Internet Engineering Task Force (IETF) K. Fujiwara
Request for Comments: 8198 JPRS
Updates: 4035 A. Kato
Category: Standards Track Keio/WIDE
ISSN: 2070-1721 W. Kumari
July 2017 Aggressive Use of DNSSEC-Validated Cache
The DNS relies upon caching to scale; however, the cache lookup
generally requires an exact match. This document specifies the use
of NSEC/NSEC3 resource records to allow DNSSEC-validating resolvers
to generate negative answers within a range and positive answers from
wildcards. This increases performance, decreases latency, decreases
resource utilization on both authoritative and recursive servers, and
increases privacy. Also, it may help increase resilience to certain
DoS attacks in some circumstances.
This document updates RFC 4035 by allowing validating resolvers to
generate negative answers based upon NSEC/NSEC3 records and positive
answers in the presence of wildcards.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(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 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
A DNS negative cache exists, and is used to cache the fact that an
RRset does not exist. This method of negative caching requires exact
matching; this leads to unnecessary additional lookups, increases
latency, leads to extra resource utilization on both authoritative
and recursive servers, and decreases privacy by leaking queries.
This document updates RFC 4035 to allow resolvers to use NSEC/NSEC3
resource records to synthesize negative answers from the information
they have in the cache. This allows validating resolvers to respond
with a negative answer immediately if the name in question falls into
a range expressed by an NSEC/NSEC3 resource record already in the
cache. It also allows the synthesis of positive answers in the
presence of wildcard records.
Aggressive negative caching was first proposed in Section 6 of DNSSEC
Lookaside Validation (DLV) [RFC5074] in order to find covering NSEC
[RFC8020] and [RES-IMPROVE] propose steps to using NXDOMAIN
information for more effective caching. This document takes this
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 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Many of the specialized terms used in this document are defined in
DNS Terminology [RFC7719].
The key words "source of synthesis" in this document are to be
interpreted as described in [RFC4592].
3. Problem Statement
The DNS negative cache caches negative (non-existent) information,
and requires an exact match in most instances [RFC2308].
Assume that the (DNSSEC-signed) "example.com" zone contains:
albatross.example.com. IN A 192.0.2.1
elephant.example.com. IN A 192.0.2.2
zebra.example.com. IN A 192.0.2.3
If a validating resolver receives a query for cat.example.com, it
contacts its resolver (which may be itself) to query the example.com
servers and will get back an NSEC record stating that there are no
records (alphabetically) between albatross and elephant, or an NSEC3
record stating there is nothing between two hashed names. The
resolver then knows that cat.example.com does not exist; however, it
does not use the fact that the proof covers a range (albatross to
elephant) to suppress queries for other labels that fall within this
range. This means that if the validating resolver gets a query for
ball.example.com (or dog.example.com) it will once again go off and
query the example.com servers for these names.
Apart from wasting bandwidth, this also wastes resources on the
recursive server (it needs to keep state for outstanding queries),
wastes resources on the authoritative server (it has to answer
additional questions), increases latency (the end user has to wait
longer than necessary to get back an NXDOMAIN answer), can be used by
attackers to cause a DoS, and also has privacy implications (e.g.,
typos leak out further than necessary).
Another example: assume that the (DNSSEC-signed) "example.org" zone
avocado.example.org. IN A 192.0.2.1
*.example.org. IN A 192.0.2.2
zucchini.example.org. IN A 192.0.2.3
If a query is received for leek.example.org, the system contacts its
resolver (which may be itself) to query the example.org servers and
will get back an NSEC record stating that there are no records
(alphabetically) between avocado and zucchini (or an NSEC3 record
stating there is nothing between two hashed names), as well as an
answer for leek.example.org, with the label count of the signature
set to two (see [RFC7129], Section 5.3 for more details).
If the validating resolver gets a query for banana.example.org, it
will once again go off and query the example.org servers for
banana.example.org (even though it already has proof that there is a
wildcard record) -- just like above, this has privacy implications,
wastes resources, can be used to contribute to a DoS, etc.
