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Wessels 3 Internet-Draft Verisign Labs 4 Intended status: Standards Track July 29, 2015 5 Expires: January 30, 2016 7 The EDNS Key Tag Option 8 draft-wessels-edns-key-tag-00 10 Abstract 12 The DNS Security Extensions (DNSSEC) were developed to provide origin 13 authentication and integrity protection for DNS data by using digital 14 signatures. These digital signatures can be verified by building a 15 chain-of-trust starting from a trust anchor and proceeding down to a 16 particular node in the DNS. This document specifies a way for 17 validating end-system resolvers to signal to a server which keys are 18 referenced in their chain-of-trust. The extensions allow zone 19 administrators to monitor the progress of rollovers in a DNSSEC- 20 signed zone. 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 http://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 January 30, 2016. 39 Copyright Notice 41 Copyright (c) 2015 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 (http://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 . . . . . . . . . . . . . . . . . . . . . . . . . 3 57 2. Requirements Terminology . . . . . . . . . . . . . . . . . . . 3 58 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3 59 4. Option Format . . . . . . . . . . . . . . . . . . . . . . . . . 4 60 5. Use By Queriers . . . . . . . . . . . . . . . . . . . . . . . . 4 61 5.1. Stub Resolvers . . . . . . . . . . . . . . . . . . . . . . 5 62 5.1.1. Validating Stub Resolvers . . . . . . . . . . . . . . . 5 63 5.1.2. Non-validating Stub Resolvers . . . . . . . . . . . . . 6 64 5.2. Recursive Resolvers . . . . . . . . . . . . . . . . . . . . 6 65 5.2.1. Validating Recursive Resolvers . . . . . . . . . . . . 6 66 5.2.2. Non-validating Recursive Resolvers . . . . . . . . . . 6 67 6. Use By Responders . . . . . . . . . . . . . . . . . . . . . . . 6 68 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7 69 8. Security Considerations . . . . . . . . . . . . . . . . . . . . 7 70 9. Privacy Considerations . . . . . . . . . . . . . . . . . . . . 7 71 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 8 72 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 73 11.1. Normative References . . . . . . . . . . . . . . . . . . . 8 74 11.2. Informative References . . . . . . . . . . . . . . . . . . 9 75 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 9 77 1. Introduction 79 The DNS Security Extensions (DNSSEC) [RFC4033], [RFC4034] and 80 [RFC4035] were developed to provide origin authentication and 81 integrity protection for DNS data by using digital signatures. 82 DNSSEC uses Key Tags to efficiently match signatures to the keys from 83 which they are generated. The Key Tag is a 16-bit value computed 84 from the RDATA portion of a DNSKEY RR using a formula not unlike a 85 ones-complement checksum. RRSIG RRs contain a Key Tag field whose 86 value is equal to the Key Tag of the DNSKEY RR that validates the 87 signature. 89 Likewise, Delegation Signer (DS) RRs also contain a Key Tag field 90 whose value is equal to the Key Tag of the DNSKEY RR to which it 91 refers. 93 This draft sets out to specify a way for validating end-system 94 resolvers to tell a server in a DNS query which DNSSEC key(s) they 95 would use to validate the expected response. This is done using the 96 new EDNS option specified below in Section 4 for use in the OPT 97 meta-RR [RFC6891]. This new EDNS option code is OPTIONAL to 98 implement and use. 100 This proposed EDNS option serves to measure the acceptance and use of 101 new trust anchors and key signing keys (KSKs). This signaling option 102 can be used by zone administrators as a gauge to measure the 103 successful deployment of new keys. This is of particular interest 104 for the DNS root zone in the event of key and/or algorithm rollovers 105 which relies on [RFC5011] to automatically update a validating end- 106 system's trust anchor. 108 This draft does not seek to introduce another process for rolling 109 keys or updating trust anchors. Rather, this document specifies a 110 means by which a client query can signal the set of keys that a 111 client uses for DNSSEC validation. 113 2. Requirements Terminology 115 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 116 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 117 document are to be interpreted as described in [RFC2119]. 119 3. Terminology 120 Trust Anchor: A configured DNSKEY RR or DS RR hash of a DNSKEY RR. 121 A validating security-aware resolver uses this public key or hash 122 as a starting point for building the authentication chain to a 123 signed DNS response. In general, a validating resolver will have 124 to obtain the initial values of its trust anchors via some secure 125 or trusted means outside the DNS protocol. Presence of a trust 126 anchor also implies that the resolver should expect the zone to 127 which the trust anchor points to be signed. (quoted from [RFC4033] 128 Section 2) 130 Key Tag: A 16-bit integer that identifies and enables efficient 131 selection of DNSSEC public keys. A Key Tag value can be computed 132 over the RDATA of a DNSKEY RR. The Key Tag field in the RRSIG and 133 DS records can be used to help select the corresponding DNSKEY RR 134 efficiently when more than one candidate DNSKEY RR is available. 