DNS Extensions Working Group R. Arends
Internet-Draft Nominet UK
Intended status: Informational P. Koch
Expires: January 15, 2009 DENIC eG
J. Schlyter
Kirei AB
July 14, 2008
Evaluating DNSSEC Transition Mechanisms
draft-ietf-dnsext-dnssec-trans-06.txt
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Abstract
This document collects and summarizes different proposals for
alternative and additional strategies for authenticated denial in DNS
responses, evaluates these proposals and gives a recommendation for a
way forward. It is a snapshot of the DNSEXT working group discussion
of June 2004.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Transition Mechanisms . . . . . . . . . . . . . . . . . . . . 3
2.1. Mechanisms With Need of Updating DNSSEC-bis . . . . . . . 4
2.1.1. Dynamic NSEC Synthesis . . . . . . . . . . . . . . . . 4
2.1.2. Add Versioning/Subtyping to Current NSEC . . . . . . . 5
2.1.3. Type Bit Map NSEC Indicator . . . . . . . . . . . . . 6
2.1.4. New Apex Type . . . . . . . . . . . . . . . . . . . . 7
2.1.5. NSEC White Lies . . . . . . . . . . . . . . . . . . . 8
2.1.6. NSEC Optional via DNSKEY Flag . . . . . . . . . . . . 9
2.1.7. New Answer Pseudo RR Type . . . . . . . . . . . . . . 9
2.2. Mechanisms Without Need of Updating DNSSEC-bis . . . . . . 10
2.2.1. Partial Type-code and Signal Rollover . . . . . . . . 10
2.2.2. A Complete Type-code and Signal Rollover . . . . . . . 11
2.2.3. Unknown (New) Algorithm in DS, DNSKEY, and RRSIG . . . 12
2.2.4. Unknown (New) Hash Algorithm in DS . . . . . . . . . . 13
3. Recommendation . . . . . . . . . . . . . . . . . . . . . . . . 13
4. Security Considerations . . . . . . . . . . . . . . . . . . . 14
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.1. Normative References . . . . . . . . . . . . . . . . . . . 14
7.2. Informative References . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . . . . 16
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1. Introduction
This report shall document the process of dealing with the NSEC zone
walking problem late in the Last Call for [RFC4033], [RFC4034], and
[RFC4035] (further referred to as DNSSEC-bis, with the obsoleted
[RFC2535] representing DNSSEC). It preserves some of the discussion
that took place in the DNSEXT WG during the first half of June 2004
as well as some additional ideas that came up subsequently.
This is an edited excerpt of the chairs' mail to the WG:
The working group consents on not including NSEC-alt in the
DNSSEC-bis documents. The working group considers to take up
"prevention of zone enumeration" as a work item.
There may be multiple mechanisms to allow for co-existence with
DNSSEC-bis. The chairs allow the working group a little over a
week (up to June 12, 2004) to come to consensus on a possible
modification to the document to enable gentle rollover. If that
consensus cannot be reached the DNSSEC-bis documents will go out
as-is.
To ease the process of getting consensus, a summary of the proposed
solutions and analysis of the pros and cons were written during the
weekend.
This summary includes:
An inventory of the proposed mechanisms to make a transition to
future work on authenticated denial of existence.
List the known Pros and Cons, possibly provide new arguments, and
possible security considerations of these mechanisms.
Provide a recommendation on a way forward that is least disruptive
to the DNSSEC-bis specifications as they stand and keep an open
path to other methods for authenticated denial of existence.
The descriptions of the proposals in this document are coarse and do
not cover every detail necessary for implementation. In any case,
documentation and further study is needed before implementation
and/or deployment, including those which seem to be solely
operational in nature.
2. Transition Mechanisms
In the light of earlier discussions and past proposals, we have found
several ways to allow for transition to future expansion of
authenticated denial. We tried to illuminate the paths and pitfalls
in these ways forward. Some proposals lead to a versioning of
DNSSEC, where DNSSEC-bis may co-exist with a future DNSSEC-ter, other
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proposals are 'clean' but may cause delay, while again others may be
plain hacks.
Some paths do not introduce versioning, and might require the current
DNSSEC-bis documents to be fully updated to allow for extensions to
authenticated denial mechanisms. Other paths introduce versioning
and do not (or minimally) require DNSSEC-bis documents to be updated,
allowing DNSSEC-bis to be deployed, while future versions can be
drafted independent from or partially depending on DNSSEC-bis.
