Internet DRAFT - draft-ietf-dnsext-dnssec-2535typecode-change
INTERNET-DRAFT Samuel Weiler
Expires: June 2004 December 15, 2003
Updates: RFC 2535, [DS]
Legacy Resolver Compatibility for Delegation Signer
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other
documents at any time. It is inappropriate to use Internet-Drafts
as reference material or to cite them other than as "work in
The list of current Internet-Drafts can be accessed at
The list of Internet-Draft Shadow Directories can be accessed at
Comments should be sent to the author or to the DNSEXT WG mailing
As the DNS Security (DNSSEC) specifications have evolved, the
syntax and semantics of the DNSSEC resource records (RRs) have
changed. Many deployed nameservers understand variants of these
semantics. Dangerous interactions can occur when a resolver that
understands an earlier version of these semantics queries an
authoritative server that understands the new delegation signer
semantics, including at least one failure scenario that will cause
an unsecured zone to be unresolvable. This document changes the
type codes and mnemonics of the DNSSEC RRs (SIG, KEY, and NXT) to
avoid those interactions.
Changes between 05 and 06:
Signifigantly reworked the IANA section -- went back to one
Removed Diffie-Hellman from the list of zone-signing algorithms
(leaving only DSA, RSA/SHA-1, and private algorithms).
Added a DNSKEY flags field registry.
Changes between 04 and 05:
IESG approved publication.
Cleaned up an internal reference in the acknowledgements section.
Retained KEY and SIG for TKEY, too. Added TKEY (2930) reference.
Changed the names of both new registries. Added algorithm
mnemonics to the new zone signing algorithm registry. Minor
rewording in the IANA section for clarity.
Cleaned up formatting of references. Replaced unknown-rr draft
references with RFC3597. Bumped DS version number.
Changes between 03 and 04:
Clarified that RRSIG(0) may be defined by standards action.
Created a new algorithm registry and renamed the old algorithm
registry for SIG(0) only. Added references to the appropriate
crypto algorithm and format specifications.
Several minor rephrasings.
Changes between 02 and 03:
KEY (as well as SIG) retained for SIG(0) use only.
Changes between 01 and 02:
SIG(0) still uses SIG, not RRSIG. Added 2931 reference.
Domain names embedded in NSECs and RRSIGs are not compressible and
are not downcased. Added unknown-rrs reference (as informative).
Simplified the last paragraph of section 3 (NSEC doesn't always
signal a negative answer).
Changed the suggested type code assignments.
Added 2119 reference.
Added definitions of "unsecure delegation" and "unsecure referral",
since they're not clearly defined elsewhere.
Moved 2065 to informative references, not normative.
The DNSSEC protocol has been through many iterations whose syntax
and semantics are not completely compatible. This has occurred as
part of the ordinary process of proposing a protocol, implementing
it, testing it in the increasingly complex and diverse environment
of the Internet, and refining the definitions of the initial
Proposed Standard. In the case of DNSSEC, the process has been
complicated by DNS's criticality and wide deployment and the need
to add security while minimizing daily operational complexity.
A weak area for previous DNS specifications has been lack of detail
in specifying resolver behavior, leaving implementors largely on
their own to determine many details of resolver function. This,
combined with the number of iterations the DNSSEC spec has been
through, has resulted in fielded code with a wide variety of
behaviors. This variety makes it difficult to predict how a
protocol change will be handled by all deployed resolvers. The
risk that a change will cause unacceptable or even catastrophic
failures makes it difficult to design and deploy a protocol change.
One strategy for managing that risk is to structure protocol
changes so that existing resolvers can completely ignore input that
might confuse them or trigger undesirable failure modes.
This document addresses a specific problem caused by Delegation
Signer's [DS] introduction of new semantics for the NXT RR that are
incompatible with the semantics in RFC 2535 [RFC2535]. Answers
provided by DS-aware servers can trigger an unacceptable failure
mode in some resolvers that implement RFC 2535, which provides a
great disincentive to sign zones with DS. The changes defined in
this document allow for the incremental deployment of DS.
