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Kumari 6 Google 7 October 26, 2017 9 A Sentinel for Detecting Trusted Keys in DNSSEC 10 draft-huston-kskroll-sentinel-02.txt 12 Abstract 14 The DNS Security Extensions (DNSSEC) were developed to provide origin 15 authentication and integrity protection for DNS data by using digital 16 signatures. These digital signatures can be verified by building a 17 chain of trust starting from a trust anchor and proceeding down to a 18 particular node in the DNS. This document specifies a mechanism that 19 will allow an end user to determine the trusted key state of the 20 resolvers that handle the user's DNS queries. 22 Status of This Memo 24 This Internet-Draft is submitted in full conformance with the 25 provisions of BCP 78 and BCP 79. 27 Internet-Drafts are working documents of the Internet Engineering 28 Task Force (IETF). Note that other groups may also distribute 29 working documents as Internet-Drafts. The list of current Internet- 30 Drafts is at https://datatracker.ietf.org/drafts/current/. 32 Internet-Drafts are draft documents valid for a maximum of six months 33 and may be updated, replaced, or obsoleted by other documents at any 34 time. It is inappropriate to use Internet-Drafts as reference 35 material or to cite them other than as "work in progress." 37 This Internet-Draft will expire on April 29, 2018. 39 Copyright Notice 41 Copyright (c) 2017 IETF Trust and the persons identified as the 42 document authors. All rights reserved. 44 This document is subject to BCP 78 and the IETF Trust's Legal 45 Provisions Relating to IETF Documents 46 (https://trustee.ietf.org/license-info) in effect on the date of 47 publication of this document. Please review these documents 48 carefully, as they describe your rights and restrictions with respect 49 to this document. Code Components extracted from this document must 50 include Simplified BSD License text as described in Section 4.e of 51 the Trust Legal Provisions and are provided without warranty as 52 described in the Simplified BSD License. 54 Table of Contents 56 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 57 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 58 2. Sentinel Mechanism . . . . . . . . . . . . . . . . . . . . . 3 59 3. Sentinel Processing . . . . . . . . . . . . . . . . . . . . . 4 60 4. Sentinel Test Result Considerations . . . . . . . . . . . . . 5 61 5. Security Considerations . . . . . . . . . . . . . . . . . . . 7 62 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 63 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7 64 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 65 8.1. Normative References . . . . . . . . . . . . . . . . . . 7 66 8.2. Informative References . . . . . . . . . . . . . . . . . 8 67 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 8 69 1. Introduction 71 The DNS Security Extensions (DNSSEC) [RFC4033], [RFC4034] and 72 [RFC4035] were developed to provide origin authentication and 73 integrity protection for DNS data by using digital signatures. 74 DNSSEC uses Key Tags to efficiently match signatures to the keys from 75 which they are generated. The Key Tag is a 16-bit value computed 76 from the RDATA portion of a DNSKEY RR using a formula not unlike a 77 ones-complement checksum. RRSIG RRs contain a Key Tag field whose 78 value is equal to the Key Tag of the DNSKEY RR that validates the 79 signature. 81 This document specifies how validating resolvers can respond to 82 certain queries in a manner that allows a querier to deduce whether a 83 particular key has been loaded into that resolver's trusted key 84 store. In particular, this response mechanism can be used to 85 determine whether a certain Root Zone KSK is ready to be used as a 86 trusted key within the context of a key roll by this resolver. 88 This new mechanism is OPTIONAL to implement and use, although for 89 reasons of supporting broad-based measurement techniques, it is 90 strongly preferred if configurations of DNSSEC-validating resolvers 91 enabled this mechanism by default, allowing for configuration 92 directives to disable this mechanism if desired. 94 1.1. Terminology 96 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 97 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 98 document are to be interpreted as described in RFC 2119. 100 2. Sentinel Mechanism 102 DNSSEC-Validating resolvers that implement this mechanism MUST be 103 performing validation of responses in accordance with the DNSSEC 104 response validation specification [RFC4035]. 106 This mechanism makes use of 2 special labels, "._is-ta-." 107 (Intended to be used in a query where the response can answer the 108 question: Is this the key tag a trust anchor which the validating DNS 109 resolver is currently trusting?) and "._not-ta-." 110 (Intended to be used in a query where the response can answer the 111 question: Is this the key tag of a key that is NOT in the resolver's 112 current trust store?). The use of the positive question and its 113 inverse allows for queries to detect whether resolvers support this 114 mechanism. 