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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Domain Name System Operations (dnsop) Working Group S. Bortzmeyer 3 Internet-Draft AFNIC 4 Intended status: Experimental June 7, 2015 5 Expires: December 9, 2015 7 DNS query name minimisation to improve privacy 8 draft-ietf-dnsop-qname-minimisation-03 10 Abstract 12 This document describes one of the techniques that could be used to 13 improve DNS privacy (see [I-D.ietf-dprive-problem-statement]), a 14 technique called "QNAME minimisation", where the DNS resolver no 15 longer sends the full original QNAME to the upstream name server. 17 REMOVE BEFORE PUBLICATION Discussions of the document should take 18 place on the DNSOP working group mailing list [dnsop]. 20 Status of This Memo 22 This Internet-Draft is submitted in full conformance with the 23 provisions of BCP 78 and BCP 79. 25 Internet-Drafts are working documents of the Internet Engineering 26 Task Force (IETF). Note that other groups may also distribute 27 working documents as Internet-Drafts. The list of current Internet- 28 Drafts is at http://datatracker.ietf.org/drafts/current/. 30 Internet-Drafts are draft documents valid for a maximum of six months 31 and may be updated, replaced, or obsoleted by other documents at any 32 time. It is inappropriate to use Internet-Drafts as reference 33 material or to cite them other than as "work in progress." 35 This Internet-Draft will expire on December 9, 2015. 37 Copyright Notice 39 Copyright (c) 2015 IETF Trust and the persons identified as the 40 document authors. All rights reserved. 42 This document is subject to BCP 78 and the IETF Trust's Legal 43 Provisions Relating to IETF Documents 44 (http://trustee.ietf.org/license-info) in effect on the date of 45 publication of this document. Please review these documents 46 carefully, as they describe your rights and restrictions with respect 47 to this document. Code Components extracted from this document must 48 include Simplified BSD License text as described in Section 4.e of 49 the Trust Legal Provisions and are provided without warranty as 50 described in the Simplified BSD License. 52 Table of Contents 54 1. Introduction and background . . . . . . . . . . . . . . . . . 2 55 2. QNAME minimisation . . . . . . . . . . . . . . . . . . . . . 3 56 3. Possible issues . . . . . . . . . . . . . . . . . . . . . . . 3 57 4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 5 58 5. Operational considerations . . . . . . . . . . . . . . . . . 5 59 6. Performance considerations . . . . . . . . . . . . . . . . . 5 60 7. Security considerations . . . . . . . . . . . . . . . . . . . 6 61 8. Implementation status - REMOVE BEFORE PUBLICATION . . . . . . 6 62 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 7 63 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 7 64 10.1. Normative References . . . . . . . . . . . . . . . . . . 7 65 10.2. Informative References . . . . . . . . . . . . . . . . . 8 66 10.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 9 67 Appendix A. An algorithm to find the zone cut . . . . . . . . . 9 68 Appendix B. Alternatives . . . . . . . . . . . . . . . . . . . . 10 69 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10 71 1. Introduction and background 73 The problem statement is exposed in 74 [I-D.ietf-dprive-problem-statement] TODO: add a reference to the 75 specific section when ietf-dprive-problem-statement will be published 76 as RFC. The terminology ("QNAME", "resolver", etc) is also defined 77 in this companion document. This specific solution is not intended 78 to fully solve the DNS privacy problem; instead, it should be viewed 79 as one tool amongst many. 81 It follows the principle explained in section 6.1 of [RFC6973]: the 82 less data you send out, the fewer privacy problems you'll get. 84 Under current practice, when a resolver receives the query "What is 85 the AAAA record for www.example.com?", it sends to the root (assuming 86 a cold resolver, whose cache is empty) the very same question. 