DNS Security Working Group Donald E. Eastlake 3rd
INTERNET-DRAFT CyberCash
OBSOLETES RFC 2065
UPDATES RFC 1034, 1035
Expires: 22 February 1998 23 August 1997
Domain Name System Security Extensions
------ ---- ------ -------- ----------
Status of This Document
This draft, file name draft-ietf-dnssec-secext2-01.txt, is intended
to become a Draft Standard RFC obsoleting Proposed Standard RFC 2065.
Distribution of this document is unlimited. Comments should be sent
to the DNS Security Working Group mailing list <dns-security@tis.com>
or to the author.
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
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To learn the current status of any Internet-Draft, please check the
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Donald E. Eastlake 3rd [Page 1]
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Abstract
Extensions to the Domain Name System (DNS) are described that provide
data integrity and authentication to security aware resolvers or
applications through the use of cryptographic digital signatures.
These digital signatures are included in secured zones as resource
records. Security can also be provided even through non-security
aware DNS servers in many cases.
The extensions provide for the storage of authenticated public keys
in the DNS. This storage of keys can support general public key
distribution services as well as DNS security. The stored keys
enable security aware resolvers to learn the authenticating key of
zones in addition to those for which they are initially configured.
Keys associated with DNS names can be retrieved to support other
protocols. Provision is made for a variety of key types and
algorithms.
In addition, the security extensions provide for the optional
authentication of DNS protocol transactions and requests.
This document incorporates feedback from implementors and potential
users to RFC 2065.
Acknowledgments
The significant contributions of the following persons (in alphabetic
order) to DNS security are gratefully acknowledged:
James M. Galvin
John Gilmore
Olafur Gudmundsson
Charlie Kaufman
Edward Lewis
Radia J. Perlman
Jeffrey I. Schiller
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Table of Contents
Status of This Document....................................1
Abstract...................................................2
Acknowledgments............................................2
Table of Contents..........................................3
1. Overview of Contents....................................5
2. Overview of the DNS Extensions..........................6
2.1 Services Not Provided..................................6
2.2 Key Distribution.......................................6
2.3 Data Origin Authentication and Integrity...............7
2.3.1 The SIG Resource Record..............................8
2.3.2 Authenticating Name and Type Non-existence...........8
2.3.3 Special Considerations With Time-to-Live.............8
2.3.4 Special Considerations at Delegation Points..........9
2.3.5 Special Considerations with CNAME....................9
2.3.6 Signers Other Than The Zone.........................10
2.4 DNS Transaction and Request Authentication............10
3. The KEY Resource Record................................12
3.1 KEY RDATA format......................................12
3.1.1 Object Types, DNS Names, and Keys...................12
3.1.2 The KEY RR Flag Field...............................13
3.1.3 The Protocol Octet..................................15
3.2 The KEY Algorithm Number Specification................15
3.2.1 The MD5/RSA Algorithm...............................15
3.3 Interaction of Flags, Algorithm, and Protocol Bytes...16
3.4 Determination of Zone Secure/Unsecured Status.........16
3.5 KEY RRs in the Construction of Responses..............18
4. The SIG Resource Record................................19
4.1 SIG RDATA Format......................................19
4.1.1 ....................................................19
4.1.2 Algorithm Number Field..............................20
4.1.3 Labels Field........................................20
4.1.4 Original TTL Field..................................20
4.1.5 Signature Expiration and Time Signed Fields.........21
4.1.6 Key Tag Field.......................................21
4.1.7 Signer's Name Field.................................21
4.1.8 Signature Field.....................................22
4.1.8.1 Signature Data....................................22
4.1.8.2 MD5/RSA Algorithm Signature Calculation...........22
4.1.8.3 Transaction and Request SIGs......................23
4.2 SIG RRs in the Construction of Responses..............24
4.3 Processing Responses and SIG RRs......................25
4.4 Signature Lifetime, Expiration, TTLs, and Validity....26
4.5 The Root Zone as Signer...............................26
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5. Non-existent Names and Types...........................27
5.1 The NXT Resource Record...............................27
5.2 NXT RDATA Format......................................28
5.3 Example...............................................28
5.4 Special Considerations at Delegation Points...........29
5.5 Zone Transfers........................................29
5.5.1 Incremental Zone Transfers..........................30
6. How to Resolve Securely and the AD and CD Bits.........31
6.1 The AD and CD Header Bits.............................31
6.2 Staticly Configured Keys..............................32
6.3 Chaining Through The DNS..............................33
6.3.1 Chaining Through KEYs...............................33
6.3.2 Conflicting Data....................................34
6.4 Secure Time...........................................35
7. ASCII Representation of Security RRs...................36
7.1 Presentation of KEY RRs...............................36
7.2 Presentation of SIG RRs...............................37
7.3 Presentation of NXT RRs...............................38
8. Canonical Form and Order of Resource Records...........39
8.1 Canonical RR Form.....................................39
8.2 Canonical DNS Name Order..............................39
8.3 Canonical RR Ordering Within An RRset.................39
9. Conformance............................................40
9.1 Server Conformance....................................40
9.2 Resolver Conformance..................................40
10. Security Considerations...............................41
References................................................42
Author's Addresses........................................44
Expiration and File Name..................................44
Appendix A: Base 64 Encoding..............................45
Appendix B: Changes from RFC 2065.........................47
Appendix C: Key Tag Calculation...........................48
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1. Overview of Contents
This document standardizes extensions of the Domain Name System (DNS)
protocol to support DNS security and public key distribution. It
assumes that the reader is familiar with the Domain Name System,
particularly as described in RFCs 1033, 1034, and 1035. An earlier
version of these extensions appears in RFC 2065. This replacement
for that RFC incorporates implementation experience and requests from
potential users.
Section 2 provides an overview of the extensions and the key
distribution, data origin authentication, and transaction and request
security they provide.
Section 3 discusses the KEY resource record, its structure, and use
in DNS responses. These resource records represent the public keys
of entities named in the DNS and are used for key distribution.
Section 4 discusses the SIG digital signature resource record, its
structure, and use in DNS responses. These resource records are used
to authenticate other resource records in the DNS and optionally to
authenticate DNS transactions and requests.
Section 5 discusses the NXT resource record (RR) and its use in DNS
responses including full and incremental zone transfers. The NXT RR
permits authenticated denial of the existence of a name or of a
particular type for an existing name.
Section 6 discusses how a resolver can be configured with a starting
key or keys and proceed to securely resolve DNS requests.
Interactions between resolvers and servers are discussed for
combinations of security aware and security non-aware. Two
additional DNS header bits are defined for signaling between
resolvers and servers.
Section 7 describes the ASCII representation of the security resource
records for use in master files and elsewhere.
Section 8 defines the canonical form and order of RRs for DNS
security purposes.
Section 9 defines levels of conformance for resolvers and servers.
Section 10 provides a few paragraphs on overall security
considerations.
Appendix A gives details of base 64 encoding which is used in the
file representation of some RR's defined in this document.
Appendix B summarizes changes between this draft and RFC 2065.
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Appendix C specified how to calculate the simple checksum used as a
key tag in the SIG RR.
2. Overview of the DNS Extensions
The Domain Name System (DNS) protocol security extensions provide
three distinct services: key distribution as described in Section 2.2
below, data origin authentication as described in Section 2.3 below,
and transaction and request authentication, described in Section 2.4
below.
Special considerations related to "time to live", CNAMEs, and
delegation points are also discussed in Section 2.3.
2.1 Services Not Provided
It is part of the design philosophy of the DNS that the data in it is
public and that the DNS gives the same answers to all inquirers.
Following this philosophy, no attempt has been made to include any
sort of access control lists or other means to differentiate
inquirers.
No effort has been made to provide for any confidentiality for
queries or responses. (This service may be available via IPSEC [RFC
1825] or TLS [draft-ietf-tls-*].)
Protection is not provided against denial of service.
2.2 Key Distribution
A resource record format is defined to associate keys with DNS names.
This permits the DNS to be used as a public key distribution
mechanism in support of the DNS data origin authentication and other
security services.
The syntax of a KEY resource record (RR) is described in Section 3.
It includes an algorithm identifier, the actual public key
parameter(s), and a variety of flags including those indicating the
type of entity the key is associated with and/or asserting that there
is no key associated with that entity.
Under conditions described in Section 3.5, security aware DNS servers
will automatically attempt to return KEY resources as additional
information, along with those resource records actually requested, to
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minimize the number of queries needed.
2.3 Data Origin Authentication and Integrity
Authentication is provided by associating with resource record sets
(RRsets) the DNS cryptographically generated digital signatures.
Commonly, there will be a single private key that signs for an entire
zone. If a security aware resolver reliably learns the public key of
the zone, it can verify, for signed data read from that zone, that it
was properly authorized and is current. The most secure
implementation is for the zone private key to be kept off-line and
used to re-sign all of the records in the zone periodically.
