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Network Working Group R. Arends
Request for Comments: 4034 Telematica Instituut
Obsoletes: 2535, 3008, 3090, 3445, 3655, 3658, R. Austein
3755, 3757, 3845 ISC
Updates: 1034, 1035, 2136, 2181, 2308, 3225, M. Larson
3007, 3597, 3226 VeriSign
Category: Standards Track D. Massey
Colorado State University
S. Rose
NIST
March 2005
Resource Records for the DNS Security Extensions
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document is part of a family of documents that describe the DNS
Security Extensions (DNSSEC). The DNS Security Extensions are a
collection of resource records and protocol modifications that
provide source authentication for the DNS. This document defines the
public key (DNSKEY), delegation signer (DS), resource record digital
signature (RRSIG), and authenticated denial of existence (NSEC)
resource records. The purpose and format of each resource record is
described in detail, and an example of each resource record is given.
This document obsoletes RFC 2535 and incorporates changes from all
updates to RFC 2535.
Arends, et al. Standards Track [Page 1]
RFC 4034 DNSSEC Resource Records March 2005
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Background and Related Documents . . . . . . . . . . . 3
1.2. Reserved Words . . . . . . . . . . . . . . . . . . . . 3
2. The DNSKEY Resource Record . . . . . . . . . . . . . . . . . 4
2.1. DNSKEY RDATA Wire Format . . . . . . . . . . . . . . . 4
2.1.1. The Flags Field. . . . . . . . . . . . . . . . 4
2.1.2. The Protocol Field . . . . . . . . . . . . . . 5
2.1.3. The Algorithm Field. . . . . . . . . . . . . . 5
2.1.4. The Public Key Field . . . . . . . . . . . . . 5
2.1.5. Notes on DNSKEY RDATA Design . . . . . . . . . 5
2.2. The DNSKEY RR Presentation Format. . . . . . . . . . . 5
2.3. DNSKEY RR Example . . . . . . . . . . . . . . . . . . 6
3. The RRSIG Resource Record . . . . . . . . . . . . . . . . . 6
3.1. RRSIG RDATA Wire Format. . . . . . . . . . . . . . . . 7
3.1.1. The Type Covered Field . . . . . . . . . . . . 7
3.1.2. The Algorithm Number Field . . . . . . . . . . 8
3.1.3. The Labels Field . . . . . . . . . . . . . . . 8
3.1.4. Original TTL Field . . . . . . . . . . . . . . 8
3.1.5. Signature Expiration and Inception Fields. . . 9
3.1.6. The Key Tag Field. . . . . . . . . . . . . . . 9
3.1.7. The Signer's Name Field. . . . . . . . . . . . 9
3.1.8. The Signature Field. . . . . . . . . . . . . . 9
3.2. The RRSIG RR Presentation Format . . . . . . . . . . . 10
3.3. RRSIG RR Example . . . . . . . . . . . . . . . . . . . 11
4. The NSEC Resource Record . . . . . . . . . . . . . . . . . . 12
4.1. NSEC RDATA Wire Format . . . . . . . . . . . . . . . . 13
4.1.1. The Next Domain Name Field . . . . . . . . . . 13
4.1.2. The Type Bit Maps Field. . . . . . . . . . . . 13
4.1.3. Inclusion of Wildcard Names in NSEC RDATA. . . 14
4.2. The NSEC RR Presentation Format. . . . . . . . . . . . 14
4.3. NSEC RR Example. . . . . . . . . . . . . . . . . . . . 15
5. The DS Resource Record . . . . . . . . . . . . . . . . . . . 15
5.1. DS RDATA Wire Format . . . . . . . . . . . . . . . . . 16
5.1.1. The Key Tag Field. . . . . . . . . . . . . . . 16
5.1.2. The Algorithm Field. . . . . . . . . . . . . . 16
5.1.3. The Digest Type Field. . . . . . . . . . . . . 17
5.1.4. The Digest Field . . . . . . . . . . . . . . . 17
5.2. Processing of DS RRs When Validating Responses . . . . 17
5.3. The DS RR Presentation Format. . . . . . . . . . . . . 17
5.4. DS RR Example. . . . . . . . . . . . . . . . . . . . . 18
6. Canonical Form and Order of Resource Records . . . . . . . . 18
6.1. Canonical DNS Name Order . . . . . . . . . . . . . . . 18
6.2. Canonical RR Form. . . . . . . . . . . . . . . . . . . 19
6.3. Canonical RR Ordering within an RRset. . . . . . . . . 20
7. IANA Considerations. . . . . . . . . . . . . . . . . . . . . 20
8. Security Considerations. . . . . . . . . . . . . . . . . . . 21
Arends, et al. Standards Track [Page 2]
RFC 4034 DNSSEC Resource Records March 2005
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 22
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
10.1. Normative References . . . . . . . . . . . . . . . . . 22
10.2. Informative References . . . . . . . . . . . . . . . . 23
A. DNSSEC Algorithm and Digest Types. . . . . . . . . . . . . . 24
A.1. DNSSEC Algorithm Types . . . . . . . . . . . . . . . . 24
A.1.1. Private Algorithm Types. . . . . . . . . . . . 25
A.2. DNSSEC Digest Types. . . . . . . . . . . . . . . . . . 25
B. Key Tag Calculation. . . . . . . . . . . . . . . . . . . . . 25
B.1. Key Tag for Algorithm 1 (RSA/MD5). . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . 29
1. Introduction
The DNS Security Extensions (DNSSEC) introduce four new DNS resource
record types: DNS Public Key (DNSKEY), Resource Record Signature
(RRSIG), Next Secure (NSEC), and Delegation Signer (DS). This
document defines the purpose of each resource record (RR), the RR's
RDATA format, and its presentation format (ASCII representation).
1.1. Background and Related Documents
This document is part of a family of documents defining DNSSEC, which
should be read together as a set.
[RFC4033] contains an introduction to DNSSEC and definition of common
terms; the reader is assumed to be familiar with this document.
