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 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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on April 21, 2016. Copyright Notice Copyright (c) 2015 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 (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect Fujiwara & Kato Expires April 21, 2016 [Page 1] Internet-Draft NSEC/NSEC3 usage October 2015 to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. 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". Fujiwara & Kato Expires April 21, 2016 [Page 2] Internet-Draft NSEC/NSEC3 usage October 2015 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. Fujiwara & Kato Expires April 21, 2016 [Page 3] Internet-Draft NSEC/NSEC3 usage October 2015 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 Fujiwara & Kato Expires April 21, 2016 [Page 4] Internet-Draft NSEC/NSEC3 usage October 2015 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. Fujiwara & Kato Expires April 21, 2016 [Page 5] Internet-Draft NSEC/NSEC3 usage October 2015 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 Fujiwara & Kato Expires April 21, 2016 [Page 6] Internet-Draft NSEC/NSEC3 usage October 2015 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. Fujiwara & Kato Expires April 21, 2016 [Page 7] Internet-Draft NSEC/NSEC3 usage October 2015 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, . [RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998, . [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, "Protocol Modifications for the DNS Security Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005, . [RFC4470] Weiler, S. and J. Ihren, "Minimally Covering NSEC Records and DNSSEC On-line Signing", RFC 4470, DOI 10.17487/ RFC4470, April 2006, . Fujiwara & Kato Expires April 21, 2016 [Page 8] Internet-Draft NSEC/NSEC3 usage October 2015 [RFC5074] Weiler, S., "DNSSEC Lookaside Validation (DLV)", RFC 5074, DOI 10.17487/RFC5074, November 2007, . [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, . [RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms for DNS (EDNS(0))", STD 75, RFC 6891, DOI 10.17487/ RFC6891, April 2013, . 13.2. Informative References [BIND9] Internet Systems Consortium, Inc., "Name Server Software", 2000, . [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, . 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]. Fujiwara & Kato Expires April 21, 2016 [Page 9] Internet-Draft NSEC/NSEC3 usage October 2015 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; Fujiwara & Kato Expires April 21, 2016 [Page 10] Internet-Draft NSEC/NSEC3 usage October 2015 } 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 Fujiwara & Kato Expires April 21, 2016 [Page 11] Internet-Draft NSEC/NSEC3 usage October 2015 } // 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 Fujiwara & Kato Expires April 21, 2016 [Page 12]