DNSEXT (Independent submission) O. Kolkman
Internet-Draft RIPE NCC
Expires: March 2, 2003 J. Ihren
Autonomica
R. Arends
A.R.E.N.D.S.
September 2002
DNSSEC Wildcard Optimization
draft-olaf-dnsext-dnssec-wildcard-optimization-01.txt
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
Secure denial of the existence of wildcards may lead to a large
number of NXT RRs and associated SIG RRs in DNS responses, even in
the common case when wildcards are not present in the zone. This
optimization uses one bit from the NXT type array to signal that
there is no closer wildcard in the zone for a given query name. This
reduces the packet size and the need for executing slow, and
complicated, code paths in common queries. In cases where there are
no wildcard RRs in the zone (i.e. the root zone) only one NXT RR and
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corresponding SIG is needed for denial of existence of the wildcard.
The key words "MAY","MAY NOT", "MUST", "MUST NOT", "REQUIRED",
"RECOMMENDED", "SHOULD", and "SHOULD NOT" in this document are to be
interpreted as described in RFC2119.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 RFC2535 Wildcard Processing . . . . . . . . . . . . . . . . 3
1.2 Signalling the Existence of a Wildcard . . . . . . . . . . . 3
2. DNSSEC Protocol Changes . . . . . . . . . . . . . . . . . . 4
2.1 Server Side . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1.1 Zone Signing . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1.2 Server Responses . . . . . . . . . . . . . . . . . . . . . . 4
2.1.3 Dynamic DNS . . . . . . . . . . . . . . . . . . . . . . . . 5
2.2 Resolver Side . . . . . . . . . . . . . . . . . . . . . . . 5
3. IANA Considerations . . . . . . . . . . . . . . . . . . . . 6
4. Security Considerations . . . . . . . . . . . . . . . . . . 6
5. Internationalization Considerations . . . . . . . . . . . . 6
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
7. Document Changes . . . . . . . . . . . . . . . . . . . . . . 6
7.1 draft 00->01 . . . . . . . . . . . . . . . . . . . . . . . . 6
Normative References . . . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 7
A. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 8
A.1 Zone without wildcards . . . . . . . . . . . . . . . . . . . 8
A.1.1 Optimized proof . . . . . . . . . . . . . . . . . . . . . . 8
A.1.2 RFC2535 proof . . . . . . . . . . . . . . . . . . . . . . . 8
A.2 Zone with wildcards . . . . . . . . . . . . . . . . . . . . 9
A.2.1 Optimized proof . . . . . . . . . . . . . . . . . . . . . . 10
A.2.2 NXDOMAIN with additional proof for no wildcard . . . . . . . 10
A.2.3 Another Optimized Proof . . . . . . . . . . . . . . . . . . 11
A.2.4 Denial of Existence of Closer Match . . . . . . . . . . . . 11
A.2.5 The NXT 'next name' Proving Existence of a Wildcard . . . . 12
Full Copyright Statement . . . . . . . . . . . . . . . . . . 13
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1. Introduction
Wildcards make authenticated denial of existence complex. Many zones
do not contain wildcards but still incur a penalty. If the NXT RR
contains an indication that a wildcard match can not exist then less
DNSSEC related RRs and less computation are needed to authoritatively
deny the existence of a name in the zone.
1.1 RFC2535 Wildcard Processing
RFC2535 [1] specifies that the non-existence of a match against a
wildcard is proven by a set of relevant NXT records. In practice
this will result to at least 2 NXT RRs and corresponding SIGs being
returned. There are cases where the denial of the existence of
wildcards will need many more than 2 NXT RRs. Even in zones that do
not use wildcards this will lead to complex answers for which the
resolvers will need to follow NXT chains and which are hard to
troubleshoot by operators.
1.2 Signalling the Existence of a Wildcard
The NXT RR, used to the prove the non-existence of data, uses a type
bit-map to track which types are available for a given name. We
propose to use one bit (see section Section 3) in the type bitmap to
signal if a wildcard is available in a zone. We refer to this bit as
the "NOWILD-bit".
If the NOWILD-bit is set to 1 then the NXT RR signals that there is
no wildcard match possible against the query name, only if the bit is
set to 0 further processing needs to be done. For zones without
wildcards the NOWILD-bit MAY always be set to 1.
