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'I-D.thaler-behave-translator-addressing' -- Obsolete informational reference (is this intentional?): RFC 2766 (Obsoleted by RFC 4966) -- Obsolete informational reference (is this intentional?): RFC 3484 (Obsoleted by RFC 6724) == Outdated reference: A later version (-11) exists of draft-iana-rfc3330bis-06 == Outdated reference: A later version (-03) exists of draft-ietf-6man-addr-select-sol-01 == Outdated reference: A later version (-04) exists of draft-wing-behave-learn-prefix-02 == Outdated reference: A later version (-02) exists of draft-venaas-behave-mcast46-00 == Outdated reference: A later version (-15) exists of draft-ietf-dnsop-default-local-zones-08 Summary: 5 errors (**), 0 flaws (~~), 24 warnings (==), 4 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 BEHAVE WG M. Bagnulo 3 Internet-Draft UC3M 4 Intended status: Standards Track A. Sullivan 5 Expires: April 22, 2010 Shinkuro 6 P. Matthews 7 Alcatel-Lucent 8 I. van Beijnum 9 IMDEA Networks 10 October 19, 2009 12 DNS64: DNS extensions for Network Address Translation from IPv6 Clients 13 to IPv4 Servers 14 draft-ietf-behave-dns64-01 16 Status of this Memo 18 This Internet-Draft is submitted to IETF in full conformance with the 19 provisions of BCP 78 and BCP 79. 21 Internet-Drafts are working documents of the Internet Engineering 22 Task Force (IETF), its areas, and its working groups. Note that 23 other groups may also distribute working documents as Internet- 24 Drafts. 26 Internet-Drafts are draft documents valid for a maximum of six months 27 and may be updated, replaced, or obsoleted by other documents at any 28 time. It is inappropriate to use Internet-Drafts as reference 29 material or to cite them other than as "work in progress." 31 The list of current Internet-Drafts can be accessed at 32 http://www.ietf.org/ietf/1id-abstracts.txt. 34 The list of Internet-Draft Shadow Directories can be accessed at 35 http://www.ietf.org/shadow.html. 37 This Internet-Draft will expire on April 22, 2010. 39 Copyright Notice 41 Copyright (c) 2009 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 in effect on the date of 46 publication of this document (http://trustee.ietf.org/license-info). 47 Please review these documents carefully, as they describe your rights 48 and restrictions with respect to this document. 50 Abstract 52 DNS64 is a mechanism for synthesizing AAAA records from A records. 53 DNS64 is used with an IPv6/IPv4 translator to enable client-server 54 communication between an IPv6-only client and an IPv4-only server, 55 without requiring any changes to either the IPv6 or the IPv4 node, 56 for the class of applications that work through NATs. This document 57 specifies DNS64, and provides suggestions on how it should be 58 deployed in conjunction with IPv6/IPv4 translators. 60 Table of Contents 62 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 63 2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 64 3. Background to DNS64 - DNSSEC interaction . . . . . . . . . . . 6 65 4. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 8 66 5. DNS64 Normative Specification . . . . . . . . . . . . . . . . 9 67 5.1. Resolving AAAA queries and the answer section . . . . . . 9 68 5.1.1. The answer when there is AAAA data available . . . . . 9 69 5.1.2. The answer when there is an error . . . . . . . . . . 9 70 5.1.3. Data for the answer when performing synthesis . . . . 9 71 5.1.4. Performing the synthesis . . . . . . . . . . . . . . . 10 72 5.1.5. Querying in parallel . . . . . . . . . . . . . . . . . 11 73 5.2. Generation of the IPv6 representations of IPv4 74 addresses . . . . . . . . . . . . . . . . . . . . . . . . 11 75 5.3. Handling other RRs . . . . . . . . . . . . . . . . . . . . 12 76 5.3.1. PTR queries . . . . . . . . . . . . . . . . . . . . . 12 77 5.3.2. Handling the additional section . . . . . . . . . . . 13 78 5.3.3. Other records . . . . . . . . . . . . . . . . . . . . 13 79 5.4. Assembling a synthesized response to a AAAA query . . . . 14 80 5.5. DNSSEC processing: DNS64 in recursive server mode . . . . 14 81 5.6. DNS64 and multihoming . . . . . . . . . . . . . . . . . . 15 82 6. Deployment notes . . . . . . . . . . . . . . . . . . . . . . . 16 83 6.1. DNS resolvers and DNS64 . . . . . . . . . . . . . . . . . 16 84 6.2. DNSSEC validators and DNS64 . . . . . . . . . . . . . . . 16 85 7. Security Considerations . . . . . . . . . . . . . . . . . . . 16 86 8. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 16 87 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17 88 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17 89 10.1. Normative References . . . . . . . . . . . . . . . . . . . 17 90 10.2. Informative References . . . . . . . . . . . . . . . . . . 18 91 Appendix A. Deployment scenarios and examples . . . . . . . . . . 20 92 A.1. Embed and Zero-Pad algorithm description . . . . . . . . . 21 93 A.2. An-IPv6-network-to-IPv4-Internet setup with DNS64 in 94 DNS server mode . . . . . . . . . . . . . . . . . . . . . 22 95 A.3. An-IPv6-network-to-IPv4-Internet setup with DNS64 in 96 stub-resolver mode . . . . . . . . . . . . . . . . . . . . 23 98 A.4. IPv6-Internet-to-an-IPv4-network setup DNS64 in DNS 99 server mode . . . . . . . . . . . . . . . . . . . . . . . 25 100 Appendix B. Motivations and Implications of synthesizing AAAA 101 RR when real AAAA RR exists . . . . . . . . . . . . . 27 102 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 28 104 1. Introduction 106 This document specifies DNS64, a mechanism that is part of the 107 toolbox for IPv6-IPv4 transition and co-existence. DNS64, used 108 together with an IPv6/IPv4 translator such as NAT64 109 [I-D.bagnulo-behave-nat64], allows an IPv6-only client to initiate 110 communications by name to an IPv4-only server. 112 DNS64 is a mechanism for synthesizing AAAA resource records (RRs) 113 from A RRs. A synthetic AAAA RR created by the DNS64 from an 114 original A RR contains the same FQDN of the original A RR but it 115 contains an IPv6 address instead of an IPv4 address. The IPv6 116 address is an IPv6 representation of the IPv4 address contained in 117 the original A RR. The IPv6 representation of the IPv4 address is 118 algorithmically generated from the IPv4 address returned in the A RR 119 and a set of parameters configured in the DNS64 (typically, an IPv6 120 prefix used by IPv6 representations of IPv4 addresses and optionally 121 other parameters). 123 Together with a IPv6/IPv4 translator, these two mechanisms allow an 124 IPv6-only client to initiate communications to an IPv4-only server 125 using the FQDN of the server. 127 These mechanisms are expected to play a critical role in the IPv4- 128 IPv6 transition and co-existence. Due to IPv4 address depletion, it 129 is likely that in the future, many IPv6-only clients will want to 130 connect to IPv4-only servers. In the typical case, the approach only 131 requires the deployment of IPv6/IPv4 translators that connect an 132 IPv6-only network to an IPv4-only network, along with the deployment 133 of one or more DNS64-enabled name servers. However, some advanced 134 features require performing the DNS64 function directly by the end- 135 hosts themselves. 137 2. Overview 139 This section provides a non-normative introduction to the DNS64 140 mechanism. 