116 If the outcome of the DNS response validation process indicates that 117 the response is authentic, and if the original query contains exactly 118 one label that matches the template "._is-ta-.", then the 119 following rule should be applied to the response: If the resolver has 120 placed a Root Zone Key Signing Key with tag index value matching the 121 value specified in the query into the local resolver's store of 122 trusted keys, then the resolver should return a response indicating 123 that the response contains authenticated data according to section 124 5.8 of [RFC6840]. Otherwise, the resolver MUST return RCODE 2 125 (server failure). Note that the is specified in the DNS 126 label using hex notation. 128 If the outcome of the DNS response validation process indicates that 129 the response is authentic, and if the original query contains exactly 130 one label that matches the template "._not-ta-.", then the 131 following rule should be applied to the response: If the resolver has 132 not placed a Root Zone Key Signing Key with tag index value matching 133 the value specified in the query into the local resolver's store of 134 trusted keys, then the resolver should return a response indicating 135 that the response contains authenticated data according to section 136 5.8 of [RFC6840]. Otherwise, the resolver MUST return RCODE 2 137 (server failure). Note that the is specified in the DNS 138 label using hex notation. 140 If a query contains one instance of both of these query templates 141 then the resolver MUST NOT alter the outcome of the DNS response 142 validation process. 144 This mechanism is to be applied only by resolvers that perform DNSSEC 145 validation, and applies only to responses to an A or AAAA query 146 (Query Type value 1 or 28) where the resolver has authenticated the 147 response according to the DNSSEC validation process and where the 148 query name contains either of the labels described in this section. 149 In this case, the resolver is to perform an additional test following 150 the conventional validation function as described in this section. 151 The result of this test directs whether the resolver is to change an 152 authentic response to a response that indicates validation failure. 154 3. Sentinel Processing 156 This proposed test that uses the DNS resolver mechanism described in 157 this document is based on three DNS names that have three distinct 158 DNS resolution behaviours. The test is intended to allow a user to 159 determine the state of their DNS resolution system, and, in 160 particular, whether or not they are using validating DNS resolvers 161 that have picked up an incoming trust anchor in a key roll. 163 The name format can be defined in a number of ways, and no name form 164 is intrinsically better than any other in terms of the test itself. 165 The critical aspect of the DNS names used in any such test is that 166 they contain the specified label for either the positive and negative 167 test. 169 The sentinel process is envisaged to use a test with three names: 171 a. a name containing the label "._is-ta-.". This is a 172 validly signed name so that responses about names in this zone 173 can be authenticated by a validating resolver. 175 b. a name containing the label "._not-ta-.". This is 176 also a validly-signed name. 178 c. a third name that is signed with a DNSSEC signature that cannot 179 be validated. 181 The responses received from queries to resolve each of these names 182 would allow us to infer a trust key state of the resolution 183 environment. 185 o Vnew: A DNSSEC-Validating resolver that includes this mechanism 186 that has loaded the nominated key into its trusted key stash will 187 respond with an A record response for "is-ta", SERVFAIL for "not- 188 ta" and SERVFAIL for the invalid name. 190 o Vold: A DNSSEC-Validating resolver that includes this mechanism 191 that has not loaded the nominated key into its trusted key stash 192 will respond with an SERVFAIL record for "is-ta", an A record 193 response for "not-ta" and SERVFAIL for the invalid name. 195 o Vleg: A DNSSEC-Validating resolver that does not include this 196 mechanism will respond with an A record response for "is-ta", an A 197 record response for "not-ta" and SERVFAIL for the invalid name. 199 o nonV: A non-DNSSEC-Validating resolver will respond with an A 200 record response for "is-ta", an A record response for "not-ta" and 201 an A record response for the invalid name. 203 Given the clear delineation amongst these three cases, if a client 204 directs these three queries to a simple resolver, the variation in 205 response to the three queries should allow the client to determine 206 the category of the resolver, and if it supports this mechanism, 207 whether or not it has loaded a particular key into its local trusted 208 key stash. 210 +-------------+----------+-----------+------------+ 211 | Type\Query | is_ta | not_ta | invalid | 212 +-------------+----------+-----------+------------+ 213 | Vnew | A | SERVFAIL | SERVFAIL | 214 | Vold | SERVFAIL | A | SERVFAIL | 215 | Vleg | A | A | SERVFAIL | 216 | nonV | A | A | A | 217 +-------------+----------+-----------+------------+ 219 A Vnew response pattern says that the nominated key is trusted by the 220 resolver and has been loaded into its local trusted key stash. A 221 Vleg response pattern says that the nominated key is not yet trusted 222 by the resolver in its own right. A Vleg response is indeterminate, 223 and a nonV response indicates that the client does not have a 224 validating resolver. 