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DNS Extensions R. Arends
Internet-Draft Telematica Instituut
Expires: August 26, 2003 R. Austein
ISC
M. Larson
VeriSign
D. Massey
USC/ISI
S. Rose
NIST
February 25, 2003
Resource Records for the DNS Security Extensions
draft-ietf-dnsext-dnssec-records-03
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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 August 26, 2003.
Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This document is part of a family of documents that describes 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
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KEY, DS, SIG, and NXT resource records. The purpose and format of
each resource record is described in detail and an example of each
resource record is given.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1 Background and Related Documents . . . . . . . . . . . . . . 4
1.2 Reserved Words . . . . . . . . . . . . . . . . . . . . . . . 4
1.3 Editors Notes . . . . . . . . . . . . . . . . . . . . . . . 4
1.3.1 Open Technical Issues . . . . . . . . . . . . . . . . . . . 4
1.3.2 Technical Changes or Corrections . . . . . . . . . . . . . . 4
1.3.3 Typos and Minor Corrections . . . . . . . . . . . . . . . . 5
2. The KEY Resource Record . . . . . . . . . . . . . . . . . . 6
2.1 KEY RDATA Wire Format . . . . . . . . . . . . . . . . . . . 6
2.1.1 The Flags Field . . . . . . . . . . . . . . . . . . . . . . 6
2.1.2 The Protocol Field . . . . . . . . . . . . . . . . . . . . . 7
2.1.3 The Algorithm Field . . . . . . . . . . . . . . . . . . . . 7
2.1.4 The Public Key Field . . . . . . . . . . . . . . . . . . . . 7
2.1.5 Notes on KEY RDATA Design . . . . . . . . . . . . . . . . . 7
2.2 The KEY RR Presentation Format . . . . . . . . . . . . . . . 7
2.3 KEY RR Example . . . . . . . . . . . . . . . . . . . . . . . 7
3. The SIG Resource Record . . . . . . . . . . . . . . . . . . 9
3.1 SIG RDATA Wire Format . . . . . . . . . . . . . . . . . . . 9
3.1.1 The Type Covered Field . . . . . . . . . . . . . . . . . . . 10
3.1.2 The Algorithm Number Field . . . . . . . . . . . . . . . . . 10
3.1.3 The Labels Field . . . . . . . . . . . . . . . . . . . . . . 10
3.1.4 Original TTL Field . . . . . . . . . . . . . . . . . . . . . 11
3.1.5 Signature Expiration and Inception Fields . . . . . . . . . 11
3.1.6 The Key Tag Field . . . . . . . . . . . . . . . . . . . . . 11
3.1.7 The Signer's Name Field . . . . . . . . . . . . . . . . . . 11
3.1.8 The Signature Field . . . . . . . . . . . . . . . . . . . . 12
3.2 The SIG RR Presentation Format . . . . . . . . . . . . . . . 12
3.3 SIG RR Example . . . . . . . . . . . . . . . . . . . . . . . 13
4. The NXT Resource Record . . . . . . . . . . . . . . . . . . 15
4.1 NXT RDATA Wire Format . . . . . . . . . . . . . . . . . . . 15
4.1.1 The Next Domain Name Field . . . . . . . . . . . . . . . . . 15
4.1.2 The Type Bit Map Field . . . . . . . . . . . . . . . . . . . 15
4.1.3 Inclusion of Wildcard Names in NXT RDATA . . . . . . . . . . 16
4.2 The NXT RR Presentation Format . . . . . . . . . . . . . . . 16
4.3 NXT RR Example . . . . . . . . . . . . . . . . . . . . . . . 16
5. The DS Resource Record . . . . . . . . . . . . . . . . . . . 18
5.1 DS RDATA Wire Format . . . . . . . . . . . . . . . . . . . . 18
5.1.1 The Key Tag Field . . . . . . . . . . . . . . . . . . . . . 18
5.1.2 The Algorithm Field . . . . . . . . . . . . . . . . . . . . 19
5.1.3 The Digest Type Field . . . . . . . . . . . . . . . . . . . 19
5.1.4 The Digest Field . . . . . . . . . . . . . . . . . . . . . . 19
5.2 The DS RR Presentation Format . . . . . . . . . . . . . . . 19
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5.3 DS RR Example . . . . . . . . . . . . . . . . . . . . . . . 20
6. Canonical Form and Order of Resource Records . . . . . . . . 21
6.1 Canonical DNS Name Order . . . . . . . . . . . . . . . . . . 21
6.2 Canonical RR Form . . . . . . . . . . . . . . . . . . . . . 21
