Network Working Group B. Laurie
Internet-Draft G. Sisson
Expires: December 3, 2005 Nominet
R. Arends
Telematica Instituut
june 2005
DNSSEC Hash Authenticated Denial of Existence
draft-ietf-dnsext-nsec3-02
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 BCP 79.
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 December 3, 2005.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
The DNS Security (DNSSEC) NSEC resource record (RR) is intended to be
used to provide authenticated denial of existence of DNS ownernames
and types; however, it permits any user to traverse a zone and obtain
a listing of all ownernames.
A complete zone file can be used either directly as a source of
Laurie, et al. Expires December 3, 2005 [Page 1]
Internet-Draft nsec3 june 2005
probable e-mail addresses for spam, or indirectly as a key for
multiple WHOIS queries to reveal registrant data which many
registries (particularly in Europe) may be under strict legal
obligations to protect. Many registries therefore prohibit copying
of their zone file; however the use of NSEC RRs renders policies
unenforceable.
This document proposes a scheme which obscures original ownernames
while permitting authenticated denial of existence of non-existent
names. Non-authoritative delegation point NS RR types may be
excluded.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Rationale . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2 Reserved Words . . . . . . . . . . . . . . . . . . . . . . 4
1.3 Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. The NSEC3 Resource Record . . . . . . . . . . . . . . . . . . 5
2.1 NSEC3 RDATA Wire Format . . . . . . . . . . . . . . . . . 5
2.1.1 The Authoritative Only Flag Field . . . . . . . . . . 6
2.1.2 The Hash Function Field . . . . . . . . . . . . . . . 6
2.1.3 The Iterations Field . . . . . . . . . . . . . . . . . 7
2.1.4 The Salt Length Field . . . . . . . . . . . . . . . . 7
2.1.5 The Salt Field . . . . . . . . . . . . . . . . . . . . 7
2.1.6 The Next Hashed Ownername Field . . . . . . . . . . . 7
2.1.7 The list of Type Bit Map(s) Field . . . . . . . . . . 8
2.2 The NSEC3 RR Presentation Format . . . . . . . . . . . . . 9
3. Creating Additional NSEC3 RRs for Empty Non Terminals . . . . 9
4. Calculation of the Hash . . . . . . . . . . . . . . . . . . . 10
5. Including NSEC3 RRs in a Zone . . . . . . . . . . . . . . . . 10
6. Special Considerations . . . . . . . . . . . . . . . . . . . . 11
6.1 Delegation Points . . . . . . . . . . . . . . . . . . . . 11
6.1.1 Unsigned Delegations . . . . . . . . . . . . . . . . . 11
6.2 Proving Nonexistence . . . . . . . . . . . . . . . . . . . 12
6.3 Salting . . . . . . . . . . . . . . . . . . . . . . . . . 13
6.4 Hash Collision . . . . . . . . . . . . . . . . . . . . . . 13
6.4.1 Avoiding Hash Collisions during generation . . . . . . 14
6.4.2 Second Preimage Requirement Analysis . . . . . . . . . 14
6.4.3 Possible Hash Value Truncation Method . . . . . . . . 14
7. Performance Considerations . . . . . . . . . . . . . . . . . . 15
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
9. Security Considerations . . . . . . . . . . . . . . . . . . . 15
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 16
10.1 Normative References . . . . . . . . . . . . . . . . . . . 16
10.2 Informative References . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 17
A. Example Zone . . . . . . . . . . . . . . . . . . . . . . . . . 18
Laurie, et al. Expires December 3, 2005 [Page 2]
Internet-Draft nsec3 june 2005
B. Example Responses . . . . . . . . . . . . . . . . . . . . . . 23
B.1 answer . . . . . . . . . . . . . . . . . . . . . . . . . . 23
B.1.1 Authenticating the Example DNSKEY RRset . . . . . . . 25
B.2 Name Error . . . . . . . . . . . . . . . . . . . . . . . . 26
B.3 No Data Error . . . . . . . . . . . . . . . . . . . . . . 28
B.3.1 No Data Error, Empty Non-Terminal . . . . . . . . . . 29
B.4 Referral to Signed Zone . . . . . . . . . . . . . . . . . 30
B.5 Referral to Unsigned Zone using Opt-In . . . . . . . . . . 31
B.6 Wildcard Expansion . . . . . . . . . . . . . . . . . . . . 32
B.7 Wildcard No Data Error . . . . . . . . . . . . . . . . . . 34
B.8 DS Child Zone No Data Error . . . . . . . . . . . . . . . 35
Intellectual Property and Copyright Statements . . . . . . . . 37
Laurie, et al. Expires December 3, 2005 [Page 3]
Internet-Draft nsec3 june 2005
1. Introduction
The DNS Security Extensions (DNSSEC) introduced the NSEC Resource
Record (RR) for authenticated denial of existence. This document
introduces a new RR as an alternative to NSEC that provides measures
against zone traversal and allows for gradual expansion of
delegation-centric zones.
1.1 Rationale
The DNS Security Extensions included the NSEC RR to provide
authenticated denial of existence. Though the NSEC RR meets the
requirements for authenticated denial of existence, it introduced a
side-effect in that the contents of a zone can be enumerated. This
property introduces undesired policy issues.
A second problem was the requirement that the existence of all record
types in a zone - including delegation point NS record types - must
be accounted for, despite the fact that delegation point NS RRsets
are not authoritative and not signed. This requirement has a side-
effect that the overhead of delegation-centric signed zones is not
related to the increase in security of subzones. This requirement
does not allow delegation-centric zones size to grow in relation to
the growth of signed subzones.
In the past, solutions have been proposed as a measure against these
side effects but at the time were regarded as secondary over the need
to have a stable DNSSEC specification. With (draft-vixie-dnssec-ter)
a graceful transition path to future enhancements is introduced,
while current DNSSEC deployment can continue. This document presents
the NSEC3 Resource Record which mitigates these issues with the NSEC
RR.
The reader is assumed to be familiar with the basic DNS concepts
described in RFC1034 [RFC1034], RFC1035 [RFC1035] and subsequent RFCs
that update them: RFC2136 [RFC2136], RFC2181 [RFC2181] and RFC2308
[RFC2308].
1.2 Reserved Words
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
1.3 Terminology
In this document the term "original ownername" refers to a standard
ownername. Because this proposal uses the result of a hash function
Laurie, et al. Expires December 3, 2005 [Page 4]
Internet-Draft nsec3 june 2005
over the original (unmodified) ownername, this result is referred to
as "hashed ownername".
"Canonical ordering of the zone" means the order in which hashed
ownernames are arranged according to their numerical value, treating
the leftmost (lowest numbered) byte as the most significant byte.
2. The NSEC3 Resource Record
The NSEC3 RR provides Authenticated Denial of Existence for DNS
Resource Record Sets.
The NSEC3 Resource Record lists RR types present at the NSEC3 RR's
original ownername. It includes the next hashed ownername in the
canonical ordering of the zone. The complete set of NSEC3 RRs in a
zone indicates which RRsets exist for the original ownername of the
RRset and form a chain of hashed ownernames in the zone. This
information is used to provide authenticated denial of existence for
DNS data, as described in RFC 4035 [RFC4035]. Unsigned delegation
point NS RRsets can optionally be excluded. To provide protection
against zone traversal, the ownernames used in the NSEC3 RR are
cryptographic hashes of the original ownername prepended to the name
of the zone. The NSEC3 RR indicates which hash function is used to
construct the hash, which salt is used, and how many iterations of
the hash function are performed over the original ownername.
The ownername for the NSEC3 RR is the base32 encoding of the hashed
ownername.
The type value for the NSEC3 RR is XX.
The NSEC3 RR RDATA format is class independent.
The NSEC3 RR SHOULD have the same TTL value as the SOA minimum TTL
field. This is in the spirit of negative caching [RFC2308].
2.1 NSEC3 RDATA Wire Format
The RDATA of the NSEC3 RR is as shown below:
Laurie, et al. Expires December 3, 2005 [Page 5]
Internet-Draft nsec3 june 2005
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|A|Hash Function| Iterations |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Salt Length | Salt /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Next Hashed Ownername /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Type Bit Maps /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2.1.1 The Authoritative Only Flag Field
The Authoritative Only Flag field indicates whether the Type Bit Maps
include delegation point NS record types.
