Internet DRAFT - draft-fujiwara-dnsop-nsec-aggressiveuse
draft-fujiwara-dnsop-nsec-aggressiveuse
Network Working Group K. Fujiwara
Internet-Draft JPRS
Intended status: Informational A. Kato
Expires: September 19, 2016 Keio/WIDE
March 18, 2016
Aggressive use of NSEC/NSEC3
draft-fujiwara-dnsop-nsec-aggressiveuse-03
Abstract
While DNS highly depends on cache, its cache usage of non-existence
information has been limited to exact matching. This draft proposes
the aggressive use of a NSEC/NSEC3 resource record, which is able to
express non-existence of a range of names authoritatively. With this
proposal, it is expected that shorter latency to many of negative
responses 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
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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time. It is inappropriate to use Internet-Drafts as reference
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This Internet-Draft will expire on September 19, 2016.
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document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4
4. Proposed Solution . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Aggressive Negative Caching . . . . . . . . . . . . . . . 4
4.2. NSEC . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.3. NSEC3 . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.4. NSEC3 Opt-Out . . . . . . . . . . . . . . . . . . . . . . 6
4.5. Wildcard . . . . . . . . . . . . . . . . . . . . . . . . 6
4.6. Consideration on TTL . . . . . . . . . . . . . . . . . . 6
5. Additional Considerations . . . . . . . . . . . . . . . . . . 6
5.1. The CD Bit . . . . . . . . . . . . . . . . . . . . . . . 6
5.2. Detecting random subdomain attacks . . . . . . . . . . . 7
6. Possible side effect . . . . . . . . . . . . . . . . . . . . 7
7. Additional proposals . . . . . . . . . . . . . . . . . . . . 7
7.1. Partial implementation . . . . . . . . . . . . . . . . . 7
7.2. Aggressive negative caching without DNSSEC validation . . 8
7.3. Aggressive negative caching flag idea . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
9. Security Considerations . . . . . . . . . . . . . . . . . . . 8
10. Implementation Status . . . . . . . . . . . . . . . . . . . . 9
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
12. Change History . . . . . . . . . . . . . . . . . . . . . . . 9
12.1. Version 01 . . . . . . . . . . . . . . . . . . . . . . . 9
12.2. Version 02 . . . . . . . . . . . . . . . . . . . . . . . 9
12.3. Version 03 . . . . . . . . . . . . . . . . . . . . . . . 9
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
13.1. Normative References . . . . . . . . . . . . . . . . . . 10
13.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Aggressive negative caching from RFC 5074 . . . . . 11
Appendix B. Detailed implementation idea . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
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
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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".
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) along with their RRSIG
records represent authentic non-existence. For a zone signed with
NSEC, it would be possible to use the information carried in NSEC
resource records to indicate that a 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 a full-
service resolver can use NSEC/NSEC3 resource records aggressively so
that the resolver responds with NXDOMAIN immediately if the name in
question falls into a range expressed by a NSEC/NSEC3 resource
record.
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 [RFC7719].
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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.
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 in ad-hoc manner.
4. Proposed Solution
4.1. 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/
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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 matching/covering NSEC/NSEC3 of
the wildcards which may match the query name, matching/covering NSEC/
NSEC3 of non-terminals which derive from the query name and matching/
covering NSEC/NSEC3 of the query name.
If the query name has the matching NSEC/NSEC3 RR and it shows the
query type does not exist, the full-service resolver is possible to
respond with NODATA (empty) answer.
4.2. NSEC
A full-service resolver implementation SHOULD support aggressive use
of NSEC and enable it by default. It SHOULD provide a configuration
knob to disable aggressive use of NSEC.
The validating resolver need to check the existence of matching
wildcards which derive from the query name, covering NSEC RRs of the
matching wildcards and covering NSEC RR of the query name.
If the full-service resolver's cache contains covering NSEC RRs of
matching wildcards and the covering NSEC RR of the query name, the
full-service resolver is possible to respond with NXDOMAIN error
immediately.
4.3. NSEC3
NSEC3 aggressive negative caching is more difficult. If the zone is
signed with NSEC3, the validating resolver need to check the
existence of non-terminals and wildcards which derive from query
names.
If the full-service resolver's cache contains covering NSEC3 RRs of
matching wildcards, the covering NSEC3 RRs of the non-terminals and
the covering NSEC3 RR of the query name, the full-service resolver is
possible to respond with NXDOMAIN error immediately.
If the validating resolver proves the non-exisence of the non-
terminal domain name of the query name, the query name does not
exist.