6.3 Canonical RR Ordering Within An RRset . . . . . . . . . . . 22
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . 23
8. Security Considerations . . . . . . . . . . . . . . . . . . 24
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . 25
Normative References . . . . . . . . . . . . . . . . . . . . 26
Informative References . . . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . 27
A. DNSSEC Algorithm and Digest Types . . . . . . . . . . . . . 29
A.1 DNSSEC Algorithm Types . . . . . . . . . . . . . . . . . . . 29
A.1.1 Private Algorithm Types . . . . . . . . . . . . . . . . . . 29
A.2 DNSSEC Digest Types . . . . . . . . . . . . . . . . . . . . 30
B. Key Tag Calculation . . . . . . . . . . . . . . . . . . . . 31
B.1 Key Tag for Algorithm 1 (RSA/MD5) . . . . . . . . . . . . . 32
Full Copyright Statement . . . . . . . . . . . . . . . . . . 33
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1. Introduction
The DNS Security Extensions (DNSSEC) introduce four new DNS resource
record types: KEY, SIG, NXT, and DS. This document defines the
purpose of each resource record (RR), the RR's RDATA format, and its
ASCII representation.
1.1 Background and Related Documents
The reader is assumed to be familiar with the basic DNS concepts
described in RFC1034 [1] and RFC1035 [2].
This document is part of a family of documents that define the DNS
security extensions. The DNS security extensions (DNSSEC) are a
collection of resource records and DNS protocol modifications that
add source authentication the Domain Name System (DNS). An
introduction to DNSSEC and definition of common terms can be found in
[10]. A description of DNS protocol modifications can be found in
[11]. This document defines the DNSSEC resource records.
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 [5].
1.3 Editors Notes
1.3.1 Open Technical Issues
The NXT section (Section 4) may be updated in the next version if
DNSSEC-Opt-In [13] becomes part of DNSSEC.
The cryptographic algorithm types (Appendix A) requires input from
the working group. The DSA algorithm was moved to OPTIONAL. This
had strong consensus in workshops and various discussions and a
separate internet draft solely to move DSA from MANDATORY to OPTIONAL
seemed excessive. This draft solicits input on that proposed change.
1.3.2 Technical Changes or Corrections
Please report technical corrections to dnssec-editors@east.isi.edu.
To assist the editors, please indicate the text in error and point
out the RFC that defines the correct behavior. For a technical
change where no RFC that defines the correct behavior, or if there's
more than one applicable RFC and the definitions conflict, please
post the issue to namedroppers.
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An example correction to dnssec-editors might be: Page X says
"DNSSEC RRs SHOULD be automatically returned in responses." This was
true in RFC 2535, but RFC 3225 (Section 3, 3rd paragraph) says the
DNSSEC RR types MUST NOT be included in responses unless the resolver
indicated support for DNSSEC.
1.3.3 Typos and Minor Corrections
Please report any typos corrections to dnssec-editors@east.isi.edu.
To assist the editors, please provide enough context for us to find
the incorrect text quickly.
An example message to dnssec-editors might be: page X says "the
DNSSEC standard has been in development for over 1 years". It
should read "over 10 years".
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2. The KEY Resource Record
DNSSEC uses public key cryptography to sign and authenticate DNS
resource record sets (RRsets). The public keys are stored in KEY
resource records and are used in the DNSSEC authentication process
described in [11]. In a typical example, a zone signs its
authoritative RRsets using a private key and stores the corresponding
public key in a KEY RR. A resolver can then use these signatures to
authenticate RRsets from the zone.