If the flag is set to 1, the NS RR type bit for a delegation point
ownername SHOULD be clear when the NSEC3 RR is generated. The NS RR
type bit MUST be ignored during processing of the NSEC3 RR. The NS
RR type bit has no meaning in this context (it is not authoritative),
hence the NSEC3 does not contest the existence of a NS RRset for this
ownername. When a delegation is not secured, there exist no DS RR
type nor any other authoritative types for this delegation, hence the
unsecured delegation has no NSEC3 record associated. Please see the
Special Consideration section for implications for unsigned
delegations.
If the flag is set to 0, the NS RR type bit for a delegation point
ownername MUST be set if the NSEC3 covers a delegation, even though
the NS RR itself is not authoritative. This implies that all
delegations, signed or unsigned, have an NSEC3 record associated.
This behaviour is identical to NSEC behaviour.
2.1.2 The Hash Function Field
The Hash Function field identifies the cryptographic hash function
used to construct the hash-value.
This document defines Value 1 for SHA-1 and Value 127 for
experimental. All other values are reserved.
On reception, a resolver MUST discard an NSEC3 RR with an unknown
hash function value.
Laurie, et al. Expires December 3, 2005 [Page 6]
Internet-Draft nsec3 june 2005
2.1.3 The Iterations Field
The Iterations field defines the number of times the hash has been
iterated. More iterations results in greater resiliency of the hash
value against dictionary attacks, but at a higher cost for both the
server and resolver.
2.1.4 The Salt Length Field
The salt length field defines the length of the salt in octets.
2.1.5 The Salt Field
The Salt field is not present when the Salt Length Field has a value
of 0.
The Salt field is prepended to the original ownername before hashing
in order to defend against precalculated dictionary attacks.
The salt is also prepended during iterations of the hash function.
Note that although it is theoretically possible to cover the entire
possible ownername space with different salt values, it is
computationally infeasible to do so, and so there MUST be at least
one salt which is the same for all NSEC3 records. This means that no
matter what name is asked for in a query, it is guaranteed to be
possible to find a covering NSEC3 record. Note that this does not
preclude the use of two different salts at the same time - indeed
this may well occur naturally, due to rolling the salt value
periodically.
The salt value SHOULD be changed from time to time - this is to
prevent the use of a precomputed dictionary to reduce the cost of
enumeration.
2.1.6 The Next Hashed Ownername Field
The Next Hashed Ownername field contains the hash of the ownername of
the next RR in the canonical ordering of the hashed ownernames of the
zone. The value of the Next Hashed Ownername Field in the last NSEC3
record in the zone is the same as the ownername of the first NSEC3 RR
in the zone in canonical order.
Hashed ownernames of RRsets not authoritative for the given zone
(such as glue records) MUST NOT be listed in the Next Hashed
Ownername unless at least one authoritative RRset exists at the same
ownername.
Laurie, et al. Expires December 3, 2005 [Page 7]
Internet-Draft nsec3 june 2005
Note that the Next Hashed Ownername field is not encoded, unlike the
NSEC3 RR's ownername. It is the unmodified binary hash value.
2.1.7 The list of Type Bit Map(s) Field
The Type Bit Maps field identifies the RRset types which exist at the
NSEC3 RR's ownername.
The Type bits for the NSEC3 RR and RRSIG RR MUST be set during
generation, and MUST be ignored during processing.
The RR type space is split into 256 window blocks, each representing
the low-order 8 bits of the 16-bit RR type space. Each block that
has at least one active RR type is encoded using a single octet
window number (from 0 to 255), a single octet bitmap length (from 1
to 32) indicating the number of octets used for the window block's
bitmap, and up to 32 octets (256 bits) of bitmap.
Blocks are present in the NSEC3 RR RDATA in increasing numerical
order.
"|" denotes concatenation
Type Bit Map(s) Field = ( Window Block # | Bitmap Length | Bitmap ) +
Each bitmap encodes the low-order 8 bits of RR types within the
window block, in network bit order. The first bit is bit 0. For
window block 0, bit 1 corresponds to RR type 1 (A), bit 2 corresponds
to RR type 2 (NS), and so forth. For window block 1, bit 1
corresponds to RR type 257, bit 2 to RR type 258. If a bit is set to
1, it indicates that an RRset of that type is present for the NSEC3
RR's ownername. If a bit is set to 0, it indicates that no RRset of
that type is present for the NSEC3 RR's ownername.
The RR type 2 (NS) is authoritative at the apex of a zone and is not
authoritative at delegation points. If the Authoritative Only Flag
is set to 1, the delegation point NS RR type MUST NOT be included in
the type bit maps. If the Authoritative Only Flag is set to 0, the
NS RR type at a delegation point MUST be included in the type bit
maps.
Since bit 0 in window block 0 refers to the non-existing RR type 0,
it MUST be set to 0. After verification, the validator MUST ignore
the value of bit 0 in window block 0.
Bits representing Meta-TYPEs or QTYPEs as specified in RFC 2929
[RFC2929] (section 3.1) or within the range reserved for assignment
only to QTYPEs and Meta-TYPEs MUST be set to 0, since they do not
Laurie, et al. Expires December 3, 2005 [Page 8]
Internet-Draft nsec3 june 2005
appear in zone data. If encountered, they must be ignored upon
reading.
Blocks with no types present MUST NOT be included. Trailing zero
octets in the bitmap MUST be omitted. The length of each block's
bitmap is determined by the type code with the largest numerical
value, within that block, among the set of RR types present at the
NSEC3 RR's actual ownername. Trailing zero octets not specified MUST
be interpreted as zero octets.
2.2 The NSEC3 RR Presentation Format
The presentation format of the RDATA portion is as follows:
The Authoritative Only Field is represented as an unsigned decimal
integer. The value are either 0 or 1.
The Hash field is presented as the name of the hash or as an unsigned
decimal integer. The value has a maximum of 127.
The Iterations field is presented as an unsigned decimal integer.
The Salt Length field is not presented.
The Salt field is represented as a sequence of case-insensitive
hexadecimal digits. Whitespace is not allowed within the sequence.
The Salt Field is represented as 00 when the Salt Length field has
value 0.
The Next Hashed Ownername field is represented as a sequence of case-
insensitive base32 digits. Whitespace is allowed within the
sequence.
The List of Type Bit Map(s) Field is represented as a sequence of RR
type mnemonics. When the mnemonic is not known, the TYPE
representation as described in RFC 3597 [RFC3597] (section 5) MUST be
used.
3. Creating Additional NSEC3 RRs for Empty Non Terminals
In order to prove the non-existence of a record that might be covered
by a wildcard, it is necessary to prove the existence of its closest
encloser. A closest encloser might be an Empty Non Terminal.
Additional NSEC3 RRs are synthesized which cover every existing
intermediate label level. Additional NSEC3 RRs are identical in
format to NSEC3 RRs that cover existing RRs in the zone. The
difference is that the type-bit-maps only indicate the existence of
Laurie, et al. Expires December 3, 2005 [Page 9]
Internet-Draft nsec3 june 2005
an NSEC3 RR type and an RRSIG RR type.
4. Calculation of the Hash
Define H(x) to be the hash of x using the hash function selected by
the NSEC3 record and || to indicate concatenation. Then define:
IH(salt,x,0)=H(x || salt)
IH(salt,x,k)=H(IH(salt,x,k-1) || salt) if k > 0
Then the calculated hash of an ownername is
IH(salt,ownername,iterations-1), where the ownername is the canonical
form.
The canonical form of the ownername is the wire format of the
ownername where:
1. The ownername is fully expanded (no DNS name compression) and
fully qualified;
2. All uppercase US-ASCII letters are replaced by the corresponding
lowercase US-ASCII letters;
3. If the ownername is a wildcard name, the ownername is in its
original unexpanded form, including the "*" label (no wildcard
substitution);
5. Including NSEC3 RRs in a Zone
Each owner name in the zone which has authoritative data or a secured
delegation point NS RRset MUST have an NSEC3 resource record.
An unsecured delegation point NS RRset MAY have an NSEC3 resource
record. This is different from NSEC records where an unsecured
delegation point NS RRset MUST have an NSEC record.
The TTL value for any NSEC3 RR SHOULD be the same as the minimum TTL
value field in the zone SOA RR.
The type bitmap of every NSEC3 resource record in a signed zone MUST
indicate the presence of both the NSEC3 RR type itself and its
corresponding RRSIG RR type.
The bitmap for the NSEC3 RR at a delegation point requires special
attention. Bits corresponding to the delegation NS RRset and any
RRsets for which the parent zone has authoritative data MUST be set;
bits corresponding to any non-NS RRset for which the parent is not
authoritative MUST be clear.
