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IETF RFC 4592
The Role of Wildcards in the Domain Name System
Last modified on Thursday, July 20th, 2006
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Network Working Group E. Lewis
Request for Comments: 4592 NeuStar
Updates: 1034, 2672 July 2006
Category: Standards Track
The Role of Wildcards
in the Domain Name System
 Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright © The Internet Society (2006).
Abstract
This is an update to the wildcard definition of RFC 1034. The
interaction with wildcards and CNAME is changed, an error condition
is removed, and the words defining some concepts central to wildcards
are changed. The overall goal is not to change wildcards, but to
refine the definition of RFC 1034.
Lewis Standards Track PAGE 1 
RFC 4592 DNSEXT WCARD July 2006
Table of Contents
1. Introduction ....................................................3
1.1. Motivation .................................................3
1.2. The Original Definition ....................................3
1.3. Roadmap to This Document ...................................4
1.3.1. New Terms ...........................................5
1.3.2. Changed Text ........................................5
1.3.3. Considerations with Special Types ...................5
1.4. Standards Terminology ......................................6
2. Wildcard Syntax .................................................6
2.1. Identifying a Wildcard .....................................6
2.1.1. Wildcard Domain Name and Asterisk Label .............6
2.1.2. Asterisks and Other Characters ......................7
2.1.3. Non-terminal Wildcard Domain Names ..................7
2.2. Existence Rules ............................................7
2.2.1. An Example ..........................................8
2.2.2. Empty Non-terminals .................................9
2.2.3. Yet Another Definition of Existence ................10
2.3. When Is a Wildcard Domain Name Not Special? ...............10
3. Impact of a Wildcard Domain Name on a Response .................10
3.1. Step 2 ....................................................11
3.2. Step 3 ....................................................11
3.3. Part 'c' ..................................................12
3.3.1. Closest Encloser and the Source of Synthesis .......12
3.3.2. Closest Encloser and Source of Synthesis Examples ..13
3.3.3. Type Matching ......................................13
4. Considerations with Special Types ..............................14
4.1. SOA RRSet at a Wildcard Domain Name .......................14
4.2. NS RRSet at a Wildcard Domain Name ........................14
4.2.1. Discarded Notions ..................................15
4.3. CNAME RRSet at a Wildcard Domain Name .....................16
4.4. DNAME RRSet at a Wildcard Domain Name .....................16
4.5. SRV RRSet at a Wildcard Domain Name .......................17
4.6. DS RRSet at a Wildcard Domain Name ........................17
4.7. NSEC RRSet at a Wildcard Domain Name ......................18
4.8. RRSIG at a Wildcard Domain Name ...........................18
4.9. Empty Non-terminal Wildcard Domain Name ...................18
5. Security Considerations ........................................18
6. References .....................................................18
6.1. Normative References ......................................18
6.2. Informative References ....................................19
7. Others Contributing to the Document ............................19
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1. Introduction
In RFC 1034 [RFC 1034], sections 4.3.2 and 4.3.3 describe the
synthesis of answers from special resource records (RRs) called
wildcards. The definition in RFC 1034 is incomplete and has proven
to be confusing. This document describes the wildcard synthesis by
adding to the discussion and making limited modifications.
Modifications are made to close inconsistencies that have led to
interoperability issues. This description does not expand the
service intended by the original definition.
Staying within the spirit and style of the original documents, this
document avoids specifying rules for DNS implementations regarding
wildcards. The intention is to only describe what is needed for
interoperability, not restrict implementation choices. In addition,
consideration is given to minimize any backward-compatibility issues
with implementations that comply with RFC 1034's definition.
This document is focused on the concept of wildcards as defined in
RFC 1034. Nothing is implied regarding alternative means of
synthesizing resource record sets (RRSets), nor are alternatives
discussed.
