RFC2535 - Domain Name System Security Extensions

Network Working Group D. Eastlake
Request for Comments: 2535 IBM
Obsoletes: 2065 March 1999
Updates: 2181, 1035, 1034
Category: Standards Track
Domain Name System Security Extensions
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (1999). All Rights Reserved.
Abstract
Extensions to the Domain Name System (DNS) are described that provide
data integrity and authentication to security aware resolvers and
applications through the use of cryptographic digital signatures.
These digital signatures are included in secured zones as resource
records. Security can also be provided through non-security aware
DNS servers in some 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 on RFC2065 from early
implementers and potential users.
Acknowledgments
The significant contributions and suggestions of the following
persons (in alphabetic order) to DNS security are gratefully
acknowledged:
James M. Galvin
John Gilmore
Olafur Gudmundsson
Charlie Kaufman
Edward Lewis
Thomas Narten
Radia J. Perlman
Jeffrey I. Schiller
Steven (Xunhua) Wang
Brian Wellington
Table of Contents
Abstract...................................................1
Acknowledgments............................................2
1. Overview of Contents....................................4
2. Overview of the DNS Extensions..........................5
2.1 Services Not Provided..................................5
2.2 Key Distribution.......................................5
2.3 Data Origin Authentication and Integrity...............6
2.3.1 The SIG Resource Record..............................7
2.3.2 Authenticating Name and Type Non-existence...........7
2.3.3 Special Considerations With Time-to-Live.............7
2.3.4 Special Considerations at Delegation Points..........8
2.3.5 Special Considerations with CNAME....................8
2.3.6 Signers Other Than The Zone..........................9
2.4 DNS Transaction and Request Authentication.............9
3. The KEY Resource Record................................10
3.1 KEY RDATA format......................................10
3.1.1 Object Types, DNS Names, and Keys...................11
3.1.2 The KEY RR Flag Field...............................11
3.1.3 The Protocol Octet..................................13
3.2 The KEY Algorithm Number Specification................14
3.3 Interaction of Flags, Algorithm, and Protocol Bytes...15
3.4 Determination of Zone Secure/Unsecured Status.........15
3.5 KEY RRs in the ConstrUCtion of Responses..............17
4. The SIG Resource Record................................17
4.1 SIG RDATA Format......................................17
4.1.1 Type Covered Field..................................18
4.1.2 Algorithm Number Field..............................18
4.1.3 Labels Field........................................18
4.1.4 Original TTL Field..................................19
4.1.5 Signature EXPiration and Inception Fields...........19
4.1.6 Key Tag Field.......................................20
4.1.7 Signer"s Name Field.................................20
4.1.8 Signature Field.....................................20
4.1.8.1 Calculating Transaction and Request SIGs..........21
4.2 SIG RRs in the Construction of Responses..............21
4.3 Processing Responses and SIG RRs......................22
4.4 Signature Lifetime, Expiration, TTLs, and Validity....23
5. Non-existent Names and Types...........................24
5.1 The NXT Resource Record...............................24
5.2 NXT RDATA Format......................................25
5.3 Additional Complexity Due to Wildcards................26
5.4 Example...............................................26
5.5 Special Considerations at Delegation Points...........27
5.6 Zone Transfers........................................27
5.6.1 Full Zone Transfers.................................28
5.6.2 Incremental Zone Transfers..........................28
6. How to Resolve Securely and the AD and CD Bits.........29
6.1 The AD and CD Header Bits.............................29
6.2 Staticly Configured Keys..............................31
6.3 Chaining Through The DNS..............................31
6.3.1 Chaining Through KEYs...............................31
6.3.2 Conflicting Data....................................33
6.4 Secure Time...........................................33
7. ASCII Representation of Security RRs...................34
7.1 Presentation of KEY RRs...............................34
7.2 Presentation of SIG RRs...............................35
7.3 Presentation of NXT RRs...............................36
8. Canonical Form and Order of Resource Records...........36
8.1 Canonical RR Form.....................................36
8.2 Canonical DNS Name Order..............................37
8.3 Canonical RR Ordering Within An RRset.................37
8.4 Canonical Ordering of RR Types........................37
9. Conformance............................................37
9.1 Server Conformance....................................37
9.2 Resolver Conformance..................................38
10. Security Considerations...............................38
11. IANA Considerations...................................39
References................................................39
Author"s Address..........................................41
Appendix A: Base 64 Encoding..............................42
Appendix B: Changes from RFC2065.........................44
Appendix C: Key Tag Calculation...........................46
Full Copyright Statement..................................47
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, 1035 and later RFCs. An
earlier version of these extensions appears in RFC2065. This
replacement for that RFCincorporates early 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 an RR
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 various
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.
