Network Working Group K. Fujiwara
Internet-Draft JPRS
Intended status: Informational A. Kato
Expires: April 21, 2016 Keio/WIDE
October 19, 2015
Aggressive use of NSEC/NSEC3
draft-fujiwara-dnsop-nsec-aggressiveuse-02
Abstract
While DNS highly depends on cache, its cache usage of non-existence
information was limited to exact matching. This draft proposes the
aggressive use of a NSEC/NSEC3 resource record, which is able to
express non-existence of range of names authoritatively. With this
proposal, shorter latency to many of negative responses is expected
as well as some level of mitigation of random sub-domain attacks
(referred to as "Water Torture" attacks). It is also expected that
non-existent TLD queries to Root DNS servers will decrease.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 21, 2016.
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to this document. Code Components extracted from this document must
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3
4. Proposed Solution: Aggressive Negative Caching . . . . . . . 4
5. Possible side effect . . . . . . . . . . . . . . . . . . . . 5
6. The CD Bit . . . . . . . . . . . . . . . . . . . . . . . . . 6
6.1. Detecting random subdomain attacks . . . . . . . . . . . 6
7. Additional proposals . . . . . . . . . . . . . . . . . . . . 6
7.1. Another option . . . . . . . . . . . . . . . . . . . . . 6
7.2. Aggressive negative caching flag idea . . . . . . . . . . 6
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
9. Security Considerations . . . . . . . . . . . . . . . . . . . 7
10. Implementation Considerations . . . . . . . . . . . . . . . . 7
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7
12. Change History . . . . . . . . . . . . . . . . . . . . . . . 7
12.1. Version 01 . . . . . . . . . . . . . . . . . . . . . . . 8
12.2. Version 02 . . . . . . . . . . . . . . . . . . . . . . . 8
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
13.1. Normative References . . . . . . . . . . . . . . . . . . 8
13.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Aggressive negative caching from RFC 5074 . . . . . 9
Appendix B. Detailed implementation idea . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
While negative (non-existence) information of DNS caching mechanism
has been known as DNS negative cache [RFC2308], it requires exact
matching in most cases. Assume that "example.com" zone doesn't have
names such as "a.example.com" and "b.example.com". When a full-
service resolver receives a query "a.example.com" , it performs a DNS
resolution process, and eventually gets NXDOMAIN and stores it into
its negative cache. When the full-service resolver receives another
query "b.example.com", it doesn't match with "a.example.com". So it
will send a query to one of the authoritative servers of
"example.com". This was because the NXDOMAIN response just says
there is no such name "a.example.com" and it doesn't tell anything
for "b.example.com".
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Section 5 of [RFC2308] seems to show that negative answers should be
cached only for the exact query name, and not (necessarily) for
anything below it.
Recently, DNSSEC [RFC4035] [RFC5155] has been practically deployed.
Two types of resource record (NSEC and NSEC3) are used for authentic
non-existence. For a zone signed with NSEC, it may be possible to
use the information carried in NSEC resource records to indicate that
the range of names where no valid name exists. Such use is
discouraged by Section 4.5 of RFC 4035, however.
This document proposes to make a minor change to RFC 4035 and the
full-service resolver can use NSEC/NSEC3 resource records
aggressively.
Aggressive Negative Caching was first proposed in Section 6 of DNSSEC
Lookaside Validation (DLV) [RFC5074] in order to find covering NSEC
records efficiently. Unbound [UNBOUND] has aggressive negative
caching code in its DLV validator. Unbound TODO file contains "NSEC/
NSEC3 aggressive negative caching".
Section 3 of [I-D.vixie-dnsext-resimprove] ("Stopping Downward Cache
Search on NXDOMAIN") 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 specification are defined
in DNS Terminology [I-D.ietf-dnsop-dns-terminology].
3. Problem Statement
Random sub-domain attacks (referred to as "Water Torture" attacks or
NXDomain attacks) send many non-existent queries to full-service
resolvers. Their query names consist of random prefixes and a target
domain name. The negative cache does not work well and target full-
service resolvers result in sending queries to authoritative DNS
servers of the target domain name.
When number of queries is large, the full-service resolvers drop
queries from both legitimate users and attackers as their outstanding
queues are filled up.
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For example, BIND 9.10.2 [BIND9] full-service resolvers answer
SERVFAIL and Unbound 1.5.2 full-service resolvers drop most of
queries under 10,000 queries per second attack.
The countermeasures implemented at this moment are rate limiting and
disabling name resolution of target domain names.
4. Proposed Solution: Aggressive Negative Caching
If the target domain names are DNSSEC signed, aggressive use of NSEC/
NSEC3 resource records mitigates the problem.
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".
To reduce non-existent queries sent to authoritative DNS servers, it
is suggested to relax this restriction as follows:
+--------------------------------------------------------------+
| DNSSEC enabled full-service resolvers MAY use |
| NSEC/NSEC3 resource records to generate negative responses |
| until their effective TTLs or signatures on the records |
| in question expire. |
+--------------------------------------------------------------+
If the full-service resolver's cache have enough information to
validate the query, the full-service resolver MAY use NSEC/NSEC3/
wildcard records aggressively. Otherwise, the full-service resolver
MUST fall back to send the query to the authoritative DNS servers.
