1 Internet Engineering Task Force (IETF) P. van Dijk
2 Request for Comments: 9077 PowerDNS
3 Updates: 4034, 4035, 5155, 8198 July 2021
4 Category: Standards Track
5 ISSN: 2070-1721
6
7
8 NSEC and NSEC3: TTLs and Aggressive Use
9
10 Abstract
11
12 Due to a combination of unfortunate wording in earlier documents,
13 aggressive use of NSEC and NSEC3 records may deny the existence of
14 names far beyond the intended lifetime of a denial. This document
15 changes the definition of the NSEC and NSEC3 TTL to correct that
16 situation. This document updates RFCs 4034, 4035, 5155, and 8198.
17
18 Status of This Memo
19
20 This is an Internet Standards Track document.
21
22 This document is a product of the Internet Engineering Task Force
23 (IETF). It represents the consensus of the IETF community. It has
24 received public review and has been approved for publication by the
25 Internet Engineering Steering Group (IESG). Further information on
26 Internet Standards is available in Section 2 of RFC 7841.
27
28 Information about the current status of this document, any errata,
29 and how to provide feedback on it may be obtained at
30 https://www.rfc-editor.org/info/rfc9077.
31
32 Copyright Notice
33
34 Copyright (c) 2021 IETF Trust and the persons identified as the
35 document authors. All rights reserved.
36
37 This document is subject to BCP 78 and the IETF Trust's Legal
38 Provisions Relating to IETF Documents
39 (https://trustee.ietf.org/license-info) in effect on the date of
40 publication of this document. Please review these documents
41 carefully, as they describe your rights and restrictions with respect
42 to this document. Code Components extracted from this document must
43 include Simplified BSD License text as described in Section 4.e of
44 the Trust Legal Provisions and are provided without warranty as
45 described in the Simplified BSD License.
46
47 Table of Contents
48
49 1. Introduction
50 2. Conventions and Definitions
51 3. NSEC and NSEC3 TTL Changes
52 3.1. Updates to RFC 4034
53 3.2. Updates to RFC 4035
54 3.3. Updates to RFC 5155
55 3.4. Updates to RFC 8198
56 4. Zone Operator Considerations
57 4.1. A Note on Wildcards
58 5. Security Considerations
59 6. IANA Considerations
60 7. Normative References
61 Acknowledgements
62 Author's Address
63
64 1. Introduction
65
66 [RFC2308] defines the TTL of the Start of Authority (SOA) record that
67 must be returned in negative answers (NXDOMAIN or NODATA):
68
69 | The TTL of this record is set from the minimum of the MINIMUM
70 | field of the SOA record and the TTL of the SOA itself, and
71 | indicates how long a resolver may cache the negative answer.
72
73 Thus, if the TTL of the SOA in the zone is lower than the SOA MINIMUM
74 value (the last number in the SOA record), the authoritative server
75 sends that lower value as the TTL of the returned SOA record. The
76 resolver always uses the TTL of the returned SOA record when setting
77 the negative TTL in its cache.
78
79 However, [RFC4034], Section 4 has this unfortunate text:
80
81 | The NSEC RR SHOULD have the same TTL value as the SOA minimum TTL
82 | field. This is in the spirit of negative caching ([RFC2308]).
83
84 This text, while referring to [RFC2308], can cause NSEC records to
85 have much higher TTLs than the appropriate negative TTL for a zone.
86 [RFC5155] contains equivalent text.
87
88 [RFC8198], Section 5.4 tries to correct this:
89
90 | Section 5 of [RFC2308] also states that a negative cache entry TTL
91 | is taken from the minimum of the SOA.MINIMUM field and SOA's TTL.
92 | This can be less than the TTL of an NSEC or NSEC3 record, since
93 | their TTL is equal to the SOA.MINIMUM field (see [RFC4035],
94 | Section 2.3 and [RFC5155], Section 3).
95 |
96 | A resolver that supports aggressive use of NSEC and NSEC3 SHOULD
97 | reduce the TTL of NSEC and NSEC3 records to match the SOA.MINIMUM
98 | field in the authority section of a negative response, if
99 | SOA.MINIMUM is smaller.
100
101 But the NSEC and NSEC3 RRs should, according to [RFC4034] and
102 [RFC5155], already be at the value of the MINIMUM field in the SOA.