The following optimizations are realized:
o Servers and resolvers will only have to execute a slow and
somewhat complicated code paths if wildcard are present in the
zone.
o Packet size of answers reduce in most common cases; for the root
zone the authority section only contains one NXT RR with
associated SIGs instead of two NXT RRs with associated SIGs.
o In case of absence of wildcards-matches answers will be easier to
interpret by human operators troubleshooting responses;
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2. DNSSEC Protocol Changes
This is an update to the RFC2535 protocol. Resolvers MUST implement
these changes. Servers MAY implement these changes.
2.1 Server Side
2.1.1 Zone Signing
Servers that implement the optimization MAY perform the following
actions at zone signing time.
At zone signing time, when the NXT RRs are generated, the NOWILD-bit
MUST be set to 0 if for an ownername 'label(j).label(j-1).label(j-2)
... label(0).' there is no wildcard name '*.label(i).label(i-1) ...
label(0).' in the zone for all i < j. In other words the label is
set to 0 if there exists a wildcard that would match QNAME=ownername
while ignoring the possible existence of a domain name between the
ownername and the wildcard domain. For all other ownernames the bit
MUST be set to 1.
If, because of implementation or policy issues, the algorithm in the
previous paragraph is not applied then the bit MUST be set to 0 for
all NXT RRs in the zone. Servers that do do not implement the
optimization have already set their NOWILD bit to 0 by virtue of the
requirements of RFC2535 section 5.2.
When the algorithm is applied a NXT RR that proves the non-existence
of a full match of the QNAME will also prove, when it's NOWILD-bit is
set to 1, that there is no match of the QNAME to any wildcard that
may exist in the zone
2.1.2 Server Responses
When queried for a name for which there is no match, i.e. no full
and no wildcard match, in the zone:
o Servers MUST return the NXT RR that proves the non-existence of
the query name in the NXDOMAIN response. If there is no match for
a wildcard and the NOWILD-bit is set to 1 at signing time and the
one NXT RR is sufficient. If the NOWILD-bit for the NXT RR that
proves non-existence of the query name is set to 0 then NXT RRs
that prove the non-existence of possible wildcard matches MUST be
returned as well.
When queried for a name for which there is a match in the zone:
o If the match is an exact match than no NXT RRs are returned in the
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additional section.
o Servers for zones that contain one or more wildcards MUST return
the NXT RRs that prove the non-existence of the exact match. They
must also provide proof that there is no closer match for the
QNAME than the match returned in the answer section.
The proof algorithm for non-existence of wildcards, an exact match or
closer matches conforms to RFC2535.
2.1.3 Dynamic DNS
When dynamically adding or removing a name that does not contains
wildcards, the 'next name' for the name immediately above the
inserted, or deleted name needs to be updated. The NOWILD bit of the
inserted name is to be set according to the procedure as described in
Section 2.1.1. Except for setting the NOWILD bit this is similar to
the RFC2535 procedure.
If a name containing a wildcard is deleted from a zone one has to
verify if, for all names in the zone with the bit set to 0, the
NOWILD bit can be toggled. If a name containing a wildcard is added
one has to verify if, for all the names in the zone, the bit needs to
be set to 0.
The NOWILD bit is not to be modified during an update of a name that
already exists in the zone.
Dynamic updates of names that contain wildcards may lead to
performance penalties for large dynamic zones and one MAY therefore
choose not to perform the NOWILD optimization for dynamic zones.
2.2 Resolver Side
When receiving an answer to a query a resolver MUST assess if the
answer is a result of a wildcard match. If the result is an exact
match then there will be no NXT RRs in the authority section.
If the answer is a wildcard match then the resolver will need to
verify that the exact name does not exist. The NXT RRs in the
additional section, which per definition have their NOWILD-bit set to
0, will need to prove that there is no closer match. ( conforming to
RFC2535).
If the response is NXDOMAIN (i.e. no match at all) then the resolver
MUST verify if the NXT RR proves the non-existence of the exact match
in the zone. No further NXT RRs are needed if the NXT RR has it's
NOWILD-bit set to 1. A DNS packet containing an NXDOMAIN response
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accompanied by a NXT RR that has it's NOWILD-bit set to 0 will need
to contain proof that there are no wildcard matches against the QNAME
(conforming to RFC2535 ).
The NXT data and the NOWILD-bit together supply the proof on the non-
existence of a wildcard. There is one situation where the NOWILD-bit
is set to 1 but the NXT's 'next name' proves that there is a
wildcard. This is when the 'next name' itself contains a wildcard.