87 Sending the full QNAME to the authoritative name server is a 88 tradition, not a protocol requirement. This tradition 89 comes[mockapetris-history] from a desire to optimize the number of 90 requests, when the same name server is authoritative for many zones 91 in a given name (something which was more common in the old days, 92 where the same name servers served .com and the root) or when the 93 same name server is both recursive and authoritative (something which 94 is strongly discouraged now). Whatever the merits of this choice at 95 this time, the DNS is quite different now. 97 2. QNAME minimisation 99 The idea is to minimise the amount of data sent from the DNS resolver 100 to the authoritative name server. In the example in the previous 101 section, sending "What are the NS records for .com?" would have been 102 sufficient (since it will be the answer from the root anyway). The 103 rest of this section describes the recommended way to do QNAME 104 minimisation, the one which maximimes privacy benefits (other 105 alternatives are discussed in appendixes). 107 A resolver which implements QNAME minimisation, and which does not 108 have already the answer in its cache, instead of sending the full 109 QNAME and the original QTYPE upstream, sends a request to the name 110 server authoritative for the closest known parent of the original 111 QNAME. The request is done with: 113 the QTYPE NS, 115 the QNAME which is the original QNAME, stripped to just one label 116 more than the zone for which the server is authoritative. 118 For example, a resolver receives a request to resolve 119 foo.bar.baz.example. Let's assume it already knows that 120 ns1.nic.example is authoritative for .example and the resolver does 121 not know a more specific authoritative name server. It will send the 122 query QTYPE=NS,QNAME=baz.example to ns1.nic.example. 124 To do such minimisation, the resolver needs to know the zone cut 125 [RFC2181]. Zone cuts do not necessarily exist at every label 126 boundary. If we take the name www.foo.bar.example, it is possible 127 that there is a zone cut between "foo" and "bar" but not between 128 "bar" and "example". So, assuming the resolver already knows the 129 name servers of .example, when it receives the query "What is the 130 AAAA record of www.foo.bar.example", it does not always know whether 131 the request should be sent to the name servers of bar.example or to 132 those of example. [RFC2181] suggests a method to find the zone cut 133 (section 6), so resolvers may try it. 135 Note that DNSSEC-validating resolvers already have access to this 136 information, since they have to find the zone cut (the DNSKEY record 137 set is just below, the DS record set just above). 139 3. Possible issues 141 QNAME minimisation is legal, since the original DNS RFC do not 142 mandate sending the full QNAME. So, in theory, it should work 143 without any problems. However, in practice, some problems may occur 144 (see an analysis in [huque-qnamemin]). 146 Some broken name servers do not react properly to qtype=NS requests. 147 For instance, some authoritative name servers embedded in load 148 balancers reply properly to A queries but send REFUSED to NS queries. 149 REMOVE THIS SENTENCE BEFORE PUBLICATION: As an example of today, look 150 at www.ratp.fr (not ratp.fr). This behaviour is a gross protocol 151 violation, and there is no need to stop improving the DNS because of 152 such brokenness. However, QNAME minimisation may still work with 153 such domains since they are only leaf domains (no need to send them 154 NS requests). Such setup breaks more than just QNAME minimisation. 155 It breaks negative answers, since the servers don't return the 156 correct SOA, and it also breaks anything dependent upon NS and SOA 157 records existing at the top of the zone. 159 A problem can also appear when a name server does not react properly 160 to ENT (Empty Non-Terminals). If ent.example.com has no resource 161 records but foobar.ent.example.com does, then ent.example.com is an 162 ENT. A query, whatever the qtype, for ent.example.com must return 163 NODATA (NOERROR / ANSWER: 0). However, some broken name servers 164 return NXDOMAIN for ENTs. REMOVE THIS SENTENCE BEFORE PUBLICATION: 165 As an example of today, look at com.akadns.net or www.upenn.edu with 166 its delegations to Akamai. If a resolver queries only 167 foobar.ent.example.com, everything will be OK but, if it implements 168 QNAME minimisation, it may query ent.example.com and get a NXDOMAIN. 169 See also section 3 of [I-D.vixie-dnsext-resimprove] for the other bad 170 consequences of this brokenness. 