2.1. Mechanisms With Need of Updating DNSSEC-bis
Mechanisms in this category demand updates to the DNSSEC-bis document
set.
2.1.1. Dynamic NSEC Synthesis
This proposal assumes that NSEC RRs and the authenticating RRSIG will
be generated dynamically to just cover the (non existent) query name.
The owner name is (the) one preceding the name queried for, the Next
Owner Name Field has the value of the Query Name Field + 1 (first
successor in canonical ordering). A separate key (the normal ZSK or
a separate ZSK per authoritative server) would be used for RRSIGs on
NSEC RRs. This is a defense against enumeration, though it has the
presumption of online signing.
2.1.1.1. Coexistence and Migration
There is no change in interpretation other than that the next owner
name might or might not exist.
2.1.1.2. Limitations
This introduces an unbalanced cost between query and response
generation due to dynamic generation of signatures.
2.1.1.3. Amendments to DNSSEC-bis
The current DNSSEC-bis documents might need to be updated to indicate
that the next owner name might not be an existing name in the zone.
This is not a real change to the spec since implementers have been
warned not to synthesize negative responses with previously cached
NSEC records. A specific bit to identify the dynamic signature
generating key might be useful as well, to prevent it from being used
to fake positive data, i.e., to limit the damage of a compromise of
the online key.
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2.1.1.4. Cons
Unbalanced cost may be abused for Denial of Service (DoS) attacks on
the synthesizing name servers. Also, this method requires all
authoritative servers to have access to a private key. While dynamic
synthesis protects against enumeration, it is not really a path for
versioning.
2.1.1.5. Pros
Only a minimal amendment to DNSSEC-bis is needed to allow "dangling"
pointers in an NSEC RR. However, implementations are not allowed to
exploit the additional knowledge that NSEC RRs provide anyway, so
this amendment is more formal in nature than actually having an
influence on complying implementations.
2.1.2. Add Versioning/Subtyping to Current NSEC
This proposal introduces versioning for the NSEC RR type (a.k.a.
subtyping) by adding a (one octet) version field to the NSEC RDATA.
Version number 0 is assigned to the current (DNSSEC-bis) meaning,
making this a 'Must Be Zero' (MBZ) for the to-be-published document
set.
2.1.2.1. Coexistence and Migration
Since the versioning is done inside the NSEC RR, different versions
may coexist in a zone. However, depending on future methods, that
may or may not be useful. Resolvers cannot ask for specific NSEC
versions but may be able to indicate version support by means of a
to-be-defined EDNS option bit.
2.1.2.2. Limitations
There are no technical limitations, though introducing this method
will cause delay to allow testing of the (currently unknown) new NSEC
interpretation.
Since the versioning and signaling is done inside the NSEC RR, future
methods will likely be restricted to a single RR type for
authenticated denial (as opposed to, e.g., NSEC-alt, which currently
proposes three RR types).
2.1.2.3. Amendments to DNSSEC-bis
Versioning or subtyping would require a full update of the current
DNSSEC-bis documents to provide for new fields in NSEC, including the
need to specify client behavior in response to unknown field values.
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2.1.2.4. Cons
Although this is a clear and clean path without versioning DNSSEC as
a whole, it would take some time to design, gain consensus, update
the current DNSSEC-bis document set, test and implement a new DNS
record type for authenticated denial.
2.1.2.5. Pros
NSEC versioning does not introduce an iteration to DNSSEC while
providing a clear and clean migration strategy.
2.1.3. Type Bit Map NSEC Indicator
Bits in the type-bit-map are reused or allocated to signify the
interpretation of NSEC.
This proposal assumes that future extensions make use of the existing
NSEC RDATA syntax, while it may need to change the interpretation of
the RDATA or introduce an alternative denial mechanism, invoked by
the specific type-bit-map-bits.
2.1.3.1. Coexistence and migration
Old and new NSEC meaning could coexist, depending how the signaling
would be defined. The bits for NXT, KEY, SIG or other outdated RR
types are available as well as those covering meta/query types or
types to be specifically allocated.
2.1.3.2. Limitations
This mechanism uses an NSEC field that was not designed for that
purpose. Similar methods were discussed during the Opt-In discussion
and the Silly-State discussion.