The following steps describe the proper construction of NSEC3
Laurie, et al. Expires December 3, 2005 [Page 10]
Internet-Draft nsec3 june 2005
records.
1. For each unique original owner name in the zone, add an NSEC3
RRset. This includes NSEC3 RRsets for unsigned delegation point
NS RRsets, unless the policy is to have Authoritative Only NSEC3
RRsets. The ownername of the NSEC3 RR is the hashed equivalent
of the original owner name, prepended to the zone name.
2. For each RRset at the original owner, set the corresponding bit
in the type bit map.
3. If the difference in number of labels between the apex and the
original ownername is greater then 1, additional NSEC3s need to
be added for every empty non-terminal between the apex and the
original ownername.
4. Sort the set of NSEC3 RRs.
5. In each NSEC3 RR, insert the Next Hashed Ownername. The Next
Hashed Ownername of the last NSEC3 in the zone contains the value
of the hashed ownername of the first NSEC3 in the zone.
6. If the policy is to have authoritative only, set the
Authoritative Only bit in those NSEC3 RRs that cover unsecured
delegation points.
6. Special Considerations
The following paragraphs clarify specific behaviour explain special
considerations for implementations.
6.1 Delegation Points
This proposal introduces the Authoritative Only Flag which indicates
whether non authoritative delegation point NS records are included in
the type bit Maps. As discussed in paragraph 2.1.1, a flag value of
0 indicates that the interpretation of the type bit maps is identical
to NSEC records.
The following subsections describe behaviour when the flag value is
1.
6.1.1 Unsigned Delegations
Delegation point NS records are not authoritative. They are
authoritative in the delegated zone. No other data exists at the
ownername of an unsigned delegation point.
Since no authoritative data exist at this ownername, it is excluded
from the NSEC3 chain. This is an optimization, since it relieves the
zone of including an NSEC3 record and its associated signature for
this name.
An NSEC3 that denies existence of ownernames between X and X' with
Laurie, et al. Expires December 3, 2005 [Page 11]
Internet-Draft nsec3 june 2005
the Authoritative Only Flag set to 1 can not be used to prove the
presence or the absence of delegation point NS records for unsigned
delegations in the interval (X, X'). The Authoritative Only Flag
effectively states No Contest on the presence of delegation point NS
resource records.
Since proof is absent, there exists a new attack vector. Unsigned
delegation point NS records can be deleted during a man in the middle
attack, effectively denying existence of the delegation. This is a
form of Denial of Service, where the victim has no information it is
under attack, since all signatures are valid and the fabricated
response form is a known type of response.
The only possible mitigation is to either not use this method, hence
proving existence or absence of unsigned delegations, or to sign all
delegations, regardless of whether the delegated zone is signed or
not.
A second attack vector exists in that an adversary is able to
successfully fabricate an (unsigned) response claiming a nonexistent
delegation exists.
The only possible mitigation is to mandate the signing of all
delegations.
6.2 Proving Nonexistence
If a wildcard resource record appears in a zone, its asterisk label
is treated as a literal symbol and is treated in the same way as any
other ownername for purposes of generating NSEC3 RRs. RFC 4035
[RFC4035] describes the impact of wildcards on authenticated denial
of existence.
In order to prove there exist no RRs for a domain, as well as no
source of synthesis, an RR must be shown for the closest encloser,
and non-existence must be shown for all closer labels and for the
wildcard at the closest encloser.
This can be done as follows. If the QNAME in the query is
omega.alfa.beta.example, and the closest encloser is beta.example
(the nearest ancestor to omega.alfa.beta.example), then the server
should return an NSEC3 that demonstrates the nonexistence of
alfa.beta.example, an NSEC3 that demonstrates the nonexistence of
*.beta.example, and an NSEC3 that demonstrates the existence of
beta.example. This takes between one and three NSEC3 records, since
a single record can, by chance, prove more than one of these facts.
When a verifier checks this response, then the existence of
Laurie, et al. Expires December 3, 2005 [Page 12]
Internet-Draft nsec3 june 2005
beta.example together with the non-existence of alfa.beta.example
proves that the closest encloser is indeed beta.example. The non-
existence of *.beta.example shows that there is no wildcard at the
closest encloser, and so no source of synthesis for
omega.alfa.beta.example. These two facts are sufficient to satisfy
the resolver that the QNAME cannot be resolved.
In practice, since the NSEC3 owner and next names are hashed, if the
server responds with an NSEC3 for beta.example, the resolver will
have to try successively longer names, starting with example, moving
to beta.example, alfa.beta.example, and so on, until one of them
hashes to a value that matches the interval (but not the ownername
nor next owner name) of one of the returned NSEC3s (this name will be
alfa.beta.example). Once it has done this, it knows the closest
encloser (i.e. beta.example), and can then easily check the other two
required proofs.
Note that it is not possible for one of the shorter names tried by
the resolver to be denied by one of the returned NSEC3s, since, by
definition, all these names exist and so cannot appear within the
range covered by an NSEC3. Note, however, that the first name that
the resolver tries MUST be the apex of the zone, since names above
the apex could be denied by one of the returned NSEC3s.
6.3 Salting
Augmenting original ownernames with salt before hashing increases the
cost of a dictionary of pre-generated hash-values. For every bit of
salt, the cost of the dictionary doubles. The NSEC3 RR can use a
maximum of 2040 bits of salt, multiplying the cost by 2^2040.
There MUST be a complete set of NSEC3s for the zone using the same
salt value. The salt value for each NSEC3 RR MUST be equal for a
single version of the zone.
The salt SHOULD be changed every time the zone is resigned to prevent
precomputation using a single salt.
6.4 Hash Collision
Hash collisions occur when different messages have the same hash
value. The expected number of domain names needed to give a 1 in 2
chance of a single collision is about 2^(n/2) for a hash of length n
bits (i.e. 2^80 for SHA-1). Though this probability is extremely
low, the following paragraphs deal with avoiding collisions and
assessing possible damage in the event of an attack using hash
collisions.
Laurie, et al. Expires December 3, 2005 [Page 13]
Internet-Draft nsec3 june 2005
6.4.1 Avoiding Hash Collisions during generation
During generation of NSEC3 RRs, hash values are supposedly unique.
In the (academic) case of a collision occurring, an alternative salt
SHOULD be chosen and all hash values SHOULD be regenerated.
If hash values are not regenerated on collision, the NSEC3 RR MUST
list all authoritative RR types that exist for both owners, to avoid
a replay attack, spoofing an existing type as non-existent.
6.4.2 Second Preimage Requirement Analysis
A cryptographic hash function has a second-preimage resistance
property. The second-preimage resistance property means that it is
computationally infeasible to find another message with the same hash
value as a given message, i.e. given preimage X, to find a second
preimage X' <> X such that hash(X) = hash(X'). The work factor for
finding a second preimage is of the order of 2^160 for SHA-1. To
mount an attack using an existing NSEC3 RR, an adversary needs to
find a second preimage.
Assuming an adversary is capable of mounting such an extreme attack,
the actual damage is that a response message can be generated which
claims that a certain QNAME (i.e. the second pre-image) does exist,
while in reality QNAME does not exist (a false positive), which will
either cause a security aware resolver to re-query for the non-
existent name, or to fail the initial query. Note that the adversary
can't mount this attack on an existing name but only on a name that
the adversary can't choose and does not yet exist.
6.4.3 Possible Hash Value Truncation Method
The previous sections outlined the low probability and low impact of
a second-preimage attack. When impact and probability are low, while
space in a DNS message is costly, truncation is tempting. Truncation
might be considered to allow for shorter ownernames and rdata for
hashed labels. In general, if a cryptographic hash is truncated to n
bits, then the expected number of domains required to give a 1 in 2
probability of a single collision is approximately 2^(n/2) and the
work factor to produce a second preimage resistance is 2^n.
An extreme hash value truncation would be truncating to the shortest
possible unique label value. Considering that hash values are
presented in base32, which represents 5 bits per label character,
truncation must be done on a 5 bit boundary. This would be unwise,
since the work factor to produce collisions would then approximate
the size of the zone.
Laurie, et al. Expires December 3, 2005 [Page 14]
Internet-Draft nsec3 june 2005
Though the mentioned truncation can be maximized to a certain
extreme, the probability of collision increases exponentially for
every truncated bit. Given the low impact of hash value collisions
and limited space in DNS messages, the balance between truncation
profit and collision damage may be determined by local policy. Of
course, the size of the corresponding RRSIG RR is not reduced, so
truncation is of limited benefit.