DNSSEC [RFC4035] and [RFC5155] both provide "authenticated denial of
existence"; this is a cryptographic proof that the queried-for name
does not exist or the type does not exist. Proof that a name does
not exist is accomplished by providing a (DNSSEC-secured) record
containing the names that appear alphabetically before and after the
queried-for name. In the first example above, if the (DNSSEC-
validating) recursive server were to query for dog.example.com, it
would receive a (signed) NSEC record stating that there are no labels
between "albatross" and "elephant" (or, for NSEC3, a similar pair of
hashed names). This is a signed, cryptographic proof that these
names are the ones before and after the queried-for label. As
dog.example.com falls within this range, the recursive server knows
that dog.example.com really does not exist. Proof that a type does
not exist is accomplished by providing a (DNSSEC-secured) record
containing the queried-for name, and a type bitmap that does not
include the requested type.
This document specifies that this NSEC/NSEC3 record should be used to
generate negative answers for any queries that the validating server
receives that fall within the range covered by the record (for the
TTL for the record). This document also specifies that a positive
answer should be generated for any queries that the validating server
receives that are proven to be covered by a wildcard record.
Section 4.5 of [RFC4035] says:
In theory, a resolver could use wildcards or NSEC RRs to generate
positive and negative responses (respectively) until the TTL or
signatures on the records in question expire. However, it seems
prudent for resolvers to avoid blocking new authoritative data or
synthesizing new data on their own. Resolvers that follow this
recommendation will have a more consistent view of the namespace.
And, earlier, Section 4.5 of [RFC4035] says:
The reason for these recommendations is that, between the initial
query and the expiration of the data from the cache, the
authoritative data might have been changed (for example, via
In other words, if a resolver generates negative answers from an NSEC
record, it will not send any queries for names within that NSEC range
(for the TTL). If a new name is added to the zone during this
interval, the resolver will not know this. Similarly, if the
resolver is generating responses from a wildcard record, it will
continue to do so (for the TTL).
We believe that this recommendation can be relaxed because, in the
absence of this technique, a lookup for the exact name could have
come in during this interval, and so a negative answer could already
be cached (see [RFC2308] for more background). This means that zone
operators should have no expectation that an added name would work
immediately. With DNSSEC and aggressive use of DNSSEC-validated
cache, the TTL of the NSEC/NSEC3 record and the SOA.MINIMUM field are
the authoritative statement of how quickly a name can start working
within a zone.
5. Aggressive Use of DNSSEC-Validated Cache
This document relaxes the restriction given in Section 4.5 of
[RFC4035]. See Section 7 for more detail.
If the negative cache of the validating resolver has sufficient
information to validate the query, the resolver SHOULD use NSEC,
NSEC3, and wildcard records to synthesize answers as described in
this document. Otherwise, it MUST fall back to send the query to the
authoritative DNS servers.
The validating resolver needs to check the existence of an NSEC RR
matching/covering the source of synthesis and an NSEC RR covering the
If denial of existence can be determined according to the rules set
out in Section 5.4 of [RFC4035], using NSEC records in the cache,
then the resolver can immediately return an NXDOMAIN or NODATA (as
NSEC3 aggressive negative caching is more difficult than NSEC
aggressive caching. If the zone is signed with NSEC3, the validating
resolver needs to check the existence of non-terminals and wildcards
that derive from query names.
If denial of existence can be determined according to the rules set
out in [RFC5155], Sections 8.4, 8.5, 8.6, and 8.7, using NSEC3
records in the cache, then the resolver can immediately return an
NXDOMAIN or NODATA response (as appropriate).
If a covering NSEC3 RR has an Opt-Out flag, the covering NSEC3 RR
does not prove the non-existence of the domain name and the
aggressive negative caching is not possible for the domain name.