2.1.3.3. Amendments to DNSSEC-bis
The specific type-bit-map-bits must be allocated and they need to be
specified as 'Must Be Zero' (MBZ) when used for standard (DNSSEC-bis)
interpretation. Also, behaviour of the resolver and validator must
be specified in case unknown values are encountered for the MBZ
field. Currently the protocol document specifies that the validator
must ignore the setting of the NSEC and the RRSIG bits, while other
bits are only used for the specific purpose of the type-bit-map
field.
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2.1.3.4. Cons
Overloading the meaning of the type-bit-map is a straightforward
hack. The type-bit-map was not only not designed for this purpose,
but the text in section 5.4 of [RFC4035] was put in place to
explicitly prevent this usage.
2.1.3.5. Pros
No change is needed to the on-the-wire protocol as specified in the
current DNSSEC-bis document set.
2.1.4. New Apex Type
This introduces a new Apex type (parallel to the zone's SOA)
indicating the DNSSEC version (or authenticated denial) used in or
for this zone.
2.1.4.1. Coexistence and Migration
Depending on the design of this new RR type multiple denial
mechanisms may coexist in a zone. Old validators will not understand
and thus ignore the new type, so interpretation of the new NSEC
scheme may fail, negative responses may appear 'bogus'.
2.1.4.2. Limitations
A record of this kind is likely to carry additional feature/
versioning indications unrelated to the current question of
authenticated denial.
2.1.4.3. Amendments to DNSSEC-bis
The current DNSSEC-bis documents need to be updated to indicate that
the absence of this type indicates DNSSEC-bis, and that the (mere)
presence of this type indicated unknown versions.
2.1.4.4. Cons
The only other 'zone' or 'apex' record is the SOA record. Adding
more RRs to the zone apex bloats QTYPE ANY responses for this apex.
Even though the proposal is not new, it is yet unknown how it might
fulfill authenticated denial extensions. This new RR type would only
provide for a generalized signaling mechanism, not the new
authenticated denial scheme. Since it is likely to be general in
nature, due to this generality consensus is not to be reached soon.
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2.1.4.5. Pros
This approach would allow for a lot of other per zone information to
be transported or signaled in band to both (slave) servers and
resolvers.
2.1.5. NSEC White Lies
This proposal disables one part of NSEC (the pointer part) by means
of a special target (root, apex, owner, ...), leaving intact only the
ability to authenticate denial of existence of RR sets, not denial of
existence of domain names (NXDOMAIN). It may be necessary to have
one working NSEC to prove the absence of a wildcard.
2.1.5.1. Coexistence and Migration
The NSEC target can be specified per RR, so standard NSEC and 'white
lie' NSEC can coexist in a zone. There is no need for migration
because no versioning is introduced or intended.
2.1.5.2. Limitations
This proposal breaks the protocol and is applicable to certain types
of zones only (no wildcard, no multi-label names, delegation only).
Most of the burden is put on the resolver side and operational
consequences are yet to be studied.
2.1.5.3. Amendments to DNSSEC-bis
The current DNSSEC-bis documents need to be updated to indicate that
the NXDOMAIN responses may be insecure.
2.1.5.4. Cons
Strictly speaking this breaks the protocol and doesn't really satisfy
the requirements for authenticated denial of existence. Security
implications need to be carefully documented: search path problems
(forged denial of existence may lead to wrong expansion of non-FQDNs
[RFC1535]) and replay attacks to deny existence of records. In
addition, this does not provide for a versioning or signalling
scheme.
2.1.5.5. Pros
Solves the enumeration problem without the need of additional RR
types.
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2.1.6. NSEC Optional via DNSKEY Flag
A new DNSKEY Flag may be defined to declare NSEC optional per zone.
2.1.6.1. Coexistence and Migration
Current resolvers/validators will not understand the Flag bit and
will have to treat negative responses as bogus. Otherwise, no
migration path is needed since NSEC is simply turned off.
2.1.6.2. Limitations
NSEC can only be made completely optional at the cost of being unable
to prove unsecure delegations (absence of a DS RR). An almost
identical approach would just disable authenticated denial for non-
existence of nodes.
2.1.6.3. Amendments to DNSSEC-bis
New DNSKEY Flag to be defined. Resolver/Validator behaviour needs to
be specified in the light of absence of authenticated denial.
2.1.6.4. Cons
DNSSEC-bis less authenticated denial doesn't fully meet the
requirements and breaks the DNSSEC protocol by not fully covering the
threat model. Existing implementations will be confused.