1.1. Motivation
Many DNS implementations diverge, in different ways, from the
original definition of wildcards. Although there is clearly a need
to clarify the original documents in light of this alone, the impetus
for this document lay in the engineering of the DNS security
extensions [RFC 4033]. With an unclear definition of wildcards, the
design of authenticated denial became entangled.
This document is intended to limit its changes, documenting only
those deemed necessary based on implementation experience, and to
remain as close to the original document as possible. To reinforce
that this document is meant to clarify and adjust and not redefine
wildcards, relevant sections of RFC 1034 are repeated verbatim to
facilitate comparison of the old and new text.
1.2. The Original Definition
The definition of the wildcard concept is comprised by the
documentation of the algorithm by which a name server prepares a
response (in RFC 1034's section 4.3.2) and the way in which a
resource record (set) is identified as being a source of synthetic
data (section 4.3.3).
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This is the definition of the term "wildcard" as it appears in RFC
1034, section 4.3.3.
# In the previous algorithm, special treatment was given to RRs with
# owner names starting with the label "*". Such RRs are called
# wildcards. Wildcard RRs can be thought of as instructions for
# synthesizing RRs. When the appropriate conditions are met, the
# name server creates RRs with an owner name equal to the query name
# and contents taken from the wildcard RRs.
This passage follows the algorithm in which the term wildcard is
first used. In this definition, wildcard refers to resource records.
In other usage, wildcard has referred to domain names, and it has
been used to describe the operational practice of relying on
wildcards to generate answers. It is clear from this that there is a
need to define clear and unambiguous terminology in the process of
discussing wildcards.
The mention of the use of wildcards in the preparation of a response
is contained in step 3, part 'c' of RFC 1034's section 4.3.2,
entitled "Algorithm". Note that "wildcard" does not appear in the
algorithm, instead references are made to the "*" label. The portion
of the algorithm relating to wildcards is deconstructed in detail in
section 3 of this document; this is the beginning of the relevant
portion of the "Algorithm".
# c. If at some label, a match is impossible (i.e., the
# corresponding label does not exist), look to see if [...]
# the "*" label exists.
The scope of this document is the RFC 1034 definition of wildcards
and the implications of updates to those documents, such as DNS
Security (DNSSEC). Alternate schemes for synthesizing answers are
not considered. (Note that there is no reference listed. No
document is known to describe any alternate schemes, although there
has been some mention of them in mailing lists.)
1.3. Roadmap to This Document
This document accomplishes these three tasks.
o Defines new terms
o Makes minor changes to avoid conflicting concepts
o Describes the actions of certain resource records as wildcards
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1.3.1. New Terms
To help in discussing what resource records are wildcards, two terms
will be defined: "asterisk label" and "wildcard domain name". These
are defined in section 2.1.1.
To assist in clarifying the role of wildcards in the name server
algorithm in RFC 1034, section 4.3.2, "source of synthesis" and
"closest encloser" are defined. These definitions are in section
3.3.1. "Label match" is defined in section 3.2.
The new terms are used to make discussions of wildcards clearer.
Terminology does not directly have an impact on implementations.
1.3.2. Changed Text
The definition of "existence" is changed superficially. This change
will not be apparent to implementations; it is needed to make
descriptions more precise. The change appears in section 2.2.3.
RFC 1034, section 4.3.3, seems to prohibit having two asterisk labels
in a wildcard owner name. With this document, the restriction is
removed entirely. This change and its implications are in section
2.1.3.
The actions when a source of synthesis owns a CNAME RR are changed to
mirror the actions if an exact match name owns a CNAME RR. This is
an addition to the words in RFC 1034, section 4.3.2, step 3, part
'c'. The discussion of this is in section 3.3.3.
Only the latter change represents an impact to implementations. The
definition of existence is not a protocol impact. The change to the
restriction on names is unlikely to have an impact, as RFC 1034
contained no specification on when and how to enforce the
restriction.
1.3.3. Considerations with Special Types
This document describes semantics of wildcard RRSets for
"interesting" types as well as empty non-terminal wildcards.