Section 11 specified IANA considerations for allocation of additional
values of paramters defined in this document.
Appendix A gives details of base 64 encoding which is used in the
file representation of some RRs defined in this document.
Appendix B summarizes changes between this memo and RFC2065.
Appendix C specified how to calculate the simple checksum used as a
key tag in most SIG RRs.
The key Words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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
2401], TLS, or other security protocols.)
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 DNS security itself and other protocols.
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
minimize the number of queries needed.
2.3 Data Origin Authentication and Integrity
Authentication is provided by associating with resource record sets
(RRsets [RFC2181]) in the DNS cryptographically generated digital
signatures. Commonly, there will be a single private key that
authenticates an entire zone but there might be multiple keys for
different algorithms, signers, etc. If a security aware resolver
reliably learns a public key of the zone, it can authenticate, for
signed data read from that zone, that it is properly authorized. The
most secure implementation is for the zone private key(s) to be kept
off-line and used to re-sign all of the records in the zone
periodically. However, there are cases, for example dynamic update
[RFCs 2136, 2137], where DNS private keys need to be on-line [RFC
2541].
The data origin authentication key(s) are associated with the zone
and not with the servers that store copies of the data. That means
compromise of a secondary server or, if the key(s) are kept off line,
even the primary server 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 could learn a public key of a zone either by reading it
from the DNS or by having it staticly configured. To reliably learn
a 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 a public key which authenticates one zone as
a starting point. From there, it can securely read 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 in 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 (see
Section 4.2).
2.3.1 The SIG Resource Record
The syntax of a SIG resource record (signature) is described in
Section 4. It cryptographicly binds the RRset 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
RRsets 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 signature inception time and a signature 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 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 manager.
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. [RFC2181]
There MUST be a zone KEY RR, signed by its superzone, for every
subzone if the superzone is secure. This will normally appear in the
subzone and may also be included in the superzone. But, 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 with the superzone signature in 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 subzone KEY RR should be
signed in the superzone if it appears there. 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 SIG, 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
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 cases where the signer in a SIG resource record is other
than one of the private key(s) used to authenticate a zone.
One is for support of dynamic update [RFC2136] (or future requests
which require secure authentication) where an entity is permitted to
authenticate/update its records [RFC2137] and the zone is operating
in a mode where the zone key is not on line. The public key of the
entity must be present in the DNS and be signed by a zone level key
but the other RR(s) may be signed with the entity"s key.
A second case is support of transaction and request authentication as
described in Section 2.4.
In additions, signatures can be included on resource records within
the DNS for use by applications other than DNS. DNS related
signatures authenticate that data originated with the authority of a
zone owner or that a request or transaction originated with the
relevant entity. Other signatures can provide other types of
assurances.
2.4 DNS Transaction and Request Authentication
The data origin authentication service described above protects
retrieved resource records and the non-existence of 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 produce error returns or may even be ignored by
many of them. However, this syntax for signing requests is defined as
a way of authenticating secure dynamic update requests [RFC2137] or
future requests requiring authentication.
The private keys used in transaction security belong to the entity
composing the reply, not to the zone involved. Request
authentication may also involve the private key of the host or other
entity composing the request or other private keys depending on the
request authority it is sought to establish. The corresponding public
key(s) are normally stored in and retrieved from the DNS for
verification.
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.
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 user, or a host or other end entity. 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.
A KEY RR is, like any other RR, authenticated by a SIG RR. KEY RRs
must be signed by a zone level key.
3.1 KEY RDATA format
The RDATA for a KEY RR consists of flags, a protocol octet, the
algorithm number octet, 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 has been developed for that purpose, defined in [RFC
2538].
