| Network Working Group | K. Fujiwara |
| Internet-Draft | JPRS |
| Updates: 4035 (if approved) | A. Kato |
| Intended status: Standards Track | Keio/WIDE |
| Expires: July 16, 2017 | W. Kumari |
| January 12, 2017 |
Aggressive use of DNSSEC-validated Cache
draft-ietf-dnsop-nsec-aggressiveuse-08
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 resilience to certain DoS attacks in some circumstances.
This document updates RFC4035 by allowing validating resolvers to generate negative answers based upon NSEC/NSEC3 records (and positive answers in the presence of wildcards).
[ 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.]
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A DNS negative cache exists, and is used to cache the fact that an RRset 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 NSEC/NSEC3 resource records to synthesize negative answers from the information they have in the cache. This allows validating resolvers to respond with a negative answer immediately if the name in question falls into a range expressed by a NSEC/NSEC3 resource record already in the cache. It also allows the synthesis of positive answers in the presence of wildcard records.
Aggressive Negative Caching was first proposed in Section 6 of DNSSEC Lookaside Validation (DLV) [RFC5074] in order to find covering NSEC records efficiently.
[RFC8020], and [I-D.vixie-dnsext-resimprove] proposes first steps to using NXDOMAIN information for more effective caching. This takes this technique further.
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 "Source of Synthesis" in this document are to be interpreted as described in [RFC4592].
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:
albatross.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 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 stating that there are no 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 (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.
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).
Another example: 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 leek.example.org, with the label count of the signature set to two (see [RFC7129], section 5.3 for more details).
If the validating resolver gets a query for banana.example.org it will once again go off and query the example.org servers for banana.example.org (even though it already has proof that there is a wildcard record) - just like above, this has privacy implications, wastes resources, can be used to contribute to a DoS, etc.
DNSSEC [RFC4035] and [RFC5155] both provide "authenticated denial of existence"; this is a cryptographic proof that the queried for name does not exist or type does not exist. Proof that a name does not exist is accomplished by providing a (DNSSEC secured) record containing the names which appear alphabetically before and after the 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. Proof that a type does not exist is accomplished by providing a (DNSSEC secured) record containing the queried for name, and a type bitmap which does not include the requested type.
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:
"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." and "The reason for these recommendations is that, between the initial query and the expiration of the data from the cache, the authoritative data might have been changed (for example, via dynamic update).". In other words, if a resolver generates negative answers from an NSEC record, it will not send any queries for names within that NSEC range (for the TTL). If a new name is added to the zone during this interval the resolver will not know this. Similarly, if the resolver is generating responses from a wildcard record, it will continue to do so (for the TTL).
We believe this recommendation can be relaxed because, in the absence 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/NSEC3 record and the SOA.MINIMUM field are the authoritative statement of how quickly a name can start working within a zone.
This document relaxes the restriction given in Section 4.5 of [RFC4035], see Section 7 for more detail.
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.
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.
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.
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 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.
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, 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 (or NODATA response) 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.
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 negative responses (NSEC/NSEC3 RRs) to this same value.
Section 5 of [RFC2308] also states that a negative cache entry TTL is taken from the minimum of the SOA.MINIMUM field and SOA's TTL. This can be less than the TTL of an NSEC or NSEC3 record, since their TTL is equal to the SOA.MINIMUM field (see [RFC4035]section 2.3 and [RFC5155] section 3.)
A resolver that supports aggressive use of NSEC and NSEC3 SHOULD reduce the TTL of NSEC and NSEC3 records to match the SOA.MINIMUM field in the authority section of a negative response, if SOA.MINIMUM is smaller.
The techniques described in this document provide a number of benefits, including (in no specific order):
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 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 at all -- but it will still provide an additional layer of defense.
As these benefits are only accrued by those using DNSSEC, it is hoped that these techniques will lead to more DNSSEC deployment.
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".
The paragraph is updated as follows:
+-----------------------------------------------------------------+ | Once the records are validated, DNSSEC enabled validating | | resolvers SHOULD use wildcards and NSEC/NSEC3 resource records | | to generate positive and negative responses until the | | effective TTLs or signatures for those records expire. | +-----------------------------------------------------------------+
This document has no IANA actions.
Use of NSEC / NSEC3 resource records without DNSSEC validation may create serious security issues, and so this technique requires DNSSEC validation.
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.
Although the TTL of NSEC/NSEC3 records is typically fairly short (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.
[ 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.
The authors gratefully acknowledge DLV [RFC5074] author Samuel Weiler and the Unbound developers.
Thanks to Mark Andrews for providing the helpful notes for implementors provided in Appendix B.
The authors would like to specifically thank Stephane Bortzmeyer (for standing next to and helping edit), Ralph Dolmans, 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, Matthijs Mekking (who even sent pull requests!) and Ondrej Sury. Mark Andrews also provided the helpful notes for implementors (https://www.ietf.org/mail-archive/web/dnsop/current/msg18332.html) which we made into Appendix B.
RFC Editor: Please remove this section prior to publication.
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From draft-fujiwara-dnsop-nsec-aggressiveuse-03 -> draft-ietf-dnsop-nsec-aggressiveuse
| [I-D.vixie-dnsext-resimprove] | Vixie, P., Joffe, R. and F. Neves, "Improvements to DNS Resolvers for Resiliency, Robustness, and Responsiveness", Internet-Draft draft-vixie-dnsext-resimprove-00, June 2010. |
| [RFC8020] | Bortzmeyer, S. and S. Huque, "NXDOMAIN: There Really Is Nothing Underneath", RFC 8020, DOI 10.17487/RFC8020, November 2016. |
| [root-servers.org] | IANA, "Root Server Technical Operations Assn" |
This procedure outlines how to determine if a given name does not exist, or is an ENT (Empty Non-Terminal, see [RFC5155] Section 1.3) with NSEC.
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.