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.
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.
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 [RFC4033, RFC4034, RFC4035] 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.
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 [RFC2136] 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".
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 [RFC2672] 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.
4.5. SRV RRSet at a Wildcard 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 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.
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 [RFC2308].
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
[RFC20] Cerf, V., "ASCII format for network interchange", RFC 20,
October 1969.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC1995] Ohta, M., "Incremental Zone Transfer in DNS", RFC 1995,
August 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, March 1998.
[RFC2672] Crawford, M., "Non-Terminal DNS Name Redirection", RFC
2672, August 1999.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
February 2000.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", RFC
4033, March 2005.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005.
[RFC4035] 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
[RFC2136] 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
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