idnits 2.17.1 draft-lewis-domain-names-10.txt: Checking boilerplate required by RFC 5378 and the IETF Trust (see https://trustee.ietf.org/license-info): ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/1id-guidelines.txt: ---------------------------------------------------------------------------- No issues found here. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- == There are 1 instance of lines with non-RFC2606-compliant FQDNs in the document. -- The document has examples using IPv4 documentation addresses according to RFC6890, but does not use any IPv6 documentation addresses. Maybe there should be IPv6 examples, too? Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year == Line 366 has weird spacing: '...on, on the ...' == Line 367 has weird spacing: '...taining a st...' == Line 368 has weird spacing: '...ed host can ...' == Line 369 has weird spacing: '... This stand...' == Line 370 has weird spacing: '...OW data is t...' -- The document date (February 1, 2018) is 2274 days in the past. Is this intentional? Checking references for intended status: Informational ---------------------------------------------------------------------------- == Missing Reference: 'IEN-116' is mentioned on line 214, but not defined == Missing Reference: 'RFC-799' is mentioned on line 214, but not defined == Missing Reference: 'RFC-819' is mentioned on line 214, but not defined == Missing Reference: 'RFC-830' is mentioned on line 214, but not defined == Missing Reference: 'RFC-882' is mentioned on line 219, but not defined ** Obsolete undefined reference: RFC 882 (Obsoleted by RFC 1034, RFC 1035) == Missing Reference: 'RFC-883' is mentioned on line 219, but not defined ** Obsolete undefined reference: RFC 883 (Obsoleted by RFC 1034, RFC 1035) -- Obsolete informational reference (is this intentional?): RFC 724 (Obsoleted by RFC 733) -- Obsolete informational reference (is this intentional?): RFC 788 (Obsoleted by RFC 821, RFC 974, RFC 1869, RFC 1870) -- Obsolete informational reference (is this intentional?): RFC 882 (Obsoleted by RFC 1034, RFC 1035) -- Obsolete informational reference (is this intentional?): RFC 883 (Obsoleted by RFC 1034, RFC 1035) -- Obsolete informational reference (is this intentional?): RFC 7719 (Obsoleted by RFC 8499) Summary: 2 errors (**), 0 flaws (~~), 13 warnings (==), 7 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Network Working Group E. Lewis 3 Internet-Draft ICANN 4 Intended status: Informational February 1, 2018 5 Expires: August 5, 2018 7 Domain Names, A Case for Clarifying 8 draft-lewis-domain-names-10.txt 10 Abstract 12 Is the concept of Domain Names owned by the DNS protocol or does the 13 DNS protocol exist to support the Domain Names concept? This 14 question has become pertinent in light of proposals to use Domain 15 Names in protocols in ways incompatible with the DNS protocol and the 16 operational environment built to runthe protocol. This document 17 looks first at the early record in the RFC series and then expands to 18 survey how Domain Names are used in protocols and reaches the 19 conclusion that Domain Names are independent of the DNS, and further, 20 that there might be a need to clarify the definition of Domain Names 21 to reinforce that notion. 23 Status of This Memo 25 This Internet-Draft is submitted in full conformance with the 26 provisions of BCP 78 and BCP 79. 28 Internet-Drafts are working documents of the Internet Engineering 29 Task Force (IETF). Note that other groups may also distribute 30 working documents as Internet-Drafts. The list of current Internet- 31 Drafts is at https://datatracker.ietf.org/drafts/current/. 33 Internet-Drafts are draft documents valid for a maximum of six months 34 and may be updated, replaced, or obsoleted by other documents at any 35 time. It is inappropriate to use Internet-Drafts as reference 36 material or to cite them other than as "work in progress." 38 This Internet-Draft will expire on August 5, 2018. 40 Copyright Notice 42 Copyright (c) 2018 IETF Trust and the persons identified as the 43 document authors. All rights reserved. 45 This document is subject to BCP 78 and the IETF Trust's Legal 46 Provisions Relating to IETF Documents 47 (https://trustee.ietf.org/license-info) in effect on the date of 48 publication of this document. Please review these documents 49 carefully, as they describe your rights and restrictions with respect 50 to this document. Code Components extracted from this document must 51 include Simplified BSD License text as described in Section 4.e of 52 the Trust Legal Provisions and are provided without warranty as 53 described in the Simplified BSD License. 55 Table of Contents 57 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 58 1.1. Goal . . . . . . . . . . . . . . . . . . . . . . . . . . 4 59 2. Early RFCs . . . . . . . . . . . . . . . . . . . . . . . . . 4 60 3. Emergence of Domain Names . . . . . . . . . . . . . . . . . . 6 61 3.1. The Term "Domain Name" Itself . . . . . . . . . . . . . . 6 62 3.2. The Term "Resolve" . . . . . . . . . . . . . . . . . . . 8 63 4. Dialects, So To Speak, of Domain Names . . . . . . . . . . . 9 64 4.1. Domain Names as Restricted for DNS . . . . . . . . . . . 9 65 4.2. Host Names . . . . . . . . . . . . . . . . . . . . . . . 10 66 4.3. URI Authority and Domain Names . . . . . . . . . . . . . 11 67 4.4. Internet Protocol Address Literals . . . . . . . . . . . 