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Network Working Group B. Laurie
Internet-Draft G. Sisson
Expires: July 2, 2005 Nominet
R. Arends
Telematica Instituut
january 2005
DNSSEC Hash Authenticated Denial of Existence
draft-ietf-dnsext-nsec3-00
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of section 3 of RFC 3667. 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 become aware will be disclosed, in accordance with
RFC 3668.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as
Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on July 2, 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.
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A complete zone file can be used either directly as a source of
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 makes 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.
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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 . . . . . . . . . . . . . . . . . 6
2.1.4 The Salt Length Field . . . . . . . . . . . . . . . . 6
2.1.5 The Salt Field . . . . . . . . . . . . . . . . . . . . 6
2.1.6 The Next Hashed Ownername Field . . . . . . . . . . . 7
2.1.7 The list of Type Bit Map(s) Field . . . . . . . . . . 7
2.2 The NSEC3 RR Presentation Format . . . . . . . . . . . . . 8
3. Creating Additional NSEC3 RR for Empty Non Terminals . . . . . 9
4. Calculation of the Hash . . . . . . . . . . . . . . . . . . . 9
5. Special Considerations . . . . . . . . . . . . . . . . . . . . 9
5.1 delegation points . . . . . . . . . . . . . . . . . . . . 10
5.1.1 Unsigned Delegations . . . . . . . . . . . . . . . . . 10
5.2 Additional Complexity Caused by Wildcards . . . . . . . . 11
5.3 Salting . . . . . . . . . . . . . . . . . . . . . . . . . 11
5.4 Hash Collision . . . . . . . . . . . . . . . . . . . . . . 11
5.4.1 Avoiding Hash Collisions during generation . . . . . . 11
5.4.2 Second Preimage Requirement Analysis . . . . . . . . . 11
5.4.3 Possible Hash Value Truncation Method . . . . . . . . 12
6. Performance Considerations . . . . . . . . . . . . . . . . . . 12
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
8. Security Considerations . . . . . . . . . . . . . . . . . . . 13
9. Requirements notation . . . . . . . . . . . . . . . . . . . . 13
10. Security Considerations . . . . . . . . . . . . . . . . . . 13
A. Example Zone . . . . . . . . . . . . . . . . . . . . . . . . . 13
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 14
11.1 Normative References . . . . . . . . . . . . . . . . . . . . 14
11.2 Informative References . . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . . 17
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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 requirement was that the existence of all record types in a
zone -including delegation point NS record types- can 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
accumulates measures against the side effects introduced by NSEC, and
presents the NSEC3 Resource Record.
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
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over the original (unmodified) ownername, this result is refered to
as "hashed ownername".
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 2535 [RFC2535]. Unsigned delegation
point NS RR sets can optionally be excluded. To provide protection
against zone traversal, the ownernames used in the NSEC3 RR are
cryptographic hash-value prepended to the name of the zone. The
NSEC3 RR record indicates which Hash Function is used to construct to
hash, which Salt is used, and how many iterations of the Hash
Function are performed over the original 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:
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 /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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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 RR type record
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 behavior is identical to NSEC behavior.
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.
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.
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The salt field is prepended to the original ownername before hashing
in order to defend against precalculated dictionary attacks.
The salt is not prepended during iterations of the hash function.
The Salt field value MUST be identical for all NSEC3 RRs generated
for the zone. If the salt were different for each NSEC3 RR,
collisions could occur where an NSEC3 denies the existence of
existing RRs due to the application of different salt values.
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.
A sender MUST NOT use DNS name compression on the Next Hashed
Ownername field when transmitting an NSEC3 RR.
Hashed ownernames of RRsets not authoritative for the given zone
(such as glue records) MUST NOT be listed in the Hash of Next Domain
Name unless at least one authoritative RRset exists at the same owner
name.
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 bit for the NSEC3 and RRSIG 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 ) +
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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 [4]
(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 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 interpretted 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.
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The Salt Field is represented as 00 when the Salt Length field has
value 0.
