DNS Extensions Working Group S. Crocker
Internet-Draft Shinkuro Inc.
Updates: 4035 (if approved) S. Rose
Intended status: Standards Track NIST
Expires: October 15, 2010 April 13, 2010
Signaling Cryptographic Algorithm Understanding in DNSSEC
draft-crocker-dnssec-algo-signal-06
Abstract
The DNS Security Extensions (DNSSEC) were developed to provide origin
authentication and integrity protection for DNS data by using digital
signatures. These digital signatures can be generated using
different algorithms. This draft sets out to specify a way for
validating end-system resolvers to signal to a server which
cryptographic algorithms they support.
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].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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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
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This Internet-Draft will expire on October 15, 2010.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Signaling Algorithm Understood (AU) Using EDNS . . . . . . . . 3
3. Client Considerations . . . . . . . . . . . . . . . . . . . . . 4
3.1. Recommendations for Stub Clients . . . . . . . . . . . . . 4
3.2. Recursive Cache Considerations . . . . . . . . . . . . . . 5
4. Intermediate Middlebox Considerations . . . . . . . . . . . . . 5
5. Server Considerations . . . . . . . . . . . . . . . . . . . . . 5
6. Traffic Analysis Considerations . . . . . . . . . . . . . . . . 5
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
8. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
9. Normative References . . . . . . . . . . . . . . . . . . . . . 6
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1. Introduction
The DNS Security Extensions (DNSSEC) [RFC4033], [RFC4034] and
[RFC4035] were developed to provide origin authentication and
integrity protection for DNS data by using digital signatures . 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.
This draft sets out to specify a way for validating end-system
resolvers to signal to a server which cryptographic algorithms they
support in a DNSSEC response. This is done using the EDNS attribute
values in the OPT meta-RR [RFC2671].
This proposed EDNS option serves to measure the acceptance and use of
new digital signing algorithms. This algorithm signaling option can
be used by zone administrators as a gauge to measure the successful
deployment of code that implements a newly deployed digital signature
algorithm used with DNSSEC. A zone administrator may be able to
determine when to stop serving the old algorithm when the server sees
that all or almost all of its clients signal that they are able to
accept the new algorithm.
This draft does not seek to include a formal process for including
new algorithms for use with DNSSEC. It also does not address the
question of which algorithms are to be included in any official list
of mandatory or recommended cryptographic algorithms for use with
DNSSEC. Rather, this document specifies a means by which a client
query can signal a set of algorithms it implements.
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 supports.
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The figure below shows how the signally attribute is defined in the
RDATA of the OPT RR specified in [RFC2671]:
0 8 16
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| OPTION-CODE (TBD) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| OPTION-LENGTH |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ALG-CODE | ... \
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
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 so this value is the number
of octets.
ALG-CODE is the list of assigned values of DNSSEC zone signing
algorithms that the client indicates as understood. The values MUST
be in descending order of preference, with the most preferred
algorithm first. For example, if a validating client implements RSA/
SHA-1, RSA/SHA-256 and prefers the latter, the value of 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
sets the value(s) in the order of preference, with the most preferred
algorithm first as described in section 2. The end-system resolver
MUST also set the DNSSEC-OK bit [RFC4035] to indicate that it wishes
to receive DNSSEC RRs in the response.
Note that when including the PRIVATEDNS (253) and/or the PRIVATEOID
(254) codes, the client only indicates that it understands one or
more private algorithms but does not indicate which algorithms.
3.1. Recommendations for Stub Clients
Typically, stub resolvers rely on an upstream recursive server (or
cache) to provide a response; any algorithm supportence on the stub
resolver's side could be overruled by the upstream recursive server.
The AU EDNS option is NOT RECOMMENDED for non-validating stub
clients.
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The exception to the above is that validating stub resolvers which
set the CD bit in queries MAY set the AU option. In the most common
scenario, the validating stub indicates that it wishes to perform its
own validation (via the CD bit) and may therefore wish to indicate
which cryptographic algorithm(s) it supports.
3.2. Recursive Cache Considerations
DNSSEC validating recursive caches MAY set the AU option on any
outgoing query from the cache when performing recursion on behalf of
a non-DNSSEC aware stub client. If the stub indicates it is DNSSEC-
aware, but does not set the AU option in the query, the DNSSEC
validating recursive cache SHOULD NOT set the AU option to avoid
conflicts.
Forwarders that do not do validation or caching SHOULD copy the AU
option seen in received queries as they represent the wishes of the
validating downstream resolver that issued the original query.
4. Intermediate Middlebox Considerations
Intermediate middleboxes SHOULD copy the AU option seen in queries
from end system resolvers. If the system is validating, it SHOULD
also check for the presence of the CD bit in the query. If present,
the intermediate middlebox SHOULD copy the AU option as seen in the
query. If not set or if the DNSSEC-OK bit is not set, then the
validating intermediate middlebox MAY chose to ignore the AU option
in the query and MAY include its own preference as the AU option.
5. 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.
If the AU option is present but the DNSSEC-OK bit is not set, then
the authoritative server ignores the ALG-CODE list and does not
include any additional DNSSEC RRs in the response.
6. Traffic Analysis Considerations
Zone administrators that are planning or are in the process of
completing a cryptographic algorithm rollover operation should
monitor DNS query traffic and record the values of the AU option in
queries. This monitoring can measure the deployment of client code
that implements (and signals) certain algorithms. Exactly how to
capture DNS traffic and measure new algorithm adoption is beyond the
scope of this document.
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Zone administrators can use this data to set plans for starting an
algorithm rollover and when older algorithms can be phased out
without disrupting the majority of clients. In order to keep this
disruption to a minimum, zone administrators should wait to complete
an algorithm rollover until a large majority of clients signal that
they understand the new algorithm. Note that clients that do not
implement the AU option may be older implementations which would also
not implement any newly deployed algorithm.
7. 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.
8. 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 signal new algorithm uptake in client code to
allow zone administrators to know when it is possible to complete an
algorithm rollover in a DNSSEC signed zone.
9. 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)",
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.
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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: scottr.nist@gmail.com
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