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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group J. Klensin 3 Internet-Draft November 2, 2008 4 Obsoletes: 3490, 3491 5 (if approved) 6 Updates: 3492 (if approved) 7 Intended status: Standards Track 8 Expires: May 6, 2009 10 Internationalized Domain Names in Applications (IDNA): Protocol 11 draft-ietf-idnabis-protocol-06.txt 13 Status of this Memo 15 By submitting this Internet-Draft, each author represents that any 16 applicable patent or other IPR claims of which he or she is aware 17 have been or will be disclosed, and any of which he or she becomes 18 aware will be disclosed, in accordance with Section 6 of BCP 79. 20 Internet-Drafts are working documents of the Internet Engineering 21 Task Force (IETF), its areas, and its working groups. Note that 22 other groups may also distribute working documents as Internet- 23 Drafts. 25 Internet-Drafts are draft documents valid for a maximum of six months 26 and may be updated, replaced, or obsoleted by other documents at any 27 time. It is inappropriate to use Internet-Drafts as reference 28 material or to cite them other than as "work in progress." 30 The list of current Internet-Drafts can be accessed at 31 http://www.ietf.org/ietf/1id-abstracts.txt. 33 The list of Internet-Draft Shadow Directories can be accessed at 34 http://www.ietf.org/shadow.html. 36 This Internet-Draft will expire on May 6, 2009. 38 Abstract 40 This document supplies the protocol definition for a revised and 41 updated specification for internationalized domain names (IDNs). The 42 rationale for these changes, the relationship to the older 43 specification, and important terminology are provided in other 44 documents. This document specifies the protocol mechanism, called 45 Internationalizing Domain Names in Applications (IDNA), for 46 registering and looking up IDNs in a way that does not require 47 changes to the DNS itself. IDNA is only meant for processing domain 48 names, not free text. 50 Table of Contents 52 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 53 1.1. Discussion Forum . . . . . . . . . . . . . . . . . . . . . 4 54 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5 55 3. Requirements and Applicability . . . . . . . . . . . . . . . . 5 56 3.1. Requirements . . . . . . . . . . . . . . . . . . . . . . . 5 57 3.2. Applicability . . . . . . . . . . . . . . . . . . . . . . 5 58 3.2.1. DNS Resource Records . . . . . . . . . . . . . . . . . 6 59 3.2.2. Non-domain-name Data Types Stored in the DNS . . . . . 6 60 4. Registration Protocol . . . . . . . . . . . . . . . . . . . . 6 61 4.1. Proposed label . . . . . . . . . . . . . . . . . . . . . . 7 62 4.2. Conversion to Unicode and Normalization . . . . . . . . . 7 63 4.3. Permitted Character and Label Validation . . . . . . . . . 8 64 4.3.1. Rejection of Characters that are not Permitted . . . . 8 65 4.3.2. Label Validation . . . . . . . . . . . . . . . . . . . 8 66 4.3.3. Registration Validation Summary . . . . . . . . . . . 9 67 4.4. Registry Restrictions . . . . . . . . . . . . . . . . . . 9 68 4.5. Punycode Conversion . . . . . . . . . . . . . . . . . . . 10 69 4.6. Insertion in the Zone . . . . . . . . . . . . . . . . . . 10 70 5. Domain Name Lookup Protocol . . . . . . . . . . . . . . . . . 10 71 5.1. Label String Input . . . . . . . . . . . . . . . . . . . . 10 72 5.2. Conversion to Unicode . . . . . . . . . . . . . . . . . . 10 73 5.3. Character Changes in Preprocessing or the User 74 Interface . . . . . . . . . . . . . . . . . . . . . . . . 11 75 5.4. A-label Input . . . . . . . . . . . . . . . . . . . . . . 12 76 5.5. Validation and Character List Testing . . . . . . . . . . 12 77 5.6. Punycode Conversion . . . . . . . . . . . . . . . . . . . 13 78 5.7. DNS Name Resolution . . . . . . . . . . . . . . . . . . . 13 79 6. Name Server Considerations . . . . . . . . . . . . . . . . . . 13 80 6.1. Processing Non-ASCII Strings . . . . . . . . . . . . . . . 14 81 6.2. DNSSEC Authentication of IDN Domain Names . . . . . . . . 14 82 6.3. Root and other DNS Server Considerations . . . . . . . . . 15 83 7. Security Considerations . . . . . . . . . . . . . . . . . . . 15 84 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 85 9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 16 86 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16 87 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17 88 11.1. Normative References . . . . . . . . . . . . . . . . . . . 17 89 11.2. Informative References . . . . . . . . . . . . . . . . . . 18 90 Appendix A. Summary of Major Changes from IDNA2003 . . . . . . . 19 91 Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 20 92 B.1. Changes between Version -00 and -01 of 93 draft-ietf-idnabis-protocol . . . . . . . . . . . . . . . 20 94 B.2. Version -02 . . . . . . . . . . . . . . . . . . . . . . . 20 95 B.3. Version -03 . . . . . . . . . . . . . . . . . . . . . . . 20 96 B.4. Version -04 . . . . . . . . . . . . . . . . . . . . . . . 21 97 B.5. Version -05 . . . . . . . . . . . . . . . . . . . . . . . 21 98 B.6. Version -06 . . . . . . . . . . . . . . . . . . . . . . . 21 99 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 21 100 Intellectual Property and Copyright Statements . . . . . . . . . . 22 102 1. Introduction 104 This document supplies the protocol definition for a revised and 105 updated specification for internationalized domain names. Essential 106 definitions and terminology for understanding this document and a 107 road map of the collection of documents that make up IDNA2008 appear 108 in [IDNA2008-Defs]. Appendix A discusses the relationship between 109 this specification and the earlier version of IDNA (referred to here 110 as "IDNA2003") and the rationale for these changes, along with 111 considerable explanatory material and advice to zone administrators 112 who support IDNs is provided in another documents, notably 113 [IDNA2008-Rationale]. 