Necessary information to validate are wildcards which match the query
name, covering NSEC/NSEC3 of the wildcards, and covering NSEC/NSEC3
of (parts of) the query name.
If the zone has a wildcard and it is in the full-service resolver's
cache, the full-service resolver MAY generate positive responses
based on the information associated with the wildcard in the cache.
This approach is effective for DNSSEC signed zones with NSEC or
NSEC3, except zones with NSEC3 Opt-Out. To identify signing types of
the zone, validating resolvers need to build special cache of NSEC
and NSEC3 resource records for each signer domain name. When a query
name is not in the cache, find closest enclosing NS RRset in the
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cache. The owner of this NS RRset may be the longest signer domain
name of the query name. If the NSEC/NSEC3 cache of the signer domain
name is empty, the aggressive negative caching is not possible.
Otherwise, there is at least one NSEC or NSEC3 resource records. The
record shows the signing type. If the resource record is NSEC3 and
with Opt-Out, the aggressive negative caching is not possible.
When the query name has a matching NSEC resource records in the cache
and there is no wildcard in the zone which the query name matches
with, the full-service resolver is allowed to respond with NXDOMAIN
error immediately.
NSEC3 aggressive negative caching is more difficult. If the zone is
signed with NSEC3, the validating resolver need to check the
existence of each label from the query name. If a label is not exist
in the zone, and there is no matching wildcard in the zone, the full-
service resolver is allowed to respond with NXDOMAIN error
immediately.
This function needs care on the TTL value of negative information
because newly added domain names cannot be used while the negative
information is effective. RFC 2308 states the maximum number of
negative cache TTL value is 10800 (3 hours). So the full-service
resolver SHOULD limit the maximum effective TTL value of negative
responses (NSEC/NSEC3 RRs) to 10800 (3 hours). It is reasonably
small but still effective for the purpose of this document as it can
eliminate significant amount of DNS attacks with randomly generated
names.
The same discussion is also applicable to wildcards. If a query name
is covered by a NSEC or a NSEC3 resource record in the cache and
there is a covering wildcard, the full-service resolver MAY use
wildcards to generate positive responses while wildcard and NSEC/
NSEC3 resource records in the cache are effective.
5. Possible side effect
Aggressive use of NSEC/NSEC3 resource records may decrease queries to
Root DNS servers.
People may generate many typos in TLD, and they will result in
unnecessary DNS queries. Some implementations leak non-existent TLD
queries whose second level domain are different each other. Well
observed TLDs are ".local" and ".belkin". With this proposal, it is
possible to return NXDOMAIN immediately to such queries without
further DNS recursive resolution process. It may reduces round trip
time, as well as reduces the DNS queries to corresponding
authoritative servers, including Root DNS servers.
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6. The CD Bit
The CD bit disables signature validation. It is one of the basic
functions of DNSSEC protocol and it SHOULD NOT be changed. However,
attackers may set the CD bit to their attack queries and the
aggressive negative caching will be of no use.
Ignoring the CD bit function may break the DNSSEC protocol.
This draft proposes that the CD bit may be ignored to support
aggressive negative caching when the full-service resolver is under
attacks with CD bit set.
6.1. Detecting random subdomain attacks
Full-service resolvers should detect conditions under random
subdomain attacks. When they are under attacks, their outstanding
queries increase. If there are some destination addresses whose
outstanding queries are many, they may contain attack target domain
names. Existing countermeasures may implement attack detection.
7. Additional proposals
There are additional proposals to the aggressive negative caching.
7.1. Another option
The proposed technique is applicable to zones where there is a NSEC
record to each owner name in the zone even without DNSSEC signed.
And it is also applicable to full-service resolvers without DNSSEC
validation. Full-service resolvers can set DNSSEC OK bit in query
packets and they will cache NSEC/NSEC3 resource records. They can
apply aggressive use of NSEC/NSEC3 resource records without DNSSEC
validation.
It is highly recommended to sign the zone, of course, and it is
recommended to apply DNSSEC validation of NSEC record prior to cache
it in the negative cache.
7.2. Aggressive negative caching flag idea
Authoritative DNS servers that dynamically generate NSEC records
normally generate minimally covering NSEC Records [RFC4470].
Aggressive negative caching does not work with minimally covering
NSEC records. DNS operators don't want zone walking and zone
information leaks. They prefer NSEC resource records with narrow
ranges. When there is a flag that show a full-service resolver
support the aggressive negative caching and a query have the
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aggressive negative caching flag, authoritative DNS servers can
generate NSEC resource records with wider range under random
subdomain attacks.
However, changing range of minimally covering NSEC Records may be
implemented by detecting attacks. Authoritative DNS servers can
answer any range of minimally covering NSEC Records.
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 will mitigate the problem and it
is not a security problem.
