nameservers; in fact, some might even refuse to pass on the DS or NXT
records.
2.4. Wire Format of the DS record
The DS (type=43) record contains these fields: key tag, algorithm,
digest type, and the digest of a public key KEY record that is
allowed and/or used to sign the child’s apex KEY RRset. Other keys
MAY sign the child’s apex KEY RRset.
1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| key tag | algorithm | Digest type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| digest (length depends on type) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (SHA-1 digest is 20 bytes) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-|
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The key tag is calculated as specified in RFC 2535. Algorithm MUST
be allowed to sign DNS data. The digest type is an identifier for
the digest algorithm used. The digest is calculated over the
canonical name of the delegated domain name followed by the whole
RDATA of the KEY record (all four fields).
digest = hash( canonical FQDN on KEY RR | KEY_RR_rdata)
KEY_RR_rdata = Flags | Protocol | Algorithm | Public Key
Digest type value 0 is reserved, value 1 is SHA-1, and reserving
other types requires IETF standards action. For interoperability
reasons, keeping number of digest algorithms low is strongly
RECOMMENDED. The only reason to reserve additional digest types is
to increase security.
DS records MUST point to zone KEY records that are allowed to
authenticate DNS data. The indicated KEY records protocol field MUST
be set to 3; flag field bit 7 MUST be set to 1. The value of other
flag bits is not significant for the purposes of this document.
The size of the DS RDATA for type 1 (SHA-1) is 24 bytes, regardless
of key size. New digest types probably will have larger digests.
2.4.1. Justifications for Fields
The algorithm and key tag fields are present to allow resolvers to
quickly identify the candidate KEY records to examine. SHA-1 is a
strong cryptographic checksum: it is computationally infeasible for
an attacker to generate a KEY record that has the same SHA-1 digest.
Combining the name of the key and the key rdata as input to the
digest provides stronger assurance of the binding. Having the key
tag in the DS record adds greater assurance than the SHA-1 digest
alone, as there are now two different mapping functions.
This format allows concise representation of the keys that the child
will use, thus keeping down the size of the answer for the
delegation, reducing the probability of DNS message overflow. The
SHA-1 hash is strong enough to uniquely identify the key and is
similar to the PGP key footprint. The digest type field is present
for possible future expansion.
The DS record is well suited to listing trusted keys for islands of
security in configuration files.
2.5. Presentation Format of the DS Record
The presentation format of the DS record consists of three numbers
(key tag, algorithm, and digest type) followed by the digest itself
presented in hex:
example. DS 12345 3 1 123456789abcdef67890123456789abcdef67890
2.6. Transition Issues for Installed Base
No backwards compatibility with RFC 2535 is provided.
RFC 2535-compliant resolvers will assume that all DS-secured
delegations are locally secure. This is bad, but the DNSEXT Working
Group has determined that rather than dealing with both RFC 2535-
secured zones and DS-secured zones, a rapid adoption of DS is
preferable. Thus, the only option for early adopters is to upgrade
to DS as soon as possible.
2.6.1. Backwards compatibility with RFC 2535 and RFC 1035
This section documents how a resolver determines the type of
delegation.
RFC 1035 delegation (in parent) has:
RFC 1035 NS
RFC 2535 adds the following two cases:
Secure RFC 2535: NS + NXT + SIG(NXT)
NXT bit map contains: NS SIG NXT
Unsecure RFC 2535: NS + KEY + SIG(KEY) + NXT + SIG(NXT)
NXT bit map contains: NS SIG KEY NXT
KEY must be a NULL key.
DNSSEC with DS has the following two states:
Secure DS: NS + DS + SIG(DS)
NXT bit map contains: NS SIG NXT DS
Unsecure DS: NS + NXT + SIG(NXT)
NXT bit map contains: NS SIG NXT
It is difficult for a resolver to determine if a delegation is secure
RFC 2535 or unsecure DS. This could be overcome by adding a flag to
the NXT bit map, but only upgraded resolvers would understand this
flag, anyway. Having both parent and child signatures for a KEY
RRset might allow old resolvers to accept a zone as secure, but the
cost of doing this for a long time is much higher than just
prohibiting RFC 2535-style signatures at child zone apexes and
forcing rapid deployment of DS-enabled nameservers and resolvers.
RFC 2535 and DS can, in theory, be deployed in parallel, but this
would require resolvers to deal with RFC 2535 configurations forever.
This document obsoletes the NULL KEY in parent zones, which is a
difficult enough change that to cause a flag day.
2.7. KEY and corresponding DS record example
This is an example of a KEY record and the corresponding DS record.
dskey.example. KEY 256 3 1 (
AQPwHb4UL1U9RHaU8qP+Ts5bVOU1s7fYbj2b3CCbzNdj
4+/ECd18yKiyUQqKqQFWW5T3iVc8SJOKnueJHt/Jb/wt
) ; key id = 28668
DS 28668 1 1 49FD46E6C4B45C55D4AC69CBD3CD34AC1AFE51DE
3. Resolver
3.1. DS Example
To create a chain of trust, a resolver goes from trusted KEY to DS to
KEY.