The Hash of Next Domain Name 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 [5] (section 5) MUST be used.
3. Creating Additional NSEC3 RR for Empty Non Terminals
In order to prove the nonexistence 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 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 an NSEC3 RR and a RRSIG 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. Special Considerations
The following paragraphs clarify specific behavior explain special
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considerations for implementations.
5.1 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 behavior when the flag value is 1.
5.1.1 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
the Authoritative Only Flag set to 1 can not be used to proof
presence nor absence of the 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 signing the
delegated zone, changing the unsigned delegation into a signed
delegation.
A second attack vector exists in that an adversary is able to
successfully fabricate a response claiming a not existent delegation
to exist, though unsigned.
The only possible mitigation is to either not use this method, hence
proving absence of unsigned delegations.
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5.2 Additional Complexity Caused by Wildcards
If a wildcard ownername appears in a zone, the wildcard 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 2535
[RFC2525] describes the impact of wildcards on authenticated denial
of existence.
In order to prove there are no wildcards for a domain, as well as no
RRs that match directly, an RR must be shown for the closest
encloser, and nonexistence must be shown for all enclosers that could
be closer.
5.3 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
maximum 2040 bits of salt, multiplying the cost by 2^2040.
The salt value for each NSEC3 RR MUST be equal for a single version
of the zone.
5.4 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 2^80. 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.
5.4.1 Avoiding Hash Collisions during generation
During generation of NSEC3 RRs, hash values are supposedly unique.
In the (academic) case of a collision occuring, 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.
5.4.2 Second Preimage Requirement Analysis
A collision resistant 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
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preimage X' <> X such that hash(X) = hash(X'). The probability of
finding a second preimage is 1 in 2^160 for SHA-1 on average. 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.
5.4.3 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.
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.
6. 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.
7. IANA Considerations
IANA has to create a new registry for NSEC3 Hash Functions. The
range for this registry is 0-127. Value 1 is marked as SHA-1.
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Values 0, 2-126 are marked as Reserved For Future Use. Value 127 is
marked as Experimental.
8. 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. The expense of this attack can be chosen by
setting the 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 nonexistence of the colliding domain - however, this is
fantastically unlikely, and, in any case, DNSSEC already relies on
SHA-1 to not collide.
9. Requirements notation
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 [RFC2119].
10. Security Considerations
Appendix A. Example Zone
This is a zone showing its NSEC3 records. They can also be used as
test vectors for the hash algorithm. RRSIG records have been elided.
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example.com. 1000 IN SOA localhost.
postmaster.localhost.example.com. (
1 ; serial
3600 ; refresh (1 hour)
1800 ; retry (30 minutes)
604800 ; expire (1 week)
3600 ; minimum (1 hour)
)
1000 NS ns1.example.com.
1000 NS ns2.example.com.
f519593e82969842a136e0f47814c881fa163833.example.com. 3600 IN NSEC3 \
SHA-1 200 31323334 4EF8239D95C18403A509D7C336A5D0FA48FD1107 \
NS SOA RRSIG DNSKEY NSEC3
a.example.com. 1000 IN A 1.2.3.4
1000 IN A 1.2.3.5
1000 TXT "An example"
bfe6ea21dee9d228889ae11fa58c4bd551d15801.example.com. 3600 IN NSEC3 \
SHA-1 200 31323334 F519593E82969842A136E0F47814C881FA163833 \
A TXT RRSIG NSEC3
b.example.com. 1000 IN A 1.2.3.7
83c06d3b7d01fbc9576c71af2bec1a1163435153.example.com. 3600 IN NSEC3 \
SHA-1 200 31323334 A33559360EECB02F36B5C1B72C109126CA4F5A0D \
A RRSIG NSEC3
a.b.c.example.com. 1000 IN A 1.2.3.6
a33559360eecb02f36b5c1b72c109126ca4f5a0d.example.com. 3600 IN NSEC3 \
SHA-1 200 31323334 BFE6EA21DEE9D228889AE11FA58C4BD551D15801 \
A RRSIG NSEC3
ns1.example.com. 1000 IN A 1.2.3.8
4ef8239d95c18403a509d7c336a5d0fa48fd1107.example.com. 3600 IN NSEC3 \
SHA-1 200 31323334 50016B56FD2F0FFC7B563C50FAF0E34259763BBB \
A RRSIG NSEC3
ns2.example.com. 1000 IN A 1.2.3.9
50016b56fd2f0ffc7b563c50faf0e34259763bbb.example.com. 3600 IN NSEC3 \
SHA-1 200 31323334 83C06D3B7D01FBC9576C71AF2BEC1A1163435153 \
A RRSIG NSEC3
11. References
11.1 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
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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.