Truncation could be signalled simply by reducing the length of the
first label in the ownername. Note that there would have to be a
corresponding reduction in the length of the Next Hashed Ownername
field.
7. Performance Considerations
Iterated hashes will obviously impose a performance penalty on both
authoritative servers and resolvers. Therefore, the number of
iterations should be carefully chosen. In particular it should be
noted that a high value for iterations gives an attacker a very good
denial of service attack, since the attacker need not bother to
verify the results of their queries, and hence has no performance
penalty of his own.
On the other hand, nameservers with low query rates and limited
bandwidth are already subject to a bandwidth based denial of service
attack, since responses are typically an order of magnitude larger
than queries, and hence these servers may choose a high value of
iterations in order to increase the difficulty of offline attempts to
enumerate their namespace without significantly increasing their
vulnerability to denial of service attacks.
8. IANA Considerations
IANA has to create a new registry for NSEC3 Hash Functions. The
range for this registry is 0-127. Value 0 is the identity function.
Value 1 is SHA-1. Values 2-126 are Reserved For Future Use. Value
127 is marked as Experimental.
9. Security Considerations
The NSEC3 records are still susceptible to dictionary attacks (i.e.
the attacker retrieves all the NSEC3 records, then calculates the
hashes of all likely domain names, comparing against the hashes found
in the NSEC3 records, and thus enumerating the zone). These are
substantially more expensive than traversing the original NSEC
records would have been, and in any case, such an attack could also
be used directly against the name server itself by performing queries
for all likely names, though this would obviously be more detectable.
Laurie, et al. Expires December 3, 2005 [Page 15]
Internet-Draft nsec3 june 2005
The expense of this off-line attack can be chosen by setting the
number of iterations in the NSEC3 RR.
High-value domains are also susceptible to a precalculated dictionary
attack - that is, a list of hashes for all likely names is computed
once, then NSEC3 is scanned periodically and compared against the
precomputed hashes. This attack is prevented by changing the salt on
a regular basis.
Walking the NSEC3 RRs will reveal the total number of records in the
zone, and also what types they are. This could be mitigated by
adding dummy entries, but certainly an upper limit can always be
found.
Hash collisions may occur. If they do, it will be impossible to
prove the non-existence of the colliding domain - however, this is
fantastically unlikely, and, in any case, DNSSEC already relies on
SHA-1 to not collide.
10. References
10.1 Normative References
[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.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2136] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, April 1997.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, March 1998.
[RFC2929] Eastlake, D., Brunner-Williams, E., and B. Manning,
"Domain Name System (DNS) IANA Considerations", BCP 42,
RFC 2929, September 2000.
[RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record
(RR) Types", RFC 3597, September 2003.
Laurie, et al. Expires December 3, 2005 [Page 16]
Internet-Draft nsec3 june 2005
[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.
10.2 Informative References
[I-D.ietf-dnsext-trustupdate-threshold]
Ihren, J., "An In-Band Rollover Mechanism and an Out-Of-
Band Priming Method for DNSSEC Trust Anchors.",
draft-ietf-dnsext-trustupdate-threshold-00 (work in
progress), October 2004.
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC 2026, October 1996.
[RFC2418] Bradner, S., "IETF Working Group Guidelines and
Procedures", BCP 25, RFC 2418, September 1998.
Authors' Addresses
Ben Laurie
Nominet
17 Perryn Road
London W3 7LR
England
Phone: +44 (20) 8735 0686
Email: ben@algroup.co.uk
Geoffrey Sisson
Nominet
Laurie, et al. Expires December 3, 2005 [Page 17]
Internet-Draft nsec3 june 2005
Roy Arends
Telematica Instituut
Brouwerijstraat 1
7523 XC Enschede
The Netherlands
Phone: +31 (53) 485 0485
Email: roy.arends@telin.nl
Appendix A. Example Zone
This is a zone showing its NSEC3 records. They can also be used as
test vectors for the hash algorithm.
example. 3600 IN SOA ns1.example. bugs.x.w.example. (
1
3600
300
3600000
3600 )
3600 RRSIG SOA 5 1 3600 20050712112304 (
20050612112304 62699 example.
RtctD6aLUU5Md5wOOItilS7JXX1tf58Ql3sK
mTXkL13jqLiUFOGg0uzqRh1U9GbydS0P7M0g
qYIt90txzE/4+g== )
3600 NS ns1.example.
3600 NS ns2.example.
3600 RRSIG NS 5 1 3600 20050712112304 (
20050612112304 62699 example.
hNyyin2JpECIFxW4vsj8RhHcWCQKUXgO+z4l
m7g2zM8q3Qpsm/gYIXSF2Rhj6lAG7esR/X9d
1SH5r/wfjuCg+g== )
3600 MX 1 xx.example.
3600 RRSIG MX 5 1 3600 20050712112304 (
20050612112304 62699 example.
L/ZDLMSZJKITmSxmM9Kni37/wKQsdSg6FT0l
NMm14jy2Stp91Pwp1HQ1hAMkGWAqCMEKPMtU
S/o/g5C8VM6ftQ== )
3600 DNSKEY 257 3 5 (
AQOnsGyJvywVjYmiLbh0EwIRuWYcDiB/8blX
cpkoxtpe19Oicv6Zko+8brVsTMeMOpcUeGB1
zsYKWJ7BvR2894hX
) ; Key ID = 21960
3600 DNSKEY 256 3 5 (
AQO0gEmbZUL6xbD/xQczHbnwYnf+jQjwz/sU
5k44rHTt0Ty+3aOdYoome9TjGMhwkkGby1TL
ExXT48OGGdbfIme5
Laurie, et al. Expires December 3, 2005 [Page 18]
Internet-Draft nsec3 june 2005
) ; Key ID = 62699
3600 RRSIG DNSKEY 5 1 3600 20050712112304 (
20050612112304 62699 example.
e6EB+K21HbyZzoLUeRDb6+g0+n8XASYe6h+Z
xtnB31sQXZgq8MBHeNFDQW9eZw2hjT9zMClx
mTkunTYzqWJrmQ== )
3600 RRSIG DNSKEY 5 1 3600 20050712112304 (
20050612112304 21960 example.
SnWLiNWLbOuiKU/F/wVMokvcg6JVzGpQ2VUk
ZbKjB9ON0t3cdc+FZbOCMnEHRJiwgqlnncik
3w7ZY2UWyYIvpw== )
5pe7ctl7pfs2cilroy5dcofx4rcnlypd.example. 3600 NSEC3 0 1 1 (
deadbeaf
7nomf47k3vlidh4vxahhpp47l3tgv7a2
NSEC3 RRSIG )
3600 RRSIG NSEC3 5 2 3600 20050712112304 (
20050612112304 62699 example.
PTWYq4WZmmtgh9UQif342HWf9DD9RuuM4ii5
Z1oZQgRi5zrsoKHAgl2YXprF2Rfk1TLgsiFQ
sb7KfbaUo/vzAg== )
7nomf47k3vlidh4vxahhpp47l3tgv7a2.example. 3600 NSEC3 0 1 1 (
deadbeaf
dw4o7j64wnel3j4jh7fb3c5n7w3js2yb
MX NSEC3 RRSIG )
3600 RRSIG NSEC3 5 2 3600 20050712112304 (
20050612112304 62699 example.
YTcqole3h8EOsTT3HKnwhR1QS8borR0XtZaA
ZrLsx6n0RDC1AAdZONYOvdqvcal9PmwtWjlo
MEFQmc/gEuxojA== )
a.example. 3600 IN NS ns1.a.example.
3600 IN NS ns2.a.example.
3600 DS 58470 5 1 3079F1593EBAD6DC121E202A8B
766A6A4837206C )
3600 RRSIG DS 5 2 3600 20050712112304 (
20050612112304 62699 example.