135 For most algorithms the Key Tag is a simple 16-bit modular sum of 136 the DNSKEY RDATA. See [RFC4034] Appendix B. 138 4. Option Format 140 The edns-key-tag option is encoded as follows: 142 0 8 16 143 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 144 | OPTION-CODE | 145 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 146 | OPTION-LENGTH | 147 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 148 | KEY-TAG | 149 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 150 | ... / 151 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 153 where: 155 OPTION-CODE: The EDNS0 option code assigned to edns-key-tag, [TBD]. 157 OPTION-LENGTH: The value 2 x number of key-tag values present. 159 KEY-TAG: One or more 16-bit Key Tag values ([RFC4034], Appendix B). 161 5. Use By Queriers 163 A validating end-system resolver sets the edns-key-tag option in the 164 OPT meta-RR when sending a DNSKEY query. The validating end-system 165 resolver SHOULD also set the DNSSEC OK bit [RFC4034] to indicate that 166 it wishes to receive DNSSEC RRs in the response. 168 A DNS client MUST NOT include the edns-key-tag option for non-DNSKEY 169 queries. 171 The KEY-TAG value(s) included in the edns-key-tag option represent 172 the Key Tag of the Trust Anchor or DNSKEY RR that will be used to 173 validate the expected response. When the client sends a DNSKEY 174 query, the edns-key-tag option represents the Key Tag(s) of the 175 KSK(s) of the zone for which the server is authoritative. A 176 validating end-system resolver learns the Key Tag(s) of the KSK(s) 177 from the zone's DS record(s) (found in the parent), or from a 178 configured trust anchor. 180 A DNS client SHOULD include the edns-key-tag option when issuing a 181 DNSKEY query for a zone corresponding to a configured Trust Anchor. 183 A DNS client MAY include the edns-key-tag option when issuing a 184 DNSKEY query for a non-Trust Anchor zone (i.e., Key Tags learned via 185 DS records). Since some DNSSEC validators implement bottom-up 186 validation, non-Trust Anchor Key Tags zone might not be known at the 187 time of the query. Such a validator can include the edns-key-tag 188 option based on previously cached data. 190 A DNS client MUST NOT include Key Tag(s) for keys which are not 191 learned via either configured Trust Anchor or DS records. 193 Since the edns-key-tag option is only set in the query, if a client 194 sees these options in the response, no action needs to be taken and 195 the client MUST ignore the option values. 197 5.1. Stub Resolvers 199 Typically, stub resolvers rely on an upstream recursive server (or 200 cache) to provide a response. Optimal setting of the edns-key-tag 201 option depends on whether the stub resolver elects to perform its own 202 validation. 204 5.1.1. Validating Stub Resolvers 206 A validating stub resolver sets the DNSSEC OK (DO) bit [RFC4034] to 207 indicate that it wishes to receive additional DNSSEC RRs (i.e., RRSIG 208 RRs) in the response. Such validating resolvers SHOULD include the 209 edns-key-tag option in the OPT RR when sending a DNSKEY query. 211 5.1.2. Non-validating Stub Resolvers 213 The edns-key-tag option MUST NOT be included by non-validating stub 214 resolvers. 216 5.2. Recursive Resolvers 218 5.2.1. Validating Recursive Resolvers 220 A validating recursive resolver sets the edns-key-tag option when 221 performing recursion based on relevant keys it knows and any edns- 222 key-tag values in the stub client query. When the recursive server 223 receives a query with the option set, the recursive server SHOULD set 224 the edns-key-tag list for any outgoing iterative queries for that 225 resolution chain to a union of the stub client's Key Tag(s) and the 226 validating recursive resolver's Key Tag(s). For example, if the 227 recursive resolver's Key Tag list is (19036, 12345) and the stub's 228 list is (19036, 34567), the final edns-key-tag list would be (19036, 229 12345, 34567). 231 If the client included the DO and Checking Disabled (CD) bits, but 232 did not include the edns-key-tag option in the query, the validating 233 recursive resolver MAY include the option with its own Key Tag values 234 in full. 236 Validating recursive resolvers MUST NOT set the edns-key-tag option 237 in the final response to the stub client. 239 5.2.2. Non-validating Recursive Resolvers 241 Recursive resolvers that do not validate responses SHOULD copy the 242 edns-key-tag option seen in received queries, as they represent the 243 wishes of the validating downstream resolver that issued the original 244 query. 246 6. Use By Responders 248 An authoritative name server receiving queries with the edns-key-tag 249 option MAY log or otherwise collect the Key Tag values to provide 250 information to the zone operator. 252 A responder MUST NOT include the edns-key-tag option in any DNS 253 response. 