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DNSEXT Working Group E. Lewis
INTERNET DRAFT NeuStar
Expiration Date: August 10, 2005 February 2005
The Role of Wildcard Domains
in the Domain Name System
draft-ietf-dnsext-wcard-clarify-05.txt
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of RFC 3668.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
This Internet-Draft will expire on August 10, 2005.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This is an update to the wildcard definition of RFC 1034. The
interaction with wildcards and CNAME is changed, an error
condition 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.
1 Introduction
In RFC 1034 [RFC1034], sections 4.3.2 and 4.3.3 describe the synthesis
of answers from special resource records 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 has been given to minimize any backwards compatibility
with implementations that have complied with RFC 1034's definition.
This document is focused on the concept of wildcards as defined in RFC
1034. Nothing is implied regarding alternative approaches, nor are
alternatives discussed.
[Note to the WG - this draft is not complete, it is presented as fodder
for the upcoming meeting. Sections 4.2.3, 4.6, 3.7, and 4.8 are
particularly incomplete. I wanted to make sure there was something
recent in the draft repository before setting out on more travel.
For 4.2.3, refer to the threads for the most recent discussions...
http://ops.ietf.org/lists/namedroppers/namedroppers.2004/msg01601.html
http://ops.ietf.org/lists/namedroppers/namedroppers.2004/msg01603.html
And you might want to check out the minutes from the last IETF meeting
as well as http://www.ietf.org/proceedings/03nov/131.htm.]
1.1 Motivation
Many DNS implementations have diverged with respect to wildcards in
different ways from the original definition, or at from least what
had been intended. 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 TBD]. With
an unclear definition of wildcards the design of authenticated denial
became entangled.
This document is intended to limit changes, only those based on
implementation experience, and to remain as close to the original
document as possible. To reinforce this, relevant sections of RFC
1034 are repeated verbatim to help compare the old and new text.
1.2 The Original Definition
The context of the wildcard concept involves 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).
The beginning of the discussion ought to start with 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 appears after 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 3c 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 passage.
# c. If at some label, a match is impossible (i.e., the
# corresponding label does not exist), look to see if a
# 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 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 This Document
This document accomplishes these three items.
o Defines new terms
o Makes minor changes to avoid conflicting concepts
o Describe the actions of certain resource records as wildcards
1.3.1 New Terms
To help in discussing what resource records are wildcards, two terms
will be defined - "asterisk label" and "wild card 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, 4.3.2, "source of synthesis" and "closest encloser" are
defined. These definitions are in section 3.3.2. "Label match" is
defined in section 3.2.
The introduction of new terms ought not have an impact on any existing
implementations. The new terms are used only to make discussions of
wildcards clearer.
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 there was no
discussion of how to enforce the restriction.
1.3.3 Considerations with Special Types
This document describes semantics of wildcard CNAME RRSets [RFC2181],
wildcard NS RRSets, wildcard SOA RRSets, wildcard DNAME RRSets
[RFC2672], wildcard DS RRSets [RFC TBD], and empty non-terminal
wildcards. Understanding these types 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.
1.4 Standards Terminology
This document does not use terms as defined in "Key words for use in
RFCs to Indicate Requirement Levels." [RFC2119]
Quotations of RFC 1034 are denoted by a '#' in the leftmost column.
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.
[RFC1995]. This feature has been underappreciated 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 "Wild Card
Domain Name."
2.1.1 Wild Card Domain Name and Asterisk Label
A "wild card domain name" is defined by having its initial
(i.e., left-most 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, the second octet is the ASCII representation [RFC20] 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 wild card domain name."
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 wild card domain
names. Labels such as 'the*' and '**' are not asterisk labels,
they do not start wild card domain names.
2.1.3 Non-terminal Wild Card 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......................
This restriction is lifted because the original documentation of it
is incomplete and the restriction does not serve any purpose given
years of operational experience.
Indirectly, the above passage raises questions about wild card domain
names having subdomains and possibly being an empty non-terminal. By
thinking of domain names such as "*.example.*.example." and
"*.*.example." and focusing on the right-most asterisk label in each,
the issues become apparent.
