Network Working Group K. Fujiwara Internet-Draft JPRS Updates: 4035 (if approved) A. Kato Intended status: Standards Track Keio/WIDE Expires: February 3, 2017 W. Kumari Google August 02, 2016 Aggressive use of NSEC/NSEC3 draft-ietf-dnsop-nsec-aggressiveuse-01 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 resilience to DoS attacks, increases performance / decreases latency, decreases resource utilization on both authoritative and recursive servers, and also increases privacy. 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. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute Fujiwara, et al. Expires February 3, 2017 [Page 1] Internet-Draft NSEC/NSEC3 usage August 2016 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 February 3, 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 (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 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 . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3 4. Background . . . . . . . . . . . . . . . . . . . . . . . . . 4 5. Proposed Solution . . . . . . . . . . . . . . . . . . . . . . 5 5.1. Aggressive Negative Caching . . . . . . . . . . . . . . . 5 5.2. NSEC . . . . . . . . . . . . . . . . . . . . . . . . . . 5 5.3. NSEC3 . . . . . . . . . . . . . . . . . . . . . . . . . . 6 5.4. Wildcard . . . . . . . . . . . . . . . . . . . . . . . . 6 5.5. Consideration on TTL . . . . . . . . . . . . . . . . . . 7 6. Update to RFC 4035 . . . . . . . . . . . . . . . . . . . . . 7 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 8. Security Considerations . . . . . . . . . . . . . . . . . . . 7 9. Implementation Status . . . . . . . . . . . . . . . . . . . . 8 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8 11. Change History . . . . . . . . . . . . . . . . . . . . . . . 8 11.1. Version draft-fujiwara-dnsop-nsec-aggressiveuse-01 . . . 9 11.2. Version draft-fujiwara-dnsop-nsec-aggressiveuse-02 . . . 9 11.3. Version draft-fujiwara-dnsop-nsec-aggressiveuse-03 . . . 9 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 12.1. Normative References . . . . . . . . . . . . . . . . . . 10 Fujiwara, et al. Expires February 3, 2017 [Page 2] Internet-Draft NSEC/NSEC3 usage August 2016 12.2. Informative References . . . . . . . . . . . . . . . . . 10 Appendix A. Detailed implementation idea . . . . . . . . . . . . 11 Appendix B. Side effect: mitigation of random subdomain attacks 11 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 1. Introduction A DNS negative cache currently 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, which have negative implications for DoS survivability, 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 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 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]. 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". 3. Problem Statement The current DNS negative cache caches negative (non-existent) information, and requires an exact match in most instances [RFC2308]. Fujiwara, et al. Expires February 3, 2017 [Page 3] Internet-Draft NSEC/NSEC3 usage August 2016 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 (currently) 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). [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- Fujiwara, et al. Expires February 3, 2017 [Page 4] Internet-Draft NSEC/NSEC3 usage August 2016 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. Proposed Solution 5.1. Aggressive 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 full-service | | resolvers MAY use NSEC/NSEC3 resource records to generate | | negative responses until their effective TTLs or signatures | | for those records expire. | +--------------------------------------------------------------+ If the full-service resolver's cache has sufficient 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. If the query name has the matching NSEC/NSEC3 RR proving the information requested does not exist, the full-service resolver may respond with a NODATA (empty) answer. 5.2. NSEC If a full-service resolver implementation supports aggressive negative caching, then it SHOULD support aggressive use of NSEC and enable it by default. It SHOULD provide a configuration switch to Fujiwara, et al. Expires February 3, 2017 [Page 5] Internet-Draft NSEC/NSEC3 usage August 2016 disable aggressive use of NSEC and allow it to be enabled or disabled for specific zones. 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 full-service resolver's cache contains an NSEC RR covering the source of synthesis and the covering NSEC RR of the query name, the full-service resolver may respond with NXDOMAIN error immediately. 5.3. NSEC3 NSEC3 aggressive 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 full-service 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 full-service 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 full-service 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. 5.4. 