--- 1/draft-ietf-dnsop-nsec-aggressiveuse-04.txt 2016-10-20 13:15:56.474791316 -0700 +++ 2/draft-ietf-dnsop-nsec-aggressiveuse-05.txt 2016-10-20 13:15:56.506792108 -0700 @@ -1,21 +1,21 @@ Network Working Group K. Fujiwara Internet-Draft JPRS Updates: 4035 (if approved) A. Kato Intended status: Standards Track Keio/WIDE -Expires: April 10, 2017 W. Kumari +Expires: April 23, 2017 W. Kumari Google - October 7, 2016 + October 20, 2016 Aggressive use of NSEC/NSEC3 - draft-ietf-dnsop-nsec-aggressiveuse-04 + draft-ietf-dnsop-nsec-aggressiveuse-05 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 allow DNSSEC validating resolvers to generate negative answers within a range, and positive answers from wildcards. This increases performance / decreases latency, decreases resource utilization on both authoritative and recursive servers, and also increases privacy. It may also help increase @@ -41,21 +41,21 @@ 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 10, 2017. + This Internet-Draft will expire on April 23, 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 @@ -64,41 +64,42 @@ 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. Aggressive Caching . . . . . . . . . . . . . . . . . . . . . 5 + 5. Aggressive Negative Caching . . . . . . . . . . . . . . . . . 5 5.1. NSEC . . . . . . . . . . . . . . . . . . . . . . . . . . 6 5.2. NSEC3 . . . . . . . . . . . . . . . . . . . . . . . . . . 6 5.3. Wildcards . . . . . . . . . . . . . . . . . . . . . . . . 7 5.4. Consideration on TTL . . . . . . . . . . . . . . . . . . 7 - 6. Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . 8 + 6. Benefits . . . . . . . . . . . . . . . . . . . . . . . . . . 7 7. Update to RFC 4035 . . . . . . . . . . . . . . . . . . . . . 8 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 9. Security Considerations . . . . . . . . . . . . . . . . . . . 9 10. Implementation Status . . . . . . . . . . . . . . . . . . . . 9 - 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10 + 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9 11.1. Change History . . . . . . . . . . . . . . . . . . . . . 10 11.1.1. Version draft-fujiwara-dnsop-nsec-aggressiveuse-01 . 12 - 11.1.2. Version draft-fujiwara-dnsop-nsec-aggressiveuse-02 . 12 + 11.1.2. Version draft-fujiwara-dnsop-nsec-aggressiveuse-02 . 13 11.1.3. Version draft-fujiwara-dnsop-nsec-aggressiveuse-03 . 13 11.2. new section . . . . . . . . . . . . . . . . . . . . . . 13 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 12.1. Normative References . . . . . . . . . . . . . . . . . . 13 12.2. Informative References . . . . . . . . . . . . . . . . . 14 Appendix A. Detailed implementation notes . . . . . . . . . . . 14 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15 + Appendix B. Procedure for determining ENT vs NXDOMAN . . . . . . 15 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16 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 @@ -132,67 +133,72 @@ "Closest Encloser" is also defined in NSEC3 [RFC5155], as is "Next closer name". 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 + albatross.example.com IN A 192.0.2.1 elephant.example.com IN A 192.0.2.2 - *.example.com IN A 192.0.2.3 - zebra.example.com IN A 192.0.2.4 + zebra.example.com IN A 192.0.2.3 If a validating resolver receives a query for cat.example.com, it contacts its resolver (which may be itself) to query the example.com servers and will get back an NSEC record starting that there are no - records (alphabetically) between apple and elephant, or an NSEC3 + records (alphabetically) between albatross and elephant, or an NSEC3 record stating there is nothing between two hashed names. The 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 + does not use the fact that the proof covers a range (albatross to elephant) to suppress queries for other labels that fall within this range. This means that if the validating resolver 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. - Further, if a query is received for lion.example.com, it contacts its - resolver (which may be itself) to query the example.com servers and - will get back an NSEC record stating that there are no records - (alphabetically) between elephant and zebra (or an NSEC3 record + Now, assume that the (DNSSEC signed) "example.org" zone contains: + + avocado.example.org IN A 192.0.2.1 + *.example.org IN A 192.0.2.2 + zucchini.example.org IN A 192.0.2.3 + + If a query is received for leek.example.org, it contacts its resolver + (which may be itself) to query the example.org servers and will get + back an NSEC record stating that there are no records + (alphabetically) between avocado and zucchini (or an NSEC3 record stating there is nothing between two hashed names), as well as an - answer for lion.example.com, with the label count of the signature + answer for leek.example.org, with the label count of the signature set to two (see [RFC7129], section 5.3 for more details). 