103 Thus, the advice from [RFC8198] would not actually change the TTL
104 used for the NSEC and NSEC3 RRs for authoritative servers that follow
105 the RFCs.
106
107 As a theoretical exercise, consider a top-level domain (TLD) named
108 .example with an SOA record like this:
109
110 example. 900 IN SOA primary.example. dnsadmin.example. (
111 1 1800 900 604800 86400 )
112
113 The SOA record has a 900-second TTL and an 86400-second MINIMUM TTL.
114 Negative responses from this zone have a 900-second TTL, but the NSEC
115 or NSEC3 records in those negative responses have an 86400-second
116 TTL. If a resolver were to use those NSEC or NSEC3 records
117 aggressively, they would be considered valid for a day instead of the
118 intended 15 minutes.
119
120 2. Conventions and Definitions
121
122 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
123 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
124 "OPTIONAL" in this document are to be interpreted as described in
125 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
126 capitals, as shown here.
127
128 3. NSEC and NSEC3 TTL Changes
129
130 [RFC4034], [RFC4035], and [RFC5155] use the SHOULD requirement level,
131 but they were written prior to the publication of [RFC8198] when
132 [RFC4035] still said:
133
134 | However, it seems prudent for resolvers to avoid blocking new
135 | authoritative data or synthesizing new data on their own.
136
137 [RFC8198] updated that text to contain:
138
139 | ...DNSSEC-enabled validating resolvers SHOULD use wildcards and
140 | NSEC/NSEC3 resource records to generate positive and negative
141 | responses until the effective TTLs or signatures for those records
142 | expire.
143
144 This means that the correctness of NSEC and NSEC3 records and their
145 TTLs has become much more important. Because of that, the updates in
146 this document upgrade the requirement level to MUST.
147
148 3.1. Updates to RFC 4034
149
150 [RFC4034] says:
151
152 | The NSEC RR SHOULD have the same TTL value as the SOA minimum TTL
153 | field. This is in the spirit of negative caching ([RFC2308]).
154
155 This is updated to say:
156
157 | The TTL of the NSEC RR that is returned MUST be the lesser of the
158 | MINIMUM field of the SOA record and the TTL of the SOA itself.
159 | This matches the definition of the TTL for negative responses in
160 | [RFC2308]. Because some signers incrementally update the NSEC
161 | chain, a transient inconsistency between the observed and expected
162 | TTL MAY exist.
163
164 3.2. Updates to RFC 4035
165
166 [RFC4035] says:
167
168 | The TTL value for any NSEC RR SHOULD be the same as the minimum
169 | TTL value field in the zone SOA RR.
170
171 This is updated to say:
172
173 | The TTL of the NSEC RR that is returned MUST be the lesser of the
174 | MINIMUM field of the SOA record and the TTL of the SOA itself.
175 | This matches the definition of the TTL for negative responses in
176 | [RFC2308]. Because some signers incrementally update the NSEC
177 | chain, a transient inconsistency between the observed and expected
178 | TTL MAY exist.
179
180 3.3. Updates to RFC 5155
181
182 [RFC5155] says:
183
184 | The NSEC3 RR SHOULD have the same TTL value as the SOA minimum TTL
185 | field. This is in the spirit of negative caching [RFC2308].
186
187 This is updated to say:
188
189 | The TTL of the NSEC3 RR that is returned MUST be the lesser of the
190 | MINIMUM field of the SOA record and the TTL of the SOA itself.
191 | This matches the definition of the TTL for negative responses in
192 | [RFC2308]. Because some signers incrementally update the NSEC3
193 | chain, a transient inconsistency between the observed and expected
194 | TTL MAY exist.
195
196 Where [RFC5155] says:
197
198 | * The TTL value for any NSEC3 RR SHOULD be the same as the
199 | minimum TTL value field in the zone SOA RR.
200
201 This is updated to say:
202
203 | * The TTL value for each NSEC3 RR MUST be the lesser of the
204 | MINIMUM field of the zone SOA RR and the TTL of the zone SOA RR
205 | itself. Because some signers incrementally update the NSEC3
206 | chain, a transient inconsistency between the observed and
207 | expected TTL MAY exist.
208
209 3.4. Updates to RFC 8198
210
211 [RFC8198], Section 5.4 ("Consideration on TTL") is completely
212 replaced by the following text:
213
214 | The TTL value of negative information is especially important,
215 | because newly added domain names cannot be used while the negative
216 | information is effective.