In this document, the term "unsecure delegation" means any
delegation for which no DS record appears at the parent. An
"unsecure referral" is an answer from the parent containing an NS
RRset and a proof that no DS record exists for that name.
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].
1.2 The Problem
Delegation Signer introduces new semantics for the NXT RR that are
incompatible with the semantics in RFC 2535. In RFC 2535, NXT
records were only required to be returned as part of a
non-existence proof. With DS, an unsecure referral returns, in
addition to the NS, a proof of non-existence of a DS RR in the form
of an NXT and SIG(NXT). RFC 2535 didn't specify how a resolver was
to interpret a response with both an NS and an NXT in the authority
section, RCODE=0, and AA=0. Some widely deployed 2535-aware
resolvers interpret any answer with an NXT as a proof of
non-existence of the requested record. This results in unsecure
delegations being invisible to 2535-aware resolvers and violates
the basic architectural principle that DNSSEC must do no harm --
the signing of zones must not prevent the resolution of unsecured
2. Possible Solutions
This section presents several solutions that were considered.
Section 3 describes the one selected.
2.1. Change SIG, KEY, and NXT type codes
To avoid the problem described above, legacy (RFC2535-aware)
resolvers need to be kept from seeing unsecure referrals that
include NXT records in the authority section. The simplest way to
do that is to change the type codes for SIG, KEY, and NXT.
The obvious drawback to this is that new resolvers will not be able
to validate zones signed with the old RRs. This problem already
exists, however, because of the changes made by DS, and resolvers
that understand the old RRs (and have compatibility issues with DS)
are far more prevalent than 2535-signed zones.
2.2. Change a subset of type codes
The observed problem with unsecure referrals could be addressed by
changing only the NXT type code or another subset of the type codes
that includes NXT. This has the virtue of apparent simplicity, but
it risks introducing new problems or not going far enough. It's
quite possible that more incompatibilities exist between DS and
earlier semantics. Legacy resolvers may also be confused by seeing
records they recognize (SIG and KEY) while being unable to find
NXTs. Although it may seem unnecessary to fix that which is not
obviously broken, it's far cleaner to change all of the type codes
at once. This will leave legacy resolvers and tools completely
blinded to DNSSEC -- they will see only unknown RRs.
2.3. Replace the DO bit
Another way to keep legacy resolvers from ever seeing DNSSEC
records with DS semantics is to have authoritative servers only
send that data to DS-aware resolvers. It's been proposed that
assigning a new EDNS0 flag bit to signal DS-awareness (tentatively
called "DA"), and having authoritative servers send DNSSEC data
only in response to queries with the DA bit set, would accomplish
this. This bit would presumably supplant the DO bit described in
This solution is sufficient only if all 2535-aware resolvers zero
out EDNS0 flags that they don't understand. If one passed through
the DA bit unchanged, it would still see the new semantics, and it
would probably fail to see unsecure delegations. Since it's
impractical to know how every DNS implementation handles unknown
EDNS0 flags, this is not a universal solution. It could, though,
be considered in addition to changing the RR type codes.
2.4. Increment the EDNS version
Another possible solution is to increment the EDNS version number
as defined in RFC 2671 [RFC2671], on the assumption that all
existing implementations will reject higher versions than they
support, and retain the DO bit as the signal for DNSSEC awareness.
This approach has not been tested.
2.5. Do nothing
There is a large deployed base of DNS resolvers that understand
DNSSEC as defined by the standards track RFC 2535 and RFC 2065
and, due to under specification in those documents, interpret any
answer with an NXT as a non-existence proof. So long as that is
the case, zone owners will have a strong incentive to not sign any
zones that contain unsecure delegations, lest those delegations be
invisible to such a large installed base. This will dramatically
slow DNSSEC adoption.
