DNS Extensions Working Group S. Crocker
Internet-Draft Shinkuro Inc.
Updates: 4035 (if approved) S. Rose
Intended status: Standards Track NIST
Expires: January 15, 2009 July 14, 2008
Signaling Cryptographic Algorithm Understanding in DNSSEC
draft-crocker-dnssec-algo-signal-01
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Copyright (C) The IETF Trust (2008).
Abstract
The DNS Security Extensions (DNSSEC) was developed to provide origin
authentication and integrity protection for DNS data by using digital
signatures. These digital signatures can be generated using
different algorithms. Each digital signature added to a response
increases the size of the response, which could result in the
response message being truncated. This draft sets out to specify a
way for validating end-system resolvers to signal to a server which
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cryptographic algorithms they prefer in a DNSSEC response by defining
an EDNS option to list a client's preferred algorithms.
Requirements Language
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 [RFC2119].
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Signaling Algorithm Understood (AU) Using EDNS . . . . . . . . 3
3. Client Considerations . . . . . . . . . . . . . . . . . . . . . 4
3.1. Recommendations for Stub Clients . . . . . . . . . . . . . 5
4. Server Considerations . . . . . . . . . . . . . . . . . . . . . 5
5. Cache and Forwarder Considerations . . . . . . . . . . . . . . 5
5.1. Intermediate Resolvers . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
8. Normative References . . . . . . . . . . . . . . . . . . . . . 6
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1. Introduction
The DNS Security Extensions (DNSSEC) was developed to provide origin
authentication and integrity protection for DNS data by using digital
signatures [RFC4033], [RFC4034] and [RFC4035]. Each digital
signature RR (RRSIG) contains an algorithm code number. These
algorithm codes help validators identify which cryptographic
algorithm was used to generate the digital signature.
RRSIG RRs can be fairly large, and increase the size of a response.
If multiple algorithms are used, then multiple RRSIGs are returned
for each RRset in a response. If the response is too large, it may
be truncated, and the client forced to resend the query using TCP.
It would be in the client and server's interests if there was a way
to limit the number of RRSIGs in a response to only those algorithms
the client was interested in (if present).
This draft sets out to specify a way for validating end-system
resolvers to signal to a server which cryptographic algorithms they
prefer in a DNSSEC response. This is done using the EDNS attribute
values in the OPT meta-RR [RFC2671]. This option could also be used
by servers to advertise which cryptographic algorithms are used in
signing a particular zone.
An additional reason for having the end-system resolver signal which
algorithms it understands is to speed the transition to a new
algorithm. A server will be able to determine when to start serving
a new algorithm when it sees a sufficient number of its clients are
able to accept the new algorithm and it will be able to determine
when to stop serving the old algorithm when it sees that all or
almost all of its clients are able to accept the new algorithm.
Information about clients can also be used to communicate to the
operators of those clients and/or the providers of their software
that it's time to upgrade.
2. Signaling Algorithm Understood (AU) Using EDNS
The ENDS0 specification outlined in [RFC2671] defines a way to
include new options using a standardized mechanism. These options
are contained in the RDATA of the OPT meta-RR. This document seeks
to define a new EDNS0 option for a client to signal which algorithms
the client prefers, and the server to advertise which algorithms are
used to sign a particular zone.
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Below shows how the signaling attribute is defined in the RDATA of
the OPT RR as specified in [RFC2671]:
0 8 16
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| OPTION-CODE (TBD) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| OPTION-LENGTH |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ALG-CODE1 | ALG-CODE2 |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ALG-CODE3 | ... \
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
OPTION-CODE is the code for the Algorithm Understood (AU) option.
Its value is fixed at TBD.
OPTION-LENGTH is the length of the data of the attribute in octets.
DNSSEC algorithm codes are 1 octet long. The value in this field is
the number of algorithm codes the client wishes to signal as
understood.
ALG-CODE is the assigned DNSSEC algorithm code(s) that the client
indicates as understood. These values SHOULD be in the order of
preference. For example, if a validating client understands RSA/
SHA-1 and RSA/SHA-256 but prefers to trust RSA/SHA-256 signatures due
to local policy, the order of the ALG-CODE would be: 8 (RSA/SHA-256),
5 (RSA/SHA-1).
3. Client Considerations
A validating end-system resolver sets the AU option in the OPT
meta-RR when sending a query. The validating end-system resolver
SHOULD set the order of the algorithms in the AU option to the
preferred order; i.e., most preferred first, followed by the least
preferred.