QavhbsSmEvJLSUzGoTpsV3SKXCpaL1UO3Ehn
cB0ObBIlex/Zs9kJyG/9uW1cYYt/1wvgzmX2
0kx7rGKTc3RQDA== )
ns1.a.example. 3600 IN A 192.0.2.5
ns2.a.example. 3600 IN A 192.0.2.6
ai.example. 3600 IN A 192.0.2.9
3600 RRSIG A 5 2 3600 20050712112304 (
20050612112304 62699 example.
plY5M26ED3Owe3YX0pBIhgg44j89NxUaoBrU
6bLRr99HpKfFl1sIy18JiRS7evlxCETZgubq
ZXW5S+1VjMZYzQ== )
3600 HINFO "KLH-10" "ITS"
3600 RRSIG HINFO 5 2 3600 20050712112304 (
Laurie, et al. Expires December 3, 2005 [Page 19]
Internet-Draft nsec3 june 2005
20050612112304 62699 example.
AR0hG/Z/e+vlRhxRQSVIFORzrJTBpdNHhwUk
tiuqg+zGqKK84eIqtrqXelcE2szKnF3YPneg
VGNmbgPnqDVPiA== )
3600 AAAA 2001:db8:0:0:0:0:f00:baa9
3600 RRSIG AAAA 5 2 3600 20050712112304 (
20050612112304 62699 example.
PNF/t7+DeosEjhfuL0kmsNJvn16qhYyLI9FV
ypSCorFx/PKIlEL3syomkYM2zcXVSRwUXMns
l5/UqLCJJ9BDMg== )
b.example. 3600 IN NS ns1.b.example.
3600 IN NS ns2.b.example.
ns1.b.example. 3600 IN A 192.0.2.7
ns2.b.example. 3600 IN A 192.0.2.8
dw4o7j64wnel3j4jh7fb3c5n7w3js2yb.example. 3600 NSEC3 0 1 1 (
deadbeaf
gmnfcccja7wkax3iv26bs75myptje3qk
MX DNSKEY NS SOA NSEC3 RRSIG )
3600 RRSIG NSEC3 5 2 3600 20050712112304 (
20050612112304 62699 example.
VqEbXiZLJVYmo25fmO3IuHkAX155y8NuA50D
C0NmJV/D4R3rLm6tsL6HB3a3f6IBw6kKEa2R
MOiKMSHozVebqw== )
gmnfcccja7wkax3iv26bs75myptje3qk.example. 3600 NSEC3 0 1 1 (
deadbeaf
jt4bbfokgbmr57qx4nqucvvn7fmo6ab6
DS NS NSEC3 RRSIG )
3600 RRSIG NSEC3 5 2 3600 20050712112304 (
20050612112304 62699 example.
ZqkdmF6eICpHyn1Cj7Yvw+nLcbji46Qpe76/
ZetqdZV7K5sO3ol5dOc0dZyXDqsJp1is5StW
OwQBGbOegrW/Zw== )
jt4bbfokgbmr57qx4nqucvvn7fmo6ab6.example. 3600 NSEC3 0 1 1 (
deadbeaf
kcll7fqfnisuhfekckeeqnmbbd4maanu
NSEC3 RRSIG )
3600 RRSIG NSEC3 5 2 3600 20050712112304 (
20050612112304 62699 example.
FXyCVQUdFF1EW1NcgD2V724/It0rn3lr+30V
IyjmqwOMvQ4G599InTpiH46xhX3U/FmUzHOK
94Zbq3k8lgdpZA== )
kcll7fqfnisuhfekckeeqnmbbd4maanu.example. 3600 NSEC3 1 1 1 (
deadbeaf
n42hbhnjj333xdxeybycax5ufvntux5d
MX NSEC3 RRSIG )
3600 RRSIG NSEC3 5 2 3600 20050712112304 (
20050612112304 62699 example.
d0g8MTOvVwByOAIwvYV9JrTHwJof1VhnMKuA
Laurie, et al. Expires December 3, 2005 [Page 20]
Internet-Draft nsec3 june 2005
IBj6Xaeney86RBZYgg7Qyt9WnQSK3uCEeNpx
TOLtc5jPrkL4zQ== )
n42hbhnjj333xdxeybycax5ufvntux5d.example. 3600 NSEC3 0 1 1 (
deadbeaf
nimwfwcnbeoodmsc6npv3vuaagaevxxu
A NSEC3 RRSIG )
3600 RRSIG NSEC3 5 2 3600 20050712112304 (
20050612112304 62699 example.
MZGzllh+YFqZbY8SkHxARhXFiMDPS0tvQYyy
91tj+lbl45L/BElD3xxB/LZMO8vQejYtMLHj
xFPFGRIW3wKnrA== )
nimwfwcnbeoodmsc6npv3vuaagaevxxu.example. 3600 NSEC3 0 1 1 (
deadbeaf
vhgwr2qgykdkf4m6iv6vkagbxozphazr
HINFO A AAAA NSEC3 RRSIG )
3600 RRSIG NSEC3 5 2 3600 20050712112304 (
20050612112304 62699 example.
c3zQdK68cYTHTjh1cD6pi0vblXwzyoU/m7Qx
z8kaPYikbJ9vgSl9YegjZukgQSwybHUC0SYG
jL33Wm1p07TBdw== )
ns1.example. 3600 A 192.0.2.1
3600 RRSIG A 5 2 3600 20050712112304 (
20050612112304 62699 example.
QLGkaqWXxRuE+MHKkMvVlswg65HcyjvD1fyb
BDZpcfiMHH9w4x1eRqRamtSDTcqLfUrcYkrr
nWWLepz1PjjShQ== )
ns2.example. 3600 A 192.0.2.2
3600 RRSIG A 5 2 3600 20050712112304 (
20050612112304 62699 example.
UoIZaC1O6XHRWGHBOl8XFQKPdYTkRCz6SYh3
P2mZ3xfY22fLBCBDrEnOc8pGDGijJaLl26Cz
AkeTJu3J3auUiA== )
vhgwr2qgykdkf4m6iv6vkagbxozphazr.example. 3600 NSEC3 0 1 1 (
deadbeaf
wbyijvpnyj33pcpi3i44ecnibnaj7eiw
HINFO A AAAA NSEC3 RRSIG )
3600 RRSIG NSEC3 5 2 3600 20050712112304 (
20050612112304 62699 example.
leFhoF5FXZAiNOxK4OBOOA0WKdbaD5lLDT/W
kLoyWnQ6WGBwsUOdsEcVmqz+1n7q9bDf8G8M
5SNSHIyfpfsi6A== )
*.w.example. 3600 MX 1 ai.example.
3600 RRSIG MX 5 3 3600 20050712112304 (
20050612112304 62699 example.
sYNUPHn1/gJ87wTHNksGdRm3vfnSFa2BbofF
xGfJLF5A4deRu5f0hvxhAFDCcXfIASj7z0wQ
gQlgxEwhvQDEaQ== )
x.w.example. 3600 MX 1 xx.example.
Laurie, et al. Expires December 3, 2005 [Page 21]
Internet-Draft nsec3 june 2005
3600 RRSIG MX 5 3 3600 20050712112304 (
20050612112304 62699 example.
s1XQ/8SlViiEDik9edYs1Ooe3XiXo453Dg7w
lqQoewuDzmtd6RaLNu52W44zTM1EHJES8ujP
U9VazOa1KEIq1w== )
x.y.w.example. 3600 MX 1 xx.example.
3600 RRSIG MX 5 4 3600 20050712112304 (
20050612112304 62699 example.
aKVCGO/Fx9rm04UUsHRTTYaDA8o8dGfyq6t7
uqAcYxU9xiXP+xNtLHBv7er6Q6f2JbOs6SGF
9VrQvJjwbllAfA== )
wbyijvpnyj33pcpi3i44ecnibnaj7eiw.example. 3600 NSEC3 0 1 1 (
deadbeaf
zjxfz5o7t4ty4u3f6fa7mhhqzjln4mui
A NSEC3 RRSIG )
3600 RRSIG NSEC3 5 2 3600 20050712112304 (
20050612112304 62699 example.
ledFAaDCqDxapQ1FvBAjjK2DP06iQj8AN6gN
ZycTeSmobKLTpzbgQp8uKYYe/DPHjXYmuEhd
oorBv4xkb0flXw== )
xx.example. 3600 A 192.0.2.10
3600 RRSIG A 5 2 3600 20050712112304 (
20050612112304 62699 example.
