Important: Read CONTRIBUTING.md before submitting feedback or contributing
Network Working Group K. Fujiwara
Internet-Draft JPRS
Updates: 4035 (if approved) A. Kato
Intended status: Standards Track Keio/WIDE
Expires: March 17, 2017 W. Kumari
Google
September 13, 2016
Aggressive use of NSEC/NSEC3
draft-ietf-dnsop-nsec-aggressiveuse-02
Abstract
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 generate negative answers within a
range. This increases performance / decreases latency, decreases
resource utilization on both authoritative and recursive servers, and
also increases privacy. It may also help increase resilience to
certain DoS attacks in some circumstances.
This document updates RFC4035 by allowing resolvers to generate
negative answers based upon NSEC/NSEC3 records.
[ Ed note: Text inside square brackets ([]) is additional background
information, answers to frequently asked questions, general musings,
etc. They will be removed before publication.This document is being
collaborated on in Github at: https://github.com/wkumari/draft-ietf-
dnsop-nsec-aggressiveuse. The most recent version of the document,
open issues, etc should all be available here. The authors
(gratefully) accept pull requests.
Known / open issues [To be moved to Github issue tracker]:
1. We say things like: "Currently the DNS does ..." - this will not
be true after this is deployed, but I'm having a hard time
rewording this. "Without the techniques described in this
document..." seems klunky. Perhaps "historically?!"
]
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on March 17, 2017.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4
4. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Aggressive Use Of Negative Caching . . . . . . . . . . . . . 5
5.1. NSEC . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.2. NSEC3 . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.3. Wildcard . . . . . . . . . . . . . . . . . . . . . . . . 6
5.4. Consideration on TTL . . . . . . . . . . . . . . . . . . 7
6. Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . 7
7. Update to RFC 4035 . . . . . . . . . . . . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
9. Security Considerations . . . . . . . . . . . . . . . . . . . 9
10. Implementation Status . . . . . . . . . . . . . . . . . . . . 9
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
12. Change History . . . . . . . . . . . . . . . . . . . . . . . 9
12.1. Version draft-fujiwara-dnsop-nsec-aggressiveuse-01 . . . 11
12.2. Version draft-fujiwara-dnsop-nsec-aggressiveuse-02 . . . 11
12.3. Version draft-fujiwara-dnsop-nsec-aggressiveuse-03 . . . 11
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13. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
13.1. Normative References . . . . . . . . . . . . . . . . . . 11
13.2. Informative References . . . . . . . . . . . . . . . . . 12
Appendix A. Detailed implementation notes . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
A DNS negative cache exists, and is used to cache the fact that a
name 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 recursive resolvers to use
NSEC/NSEC3 resource records to aggressively use cache negative
answers. This would allow such resolvers to respond with NXDOMAIN
immediately if the name in question falls into a range expressed by a
NSEC/NSEC3 resource record already in the cache.
Aggressive Use Of Negative Caching was first proposed in Section 6 of
DNSSEC Lookaside Validation (DLV) [RFC5074] in order to find covering
NSEC records efficiently.
Section 3 of [I-D.vixie-dnsext-resimprove] "Stopping Downward Cache
Search on NXDOMAIN" and [I-D.ietf-dnsop-nxdomain-cut] proposed
another approach to use NXDOMAIN information effectively.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Many of the specialized terms used in this document are defined in
DNS Terminology [RFC7719]. In this document we are using the terms
"recursive resolver" or "recursive server" as a more readable
alternative to the more formal[RFC7719] "full-service resolver"
The key words "Closest Encloser" and "Source of Synthesis" in this
document are to be interpreted as described in [RFC4592].
"Closest Encloser" is also defined in NSEC3 [RFC5155], as is "Next
closer name".
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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:
apple.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 recursive resolver gets a query for cat.example.com, it will
query the example.com authoritative servers and will get back an NSEC
(or NSEC3) record starting that there are no records between apple
and elephant. The recursive resolver then knows that cat.example.com
does not exist; however, it does not use the fact that the proof
covers a range (apple to elephant) to suppress queries for other
labels that fall within this range. This means that if the recursive
resolvers 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 (see additional resources), and also has
privacy implications (e.g: typos leak out further than necessary).
