draft-ietf-dnsop-nsec-aggressiveuse-06.txt   draft-ietf-dnsop-nsec-aggressiveuse-07.txt 
Network Working Group K. Fujiwara Network Working Group K. Fujiwara
Internet-Draft JPRS Internet-Draft JPRS
Updates: 4035 (if approved) A. Kato Updates: 4035 (if approved) A. Kato
Intended status: Standards Track Keio/WIDE Intended status: Standards Track Keio/WIDE
Expires: May 20, 2017 W. Kumari Expires: June 16, 2017 W. Kumari
Google Google
November 16, 2016 December 13, 2016
Aggressive use of NSEC/NSEC3 Aggressive use of NSEC/NSEC3
draft-ietf-dnsop-nsec-aggressiveuse-06 draft-ietf-dnsop-nsec-aggressiveuse-07
Abstract Abstract
The DNS relies upon caching to scale; however, the cache lookup The DNS relies upon caching to scale; however, the cache lookup
generally requires an exact match. This document specifies the use generally requires an exact match. This document specifies the use
of NSEC/NSEC3 resource records to allow DNSSEC validating resolvers of NSEC/NSEC3 resource records to allow DNSSEC validating resolvers
to generate negative answers within a range, and positive answers to generate negative answers within a range, and positive answers
from wildcards. This increases performance / decreases latency, from wildcards. This increases performance / decreases latency,
decreases resource utilization on both authoritative and recursive decreases resource utilization on both authoritative and recursive
servers, and also increases privacy. It may also help increase servers, and also increases privacy. It may also help increase
resilience to certain DoS attacks in some circumstances. resilience to certain DoS attacks in some circumstances.
This document updates RFC4035 by allowing validating resolvers to This document updates RFC4035 by allowing validating resolvers to
generate negative based upon NSEC/NSEC3 records (and positive answers generate negative answers based upon NSEC/NSEC3 records (and positive
in the presence of wildcards). answers in the presence of wildcards).
[ Ed note: Text inside square brackets ([]) is additional background [ Ed note: Text inside square brackets ([]) is additional background
information, answers to frequently asked questions, general musings, information, answers to frequently asked questions, general musings,
etc. They will be removed before publication.This document is being etc. They will be removed before publication.This document is being
collaborated on in Github at: https://github.com/wkumari/draft-ietf- collaborated on in Github at: https://github.com/wkumari/draft-ietf-
dnsop-nsec-aggressiveuse. The most recent version of the document, dnsop-nsec-aggressiveuse. The most recent version of the document,
open issues, etc should all be available here. The authors open issues, etc should all be available here. The authors
(gratefully) accept pull requests.] (gratefully) accept pull requests.]
Status of This Memo Status of This Memo
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 20, 2017. This Internet-Draft will expire on June 16, 2017.
Copyright Notice Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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include Simplified BSD License text as described in Section 4.e of include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3 3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3
4. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4 4. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Aggressive use of Cache . . . . . . . . . . . . . . . . . . . 5 5. Aggressive use of Cache . . . . . . . . . . . . . . . . . . . 6
5.1. NSEC . . . . . . . . . . . . . . . . . . . . . . . . . . 6 5.1. NSEC . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.2. NSEC3 . . . . . . . . . . . . . . . . . . . . . . . . . . 6 5.2. NSEC3 . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.3. Wildcards . . . . . . . . . . . . . . . . . . . . . . . . 6 5.3. Wildcards . . . . . . . . . . . . . . . . . . . . . . . . 6
5.4. Consideration on TTL . . . . . . . . . . . . . . . . . . 7 5.4. Consideration on TTL . . . . . . . . . . . . . . . . . . 7
6. Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . 7 6. Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7. Update to RFC 4035 . . . . . . . . . . . . . . . . . . . . . 8 7. Update to RFC 4035 . . . . . . . . . . . . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
9. Security Considerations . . . . . . . . . . . . . . . . . . . 9 9. Security Considerations . . . . . . . . . . . . . . . . . . . 9