11 68 4.5. Internationalized Domain Names in Applications . . . . . 11 69 4.6. Restricted for DNS Registration . . . . . . . . . . . . . 12 70 4.7. Tor Network Names . . . . . . . . . . . . . . . . . . . . 12 71 4.8. X.509 . . . . . . . . . . . . . . . . . . . . . . . . . . 12 72 4.9. Multicast DNS . . . . . . . . . . . . . . . . . . . . . . 12 73 4.10. /etc/hosts . . . . . . . . . . . . . . . . . . . . . . . 13 74 4.11. Other Protocols . . . . . . . . . . . . . . . . . . . . . 13 75 4.12. Other Others . . . . . . . . . . . . . . . . . . . . . . 14 76 5. Interoperability Considerations . . . . . . . . . . . . . . . 14 77 6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 15 78 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 79 8. Security Considerations . . . . . . . . . . . . . . . . . . . 15 80 9. Informational References . . . . . . . . . . . . . . . . . . 15 81 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 20 83 1. Introduction 85 Which came first, the concept of Domain Names or the protocol called 86 DNS? This question is at the heart of whether or how Domain Names 87 are put to use in ways avoiding the DNS protocol. 89 The discussion leading to "The '.onion' Special-Use Domain Name" 90 [RFC7686], a document designating "onion" as a top-level domain in 91 the Special Use Domain Names registry (see "Special Use Domain Names" 92 [RFC6761]), opened the question of how to treat Domain Names that 93 were designed to be used external to the DNS. The history of Domain 94 Names and DNS had become intertwined to the point over time to the 95 point that what is essentially a case of permission-less innovation 96 led to a contentious discussion on the IETF's DNS Operations working 97 group mail list. 99 A portion of the discussion centered around a seeming conflict among 100 processes to register Domain Names, such as the process launched from 101 "Memorandum of Understanding Concerning the Technical Work of the 102 Internet Assigned Numbers Authority" [RFC2860], for registering a 103 name in the global, public DNS and the process for registering a name 104 in the Special Use Domain Names registry. 106 To help establish a way forward, a look backward is needed. A 107 document search, sticking to RFC documents, reveals evidence of 108 discussions on domains prior to the DNS, with the DNS protocol's base 109 documents indicating that the DNS is based on some simplifying 110 assumptions, implying there is a larger cconcept in play. 112 To help bolster the idea that Domain Names came first, a look at how 113 other protocols have treated identifying names, how Domain Names are 114 put to use, how what a name is further restricted for the protocol's 115 needs. From this it has become apparent that the concept of Domain 116 Names has drifted over time, which leads to some uncertainty when it 117 comes to looking forward. 119 During reviews of this document, documented studies of other 120 difficulties have surfaced. "IAB Thoughts on Encodings for 121 Internationalized Domain Names" [RFC6055] documents issues related to 122 converting human-readable forms of Domain Names in forms useful to 123 automated applications when there is no clear architecture or precise 124 definition of how to handle Domain Names. "Issues in Identifier 125 Comparison for Security Purposes" [RFC6943] documents issues related 126 to the same conversion as related to evaluating security policies. 127 The presence of these studies suggests a need to examine the 128 architecture of naming and identifiers. 130 The most glaring omission in the document survey is a definitive 131 foundation for Domain Names. There are abstract descriptions of the 132 concept that come close to being a definition. The descriptions 133 though are too loose to be something that can be tested objectively, 134 frustrating discussions when it comes to innovations in the use of 135 Domain Names. 137 This document settles being a literature search covering the RFC 138 series and comes to making a case for clarications to be made. There 139 are obvious continuations to this work, such as the earlier Internet 140 Engineering Notes, other published works, and interviews with 141 participants in the early days. That will be conducted as follow on 142 work to this document. 144 1.1. Goal 146 To establish a solid foundation accommodating an installed based and 147 permission-less innovation, having a clear definition of Domain Names 148 would be great. This document, however, does not attempt to achieve 149 a definition. This document's goal has settled into compiling a 150 narrative on the history, within perhaps artificial bounds (the RFC 151 series), and declaring that there is a need to clarify Domain Names. 153 In this document are criteria for performing a clarification, 154 recognizing from experience in preparing "The Role of Wildcards in 155 the Domain Name System" [RFC4592] and "DNS Zone Transfer Protocol 156 (AXFR)" [RFC5936] that clarifications may have adverse impacts on 157 deployed software, thus entering into a clarifications activity is 158 not to be taken without considerations. 160 There is one deviation from the strict rules of relying on the RFC 161 series which has been included, that is the section on determining 162 the origin of the term "resolve" in the context of Domain Names and 163 the DNS. This work is interesting and adds to the larger picture of 164 what needs to be done. The experience of that side route illustrates 165 the need to expand the literature search beyond the RFC series and to 166 include other publications and recollections. 