Network Working Group S. Weiler
Internet-Draft SPARTA, Inc
Updates: [-records] [-protocol] March 7, 2005
(if approved)
Expires: September 8, 2005
Clarifications and Implementation Notes for DNSSECbis
draft-weiler-dnsext-dnssec-bis-updates-00
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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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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 (-00), feedback is particularly solicited on the
structure of the document and about what query type(s) should be used
to find delegation points (see Section 4).
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. Unknown DS Message Digest Algorithms
Section 5.2 of -protocol 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.
To paraphrase the above, when determining the security status of a
zone, a resolver 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, that text becomes:
a resolver discards any DS records listing unknown or unsupported
algorithms or using unknown or unsupported message digest algorithms.
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3. Private Algorithms
As discussed above, section 5.2 of -protocol 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 -records 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 2). 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
-protocol.
This clarification facilitates the broader use of private algorithms,
as suggested by [5] .
4. Finding Zone Cuts
As explained in Section 3.1.4.1 of -protocol, 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 -protocol suggests using NS queries for this purpose.
Appendix C.8 of -protocol suggests using DS queries.
*** Which is correct? Does it matter?
5. 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
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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.
Futhermore, note that the SEP bit setting has no effect on how a
DNSKEY may be used -- the validation process is specifically
prohibitted from using that bit by -records section 2.1.2. It
possible to use a DNSKEY without the SEP bit set as the sole secure
entry 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.
6. IANA Considerations
This document specifies no IANA Actions.
7. Security Considerations
8. References
8.1 Normative References
[1] Arends, R., Austein, R., Massey, D., Larson, M. and S. Rose,
"DNS Security Introduction and Requirements",
Internet-Draft draft-ietf-dnsext-dnssec-intro-13, October 2004.
[2] Arends, R., "Resource Records for the DNS Security Extensions",
Internet-Draft draft-ietf-dnsext-dnssec-records-11, October
2004.
[3] Arends, R., "Protocol Modifications for the DNS Security
Extensions",
Internet-Draft draft-ietf-dnsext-dnssec-protocol-09, October
2004.
[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
8.2 Informative References
[5] Blacka, D., "DNSSEC Experiments",
Internet-Draft draft-blacka-dnssec-experiments-00, December
2004.
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Author's Address
Samuel Weiler
SPARTA, Inc
7075 Samuel Morse Drive
Columbia, Maryland 21046
US
Email: weiler@tislabs.com
Appendix A. Acknowledgments
The lack of specificity about handling private algorithms, as
described in Section 3, was discovered by David Blacka.
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