10. Implementation Status . . . . . . . . . . . . . . . . . . . . 9 10. Implementation Status . . . . . . . . . . . . . . . . . . . . 9
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
11.1. Change History . . . . . . . . . . . . . . . . . . . . . 10 11.1. Change History . . . . . . . . . . . . . . . . . . . . . 10
11.1.1. Version draft-fujiwara-dnsop-nsec-aggressiveuse-01 . 12 11.1.1. Version draft-fujiwara-dnsop-nsec-aggressiveuse-01 . 13
11.1.2. Version draft-fujiwara-dnsop-nsec-aggressiveuse-02 . 13 11.1.2. Version draft-fujiwara-dnsop-nsec-aggressiveuse-02 . 13
11.1.3. Version draft-fujiwara-dnsop-nsec-aggressiveuse-03 . 13 11.1.3. Version draft-fujiwara-dnsop-nsec-aggressiveuse-03 . 13
11.2. new section . . . . . . . . . . . . . . . . . . . . . . 13 11.2. new section . . . . . . . . . . . . . . . . . . . . . . 13
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
12.1. Normative References . . . . . . . . . . . . . . . . . . 13 12.1. Normative References . . . . . . . . . . . . . . . . . . 13
12.2. Informative References . . . . . . . . . . . . . . . . . 14 12.2. Informative References . . . . . . . . . . . . . . . . . 14
Appendix A. Detailed implementation notes . . . . . . . . . . . 14 Appendix A. Detailed implementation notes . . . . . . . . . . . 14
Appendix B. Procedure for determining ENT vs NXDOMAN . . . . . . 15 Appendix B. Procedure for determining ENT vs NXDOMAN with NSEC . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
1. Introduction 1. Introduction
A DNS negative cache exists, and is used to cache the fact that a A DNS negative cache exists, and is used to cache the fact that an
name does not exist. This method of negative caching requires exact RRset does not exist. This method of negative caching requires exact
matching; this leads to unnecessary additional lookups, increases matching; this leads to unnecessary additional lookups, increases
latency, leads to extra resource utilization on both authoritative latency, leads to extra resource utilization on both authoritative
and recursive servers, and decreases privacy by leaking queries. and recursive servers, and decreases privacy by leaking queries.
This document updates RFC 4035 to allow recursive resolvers to use This document updates RFC 4035 to allow recursive resolvers to use
NSEC/NSEC3 resource records to synthesize negative answers from the NSEC/NSEC3 resource records to synthesize negative answers from the
information they have in the cache. This allows validating resolvers information they have in the cache. This allows validating resolvers
to respond with NXDOMAIN immediately if the name in question falls to respond with a negative answer immediately if the name in question
into a range expressed by a NSEC/NSEC3 resource record already in the falls into a range expressed by a NSEC/NSEC3 resource record already
cache. It also allows the synthesis of positive answers in the in the cache. It also allows the synthesis of positive answers in
presence of wildcard records. the presence of wildcard records.
Aggressive Negative Caching was first proposed in Section 6 of DNSSEC Aggressive Negative Caching was first proposed in Section 6 of DNSSEC
Lookaside Validation (DLV) [RFC5074] in order to find covering NSEC Lookaside Validation (DLV) [RFC5074] in order to find covering NSEC
records efficiently. records efficiently.
[RFC8020] proposes a first step to using NXDOMAIN information for [RFC8020], and [I-D.vixie-dnsext-resimprove] proposes first steps to
more effective caching. This takes this technique further. using NXDOMAIN information for more effective caching. This takes
this technique further.
2. Terminology 2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
Many of the specialized terms used in this document are defined in Many of the specialized terms used in this document are defined in
DNS Terminology [RFC7719]. DNS Terminology [RFC7719].
The key words "Closest Encloser" and "Source of Synthesis" in this The key words "Source of Synthesis" in this document are to be
document are to be interpreted as described in [RFC4592]. interpreted as described in [RFC4592].
"Closest Encloser" is also defined in NSEC3 [RFC5155], as is "Next
closer name".
3. Problem Statement 3. Problem Statement
The DNS negative cache caches negative (non-existent) information, The DNS negative cache caches negative (non-existent) information,
and requires an exact match in most instances [RFC2308]. and requires an exact match in most instances [RFC2308].