168 2. Early RFCs 170 Two or three decades into the history of Domain Names, a popular 171 notion has taken hold that Domains Names were defined and specified 172 in the definition of the Domain Name System (DNS). There are two 173 documents that form the basic definition of the DNS, "Domain names - 174 concepts and facilities" [RFC1034] and "Domain names - implementation 175 and specification" [RFC1035] referred to as RFC 1034 and RFC 1035, 176 respectively. (Note that there is another pair of Request for 177 Comments documents with the same titles [RFC0882] [RFC0883] that 178 precede RFC 1034 and RFC 1035, those were declared obsolete in favor 179 of the newer documents.) Together RFC 1034 and RFC 1035 form STD 13, 180 a full standard cataloged by the RFC Editor. The definitions of DNS 181 domain names within RFC 1034 and RFC 1035 have become the apparently- 182 authoritative source for discussions on what is a Domain Name. 184 Throughout this document the term "Domain Names" is capitalized to 185 emphasize the concept of the names and DNS is used to describe the 186 protocol and algorithms described in STD 13, including any applicable 187 updates, related standards track documents and experimental track 188 documents. 190 The term "domain" is a generic term, there are many naming systems in 191 existence. The use of the term Domain Names in this document refers 192 to the roughly-defined set of protocols and their applications' use 193 of a naming structure that is prevalent amongst many protocols 194 defined in IETF RFC documents. 196 The truth is, STD 13 does not define Domain Names, the documents 197 define only how Domain Names are used and processed in the DNS. 198 However, the way in which the RFC documents read seem to lend to the 199 confusion. 201 RFC 1034, section 2 begins with this text: 203 "This RFC introduces domain style names, their use for Internet mail 204 and host address support, and the protocols and servers used to 205 implement domain name facilities." 207 Which seems to indicate that RFC 1034 is the origin of Domain Names. 208 Immediately following is section 2.1, entitled "The history of domain 209 names" which includes the following text. (The text differs from the 210 original presentation only in wrapping of text to fit current 211 formatting rules.) 213 "The result was several ideas about name spaces and their management 214 [IEN-116, RFC-799, RFC-819, RFC-830]. The proposals varied, but a 215 common thread was the idea of a hierarchical name space, with the 216 hierarchy roughly corresponding to organizational structure, and 217 names using "." as the character to mark the boundary between 218 hierarchy levels. A design using a distributed database and 219 generalized resources was described in [RFC-882, RFC-883]. Based on 220 experience with several implementations, the system evolved into the 221 scheme described in this memo." 223 The only reference included in that text not otherwise mentioened in 224 this document is IEN-116. The reference for that is defined as:" 226 "[IEN-116] J. Postel, "Internet Name Server", IEN-116, 227 USC/Information Sciences Institute, August 1979. 229 A name service obsoleted by the Domain Name System, but 230 still in use." 232 The DNS as it is known today did not invent Domain Names. Work on 233 the Simple Mail Transfer Protocol preceding the DNS mentions domain 234 names, and even it was not the origin of the concept. The DNS is not 235 even the first attempt at an Internet naming system, see "The Domain 236 Naming Convention for Internet User Applications" [RFC0819] and "A 237 Distributed System for Internet Name Service" [RFC0830]. 239 One important phrase to keep in mind is: 241 "To simplify implementations," 243 which appears in both RFC 1034 and RFC 1035 as well as their 244 predecessors RFC 882 and RFC 883. This gives credence to the notion 245 that Domain Names exist beyond the DNS. 247 3. Emergence of Domain Names 249 The first effort taken, in preparation for writing this document, was 250 to scan for the earliest use of the term "domain name" or "name 251 domain". This work is detailed in the following section, but, as 252 noted in private email by reviews of early versions of the document, 253 gave the impression that Domain Names were somehow a by-product of 254 the effort to develop electronic mail. To challenge the notion that 255 email begat domain names, a search through RFC documents for the use 256 of the term resolve as it applies to Domain Names was also done. 258 3.1. The Term "Domain Name" Itself 260 Domain Names emerged from the need to build a hierarchy around the 261 growing number of identified hosts exchanging email. "SIMPLE MAIL 262 TRANSFER PROTOCOL" [RFC0788], explains, in its section 3.7: 264 "At some not too distant future time it might be necessary to 265 expand the mailbox format to include a region or name domain 266 identifier. There is quite a bit of discussion on this at 267 present, and is likely that SMTP will be revised in the future to 268 take into account naming domains." 270 Knowing the origins of a concept helps setting the correct boundaries 271 for discussion. The past isn't meant to restrict the future but 272 meant to help provide a context, include forgotten ideas, and help 273 identify rational for scope creep. 