115 IDNA works by allowing applications to use certain ASCII string 116 labels (beginning with a special prefix) to represent non-ASCII name 117 labels. Lower-layer protocols need not be aware of this; therefore 118 IDNA does not depend on changes to any infrastructure. In 119 particular, IDNA does not depend on any changes to DNS servers, 120 resolvers, or protocol elements, because the ASCII name service 121 provided by the existing DNS is entirely sufficient for IDNA. 123 IDNA is applied only to DNS labels. Standards for combining labels 124 into fully-qualified domain names and parsing labels out of those 125 names are covered in the base DNS standards [RFC1034] [RFC1035] and 126 their various updates. An application may, of course, apply locally- 127 appropriate conventions to the presentation forms of domain names as 128 discussed in [IDNA2008-Rationale]. 130 While they share terminology, reference data, and some operations, 131 this document describes two separate protocols, one for IDN 132 registration (Section 4) and one for IDN lookup (Section 5). 134 A good deal of the background material that appeared in RFC 3490 135 [RFC3490] has been removed from this update. That material is either 136 of historical interest only or has been covered from a more recent 137 perspective in RFC 4690 [RFC4690] and [IDNA2008-Rationale]. 138 [[anchor2: This paragraph is not normative and not required to 139 understand this spec. It will be removed in version -07 unless 140 someone provides a convincing rationale for retaining it.]] 142 1.1. Discussion Forum 144 [[anchor4: RFC Editor: please remove this section.]] 146 This work is being discussed in the IETF IDNABIS WG and on the 147 mailing list idna-update@alvestrand.no 149 2. Terminology 151 General terminology applicable to IDNA, but with meanings familiar to 152 those who have worked with Unicode or other character set standards 153 and the DNS, appears in [IDNA2008-Defs]. Terminology that is an 154 integral, normative, part of the IDNA definition, including the 155 definitions of "ACE", appears in that document as well. Familiarity 156 with the terminology materials in that document is assumed for 157 reading this one. The reader of this document is assumed to be 158 familiar with DNS-specific terminology as defined in RFC 1034 159 [RFC1034]. 161 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 162 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 163 document are to be interpreted as described in BCP 14, RFC 2119 164 [RFC2119]. 166 3. Requirements and Applicability 168 3.1. Requirements 170 IDNA conformance means adherence to the following requirements: 172 1. Whenever a domain name is put into an IDN-unaware domain name 173 slot (see Section 2 and [IDNA2008-Rationale]), it MUST contain 174 only ASCII characters (i.e., must be either an A-label or an LDH- 175 label), or must be a label associated with a DNS application that 176 is not subject to either IDNA or the historical recommendations 177 for "hostname"-style names [RFC1034]. 179 2. Comparison of labels SHOULD be done on the A-label form, using an 180 ASCII case-insensitive comparison as with all comparisons of DNS 181 labels. Because A-labels and U-labels can be transformed into 182 each other without loss of information, comparison of native 183 character labels is possible if the application first carefully 184 verifies that the strings are U-labels. 186 3. Labels being registered MUST conform to the requirements of 187 Section 4. Labels being looked up and the lookup process MUST 188 conform to the requirements of Section 5. 190 3.2. Applicability 192 IDNA is applicable to all domain names in all domain name slots 193 except where it is explicitly excluded. It is not applicable to 194 domain name slots which do not use the LDH syntax rules. 196 This implies that IDNA is applicable to many protocols that predate 197 IDNA. Note that IDNs occupying domain name slots in those older 198 protocols MUST be in A-label form until and unless those protocols 199 and implementations of them are upgraded and that IDNs actually 200 appearing in DNS queries or responses MUST be in A-label form. 202 3.2.1. DNS Resource Records 204 IDNA applies only to domain names in the NAME and RDATA fields of DNS 205 resource records whose CLASS is IN. 207 There are currently no other exclusions on the applicability of IDNA 208 to DNS resource records. Applicability depends entirely on the 209 CLASS, and not on the TYPE except as noted below. This will remain 210 true, even as new types are defined, unless there is a compelling 211 reason for a new type that requires type-specific rules. The special 212 naming conventions applicable to SRV records are examples of type- 213 specific rules that are incompatible with IDNA coding. Hence the 214 first two labels (the ones required to start in "_") on a record with 215 TYPE SRV MUST NOT be A-labels or U-labels (while it would be possible 216 to write a non-ASCII string with a leading underscore, conversion to 217 an A-label would be impossible without loss of information because 218 the underscore is not a letter, digit, or hyphen and is consequently 219 DISALLOWED in IDNs). Of course, those labels may be part of a domain 220 that uses IDN labels at higher levels in the tree. 222 3.2.2. Non-domain-name Data Types Stored in the DNS 224 Although IDNA enables the representation of non-ASCII characters in 225 domain names, that does not imply that IDNA enables the 226 representation of non-ASCII characters in other