|Title||Signaling Cryptographic Algorithm Understanding in DNS Security
|Author||S. Crocker, S. Rose
Internet Engineering Task Force (IETF) S. Crocker
Request for Comments: 6975 Shinkuro Inc.
Category: Standards Track S. Rose
ISSN: 2070-1721 NIST
Signaling Cryptographic Algorithm Understanding in
DNS Security Extensions (DNSSEC)
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 document specifies a way for validating
end-system resolvers to signal to a server which digital signature
and hash algorithms they support. The extensions allow the signaling
of new algorithm uptake in client code to allow zone administrators
to know when it is possible to complete an algorithm rollover in a
Status of This Memo
This is an Internet Standards Track document.
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(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Language . . . . . . . . . . . . . . . . . . . . . 4
3. Signaling DNSSEC Algorithm Understood (DAU), DS Hash
Understood (DHU), and NSEC3 Hash Understood (N3U) Using EDNS . 4
4. Client Considerations . . . . . . . . . . . . . . . . . . . . . 5
4.1. Stub Resolvers . . . . . . . . . . . . . . . . . . . . . . 5
4.1.1. Validating Stub Resolvers . . . . . . . . . . . . . . . 5
4.1.2. Non-validating Stub Resolvers . . . . . . . . . . . . . 6
4.2. Recursive Resolvers . . . . . . . . . . . . . . . . . . . . 6
4.2.1. Validating Recursive Resolvers . . . . . . . . . . . . 6
4.2.2. Non-validating Recursive Resolvers . . . . . . . . . . 6
5. Intermediate System Considerations . . . . . . . . . . . . . . 6
6. Server Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. Traffic Analysis Considerations . . . . . . . . . . . . . . . . 7
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
9. Security Considerations . . . . . . . . . . . . . . . . . . . . 8
10. Normative References . . . . . . . . . . . . . . . . . . . . . 8
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 (RRSIG) Resource Record (RR) contains an algorithm
code number that corresponds to a DNSSEC public key (DNSKEY) RR.
These algorithm codes tell validators which cryptographic algorithm
was used to generate the digital signature.
Likewise, the Delegation Signer (DS) RRs and Hashed Authenticated
Denial of Existence (NSEC3) RRs use a hashed value as part of their
resource record data (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
This document sets specifies 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
Extension Mechanisms for DNS (EDNS0) options specified below in
Section 2 for use in the OPT meta-RR [RFC6891]. These three new
EDNS0 option codes are all OPTIONAL to implement and use.
These proposed EDNS0 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 the newly deployed digital
signature algorithm, DS hash, and the NSEC3 hash algorithm used with
DNSSEC. A zone administrator is 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 a
period of years before a tipping point is seen.
This document 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 the set of algorithms and hashes that
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC
3. Signaling DNSSEC Algorithm Understood (DAU), DS Hash Understood
(DHU), and NSEC3 Hash Understood (N3U) Using EDNS
The EDNS0 specification outlined in [RFC6891] 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 EDNS0 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. Each option code can appear only once in an OPT RR.
The figure below shows how each option is defined in the RDATA of the
OPT RR specified in [RFC6891]:
0 8 16
| OPTION-CODE |
| LIST-LENGTH |
| ALG-CODE | ... /
OPTION-CODE is the code for the given signaling option. The options
o DNSSEC Algorithm Understood (DAU) option for DNSSEC digital
signing algorithms. Its value is fixed at 5.
o DS Hash Understood (DHU) option for DS RR hash algorithms. Its
value is fixed at 6.
o NSEC3 Hash Understood (N3U) option for NSEC3 hash algorithms. Its
value is fixed at 7.
LIST-LENGTH is the length of the list of digital signatures or hash
algorithm codes in octets. Each algorithm code occupies a single
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 order of the code values can be arbitrary and MUST NOT be used to
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 22-32 octets (average of 6-10 digital signature algorithms,
3-5 DS hash algorithms, and 1-5 NSEC3 hash algorithms) including the
EDNS0 option codes and option lengths in potential future examples.
4. Client Considerations
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 MUST also set the DNSSEC
OK bit [RFC4035] to indicate that it wishes to receive DNSSEC RRs in
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.
Additionally, a client is under no obligation to list every algorithm
it implements and MAY choose to only list algorithms the client
wishes to signal as understood.
Since the DAU, DHU, and/or N3U options are only set in the query, if
a client sees these options in the response, no action needs to be
taken and the client MUST ignore the option values.
4.1. Stub Resolvers
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.
4.1.1. Validating Stub Resolvers
A validating stub resolver sets the DNSSEC OK (DO) bit [RFC4035] to
indicate that it wishes to receive additional DNSSEC RRs (i.e., RRSIG
RRs) in the response. Such validating resolvers SHOULD include the
DAU, DHU, and/or the N3U option(s) in the OPT RR when sending a
4.1.2. Non-validating Stub Resolvers
The DAU, DHU, and N3U EDNS0 options MUST NOT be included by
non-validating stub resolvers.
4.2. Recursive Resolvers
4.2.1. Validating Recursive Resolvers
A validating recursive resolver sets the DAU, DHU, and/or N3U
option(s) when performing recursion based on its list of algorithms
and any DAU, DHU, and/or N3U option lists in the stub client query.
When the recursive server receives a query with one or more of the
options set, the recursive server MUST set the algorithm list for any
outgoing iterative queries for that resolution chain to a union of
the stub client's list and the validating recursive resolver's list.
For example, if the recursive resolver's algorithm list for the DAU
option is (3, 5, 7) and the stub's algorithm list is (7, 8), the
final DAU algorithm list would be (3, 5, 7, 8).
If the client included the DO and Checking Disabled (CD) bits, but
did not include the DAU, DHU, and/or N3U option(s) in the query, the
validating recursive resolver MAY include the option(s) with its own
list in full. If one or more of the options are missing, the
validating recursive resolver MAY include the missing options with
its own list in full.
Validating recursive resolvers MUST NOT set the DAU, DHU, and/or N3U
option(s) in the final response to the stub client.
4.2.2. Non-validating Recursive Resolvers
Recursive resolvers that do not do validation MUST 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
5. Intermediate System Considerations
Intermediate proxies (see Section 4.4.2 of [RFC5625]) that understand
DNS are RECOMMENDED to behave like a comparable recursive resolver
when dealing with the DAU, DHU, and N3U options.
6. Server Considerations
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 DO bit is not set, the server does
not do any DNSSEC processing, which includes any recording of the
If the server sees one (or more) of the options set with RESERVED
values, the server MAY ignore recoding of those values.
Authoritative servers MUST NOT set the DAU, DHU, and/or N3U option(s)
on any responses. These values are only set in queries.
7. Traffic Analysis Considerations
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 presence 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. A 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 that would also not implement any newly
8. IANA Considerations
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
IANA has allocated option codes 5, 6, and 7 for the DAU, DHU, and N3U
options, respectively, in the "DNS EDNS0 Option Codes (OPT)"
registry. The three options have a status of "standard".
9. Security Considerations
This document specifies a way for a client to signal its digital
signature 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 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.
There is a possibility that an eavesdropper or server could infer the
validator in use by a client by the presence of the AU options and/or
algorithm code list. This information leakage in itself is not very
useful to a potential attacker, but it could be used to identify the
validator or narrow down the possible validator implementations in
use by a client, which could have a known vulnerability that could be
exploited by the attacker.
10. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[RFC5625] Bellis, R., "DNS Proxy Implementation Guidelines",
BCP 152, RFC 5625, August 2009.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891, April 2013.
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