XSuMVjNxovbZUsnKU6oQDygaK+WB+O5HYQG9
tJgphHIX7TM4uZggfR3pNM+4jeC8nt2OxZZj
cxwCXWj82GVGdw== )
3600 HINFO "KLH-10" "TOPS-20"
3600 RRSIG HINFO 5 2 3600 20050712112304 (
20050612112304 62699 example.
ghS2DimOqPSacG9j6KMgXSfTMSjLxvoxvx3q
OKzzPst4tEbAmocF2QX8IrSHr67m4ZLmd2Fk
KMf4DgNBDj+dIQ== )
3600 AAAA 2001:db8:0:0:0:0:f00:baaa
3600 RRSIG AAAA 5 2 3600 20050712112304 (
20050612112304 62699 example.
rto7afZkXYB17IfmQCT5QoEMMrlkeOoAGXzo
w8Wmcg86Fc+MQP0hyXFScI1gYNSgSSoDMXIy
rzKKwb8J04/ILw== )
zjxfz5o7t4ty4u3f6fa7mhhqzjln4mui.example. 3600 NSEC3 0 1 1 (
deadbeaf
5pe7ctl7pfs2cilroy5dcofx4rcnlypd
MX NSEC3 RRSIG )
3600 RRSIG NSEC3 5 2 3600 20050712112304 (
20050612112304 62699 example.
eULkdWjcjmM+wXQcr7zXNfnGLgHjZSJINGkt
7Zmvp7WKVAqoHMm1RXV8IfBH1aRgv5+/Lgny
OcFlrPGPMm48/A== )
Laurie, et al. Expires December 3, 2005 [Page 22]
Internet-Draft nsec3 june 2005
Appendix B. Example Responses
The examples in this section show response messages using the signed
zone example in Appendix A.
B.1 answer
A successful query to an authoritative server.
Laurie, et al. Expires December 3, 2005 [Page 23]
Internet-Draft nsec3 june 2005
;; Header: QR AA DO RCODE=0
;;
;; Question
x.w.example. IN MX
;; Answer
x.w.example. 3600 IN MX 1 xx.example.
x.w.example. 3600 IN RRSIG MX 5 3 3600 20050712112304 (
20050612112304 62699 example.
s1XQ/8SlViiEDik9edYs1Ooe3XiXo453Dg7w
lqQoewuDzmtd6RaLNu52W44zTM1EHJES8ujP
U9VazOa1KEIq1w== )
;; Authority
example. 3600 IN NS ns1.example.
example. 3600 IN NS ns2.example.
example. 3600 IN RRSIG NS 5 1 3600 20050712112304 (
20050612112304 62699 example.
hNyyin2JpECIFxW4vsj8RhHcWCQKUXgO+z4l
m7g2zM8q3Qpsm/gYIXSF2Rhj6lAG7esR/X9d
1SH5r/wfjuCg+g== )
;; Additional
xx.example. 3600 IN A 192.0.2.10
xx.example. 3600 IN RRSIG A 5 2 3600 20050712112304 (
20050612112304 62699 example.
XSuMVjNxovbZUsnKU6oQDygaK+WB+O5HYQG9
tJgphHIX7TM4uZggfR3pNM+4jeC8nt2OxZZj
cxwCXWj82GVGdw== )
xx.example. 3600 IN AAAA 2001:db8::f00:baaa
xx.example. 3600 IN RRSIG AAAA 5 2 3600 20050712112304 (
20050612112304 62699 example.
rto7afZkXYB17IfmQCT5QoEMMrlkeOoAGXzo
w8Wmcg86Fc+MQP0hyXFScI1gYNSgSSoDMXIy
rzKKwb8J04/ILw== )
ns1.example. 3600 IN A 192.0.2.1
ns1.example. 3600 IN RRSIG A 5 2 3600 20050712112304 (
20050612112304 62699 example.
QLGkaqWXxRuE+MHKkMvVlswg65HcyjvD1fyb
BDZpcfiMHH9w4x1eRqRamtSDTcqLfUrcYkrr
nWWLepz1PjjShQ== )
ns2.example. 3600 IN A 192.0.2.2
ns2.example. 3600 IN RRSIG A 5 2 3600 20050712112304 (
20050612112304 62699 example.
UoIZaC1O6XHRWGHBOl8XFQKPdYTkRCz6SYh3
P2mZ3xfY22fLBCBDrEnOc8pGDGijJaLl26Cz
AkeTJu3J3auUiA== )
Laurie, et al. Expires December 3, 2005 [Page 24]
Internet-Draft nsec3 june 2005
The query returned an MX RRset for "x.w.example". The corresponding
RRSIG RR indicates that the MX RRset was signed by an "example"
DNSKEY with algorithm 5 and key tag 62699. The resolver needs the
corresponding DNSKEY RR in order to authenticate this answer. The
discussion below describes how a resolver might obtain this DNSKEY
RR.
The RRSIG RR indicates the original TTL of the MX RRset was 3600,
and, for the purpose of authentication, the current TTL is replaced
by 3600. The RRSIG RR's labels field value of 3 indicates that the
answer was not the result of wildcard expansion. The "x.w.example"
MX RRset is placed in canonical form, and, assuming the current time
falls between the signature inception and expiration dates, the
signature is authenticated.
B.1.1 Authenticating the Example DNSKEY RRset
This example shows the logical authentication process that starts
from a configured root DNSKEY RRset (or DS RRset) and moves down the
tree to authenticate the desired "example" DNSKEY RRset. Note that
the logical order is presented for clarity. An implementation may
choose to construct the authentication as referrals are received or
to construct the authentication chain only after all RRsets have been
obtained, or in any other combination it sees fit. The example here
demonstrates only the logical process and does not dictate any
implementation rules.
We assume the resolver starts with a configured DNSKEY RRset for the
root zone (or a configured DS RRset for the root zone). The resolver
checks whether this configured DNSKEY RRset is present in the root
DNSKEY RRset (or whether a DS RR in the DS RRset matches some DNSKEY
RR in the root DNSKEY RRset), whether this DNSKEY RR has signed the
root DNSKEY RRset, and whether the signature lifetime is valid. If
all these conditions are met, all keys in the DNSKEY RRset are
considered authenticated. The resolver then uses one (or more) of
the root DNSKEY RRs to authenticate the "example" DS RRset. Note
that the resolver may have to query the root zone to obtain the root
DNSKEY RRset or "example" DS RRset.
Once the DS RRset has been authenticated using the root DNSKEY, the
resolver checks the "example" DNSKEY RRset for some "example" DNSKEY
RR that matches one of the authenticated "example" DS RRs. If such a
matching "example" DNSKEY is found, the resolver checks whether this
DNSKEY RR has signed the "example" DNSKEY RRset and the signature
lifetime is valid. If these conditions are met, all keys in the
"example" DNSKEY RRset are considered authenticated.
Finally, the resolver checks that some DNSKEY RR in the "example"
Laurie, et al. Expires December 3, 2005 [Page 25]
Internet-Draft nsec3 june 2005
DNSKEY RRset uses algorithm 5 and has a key tag of 62699. This
DNSKEY is used to authenticate the RRSIG included in the response.
If multiple "example" DNSKEY RRs match this algorithm and key tag,
then each DNSKEY RR is tried, and the answer is authenticated if any
of the matching DNSKEY RRs validate the signature as described above.
B.2 Name Error
An authoritative name error. The NSEC3 RRs prove that the name does
not exist and that no covering wildcard exists.
Laurie, et al. Expires December 3, 2005 [Page 26]
Internet-Draft nsec3 june 2005
;; Header: QR AA DO RCODE=3
;;
;; Question
a.c.x.w.example. IN A
;; Answer
;; (empty)
;; Authority
example. 3600 IN SOA ns1.example. bugs.x.w.example. (
1
3600
300
3600000
3600
)
example. 3600 IN RRSIG SOA 5 1 3600 20050712112304 (
20050612112304 62699 example.
RtctD6aLUU5Md5wOOItilS7JXX1tf58Ql3sK
mTXkL13jqLiUFOGg0uzqRh1U9GbydS0P7M0g
qYIt90txzE/4+g== )
7nomf47k3vlidh4vxahhpp47l3tgv7a2.example. 3600 IN NSEC3 0 1 1 (
deadbeaf
dw4o7j64wnel3j4jh7fb3c5n7w3js2yb
MX NSEC3 RRSIG )
7nomf47k3vlidh4vxahhpp47l3tgv7a2.example. 3600 IN RRSIG NSEC3 (
5 2 3600 20050712112304
20050612112304 62699 example.
