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Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Unused Reference: 'Unicode-PropertyValueAliases' is defined on line 778, but no explicit reference was found in the text == Unused Reference: 'Unicode-RegEx' is defined on line 783, but no explicit reference was found in the text == Unused Reference: 'Unicode-Scripts' is defined on line 788, but no explicit reference was found in the text == Unused Reference: 'ASCII' is defined on line 800, but no explicit reference was found in the text -- Possible downref: Non-RFC (?) normative reference: ref. 'IDNA2008-BIDI' -- Possible downref: Non-RFC (?) normative reference: ref. 'Unicode-PropertyValueAliases' -- Possible downref: Non-RFC (?) normative reference: ref. 'Unicode-RegEx' -- Possible downref: Non-RFC (?) normative reference: ref. 'Unicode-Scripts' -- Possible downref: Non-RFC (?) normative reference: ref. 'Unicode-UAX15' -- Obsolete informational reference (is this intentional?): RFC 2535 (Obsoleted by RFC 4033, RFC 4034, RFC 4035) -- Obsolete informational reference (is this intentional?): RFC 2671 (Obsoleted by RFC 6891) -- Obsolete informational reference (is this intentional?): RFC 3490 (Obsoleted by RFC 5890, RFC 5891) -- Obsolete informational reference (is this intentional?): RFC 3491 (Obsoleted by RFC 5891) -- Obsolete informational reference (is this intentional?): RFC 4952 (Obsoleted by RFC 6530) Summary: 1 error (**), 0 flaws (~~), 5 warnings (==), 19 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group J. Klensin 3 Internet-Draft December 7, 2008 4 Obsoletes: 3490, 3491 5 (if approved) 6 Updates: 3492 (if approved) 7 Intended status: Standards Track 8 Expires: June 10, 2009 10 Internationalized Domain Names in Applications (IDNA): Protocol 11 draft-ietf-idnabis-protocol-08.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 June 10, 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 . . . . . . . . . . . . . . . . . . . . . . . . . 4 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 . . . . . . . . . 7 64 4.3.1. Input Format . . . . . . . . . . . . . . . . . . . . . 7 65 4.3.2. Rejection of Characters that are not Permitted . . . . 8 66 4.3.3. Label Validation . . . . . . . . . . . . . . . . . . . 8 67 4.3.4. Registration Validation Summary . . . . . . . . . . . 9 68 4.4. Registry Restrictions . . . . . . . . . . . . . . . . . . 9 69 4.5. Punycode Conversion . . . . . . . . . . . . . . . . . . . 10 70 4.6. Insertion in the Zone . . . . . . . . . . . . . . . . . . 10 71 5. Domain Name Lookup Protocol . . . . . . . . . . . . . . . . . 10 72 5.1. Label String Input . . . . . . . . . . . . . . . . . . . . 10 73 5.2. Conversion to Unicode . . . . . . . . . . . . . . . . . . 11 74 5.3. Character Changes in Preprocessing or the User 75 Interface . . . . . . . . . . . . . . . . . . . . . . . . 11 76 5.4. A-label Input . . . . . . . . . . . . . . . . . . . . . . 12 77 5.5. Validation and Character List Testing . . . . . . . . . . 12 78 5.6. Punycode Conversion . . . . . . . . . . . . . . . . . . . 14 79 5.7. DNS Name Resolution . . . . . . . . . . . . . . . . . . . 14 80 6. Name Server Considerations . . . . . . . . . . . . . . . . . . 14 81 6.1. Processing Non-ASCII Strings . . . . . . . . . . . . . . . 14 82 6.2. DNSSEC Authentication of IDN Domain Names . . . . . . . . 15 83 6.3. Root and other DNS Server Considerations . . . . . . . . . 15 84 7. Security Considerations . . . . . . . . . . . . . . . . . . . 15 85 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 86 9. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 16 87 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17 88 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17 89 11.1. Normative References . . . . . . . . . . . . . . . . . . . 17 90 11.2. Informative References . . . . . . . . . . . . . . . . . . 18 91 Appendix A. Summary of Major Changes from IDNA2003 . . . . . . . 19 92 Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 20 93 B.1. Changes between Version -00 and -01 of 94 draft-ietf-idnabis-protocol . . . . . . . . . . . . . . . 20 95 B.2. Version -02 . . . . . . . . . . . . . . . . . . . . . . . 20 96 B.3. Version -03 . . . . . . . . . . . . . . . . . . . . . . . 21 97 B.4. Version -04 . . . . . . . . . . . . . . . . . . . . . . . 21 98 B.5. Version -05 . . . . . . . . . . . . . . . . . . . . . . . 21 99 B.6. Version -06 . . . . . . . . . . . . . . . . . . . . . . . 21 100 B.7. Version -07 . . . . . . . . . . . . . . . . . . . . . . . 22 101 B.8. Version -08 . . . . . . . . . . . . . . . . . . . . . . . 22 102 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 22 103 Intellectual Property and Copyright Statements . . . . . . . . . . 23 105 1. Introduction 107 This document supplies the protocol definition for a revised and 108 updated specification for internationalized domain names. Essential 109 definitions and terminology for understanding this document and a 110 road map of the collection of documents that make up IDNA2008 appear 111 in [IDNA2008-Defs]. Appendix A discusses the relationship between 112 this specification and the earlier version of IDNA (referred to here 113 as "IDNA2003") and the rationale for these changes, along with 114 considerable explanatory material and advice to zone administrators 115 who support IDNs is provided in another documents, notably 116 [IDNA2008-Rationale]. 