142 We assume that we have an IPv6/IPv4 translator box connecting an IPv4 143 network and an IPv6 network. The IPv6/IPv4 translator device 144 provides translation services between the two networks enabling 145 communication between IPv4-only hosts and IPv6-only hosts. (NOTE: By 146 IPv6-only hosts we mean hosts running IPv6-only applications, hosts 147 that can only use IPv6, as well as the cases where only IPv6 148 connectivity is available to the client. By IPv4-only servers we 149 mean servers running IPv4-only applications, servers that can only 150 use IPv4, as well as the cases where only IPv4 connectivity is 151 available to the server). The IPv6/IPv4 translator used in 152 conjunction with DNS64 must allow communications initiated from the 153 IPv6-only host to the IPv4-only host. 155 To allow an IPv6 initiator to do a standard AAAA RR DNS lookup to 156 learn the address of the responder, DNS64 is used to synthesize a 157 AAAA record from an A record containing a real IPv4 address of the 158 responder, whenever the DNS64 service cannot retrieve a AAAA record 159 for the requested host name. The DNS64 device appears as a regular 160 recursive resolver for the IPv6 initiator. The DNS64 box receives an 161 AAAA DNS query generated by the IPv6 initiator. It first attempts a 162 recursive resolution for the requested AAAA records. If there is no 163 AAAA record available for the target node (which is the normal case 164 when the target node is an IPv4-only node), DNS64 performs a query 165 for A records. If any A records are discovered, DNS64 creates a 166 synthetic AAAA RR from the information retrieved in each A RR. 168 The FQDN of a synthetic AAAA RR is the same as that of the original A 169 RR, but an IPv6 representation of the IPv4 address contained in the 170 original A RR is included in the AAAA RR. The IPv6 representation of 171 the IPv4 address is algorithmically generated from the IPv4 address 172 and additional parameters configured in the DNS64. Among those 173 parameters configured in the DNS64, there is at least one IPv6 174 prefix, called Pref64::/n. The IPv6 address representing IPv4 175 addresses included in the AAAA RR synthesized by the DNS64 function 176 contain Pref64::/n and they also embed the original IPv4 address. 178 The same algorithm and the same Pref64::/n prefix or prefixes must be 179 configured both in the DNS64 device and the IPv6/IPv4 translator, so 180 that both can algorithmically generate the same IPv6 representation 181 for a given IPv4 address. In addition, it is required that IPv6 182 packets addressed to an IPv6 destination that contains the Pref64::/n 183 be delivered to the IPv6/IPv4 translator, so they can be translated 184 into IPv4 packets. 186 Once the DNS64 has synthesized the AAAA RR, the synthetic AAAA RR is 187 passed back to the IPv6 initiator, which will initiate an IPv6 188 communication with the IPv6 address associated with the IPv4 189 receiver. The packet will be routed to the IPv6/IPv4 translator 190 which will forward it to the IPv4 network . 192 In general, the only shared state between the DNS64 and the IPv6/IPv4 193 translator is the Pref64::/n and an optional set of static 194 parameters. The Pref64::/n and the set of static parameters must be 195 configured to be the same on both; there is no communication between 196 the DNS64 device and IPv6/IPv4 translator functions. The mechanism 197 to be used for configuring the parameters of the DNS64 is beyond the 198 scope of this memo. 200 The DNS64 function can be performed in two places. 202 One option is to locate the DNS64 function in recursive name 203 servers serving end hosts. In this case, when an IPv6-only host 204 queries the name server for AAAA RRs for an IPv4-only host, the 205 name server can perform the synthesis of AAAA RRs and pass them 206 back to the IPv6 only initiator. The main advantage of this mode 207 is that current IPv6 nodes can use this mechanism without 208 requiring any modification. This mode is called "DNS64 in DNS 209 server mode". 211 The other option is to place the DNS64 function in the end hosts 212 themselves, coupled to the local stub resolver. In this case, the 213 stub resolver will try to obtain (real) AAAA RRs and in case they 214 are not available, the DNS64 function will synthesize AAAA RRs for 215 internal usage. This mode is compatible with some advanced 216 functions like DNSSEC validation in the end host. The main 217 drawback of this mode is its deployability, since it requires 218 changes in the end hosts. This mode is called "DNS64 in stub- 219 resolver mode"". 221 3. Background to DNS64 - DNSSEC interaction 223 DNSSEC presents a special challenge for DNS64, because DNSSEC is 224 designed to detect changes to DNS answers, and DNS64 may alter 225 answers coming from an authoritative server. 227 A recursive resolver can be security-aware or security-oblivious. 228 Moreover, a security-aware recursive name server can be validating or 229 non-validating, according to operator policy. In the cases below, 230 the recursive server is also performing DNS64, and has a local policy 231 to validate. We call this general case vDNS64, but in all the cases 232 below the DNS64 functionality should be assumed needed. 234 DNSSEC includes some signaling bits that offer some indicators of 235 what the query originator understands. 237 If a query arrives at a vDNS64 device with the DO bit set, the query 238 originator is signaling that it understands DNSSEC. The DO bit does 239 not indicate that the query originator will validate the response. 240 It only means that the query originator can understand responses 241 containing DNSSEC data. Conversely, if the DO bit is clear, that is 242 evidence that the querying agent is not aware of DNSSEC. 244 If a query arrives at a vDNS64 device with the CD bit set, it is an 245 indication that the querying agent wants all the validation data so 246 it can do checking itself. By local policy, vDNS64 could still 247 validate, but it must return all data to the querying agent anyway. 249 Here are the possible cases: 251 1. A security-oblivious DNS64 node receives a query with the DO bit 252 clear. In this case, DNSSEC is not a concern, because the 253 querying agent does not understand DNSSEC responses. 255 2. A security-oblivious DNS64 node receives a query with the DO bit 256 set, and the CD bit clear. This is just like the case of a non- 257 DNS64 case: the server doesn't support it, so the querying agent 258 is out of luck. 260 3. A security-aware and non-validating DNS64 node receives a query 261 with the DO bit set and the CD bit clear. Such a resolver is not 262 validating responses, likely due to local policy (see [RFC4035], 263 section 4.2). For that reason, this case amounts to the same as 264 the previous case, and no validation happens. 266 4. A security-aware and non-validating DNS64 node receives a query 267 with the DO bit set and the CD bit set. In this case, the 268 resolver is supposed to pass on all the data it gets to the query 269 initiator (see section 3.2.2 of [RFC4035]). This case will be 270 problematic with DNS64. If the DNS64 server modifies the record, 271 the client will get the data back and try to validate it, and the 272 data will be invalid as far as the client is concerned. 274 5. A security-aware and validating DNS64 node receives a query with 275 the DO bit clear and CD clear. In this case, the resolver 276 validates the data. If it fails, it returns RCODE 2 (SERVFAIL); 277 otherwise, it returns the answer. This is the ideal case for 278 vDNS64. The resolver validates the data, and then synthesizes 279 the new record and passes that to the client. The client, which 280 is presumably not validating (else it would have set DO and CD), 281 cannot tell that DNS64 is involved. 283 6. A security-aware and validating DNS64 node receives a query with 284 the DO bit set and CD clear. In principle, this ought to work 285 like the previous case, except that the resolver should also set 286 the AD bit on the response. 