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 (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. Use of the
opt-out feature 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 NSEC3PARAM flags field currently contains no flags, but individual
NSEC3 records contain the “Opt-Out” flag , which specifies
whether or not that NSEC3 record provides 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 fewer 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 or when using
memory-constrained hardware. 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 .Operators are encouraged to forget the salt entirely by using a
zero-length salt value instead (represented as a “-“ in the
presentation format).NSEC3 records provide an an additional salt value, which can be
combined with an FQDN to influence the resulting hash, but properties
of this extra salt are complicated.In cryptography, salts generally add a layer of protection against
offline, stored dictionary attacks by combining the value to be hashed
with a unique “salt” value. This prevents adversaries from building up
and remembering a single dictionary of values that can translate a
hash output back to the value that it derived from.In the case of DNS, the situation is different because the hashed
names placed in NSEC3 records are always implicitly “salted” by
hashing 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 always required to
compute a complete dictionary per zone, which is expensive in both
storage and CPU time.To understand the role of the additional NSEC3 salt field, we have to
consider how a typical zone walking attack works. Typically the attack
has two phases - online and offline. In the online phase, an attacker
“walks the zone” by enumerating (almost) all hashes listed in NSEC3
records and storing them for the offline phase. Then, in the offline
cracking phase, the attacker attempts to crack the underlying hash. In
this phase, the additional salt value raises the cost of the attack
only if the salt value changes during the online phase of the
attack. In other words, an additional, constant salt value does not
change the cost of the attack.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 when incrementally signing it in the background where the new
salt is only activated once every name in the chain has been
completed. As a result, re-salting a is very complex operation, with
significant CPU time, memory, and bandwidth consumption. This makes
very frequent re-salting impractical, and renders the additional salt
field functionally useless.The following subsections describe recommendations for the different
operating realms within the DNS.First, if the operational or security features of NSEC3 are not
needed, then NSEC SHOULD be used in preference to NSEC3. NSEC3
requires greater computational power (see )
for both authoritative servers and validating clients. Specifically,
there is a non trivial complexity in finding matching NSEC3 records to
randomly generated prefixes within a DNS zone. NSEC mitigates this
concern. If NSEC3 must be used, then an iterations count of 0 MUST be
used to alleviate computational burdens. Please note that extra
iteration counts other than 0 increase impact of resource
CPU-exhausting DoS attacks, and also increase risk of interoperability
problems.Note that deploying NSEC with minimally covering NSEC records
also incurs a cost, and zone owners should measure the
computational difference in deploying both RFC4470 or NSEC3.In short, for all zones, the recommended NSEC3 parameters are as shown
below:For small zones, the use of opt-out based NSEC3 records is NOT
RECOMMENDED.For very large and sparsely signed zones, where the majority of the
records are insecure delegations, opt-out MAY be used.Since the NSEC3PARAM RR is not used by validating resolvers (see
section 4), the iterations and salt parameters can be changed
without the need to wait for RRsets to expire from caches. A complete
new NSEC3 chain needs to be constructed and the zone resigned.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 change their behavior with
respect to large iteration values. Specifically, validating
resolver operators and validating resolver software implementers are
encouraged to continue evaluating NSEC3 iteration count deployments
and lower their default acceptable limits over time. Similarly, because
treating a high iterations count as insecure leaves zones subject to
attack, validating resolver operators and validating resolver software
implementers are further encouraged to lower their default and acceptable
limit for returning SERVFAIL when processing NSEC3 parameters
containing large iteration count values. See
for measurements taken near the time of
publication and potential starting points.Validating resolvers MAY return an insecure response when processing
NSEC3 records with iterations larger than 0. Validating resolvers MAY
also return SERVFAIL when processing NSEC3 records with iterations
larger than 0. Validating resolvers MAY choose to not respond to NSEC3
records with iterations larger than 0. This significantly decreases
the requirements originally specified in Section 10.3 of
. See the Security Considerations for arguments on how to
handle responses with non-zero iteration count.Validating resolvers returning an insecure or SERVFAIL answer because
of unsupported NSEC3 parameter values SHOULD return an Extended DNS
Error (EDE) {RFC8914} EDNS0 option of value (RFC EDITOR: TBD).Note that a validating resolver returning an insecure response MUST
still validate the signature over the NSEC3 record to ensure the
iteration count was not altered since record publication (see
section 10.3).Primary and secondary authoritative servers for a zone that are not
being run by the same operational staff and/or using the same software
and configuration must take into account the potential differences in
NSEC3 iteration support.Operators of secondary services should advertise the parameter limits
that their servers support. Correspondingly, operators of primary
servers need to ensure that their secondaries support the NSEC3
parameters they expect to use in their zones. To ensure reliability,
after primaries change their iteration counts, they should query their
secondaries with known non-existent labels to verify the secondary
servers are responding as expected.This entire document discusses security considerations with various
parameters selections of NSEC3 and NSEC3PARAM fields.The point where a validating resolver returns insecure vs the point
where it returns SERVFAIL must be considered carefully. Specifically,
when a validating resolver treats a zone as insecure above a
particular value (say 100) and returns SERVFAIL above a higher point
(say 500), it leaves the zone subject to man-it-the-middle attacks as
if it was unsigned between these values. Thus, validating resolver
operators and software implementers SHOULD set the point above which a zone
is treated for certain values of NSEC3 iterations counts to the same
as the point where a validating resolver begins returning SERVFAIL.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: (RFC EDITOR: TBD)Purpose: Unsupported NSEC3 iterations valueReference: (RFC EDITOR: this document)Key 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]Minimally Covering NSEC Records and DNSSEC On-line SigningThis document describes how to construct DNSSEC NSEC resource records that cover a smaller range of names than called for by RFC 4034. By generating and signing these records on demand, authoritative name servers can effectively stop the disclosure of zone contents otherwise made possible by walking the chain of NSEC records in a signed zone. [STANDARDS-TRACK]GPU-Based NSEC3 Hash BreakingAn efficient DNSSEC zone enumeration algorithmAmbiguity 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.At the time of publication, setting an upper limit of 100 iterations
for treating a zone as insecure is interoperable without significant
problems, but at the same time still enables CPU-exhausting DoS
attacks.As the time of publication, returning SERVFAIL beyond 500 iterations
appears to be interoperable without significant problems.The Queries Per Second (QPS) of authoritative servers will decrease due
to computational overhead when processing DNS requests for zones
containing higher NSEC3 iteration counts. The table () below
shows the drop in QPS for various iteration counts.The authors would like to thank the dns-operations discussion
participants, which took place on mattermost hosted by DNS-OARC.Additionally, the following people contributed text or review comments
to the draft:Vladimír ČunátTony FinchPaul HoffmanAlexander MayrhoferMatthijs MekkingFlorian ObserPetr ŠpačekPaul VixieWhile this document is under development, it can be viewed, tracked,
issued, pushed with PRs, … here:https://github.com/hardaker/draft-hardaker-dnsop-nsec3-guidanceThe following implementations have implemented the guidance in this
document. They have graciously provided notes about the details of
their implementation below.The OpenDNSSEC configuration checking utility will alert the user
about nsec3 iteration values larger than 100.PowerDNS 4.5.2 changed the default value of nsec3-max-iterations to 150.Knot DNS 3.0.6 warns when signing with more than 20 NSEC3 iterations.
Knot Resolver 5.3.1 treats NSEC3 iterations above 150 as insecure.Google Public DNS treats NSEC3 iterations above 100 as insecure since
September 2021.Google Cloud DNS uses 1 iteration and 64-bits of fixed random salt for
all zones using NSEC3. These parameters cannot be adjusted by users.