This data origin authentication key belongs to the zone and not to
the servers that store copies of the data. That means compromise of
a server or even all servers for a zone will not necessarily affect
the degree of assurance that a resolver has that it can determine
whether data is genuine.
A resolver can learn the public key of a zone either by reading it
from the DNS or by having it staticly configured. To reliably learn
the public key by reading it from the DNS, the key itself must be
signed with a key the resolver trusts. The resolver must be
configured with at least the public key of one zone as a starting
point. From there, it can securely read the public keys of other
zones, if the intervening zones in the DNS tree are secure and their
signed keys accessible.
Adding data origin authentication and integrity requires no change to
the "on-the-wire" DNS protocol beyond the addition of the signature
resource type and the key resource type needed for key distribution.
(Data non-existence authentication also requires the NXT RR as
described in 2.3.2.) This service can be supported by existing
resolver and caching server implementations so long as they can
support the additional resource types (see Section 9). The one
exception is that CNAME referrals from a secure zone can not be
authenticated if they are from non-security aware servers (see
Section 2.3.5).
If signatures are separately retrieved and verified when retrieving
the information they authenticate, there will be more trips to the
server and performance will suffer. Security aware servers mitigate
that degradation by attempting to send the signature(s) needed.
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2.3.1 The SIG Resource Record
The syntax of a SIG resource record (signature) is described in
Section 4. It cryptographically binds the RR set being signed to the
signer and a validity interval.
Every name in a secured zone will have associated with it at least
one SIG resource record for each resource type under that name except
for glue address RRs and delegation point NS RRs. A security aware
server will attempt to return, with RRs retrieved, the corresponding
SIGs. If a server is not security aware, the resolver must retrieve
all the SIG records for a name and select the one or ones that sign
the resource record set(s) that resolver is interested in.
2.3.2 Authenticating Name and Type Non-existence
The above security mechanism only provides a way to sign existing RR
sets in a zone. "Data origin" authentication is not obviously
provided for the non-existence of a domain name in a zone or the
non-existence of a type for an existing name. This gap is filled by
the NXT RR which authenticatably asserts a range of non-existent
names in a zone and the non-existence of types for the existing name
just before that range.
Section 5 below covers the NXT RR.
2.3.3 Special Considerations With Time-to-Live
A digital signature will fail to verify if any change has occurred to
the data between the time it was originally signed and the time the
signature is verified. This conflicts with our desire to have the
time-to-live (TTL) field of resource records tick down while they are
cached.
This could be avoided by leaving the time-to-live out of the digital
signature, but that would allow unscrupulous servers to set
arbitrarily long TTL values undetected. Instead, we include the
"original" TTL in the signature and communicate that data along with
the current TTL. Unscrupulous servers under this scheme can
manipulate the TTL but a security aware resolver will bound the TTL
value it uses at the original signed value. Separately, signatures
include a time signed and an expiration time. A resolver that knows
the absolute time can determine securely whether a signature is in
effect. It is not possible to rely solely on the signature
expiration as a substitute for the TTL, however, since the TTL is
primarily a database consistency mechanism and non-security aware
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servers that depend on TTL must still be supported.
2.3.4 Special Considerations at Delegation Points
DNS security would like to view each zone as a unit of data
completely under the control of the zone owner with each entry
(RRset) signed by a special private key held by the zone. But the
DNS protocol views the leaf nodes in a zone, which are also the apex
nodes of a subzone (i.e., delegation points), as "really" belonging
to the subzone. These nodes occur in two master files and might have
RRs signed by both the upper and lower zone's keys. A retrieval
could get a mixture of these RRs and SIGs, especially since one
server could be serving both the zone above and below a delegation
point.
There MUST be a zone KEY RR, signed by its superzone, for every
subzone if the superzone is secure. In the case of an unsecured
subzone which can not or will not be modified to add any security
RRs, a KEY declaring the subzone to be unsecured MUST appear in and
be signed by the superzone, if the superzone is secure. For all but
one other RR type the data from the subzone is more authoritative so
only the KEY RR in the superzone should be signed and the NS and any
glue address RRs should only be signed in the subzone. The SOA and
any other RRs that have the zone name as owner should appear only in
the subzone and thus are signed only there. The NXT RR type is the
exceptional case that will always appear differently and
authoritatively in both the superzone and subzone, if both are
secure, as described in Section 5.
2.3.5 Special Considerations with CNAME
There is a problem when security related RRs with the same owner name
as a CNAME RR are retrieved from a non-security-aware server. In
particular, an initial retrieval for the CNAME or any other type may
not retrieve any associated signature, KEY, or NXT RR. For retrieved
types other than CNAME, it will retrieve that type at the target name
of the CNAME (or chain of CNAMEs) and will also return the CNAME. In
particular, a specific retrieval for type SIG will not get the SIG,
if any, at the original CNAME domain name but rather a SIG at the
target name.
Security aware servers must be used to securely CNAME in DNS.
Security aware servers MUST (1) allow KEY, SIG, and NXT RRs along
with CNAME RRs, (2) suppress CNAME processing on retrieval of these
types as well as on retrieval of the type CNAME, and (3)
automatically return SIG RRs authenticating the CNAME or CNAMEs
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encountered in resolving a query. This is a change from the previous
DNS standard [RFCs 1034/1035] which prohibited any other RR type at a
node where a CNAME RR was present.
2.3.6 Signers Other Than The Zone
There are two cases where a SIG resource record is signed by other
than the zone private key.
One is for support of dynamic update [RFC 2136], or future requests
which require authentication, where an entity is permitted to
authenticate/update its records [RFC tbd]. The public key of the
entity must be present in the DNS and be appropriately signed but the
other RR(s) may be signed with the entity's key.
The second case is support of transaction and request authentication
as described in Section 2.4 immediately below.
2.4 DNS Transaction and Request Authentication
The data origin authentication service described above protects
retrieved resource records but provides no protection for DNS
requests or for message headers.
If header bits are falsely set by a bad server, there is little that
can be done. However, it is possible to add transaction
authentication. Such authentication means that a resolver can be
sure it is at least getting messages from the server it thinks it
queried and that the response is from the query it sent (i.e., that
these messages have not been diddled in transit). This is
accomplished by optionally adding a special SIG resource record at
the end of the reply which digitally signs the concatenation of the
server's response and the resolver's query.
Requests can also be authenticated by including a special SIG RR at
the end of the request. Authenticating requests serves no function
in older DNS servers and requests with a non-empty additional
information section are ignored by many older and current DNS
servers. However, this syntax for signing requests is defined in
connection with authenticating secure dynamic update requests [RFC
tbd] or future requests requiring authentication.
The private keys used in transaction and request security belong to
the host composing the request or reply, not to the zone involved.
The corresponding public key is normally stored in and retrieved from
the DNS for verification.
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Because requests and replies are highly variable, message
authentication SIGs can not be pre-calculated. Thus it will be
necessary to keep the private key on-line, for example in software or
in a directly connected piece of hardware.
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3. The KEY Resource Record
The KEY resource record (RR) is used to store a public key that is
associated with a Domain Name System (DNS) name. This can be the
public key of a zone, a host or other end entity, or a user. A KEY
RR is, like any other RR, authenticated by a SIG RR. Security aware
DNS implementations MUST be designed to handle at least two
simultaneously valid keys of the same type associated with the same
name.
The type number for the KEY RR is 25.
3.1 KEY RDATA format
The RDATA for a KEY RR consists of flags, a protocol octet, the
algorithm number, and the public key itself. The format is as
follows:
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 /
/ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
The KEY RR is not intended for storage of certificates and a separate
certificate RR is being considered, to be defined in a separate
document.
The meaning of the KEY RR owner name, flags, and protocol octet are
described in Sections 3.1.1 through 3.1.5 below. The flags and
algorithm must be examined before any data following the algorithm
octet as they control the existence and format of any following data.
The algorithm and public key fields are described in Section 3.2.
The format of the public key is algorithm dependent.
KEY RRs do not expire but their authenticating SIG RR does as
described in Section 4 below.
3.1.1 Object Types, DNS Names, and Keys
The public key in a KEY RR is for the object named in the owner name.
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This DNS name may refer to up to three different categories of
things. For example, foo.host.example could be (1) a zone, (2) a
host or other end entity , or (3) the mapping into a DNS name of the
user or account foo@host.example. Thus, there are flag bits, as
described below, in the KEY RR to indicate with which of these roles
the owner name and public key are associated. Note that an
appropriate zone KEY RR MUST occur at the apex node of a secure zone.