217 |
218 | Section 5 of [RFC2308] suggests a maximum default negative cache
219 | TTL value of 3 hours (10800). It is RECOMMENDED that validating
220 | resolvers limit the maximum effective TTL value of negative
221 | responses (NSEC/NSEC3 RRs) to this same value.
222 |
223 | A resolver that supports aggressive use of NSEC and NSEC3 MAY
224 | limit the TTL of NSEC and NSEC3 records to the lesser of the
225 | SOA.MINIMUM field and the TTL of the SOA in a response, if
226 | present. It MAY also use a previously cached SOA for a zone to
227 | find these values.
228
229 (The third paragraph of the original is removed, and the fourth
230 paragraph is updated to allow resolvers to also take the lesser of
231 the SOA TTL and SOA MINIMUM.)
232
233 4. Zone Operator Considerations
234
235 If signers and DNS servers for a zone cannot immediately be updated
236 to conform to this document, zone operators are encouraged to
237 consider setting their SOA record TTL and the SOA MINIMUM field to
238 the same value. That way, the TTL used for aggressive NSEC and NSEC3
239 use matches the SOA TTL for negative responses.
240
241 Note that some signers might use the SOA TTL or MINIMUM as a default
242 for other values, such as the TTL for DNSKEY records. Operators
243 should consult documentation before changing values.
244
245 4.1. A Note on Wildcards
246
247 Validating resolvers consider an expanded wildcard valid for the
248 wildcard's TTL, capped by the TTLs of the NSEC or NSEC3 proof that
249 shows that the wildcard expansion is legal. Thus, changing the TTL
250 of NSEC or NSEC3 records (explicitly, or by implementation of this
251 document implicitly) might affect (shorten) the lifetime of
252 wildcards.
253
254 5. Security Considerations
255
256 An attacker can delay future records from appearing in a cache by
257 seeding the cache with queries that cause NSEC or NSEC3 responses to
258 be cached for aggressive use purposes. This document reduces the
259 impact of that attack in cases where the NSEC or NSEC3 TTL is higher
260 than the zone operator intended.
261
262 6. IANA Considerations
263
264 IANA has added a reference to this document in the "Resource Record
265 (RR) TYPEs" subregistry of the "Domain Name System (DNS) Parameters"
266 registry for the NSEC and NSEC3 types.
267
268 7. Normative References
269
270 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
271 Requirement Levels", BCP 14, RFC 2119,
272 DOI 10.17487/RFC2119, March 1997,
273 <https://www.rfc-editor.org/info/rfc2119>.
274
275 [RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
276 NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,
277 <https://www.rfc-editor.org/info/rfc2308>.
278
279 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S.
280 Rose, "Resource Records for the DNS Security Extensions",
281 RFC 4034, DOI 10.17487/RFC4034, March 2005,
282 <https://www.rfc-editor.org/info/rfc4034>.
283
284 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S.
285 Rose, "Protocol Modifications for the DNS Security
286 Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005,
287 <https://www.rfc-editor.org/info/rfc4035>.
288
289 [RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
290 Security (DNSSEC) Hashed Authenticated Denial of
291 Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008,
292 <https://www.rfc-editor.org/info/rfc5155>.
293
294 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
295 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
296 May 2017, <https://www.rfc-editor.org/info/rfc8174>.
297
298 [RFC8198] Fujiwara, K., Kato, A., and W. Kumari, "Aggressive Use of
299 DNSSEC-Validated Cache", RFC 8198, DOI 10.17487/RFC8198,
300 July 2017, <https://www.rfc-editor.org/info/rfc8198>.
301
302 Acknowledgements
303
304 This document was made possible with the help of the following
305 people:
306
307 * Ralph Dolmans
308
309 * Warren Kumari
310
311 * Matthijs Mekking
312
313 * Vladimir Cunat
314
315 * Matt Nordhoff
316
317 * Josh Soref
318
319 * Tim Wicinski
320
321 The author would like to explicitly thank Paul Hoffman for the
322 extensive reviews, text contributions, and help in navigating WG
323 comments.
324
325 Author's Address
326
327 Peter van Dijk
328 PowerDNS
329 Den Haag
330 Netherlands
331
332 Email: peter.van.dijk@powerdns.com
333
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