To identify signing types of the zone, validating resolvers need to
build separated cache of NSEC and NSEC3 resource records for each
signer domain name.
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When a query name is not in the regular cache, find closest enclosing
NS RRset in the regular cache. The owner of the closest enclosing NS
RRset may be the longest signer domain name of the query name. If
there is no entry in the NSEC/NSEC3 cache of the signer domain name,
aggressive negative caching is not possible at this moment.
Otherwise, there is at least one NSEC or NSEC3 resource records. The
record shows the signing type.
A full-service resolver implementation MAY support aggressive use of
NSEC3. It SHOULD provide a configuration knob to disable aggressive
use NSEC3 in this case.
4.4. NSEC3 Opt-Out
If the zone is signed with NSEC3 and with Opt-Out flag set to 1, the
aggressive negative caching is not possible at the zone.
4.5. Wildcard
Even if a wildcard is cached, it is necessary to send a query to an
authoritative server to ensure that the name in question doesn't
exist as long as the name is not in the negative cache.
When aggressive use is enabled, regardless of description of
Section 4.5 of [RFC4035], it is possible to send a positive response
immediately when the name in question matches a NSEC/NSEC3 RRs in the
negative cache.
4.6. Consideration on TTL
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.
5. Additional Considerations
5.1. 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.
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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.
5.2. 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.
6. 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.
7. Additional proposals
There are additional proposals to the aggressive negative caching.
7.1. Partial implementation
It is possible to implement aggressive negative caching partially.
DLV aggressive negative caching [RFC5074] is an implementation of
NSEC aggressive negative caching which targets DLV domain names.
NSEC only aggressive negative caching is easier to implement NSEC/
NSEC3 aggressive negative caching (full implantation) because NSEC3
handling is hard to implement.
Root only aggressive negative caching is possible. It uses NSEC and
RRSIG resource records whose signer domain name is root.
An implementation without detecting attacks is possible. It cannot
ignore the CD bit and the effectiveness may be limited.
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7.2. Aggressive negative caching without DNSSEC validation
Aggressive negative caching may be applicable to full-service
resolvers without DNSSEC validation. They can set DNSSEC OK bit in
query packets to obtain corresponding NSEC/NSEC3 resource records.
While the full-service resolvers SHOULD validate the NSEC/NSEC3
resource records, they MAY use the records to respond NXDOMAIN error
immediately without DNSSEC validation.
However, it is highly recommended to apply DNSSEC validation.
7.3. 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. Most of DNS operators don't want zone enumeration 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
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.
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10. Implementation Status
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.
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 Additional
Proposals
12.3. Version 03
o Added "Partial implementation"
o Section 4,5,6 reorganized for better representation
o Added NODATA answer in Section 4
o Trivial updates
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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>.
[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>.
[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
[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.
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[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].
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.
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To implement aggressive negative caching, resolver algorithm near
cache lookup will be changed as follows:
QNAME = the query name;
QTYPE = the query type;
if ({QNAME,QTYPE} entry exists in the cache) {
// the resolver responds the RRSet from the cache
resolve the query as usual;
}
// if NSEC* exists, QTYPE existence is proved by type bitmap
if (matching NSEC/NSEC3 of QNAME exists in the cache) {
if (QTYPE exists in type bitmap of NSEC/NSEC3 of QNAME) {
// the entry exists, however, it is not in the cache.
// need to iterate QNAME/QTYPE.
resolve the query as usual;
} else {
// QNAME exists, QTYPE does not exist.
the resolver can generate NODATA response;
}
}
// 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;
}
if (SIGNER zone is signed with NSEC) { // NSEC mode
// Check the non-existence of QNAME
CoveringNSEC = Find the covering NSEC of QNAME;
if (Covering NSEC doesn't exist in the cache) {
Resolve the query as usual.
}
// Select the longest existing name of QNAME from covering NSEC
LongestExistName = common part of both owner name and
next domain name of CoveringNSEC;
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if (*.LongestExistName entry exists in the cache) {
the resolver can generate positive response
// synthesize the wildcard *.TEST
}
if covering NSEC RR of "*.LongestExistName" at SIGNER zone exists
in the cache {
the resolver can generate negative response;
}
//*.LongestExistName may exist. cannot generate negative response
Resolve the query as usual.
} 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
|| matching NSEC3 of TEST exist in the cache) {
// TEST exist
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 (No *.UPPER information)
}
break; // Lack of information (No TEST information)
}
// no matching/covering NSEC3 of QNAME information
Resolve the query as usual
}
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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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