data types that are 227 stored in domain names, specifically in the RDATA field for types 228 that have structured RDATA format. For example, an email address 229 local part is stored in a domain name in the RNAME field as part of 230 the RDATA of an SOA record (hostmaster@example.com would be 231 represented as hostmaster.example.com). IDNA specifically does not 232 update the existing email standards, which allow only ASCII 233 characters in local parts. Even though work is in progress to define 234 internationalization for email addresses [RFC4952], changes to the 235 email address part of the SOA RDATA would require action in, or 236 updates to, other standards, specifically those that specify the 237 format of the SOA RR. 239 4. Registration Protocol 241 This section defines the procedure for registering an IDN. The 242 procedure is implementation independent; any sequence of steps that 243 produces exactly the same result for all labels is considered a valid 244 implementation. 246 Note that, while the registration and lookup protocols (Section 5) 247 are very similar in most respects, they are different and 248 implementers should carefully follow the steps they are implementing. 250 4.1. Proposed label 252 The registrant submits a request for an IDN. The user typically 253 produces the request string by the keyboard entry of a character 254 sequence in the local native character set (which might, of course, 255 be Unicode). 257 The registry MAY permit submission of labels in A-label form. If it 258 does so, it SHOULD perform a conversion to a U-label, perform the 259 steps and tests described below, and verify that the A-label produced 260 by the step in Section 4.5 matches the one provided as input. If, 261 for some reason, it does not, the registration MUST be rejected. If 262 the conversion to a U-label is not performed, the registry MUST 263 verify that the A-label is superficially valid, i.e., that it does 264 not violate any of the rules of Punycode [RFC3492] encoding such as 265 the prohibition on trailing hyphen-minus, appearance of non-basic 266 characters before the delimiter, and so on. Invalid strings that 267 appear to be A-labels MUST NOT be placed in DNS zones. 268 [[anchor9: Editorial: Should the sentences starting with "The 269 registry" be moved to 4.3? I.e., would they be more in sequence 270 there? Note that A-labels are, by definition, in ASCII, so section 271 4.2 does not apply to them. The tone of this recommendation also 272 seems slightly at odds with the statements at the end of 4.2. 273 Suggested text for cleaning this up, harmonizing it, and reducing 274 redundancy would be appreciated.]] 276 4.2. Conversion to Unicode and Normalization 278 Some system routine, or a localized front-end to the IDNA process, 279 ensures that the proposed label is a Unicode string or converts it to 280 one as appropriate. That string MUST be in Unicode Normalization 281 Form C (NFC [Unicode-UAX15]). 283 As a local implementation choice, the implementation MAY choose to 284 map some forbidden characters to permitted characters (for instance 285 mapping uppercase characters to lowercase ones), displaying the 286 result to the user, and allowing processing to continue. However, it 287 is strongly recommended that, to avoid any possible ambiguity, 288 entities responsible for zone files ("registries") accept 289 registrations only for A-labels (to be converted to U-labels by the 290 registry as discussed above) or U-labels actually produced from 291 A-labels, not forms expected to be converted by some other process. 293 4.3. Permitted Character and Label Validation 295 4.3.1. Rejection of Characters that are not Permitted 297 The Unicode string is checked to verify that no characters that IDNA 298 does not permit in input appear in it. Those characters are 299 identified in the "DISALLOWED" and "UNASSIGNED" lists that are 300 specified in [IDNA2008-Tables] and described informally in 301 [IDNA2008-Rationale]. Characters that are either DISALLOWED or 302 UNASSIGNED MUST NOT be part of labels to be processed for 303 registration in the DNS. 305 4.3.2. Label Validation 307 The proposed label (in the form of a Unicode string, i.e., a putative 308 U-label) is then examined, performing tests that require examination 309 of more than one character. 311 4.3.2.1. Rejection of Confusing or Hostile Sequences in U-labels 313 The Unicode string MUST NOT contain "--" (two consecutive hyphens) in 314 the third and fourth character positions. 316 4.3.2.2. Leading Combining Marks 318 The first character of the string is examined to verify that it is 319 not a combining mark. If it is a combining mark, the string MUST NOT 320 be registered. 322 4.3.2.3. Contextual Rules 324 Each code point is checked for its identification as a character 325 requiring contextual processing for registration (the list of 326 characters appears as the combination of CONTEXTJ and CONTEXTO in 327 [IDNA2008-Tables] as do the contextual rules themselves). If that 328 indication appears, the table of contextual rules is checked for a 329 rule for that character. If no rule is found, the proposed label is 330 rejected and MUST NOT be installed in a zone file. If one is found, 331 it is applied (typically as a test on the entire label or on adjacent 332 characters within the label). If the application of the rule does 333 not conclude that the character is valid in context, the proposed 334 label MUST BE rejected. (See the IANA Considerations: IDNA Context 335 Registry section of [IDNA2008-Tables].) 