172 Another way to deal with such broken name servers would be to try 173 with QTYPE=A requests (A being chosen because it is the most common 174 and hence a qtype which will be always accepted, while a qtype NS may 175 ruffle the feathers of some middleboxes). Instead of querying name 176 servers with a query "NS example.com", we could use "A _.example.com" 177 and see if we get a referral. 179 Other strange and non-conformant practices may pose a problem: there 180 is a common DNS anti-pattern used by low-end web hosters that also do 181 DNS hosting that exploits the fact that the DNS protocol (pre-DNSSEC) 182 allows certain serious misconfigurations, such as parent and child 183 zones disagreeing on the location of a zone cut. Basically, they 184 have a single zone with wildcards for each TLD like: 186 *.example. 60 IN A 192.0.2.6 188 (It is not known why they don't just wildcard all of "*." and be done 189 with it.) 191 This lets them turn up many web hosting customers without having to 192 configure thousands of individual zones on their nameservers. They 193 just tell the prospective customer to point their NS records at the 194 hoster's nameservers, and the Web hoster doesn't have to provision 195 anything in order to make the customer's domain resolve. NS queries 196 to the hoster will therefore do not give the right result, which may 197 endanger QNAME minimisation (it will be a problem for DNSSEC, too). 199 4. Discussion 201 QNAME minimisation is compatible with the current DNS system and 202 therefore can easily be deployed; since it is a unilateral change to 203 the resolver, it does not change the protocol. (Because of that, 204 resolver implementers may do QNAME minimisation in slightly different 205 ways, see the appendices for examples.). 207 One should note that the behaviour suggested here (minimising the 208 amount of data sent in QNAMEs from the resolver) is NOT forbidden by 209 the [RFC1034] (section 5.3.3) or [RFC1035] (section 7.2). As said in 210 Section 1, the current method, sending the full QNAME, is not 211 mandated by the DNS protocol. 213 It may be noticed that many documents explaining the DNS and intended 214 for a wide audience, incorrectly describe the resolution process as 215 using QNAME minimisation, for instance by showing a request going to 216 the root, with just the TLD in the query. As a result, these 217 documents may confuse the privacy analysis of the users who see them. 219 5. Operational considerations 221 The administrators of the forwarders, and of the authoritative name 222 servers, will get less data, which will reduce the utility of the 223 statistics they can produce (such as the percentage of the various 224 QTYPEs) [kaliski-minimum]. 226 DNS administrators are reminded that the data on DNS requests that 227 they store may have legal consequences, depending on your 228 jurisdiction (check with your local lawyer). 230 6. Performance considerations 232 The main goal of QNAME minimisation is to improve privacy by sending 233 less data. However, it may have other advantages. For instance, if 234 a root name server receives a query from some resolver for A.example 235 followed by B.example followed by C.example, the result will be three 236 NXDOMAINs, since .example does not exist in the root zone. Under 237 query name minimisation, the root name servers would hear only one 238 question (for .example itself) to which they could answer NXDOMAIN, 239 thus opening up a negative caching opportunity in which the full 240 resolver could know a priori that neither B.example or C.example 241 could exist. Thus in this common case the total number of upstream 242 queries under QNAME minimisation would be counter-intuitively less 243 than the number of queries under the traditional iteration (as 244 described in the DNS standard). 246 QNAME minimisation may also improve look-up performance for TLD 247 operators. For a typical TLD, delegation-only, and with delegations 248 just under the TLD, a 2-label QNAME query is optimal for finding the 249 delegation owner name. 251 QNAME minimisation can decrease performance in some cases, for 252 instance for a deep domain name (like 253 www.host.group.department.example.com where 254 host.group.department.example.com is hosted on example.com's name 255 servers). For such a name, a cold resolver will, depending how QNAME 256 minimisation is implemented, send more queries. Once the cache is 257 warm, there will be no difference with a traditional resolver. A 258 possible solution is to always use the traditional algorithm when the 259 cache is cold and then to move to QNAME minimisation. This will 260 decrease the privacy a bit but will guarantee no degradation of 261 performance. Actual testing is described in [huque-qnamemin]. Such 262 deep domains are specially common under ip6.arpa. 264 7. Security considerations 266 QNAME minimisation's benefits are clear in the case where you want to 267 decrease exposure to the authoritative name server. But minimising 268 the amount of data sent also, in part, addresses the case of a wire 269 sniffer as well the case of privacy invasion by the servers. 