Understanding these situations in the context of wildcards has been
clouded because these types incur special processing if they are the
result of an exact match. This discussion is in section 4.
These discussions do not have an implementation impact; they cover
existing knowledge of the types, but to a greater level of detail.
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1.4. Standards Terminology
This document does not use terms as defined in "Key words for use in
RFCs to Indicate Requirement Levels" [RFC 2119].
Quotations of RFC 1034 are denoted by a '#' at the start of the line.
References to section "4.3.2" are assumed to refer to RFC 1034's
section 4.3.2, simply titled "Algorithm".
2. Wildcard Syntax
The syntax of a wildcard is the same as any other DNS resource
record, across all classes and types. The only significant feature
is the owner name.
Because wildcards are encoded as resource records with special names,
they are included in zone transfers and incremental zone transfers
[RFC 1995] just as non-wildcard resource records are. This feature
has been under appreciated until discussions on alternative
approaches to wildcards appeared on mailing lists.
2.1. Identifying a Wildcard
To provide a more accurate description of wildcards, the definition
has to start with a discussion of the domain names that appear as
owners. Two new terms are needed, "asterisk label" and "wildcard
domain name".
2.1.1. Wildcard Domain Name and Asterisk Label
A "wildcard domain name" is defined by having its initial (i.e.,
leftmost or least significant) label be, in binary format:
0000 0001 0010 1010 (binary) = 0x01 0x2a (hexadecimal)
The first octet is the normal label type and length for a 1-octet-
long label, and the second octet is the ASCII representation [RFC 20]
for the '*' character.
A descriptive name of a label equaling that value is an "asterisk
label".
RFC 1034's definition of wildcard would be "a resource record owned
by a wildcard domain name".
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2.1.2. Asterisks and Other Characters
No label values other than that in section 2.1.1 are asterisk labels,
hence names beginning with other labels are never wildcard domain
names. Labels such as 'the*' and '**' are not asterisk labels, so
these labels do not start wildcard domain names.
2.1.3. Non-terminal Wildcard Domain Names
In section 4.3.3, the following is stated:
# .......................... The owner name of the wildcard RRs is
# of the form "*.<anydomain>", where <anydomain> is any domain name.
# <anydomain> should not contain other * labels......................
The restriction is now removed. The original documentation of it is
incomplete and the restriction does not serve any purpose given years
of operational experience.
There are three possible reasons for putting the restriction in
place, but none of the three has held up over time. One is that the
restriction meant that there would never be subdomains of wildcard
domain names, but the restriction as stated still permits
"example.*.example." for instance. Another is that wildcard domain
names are not intended to be empty non-terminals, but this situation
does not disrupt the algorithm in 4.3.2. Finally, "nested" wildcard
domain names are not ambiguous once the concept of the closest
encloser had been documented.
A wildcard domain name can have subdomains. There is no need to
inspect the subdomains to see if there is another asterisk label in
any subdomain.
A wildcard domain name can be an empty non-terminal. (See the
upcoming sections on empty non-terminals.) In this case, any lookup
encountering it will terminate as would any empty non-terminal match.
2.2. Existence Rules
The notion that a domain name 'exists' is mentioned in the definition
of wildcards. In section 4.3.3 of RFC 1034:
# Wildcard RRs do not apply:
#
...
# - When the query name or a name between the wildcard domain and
# the query name is know[n] to exist. . . .
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"Existence" is therefore an important concept in the understanding of
wildcards. Unfortunately, the definition of what exists, in RFC
1034, is unclear. So, in sections 2.2.2. and 2.2.3, another look is
taken at the definition of existence.
2.2.1. An Example
To illustrate what is meant by existence consider this complete zone:
$ORIGIN example.
example. 3600 IN SOA <SOA RDATA>
example. 3600 NS ns.example.com.
example. 3600 NS ns.example.net.
