draft-ietf-dnsext-dnssec-bis-updates-01.txt [plain text]
Network Working Group S. Weiler
Internet-Draft SPARTA, Inc
Updates: 4034, 4035 (if approved) May 23, 2005
Expires: November 24, 2005
Clarifications and Implementation Notes for DNSSECbis
draft-ietf-dnsext-dnssec-bis-updates-01
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document is a collection of minor technical clarifications to
the DNSSECbis document set. It is meant to serve as a resource to
implementors as well as an interim repository of possible DNSSECbis
errata.
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Proposed additions in future versions
An index sorted by the section of DNSSECbis being clarified.
A list of proposed protocol changes being made in other documents,
such as NSEC3 and Epsilon. This document would not make those
changes, merely provide an index into the documents that are making
changes.
Changes between -00 and -01
Document significantly restructured.
Added section on QTYPE=ANY.
Changes between personal submission and first WG draft
Added Section 2.1 based on namedroppers discussions from March 9-10,
2005.
Added Section 3.4, Section 3.3, Section 4.3, and Section 2.2.
Added the DNSSECbis RFC numbers.
Figured out the confusion in Section 4.1.
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Table of Contents
1. Introduction and Terminology . . . . . . . . . . . . . . . . . 4
1.1 Structure of this Document . . . . . . . . . . . . . . . . 4
1.2 Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Significant Concerns . . . . . . . . . . . . . . . . . . . . . 4
2.1 Clarifications on Non-Existence Proofs . . . . . . . . . . 4
2.2 Empty Non-Terminal Proofs . . . . . . . . . . . . . . . . 5
2.3 Validating Responses to an ANY Query . . . . . . . . . . . 5
3. Interoperability Concerns . . . . . . . . . . . . . . . . . . 5
3.1 Unknown DS Message Digest Algorithms . . . . . . . . . . . 5
3.2 Private Algorithms . . . . . . . . . . . . . . . . . . . . 6
3.3 Caution About Local Policy and Multiple RRSIGs . . . . . . 6
3.4 Key Tag Calculation . . . . . . . . . . . . . . . . . . . 7
4. Minor Corrections and Clarifications . . . . . . . . . . . . . 7
4.1 Finding Zone Cuts . . . . . . . . . . . . . . . . . . . . 7
4.2 Clarifications on DNSKEY Usage . . . . . . . . . . . . . . 7
4.3 Errors in Examples . . . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1 Normative References . . . . . . . . . . . . . . . . . . . 8
7.2 Informative References . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 9
A. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . . 11
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1. Introduction and Terminology
This document lists some minor clarifications and corrections to
DNSSECbis, as described in [1], [2], and [3].
It is intended to serve as a resource for implementors and as a
repository of items that need to be addressed when advancing the
DNSSECbis documents from Proposed Standard to Draft Standard.
In this version (-01 of the WG document), feedback is particularly
solicited on the structure of the document and whether the text in
the recently added sections is correct and sufficient.
Proposed substantive additions to this document should be sent to the
namedroppers mailing list as well as to the editor of this document.
The editor would greatly prefer text suitable for direct inclusion in
this document.
1.1 Structure of this Document
The clarifications to DNSSECbis are sorted according to the editor's
impression of their importance, starting with ones which could, if
ignored, lead to security and stability problems and progressing down
to clarifications that are likely to have little operational impact.
Mere typos and awkward phrasings are not addressed unless they could
lead to misinterpretation of the DNSSECbis documents.
1.2 Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [4].
2. Significant Concerns
This section provides clarifications that, if overlooked, could lead
to security issues or major interoperability problems.
2.1 Clarifications on Non-Existence Proofs
RFC4035 Section 5.4 slightly underspecifies the algorithm for
checking non-existence proofs. In particular, the algorithm there
might incorrectly allow the NSEC from the parent side of a zone cut
to prove the non-existence of either other RRs at that name in the
child zone or other names in the child zone. It might also allow a
NSEC at the same name as a DNAME to prove the non-existence of names
beneath that DNAME.
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A parent-side delegation NSEC (one with the NS bit set, but no SOA
bit set, and with a singer field that's shorter than the owner name)
must not be used to assume non-existence of any RRs below that zone
cut (both RRs at that ownername and at ownernames with more leading
labels, no matter their content). Similarly, an NSEC with the DNAME
bit set must not be used to assume the non-existence of any
descendant of that NSEC's owner name.