4. Background
DNSSEC [RFC4035] and [RFC5155] both provide "authenticated denial of
existence"; this is a cryptographic proof that the queried for name
does not exist, accomplished by providing a (DNSSEC secured) record
containing the names which appear alphabetically before and after the
queried for name. In the example above, if the (DNSSEC validating)
recursive server were to query for lion.example.com it would receive
a (signed) NSEC/NSEC3 record stating that there are no labels between
"elephant" and "zebra". This is a signed, cryptographic proof that
these names are the ones before and after the queried for label. As
lion.example.com falls within this range, the recursive server knows
that lion.example.com really does not exist. This document specifies
that this NSEC/NSEC3 record should be used to generate negative
answers for any queries that the recursive server receives that fall
within the range covered by the record (for the TTL for the record).
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[RFC4035]; Section 4.5 states:
For a zone signed with NSEC, it would be possible to use the
information carried in NSEC resource records to indicate the non-
existence of a range of names. However, such use is discouraged by
Section 4.5 of RFC4035. It is recommended that readers read RFC4035
in its entirety for a better understanding. At the root of the
concern is that new records could have been added to the zone during
the TTL of the NSEC record, and that generating negative responses
from the NSEC record would hide these. We believe this
recommendation can be relaxed because lookups for the specific name
could have come in during the normal negative cache time and so
operators should have no expectation that an added name would work
immediately. We think that the TTL of the NSEC record is the
authoritative statement of how quickly a name can start working
within a zone.
5. Aggressive Use Of Negative Caching
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".
This document relaxes this this restriction, as follows:
+--------------------------------------------------------------+
| Once the records are validated, DNSSEC enabled validating |
| resolvers MAY use NSEC/NSEC3 resource records to generate |
| negative responses until their effective TTLs or signatures |
| for those records expire. |
+--------------------------------------------------------------+
If the validating resolver's cache has sufficient information to
validate the query, the resolver SHOULD use NSEC/NSEC3/wildcard
records aggressively. Otherwise, it MUST fall back to send the query
to the authoritative DNS servers.
If the query name has the matching NSEC/NSEC3 RR proving the
information requested does not exist, the resolver may respond with a
NODATA (empty) answer.
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5.1. NSEC
Implementations SHOULD enable aggressive use of NSEC by default.
Implementations SHOULD provide a configuration switch to disable
aggressive use of NSEC and allow it to be enabled or disabled per
domain.
The validating resolver needs to check the existence of an NSEC RR
matching/covering the source of synthesis and an NSEC RR covering the
query name.
If the validating resolver's cache contains an NSEC RR covering the
source of synthesis and the covering NSEC RR of the query name, the
resolver may respond with NXDOMAIN error immediately.
5.2. NSEC3
NSEC3 aggressive use of negative caching is more difficult. If the
zone is signed with NSEC3, the validating resolver needs to check the
existence of non-terminals and wildcards which derive from query
names.
If the validating resolver's cache contains an NSEC3 RR matching the
closest encloser, an NSEC3 RR covering the next closer name, and an
NSEC3 RR covering the source of synthesis, it is possible for the
resolver to respond with NXDOMAIN immediately.
If a covering NSEC3 RR has 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.
A validating resolver implementation MAY support aggressive use of
NSEC3. If it does aggressive use of NSEC3, it SHOULD provide a
configuration switch to disable aggressive use of NSEC3 and allow it
to be enabled or disabled for specific zones.
5.3. Wildcard
The last paragraph of RFC 4035 Section 4.5 discusses aggressive use
of a cached deduced wildcard (as well as aggressive use of NSEC) and
recommends that it is not relied upon.
Just like the case for the aggressive use of NSEC discussed in this
draft, we revise this recommendation. As long as the resolver knows
a name would not exist without the wildcard match, it can answer a
query for that name using the cached deduced wildcard, and it may be
justified for performance and other benefits.
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Such aggressive use of cached deduced wildcard can be employed
independently from aggressive use of NSEC. But, it will be more
effective when both are enabled since the resolver can determine the
name subject to wildcard would not otherwise exist more efficiently.