118 IDNA works by allowing applications to use certain ASCII string 119 labels (beginning with a special prefix) to represent non-ASCII name 120 labels. Lower-layer protocols need not be aware of this; therefore 121 IDNA does not depend on changes to any infrastructure. In 122 particular, IDNA does not depend on any changes to DNS servers, 123 resolvers, or protocol elements, because the ASCII name service 124 provided by the existing DNS is entirely sufficient for IDNA. 126 IDNA is applied only to DNS labels. Standards for combining labels 127 into fully-qualified domain names and parsing labels out of those 128 names are covered in the base DNS standards [RFC1034] [RFC1035] and 129 their various updates. An application may, of course, apply locally- 130 appropriate conventions to the presentation forms of domain names as 131 discussed in [IDNA2008-Rationale]. 133 While they share terminology, reference data, and some operations, 134 this document describes two separate protocols, one for IDN 135 registration (Section 4) and one for IDN lookup (Section 5). 137 1.1. Discussion Forum 139 [[anchor3: RFC Editor: please remove this section.]] 141 This work is being discussed in the IETF IDNABIS WG and on the 142 mailing list idna-update@alvestrand.no 144 2. Terminology 146 General terminology applicable to IDNA, but with meanings familiar to 147 those who have worked with Unicode or other character set standards 148 and the DNS, appears in [IDNA2008-Defs]. Terminology that is an 149 integral, normative, part of the IDNA definition, including the 150 definitions of "ACE", appears in that document as well. Familiarity 151 with the terminology materials in that document is assumed for 152 reading this one. The reader of this document is assumed to be 153 familiar with DNS-specific terminology as defined in RFC 1034 154 [RFC1034]. 156 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 157 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 158 document are to be interpreted as described in BCP 14, RFC 2119 159 [RFC2119]. 161 3. Requirements and Applicability 163 3.1. Requirements 165 IDNA conformance means adherence to the following requirements: 167 1. Whenever a domain name is put into an IDN-unaware domain name 168 slot (see Section 2 and [IDNA2008-Defs]), it MUST contain only 169 ASCII characters (i.e., must be either an A-label or an LDH- 170 label), or must be a label associated with a DNS application that 171 is not subject to either IDNA or the historical recommendations 172 for "hostname"-style names [RFC1034]. 174 2. Comparison of labels MUST be done on equivalent forms: either 175 both A-Label forms or both U-Label forms. Because A-labels and 176 U-labels can be transformed into each other without loss of 177 information, these comparisons are equivalent. However, when a 178 pair of putative A-labels are compared, the comparison MUST use 179 an ASCII case-insensitive comparison (as with all comparisons of 180 ASCII DNS labels). Comparisons on putative U-labels must test 181 that the two strings are identical, without case-folding or other 182 intermediate steps. Note that it is not necessary to verify that 183 labels are valid in order to compare them. In many cases, 184 verification of validity (that the strings actually are A-labels 185 or U-labels) may be important for other reasons and SHOULD be 186 performed. 188 3. Labels being registered MUST conform to the requirements of 189 Section 4. Labels being looked up and the lookup process MUST 190 conform to the requirements of Section 5. 192 3.2. Applicability 194 IDNA is applicable to all domain names in all domain name slots 195 except where it is explicitly excluded. It is not applicable to 196 domain name slots which do not use the LDH syntax rules. 198 This implies that IDNA is applicable to many protocols that predate 199 IDNA. Note that IDNs occupying domain name slots in those older 200 protocols MUST be in A-label form until and unless those protocols 201 and implementations of them are upgraded to be IDN-aware. IDNs 202 actually appearing in DNS queries or responses MUST be A-labels. 204 3.2.1. DNS Resource Records 206 IDNA applies only to domain names in the NAME and RDATA fields of DNS 207 resource records whose CLASS is IN. 209 There are currently no other exclusions on the applicability of IDNA 210 to DNS resource records. Applicability depends entirely on the 211 CLASS, and not on the TYPE except as noted below. This will remain 212 true, even as new types are defined, unless there is a compelling 213 reason for a new type that requires type-specific rules. The special 214 naming conventions applicable to SRV records are examples of type- 215 specific rules that are incompatible with IDNA coding. Hence the 216 first two labels (the ones required to start in "_") on a record with 217 TYPE SRV MUST NOT be A-labels or U-labels (while it would be possible 218 to write a non-ASCII string with a leading underscore, conversion to 219 an A-label would be impossible without loss of information because 220 the underscore is not a letter, digit, or hyphen and is consequently 221 DISALLOWED in IDNs). Of course, those labels may be part of a domain 222 that uses IDN labels at higher levels in the tree. 224 3.2.2. Non-domain-name Data Types Stored in the DNS 226 Although IDNA enables the representation of non-ASCII characters in 227 domain names, that does not imply that IDNA enables the 228 representation of non-ASCII characters in other data types that are 229 stored in domain names, specifically in the RDATA field for types 230 that have structured RDATA format. For example, an email address 231 local part is stored in a domain name in the RNAME field as part of 232 the RDATA of an SOA record (hostmaster@example.com would be 233 represented as hostmaster.example.com). IDNA specifically does not 234 update the existing email standards, which allow only ASCII 235 characters in local parts. Even though work is in progress to define 236 internationalization for email addresses [RFC4952], changes to the 237 email address part of the SOA RDATA would require action in, or 238 updates to, other standards, specifically those that specify the 239 format of the SOA RR. 241 4. Registration Protocol 243 This section defines the procedure for registering an IDN. The 244 procedure is implementation independent; any sequence of steps that 245 produces exactly the same result for all labels is considered a valid 246 implementation. 