Although those example names have been restricted per RFC 1034, a name
such as "example.*.example." illustrates the same problems. The
sticky issue of subdomains and empty non-terminals is not removed by
the restriction. With that conclusion, the restriction appears to
be meaningless, worse yet, it implies that an implementation would have
to perform checks that do little more than waste CPU cycles.
A wild card 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 wild card 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. For example, if a wildcard
RFC 1034 also refers to non-existence in the process of generating
a response that results in a return code of "name error." NXDOMAIN
is introduced in RFC 2308, section 2.1 says "In this case the domain
... does not exist." The overloading of the term "existence" is
confusing.
For the purposes of this document, a domain name is said to exist if
it plays a role in the execution of the algorithms in RFC 1034. This
document avoids discussion determining when an authoritative name
error has occurred.
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 wild card"
*.example. 3600 MX 10 host1.example.
sub.*.example. 3600 TXT "this is not a wild card"
host1.example. 3600 A 192.0.4.1
_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 queries would be synthesized from one of the wildcards:
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.'
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 queries would not be synthesized from any of the wildcards:
QNAME=host1.example., QTYPE=MX, QCLASS=IN
because host1.example. exists
QNAME=ghost.*.example., QTYPE=MX, QCLASS=IN
because *.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)
2.2.2 Empty Non-terminals
Empty non-terminals [RFC2136, 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 a 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."
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 [RFC1035]. 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, this is not the intent of
the original document.
2.2.3 Yet Another Definition of Existence
RFC1034'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 on RRSet. A node may have no RRSets if it has descendents that
do, this node is a empty non-terminal. A node may have its own RRSets
and have descendants with RRSets too.
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. At the delegating server, 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 does a Wild Card Domain Name is not Special
When a wild card domain name appears in a message's query section,
no special processing occurs. An asterisk label in a query name
only (label) matches an asterisk label in the existing zone tree
when the 4.3.2 algorithm is being followed.
When a wild card 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 Wild Card Domain Name On a Response
The description of how wildcards impact response generation is in
RFC 1034, 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 wild card.
The algorithm in RFC 1034, section 4.3.2. is not intended to be pseudo
code, i.e., its steps are not intended to be followed in strict
order. The "algorithm" is a suggestion. As such, in step 3, parts
a, b, and c, do not have to be implemented in that order.
3.1 Step 2
Step 2 of the RFC 1034's 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 be ever be used for synthesis.
3.2 Step 3
Step 3 is dominated by three parts, labelled '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. It is possible, from the description
given, that a query might fit into both part a and part b, this is
not within the scope of this document.
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 a
# the "*" label exists.
To help describe the process of looking 'to see if a [sic] the "*"
label exists' a term has been coined to describe the last label
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 wild card
domain name itself. In no circumstances is the closest encloser
the used to synthesize records for the current query.
The source of synthesis is defined in the context of a query process
as that wild card domain name immediately descending from the
closest encloser, provided that this wild card 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, the lookup does not look for other wildcard records.
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
_telnet._tcp.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 step 'a' and
step 'c' a change to this passage a change has been made.
The change is to add the following text to step 'c':
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 step a covering the processing of
CNAME RRSets.
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 are
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," i.e., empty non-terminal wild card
domain names.
4.1 SOA RRSet at a Wild Card Domain Name
A wild card domain name owning an SOA RRSet means that the domain
is at the root of the zone (apex). The domain can not be a source of
synthesis because that is, by definition, a descendent node (of
the closest encloser) and a zone apex is at the top of the zone.
Although a wild card 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.
E.g., 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 reason is that the asterisk label only becomes
significant when RFC 1034's 4.3.2, step 3 part 'c' in 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 Wild Card Domain Name
The semantics of a wild card domain name's ownership of a NS RRSet
has been unclear. There are three considerations to cover. One is
is that if the query processing lands in part 'a' or part 'b' of
RFC 1034's 4.3.2, step 3, the incidence of the wild card domain name
owning an NS RRset has no special meaning. Second, synthesized
records never appear in the authority section of a response, meaning
that referrals are never synthesized. And finally, DNSSEC validators
will have to be aware of a quirk in ownership rules.
4.2.1 NS, *, and answers
If the NS RRSet in question is at the top of the zone, i.e., the
name also owns an SOA RRSet, the QNAME equals the zone name. This
would trigger part 'a' of step 3.