[RFC4033] also contains a list of other documents updated by and
obsoleted by this document set.
[RFC4035] defines the DNSSEC protocol operations.
The reader is also assumed to be familiar with the basic DNS concepts
described in [RFC1034], [RFC1035], and the subsequent documents that
update them, particularly [RFC2181] and [RFC2308].
This document defines the DNSSEC resource records. All numeric DNS
type codes given in this document are decimal integers.
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 [RFC2119].
Arends, et al. Standards Track [Page 3]
RFC 4034 DNSSEC Resource Records March 2005
2. The DNSKEY Resource Record
DNSSEC uses public key cryptography to sign and authenticate DNS
resource record sets (RRsets). The public keys are stored in DNSKEY
resource records and are used in the DNSSEC authentication process
described in [RFC4035]: A zone signs its authoritative RRsets by
using a private key and stores the corresponding public key in a
DNSKEY RR. A resolver can then use the public key to validate
signatures covering the RRsets in the zone, and thus to authenticate
them.
The DNSKEY RR is not intended as a record for storing arbitrary
public keys and MUST NOT be used to store certificates or public keys
that do not directly relate to the DNS infrastructure.
The Type value for the DNSKEY RR type is 48.
The DNSKEY RR is class independent.
The DNSKEY RR has no special TTL requirements.
2.1. DNSKEY RDATA Wire Format
The RDATA for a DNSKEY RR consists of a 2 octet Flags Field, a 1
octet Protocol Field, a 1 octet Algorithm Field, and the Public Key
Field.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Protocol | Algorithm |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
/ Public Key /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
2.1.1. The Flags Field
Bit 7 of the Flags field is the Zone Key flag. If bit 7 has value 1,
then the DNSKEY record holds a DNS zone key, and the DNSKEY RR's
owner name MUST be the name of a zone. If bit 7 has value 0, then
the DNSKEY record holds some other type of DNS public key and MUST
NOT be used to verify RRSIGs that cover RRsets.
Bit 15 of the Flags field is the Secure Entry Point flag, described
in [RFC3757]. If bit 15 has value 1, then the DNSKEY record holds a
key intended for use as a secure entry point. This flag is only
Arends, et al. Standards Track [Page 4]
RFC 4034 DNSSEC Resource Records March 2005
intended to be a hint to zone signing or debugging software as to the
intended use of this DNSKEY record; validators MUST NOT alter their
behavior during the signature validation process in any way based on
the setting of this bit. This also means that a DNSKEY RR with the
SEP bit set would also need the Zone Key flag set in order to be able
to generate signatures legally. A DNSKEY RR with the SEP set and the
Zone Key flag not set MUST NOT be used to verify RRSIGs that cover
RRsets.
Bits 0-6 and 8-14 are reserved: these bits MUST have value 0 upon
creation of the DNSKEY RR and MUST be ignored upon receipt.
2.1.2. The Protocol Field
The Protocol Field MUST have value 3, and the DNSKEY RR MUST be
treated as invalid during signature verification if it is found to be
some value other than 3.
2.1.3. The Algorithm Field
The Algorithm field identifies the public key's cryptographic
algorithm and determines the format of the Public Key field. A list
of DNSSEC algorithm types can be found in Appendix A.1
2.1.4. The Public Key Field
The Public Key Field holds the public key material. The format
depends on the algorithm of the key being stored and is described in
separate documents.
2.1.5. Notes on DNSKEY RDATA Design
Although the Protocol Field always has value 3, it is retained for
backward compatibility with early versions of the KEY record.
2.2. The DNSKEY RR Presentation Format
The presentation format of the RDATA portion is as follows:
The Flag field MUST be represented as an unsigned decimal integer.
Given the currently defined flags, the possible values are: 0, 256,
and 257.
The Protocol Field MUST be represented as an unsigned decimal integer
with a value of 3.
The Algorithm field MUST be represented either as an unsigned decimal
integer or as an algorithm mnemonic as specified in Appendix A.1.
Arends, et al. Standards Track [Page 5]
RFC 4034 DNSSEC Resource Records March 2005
The Public Key field MUST be represented as a Base64 encoding of the
Public Key. Whitespace is allowed within the Base64 text. For a
definition of Base64 encoding, see [RFC3548].
2.3. DNSKEY RR Example
The following DNSKEY RR stores a DNS zone key for example.com.
example.com. 86400 IN DNSKEY 256 3 5 ( AQPSKmynfzW4kyBv015MUG2DeIQ3
Cbl+BBZH4b/0PY1kxkmvHjcZc8no
kfzj31GajIQKY+5CptLr3buXA10h
WqTkF7H6RfoRqXQeogmMHfpftf6z
Mv1LyBUgia7za6ZEzOJBOztyvhjL
742iU/TpPSEDhm2SNKLijfUppn1U
aNvv4w== )
The first four text fields specify the owner name, TTL, Class, and RR
type (DNSKEY). Value 256 indicates that the Zone Key bit (bit 7) in
the Flags field has value 1. Value 3 is the fixed Protocol value.
Value 5 indicates the public key algorithm. Appendix A.1 identifies
algorithm type 5 as RSA/SHA1 and indicates that the format of the
RSA/SHA1 public key field is defined in [RFC3110]. The remaining
text is a Base64 encoding of the public key.
3. The RRSIG Resource Record
DNSSEC uses public key cryptography to sign and authenticate DNS
resource record sets (RRsets). Digital signatures are stored in
RRSIG resource records and are used in the DNSSEC authentication
process described in [RFC4035]. A validator can use these RRSIG RRs
to authenticate RRsets from the zone. The RRSIG RR MUST only be used
to carry verification material (digital signatures) used to secure
DNS operations.
An RRSIG record contains the signature for an RRset with a particular
name, class, and type. The RRSIG RR specifies a validity interval
for the signature and uses the Algorithm, the Signer's Name, and the
Key Tag to identify the DNSKEY RR containing the public key that a
validator can use to verify the signature.