226 4. Sentinel Test Result Considerations 228 The description in the previous section describes a simple situation 229 where the test queries were being passed to a single recursive 230 resolver that directly queried authoritative name servers, including 231 the root servers. 233 There is also the common case where the end client is configured to 234 use multiple resolvers. In these cases the SERVFAIL responses from 235 one resolver will prompt the end client to repeat the query against 236 one of the other configured resolvers. 238 If any of the client's resolvers are non-validating resolvers, the 239 tests will result in the client reporting that it has a non- 240 validating DNS environment (nonV), which is effectively the case. 242 If all of the client resolvers are DNSSEC-validating resolvers, but 243 some do not support this trusted key mechanism, then the result will 244 be indeterminate with respect to trusted key status (Vleg). 245 Simlarly, if all the client's resolvers support this mechanism, but 246 some have loaded the key into the trusted key stash and some have 247 not, then the result is indeterminate (Vleg). 249 There is also the common case of a recursive resolver using a 250 forwarder. 252 If the resolver is non-validating, and it has a single forwarder 253 clause, then the resolver will presumably mirror the capabilities of 254 the forwarder target resolver. If this non-validating resolver it 255 has multiple forwarders, then the above considerations will apply. 257 If the validating resolver has a forwarding configuration, and uses 258 the CD flag on all forwarded queries, then this resolver is acting in 259 a manner that is identical to a standalone resolver. The same 260 consideration applies if any one one of the forwarder targets is a 261 non-validating resolver. Similarly, if all the forwarder targets do 262 not apply this trusted key mechanism, the same considerations apply. 264 A more complex case is where the following conditions all hold: 266 both the validating resolver and the forwarder target resolver 267 support this trusted key sentinel mechanism, and 269 the local resolver's queries do not carry the CD bit, and 271 the trusted key state differs between the forwarding resolver and 272 the forwarder target resolver 274 then either the outcome is indeterminate validating (Vleg), or a case 275 of mixed signals (SERVFAIL in all three responses), which is 276 similarly an indeterminate response with respect to the trusted key 277 state. 279 5. Security Considerations 281 This document describes a mechanism to allow users to determine the 282 trust state of root zone key signing keys in the DNS resolution 283 system that they use. 285 The mechanism does not require resolvers to set otherwise 286 unauthenticated responses to be marked as authenticated, and does not 287 alter the security properties of DNSSEC with respect to the 288 interpretation of the authenticity of responses that are so marked. 290 The mechanism does not require any further significant processing of 291 DNS responses, and queries of the form described in this document do 292 not impose any additional load that could be exploited in an attack 293 over the the normal DNSSEC validation processing load. 295 6. IANA Considerations 297 [Note to IANA, to be removed prior to publication: there are no IANA 298 considerations stated in this version of the document.] 300 7. Acknowledgements 302 This document has borrowed extensively from RFC8145 for the 303 introductory text, and the authors would like to acknowledge and 304 thank the authors of that document both for some text excerpts and 305 for the more general stimulation of thoughts about monitoring the 306 progress of a roll of the Key Signing Key of the Root Zone of the 307 DNS. 309 8. References 311 8.1. Normative References 313 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 314 Rose, "DNS Security Introduction and Requirements", 315 RFC 4033, DOI 10.17487/RFC4033, March 2005, 316 . 318 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. 319 Rose, "Resource Records for the DNS Security Extensions", 320 RFC 4034, DOI 10.17487/RFC4034, March 2005, 321 . 323 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 324 Rose, "Protocol Modifications for the DNS Security 325 Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005, 326 . 328 [RFC6840] Weiler, S., Ed. and D. Blacka, Ed., "Clarifications and 329 Implementation Notes for DNS Security (DNSSEC)", RFC 6840, 330 DOI 10.17487/RFC6840, February 2013, 331 . 333 8.2. Informative References 335 [RFC8145] Wessels, D., Kumari, W., and P. Hoffman, "Signaling Trust 336 Anchor Knowledge in DNS Security Extensions (DNSSEC)", 337 RFC 8145, DOI 10.17487/RFC8145, April 2017, 338 . 340 Authors' Addresses 342 Geoff Huston 344 Email: gih@apnic.net 345 URI: http://www.apnic.net 347 Joao Silva Damas 349 Email: joao@apnic.net 350 URI: http://www.apnic.net 352 Warren Kumari 354 Email: warren@kumari.net