5Internet Engineering Task Force (IETF) P. van Dijk
6Request for Comments: 9077 PowerDNS
7Updates: 4034, 4035, 5155, 8198 July 2021
8Category: Standards Track
12 NSEC and NSEC3: TTLs and Aggressive Use
16 Due to a combination of unfortunate wording in earlier documents,
17 aggressive use of NSEC and NSEC3 records may deny the existence of
18 names far beyond the intended lifetime of a denial. This document
19 changes the definition of the NSEC and NSEC3 TTL to correct that
20 situation. This document updates RFCs 4034, 4035, 5155, and 8198.
24 This is an Internet Standards Track document.
26 This document is a product of the Internet Engineering Task Force
27 (IETF). It represents the consensus of the IETF community. It has
28 received public review and has been approved for publication by the
29 Internet Engineering Steering Group (IESG). Further information on
30 Internet Standards is available in Section 2 of RFC 7841.
32 Information about the current status of this document, any errata,
33 and how to provide feedback on it may be obtained at
34 https://www.rfc-editor.org/info/rfc9077.
38 Copyright (c) 2021 IETF Trust and the persons identified as the
39 document authors. All rights reserved.
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47 include Simplified BSD License text as described in Section 4.e of
48 the Trust Legal Provisions and are provided without warranty as
49 described in the Simplified BSD License.
54 2. Conventions and Definitions
55 3. NSEC and NSEC3 TTL Changes
56 3.1. Updates to RFC 4034
57 3.2. Updates to RFC 4035
58 3.3. Updates to RFC 5155
59 3.4. Updates to RFC 8198
60 4. Zone Operator Considerations
61 4.1. A Note on Wildcards
62 5. Security Considerations
63 6. IANA Considerations
64 7. Normative References
70 [RFC2308] defines the TTL of the Start of Authority (SOA) record that
71 must be returned in negative answers (NXDOMAIN or NODATA):
73 | The TTL of this record is set from the minimum of the MINIMUM
74 | field of the SOA record and the TTL of the SOA itself, and
75 | indicates how long a resolver may cache the negative answer.
77 Thus, if the TTL of the SOA in the zone is lower than the SOA MINIMUM
78 value (the last number in the SOA record), the authoritative server
79 sends that lower value as the TTL of the returned SOA record. The
80 resolver always uses the TTL of the returned SOA record when setting
81 the negative TTL in its cache.
83 However, [RFC4034], Section 4 has this unfortunate text:
85 | The NSEC RR SHOULD have the same TTL value as the SOA minimum TTL
86 | field. This is in the spirit of negative caching ([RFC2308]).
88 This text, while referring to [RFC2308], can cause NSEC records to
89 have much higher TTLs than the appropriate negative TTL for a zone.
90 [RFC5155] contains equivalent text.
92 [RFC8198], Section 5.4 tries to correct this:
94 | Section 5 of [RFC2308] also states that a negative cache entry TTL
95 | is taken from the minimum of the SOA.MINIMUM field and SOA's TTL.
96 | This can be less than the TTL of an NSEC or NSEC3 record, since
97 | their TTL is equal to the SOA.MINIMUM field (see [RFC4035],
98 | Section 2.3 and [RFC5155], Section 3).
100 | A resolver that supports aggressive use of NSEC and NSEC3 SHOULD
101 | reduce the TTL of NSEC and NSEC3 records to match the SOA.MINIMUM
102 | field in the authority section of a negative response, if
103 | SOA.MINIMUM is smaller.
105 But the NSEC and NSEC3 RRs should, according to [RFC4034] and
106 [RFC5155], already be at the value of the MINIMUM field in the SOA.
107 Thus, the advice from [RFC8198] would not actually change the TTL
108 used for the NSEC and NSEC3 RRs for authoritative servers that follow
111 As a theoretical exercise, consider a top-level domain (TLD) named
112 .example with an SOA record like this:
114 example. 900 IN SOA primary.example. dnsadmin.example. (
115 1 1800 900 604800 86400 )
117 The SOA record has a 900-second TTL and an 86400-second MINIMUM TTL.
118 Negative responses from this zone have a 900-second TTL, but the NSEC
119 or NSEC3 records in those negative responses have an 86400-second
120 TTL. If a resolver were to use those NSEC or NSEC3 records
121 aggressively, they would be considered valid for a day instead of the
1242. Conventions and Definitions
126 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
127 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
128 "OPTIONAL" in this document are to be interpreted as described in
129 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
130 capitals, as shown here.
1323. NSEC and NSEC3 TTL Changes
134 [RFC4034], [RFC4035], and [RFC5155] use the SHOULD requirement level,
135 but they were written prior to the publication of [RFC8198] when
136 [RFC4035] still said:
138 | However, it seems prudent for resolvers to avoid blocking new
139 | authoritative data or synthesizing new data on their own.
141 [RFC8198] updated that text to contain:
143 | ...DNSSEC-enabled validating resolvers SHOULD use wildcards and
144 | NSEC/NSEC3 resource records to generate positive and negative
145 | responses until the effective TTLs or signatures for those records
148 This means that the correctness of NSEC and NSEC3 records and their
149 TTLs has become much more important. Because of that, the updates in
150 this document upgrade the requirement level to MUST.
1523.1. Updates to RFC 4034
156 | The NSEC RR SHOULD have the same TTL value as the SOA minimum TTL
157 | field. This is in the spirit of negative caching ([RFC2308]).
159 This is updated to say:
161 | The TTL of the NSEC RR that is returned MUST be the lesser of the
162 | MINIMUM field of the SOA record and the TTL of the SOA itself.