3.1.2 The KEY RR Flag Field
In the "flags" field:
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
| A/C | Z | XT| Z | Z | NAMTYP| IP| Z | Z | Z | SIG |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
Bit 0 and 1 are the key "type" bits. Bit 0 a one indicates that use
of the key is prohibited for authentication. Bit 1 a one
indicates that use of the key is prohibited for confidentiality.
If this field is zero, then use of the key for authentication
and/or confidentiality is permitted. Note that DNS security makes
use of keys for authentication only. Confidentiality use flagging
is provided for use of keys in other protocols. Implementations
not intended to support key distribution for confidentiality MAY
require that the confidentiality use prohibited bit be on for keys
they serve. If both bits are one, the "no key" value, there is no
key information and the RR stops after the algorithm octet. By
the use of this "no key" value, a signed KEY RR can
authenticatably assert that, for example, a zone is not secured.
See section 3.4 below.
Bits 2 is reserved and must be zero.
Bits 3 is reserved as a flag extension bit. If it is a one, a second
16 bit flag field is added after the algorithm octet and before
the key data. This bit MUST NOT be set unless one or more such
additional bits have been defined and are non-zero.
Bits 4-5 are reserved and must be zero.
Bits 6 and 7 form a field that encodes the name type.
A value of 0 indicates that this is a key associated with a
"user" or "account" at an end entity, usually a host. The coding
of the owner name is that used for the responsible individual
mailbox in the SOA and RP RRs: The owner name is the user name as
the name of a node under the entity name. For example,
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"j_random_user" on host.subdomain.example could have a public key
associated through a KEY RR with name
j_random_user.host.subdomain.example. It could be used in a
security protocol where authentication of a user was desired.
This key might be useful in IP or other security for a user level
service such a telnet, ftp, rlogin, etc.
A value of 1 indicates that this is a zone key for the zone
whose name is the KEY RR owner name. This is the public key used
for the primary DNS security feature of data origin
authentication.
A value of 2 indicates that this is a key associated with the
non-zone "entity" whose name is the RR owner name. This will
commonly be a host but could, in some parts of the DNS tree, be
some other type of entity such as a telephone number [RFC 1530] or
numeric IP address. This is the public key used in connection
with DNS request and transaction authentication services if the
owner name designates a DNS resolver or server host. It could
also be used in an IP-security protocol where authentication at
the host, rather than user, level was desired, such as routing,
NTP, etc.
The value of 3 is reserved.
Bit 8 is reserved to be the Oakley/IPSEC [RFC 1825] bit and indicates
that this key is valid for use in conjunction with that security
standard. This key could be used in connection with secured
communication on behalf of an end entity or user whose name is the
owner name of the KEY RR if the entity or user bits are on. The
presence of a KEY resource with the Oakley/IPSEC bit on is an
assertion that the host speaks Oakley/IPSEC.
Bits 9 through 11 are reserved and must be zero.
Bits 12-15 are the "signatory" field. If non-zero, they indicate
that the key can validly sign RRs or updates of the same name. If
the owner name is a wildcard, then RRs or updates with any name
which is in the wildcard's scope can, in some cases, be signed.
Fifteen different non-zero values are possible for this field and
any differences in their meaning are reserved for definition in
connection with DNS dynamic update [RFC tbd] or other new DNS
commands. Zone keys (see bits 6 and 7 above) always have
authority to sign any RRs in the zone regardless of the value of
the signatory field. The signatory field, like all other aspects
of the KEY RR, is only effective if the KEY RR is appropriately
signed by a SIG RR.
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3.1.3 The Protocol Octet
It is anticipated that some keys stored in DNS will be used in
conjunction with Internet protocols other than DNS (keys with the
zone name type or with their signatory field non-zero). It is
intended that the protocol octet and possibly some of the unused
(must be zero) bits in the KEY RR flags will be used for this
purpose.
The following values of the Protocol Octet are reserved as indicated:
VALUE Protocol
1 TLS
2 Email
3.2 The KEY Algorithm Number Specification
This octet is the key algorithm parallel to the same field for the
SIG resource. The MD5/RSA algorithm described in this document is
number 1. Numbers 2 through 253 are available for assignment should
sufficient reason arise. However, the designation of a new algorithm
could have a major impact on interoperability and requires an IETF
standards action. Number 254 is reserved for private use and will
never be assigned a specific algorithm. For number 254, the public
key area for the KEY RR and the signature will actually begin with a
length byte followed by an 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.
Values 0 and 255 are reserved.
3.2.1 The MD5/RSA Algorithm
If the type field does not have the "no key" value and the algorithm
field is 1, indicating the MD5/RSA algorithm, the public key field is
structured as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| pub exp length| public key exponent /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| /
+- modulus /
| /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-/
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For interoperability, the exponent and modulus are each currently
limited to 4096 bits in length. The public key exponent is a
variable length unsigned integer. Its length in octets is
represented as one octet if it is in the range of 1 to 255 and by a
zero octet followed by a two octet unsigned length if it is longer
than 255 bytes. The public key modulus field is a multiprecision
unsigned integer. The length of the modulus can be determined from
the RDLENGTH and the preceding RDATA fields including the exponent.
Leading zero octets are prohibited in the exponent and modulus.
3.3 Interaction of Flags, Algorithm, and Protocol Bytes
Various combinations of the no-key type flags, algorithm byte,
protocol byte, and any future assigned protocol indicating flags are
possible. (Note that the zone value of the name type flags or the
signatory field being non-zero means usability in the DNS protocol.)
The meaning of these combinations is indicated below:
NK = no key type flags (bits 0 and 1 on)
AL = algorithm byte
PR = protocols indicated by protocol byte or future assigned flags
x represents any valid non-zero value(s).
AL PR NK Meaning
0 0 0 Illegal, claims key but has bad algorithm field.
0 0 1 Specifies total lack of security for owner.
0 x 0 Illegal, claims key but has bad algorithm field.
0 x 1 Specified protocols unsecured, others may be secure.
x 0 0 Useless. Gives key but no protocols to use it.
x 0 1 Useless. Denies key but for no protocols.
x x 0 Specifies key for protocols.
x x 1 Algorithm not understood for protocol.
3.4 Determination of Zone Secure/Unsecured Status
A zone KEY RR with the "no-key" type field value (both bits 0 and 1
on) indicates that the zone named is unsecured while a zone KEY RR
with a key present indicates that the zone named is secure. It is
possible for conflicting zone KEY RRs to be present.
Zone KEY RRs, like all RRs, are only trusted if they are
authenticated by a SIG RR whose signer field is a signer for which
the resolver has a public key they trust and where resolver policy
permits that signer to sign for the KEY owner name. Untrusted zone
KEY RRs can be ignored in determining the security status of the
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zone. There can be multiple sets of trusted zone KEY RRs for a zone
with each set having a different signer.
Zones can be (1) secure, indicating that any retrieved RR must be
authenticated by a SIG RR or it will be discarded as bogus, (2)
unsecured, indicating that SIG RRs are not expected or required for
RRs retrieved from the zone, or (3) experimentally secure, which
indicates that SIG RRs might or might not be present but must be
checked if found. The status of a zone is determined as follows:
1. If, for a zone, every zone KEY RR signed by a signer trusted by
the resolver and authorized by resolver policy to sign says there
is no key for that zone, it is unsecured.
2. If, for every trusted and resolver policy authorized zone KEY RR
signer for a zone, there is at least one no-key KEY RR and at
least one key specifying KEY RR(s), then that zone is only
experimentally secure. Both authenticated and non-authenticated
RRs for it should be accepted by the resolver.
3. If every trusted and resolver policy authorized zone KEY RR signer
for the zone has only key specifying KEY RR(s) for the zone, then
it is secure and only authenticated RRs from it will be accepted.
Examples:
(1) A resolver only trusts signatures by the superzone within the
DNS hierarchy so it will look only at the KEY RRs that are signed by
the superzone. If it finds only no-key KEY RRs, it will assume the
zone is not secure. If it finds only key specifying KEY RRs, it will
assume the zone is secure and reject any unsigned responses. If it
finds both, it will assume the zone is experimentally secure
(2) A resolver trusts the superzone of zone Z (to which it got
securely from its local zone) and a third party, cert-auth.xx. When
considering data from zone Z, it may be signed by the superzone of Z,
by cert-auth.xx, by both, or by neither. The following table
indicates whether zone Z will be considered secure, experimentally
secure, or unsecured, depending on the signed zone KEY RRs for Z;
c e r t - a u t h . x x
| None | NoKeys | Mixed | Keys |
S --+-----------+-----------+----------+----------+
u None | illegal | unsecured | experim. | secure |
p +-----------+-----------+----------+----------+
e NoKeys | unsecured | unsecured | experim. | secure |
r +-----------+-----------+----------+----------+
Z Mixed | experim. | experim. | experim. | secure |
o +-----------+-----------+----------+----------+
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n Keys | secure | secure | secure | secure |
e +-----------+-----------+----------+----------+
3.5 KEY RRs in the Construction of Responses
An explicit request for KEY RRs does not cause any special additional
information processing except, of course, for the corresponding SIG
RR from a security aware server.