4.2.2 NS, *, and referrals
If the NS RRset is not at the top of the zone and part 'b' is triggered,
this implies that the labels being matched are an asterisk label in
the QNAME and the asterisk label owning the NS RRset. In either case,
what is copied to the response will have the asterisk label in it - no
synthesis, no name substitution.
E.g., consider the parent zone for the example in section 4.1.
$ORIGIN example.
@ 3600 IN SOA <SOA RDATA>
3600 NS ns0.example.com.
3600 NS ns0.example.net.
* 3600 NS ns1.example.com.
3600 NS ns1.example.net.
If the query for www.*.example.'s TXT set arrived here, the response
would be a referral as in part 'b'.
Response, non-authoritative, no error rcode
ANSWER: (empty)
AUTHORITY:
* 3600 NS ns1.example.com.
3600 NS ns1.example.net.
ADDITIIONAL: (empty, or with OPT RR)
The same response message would be sent to a query for *.example.'s NS
set. Note that the NS records in the response are not expanded, simply
copied verbatim. (Compare this the case where "*" is "star".)
There is no synthesis of records in the authority section because part
'b' does not specify synthesis. The referral returned would have the
wild card domain name in the authority section unchanged.
4.2.3 NS, *, and synthesis
If the QNAME is not the same as the wild card domain name nor a
subdomain of it, then part 'c' of step 3 has been triggered. Assuming
that "a match is impossible" a source of synthesis is sought. If
the source of synthesis owns an NS RRset and the QTYPE is NS, then
a NS RRset is synthesized and put into the answer section and marked
as an authoritative answer. If the QTYPE is not NS, then the NS RRset
is ignored, as it would have been if it were an A RR and the QTYPE was
AAAA. An NS RRSet at a wild card domain name will not result in
the generation of referral messages for non-existent domains because
part 'c' does not write anything into the authority section.
(If we choose this, then we have to have a section 4.2.4 on DNSSEC
implications.)
OR
If the QNAME is not the same as the wild card domain name nor a
subdomain of it, then part 'c' of step 3 has been triggered. Assuming
that "a match is impossible" a source of synthesis is sought. If
the source of synthesis owns an NS RRset and the QTYPE is NS, then
no synthesis happens. A NS RRset is never synthesized. The proper
response is, what, no error/no data? Name error?
OR
If the QNAME is not the same as the wild card domain name nor a
subdomain of it, then part 'c' of step 3 has been triggered. Assuming
that "a match is impossible" a source of synthesis is sought. If
the source of synthesis owns an NS RRset then no synthesis happens.
A cut point is never a source of synthesis. The proper response is,
what, no error/no data? Name error?
4.3 CNAME RRSet at a Wild Card Domain Name
The issue of a CNAME RRSet owned by wild card domain names 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 Wild Card Domain Name
A DNAME RRset at a wild card domain name is effectively the same
as a CNAME at a wild card domain name.
4.5 SRV RRSet at a Wild Card Domain Name
The definition of the SRV RRset is RFC 2782 [RFC2782]. 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 a domain name. I.e., once
removing the _Service and _Proto labels from the owner name of the
SRV RRSet, what remains could be a wild card domain name but this is
immaterial to the SRV RRSet.
E.g., If an SRV record is:
_foo._udp.*.example. 10800 IN SRV 0 1 9 old-slow-box.example.
*.example is a wild card domain name and although it it 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 wild card domain name.
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 Wild Card Domain Name
...probably harmless...
4.7 NSEC RRSet at a Wild Card Domain Name
...will be present, don't know if it should be synthesized...
4.8 RRSIG at a Wild Card Domain Name
...need to cross check with DNSSECbis to see what is said about querying
for RRSIG...
4.9 Empty Non-terminal Wild Card 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
Normative References
[RFC20] ASCII Format for Network Interchange, V.G. Cerf, Oct-16-1969
[RFC1034] Domain Names - Concepts and Facilities, P.V. Mockapetris,
Nov-01-1987
[RFC1035] Domain Names - Implementation and Specification, P.V
Mockapetris, Nov-01-1987
[RFC1995] IXFR ... Ohta
[RFC2119] Key Words for Use in RFCs to Indicate Requirement Levels, S
Bradner, March 1997
[RFC2181] Clarifications to the DNS Specification, R. Elz and R. Bush,
July 1997.