The last paragraph of [RFC4035], Section 4.5 also discusses the use
of wildcards and NSEC RRs to generate positive responses and
recommends that it not be relied upon. Just like the case for the
aggressive use of NSEC/NSEC3 for negative answers, we revise this
As long as the validating resolver can determine that a name would
not exist without the wildcard match, determined according to the
rules set out in Section 5.3.4 of [RFC4035] (NSEC), or in Section 8.8
of [RFC5155], it SHOULD synthesize an answer (or NODATA response) for
that name using the cache-deduced wildcard. If the corresponding
wildcard record is not in the cache, it MUST fall back to send the
query to the authoritative DNS servers.
5.4. Consideration on TTL
The TTL value of negative information is especially important,
because newly added domain names cannot be used while the negative
information is effective.
Section 5 of [RFC2308] suggests a maximum default negative cache TTL
value of 3 hours (10800). It is RECOMMENDED that validating
resolvers limit the maximum effective TTL value of negative responses
(NSEC/NSEC3 RRs) to this same value.
Section 5 of [RFC2308] also states that a negative cache entry TTL is
taken from the minimum of the SOA.MINIMUM field and SOA's TTL. This
can be less than the TTL of an NSEC or NSEC3 record, since their TTL
is equal to the SOA.MINIMUM field (see [RFC4035], Section 2.3 and
[RFC5155], Section 3).
A resolver that supports aggressive use of NSEC and NSEC3 SHOULD
reduce the TTL of NSEC and NSEC3 records to match the SOA.MINIMUM
field in the authority section of a negative response, if SOA.MINIMUM
The techniques described in this document provide a number of
benefits, including (in no specific order):
Reduced latency: By answering directly from cache, validating
resolvers can immediately inform clients that the name they are
looking for does not exist, improving the user experience.
Decreased recursive server load: By answering queries from the cache
by synthesizing answers, validating servers avoid having to send a
query and wait for a response. In addition to decreasing the
bandwidth used, it also means that the server does not need to
allocate and maintain state, thereby decreasing memory and CPU
Decreased authoritative server load: Because recursive servers can
answer queries without asking the authoritative server, the
authoritative servers receive fewer queries. This decreases the
authoritative server bandwidth, queries per second, and CPU
The scale of the benefit depends upon multiple factors, including the
query distribution. For example, at the time of this writing, around
65% of queries to root name servers result in NXDOMAIN responses (see
statistics from [ROOT-SERVERS]); this technique will eliminate a
sizable quantity of these.
The technique described in this document may also mitigate so-called
"random QNAME attacks", in which attackers send many queries for
random subdomains to resolvers. As the resolver will not have the
answers cached, it has to ask external servers for each random query,
leading to a DoS on the authoritative servers (and often resolvers).
The technique may help mitigate these attacks by allowing the
resolver to answer directly from the cache for any random queries
that fall within already requested ranges. It will not always work
as an effective defense, not least because not many zones are DNSSEC
signed at all -- but it will still provide an additional layer of
As these benefits are only accrued by those using DNSSEC, it is hoped
that these techniques will lead to more DNSSEC deployment.
7. Update to RFC 4035
Section 4.5 of [RFC4035] shows that "In theory, a resolver could use
wildcards or NSEC RRs to generate positive and negative responses
(respectively) until the TTL or signatures on the records in question
expire. However, it seems prudent for resolvers to avoid blocking
new authoritative data or synthesizing new data on their own.
Resolvers that follow this recommendation will have a more consistent
view of the namespace".
The paragraph is updated as follows:
| Once the records are validated, DNSSEC-enabled validating |
| resolvers SHOULD use wildcards and NSEC/NSEC3 resource records |
| to generate positive and negative responses until the |
| effective TTLs or signatures for those records expire. |
8. IANA Considerations
This document does not require any IANA actions.
9. Security Considerations
Use of NSEC/NSEC3 resource records without DNSSEC validation may
create serious security issues, and so this technique requires DNSSEC
Newly registered resource records may not be used immediately.