The KEY RR may also be used to store public keys associated with
other DNS operations such as TKEY [15]. In all cases, the KEY RR
plays a special role in secure DNS resolution and DNS message
processing. The KEY RR is not intended as a record for storing
arbitrary public keys. The KEY RR MUST NOT be used to store
certificates or public keys that do not directly relate to the DNS
infrastructure. Examples of certificates and public keys that MUST
NOT be stored in the KEY RR include X.509 certificates, IPSEC public
keys, and SSH public keys.
The Type value for the KEY RR type is 25.
The KEY RR is class independent.
There are no special TTL requirements on the KEY record.
2.1 KEY RDATA Wire Format
The RDATA for a KEY 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 KEY record holds a DNS zone key and the KEY's owner name
MUST be the name of a zone. If bit 7 has value 0, then the KEY
record holds some other type of DNS public key, such as a public key
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used by TKEY.
Bits 0-6 and 8-15 are reserved and MUST have value 0 upon creation of
the KEY RR, and MUST be ignored upon reception.
Editors' Note: draft-ietf-dnsext-keyrr-key-signing-flag changes this
by allocating bit 15 as the KSK bit.
2.1.2 The Protocol Field
The Protocol Field MUST have value 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.
2.1.5 Notes on KEY RDATA Design
Although the Protocol Field always has value 3, it is retained for
backward compatibility with an earlier version of the KEY record.
2.2 The KEY RR Presentation Format
The presentation format of the RDATA portion is as follows:
The Flag field is represented as an unsigned decimal integer with a
value of either 0 or 256.
The Protocol Field is represented as an unsigned decimal integer with
a value of 3.
The Algorithm field is represented either as an unsigned decimal
integer or as an algorithm mnemonic as specified in Appendix A.1.
The Public Key field is represented as a Base64 encoding of the
Public Key. Whitespace is allowed within the Base64 text. For a
definition of Base64 encoding, see [3] Section 5.2.
2.3 KEY RR Example
The following KEY RR stores a DNS zone key for example.com.
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example.com. 86400 IN KEY 256 3 5 ( AQPSKmynfzW4kyBv015MUG2DeIQ3Cbl
+BBZH4b/0PY1kxkmvHjcZc8nokfzj31
GajIQKY+5CptLr3buXA10hWqTkF7H6R
foRqXQeogmMHfpftf6zMv1LyBUgia7z
a6ZEzOJBOztyvhjL742iU/TpPSEDhm2
SNKLijfUppn1UaNvv4w== )
The first four text fields specify the owner name, TTL, Class, and RR
type (KEY). 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 [8]. The remaining text is a
base 64 encoding of the public key.
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3. The SIG Resource Record
DNSSEC uses public key cryptography to sign and authenticate DNS
resource record sets (RRsets). Signatures are stored in SIG resource
records and are used in the DNSSEC authentication process described
in [11]. In a typical example, a zone signs its authoritative RRsets
using a private key and stores the corresponding signatures in SIG
RRs. A resolver can then use these SIG RRs to authenticate RRsets
from the zone.
A SIG record contains the signature for an RRset with a particular
name, class, and type. The SIG RR specifies a validity interval for
the signature and uses the Algorithm, the Signer's Name, and the Key
Tag to identify the public key (KEY RR) that can be used to verify
the signature.
The SIG RR may cover a transaction instead of an RRset. In this
case, the "Type Covered" field value is 0, the SIG RR MUST NOT appear
in any zone, and its use and processing are outside the scope of this
document. Please see [7] for further details.
The Type value for the SIG RR type is 24.
The SIG RR MUST have the same class as the RRset it covers.
The SIG RR TTL value SHOULD match the TTL value of the RRset it
covers.
3.1 SIG RDATA Wire Format
The RDATA for a SIG 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 | /
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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Signer's Name /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
/ /
/ Signature /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
3.1.1 The Type Covered Field
The Type Covered field identifies the type of the RRset which is
covered by this SIG record.