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 specify their validity period but their authenticating
SIG RR(s) do 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.
A DNS name may refer 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 and zone KEY RRs
occur only at delegation points.
3.1.2 The KEY RR Flag Field
In the "flags" field:
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
A/C Z XT Z Z NAMTYP Z Z Z Z SIG
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
Bit 0 and 1 are the key "type" bits whose values have the following
meanings:
10: Use of the key is prohibited for authentication.
01: Use of the key is prohibited for confidentiality.
00: 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.
11: 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. Field values
have the following meanings:
00: 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, "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.
01: 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. Zone KEY RRs occur only at delegation
points.
10: 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 [RFC1530] or numeric IP address. This is the
public key used in connection with DNS request and
transaction authentication services. 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.
11: reserved.
Bits 8-11 are reserved and must be zero.
Bits 12-15 are the "signatory" field. If non-zero, they indicate
that the key can validly sign things as specified in DNS
dynamic update [RFC2137]. Note that 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.
3.1.3 The Protocol Octet
It is anticipated that keys stored in DNS will be used in conjunction
with a variety of Internet protocols. It is intended that the
protocol octet and possibly some of the currently unused (must be
zero) bits in the KEY RR flags as specified in the future will be
used to indicate a key"s validity for different protocols.
The following values of the Protocol Octet are reserved as indicated:
VALUE Protocol
0 -reserved
1 TLS
2 email
3 dnssec
4 IPSEC
5-254 - available for assignment by IANA
255 All
In more detail:
1 is reserved for use in connection with TLS.
2 is reserved for use in connection with email.
3 is used for DNS security. The protocol field SHOULD be set to
this value for zone keys and other keys used in DNS security.
Implementations that can determine that a key is a DNS
security key by the fact that flags label it a zone key or the
signatory flag field is non-zero are NOT REQUIRED to check the
protocol field.
4 is reserved to refer to the Oakley/IPSEC [RFC2401] protocol
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 flag bits are set. The presence of a KEY
resource with this protocol value is an assertion that the
host speaks Oakley/IPSEC.
255 indicates that the key can be used in connection with any
protocol for which KEY RR protocol octet values have been
defined. The use of this value is discouraged and the use of
different keys for different protocols is encouraged.
3.2 The KEY Algorithm Number Specification
This octet is the key algorithm parallel to the same field for the
SIG resource as described in Section 4.1. The following values are
assigned:
VALUE Algorithm
0 - reserved, see Section 11
1 RSA/MD5 [RFC2537] - recommended
2 Diffie-Hellman [RFC2539] - optional, key only
3 DSA [RFC2536] - MANDATORY
4 reserved for elliptic curve crypto
5-251 - available, see Section 11
252 reserved for indirect keys
253 private - domain name (see below)
254 private - OID (see below)
255 - reserved, see Section 11
Algorithm specific formats and procedures are given in separate
documents. The mandatory to implement for interoperability algorithm
is number 3, DSA. It is recommended that the RSA/MD5 algorithm,
number 1, also be implemented. Algorithm 2 is used to indicate
Diffie-Hellman keys and algorithm 4 is reserved for elliptic curve.
Algorithm number 252 indicates an indirect key format where the
actual key material is elsewhere. This format is to be defined in a
separate document.
Algorithm numbers 253 and 254 are reserved for private use and will
never be assigned a specific algorithm. For number 253, the public
key area and the signature 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 whatever is required by that algorithm. For
number 254, the public key area for the KEY RR and the signature
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 domain names
and OIDs they control to designate their private algorithms.
Values 0 and 255 are reserved but the value 0 is used in the
algorithm field when that field is not used. An example is in a KEY
RR with the top two flag bits on, the "no-key" value, where no key is
present.
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. 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 zone.
0 x 0 Illegal, claims key but has bad algorithm field.
0 x 1 Specified protocols unsecured, others may be secure.
x 0 0 Gives key but no protocols to use it.
x 0 1 Denies key for specific algorithm.