A validating end-system resolver SHOULD only list algorithm codes
that the client has implemented or has a trust anchor for.
Conversely, a validating end-system resolver SHOULD NOT include the
algorithm code for cryptographic algorithms for which they have not
implemented and do not have an associated trust anchor.
The end-system resolver MUST also set the DNSSEC-OK bit [RFC4035] to
indicate that it wishes to receive DNSSEC RRs in the response.
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3.1. Recommendations for Stub Clients
Stub resolvers rely on an upstream recursive server (or cache) to
provide a response, any algorithm preference on the stub resolver's
side can be overruled by the upstream recursive server. The AU EDNS
option is NOT RECOMMENDED for non-validating stub clients.
The only exception is for validating sub resolvers, which set the CD
bit in queries. In this scenario, the validating stub indicates that
it wishes to perform its own validation and may wish to indicate
which cryptographic algorithm it prefers.
4. Server Considerations
When an authoritative server sees the AU option in the OPT meta-RR in
a request the normal algorithm for servicing requests is followed.
The only difference is what DNSSEC RRs are included in the final
response.
If the AU option is present but the DNSSEC-OK bit is not set, then
the authoritative server does not include any additional DNSSEC RRs
in the response. If the DNSSEC-OK bit is set, the authoritative
server looks at the list of algorithms in the OPT meta-RR, selects
the most preferred by the client and adds the appropriate RRSIGs (as
per the rules in [RFC4035])of the algorithms listed in the ALG-CODE
list.
If the zone containing the QNAME is not signed, the authoritative
server sends a traditional non-DNSSEC response. If the zone
containing the QNAME is signed with a cryptographic algorithm that
does not appear in the ALG-CODE list in the client query the
authoritative server SHOULD include any or all RRSIGs in the response
regardless of algorithm used to generate them.
5. Cache and Forwarder Considerations
Caches MUST NOT set the AU option on any outgoing query from the
cache when performing recursion on behalf of a stub client. A cache
MUST follow the guidelines in the DNSSEC specification ([RFC4033],
[RFC4034], [RFC4035] and any updating documents).
If a cache receives a query with the AU option set and the response
can be answered by data out of the cache, the cache SHOULD follow the
AU option request and only include the RRSIGs generated using the
algorithms found in the query AU list.
Forwarders that do not do validation or caching MAY copy the AU
option seen in received queries as they represent the wishes of the
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validating downstream resolver that issued the original query.
5.1. Intermediate Resolvers
Intermediate resolvers SHOULD copy the AU option seen in queries from
end- system resolvers. If the intermediate resolver is validating,
it SHOULD also check for the presence of the CD bit in the query. If
present, the intermediate resolvers SHOULD copy the AU option as seen
in the query. If not or if the DNSSEC-OK bit is not set, then the
validating intermediate resolver MAY chose to ignore the AU option in
the query and MAY include its own preference as the AU option.
6. IANA Considerations
The algorithm codes used to identify DNSSEC algorithms has already
been established by IANA. This document does not seek to alter that
registry in any way.
This draft seeks to update the "DNS EDNS0 Options" registry by adding
the AU option and referencing this document. The code for the option
should be TBD.
7. Security Considerations
This document specifies a way for a client to signal its digital
signature algorithm preference to a cache or server. It is not meant
to be a discussion on algorithm superiority. The signal is an
optional code contained in the OPT meta-RR used with EDNS0. The goal
of this option is to reduce response size by having the client signal
with digital signature algorithms it prefers and that it may not care
about other algorithms used to sign zone data.
It is possible that an attacker can attempt to conduct a downgrade
attack by intercepting the query and altering the AU option code. An
attacker could alter the algorithm list to force the client to rely
on a weaker digital signature algorithm even though the zone is
signed using a stronger algorithm the client prefers. In these cases
a client might be able to detect an attack if the target zone has a
DS RR in its delegating parent with the desired algorithm. The DS
cannot be deleted without making the parent's RRSIG over that RRset
invalid.
8. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
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RFC 2671, August 1999.
[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.
Authors' Addresses
Steve Crocker
Shinkuro Inc.
5110 Edgemoor Lane
Bethesda, MD 20814
USA
EMail: steve@shinkuro.com
Scott Rose
NIST
100 Bureau Dr.
Gaithersburg, MD 20899
USA
Phone: +1-301-975-8439
EMail: scott.rose@nist.gov
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