Guidance for NSEC3 parameter settingsUSC/ISIietf@hardakers.netBloomberg, L.P.ietf-dane@dukhovni.orgNSEC3 is a DNSSEC mechanism providing proof of non-existence by
promising there are no names that exist between two domainnames within
a zone. Unlike its counterpart NSEC, NSEC3 avoids directly disclosing
the bounding domainname pairs. This document provides guidance on
setting NSEC3 parameters based on recent operational deployment
experience.As with NSEC , NSEC3 provides proof of
non-existence that consists of signed DNS records establishing the
non-existence of a given name or associated Resource Record Type
(RRTYPE) in a DNSSEC signed zone. In the case of NSEC3,
however, the names of valid nodes in the zone are obfuscated through
(possibly multiple iterations of) hashing via SHA-1. (currently only
SHA-1 is in use within the Internet).NSEC3 also provides “opt-out support”, allowing for blocks of unsigned
delegations to be covered by a single NSEC3 record. Opt-out blocks
allow large registries to only sign as many NSEC3 records as there are
signed DS or other RRsets in the zone – with opt-out, unsigned
delegations don’t require additional NSEC3 records. This sacrifices
the tamper-resistance proof of non-existence offered by NSEC3 in order
to reduce memory and CPU overheads.NSEC3 records have a number of tunable parameters that are specified
via an NSEC3PARAM record at the zone apex. These parameters are the
Hash Algorithm, processing Flags, the number of hash Iterations and
the Salt. Each of these has security and operational considerations
that impact both zone owners and validating resolvers. This document
provides some best-practice recommendations for setting the NSEC3
parameters.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 BCP 14 when, and only when, they appear
in all capitals, as shown here.The following sections describe recommendations for setting parameters
for NSEC3 and NSEC3PARAM.The algorithm field is not discussed by this document.The flags field currently contains a single flag, that of the
“Opt-Out” flag , which specifies whether or not NSEC3
records provide proof of non-existence or not. In general, NSEC3 with
the Opt-Out flag enabled should only be used in large, highly dynamic
zones with a small percentage of signed delegations. Operationally,
this allows for less signature creations when new delegations are
inserted into a zone. This is typically only necessary for extremely
large registration points providing zone updates faster than
real-time signing allows. Smaller zones, or large but relatively
static zones, are encouraged to use a Flags value of 0 (zero) and take
advantage of DNSSEC’s proof-of-non-existence support.NSEC3 records are created by first hashing the input domain and then
repeating that hashing algorithm a number of times based on the
iterations parameter in the NSEC3PARM and NSEC3 records. The first
hash is typically sufficient to discourage zone enumeration performed
by “zone walking” an NSEC or NSEC3 chain. Only determined parties
with significant resources are likely to try and uncover hashed
values, regardless of the number of additional iterations performed.
If an adversary really wants to expend significant CPU resources to
mount an offline dictionary attack on a zone’s NSEC3 chain, they’ll
likely be able to find most of the “guessable” names despite any
level of additional hashing iterations.Most names published in the DNS are rarely secret or unpredictable.