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 key type flag
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. The secured versus unsecured status of a zone may vary with
different cryptographic algorithms. Even for the same algorithm,
conflicting zone KEY RRs may 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 MUST be ignored in determining the security status of the
zone. However, there can be multiple sets of trusted zone KEY RRs
for a zone with different algorithms, signers, etc.
For any particular algorithm, 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 and algorithm, every trusted zone KEY RR for the
zone says there is no key for that zone, it is unsecured for that
algorithm.
2. If, there is at least one trusted no-key zone KEY RR and one
trusted key specifying zone KEY RR, then that zone is only
experimentally secure for the algorithm. Both authenticated and
non-authenticated RRs for it should be accepted by the resolver.
3. If every trusted zone KEY RR that the zone and algorithm has is
key specifying, then it is secure for that algorithm and only
authenticated RRs from it will be accepted.
Examples:
(1) A resolver initially trusts only signatures by the superzone of
zone Z within the DNS hierarchy. Thus 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.example.
When considering data from zone Z, it may be signed by the superzone
of Z, by cert-auth.example, 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 . e x a m p l e
KEY RRs 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 --+-----------+-----------+----------+----------+
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 (see Section 4.2).
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 SOA or NS RRs, the KEY RRset with the same
name (perhaps just a zone key) SHOULD be included as additional
information if space is available. If not all additional information
will fit, type A and AAAA glue RRs have higher priority than KEY
RR(s).
(2) On retrieval of type A or AAAA RRs, the KEY RRset with the same
name (usually just a host RR and NOT the zone key (which usually
would have a different name)) SHOULD be included if space is
available. On inclusion of A or AAAA RRs as additional information,
the KEY RRset with the same name should also be included but with
lower priority than the A or AAAA RRs.
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 [RFC2181] 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 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
signature inception
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
key tag
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ signer"s name +
/
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-/
/ /
/ signature /
/ /
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4.1.1 Type Covered Field
The "type covered" is the type of the other RRs covered by this SIG.
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 generaly implies
that all RRs for a particular type, name, and class, that is, all the
RRs in any particular RRset, must have the same TTL to start with.
4.1.5 Signature Expiration and Inception Fields
The SIG is valid from the "signature inception" time until the
"signature expiration" time. Both are unsigned numbers 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 [RFC1982] which means that these times can never be more
than about 68 years in the past or the future. This means that these
times are ambiguous modulo ~136.09 years. However there is no
security flaw because keys are required to be changed to new random
keys by [RFC2541] at least every five years. This means that the
probability that the same key is in use N*136.09 years later should
be the same as the probability that a random guess will work.
A SIG RR may have an expiration time numerically less than the
inception time if the expiration time is near the 32 bit wrap around
point and/or the signature is long lived.
(To prevent misordering of network requests to update a zone
dynamically, monotonically increasing "signature inception" times may
be necessary.)
A secure zone must be considered changed for SOA serial number
purposes not only when its data is updated but also when new SIG RRs
are inserted (ie, the zone or any part of it is re-signed).
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 in [RFC2537], 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 least
significant 24 bits of the modulus in network (big endian) 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 name of the public KEY RR that can be
used to verify the signature. It is frequently the zone which
contained the RRset being authenticated. Which 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.
4.1.8 Signature Field
The actual signature portion of the SIG RR binds the other RDATA
fields to the RRset 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 the wire format of all the RDATA fields in the SIG RR itself
(including the canonical form of the signer"s name) before but not
including the signature, and
RR(s) is the RRset of 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. These algorithm dependent formats and procedures are
described in separate documents (Section 3.2).
SIGs SHOULD NOT be included in a zone for any "meta-type" such as
ANY, AXFR, etc. (but see section 5.6.2 with regard to IXFR).
4.1.8.1 Calculating 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) 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 any currently defined
request other than update [RFC2136, 2137] and will cause some old
DNS servers to give an error return or ignore a query. However, such
SIGs may in the future be 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 RRset the
query will return, attempt to send the available SIG RRs which
authenticate the requested RRset. 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
the SIG RR with the additional information.
3. SIGs to authenticate glue records and NS RRs for subzones at a
delegation point are unnecessary and MUST NOT be sent.
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 1034,
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 4.1.8.1). Such SIG RRs are signed by the DNS server
originating the response. Although the signer field MUST be a
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
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 a 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 directly 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
directly 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.