Furthermore, when aggressive use of NSEC is enabled, the aggressive
use of cached deduced wildcard will be more effective.
An implementation MAY support aggressive use of wildcards. It SHOULD
provide a configuration switch to disable aggressive use of
wildcards.
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 RFC 2308 states that the
maximum number of negative cache TTL value is 3 hours (10800). It is
RECOMMENDED that resolvers limit the maximum effective TTL value of
negative responses (NSEC/NSEC3 RRs) to this same value.
6. Benefits
The techniques described in this document provide a number of
benefits, including (in no specific order):
Reduced latency By answering directly from cache, recursive
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 negative queries from
the cache, recursive servers avoid having 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 load.
Decreased authorative server load Because recursive servers can
answer (negative) queries without asking the authoritative server,
the authoritative servers receive less queries. This decreases
the authoritative server bandwidth, queries per second and CPU
utilization.
The scale of the benefit depends upon multiple factors, including the
query distribution. For example, currently around 65% of queries to
Root Name servers result in NXDOMAIN responses; this technique will
eliminate a sizable quantity of these.
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[ Editor note: There has been some discussion on if this document
should discuss this attack and mitigation. The authors think that
this is useful / important, but some participants feel that it
oversells the DoS mitigation benefit. Please let us know if the
below is helpful. Also, the below description is not as clear as it
could be - it's been tricky to balance readability, correctness and
conciseness. Text gratefully accepted... ]
The technique described in this document may also mitigate so-called
"random QNAME attacks", in which attackers send many queries for
random sub-domains to recursive resolvers. As the recursive server
will not have the answers cached it has to ask the authoritative
servers for each random query, leading to a DoS on the authoritative
(and often recursive) servers. Aggressive NSEC may help mitigate
these attacks by allowing the recursive to answer directly from cache
for any random queries which fall within already requested ranges.
The effectiveness of this depends upon a number of factors, including
if the attacker is making his queries through recursive resolvers
(e.g to hide his source), the number of entries in the zone, the TTL,
if the zone is using NSEC, if the attacker is setting the CD bit,
etc. In the ideal case, authoritative servers under attack will need
to answer somewhere between number_of_entries_in_zone queries and 2 *
number_of_entries_in_zone queries from each recursive server. This
is because there are as many "holes" between labels as there are
labels in a zone. If the random query falls in range for which
recursive server does not have an NSEC record cached, it will send a
query to the authoritative server, and so it will send approximately
the same number of queries as there are "holes" between entries. If
the random queries happen to be for names which exist in the zone,
the recursive will send those as well.
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:
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+--------------------------------------------------------------+
| Once the records are validated, DNSSEC enabled recursive |
| resolvers MAY use wildcards and NSEC/NSEC3 resource records |
| to generate (positive and) negative responses until their |
| effective TTLs or signatures for those records expire. |
+--------------------------------------------------------------+
8. IANA Considerations
This document has no IANA actions.
9. Security Considerations
Newly registered resource records may not be used immediately.
However, choosing suitable TTL value and negative cache TTL value
(SOA MINIMUM field) will mitigate the delay concern, and it is not a
security problem.
It is also suggested to limit the maximum TTL value of NSEC / NSEC3
resource records in the negative cache to, for example, 10800 seconds
(3hrs), to mitigate this issue. Implementations which comply with
this proposal are recommended to have a configurable maximum value of
NSEC RRs in the negative cache.
Aggressive use of NSEC / NSEC3 resource records without DNSSEC
validation may cause security problems. It is highly recommended to
apply DNSSEC validation.
10. Implementation Status
Unbound supports aggressive use of negative caching.
11. Acknowledgments
The authors gratefully acknowledge DLV [RFC5074] author Samuel Weiler
and the Unbound developers.
The authors would like to specifically thank Tatuya JINMEI for
extensive review and comments, and also Mark Andrews, Stephane
Bortzmeyer, Casey Deccio, Alexander Dupuy, Olafur Gudmundsson, Bob
Harold, Shumon Huque, Pieter Lexis and Matthijs Mekking.
12. Change History
RFC Editor: Please remove this section prior to publication.