If Type Covered field has a value of 0, the record is referred to as
a transaction signature; please see [7] for further details.
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 SIG
RR owner name. It is included to handle signatures associated with
wildcard owner names.
To validate a signature, the validator requires the original owner
name that was used when the signature was created. 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 need to reconstruct the original owner name
in order to validate the signature. [11] describes how to use the
Labels field to reconstruct the original owner name.
The value of the Label field MUST NOT count either the null (root)
label that terminates the owner name or the wildcard label (if
present). The value of the Label field MUST be less than or equal to
the number of labels in the SIG owner name. For example,
"www.example.com." has a Label field value of 3, and "*.example.com."
has a Label field value of 2. Root (".") has a Label field value of
0.
Note that, although the wildcard label is not included in the count
stored in the Label field of the SIG RR, the wildcard label is part
of the RRset's owner name when generating or verifying the signature.
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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 resolver requires the original TTL. [11] describes how
to use the Original TTL field value to reconstruct the original TTL.
The Original TTL value MUST be greater than or equal to the TTL value
of the SIG record itself.
3.1.5 Signature Expiration and Inception Fields
The Signature Expiration and Inception fields specify a validity
period for the signature. The SIG record MUST NOT be used for
authentication prior to the inception date and MUST NOT be used for
authentication after the expiration date.
Signature Expiration and Inception field values are in POSIX.1 time
format, 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 which can be expressed by this format without
wrapping is approximately 136 years. A SIG RR can have an Expiration
field value which 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 [4]. 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 KEY RR that
validates this signature. The process of calculating the Key Tag
value is given in Appendix B.
3.1.7 The Signer's Name Field
The Signer's Name field value identifies the owner name of the KEY RR
used to authenticate this signature. The Signer's Name field MUST
contain the name of the zone of the covered RRset, unless the Type
Covered field value is 0. A sender MUST NOT use DNS name compression
on the Signer's Name field when transmitting a SIG RR. A receiver
which receives a SIG RR containing a compressed Signer's Name field
SHOULD decompress the field value.
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3.1.8 The Signature Field
The Signature field contains the cryptographic signature which covers
the SIG RDATA (excluding the Signature field) and the RRset specified
by the SIG owner name, SIG class, and SIG Type Covered field.
3.1.8.1 Signature Calculation
A signature covers the SIG RDATA (excluding the Signature Field) and
covers the RRset specified by the SIG owner name, SIG class, and SIG
Type Covered field. The RRset is in canonical form (see Section 6)
and the set RR(1),...RR(n) is signed as follows:
signature = sign(SIG_RDATA | RR(1) | RR(2)... ) where
"|" denotes concatenation;
SIG_RDATA is the wire format of the SIG RDATA fields with
the Signer's Name field in canonical form and
the Signature field excluded;
RR(i) = owner | class | type | 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 SIG RR;
Each RR MUST have the same class as the SIG RR;
Each RR in the RRset MUST have the RR type listed in the
SIG RR's Type Covered field;
Each RR in the RRset MUST have the TTL listed in the SIG
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.
3.2 The SIG RR Presentation Format
The presentation format of the RDATA portion is as follows:
The Type Covered field value is represented either as an unsigned
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decimal integer or as the mnemonic for the covered RR type.
The Algorithm field value is represented either as an unsigned
decimal integer or as an algorithm mnemonic as specified in Appendix
A.1.
The Labels field value is represented as an unsigned decimal integer.
The Original TTL field value is represented as an unsigned decimal
integer.
The Signature Inception Time and Expiration Time field values are
represented in the form YYYYMMDDHHmmSS in UTC, where:
YYYY is the year (0000-9999, 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 hours notation (00-23);
mm is the minute (00-59);
SS is the second (00-59).
The Key Tag field is represented as an unsigned decimal integer.
The Signer's Name field value is represented as a fully qualified
domain name.