It is also suggested to limit the maximum TTL value of NSEC resource
records in the negative cache to, for example, 10800 seconds (3hrs),
to mitigate the issue. Implementations which comply with this
proposal is suggested 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.
10. Implementation Considerations
Unbound has aggressive negative caching code in its DLV validator.
The author implemented NSEC aggressive caching using Unbound and its
DLV validator code.
11. Acknowledgments
The authors gratefully acknowledge DLV [RFC5074] author Samuel Weiler
and Unbound developers. Olafur Gudmundsson and Pieter Lexis proposed
aggressive negative caching flag idea. Valuable comments were
provided by Bob Harold, Tatuya JINMEI, Shumon Huque, Mark Andrews,
and Casey Deccio.
12. Change History
This section is used for tracking the update of this document. Will
be removed after finalize.
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12.1. Version 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 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 Another Option.
13. References
13.1. Normative References
[I-D.ietf-dnsop-dns-terminology]
Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", draft-ietf-dnsop-dns-terminology-05 (work in
progress), September 2015.
[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>.
[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>.
[RFC4470] Weiler, S. and J. Ihren, "Minimally Covering NSEC Records
and DNSSEC On-line Signing", RFC 4470, DOI 10.17487/
RFC4470, April 2006,
<http://www.rfc-editor.org/info/rfc4470>.
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[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>.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891, DOI 10.17487/
RFC6891, April 2013,
<http://www.rfc-editor.org/info/rfc6891>.
13.2. Informative References
[BIND9] Internet Systems Consortium, Inc., "Name Server Software",
2000, <https://www.isc.org/downloads/bind/>.
[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.
[UNBOUND] NLnet Labs, "Unbound DNS validating resolver", 2006,
<http://www.unbound.net/>.
Appendix A. Aggressive negative caching from RFC 5074
Imported from Section 6 of [RFC5074].
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 whenever it has an applicable validated
NSEC in its cache unless the CD bit was set on the incoming query.
Imported from Section 6.1 of [RFC5074].
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Implementing aggressive negative caching suggests that a validator
will need to build an ordered data structure of NSEC records in order
to efficiently find covering NSEC records. Only NSEC records from
DLV domains need to be included in this data structure.
Appendix B. Detailed implementation idea
Section 6.1 of [RFC5074] is expanded as follows.
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.
The aggressive negative caching may be inserted at the cache lookup
part of the full-service resolvers.
If errors happen in aggressive negative caching algorithm, resolvers
MUST fall back to resolve the query as usual. "Resolve the query as
usual" means that the full-resolver resolve the query in Recursive-
mode as if the full-service resolver does not implement aggressive
negative caching.
To implement aggressive negative caching, resolver algorithm near
cache lookup will be changed as follows:
QNAME = the query name;
if (QNAME name entry exists in the cache) {
resolve the query as usual;
// if RRSet (query name and query type) exists in the cache,
// the resolver responds the RRSet from the cache
// Otherwise, the resolver needs to iterate the query.
}
// Find closest enclosing NS RRset in the cache.
// The owner of this NS RRset will be a suffix of the QNAME
// - the longest suffix of any NS RRset in the cache.
SIGNER = closest enclosing NS RRSet of QNAME in the cache;
// Check the SOA RR of the SIGNER
if (SOA RR of SIGNER does not exist in the cache
or SIGNER zone is not signed or not validated) {
Resolve the query as usual;
}
if (SIGNER zone does not have NSEC_TABLE) {
Resolve the query as usual;
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}
if (SIGNER zone is signed with NSEC) {
// NSEC mode
if (covering NSEC RR of QNAME at SIGNER zone
doesn't exist in the cache) {
Resolve the query as usual.
}
TEST = Find the closest encloser domain name of QNAME and
the covering NSEC RR of QNAME
if (*.TEST name entry exists in the cache) {
the resolver can generate positive response
// synthesize the wildcard *.TEST
}
if covering NSEC RR of "*.TEST" at SIGNER zone exists
in the cache {
the resolver can generate negative response;
}
// Lack of information
} else
if (SIGNER zone is signed with NSEC3 and does not use Opt-Out) {
// NSEC3 mode
TEST = SIGNER;
while (TEST != QNAME) {
// if any error happens in this loop, break this loop
UPPER = TEST;
add a label from the QNAME to the start of TEST;
// TEST = label.UPPER
if (TEST name entry exist in the cache) {
continue; // need to check rest of QNAME
}
if (covering NSEC3 of TEST exist in the cache) {
// (non-)terminal name TEST does not exist
if (*.UPPER name entry exist in the cache) {
// TEST does not exist and *.UPPER exist
the resolver can generate positive response;
} else
if (covering NSEC3 of *.UPPER exist in the cache) {
// TEST does not exist and *.UPPER does not exist
the resolver can generate negative response;
}
break; // Lack of information
} else
if (NSEC3 of TEST does not exist in the cache) {
break; // Lack of information
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}
// TEST label exist, then need to check rest of QNAME
}
// Lack of information, need to resolve the query as usual
}
Resolve the query as usual
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: fujiwara@jprs.co.jp
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: kato@wide.ad.jp
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