337 These contextual rules are required to permit the use of characters 338 that would otherwise risk causing considerable harm. For example, 339 labels containing invisible ("zero-width") characters may be 340 permitted in context with characters whose presentation forms are 341 significantly changed by the presence or absence of the zero-width 342 characters, while other labels in which zero-width characters appear 343 may be rejected. 344 [[anchor14: Should this paragraph be removed? Note that I've been 345 strongly encouraged to supply specific examples to reduce abstraction 346 and questions about the appropriateness of the text. -JcK]] 348 4.3.2.4. Labels Containing Characters Written Right to Left 350 Additional special tests for right-to-left strings are applied (See 351 [IDNA2008-BIDI]). Strings that contain right to left characters that 352 do not conform to the rule(s) identified there MUST NOT be inserted 353 as labels in zone files. 355 4.3.3. Registration Validation Summary 357 Strings that have been produced by the steps above, and whose 358 contents pass the above tests, are U-labels. 360 To summarize, tests are made in Section 4.3 for invalid characters, 361 invalid combinations of characters, and for labels that are invalid 362 even if the characters they contain are valid individually. 364 4.4. Registry Restrictions 366 Registries at all levels of the DNS, not just the top level, are 367 expected to establish policies about the labels that may be 368 registered, and for the processes associated with that action. While 369 exact policies are not specified as part of IDNA2008 and it is 370 expected that different registries may specify different policies, 371 there SHOULD be policies. Even a trivial policy (e.g., "anything can 372 be registered in this zone that can be represented as an A-label - 373 U-label pair") has value because it provides notice to users and 374 applications implementers that the registry cannot be relied upon to 375 provide even minimal user-protection restrictions. These per- 376 registry policies and restrictions are an essential element of the 377 IDNA registration protocol even for registries (and corresponding 378 zone files) deep in the DNS hierarchy. As discussed in 379 [IDNA2008-Rationale], such restrictions have always existed in the 380 DNS. That document also contains a discussion and recommendations 381 about possible types of rules. 383 The string produced by the above steps is checked and processed as 384 appropriate to local registry restrictions. Application of those 385 registry restrictions may result in the rejection of some labels or 386 the application of special restrictions to others. 388 4.5. Punycode Conversion 390 The resulting U-label is converted to an A-label. The A-label, more 391 precisely defined elsewhere, is the encoding of the U-label according 392 to the Punycode algorithm [RFC3492] with the ACE prefix "xn--" added 393 at at the beginning of the string. This document updates RFC 3492 394 only to the extent of replacing the reference to the discussion of 395 the ACE prefix. The ACE prefix is now specified in this document 396 rather than as part of RFC 3490 or Nameprep [RFC3491]. 398 The failure conditions identified in the Punycode encoding procedure 399 cannot occur if the input is a U-label as determined by the steps 400 above. 402 4.6. Insertion in the Zone 404 The A-label is registered in the DNS by insertion into a zone. 406 5. Domain Name Lookup Protocol 408 Lookup is conceptually different from registration and different 409 tests are applied on the client. Although some validity checks are 410 necessary to avoid serious problems with the protocol (see 411 Section 5.5ff.), the lookup-side tests are more permissive and rely 412 on the assumption that names that are present in the DNS are valid. 413 That assumption is, however, a weak one because the presence of wild 414 cards in the DNS might cause a string that is not actually registered 415 in the DNS to be successfully looked up. 417 5.1. Label String Input 419 The user supplies a string in the local character set, typically by 420 typing it or clicking on, or copying and pasting, a resource 421 identifier, e.g., a URI [RFC3986] or IRI [RFC3987] from which the 422 domain name is extracted. Alternately, some process not directly 423 involving the user may read the string from a file or obtain it in 424 some other way. Processing in this step and the next two are local 425 matters, to be accomplished prior to actual invocation of IDNA, but 426 at least the two steps in Section 5.2 and Section 5.3 must be 427 accomplished in some way. 429 5.2. Conversion to Unicode 431 The string is converted from the local character set into Unicode, if 432 it is not already Unicode. The exact nature of this conversion is 433 beyond the scope of this document, but may involve normalization as 434 described in Section 4.2. The result MUST be a Unicode string in NFC 435 form. 437 5.3. Character Changes in Preprocessing or the User Interface 439 The Unicode string MAY then be processed to prevent confounding of 440 user expectations. For instance, it might be reasonable, at this 441 step, to convert all upper case characters to lower case, if this 442 makes sense in the user's environment, but even this should be 443 approached with caution due to some edge cases: in the long term, it 444 is probably better for users to understand IDNs strictly in lower- 445 case, U-label, form. More generally, preprocessing may be useful to 446 smooth the transition from IDNA2003, especially for direct user 447 input, but with similar cautions. In general, IDNs appearing in 448 files and those transmitted across the network as part of protocols 449 are expected to be in either ASCII form (including A-labels) or to 450 contain U-labels, rather than being in forms requiring mapping or 451 other conversions. 