Assume that the (DNSSEC signed) "example.com" zone contains: Assume that the (DNSSEC signed) "example.com" zone contains:
albatross.example.com IN A 192.0.2.1 albatross.example.com IN A 192.0.2.1
elephant.example.com IN A 192.0.2.2 elephant.example.com IN A 192.0.2.2
zebra.example.com IN A 192.0.2.3 zebra.example.com IN A 192.0.2.3
If a validating resolver receives a query for cat.example.com, it If a validating resolver receives a query for cat.example.com, it
contacts its resolver (which may be itself) to query the example.com contacts its resolver (which may be itself) to query the example.com
servers and will get back an NSEC record starting that there are no servers and will get back an NSEC record stating that there are no
records (alphabetically) between albatross and elephant, or an NSEC3 records (alphabetically) between albatross and elephant, or an NSEC3
record stating there is nothing between two hashed names. The record stating there is nothing between two hashed names. The
resolver then knows that cat.example.com does not exist; however, it 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 does not use the fact that the proof covers a range (albatross to
elephant) to suppress queries for other labels that fall within this elephant) to suppress queries for other labels that fall within this
range. This means that if the validating resolver gets a query for 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 ball.example.com (or dog.example.com) it will once again go off and
query the example.com servers for these names. query the example.com servers for these names.
Apart from wasting bandwidth, this also wastes resources on the Apart from wasting bandwidth, this also wastes resources on the
recursive server (it needs to keep state for outstanding queries), recursive server (it needs to keep state for outstanding queries),
wastes resources on the authoritative server (it has to answer wastes resources on the authoritative server (it has to answer
additional questions), increases latency (the end user has to wait additional questions), increases latency (the end user has to wait
longer than necessary to get back an NXDOMAIN answer), can be used by longer than necessary to get back an NXDOMAIN answer), can be used by
attackers to cause a DoS (see additional resources), and also has attackers to cause a DoS (see additional resources), and also has
privacy implications (e.g: typos leak out further than necessary). privacy implications (e.g: typos leak out further than necessary).
Now, assume that the (DNSSEC signed) "example.org" zone contains: Another example: assume that the (DNSSEC signed) "example.org" zone
contains:
avocado.example.org IN A 192.0.2.1 avocado.example.org IN A 192.0.2.1
*.example.org IN A 192.0.2.2 *.example.org IN A 192.0.2.2
zucchini.example.org IN A 192.0.2.3 zucchini.example.org IN A 192.0.2.3
If a query is received for leek.example.org, it contacts its resolver If a query is received for leek.example.org, it contacts its resolver
(which may be itself) to query the example.org servers and will get (which may be itself) to query the example.org servers and will get
back an NSEC record stating that there are no records back an NSEC record stating that there are no records
(alphabetically) between avocado and zucchini (or an NSEC3 record (alphabetically) between avocado and zucchini (or an NSEC3 record
stating there is nothing between two hashed names), as well as an stating there is nothing between two hashed names), as well as an
answer for leek.example.org, with the label count of the signature answer for leek.example.org, with the label count of the signature
set to two (see [RFC7129], section 5.3 for more details). set to two (see [RFC7129], section 5.3 for more details).
If the validating resolver gets a query for banana.example.org it If the validating resolver gets a query for banana.example.org it
will once again go off and query the example.com servers for will once again go off and query the example.org servers for
banana.example.com (even though it already has proof that there is a banana.example.org (even though it already has proof that there is a
wildcard record) - just like above, this has privacy implications, wildcard record) - just like above, this has privacy implications,
wastes resources, can be used to contribute to a DoS, etc. wastes resources, can be used to contribute to a DoS, etc.
4. Background 4. Background
DNSSEC [RFC4035] and [RFC5155] both provide "authenticated denial of DNSSEC [RFC4035] and [RFC5155] both provide "authenticated denial of
existence"; this is a cryptographic proof that the queried for name existence"; this is a cryptographic proof that the queried for name
does not exist, accomplished by providing a (DNSSEC secured) record does not exist or type does not exist. Proof that a name does not
exist is accomplished by providing a (DNSSEC secured) record
containing the names which appear alphabetically before and after the containing the names which appear alphabetically before and after the
queried for name. In the first example above, if the (DNSSEC queried for name. In the first example above, if the (DNSSEC
validating) recursive server were to query for dog.example.com it validating) recursive server were to query for dog.example.com it
would receive a (signed) NSEC record stating that there are no labels would receive a (signed) NSEC record stating that there are no labels
between "albatross" and "elephant" (or, for NSEC3, a similar pair of between "albatross" and "elephant" (or, for NSEC3, a similar pair of
hashed names). This is a signed, cryptographic proof that these hashed names). This is a signed, cryptographic proof that these
names are the ones before and after the queried for label. As names are the ones before and after the queried for label. As
dog.example.com falls within this range, the recursive server knows dog.example.com falls within this range, the recursive server knows
that dog.example.com really does not exist. 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 which does not
include the requested type.