2.2 Empty Non-Terminal Proofs
To be written, based on Roy Arends' May 11th message to namedroppers.
2.3 Validating Responses to an ANY Query
RFC4035 does not address now to validate responses when QTYPE=*. As
described in Section 6.2.2 of RFC1034, a proper response to QTYPE=*
may include a subset of the RRsets at a given name -- it is not
necessary to include all RRsets at the QNAME in the response.
When validating a response to QTYPE=*, validate all received RRsets
that match QNAME and QCLASS. If any of those RRsets fail validation,
treat the answer as Bogus. If there are no RRsets matching QNAME and
QCLASS, validate that fact using the rules in RFC4035 Section 5.4 (as
clarified in this document). To be clear, a validator must not
insist on receiving all records at the QNAME in response to QTYPE=*.
3. Interoperability Concerns
3.1 Unknown DS Message Digest Algorithms
Section 5.2 of RFC4035 includes rules for how to handle delegations
to zones that are signed with entirely unsupported algorithms, as
indicated by the algorithms shown in those zone's DS RRsets. It does
not explicitly address how to handle DS records that use unsupported
message digest algorithms. In brief, DS records using unknown or
unsupported message digest algorithms MUST be treated the same way as
DS records referring to DNSKEY RRs of unknown or unsupported
algorithms.
The existing text says:
If the validator does not support any of the algorithms listed
in an authenticated DS RRset, then the resolver has no supported
authentication path leading from the parent to the child. The
resolver should treat this case as it would the case of an
authenticated NSEC RRset proving that no DS RRset exists, as
described above.
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To paraphrase the above, when determining the security status of a
zone, a validator discards (for this purpose only) any DS records
listing unknown or unsupported algorithms. If none are left, the
zone is treated as if it were unsigned.
Modified to consider DS message digest algorithms, a validator also
discards any DS records using unknown or unsupported message digest
algorithms.
3.2 Private Algorithms
As discussed above, section 5.2 of RFC4035 requires that validators
make decisions about the security status of zones based on the public
key algorithms shown in the DS records for those zones. In the case
of private algorithms, as described in RFC4034 Appendix A.1.1, the
eight-bit algorithm field in the DS RR is not conclusive about what
algorithm(s) is actually in use.
If no private algorithms appear in the DS set or if any supported
algorithm appears in the DS set, no special processing will be
needed. In the remaining cases, the security status of the zone
depends on whether or not the resolver supports any of the private
algorithms in use (provided that these DS records use supported hash
functions, as discussed in Section 3.1). In these cases, the
resolver MUST retrieve the corresponding DNSKEY for each private
algorithm DS record and examine the public key field to determine the
algorithm in use. The security-aware resolver MUST ensure that the
hash of the DNSKEY RR's owner name and RDATA matches the digest in
the DS RR. If they do not match, and no other DS establishes that
the zone is secure, the referral should be considered BAD data, as
discussed in RFC4035.
This clarification facilitates the broader use of private algorithms,
as suggested by [5].
3.3 Caution About Local Policy and Multiple RRSIGs
When multiple RRSIGs cover a given RRset, RFC4035 Section 5.3.3
suggests that "the local resolver security policy determines whether
the resolver also has to test these RRSIG RRs and how to resolve
conflicts if these RRSIG RRs lead to differing results." In most
cases, a resolver would be well advised to accept any valid RRSIG as
sufficient. If the first RRSIG tested fails validation, a resolver
would be well advised to try others, giving a successful validation
result if any can be validated and giving a failure only if all
RRSIGs fail validation.
If a resolver adopts a more restrictive policy, there's a danger that
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properly-signed data might unnecessarily fail validation, perhaps
because of cache timing issues. Furthermore, certain zone management
techniques, like the Double Signature Zone-signing Key Rollover
method described in section 4.2.1.2 of [6] might not work reliably.
3.4 Key Tag Calculation
RFC4034 Appendix B.1 incorrectly defines the Key Tag field
calculation for algorithm 1. It correctly says that the Key Tag is
the most significant 16 of the least significant 24 bits of the
public key modulus. However, RFC4034 then goes on to incorrectly say
that this is 4th to last and 3rd to last octets of the public key
modulus. It is, in fact, the 3rd to last and 2nd to last octets.
4. Minor Corrections and Clarifications
4.1 Finding Zone Cuts
Appendix C.8 of RFC4035 discusses sending DS queries to the servers
for a parent zone. To do that, a resolver may first need to apply
special rules to discover what those servers are.