275 "Internet Name Domains" [RFC0799] has (arguably) the first formation 276 of what is a Domain Name: 278 "In its most general form, a standard internet mailbox name has 279 the syntax 281 .@ , 283 where is the name of a user known at the host in the 284 name domain ." 286 Prior to this, domain referred to principally an administrative 287 domain, such as the initial organizations involved in networks at the 288 time. 290 "NCP/TCP TRANSITION PLAN" [RFC0801] contains this, indicating the 291 passage from the host tables: 293 "It might be advantageous to do away with the host name table and 294 use a Name Server instead, or to keep a relatively small table as 295 a cache of recently used host names." 297 "Computer Mail Meeting Notes" [RFC0805] contains this: 299 "The conclusion in this area was that the current "user@host" mailbox 300 identifier should be extended to 'user@host.domain' where 'domain' 301 could be a hierarchy of domains." 303 "The Domain Naming Convention for Internet User Applications" 304 [RFC0819] contains this: 306 "A decision has recently been reached to 307 replace the simple name field, "", by a composite name field, 308 '' " 310 A domain name began to take on its current form: 312 "Internet Convention: Fred@F.ISI.ARPA" 314 In addition, "simple name" is defined as what we now call a label, 315 and a "complete (fully qualified) name" is defined as "concatenation 316 of the simple names of the domain structure tree nodes starting with 317 its own name and ending with the top level node name". Noticeably 318 absent is a terminating dot or any mention or representation of a 319 root. 321 "The Domain Naming Convention for Internet User Applications" (RFC 322 819) also defines ARPA as a top-level name (as opposed to top-level 323 domain name). This is an early mention of the role of top-level 324 names. Additionally, the use of "." [RFC0020][ANSIX34] as a 325 separating character is mentioned. 327 This walk thru history relies solely on the record left behind inside 328 RFC documents. The precise chain of events is likely slightly 329 different and nuanced. The point of the exercise is to show that 330 Domain Names are a concept the emerged over time, spawned the DNS 331 with its domain names, a definition of host names derived from the 332 host tables, and was heavily influenced by SMTP as the driving 333 application. The definition of the FTP protocol, originally defined 334 in "FILE TRANSFER PROTOCOL" [RFC0959], never mentions hosts, domains 335 or host names. But no formal definition of Domain Names has been 336 written and recorded. 338 Note: Concurrent with the writing of this document, the Domain Name 339 Systems Operations working group is documenting a definition for 340 "Domain Names". The first edition of "DNS Terminology" [RFC7719] has 341 a recitation of the original definition from STD 13, the successor 342 edition (still in preparation) has a new, further reaching 343 definition. 345 3.2. The Term "Resolve" 347 As much as Domain Names were influenced by SMTP, electronic mail was 348 not the origin of the Domain Names concepts, this was a hypothesis 349 came from a personal view of the early days of Internet work. To 350 test this, a look for the use of the term "resolve" or "resolution" 351 was conducted in early (arbitrarily defined as pre-1000) RFC 352 documents. 354 The term "resolve" appears numerous times, but in many different 355 contexts. "Resolve" has many meanings, consulting a dictionary, such 356 as Merriam Webster's dictionary [MWDICT], none which seem to match 357 the use associated with domain names. For example, a committee can 358 resolve to solve a certain question. This use of "resolve" occurred 359 numerous times in early RFC documents unrelated to Domain Names. 361 In "Proposed Official Standard for the Format of ARPA Network 362 Messages" [RFC0724] the term resolve was used in the sense of mapping 363 an identifier into an address or something actionable. A section on 364 Semantics (C), Address fields (1.), General (a.), bullet 1 states: 366 "s are used to refer to a location, on the ARPANET, 367 containing a stored address list. The should 368 contain text which the referenced host can resolve to a 369 file. This standard is not a protocol and so does not 370 prescribe HOW data is to be retrieved from the file." 372 Private email to the (reachable) authors of the document pointed to 373 the use of "resolve" stemming from work on programming languages and 374 compiler theory. In that field of work, variables are associated 375 with machine addresses when linking code. There are formal papers 376 including "A Theory of Name Resolution" [TONR15] using the term and 377 the term resolution is used in the field of "Automated Reasoning" 378 [WIKIAR]. 380 The exercise of determining how the term "resolve" came to be part of 381 Domain Names and DNS shows tthat there are influences, topics, terms 382 and concepts from technologies preceeding Domain Name and DNS that 383 can be researched to help establish a foundation from which to build. 384 There is more work to do here. 386 4. Dialects, So To Speak, of Domain Names 388 Subtypes of Domain Names have come to be defined for different 389 protocols, evolving and sometimes building on previous definitions. 