248 Note that, while the registration and lookup protocols (Section 5) 249 are very similar in most respects, they are different and 250 implementers should carefully follow the steps they are implementing. 252 4.1. Proposed label 254 The registrant submits a request for an IDN. The user typically 255 produces the request string by the keyboard entry of a character 256 sequence in the local native character set (which might, of course, 257 be Unicode). 259 4.2. Conversion to Unicode and Normalization 261 Some system routine, or a localized front-end to the IDNA process, 262 ensures that the proposed label is a Unicode string or converts it to 263 one as appropriate. Independent of its source form, the string MUST 264 be in Unicode Normalization Form C (NFC [Unicode-UAX15]) before 265 further processing in this protocol. 267 As a local implementation choice, the implementation MAY choose to 268 map some forbidden characters to permitted characters (for instance 269 mapping uppercase characters to lowercase ones), displaying the 270 result to the user, and allowing processing to continue. This should 271 be done very conservatively to prevent interoperability problems with 272 lookup applications that do not follow exactly the same rules. In 273 particular, it is strongly recommended that, to avoid any possible 274 ambiguity, entities responsible for zone files ("registries") accept 275 registrations only for A-labels (to be converted to U-labels by the 276 registry as discussed above) or U-labels actually produced from 277 A-labels, not forms expected to be converted by some other process. 279 4.3. Permitted Character and Label Validation 281 4.3.1. Input Format 283 [[anchor8: Note in -07 -- this section was formerly the second 284 paragraph of Section 4.1. It may need additional work; suggestions 285 welcome.]] 287 The registry MAY permit submission of labels in A-label form. If it 288 does so, it MUST perform a conversion to a U-label, perform the steps 289 and tests described below, and verify that the A-label produced by 290 the step in Section 4.5 matches the one provided as input. If, for 291 some reason, it does not, the registration MUST be rejected. If the 292 conversion to a U-label is not performed, the registry MUST verify 293 that the A-label is superficially valid, i.e., that it does not 294 violate any of the rules of Punycode [RFC3492] encoding such as the 295 prohibition on trailing hyphen-minus, appearance of non-basic 296 characters before the delimiter, and so on. Invalid strings that 297 appear to be A-labels MUST NOT be placed in DNS zones. 299 4.3.2. Rejection of Characters that are not Permitted 301 The candidate Unicode string is checked to verify that characters 302 that IDNA does not permit do not appear in it. Those characters are 303 identified in the "DISALLOWED" and "UNASSIGNED" lists that are 304 specified in [IDNA2008-Tables] and described informally in 305 [IDNA2008-Rationale]. Characters that are either DISALLOWED or 306 UNASSIGNED MUST NOT be part of labels to be processed for 307 registration in the DNS. 309 4.3.3. Label Validation 311 The proposed label (in the form of a Unicode string, i.e., a putative 312 U-label) is then examined, performing tests that require examination 313 of more than one character. 315 4.3.3.1. Rejection of Hyphen Sequences in U-labels 317 The Unicode string MUST NOT contain "--" (two consecutive hyphens) in 318 the third and fourth character positions when the label is considered 319 in "on the wire" order. 321 4.3.3.2. Leading Combining Marks 323 The first character of the string (when the label is considered in 324 "on the wire" order) is examined to verify that it is not a combining 325 mark (or combining character) (see The Unicode Standard, Section 2.11 326 [Unicode] for an exact definition). If it is a combining mark, the 327 string MUST NOT be registered. 329 4.3.3.3. Contextual Rules 331 Each code point is checked for its identification as a character 332 requiring contextual processing for registration (the list of 333 characters appears as the combination of CONTEXTJ and CONTEXTO in 334 [IDNA2008-Tables] as do the contextual rules themselves). If that 335 indication appears, the table of contextual rules is checked for a 336 rule for that character. If no rule is found, the proposed label is 337 rejected and MUST NOT be installed in a zone file. If one is found, 338 it is applied (typically as a test on the entire label or on adjacent 339 characters within the label). If the application of the rule does 340 not conclude that the character is valid in context, the proposed 341 label MUST BE rejected. (See the IANA Considerations: IDNA Context 342 Registry section of [IDNA2008-Tables].) 344 These contextual rules are required to support the use of characters 345 that could be used, under other conditions, to produce misleading 346 labels or to cause unacceptable ambiguity in label matching and 347 interpretation. For example, labels containing invisible ("zero- 348 width") characters may be permitted in context with characters whose 349 presentation forms are significantly changed by the presence or 350 absence of the zero-width characters, while other labels in which 351 zero-width characters appear may be rejected. 353 4.3.3.4. Labels Containing Characters Written Right to Left 355 Special tests are required for strings containing characters that are 356 normally written from right to left. The criteria for classifying 357 characters in terms of directionality are identified in the "Bidi" 358 document [IDNA2008-BIDI] in this series. That document also 359 describes conditions for strings that contain one or more of those 360 characters to be U-labels. The tests for those conditions, specified 361 there, are applied. Strings that contain right to left characters 362 that do not conform to the IDNA Bidi rules MUST NOT be inserted as 363 labels in zone files. 