Unfortunately, without signed zones there's no clear incentive for
operators of resolvers to upgrade their software to support the new
version of DNSSEC, as defined in [DS]. Historical data suggests
that resolvers are rarely upgraded, and that old nameserver code
Rather than wait years for resolvers to be upgraded through natural
processes before signing zones with unsecure delegations,
addressing this problem with a protocol change will immediately
remove the disincentive for signing zones and allow widespread
deployment of DNSSEC.
3. Protocol changes
This document changes the type codes of SIG, KEY, and NXT. This
approach is the cleanest and safest of those discussed above,
largely because the behavior of resolvers that receive unknown type
codes is well understood. This approach has also received the most
To avoid operational confusion, it's also necessary to change the
mnemonics for these RRs. DNSKEY will be the replacement for KEY,
with the mnemonic indicating that these keys are not for
application use, per [RFC3445]. RRSIG (Resource Record SIGnature)
will replace SIG, and NSEC (Next SECure) will replace NXT. These
new types completely replace the old types, except that SIG(0)
[RFC2931] and TKEY [RFC2930] will continue to use SIG and KEY.
The new types will have exactly the same syntax and semantics as
specified for SIG, KEY, and NXT in RFC 2535 and [DS] except for
1) Consistent with [RFC3597], domain names embedded in
RRSIG and NSEC RRs MUST NOT be compressed,
2) Embedded domain names in RRSIG and NSEC RRs are not downcased
for purposes of DNSSEC canonical form and ordering nor for
equality comparison, and
3) An RRSIG with a type-covered field of zero has undefined
semantics. The meaning of such a resource record may only be
defined by IETF Standards Action.
If a resolver receives the old types, it SHOULD treat them as
unknown RRs and SHOULD NOT assign any special meaning to them or
give them any special treatment. It MUST NOT use them for DNSSEC
validations or other DNS operational decision making. For example,
a resolver MUST NOT use DNSKEYs to validate SIGs or use KEYs to
validate RRSIGs. If SIG, KEY, or NXT RRs are included in a zone,
they MUST NOT receive special treatment. As an example, if a SIG
is included in a signed zone, there MUST be an RRSIG for it.
Authoritative servers may wish to give error messages when loading
zones containing SIG or NXT records (KEY records may be included
for SIG(0) or TKEY).
As a clarification to previous documents, some positive responses,
particularly wildcard proofs and unsecure referrals, will contain
NSEC RRs. Resolvers MUST NOT treat answers with NSEC RRs as
negative answers merely because they contain an NSEC.
4. IANA Considerations
4.1 DNS Resource Record Types
This document updates the IANA registry for DNS Resource Record
Types by assigning types 46, 47, and 48 to the RRSIG, NSEC, and
DNSKEY RRs, respectively.
Types 24 and 25 (SIG and KEY) are retained for SIG(0) [RFC2931] and
TKEY [RFC2930] use only.
Type 30 (NXT) should be marked as Obsolete.
4.2 DNS Security Algorithm Numbers
To allow zone signing (DNSSEC) and transaction security mechanisms
(SIG(0) and TKEY) to use different sets of algorithms, the existing
"DNS Security Algorithm Numbers" registry is modified to include
the applicability of each algorithm. Specifically, two new columns
are added to the registry, showing whether each algorithm may be
used for zone signing, transaction security mechanisms, or both.
Only algorithms usable for zone signing may be used in DNSKEY,
RRSIG, and DS RRs. Only algorithms usable for SIG(0) and/or TSIG
may be used in SIG and KEY RRs.
All currently defined algorithms remain usable for transaction
security mechanisms. Only RSA/SHA-1, DSA/SHA-1, and private
algorithms (types 253 and 254) may be used for zone signing. Note
that the registry does not contain the requirement level of each
algorithm, only whether or not an algorithm may be used for the
given purposes. For example, RSA/MD5, while allowed for
transaction security mechanisms, is NOT RECOMMENDED, per RFC3110.