Security aware DNS servers include KEY RRs as additional information
in responses, where a KEY is available, in the following cases:
(1) On the retrieval of NS RRs, the zone key KEY RR(s) for the zone
served by these name servers must be included as additional
information if space is available. There will always be at least one
such KEY RR in a secure zone, even if it has the no-key type value to
indicate that the subzone is unsecured. If not all additional
information will fit, the KEY RR(s) have higher priority than type A
or AAAA glue RRs. If such a KEY RR does not fit on a retrieval, the
retrieval must be considered truncated.
(2) On retrieval of type A or AAAA RRs, the end entity KEY RR(s) MUST
be included if space is available. On inclusion of A or AAAA RRs as
additional information, KEY RRs with the same name will also be
included but with lower priority than the A or AAAA RRs.
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4. The SIG Resource Record
The SIG or "signature" resource record (RR) is the fundamental way
that data is authenticated in the secure Domain Name System (DNS). As
such it is the heart of the security provided.
The SIG RR unforgably authenticates an RRset of a particular type,
class, and name and binds it to a time interval and the signer's
domain name. This is done using cryptographic techniques and the
signer's private key. The signer is frequently the owner of the zone
from which the RR originated. The SIG RR is only intended to be
meaningful to DNS security.
The type number for the SIG RR type is 24.
4.1 SIG RDATA Format
The RDATA portion of a SIG RR is as shown below. The integrity of
the RDATA information is protected by 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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| time signed |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| key tag | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ signer's name +
| /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-/
/ /
/ signature /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4.1.1
The "type covered" is the type of the other RRs covered by this SIG.
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4.1.2 Algorithm Number Field
This octet is as described in section 3.2.
4.1.3 Labels Field
The "labels" octet is an unsigned count of how many labels there are
in the original SIG RR owner name not counting the null label for
root and not counting any initial "*" for a wildcard. If a secured
retrieval is the result of wild card substitution, it is necessary
for the resolver to use the original form of the name in verifying
the digital signature. This field makes it easy to determine the
original form.
If, on retrieval, the RR appears to have a longer name than indicated
by "labels", the resolver can tell it is the result of wildcard
substitution. If the RR owner name appears to be shorter than the
labels count, the SIG RR must be considered corrupt and ignored. The
maximum number of labels allowed in the current DNS is 127 but the
entire octet is reserved and would be required should DNS names ever
be expanded to 255 labels. The following table gives some examples.
The value of "labels" is at the top, the retrieved owner name on the
left, and the table entry is the name to use in signature
verification except that "bad" means the RR is corrupt.
labels= | 0 | 1 | 2 | 3 | 4 |
--------+-----+------+--------+----------+----------+
.| . | bad | bad | bad | bad |
d.| *. | d. | bad | bad | bad |
c.d.| *. | *.d. | c.d. | bad | bad |
b.c.d.| *. | *.d. | *.c.d. | b.c.d. | bad |
a.b.c.d.| *. | *.d. | *.c.d. | *.b.c.d. | a.b.c.d. |
4.1.4 Original TTL Field
The "original TTL" field is included in the RDATA portion to avoid
(1) authentication problems that caching servers would otherwise
cause by decrementing the real TTL field and (2) security problems
that unscrupulous servers could otherwise cause by manipulating the
real TTL field. This original TTL is protected by the signature
while the current TTL field is not.
NOTE: The "original TTL" must be restored into the covered RRs when
the signature is verified (see Section 8). This implies that all RRs
for a particular type, name, and class must have the same TTL to
start with.
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4.1.5 Signature Expiration and Time Signed Fields
The SIG is valid until the "signature expiration" time which is an
unsigned number of seconds since the start of 1 January 1970, GMT,
ignoring leap seconds. (See also Section 4.4.) Ring arithmetic is
used as for DNS SOA serial numbers [RFC 1982] which means that the
expiration date can never be more than ~136.09 years in the future.
The "time signed" field is an unsigned number of seconds since the
start of 1 January 1970, GMT, ignoring leap seconds. SIG RRs SHOULD
NOT have a date signed more than a few days in the future. To
prevent misordering of network requests to update a zone dynamically,
monotonically increasing "time signed" dates may be necessary.
SOA serial numbers for secure zones MUST not only be advanced when
their data is updated but also when new SIG RRs are inserted (ie, the
zone or any part of it is re-signed).
A SIG RR may have an expiration date numerically less than the time
signed if time is near the 32 bit wrap around point and/or the
signature is long lived.
4.1.6 Key Tag Field
The "key Tag" is a two octet quantity that is used to efficiently
select between multiple keys which may be applicable and thus check
that a public key about to be used for the computationally expensive
effort to check the signature is possibly valid. For algorithm 1
(MD5/RSA) as defined below, it is the next to the bottom two octets
of the public key modulus needed to decode the signature field. That
is to say, the most significant 16 of the lest significant 24 bits of
the modulus in network order. For all other algorithms, including
private algorithms, it is calculated as a simple checksum of the KEY
RR as described in Appendix C.
4.1.7 Signer's Name Field
The "signer's name" field is the domain name of the signer generating
the SIG RR. This is the owner of the public KEY RR that can be used
to verify the signature. It is frequently the zone which contained
the RRset being authenticated. What signers signers should be
authorized to sign what is a significant resolver policy question as
discussed in Section 6. The signer's name may be compressed with
standard DNS name compression when being transmitted over the
network.
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4.1.8 Signature Field
The structure of the "signature" field is described below.
4.1.8.1 Signature Data
The actual signature portion of the SIG RR binds the other RDATA
fields to all of the "type covered" RRs with that owner name and
class. This covered RRset is thereby authenticated. To accomplish
this, a data sequence is constructed as follows:
data = RDATA | RR(s)...
where "|" is concatenation, RDATA is wire format of all the RDATA
fields in the SIG RR itself including the canonical form of the
signers name before and not including the signature, and RR(s) are
all the RR(s) of the type covered with the same owner name and class
as the SIG RR in canonical form and order as defined in section 8.
How this data sequence is processed into the signature is algorithm
dependent.
SIGs SHOULD NOT be included in a zone for any "meta-type" such as
ANY, AXFR, etc.
4.1.8.2 MD5/RSA Algorithm Signature Calculation
For the MD5/RSA algorithm, the signature is as follows
hash = MD5 ( data )
signature = ( 01 | FF* | 00 | prefix | hash ) ** e (mod n)
where MD5 is the message digest algorithm documented in RFC 1321, "|"
is concatenation, "e" is the private key exponent of the signer, and
"n" is the modulus of the signer's public key. 01, FF, and 00 are
fixed octets of the corresponding hexadecimal value. "prefix" is the
ASN.1 BER MD5 algorithm designator prefix specified in PKCS1, that
is,
hex 3020300c06082a864886f70d020505000410 [NETSEC].
This prefix is included to make it easier to use RSAREF (or similar
packages such as EuroRef). The FF octet MUST be repeated the maximum
number of times such that the value of the quantity being
exponentiated is one octet shorter than the value of n.
(The above specifications are identical to the corresponding part of
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Public Key Cryptographic Standard #1 [PKCS1].)
The size of n, including most and least significant bits (which will
be 1) MUST be not less than 512 bits and not more than 4096 bits. n
and e SHOULD be chosen such that the public exponent is small.
Leading zero bytes are permitted in the MD5/RSA algorithm signature.
A public exponent of 3 minimizes the effort needed to decode a
signature. Use of 3 as the public exponent may be weak for
confidentiality uses since, if the same data can be collected
encrypted under three different keys with an exponent of 3 then,
using the Chinese Remainder Theorem [NETSEC], the original plain text
can be easily recovered. This weakness is not significant for DNS
security because we seek only authentication, not confidentiality.
4.1.8.3 Transaction and Request SIGs
A response message from a security aware server may optionally
contain a special SIG at the end of the additional information
section to authenticate the transaction.
This SIG has a "type covered" field of zero, which is not a valid RR
type. It is calculated by using a "data" (see Section 4.1.8.1) of
the entire preceding DNS reply message, including DNS header but not
the IP header and before the reply RR counts have been adjusted for
the inclusion of any transaction SIG, concatenated with the entire
DNS query message that produced this response, including the query's
DNS header and any request SIGs but not its IP header. That is
data = full response (less transaction SIG) | full query
Verification of the transaction SIG (which is signed by the server
host key, not the zone key) by the requesting resolver shows that the
query and response were not tampered with in transit, that the
response corresponds to the intended query, and that the response
comes from the queried server.