365 4.3.4. Registration Validation Summary 367 Strings that contain at least one non-ASCII character, have been 368 produced by the steps above, whose contents pass the above tests, and 369 are 63 or fewer characters long in ACE form (see Section 4.5), are 370 U-labels. 372 To summarize, tests are made in Section 4.3 for invalid characters, 373 invalid combinations of characters, and for labels that are invalid 374 even if the characters they contain are valid individually. 376 4.4. Registry Restrictions 378 Registries at all levels of the DNS, not just the top level, are 379 expected to establish policies about the labels that may be 380 registered, and for the processes associated with that action. While 381 exact policies are not specified as part of IDNA2008 and it is 382 expected that different registries may specify different policies, 383 there SHOULD be policies. Even a trivial policy (e.g., "anything can 384 be registered in this zone that can be represented as an A-label - 385 U-label pair") has value because it provides notice to users and 386 applications implementers that the registry cannot be relied upon to 387 provide even minimal user-protection restrictions. These per- 388 registry policies and restrictions are an essential element of the 389 IDNA registration protocol even for registries (and corresponding 390 zone files) deep in the DNS hierarchy. As discussed in 391 [IDNA2008-Rationale], such restrictions have always existed in the 392 DNS. That document also contains a discussion and recommendations 393 about possible types of rules. 395 The string produced by the above steps is checked and processed as 396 appropriate to local registry restrictions. Application of those 397 registry restrictions may result in the rejection of some labels or 398 the application of special restrictions to others. 400 4.5. Punycode Conversion 402 The resulting U-label is converted to an A-label. The A-label, more 403 precisely defined elsewhere, is the encoding of the U-label according 404 to the Punycode algorithm [RFC3492] with the ACE prefix "xn--" added 405 at the beginning of the string. The resulting string much, of 406 course, conform to the length limits imposed by the DNS. This 407 document updates RFC 3492 only to the extent of replacing the 408 reference to the discussion of the ACE prefix. The ACE prefix is now 409 specified in this document rather than as part of RFC 3490 or 410 Nameprep [RFC3491] but is the same in both sets of documents. 412 The failure conditions identified in the Punycode encoding procedure 413 cannot occur if the input is a U-label as determined by the steps 414 above. 416 4.6. Insertion in the Zone 418 The A-label is registered in the DNS by insertion into a zone. 420 5. Domain Name Lookup Protocol 422 Lookup is conceptually different from registration and different 423 tests are applied on the client. Although some validity checks are 424 necessary to avoid serious problems with the protocol (see 425 Section 5.5ff.), the lookup-side tests are more permissive and rely 426 on the assumption that names that are present in the DNS are valid. 427 That assumption is, however, a weak one because the presence of wild 428 cards in the DNS might cause a string that is not actually registered 429 in the DNS to be successfully looked up. 431 5.1. Label String Input 433 The user supplies a string in the local character set, typically by 434 typing it or clicking on, or copying and pasting, a resource 435 identifier, e.g., a URI [RFC3986] or IRI [RFC3987] from which the 436 domain name is extracted. Alternately, some process not directly 437 involving the user may read the string from a file or obtain it in 438 some other way. Processing in this step and the next two are local 439 matters, to be accomplished prior to actual invocation of IDNA, but 440 at least the two steps in Section 5.2 and Section 5.3 must be 441 accomplished in some way. 443 5.2. Conversion to Unicode 445 The string is converted from the local character set into Unicode, if 446 it is not already Unicode. The exact nature of this conversion is 447 beyond the scope of this document, but may involve normalization as 448 described in Section 4.2. The result MUST be a Unicode string in NFC 449 form. 451 5.3. Character Changes in Preprocessing or the User Interface 453 The Unicode string MAY then be processed to prevent confounding of 454 user expectations. For instance, it might be reasonable, at this 455 step, to convert all upper case characters to lower case, if this 456 makes sense in the user's environment, but even this should be 457 approached with caution due to some edge cases: in the long term, it 458 is probably better for users to understand IDNs strictly in lower- 459 case, U-label, form. More generally, preprocessing may be useful to 460 smooth the transition from IDNA2003, especially for direct user 461 input, but with similar cautions. In general, IDNs appearing in 462 files and those transmitted across the network as part of protocols 463 are expected to be in either ASCII form (including A-labels) or to 464 contain U-labels, rather than being in forms requiring mapping or 465 other conversions. 467 Other examples of processing for localization might be applied, 468 especially to direct user input, at this point. They include 469 interpreting various characters as separating domain name components 470 from each other (label separators) because they either look like 471 periods or are used to separate sentences, mapping halfwidth or 472 fullwidth East Asian characters to the common form permitted in 473 labels, or giving special treatment to characters whose presentation 474 forms are dependent only on placement in the label. Such 475 localization changes are also outside the scope of this 476 specification. 