . . . . . 9
5. Why adding a new Resource Record Type is the preferred
solution . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6. Conclusion and Recommendation . . . . . . . . . . . . . . . . 13
7. Creating A New Resource Record Type . . . . . . . . . . . . . 14
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
9. Security Considerations . . . . . . . . . . . . . . . . . . . 14
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
11.1. Normative References . . . . . . . . . . . . . . . . . . . 16
11.2. Informative References . . . . . . . . . . . . . . . . . . 16
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
Intellectual Property and Copyright Statements . . . . . . . . . . 18
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1. Introduction
The DNS stores multiple categories of data. The two most commonly
used categories are infrastructure data for the DNS system itself (NS
and SOA Resource Records) and data which have to do with mappings
between domain names and IP addresses (A, AAAA and PTR Resource
Records). There are other categories as well, some of which are tied
to specific applications like email (MX Resource Records), while
others are generic Resource Record Types used to convey information
for multiple protocols (SRV and NAPTR Resource Records).
When storing data in the DNS for a new application, the goal must be
to store data in such a way that the application can query for the
data it wants, while minimizing the impact on both existing
applications and the amount of extra data transfered to the client.
This implies a number of design choices have to be made, where the
most important is to ensure that an as precise selection of what data
to return must be made already in the query. A query that consists
of the triple Owner, Resource Record Type and Resource Record Class.
Historically, extending DNS to store application data tied to a
domain name has been done in different ways at different times. MX
Resource Records were created as a new Resource Record Type
specifically designed to support electronic mail. SRV records are a
generic type which use a prefixing scheme in combination with a base
domain name. NAPTR records add selection data inside the RDATA. It
is clear that the methods used to add new data types to the DNS have
been inconsistent, and the purpose of this document is to attempt to
clarify the implications of each of these methods, both for the
applications that use them and for the rest of the DNS.
This document talks extensively about use of DNS wildcards. Many
people might think use of wildcards is not something that happens
today. In reality though, wildcards are in use, especially for
certain application-specific data such as MX Resource Records.
Because of this, the choice has to be made with existence of
wildcards in mind.
Another overall issue that must be taken into account is what the new
data in the DNS are to describe. In some cases they might be
completely new data. In other cases they might be metadata tied to
data that already exist in the DNS. An example of new data is key
information for SSH and data used for authenticating sender of email
messages (metadata tied to MX Resource Records). If the new data are
tied to data that already exist in the DNS, an analysis should be
made as to whether having (for example) address records and SSH key
information in different DNS zones is a problem, or if it is a bonus,
and if it is a problem, whether the specification must require all of
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the related data to be in the same zone. One specific difference
between having the records in the same zone or not have to do with
maintenance of the records. If they are in the same zone, the same
maintainer (from a DNS perspective) manages the two records.
Specifically, they must be signed with the same DNSSEC keys if DNSSEC
is in use.
This document does not talk about what one should store in the DNS.
It also doesn't discuss whether DNS should be used for service
discovery, or whether DNS should be used for storage of data specific
for the service. In general, DNS is a protocol that, apart from
holding metadata that makes the DNS itself function (NS, SOA, DNSSEC
Resource Record Types, etc), only holds references to service
locations (SRV, NAPTR, A, AAAA Resource Record Types), but there are
exceptions (such as MX Resource Records).
2. Background
See RFC 2929 [RFC2929] for a brief summary of DNS query structure.
Readers interested in the full story should start with the base DNS
specification in RFC 1035 [RFC1035], and continue with the various
documents that update, clarify, and extend the base specification.
When composing a DNS query, the parameters used by the protocol are a
triple: a DNS name, a DNS class, and a DNS Resource Record Type.
Every Resource Record matching a particular name, type and class is
said to belong to the same Resource Record Set (RRSet), and the whole
RRSet is always returned to the client that queries for it.
Splitting an RRSet is a protocol violation (sending a partial RRSet,
not truncating the DNS response), because it can result in coherency
problems with the DNS caching mechanism. See RFC 2181 section 5
[RFC2181] for more information.
Some discussions around extensions to the DNS include arguments
around MTU size. Note that most discussions about DNS and MTU size
are about the size of the whole DNS packet, not about the size of a
single RRSet.