Operational consequences need to be studied.
2.1.6.5. Pros
Positive responses can still be validated.
2.1.7. New Answer Pseudo RR Type
A new pseudo RR type may be defined that will be dynamically created
(and signed) by the responding authoritative server. The RR in the
response will cover the QNAME, QCLASS and QTYPE and will authenticate
both denial of existence of name (NXDOMAIN) or RRset.
2.1.7.1. Coexistence and Migration
Current resolvers/validators will not understand the pseudo RR and
will thus not be able to process negative responses so testified. A
signaling or solicitation method would have to be specified.
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2.1.7.2. Limitations
This method can only be used with online keys and online signing
capacity.
2.1.7.3. Amendments to DNSSEC-bis
Signaling method needs to be defined.
2.1.7.4. Cons
Keys have to be held and processed online with all security
implications. An additional flag for those keys identifying them as
online or negative answer only keys should be considered, for the
same reasons given in Section 2.1.1.
2.1.7.5. Pros
Expands DNSSEC authentication to the RCODE.
2.2. Mechanisms Without Need of Updating DNSSEC-bis
2.2.1. Partial Type-code and Signal Rollover
Carefully crafted type code/signal rollover to define a new
authenticated denial space that extends/replaces DNSSEC-bis
authenticated denial space. This particular path is illuminated by
Paul Vixie in a Message-Id <20040602070859.0F50913951@sa.vix.com>
posted to <namedroppers@ops.ietf.org> 2004-06-02.
2.2.1.1. Coexistence and Migration
To protect the current resolver for future versions, a new DNSSEC-OK
bit must be allocated to make clear it does or does not understand
the future version. Also, a new DS type needs to be allocated to
allow differentiation between a current signed delegation and a
'future' signed delegation. Also, current NSEC needs to be rolled
into a new authenticated denial type.
2.2.1.2. Limitations
None.
2.2.1.3. Amendments to DNSSEC-bis
None.
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2.2.1.4. Cons
It is cumbersome to carefully craft a type code roll (TCR) that 'just
fits'. The DNSSEC-bis protocol has many 'borderline' cases that need
special consideration. It might be easier to do a full TCR, since a
few of the types and signals need upgrading anyway.
2.2.1.5. Pros
Graceful adoption of future versions of NSEC, while there are no
amendments to DNSSEC-bis.
2.2.2. A Complete Type-code and Signal Rollover
A new DNSSEC type code space is defined which can exist independent
of the current DNSSEC-bis type code space.
This proposal assumes that all current DNSSEC type-codes (RRSIG/
DNSKEY/NSEC/DS) and signals (DNSSEC-OK) are not used in any future
versions of DNSSEC. Any future version of DNSSEC has its own types
to allow for keys, signatures, authenticated denial, etcetera.
2.2.2.1. Coexistence and Migration
Both spaces can co-exist. They can be made completely orthogonal.
2.2.2.2. Limitations
None.
2.2.2.3. Amendments to DNSSEC-bis
None.
2.2.2.4. Cons
With this path we abandon the current DNSSEC-bis. Although it is
easy to roll specific well-known and well-tested parts into the re-
write, once deployment has started, this path is very expensive for
implementers, registries, registrars and registrants as well as
resolver operators and users. A TCR is not to be expected to occur
frequently, so while a next generation authenticated denial may be
enabled by a TCR, it is likely that that TCR will only be agreed upon
if it serves a whole basket of changes or additions. A quick
introduction of NSEC-ng should not be expected from this path.
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2.2.2.5. Pros
No amendments/changes to current DNSSEC-bis docset needed. It is
always there as last resort.
2.2.3. Unknown (New) Algorithm in DS, DNSKEY, and RRSIG
This proposal assumes that future extensions make use of the existing
NSEC RDATA syntax, while they may need to change the interpretation
of the RDATA or introduce an alternative denial mechanism, invoked by
the specific unknown (new) signing algorithm. The different
interpretation would be signaled by use of different signature
algorithms in the DS RR at the parent. Consequently, the DNSKEY RR
for the child zone's KSK would contain a matching algorithm field.
2.2.3.1. Coexistence and migration
Old and new NSEC RDATA interpretation or known and unknown signatures
cannot coexist in a zone. While DS RRs with both new and well known
algorithm designation could both exist at the parent, that would not
lead to an unambiguous interpretation of the NSEC RRs in the zone.