[RFC2535] Eastlake, D., "Domain Name System Security Extensions",
RFC 2535, March 1999.
11.2 Informative References
[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.
[rollover]
Ihren, J., Kolkman, O. and B. Manning, "An In-Band
Rollover Algorithm and a Out-Of-Band Priming Method for
DNS Trust Anchors.", July 2004.
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
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Roy Arends
Telematica Instituut
Brouwerijstraat 1
7523 XC Enschede
The Netherlands
Phone: +31 (53) 485 0485
EMail: roy.arends@telin.nl
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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.
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Network Working Group S. Josefsson
Internet-Draft January 24, 2005
Expires: July 25, 2005
Storing Certificates in the Domain Name System (DNS)
draft-ietf-dnsext-rfc2538bis-00
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of section 3 of RFC 3667. 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 become aware will be disclosed, in accordance with
RFC 3668.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as
Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on July 25, 2005.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
Cryptographic public key are frequently published and their
authenticity demonstrated by certificates. A CERT resource record
(RR) is defined so that such certificates and related certificate
revocation lists can be stored in the Domain Name System (DNS).
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. The CERT Resource Record . . . . . . . . . . . . . . . . . . . 3
2.1 Certificate Type Values . . . . . . . . . . . . . . . . . 4
2.2 Text Representation of CERT RRs . . . . . . . . . . . . . 5
2.3 X.509 OIDs . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Appropriate Owner Names for CERT RRs . . . . . . . . . . . . . 6
3.1 Content-based X.509 CERT RR Names . . . . . . . . . . . . 7
3.2 Purpose-based X.509 CERT RR Names . . . . . . . . . . . . 8
3.3 Content-based OpenPGP CERT RR Names . . . . . . . . . . . 8
3.4 Purpose-based OpenPGP CERT RR Names . . . . . . . . . . . 9
4. Performance Considerations . . . . . . . . . . . . . . . . . . 9
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Changes since RFC 2538 . . . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 12
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1 Normative References . . . . . . . . . . . . . . . . . . . . 11
9.2 Informative References . . . . . . . . . . . . . . . . . . . 12
A. Copying conditions . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . 13
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1. Introduction
Public keys are frequently published in the form of a certificate and
their authenticity is commonly demonstrated by certificates and
related certificate revocation lists (CRLs). A certificate is a
binding, through a cryptographic digital signature, of a public key,
a validity interval and/or conditions, and identity, authorization,
or other information. A certificate revocation list is a list of
certificates that are revoked, and incidental information, all signed
by the signer (issuer) of the revoked certificates. Examples are
X.509 certificates/CRLs in the X.500 directory system or OpenPGP
certificates/revocations used by OpenPGP software.
Section 2 below specifies a CERT resource record (RR) for the storage
of certificates in the Domain Name System.
Section 3 discusses appropriate owner names for CERT RRs.
Sections 4, 5, and 6 below cover performance, IANA, and security
considerations, respectively.
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 [10].
2. The CERT Resource Record
The CERT resource record (RR) has the structure given below. Its RR
type code is 37.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| type | key tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| algorithm | /
+---------------+ certificate or CRL /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
The type field is the certificate type as define in section 2.1
below.
The algorithm field has the same meaning as the algorithm field in
DNSKEY and RRSIG RRs [9] except that a zero algorithm field indicates
the algorithm is unknown to a secure DNS, which may simply be the
result of the algorithm not having been standardized for DNSSEC.