478 Recommendations for preprocessing for global contexts (i.e., when 479 local considerations do not apply or cannot be used) and for maximum 480 interoperability with labels that might have been specified under 481 liberal readings of IDNA2003 are given in [IDNA2008-Rationale]. It 482 is important to note that the intent of these specifications is that 483 labels in application protocols, files, or links are intended to be 484 in U-label or A-label form. Preprocessing MUST NOT map a character 485 that is valid in a label as specified elsewhere in this document or 486 in [IDNA2008-Tables] into another character. Excessively liberal use 487 of preprocessing, especially to strings stored in files, poses a 488 threat to consistent and predictable behavior for the user even if 489 not to actual interoperability. 491 Because these transformations are local, it is important that domain 492 names that might be passed between systems (e.g., in IRIs) be 493 U-labels or A-labels and not forms that might be accepted locally as 494 a consequence of this step. This step is not standardized as part of 495 IDNA, and is not further specified here. 497 5.4. A-label Input 499 If the input to this procedure appears to be an A-label (i.e., it 500 starts in "xn--"), the lookup application MAY attempt to convert it 501 to a U-label and apply the tests of Section 5.5 and the conversion of 502 Section 5.6 to that form. If the label is converted to Unicode 503 (i.e., to U-label form) using the Punycode decoding algorithm, then 504 the processing specified in those two sections MUST be performed, and 505 the label MUST be rejected if the resulting label is not identical to 506 the original. See also Section 6.1. 508 That conversion and testing SHOULD be performed if the domain name 509 will later be presented to the user in native character form (this 510 requires that the lookup application be IDNA-aware). If those steps 511 are not performed, the lookup process SHOULD at least make tests to 512 determine that the string is actually an A-label, examining it for 513 the invalid formats specified in the Punycode decoding specification. 514 Applications that are not IDNA-aware will obviously omit that 515 testing; others MAY treat the string as opaque to avoid the 516 additional processing at the expense of providing less protection and 517 information to users. 519 5.5. Validation and Character List Testing 521 As with the registration procedure described in Section 4, the 522 Unicode string is checked to verify that all characters that appear 523 in it are valid as input to IDNA lookup processing. As discussed 524 above and in [IDNA2008-Rationale], the lookup check is more liberal 525 than the registration one. Putative labels with any of the following 526 characteristics MUST BE rejected prior to DNS lookup: 528 o Labels containing code points that are unassigned in the version 529 of Unicode being used by the application, i.e.,in the UNASSIGNED 530 category of [IDNA2008-Tables]. 532 o Labels that are not in NFC form as defined in [Unicode-UAX15]. 534 o Labels containing prohibited code points, i.e., those that are 535 assigned to the "DISALLOWED" category in the permitted character 536 table [IDNA2008-Tables]. 538 o Labels containing code points that are identified in 539 [IDNA2008-Tables] as "CONTEXTJ", i.e., requiring exceptional 540 contextual rule processing on lookup, but that do not conform to 541 that rule. Note that this implies that a rule much be defined, 542 not missing: a character that requires a contextual rule but for 543 which the rule is missing is treated in this step as having failed 544 to conform to the rule. 546 o Labels containing code points that are identified in in 547 [IDNA2008-Tables] as "CONTEXTO", but for which no such rule 548 appears in the table of rules. Applications resolving DNS names 549 or carrying out equivalent operations are not required to test 550 contextual rules for "CONTEXTO" characters, only to verify that a 551 rule exists (although they MAY make such tests to give better 552 information to the user). 554 o Labels whose first character is a combining mark (see 555 Section 4.3.3.2. 557 In addition, the application SHOULD apply the following test. The 558 test may be omitted in special circumstances, such as when the lookup 559 application knows that the conditions are enforced elsewhere, because 560 an attempt to look up and resolve such strings will almost certainly 561 lead to a DNS lookup failure except when wildcards are present in the 562 zone. However, applying the test is likely to give much better 563 information about the reason for a lookup failure -- information that 564 may be usefully passed to the user when that is feasible -- than DNS 565 resolution failure information alone. In any event, lookup 566 applications should avoid attempting to resolve labels that are 567 invalid under that test. 569 o Verification that the string is compliant with the requirements 570 for right to left characters, specified in [IDNA2008-BIDI]. 572 For all other strings, the lookup application MUST rely on the 573 presence or absence of labels in the DNS to determine the validity of 574 those labels and the validity of the characters they contain. If 575 they are registered, they are presumed to be valid; if they are not, 576 their possible validity is not relevant. A lookup application that 577 declines to process a string that conforms to the rules above and 578 does not look it up in the DNS is not in conformance with this 579 protocol. 