*.example. 3600 TXT "this is a wildcard"
*.example. 3600 MX 10 host1.example.
sub.*.example. 3600 TXT "this is not a wildcard"
host1.example. 3600 A 192.0.2.1
_ssh._tcp.host1.example. 3600 SRV <SRV RDATA>
_ssh._tcp.host2.example. 3600 SRV <SRV RDATA>
subdel.example. 3600 NS ns.example.com.
subdel.example. 3600 NS ns.example.net.
A look at the domain names in a tree structure is helpful:
|
-------------example------------
/ / \ \
/ / \ \
/ / \ \
* host1 host2 subdel
| | |
| | |
sub _tcp _tcp
| |
| |
_ssh _ssh
The following responses would be synthesized from one of the
wildcards in the zone:
QNAME=host3.example. QTYPE=MX, QCLASS=IN
the answer will be a "host3.example. IN MX ..."
QNAME=host3.example. QTYPE=A, QCLASS=IN
the answer will reflect "no error, but no data"
because there is no A RR set at '*.example.'
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QNAME=foo.bar.example. QTYPE=TXT, QCLASS=IN
the answer will be "foo.bar.example. IN TXT ..."
because bar.example. does not exist, but the wildcard
does.
The following responses would not be synthesized from any of the
wildcards in the zone:
QNAME=host1.example., QTYPE=MX, QCLASS=IN
because host1.example. exists
QNAME=sub.*.example., QTYPE=MX, QCLASS=IN
because sub.*.example. exists
QNAME=_telnet._tcp.host1.example., QTYPE=SRV, QCLASS=IN
because _tcp.host1.example. exists (without data)
QNAME=host.subdel.example., QTYPE=A, QCLASS=IN
because subdel.example. exists (and is a zone cut)
QNAME=ghost.*.example., QTYPE=MX, QCLASS=IN
because *.example. exists
The final example highlights one common misconception about
wildcards. A wildcard "blocks itself" in the sense that a wildcard
does not match its own subdomains. That is, "*.example." does not
match all names in the "example." zone; it fails to match the names
below "*.example.". To cover names under "*.example.", another
wildcard domain name is needed--"*.*.example."--which covers all but
its own subdomains.
2.2.2. Empty Non-terminals
Empty non-terminals [RFC 2136, section 7.16] are domain names that own
no resource records but have subdomains that do. In section 2.2.1,
"_tcp.host1.example." is an example of an empty non-terminal name.
Empty non-terminals are introduced by this text in section 3.1 of RFC
1034:
# The domain name space is a tree structure. Each node and leaf on
# the tree corresponds to a resource set (which may be empty). The
# domain system makes no distinctions between the uses of the
# interior nodes and leaves, and this memo uses the term "node" to
# refer to both.
The parenthesized "which may be empty" specifies that empty non-
terminals are explicitly recognized and that empty non-terminals
"exist".
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Pedantically reading the above paragraph can lead to an
interpretation that all possible domains exist--up to the suggested
limit of 255 octets for a domain name [RFC 1035]. For example,
www.example. may have an A RR, and as far as is practically
concerned, is a leaf of the domain tree. But the definition can be
taken to mean that sub.www.example. also exists, albeit with no data.
By extension, all possible domains exist, from the root on down.
As RFC 1034 also defines "an authoritative name error indicating that
the name does not exist" in section 4.3.1, so this apparently is not
the intent of the original definition, justifying the need for an
updated definition in the next section.
2.2.3. Yet Another Definition of Existence
RFC 1034's wording is fixed by the following paragraph:
The domain name space is a tree structure. Nodes in the tree either
own at least one RRSet and/or have descendants that collectively own
at least one RRSet. A node may exist with no RRSets only if it has
descendants that do; this node is an empty non-terminal.
A node with no descendants is a leaf node. Empty leaf nodes do not
exist.