This document specifies that this NSEC/NSEC3 record should be used to This document specifies that this NSEC/NSEC3 record should be used to
generate negative answers for any queries that the validating server generate negative answers for any queries that the validating server
receives that fall within the range covered by the record (for the receives that fall within the range covered by the record (for the
TTL for the record). This document also specifies that a positive TTL for the record). This document also specifies that a positive
answer should be generated for any queries that the validating server answer should be generated for any queries that the validating server
receives that are proven to be covered by a wildcard record. receives that are proven to be covered by a wildcard record.
Section 4.5 of [RFC4035] says: Section 4.5 of [RFC4035] says:
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synthesizing new data on their own. Resolvers that follow this synthesizing new data on their own. Resolvers that follow this
recommendation will have a more consistent view of the namespace." recommendation will have a more consistent view of the namespace."
and "The reason for these recommendations is that, between the and "The reason for these recommendations is that, between the
initial query and the expiration of the data from the cache, the initial query and the expiration of the data from the cache, the
authoritative data might have been changed (for example, via dynamic authoritative data might have been changed (for example, via dynamic
update).". In other words, if a resolver generates negative answers update).". In other words, if a resolver generates negative answers
from an NSEC record, it will not send any queries for names within 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 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 during this interval the resolver will not know this. Similarly, if
the resolver is generating responses from a wildcard record, it will the resolver is generating responses from a wildcard record, it will
continue to do so (for the continue to do so (for the TTL).
We believe this recommendation can be relaxed because, in the absense We believe this recommendation can be relaxed because, in the absence
of this technique, a lookup for the exact name could have come in of this technique, a lookup for the exact name could have come in
during this interval, and so a negative answer could already be during this interval, and so a negative answer could already be
cached (see [RFC2308] for more background). This means that zone cached (see [RFC2308] for more background). This means that zone
operators should have no expectation that an added name would work operators should have no expectation that an added name would work
immediately. With DNSSEC and Aggressive NSEC, the TTL of the NSEC immediately. With DNSSEC and Aggressive NSEC, the TTL of the NSEC/
record is the authoritative statement of how quickly a name can start NSEC3 record and the SOA.MINIMUM field are the authoritative
working within a zone. statement of how quickly a name can start working within a zone.
5. Aggressive use of Cache 5. Aggressive use of Cache
Section 4.5 of [RFC4035] says that "In theory, a resolver could use Section 4.5 of [RFC4035] says that "In theory, a resolver could use
wildcards or NSEC RRs to generate positive and negative responses wildcards or NSEC RRs to generate positive and negative responses
(respectively) until the TTL or signatures on the records in question (respectively) until the TTL or signatures on the records in question
expire. However, it seems prudent for resolvers to avoid blocking expire. However, it seems prudent for resolvers to avoid blocking
new authoritative data or synthesizing new data on their own. new authoritative data or synthesizing new data on their own.
Resolvers that follow this recommendation will have a more consistent Resolvers that follow this recommendation will have a more consistent
view of the namespace". This document relaxes this this restriction, view of the namespace". This document relaxes this this restriction,
see Section 7 for more detail. see Section 7 for more detail.
If the negative cache of the validating resolver has sufficient If the negative cache of the validating resolver has sufficient
information to validate the query, the resolver SHOULD use NSEC, information to validate the query, the resolver SHOULD use NSEC,
NSEC3 and wildcard records aggressively. Otherwise, it MUST fall NSEC3 and wildcard records aggressively. Otherwise, it MUST fall
back to send the query to the authoritative DNS servers. back to send the query to the authoritative DNS servers.
It is recommended that resolvers that implement Aggressive Negative
Caching provide a configuration switch to disable the feature.
Separate configuration switches may be implemented for the aggressive
use of NSEC, NSEC3 and wildcard records, and it is recommended to
enable aggressive negative caching by default.
5.1. NSEC 5.1. NSEC
The validating resolver needs to check the existence of an NSEC RR The validating resolver needs to check the existence of an NSEC RR
matching/covering the source of synthesis and an NSEC RR covering the matching/covering the source of synthesis and an NSEC RR covering the
query name. query name.