The Signature field is represented as a Base64 encoding of the
signature. Whitespace is allowed within the Base64 text. For a
definition of Base64 encoding see [3] Section 5.2.
3.3 SIG RR Example
The following a SIG RR stores the signature for the A RRset of
host.example.com:
host.example.com. 86400 IN SIG A 5 3 86400 20030322173103 (
20030220173103 2642 example.com.
oJB1W6WNGv+ldvQ3WDG0MQkg5IEhjRip8WTr
PYGv07h108dUKGMeDPKijVCHX3DDKdfb+v6o
B9wfuh3DTJXUAfI/M0zmO/zz8bW0Rznl8O3t
GNazPwQKkRN20XPXV6nwwfoXmJQbsLNrLfkG
J5D6fwFm8nN+6pBzeDQfsS3Ap3o= )
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The first four fields specify the owner name, TTL, Class, and RR type
(SIG). 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 SIG RDATA is the Original TTL for the covered A
RRset. 20030322173103 and 20030220173103 are the expiration and
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 SIG RR owner name, class, and Type Covered
indicate that this SIG 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 this signature can be
authenticated using an example.com zone KEY RR whose algorithm is 5
and key tag is 2642.
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4. The NXT Resource Record
The NXT resource record lists two separate things: the owner name of
the next authoritative RRset in the canonical ordering of the zone,
and the set of RR types present at the NXT RR's owner name. The
complete set of NXT RRs in a zone both indicate 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 [11].
The type value for the NXT RR is 30.
The NXT RR is class independent.
4.1 NXT RDATA Wire Format
The RDATA of the NXT 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 Map /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4.1.1 The Next Domain Name Field
The Next Domain Name field contains the owner name of the next
authoritative RRset in the canonical ordering of the zone; see
Section 6.1 for an explanation of canonical ordering. The value of
the Next Domain Name field in the last NXT record in the zone is the
name of the zone apex (the owner name name of the zone's SOA RR).
A sender MUST NOT use DNS name compression on the Next Domain Name
field when transmitting an NXT RR. A receiver which receives an NXT
RR containing a compressed Next Domain Name field SHOULD decompress
the field value.
Owner names of non-authoritative RRsets (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 Map Field
The Type Bit Map field identifies the RRset types which exist at the
NXT RR's owner name.
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Each bit in the Type Bit Map field corresponds to an RR type. Bit 1
corresponds to RR type 1 (A), bit 2 corresponds to RR type 2 (NS),
and so forth. If a bit is set to 1, it indicates that an RRset of
that type is present for the NXT's owner name. If a bit is set to 0,
it indicates that no RRset of that type present for the NXT's owner
name.
Bit 1 MUST NOT indicate glue address records.
Bit 41 MUST have the value of 0, since the OPT pseudo-RR [6] can
never appear in zone data.
Trailing zero octets MUST be omitted. The length of the Type Bit Map
field varies, and is determined by the type code with the largest
numerical value among the set of RR types present at the NXT RR's
owner name. Trailing zero octets not specified MUST be interpreted
as zero octets.
The above Type Bit Map format MUST NOT be used when an RR type code
with numerical value greater than 127 is present.
Bit 0 in the Type Bit Map field indicates the Type Bit Map format. A
value of 0 in bit 0 denotes the format described above, therefore bit
0 MUST have a value of 0. The format and meaning of a Type Bit Map
with a value of 1 in bit 0 is undefined.
4.1.3 Inclusion of Wildcard Names in NXT 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 purposes of generating NXT RRs. Wildcard owner names
appear in the Next Domain Name field without any wildcard expansion.
[11] describes the impact of wildcards on authenticated denial of
existence.
4.2 The NXT 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 Map field is represented either as a sequence of RR type
mnemonics or as a sequence of unsigned decimal integers denoting the
RR type codes.