453 Other examples of processing for localization might be applied, 454 especially to direct user input, at this point. They include 455 interpreting various characters as separating domain name components 456 from each other (label separators) because they either look like 457 periods or are used to separate sentences, mapping halfwidth or 458 fullwidth East Asian characters to the common form permitted in 459 labels, or giving special treatment to characters whose presentation 460 forms are dependent only on placement in the label. Such 461 localization changes are also outside the scope of this 462 specification. 464 Recommendations for preprocessing for global contexts (i.e., when 465 local considerations do not apply or cannot be used) and for maximum 466 interoperability with labels that might have been specified under 467 liberal readings of IDNA2003 are given in [IDNA2008-Rationale]. It 468 is important to note that the intent of these specifications is that 469 labels in application protocols, files, or links are intended to be 470 in U-label or A-label form. Preprocessing MUST NOT map a character 471 that is valid in a label as specified elsewhere in this document or 472 in [IDNA2008-Tables] into another character. Excessively liberal use 473 of preprocessing, especially to strings stored in files, poses a 474 threat to consistent and predictable behavior for the user even if 475 not to actual interoperability. 477 Because these transformations are local, it is important that domain 478 names that might be passed between systems (e.g., in IRIs) be 479 U-labels or A-labels and not forms that might be accepted locally as 480 a consequence of this step. This step is not standardized as part of 481 IDNA, and is not further specified here. 483 5.4. A-label Input 485 If the input to this procedure appears to be an A-label (i.e., it 486 starts in "xn--"), the lookup application MAY attempt to convert it 487 to a U-label and apply the tests of Section 5.5 and the conversion of 488 Section 5.6 to that form. If the label is converted to Unicode 489 (i.e., to U-label form) using the Punycode decoding algorithm, then 490 the processing specified in those two sections MUST be performed, and 491 the label MUST be rejected if the resulting label is not identical to 492 the original. See also Section 6.1. 494 That conversion and testing SHOULD be performed if the domain name 495 will later be presented to the user in native character form (this 496 requires that the lookup application be IDNA-aware). If those steps 497 are not performed, the lookup process SHOULD at least make tests to 498 determine that the string is actually an A-label, examining it for 499 the invalid formats specified in the Punycode decoding specification. 500 Applications that are not IDNA-aware will obviously omit that 501 testing; others MAY treat the string as opaque to avoid the 502 additional processing at the expense of providing less protection and 503 information to users. 505 5.5. Validation and Character List Testing 507 As with the registration procedure, the Unicode string is checked to 508 verify that all characters that appear in it are valid as input to 509 IDNA lookup processing. As discussed above and in 510 [IDNA2008-Rationale], the lookup check is more liberal than the 511 registration one. Putative labels with any of the following 512 characteristics MUST BE rejected prior to DNS lookup: 514 o Labels containing code points that are unassigned in the version 515 of Unicode being used by the application, i.e., in the 516 "Unassigned" Unicode category or the UNASSIGNED category of 517 [IDNA2008-Tables]. 519 o Labels that are not in NFC form. 521 o Labels containing prohibited code points, i.e., those that are 522 assigned to the "DISALLOWED" category in the permitted character 523 table [IDNA2008-Tables]. 525 o Labels containing code points that are shown in the permitted 526 character table as requiring a contextual rule and that are 527 flagged as requiring exceptional special processing on lookup 528 ("CONTEXTJ" in the Tables) but do not conform to that rule. 530 o Labels containing other code points that are shown in the 531 permitted character table as requiring a contextual rule 532 ("CONTEXTO" in the tables), but for which no such rule appears in 533 the table of rules. With the exception in the rule immediately 534 above, applications resolving DNS names or carrying out equivalent 535 operations are not required to test contextual rules, only to 536 verify that a rule exists. 538 o Labels whose first character is a combining mark. 540 In addition, the application SHOULD apply the following test. The 541 test may be omitted in special circumstances, such as when the lookup 542 application knows that the conditions are enforced elsewhere, because 543 an attempt to look up and resolve such strings will almost certainly 544 lead to a DNS lookup failure except when wildcards are present in the 545 zone. However, applying the test is likely to give much better 546 information about the reason for a lookup failure -- information that 547 may be usefully passed to the user when that is feasible -- then DNS 548 resolution failure information alone. In any event, lookup 549 applications should avoid attempting to resolve labels that are 550 invalid under that test. 552 o Verification that the string is compliant with the requirements 553 for right to left characters, specified in [IDNA2008-BIDI]. 555 For all other strings, the lookup application MUST rely on the 556 presence or absence of labels in the DNS to determine the validity of 557 those labels and the validity of the characters they contain. If 558 they are registered, they are presumed to be valid; if they are not, 559 their possible validity is not relevant. A lookup application that 560 declines to process a string that conforms to the above rules and 561 look it up in the DNS is not in conformance with this protocol. 