If denial of existence can be determined according to the rules set 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, out in Section 5.4 of [RFC4035], using NSEC records in the cache,
then the resolver can immediately return an NXDOMAIN or NODATA (as then the resolver can immediately return an NXDOMAIN or NODATA (as
appropriate) response. appropriate) response.
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The last paragraph of [RFC4035] Section 4.5 also discusses the use of The last paragraph of [RFC4035] Section 4.5 also discusses the use of
wildcards and NSEC RRs to generate positive responses and recommends wildcards and NSEC RRs to generate positive responses and recommends
that it not be relied upon. Just like the case for the aggressive that it not be relied upon. Just like the case for the aggressive
use of NSEC/NSEC3 for negative answers, we revise this use of NSEC/NSEC3 for negative answers, we revise this
recommendation. recommendation.
As long as the validating resolver can determine that a name would As long as the validating resolver can determine that a name would
not exist without the wildcard match, determined according to the 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 rules set out in Section 5.3.4 of [RFC4035] (NSEC), or in Section 8.8
of [RFC5155], it SHOULD synthesize an answer for that name using the of [RFC5155], it SHOULD synthesize an answer (or NODATA response) for
cached deduced wildcard. If the corresponding wildcard record is not that name using the cached deduced wildcard. If the corresponding
in the cache, it MUST fall back to send the query to the wildcard record is not in the cache, it MUST fall back to send the
authoritative DNS servers. query to the authoritative DNS servers.
5.4. Consideration on TTL 5.4. Consideration on TTL
The TTL value of negative information is especially important, The TTL value of negative information is especially important,
because newly added domain names cannot be used while the negative because newly added domain names cannot be used while the negative
information is effective. information is effective.
Section 5 of [RFC2308] states that the maximum number of negative Section 5 of [RFC2308] states that the maximum number of negative
cache TTL value is 3 hours (10800). It is RECOMMENDED that cache TTL value is 3 hours (10800). It is RECOMMENDED that
validating resolvers limit the maximum effective TTL value of validating resolvers limit the maximum effective TTL value of
negative responses (NSEC/NSEC3 RRs) to this same value. negative responses (NSEC/NSEC3 RRs) to this same value.
Section 5 of [RFC2308]also states that a negative cache entry TTL is 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 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 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 is equal to the SOA.MINIMUM field (see [RFC4035]section 2.3 and
[RFC5155] section 3.) [RFC5155] section 3.)
A resolver that supports aggressive use of NSEC and NSEC3 should A resolver that supports aggressive use of NSEC and NSEC3 SHOULD
reduce the TTL of NSEC and NSEC3 records to match the SOA.MINIMUM 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 field in the authority section of a negative response, if SOA.MINIMUM
is smaller. is smaller.
6. Benefits 6. Benefits
The techniques described in this document provide a number of The techniques described in this document provide a number of
benefits, including (in no specific order): benefits, including (in no specific order):
Reduced latency: By answering directly from cache, validating Reduced latency: By answering directly from cache, validating
resolvers can immediately inform clients that the name they are resolvers can immediately inform clients that the name they are
looking for does not exist, improving the user experience. looking for does not exist, improving the user experience.
Decreased recursive server load: By answering queries from the cache Decreased recursive server load: By answering queries from the cache
by synthesizing answers, validating servers avoid having to send a by synthesizing answers, validating servers avoid having to send a
query and wait for a response. In addition to decreasing the query and wait for a response. In addition to decreasing the
bandwidth used, it also means that the server does not need to bandwidth used, it also means that the server does not need to
allocate and maintain state, thereby decreasing memory and CPU allocate and maintain state, thereby decreasing memory and CPU
load. load.
Decreased authorative server load: Because recursive servers can Decreased authoritative server load: Because recursive servers can
answer queries without asking the authoritative server, the answer queries without asking the authoritative server, the
authoritative servers receive fewer queries. This decreases the authoritative servers receive fewer queries. This decreases the
authoritative server bandwidth, queries per second and CPU authoritative server bandwidth, queries per second and CPU
utilization. utilization.