YTcqole3h8EOsTT3HKnwhR1QS8borR0XtZaA
ZrLsx6n0RDC1AAdZONYOvdqvcal9PmwtWjlo
MEFQmc/gEuxojA== )
nimwfwcnbeoodmsc6npv3vuaagaevxxu.example. 3600 IN NSEC3 0 1 1 (
deadbeaf
vhgwr2qgykdkf4m6iv6vkagbxozphazr
HINFO A AAAA NSEC3 RRSIG )
nimwfwcnbeoodmsc6npv3vuaagaevxxu.example. 3600 IN RRSIG NSEC3 (
5 2 3600 20050712112304
20050612112304 62699 example.
c3zQdK68cYTHTjh1cD6pi0vblXwzyoU/m7Qx
z8kaPYikbJ9vgSl9YegjZukgQSwybHUC0SYG
jL33Wm1p07TBdw== )
;; Additional
;; (empty)
The query returned two NSEC3 RRs that prove that the requested data
does not exist and no wildcard applies. The negative reply is
authenticated by verifying both NSEC3 RRs. The NSEC3 RRs are
authenticated in a manner identical to that of the MX RRset discussed
Laurie, et al. Expires December 3, 2005 [Page 27]
Internet-Draft nsec3 june 2005
above. At least one of the owner names of the NSEC3 RRs will match
the closest encloser. At least one of the NSEC3 RRs prove that there
exists no longer name. At least one of the NSEC3 RRs prove that
there exists no wildcard RRsets that should have been expanded. The
closest encloser can be found by hasing the apex ownername (The SOA
RR's ownername, or the ownername of the DNSKEY RRset referred by an
RRSIG RR), matching it to the ownername of one of the NSEC3 RRs, and
if that fails, continue by adding labels.
In the above example, the name 'x.w.example' hashes to
'7nomf47k3vlidh4vxahhpp47l3tgv7a2'. This indicates that this might
be the closest encloser. To prove that 'c.x.w.example' and
'*.x.w.example' do not exists, these names are hashed to respectively
'qsgoxsf2lanysajhtmaylde4tqwnqppl' and
'cvljzyf6nsckjowghch4tt3nohocpdka'. The two NSEC3 records prove that
these hashed ownernames do not exists, since the names are within the
given intervals.
B.3 No Data Error
A "no data" response. The NSEC3 RR proves that the name exists and
that the requested RR type does not.
Laurie, et al. Expires December 3, 2005 [Page 28]
Internet-Draft nsec3 june 2005
;; Header: QR AA DO RCODE=0
;;
;; Question
ns1.example. IN MX
;; Answer
;; (empty)
;; Authority
example. 3600 IN SOA ns1.example. bugs.x.w.example. (
1
3600
300
3600000
3600
)
example. 3600 IN RRSIG SOA 5 1 3600 20050712112304 (
20050612112304 62699 example.
RtctD6aLUU5Md5wOOItilS7JXX1tf58Ql3sK
mTXkL13jqLiUFOGg0uzqRh1U9GbydS0P7M0g
qYIt90txzE/4+g== )
wbyijvpnyj33pcpi3i44ecnibnaj7eiw.example. 3600 IN NSEC3 0 1 1 (
deadbeaf
zjxfz5o7t4ty4u3f6fa7mhhqzjln4mui
A NSEC3 RRSIG )
wbyijvpnyj33pcpi3i44ecnibnaj7eiw.example. 3600 IN RRSIG NSEC3 (
5 2 3600 20050712112304
20050612112304 62699 example.
ledFAaDCqDxapQ1FvBAjjK2DP06iQj8AN6gN
ZycTeSmobKLTpzbgQp8uKYYe/DPHjXYmuEhd
oorBv4xkb0flXw== )
;; Additional
;; (empty)
The query returned an NSEC3 RR that proves that the requested name
exists ("ns1.example." hashes to "wbyijvpnyj33pcpi3i44ecnibnaj7eiw"),
but the requested RR type does not exist (type MX is absent in the
type code list of the NSEC RR). The negative reply is authenticated
by verifying the NSEC3 RR. The NSEC3 RR is authenticated in a manner
identical to that of the MX RRset discussed above.
B.3.1 No Data Error, Empty Non-Terminal
A "no data" response because of an empty non-terminal. The NSEC3 RR
proves that the name exists and that the requested RR type does not.
Laurie, et al. Expires December 3, 2005 [Page 29]
Internet-Draft nsec3 june 2005
;; Header: QR AA DO RCODE=0
;;
;; Question
y.w.example. IN A
;; Answer
;; (empty)
;; Authority
example. 3600 IN SOA ns1.example. bugs.x.w.example. (
1
3600
300
3600000
3600
)
example. 3600 IN RRSIG SOA 5 1 3600 20050712112304 (
20050612112304 62699 example.
RtctD6aLUU5Md5wOOItilS7JXX1tf58Ql3sK
mTXkL13jqLiUFOGg0uzqRh1U9GbydS0P7M0g
qYIt90txzE/4+g== )
jt4bbfokgbmr57qx4nqucvvn7fmo6ab6.example. 3600 IN NSEC3 0 1 1 (
deadbeaf
kcll7fqfnisuhfekckeeqnmbbd4maanu
NSEC3 RRSIG )
jt4bbfokgbmr57qx4nqucvvn7fmo6ab6.example. 3600 IN RRSIG NSEC3 (
5 2 3600 20050712112304
20050612112304 62699 example.
FXyCVQUdFF1EW1NcgD2V724/It0rn3lr+30V
IyjmqwOMvQ4G599InTpiH46xhX3U/FmUzHOK
94Zbq3k8lgdpZA== )
The query returned an NSEC3 RR that proves that the requested name
exists ("y.w.example." hashes to "jt4bbfokgbmr57qx4nqucvvn7fmo6ab6"),
but the requested RR type does not exist (Type A is absent in the
type-bit-maps of the NSEC3 RR). The negative reply is authenticated
by verifying the NSEC3 RR. The NSEC3 RR is authenticated in a manner
identical to that of the MX RRset discussed above. Note that, unlike
generic empty non terminal proof using NSECs, this is identical to
proving a No Data Error. This example is solely mentioned to be
complete.
B.4 Referral to Signed Zone
Referral to a signed zone. The DS RR contains the data which the
resolver will need to validate the corresponding DNSKEY RR in the
child zone's apex.
Laurie, et al. Expires December 3, 2005 [Page 30]
Internet-Draft nsec3 june 2005
;; Header: QR DO RCODE=0
;;
;; Question
mc.a.example. IN MX
;; Answer
;; (empty)
;; Authority
a.example. 3600 IN NS ns1.a.example.
a.example. 3600 IN NS ns2.a.example.
a.example. 3600 IN DS 58470 5 1 (
3079F1593EBAD6DC121E202A8B766A6A4837
206C )
a.example. 3600 IN RRSIG DS 5 2 3600 20050712112304 (
20050612112304 62699 example.
QavhbsSmEvJLSUzGoTpsV3SKXCpaL1UO3Ehn
cB0ObBIlex/Zs9kJyG/9uW1cYYt/1wvgzmX2
0kx7rGKTc3RQDA== )
;; Additional
ns1.a.example. 3600 IN A 192.0.2.5
ns2.a.example. 3600 IN A 192.0.2.6
The query returned a referral to the signed "a.example." zone. The
DS RR is authenticated in a manner identical to that of the MX RRset
discussed above. This DS RR is used to authenticate the "a.example"
DNSKEY RRset.
Once the "a.example" DS RRset has been authenticated using the
"example" DNSKEY, the resolver checks the "a.example" DNSKEY RRset
for some "a.example" DNSKEY RR that matches the DS RR. If such a
matching "a.example" DNSKEY is found, the resolver checks whether
this DNSKEY RR has signed the "a.example" DNSKEY RRset and whether
the signature lifetime is valid. If all these conditions are met,
all keys in the "a.example" DNSKEY RRset are considered
authenticated.