255 7. IANA Considerations 257 The IANA is directed to assign an EDNS0 option code for the edns-key- 258 tag option from the DNS EDNS0 Option Codes (OPT) registry as follows: 260 +-------+--------------+----------+-----------------+ 261 | Value | Name | Status | Reference | 262 +-------+--------------+----------+-----------------+ 263 | [TBA] | edns-key-tag | Optional | [This document] | 264 +-------+--------------+----------+-----------------+ 266 8. Security Considerations 268 This document specifies a way for a client to signal its trust anchor 269 knowledge to a cache or server. The signals are optional codes 270 contained in the OPT meta-RR used with EDNS. The goal of these 271 options is to signal new trust anchor uptake in client code to allow 272 zone administrators to know when it is possible to complete a key 273 rollover in a DNSSEC-signed zone. 275 There is a possibility that an eavesdropper or server could infer the 276 validator in use by a client by the Key Tag list seen in queries. 277 This may allow an attacker to find validators using old, possibly 278 broken, keys. It could also be used to identify the validator or 279 narrow down the possible validator implementations in use by a 280 client, which could have a known vulnerability that could be 281 exploited by the attacker. 283 Consumers of data collected from the edns-key-tag option are advised 284 that provided Key Tag values might be "made up" by some DNS clients 285 with malicious or at least mischievous intentions. 287 DNSSEC does not require keys in a zone to have unique Key Tags. 288 During a rollover there is a small possibility that an old key and a 289 new key will have identical Key Tag values. Zone operators relying 290 on the edns-key-tag mechanism SHOULD take care to ensure that new 291 keys have unique Key Tag values. 293 9. Privacy Considerations 295 This proposal adds additional "signaling" to DNS queries in the form 296 of Key Tag values. While Key Tag values themselves are not 297 considered private information, it may be possible for an 298 eavesdropper to use Key Tag values as a fingerprinting technique to 299 identify particular DNS validating clients. This may be especially 300 true if the validator is configured with trust anchor for zones in 301 addition to the root zone. 303 A validating end-system resolver need not transmit the edns-key-tag 304 option in every applicable query. Due to privacy concerns, such a 305 resolver MAY choose to transmit the edns-key-tag option for a subset 306 of queries (e.g., every 25th time), or by random chance with a 307 certain probability (e.g., 5%). 309 Implementations of this specification MAY be administratively 310 configured to only transmit the edns-key-tag option for certain 311 zones. For example, the software's configuration file may specify a 312 list of zones for which use of the option is allowed or denied. 313 Since the primary motivation for this specification is to provide 314 operational measurement data for root zone key rollovers, it is 315 RECOMMENDED that implementations at least include the edns-key-tag 316 option for root zone DNSKEY queries. 318 10. Acknowledgments 320 This document was inspired by and borrows heavily from [RFC6975] by 321 Scott Rose and Steve Crocker. The author would like to thank to 322 Casey Deccio and Burt Kalisky for early feedback. 324 11. References 326 11.1. Normative References 328 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 329 STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987, 330 . 332 [RFC1035] Mockapetris, P., "Domain names - implementation and 333 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 334 November 1987, . 336 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 337 Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ 338 RFC2119, March 1997, 339 . 341 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 342 Rose, "DNS Security Introduction and Requirements", 343 RFC 4033, DOI 10.17487/RFC4033, March 2005, 344 . 346 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. 348 Rose, "Resource Records for the DNS Security Extensions", 349 RFC 4034, DOI 10.17487/RFC4034, March 2005, 350 . 352 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 353 Rose, "Protocol Modifications for the DNS Security 354 Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005, 355 . 357 [RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms 358 for DNS (EDNS(0))", STD 75, RFC 6891, DOI 10.17487/ 359 RFC6891, April 2013, 360 . 362 11.2. Informative References 364 [RFC5011] StJohns, M., "Automated Updates of DNS Security (DNSSEC) 365 Trust Anchors", STD 74, RFC 5011, DOI 10.17487/RFC5011, 366 September 2007, . 368 [RFC6975] Crocker, S. and S. Rose, "Signaling Cryptographic 369 Algorithm Understanding in DNS Security Extensions 370 (DNSSEC)", RFC 6975, DOI 10.17487/RFC6975, July 2013, 371 . 373 Author's Address 375 Duane Wessels 376 Verisign Labs 377 12061 Bluemont Way 378 Reston, VA 20190 380 Phone: +1 703 948-3200 381 Email: dwessels@verisign.com 382 URI: http://verisigninc.com