The scale of the benefit depends upon multiple factors, including the The scale of the benefit depends upon multiple factors, including the
query distribution. For example, at the time of this writing, around query distribution. For example, at the time of this writing, around
65% of queries to Root Name servers result in NXDOMAIN responses (see 65% of queries to Root Name servers result in NXDOMAIN responses (see
statistics from [root-servers.org]); this technique will eliminate a statistics from [root-servers.org]); this technique will eliminate a
sizable quantity of these. sizable quantity of these.
skipping to change at page 8, line 28 skipping to change at page 8, line 28
"random QNAME attacks", in which attackers send many queries for "random QNAME attacks", in which attackers send many queries for
random sub-domains to resolvers. As the resolver will not have the random sub-domains to resolvers. As the resolver will not have the
answers cached, it has to ask external servers for each random query, answers cached, it has to ask external servers for each random query,
leading to a DoS on the authoritative servers (and often resolvers). leading to a DoS on the authoritative servers (and often resolvers).
Aggressive NSEC may help mitigate these attacks by allowing the Aggressive NSEC may help mitigate these attacks by allowing the
resolver to answer directly from cache for any random queries which resolver to answer directly from cache for any random queries which
fall within already requested ranges. It will not always work as an fall within already requested ranges. It will not always work as an
effective defense, not least because not many zones are DNSSEC signed effective defense, not least because not many zones are DNSSEC signed
at all -- but it will still provide an additional layer of defense. at all -- but it will still provide an additional layer of defense.
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 7. Update to RFC 4035
Section 4.5 of [RFC4035] shows that "In theory, a resolver could use Section 4.5 of [RFC4035] shows that "In theory, a resolver could use
wildcards or NSEC RRs to generate positive and negative responses wildcards or NSEC RRs to generate positive and negative responses
(respectively) until the TTL or signatures on the records in question (respectively) until the TTL or signatures on the records in question
expire. However, it seems prudent for resolvers to avoid blocking expire. However, it seems prudent for resolvers to avoid blocking
new authoritative data or synthesizing new data on their own. new authoritative data or synthesizing new data on their own.
Resolvers that follow this recommendation will have a more consistent Resolvers that follow this recommendation will have a more consistent
view of the namespace". view of the namespace".
The paragraph is updated as follows: The paragraph is updated as follows:
+--------------------------------------------------------------+ +-----------------------------------------------------------------+
| Once the records are validated, DNSSEC enabled validating | | Once the records are validated, DNSSEC enabled validating |
| resolvers MAY use wildcards and NSEC/NSEC3 resource records | | resolvers SHOULD use wildcards and NSEC/NSEC3 resource records |
| to generate positive and negative responses until the | | to generate positive and negative responses until the |
| effective TTLs or signatures for those records expire. | | effective TTLs or signatures for those records expire. |
+--------------------------------------------------------------+ +-----------------------------------------------------------------+
8. IANA Considerations 8. IANA Considerations
This document has no IANA actions. This document has no IANA actions.
9. Security Considerations 9. Security Considerations
Use of NSEC / NSEC3 resource records without DNSSEC validation may Use of NSEC / NSEC3 resource records without DNSSEC validation may
create serious security issues, and so this technique requires DNSSEC create serious security issues, and so this technique requires DNSSEC
validation. validation.
Newly registered resource records may not be used immediately. Newly registered resource records may not be used immediately.
However, choosing suitable TTL value and negative cache TTL value However, choosing suitable TTL value and negative cache TTL value
(SOA MINIMUM field) will mitigate the delay concern, and it is not a (SOA MINIMUM field) will mitigate the delay concern, and it is not a
security problem. security problem.
It is also suggested to limit the maximum TTL value of NSEC / NSEC3 It is also suggested to limit the maximum TTL value of NSEC / NSEC3
resource records in the negative cache to, for example, 10800 seconds resource records in the negative cache to, for example, 10800 seconds
(3hrs), to mitigate this issue. Implementations which comply with (3hrs), to mitigate this issue.
this proposal are recommended to have a configurable maximum value of
NSEC RRs in the negative cache.