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 full-service 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. (Note that, so far, this is orthogonal to "when aggressive use (of NSEC) is enabled"). Furthermore, when aggressive use of NSEC is enabled, the aggressive use of cached deduced wildcard will be more effective. Fujiwara, et al. Expires February 3, 2017 [Page 6] Internet-Draft NSEC/NSEC3 usage August 2016 A full-service resolver implementation MAY support aggressive use of wildcards. It SHOULD provide a configuration switch to disable aggressive use of wildcards. 5.5. 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 full-service resolvers limit the maximum effective TTL value of negative responses (NSEC/NSEC3 RRs) to this same value. 6. 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". (If approved, ) The paragraph is updated as follows: +--------------------------------------------------------------+ | Once the records are validated, DNSSEC enabled full-service | | 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. | +--------------------------------------------------------------+ 7. IANA Considerations This document has no IANA actions. 8. 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. Fujiwara, et al. Expires February 3, 2017 [Page 7] Internet-Draft NSEC/NSEC3 usage August 2016 Aggressive use of NSEC / NSEC3 resource records without DNSSEC validation may cause security problems. It is highly recommended to apply DNSSEC validation. 9. 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. 10. Acknowledgments The authors gratefully acknowledge DLV [RFC5074] author Samuel Weiler and Unbound developers. Valuable comments were provided by Alexander Dupuy, Olafur Gudmundsson, Pieter Lexis, Bob Harold, Tatuya JINMEI, Shumon Huque, Mark Andrews, Casey Deccio, Bob Harold, Stephane Bortzmeyer and Matthijs Mekking. 11. Change History RFC Editor: Please remove this section prior to publication. -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 "mitigaton 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 Fujiwara, et al. Expires February 3, 2017 [Page 8] Internet-Draft NSEC/NSEC3 usage August 2016 o Use NSEC3/Wildcard keywords o Improved wordings (from good comments) o Simplified pseudo code for NSEC3 o Added Warren as co-author. o Reworded much of the problem statement o Reworked examples to better explain the problem / solution. 11.1. 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. 11.2. 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 11.3. 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 12. References Fujiwara, et al. Expires February 3, 2017 [Page 9] Internet-Draft NSEC/NSEC3 usage August 2016 12.1. 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, . [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, . [RFC4592] Lewis, E., "The Role of Wildcards in the Domain Name System", RFC 4592, DOI 10.17487/RFC4592, July 2006, . [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, . [RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS Terminology", RFC 7719, DOI 10.17487/RFC7719, December 2015, . 12.2. Informative References [I-D.ietf-dnsop-nxdomain-cut] Bortzmeyer, S. and S. Huque, "NXDOMAIN really means there is nothing underneath", draft-ietf-dnsop-nxdomain-cut-03 (work in progress), May 2016. [I-D.vixie-dnsext-resimprove] 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. Fujiwara, et al. Expires February 3, 2017 [Page 10] Internet-Draft NSEC/NSEC3 usage August 2016 Appendix A. Detailed implementation idea 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 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 of [RFC5074]). o 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. (Imported from Section 6.1 of [RFC5074] and expanded.) o The aggressive negative caching may be inserted at the cache lookup part of the full-service resolvers. o 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. Appendix B. Side effect: mitigation of random subdomain attacks Random sub-domain attacks (referred to as "Water Torture" attacks or NXDomain attacks) send many queries for non-existent information to full-service resolvers. Their query names consist of random prefixes and a target domain name. The negative cache does not work well, and thus targeted full-service resolvers end up sending queries to authoritative DNS servers of the target domain name. The aggressive negative caching is one of possible countermeasures to random subdomain attacks. If the full-service resolver supports aggressive negative caching and the target domain name is signed with NSEC/NSEC3 (without Opt-Out), the aggressive negative caching is one of countermeasures of random subdomain attacks. Fujiwara, et al. Expires February 3, 2017 [Page 11] Internet-Draft NSEC/NSEC3 usage August 2016 However, attackers can set the CD bit to their attack queries. The CD bit disables signature validation and the aggressive negative caching will be of no use. 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 Warren Kumari Google 1600 Amphitheatre Parkway Mountain View, CA 94043 US Email: warren@kumari.net Fujiwara, et al. Expires February 3, 2017 [Page 12]