Because every authoritative RRset in a zone must be protected by a
digital signature, RRSIG RRs must be present for names containing a
CNAME RR. This is a change to the traditional DNS specification
[RFC1034], which stated that if a CNAME is present for a name, it is
the only type allowed at that name. A RRSIG and NSEC (see Section 4)
MUST exist for the same name as a CNAME resource record in a signed
zone.
Arends, et al. Standards Track [Page 6]
RFC 4034 DNSSEC Resource Records March 2005
The Type value for the RRSIG RR type is 46.
The RRSIG RR is class independent.
An RRSIG RR MUST have the same class as the RRset it covers.
The TTL value of an RRSIG RR MUST match the TTL value of the RRset it
covers. This is an exception to the [RFC2181] rules for TTL values
of individual RRs within a RRset: individual RRSIG RRs with the same
owner name will have different TTL values if the RRsets they cover
have different TTL values.
3.1. RRSIG RDATA Wire Format
The RDATA for an RRSIG RR consists of a 2 octet Type Covered field, a
1 octet Algorithm field, a 1 octet Labels field, a 4 octet Original
TTL field, a 4 octet Signature Expiration field, a 4 octet Signature
Inception field, a 2 octet Key tag, the Signer's Name field, and the
Signature field.
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 Covered | Algorithm | Labels |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Original TTL |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Signature Expiration |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Signature Inception |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key Tag | /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Signer's Name /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
/ Signature /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.1.1. The Type Covered Field
The Type Covered field identifies the type of the RRset that is
covered by this RRSIG record.
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RFC 4034 DNSSEC Resource Records March 2005
3.1.2. The Algorithm Number Field
The Algorithm Number field identifies the cryptographic algorithm
used to create the signature. A list of DNSSEC algorithm types can
be found in Appendix A.1
3.1.3. The Labels Field
The Labels field specifies the number of labels in the original RRSIG
RR owner name. The significance of this field is that a validator
uses it to determine whether the answer was synthesized from a
wildcard. If so, it can be used to determine what owner name was
used in generating the signature.
To validate a signature, the validator needs the original owner name
that was used to create the signature. If the original owner name
contains a wildcard label ("*"), the owner name may have been
expanded by the server during the response process, in which case the
validator will have to reconstruct the original owner name in order
to validate the signature. [RFC4035] describes how to use the Labels
field to reconstruct the original owner name.
The value of the Labels field MUST NOT count either the null (root)
label that terminates the owner name or the wildcard label (if
present). The value of the Labels field MUST be less than or equal
to the number of labels in the RRSIG owner name. For example,
"www.example.com." has a Labels field value of 3, and
"*.example.com." has a Labels field value of 2. Root (".") has a
Labels field value of 0.
Although the wildcard label is not included in the count stored in
the Labels field of the RRSIG RR, the wildcard label is part of the
RRset's owner name when the signature is generated or verified.
3.1.4. Original TTL Field
The Original TTL field specifies the TTL of the covered RRset as it
appears in the authoritative zone.
The Original TTL field is necessary because a caching resolver
decrements the TTL value of a cached RRset. In order to validate a
signature, a validator requires the original TTL. [RFC4035]
describes how to use the Original TTL field value to reconstruct the
original TTL.
Arends, et al. Standards Track [Page 8]
RFC 4034 DNSSEC Resource Records March 2005
3.1.5. Signature Expiration and Inception Fields
The Signature Expiration and Inception fields specify a validity
period for the signature. The RRSIG record MUST NOT be used for
authentication prior to the inception date and MUST NOT be used for
authentication after the expiration date.
The Signature Expiration and Inception field values specify a date
and time in the form of a 32-bit unsigned number of seconds elapsed
since 1 January 1970 00:00:00 UTC, ignoring leap seconds, in network
byte order. The longest interval that can be expressed by this
format without wrapping is approximately 136 years. An RRSIG RR can
have an Expiration field value that is numerically smaller than the
Inception field value if the expiration field value is near the
32-bit wrap-around point or if the signature is long lived. Because
of this, all comparisons involving these fields MUST use "Serial
number arithmetic", as defined in [RFC1982]. As a direct
consequence, the values contained in these fields cannot refer to
dates more than 68 years in either the past or the future.
3.1.6. The Key Tag Field
The Key Tag field contains the key tag value of the DNSKEY RR that
validates this signature, in network byte order. Appendix B explains
how to calculate Key Tag values.
3.1.7. The Signer's Name Field
The Signer's Name field value identifies the owner name of the DNSKEY
RR that a validator is supposed to use to validate this signature.
The Signer's Name field MUST contain the name of the zone of the
covered RRset. A sender MUST NOT use DNS name compression on the
Signer's Name field when transmitting a RRSIG RR.
3.1.8. The Signature Field
The Signature field contains the cryptographic signature that covers
the RRSIG RDATA (excluding the Signature field) and the RRset
specified by the RRSIG owner name, RRSIG class, and RRSIG Type
Covered field. The format of this field depends on the algorithm in
use, and these formats are described in separate companion documents.
3.1.8.1. Signature Calculation
A signature covers the RRSIG RDATA (excluding the Signature Field)
and covers the data RRset specified by the RRSIG owner name, RRSIG
class, and RRSIG Type Covered fields. The RRset is in canonical form
(see Section 6), and the set RR(1),...RR(n) is signed as follows:
Arends, et al. Standards Track [Page 9]
RFC 4034 DNSSEC Resource Records March 2005
signature = sign(RRSIG_RDATA | RR(1) | RR(2)... ) where
"|" denotes concatenation;
RRSIG_RDATA is the wire format of the RRSIG RDATA fields
with the Signer's Name field in canonical form and
the Signature field excluded;
RR(i) = owner | type | class | TTL | RDATA length | RDATA
"owner" is the fully qualified owner name of the RRset in
canonical form (for RRs with wildcard owner names, the
wildcard label is included in the owner name);
Each RR MUST have the same owner name as the RRSIG RR;
Each RR MUST have the same class as the RRSIG RR;
Each RR in the RRset MUST have the RR type listed in the
RRSIG RR's Type Covered field;
Each RR in the RRset MUST have the TTL listed in the
RRSIG Original TTL Field;
Any DNS names in the RDATA field of each RR MUST be in
canonical form; and
The RRset MUST be sorted in canonical order.