Although the TTL of NSEC/NSEC3 records is typically fairly short Although the TTL of NSEC/NSEC3 records is typically fairly short
(minutes or hours), their RRSIG expiration time can be much further (minutes or hours), their RRSIG expiration time can be much further
in the future (weeks). An attacker who is able to successfully spoof in the future (weeks). An attacker who is able to successfully spoof
responses might poison a cache with old NSEC/NSEC3 records. If the responses might poison a cache with old NSEC/NSEC3 records. If the
resolver is NOT making aggressive use of NSEC/NSEC3, the attacker has resolver is NOT making aggressive use of NSEC/NSEC3, the attacker has
to repeat the attack for every query. If the resolver IS making to repeat the attack for every query. If the resolver IS making
aggressive use of NSEC/NSEC3, one successful attack would be able to aggressive use of NSEC/NSEC3, one successful attack would be able to
suppress many queries for new names, up to the negative TTL. suppress many queries for new names, up to the negative TTL.
skipping to change at page 9, line 45 skipping to change at page 9, line 47
it is inappropriate for inclusion in a published RFC." ] it is inappropriate for inclusion in a published RFC." ]
Unbound currently implements aggressive negative caching, as does Unbound currently implements aggressive negative caching, as does
Google Public DNS. Google Public DNS.
11. Acknowledgments 11. Acknowledgments
The authors gratefully acknowledge DLV [RFC5074] author Samuel Weiler The authors gratefully acknowledge DLV [RFC5074] author Samuel Weiler
and the Unbound developers. 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 The authors would like to specifically thank Stephane Bortzmeyer (for
standing next to and helping edit), Tony Finch, Tatuya JINMEI for standing next to and helping edit), Ralph Dolmans, Tony Finch, Tatuya
extensive review and comments, and also Mark Andrews, Casey Deccio, JINMEI for extensive review and comments, and also Mark Andrews,
Alexander Dupuy, Olafur Gudmundsson, Bob Harold, Shumon Huque, John Casey Deccio, Alexander Dupuy, Olafur Gudmundsson, Bob Harold, Shumon
Levine, Pieter Lexis and Matthijs Mekking (who even sent pull Huque, John Levine, Pieter Lexis, Matthijs Mekking (who even sent
requests!). Mark Andrews also provided the text pull requests!) and Ondrej Sury. Mark Andrews also provided the
(https://www.ietf.org/mail-archive/web/dnsop/current/msg18332.html) helpful notes for implementors (https://www.ietf.org/mail-
which we made into Appendix B. archive/web/dnsop/current/msg18332.html) which we made into
Appendix B.
11.1. Change History 11.1. Change History
RFC Editor: Please remove this section prior to publication. RFC Editor: Please remove this section prior to publication.
-05 to -06: -05 to -06:
o Moved some dangling text around - when the examples were added o Moved some dangling text around - when the examples were added
some text added in the wrong place. some text added in the wrong place.
o There were some bits which mentioned "negative" in the title. o There were some bits which mentioned "negative" in the title.
o We had the cut-and-paste of what changed in 4035 twice. o We had the cut-and-paste of what changed in 4035 twice.
o Clarified that this also allows NODATA responses to be
synthesized.
-04 to -05: -04 to -05:
o Bob pointed out that I did a stupid - when I added the wildcard to o Bob pointed out that I did a stupid - when I added the wildcard to
'example.com' I made the example wrong / confusing. I have 'example.com' I made the example wrong / confusing. I have
attempted to fix this by adding a second example zone attempted to fix this by adding a second example zone
(example.org) with the wildcard instead. (example.org) with the wildcard instead.
o More helpful changes (in a pull request, thanks!) from Matthijs o More helpful changes (in a pull request, thanks!) from Matthijs
o Included Mark Andrew's useful explanation of how to tell ENT from o Included Mark Andrew's useful explanation of how to tell ENT from
skipping to change at page 14, line 24 skipping to change at page 14, line 33
[RFC7129] Gieben, R. and W. Mekking, "Authenticated Denial of [RFC7129] Gieben, R. and W. Mekking, "Authenticated Denial of
Existence in the DNS", RFC 7129, DOI 10.17487/RFC7129, Existence in the DNS", RFC 7129, DOI 10.17487/RFC7129,
February 2014, <http://www.rfc-editor.org/info/rfc7129>. February 2014, <http://www.rfc-editor.org/info/rfc7129>.
[RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS [RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", RFC 7719, DOI 10.17487/RFC7719, December Terminology", RFC 7719, DOI 10.17487/RFC7719, December
2015, <http://www.rfc-editor.org/info/rfc7719>. 2015, <http://www.rfc-editor.org/info/rfc7719>.
12.2. Informative References 12.2. Informative References
[I-D.ietf-dnsop-nxdomain-cut]
Bortzmeyer, S. and S. Huque, "NXDOMAIN really means there
is nothing underneath", draft-ietf-dnsop-nxdomain-cut-03
(work in progress), May 2016.