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 dog.example.com it would receive a - (signed) NSEC record stating that there are no labels between "apple" - and "elephant" (or, for NSEC3, a similar pair of hashed names). This - is a signed, cryptographic proof that these names are the ones before - and after the queried for label. As dog.example.com falls within - this range, the recursive server knows that dog.example.com really - does not exist. + queried for name. In the first example above, if the (DNSSEC + validating) recursive server were to query for dog.example.com it + would receive a (signed) NSEC record stating that there are no labels + between "albatross" and "elephant" (or, for NSEC3, a similar pair of + hashed names). This is a signed, cryptographic proof that these + names are the ones before and after the queried for label. As + dog.example.com falls within this range, the recursive server knows + that dog.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 validating server receives that fall within the range covered by the record (for the TTL for the record). This document also specifies that a positive answer should be generated for any queries that the validating server receives that are proven to be covered by a wildcard record. Section 4.5 of [RFC4035] says: @@ -214,99 +220,92 @@ We believe this recommendation can be relaxed because, in the absense of this technique, a lookup for the exact name could have come in during this interval, and so a negative answer could already be cached (see [RFC2308] for more background). This means that zone operators should have no expectation that an added name would work immediately. With DNSSEC and Aggressive NSEC, the TTL of the NSEC record is the authoritative statement of how quickly a name can start working within a zone. -5. Aggressive Caching +5. Aggressive Negative Caching Section 4.5 of [RFC4035] says 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 wildcards and NSEC/NSEC3 resource records | | to generate positive and negative responses until the | | 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 negative cache of the validating resolver has sufficient + information to validate the query, the resolver SHOULD use NSEC, + NSEC3 and wildcard records aggressively. Otherwise, it MUST fall + back to send the query to the authoritative DNS servers. -5.1. NSEC + It is recommended that resolvers that implement Aggressive Negative + Caching provide a configuration switch to disable the feature. + Separate configuration switches may be implemented for the aggressive + use of NSEC, NSEC3 and wildcard records, and it is recommended to + enable aggressive negative caching by default. - Implementations which support aggressive use of NSEC SHOULD enable - this by default. Implementations MAY provide a configuration switch - to disable aggressive use of NSEC and allow it to be enabled or - disabled per domain. +5.1. NSEC 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 denial of existence can be determined according to the rules set out in Section 5.4 of [RFC4035], using NSEC records in the cache, then the resolver can immediately return an NXDOMAIN or NODATA (as appropriate) response. 5.2. NSEC3 NSEC3 aggressive negative caching is more difficult than NSEC aggressive caching. 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. - A validating resolver implementation MAY support aggressive use of - NSEC3. If it does support aggressive use of NSEC3, it SHOULD enable - this by default. It MAY provide a configuration switch to disable - aggressive use of NSEC3 and allow it to be enabled or disabled for - specific zones. - If denial of existence can be determined according to the rules set - out in [RFC5155] sections 8.4, 8.5, 8.6, 8.7,using NSEC3 records in + out in [RFC5155] Sections 8.4, 8.5, 8.6, 8.7, using NSEC3 records in the cache, then the resolver can immediately return an NXDOMAIN or NODATA response (as appropriate). 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. 5.3. Wildcards The last paragraph of [RFC4035] Section 4.5 also discusses the use of wildcards and NSEC RRs to generate positive responses and recommends that it not be relied upon. Just like the case for the aggressive use of NSEC/NSEC3 for negative answers, we revise this recommendation. As long as the validating resolver can determine that a name would - not exist without the wildcard match, it MAY synthesize an answer for - that name using the cached deduced wildcard. If the corresponding - wildcard record is not in the cache, it MUST fall back to send the - query to the authoritative DNS servers. - - An implementation MAY support aggressive use of wildcards. It SHOULD - provide a configuration switch to disable aggressive use of - wildcards. + not exist without the wildcard match, determined according to the + rules set out in Section 5.3.4 of [RFC4035] (NSEC), or in Section 8.8 + of [RFC5155], it SHOULD synthesize an answer for that name using the + cached deduced wildcard. If the corresponding wildcard record is not + in the cache, it MUST fall back to send the query to the + authoritative DNS servers. 