RRSIG RRs need to cover complete RRSets, so it is not possible to
sign an 'old' NSEC RR with an RRSIG using an 'old' algorithm and
then, at the same owner, sign another 'new' NSEC RR with an RRSIG of
the 'new' algorithm type. A similar approach was subsequently
standardized in [I-D.ietf-dnsext-dnssec-experiments].
2.2.3.2. Limitations
Validating resolvers agnostic of the 'new' signing algorithm (which
may be a well known algorithm, but might not be recognized due to the
new code) will treat the entire zone as insecure.
The algorithm number space might be split for each future version of
DNSSEC. Violation of the 'modular components' concept. We use the
'validator' to protect the 'resolver' from unknown interpretations.
2.2.3.3. Amendments to DNSSEC-bis
None.
2.2.3.4. Cons
The algorithm field was not designed for this purpose. This is a
straightforward hack.
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2.2.3.5. Pros
No amendments/changes to current DNSSEC-bis docset needed.
2.2.4. Unknown (New) Hash Algorithm in DS
Similar to the previous method this one uses the DS RR at the parent
to signal child zone properties. Here, the digest type field of the
DS RR would be used to signal presence of a different (than DNSSEC-
bis) authenticated denial scheme at the child.
2.2.4.1. Coexistence and migration
Old and new NSEC RDATA interpretation or known and unknown signatures
can NOT coexist in a zone.
2.2.4.2. Limitations
Validating resolvers agnostic of the 'new' hashing algorithm (which
may be a well known algorithm, but might not be recognized due to the
new code) will treat the entire zone as insecure.
The digest type space might be split for each future version of
DNSSEC. Violation of the 'modular components' concept. We use the
'validator' to protect the 'resolver' from unknown interpretations.
2.2.4.3. Amendments to DNSSEC-bis
None.
2.2.4.4. Cons
The digest type field was not designed for this purpose. This is a
straightforward hack.
2.2.4.5. Pros
No amendments/changes to current DNSSEC-bis docset needed.
3. Recommendation
The authors recommend that the working group commits to and starts
work on a partial TCR, allowing graceful transition towards a future
version of NSEC. Meanwhile, to accomodate the need for an
immediately, temporary, solution against zone-traversal, we recommend
On-Demand NSEC synthesis.
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This approach does not require any mandatory changes to DNSSEC-bis,
does not violate the protocol and fulfills the requirements. As a
side effect, it moves the cost of implementation and deployment to
the users (zone owners) of this mechanism.
4. Security Considerations
This document deals with transition mechanisms for new versions of
the DNS Security Extensions. The particular considerations for the
methods studied are listed in the respective sections, most
importantly the requirement for keeping private keys online in
Section 2.1.1 and Section 2.1.7 and the full or partial abandoning of
authenticated denial in Section 2.1.5 and Section 2.1.6.
5. IANA Considerations
[[Note to the RFC Editor: This section may be removed prior to
publication.]]
This document does not create any new IANA registry nor does it ask
for any allocation from an existing IANA registry.
6. Acknowledgements
The authors would like to thank Sam Weiler, Mark Andrews, and Stuart
Schechter for their input and constructive comments.
7. References
7.1. Normative References
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
[RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Resource Records for the DNS Security Extensions",
RFC 4034, March 2005.
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, March 2005.
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7.2. Informative References
[I-D.ietf-dnsext-dnssec-experiments]
Blacka, D., "DNSSEC Experiments",
draft-ietf-dnsext-dnssec-experiments-04 (work in
progress), March 2007.
[RFC1535] Gavron, E., "A Security Problem and Proposed Correction
With Widely Deployed DNS Software", RFC 1535,
October 1993.
[RFC2535] Eastlake, D., "Domain Name System Security Extensions",
RFC 2535, March 1999.
Authors' Addresses
Roy Arends
Nominet UK
Edmund Halley Road
Oxford OX4 4DQ
United Kingdom
Email: roy@nominet.org.uk
Peter Koch
DENIC eG
Kaiserstrasse 75-77
Frankfurt 60329
DE
Phone: +49 69 27235 0
Email: pk@DENIC.DE
Jakob Schlyter
Kirei AB
P.O. Box 53204
Goteborg SE-400 16
Sweden
Email: jakob@kirei.se
URI: http://www.kirei.se/
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