Almost all DNS query traffic is carried over UDP, where a DNS message
must fit within a single UDP packet. DNS response messages are
almost always larger than DNS query messages, so message size issues
are almost always about responses, not queries. The base DNS
specification limits DNS messages over UDP to 512 octets; EDNS0
[RFC2671] specifies a mechanism by which a client can signal its
willingness to receive larger responses, but deployment of EDNS0 is
not universal, in part because of firewalls that block fragmented UDP
packets or EDNS0. If a response message won't fit in a single
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packet, the name server returns a truncated response, at which point
the client may retry using TCP. DNS queries over TCP are not subject
to this length limitation, but TCP imposes significantly higher per-
query overhead on name servers than UDP. It is also the case that
the policies in deployed firewalls far too often are such that it
blocks DNS over TCP, so using TCP might not in reality be an option.
There are also risks (although possibly small) that a change of
routing while a TCP flow is open creates problems when the DNS
servers are deployed in an anycast environment.
3. Extension mechanisms
The DNS protocol is intended to be extensible to support new kinds of
data. This section examines the various ways in which this sort of
extension can be accomplished.
3.1. Place selectors inside the RDATA of existing Resource Record Types
For a given query name, one might choose to have a single RRSet (all
Resource Records sharing the same name, type, and class) shared by
multiple applications, and have the different applications use
selectors within the Resource Record data (RDATA) to determine which
records are intended for which applications. This sort of selector
mechanism is usually referred to "subtyping", because it is in effect
creating an additional type subsystem within a single DNS Resource
Record Type.
Examples of subtyping include NAPTR Resource Records [RFC3761] and
the original DNSSEC KEY Resource Record Type [RFC2535] (which was
later updated by RFC 3445 [RFC3445]).
All DNS subtyping schemes share a common weakness: With subtyping
schemes it is impossible for a client to query for just the data it
wants. Instead, the client must fetch the entire RRSet, then select
the Resource Records in which it is interested. Furthermore, since
DNSSEC signatures operate on complete RRSets, the entire RRSet must
be re-signed if any Resource Record in it changes. As a result, each
application that uses a subtyped Resource Record incurs higher
overhead than any of the applications would have incurred had they
not been using a subtyping scheme. The fact the RRSet is always
passed around as an indivisible unit increases the risk the RRSet
will not fit in a UDP packet, which in turn increases the risk that
the client will have to retry the query with TCP, which substantially
increases the load on the name server. More precisely: having one
query fail over to TCP is not a big deal, but since the typical ratio
of clients to servers in today's deployed DNS is very high, having a
substantial number of DNS messages fail over to TCP may cause the
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queried name servers to be overloaded by TCP overhead.
Because of the size limitations, using a subtyping scheme to list a
large number of services for a single domain name risks triggering
truncation and fallback to TCP, which may in turn force the zone
administrator to announce only a subset of available services.
3.2. Add a prefix to the owner name
By adding an application-specific prefix to a domain name, we get a
different name/class/type triple, and therefore a different RRSet.
One problem with adding prefixes has to do with wildcards, especially
if one has records like
*.example.com. IN MX 1 mail.example.com.
and one wants records tied to those names. Suppose one creates the
prefix "_mail". One would then have to say something like
_mail.*.example.com. IN X-FOO A B C D
but DNS wildcards only work with the "*" as the leftmost token in the
domain name (see also RFC 4592 [RFC4592]).
There have been proposals to deal with the problem that DNS wild-
cards are always terminal records. These proposals introduce an
additional set of trade-offs that would need to be taken into account
when assessing which extension mechanism to choose. Aspects of extra
response time needed to perform the extra queries, costs of pre-
calculation of possible answers, or the costs induced to the system
as a whole come to mind. At the time of writing none of these
proposals has been published as standards tracks RFCs.
Even when a specific prefix is chosen, the data will still have to be
stored in some Resource Record Type. This Resource Record Type can
either be an existing Resource Record Type that has an appropriate
format to store the data or a new Resource Record Type. One also
might nee some other selection mechanism, such as ability to
distinguish between the records in an RRSet given they have the same
Resource Record Type. Because of this, one needs to both register a
unique prefix and define what Resource Record Type is to be used for
this specific service.