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The key tag field is the 16 bit value computed for the key embedded
in the certificate, using the RRSIG Key Tag Algorithm described in
Appendix B of [9]. This field is used as an efficiency measure to
pick which CERT RRs may be applicable to a particular key. The key
tag can be calculated for the key in question and then only CERT RRs
with the same key tag need be examined. However, the key must always
be transformed to the format it would have as the public key portion
of a DNSKEY RR before the key tag is computed. This is only possible
if the key is applicable to an algorithm (and limits such as key size
limits) defined for DNS security. If it is not, the algorithm field
MUST BE zero and the tag field is meaningless and SHOULD BE zero.
2.1 Certificate Type Values
The following values are defined or reserved:
Value Mnemonic Certificate Type
----- -------- ----------- ----
0 reserved
1 PKIX X.509 as per PKIX
2 SPKI SPKI certificate
3 PGP OpenPGP packet
4-252 available for IANA assignment
253 URI URI private
254 OID OID private
255-65534 available for IANA assignment
65535 reserved
The PKIX type is reserved to indicate an X.509 certificate conforming
to the profile being defined by the IETF PKIX working group. The
certificate section will start with a one byte unsigned OID length
and then an X.500 OID indicating the nature of the remainder of the
certificate section (see 2.3 below). (NOTE: X.509 certificates do
not include their X.500 directory type designating OID as a prefix.)
The SPKI type is reserved to indicate a certificate formated as to be
specified by the IETF SPKI working group.
The PGP type indicates an OpenPGP packet as described in [5] and its
extensions and successors. Two uses are to transfer public key
material and revocation signatures. The data is binary, and MUST NOT
be encoded into an ASCII armor. An implementation SHOULD process
transferable public keys as described in section 10.1 of [5], but it
MAY handle additional OpenPGP packets.
The URI private type indicates a certificate format defined by an
absolute URI. The certificate portion of the CERT RR MUST begin with
a null terminated URI [4] and the data after the null is the private
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format certificate itself. The URI SHOULD be such that a retrieval
from it will lead to documentation on the format of the certificate.
Recognition of private certificate types need not be based on URI
equality but can use various forms of pattern matching so that, for
example, subtype or version information can also be encoded into the
URI.
The OID private type indicates a private format certificate specified
by a an ISO OID prefix. The certificate section will start with a
one byte unsigned OID length and then a BER encoded OID indicating
the nature of the remainder of the certificate section. This can be
an X.509 certificate format or some other format. X.509 certificates
that conform to the IETF PKIX profile SHOULD be indicated by the PKIX
type, not the OID private type. Recognition of private certificate
types need not be based on OID equality but can use various forms of
pattern matching such as OID prefix.
2.2 Text Representation of CERT RRs
The RDATA portion of a CERT RR has the type field as an unsigned
decimal integer or as a mnemonic symbol as listed in section 2.1
above.
The key tag field is represented as an unsigned decimal integer.
The algorithm field is represented as an unsigned decimal integer or
a mnemonic symbol as listed in [9].
The certificate / CRL portion is represented in base 64 [11] and may
be divided up into any number of white space separated substrings,
down to single base 64 digits, which are concatenated to obtain the
full signature. These substrings can span lines using the standard
parenthesis.
Note that the certificate / CRL portion may have internal sub-fields
but these do not appear in the master file representation. For
example, with type 254, there will be an OID size, an OID, and then
the certificate / CRL proper. But only a single logical base 64
string will appear in the text representation.
2.3 X.509 OIDs
OIDs have been defined in connection with the X.500 directory for
user certificates, certification authority certificates, revocations
of certification authority, and revocations of user certificates.