288 7. A security-aware and validating DNS64 node receives a query with 289 the DO bit set and CD set. This is effectively the same as the 290 case where a security-aware and non-validating recursive resolver 291 receives a similar query, and the same thing will happen: the 292 downstream validator will mark the data as invalid if DNS64 has 293 performed synthesis. 295 4. Terminology 297 This section provides definitions for the special terms used in the 298 document. 300 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 301 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 302 document are to be interpreted as described in RFC 2119 [RFC2119]. 304 Authoritative server: A DNS server that can answer authoritatively a 305 given DNS question. 307 DNS64: A logical function that synthesizes DNS resource records (e.g 308 AAAA records containing IPv6 addresses) from DNS resource records 309 actually contained in the global DNS (e.g. A records containing 310 IPv4 addresses). 312 DNS64 recursor: A recursive resolver that provides the DNS64 313 functionality as part of its operation. 315 Recursive resolver: A DNS server that accepts requests from one 316 resolver, and asks another resolver for the answer on behalf of 317 the first resolver. In the context of this document, "the 318 recursive resolver" means a recursive resolver immediately next in 319 the DNS resolution chain from an end point. The end point usually 320 has only a stub resolver available.[[anchor5: I can't actually 321 remember why we needed the sentences following "In the context of 322 this document. . ." Unless someone has a reason, I'll take it 323 out. --ajs@shinkuro.com]] 325 Synthetic RR: A DNS resource record (RR) that is not contained in 326 any zone data file, but has been synthesized from other RRs. An 327 example is a synthetic AAAA record created from an A record. 329 Stub resolver: A resolver with minimum functionality, typically for 330 use in end points that depend on a recursive resolver. Most end 331 points on the Internet as of this writing use stub 332 resolvers.[[anchor6: Do we need this in the document? I don't 333 think so. 1034 defines this term. --ajs@shinkuro.com]] 335 IPv6/IPv4 translator: A device that translates IPv6 packets to IPv4 336 packets and vice-versa. It is only required that the 337 communication initiated from the IPv6 side be supported. 339 For a detailed understanding of this document, the reader should also 340 be familiar with DNS terminology from [RFC1034],[RFC1035] and current 341 NAT terminology from [RFC4787]. Some parts of this document assume 342 familiarity with the terminology of the DNS security extensions 343 outlined in [RFC4035]. 345 5. DNS64 Normative Specification 347 A DNS64 is a logical function that synthesizes AAAA records from A 348 records. The DNS64 function may be implemented in a stub resolver, 349 in a recursive resolver, or in an authoritative name server. 351 The implementation SHOULD support mapping of IPv4 address ranges to 352 separate IPv6 prefixes for AAAA record synthesis. This allows 353 handling of special use IPv4 addresses [I-D.iana-rfc3330bis]. 354 Multicast address handling is further specified in 355 [I-D.venaas-behave-mcast46]. 357 5.1. Resolving AAAA queries and the answer section 359 When the DNS64 receives a query for RRs of type AAAA and class IN, it 360 first attempts to retrieve non-synthetic RRs of this type and class, 361 either by performing a query or, in the case of an authoritative 362 server, by examining its own results. 364 5.1.1. The answer when there is AAAA data available 366 If the query results in one or more AAAA records in the answer 367 section, the result is returned to the requesting client as per 368 normal DNS semantics (except in the case where the AAAA falls in the 369 ::ffff/96 network; see below for treatment of that network). In this 370 case, DNS64 SHOULD NOT include synthetic AAAA RRs in the response 371 (see Appendix B for an analysis of the motivations for and the 372 implications of not complying with this recommendation). By default 373 DNS64 implementations MUST NOT synthesize AAAA RRs when real AAAA RRs 374 exist. 376 5.1.2. The answer when there is an error 378 If the query results in a response with an error code other than 0, 379 the result is handled according to normal DNS operation -- that is, 380 either the resolver tries again using a different server from the 381 authoritative NS RRSet, or it returns the error to the client. This 382 stage is still prior to any synthesis having happened, so a response 383 to be returned to the client does not need any special assembly than 384 would usually happen in DNS operation. 386 5.1.3. Data for the answer when performing synthesis 388 If the query results in no error but an empty answer section in the 389 response, the DNS64 resolver attempts to retrieve A records for the 390 name in question. If this new A RR query results in an empty answer 391 or in an error, then the empty result or error is used as the basis 392 for the answer returned to the querying client. (Transient errors 393 may result in retrying the query, depening on the operation of the 394 resolver; this is just as in Section 5.1.2.) If instead the query 395 results in one or more A RRs, the DNS64 synthesizes AAAA RRs based on 396 the A RRs according to the procedure outlined in Section 5.1.4. The 397 DNS64 resolver then returns the synthesized AAAA records in the 398 answer section to the client, removing the A records that form the 399 basis of the synthesis. 401 As an exception to the general rule about always returning the AAAA 402 records if they are returned in the answer, AAAA records with 403 addresses in the ::ffff/96 network are treated just like the case 404 where there is neither an error nor an empty answer section. This is 405 because a real IPv6-only node will not be any more able to reach the 406 addresses in ::ffff/96 than it is able to reach an IPv4 address 407 without assistance. An implementation MAY use the address in 408 ::ffff/96 as the basis of synthesis without querying for an A record, 409 by using the last 32 bits of the address provided in the AAAA record. 410 [[anchor10: I changed this to say "neither. . .nor" because the 411 previous version suggested that it would return the error-or-empty- 412 answer to the querying client, and that can't be right. Correct? 413 --ajs@shinkuro.com]] 415 5.1.4. Performing the synthesis 417 A synthetic AAAA record is created from an A record as follows: 419 o The NAME field is set to the NAME field from the A record 421 o The TYPE field is set to 28 (AAAA) 423 o The CLASS field is set to 1 (IN) 425 o The TTL field is set to the minimum of the TTL of the original A 426 RR and the SOA RR for the queried domain. (Note that in order to 427 obtain the TTL of the SOA RR the DNS64 does not need to perform a 428 new query, but it can remember the TTL from the SOA RR in the 429 negative response to the AAAA query). 431 o The RDLENGTH field is set to 16 433 o The RDATA field is set to the IPv6 representation of the IPv4 434 address from the RDATA field of the A record. The DNS64 SHOULD 435 check each A RR against IPv4 address ranges and select the 436 corresponding IPv6 prefix to use in synthesizing the AAAA RR. See 437 Section 5.2 for discussion of the algorithms to be used in 438 effecting the transformation. 440 5.1.5. Querying in parallel 442 DNS64 MAY perform the query for the AAAA RR and for the A RR in 443 parallel, in order to minimize the delay. However, this would result 444 in performing unnecessary A RR queries in the case no AAAA RR 445 synthesis is required. A possible trade-off would be to perform them 446 sequentially but with a very short interval between them, so if we 447 obtain a fast reply, we avoid doing the additional query. (Note that 448 this discussion is relevant only if the DNS64 function needs to 449 perform external queries to fetch the RR. If the needed RR 450 information is available locally, as in the case of an authoritative 451 server, the issue is no longer relevant.) 