They are published to be memorable, used and consumed by humans. They
are often recorded in many other network logs such as email logs,
certificate transparency logs, web page links, intrusion detection
systems, malware scanners, email archives, etc. Many times a simple
dictionary of commonly used domain names prefixes (www, ftp, mail,
imap, login, database, etc) can be used to quickly reveal a large
number of labels within a zone. Because of this, there are increasing
performance costs yet diminishing returns associated with applying
additional hash iterations beyond the first.Although Section 10.3 of specifies upper bounds for the
number of hash iterations to use, there is no published guidance for
zone owners about good values to select. Because hashing provides
only moderate protection, as shown recently in academic studies of
NSEC3 protected zones (tbd: insert ref), this document recommends that
zone owners SHOULD use an iteration value of 0 (zero), indicating that
only the initial hash value should be placed into a DNS zone’s NSEC3
records.Salts add yet another layer of protection against offline, stored
dictionary attacks by combining the value to be hashed (in our case, a
DNS domainname) with a randomly generated value. This prevents
adversaries from building up and remembering a dictionary of values
that can translate a hash output back to the value that it derived from.In the case of DNS, it should be noted the hashed names placed in
NSEC3 records already include the fully-qualified domain name from
each zone. Thus, no single pre-computed table works to speed up
dictionary attacks against multiple target zones. An attacker is
required to compute a complete dictionary per zone, which is expensive
in both storage and CPU time.To protect against a dictionary being built and used for a target
zone, an additional salt field can be included and changed on a
regular basis, forcing a would-be attacker to repeatedly compute a new
dictionary (or just do trial and error without the benefits of
precomputation).Changing a zone’s salt value requires the construction of a complete
new NSEC3 chain. This is true both when resigning the entire zone at
once, or incrementally signing it in the background where the new salt
is only activated once every name in the chain has been completed.Most users of NSEC3 publish static salt values that never change.
This provides no added security benefit (because the complete fully
qualified domain name is already unique). If no rotation is planned,
operators are encouraged to forgo the salt entirely by using a
zero-length salt value instead (represented as a “-“ in the
presentation format).In short, for most zones, the recommended NSEC3 parameters are as
shown below:For very large (e.g. 10 million plus unsigned delegations) and only sparsely
signed zones, where the majority of the records are insecure delegations, use
of opt-out may be justified. In such (large TLD or similar) zones the
alternative parameters are:Because there has been a large growth of open (public) DNSSEC
validating resolvers that are subject to compute resource constraints
when handling requests from anonymous clients, this document
recommends that validating resolvers should change their behaviour
with respect to large iteration values. Validating resolvers SHOULD
return a SERVFAIL when processing NSEC3 records with iterations larger
than 100. Note that this significantly decreases the requirements
originally specified in Section 10.3 of .Validating resolvers returning a SERVFAIL in this situation SHOULD
return an Extended DNS Error {RFC8914} EDNS0 option of value [TBD].This entire document discusses security considerations with various
parameters selections of NSEC3 and NSEC3PARAM fields.This entire document discusses operational considerations with various
parameters selections of NSEC3 and NSEC3PARAM fields.This document requests a new allocation in the “Extended DNS Error
Codes” of the “Domain Name System (DNS) Parameters” registration
table with the following characteristics:INFO-CODE: TBDPurpose: Unsupported NSEC3 iterations valueReference: this documentKey words for use in RFCs to Indicate Requirement LevelsIn many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements.DNS Security (DNSSEC) Hashed Authenticated Denial of ExistenceThe Domain Name System Security (DNSSEC) Extensions introduced the NSEC resource record (RR) for authenticated denial of existence. This document introduces an alternative resource record, NSEC3, which similarly provides authenticated denial of existence. However, it also provides measures against zone enumeration and permits gradual expansion of delegation-centric zones. [STANDARDS-TRACK]Protocol Modifications for the DNS Security ExtensionsThis document is part of a family of documents that describe the DNS Security Extensions (DNSSEC). The DNS Security Extensions are a collection of new resource records and protocol modifications that add data origin authentication and data integrity to the DNS. This document describes the DNSSEC protocol modifications. This document defines the concept of a signed zone, along with the requirements for serving and resolving by using DNSSEC. These techniques allow a security-aware resolver to authenticate both DNS resource records and authoritative DNS error indications. This document obsoletes RFC 2535 and incorporates changes from all updates to RFC 2535. [STANDARDS-TRACK]Ambiguity of Uppercase vs Lowercase in RFC 2119 Key WordsRFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.dns-operations discussion participantsWhile this document is under development, it can be viewed, tracked,
issued, pushed with PRs, … here:https://github.com/hardaker/draft-hardaker-dnsop-nsec3-guidance