[RFC2782] A DNS RR for specifying the location of services (DNS SRV),
A. Gulbrandsen, et.al., February 2000.
Informative References
[RFC2136] Dynamic Updates in the Domain Name System (DNS UPDATE), P.
Vixie, Ed., S. Thomson, Y. Rekhter, J. Bound, April 1997
[RFC2535] Domain Name System Security Extensions, D. Eastlake, March 1999
[RFC2672] Non-Terminal DNS Name Redirection, M. Crawford, August 1999
7. Others Contributing to This Document
Others who have been editors of this document: Bob Halley.
Others who have directly caused text to appear in the document: Alex
Bligh, Robert Elz, Paul Vixie, David Blacka and Olaf Kolkman.
Many others have indirect influences on the content.
8. Editor
Name: Edward Lewis
Affiliation: NeuStar
Address: 46000 Center Oak Plaza, Sterling, VA, 20166, US
Phone: +1-571-434-5468
Email: ed.lewis@neustar.biz
Comments on this document can be sent to the editor or the mailing
list for the DNSEXT WG, namedroppers@ops.ietf.org.
9. Trailing Boilerplate
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78 and
except as set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
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Expiration
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DNSEXT Working Group E. Lewis
INTERNET DRAFT NeuStar
Expiration Date: November 11, 2005 May 11 2005
The Role of Wildcards
in the Domain Name System
draft-ietf-dnsext-wcard-clarify-06.txt
Status of this Memo
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any applicable patent or other IPR claims of which he or she is
aware have been or will be disclosed, and any of which he or she
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BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on November 11, 2005.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This is an update to the wildcard definition of RFC 1034. The
interaction with wildcards and CNAME is changed, an error
condition 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.
1 Introduction
In RFC 1034 [RFC1034], sections 4.3.2 and 4.3.3 describe the synthesis
of answers from special resource records 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 has been given to minimize any backwards compatibility
with implementations that have complied with RFC 1034's definition.
This document is focused on the concept of wildcards as defined in RFC
1034. Nothing is implied regarding alternative approaches, nor are
alternatives discussed.
1.1 Motivation
Many DNS implementations have diverged with respect to wildcards in
different ways from the original definition, or at from least what
had been intended. 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
[RFC4033]. With an unclear definition of wildcards the design of
authenticated denial became entangled.
This document is intended to limit changes, only those based on
implementation experience, and to remain as close to the original
document as possible. To reinforce this, relevant sections of RFC
1034 are repeated verbatim to help compare the old and new text.
1.2 The Original Definition
The context of the wildcard concept involves 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).
The beginning of the discussion ought to start with 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 appears after 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 3c 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 passage.
# 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 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 This Document
This document accomplishes these three items.
o Defines new terms
o Makes minor changes to avoid conflicting concepts
o Describes the actions of certain resource records as wildcards
1.3.1 New Terms
To help in discussing what resource records are wildcards, two terms
will be defined - "asterisk label" and "wild card 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, 4.3.2, "source of synthesis" and "closest
encloser" are defined. These definitions are in section 3.3.2.
"Label match" is defined in section 3.2.
The introduction of new terms ought not have an impact on any existing
implementations. The new terms are used only to make discussions of
wildcards clearer.
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 there was no
discussion of how to enforce the restriction.
1.3.3 Considerations with Special Types
This document describes semantics of wildcard CNAME RRSets [RFC2181],
wildcard NS RRSets, wildcard SOA RRSets, wildcard DNAME RRSets
[RFC2672], wildcard DS RRSets [RFC TBD], and empty non-terminal
wildcards. Understanding these types 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.
1.4 Standards Terminology
This document does not use terms as defined in "Key words for use in
RFCs to Indicate Requirement Levels." [RFC2119]
Quotations of RFC 1034 are denoted by a '#' in the leftmost column.
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.
[RFC1995]. This feature has been underappreciated 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 "Wild Card
Domain Name."