581 5.6. Punycode Conversion 583 The validated string, a U-label, is converted to an A-label using the 584 Punycode algorithm with the ACE prefix added. 586 5.7. DNS Name Resolution 588 The A-label is looked up in the DNS, using normal DNS resolver 589 procedures. 591 6. Name Server Considerations 593 [[anchor15: Note in draft: If we really want this document to contain 594 only information that is necessary to proper implementation of IDNA 595 by implementers who are familiar with the DNS, the material in this 596 section is either tutorial, explanatory, or totally unnecessary. 597 Should some or all of it be moved back to Rationale?]] 599 6.1. Processing Non-ASCII Strings 601 Existing DNS servers do not know the IDNA rules for handling non- 602 ASCII forms of IDNs, and therefore need to be shielded from them. 603 All existing channels through which names can enter a DNS server 604 database (for example, master files (as described in RFC 1034) and 605 DNS update messages [RFC2136]) are IDN-unaware because they predate 606 IDNA. Other sections of this document provide the needed shielding 607 by ensuring that internationalized domain names entering DNS server 608 databases through such channels have already been converted to their 609 equivalent ASCII A-label forms. 611 Because of the design of the algorithms in Section 4 and Section 5 (a 612 domain name containing only ASCII codepoints can not be converted to 613 an A-label), there can not be more than one A-label form for any 614 given U-label. 616 As specified in RFC 2181 [RFC2181], the DNS protocol explicitly 617 allows domain labels to contain octets beyond the ASCII range 618 (0000..007F), and this document does not change that. Note, however, 619 that there is no defined interpretation of octets 0080..00FF as 620 characters. If labels containing these octets are returned to 621 applications, unpredictable behavior could result. The A-label form, 622 which cannot contain those characters, is the only standard 623 representation for internationalized labels in the DNS protocol. 625 6.2. DNSSEC Authentication of IDN Domain Names 627 DNS Security (DNSSEC) [RFC2535] is a method for supplying 628 cryptographic verification information along with DNS messages. 629 Public Key Cryptography is used in conjunction with digital 630 signatures to provide a means for a requester of domain information 631 to authenticate the source of the data. This ensures that it can be 632 traced back to a trusted source, either directly or via a chain of 633 trust linking the source of the information to the top of the DNS 634 hierarchy. 636 IDNA specifies that all internationalized domain names served by DNS 637 servers that cannot be represented directly in ASCII MUST use the 638 A-label form. Conversion to A-labels MUST be performed prior to a 639 zone being signed by the private key for that zone. Because of this 640 ordering, it is important to recognize that DNSSEC authenticates a 641 domain name containing A-labels or conventional LDH-labels, not 642 U-labels. In the presence of DNSSEC, no form of a zone file or query 643 response that contains a U-label may be signed or the signature 644 validated. 646 One consequence of this for sites deploying IDNA in the presence of 647 DNSSEC is that any special purpose proxies or forwarders used to 648 transform user input into IDNs must be earlier in the lookup flow 649 than DNSSEC authenticating nameservers for DNSSEC to work. 651 6.3. Root and other DNS Server Considerations 653 IDNs in A-label form will generally be somewhat longer than current 654 domain names, so the bandwidth needed by the root servers is likely 655 to go up by a small amount. Also, queries and responses for IDNs 656 will probably be somewhat longer than typical queries historically, 657 so EDNS0 [RFC2671] support may be more important (otherwise, queries 658 and responses may be forced to go to TCP instead of UDP). 660 7. Security Considerations 662 The general security principles and issues for IDNA appear in 663 [IDNA2008-Defs] with additional explanation in [IDNA2008-Rationale]. 664 The comments below are specific to the registration and loopup 665 protocols specified in this document, but should be read in the 666 context of the material in the first of those documents and the 667 definitions and specifications, identified there, on which this one 668 depends. 670 This memo describes procedures for registering and looking up labels 671 that are not compatible with the preferred syntax described in the 672 base DNS specifications (STD13 [RFC1034] [RFC1035] and Host 673 Requirements [RFC1123]) because they contain non-ASCII characters. 674 These procedures depend on the use of a special ASCII-compatible 675 encoding form that contains only characters permitted in host names 676 by those earlier specifications. The encoding used is Punycode 677 [RFC3492]. No security issues such as string length increases or new 678 allowed values are introduced by the encoding process or the use of 679 these encoded values, apart from those introduced by the ACE encoding 680 itself. 682 Domain names (or portions of them) are sometimes compared against a 683 set of domains to be given special treatment if a match occurs, e.g., 684 treated as more privileged than others or blocked in some way. In 685 such situations, it is especially important that the comparisons be 686 done properly, as specified in Requirement 2 of Section 3.1. For 687 labels already in ASCII form (i.e., are LDH-labels or A-labels), the 688 proper comparison reduces to the same case-insensitive ASCII 689 comparison that has always been used for ASCII labels. 