Resolvers that verify NXDOMAIN replies MUST verify the NXT 'next
name' first before the NOWILD-bit. Also see example Appendix A.2.5.
The fact that resolvers that obtain an answer with a NXT RR's NOWILD
set to 1 do not receive additional proof for the non-existence of
wildcards is incompatible with RFC2535.
3. IANA Considerations
Although there is no RR record associated the NOWILD-bit. The value
of the bit must be registered as a DNS RR-type. To not cause the NXT
type bitmap to grow beyond 4 octets unnecessary we propose to reuse
type code 31 (the EID type code is undocumented).
4. Security Considerations
The draft provides an optimization for wildcard handling. Resolvers
MUST verify for the denial of existence of matches or the denial of
existence of closer matches when an answer is returned and the
NOWILD-bit is set to 0.
5. Internationalization Considerations
There are no internationalization considerations.
6. Acknowledgements
Olafur Gudmundsson, Daniel Karrenberg and Ed Lewis for providing
critique and input on earlier versions of this document.
7. Document Changes
7.1 draft 00->01
Reordered and reworded the 'protocol changes' section. We tried
to make the fact that resolvers must and servers may implement
this optimization more explicit.
Change from using the SIG bit to another bit in the NXT type-
bitmap, changed the name of the bit and added IANA considerations.
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Note that the meaning of the bit being set and unset are changed
because of the default setting. Because of the fact that we want
to maintain backward compatibility with servers that do not
implement this bit and the bit in the typemap is currently set to
0 the default behaviour should be follow old-style NXT proof.
Corrected mistakes in the examples.
Various style and spelling corrections.
Normative References
[1] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
Authors' Addresses
Olaf M. Kolkman
RIPE NCC
Singel 256
1016 AB Amsterdam
NL
Phone: +31 20 535 4444
EMail: olaf@ripe.net
URI: http://www.ripe.net/
Johan Ihren
Autonomica
Bellmansgatan 30
SE-118 47 Stockholm
SE
EMail: johani@autonomica.se
Roy Arends
A.R.E.N.D.S.
Bankastraat 41-e
1094 EB Amsterdam
NL
Phone: +31206931681
EMail: Roy@logmess.com
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Appendix A. Examples
A.1 Zone without wildcards
In the following example zone file there are no wildcards. All
NOWILD bits are set to 1. The actual SIG RRs and the KEY RRs are
left out from the zone data and type bitmaps for clarity only.
$ORIGIN example.
@ IN SOA
@ NXT a SOA NXT NOWILD ; NOWILD-bit set to 1
a A 10.0.0.1
a NXT a.b A NXT NOWILD ; NOWILD-bit set to 1
a.b A 10.0.0.2
a.b NXT a.c A NXT NOWILD ; NOWILD-bit set to 1
a.c A 10.0.0.4
a.c NXT a.b.c A NXT NOWILD ; NOWILD-bit set to 1
a.b.c A 10.0.0.5
a.b.c NXT f A NXT NOWILD ; NOWILD-bit set to 1
f A 10.0.0.6
f NXT @ A NXT NOWILD ; NOWILD-bit set to 1
A.1.1 Optimized proof
A query for any existing name will return a signed answer without NXT
RRs in the authority section. A query for any non existing name will
only return 1 NXT RR proving the non-existence of the QNAME in the
zone and, by virtue of the NOWILD-bit being 1, this is sufficient
proof there is no wildcard.
QNAME= d.b.c.example. QTYPE=A
RCODE=NXDOMAIN
;; Authority
example. SOA
SIG SOA
a.b.c.example. NXT f.example. A NXT NOWILD
SIG NXT
;; Additional
(... skipped ... )
A.1.2 RFC2535 proof
For comparison we supply the same answer without the optimization
applied i.e. NOWILD set to 0 for all NXT RRs in the zone. The
answer needs to contain prove that *.b.c.example, *.c.example and
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*.example do not exist, unless a name that exists in the zone
terminates the possible match of those wildcards against the QNAME.
QNAME= d.b.c.example. QTYPE=A
RCODE=NXDOMAIN
;; Authority
example. SOA
SIG SOA
a.b.c.example. NXT f.example. A NXT
SIG NXT
; proofs non-existence of exact match.
a.c.example. NXT a.b.c.example. A NXT
SIG NXT
; proofs non-existence of *.b.c.example.
