DNS Extensions Working Group R. Arends
Internet-Draft Telematica Instituut
Expires: April 25, 2005 P. Koch
Universitaet Bielefeld
J. Schlyter
NIC-SE
October 25, 2004
Evaluating DNSSEC Transition Mechanisms
draft-ietf-dnsext-dnssec-trans-01.txt
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Copyright (C) The Internet Society (2004).
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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Transition Mechanisms . . . . . . . . . . . . . . . . . . . . 3
2.1 Mechanisms Updating DNSSEC-bis . . . . . . . . . . . . . . 4
2.1.1 Dynamic NSEC Synthesis . . . . . . . . . . . . . . . . 4
2.1.2 Add Versioning/Subtyping to Current NSEC . . . . . . . 4
2.1.3 Type Bit Map NSEC Indicator . . . . . . . . . . . . . 5
2.1.4 New Apex Type . . . . . . . . . . . . . . . . . . . . 6
2.1.5 NSEC White Lies . . . . . . . . . . . . . . . . . . . 7
2.1.6 NSEC Optional via DNSSKEY Flag . . . . . . . . . . . . 8
2.2 Mechanisms not Updating DNSSEC-bis . . . . . . . . . . . . 9
2.2.1 Partial Type-code and Signal Rollover . . . . . . . . 9
2.2.2 A Complete Type-code and Signal Rollover . . . . . . . 9
2.2.3 Unknown Algorithm in RRSIG . . . . . . . . . . . . . . 10
3. Recommendation . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . 13
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1. Introduction
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) 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 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 implementaion and/or
deployment, including those which seem to be solely operational in
nature.
2. Transition Mechanisms
In the light of recent 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 DNSSEC-ter, other
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.
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2.1 Mechanisms Updating DNSSEC-bis
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 then 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 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.
2.1.1.4 Cons
Unbalanced cost is a ground for DDoS. Though this protects against
enumeration, it is not really a path for versioning.
2.1.1.5 Pros
Hardly any amendments to DNSSEC-bis.
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 an 'Must Be Zero' (MBZ) for the to be published docset.
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2.1.2.1 Coexistence and Migration
Since the versioning is done inside the NSEC RR, different versions
may coexist. However, depending on future methods, that may or may
not be useful inside a single zone. 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 it 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 authenticated
denial (as opposed to e.g. NSEC-alt, which currently proposes three
RR types).
2.1.2.3 Amendments to DNSSEC-bis
Full Update of the current DNSSEC-bis documents to provide for new
fields in NSEC, while specifying behavior in case of unknown field
values.
2.1.2.4 Cons
Though this is a clean and clear path without versioning DNSSEC, it
takes some time to design, gain consensus, update the current
dnssec-bis document, test and implement a new authenticated denial
record.
2.1.2.5 Pros
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.
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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, NSEC, RRSIG 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 documented 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
2.1.3.4 Cons
The type-bit-map was not designed for this purpose. It is a
straightforward hack. Text in protocol section 5.4 was put in
specially to defend against this usage.
2.1.3.5 Pros
No change needed to the on-the-wire protocol as specified in the
current docset.
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'.
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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. Though
this 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.
2.1.4.5 Pros
This approach would allow for a lot of other per zone information to
be transported or signaled 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 deep names, delegation only). Most of
the burden is put on the resolver side and operational consequences
are yet to be studied.
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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 fully fulfill
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,
cf. RFC 1535); replay attacks to deny existence of records
2.1.5.5 Pros
Hardly any amendments to DNSSEC-bis. Operational "trick" that is
available anyway.
2.1.6 NSEC Optional via DNSSKEY Flag
A new DNSKEY 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 DS RR). A next to this
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
Doesn't fully meet requirements. Operational consequences to be
studied.
2.1.6.5 Pros
Official version of the "trick" presented in (8). Operational
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problems can be addressed during future work on validators.
2.2 Mechanisms not 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.
2.2.1.4 Cons
It is cumbersome to carefully craft an TCR that 'just fits'. The
DNSSEC-bis protocol has many 'borderline' cases that needs 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 space is defined which can exist independent of current
DNSSEC-bis space.
This proposal assumes that all current DNSSEC type-codes
(RRSIG/DNSKEY/NSEC/DS) and signals (DNSSEC-OK) are not used in any
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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. Though it is easy
to role 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
resolvers/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.
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 Algorithm in RRSIG
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 unknown signing algorithm. The different interpretation
would be signaled by use of different signature algorithms in the
RRSIG records covering the NSEC RRs.
When an entire zone is signed with a single unknown algorithm, it
will cause implementations that follow current dnssec-bis documents
to treat individual RRsets as unsigned.
2.2.3.1 Coexistence and migration
Old and new NSEC RDATA interpretation or known and unknown Signatures
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can NOT coexist in a zone since signatures cover complete (NSEC)
RRSets.
2.2.3.2 Limitations
Validating resolvers agnostic of new interpretation will treat the
NSEC RRset as "not signed". This affects wildcard and non-existence
proof, as well as proof for (un)secured delegations. Also, all
positive signatures (RRSIGs on RRSets other than DS, NSEC) appear
insecure/bogus to an old validator.
The algorithm version space is 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.
2.2.3.5 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 gracefull 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.
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.
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Authors' Addresses
Roy Arends
Telematica Instituut
Drienerlolaan 5
Enschede 7522 NB
Netherlands
Phone: +31 534850485
EMail: roy.arends@telin.nl
Peter Koch
Universitaet Bielefeld
Bielefeld 33594
Germany
Phone: +49 521 106 2902
EMail: pk@TechFak.Uni-Bielefeld.DE
Jakob Schlyter
NIC-SE
Box 5774
Stockholm SE-114 87
Sweden
EMail: jakob@nic.se
URI: http://www.nic.se/
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