B.5 Referral to Unsigned Zone using Opt-In
Referral to an unsigned zone using Opt-In. The NSEC3 RR proves that
nothing for this delegation was signed in the parent zone. There is
no proof that the delegation exists
Laurie, et al. Expires December 3, 2005 [Page 31]
Internet-Draft nsec3 june 2005
;; Header: QR DO RCODE=0
;;
;; Question
mc.b.example. IN MX
;; Answer
;; (empty)
;; Authority
b.example. 3600 IN NS ns1.b.example.
b.example. 3600 IN NS ns2.b.example.
kcll7fqfnisuhfekckeeqnmbbd4maanu.example. 3600 IN NSEC3 1 1 1 (
deadbeaf
n42hbhnjj333xdxeybycax5ufvntux5d
MX NSEC3 RRSIG )
kcll7fqfnisuhfekckeeqnmbbd4maanu.example. 3600 IN RRSIG NSEC3 (
5 2 3600 20050712112304
20050612112304 62699 example.
d0g8MTOvVwByOAIwvYV9JrTHwJof1VhnMKuA
IBj6Xaeney86RBZYgg7Qyt9WnQSK3uCEeNpx
TOLtc5jPrkL4zQ== )
;; Additional
ns1.b.example. 3600 IN A 192.0.2.7
ns2.b.example. 3600 IN A 192.0.2.8
The query returned a referral to the unsigned "b.example." zone. The
NSEC3 proves that no authentication leads from "example" to
"b.example", since the hash of "b.example"
("ldjpfcucebeks5azmzpty4qlel4cftzo") is within the NSEC3 interval and
the NSEC3 opt-in bit is set. The NSEC3 RR is authenticated in a
manner identical to that of the MX RRset discussed above.
B.6 Wildcard Expansion
A successful query that was answered via wildcard expansion. The
label count in the answer's RRSIG RR indicates that a wildcard RRset
was expanded to produce this response, and the NSEC3 RR proves that
no closer match exists in the zone.
Laurie, et al. Expires December 3, 2005 [Page 32]
Internet-Draft nsec3 june 2005
;; Header: QR AA DO RCODE=0
;;
;; Question
a.z.w.example. IN MX
;; Answer
a.z.w.example. 3600 IN MX 1 ai.example.
a.z.w.example. 3600 IN RRSIG MX 5 3 3600 20050712112304 (
20050612112304 62699 example.
sYNUPHn1/gJ87wTHNksGdRm3vfnSFa2BbofF
xGfJLF5A4deRu5f0hvxhAFDCcXfIASj7z0wQ
gQlgxEwhvQDEaQ== )
;; Authority
example. 3600 NS ns1.example.
example. 3600 NS ns2.example.
example. 3600 IN RRSIG NS 5 1 3600 20050712112304 (
20050612112304 62699 example.
hNyyin2JpECIFxW4vsj8RhHcWCQKUXgO+z4l
m7g2zM8q3Qpsm/gYIXSF2Rhj6lAG7esR/X9d
1SH5r/wfjuCg+g== )
zjxfz5o7t4ty4u3f6fa7mhhqzjln4mui.example. 3600 IN NSEC3 0 1 1 (
deadbeaf
5pe7ctl7pfs2cilroy5dcofx4rcnlypd
MX NSEC3 RRSIG )
zjxfz5o7t4ty4u3f6fa7mhhqzjln4mui.example. 3600 IN RRSIG NSEC3 (
5 2 3600 20050712112304
20050612112304 62699 example.
eULkdWjcjmM+wXQcr7zXNfnGLgHjZSJINGkt
7Zmvp7WKVAqoHMm1RXV8IfBH1aRgv5+/Lgny
OcFlrPGPMm48/A== )
;; Additional
ai.example. 3600 IN A 192.0.2.9
ai.example. 3600 IN RRSIG A 5 2 3600 20050712112304 (
20050612112304 62699 example.
plY5M26ED3Owe3YX0pBIhgg44j89NxUaoBrU
6bLRr99HpKfFl1sIy18JiRS7evlxCETZgubq
ZXW5S+1VjMZYzQ== )
ai.example. 3600 AAAA 2001:db8::f00:baa9
ai.example. 3600 IN RRSIG AAAA 5 2 3600 20050712112304 (
20050612112304 62699 example.
PNF/t7+DeosEjhfuL0kmsNJvn16qhYyLI9FV
ypSCorFx/PKIlEL3syomkYM2zcXVSRwUXMns
l5/UqLCJJ9BDMg== )
The query returned an answer that was produced as a result of
wildcard expansion. The answer section contains a wildcard RRset
expanded as it would be in a traditional DNS response, and the
corresponding RRSIG indicates that the expanded wildcard MX RRset was
Laurie, et al. Expires December 3, 2005 [Page 33]
Internet-Draft nsec3 june 2005
signed by an "example" DNSKEY with algorithm 5 and key tag 62699.
The RRSIG indicates that the original TTL of the MX RRset was 3600,
and, for the purpose of authentication, the current TTL is replaced
by 3600. The RRSIG labels field value of 2 indicates that the answer
is the result of wildcard expansion, as the "a.z.w.example" name
contains 4 labels. The name "a.z.w.example" is replaced by
"*.w.example", the MX RRset is placed in canonical form, and,
assuming that the current time falls between the signature inception
and expiration dates, the signature is authenticated.
The NSEC3 proves that no closer match (exact or closer wildcard)
could have been used to answer this query, and the NSEC3 RR must also
be authenticated before the answer is considered valid.
B.7 Wildcard No Data Error
A "no data" response for a name covered by a wildcard. The NSEC3 RRs
prove that the matching wildcard name does not have any RRs of the
requested type and that no closer match exists in the zone.
Laurie, et al. Expires December 3, 2005 [Page 34]
Internet-Draft nsec3 june 2005
;; Header: QR AA DO RCODE=0
;;
;; Question
a.z.w.example. IN AAAA
;; Answer
;; (empty)
;; Authority
example. 3600 IN SOA ns1.example. bugs.x.w.example. (
1
3600
300
3600000
3600
)
example. 3600 IN RRSIG SOA 5 1 3600 20050712112304 (
20050612112304 62699 example.
RtctD6aLUU5Md5wOOItilS7JXX1tf58Ql3sK
mTXkL13jqLiUFOGg0uzqRh1U9GbydS0P7M0g
qYIt90txzE/4+g== )
zjxfz5o7t4ty4u3f6fa7mhhqzjln4mui.example. 3600 IN NSEC3 0 1 1 (
deadbeaf
5pe7ctl7pfs2cilroy5dcofx4rcnlypd
MX NSEC3 RRSIG )
zjxfz5o7t4ty4u3f6fa7mhhqzjln4mui.example. 3600 IN RRSIG NSEC3 (
5 2 3600 20050712112304
20050612112304 62699 example.
eULkdWjcjmM+wXQcr7zXNfnGLgHjZSJINGkt
7Zmvp7WKVAqoHMm1RXV8IfBH1aRgv5+/Lgny
OcFlrPGPMm48/A== )
;; Additional
;; (empty)
The query returned NSEC3 RRs that prove that the requested data does
not exist and no wildcard applies. The negative reply is
authenticated by verifying both NSEC3 RRs.
B.8 DS Child Zone No Data Error
A "no data" response for a QTYPE=DS query that was mistakenly sent to
a name server for the child zone.
Laurie, et al. Expires December 3, 2005 [Page 35]
Internet-Draft nsec3 june 2005
;; Header: QR AA DO RCODE=0
;;
;; Question
example. IN DS
;; Answer
;; (empty)
;; Authority
example. 3600 IN SOA ns1.example. bugs.x.w.example. (
1
3600
300
3600000
3600
)
example. 3600 IN RRSIG SOA 5 1 3600 20050712112304 (
20050612112304 62699 example.
RtctD6aLUU5Md5wOOItilS7JXX1tf58Ql3sK
mTXkL13jqLiUFOGg0uzqRh1U9GbydS0P7M0g
qYIt90txzE/4+g== )
dw4o7j64wnel3j4jh7fb3c5n7w3js2yb.example. 3600 IN NSEC3 0 1 1 (
deadbeaf
gmnfcccja7wkax3iv26bs75myptje3qk
MX DNSKEY NS SOA NSEC3 RRSIG )
dw4o7j64wnel3j4jh7fb3c5n7w3js2yb.example. 3600 IN RRSIG NSEC3 (
5 2 3600 20050712112304
20050612112304 62699 example.
VqEbXiZLJVYmo25fmO3IuHkAX155y8NuA50D
C0NmJV/D4R3rLm6tsL6HB3a3f6IBw6kKEa2R
MOiKMSHozVebqw== )
;; Additional
;; (empty)
The query returned NSEC RRs that shows the requested was answered by
a child server ("example" server). The NSEC RR indicates the
presence of an SOA RR, showing that the answer is from the child .
Queries for the "example" DS RRset should be sent to the parent
servers ("root" servers).
Laurie, et al. Expires December 3, 2005 [Page 36]
Internet-Draft nsec3 june 2005
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Disclaimer of Validity
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,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2005). 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.
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Laurie, et al. Expires December 3, 2005 [Page 37]