Note that at a zone boundary, the domain name owns data, including
the NS RR set. In the delegating zone, the NS RR set is not
authoritative, but that is of no consequence here. The domain name
owns data; therefore, it exists.
2.3. When Is a Wildcard Domain Name Not Special?
When a wildcard domain name appears in a message's query section, no
special processing occurs. An asterisk label in a query name only
matches a single, corresponding asterisk label in the existing zone
tree when the 4.3.2 algorithm is being followed.
When a wildcard domain name appears in the resource data of a record,
no special processing occurs. An asterisk label in that context
literally means just an asterisk.
3. Impact of a Wildcard Domain Name on a Response
RFC 1034's description of how wildcards impact response generation is
in its section 4.3.2. That passage contains the algorithm followed
by a server in constructing a response. Within that algorithm, step
3, part 'c' defines the behavior of the wildcard.
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The algorithm in section 4.3.2 is not intended to be pseudo-code;
that is, its steps are not intended to be followed in strict order.
The "algorithm" is a suggested means of implementing the
requirements. As such, in step 3, parts 'a', 'b', and 'c' do not
have to be implemented in that order, provided that the result of the
implemented code is compliant with the protocol's specification.
3.1. Step 2
Step 2 of section 4.3.2 reads:
# 2. Search the available zones for the zone which is the nearest
# ancestor to QNAME. If such a zone is found, go to step 3,
# otherwise step 4.
In this step, the most appropriate zone for the response is chosen.
The significance of this step is that it means all of step 3 is being
performed within one zone. This has significance when considering
whether or not an SOA RR can ever be used for synthesis.
3.2. Step 3
Step 3 is dominated by three parts, labeled 'a', 'b', and 'c'. But
the beginning of the step is important and needs explanation.
# 3. Start matching down, label by label, in the zone. The
# matching process can terminate several ways:
The word 'matching' refers to label matching. The concept is based
in the view of the zone as the tree of existing names. The query
name is considered to be an ordered sequence of labels--as if the
name were a path from the root to the owner of the desired data
(which it is--3rd paragraph of RFC 1034, section 3.1).
The process of label matching a query name ends in exactly one of
three choices, the parts 'a', 'b', and 'c'. Either the name is
found, the name is below a cut point, or the name is not found.
Once one of the parts is chosen, the other parts are not considered
(e.g., do not execute part 'c' and then change the execution path to
finish in part 'b'). The process of label matching is also done
independent of the query type (QTYPE).
Parts 'a' and 'b' are not an issue for this clarification as they do
not relate to record synthesis. Part 'a' is an exact match that
results in an answer; part 'b' is a referral.
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3.3. Part 'c'
The context of part 'c' is that the process of label matching the
labels of the query name has resulted in a situation in which there
is no corresponding label in the tree. It is as if the lookup has
"fallen off the tree".
# c. If at some label, a match is impossible (i.e., the
# corresponding label does not exist), look to see if [...]
# the "*" label exists.
To help describe the process of looking 'to see if [...] the "*"
label exists' a term has been coined to describe the last domain
(node) matched. The term is "closest encloser".
3.3.1. Closest Encloser and the Source of Synthesis
The closest encloser is the node in the zone's tree of existing
domain names that has the most labels matching the query name
(consecutively, counting from the root label downward). Each match
is a "label match" and the order of the labels is the same.
The closest encloser is, by definition, an existing name in the zone.
The closest encloser might be an empty non-terminal or even be a
wildcard domain name itself. In no circumstances is the closest
encloser to be used to synthesize records for the current query.
The source of synthesis is defined in the context of a query process
as that wildcard domain name immediately descending from the closest
encloser, provided that this wildcard domain name exists.
"Immediately descending" means that the source of synthesis has a
name of the form:
<asterisk label>.<closest encloser>.
A source of synthesis does not guarantee having a RRSet to use for
synthesis. The source of synthesis could be an empty non-terminal.
If the source of synthesis does not exist (not on the domain tree),
there will be no wildcard synthesis. There is no search for an
alternate.