A DNS request may be optionally signed by including one or more SIGs
at the end of the query. Such SIGs are identified by having a "type
covered" field of zero. They sign the preceding DNS request message
including DNS header but not including the IP header or any request
SIGs at the end and before the request RR counts have been adjusted
for the inclusions of any request SIG(s).
WARNING: Request SIGs are unnecessary for currently defined queries
and will cause almost all existing DNS servers to completely ignore a
query. However, such SIGs are needed to authenticate some DNS secure
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dynamic update requests [RFC tbd] and may in the future been needed
for other requests.
Except where needed to authenticate an update or similar privileged
request, servers are not required to check request SIGs.
4.2 SIG RRs in the Construction of Responses
Security aware DNS servers SHOULD, for every authenticated RR the
query will return, attempt to send the available SIG RRs which
authenticate the requested RR. The following rules apply to the
inclusion of SIG RRs in responses:
1. when an RRset is placed in a response, its SIG RR has a higher
priority for inclusion than additional RRs that may need to be
included. If space does not permit its inclusion, the response
MUST be considered truncated except as provided in 2 below.
2. when a SIG RR is present in the zone for an additional
information section RR, the response MUST NOT be considered
truncated merely because space does not permit the inclusion of
its SIG RR.
3. SIGs to authenticate non-authoritative data (glue records and NS
RRs for subzones) are unnecessary and MUST NOT be sent. (Note
that KEYs given for a subzone in that subzone's superzone is
controlling so the superzone's signature on the KEY MUST be
included (unless the KEY was additional information and the SIG
did not fit).)
4. If a SIG covers any RR that would be in the answer section of
the response, its automatic inclusion MUST be in the answer
section. If it covers an RR that would appear in the authority
section, its automatic inclusion MUST be in the authority
section. If it covers an RR that would appear in the additional
information section it MUST appear in the additional information
section. This is a change in the existing standard [RFCs
10334/1035] which contemplates only NS and SOA RRs in the
authority section.
5. Optionally, DNS transactions may be authenticated by a SIG RR at
the end of the response in the additional information section
(Section 4.1.8.3). Such SIG RRs are signed by the DNS server
originating the response. Although the signer field MUST be the
name of the originating server host, the owner name, class, TTL,
and original TTL, are meaningless. The class and TTL fields
SHOULD be zero. To conserve space, the owner name SHOULD be
root (a single zero octet). If transaction authentication is
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desired, that SIG RR must be considered the highest priority for
inclusion.
4.3 Processing Responses and SIG RRs
The following rules apply to the processing of SIG RRs included in a
response:
1. a security aware resolver that receives a response from a
security aware server via a secure communication with the AD bit
(see Section 6.1) set, MAY choose to accept the RRs as received
without verifying the zone SIG RRs.
2. in other cases, a security aware resolver SHOULD verify the SIG
RRs for the RRs of interest. This may involve initiating
additional queries for SIG or KEY RRs, especially in the case of
getting a response from an server that does not implement
security. (As explained in 2.3.5 above, it will not be possible
to secure CNAMEs being served up by non-secure resolvers.)
NOTE: Implementers might expect the above SHOULD to be a MUST.
However, local policy or the calling application may not require
the security services.
3. If SIG RRs are received in response to a user query explicitly
specifying the SIG type, no special processing is required.
If the message does not pass integrity checks or the SIG does not
check against the signed RRs, the SIG RR is invalid and should be
ignored. If all of the SIG RR(s) purporting to authenticate an RRset
are invalid, then the RRset is not authenticated.
If the SIG RR is the last RR in a response in the additional
information section and has a type covered of zero, it is a
transaction signature of the response and the query that produced the
response. It MAY be optionally checked and the message rejected if
the checks fail. But even if the checks succeed, such a transaction
authentication SIG does NOT authenticate any RRs in the message.
Only a proper SIG RR signed by the zone or a key tracing its
authority to the zone or to static resolver configuration can
authenticate RRs depending on resolver policy (see Section 6). If a
resolver does not implement transaction and/or request SIGs, it MUST
ignore them without error.
If all checks indicate that the SIG RR is valid then RRs verified by
it should be considered authenticated.
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4.4 Signature Lifetime, Expiration, TTLs, and Validity
Security aware servers MUST NOT consider SIG RRs to authenticate
anything after their expiration time and NOT consider any RR to be
authenticated after all its signatures have expired. Within that
constraint, servers should continue to follow DNS TTL aging. Thus
authoritative servers should continue to follow the zone refresh and
expire parameters and a non-authoritative server should count down
the TTL and discard RRs when the TTL is zero. In addition, when RRs
are transmitted in a query response, the TTL should be trimmed so
that current time plus the TTL does not extend beyond the signature
expiration time. Thus, in general, the TTL on a transmitted RR would
be
min(sigExpTim,max(zoneMinTTL,min(originalTTL,currentTTL)))
When signatures are generated, signature expiration times should be
set far enough in the future that it is quite certain that new
signatures can be generated before the old ones expire. However,
setting expiration too far into the future could, if bad data or
signatures were ever generated, mean a long time to flush such
badness.
It is recommended that signature lifetime be a small multiple of the
TTL (ie, 4 to 16 times the TTL) but not less than a reasonable
maximum re-signing interval and not less than the zone expiry time.
4.5 The Root Zone as Signer
It should be noted that in DNS the root is a zone unto itself. Thus
the root zone key should only be seen signing itself or signing RRs
with names one level below root, such as .aq, .edu, or .arpa.
Implementations SHOULD reject as bogus any purported root signature
of records with a name more than one level below root. The root zone
contains the root KEY RR signed by a SIG RR under the root key
itself.
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5. Non-existent Names and Types
The SIG RR mechanism described in Section 4 above provides strong
authentication of RRs that exist in a zone. But is it not clear
above how to authenticatably deny the existence of a name in a zone
or a type for an existent name.
The nonexistence of a name in a zone is indicated by the NXT ("next")
RR for a name interval containing the nonexistent name. A NXT RR and
its SIG are returned in the authority section, along with the error,
if the server is security aware. The same is true for a non-existent
type under an existing name. This is a change in the existing
standard [RFCs 1034/1035] which contemplates only NS and SOA RRs in
the authority section. NXT RRs will also be returned if an explicit
query is made for the NXT type.
The existence of a complete set of NXT records in a zone means that
any query for any name and any type to a security aware server
serving the zone will result in an reply containing at least one
signed RR unless it is a query for delegation point NS or glue A or
AAAA RRs.
5.1 The NXT Resource Record
The NXT resource record is used to securely indicate that RRs with an
owner name in a certain name interval do not exist in a zone and to
indicate what RR types are present for an existing name.
The owner name of the NXT RR is an existing name in the zone. It's
RDATA is a "next" name and a type bit map. The presence of the NXT RR
means that no name between its owner name and the name in its RDATA
area exists and that no other types exist under its owner name. This
implies a canonical ordering of all domain names in a zone as
described in Section 8.
There is a potential problem with the last NXT in a zone as it wants
to have an owner name which is the last existing name in canonical
order, which is easy, but it is not obvious what name to put in its
RDATA to indicate the entire remainder of the name space. This is
handled by treating the name space as circular and putting the zone
name in the RDATA of the last NXT in a zone.
The NXT RRs for a zone SHOULD be automatically calculated and added
to the zone when SIGs are added. The NXT RR's TTL SHOULD NOT exceed
the zone minimum TTL.
The type number for the NXT RR is 30.
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5.2 NXT RDATA Format
The RDATA for an NXT RR consists simply of a domain name followed by
a bit map, 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 /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The NXT RR type bit map format currently defined is one bit per RR
type present for the owner name similar to the WKS RR socket bit map.
The first bit represents RR type zero (an illegal type which can not
be present.) A one bit indicates that at least one RR of that type
is present for the owner name. A zero indicates that no such RR is
present. All bits not specified because they are beyond the end of
the bit map are assumed to be zero. Note that bit 30, for NXT, will
always be on so the minimum bit map length is actually four octets.
Trailing zero octets are prohibited in this format. This format must
be used unless there are RRs with a type number greater than 127. If
the zero bit of the type bit map is a one, it indicates that a
different format is in use which is to be defined.
The NXT bit map should be printed as a list of RR type mnemonics or
decimal numbers similar to the WKS RR.
The domain name may be compressed with standard DNS name compression
when being transmitted over the network. The size of the bit map can
be inferred from the RDLENGTH and the length of the next domain name.
5.3 Example
Assume zone foo.nil has entries for
big.foo.nil,
medium.foo.nil.
small.foo.nil.
tiny.foo.nil.