Assume the key for domain "example." is trusted. Zone "example."
contains at least the following records:
example. SOA <soa stuff>
example. NS ns.example.
example. KEY <stuff>
example. NXT secure.example. NS SOA KEY SIG NXT
example. SIG(SOA)
example. SIG(NS)
example. SIG(NXT)
example. SIG(KEY)
secure.example. NS ns1.secure.example.
secure.example. DS tag=12345 alg=3 digest_type=1 <foofoo>
secure.example. NXT unsecure.example. NS SIG NXT DS
secure.example. SIG(NXT)
secure.example. SIG(DS)
unsecure.example NS ns1.unsecure.example.
unsecure.example. NXT example. NS SIG NXT
unsecure.example. SIG(NXT)
In zone "secure.example." following records exist:
secure.example. SOA <soa stuff>
secure.example. NS ns1.secure.example.
secure.example. KEY <tag=12345 alg=3>
secure.example. KEY <tag=54321 alg=5>
secure.example. NXT <nxt stuff>
secure.example. SIG(KEY) <key-tag=12345 alg=3>
secure.example. SIG(SOA) <key-tag=54321 alg=5>
secure.example. SIG(NS) <key-tag=54321 alg=5>
secure.example. SIG(NXT) <key-tag=54321 alg=5>
In this example, the private key for "example." signs the DS record
for "secure.example.", making that a secure delegation. The DS
record states which key is expected to sign the KEY RRset at
"secure.example.". Here "secure.example." signs its KEY RRset with
the KEY identified in the DS RRset, thus the KEY RRset is validated
and trusted.
This example has only one DS record for the child, but parents MUST
allow multiple DS records to facilitate key roll-over and multiple
KEY algorithms.
The resolver determines the security status of "unsecure.example." by
examining the parent zone’s NXT record for this name. The absence of
the DS bit indicates an unsecure delegation. Note the NXT record
SHOULD only be examined after verifying the corresponding signature.
3.2. Resolver Cost Estimates for DS Records
From a RFC 2535 recursive resolver point of view, for each delegation
followed to chase down an answer, one KEY RRset has to be verified.
Additional RRsets might also need to be verified based on local
policy (e.g., the contents of the NS RRset). Once the resolver gets
to the appropriate delegation, validating the answer might require
verifying one or more signatures. A simple A record lookup requires
at least N delegations to be verified and one RRset. For a DS-
enabled recursive resolver, the cost is 2N+1. For an MX record,
where the target of the MX record is in the same zone as the MX
record, the costs are N+2 and 2N+2, for RFC 2535 and DS,
respectively. In the case of a negative answer, the same ratios hold
true.
The recursive resolver has to do an extra query to get the DS record,
which will increase the overall cost of resolving this question, but
it will never be worse than chasing down NULL KEY records from the
parent in RFC 2535 DNSSEC.
DS adds processing overhead on resolvers and increases the size of
delegation answers, but much less than storing signatures in the
parent zone.
4. Security Considerations
This document proposes a change to the validation chain of KEY
records in DNSSEC. The change is not believed to reduce security in
the overall system. In RFC 2535 DNSSEC, the child zone has to
communicate keys to its parent and prudent parents will require some
authentication with that transaction. The modified protocol will
require the same authentication, but allows the child to exert more
local control over its own KEY RRset.
There is a remote possibility that an attacker could generate a valid
KEY that matches all the DS fields, of a specific DS set, and thus
forge data from the child. This possibility is considered
impractical, as on average more than
2 ^ (160 - <Number of keys in DS set>)
keys would have to be generated before a match would be found.
An attacker that wants to match any DS record will have to generate
on average at least 2^80 keys.
The DS record represents a change to the DNSSEC protocol and there is
an installed base of implementations, as well as textbooks on how to
set up secure delegations. Implementations that do not understand
the DS record will not be able to follow the KEY to DS to KEY chain
and will consider all zones secured that way as unsecure.
5. IANA Considerations
IANA has allocated an RR type code for DS from the standard RR type
space (type 43).
IANA has established a new registry for the DS RR type for digest
algorithms. Defined types are:
0 is Reserved,
1 is SHA-1.
Adding new reservations requires IETF standards action.
6. Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF’s procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
7. Acknowledgments
Over the last few years a number of people have contributed ideas
that are captured in this document. The core idea of using one key
to sign only the KEY RRset comes from discussions with Bill Manning
and Perry Metzger on how to put in a single root key in all
resolvers. Alexis Yushin, Brian Wellington, Sam Weiler, Paul Vixie,
Jakob Schlyter, Scott Rose, Edward Lewis, Lars-Johan Liman, Matt
Larson, Mark Kosters, Dan Massey, Olaf Kolman, Phillip Hallam-Baker,
Miek Gieben, Havard Eidnes, Donald Eastlake 3rd., Randy Bush, David
Blacka, Steve Bellovin, Rob Austein, Derek Atkins, Roy Arends, Mark
Andrews, Harald Alvestrand, and others have provided useful comments.
8. References
8.1. Normative References
[RFC1035] Mockapetris, P., "Domain Names - Implementation and
Specification", STD 13, RFC 1035, November 1987.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2535] Eastlake, D., "Domain Name System Security Extensions",
RFC 2535, March 1999.
[RFC3008] Wellington, B., "Domain Name System Security (DNSSEC)
Signing Authority", RFC 3008, November 2000.
[RFC3090] Lewis, E., "DNS Security Extension Clarification on Zone
Status", RFC 3090, March 2001.
[RFC3225] Conrad, D., "Indicating Resolver Support of DNSSEC", RFC
3225, December 2001.
[RFC3445] Massey, D. and S. Rose, "Limiting the scope of the KEY
Resource Record (RR)", RFC 3445, December 2002.
8.2. Informational References
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, July 1997.
[RFC3226] Gudmundsson, O., "DNSSEC and IPv6 A6 aware server/resolver
message size requirements", RFC 3226, December 2001.
9. Author’s Address
Olafur Gudmundsson
3821 Village Park Drive
Chevy Chase, MD, 20815
EMail: ds-rfc@ogud.com
10. Full Copyright Statement
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