453 5.2. Generation of the IPv6 representations of IPv4 addresses 455 DNS64 supports multiple algorithms for the generation of the IPv6 456 representation of an IPv4 address. The constraints imposed on the 457 generation algorithms are the following: 459 The same algorithm to create an IPv6 address from an IPv4 address 460 MUST be used by both the DNS64 to create the IPv6 address to be 461 returned in the synthetic AAAA RR from the IPv4 address contained 462 in original A RR, and by the IPv6/IPv4 translator to create the 463 IPv6 address to be included in the destination address field of 464 the outgoing IPv6 packets from the IPv4 address included in the 465 destination address field of the incoming IPv4 packet. 467 The algorithm MUST be reversible, i.e. it MUST be possible to 468 extract the original IPv4 address from the IPv6 representation. 470 The input for the algorithm MUST be limited to the IPv4 address, 471 the IPv6 prefix (denoted Pref64::/n) used in the IPv6 472 representations and optionally a set of stable parameters that are 473 configured in the DNS64 (such as fixed string to be used as a 474 suffix). 476 If we note n the length of the prefix Pref64::/n, then n MUST 477 the less or equal than 96. If a Pref64::/n is configured 478 through any means in the DNS64 (such as manually configured, or 479 other automatic mean not specified in this document), the 480 default algorithm MUST use this prefix. If no prefix is 481 available, the algorithm MUST use the Well-Known prefix TBD1 482 defined in [I-D.thaler-behave-translator-addressing] 484 [[anchor12: Note in document: TBD1 in the passage above is to be 485 substituted by whatever prefix is assigned by IANA to be the well- 486 known prefix.]] 488 DNS64 MUST support the following algorithms for generating IPv6 489 representations of IPv4 addresses defined in 490 [I-D.thaler-behave-translator-addressing]: 492 Zero-Pad And Embed, defined in section 3.2.3 of 493 [I-D.thaler-behave-translator-addressing] 495 Compensation-Pad And Embed, defined in section of 3.2.4 of 496 [I-D.thaler-behave-translator-addressing] 498 Embed And Zero-Pad, defined in section of 3.2.5 of 499 [I-D.thaler-behave-translator-addressing] 501 Preconfigured Mapping Table, defined in section of 3.2.6 of 502 [I-D.thaler-behave-translator-addressing] 504 The default algorithm used by DNS64 must be Embed and Zero-Pad. 505 While the normative description of the algorithms is provided in 506 [I-D.thaler-behave-translator-addressing], an sample description of 507 the algorithm and its application to different scenarios is provided 508 in Appendix A for illustration purposes. 510 5.3. Handling other RRs 512 5.3.1. PTR queries 514 If a DNS64 nameserver receives a PTR query for a record in the 515 IP6.ARPA domain, it MUST strip the IP6.ARPA labels from the QNAME, 516 reverse the address portion of the QNAME according to the encoding 517 scheme outlined in section 2.5 of [RFC3596] , and examine the 518 resulting address to see whether its prefix matches the locally- 519 configured Pref64::/n. There are two alternatives for a DNS64 520 nameserver to respond to such PTR queries. A DNS64 node MUST provide 521 one of these, and SHOULD NOT provide both at the same time unless 522 different IP6.ARPA zones require answers of different sorts. 524 The first option is for the DNS64 nameserver to respond 525 authoritatively for its prefixes. If the address prefix matches any 526 Pref64::/n used in the site, either a LIR prefix or a well-known 527 prefix used for NAT64 as defined in 528 [I-D.thaler-behave-translator-addressing], then the DNS64 server MAY 529 answer the query using locally-appropriate RDATA. The DNS64 server 530 MAY use the same RDATA for all answers. Note that the requirement is 531 to match any Pref64::/n used at the site, and not merely the locally- 532 configured Pref64::/n. This is because end clients could ask for a 533 PTR record matching an address received through a different (site- 534 provided) DNS64, and if this strategy is in effect, those queries 535 should never be sent to the global DNS. The advantage of this 536 strategy is that it makes plain to the querying client that the 537 prefix is one operated by the DNS64 site, and that the answers the 538 client is getting are generated by the DNS64. The disadvantage is 539 that any useful reverse-tree information that might be in the global 540 DNS is unavailable to the clients querying the DNS64. 542 The second option is for the DNS64 nameserver to synthesize a CNAME 543 mapping the IP6.ARPA namespace to the corresponding IN-ADDR.ARPA 544 name. The rest of the response would be the normal DNS processing. 545 The CNAME can be signed on the fly if need be. The advantage of this 546 approach is that any useful information in the reverse tree is 547 available to the querying client. The disadvantage is that it adds 548 additional load to the DNS64 (because CNAMEs have to be synthesized 549 for each PTR query that matches the Pref64::/n), and that it may 550 require signing on the fly. [[anchor15: what are we supposed to do 551 here when the in-addr.arpa zone is unmaintained, as it may be. If 552 there is no data at the target name, then we'll get a CNAME with a 553 map to an empty namespace, I think? Isn't that bad? 554 --ajs@shinkuro.com]] 556 If the address prefix does not match any of the Pref64::/n, then the 557 DNS64 server MUST process the query as though it were any other query 558 -- i.e. a recursive nameserver MUST attempt to resolve the query as 559 though it were any other (non-A/AAAA) query, and an authoritative 560 server MUST respond authoritatively or with a referral, as 561 appropriate. 563 5.3.2. Handling the additional section 565 DNS64 synthesis MUST NOT be performed on any records in the 566 additional section of synthesized answers. The DNS64 MUST pass the 567 additional section unchanged. 569 [[anchor16: We had some discussion, as an alternative to the above, 570 of allowing the DNS64 to truncate the additional section completely, 571 on the grounds that the additional section could break mixed-mode 572 iterative/forwarding resolvers that happen to end up behind DNS64. 573 Nobody else seemed to like that plan, so I haven't included it. 574 --ajs@shinkuro.com]] 576 5.3.3. Other records 578 If the DNS64 is in recursive resolver mode, then it SHOULD also serve 579 the zones specified in [I-D.ietf-dnsop-default-local-zones], rather 580 than forwarding those queries elsewhere to be handled. 582 All other RRs MUST be returned unchanged. 584 5.4. Assembling a synthesized response to a AAAA query 586 The DNS64 uses different pieces of data to build the response 587 returned to the querying client. 589 The query that is used as the basis for synthesis results either in 590 an error, an answer that can be used as a basis for synthesis, or an 591 empty (authoritative) answer. If there is an empty answer, then the 592 DNS64 responds to the original querying client with the answer the 593 DNS64 received to the original AAAA query. Otherwise, the response 594 is assembled as follows. 596 The header fields are set according to the usual rules for recursive 597 or authoritative servers, depending on the role that the DNS64 is 598 serving. The question section is copied from the original AAAA 599 query. The answer section is populated according to the rules in 600 Section 5.1.4. The authority section is copied from the response to 601 the A query that the DNS64 performed. The additional section is 602 populated according to the rules in Section 5.3.2. 