The important concept is that for any given lookup process, there is
at most one place at which wildcard synthetic records can be
obtained. If the source of synthesis does not exist, the lookup
terminates, and the lookup does not look for other wildcard records.
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3.3.2. Closest Encloser and Source of Synthesis Examples
To illustrate, using the example zone in section 2.2.1 of this
document, the following chart shows QNAMEs and the closest enclosers.
QNAME Closest Encloser Source of Synthesis
host3.example. example. *.example.
_telnet._tcp.host1.example. _tcp.host1.example. no source
_dns._udp.host2.example. host2.example. no source
_telnet._tcp.host3.example. example. *.example.
_chat._udp.host3.example. example. *.example.
foobar.*.example. *.example. no source
3.3.3. Type Matching
RFC 1034 concludes part 'c' with this:
# If the "*" label does not exist, check whether the name
# we are looking for is the original QNAME in the query
# or a name we have followed due to a CNAME. If the name
# is original, set an authoritative name error in the
# response and exit. Otherwise just exit.
#
# If the "*" label does exist, match RRs at that node
# against QTYPE. If any match, copy them into the answer
# section, but set the owner of the RR to be QNAME, and
# not the node with the "*" label. Go to step 6.
The final paragraph covers the role of the QTYPE in the lookup
process.
Based on implementation feedback and similarities between part 'a'
and part 'c', a change to this passage has been made.
The change is to add the following text to part 'c' prior to the
instructions to "go to step 6":
If the data at the source of synthesis is a CNAME, and QTYPE
doesn't match CNAME, copy the CNAME RR into the answer section of
the response changing the owner name to the QNAME, change QNAME to
the canonical name in the CNAME RR, and go back to step 1.
This is essentially the same text in part 'a' covering the processing
of CNAME RRSets.
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4. Considerations with Special Types
Sections 2 and 3 of this document discuss wildcard synthesis with
respect to names in the domain tree and ignore the impact of types.
In this section, the implication of wildcards of specific types is
discussed. The types covered are those that have proven to be the
most difficult to understand. The types are SOA, NS, CNAME, DNAME,
SRV, DS, NSEC, RRSIG, and "none", that is, empty non-terminal
wildcard domain names.
4.1. SOA RRSet at a Wildcard Domain Name
A wildcard domain name owning an SOA RRSet means that the domain is
at the root of the zone (apex). The domain cannot be a source of
synthesis because that is, by definition, a descendant node (of the
closest encloser) and a zone apex is at the top of the zone.
Although a wildcard domain name owning an SOA RRSet can never be a
source of synthesis, there is no reason to forbid the ownership of an
SOA RRSet.
For example, given this zone:
$ORIGIN *.example.
@ 3600 IN SOA <SOA RDATA>
3600 NS ns1.example.com.
3600 NS ns1.example.net.
www 3600 TXT "the www txt record"
A query for www.*.example.'s TXT record would still find the "the www
txt record" answer. The asterisk label only becomes significant when
section 4.3.2, step 3, part 'c' is in effect.
Of course, there would need to be a delegation in the parent zone,
"example." for this to work too. This is covered in the next
section.
4.2. NS RRSet at a Wildcard Domain Name
With the definition of DNSSEC [RFC 4033, RFC 4034, RFC 4035] now in
place, the semantics of a wildcard domain name owning an NS RRSet has
come to be poorly defined. The dilemma relates to a conflict between
the rules for synthesis in part 'c' and the fact that the resulting
synthesis generates a record for which the zone is not authoritative.
In a DNSSEC signed zone, the mechanics of signature management
(generation and inclusion in a message) have become unclear.
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Salient points of the working group discussion on this topic are
summarized in section 4.2.1.
As a result of these discussions, there is no definition given for
wildcard domain names owning an NS RRSet. The semantics are left
undefined until there is a clear need to have a set defined, and
until there is a clear direction to proceed. Operationally,
inclusion of wildcard NS RRSets in a zone is discouraged, but not
barred.