Additionally, the presentation format algorithm mnemonics from
RFC2535 Section 7 are added to the registry. This document assigns
RSA/SHA-1 the mnemonic RSASHA1.
As before, assignment of new algorithms in this registry requires
IETF Standards Action. Additionally, modification of algorithm
mnemonics or applicability requires IETF Standards Action.
Documents defining a new algorithm must address the applicability
of the algorithm and should assign a presentation mnemonic to the
4.3 DNSKEY Flags
Like the KEY resource record, DNSKEY contains a 16-bit flags field.
This document creates a new registry for the DNSKEY flags field.
Initially, this registry only contains an assignment for bit 7 (the
ZONE bit). Bits 0-6 and 8-15 are available for assignment by IETF
4.4 DNSKEY Protocol Octet
Like the KEY resource record, DNSKEY contains an eight bit protocol
field. The only defined value for this field is 3 (DNSSEC). No
other values are allowed, hence no IANA registry is needed for this
5. Security Considerations
The changes introduced here do not materially affect security.
The implications of trying to use both new and legacy types
together are not well understood, and attempts to do so would
probably lead to unintended and dangerous results.
Changing type codes will leave code paths in legacy resolvers that
are never exercised. Unexercised code paths are a frequent source
of security holes, largely because those code paths do not get
Doing nothing, as described in section 2.5, will slow DNSSEC
deployment. While this does not decrease security, it also fails
to increase it.
6. Normative references
[RFC2535] Eastlake, D., "Domain Name System Security Extensions",
RFC 2535, March 1999.
[DS] Gudmundsson, O., "Delegation Signer Resource Record",
draft-ietf-dnsext-delegation-signer-15.txt, work in
progress, June 2003.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2931] Eastlake, D., "DNS Request and Transaction Signatures
(SIG(0)s)", RFC 2931, September 2000.
[RFC2930] Eastlake, D., "Secret Key Establishment for DNS (TKEY
RR)", RFC 2930, September 2000.
[RFC2536] Eastlake, D., "DSA KEYs and SIGs in the Domain Name
System (DNS)", RFC 2436, March 1999.
[RFC2539] Eastlake, D., "Storage of Diffie-Hellman Keys in the
Domain Name System (DNS)", RFC 2539, March 1999.
[RFC3110] Eastlake, D., "RSA/SHA-1 SIGs and RSA KEYs in the
Domain Name System (DNS)", RFC 3110, May 2001.
7. Informative References
[RFC2065] Eastlake, D. and C. Kaufman, "Domain Name System Security
Extensions", RFC 2065, January 1997.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC
2671, August 1999.
[RFC3225] Conrad, D., "Indicating Resolver Support of DNSSEC", RFC
3225, December 2001.
[RFC2929] Eastlake, D., E. Brunner-Williams, and B. Manning,
"Domain Name System (DNS) IANA Considerations", BCP 42,
RFC 2929, September 2000.
[RFC3445] Massey, D., and S. Rose, "Limiting the Scope of the KEY
Resource Record (RR)", RFC 3445, December 2002.
[RFC3597] Gustafsson, A., "Handling of Unknown DNS Resource
Record (RR) Types", RFC 3597, September 2003.
The changes introduced here and the analysis of alternatives had
many contributors. With apologies to anyone overlooked, those
include: Micheal Graff, John Ihren, Olaf Kolkman, Mark Kosters, Ed
Lewis, Bill Manning, and Suzanne Woolf.
Thanks to Jakob Schlyter and Mark Andrews for identifying the
incompatibility described in section 1.2.
In addition to the above, the author would like to thank Scott
Rose, Olafur Gudmundsson, and Sandra Murphy for their substantive
9. Author's Address
7075 Samuel Morse Drive
Columbia, MD 21046