604 [[anchor18: The cross-reference to how to do the additional section 605 can be removed, and replaced by "copied from the response to the A 606 query that the DNS64 performed" if we don't want to allow the DNS64 607 to truncate the additional section. See the note above. If I hear 608 no more feedback on this topic, then I'll make this change in the 609 next version. --ajs@shinkuro.com]] 611 5.5. DNSSEC processing: DNS64 in recursive server mode 613 We consider the case where the recursive server that is performing 614 DNS64 also has a local policy to validate the answers according to 615 the procedures outlined in [RFC4035] Section 5. We call this general 616 case vDNS64. 618 The vDNS64 uses the presence of the DO and CD bits to make some 619 decisions about what the query originator needs, and can react 620 accordingly: 622 1. If CD is not set and DO is not set, vDNS64 SHOULD perform 623 validation and do synthesis as needed. 625 2. If CD is not set and DO is set, then vDNS64 SHOULD perform 626 validation. Whenever vDNS64 performs validation, it MUST 627 validate the negative answer for AAAA queries before proceeding 628 to query for A records for the same name, in order to be sure 629 that there is not a legitimate AAAA record on the Internet. 631 Failing to observe this step would allow an attacker to use DNS64 632 as a mechanism to circumvent DNSSEC. If the negative response 633 validates, and the response to the A query validates, then the 634 vDNS64 MAY perform synthesis and SHOULD set the AD bit in the 635 answer to the client. This is acceptable, because [RFC4035], 636 section 3.2.3 says that the AD bit is set by the name server side 637 of a security-aware recursive name server if and only if it 638 considers all the RRSets in the Answer and Authority sections to 639 be authentic. In this case, the name server has reason to 640 believe the RRSets are all authentic, so it SHOULD set the AD 641 bit. If the data does not validate, the vDNS64 MUST respond with 642 RCODE=2 (server failure). 643 A security-aware end point might take the presence of the AD bit 644 as an indication that the data is valid, and may pass the DNS 645 (and DNSSEC) data to an application. If the application attempts 646 to validate the synthesized data, of course, the validation will 647 fail. One could argue therefore that this approach is not 648 desirable. But security aware stub resolvers MUST NOT place any 649 reliance on data received from resolvers and validated on their 650 behalf without certain criteria established by [RFC4035], section 651 4.9.3. An application that wants to perform validation on its 652 own should use the CD bit. 654 3. If the CD bit is set and DO is set, then vDNS64 MAY perform 655 validation, but MUST NOT perform synthesis. It MUST hand the 656 data back to the query initiator, just like a regular recursive 657 resolver, and depend on the client to do the validation and the 658 synthesis itself. 659 The disadvantage to this approach is that an end point that is 660 translation-oblivious but security-aware and validating will not 661 be able to use the DNS64 functionality. In this case, the end 662 point will not have the desired benefit of NAT64. In effect, 663 this strategy means that any end point that wishes to do 664 validation in a NAT64 context must be upgraded to be translation- 665 aware as well. 667 5.6. DNS64 and multihoming 669 Synthetic AAAA records may be constructed on the basis of the network 670 context in which they were constructed. Therefore, a synthetic AAAA 671 received from one interface MUST NOT be used to resolve hosts via 672 another network interface. [[anchor21: This seems to be the result of 673 the discussion on-list starting with message id 18034D4D7FE9AE48BF19A 674 B1B0EF2729F3EF0E69687@NOK-EUMSG-01.mgdnok.nokia.com, but it's pretty 675 strange when stated baldly. In particular, how is the multi-homed 676 host supposed to know that a given AAAA is synthetic? 677 --ajs@shinkuro.com]] 679 6. Deployment notes 681 While DNS64 is intended to be part of a strategy for aiding IPv6 682 deployment in an internetworking environment with some IPv4-only and 683 IPv6-only networks, it is important to realise that it is 684 incompatible with some things that may be deployed in an IPv4-only or 685 dual-stack context. 687 6.1. DNS resolvers and DNS64 689 Full-service resolvers that are unaware of the DNS64 function can be 690 (mis)configured to act as mixed-mode iterative and forwarding 691 resolvers. In a native-IPv4 context, this sort of configuration may 692 appear to work. It is impossible to make it work properly without it 693 being aware of the DNS64 function, because it will likely at some 694 point obtain IPv4-only glue records and attempt to use them for 695 resolution. The result that is returned will contain only A records, 696 and without the ability to perform the DNS64 function the resolver 697 will simply be unable to answer the necessary AAAA queries. 699 6.2. DNSSEC validators and DNS64 701 Existing DNSSEC validators (i.e. that are unaware of DNS64) will 702 reject all the data that comes from the DNS64 as having been tampered 703 with. If it is necessary to have validation behind the DNS64, then 704 the validator must know how to perform the DNS64 function itself. 705 Alternatively, the validating host may establish a trusted connection 706 with the DNS64, and allow the DNS64 to do all validation on its 707 behalf. 709 7. Security Considerations 711 See the discussion on the usage of DNSSEC and DNS64 described in the 712 document. 714 8. Contributors 716 Dave Thaler 718 Microsoft 720 dthaler@windows.microsoft.com 722 9. Acknowledgements 724 This draft contains the result of discussions involving many people, 725 including the participants of the IETF BEHAVE Working Group. The 726 following IETF participants made specific contributions to parts of 727 the text, and their help is gratefully acknowledged: Mark Andrews, 728 Jari Arkko, Rob Austein, Timothy Baldwin, Fred Baker, Marc Blanchet, 729 Cameron Byrne, Brian Carpenter, Hui Deng, Francis Dupont, Ed 730 Jankiewicz, Peter Koch, Suresh Krishnan, Ed Lewis, Xing Li, Matthijs 731 Mekking, Hiroshi Miyata, Simon Perrault, Teemu Savolainen, Jyrki 732 Soini, Dave Thaler, Mark Townsley, Stig Venaas, Magnus Westerlund, 733 Florian Weimer, Dan Wing, Xu Xiaohu. 735 Marcelo Bagnulo and Iljitsch van Beijnum are partly funded by 736 Trilogy, a research project supported by the European Commission 737 under its Seventh Framework Program. 739 10. References 741 10.1. Normative References 743 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 744 Requirement Levels", BCP 14, RFC 2119, March 1997. 746 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 747 STD 13, RFC 1034, November 1987. 749 [RFC1035] Mockapetris, P., "Domain names - implementation and 750 specification", STD 13, RFC 1035, November 1987. 752 [RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", 753 RFC 2671, August 1999. 755 [RFC2672] Crawford, M., "Non-Terminal DNS Name Redirection", 756 RFC 2672, August 1999. 758 [RFC2765] Nordmark, E., "Stateless IP/ICMP Translation Algorithm 759 (SIIT)", RFC 2765, February 2000. 761 [RFC4787] Audet, F. and C. Jennings, "Network Address Translation 762 (NAT) Behavioral Requirements for Unicast UDP", BCP 127, 763 RFC 4787, January 2007. 765 [I-D.ietf-behave-tcp] 766 Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P. 767 Srisuresh, "NAT Behavioral Requirements for TCP", 768 draft-ietf-behave-tcp-08 (work in progress), 769 September 2008. 771 [I-D.ietf-behave-nat-icmp] 772 Srisuresh, P., Ford, B., Sivakumar, S., and S. Guha, "NAT 773 Behavioral Requirements for ICMP protocol", 774 draft-ietf-behave-nat-icmp-12 (work in progress), 775 January 2009. 