563 5.6. Punycode Conversion 565 The validated string, a U-label, is converted to an A-label using the 566 Punycode algorithm with the ACE prefix added. 568 5.7. DNS Name Resolution 570 The A-label is looked up in the DNS, using normal DNS resolver 571 procedures. 573 6. Name Server Considerations 575 [[anchor18: Note in draft: If we really want this document to contain 576 only information that is necessary to proper implementation of IDNA 577 by implementers who are familiar with the DNS, the material in this 578 section is either tutorial, explanatory, or totally unnecessary. 579 Should some or all of it be moved back to Rationale?]] 581 6.1. Processing Non-ASCII Strings 583 Existing DNS servers do not know the IDNA rules for handling non- 584 ASCII forms of IDNs, and therefore need to be shielded from them. 585 All existing channels through which names can enter a DNS server 586 database (for example, master files (as described in RFC 1034) and 587 DNS update messages [RFC2136]) are IDN-unaware because they predate 588 IDNA. Other sections of this document provide the needed shielding 589 by ensuring that internationalized domain names entering DNS server 590 databases through such channels have already been converted to their 591 equivalent ASCII A-label forms. 593 Because of the design of the algorithms in Section 4 and Section 5 (a 594 domain name containing only ASCII codepoints can not be converted to 595 an A-label), there can not be more than one A-label form for any 596 given U-label. 598 As specified in RFC 2181 [RFC2181], the DNS protocol explicitly 599 allows domain labels to contain octets beyond the ASCII range 600 (0000..007F), and this document does not change that. Note, however, 601 that there is no defined interpretation of octets 0080..00FF as 602 characters. If labels containing these octets are returned to 603 applications, unpredictable behavior could result. The A-label form, 604 which cannot contain those characters, is the only standard 605 representation for internationalized labels in the DNS protocol. 607 6.2. DNSSEC Authentication of IDN Domain Names 609 DNS Security [RFC2535] is a method for supplying cryptographic 610 verification information along with DNS messages. Public Key 611 Cryptography is used in conjunction with digital signatures to 612 provide a means for a requester of domain information to authenticate 613 the source of the data. This ensures that it can be traced back to a 614 trusted source, either directly or via a chain of trust linking the 615 source of the information to the top of the DNS hierarchy. 617 IDNA specifies that all internationalized domain names served by DNS 618 servers that cannot be represented directly in ASCII must use the 619 A-label form. Conversion to A-labels must be performed prior to a 620 zone being signed by the private key for that zone. Because of this 621 ordering, it is important to recognize that DNSSEC authenticates a 622 domain name containing A-labels or conventional LDH-labels, not 623 U-labels. In the presence of DNSSEC, no form of a zone file or query 624 response that contains a U-label may be signed or the signature 625 validated. 627 One consequence of this for sites deploying IDNA in the presence of 628 DNSSEC is that any special purpose proxies or forwarders used to 629 transform user input into IDNs must be earlier in the lookup flow 630 than DNSSEC authenticating nameservers for DNSSEC to work. 632 6.3. Root and other DNS Server Considerations 634 IDNs in A-label form will generally be somewhat longer than current 635 domain names, so the bandwidth needed by the root servers is likely 636 to go up by a small amount. Also, queries and responses for IDNs 637 will probably be somewhat longer than typical queries historically, 638 so EDNS0 [RFC2671] support may be more important (otherwise, queries 639 and responses may be forced to go to TCP instead of UDP). 641 7. Security Considerations 643 The general security principles and issues for IDNA appear in 644 [IDNA2008-Defs] with additional explanation in [IDNA2008-Rationale]. 645 The comments below are specific to the registration and loopup 646 protocols specified in this document, but should be read in the 647 context of the material in the first of those documents and the 648 definitions and specifications, identified there, on which this one 649 depends. 651 This memo describes procedures for registering and looking up labels 652 that are not compatible with the preferred syntax described in the 653 base DNS specifications (STD13 [RFC1034] [RFC1035] and Host 654 Requirements [RFC1123]) because they contain non-ASCII characters. 655 These procedures depend on the use of a special ASCII-compatible 656 encoding form that contains only characters permitted in host names 657 by those earlier specifications. The encoding used is Punycode 658 [RFC3492]. No security issues such as string length increases or new 659 allowed values are introduced by the encoding process or the use of 660 these encoded values, apart from those introduced by the ACE encoding 661 itself. 663 Domain names (or portions of them) are sometimes compared against a 664 set of domains to be given special treatment if a match occurs, e.g., 665 treated as more privileged than others or blocked in some way. In 666 such situations, it is especially important that the comparisons be 667 done properly, as specified in Requirement 2 of Section 3.1. For 668 labels already in ASCII form (i.e., are LDH-labels or A-labels), the 669 proper comparison reduces to the same case-insensitive ASCII 670 comparison that has always been used for ASCII labels. 