691 The introduction of IDNA means that any existing labels that start 692 with the ACE prefix would be construed as A-labels, at least until 693 they failed one of the relevant tests, whether or not that was the 694 intent of the zone administrator or registrant. There is no evidence 695 that this has caused any practical problems since RFC 3490 was 696 adopted, but the risk still exists in principle. 698 8. IANA Considerations 700 IANA actions for this version of IDNA are specified in 701 [IDNA2008-Tables] and discussed informally in [IDNA2008-Rationale]. 702 The components of IDNA described in this document do not require any 703 IANA actions. 705 9. Contributors 707 While the listed editor held the pen, the original versions of this 708 document represent the joint work and conclusions of an ad hoc design 709 team consisting of the editor and, in alphabetic order, Harald 710 Alvestrand, Tina Dam, Patrik Faltstrom, and Cary Karp. This document 711 draws significantly on the original version of IDNA [RFC3490] both 712 conceptually and for specific text. This second-generation version 713 would not have been possible without the work that went into that 714 first version and its authors, Patrik Faltstrom, Paul Hoffman, and 715 Adam Costello. While Faltstrom was actively involved in the creation 716 of this version, Hoffman and Costello were not and should not be held 717 responsible for any errors or omissions. 719 10. Acknowledgements 721 This revision to IDNA would have been impossible without the 722 accumulated experience since RFC 3490 was published and resulting 723 comments and complaints of many people in the IETF, ICANN, and other 724 communities, too many people to list here. Nor would it have been 725 possible without RFC 3490 itself and the efforts of the Working Group 726 that defined it. Those people whose contributions are acknowledged 727 in RFC 3490, [RFC4690], and [IDNA2008-Rationale] were particularly 728 important. 730 Specific textual changes were incorporated into this document after 731 suggestions from the other contributors, Stephane Bortzmeyer, Vint 732 Cerf, Mark Davis, Paul Hoffman, Kent Karlsson, Erik van der Poel, 733 Marcos Sanz, Andrew Sullivan, Ken Whistler, and other WG 734 participants. Special thanks are due to Paul Hoffman for permission 735 to extract material from his Internet-Draft to form the basis for 736 Appendix A 738 11. References 740 11.1. Normative References 742 [IDNA2008-BIDI] 743 Alvestrand, H. and C. Karp, "An updated IDNA criterion for 744 right-to-left scripts", July 2008, . 747 [IDNA2008-Defs] 748 Klensin, J., "Internationalized Domain Names for 749 Applications (IDNA): Definitions and Document Framework", 750 November 2008, . 753 [IDNA2008-Tables] 754 Faltstrom, P., "The Unicode Codepoints and IDNA", 755 July 2008, . 758 A version of this document is available in HTML format at 759 http://stupid.domain.name/idnabis/ 760 draft-ietf-idnabis-tables-02.html 762 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 763 STD 13, RFC 1034, November 1987. 765 [RFC1035] Mockapetris, P., "Domain names - implementation and 766 specification", STD 13, RFC 1035, November 1987. 768 [RFC1123] Braden, R., "Requirements for Internet Hosts - Application 769 and Support", STD 3, RFC 1123, October 1989. 771 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 772 Requirement Levels", BCP 14, RFC 2119, March 1997. 774 [RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode 775 for Internationalized Domain Names in Applications 776 (IDNA)", RFC 3492, March 2003. 778 [Unicode-PropertyValueAliases] 779 The Unicode Consortium, "Unicode Character Database: 780 PropertyValueAliases", March 2008, . 783 [Unicode-RegEx] 784 The Unicode Consortium, "Unicode Technical Standard #18: 785 Unicode Regular Expressions", May 2005, 786 . 788 [Unicode-Scripts] 789 The Unicode Consortium, "Unicode Standard Annex #24: 790 Unicode Script Property", February 2008, 791 . 793 [Unicode-UAX15] 794 The Unicode Consortium, "Unicode Standard Annex #15: 795 Unicode Normalization Forms", 2006, 796 . 798 11.2. Informative References 800 [ASCII] American National Standards Institute (formerly United 801 States of America Standards Institute), "USA Code for 802 Information Interchange", ANSI X3.4-1968, 1968. 804 ANSI X3.4-1968 has been replaced by newer versions with 805 slight modifications, but the 1968 version remains 806 definitive for the Internet. 808 [IDNA2008-Rationale] 809 Klensin, J., Ed., "Internationalizing Domain Names for 810 Applications (IDNA): Issues, Explanation, and Rationale", 811 November 2008, . 814 [RFC2136] Vixie, P., Thomson, S., Rekhter, Y., and J. Bound, 815 "Dynamic Updates in the Domain Name System (DNS UPDATE)", 816 RFC 2136, April 1997. 818 [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS 819 Specification", RFC 2181, July 1997. 821 [RFC2535] Eastlake, D., "Domain Name System Security Extensions", 822 RFC 2535, March 1999. 824 [RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", 825 RFC 2671, August 1999. 827 [RFC3490] Faltstrom, P., Hoffman, P., and A. Costello, 828 "Internationalizing Domain Names in Applications (IDNA)", 829 RFC 3490, March 2003. 831 [RFC3491] Hoffman, P. and M. Blanchet, "Nameprep: A Stringprep 832 Profile for Internationalized Domain Names (IDN)", 833 RFC 3491, March 2003. 835 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 836 Resource Identifier (URI): Generic Syntax", STD 66, 837 RFC 3986, January 2005. 839 [RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource 840 Identifiers (IRIs)", RFC 3987, January 2005. 842 [RFC4690] Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review and 843 Recommendations for Internationalized Domain Names 844 (IDNs)", RFC 4690, September 2006. 846 [RFC4952] Klensin, J. and Y. Ko, "Overview and Framework for 847 Internationalized Email", RFC 4952, July 2007. 849 [Unicode] The Unicode Consortium, "The Unicode Standard, Version 850 5.0", 2007. 