See Sections 6.2 and 6.3 for details on canonical form and ordering
of RRsets.
3.2. The RRSIG RR Presentation Format
The presentation format of the RDATA portion is as follows:
The Type Covered field is represented as an RR type mnemonic. When
the mnemonic is not known, the TYPE representation as described in
[RFC3597], Section 5, MUST be used.
The Algorithm field value MUST be represented either as an unsigned
decimal integer or as an algorithm mnemonic, as specified in Appendix
A.1.
The Labels field value MUST be represented as an unsigned decimal
integer.
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RFC 4034 DNSSEC Resource Records March 2005
The Original TTL field value MUST be represented as an unsigned
decimal integer.
The Signature Expiration Time and Inception Time field values MUST be
represented either as an unsigned decimal integer indicating seconds
since 1 January 1970 00:00:00 UTC, or in the form YYYYMMDDHHmmSS in
UTC, where:
YYYY is the year (0001-9999, but see Section 3.1.5);
MM is the month number (01-12);
DD is the day of the month (01-31);
HH is the hour, in 24 hour notation (00-23);
mm is the minute (00-59); and
SS is the second (00-59).
Note that it is always possible to distinguish between these two
formats because the YYYYMMDDHHmmSS format will always be exactly 14
digits, while the decimal representation of a 32-bit unsigned integer
can never be longer than 10 digits.
The Key Tag field MUST be represented as an unsigned decimal integer.
The Signer's Name field value MUST be represented as a domain name.
The Signature field is represented as a Base64 encoding of the
signature. Whitespace is allowed within the Base64 text. See
Section 2.2.
3.3. RRSIG RR Example
The following RRSIG RR stores the signature for the A RRset of
host.example.com:
host.example.com. 86400 IN RRSIG A 5 3 86400 20030322173103 (
20030220173103 2642 example.com.
oJB1W6WNGv+ldvQ3WDG0MQkg5IEhjRip8WTr
PYGv07h108dUKGMeDPKijVCHX3DDKdfb+v6o
B9wfuh3DTJXUAfI/M0zmO/zz8bW0Rznl8O3t
GNazPwQKkRN20XPXV6nwwfoXmJQbsLNrLfkG
J5D6fwFm8nN+6pBzeDQfsS3Ap3o= )
The first four fields specify the owner name, TTL, Class, and RR type
(RRSIG). The "A" represents the Type Covered field. The value 5
identifies the algorithm used (RSA/SHA1) to create the signature.
The value 3 is the number of Labels in the original owner name. The
value 86400 in the RRSIG RDATA is the Original TTL for the covered A
RRset. 20030322173103 and 20030220173103 are the expiration and
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RFC 4034 DNSSEC Resource Records March 2005
inception dates, respectively. 2642 is the Key Tag, and example.com.
is the Signer's Name. The remaining text is a Base64 encoding of the
signature.
Note that combination of RRSIG RR owner name, class, and Type Covered
indicates that this RRSIG covers the "host.example.com" A RRset. The
Label value of 3 indicates that no wildcard expansion was used. The
Algorithm, Signer's Name, and Key Tag indicate that this signature
can be authenticated using an example.com zone DNSKEY RR whose
algorithm is 5 and whose key tag is 2642.
4. The NSEC Resource Record
The NSEC resource record lists two separate things: the next owner
name (in the canonical ordering of the zone) that contains
authoritative data or a delegation point NS RRset, and the set of RR
types present at the NSEC RR's owner name [RFC3845]. The complete
set of NSEC RRs in a zone indicates which authoritative RRsets exist
in a zone and also form a chain of authoritative owner names in the
zone. This information is used to provide authenticated denial of
existence for DNS data, as described in [RFC4035].
Because every authoritative name in a zone must be part of the NSEC
chain, NSEC RRs must be present for names containing a CNAME RR.
This is a change to the traditional DNS specification [RFC1034],
which stated that if a CNAME is present for a name, it is the only
type allowed at that name. An RRSIG (see Section 3) and NSEC MUST
exist for the same name as does a CNAME resource record in a signed
zone.
See [RFC4035] for discussion of how a zone signer determines
precisely which NSEC RRs it has to include in a zone.
The type value for the NSEC RR is 47.
The NSEC RR is class independent.
The NSEC RR SHOULD have the same TTL value as the SOA minimum TTL
field. This is in the spirit of negative caching ([RFC2308]).
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RFC 4034 DNSSEC Resource Records March 2005
4.1. NSEC RDATA Wire Format
The RDATA of the NSEC 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Next Domain Name /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ Type Bit Maps /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4.1.1. The Next Domain Name Field
The Next Domain field contains the next owner name (in the canonical
ordering of the zone) that has authoritative data or contains a
delegation point NS RRset; see Section 6.1 for an explanation of
canonical ordering. The value of the Next Domain Name field in the
last NSEC record in the zone is the name of the zone apex (the owner
name of the zone's SOA RR). This indicates that the owner name of
the NSEC RR is the last name in the canonical ordering of the zone.
A sender MUST NOT use DNS name compression on the Next Domain Name
field when transmitting an NSEC RR.
Owner names of RRsets for which the given zone is not authoritative
(such as glue records) MUST NOT be listed in the Next Domain Name
unless at least one authoritative RRset exists at the same owner
name.
4.1.2. The Type Bit Maps Field
The Type Bit Maps field identifies the RRset types that exist at the
NSEC RR's owner name.
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 NSEC RR RDATA in increasing numerical
order.
Type Bit Maps Field = ( Window Block # | Bitmap Length | Bitmap )+
where "|" denotes concatenation.