777 [I-D.thaler-behave-translator-addressing] 778 Thaler, D., "IPv6 Addressing of IPv6/IPv4 Translators", 779 draft-thaler-behave-translator-addressing-00 (work in 780 progress), July 2009. 782 10.2. Informative References 784 [I-D.bagnulo-behave-nat64] 785 Bagnulo, M., Matthews, P., and I. Beijnum, "NAT64: Network 786 Address and Protocol Translation from IPv6 Clients to IPv4 787 Servers", draft-bagnulo-behave-nat64-03 (work in 788 progress), March 2009. 790 [RFC2766] Tsirtsis, G. and P. Srisuresh, "Network Address 791 Translation - Protocol Translation (NAT-PT)", RFC 2766, 792 February 2000. 794 [RFC2136] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound, 795 "Dynamic Updates in the Domain Name System (DNS UPDATE)", 796 RFC 2136, April 1997. 798 [RFC1858] Ziemba, G., Reed, D., and P. Traina, "Security 799 Considerations for IP Fragment Filtering", RFC 1858, 800 October 1995. 802 [RFC3128] Miller, I., "Protection Against a Variant of the Tiny 803 Fragment Attack (RFC 1858)", RFC 3128, June 2001. 805 [RFC3022] Srisuresh, P. and K. Egevang, "Traditional IP Network 806 Address Translator (Traditional NAT)", RFC 3022, 807 January 2001. 809 [RFC3484] Draves, R., "Default Address Selection for Internet 810 Protocol version 6 (IPv6)", RFC 3484, February 2003. 812 [RFC3596] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi, 813 "DNS Extensions to Support IP Version 6", RFC 3596, 814 October 2003. 816 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 818 Rose, "DNS Security Introduction and Requirements", 819 RFC 4033, March 2005. 821 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. 822 Rose, "Resource Records for the DNS Security Extensions", 823 RFC 4034, March 2005. 825 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 826 Rose, "Protocol Modifications for the DNS Security 827 Extensions", RFC 4035, March 2005. 829 [RFC4966] Aoun, C. and E. Davies, "Reasons to Move the Network 830 Address Translator - Protocol Translator (NAT-PT) to 831 Historic Status", RFC 4966, July 2007. 833 [I-D.iana-rfc3330bis] 834 Cotton, M. and L. Vegoda, "Special Use IPv4 Addresses", 835 draft-iana-rfc3330bis-06 (work in progress), 836 February 2009. 838 [I-D.ietf-mmusic-ice] 839 Rosenberg, J., "Interactive Connectivity Establishment 840 (ICE): A Protocol for Network Address Translator (NAT) 841 Traversal for Offer/Answer Protocols", 842 draft-ietf-mmusic-ice-19 (work in progress), October 2007. 844 [I-D.ietf-6man-addr-select-sol] 845 Matsumoto, A., Fujisaki, T., Hiromi, R., and K. Kanayama, 846 "Solution approaches for address-selection problems", 847 draft-ietf-6man-addr-select-sol-01 (work in progress), 848 June 2008. 850 [RFC3498] Kuhfeld, J., Johnson, J., and M. Thatcher, "Definitions of 851 Managed Objects for Synchronous Optical Network (SONET) 852 Linear Automatic Protection Switching (APS) 853 Architectures", RFC 3498, March 2003. 855 [I-D.wing-behave-learn-prefix] 856 Wing, D., Wang, X., and X. Xu, "Learning the IPv6 Prefix 857 of an IPv6/IPv4 Translator", 858 draft-wing-behave-learn-prefix-02 (work in progress), 859 May 2009. 861 [I-D.miyata-behave-prefix64] 862 Miyata, H. and M. Bagnulo, "PREFIX64 Comparison", 863 draft-miyata-behave-prefix64-02 (work in progress), 864 March 2009. 866 [I-D.venaas-behave-mcast46] 867 Venaas, S., "An IPv4 - IPv6 multicast translator", 868 draft-venaas-behave-mcast46-00 (work in progress), 869 December 2008. 871 [I-D.ietf-dnsop-default-local-zones] 872 Andrews, M., "Locally-served DNS Zones", 873 draft-ietf-dnsop-default-local-zones-08 (work in 874 progress), February 2009. 876 Appendix A. Deployment scenarios and examples 878 In this section, we first provide a description of the default 879 address transformation algorithm and then we walk through some sample 880 scenarios that are expected to be common deployment cases. It should 881 be noted that is provided for illustrative purposes and this section 882 is not normative. The normative definition of DNS64 is provided in 883 Section 5 and the normative definition of the address transformation 884 algorithm is provided in [I-D.thaler-behave-translator-addressing]. 886 There are two main different setups where DNS64 is expected to be 887 used (other setups are possible as well, but these two are the main 888 ones identified at the time of this writing). 890 One possible setup that is expected to be common is the case of an 891 end site or an ISP that is providing IPv6-only connectivity or 892 connectivity to IPv6-only hosts that wants to allow the 893 communication from these IPv6-only connected hosts to the IPv4 894 Internet. This case is called An-IPv6-network-to-IPv4-Internet. 895 In this case, the IPv6/IPv4 Translator is used to connect the end 896 site or the ISP to the IPv4 Internet and the DNS64 function is 897 provided by the end site or the ISP. 899 The other possible setup that is expected is an IPv4 site that 900 wants that its IPv4 servers to be reachable from the IPv6 901 Internet. This case is called IPv6-Internet-to-an-IPv4-network. 902 It should be noted that the IPv4 addresses used in the IPv4 site 903 can be either public or private. In this case, the IPv6/IPv4 904 Translator is used to connect the IPv4 end site to the IPv6 905 Internet and the DNS64 function is provided by the end site 906 itself. 908 In this section we illustrate how the DNS64 behaves in the different 909 scenarios that are expected to be common. We consider then 3 910 possible scenarios, namely: 912 1. An-IPv6-network-to-IPv4-Internet setup with DNS64 in DNS server 913 mode 915 2. An-IPv6-network-to-IPv4-Internet setup with DNS64 in stub- 916 resolver mode 918 3. IPv6-Internet-to-an-IPv4-network setup with DNS64 in DNS server 919 mode 921 The notation used is the following: upper case letters are IPv4 922 addresses; upper case letters with a prime(') are IPv6 addresses; 923 lower case letters are ports; prefixes are indicated by "P::X", which 924 is an IPv6 address built from an IPv4 address X by adding the prefix 925 P, mappings are indicated as "(X,x) <--> (Y',y)". 927 A.1. Embed and Zero-Pad algorithm description 929 In this section we describe the default algorithm for the generation 930 of IPv6 address from IPv4 address to be implemented in the DNS64. 932 The only parameter required by the default algorithm is an IPv6 933 prefix. This prefix is used to map IPv4 addresses into IPv6 934 addresses, and is denoted Pref64. If we note n the length of the 935 prefix Pref64, then n must the less or equal than 96. If an Pref64 936 is configured through any means in the DNS64 (such as manually 937 configured, or other automatic mean not specified in this document), 938 the default algorithm must use this prefix. If no prefix is 939 available the algorithm must use the Well-Know prefix (include here 940 the prefix to be assigned by IANA) defined in 941 [I-D.thaler-behave-translator-addressing] 943 The input for the algorithm are: 945 The IPv4 address: X 947 The IPv6 prefix: Pref64::/n 949 The IPv6 address is generated by concatenating the prefix Pref64::/n, 950 the IPv4 address X and optionally (in case n is strictly smaller than 951 96) an all-zero suffix. So, the resulting IPv6 address would be 952 Pref64:X:: 954 Reverse algorithm 956 We next describe the reverse algorithm of the algorithm described in 957 the previous section. This algorithm allows to generate and IPv4 958 address from an IPv6 address. This reverse algorithm is NOT 959 implemented by the DNS64 but it is implemented in the IPv6/IPv4 960 translator that is serving the same domain the DNS64. 962 The only parameter required by the default algorithm is an IPv6 963 prefix. This prefix is the one originally used to map IPv4 addresses 964 into IPv6 addresses, and is denoted Pref64. 