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 [RFC2308] states that the maximum number of negative cache TTL value is 3 hours (10800). It is RECOMMENDED that validating resolvers limit the maximum effective TTL value of @@ -340,22 +339,22 @@ Decreased authorative server load: Because recursive servers can answer (negative) queries without asking the authoritative server, the authoritative servers receive fewer 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, at the time of this writing, around 65% of queries to Root Name servers result in NXDOMAIN responses (see - statis [root-servers.org]); this technique will eliminate a sizable - quantity of these. + statistics from [root-servers.org]); this technique will eliminate a + sizable quantity of these. 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 resolvers. As the resolver will not have the answers cached, it has to ask external servers for each random query, leading to a DoS on the authoritative servers (and often resolvers). Aggressive NSEC may help mitigate these attacks by allowing the resolver to answer directly from cache for any random queries which fall within already requested ranges. It will not always work as an effective defense, not least because not many zones are DNSSEC signed @@ -405,42 +404,60 @@ (minutes or hours), their RRSIG expiration time can be much further in the future (weeks). An attacker who is able to successfully spoof responses might poison a cache with old NSEC/NSEC3 records. If the resolver is NOT making aggressive use of NSEC/NSEC3, the attacker has to repeat the attack for every query. If the resolver IS making aggressive use of NSEC/NSEC3, one successful attack would be able to suppress many queries for new names, up to the negative TTL. 10. Implementation Status - Unbound currenty implements aggressive negative caching, as does + [ Editor note: RFC Editor, please remove this entire section. + RFC6982 says: "Since this information is necessarily time dependent, + it is inappropriate for inclusion in a published RFC." ] + + Unbound currently implements aggressive negative caching, as does Google Public DNS. 11. Acknowledgments The authors gratefully acknowledge DLV [RFC5074] author Samuel Weiler and the Unbound developers. The authors would like to specifically thank Stephane Bortzmeyer, Tony Finch, Tatuya JINMEI for extensive review and comments, and also Mark Andrews, Casey Deccio, Alexander Dupuy, Olafur Gudmundsson, Bob Harold, Shumon Huque, John Levine, Pieter Lexis and Matthijs Mekking - (who even sent pull requests!). + (who even sent pull requests!). Mark Andrews also provided the text + (https://www.ietf.org/mail-archive/web/dnsop/current/msg18332.html) + which we made into Appendix B 11.1. Change History RFC Editor: Please remove this section prior to publication. + -04 to -05: + + o Bob pointed out that I did a stupid - when I added the wildcard to + 'example.com' I made the example wrong / confusing. I have + attempted to fix this by adding a second example zone + (example.org) with the wildcard instead. + + o More helpful changes (in a pull request, thanks!) from Matthijs + + o Included Mark Andrew's useful explanation of how to tell ENT from + NXD as an Appendix. + -03 to -04: o Working group does want the "positive" answers, not just negative - ones. This requires readding what used to be Section 7, and a + ones. This requires reading what used to be Section 7, and a bunch of cleanup, including: * Additional text in the Problem Statement * Added a wildcard record to the zone. * Added "or positive answers from wildcards" type text (where appropriate) to explain that this isn't just for negative answers. @@ -653,20 +671,48 @@ expanded.) o The aggressive negative caching may be inserted at the cache lookup part of the recursive 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 resolver must process the query as though it does not implement aggressive negative caching. +Appendix B. Procedure for determining ENT vs NXDOMAN + + Thanks to Mark Andrews for providing these helpful notes for + implementors. As they are more general than for Aggressive NSEC we + have placed them in an appendix. + + If the NSEC record has not been verified as secure discard it. + + If the given name sorts before or matches the NSEC owner name discard + it as it does not prove the NXDOMAIN or ENT. + + If the given name is a subdomain of the NSEC owner name and the NS + bit is present and the SOA bit is absent then discard the NSEC as it + is from a parent zone. + + If the next domain name sorts after the NSEC owner name and the given + name sorts after or matches next domain name then discard the NSEC + record as it does not prove the NXDOMAIN or ENT. + + If the next domain name sorts before or matches the NSEC owner name + and the given name is not a subdomain of the next domain name then + discard the NSEC as it does not prove the NXDOMAIN or ENT. + + You now have a NSEC record that proves the NXDOMAIN or ENT. + + If the next domain name is a subdomain of the given name you have a + ENT otherwise you have a NXDOMAIN. + 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