163 | This matches the definition of the TTL for negative responses in
164 | [RFC2308]. Because some signers incrementally update the NSEC
165 | chain, a transient inconsistency between the observed and expected
1683.2. Updates to RFC 4035
172 | The TTL value for any NSEC RR SHOULD be the same as the minimum
173 | TTL value field in the zone SOA RR.
175 This is updated to say:
177 | The TTL of the NSEC RR that is returned MUST be the lesser of the
178 | MINIMUM field of the SOA record and the TTL of the SOA itself.
179 | This matches the definition of the TTL for negative responses in
180 | [RFC2308]. Because some signers incrementally update the NSEC
181 | chain, a transient inconsistency between the observed and expected
1843.3. Updates to RFC 5155
188 | The NSEC3 RR SHOULD have the same TTL value as the SOA minimum TTL
189 | field. This is in the spirit of negative caching [RFC2308].
191 This is updated to say:
193 | The TTL of the NSEC3 RR that is returned MUST be the lesser of the
194 | MINIMUM field of the SOA record and the TTL of the SOA itself.
195 | This matches the definition of the TTL for negative responses in
196 | [RFC2308]. Because some signers incrementally update the NSEC3
197 | chain, a transient inconsistency between the observed and expected
200 Where [RFC5155] says:
202 | * The TTL value for any NSEC3 RR SHOULD be the same as the
203 | minimum TTL value field in the zone SOA RR.
205 This is updated to say:
207 | * The TTL value for each NSEC3 RR MUST be the lesser of the
208 | MINIMUM field of the zone SOA RR and the TTL of the zone SOA RR
209 | itself. Because some signers incrementally update the NSEC3
210 | chain, a transient inconsistency between the observed and
211 | expected TTL MAY exist.
2133.4. Updates to RFC 8198
215 [RFC8198], Section 5.4 ("Consideration on TTL") is completely
216 replaced by the following text:
218 | The TTL value of negative information is especially important,
219 | because newly added domain names cannot be used while the negative
220 | information is effective.
222 | Section 5 of [RFC2308] suggests a maximum default negative cache
223 | TTL value of 3 hours (10800). It is RECOMMENDED that validating
224 | resolvers limit the maximum effective TTL value of negative
225 | responses (NSEC/NSEC3 RRs) to this same value.
227 | A resolver that supports aggressive use of NSEC and NSEC3 MAY
228 | limit the TTL of NSEC and NSEC3 records to the lesser of the
229 | SOA.MINIMUM field and the TTL of the SOA in a response, if
230 | present. It MAY also use a previously cached SOA for a zone to
233 (The third paragraph of the original is removed, and the fourth
234 paragraph is updated to allow resolvers to also take the lesser of
235 the SOA TTL and SOA MINIMUM.)
2374. Zone Operator Considerations
239 If signers and DNS servers for a zone cannot immediately be updated
240 to conform to this document, zone operators are encouraged to
241 consider setting their SOA record TTL and the SOA MINIMUM field to
242 the same value. That way, the TTL used for aggressive NSEC and NSEC3
243 use matches the SOA TTL for negative responses.
245 Note that some signers might use the SOA TTL or MINIMUM as a default
246 for other values, such as the TTL for DNSKEY records. Operators
247 should consult documentation before changing values.
2494.1. A Note on Wildcards
251 Validating resolvers consider an expanded wildcard valid for the
252 wildcard's TTL, capped by the TTLs of the NSEC or NSEC3 proof that
253 shows that the wildcard expansion is legal. Thus, changing the TTL
254 of NSEC or NSEC3 records (explicitly, or by implementation of this
255 document implicitly) might affect (shorten) the lifetime of
2585. Security Considerations
260 An attacker can delay future records from appearing in a cache by
261 seeding the cache with queries that cause NSEC or NSEC3 responses to
262 be cached for aggressive use purposes. This document reduces the
263 impact of that attack in cases where the NSEC or NSEC3 TTL is higher
264 than the zone operator intended.
2666. IANA Considerations
268 IANA has added a reference to this document in the "Resource Record
269 (RR) TYPEs" subregistry of the "Domain Name System (DNS) Parameters"
270 registry for the NSEC and NSEC3 types.
2727. Normative References
274 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
275 Requirement Levels", BCP 14, RFC 2119,
276 DOI 10.17487/RFC2119, March 1997,
277 <https://www.rfc-editor.org/info/rfc2119>.
279 [RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
280 NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,
281 <https://www.rfc-editor.org/info/rfc2308>.
283 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
284 Rose, "Resource Records for the DNS Security Extensions",
285 RFC 4034, DOI 10.17487/RFC4034, March 2005,
286 <https://www.rfc-editor.org/info/rfc4034>.
288 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
289 Rose, "Protocol Modifications for the DNS Security
290 Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
291 <https://www.rfc-editor.org/info/rfc4035>.
293 [RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
294 Security (DNSSEC) Hashed Authenticated Denial of
295 Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008,
296 <https://www.rfc-editor.org/info/rfc5155>.
298 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
299 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
300 May 2017, <https://www.rfc-editor.org/info/rfc8174>.
302 [RFC8198] Fujiwara, K., Kato, A., and W. Kumari, "Aggressive Use of
303 DNSSEC-Validated Cache", RFC 8198, DOI 10.17487/RFC8198,
304 July 2017, <https://www.rfc-editor.org/info/rfc8198>.
308 This document was made possible with the help of the following
325 The author would like to explicitly thank Paul Hoffman for the
326 extensive reviews, text contributions, and help in navigating WG
336 Email: peter.van.dijk@powerdns.com