4.3 NXT RR Example
The following NXT RR identifies the RRsets associated with
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alfa.example.com. and identifies the next authoritative name after
alfa.example.com.
alfa.example.com. 86400 IN NXT host.example.com. A MX SIG NXT
The first four text fields specify the name, TTL, Class, and RR type
(NXT). The entry host.example.com. is the next authoritative name
after alfa.example.com. (in canonical order). The A, MX, SIG and
NXT mnemonics indicate there are A, MX, SIG and NXT RRsets associated
with the name alfa.example.com.
Note the NXT record can be used for authenticated denial of
existence. If the example NXT record were authenticated, it could be
used to prove that beta.example.com. does not exist, or could be
used to prove there is no AAAA record associated with
alfa.example.com. Authenticated denial of existence is discussed in
[11]
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5. The DS Resource Record
The DS Resource Record refers to a KEY RR and is used in the DNS KEY
authentication process. A DS RR refers to a KEY RR by storing the
key tag, algorithm number, and a digest of KEY RR. Note that while
the digest should be sufficient to identify the 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 KEY RR to which the DS record points. The key
authentication process is described in [11].
The DS RR and its corresponding KEY 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
"example.com" zone (the child zone). The corresponding KEY 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 [11].
The type number for the DS record is 43.
The DS resource record is class independent.
There are no special TTL requirements on the DS resource record.
5.1 DS RDATA Wire Format
The RDATA for a DS RR consists of 2 octet Key Tag field, a one octet
Algorithm field, a one 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 KEY RR referred to by the
DS record. The KEY RR MUST be a zone key. The KEY RR Flags MUST
have Flags bit 7 set to value 1.
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The Key Tag used by the DS RR is identical to the Key Tag used by the
SIG RR and Appendix B describes how to compute a Key Tag.
5.1.2 The Algorithm Field
The Algorithm field lists the algorithm number of the KEY RR referred
to by the DS record.
The algorithm number used by the DS RR is identical to the algorithm
number used by the SIG RR and KEY RR. Appendix A.1 lists the
algorithm number types.
5.1.3 The Digest Type Field
The DS RR refers to a KEY RR by including a digest of that KEY RR.
The Digest Type field identifies the algorithm used to construct the
digest and Appendix A.2 lists the possible digest algorithm types.
5.1.4 The Digest Field
The DS record refers to a KEY RR by including a digest of that KEY
RR. The Digest field holds the digest.
The digest is calculated by concatenating the canonical form of the
fully qualified owner name of the KEY RR (abbreviated below as "key
RR name") with the KEY RDATA, and then applying the digest algorithm.
digest = digest_algorithm( KEY RR name | KEY RDATA);
"|" denotes concatenation
KEY_RR_rdata = Flags | Protocol | Algorithm | Public Key.
The size of the digest may vary depending on the digest algorithm and
KEY RR size. Currently, the defined digest algorithm is SHA-1, which
produces a 20 octet digest.
5.2 The DS RR Presentation Format
The presentation format of the RDATA portion is as follows:
The Key Tag field is represented as an unsigned decimal integer.
The Algorithm field is represented either as an unsigned decimal
integer or as an algorithm mnemonic specified in Appendix A.1.
The Digest Type field is represented as an unsigned decimal integer.
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The Digest is represented as a sequence of case-insensitive
hexadecimal digits. Whitespace is allowed within the hexadecimal
text.
5.3 DS RR Example
The following example shows a KEY RR and its corresponding DS RR.
dskey.example.com. 86400 IN KEY 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." KEY RR, and value 5 denotes the algorithm used
by this "dskey.example.com." KEY RR. The value 1 is the algorithm
used to construct the digest, and the rest of the RDATA text is the
digest in hexadecimal.
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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 NXT name chain. A canonical RR form and ordering
within an RRset are required to construct and verify SIG RRs.
6.1 Canonical DNS Name Order
For 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 upper case
US-ASCII letters are treated as if they were lower case US-ASCII
letters.
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
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 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
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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, or A6, all uppercase US-ASCII letters in the DNS
names within the RDATA of the RR 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
authoritative zone containing the RR or the Original TTL field of
the covering SIG RR.
Editors' Note: the above definition sacrifices readability for an
attempt at precision. Please send better text!