270 (Encryption is of course a better defense against wire sniffers but, 271 unlike QNAME minimisation, it changes the protocol and cannot be 272 deployed unilaterally. Also, the effect of QNAME minimisation on 273 wire sniffers depend on whether the sniffer is, on the DNS path.) 275 QNAME minimisation offers zero protection against the recursive 276 resolver, which still sees the full request coming from the stub 277 resolver. 279 8. Implementation status - REMOVE BEFORE PUBLICATION 281 This section records the status of known implementations of the 282 protocol defined by this specification at the time of posting of this 283 Internet-Draft, and is based on a proposal described in [RFC6982]. 284 The description of implementations in this section is intended to 285 assist the IETF in its decision processes in progressing drafts to 286 RFCs. Please note that the listing of any individual implementation 287 here does not imply endorsement by the IETF. Furthermore, no effort 288 has been spent to verify the information presented here that was 289 supplied by IETF contributors. This is not intended as, and must not 290 be construed to be, a catalog of available implementations or their 291 features. Readers are advised to note that other implementations may 292 exist. 294 According to [RFC6982], "this will allow reviewers and working groups 295 to assign due consideration to documents that have the benefit of 296 running code, which may serve as evidence of valuable experimentation 297 and feedback that have made the implemented protocols more mature. 298 It is up to the individual working groups to use this information as 299 they see fit". 301 As of today, no production resolver implements QNAME minimisation but 302 it has been publically announced for the future Knot DNS resolver 303 [1]. For Unbound, see ticket 648 [2] and for PowerDNS [3]. 305 The algorithm to find the zone cuts described in Appendix A is 306 implemented with QNAME minimisation in the sample code zonecut.go 307 [4]. It is also implemented, for a much longer time, in an option of 308 dig, "dig +trace", but without QNAME minimisation. 310 Another implementation was done by Shumon Huque for testing, and is 311 described in [huque-qnamemin]. 313 9. Acknowledgments 315 Thanks to Olaf Kolkman for the original idea although the concept is 316 probably much older [5]. Thanks for Shumon Huque for implementation 317 and testing. Thanks to Mark Andrews and Francis Dupont for the 318 interesting discussions. Thanks to Brian Dickson, Warren Kumari, 319 Evan Hunt and David Conrad for remarks and suggestions. Thanks to 320 Mohsen Souissi for proofreading. Thanks to Tony Finch for the zone 321 cut algorithm in Appendix A. Thanks to Paul Vixie for pointing out 322 that there are practical advantages (besides privacy) to QNAME 323 minimisation. Thanks to Phillip Hallam-Baker for the fallback on A 324 queries, to deal with broken servers. Thanks to Robert Edmonds for 325 an interesting anti-pattern. 327 10. References 329 10.1. Normative References 331 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 332 STD 13, RFC 1034, November 1987. 334 [RFC1035] Mockapetris, P., "Domain names - implementation and 335 specification", STD 13, RFC 1035, November 1987. 337 [RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., 338 Morris, J., Hansen, M., and R. Smith, "Privacy 339 Considerations for Internet Protocols", RFC 6973, July 340 2013. 342 [I-D.ietf-dprive-problem-statement] 343 Bortzmeyer, S., "DNS privacy considerations", draft-ietf- 344 dprive-problem-statement-05 (work in progress), May 2015. 346 10.2. Informative References 348 [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS 349 Specification", RFC 2181, July 1997. 351 [RFC6982] Sheffer, Y. and A. Farrel, "Improving Awareness of Running 352 Code: The Implementation Status Section", RFC 6982, July 353 2013. 