2.1.1 Wild Card Domain Name and Asterisk Label
A "wild card domain name" is defined by having its initial
(i.e., left-most 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, the second octet is the ASCII representation [RFC20] 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 wild card domain name."
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 wild card domain
names. Labels such as 'the*' and '**' are not asterisk labels,
they do not start wild card domain names.
2.1.3 Non-terminal Wild Card 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......................
This restriction is lifted because the original documentation of it
is incomplete and the restriction does not serve any purpose given
years of operational experience.
Indirectly, the above passage raises questions about wild card domain
names having subdomains and possibly being an empty non-terminal. By
thinking of domain names such as "*.example.*.example." and
"*.*.example." and focusing on the right-most asterisk label in each,
the issues become apparent.
Although those example names have been restricted per RFC 1034, a name
such as "example.*.example." illustrates the same problems. The
sticky issue of subdomains and empty non-terminals is not removed by
the restriction. With that conclusion, the restriction appears to
be meaningless, worse yet, it implies that an implementation would
have to perform checks that do little more than waste CPU cycles.
A wild card 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 wild card 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. For example, if a wildcard
RFC 1034 also refers to non-existence in the process of generating
a response that results in a return code of "name error." NXDOMAIN
is introduced in RFC 2308, section 2.1 says "In this case the domain
... does not exist." The overloading of the term "existence" is
confusing.
For the purposes of this document, a domain name is said to exist if
it plays a role in the execution of the algorithms in RFC 1034. This
document avoids discussion determining when an authoritative name
error has occurred.
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 wild card"
*.example. 3600 MX 10 host1.example.
sub.*.example. 3600 TXT "this is not a wild card"
host1.example. 3600 A 192.0.4.1
_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 queries would be synthesized from one of the wildcards:
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.'
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 queries would not be synthesized from any of the
wildcards:
QNAME=host1.example., QTYPE=MX, QCLASS=IN
because host1.example. exists
QNAME=ghost.*.example., QTYPE=MX, QCLASS=IN
because *.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)
2.2.2 Empty Non-terminals
Empty non-terminals [RFC2136, 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 a 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."
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 [RFC1035]. 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, this is not the intent of
the original document.
2.2.3 Yet Another Definition of Existence
RFC1034'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 on RRSet. A node may have no RRSets if it has descendents
that do, this node is a empty non-terminal. A node may have its own
RRSets and have descendants with RRSets too.
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. At the delegating server, 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 does a Wild Card Domain Name is not Special
When a wild card domain name appears in a message's query section,
no special processing occurs. An asterisk label in a query name
only (label) matches an asterisk label in the existing zone tree
when the 4.3.2 algorithm is being followed.
When a wild card 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 Wild Card Domain Name On a Response
The description of how wildcards impact response generation is in
RFC 1034, 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 wild card.
The algorithm in RFC 1034, section 4.3.2. is not intended to be pseudo
code, i.e., its steps are not intended to be followed in strict
order. The "algorithm" is a suggestion. As such, in step 3, parts
a, b, and c, do not have to be implemented in that order.
3.1 Step 2
Step 2 of the RFC 1034's 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 be ever be used for synthesis.
3.2 Step 3
Step 3 is dominated by three parts, labelled '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. It is possible, from
the description given, that a query might fit into both part a and
part b, this is not within the scope of this document.
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 wild
card 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 wild card domain name immediately descending from the
closest encloser, provided that this wild card 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, the lookup does not look for other wildcard records.
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
_telnet._tcp.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 step 'a' and
step 'c' a change to this passage has been made.
The change is to add the following text to step 'c':
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 step a covering the processing of
CNAME RRSets.
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 are
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," i.e., empty non-terminal wild card
domain names.
4.1 SOA RRSet at a Wild Card Domain Name
A wild card domain name owning an SOA RRSet means that the domain
is at the root of the zone (apex). The domain can not be a source of
synthesis because that is, by definition, a descendent node (of
the closest encloser) and a zone apex is at the top of the zone.
Although a wild card 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.
E.g., 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 reason is that the asterisk label only
becomes significant when RFC 1034's 4.3.2, step 3 part 'c' in 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 Wild Card Domain Name
With the definition of DNSSEC [RFC4033, RFC4034, RFC4035] now in
place, the semantics of a wild card domain name owning an NS RR 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) become unclear.