391 4.1. Domain Names as Restricted for DNS 393 The DNS protocol places size restrictions on Domain Names and defines 394 rules for matching domain names, treating sets of Domain Names as 395 equivalent to each other. (This matching refers to treating upper 396 case and lower case ASCII letters as equivalent.) The DNS defines 397 the format used to transmit the names across the network as well as 398 rules for displaying them inside text zone files. The DNS creates 399 the notion that names are assigned by an authority per zone. 401 Placing size restrictions on Domain Names is significant in reducing 402 the overall population of names that can be represented in the DNS. 403 The matching rules have the effect of creating (to use a term from 404 graph theory) cliques, distorting the tree-nature of the Domain Name 405 graph. A clique is a completely connected sub-graph implying cyclic 406 paths, a tree is a graph that is acyclic. In sum, the treatment of 407 ASCII (and only ASCII) cases as equivalent is a distortion of the 408 Domain Name hierarchy. 410 DNS defines two formats for domain names. One is the "on-the-wire" 411 format used inside messages, a flags-and-length octet followed by 412 some count of octets for each label with the final length of 0 413 representing the root. The other is a version that can be rendered 414 in printable ASCII characters, complete with a means to represent 415 other characters via an escape sequence. This does not alter the 416 Domain Name concept but has implications when it comes to 417 interoperating with other protocol definitions of their domain name 418 use. 420 DNS assumes that there is, in concept, a central authority creating 421 names within the DNS management structure (called a zone). Although 422 the DNS does not define how a central authority is implemented nor 423 how it coins names, the names have to come from a single point to 424 appear in a zone. There are other means for claiming names, an 425 example will be mentioned later. 427 DNS domain names could appear to be the same as address literals, 428 such as "192.0.2.1" or "0:0:0:0:0:FFFF::192.0.2.1". Such DNS domain 429 names are not used for two reasons. Applications expecting a Domain 430 Name (as a comment line parameter as an example) would opt to treat 431 the string as an address literal and would therefore not look for the 432 string in the DNS domain name space. The management model of the DNS 433 would prefer to aggregate (as in routing) addresses belonging 434 together in the same zone, resulting in labels appearing in reverse 435 order. E.g., the network address 192.0.2.1 would be represented by a 436 DNS domain name as "1.2.0.192.in-addr.arpa." as described in RFC 437 1035. For IPv6, the convention used is documented in "DNS Extensions 438 to Support IP Version 6" [RFC3596], section 2.5. 440 See also "Issues in Identifier Comparison for Security Purposes" 441 [RFC6943] section 3.1, "Host Names", in particular, section 3.1.1 and 442 3.1.2 on address literals, and section 4.1, "Conflation." 444 DNS domain names have become the dominant definition of domain names 445 due to the success (scale) of the DNS on the public Internet. Many 446 protocols interact with the DNS but instead of supporting the 447 complete definition of DNS domain names, the protocols rely on a 448 subset more commonly called host names. 450 4.2. Host Names 452 Work on the definition of a host name began well before the issuance 453 of the STD 13 documents defining DNS. The rules for the Preferred 454 Syntax in RFC 1034 conform to the host name rules outlined in "DoD 455 Internet host table specification" [RFC0952]. The host name 456 definition was presented again in "Requirements for Internet Hosts -- 457 Application and Support" [RFC1123] (which is part of STD 3). In 458 section 2.1 of RFC 1123, one (of two mentions) definition of host 459 name is presented, noting that the definition is a relaxation of what 460 is in RFC 952. 462 Host names are subsets of DNS domain names in the sense that the 463 character set is limited. In particular, only "let" (i.e., 464 presumably letters a-z), "digits" and "hyphen" can be used, with 465 hyphen only internal to a label. (This description is meant to be 466 illustrative, not normative. See the grammar presented on page 5 of 467 RFC 952 for specifics.) "Hypertext Transfer Protocol -- HTTP/1.0" 468 [RFC1945], Section 3.2.2 "http URL" specifically references section 469 2.1 of RFC 1123. The reference is explicit. 471 "Simple Mail Transfer Protocol" [RFC5321] refers to RFC 1035 for a 472 definition of domain names but includes text close to what is in the 473 previous paragraph, noting that domain names as used in SMTP refer to 474 both hosts and to other entities. RFC 5321 updates RFC 1123, but 475 does not cite the latter for a definition of host names. RFC 5321 476 additionally requires brackets to surround address literals, 477 referring to the use case as an "alternative to a domain name." 479 See also "IAB Thoughts on Encodings for Internationalized Domain 480 Names" [RFC6055], particularly section 3 entitled "Use of Non-ASCII 481 in DNS" for more thoughts on host names. 483 4.3. URI Authority and Domain Names 485 In "Uniform Resource Identifier (URI): Generic Syntax" [RFC3986], 486 also known as STD 66, mentions in its section 3.2.2 (page 20) that 487 the host subcomponent of the URI Authority (section 3.2) "should 488 conform to the DNS syntax". This comes after discussion that the 489 host subcomponent is not strongly tied to the DNS, i.e., names can be 490 managed via a concept other than the DNS. There's no discussion on 491 the rationale but this enables the reuse of code parsing and 492 marshalling the host subcomponent between different Domain Name 493 environments. 