The following table lists the OIDs, their BER encoding, and their
length prefixed hex format for use in CERT RRs:
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id-at-userCertificate
= { joint-iso-ccitt(2) ds(5) at(4) 36 }
== 0x 03 55 04 24
id-at-cACertificate
= { joint-iso-ccitt(2) ds(5) at(4) 37 }
== 0x 03 55 04 25
id-at-authorityRevocationList
= { joint-iso-ccitt(2) ds(5) at(4) 38 }
== 0x 03 55 04 26
id-at-certificateRevocationList
= { joint-iso-ccitt(2) ds(5) at(4) 39 }
== 0x 03 55 04 27
3. Appropriate Owner Names for CERT RRs
It is recommended that certificate CERT RRs be stored under a domain
name related to their subject, i.e., the name of the entity intended
to control the private key corresponding to the public key being
certified. It is recommended that certificate revocation list CERT
RRs be stored under a domain name related to their issuer.
Following some of the guidelines below may result in the use in DNS
names of characters that require DNS quoting which is to use a
backslash followed by the octal representation of the ASCII code for
the character such as \000 for NULL.
The choice of name under which CERT RRs are stored is important to
clients that perform CERT queries. In some situations, the client
may not know all information about the CERT RR object it wishes to
retrieve. For example, a client may not know the subject name of an
X.509 certificate, or the e-mail address of the owner of an OpenPGP
key. Further, the client might only know the hostname of a service
that uses X.509 certificates or the Key ID of an OpenPGP key.
This motivate describing two different owner name guidelines. We
call the two rules content-based owner names and purpose-based owner
names. A content-based owner name is derived from the content of the
CERT RR data; for example the Subject field in an X.509 certificate
or the User ID field in OpenPGP keys. A purpose-based owner name is
selected to be a name that clients that wishes to retrieve CERT RRs
are expected to know; for example the host name of a X.509 protected
service or a Key ID of an OpenPGP key. Note that in some situations,
the content-based and purpose-based owner name can be the same; for
example when a client look up keys based on e-mail addresses for
incoming e-mail.
Implementations SHOULD use the purpose-based owner name guidelines
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described in this document, and MAY use CNAMEs at content-based owner
names (or other names), pointing to the purpose-based owner name.
3.1 Content-based X.509 CERT RR Names
Some X.509 versions permit multiple names to be associated with
subjects and issuers under "Subject Alternate Name" and "Issuer
Alternate Name". For example, x.509v3 has such Alternate Names with
an ASN.1 specification as follows:
GeneralName ::= CHOICE {
otherName [0] INSTANCE OF OTHER-NAME,
rfc822Name [1] IA5String,
dNSName [2] IA5String,
x400Address [3] EXPLICIT OR-ADDRESS.&Type,
directoryName [4] EXPLICIT Name,
ediPartyName [5] EDIPartyName,
uniformResourceIdentifier [6] IA5String,
iPAddress [7] OCTET STRING,
registeredID [8] OBJECT IDENTIFIER
}
The recommended locations of CERT storage are as follows, in priority
order:
1. If a domain name is included in the identification in the
certificate or CRL, that should be used.
2. If a domain name is not included but an IP address is included,
then the translation of that IP address into the appropriate
inverse domain name should be used.
3. If neither of the above it used but a URI containing a domain
name is present, that domain name should be used.
4. If none of the above is included but a character string name is
included, then it should be treated as described for PGP names
below.
5. If none of the above apply, then the distinguished name (DN)
should be mapped into a domain name as specified in [3].
Example 1: Assume that an X.509v3 certificate is issued to /CN=John
Doe/DC=Doe/DC=com/DC=xy/O=Doe Inc/C=XY/ with Subject Alternative
names of (a) string "John (the Man) Doe", (b) domain name john-
doe.com, and (c) uri <https://www.secure.john-doe.com:8080/>. Then
the storage locations recommended, in priority order, would be
1. john-doe.com,
2. www.secure.john-doe.com, and
3. Doe.com.xy.
Example 2: Assume that an X.509v3 certificate is issued to /CN=James
Hacker/L=Basingstoke/O=Widget Inc/C=GB/ with Subject Alternate names
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of (a) domain name widget.foo.example, (b) IPv4 address
10.251.13.201, and (c) string "James Hacker
<hacker@mail.widget.foo.example>". Then the storage locations
recommended, in priority order, would be
1. widget.foo.example,
2. 201.13.251.10.in-addr.arpa, and
3. hacker.mail.widget.foo.example.