If the record has some relationship with another record in the zone,
the fact that the two records can be in different zones might have
implications on the trust the application has in the records. For
example:
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example.com. IN MX 10 mail.example.com.
_foo.example.com. IN X-BAR "metadata for the mail service"
In this example, the two records might be in two different zones, and
because of this might be administered by two different organisations,
and signed by two different entities when using DNSSEC. Prefix has
lately because of these two reasons been a very interesting solution
for many protocol designers. In some cases when using TXT records
(add reference to DKIM), in other cases when adding new Resource
Record Types (SRV).
3.3. Add a suffix to the owner name
Adding a suffix to a domain name changes the name/class/type triple,
and therefore the RRSet. In this case, since the query name can be
set to exactly the data one wants the size of the RRSet is minimized.
The problem with adding a suffix is that it creates a parallel tree
within the IN class. Further, there is no technical mechanism to
ensure that the delegation for "example.com" and "example.com._bar"
are made to the same organization. Furthermore, data associated with
a single entity will now be stored in two different zones, such as
"example.com" and "example.com._bar", which, depending on who
controls "_bar", can create new synchronization and update
authorization issues.
One way of solving the administrative issues is by using the DNAME
Resource Record Type specified in RFC 2672 [RFC2672].
Even when using a different name, the data will still have to be
stored in some Resource Record Type that has an appropriate format to
store the data. This implies that one might have to mix the prefix
based selection mechanism with some other mechanism so that the right
Resource Record can be found out of many in a potential larger RRSet.
In RFC 2163 [RFC2163] an infix token is inserted directly below the
TLD, but the result is equivalent to adding a suffix to the owner
name (instead of creating a TLD one is creating a second level
domain).
3.4. Add a new Class
DNS zones are class-specific in the sense that all the records in
that zone share the same class as the zone's SOA record and the
existence of a zone in one class does not guarantee the existence of
the zone in any other class. In practice, only the IN class has ever
seen widespread deployment, and the administrative overhead of
deploying an additional class would almost certainly be prohibitive.
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Nevertheless, one could in theory use the DNS class mechanism to
distinguish between different kinds of data. However, since the DNS
delegation tree (represented by NS Resource Records) is itself tied
to a specific class, attempting to resolve a query by crossing a
class boundary may produce unexpected results because there is no
guarantee that the name servers for the zone in the new class will be
the same as the name servers in the IN class. The MIT Hesiod system
used a scheme like this for storing data in the HS class, but only on
a very small scale (within a single institution), and with an
administrative fiat requiring that the delegation trees for the IN
and HS trees be identical. The use of the HS class for such storage
of non-sensitive data was over time replaced by use of LDAP.
Even when using a different class, the data will still have to be
stored in some Resource Record Type that has an appropriate format to
store the data. This implies that one might have to mix the prefix
based selection mechanism with some other mechanism so that the right
Resource Record can be found out of many in a potential larger RRSet.
3.5. Add a new Resource Record Type
When adding a new Resource Record Type to the system, entities in
four different roles have to be able to handle the new Type:
1. There must be a way to insert the new Resource Records into the
zone of the Primary Master name server. For some server
implementations, the user interface only accepts Resource Record
Types which it understands (perhaps so that the implementation
can attempt to validate the data). Other implementations allow
the zone administrator to enter an integer for the Resource
Record Type code and the RDATA in Base64 or hexadecimal encoding
(or even as raw data). RFC 3597 [RFC3597] specifies a standard
generic encoding for this purpose.
2. A slave authoritative name server must be able to do a zone
transfer, receive the data from some other authoritative name
server, and serve data from the zone even though the zone
includes records of unknown Types. Historically, some
implementations have had problems parsing stored copies of the
zone file after restarting, but those problems have not been seen
for a few years.
3. A caching resolver (most commonly a recursive name server) will
cache the records which are responses to queries. As mentioned
in RFC 3597 [RFC3597],there are various pitfalls where a
recursive name server might end up having problems.
4. The application must be able to get the RRSet with a new Resource
Record Type. The application itself may understand the RDATA,
but the resolver library might not. Support for a generic
interface for retrieving arbitrary DNS Resource Record Types has
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been a requirement since 1989 (see RFC 1123 [RFC1123] Section
6.1.4.2). Some stub resolver library implementations neglect to
provide this functionality and cannot handle unknown Resource
Record Types, but implementation of a new stub resolver library
is not particularly difficult, and open source libraries that
already provide this functionality are available.