495 This reinforces the notion that there's a need to understand how 496 Domain Names interoperate amongst protocols and applications. And 497 reinforces the need to derive or make explicit a way for client 498 software to know how to resolve a name, that is, convert a name into 499 a network address. 501 4.4. Internet Protocol Address Literals 503 The above definition includes address literals such as 192.0.2.1 for 504 IPv4 and even IPv6 literals such as ::ffff:192.0.2.1. Yes, these 505 might qualify as Domain Names. The addresses might be encased in 506 square brackets "[" and "]" (SMTP mentioned already). In the DNS, as 507 previously described in section 3.1, they are represented per 508 appropriate conventions. 510 4.5. Internationalized Domain Names in Applications 512 The original uses of Domain Names (such as DNS domain names and host 513 names) assumed the ASCII character set. Specifically, making the 514 labels case insensitive prohibited a straightforward use of any 515 method of representation of non-ASCII characters. 517 "Internationalized Domain Names for Applications (IDNA): Definitions 518 and Document Framework" [RFC5890], with associated other documents, 519 defines IDNA2008 as a convention for handling non-ASCII characters in 520 DNS domain names. In figure 1 of that document, the sets of legal 521 DNS domain name formats are defined. Noted in the footnotes of the 522 figure, applications unaware of IDNA2008 cannot distinguish the 523 subsets defined by the document meaning this definition is not an 524 alteration of Domain Names, but, like host names, yet another subset 525 of DNS domain names. 527 4.6. Restricted for DNS Registration 529 "Suggested Practices for Registration of Internationalized Domain 530 Names (IDN)" [RFC4290] presents reasons why DNS domain name 531 registration is restricted in the context of IDN. (That RFC refers 532 to an older form than IDNA2008, but the concepts still apply.) This 533 is yet another convention related to DNS domain names, excluding 534 names that would lead to undesirable outcomes. 536 4.7. Tor Network Names 538 The Tor network is an activity organized by the Tor Project, Inc., 539 described on its main web page "https://www.torproject.org/ 540 index.html.en". 542 One component of the Tor network name space are Domain Names ending 543 in ".onion". (There are other suffixes in use, but it isn't very 544 clear how they are used, defined or whether they are active.) 546 The way in which Domain Names are used in Tor is described in two web 547 documents "Tor Rendezvous Specification" [RENDEV] and "Special 548 Hostnames in Tor" [OHOST] available from the project's website. 550 Syntactically, a Tor domain name fits within the DNS domain name 551 definition but the manner of assignment is different in a manner 552 incompatible with the DNS. (Not better or worse, still significantly 553 different.) Tor domain names are derived from cryptographic keys and 554 organized by distributed hash tables, instead of assigned by a 555 central authority per zone. 557 4.8. X.509 559 "Internet X.509 Public Key Infrastructure Certificate and Certificate 560 Revocation List (CRL) Profile" [RFC5280], section 4.2.1.6 "Subject 561 Alternative Name" a dNSName is defined to be a host name, with the 562 further restriction that the name " " cannot be used. (The subtle 563 irony is that a name consisting of just a blank would hardly qualify 564 as a Domain Name.) 566 4.9. Multicast DNS 568 Multicast DNS uses a name space ending with ".local." as described in 569 "Multicast DNS" [RFC6762]. The rules for Multicast DNS domain names 570 differ from DNS domain names. Multicast DNS domain names are encoded 571 as Net-Unicode as defined in " Unicode Format for Network 572 Interchange" [RFC5198] with the DNS domain name tradition of case 573 folding the ASCII letters when matching names. Appendix F of RFC 574 6762 gives an explanation of why the punycode algorithm, defined in 575 "Punycode: A Bootstring encoding of Unicode for Internationalized 576 Domain Names in Applications (IDNA)" [RFC3492], is not used. 578 4.10. /etc/hosts 580 The precursor to DNS, host tables, still exists in remnants in many 581 operating systems. There are library functions, used by applications 582 to resolve DNS domain names, that can return names of arbitrary 583 length (meaning, for example longer than what DNS domain names are 584 defined to be). 586 "Basic Socket Interface Extensions for IPv6" [RFC3493], addresses 587 this in Section 6, further documentation can be found as part of "The 588 Open Group Base Specifications Issue 7" [IEEE1003] and "Microsoft 589 Winsock Functions" [WINSOCK]. 591 4.11. Other Protocols 593 This section is used to list (some) other protocols that use Domain 594 Names but in general do not impose any other restrictions that what 595 has been mentioned above. 