852 Boston, MA, USA: Addison-Wesley. ISBN 0-321-48091-0 854 Appendix A. Summary of Major Changes from IDNA2003 856 1. Update base character set from Unicode 3.2 to Unicode version- 857 agnostic. 859 2. Separate the definitions for the "registration" and "lookup" 860 activities. 862 3. Disallow symbol and punctuation characters except where special 863 exceptions are necessary. 865 4. Remove the mapping and normalization steps from the protocol and 866 have them instead done by the applications themselves, possibly 867 in a local fashion, before invoking the protocol. 869 5. Change the way that the protocol specifies which characters are 870 allowed in labels from "humans decide what the table of 871 codepoints contains" to "decision about codepoints are based on 872 Unicode properties plus a small exclusion list created by 873 humans". 875 6. Introduce the new concept of characters that can be used only in 876 specific contexts. 878 7. Allow typical words and names in languages such as Dhivehi and 879 Yiddish to be expressed. 881 8. Make bidirectional domain names (delimited strings of labels, 882 not just labels standing on their own) display in a less 883 surprising fashion whether they appear in obvious domain name 884 contexts or as part of running text in paragraphs. 886 9. Remove the dot separator from the mandatory part of the 887 protocol. 889 10. Make some currently-valid labels that are not actually IDNA 890 labels invalid. 892 Appendix B. Change Log 894 [[anchor24: RFC Editor: Please remove this appendix.]] 896 B.1. Changes between Version -00 and -01 of draft-ietf-idnabis-protocol 898 o Corrected discussion of SRV records. 900 o Several small corrections for clarity. 902 o Inserted more "open issue" placeholders. 904 B.2. Version -02 906 o Rewrote the "conversion to Unicode" text in Section 5.2 as 907 requested on-list. 909 o Added a comment (and reference) about EDNS0 to the "DNS Server 910 Conventions" section, which was also retitled. 912 o Made several editorial corrections and improvements in response to 913 various comments. 915 o Added several new discussion placeholder anchors and updated some 916 older ones. 918 B.3. Version -03 920 o Trimmed change log, removing information about pre-WG drafts. 922 o Incorporated a number of changes suggested by Marcos Sanz in his 923 note of 2008.07.17 and added several more placeholder anchors. 925 o Several minor editorial corrections and improvements. 927 o "Editor" designation temporarily removed because the automatic 928 posting machinery does not accept it. 930 B.4. Version -04 932 o Removed Contextual Rule appendices for transfer to Tables. 934 o Several changes, including removal of discussion anchors, based on 935 discussions at IETF 72 (Dublin) 937 o Rewrote the preprocessing material (Section 5.3) somewhat. 939 B.5. Version -05 941 o Updated part of the A-label input explanation (Section 5.4) per 942 note from Erik van der Poel. 944 B.6. Version -06 946 o Corrected a few typographical errors. 948 o Incorporated the material (formerly in Rationale) on the 949 relationship between IDNA2003 and IDNA2008 as an appendix and 950 pointed to the new definitions document. 952 o Text modified in several places to recognize the dangers of 953 interaction between DNS wildcards and IDNs. 955 o Text added to be explicit about the handling of edge and failure 956 cases in Punycode encoding and decoding. 958 o Revised for consistency with the new Definitions document and to 959 make the text read more smoothly. 961 B.7. Version -07 963 o Multiple small textual and editorial changes and clarifications. 965 o Requirement for normalization clarified to apply to all cases and 966 conditions for preprocessing further clarified. 968 o Substantive change to Section 4.3.1, turning a SHOULD to a MUST 969 (see note from Mark Davis, 19 November, 2008 18:14 -0800). 971 B.8. Version -08 973 o Added some references and altered text to improve clarity. 975 o Changed the description of CONTEXTJ/CONTEXTO to conform to that in 976 Tables. In other words, these are now treated as distinction 977 categories (again), rather than as specially-flagged subsets of 978 PROTOCOL VALID. 980 o The discussion of label comparisons has been rewritten to make it 981 more precise and to clarify that one does not need to verify that 982 a string is a [valid] A-label or U-label in order to test it for 983 equality with another string. The WG should verify that the 984 current text is what is desired. 986 o Other changes to reflect post-IETF discussions or editorial 987 improvements. 989 Author's Address 991 John C Klensin 992 1770 Massachusetts Ave, Ste 322 993 Cambridge, MA 02140 994 USA 996 Phone: +1 617 245 1457 997 Email: john+ietf@jck.com 999 Full Copyright Statement 1001 Copyright (C) The IETF Trust (2008). 1003 This document is subject to the rights, licenses and restrictions 1004 contained in BCP 78, and except as set forth therein, the authors 1005 retain all their rights. 1007 This document and the information contained herein are provided on an 1008 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS 1009 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND 1010 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS 1011 OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF 1012 THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED 1013 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 1015 Intellectual Property 1017 The IETF takes no position regarding the validity or scope of any 1018 Intellectual Property Rights or other rights that might be claimed to 1019 pertain to the implementation or use of the technology described in 1020 this document or the extent to which any license under such rights 1021 might or might not be available; nor does it represent that it has 1022 made any independent effort to identify any such rights. 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