4.4 Signature Lifetime, Expiration, TTLs, and Validity
Security aware servers MUST NOT consider SIG RRs to authenticate
anything before their signature inception or after its expiration
time (see also Section 6). Security aware servers MUST NOT consider
any RR to be authenticated after all its signatures have expired.
When a secure server caches authenticated data, if the TTL would
expire at a time further in the future than the authentication
expiration time, the server SHOULD trim the TTL in the cache entry
not to extent beyond the authentication expiration time. Within
these constraints, 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 (even for a SIG
that has not yet reached its authentication expiration time). 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 authentication expiration time. Thus, in general, the TTL
on a transmitted RR would be
min(authExpTim,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 mean a long time to
flush any bad data or signatures that may have been generated.
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.
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 it is not clear
above how to verifiably 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. An NXT RR or
RRs and its or their SIG(s) 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 except that there
is no error indication other than an empty answer section
accompanying the NXT(s). 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. Thus the NXT RRs in a zone
create a chain of all of the literal owner names in that zone,
including unexpanded wildcards but omitting the owner name of glue
address records unless they would otherwise be included. This implies
a canonical ordering of all domain names in a zone as described in
Section 8. 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.
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.
NXT RRs are only signed by zone level keys.
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. 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. The first bit represents RR type zero (an illegal type which
can not be present) and so will be zero in this format. This format
is not used if there exists an RR 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 being used which will always be the case if a
type number greater than 127 is present.
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 Additional Complexity Due to Wildcards
Proving that a non-existent name response is correct or that a
wildcard expansion response is correct makes things a little more
complex.
In particular, when a non-existent name response is returned, an NXT
must be returned showing that the exact name queried did not exist
and, in general, one or more additional NXT"s need to be returned to
also prove that there wasn"t a wildcard whose expansion should have
been returned. (There is no need to return multiple copies of the
same NXT.) These NXTs, if any, are returned in the authority section
of the response.
Furthermore, if a wildcard expansion is returned in a response, in
general one or more NXTs needs to also be returned in the authority
section to prove that no more specific name (including possibly more
specific wildcards in the zone) existed on which the response should
have been based.
5.4 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:
foo.nil. NXT big.foo.nil NS KEY SOA NXT ;prove no *.foo.nil
foo.nil. SIG NXT 1 2 ( ;type-cov=NXT, alg=1, labels=2
19970102030405 ;signature expiration
19961211100908 ;signature inception
2143 ;key identifier
foo.nil. ;signer
AIYADP8d3zYNyQwW2EM4wXVFdslEJcUx/fxkfBeH1El4ixPFhpfHFElxbvKoWmvjDTCm
fiYy2X+8XpFjwICHc398kzWsTMKlxovpz2FnCTM= ;signature (640 bits)
)
big.foo.nil. NXT medium.foo.nil. A MX SIG NXT ;prove no huge.foo.nil
big.foo.nil. SIG NXT 1 3 ( ;type-cov=NXT, alg=1, labels=3
19970102030405 ;signature expiration
19961211100908 ;signature inception
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.
5.5 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 easily
distinguished by their signers, the next domain name fields, the
presence of the SOA type bit, etc. 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.
5.6 Zone Transfers
The subsections below describe how full and incremental zone
transfers are secured.
SIG RRs secure all authoritative RRs transferred for both full and
incremental [RFC1995] zone transfers. NXT RRs are an essential
element 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 or
simply a modified set could be sent as long as at least one of each
type is included.
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 RRset verifying that SOA RR.
The complete NXT chains specified in this document enable a resolver
to oBTain, by successive queries chaining through NXTs, all of the
names in a zone even if zone transfers are prohibited. Different
format NXTs may be specified in the future to avoid this.
5.6.1 Full Zone Transfers
To provide server authentication that a complete transfer has
occurred, transaction authentication SHOULD be used on full zone
transfers. This provides strong server based protection for the
entire zone in transit.
5.6.2 Incremental Zone Transfers
Individual RRs in an incremental (IXFR) transfer [RFC1995] 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 zone 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. Within each section,
RRs must be ordered as spe