4.2.1. Discarded Notions
Prior to DNSSEC, a wildcard domain name owning a NS RRSet appeared to
be workable, and there are some instances in which it is found in
deployments using implementations that support this. Continuing to
allow this in the specification is not tenable with DNSSEC. The
reason is that the synthesis of the NS RRSet is being done in a zone
that has delegated away the responsibility for the name. This
"unauthorized" synthesis is not a problem for the base DNS protocol,
but DNSSEC in affirming the authorization model for DNS exposes the
problem.
Outright banning of wildcards of type NS is also untenable as the DNS
protocol does not define how to handle "illegal" data.
Implementations may choose not to load a zone, but there is no
protocol definition. The lack of the definition is complicated by
having to cover dynamic update [RFC 2136] and zone transfers, as well
as loading at the master server. The case of a client (resolver,
caching server) getting a wildcard of type NS in a reply would also
have to be considered.
Given the daunting challenge of a complete definition of how to ban
such records, dealing with existing implementations that permit the
records today is a further complication. There are uses of wildcard
domain name owning NS RRSets.
One compromise proposed would have redefined wildcards of type NS to
not be used in synthesis, this compromise fell apart because it would
have required significant edits to the DNSSEC signing and validation
work. (Again, DNSSEC catches unauthorized data.)
With no clear consensus forming on the solution to this dilemma, and
the realization that wildcards of type NS are a rarity in operations,
the best course of action is to leave this open-ended until "it
matters".
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4.3. CNAME RRSet at a Wildcard Domain Name
The issue of a CNAME RRSet owned by a wildcard domain name has
prompted a suggested change to the last paragraph of step 3c of the
algorithm in 4.3.2. The changed text appears in section 3.3.3 of
this document.
4.4. DNAME RRSet at a Wildcard Domain Name
Ownership of a DNAME [RFC 2672] RRSet by a wildcard domain name
represents a threat to the coherency of the DNS and is to be avoided
or outright rejected. Such a DNAME RRSet represents non-
deterministic synthesis of rules fed to different caches. As caches
are fed the different rules (in an unpredictable manner) the caches
will cease to be coherent. ("As caches are fed" refers to the
storage in a cache of records obtained in responses by recursive or
iterative servers.)
For example, assume one cache, responding to a recursive request,
obtains the following record:
"a.b.example. DNAME foo.bar.example.net."
and another cache obtains:
"b.example. DNAME foo.bar.example.net."
both generated from the record:
"*.example. DNAME foo.bar.example.net."
by an authoritative server.
The DNAME specification is not clear on whether DNAME records in a
cache are used to rewrite queries. In some interpretations, the
rewrite occurs; in others, it does not. Allowing for the occurrence
of rewriting, queries for "sub.a.b.example. A" may be rewritten as
"sub.foo.bar.tld. A" by the former caching server and may be
rewritten as "sub.a.foo.bar.tld. A" by the latter. Coherency is
lost, and an operational nightmare ensues.
Another justification for a recommendation to avoid the use of
wildcard DNAME records is the observation that such a record could
synthesize a DNAME owned by "sub.foo.bar.example." and
"foo.bar.example.". There is a restriction in the DNAME definition
that no domain exist below a DNAME-owning domain; hence, the wildcard
DNAME is to be avoided.
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RFC 4592 DNSEXT WCARD July 2006
4.5. SRV RRSet at a Wildcard Domain Name
The definition of the SRV RRset is RFC 2782 [RFC 2782]. In the
definition of the record, there is some confusion over the term
"Name". The definition reads as follows:
# The format of the SRV RR
...
# _Service._Proto.Name TTL Class SRV Priority Weight Port Target
...