672 The introduction of IDNA means that any existing labels that start 673 with the ACE prefix would be construed as A-labels, at least until 674 they failed one of the relevant tests, whether or not that was the 675 intent of the zone administrator or registrant. There is no evidence 676 that this has caused any practical problems since RFC 3490 was 677 adopted, but the risk still exists in principle. 679 8. IANA Considerations 681 IANA actions for this version of IDNA are specified in 682 [IDNA2008-Tables] and discussed informally in [IDNA2008-Rationale]. 683 The components of IDNA described in this document do not require any 684 IANA actions. 686 9. Contributors 688 While the listed editor held the pen, the original versions of this 689 document represent the joint work and conclusions of an ad hoc design 690 team consisting of the editor and, in alphabetic order, Harald 691 Alvestrand, Tina Dam, Patrik Faltstrom, and Cary Karp. This document 692 draws significantly on the original version of IDNA [RFC3490] both 693 conceptually and for specific text. This second-generation version 694 would not have been possible without the work that went into that 695 first version and its authors, Patrik Faltstrom, Paul Hoffman, and 696 Adam Costello. While Faltstrom was actively involved in the creation 697 of this version, Hoffman and Costello were not and should not be held 698 responsible for any errors or omissions. 700 10. Acknowledgements 702 This revision to IDNA would have been impossible without the 703 accumulated experience since RFC 3490 was published and resulting 704 comments and complaints of many people in the IETF, ICANN, and other 705 communities, too many people to list here. Nor would it have been 706 possible without RFC 3490 itself and the efforts of the Working Group 707 that defined it. Those people whose contributions are acknowledged 708 in RFC 3490, [RFC4690], and [IDNA2008-Rationale] were particularly 709 important. 711 Specific textual changes were incorporated into this document after 712 suggestions from the other contributors, Stephane Bortzmeyer, Mark 713 Davis, Paul Hoffman, Kent Karlsson, Erik van der Poel, Marcos Sanz, 714 Andrew Sullivan, Ken Whistler, and other WG participants. Special 715 thanks are due to Paul Hoffman for permission to extract material 716 from his Internet-Draft to form the basis for Appendix A 718 11. References 720 11.1. Normative References 722 [IDNA2008-BIDI] 723 Alvestrand, H. and C. Karp, "An updated IDNA criterion for 724 right-to-left scripts", July 2008, . 727 [IDNA2008-Defs] 728 Klensin, J., "Internationalized Domain Names for 729 Applications (IDNA): Definitions and Document Framework", 730 November 2008, . 733 [IDNA2008-Tables] 734 Faltstrom, P., "The Unicode Codepoints and IDNA", 735 July 2008, . 738 A version of this document is available in HTML format at 739 http://stupid.domain.name/idnabis/ 740 draft-ietf-idnabis-tables-02.html 742 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 743 STD 13, RFC 1034, November 1987. 745 [RFC1035] Mockapetris, P., "Domain names - implementation and 746 specification", STD 13, RFC 1035, November 1987. 748 [RFC1123] Braden, R., "Requirements for Internet Hosts - Application 749 and Support", STD 3, RFC 1123, October 1989. 751 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 752 Requirement Levels", BCP 14, RFC 2119, March 1997. 754 [RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode 755 for Internationalized Domain Names in Applications 756 (IDNA)", RFC 3492, March 2003. 758 [Unicode-PropertyValueAliases] 759 The Unicode Consortium, "Unicode Character Database: 760 PropertyValueAliases", March 2008, . 763 [Unicode-RegEx] 764 The Unicode Consortium, "Unicode Technical Standard #18: 765 Unicode Regular Expressions", May 2005, 766 . 768 [Unicode-Scripts] 769 The Unicode Consortium, "Unicode Standard Annex #24: 770 Unicode Script Property", February 2008, 771 . 773 [Unicode-UAX15] 774 The Unicode Consortium, "Unicode Standard Annex #15: 775 Unicode Normalization Forms", 2006, 776 . 778 11.2. Informative References 780 [ASCII] American National Standards Institute (formerly United 781 States of America Standards Institute), "USA Code for 782 Information Interchange", ANSI X3.4-1968, 1968. 784 ANSI X3.4-1968 has been replaced by newer versions with 785 slight modifications, but the 1968 version remains 786 definitive for the Internet. 788 [IDNA2008-Rationale] 789 Klensin, J., Ed., "Internationalizing Domain Names for 790 Applications (IDNA): Issues, Explanation, and Rationale", 791 November 2008, . 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 [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS 799 Specification", RFC 2181, July 1997. 801 [RFC2535] Eastlake, D., "Domain Name System Security Extensions", 802 RFC 2535, March 1999. 804 [RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", 805 RFC 2671, August 1999. 807 [RFC3490] Faltstrom, P., Hoffman, P., and A. Costello, 808 "Internationalizing Domain Names in Applications (IDNA)", 809 RFC 3490, March 2003. 811 [RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep 812 Profile for Internationalized Domain Names (IDN)", 813 RFC 3491, March 2003. 815 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 816 Resource Identifier (URI): Generic Syntax", STD 66, 817 RFC 3986, January 2005. 819 [RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource 820 Identifiers (IRIs)", RFC 3987, January 2005. 822 [RFC4690] Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and 823 Recommendations for Internationalized Domain Names 824 (IDNs)", RFC 4690, September 2006. 