597 SSH, documented in "The Secure Shell (SSH) Protocol Architecture" 598 [RFC4251] uses host names, using the name when storing public keys of 599 hosts. SSH clients, not necessarily the protocol, illustrate how 600 applications juggle the different forms of Domain Names. SSH can be 601 invoked to open a secure shell with a host via its DNS domain name/ 602 host name or it can be used to open a secure shell with a host via 603 its Multicast DNS domain name. Or, many others, including name of a 604 purely local, per-user scope. (Note that SSH does not distinguish 605 between DNS names and Multicast DNS domain names in the protocol 606 definition, the difference is handled in resolution libraries 607 belonging to the computing platform.) 609 FTP, defined in "FILE TRANSFER PROTOCOL (FTP)" [RFC0959], is silent 610 on domain names but client implementations of the protocol behave as 611 SSH clients, being un aware the differences between definitions of 612 Domain Names. 614 DHCP, defined in "Dynamic Host Configuration Protocol" [RFC2131], 615 includes domain names in its Domain Search Option as described in 616 "Dynamic Host Configuration Protocol (DHCP) Domain Search Option" 617 [RFC3397]. The encoding of Domain Names used is the on-the-wire 618 format of the DNS, using DNS-defined message compression. DHCP 619 handles Domain Names in other options, such as defined in "The DHCP 620 Client FQDN Option" [RFC4702], in the same format. The significance 621 of this is that most other protocols represent DNS domain names or 622 host names in a human readable form, DHCP is using the machine- 623 friendly format. 625 4.12. Other Others 627 If there is a use of Domain Names not listed here it is merely an 628 omission. The goal in this document is to provide a survey that is 629 sufficient to avoid hand-waving arguments, recognizing the 630 diminishing return in trying to build a complete roster of uses of 631 Domain Names. If there are omissions that ought to be included, 632 please send references for the use case to the author (while this is 633 an Internet Draft, that is). 635 5. Interoperability Considerations 637 Any single protocol can define a format for a conceptual Domain Name. 638 Examples given above show that many protocols have done so. From the 639 examples, it is clear that the way in which protocols have 640 interpreted Domain Names has varied, leading to, at least, user 641 interfaces having to have built-in intelligence when handling names 642 and, at worst, a growing confusion over how the Domain Name space is 643 to be managed. 645 When protocols having different formats and rules for Domain Names 646 interact, software implementing the protocols translate one 647 protocol's domain name format to another's format. Even when the 648 translation is straightforward, it is predictable that software will 649 fail to handle this situation well. 651 Often the clash of definitions impacts the design of a new protocol 652 and/or an extension of a protocol. For example, adding non-ASCII 653 domain names has to be done with backwards compatibility with an 654 installed base of ASCII-assuming code. This clash can inhibit new 655 uses of Domain Names. 657 Search lists are a Domain Name mechanism studied in "SSAC Advisory on 658 DNS 'Search List' Processing" [SSAC064]. One of the particular use 659 cases related to this topic is the issuance of search lists via DHCP 660 and then used by any user-client protocol implementation. This 661 emphasizes an interoperability consideration for how Domain Names are 662 treated in different protocols, not just among implementations of one 663 protocol. 665 The detection and handling of Fully Qualified Domain Names is an 666 interoperability issue as well. At issue is the significance of the 667 terminating separation character in a printed version of a Domain 668 Name. Many clients, when passed a Domain Name as an identifier will 669 add a dot at the end of the argument if the argument does not already 670 end in a dot. [TRAILDOT1] Some do this only after applying the 671 aforementioned search list. As mentioned in the SSAC document in the 672 previous paragraph, inconsistency leads to surprising results. 674 The Special Use Domain Names registry lists Domain Names that are to 675 be treated in a manner inconsistent with the DNS normal processing 676 rules. This registry contains Domain Names regardless of whether the 677 name is a DNS domain name and regardless whether the name is a top- 678 level (domain) name or is positioned elsewhere in the tree structure. 680 These are reasons this document is needed. The reason for the 681 confusion over what's a legal domain name stems from application- 682 defined restrictions. For example, using a one-label domain name 683 ("dotless") for sending email is not a problem with the DNS nor the 684 name in concept, but is a problem for mail implementations that 685 expect more than one label. (One-label names may be assumed to be in 686 ARPA host table format.) The "IAB Statement: Dotless Domains 687 Considered Harmful" [IABSTMT] elaborates. 689 6. Acknowledgements 691 Comments or contributions from Andrew Sullivan, Paul Hoffman, George 692 Michaelson, Kevin Darcy, Joe Abley, Jim Reid, Tony Finch, Robert 693 Edmonds, hellekin, Stephane Bortzmeyer, Ray Bellis, Bob Harold, Alec 694 Muffett, Stuart Cheshire, Dave Thaler, Niall O'Reilly, John Klensin, 695 Dave Crocker, Ken Pogran, John Vittal and a growing list of others I 696 am losing track of. Not to imply endorsement. 698 7. IANA Considerations 700 None. 702 8. Security Considerations 704 Nothing direct. This document proposes a definition of the term 705 "Domain Name" and surveys how it has been variously applied. In some 706 sense, loosely defined terms give rise to security hazards. Beyond 707 that, there is no impact of "security." 