3.2 Purpose-based X.509 CERT RR Names
It is difficult for clients that do not already posses a certificate
to reconstruct the content-based owner name that should be used to
retrieve the certificate. For this reason, purpose-based owner names
are recommended in this section. Because purpose-based owner names
by nature depend on the specific scenario, or purpose, for which the
certificate will be used, there are more than one recommendation.
The following table summarize the purpose-based X.509 CERT RR owner
name guidelines.
Scenario Owner name
-------------------------------------------------------------------
S/MIME Certificate Standard translation of RFC 822 email address.
Example: A S/MIME certificate for
"postmaster@example.org" will use a standard
hostname translation of the owner name,
i.e. "postmaster.example.org".
SSL Certificate Hostname of the SSL server.
IPSEC Certificate Hostname of the IPSEC machine, and/or
for the in-addr.arpa reverse lookup IP address.
CRLs Hostname of the issuing CA.
3.3 Content-based OpenPGP CERT RR Names
OpenPGP signed keys (certificates) use a general character string
User ID [5]. However, it is recommended by OpenPGP that such names
include the RFC 2822 [7] email address of the party, as in "Leslie
Example <Leslie@host.example>". If such a format is used, the CERT
should be under the standard translation of the email address into a
domain name, which would be leslie.host.example in this case. If no
RFC 2822 name can be extracted from the string name no specific
domain name is recommended.
If a user has more than one email address, the CNAME type can be used
to reduce the amount of data stored in the DNS. For example:
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$ORIGIN example.org.
smith IN CERT PGP 0 0 <OpenPGP binary>
john.smith IN CNAME smith
js IN CNAME smith
3.4 Purpose-based OpenPGP CERT RR Names
Applications that receive an OpenPGP packet but do not know the email
address of the sender will have difficulties constructing the correct
owner name, and cannot use the content-based owner name guidelines.
However, these clients commonly know the key fingerprint or the Key
ID. The key ID is found in OpenPGP packets, and the key fingerprint
is commonly found in auxilliary data that may be available. For
these situations, it is recommended to use an owner name identical to
the key fingerprint and key ID expressed in hexadecimal [11]. For
example:
$ORIGIN example.org.
0424D4EE81A0E3D119C6F835EDA21E94B565716F IN CERT PGP ...
F835EDA21E94B565716F IN CERT PGP ...
B565716F IN CERT PGP ...
If the same key material is stored at several owner names, the use of
CNAME may be used to avoid data duplication. Note that CNAME is not
always applicable, because it map an owner names to the other for all
purposes, and this may be sub-optimal when two keys with the same Key
ID are stored.
4. Performance Considerations
Current Domain Name System (DNS) implementations are optimized for
small transfers, typically not more than 512 bytes including
overhead. While larger transfers will perform correctly and work is
underway to make larger transfers more efficient, it is still
advisable at this time to make every reasonable effort to minimize
the size of certificates stored within the DNS. Steps that can be
taken may include using the fewest possible optional or extensions
fields and using short field values for variable length fields that
must be included.
The RDATA field in the DNS protocol may only hold data of size 65535
octets (64kb) or less. This means that each CERT RR cannot contain
more than 64kb worth of payload, even if the corresponding
certificate or certificate revocation list is larger. This document
do not address this limitation.
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5. Acknowledgements
The majority of this document is copied verbatim from RFC 2538, by
Donald Eastlake 3rd and Olafur Gudmundsson.
The author wishes to thank David Shaw and Michael Graff for their
contributions to the earlier work that motivated this revised
document.
Florian Weimer suggested to clarify wording regarding what data can
be stored in RRDATA portion of OpenPGP CERT RRs, and that the URI
type may include hashes to secure the indirection. Olivier Dubuisson
confirmed that the X.509 OID were indeed correct.