;; Additional
(... skipped ... )
Note that the existence of 'a.b.c.example NXT' RR terminates a
wildcard match of QNAME against *.c.example. and *.example. So the
answer packet does not need to contain further proof for the non-
existence of those wildcards. However, a resolver will have to
execute logic to verify that the existence of 'a.b.c.example.'
terminates the possible match of the QNAME against the possible
wildcards and that the answer is therefore complete.
A.2 Zone with wildcards
In the following example zone file there is a wildcard. Some NOWILD
bits are set to 1, others for which there is no wildcard in the zone
if the leftmost labels are chopped off, have there NOWILD-bit set to
0. The actual SIG RRs and the KEY RRs at the apex are left out for
clarity. The queries for which a wildcard match is returned will
have the NOWILD-bit set to 0, there proof for the non-existing closer
match is to be supplied and checked by the resolver.
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$ORIGIN example.
@ IN SOA
@ NXT a SOA NXT NOWILD ; NOWILD-bit set to 1
a A 10.0.0.1
a NXT a.b A NXT NOWILD ; NOWILD-bit set to 1
a.b A 10.0.0.2
a.b NXT *.c A NXT NOWILD ; NOWILD-bit set to 1
*.c A 10.0.0.3
*.c NXT a.c A NXT SIG ; NOWILD-bit set to 0
a.c A 10.0.0.4
a.c NXT a.b.c A NXT SIG ; NOWILD-bit set to 0
a.b.c A 10.0.0.5
a.b.c NXT f A NXT SIG ; NOWILD-bit set to 0
f A 10.0.0.6
f NXT @ A NXT NOWILD ; NOWILD-bit set to 1
A.2.1 Optimized proof
QNAME= c.a.a.example. QTYPE=A
RCODE=NXDOMAIN
;; Authority
example. SOA
SIG SOA
a.example. NXT a.b.example. A NXT SIG NOWILD
; NOWILD-bit set to 1 proves no full
; match and no wildcards that match
; QNAME
SIG NXT
;; Additional
(... skipped ... )
A.2.2 NXDOMAIN with additional proof for no wildcard
The following example contains a NXDOMAIN answer and the proof that
there is no wildcard match.
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QNAME= e.example. QTYPE=A
RCODE=NXDOMAIN
;; Authority
example.example SOA
SIG SOA
a.b.c.example. NXT f.example. A NXT SIG ; NOWILD-bit set to 0,
; proves no full match
SIG NXT
example. NXT a.example A NXT SIG NOWILD ; NOWILD-bit set to 1,
; proves no *.example.
;; Additional
(... skipped ... )
A.2.3 Another Optimized Proof
The following example contains a NXDOMAIN answer and the proof that
there is no wildcard match. In this particular case the proof is
optimized because of the NOWILD-bit on the f NXT RR being set to
zero.
QNAME= g.example. QTYPE=A
RCODE=NXDOMAIN
;; Authority
example.example SOA
SIG SOA
f.example. NXT example. A NXT NOWILD ; NOWILD-bit set to 1
; proves no full match
;; Additional
(... skipped ... )
A.2.4 Denial of Existence of Closer Match
The following example contains an answer with wildcard expansion and
the proof that there is no closer match. This is similar to a
RFC2535 proof of non-existence.
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QNAME= d.b.c.example. QTYPE=A
RCODE=ANSWER
;; Answer
d.b.c.example. A 10.0.0.3 ; expansion of *.c
SIG A (labelcount=2) ; labelcount proofs wildcard
; example
;; Authority
example.example. SOA
SIG SOA
a.b.c.example. NXT f.example. A NXT SIG ; NOWILD-bit set to 0,
; proves no exact match,
SIG NXT
a.c.example. NXT a.b.c.example. A NXT SIG ; NOWILD-bit set to 0
; proves non-existence of
; *.b.c.example.
; No further proofs needed
;; Additional
(... skipped ... )
A.2.5 The NXT 'next name' Proving Existence of a Wildcard
In the zone above the a.b NXT RR has it's NOWILD-bit set to 1. If
one would query for '#.c' which canonically orders between a.b and
*.c one would get back "a.b NXT *.c". A attacker can use the this
NXT RR in a malformed NXDOMAIN response.
QNAME= #.c.example. QTYPE=A
RCODE=NXDOMAIN ; Black hat answer !!!!
;; Authority
example.example SOA
SIG SOA
a.b.example. NXT *.c.example. A NXT NOWILD ; NOWILD-bit set to 1
; but *.c exists
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