-01 to -02:
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o Added Section 6 - Benefits (as suggested by Jinmei).
o Removed Appendix B (Jinmei)
o Replaced "full-service" with "validating" (where applicable)
o Integrated other comments from Jinmei from https://www.ietf.org/
mail-archive/web/dnsop/current/msg17875.html
o Integrated comment from co-authors, including re-adding parts of
Appendix B, terminology, typos.
o Tried to explain under what conditions this may actually mitigate
attacks.
-00 to -01:
o Comments from DNSOP meeting in Berlin.
o Changed intended status to Standards Track (updates RFC 4035)
o Added a section "Updates to RFC 4035"
o Some language clarification / typo / cleanup
o Cleaned up the TTL section a bit.
o Removed Effects section, Additional proposal section, and pseudo
code.
o Moved "mitigation of random subdomain attacks" to Appendix.
From draft-fujiwara-dnsop-nsec-aggressiveuse-03 -> draft-ietf-dnsop-
nsec-aggressiveuse
o Document adopted by DNSOP WG.
o Adoption comments
o Changed main purpose to performance
o Use NSEC3/Wildcard keywords
o Improved wordings (from good comments)
o Simplified pseudo code for NSEC3
o Added Warren as co-author.
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o Reworded much of the problem statement
o Reworked examples to better explain the problem / solution.
12.1. Version draft-fujiwara-dnsop-nsec-aggressiveuse-01
o Added reference to DLV [RFC5074] and imported some sentences.
o Added Aggressive Negative Caching Flag idea.
o Added detailed algorithms.
12.2. Version draft-fujiwara-dnsop-nsec-aggressiveuse-02
o Added reference to [I-D.vixie-dnsext-resimprove]
o Added considerations for the CD bit
o Updated detailed algorithms.
o Moved Aggressive Negative Caching Flag idea into Additional
Proposals
12.3. Version draft-fujiwara-dnsop-nsec-aggressiveuse-03
o Added "Partial implementation"
o Section 4,5,6 reorganized for better representation
o Added NODATA answer in Section 4
o Trivial updates
o Updated pseudo code
13. References
13.1. 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,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,
<http://www.rfc-editor.org/info/rfc2308>.
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[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,
<http://www.rfc-editor.org/info/rfc4035>.
[RFC4592] Lewis, E., "The Role of Wildcards in the Domain Name
System", RFC 4592, DOI 10.17487/RFC4592, July 2006,
<http://www.rfc-editor.org/info/rfc4592>.
[RFC5074] Weiler, S., "DNSSEC Lookaside Validation (DLV)", RFC 5074,
DOI 10.17487/RFC5074, November 2007,
<http://www.rfc-editor.org/info/rfc5074>.
[RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
Security (DNSSEC) Hashed Authenticated Denial of
Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008,
<http://www.rfc-editor.org/info/rfc5155>.
[RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", RFC 7719, DOI 10.17487/RFC7719, December
2015, <http://www.rfc-editor.org/info/rfc7719>.
13.2. 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]
Vixie, P., Joffe, R., and F. Neves, "Improvements to DNS
Resolvers for Resiliency, Robustness, and Responsiveness",
draft-vixie-dnsext-resimprove-00 (work in progress), June
2010.
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 use of 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). Aggressive use of 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 whenever it has an
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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 use of 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 expanded.)
o The aggressive use of negative caching may be inserted at the
cache lookup part of the recursive resolvers.
o If errors happen in aggressive use of 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 use of negative
caching.
Authors' Addresses
Kazunori Fujiwara
Japan Registry Services Co., Ltd.
Chiyoda First Bldg. East 13F, 3-8-1 Nishi-Kanda
Chiyoda-ku, Tokyo 101-0065
Japan
Phone: +81 3 5215 8451
Email: [email protected]
Akira Kato
Keio University/WIDE Project
Graduate School of Media Design, 4-1-1 Hiyoshi
Kohoku, Yokohama 223-8526
Japan
Phone: +81 45 564 2490
Email: [email protected]
Warren Kumari
Google
1600 Amphitheatre Parkway
Mountain View, CA 94043
US
Email: [email protected]
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