Then a query to a security aware server for huge.foo.nil would
produce an error reply with an RCODE of NXDOMAIN and the authority
section data including something like the following:
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big.foo.nil. NXT medium.foo.nil. A MX SIG NXT
big.foo.nil. SIG NXT 1 3 ( ;type-cov=NXT, alg=1, labels=3
19970102030405 ;signature expiration
19961211100908 ;time signed
2143 ;key identifier
foo.nil. ;signer
MxFcby9k/yvedMfQgKzhH5er0Mu/vILz45IkskceFGgiWCn/GxHhai6VAuHAoNUz4YoU
1tVfSCSqQYn6//11U6Nld80jEeC8aTrO+KKmCaY= ;signature (640 bits)
)
Note that this response implies that big.foo.nil is an existing name
in the zone and thus has other RR types associated with it than NXT.
However, only the NXT (and its SIG) RR appear in the response to this
query for huge.foo.nil, which is a non-existent name.
5.4 Special Considerations at Delegation Points
A name (other than root) which is the head of a zone also appears as
the leaf in a superzone. If both are secure, there will always be
two different NXT RRs with the same name. They can be distinguished
by their signers and next domain name fields. Security aware servers
should return the correct NXT automatically when required to
authenticate the non-existence of a name and both NXTs, if available,
on explicit query for type NXT.
Non-security aware servers will never automatically return an NXT and
some old implementations may only return the NXT from the subzone on
explicit queries.
5.5 Zone Transfers
The sections below describe how full and incremental zone transfers
are secured.
SIG RRs secure all authoritative RRs transferred for both full and
incremental [RFC 1995] zone transfers. NXT RRs are an essential
elements in secure zone transfers and assure that every authoritative
name and type will be present; however, if there are multiple SIGs
with the same name and type covered a subset of the SIGs could be
sent as long as at least one is present and, in the case of unsigned
delegation point NS or glue A or AAAA RRs a subset of these RRs could
be sent as long as at least one of each type is included.
To provide server authentication that a complete transfer has
occurred, transaction authentication SHOULD be used on all full zone
transfers. This provides strong protection for the entire zone in
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transit.
When an incremental or full zone transfer request is received with
the same or newer version number than that of the server's copy of
the zone, it is replied to with just the SOA RR of the server's
current version and the SIG(s) verifying that SOA RR.
5.5.1 Incremental Zone Transfers
Individual RRs in an incremental (IXFR) transfer [RFC 1995] can be
verified in the same way as for a full zone transfer and the
integrity of the NXT name chain and correctness of the NXT type bits
for the zone after the incremental RR deletes and adds can check each
disjoint area of the zone updated. But the completeness of an
incremental transfer can not be confirmed because usually neither the
deleted RR section nor the added RR section has a compete NXT chain.
As a result, a server which securely supports IXFR must handle IXFR
SIG RRs for each incremental transfer set that it maintains.
The IXFR SIG is calculated over the incremental zone update
collection of RRs in the order in which it is transmitted: old SOA,
then deleted RRs, then new SOA and added RRs. It is recommended
that, within each section, RRs be ordered as specified in Section 8.
If condensation of adjacent incremental update sets is done by the
zone owner, the original IXFR SIG for each set included in the
condensation must be discarded and a new on IXFR SIG calculated to
cover the resulting condensed set.
The IXFR SIG really belongs to the zone as a whole, not to the zone
name. Although it should be correct for the zone name, the labels
field of an IXFR SIG is otherwise meaningless. The IXFR SIG is only
sent as part of an incremental zone transfer. After validation of
the IXFR SIG, the transferred RRs MAY be considered valid without
verification of the internal SIGs.
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6. How to Resolve Securely and the AD and CD Bits
Retrieving or resolving secure data from the Domain Name System (DNS)
involves starting with one or more trusted public keys that have been
staticly configured at the resolver. With starting trusted keys, a
resolver willing to perform cryptography can progress securely
through the secure DNS zone structure to the zone of interest as
described in Section 6.3. Such trusted public keys would normally be
configured in a manner similar to that described in Section 6.2.
However, as a practical matter, a security aware resolver would still
gain some confidence in the results it returns even if it was not
configured with any keys but trusted what it got from a local well
known server as a starting point.
Data stored at a security aware server needs to be internally
categorized as Authenticated, Pending, or Insecure. There is also a
fourth transient state of Bad which indicates that all SIG checks
have explicitly failed on the data. Such Bad data is not retained at
a security aware server. Authenticated means that the data has a
valid SIG under a KEY traceable via a chain of zero or more SIG and
KEY RRs allowed by the resolvers policies to a KEY staticly
configured at the resolver. Pending data has no authenticated SIGs
and at least one additional SIG the resolver is still trying to
authenticate. Insecure data is data which it is known can never be
either Authenticated or found Bad because it is in or has been
reached via a unsecured zone. Behavior in terms of control of and
flagging based on such data labels is described in Section 6.1.
The proper validation of signatures requires a reasonably secure
shared opinion of the absolute time between resolvers and servers as
described in Section 6.4.
6.1 The AD and CD Header Bits
Two previously unused bits are allocated out of the DNS
query/response format header. The AD (authentic data) bit indicates
in a response that the data included has been verified by the server
providing it according to the policies of that server. The CD
(checking disabled) bit indicates in a query that Pending (non-
verified) data is acceptable to the resolver sending the query.
These bits are allocated from the previously must-be-zero Z field as
follows:
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1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ID |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|QR| Opcode |AA|TC|RD|RA| Z|AD|CD| RCODE |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| QDCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ANCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| NSCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ARCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
These bits are zero in old servers and resolvers. Thus the responses
of old servers are not flagged as authenticated to security aware
resolvers and queries from non-security aware resolvers do not assert
the checking disabled bit and thus will be answered by security aware
servers only with Authenticated or Insecure data. Aware resolvers
MUST NOT trust the AD bit unless they trust the server they are
talking to and either have a secure path to it or use DNS transaction
security.
Any security aware resolver willing to do cryptography SHOULD assert
the CD bit on all queries to permit it to impose its own policies and
to reduce DNS latency time by allowing security aware servers to
answer with Pending data.
Security aware servers NEVER return Bad data. For non-security aware
resolvers or security aware resolvers requesting service by having
the CD bit clear, security aware servers MUST return only
Authenticated or Insecure data with the AD bit set in the response.
Security aware servers SHOULD return Pending data, with the AD bit
clear in the response, to security aware resolvers requesting the
service by asserting the CD bit in their request. The AD bit MUST
NOT be set on a response unless all of the RRs in the response are
either Authenticated or Insecure.
6.2 Staticly Configured Keys
The public key to authenticate a zone SHOULD be defined in local
configuration files before that zone is loaded at the primary server
so the zone can be authenticated.
While it might seem logical for everyone to start with a key for the
root zone and staticly configure this in every resolver, this has
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problems. The logistics of updating every DNS resolver in the world
when the root key changes would be excessive. Furthermore, many
organizations will explicitly wish their "interior" DNS
implementations to completely trust only their own zone. Such
interior resolvers can then go through the organization's zone
servers to access data outsize the organization's domain and should
not be configured with keys above the organization's DNS apex.
Host resolvers that are not part of a larger organization will likely
be configures with a key for the domain of their local ISP whose
recursive secure DNS caching server they use.
6.3 Chaining Through The DNS
Starting with one or more trusted keys for any zone, it should be
possible to retrieve signed keys for its subzones which have a key
and, if the zone is not root, for its superzone. Every authoritative
secure zone server MUST also include the KEY RR for one or more
super-zones (possibly including root) signed by the secure zone via
static configuration. This makes it possible to climb the tree of
zones if one starts below root. A secure sub-zone is indicated by a
KEY RR with non-null key information appearing with the NS RRs for
the sub-zone. These make it possible to descend within the tree of
zones.
6.3.1 Chaining Through KEYs
In general, some RRset in the secure DNS will be signed by one or
more SIG RRs. Each of these SIG RRs has a signer under whose name is
stored the public KEY to use in verifying the SIG. Each of those
KEYs will, generally, also be signed with a SIG. And those SIGs will
also refer to KEYs. And so on. As a result, verifying leads to
chains of alternating SIG and KEY RRs with the first SIG signing the
original data whose validity is to be shown and the final KEY being
some key staticly configured at the resolver performing the
verification.
In testing such a chain, the validation of a SIG over some data with
reference to a KEY is an objective cryptographic test; however, the
judgement that a SIG with a particular signer name can authenticate
data (possibly a KEY RRset) with a particular owner name is a policy
question. Ultimately, this is a policy local to the resolver and any
clients that depend on that resolver's decisions. It is, however,
strongly recommended, that the following policy be adopted:
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Let A < B mean that A is a shorter domain name than B formed by
dropping one or more whole labels from the left end of B. Let A
= B mean that A and B are the same domain name (i.e., are
identical after letter case canonicalization). Let A > B mean
that A is a longer domain name than B formed by adding one or
more whole labels on the left end of B.
Let K be the owner names of the set of staticly configured
trusted keys at a resolver.