6.3 Canonical RR Ordering Within An RRset
For purposes of DNS security, RRs with same owner name, same class,
and same type are sorted by sorting the canonical forms of the RRs
while treating the RDATA portion of the canonical form of each RR as
a left justified unsigned octet sequence. The absence of an octet
sorts before the zero octet.
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7. IANA Considerations
This document introduces one new IANA consideration. RFC 2535 [14]
created an IANA registry for DNS Security Algorithm Numbers. This
document re-assigns DNS Security Algorithm Number 252 to be
"reserved". This value is no longer available for assignment by
IANA.
This document clarifies the use of existing DNS resource records.
For completeness, the IANA considerations from the previous documents
which defined these resource records are summarized below. No IANA
changes are made by this document other than the one change described
in the first paragraph of this section.
[14] updated the IANA registry for DNS Resource Record Types, and
assigned types 24,25, and 30 to the SIG, KEY, and NXT RRs,
respectively. [9] assigned DNS Resource Record Type 43 to DS.
[14] created an IANA registry for DNSSEC Resource Record Algorithm
Numbers. Values to 1-4, and 252-255 were assigned by [14]. Value 5
was assigned by [8]. Value 252 is re-assigned by this document, as
noted above.
[9] created an IANA registry for DNSSEC DS Digest Types, and assigned
value 0 to reserved and value 1 to SHA-1.
[14] created an IANA Registry for KEY Protocol Values, but [16] re-
assigned all assigned values other than 3 to reserved and closed this
IANA registry. The registry remains closed, and all KEY records are
required to have Protocol Octet value of 3.
The Flag bits in the KEY RR are not assigned by IANA, and there is no
IANA registry for these flags. All changes to the meaning of the KEY
RR Flag bits require a standards action.
The meaning of a value of 1 in bit zero of the Type Bit Map of an NXT
RR can only be assigned by a standards action.
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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. The use of these records is specified in a
separate document, and security considerations related to the use
these resource records are discussed in that document.
The DS record points to a KEY RR using a cryptographic digest, the
key algorithm type and a key tag. The DS record is intended to
identify an existing KEY RR, but it is theoretically possible for an
attacker to generate a KEY that matches all the DS fields. The
probability of constructing such a matching KEY 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 which 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 KEY resource records efficiently,
but it does not uniquely identify a single KEY resource record. It
is possible for two distinct KEY RRs to have the same owner name, the
same algorithm type, and the same key tag. An implementation which
used only the key tag to select a KEY RR might select the wrong
public key in some circumstances. Implementations MUST NOT assume
the key tag is unique public key identifier; this is clearly stated
in Appendix B.
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9. Acknowledgments
This document was created from the input and ideas of several members
of the DNS Extensions Working Group and working group mailing list.
The co-authors of this draft would like to express their thanks for
the comments and suggestions received during the revision of these
security extension specifications.
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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] Borenstein, N. and N. Freed, "MIME (Multipurpose Internet Mail
Extensions) Part One: Mechanisms for Specifying and Describing
the Format of Internet Message Bodies", RFC 1521, September
1993.
[4] Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
August 1996.
[5] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[6] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671,
August 1999.
[7] Eastlake, D., "DNS Request and Transaction Signatures (
SIG(0)s)", RFC 2931, September 2000.
[8] Eastlake, D., "RSA/SHA-1 SIGs and RSA KEYs in the Domain Name
System (DNS)", RFC 3110, May 2001.
[9] Gudmundsson, O., "Delegation Signer Resource Record", draft-
ietf-dnsext-delegation-signer-12 (work in progress), December
2002.
[10] Arends, R., Austein, R., Larson, M., Massey, D. and S. Rose,
"DNS Security Introduction and Requirements", draft-ietf-
dnsext-dnssec-intro-05 (work in progress), February 2003.
[11] Arends, R., Austein, R., Larson, M., Massey, D. and S. Rose,
"Protocol Modifications for the DNS Security Extensions",
draft-ietf-dnsext-dnssec-protocol-00 (work in progress),
Februari 2003.