Historically adding a new Resource Record Type as been very
problematic. Review process has been cumbersome, DNS servers have
not been able to handle new Resource Record Types, and firewalls has
dropped queries or responses with for the firewall unknown Resource
Record Types. This is for example one of the reasons the ENUM
standard reuse the NAPTR Resource Record. A choice that today might
have been wrong, and a new resource record type could have been a
better choice.
Today, there is a requirement that DNS software can handle unknown
Resource Record Types, and investigations have shown that software
that is deployed in general do support it. Also the approval process
for new Resource Record Types has been updated so it is more
predictable on what effort is needed for various Resource Record
Types.
4. Zone boundaries are invisible to applications
Regardless of the possible choices above we have seen a number of
cases where the application made assumptions about the structure of
the namespace and the location where specific information resides.
We take a small sidestep to argue against such approaches.
The DNS namespace is a hierarchy, technically speaking. However,
this only refers to the way names are built from multiple labels.
DNS hierarchy neither follows nor implies administrative hierarchy.
That said, it cannot be assumed that data attached to a node in the
DNS tree is valid for the whole subtree. Technically, there are zone
boundaries partitioning the namespace and administrative boundaries
(or policy boundaries) may even exist elsewhere.
The false assumption has lead to an approach called "tree climbing",
where a query that does not receive a positive response (either the
requested RRSet was missing or the name did not exist) is retried by
repeatedly stripping off the leftmost label (climbing towards the
root) until the root domain is reached. Sometimes these proposals
try to avoid the query for the root or the TLD level, but still this
approach has severe drawbacks:
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o Technically, the DNS was built as a query - response tool without
any search capability [RFC3467]. Adding the search mechanism
imposes additional burden on the technical infrastructure, in the
worst case on TLD and root name servers.
o For reasons similar to those outlined in RFC 1535 [RFC1535],
querying for information in a domain outside the control of the
intended entity may lead to incorrect results and may also put
security at risk. Finding the exact policy boundary is impossible
without an explicit marker which does not exist at present. At
best, software can detect zone boundaries (e.g., by looking for
SOA Resource Records), but some TLD registries register names
starting at the second level (e.g., CO.UK), and there are various
other "registry" types at second, third or other level domains
that cannot be identified as such without policy knowledge
external to the DNS.
To restate, the zone boundary is purely a boundary that exists in the
DNS for administrative purposes, and applications should be careful
not to draw unwarranted conclusions from zone boundaries. A
different way of stating this is that the DNS does not support
inheritance, e.g. a wildcard MX RRSet for a TLD will not be valid for
any subdomain of that particular TLD.
5. Why adding a new Resource Record Type is the preferred solution
By now, the astute reader might be wondering what conclusions to draw
from the issues presented so far. We will now attempt to clear up
the reader's confusion by following the thought processes of a
typical application designer who wishes to store data in the DNS,
showing how such a designer almost inevitably hits upon the idea of
just using a TXT Resource Records, why this is a bad thing, and why a
new Resource Record Type should be allocated instead, but also
explain how to reuse an existing resource record, including TXT, can
be made less harmful.
The overall problem with most solutions has to do with two main
issues:
o No semantics to prevent collision with other use
o Space considerations in the DNS message
A typical application designer is not interested in the DNS for its
own sake, but rather regards it as a distributed database in which
application data can be stored. As a result, the designer of a new
application is usually looking for the easiest way to add whatever
new data the application needs to the DNS in a way that naturally
associates the data with a DNS name.
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As explained in Section 3.4, using the DNS class system as an
extension mechanism is not really an option, and in fact most users
of the system don't even realize that the mechanism exists. As a
practical matter, therefore any extension is likely to be within the
IN class.
Adding a new Resource Record Type is the technically correct answer
from the DNS protocol standpoint (more on this below), but doing so
requires some DNS expertise, due to the issues listed in Section 3.5.
Consequently, this option is usually not considered. Note that
according to RFC 2929 [RFC2929], some Types require IETF Consensus,
while others only require a specification.