6. Security Considerations
By definition, certificates contain their own authenticating
signature. Thus it is reasonable to store certificates in non-secure
DNS zones or to retrieve certificates from DNS with DNS security
checking not implemented or deferred for efficiency. The results MAY
be trusted if the certificate chain is verified back to a known
trusted key and this conforms with the user's security policy.
Alternatively, if certificates are retrieved from a secure DNS zone
with DNS security checking enabled and are verified by DNS security,
the key within the retrieved certificate MAY be trusted without
verifying the certificate chain if this conforms with the user's
security policy.
When the URI type is used, it should be understood that is introduce
an additional indirection that may allow for a new attack vector.
One method to secure that indirection is to include a hash of the
certificate in the URI itself.
CERT RRs are not used in connection with securing the DNS security
additions so there are no security considerations related to CERT RRs
and securing the DNS itself.
7. IANA Considerations
Certificate types 0x0000 through 0x00FF and 0xFF00 through 0xFFFF can
only be assigned by an IETF standards action [6]. This document
assigns 0x0001 through 0x0003 and 0x00FD and 0x00FE. Certificate
types 0x0100 through 0xFEFF are assigned through IETF Consensus [6]
based on RFC documentation of the certificate type. The availability
of private types under 0x00FD and 0x00FE should satisfy most
requirements for proprietary or private types.
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8. Changes since RFC 2538
1. Editorial changes to conform with new document requirements,
including splitting reference section into two parts and updating
the references to point at latest versions, and to add some
additional references.
2. Improve terminology. For example replace "PGP" with "OpenPGP",
to align with RFC 2440.
3. In section 2.1, clarify that OpenPGP public key data are binary,
not the ASCII armored format, and reference 10.1 in RFC 2440 on
how to deal with OpenPGP keys, and acknowledge that
implementations may handle additional packet types.
4. Clarify that integers in the representation format are decimal.
5. Replace KEY/SIG with DNSKEY/RRSIG etc, to align with DNSSECbis
terminology. Improve reference for Key Tag Algorithm
calculations.
6. Add examples that suggest use of CNAME to reduce bandwidth.
7. In section 3, appended the last paragraphs that discuss
"content-based" vs "purpose-based" owner names. Add section 3.2
for purpose-based X.509 CERT owner names, and section 3.4 for
purpose-based OpenPGP CERT owner names.
8. Added size considerations.
9. References
9.1 Normative References
[1] Mockapetris, P., "Domain names - concepts and facilities", STD
13, RFC 1034, November 1987.
[2] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[3] Kille, S., Wahl, M., Grimstad, A., Huber, R. and S. Sataluri,
"Using Domains in LDAP/X.500 Distinguished Names", RFC 2247,
January 1998.
[4] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource
Identifiers (URI): Generic Syntax", RFC 2396, August 1998.
[5] Callas, J., Donnerhacke, L., Finney, H. and R. Thayer, "OpenPGP
Message Format", RFC 2440, November 1998.
[6] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October 1998.
[7] Resnick, P., "Internet Message Format", RFC 2822, April 2001.
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[8] Arends, R., Austein, R., Massey, D., Larson, M. and S. Rose,
"DNS Security Introduction and Requirements",
draft-ietf-dnsext-dnssec-intro-13 (work in progress), October
2004.
[9] Arends, R., "Resource Records for the DNS Security Extensions",
draft-ietf-dnsext-dnssec-records-11 (work in progress), October
2004.
9.2 Informative References
[10] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[11] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings",
RFC 3548, July 2003.
Author's Address
Simon Josefsson
EMail: simon@josefsson.org
Appendix A. Copying conditions
Regarding the portion of this document that was written by Simon
Josefsson ("the author", for the remainder of this section), the
author makes no guarantees and is not responsible for any damage
resulting from its use. The author grants irrevocable permission to
anyone to use, modify, and distribute it in any way that does not
diminish the rights of anyone else to use, modify, and distribute it,
provided that redistributed derivative works do not contain
misleading author or version information. Derivative works need not
be licensed under similar terms.
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Internet-Draft Storing Certificates in the DNS January 2005
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