[12] Gustafsson, A., "Handling of Unknown DNS RR Types", draft-ietf-
dnsext-unknown-rrs-04 (work in progress), September 2002.
[13] Kosters, M., Blacka, D. and R. Arends, "DNSSEC Opt-in", draft-
ietf-dnsext-dnssec-opt-in-04 (work in progress), February 2003.
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Informative References
[14] Eastlake, D., "Domain Name System Security Extensions", RFC
2535, March 1999.
[15] Eastlake, D., "Secret Key Establishment for DNS (TKEY RR)", RFC
2930, September 2000.
[16] Massey, D. and S. Rose, "Limiting the Scope of the KEY Resource
Record (RR)", RFC 3445, December 2002.
Authors' Addresses
Roy Arends
Telematica Instituut
Drienerlolaan 5
7522 NB Enschede
NL
EMail: roy.arends@telin.nl
Rob Austein
Internet Software Consortium
40 Gavin Circle
Reading, MA 01867
USA
EMail: sra@isc.org
Matt Larson
VeriSign, Inc.
21345 Ridgetop Circle
Dulles, VA 20166-6503
USA
EMail: mlarson@verisign.com
Dan Massey
USC Information Sciences Institute
3811 N. Fairfax Drive
Arlington, VA 22203
USA
EMail: masseyd@isi.edu
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Scott Rose
National Institute for Standards and Technology
100 Bureau Drive
Gaithersburg, MD 20899-8920
USA
EMail: scott.rose@nist.gov
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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 KEY, SIG, 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.
A DNSSEC aware resolver or name server MUST implement all MANDATORY
algorithms.
A.1 DNSSEC Algorithm Types
An "Algorithm Number" field in the KEY, SIG, and DS resource record
types identifies the cryptographic algorithm used by the resource
record. Algorithm specific formats are described in separate
documents. The following table lists the currently defined algorithm
types and provides references to their supporting documents:
VALUE Algorithm RFC STATUS
0 Reserved - -
1 RSA/MD5 RFC 2537 NOT RECOMMENDED
2 Diffie-Hellman RFC 2539 OPTIONAL
3 DSA RFC 2536 OPTIONAL
4 elliptic curve TBA OPTIONAL
5 RSA/SHA1 RFC 3110 MANDATORY
6-251 available for assignment -
252 reserved -
253 private see below OPTIONAL
254 private see below OPTIONAL
255 reserved - -
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 KEY RR
and the signature area in the SIG RR begin with a wire encoded domain
name. Only local domain name compression is permitted. 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.
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Algorithm number 254 is reserved for private use and will never be
assigned to a specific algorithm. The public key area in the KEY RR
and the signature area in the SIG 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 -
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Appendix B. Key Tag Calculation
The Key Tag field in the SIG 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 KEY record. Both the SIG and DS resource records have
corresponding KEY records. The Key Tag field in the SIG and DS
records can be used to help select the corresponding KEY RR
efficiently when more than one candidate KEY 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 KEY 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 KEY record. Implementations MUST NOT assume
that the key tag uniquely identifies a KEY RR.
The key tag is the same for all KEY algorithm types except algorithm
1 (please see Appendix B.1 for the definition of the key tag for
algorithm 1). For all algorithms other than algorithm 1, the key tag
is defined to be the output which would be generated by running the
ANSI C function shown below with the RDATA portion of the KEY RR 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 below 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 KEY RR. The code is written for
clarity, not efficiency.
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/*
* 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 KEY 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;
}
B.1 Key Tag for Algorithm 1 (RSA/MD5)
The key tag for algorithm 1 (RSA/MD5) is defined differently than the
key tag for all other algorithms, for historical reasons. For a KEY
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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Full Copyright Statement
Copyright (C) The Internet Society (2003). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
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included on all such copies and derivative works. However, this
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the copyright notice or references to the Internet Society or other
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The limited permissions granted above are perpetual and will not be
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
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