966 The input for the algorithm are: 968 The IPv6 address: X' 970 The IPv6 prefix: Pref64::/n 972 First, the algorithm checks that the fist n bits of the IPv6 address 973 X' match with the prefix Pref64::/n i.e. verifies that Pref64::/n = 974 X'/n. 976 If this is not the case, the algorithm ends and no IPv4 address is 977 generated. 979 If the verification is successful, then the bits between the n+1 980 and the n+32 of the IPv6 address X' are extracted to form the IPv4 981 address. 983 A.2. An-IPv6-network-to-IPv4-Internet setup with DNS64 in DNS server 984 mode 986 In this example, we consider an IPv6 node located in an IPv6-only 987 site that initiates a communication to an IPv4 node located in the 988 IPv4 Internet. 990 The scenario for this case is depicted in the following figure: 992 +---------------------------------------+ +-----------+ 993 |IPv6 site +-------------+ |IP Addr: | | 994 | +----+ | Name server | +-------+ T | IPv4 | 995 | | H1 | | with DNS64 | |64Trans|------| Internet | 996 | +----+ +-------------+ +-------+ +-----------+ 997 | |IP addr: Y' | | | |IP addr: X 998 | --------------------------------- | +----+ 999 +---------------------------------------+ | H2 | 1000 +----+ 1002 The figure shows an IPv6 node H1 which has an IPv6 address Y' and an 1003 IPv4 node H2 with IPv4 address X. 1005 A IPv6/IPv4 Translator connects the IPv6 network to the IPv4 1006 Internet. This IPv6/IPv4 Translator has a prefix (called Pref64::/n) 1007 an IPv4 address T assigned to its IPv4 interface. 1009 The other element involved is the local name server. The name server 1010 is a dual-stack node, so that H1 can contact it via IPv6, while it 1011 can contact IPv4-only name servers via IPv4. 1013 The local name server needs to know the prefix assigned to the local 1014 IPv6/IPv4 Translator (Pref64::/n). For the purpose of this example, 1015 we assume it learns this through manual configuration. 1017 For this example, assume the typical DNS situation where IPv6 hosts 1018 have only stub resolvers, and always query a name server that 1019 performs recursive lookups (henceforth called "the recursive 1020 nameserver"). 1022 The steps by which H1 establishes communication with H2 are: 1024 1. H1 does a DNS lookup for FQDN(H2). H1 does this by sending a DNS 1025 query for an AAAA record for H2 to the recursive name server. 1026 The recursive name server implements DNS64 functionality. 1028 2. The recursive name server resolves the query, and discovers that 1029 there are no AAAA records for H2. 1031 3. The recursive name server queries for an A record for H2 and gets 1032 back an A record containing the IPv4 address X. The name server 1033 then synthesizes an AAAA record. The IPv6 address in the AAAA 1034 record contains the prefix assigned to the IPv6/IPv4 Translator 1035 in the upper n bits then the IPv4 address X and then an all-zero 1036 padding i.e. the resulting IPv6 address is Pref64:X:: 1038 4. H1 receives the synthetic AAAA record and sends a packet towards 1039 H2. The packet is sent from a source transport address of (Y',y) 1040 to a destination transport address of (Pref64:X::,x), where y and 1041 x are ports chosen by H2. 1043 5. The packet is routed to the IPv6 interface of the IPv6/IPv4 1044 Translator and the subsequent communication flows by means of the 1045 IPv6/IPv4 Translator mechanisms. 1047 A.3. An-IPv6-network-to-IPv4-Internet setup with DNS64 in stub-resolver 1048 mode 1050 The scenario for this case is depicted in the following figure: 1052 +---------------------------------------+ +-----------+ 1053 |IPv6 site +-------+ |IP addr: | | 1054 | +---------------+ | Name | +-------+ T | IPv4 | 1055 | | H1 with DNS64 | | Server| |64Trans|------| Internet | 1056 | +---------------+ +-------+ +-------+ +-----------+ 1057 | |IP addr: Y' | | | |IP addr: X 1058 | --------------------------------- | +----+ 1059 +---------------------------------------+ | H2 | 1060 +----+ 1062 The figure shows an IPv6 node H1 which has an IPv6 address Y' and an 1063 IPv4 node H2 with IPv4 address X. Node H1 is implementing the DNS64 1064 function. 1066 A IPv6/IPv4 Translator connects the IPv6 network to the IPv4 1067 Internet. This IPv6/IPv4 Translator has a prefix (called Pref64::/n) 1068 and an IPv4 address T assigned to its IPv4 interface. 1070 H1 needs to know the prefix assigned to the local IPv6/IPv4 1071 Translator (Pref64::/n). For the purpose of this example, we assume 1072 it learns this through manual configuration. 1074 Also shown is a name server. For the purpose of this example, we 1075 assume that the name server is a dual-stack node, so that H1 can 1076 contact it via IPv6, while it can contact IPv4-only name servers via 1077 IPv4. 1079 For this example, assume the typical situation where IPv6 hosts have 1080 only stub resolvers and always query a name server that provides 1081 recursive lookups (henceforth called "the recursive name server"). 1082 The recursive name server does not perform the DNS64 function. 1084 The steps by which H1 establishes communication with H2 are: 1086 1. H1 does a DNS lookup for FQDN(H2). H1 does this by sending a DNS 1087 query for a AAAA record for H2 to the recursive name server. 1089 2. The recursive DNS server resolves the query, and returns the 1090 answer to H1. Because there are no AAAA records in the global 1091 DNS for H2, the answer is empty. 1093 3. The stub resolver at H1 then queries for an A record for H2 and 1094 gets back an A record containing the IPv4 address X. The DNS64 1095 function within H1 then synthesizes a AAAA record. The IPv6 1096 address in the AAAA record contains the prefix assigned to the 1097 IPv6/IPv4 Translator in the upper n bits, then the IPv4 address X 1098 and then an all-zero padding i.e. the resulting IPv6 address is 1099 Pref64:X::. 1101 4. H1 sends a packet towards H2. The packet is sent from a source 1102 transport address of (Y',y) to a destination transport address of 1103 (Pref64:X::,x), where y and x are ports chosen by H2. 1105 5. The packet is routed to the IPv6 interface of the IPv6/IPv4 1106 Translator and the subsequent communication flows using the IPv6/ 1107 IPv4 Translator mechanisms. 1109 A.4. IPv6-Internet-to-an-IPv4-network setup DNS64 in DNS server mode 1111 In this example, we consider an IPv6 node located in the IPv6 1112 Internet site that initiates a communication to a IPv4 node located 1113 in the IPv4 site. 1115 This scenario can be addressed without using any form of DNS64 1116 function. This is so because it is possible to assign a fixed IPv6 1117 address to each of the IPv4 servers. Such an IPv6 address would be 1118 constructed as the Pref64::/n concatenated with the IPv4 address of 1119 the IPv4 server and an all-zero padding. Note that the IPv4 address 1120 can be a public or a private address; the latter does not present any 1121 additional difficulty, since the LIR prefix must be used a Pref64 (in 1122 this scenario the usage of the WK prefix is not supported). Once 1123 these IPv6 addresses have been assigned to represent the IPv4 servers 1124 in the IPv6 Internet, real AAAA RRs containing these addresses can be 1125 published in the DNS under the site's domain. This is the 1126 recommended approach to handle this scenario, because it does not 1127 involve synthesizing AAAA records at the time of query. Such a 1128 configuration is easier to troubleshoot in the event of problems, 1129 because it always provides the same answer to every query. 1131 However, there are some more dynamic scenarios, where synthesizing 1132 AAAA RRs in this setup may be needed. In particular, when DNS Update 1133 [RFC2136] is used in the IPv4 site to update the A RRs for the IPv4 1134 servers, there are two options: One option is to modify the server 1135 that receives the dynamic DNS updates. That would normally be the 1136 authoritative server for the zone. So the authoritative zone would 1137 have normal AAAA RRs that are synthesized as dynamic updates occur. 