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RFC 4034 DNSSEC Resource Records March 2005
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, and bit 2 to RR type 258. If a bit is
set, it indicates that an RRset of that type is present for the NSEC
RR's owner name. If a bit is clear, it indicates that no RRset of
that type is present for the NSEC RR's owner name.
Bits representing pseudo-types MUST be clear, as they do not appear
in zone data. If encountered, they MUST be ignored upon being read.
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
NSEC RR's owner name. Trailing zero octets not specified MUST be
interpreted as zero octets.
The bitmap for the NSEC RR at a delegation point requires special
attention. Bits corresponding to the delegation NS RRset and the RR
types 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.
A zone MUST NOT include an NSEC RR for any domain name that only
holds glue records.
4.1.3. Inclusion of Wildcard Names in NSEC RDATA
If a wildcard owner name appears in a zone, the wildcard label ("*")
is treated as a literal symbol and is treated the same as any other
owner name for the purposes of generating NSEC RRs. Wildcard owner
names appear in the Next Domain Name field without any wildcard
expansion. [RFC4035] describes the impact of wildcards on
authenticated denial of existence.
4.2. The NSEC RR Presentation Format
The presentation format of the RDATA portion is as follows:
The Next Domain Name field is represented as a domain name.
The Type Bit Maps field is represented as a sequence of RR type
mnemonics. When the mnemonic is not known, the TYPE representation
described in [RFC3597], Section 5, MUST be used.
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4.3. NSEC RR Example
The following NSEC RR identifies the RRsets associated with
alfa.example.com. and identifies the next authoritative name after
alfa.example.com.
alfa.example.com. 86400 IN NSEC host.example.com. (
A MX RRSIG NSEC TYPE1234 )
The first four text fields specify the name, TTL, Class, and RR type
(NSEC). The entry host.example.com. is the next authoritative name
after alfa.example.com. in canonical order. The A, MX, RRSIG, NSEC,
and TYPE1234 mnemonics indicate that there are A, MX, RRSIG, NSEC,
and TYPE1234 RRsets associated with the name alfa.example.com.
The RDATA section of the NSEC RR above would be encoded as:
0x04 'h' 'o' 's' 't'
0x07 'e' 'x' 'a' 'm' 'p' 'l' 'e'
0x03 'c' 'o' 'm' 0x00
0x00 0x06 0x40 0x01 0x00 0x00 0x00 0x03
0x04 0x1b 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00
0x00 0x00 0x00 0x00 0x20
Assuming that the validator can authenticate this NSEC record, it
could be used to prove that beta.example.com does not exist, or to
prove that there is no AAAA record associated with alfa.example.com.
Authenticated denial of existence is discussed in [RFC4035].
5. The DS Resource Record
The DS Resource Record refers to a DNSKEY RR and is used in the DNS
DNSKEY authentication process. A DS RR refers to a DNSKEY RR by
storing the key tag, algorithm number, and a digest of the DNSKEY RR.
Note that while the digest should be sufficient to identify the
public key, storing the key tag and key algorithm helps make the
identification process more efficient. By authenticating the DS
record, a resolver can authenticate the DNSKEY RR to which the DS
record points. The key authentication process is described in
[RFC4035].
The DS RR and its corresponding DNSKEY RR have the same owner name,
but they are stored in different locations. The DS RR appears only
on the upper (parental) side of a delegation, and is authoritative
data in the parent zone. For example, the DS RR for "example.com" is
stored in the "com" zone (the parent zone) rather than in the
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"example.com" zone (the child zone). The corresponding DNSKEY RR is
stored in the "example.com" zone (the child zone). This simplifies
DNS zone management and zone signing but introduces special response
processing requirements for the DS RR; these are described in
[RFC4035].
The type number for the DS record is 43.
The DS resource record is class independent.
The DS RR has no special TTL requirements.
5.1. DS RDATA Wire Format
The RDATA for a DS RR consists of a 2 octet Key Tag field, a 1 octet
Algorithm field, a 1 octet Digest Type field, and a Digest field.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Key Tag | Algorithm | Digest Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
/ Digest /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5.1.1. The Key Tag Field
The Key Tag field lists the key tag of the DNSKEY RR referred to by
the DS record, in network byte order.
The Key Tag used by the DS RR is identical to the Key Tag used by
RRSIG RRs. Appendix B describes how to compute a Key Tag.
5.1.2. The Algorithm Field
The Algorithm field lists the algorithm number of the DNSKEY RR
referred to by the DS record.
The algorithm number used by the DS RR is identical to the algorithm
number used by RRSIG and DNSKEY RRs. Appendix A.1 lists the
algorithm number types.
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5.1.3. The Digest Type Field
The DS RR refers to a DNSKEY RR by including a digest of that DNSKEY
RR. The Digest Type field identifies the algorithm used to construct
the digest. Appendix A.2 lists the possible digest algorithm types.
5.1.4. The Digest Field
The DS record refers to a DNSKEY RR by including a digest of that
DNSKEY RR.
The digest is calculated by concatenating the canonical form of the
fully qualified owner name of the DNSKEY RR with the DNSKEY RDATA,
and then applying the digest algorithm.
digest = digest_algorithm( DNSKEY owner name | DNSKEY RDATA);
"|" denotes concatenation
DNSKEY RDATA = Flags | Protocol | Algorithm | Public Key.
The size of the digest may vary depending on the digest algorithm and
DNSKEY RR size. As of the time of this writing, the only defined
digest algorithm is SHA-1, which produces a 20 octet digest.
5.2. Processing of DS RRs When Validating Responses
The DS RR links the authentication chain across zone boundaries, so
the DS RR requires extra care in processing. The DNSKEY RR referred
to in the DS RR MUST be a DNSSEC zone key. The DNSKEY RR Flags MUST
have Flags bit 7 set. If the DNSKEY flags do not indicate a DNSSEC
zone key, the DS RR (and the DNSKEY RR it references) MUST NOT be
used in the validation process.
5.3. The DS RR Presentation Format
The presentation format of the RDATA portion is as follows:
The Key Tag field MUST be represented as an unsigned decimal integer.