355 [I-D.wkumari-dnsop-hammer] 356 Kumari, W., Arends, R., Woolf, S., and D. Migault, "Highly 357 Automated Method for Maintaining Expiring Records", draft- 358 wkumari-dnsop-hammer-01 (work in progress), July 2014. 360 [I-D.vixie-dnsext-resimprove] 361 Vixie, P., Joffe, R., and F. Neves, "Improvements to DNS 362 Resolvers for Resiliency, Robustness, and Responsiveness", 363 draft-vixie-dnsext-resimprove-00 (work in progress), June 364 2010. 366 [dnsop] IETF, , "The DNSOP working group of IETF", March 2014, 367 . 369 [mockapetris-history] 370 Mockapetris, P., "Private discussion", January 2015. 372 [kaliski-minimum] 373 Kaliski, B., "Minimum Disclosure: What Information Does a 374 Name Server Need to Do Its Job?", March 2015, 375 . 378 [huque-qnamemin] 379 Huque, S., "Query name minimization and authoritative 380 server behavior", May 2015, . 383 [huque-qnamestorify] 384 Huque, S., "Qname Minimization @ DNS-OARC", May 2015, 385 . 387 10.3. URIs 389 [1] https://ripe70.ripe.net/presentations/121-knot-resolver- 390 ripe70.pdf 392 [2] https://www.nlnetlabs.nl/bugs-script/show_bug.cgi?id=648 394 [3] https://github.com/PowerDNS/pdns/issues/2311 396 [4] https://github.com/bortzmeyer/my-IETF-work/blob/master/draft- 397 ietf-dnsop-QNAME-minimisation/zonecut.go 399 [5] https://lists.dns-oarc.net/pipermail/dns- 400 operations/2010-February/005003.html 402 Appendix A. An algorithm to find the zone cut 404 Although a validating resolver already has the logic to find the zone 405 cut, other resolvers may be interested by this algorithm to follow in 406 order to locate this cut: 408 (0) If the query can be answered from the cache, do so, otherwise 409 iterate as follows: 411 (1) Find closest enclosing NS RRset in your cache. The owner of 412 this NS RRset will be a suffix of the QNAME - the longest suffix 413 of any NS RRset in the cache. Call this PARENT. 415 (2) Initialize CHILD to the same as PARENT. 417 (3) If CHILD is the same as the QNAME, resolve the original query 418 using PARENT's name servers, and finish. 420 (4) Otherwise, add a label from the QNAME to the start of CHILD. 422 (5) If you have a negative cache entry for the NS RRset at CHILD, 423 go back to step 3. 425 (6) Query for CHILD IN NS using PARENT's name servers. The 426 response can be: 428 (6a) A referral. Cache the NS RRset from the authority section 429 and go back to step 1. 431 (6b) An authoritative answer. Cache the NS RRset from the 432 answer section and go back to step 1. 434 (6c) An NXDOMAIN answer. Return an NXDOMAIN answer in response 435 to the original query and stop. 437 (6d) A NOERROR/NODATA answer. Cache this negative answer and 438 go back to step 3. 440 Appendix B. Alternatives 442 Remember that QNAME minimisation is unilateral so a resolver is not 443 forced to implement it exactly as described here. 445 There are several ways to perform QNAME minimisation. The one in 446 Section 2 is the suggested one. It can be called the aggressive 447 algorithm, since the resolver only sends NS queries as long as it 448 does not know the zone cuts. This is the safest, from a privacy 449 point of view. Another possible algorithm, not fully studied at this 450 time, could be to "piggyback" on the traditional resolution code. At 451 startup, it sends traditional full QNAMEs and learns the zone cuts 452 from the referrals received, then switches to NS queries asking only 453 for the minimum domain name. This leaks more data but could require 454 fewer changes in the existing resolver codebase. 456 In the above specification, the original QTYPE is replaced by NS (or 457 may be A, if too many servers react incorrectly to NS requests), 458 which is the best approach to preserve privacy. But this erases 459 information about the relative use of the various QTYPEs, which may 460 be interesting for researchers (for instance if they try to follow 461 IPv6 deployment by counting the percentage of AAAA vs. A queries). A 462 variant of QNAME minimisation would be to keep the original QTYPE. 464 Another useful optimisation may be, in the spirit of the HAMMER idea 465 [I-D.wkumari-dnsop-hammer] to probe in advance for the introduction 466 of zone cuts where none previously existed (i.e. confirm their 467 continued absence, or discover them.) 469 Author's Address 470 Stephane Bortzmeyer 471 AFNIC 472 1, rue Stephenson 473 Montigny-le-Bretonneux 78180 474 France 476 Phone: +33 1 39 30 83 46 477 Email: bortzmeyer+ietf@nic.fr 478 URI: http://www.afnic.fr/