826 [RFC4952] Klensin, J. and Y. Ko, "Overview and Framework for 827 Internationalized Email", RFC 4952, July 2007. 829 [Unicode] The Unicode Consortium, "The Unicode Standard, Version 830 5.0", 2007. 832 Boston, MA, USA: Addison-Wesley. ISBN 0-321-48091-0 834 Appendix A. Summary of Major Changes from IDNA2003 836 1. Update base character set from Unicode 3.2 to Unicode version- 837 agnostic. 839 2. Separate the definitions for the "registration" and "lookup" 840 activities. 842 3. Disallow symbol and punctuation characters except where special 843 exceptions are necessary. 845 4. Remove the mapping and normalization steps from the protocol and 846 have them instead done by the applications themselves, possibly 847 in a local fashion, before invoking the protocol. 849 5. Change the way that the protocol specifies which characters are 850 allowed in labels from "humans decide what the table of 851 codepoints contains" to "decision about codepoints are based on 852 Unicode properties plus a small exclusion list created by 853 humans". 855 6. Introduce the new concept of characters that can be used only in 856 specific contexts. 858 7. Allow typical words and names in languages such as Dhivehi and 859 Yiddish to be expressed. 861 8. Make bidirectional domain names (delimited strings of labels, 862 not just labels standing on their own) display in a non- 863 surprising fashion whether they appear in obvious domain name 864 contexts or as part of running text in paragraphs. 866 9. Remove the dot separator from the mandatory part of the 867 protocol. 869 10. Make some currently-valid labels that are not actually IDNA 870 labels invalid. 872 Appendix B. Change Log 874 [[anchor27: RFC Editor: Please remove this appendix.]] 876 B.1. Changes between Version -00 and -01 of draft-ietf-idnabis-protocol 878 o Corrected discussion of SRV records. 880 o Several small corrections for clarity. 882 o Inserted more "open issue" placeholders. 884 B.2. Version -02 886 o Rewrote the "conversion to Unicode" text in Section 5.2 as 887 requested on-list. 889 o Added a comment (and reference) about EDNS0 to the "DNS Server 890 Conventions" section, which was also retitled. 892 o Made several editorial corrections and improvements in response to 893 various comments. 895 o Added several new discussion placeholder anchors and updated some 896 older ones. 898 B.3. Version -03 900 o Trimmed change log, removing information about pre-WG drafts. 902 o Incorporated a number of changes suggested by Marcos Sanz in his 903 note of 2008.07.17 and added several more placeholder anchors. 905 o Several minor editorial corrections and improvements. 907 o "Editor" designation temporarily removed because the automatic 908 posting machinery does not accept it. 910 B.4. Version -04 912 o Removed Contextual Rule appendices for transfer to Tables. 914 o Several changes, including removal of discussion anchors, based on 915 discussions at IETF 72 (Dublin) 917 o Rewrote the preprocessing material (Section 5.3) somewhat. 919 B.5. Version -05 921 o Updated part of the A-label input explanation (Section 5.4) per 922 note from Erik van der Poel. 924 B.6. Version -06 926 o Corrected a few typographical errors. 928 o Incorporated the material (formerly in Rationale) on the 929 relationship between IDNA2003 and IDNA2008 as an appendix and 930 pointed to the new definitions document. 932 o Text modified in several places to recognize the dangers of 933 interaction between DNS wildcards and IDNs. 935 o Text added to be explicit about the handling of edge and failure 936 cases in Punycode encoding and decoding. 938 o Revised for consistency with the new Definitions document and to 939 make the text read more smoothly. 941 Author's Address 943 John C Klensin 944 1770 Massachusetts Ave, Ste 322 945 Cambridge, MA 02140 946 USA 948 Phone: +1 617 245 1457 949 Email: john+ietf@jck.com 951 Full Copyright Statement 953 Copyright (C) The IETF Trust (2008). 955 This document is subject to the rights, licenses and restrictions 956 contained in BCP 78, and except as set forth therein, the authors 957 retain all their rights. 959 This document and the information contained herein are provided on an 960 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 961 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND 962 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS 963 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 964 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 965 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 967 Intellectual Property 969 The IETF takes no position regarding the validity or scope of any 970 Intellectual Property Rights or other rights that might be claimed to 971 pertain to the implementation or use of the technology described in 972 this document or the extent to which any license under such rights 973 might or might not be available; nor does it represent that it has 974 made any independent effort to identify any such rights. Information 975 on the procedures with respect to rights in RFC documents can be 976 found in BCP 78 and BCP 79. 978 Copies of IPR disclosures made to the IETF Secretariat and any 979 assurances of licenses to be made available, or the result of an 980 attempt made to obtain a general license or permission for the use of 981 such proprietary rights by implementers or users of this 982 specification can be obtained from the IETF on-line IPR repository at 983 http://www.ietf.org/ipr. 985 The IETF invites any interested party to bring to its attention any 986 copyrights, patents or patent applications, or other proprietary 987 rights that may cover technology that may be required to implement 988 this standard. Please address the information to the IETF at 989 ietf-ipr@ietf.org.