However, choosing a suitable TTL value and a negative cache TTL value
(SOA.MINIMUM field) will mitigate the delay concern, and it is not a
It is also suggested to limit the maximum TTL value of NSEC/NSEC3
resource records in the negative cache to, for example, 10800 seconds
(3 hours), to mitigate this issue.
Although the TTL of NSEC/NSEC3 records is typically fairly short
(minutes or hours), their RRSIG expiration time can be much further
in the future (weeks). An attacker who is able to successfully spoof
responses might poison a cache with old NSEC/NSEC3 records. If the
resolver is not making aggressive use of NSEC/NSEC3, the attacker has
to repeat the attack for every query. If the resolver is making
aggressive use of NSEC/NSEC3, one successful attack would be able to
suppress many queries for new names, up to the negative TTL.
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
[RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,
[RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "Protocol Modifications for the DNS Security
Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
Appendix A. Detailed Implementation Notes
o Previously, cached negative responses were indexed by QNAME,
QCLASS, QTYPE, and the setting of the CD bit (see RFC 4035,
Section 4.7), and only queries matching the index key would be
answered from the cache. With aggressive negative caching, the
validator, in addition to checking to see if the answer is in its
cache before sending a query, checks to see whether any cached and
validated NSEC record denies the existence of the sought
record(s). Using aggressive negative caching, a validator will
not make queries for any name covered by a cached and validated
NSEC record. Furthermore, a validator answering queries from
clients will synthesize a negative answer (or NODATA response)
whenever it has an applicable validated NSEC in its cache unless
the CD bit was set on the incoming query. (Imported from
Section 6 of [RFC5074].)
o Implementing aggressive negative caching suggests that a validator
will need to build an ordered data structure of NSEC and NSEC3
records for each signer domain name of NSEC/NSEC3 records in order
to efficiently find covering NSEC/NSEC3 records. Call the table
as "NSEC_TABLE". (Imported from Section 6.1 of [RFC5074] and
o The aggressive negative caching may be inserted at the cache
lookup part of the recursive resolvers.
o If errors happen in an aggressive negative caching algorithm,
resolvers MUST fall back to resolve the query as usual. "Resolve
the query as usual" means that the resolver must process the query
as though it does not implement aggressive negative caching.
Appendix B. Procedure for Determining ENT vs. NXDOMAIN with NSEC
This procedure outlines how to determine if a given name does not
exist, or is an ENT (empty non-terminal; see [RFC5155], Section 1.3)
If the NSEC record has not been verified as secure, discard it.
If the given name sorts before or matches the NSEC owner name,
discard it as it does not prove the NXDOMAIN or ENT.
If the given name is a subdomain of the NSEC owner name and the NS
bit is present and the SOA bit is absent, then discard the NSEC as it
is from a parent zone.
If the next domain name sorts after the NSEC owner name and the given
name sorts after or matches next domain name, then discard the NSEC
record as it does not prove the NXDOMAIN or ENT.
If the next domain name sorts before or matches the NSEC owner name
and the given name is not a subdomain of the next domain name, then
discard the NSEC as it does not prove the NXDOMAIN or ENT.
You now have an NSEC record that proves the NXDOMAIN or ENT.
If the next domain name is a subdomain of the given name, you have an
ENT. Otherwise, you have an NXDOMAIN.
The authors gratefully acknowledge DNSSEC Lookaside Validation (DLV)
[RFC5074] author Samuel Weiler and the Unbound developers.
Thanks to Mark Andrews for providing the helpful notes for
implementors provided in Appendix B.
The authors would like to specifically thank Stephane Bortzmeyer (for
standing next to and helping edit), Ralph Dolmans, Tony Finch, Tatuya
JINMEI for extensive review and comments, and also Mark Andrews,
Casey Deccio, Alexander Dupuy, Olafur Gudmundsson, Bob Harold, Shumon
Huque, John Levine, Pieter Lexis, Matthijs Mekking (who even sent
pull requests!), and Ondrej Sury.
Japan Registry Services Co., Ltd.
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Phone: +81 3 5215 8451
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