There is a drawback to defining new RR types that is worth
mentioning. The RRTYPE is a 16 bit value and hence a a limited
resource. In order to prevent herding the registry has a review
based allocation policy [RFC2929], however this may not be sufficient
if extension of the DNS by addition of new RR types takes up
significantly and the registry starts nearing completion. In that
case the trade-offs with respect to choosing an extension mechanism
may need to change.
The application designer is thus left with the prospect of reusing
some existing DNS Type within the IN class, but when the designer
looks at the existing Types, almost all of them have well-defined
semantics, none of which quite match the needs of the new
application. This has not completely prevented proposals from
reusing existing Resource Record Types in ways incompatible with
their defined semantics, but it does tend to steer application
designers away from this approach.
For example, Resource Record Type 40 was registered for the SINK
Resource Record Type. This Resource Record Type was discussed in the
DNSIND working group of the IETF, and it was decided at the 46th IETF
to not move the I-D forward to become an RFC because of the risk of
encouraging application designers to use the SINK Resource Record
Type instead of registering a new Resource Record Type, which would
result in infeasibly large SINK RRsets.
Eliminating all of the above leaves the TXT Resource Record Type in
the IN class. The TXT RDATA format is free form text, and there are
no existing semantics to get in the way. Some attempts have been
made, for example in draft-cheshire-dnsext-dns-sd
[I-D.cheshire-dnsext-dns-sd], to specify a structured format for TXT
Resource Record Types, but no such attempt has reached RFC status.
Furthermore, the TXT Resource Record can obviously just be used as a
bucket in which to carry around data to be used by some higher level
parser, perhaps in some human readable programming or markup
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language. Thus, for many applications, TXT Resource Records are the
"obvious" choice. Unfortunately, this conclusion, while
understandable, is also wrong, for several reasons.
The first reason why TXT Resource Records are not well suited to such
use is precisely the lack of defined semantics that make them so
attractive. Arguably, the TXT Resource Record is misnamed, and
should have been called the Local Container record, because the lack
of defined semantics means that a TXT Resource Record means precisely
what the data producer says it means. This is fine, so long as TXT
Resource Records are being used by human beings or by private
agreement between data producer and data consumer. However, it
becomes a problem once one starts using them for standardized
protocols in which there is no prior relationship between data
producer and data consumer. The reason for this is that, if TXT
records are used without one of the naming modifications discussed
earlier (and in some cases even if one use such naming mechanisms),
there is nothing to prevent collisions with some other incompatible
use of TXT Resource Records. This is even worse than the general
subtyping problem described in Section 3.1, because TXT Resource
Records don't even have a standardized selector field in which to
store the subtype. RFC 1464 [RFC1464] tried, but it was not a
success. At best a definition of a subtype is reduced to hoping that
whatever scheme one has come up with will not accidently conflict
with somebody else's subtyping scheme, and that it will not be
possible to mis-parse one application's use of TXT Resource Records
as data intended for a different application. Any attempt to impose
a standardized format within the TXT Resource Record format would be
at least fifteen years too late even if it were put into effect
immediately; at best, one can restrict the syntax that a particular
application uses within a TXT Resource Record and accept the risk
that unrelated TXT Resource Record uses will collide with it.
Using one of the naming modifications discussed in Section 3.2 and
Section 3.3 would address the subtyping problem, (and have been used
in combinations with reuse of TXT record, such as for the dns/txt
lookup mechanism in DKIM) but each of these approaches brings in new
problems of its own. The prefix approach (that for example SRV
Resource Records use) does not work well with wildcards, which is a
particular problem for mail-related applications, since MX Resource
Records are probably the most common use of DNS wildcards. The
suffix approach doesn't have wildcard issues, but, as noted
previously, it does have synchronization and update authorization
issues, since it works by creating a second subtree in a different
part of the global DNS name space.
The next reason why TXT Resource Records are not well suited to
protocol use has to do with the limited data space available in a DNS
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message. As alluded to briefly in Section 3.1, typical DNS query
traffic patterns involve a very large number of DNS clients sending
queries to a relatively small number of DNS servers. Normal path MTU
discovery schemes do little good here because, from the server's
perspective, there isn't enough repeat traffic from any one client
for it to be worth retaining state. UDP-based DNS is an idempotent
query, whereas TCP-based DNS requires the server to keep state (in
the form of TCP connection state, usually in the server's kernel) and
roughly triples the traffic load. Thus, there's a strong incentive
to keep DNS messages short enough to fit in a UDP datagram,
preferably a UDP datagram short enough not to require IP
fragmentation.