As explained in Section 3.1.4.1 of RFC4035, security-aware name
servers need to apply special processing rules to handle the DS RR,
and in some situations the resolver may also need to apply special
rules to locate the name servers for the parent zone if the resolver
does not already have the parent's NS RRset. Section 4.2 of RFC4035
specifies a mechanism for doing that.
4.2 Clarifications on DNSKEY Usage
Questions of the form "can I use a different DNSKEY for signing the
X" have occasionally arisen.
The short answer is "yes, absolutely". You can even use a different
DNSKEY for each RRset in a zone, subject only to practical limits on
the size of the DNSKEY RRset. However, be aware that there is no way
to tell resolvers what a particularly DNSKEY is supposed to be used
for -- any DNSKEY in the zone's signed DNSKEY RRset may be used to
authenticate any RRset in the zone. For example, if a weaker or less
trusted DNSKEY is being used to authenticate NSEC RRsets or all
dynamically updated records, that same DNSKEY can also be used to
sign any other RRsets from the zone.
Furthermore, note that the SEP bit setting has no effect on how a
DNSKEY may be used -- the validation process is specifically
prohibited from using that bit by RFC4034 section 2.1.2. It possible
to use a DNSKEY without the SEP bit set as the sole secure entry
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point to the zone, yet use a DNSKEY with the SEP bit set to sign all
RRsets in the zone (other than the DNSKEY RRset). It's also possible
to use a single DNSKEY, with or without the SEP bit set, to sign the
entire zone, including the DNSKEY RRset itself.
4.3 Errors in Examples
The text in RFC4035 Section C.1 refers to the examples in B.1 as
"x.w.example.com" while B.1 uses "x.w.example". This is painfully
obvious in the second paragraph where it states that the RRSIG labels
field value of 3 indicates that the answer was not the result of
wildcard expansion. This is true for "x.w.example" but not for
"x.w.example.com", which of course has a label count of 4
(antithetically, a label count of 3 would imply the answer was the
result of a wildcard expansion).
The first paragraph of RFC4035 Section C.6 also has a minor error:
the reference to "a.z.w.w.example" should instead be "a.z.w.example",
as in the previous line.
5. IANA Considerations
This document specifies no IANA Actions.
6. Security Considerations
This document does not make fundamental changes to the DNSSEC
protocol, as it was generally understood when DNSSECbis was
published. It does, however, address some ambiguities and omissions
in those documents that, if not recognized and addressed in
implementations, could lead to security failures. In particular, the
validation algorithm clarifications in Section 2 are critical for
preserving the security properties DNSSEC offers. Furthermore,
failure to address some of the interoperability concerns in Section 3
could limit the ability to later change or expand DNSSEC, including
by adding new algorithms.
7. References
7.1 Normative References
[1] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"DNS Security Introduction and Requirements", RFC 4033,
March 2005.
[2] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"Resource Records for the DNS Security Extensions", RFC 4034,
March 2005.
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[3] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"Protocol Modifications for the DNS Security Extensions",
RFC 4035, March 2005.
[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
7.2 Informative References
[5] Blacka, D., "DNSSEC Experiments",
draft-blacka-dnssec-experiments-00 (work in progress),
December 2004.
[6] Gieben, R. and O. Kolkman, "DNSSEC Operational Practices",
draft-ietf-dnsop-dnssec-operational-practices-04 (work in
progress), May 2005.
Author's Address
Samuel Weiler
SPARTA, Inc
7075 Samuel Morse Drive
Columbia, Maryland 21046
US
Email: weiler@tislabs.com
Appendix A. Acknowledgments
The editor is extremely grateful to those who, in addition to finding
errors and omissions in the DNSSECbis document set, have provided
text suitable for inclusion in this document.
The lack of specificity about handling private algorithms, as
described in Section 3.2, and the lack of specificity in handling ANY
queries, as described in Section 2.3, were discovered by David
Blacka.
The error in algorithm 1 key tag calculation, as described in
Section 3.4, was found by Abhijit Hayatnagarkar. Donald Eastlake
contributed text for Section 3.4.
The bug relating to delegation NSEC RR's in Section 2.1 was found by
Roy Badami. Roy Arends found the related problem with DNAME.
The errors in the RFC4035 examples were found by Roy Arends, who also
contributed text for Section 4.3 of this document.
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The editor would like to thank Olafur Gudmundsson and Scott Rose for
their substantive comments on the text of this document.
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