The Algorithm field MUST be represented either as an unsigned decimal
integer or as an algorithm mnemonic specified in Appendix A.1.
The Digest Type field MUST be represented as an unsigned decimal
integer.
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The Digest MUST be represented as a sequence of case-insensitive
hexadecimal digits. Whitespace is allowed within the hexadecimal
text.
5.4. DS RR Example
The following example shows a DNSKEY RR and its corresponding DS RR.
dskey.example.com. 86400 IN DNSKEY 256 3 5 ( AQOeiiR0GOMYkDshWoSKz9Xz
fwJr1AYtsmx3TGkJaNXVbfi/
2pHm822aJ5iI9BMzNXxeYCmZ
DRD99WYwYqUSdjMmmAphXdvx
egXd/M5+X7OrzKBaMbCVdFLU
Uh6DhweJBjEVv5f2wwjM9Xzc
nOf+EPbtG9DMBmADjFDc2w/r
ljwvFw==
) ; key id = 60485
dskey.example.com. 86400 IN DS 60485 5 1 ( 2BB183AF5F22588179A53B0A
98631FAD1A292118 )
The first four text fields specify the name, TTL, Class, and RR type
(DS). Value 60485 is the key tag for the corresponding
"dskey.example.com." DNSKEY RR, and value 5 denotes the algorithm
used by this "dskey.example.com." DNSKEY RR. The value 1 is the
algorithm used to construct the digest, and the rest of the RDATA
text is the digest in hexadecimal.
6. Canonical Form and Order of Resource Records
This section defines a canonical form for resource records, a
canonical ordering of DNS names, and a canonical ordering of resource
records within an RRset. A canonical name order is required to
construct the NSEC name chain. A canonical RR form and ordering
within an RRset are required in order to construct and verify RRSIG
RRs.
6.1. Canonical DNS Name Order
For the purposes of DNS security, owner names are ordered by treating
individual labels as unsigned left-justified octet strings. The
absence of a octet sorts before a zero value octet, and uppercase
US-ASCII letters are treated as if they were lowercase US-ASCII
letters.
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To compute the canonical ordering of a set of DNS names, start by
sorting the names according to their most significant (rightmost)
labels. For names in which the most significant label is identical,
continue sorting according to their next most significant label, and
so forth.
For example, the following names are sorted in canonical DNS name
order. The most significant label is "example". At this level,
"example" sorts first, followed by names ending in "a.example", then
by names ending "z.example". The names within each level are sorted
in the same way.
example
a.example
yljkjljk.a.example
Z.a.example
zABC.a.EXAMPLE
z.example
\001.z.example
*.z.example
\200.z.example
6.2. Canonical RR Form
For the purposes of DNS security, the canonical form of an RR is the
wire format of the RR where:
1. every domain name in the RR is fully expanded (no DNS name
compression) and fully qualified;
2. all uppercase US-ASCII letters in the owner name of the RR are
replaced by the corresponding lowercase US-ASCII letters;
3. if the type of the RR is NS, MD, MF, CNAME, SOA, MB, MG, MR, PTR,
HINFO, MINFO, MX, HINFO, RP, AFSDB, RT, SIG, PX, NXT, NAPTR, KX,
SRV, DNAME, A6, RRSIG, or NSEC, all uppercase US-ASCII letters in
the DNS names contained within the RDATA are replaced by the
corresponding lowercase US-ASCII letters;
4. if the owner name of the RR is a wildcard name, the owner name is
in its original unexpanded form, including the "*" label (no
wildcard substitution); and
5. the RR's TTL is set to its original value as it appears in the
originating authoritative zone or the Original TTL field of the
covering RRSIG RR.
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6.3. Canonical RR Ordering within an RRset
For the purposes of DNS security, RRs with the same owner name,
class, and type are sorted by treating the RDATA portion of the
canonical form of each RR as a left-justified unsigned octet sequence
in which the absence of an octet sorts before a zero octet.
[RFC2181] specifies that an RRset is not allowed to contain duplicate
records (multiple RRs with the same owner name, class, type, and
RDATA). Therefore, if an implementation detects duplicate RRs when
putting the RRset in canonical form, it MUST treat this as a protocol
error. If the implementation chooses to handle this protocol error
in the spirit of the robustness principle (being liberal in what it
accepts), it MUST remove all but one of the duplicate RR(s) for the
purposes of calculating the canonical form of the RRset.
7. IANA Considerations
This document introduces no new IANA considerations, as all of the
protocol parameters used in this document have already been assigned
by previous specifications. However, since the evolution of DNSSEC
has been long and somewhat convoluted, this section attempts to
describe the current state of the IANA registries and other protocol
parameters that are (or once were) related to DNSSEC.
Please refer to [RFC4035] for additional IANA considerations.
DNS Resource Record Types: [RFC2535] assigned types 24, 25, and 30 to
the SIG, KEY, and NXT RRs, respectively. [RFC3658] assigned DNS
Resource Record Type 43 to DS. [RFC3755] assigned types 46, 47,
and 48 to the RRSIG, NSEC, and DNSKEY RRs, respectively.
[RFC3755] also marked type 30 (NXT) as Obsolete and restricted use
of types 24 (SIG) and 25 (KEY) to the "SIG(0)" transaction
security protocol described in [RFC2931] and to the transaction
KEY Resource Record described in [RFC2930].
DNS Security Algorithm Numbers: [RFC2535] created an IANA registry
for DNSSEC Resource Record Algorithm field numbers and assigned
values 1-4 and 252-255. [RFC3110] assigned value 5. [RFC3755]
altered this registry to include flags for each entry regarding
its use with the DNS security extensions. Each algorithm entry
could refer to an algorithm that can be used for zone signing,
transaction security (see [RFC2931]), or both. Values 6-251 are
available for assignment by IETF standards action ([RFC3755]).
See Appendix A for a full listing of the DNS Security Algorithm
Numbers entries at the time of this writing and their status for
use in DNSSEC.