[I-D.vixie-dnsext-resimprove] [I-D.vixie-dnsext-resimprove]
Vixie, P., Joffe, R., and F. Neves, "Improvements to DNS Vixie, P., Joffe, R., and F. Neves, "Improvements to DNS
Resolvers for Resiliency, Robustness, and Responsiveness", Resolvers for Resiliency, Robustness, and Responsiveness",
draft-vixie-dnsext-resimprove-00 (work in progress), June draft-vixie-dnsext-resimprove-00 (work in progress), June
2010. 2010.
[RFC8020] Bortzmeyer, S. and S. Huque, "NXDOMAIN: There Really Is [RFC8020] Bortzmeyer, S. and S. Huque, "NXDOMAIN: There Really Is
Nothing Underneath", RFC 8020, DOI 10.17487/RFC8020, Nothing Underneath", RFC 8020, DOI 10.17487/RFC8020,
November 2016, <http://www.rfc-editor.org/info/rfc8020>. November 2016, <http://www.rfc-editor.org/info/rfc8020>.
skipping to change at page 15, line 6 skipping to change at page 15, line 10
o Previously, cached negative responses were indexed by QNAME, o Previously, cached negative responses were indexed by QNAME,
QCLASS, QTYPE, and the setting of the CD bit (see RFC 4035, QCLASS, QTYPE, and the setting of the CD bit (see RFC 4035,
Section 4.7), and only queries matching the index key would be Section 4.7), and only queries matching the index key would be
answered from the cache. With aggressive negative caching, the answered from the cache. With aggressive negative caching, the
validator, in addition to checking to see if the answer is in its 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 cache before sending a query, checks to see whether any cached and
validated NSEC record denies the existence of the sought validated NSEC record denies the existence of the sought
record(s). Using aggressive negative caching, a validator will record(s). Using aggressive negative caching, a validator will
not make queries for any name covered by a cached and validated not make queries for any name covered by a cached and validated
NSEC record. Furthermore, a validator answering queries from NSEC record. Furthermore, a validator answering queries from
clients will synthesize a negative answer whenever it has an clients will synthesize a negative answer (or NODATA response)
applicable validated NSEC in its cache unless the CD bit was set whenever it has an applicable validated NSEC in its cache unless
on the incoming query. (Imported from Section 6 of [RFC5074]). the CD bit was set on the incoming query. (Imported from
Section 6 of [RFC5074]).
o Implementing aggressive negative caching suggests that a validator o Implementing aggressive negative caching suggests that a validator
will need to build an ordered data structure of NSEC and NSEC3 will need to build an ordered data structure of NSEC and NSEC3
records for each signer domain name of NSEC / NSEC3 records in records for each signer domain name of NSEC / NSEC3 records in
order to efficiently find covering NSEC / NSEC3 records. Call the order to efficiently find covering NSEC / NSEC3 records. Call the
table as NSEC_TABLE. (Imported from Section 6.1 of [RFC5074] and table as NSEC_TABLE. (Imported from Section 6.1 of [RFC5074] and
expanded.) expanded.)
o The aggressive negative caching may be inserted at the cache o The aggressive negative caching may be inserted at the cache
lookup part of the recursive resolvers. lookup part of the recursive resolvers.
o If errors happen in aggressive negative caching algorithm, o If errors happen in aggressive negative caching algorithm,
resolvers MUST fall back to resolve the query as usual. "Resolve resolvers MUST fall back to resolve the query as usual. "Resolve
the query as usual" means that the resolver must process the query the query as usual" means that the resolver must process the query
as though it does not implement aggressive negative caching. as though it does not implement aggressive negative caching.
Appendix B. Procedure for determining ENT vs NXDOMAN Appendix B. Procedure for determining ENT vs NXDOMAN with NSEC
Thanks to Mark Andrews for providing these helpful notes for This procedure outlines how to determine if a given name does not
implementors. As they are more general than for Aggressive NSEC we exist, or is an ENT (Empty Non-Terminal, see [RFC5155] Section 1.3)
have placed them in an appendix. with NSEC.
If the NSEC record has not been verified as secure discard it. 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 If the given name sorts before or matches the NSEC owner name discard
it as it does not prove the NXDOMAIN or ENT. 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 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 bit is present and the SOA bit is absent then discard the NSEC as it
is from a parent zone. is from a parent zone.
 End of changes. 37 change blocks. 
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