After some lengthy discussions, there has been no clear "best answer"
on how to document the semantics of such a situation. Barring such
records from the DNS would require definition of rules for that, as
well as introducing a restriction on records that were once legal.
Allowing such records and amending the process of signature
management would entail complicating the DNSSEC definition.
Combining these observations with thought that a wild card domain name
owning an NS record is an operationally uninteresting scenario, i.e.,
it won't happen in the normal course of events, accomodating this
situation in the specification would also be categorized as
"needless complication." Further, expending more effort on this
topic has proven to be an exercise in diminishing returns.
In summary, there is no definition given for wild card 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 wild card NS
RRSets in a zone is discouraged, but not barred.
4.3 CNAME RRSet at a Wild Card Domain Name
The issue of a CNAME RRSet owned by a wild card 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 Wild Card Domain Name
Ownership of a DNAME RRSet by a wild card 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 record "a.b.example. DNAME foo.bar.tld." and another
cache obtains "b.example. DNAME foo.bar.tld.", both generated from
the record "*.example. DNAME foo.bar.tld." 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 some, it is 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, an
operational nightmare ensues.
Another justification for banning or avoiding 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 not to be permitted.
4.5 SRV RRSet at a Wild Card Domain Name
The definition of the SRV RRset is RFC 2782 [RFC2782]. 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 a domain name. I.e., once
removing the _Service and _Proto labels from the owner name of the
SRV RRSet, what remains could be a wild card domain name but this is
immaterial to the SRV RRSet.
E.g., If an SRV record is:
_foo._udp.*.example. 10800 IN SRV 0 1 9 old-slow-box.example.
*.example is a wild card domain name and although it it 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 wild card domain name.
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 Wild Card Domain Name
A DS RRSet owned by a wild card domain name is meaningless and
harmless.
4.7 NSEC RRSet at a Wild Card Domain Name
Wild card 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 RR's will not be harmful as
they will never be used in negative caching or to generate a negative
response.
4.8 RRSIG at a Wild Card Domain Name
RRSIG records will be present at a wild card 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 Wild Card 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. IANA Considerations
None.
7. References
Normative References
[RFC20] ASCII Format for Network Interchange, V.G. Cerf, Oct-16-1969
[RFC1034] Domain Names - Concepts and Facilities, P.V. Mockapetris,
Nov-01-1987
[RFC1035] Domain Names - Implementation and Specification, P.V
Mockapetris, Nov-01-1987
[RFC1995] Incremental Zone Transfer in DNS, M. Ohta, August 1996
[RFC2119] Key Words for Use in RFCs to Indicate Requirement Levels, S
Bradner, March 1997
[RFC2181] Clarifications to the DNS Specification, R. Elz and R. Bush,
July 1997
[RFC2308] Negative Caching of DNS Queries (DNS NCACHE), M. Andrews,
March 1998
[RFC2782] A DNS RR for specifying the location of services (DNS SRV),
A. Gulbrandsen, et.al., February 2000
[RFC4033] DNS Security Introduction and Requirements, R. Arends,
et.al., March 2005
[RFC4034] Resource Records for the DNS Security Extensions, R. Arends,
et.al., March 2005
[RFC4035] Protocol Modifications for the DNS Security Extensions,
R. Arends, et.al., March 2005
[RFC2672] Non-Terminal DNS Name Redirection, M. Crawford, August 1999
Informative References
[RFC2136] Dynamic Updates in the Domain Name System (DNS UPDATE), P.
Vixie, Ed., S. Thomson, Y. Rekhter, J. Bound, April 1997
8. Editor
Name: Edward Lewis
Affiliation: NeuStar
Address: 46000 Center Oak Plaza, Sterling, VA, 20166, US
Phone: +1-571-434-5468
Email: ed.lewis@neustar.biz
Comments on this document can be sent to the editor or the mailing
list for the DNSEXT WG, namedroppers@ops.ietf.org.
9. Others Contributing to the Document
This document represents the work of a large working group. The
editor merely recorded the collective wisdom of the working group.
10. Trailing Boilerplate
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Expiration
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