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[RFC3658] created an IANA registry for DNSSEC DS Digest Types and
assigned value 0 to reserved and value 1 to SHA-1.
KEY Protocol Values: [RFC2535] created an IANA Registry for KEY
Protocol Values, but [RFC3445] reassigned all values other than 3
to reserved and closed this IANA registry. The registry remains
closed, and all KEY and DNSKEY records are required to have a
Protocol Octet value of 3.
Flag bits in the KEY and DNSKEY RRs: [RFC3755] created an IANA
registry for the DNSSEC KEY and DNSKEY RR flag bits. Initially,
this registry only contains assignments for bit 7 (the ZONE bit)
and bit 15 (the Secure Entry Point flag (SEP) bit; see [RFC3757]).
As stated in [RFC3755], bits 0-6 and 8-14 are available for
assignment by IETF Standards Action.
8. Security Considerations
This document describes the format of four DNS resource records used
by the DNS security extensions and presents an algorithm for
calculating a key tag for a public key. Other than the items
described below, the resource records themselves introduce no
security considerations. Please see [RFC4033] and [RFC4035] for
additional security considerations related to the use of these
records.
The DS record points to a DNSKEY RR by using a cryptographic digest,
the key algorithm type, and a key tag. The DS record is intended to
identify an existing DNSKEY RR, but it is theoretically possible for
an attacker to generate a DNSKEY that matches all the DS fields. The
probability of constructing a matching DNSKEY depends on the type of
digest algorithm in use. The only currently defined digest algorithm
is SHA-1, and the working group believes that constructing a public
key that would match the algorithm, key tag, and SHA-1 digest given
in a DS record would be a sufficiently difficult problem that such an
attack is not a serious threat at this time.
The key tag is used to help select DNSKEY resource records
efficiently, but it does not uniquely identify a single DNSKEY
resource record. It is possible for two distinct DNSKEY RRs to have
the same owner name, the same algorithm type, and the same key tag.
An implementation that uses only the key tag to select a DNSKEY RR
might select the wrong public key in some circumstances. Please see
Appendix B for further details.
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The table of algorithms in Appendix A and the key tag calculation
algorithms in Appendix B include the RSA/MD5 algorithm for
completeness, but the RSA/MD5 algorithm is NOT RECOMMENDED, as
explained in [RFC3110].
9. Acknowledgements
This document was created from the input and ideas of the members of
the DNS Extensions Working Group and working group mailing list. The
editors would like to express their thanks for the comments and
suggestions received during the revision of these security extension
specifications. Although explicitly listing everyone who has
contributed during the decade in which DNSSEC has been under
development would be impossible, [RFC4033] includes a list of some of
the participants who were kind enough to comment on these documents.
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.
[RFC1982] Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
August 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 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.
[RFC2536] Eastlake 3rd, D., "DSA KEYs and SIGs in the Domain Name
System (DNS)", RFC 2536, March 1999.
[RFC2931] Eastlake 3rd, D., "DNS Request and Transaction Signatures
( SIG(0)s )", RFC 2931, September 2000.
[RFC3110] Eastlake 3rd, D., "RSA/SHA-1 SIGs and RSA KEYs in the
Domain Name System (DNS)", RFC 3110, May 2001.
Arends, et al. Standards Track [Page 22]
RFC 4034 DNSSEC Resource Records March 2005
[RFC3445] Massey, D. and S. Rose, "Limiting the Scope of the KEY
Resource Record (RR)", RFC 3445, December 2002.
[RFC3548] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 3548, July 2003.
[RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource Record
(RR) Types", RFC 3597, September 2003.
[RFC3658] Gudmundsson, O., "Delegation Signer (DS) Resource Record
(RR)", RFC 3658, December 2003.
[RFC3755] Weiler, S., "Legacy Resolver Compatibility for Delegation
Signer (DS)", RFC 3755, May 2004.
[RFC3757] Kolkman, O., Schlyter, J., and E. Lewis, "Domain Name
System KEY (DNSKEY) Resource Record (RR) Secure Entry
Point (SEP) Flag", RFC 3757, April 2004.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", RFC
4033, 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
[RFC2535] Eastlake 3rd, D., "Domain Name System Security
Extensions", RFC 2535, March 1999.
[RFC2537] Eastlake 3rd, D., "RSA/MD5 KEYs and SIGs in the Domain
Name System (DNS)", RFC 2537, March 1999.
[RFC2539] Eastlake 3rd, D., "Storage of Diffie-Hellman Keys in the
Domain Name System (DNS)", RFC 2539, March 1999.
[RFC2930] Eastlake 3rd, D., "Secret Key Establishment for DNS (TKEY
RR)", RFC 2930, September 2000.
[RFC3845] Schlyter, J., "DNS Security (DNSSEC) NextSECure (NSEC)
RDATA Format", RFC 3845, August 2004.
Arends, et al. Standards Track [Page 23]
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Appendix A. DNSSEC Algorithm and Digest Types
The DNS security extensions are designed to be independent of the
underlying cryptographic algorithms. The DNSKEY, RRSIG, and DS
resource records all use a DNSSEC Algorithm Number to identify the
cryptographic algorithm in use by the resource record. The DS
resource record also specifies a Digest Algorithm Number to identify
the digest algorithm used to construct the DS record. The currently
defined Algorithm and Digest Types are listed below. Additional
Algorithm or Digest Types could be added as advances in cryptography
warrant them.
A DNSSEC aware resolver or name server MUST implement all MANDATORY
algorithms.
A.1. DNSSEC Algorithm Types
The DNSKEY, RRSIG, and DS RRs use an 8-bit number to identify the
security algorithm being used. These values are stored in the
"Algorithm number" field in the resource record RDATA.
Some algorithms are usable only for zone signing (DNSSEC), some only
for transaction security mechanisms (SIG(0) and TSIG), and some for
both. Those usable for zone signing may appear in DNSKEY, RRSIG, and
DS RRs. Those usable for transaction security would be present in
SIG(0) and KEY RRs, as described in [RFC2931].