Subtyping schemes are therefore again problematic because they
produce larger Resource RRSets than necessary, but verbose text
encodings of data are also wasteful, since the data they hold can
usually be represented more compactly in a Resource Record designed
specifically to support the application's particular data needs. If
the data that need to be carried are so large that there is no way to
make them fit comfortably into the DNS regardless of encoding, it is
probably better to move the data somewhere else, and just use the DNS
as a pointer to the data, as with NAPTR.
6. Conclusion and Recommendation
Given the problems detailed in Section 5, it is worth reexamining the
oft-jumped-to conclusion that specifying a new Resource Record Type
is hard. Historically, this was indeed the case, but recent surveys
suggest that support for unknown Resource Record Types [RFC3597] is
now widespread, and because of that the DNS infrastructure can handle
new resource record types. The lack of support for unknown Types is
mostly an issue for relatively old provision software and
applications that would probably need to be upgraded in any case as
part of supporting a new feature (that require the new Resource
Record Type). One should also remember that deployed DNS software
today should support DNSSEC, and software recent enough to do so will
likely support both unknown Resource Record Types [RFC3597] and EDNS0
[RFC2671].
Of all the issues detailed in Section 3.5, provisioning the data is
in some respects the most difficult. The problems can be divided in
two, the ability to manage the zone on the master server, and the
ability for secondary servers to do zone transfers (AXFR or IXFR)
with the new data. Investigations show that the problem here is less
difficult for the authoritative name servers themselves than the
front-end systems used to enter (and perhaps validate) the data.
Hand editing does not work well for maintenance of large zones, so
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some sort of tool is necessary, and the tool may not be tightly
coupled to the name server implementation itself. Note, however,
that this provisioning problem exists to some degree with any new
form of data to be stored in the DNS, regardless of data format,
Resource Record type (even if TXT Resource Record Types are in use),
or naming scheme. Adapting front-end systems to support a new
Resource Record Type may be a bit more difficult than reusing an
existing type, but this appears to be a minor difference in degree
rather than a difference in kind.
Given the various issues described in this note, we believe that:
o there is no magic solution which allows a completely painless
addition of new data to the DNS, but
o on the whole, the best solution is still to use the DNS Resource
Record Type mechanism designed for precisely this purpose, and
o of all the alternate solutions, the "obvious" approach of using
TXT Resource Records is almost certainly the worst.
This especially for the two reasons outlined above (lack of semantics
and its implications, and size leading to the need to use TCP).
7. Creating A New Resource Record Type
The process for creating a new Resource Record Type is specified in
draft-ietf-dnsext-2929bis [I-D.ietf-dnsext-2929bis].
8. IANA Considerations
This document does not require any IANA actions.
9. Security Considerations
DNS RRSets can be signed using DNSSEC. DNSSEC is almost certainly
necessary for any application mechanism that stores authorization
data in the DNS. DNSSEC signatures significantly increase the size
of the messages transported, and because of this, the DNS message
size issues discussed in Section 3.1 and Section 5 are more serious
than they might at first appear.
Adding new Resource Record Types (as discussed in Section 3.5) can
create two different kinds of problems. In DNS software and in
applications. In the DNS software, it might conceivably trigger bugs
and other bad behavior in software that is not compliant with RFC
3597 [RFC3597], but most such DNS software is old enough and insecure
enough that it should be updated for other reasons in any case. In
applications and provisioning software, the changes for the new
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features that need the new data in DNS can be updated to understand
the structure of the new data format (regardless of whether a new
Resource Record Type is used or some other mechanism is chosen.
Basic API support for retrieving arbitrary Resource Record Types has
been a requirement since 1989[RFC1123].
Any new protocol that proposes to use the DNS to store data used to
make authorization decisions would be well advised not only to use
DNSSEC but also to encourage upgrades to DNS server software recent
enough not to be riddled with well-known exploitable bugs. Because
of this, support for new Resource Record Types will not be as hard as
people might think at first.