Then SG is a valid signer name for a SIG authenticating data
(possibly a KEY RRset) with owner name OW at a resolver if any
of the following three rules apply:
(1) OW > SG except that if SG is root (.), OW must be a top
level domain name.
(2) ( OW < or = SG ) and ( SG > some K ).
(3) SG = some K.
Rule 1 is the rule for descending the DNS tree and includes a special
prohibition on the root zone key due to the restriction that the root
zone be only one label deep.
Rule 2 is the rule for ascending the DNS tree from one or more
staticly configured keys. Rule 2 has no effect if only root keys are
staticly configured.
Rule 3 is a rule permitting direct cross certification.
Great care should be taken that the consequences have been fully
considered before making any local policy adjustments to these rules.
6.3.2 Conflicting Data
It is possible that there will be multiple SIG-KEY chains that appear
to authenticate conflicting RRset answers to the same query. A
resolver should choose only the most reliable answer to return and
discard other data. This choice of most reliable is a matter of
local policy which could take into account differing trust in
algorithms, key sizes, staticly configured keys, zones traversed,
etc. The technique given below is recommended for taking into
account SIG-KEY chain length.
A resolver should keep track of the number of successive secure zones
traversed from a staticly configured zone key starting point to any
secure zone it can reach. In general, the lower such a distance
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number is, the greater the confidence in the data. Staticly
configured data should be given a distance number of zero. If a
query encounters different Authenticated data for the same query with
different distance values, that with a larger value should be ignored
unless some other local policy covers the case.
A security conscious resolver should completely refuse to step from a
secure zone into a unsecured zone unless the unsecured zone is
certified to be non-secure by the presence of an authenticated KEY RR
for the unsecured zone with the no-key type value. Otherwise the
resolver is getting bogus or spoofed data.
If legitimate unsecured zones are encountered in traversing the DNS
tree, then no zone can be trusted as secure that can be reached only
via information from such non-secure zones. Since the unsecured zone
data could have been spoofed, the "secure" zone reach via it could be
counterfeit. The "distance" to data in such zones or zones reached
via such zones could be set to 256 or more as this exceeds the
largest possible distance through secure zones in the DNS.
Nevertheless, continuing to apply secure checks within "secure" zones
reached via unsecured zones is a good practice and will, as a
practical matter, provide some small increase in confidence.
6.4 Secure Time
Coordinated interpretation of the time fields in SIG RRs requires
that reasonably consistent time be available to the hosts
implementing the DNS security extensions.
A variety of time synchronization protocols exist including the
Network Time Protocol (NTP, RFC 1305). If such protocols are used,
they MUST be used securely so that time can not be spoofed.
Otherwise, for example, a host could get its clock turned back and
might then believe old SIG RRs, and the data they authenticate, which
were valid but are no longer.
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7. ASCII Representation of Security RRs
This section discusses the format for master file and other ASCII
presentation of the three DNS security resource records.
The algorithm field in KEY and SIG RRs can be represented as either
an unsigned integer or symbolicly. The following initial symbols are
defined as indicated:
value symbol
001 RSAMD5
253 NULL (obsolete, see RFC 2065)
254 PRIVATE
7.1 Presentation of KEY RRs
KEY RRs may appear as single logical lines in a zone data master file
[RFC 1033].
The flag field is represented as an unsigned integer or a sequence of
mnemonics as follows:
BIT Mnemonic Explanation
0 NOAUTH authentication use prohibited
1 NOCONF confidentiality use prohibited
2 FLAG2 - reserved
3 EXTEND flags extension
4 FLAG4 - reserved
5 FLAG5 -reserved
6-7 name type
USER =0
ZONE =1
HOST =2 (host or other end entity)
NTYP3 - reserved
8 OAKLEY key usable for Oakley/IPSEC
9 FLAG9 - reserved
10 FLAG10 - reserved
11 FLAG11 - reserved
12-15 signatory field, values 0 to 15
can be represented by SIG0, SIG1, ... SIG15
The protocol octet can be represented as either an unsigned integer
or symbolicly. The following initial symbols are defined:
000 NONE
001 TLS 002 EMAIL
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Note that if the type field has the "no key" value (ie, both NOAUTH
and NOCONF are on), nothing appears after the algorithm octet.
The remaining public key portion is represented in base 64 (see
Appendix A) and may be divided up into any number of white space
separated substrings, down to single base 64 digits, which are
concatenated to obtain the full signature. These substrings can span
lines using the standard parenthesis.
Note that the public key may have internal sub-fields but these do
not appear in the master file representation. For example, with
algorithm 1 there is a public exponent size, then a public exponent,
and then a modulus. With algorithm 254, there will be an OID size,
an OID, and algorithm dependent information. But in both cases only a
single logical base 64 string will appear in the master file.
7.2 Presentation of SIG RRs
A SIG RR may be represented as a single logical line in a zone data
file [RFC 1033] but there are some special considerations as
described below. (It does not make sense to include a transaction or
request authenticating SIG RR in a file as they are a transient
authentication that covers data including an ephemeral transaction
number and so must be calculated in real time.)
There is no particular problem with the signer, covered type, and
times. The time fields appears in the form YYYYMMDDHHMMSS where YYYY
is the year, the first 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),
the second MM is the minute (00-59), and SS is the second (00-59).
The original TTL field appears as an unsigned integer.
If the original TTL, which applies to the type signed, is the same as
the TTL of the SIG RR itself, it may be omitted. The date field
which follows it is larger than the maximum possible TTL so there is
no ambiguity.
The "labels" field appears as an unsigned integer.
The key tag appears as an unsigned number.
However, the signature itself can be very long. It is the last data
field and is represented in base 64 (see Appendix A) and may be
divided up into any number of white space separated substrings, down
to single base 64 digits, which are concatenated to obtain the full
signature. These substrings can be split between lines using the
standard parenthesis.
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7.3 Presentation of NXT RRs
NXT RRs do not appear in original unsigned zone master files since
they should be derived from the zone as it is being signed. If a
signed file with NXTs added is printed or NXTs are printed by
debugging code, they appear as the next domain name followed by the
RR type present bits in the same format as the WKS RR.
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8. Canonical Form and Order of Resource Records
This section describes the canonical form of resource records (RRs),
their default name order, and their intra-RRset order, for purposes
of domain name system (DNS) security. A canonical name order is
necessary to construct the NXT name chain. A canonical form and
ordering within an RRset is necessary in constructing SIG RRs. There
is no requirement in DNS security for a canonical ordering of types
within a name so none is defined.
8.1 Canonical RR Form
For purposes of DNS security, the canonical form for an RR is the
wire format of the RR with domain names (1) fully expanded (no name
compression via pointers), (2) all domain name letters set to lower
case, and (3) the original TTL substituted for the current TTL.
8.2 Canonical DNS Name Order
For purposes of DNS security, the canonical ordering of owner names
is to sort labels as unsigned left justified octet strings where the
absence of a octet sorts before a zero value octet and upper case
letters are treated as lower case letters. Names are then sorted by
sorting on the highest level label and then, within those names with
the same highest level label by the next lower label, etc. down to
leaf node labels. Within a zone, the zone name itself always exists
and all other names are the zone name with some prefix of lower level
labels. Thus the zone name itself always sorts first.
Example:
foo.example
a.foo.example
yljkjljk.a.foo.example
Z.a.foo.example
zABC.a.FOO.EXAMPLE
z.foo.example
*.z.foo.example
\200.z.foo.example
8.3 Canonical RR Ordering Within An RRset
Within any particular owner name and type, RRs are sorted by RDATA as
a left justified unsigned octet sequence where the absence of an
octet sorts before the zero octet.
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9. Conformance
Levels of server and resolver conformance are defined below.
9.1 Server Conformance
Two levels of server conformance for DNS security are defined as
follows:
BASIC: Basic server compliance is the ability to store and retrieve
(including zone transfer) SIG, KEY, and NXT RRs. Any secondary or
caching server for a secure zone MUST have at least basic compliance
and even then some things, such as secure CNAMEs, will not work
without full compliance.
FULL: Full server compliance adds the following to basic compliance:
(1) ability to read SIG, KEY, and NXT RRs in zone files and (2)
ability, given a zone file and private key, to add appropriate SIG
and NXT RRs, possibly via a separate application, (3) proper
automatic inclusion of SIG, KEY, and NXT RRs in responses, (4)
suppression of CNAME following on retrieval of the security type RRs,
(5) recognize the CD query header bit and set the AD query header
bit, as appropriate, and (6) proper handling of the two NXT RRs at
delegation points. Primary servers for secure zones MUST be fully
compliant and for complete secure operation, all secondary, caching,
and other servers handling the zone SHOULD be fully compliant as
well.