Zone
Value Algorithm [Mnemonic] Signing References Status
----- -------------------- --------- ---------- ---------
0 reserved
1 RSA/MD5 [RSAMD5] n [RFC2537] NOT RECOMMENDED
2 Diffie-Hellman [DH] n [RFC2539] -
3 DSA/SHA-1 [DSA] y [RFC2536] OPTIONAL
4 Elliptic Curve [ECC] TBA -
5 RSA/SHA-1 [RSASHA1] y [RFC3110] MANDATORY
252 Indirect [INDIRECT] n -
253 Private [PRIVATEDNS] y see below OPTIONAL
254 Private [PRIVATEOID] y see below OPTIONAL
255 reserved
6 - 251 Available for assignment by IETF Standards Action.
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A.1.1. Private Algorithm Types
Algorithm number 253 is reserved for private use and will never be
assigned to a specific algorithm. The public key area in the DNSKEY
RR and the signature area in the RRSIG RR begin with a wire encoded
domain name, which MUST NOT be compressed. The domain name indicates
the private algorithm to use, and the remainder of the public key
area is determined by that algorithm. Entities should only use
domain names they control to designate their private algorithms.
Algorithm number 254 is reserved for private use and will never be
assigned to a specific algorithm. The public key area in the DNSKEY
RR and the signature area in the RRSIG RR begin with an unsigned
length byte followed by a BER encoded Object Identifier (ISO OID) of
that length. The OID indicates the private algorithm in use, and the
remainder of the area is whatever is required by that algorithm.
Entities should only use OIDs they control to designate their private
algorithms.
A.2. DNSSEC Digest Types
A "Digest Type" field in the DS resource record types identifies the
cryptographic digest algorithm used by the resource record. The
following table lists the currently defined digest algorithm types.
VALUE Algorithm STATUS
0 Reserved -
1 SHA-1 MANDATORY
2-255 Unassigned -
Appendix B. Key Tag Calculation
The Key Tag field in the RRSIG and DS resource record types provides
a mechanism for selecting a public key efficiently. In most cases, a
combination of owner name, algorithm, and key tag can efficiently
identify a DNSKEY record. Both the RRSIG and DS resource records
have corresponding DNSKEY records. The Key Tag field in the RRSIG
and DS records can be used to help select the corresponding DNSKEY RR
efficiently when more than one candidate DNSKEY RR is available.
However, it is essential to note that the key tag is not a unique
identifier. It is theoretically possible for two distinct DNSKEY RRs
to have the same owner name, the same algorithm, and the same key
tag. The key tag is used to limit the possible candidate keys, but
it does not uniquely identify a DNSKEY record. Implementations MUST
NOT assume that the key tag uniquely identifies a DNSKEY RR.
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RFC 4034 DNSSEC Resource Records March 2005
The key tag is the same for all DNSKEY algorithm types except
algorithm 1 (please see Appendix B.1 for the definition of the key
tag for algorithm 1). The key tag algorithm is the sum of the wire
format of the DNSKEY RDATA broken into 2 octet groups. First, the
RDATA (in wire format) is treated as a series of 2 octet groups.
These groups are then added together, ignoring any carry bits.
A reference implementation of the key tag algorithm is as an ANSI C
function is given below, with the RDATA portion of the DNSKEY RR is
used as input. It is not necessary to use the following reference
code verbatim, but the numerical value of the Key Tag MUST be
identical to what the reference implementation would generate for the
same input.
Please note that the algorithm for calculating the Key Tag is almost
but not completely identical to the familiar ones-complement checksum
used in many other Internet protocols. Key Tags MUST be calculated
using the algorithm described here rather than the ones complement
checksum.
The following ANSI C reference implementation calculates the value of
a Key Tag. This reference implementation applies to all algorithm
types except algorithm 1 (see Appendix B.1). The input is the wire
format of the RDATA portion of the DNSKEY RR. The code is written
for clarity, not efficiency.
/*
* Assumes that int is at least 16 bits.
* First octet of the key tag is the most significant 8 bits of the
* return value;
* Second octet of the key tag is the least significant 8 bits of the
* return value.
*/
unsigned int
keytag (
unsigned char key[], /* the RDATA part of the DNSKEY RR */
unsigned int keysize /* the RDLENGTH */
)
{
unsigned long ac; /* assumed to be 32 bits or larger */
int i; /* loop index */
for ( ac = 0, i = 0; i < keysize; ++i )
ac += (i & 1) ? key[i] : key[i] << 8;
ac += (ac >> 16) & 0xFFFF;
return ac & 0xFFFF;
}
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B.1. Key Tag for Algorithm 1 (RSA/MD5)
The key tag for algorithm 1 (RSA/MD5) is defined differently from the
key tag for all other algorithms, for historical reasons. For a
DNSKEY RR with algorithm 1, the key tag is defined to be the most
significant 16 bits of the least significant 24 bits in the public
key modulus (in other words, the 4th to last and 3rd to last octets
of the public key modulus).
Please note that Algorithm 1 is NOT RECOMMENDED.
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Authors' Addresses
Roy Arends
Telematica Instituut
Brouwerijstraat 1
7523 XC Enschede
NL
EMail: roy.arends@telin.nl
Rob Austein
Internet Systems Consortium
950 Charter Street
Redwood City, CA 94063
USA
EMail: sra@isc.org
Matt Larson
VeriSign, Inc.
21345 Ridgetop Circle
Dulles, VA 20166-6503
USA
EMail: mlarson@verisign.com
Dan Massey
Colorado State University
Department of Computer Science
Fort Collins, CO 80523-1873
EMail: massey@cs.colostate.edu
Scott Rose
National Institute for Standards and Technology
100 Bureau Drive
Gaithersburg, MD 20899-8920
USA
EMail: scott.rose@nist.gov
Arends, et al. Standards Track [Page 28]
RFC 4034 DNSSEC Resource Records March 2005
Full 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.
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.
Intellectual Property
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
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Arends, et al. Standards Track [Page 29]