Signaling Cryptographic Algorithm Understanding in DNSSECShinkuro Inc.5110 Edgemoor LaneBethesda20814MDUSAsteve@shinkuro.com NIST 100 Bureau Dr.Gaithersburg20899MDUSA+1-301-975-8439 scottr.nist@gmail.com
Internet Area
DNS Extensions Working Group DNSDNSSECEDNS
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 and hash algorithms they support.
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.
The DNS Security Extensions (DNSSEC) ,
and 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 tells validators
which cryptographic algorithm was used to generate the digital signature.
Likewise, Delegation Signer (DS) RR's and NSEC3 RR's use a hashed value as
part of their RDATA and, like digital signature algorithms, these hash algorithms
have code numbers. All three algorithm codes (RRSIG/DNSKEY, DS and NSEC3)
are maintained in unique IANA registries.
This draft sets out to specify a way for validating end-system
resolvers to tell a server in a DNS query which digital signature
and/or hash algorithms they support. This is done using the new
EDNS options specified below in Section 2 for use in the OPT meta-RR .
These proposed EDNS options serve to measure the
acceptance and use of new digital signing algorithms.
These signaling options can be used by zone administrators as a gauge
to measure the successful deployment of code that implements a newly deployed
digital signature algorithm, DS hash and NSEC3 hash algorithm used with DNSSEC.
A zone administrator may be able
to determine when to stop signing with a superseded algorithm when
the server sees that a significant number of its clients signal that they
are able to accept the new algorithm. Note that this survey
may be conducted over the period of years before a tipping point is seen.
This draft does not seek to introduce another 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 and hashes it implements.
The ENDS0 specification outlined in 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 defines
three new EDNS options for a client to signal which digital signature and/or hash
algorithms the client supports. These options can be used independently of each other
and MAY appear in any order in the OPT RR.
OPTION-CODE is the code for the given signaling option. They are:
DNSSEC Algorithm Understood (DAU) option for DNSSEC digital
signing algorithms. Its value is fixed at TBD1.DS Hash Understood (DHU) option for DS RR hash algorithms. Its value is fixed at TBD2.
NSEC3 Hash Understood (N3U) option for NSEC3 hash algorithms. Its value is fixed at TBD3.
LIST-LENGTH is the length of the list of digital signature or hash algorithm codes in octets.
Each algorithm code occupies a single octet.
ALG-CODE is the list of assigned values of DNSSEC zone signing algorithms, DS hash algorithms, or
NSEC3 hash algorithms (depending on the OPTION-CODE in use) that
the client declares to be supported. The values SHOULD be in descending order of
preference, with the most preferred algorithm first. For example, if
a validating client signals the DAU option and RSA/SHA-1, RSA/SHA-256 and prefers the latter,
the values of ALG-CODE would be: 8 (RSA/SHA-256), 5 (RSA/SHA-1).
If all three options are included in the OPT RR, there is a potential for the OPT RR to take up
considerable size in the DNS message. However, in practical terms, including all three options
is likely to take up 22-34 octets (average of 6-10 digital signature algorithms, 3-5 DS hash algorithms
and 1-5 NSEC3 hash algorithms) including the EDNS option codes and option lengths in a reasonable
potential future example.
A validating end-system resolver sets the DAU, DHU and/or N3U option, or combination thereof 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(s) first as described in section 2. The end-system resolver
SHOULD also set the DNSSEC-OK bit to
indicate that it wishes to receive DNSSEC RRs in the response.
Note that the PRIVATEDNS (253) and/or the PRIVATEOID (254) digital signature codes both cover a potentially wide range
of algorithms and are likely not useful to a server. There is no compelling reason for a client
to include these codes in its list of the DAU. Likewise, clients MUST NOT include RESERVED codes
in any of the options.
Typically, stub resolvers rely on an upstream recursive server (or cache) to provide
a response. So optimal setting of the DAU, DSU and N3U options depends on whether the stub resolver elects to perform its own validation.
A validating stub resolver already (usually) sets the DO bit to indicate that it wishes
to receive additional DNSSEC RRs (i.e. RRSIG RR's) in the response. Such validating resolvers SHOULD include the DAU,
DHU and/or the N3U option(s) in the OPT RR
when sending a query. This way the validating stub resolver indicates which cryptographic algorithm(s)
it supports by setting the values in the order of preference, with the most preferred
algorithm first as described in Section 2.
The DAU, DHU and N3U EDNS options are NOT
RECOMMENDED for non-validating stub resolvers.
A validating recursive resolver sets the DAU, DHU and/or N3U option(s) when performing recursion based on the DO and CD flags in the client request .
If the client of the recursive resolver did
not include the DO bit in the query the recursive resolver SHOULD include the option(s) according
to its own local policy.
If the client did include the DO and CD bits, but did not include the DAU, DHU and/or N3U option(s) in the query,
the validating recursive resolver SHOULD NOT include the option(s) to avoid conflicts.
If the client did set the DO bit and the option(s) in the query, the validating
recursive resolver SHOULD include the option(s) based on the setting of the CD bit. If the CD bit
is set, the validating recursive resolver SHOULD include the option(s) based on the client query or
a superset of the client option(s) list and the validator's own list (if different). If the CD bit is not set,
the validating recursive resolver MAY copy the client option(s) or substitute its own option list.
Recursive resolvers that do not do validation SHOULD copy the DAU, DHU and/or N3U option(s)
seen in received queries as they represent the wishes of the validating downstream resolver that issued the original query.
Intermediate proxies that understand DNS SHOULD behave like a comparable recursive resolver when
dealing with the DAU, DHU and N3U options.
When an authoritative server sees the DAU, DHU and/or N3U option(s) in the OPT meta-RR in a
request the normal algorithm for servicing requests is followed. The options
MUST NOT trigger any special processing (e.g. RRSIG filtering in responses) on the server side.
If the options are present but the DNSSEC-OK (OK) bit is not set, the
server does not do any DNSSEC processing, including any recording of the option(s).
Zone administrators that are planning or are in the process of a
cryptographic algorithm rollover operation should monitor DNS query traffic
and record the number of queries, the presense of the OPT RR in queries and the values of the DAU/DHU/N3U option(s) (if present).
This monitoring can be used to
measure the deployment of client code that implements (and signals) specific
algorithms. Description of the techniques used to capture DNS traffic and measure new
algorithm adoption is beyond the scope of this document.
Zone administrators that need to comply with changes to their organization's
security policy (with regards to cryptographic algorithm use) can use this data to set
milestone dates for performing an algorithm
rollover. For example, zone administrators can use the data to determine when older algorithms can be phased out without disrupting a
significant number 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 recognize the new algorithm. This may be
in the order of years rather than months.
Note that clients that do not implement these options are likely to be older
implementations which would also not implement any newly deployed algorithm.
The algorithm codes used to identify DNSSEC algorithms, DS RR hash algorithms and NSEC3 hash
algorithms have already been established by IANA. This document does not seek to alter
that registry in any way.
This draft seeks to update the "DNS EDNS Options" registry by adding the
DAU, DHU and N3U options and referencing this document. The code for these options are TBD1,
TBD2 and TBD3 respectively.
This document specifies a way for a client to signal its cryptographic and hash
algorithm knowledge to a cache or server. It is not meant to be a
discussion
on algorithm superiority. The signals are optional codes contained in
the OPT meta-RR used with EDNS. The goal of these options are 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.