9.2 Resolver Conformance
Two levels of resolver compliance (including the resolver portion of
a server) are defined for DNS Security:
BASIC: A basic compliance resolver can handle SIG, KEY, and NXT RRs
when they are explicitly requested.
FULL: A fully compliant resolver (1) understands KEY, SIG, and NXT
RRs including verification of SIGs, (2) maintains appropriate
information in its local caches and database to indicate which RRs
have been authenticated and to what extent they have been
authenticated, (3) performs additional queries as necessary to
attempt to obtain KEY, SIG, or NXT RRs from non-security aware
servers, (4) normally sets the CD query header bit on its queries.
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10. Security Considerations
This document describes extensions to the Domain Name System (DNS)
protocol to provide data integrity and origin authentication, public
key distribution, and optional transaction and request security.
It should be noted that, at most, these extensions guarantee the
validity of resource records, including KEY resource records,
retrieved from the DNS. They do not magically solve other security
problems. For example, using secure DNS you can have high confidence
in the IP address you retrieve for a host name; however, this does
not stop someone for substituting an unauthorized host at that
address or capturing packets sent to that address and falsely
responding with packets apparently from that address. Any reasonably
complete security system will require the protection of many
additional facets of the Internet.
Donald E. Eastlake 3rd [Page 41]
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References
[NETSEC] - Network Security: PRIVATE Communications in a PUBLIC
World, Charlie Kaufman, Radia Perlman, & Mike Speciner, Prentice Hall
Series in Computer Networking and Distributed Communications, 1995.
[PKCS1] - PKCS #1: RSA Encryption Standard, RSA Data Security, Inc.,
3 June 1991, Version 1.4.
[RFC 1032] - M. Stahl, "Domain Administrators Guide", November 1987.
[RFC 1033] - M. Lottor, "Domain Administrators Operations Guide",
November 1987.
[RFC 1034] - P. Mockapetris, "Domain Names - Concepts and
Facilities", STD 13, November 1987.
[RFC 1035] - P. Mockapetris, "Domain Names - Implementation and
Specifications", STD 13, November 1987.
[RFC 1305] - Mills, D., "Network Time Protocol (v3)", March 1992.
[RFC 1321] - R. Rivest, "The MD5 Message-Digest Algorithm", April
1992.
[RFC 1530] - Malamud, C., and M. Rose, "Principles of Operation for
the TPC.INT Subdomain: General Principles and Policy", October 1993.
[RFC 1750] - D. Eastlake, S. Crocker, and J. Schiller, "Randomness
Requirements for Security", December 1994.
[RFC 1825] - Atkinson, R., "Security Architecture for the Internet
Protocol", August 1995.
[RFC 1982] - R. Elz, R. Bush, "Serial Number Arithmetic", 09/03/1996.
[RFC 1995] - Ohta, M., "Incremental Zone Transfer in DNS", August
1996.
[RFC 2065] - D. Eastlake, C. Kaufman, "Domain Name System Security
Extensions", 01/03/1997.
[RFC 2136] - P. Vixie, S. Thomson, Y. Rekhter, J. Bound, "Dynamic
Updates in the Domain Name System (DNS UPDATE)", 04/21/1997.
[RFC 2137] - D. Eastlake, "Secure Domain Name System Dynamic Update",
04/21/1997.
[RSA FAQ] - RSADSI Frequently Asked Questions periodic posting.
Donald E. Eastlake 3rd [Page 42]
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draft-ietf-tls-*.txt
Donald E. Eastlake 3rd [Page 43]
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Author's Addresses
Donald E. Eastlake 3rd
CyberCash, Inc.
318 Acton Street
Carlisle, MA 01741 USA
Telephone: +1 508-287-4877
+1 508-371-7148(fax)
+1 703-620-4200(main office, Reston, Virginia, USA)
EMail: dee@cybercash.com
Expiration and File Name
This draft expires 19 February 1998.
Its file name is draft-ietf-dnssec-secext2-01.txt.
Donald E. Eastlake 3rd [Page 44]
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Appendix A: Base 64 Encoding
The following encoding technique is taken from RFC 1521 by N.
Borenstein and N. Freed. It is reproduced here in an edited form for
convenience.
A 65-character subset of US-ASCII is used, enabling 6 bits to be
represented per printable character. (The extra 65th character, "=",
is used to signify a special processing function.)
The encoding process represents 24-bit groups of input bits as output
strings of 4 encoded characters. Proceeding from left to right, a
24-bit input group is formed by concatenating 3 8-bit input groups.
These 24 bits are then treated as 4 concatenated 6-bit groups, each
of which is translated into a single digit in the base 64 alphabet.
Each 6-bit group is used as an index into an array of 64 printable
characters. The character referenced by the index is placed in the
output string.
Table 1: The Base 64 Alphabet
Value Encoding Value Encoding Value Encoding Value Encoding
0 A 17 R 34 i 51 z
1 B 18 S 35 j 52 0
2 C 19 T 36 k 53 1
3 D 20 U 37 l 54 2
4 E 21 V 38 m 55 3
5 F 22 W 39 n 56 4
6 G 23 X 40 o 57 5
7 H 24 Y 41 p 58 6
8 I 25 Z 42 q 59 7
9 J 26 a 43 r 60 8
10 K 27 b 44 s 61 9
11 L 28 c 45 t 62 +
12 M 29 d 46 u 63 /
13 N 30 e 47 v
14 O 31 f 48 w (pad) =
15 P 32 g 49 x
16 Q 33 h 50 y
Special processing is performed if fewer than 24 bits are available
at the end of the data being encoded. A full encoding quantum is
always completed at the end of a quantity. When fewer than 24 input
bits are available in an input group, zero bits are added (on the
right) to form an integral number of 6-bit groups. Padding at the
end of the data is performed using the '=' character. Since all base
64 input is an integral number of octets, only the following cases
can arise: (1) the final quantum of encoding input is an integral
multiple of 24 bits; here, the final unit of encoded output will be
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an integral multiple of 4 characters with no "=" padding, (2) the
final quantum of encoding input is exactly 8 bits; here, the final
unit of encoded output will be two characters followed by two "="
padding characters, or (3) the final quantum of encoding input is
exactly 16 bits; here, the final unit of encoded output will be three
characters followed by one "=" padding character.
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Appendix B: Changes from RFC 2065
This section summarizes the most important changes that have been
made since RFC 2065.
1. Most of Section 7 of RFC 2065 called "Operational Considerations",
has been split off into a separate document.
2. The KEY RR has been changed by (2a) eliminating the "experimental"
flag as unnecessary, (2b) reserving a flag bit for flags
expansion, (2c) more compactly encoding a number of bit fields in
such a way as to leave unchanged bits actually used by the limited
code currently deployed, and (2d) for the RSA MD5 algorithm
increasing the maximum required key modulus size implementation to
4096 bits. Section 3.4 describing the meaning of various
combinations of "no-key" and key present KEY RRs has been added.
3. The SIG RR has been changed by (3a) clarifying that signature
expiration and date signed used serial number ring arithmetic, and
(3b) changing the definition of the key footprint/tag for
algorithms other than 1 (i.e., algorithms to be defined in the
future) and adding Appendix C to document its calculation. In
addition, the SIG covering type AXFR has been eliminated.
5. Both the KEY and SIG RR definitions have been simplified by
eliminating the "null" algorithm 253 as defined in RFC 2065. That
algorithm had been included because at the time it was thought it
might be useful in DNS dynamic update [RFC 2136]. It was in fact
not so used and it is dropped to simplify DNS security.
6. The NXT RR has been changed so that (6a) the NXT RRs in a zone
cover all names, including wildcards as literal names without
expansion and (6b) all NXT bit map areas whose first octet has bit
zero set have been reserved for future definition, and (6c)
additional minor changes made to assure a unique encoding of RR
type combinations currently existing int he DNS.
7. Information on the canonical form and ordering of RRs has been
moved into a separate section, number 8.
8. A subsection covering incremental and full zone transfer has been
added in Section 5.
9. Further specification and policy recommendations on secure
resolution have been added, primarily in section 6.3.1.
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Appendix C: Key Tag Calculation
The key tag field in the SIG RR is just a means of more efficiently
selecting the correct KEY RR to use in verifying the signature when
there is more than one KEY RR candidate. It is possible for more
than one candidate key to have the same tag, in which case each must
be tried in verifying the signature until one works or all fail. The
following reference implementation is in ANSI C.
void keytag (
unsigned char key[], /* the RDATA part of the KEY RR */
unsigned int keysize, /* the RDLENGTH */
unsigned char tag[2] /* return calculated tag */
)
{
long int ac; /* assumed to be 32 bits or larger */
for ( ac = 0, i = 0; i < keysize; ++i )
ac += (i&1) ? key[i] : key[i]<<8;
ac += (ac>>16) & 0xFF;
tag[0] = ac>>8;
tag[1] = ac;
}
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