# Name
# The domain this RR refers to. The SRV RR is unique in that the
# name one searches for is not this name; the example near the end
# shows this clearly.
Do not confuse the definition "Name" with the owner name. That is,
once removing the _Service and _Proto labels from the owner name of
the SRV RRSet, what remains could be a wildcard domain name but this
is immaterial to the SRV RRSet.
For example, if an SRV record is the following:
_foo._udp.*.example. 10800 IN SRV 0 1 9 old-slow-box.example.
*.example is a wildcard domain name and although it is the Name of
the SRV RR, it is not the owner (domain name). The owner domain name
is "_foo._udp.*.example.", which is not a wildcard domain name.
A query for the SRV RRSet of "_foo._udp.bar.example." (class IN),
will result in a match of the name "*.example." (assuming there is no
bar.example.) and not a match of the SRV record shown. If there is
no SRV RRSet at "*.example.", the answer section will reflect that
(be empty or a CNAME RRset).
The confusion is likely based on the mixture of the specification of
the SRV RR and the description of a "use case".
4.6. DS RRSet at a Wildcard Domain Name
A DS RRSet owned by a wildcard domain name is meaningless and
harmless. This statement is made in the context that an NS RRSet at
a wildcard domain name is undefined. At a non-delegation point, a DS
RRSet has no value (no corresponding DNSKEY RRSet will be used in
DNSSEC validation). If there is a synthesized DS RRSet, it alone
will not be very useful as it exists in the context of a delegation
point.
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RFC 4592 DNSEXT WCARD July 2006
4.7. NSEC RRSet at a Wildcard Domain Name
Wildcard domain names in DNSSEC signed zones will have an NSEC RRSet.
Synthesis of these records will only occur when the query exactly
matches the record. Synthesized NSEC RRs will not be harmful as they
will never be used in negative caching or to generate a negative
response [RFC 2308].
4.8. RRSIG at a Wildcard Domain Name
RRSIG records will be present at a wildcard domain name in a signed
zone and will be synthesized along with data sought in a query. The
fact that the owner name is synthesized is not a problem as the label
count in the RRSIG will instruct the verifying code to ignore it.
4.9. Empty Non-terminal Wildcard Domain Name
If a source of synthesis is an empty non-terminal, then the response
will be one of no error in the return code and no RRSet in the answer
section.
5. Security Considerations
This document is refining the specifications to make it more likely
that security can be added to DNS. No functional additions are being
made, just refining what is considered proper to allow the DNS,
security of the DNS, and extending the DNS to be more predictable.
6. References
6.1. Normative References
[RFC 20] Cerf, V., "ASCII format for network interchange", RFC 20,
October 1969.
[RFC 1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC 1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC 1995] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
August 1996.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Lewis Standards Track PAGE 18
RFC 4592 DNSEXT WCARD July 2006
[RFC 2308] Andrews, M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, March 1998.
[RFC 2672] Crawford, M., "Non-Terminal DNS Name Redirection", RFC
2672, August 1999.
[RFC 2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[RFC 4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", RFC
4033, March 2005.
[RFC 4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005.
[RFC 4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005.
6.2. Informative References
[RFC 2136] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound, "Dynamic
Updates in the Domain Name System (DNS UPDATE)", RFC 2136,
April 1997.
7. Others Contributing to the Document
This document represents the work of a large working group. The
editor merely recorded its collective wisdom.
Comments on this document can be sent to the editor or the mailing
list for the DNSEXT WG, namedroppers@ops.ietf.org.
Editor's Address
Edward Lewis
NeuStar
46000 Center Oak Plaza
Sterling, VA
20166, US
Phone: +1-571-434-5468
EMail: ed.lewis@neustar.biz
Lewis Standards Track PAGE 19
RFC 4592 DNSEXT WCARD July 2006
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Lewis Standards Track PAGE 20
The Role of Wildcards in the Domain Name System
RFC TOTAL SIZE: 43991 bytes
PUBLICATION DATE: Thursday, July 20th, 2006
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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