10. Acknowledgements
This document has been created during a number of years, with input
from many people. The question on how to expand and use the DNS is
sensitive, and a document like this can not please everyone. The
goal is instead to describe the architecture and tradeoffs, and make
some recommendations about best practices.
People that have helped include: Dean Andersson, Loa Andersson, Mark
Andrews, John Angelmo, Roy Badami, Dan Bernstein, Alex Bligh,
Nathaniel Borenstein, Stephane Bortzmeyer, Brian Carpenter, Leslie
Daigle, Elwyn Davies, Mark Delany, Richard Draves, Martin Duerst,
Donald Eastlake, Robert Elz, Jim Fenton, Tony Finch, Jim Gilroy,
Olafur Gudmundsson, Eric Hall, Philip Hallam-Baker, Ted Hardie, Bob
Hinden, Paul Hoffman, Geoff Houston, Christian Huitema, Johan Ihren,
John Klensin, Olaf Kolkman, Ben Laurie, William Leibzon, John Levine,
Edward Lewis, David MacQuigg, Allison Manking, Bill Manning, Danny
McPherson, David Meyer, Pekka Nikander, Mans Nilsson, Masataka Ohta,
Douglas Otis, Michael Patton, Jonathan Rosenberg, Anders Rundgren,
Miriam Sapiro, Pekka Savola, Chip Sharp, James Snell, Dave Thaler,
Michael Thomas, Paul Vixie, Sam Weiler, Florian Weimer, Bert Wijnen,
and Dan Wing.
Members of the IAB when this document was made available were: Loa
Andersson, Gonzalo Camarillo, Stuart Cheshire Russ Housley, Olaf
Kolkman, Gregory Lebovitz, Barry Leiba, Kurtis Lindqvist, Andrew
Malis, Danny McPherson, David Oran, Dave Thaler, and Lixia Zhang.
11. References
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11.1. Normative References
[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.
[RFC2535] Eastlake, D., "Domain Name System Security Extensions",
RFC 2535, March 1999.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, August 1999.
[RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record
(RR) Types", RFC 3597, September 2003.
11.2. Informative References
[I-D.cheshire-dnsext-dns-sd]
Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", draft-ietf-dnsext-2929bis-06 (work in
progress), August 2006.
[I-D.ietf-dnsext-2929bis]
Eastlake 3rd, D., "Domain Name System (DNS) IANA
Considerations", draft-cheshire-dnsext-dns-sd-03 (work in
progress), August 2007.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989.
[RFC1535] Gavron, E., "A Security Problem and Proposed Correction
With Widely Deployed DNS Software", RFC 1535,
October 1993.
[RFC2163] Allocchio, C., "Using the Internet DNS to Distribute MIXER
Conformant Global Address Mapping (MCGAM)", RFC 2163,
January 1998.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC2672] Crawford, M., "Non-Terminal DNS Name Redirection",
RFC 2672, August 1999.
[RFC2929] Eastlake, D., Brunner-Williams, E., and B. Manning,
"Domain Name System (DNS) IANA Considerations", BCP 42,
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Internet-Draft Design Choices When Expanding DNS July 2008
RFC 2929, September 2000.
[RFC3445] Massey, D. and S. Rose, "Limiting the Scope of the KEY
Resource Record (RR)", RFC 3445, December 2002.
[RFC3467] Klensin, J., "Role of the Domain Name System (DNS)",
RFC 3467, February 2003.
[RFC3761] Faltstrom, P. and M. Mealling, "The E.164 to Uniform
Resource Identifiers (URI) Dynamic Delegation Discovery
System (DDDS) Application (ENUM)", RFC 3761, April 2004.
[RFC4592] Lewis, E., "The Role of Wildcards in the Domain Name
System", RFC 4592, July 2006.
Authors' Addresses
Internet Architecture Board
Email: iab@iab.org
Patrik Faltstrom (editor)
Email: paf@cisco.com
Rob Austein (editor)
Email: sra@isc.org
Peter Koch (editor)
Email: pk@denic.de
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Full Copyright Statement
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contained in BCP 78, and except as set forth therein, the authors
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IAB, et al. Expires January 15, 2009 [Page 18]