1138 The other option is modify the authoritative server to generate 1139 synthetic AAAA records for a zone, possibly based on additional 1140 constraints, upon the receipt of a DNS query for the AAAA RR. The 1141 first option -- in which the AAAA is synthesized when the DNS update 1142 message is received, and the data published in the relevant zone -- 1143 is recommended over the second option (i.e. the synthesis upon 1144 receipt of the AAAA DNS query). This is because it is usually easier 1145 to solve problems of misconfiguration and so on when the DNS 1146 responses are not being generated dynamically. For completeness, the 1147 DNS64 behavior that we describe in this section covers the case of 1148 synthesizing the AAAA RR when the DNS query arrives. Nevertheless, 1149 such a configuration is NOT RECOMMENDED. Troubleshooting 1150 configurations that change the data depending on the query they 1151 receive is notoriously hard, and the IPv4/IPv6 translation scenario 1152 is complicated enough without adding additional opportunities for 1153 possible malfunction. 1155 The scenario for this case is depicted in the following figure: 1157 +-----------+ +----------------------------------------+ 1158 | | | IPv4 site +-------------+ | 1159 | IPv6 | +-------+ +----+ | Name server | | 1160 | Internet |------|64Trans| | H2 | | with DNS64 | | 1161 +-----------+ +-------+ +----+ +-------------+ | 1162 |IP addr: Y' | | |IP addr: X | | 1163 +----+ | ----------------------------------- | 1164 | H1 | +----------------------------------------+ 1165 +----+ 1167 The figure shows an IPv6 node H1 which has an IPv6 address Y' and an 1168 IPv4 node H2 with IPv4 address X. 1170 A IPv6/IPv4 Translator connects the IPv4 network to the IPv6 1171 Internet. This IPv6/IPv4 Translator has a prefix (called 1172 Pref64::/n). 1174 Also shown is the authoritative name server for the local domain with 1175 DNS64 functionality. For the purpose of this example, we assume that 1176 the name server is a dual-stack node, so that H1 or a recursive 1177 resolver acting on the request of H1 can contact it via IPv6, while 1178 it can be contacted by IPv4-only nodes to receive dynamic DNS updates 1179 via IPv4. 1181 The local name server needs to know the prefix assigned to the local 1182 IPv6/IPv4 Translator (Pref64::/n). For the purpose of this example, 1183 we assume it learns this through manual configuration. 1185 The steps by which H1 establishes communication with H2 are: 1187 1. H1 does a DNS lookup for FQDN(H2). H1 does this by sending a DNS 1188 query for an AAAA record for H2. The query is eventually 1189 forwarded to the server in the IPv4 site. 1191 2. The local DNS server resolves the query (locally), and discovers 1192 that there are no AAAA records for H2. 1194 3. The name server verifies that FQDN(H2) and its A RR are among 1195 those that the local policy defines as allowed to generate a AAAA 1196 RR from. If that is the case, the name server synthesizes an 1197 AAAA record from the A RR and the relevant Pref64::/n. The IPv6 1198 address in the AAAA record contains the prefix assigned to the 1199 IPv6/IPv4 Translator in the first n bits and the IPv4 address X 1200 and then an all-zero padding. 1202 4. H1 receives the synthetic AAAA record and sends a packet towards 1203 H2. The packet is sent from a source transport address of (Y',y) 1204 to a destination transport address of (Pref64:X::,x), where y and 1205 x are ports chosen by H2. 1207 5. The packet is routed through the IPv6 Internet to the IPv6 1208 interface of the IPv6/IPv4 Translator and the communication flows 1209 using the IPv6/IPv4 Translator mechanisms. 1211 Appendix B. Motivations and Implications of synthesizing AAAA RR when 1212 real AAAA RR exists 1214 The motivation for synthesizing AAAA RR when a real AAAA RR exists is 1215 to support the following scenario: 1217 An IPv4-only server application (e.g. web server software) is 1218 running on a dual-stack host. There may also be dual-stack server 1219 applications also running on the same host. That host has fully 1220 routable IPv4 and IPv6 addresses and hence the authoritative DNS 1221 server has an A and a AAAA record as a result. 1223 An IPv6-only client (regardless of whether the client application 1224 is IPv6-only, the client stack is IPv6-only, or it only has an 1225 IPv6 address) wants to access the above server. 1227 The client issues a DNS query to a DNS64 recursor. 1229 If the DNS64 only generates a synthetic AAAA if there's no real AAAA, 1230 then the communication will fail. Even though there's a real AAAA, 1231 the only way for communication to succeed is with the translated 1232 address. So, in order to support this scenario, the administrator of 1233 a DNS64 service may want to enable the synthesis of AAAA RR even when 1234 real AAAA RR exist. 1236 The implication of including synthetic AAAA RR when real AAAA RR 1237 exist is that translated connectivity may be preferred over native 1238 connectivity in some cases where the DNS64 is operated in DNS server 1239 mode. 1241 RFC3484 [RFC3484] rules use longest prefix match to select which is 1242 the preferred destination address to use. So, if the DNS64 recursor 1243 returns both the synthetic AAAA RR and the real AAAA RR, then if the 1244 DNS64 is operated by the same domain as the initiating host, and a 1245 global unicast prefix (called the LIR prefix as defined in 1246 [I-D.thaler-behave-translator-addressing]) is used, then the 1247 synthetic AAAA RR is likely to be preferred. 1249 This means that without further configuration: 1251 In the case of An IPv6 network to the IPv4 internet, the host will 1252 prefer translated connectivity if LIR prefix is used. If the 1253 Well-Known (WK) prefix defined in 1254 [I-D.thaler-behave-translator-addressing] is used, it will 1255 probably prefer native connectivity. 1257 In the case of the IPv6 Internet to an IPv4 network, it is 1258 possible to bias the selection towards the real AAAA RR if the 1259 DNS64 recursor returns the real AAAA first in the DNS reply, when 1260 the LIR prefix is used (the WK prefix usage is not recommended in 1261 this case) 1263 In the case of the IPv6 to IPv4 in the same network, for local 1264 destinations (i.e., target hosts inside the local site), it is 1265 likely that the LIR prefix and the destination prefix are the 1266 same, so we can use the order of RR in the DNS reply to bias the 1267 selection through native connectivity. If a WK prefix is used, 1268 the longest prefix match rule will select native connectivity. 1270 So this option introduces problems in the following cases: 1272 An IPv6 network to the IPv4 internet with the LIR prefix 1274 IPv6 to IPv4 in the same network when reaching external 1275 destinations and the LIR prefix is used. 1277 In any case, the problem can be solved by properly configuring the 1278 RFC3484 [RFC3484] policy table, but this requires effort on the part 1279 of the site operator. 1281 Authors' Addresses 1283 Marcelo Bagnulo 1284 UC3M 1285 Av. Universidad 30 1286 Leganes, Madrid 28911 1287 Spain 1289 Phone: +34-91-6249500 1290 Fax: 1291 Email: marcelo@it.uc3m.es 1292 URI: http://www.it.uc3m.es/marcelo 1294 Andrew Sullivan 1295 Shinkuro 1296 4922 Fairmont Avenue, Suite 250 1297 Bethesda, MD 20814 1298 USA 1300 Phone: +1 301 961 3131 1301 Email: ajs@shinkuro.com 1303 Philip Matthews 1304 Unaffiliated 1305 600 March Road 1306 Ottawa, Ontario 1307 Canada 1309 Phone: +1 613-592-4343 x224 1310 Fax: 1311 Email: philip_matthews@magma.ca 1312 URI: 1314 Iljitsch van Beijnum 1315 IMDEA Networks 1316 Av. Universidad 30 1317 Leganes, Madrid 28911 1318 Spain 1320 Phone: +34-91-6246245 1321 Email: iljitsch@muada.com