709 9. Informational References 711 [ANSIX34] American National Standards Institute (formerly United 712 States of America Standards Institute), "USA Code for 713 Information Interchange, ANSI X3.4-1968", 1968. 715 [IABSTMT] "IAB Statement: Dotless Domains Considered Harmful", 2013, 716 . 720 [IEEE1003] 721 "The Open Group Base Specifications Issue 7, IEEE Std 722 1003.1, 2013 Edition, Copyright 2001-2013 The IEEE and The 723 Open Group", 2013, 724 . 727 [MWDICT] Merriam-Webster, Incorporated, "Merriam-Webster's Online 728 Dictionary, 11th Edition (Merriam-Webster's Collegiate 729 Dictionary)", 2003, . 731 [OHOST] "Special Hostnames in Tor", undated, 732 . 735 [RENDEV] "Tor Rendezvous Specification", undated, 736 . 738 [RFC0020] Cerf, V., "ASCII format for network interchange", STD 80, 739 RFC 20, DOI 10.17487/RFC0020, October 1969, 740 . 742 [RFC0724] Crocker, D., Pogran, K., Vittal, J., and D. Henderson, 743 "Proposed official standard for the format of ARPA Network 744 messages", RFC 724, DOI 10.17487/RFC0724, May 1977, 745 . 747 [RFC0788] Postel, J., "Simple Mail Transfer Protocol", RFC 788, 748 DOI 10.17487/RFC0788, November 1981, 749 . 751 [RFC0799] Mills, D., "Internet name domains", RFC 799, 752 DOI 10.17487/RFC0799, September 1981, 753 . 755 [RFC0801] Postel, J., "NCP/TCP transition plan", RFC 801, 756 DOI 10.17487/RFC0801, November 1981, 757 . 759 [RFC0805] Postel, J., "Computer mail meeting notes", RFC 805, 760 DOI 10.17487/RFC0805, February 1982, 761 . 763 [RFC0819] Su, Z. and J. Postel, "The Domain Naming Convention for 764 Internet User Applications", RFC 819, 765 DOI 10.17487/RFC0819, August 1982, 766 . 768 [RFC0830] Su, Z., "Distributed system for Internet name service", 769 RFC 830, DOI 10.17487/RFC0830, October 1982, 770 . 772 [RFC0882] Mockapetris, P., "Domain names: Concepts and facilities", 773 RFC 882, DOI 10.17487/RFC0882, November 1983, 774 . 776 [RFC0883] Mockapetris, P., "Domain names: Implementation 777 specification", RFC 883, DOI 10.17487/RFC0883, November 778 1983, . 780 [RFC0952] Harrenstien, K., Stahl, M., and E. Feinler, "DoD Internet 781 host table specification", RFC 952, DOI 10.17487/RFC0952, 782 October 1985, . 784 [RFC0959] Postel, J. and J. Reynolds, "File Transfer Protocol", 785 STD 9, RFC 959, DOI 10.17487/RFC0959, October 1985, 786 . 788 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 789 STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987, 790 . 792 [RFC1035] Mockapetris, P., "Domain names - implementation and 793 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 794 November 1987, . 796 [RFC1123] Braden, R., Ed., "Requirements for Internet Hosts - 797 Application and Support", STD 3, RFC 1123, 798 DOI 10.17487/RFC1123, October 1989, 799 . 801 [RFC1945] Berners-Lee, T., Fielding, R., and H. Frystyk, "Hypertext 802 Transfer Protocol -- HTTP/1.0", RFC 1945, 803 DOI 10.17487/RFC1945, May 1996, 804 . 806 [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", 807 RFC 2131, DOI 10.17487/RFC2131, March 1997, 808 . 810 [RFC2860] Carpenter, B., Baker, F., and M. Roberts, "Memorandum of 811 Understanding Concerning the Technical Work of the 812 Internet Assigned Numbers Authority", RFC 2860, 813 DOI 10.17487/RFC2860, June 2000, 814 . 816 [RFC3397] Aboba, B. and S. Cheshire, "Dynamic Host Configuration 817 Protocol (DHCP) Domain Search Option", RFC 3397, 818 DOI 10.17487/RFC3397, November 2002, 819 . 821 [RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode 822 for Internationalized Domain Names in Applications 823 (IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003, 824 . 826 [RFC3493] Gilligan, R., Thomson, S., Bound, J., McCann, J., and W. 827 Stevens, "Basic Socket Interface Extensions for IPv6", 828 RFC 3493, DOI 10.17487/RFC3493, February 2003, 829 . 831 [RFC3596] Thomson, S., Huitema, C., Ksinant, V., and M. Souissi, 832 "DNS Extensions to Support IP Version 6", RFC 3596, 833 DOI 10.17487/RFC3596, October 2003, 834 . 836 [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform 837 Resource Identifier (URI): Generic Syntax", STD 66, 838 RFC 3986, DOI 10.17487/RFC3986, January 2005, 839 . 841 [RFC4251] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) 842 Protocol Architecture", RFC 4251, DOI 10.17487/RFC4251, 843 January 2006, . 845 [RFC4290] Klensin, J., "Suggested Practices for Registration of 846 Internationalized Domain Names (IDN)", RFC 4290, 847 DOI 10.17487/RFC4290, December 2005, 848 . 850 [RFC4592] Lewis, E., "The Role of Wildcards in the Domain Name 851 System", RFC 4592, DOI 10.17487/RFC4592, July 2006, 852 . 854 [RFC4702] Stapp, M., Volz, B., and Y. Rekhter, "The Dynamic Host 855 Configuration Protocol (DHCP) Client Fully Qualified 856 Domain Name (FQDN) Option", RFC 4702, 857 DOI 10.17487/RFC4702, October 2006, 858 . 860 [RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network 861 Interchange", RFC 5198, DOI 10.17487/RFC5198, March 2008, 862 . 864 [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., 865 Housley, R., and W. Polk, "Internet X.509 Public Key 866 Infrastructure Certificate and Certificate Revocation List 867 (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, 868 . 870 [RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321, 871 DOI 10.17487/RFC5321, October 2008, 872 . 874 [RFC5890] Klensin, J., "Internationalized Domain Names for 875 Applications (IDNA): Definitions and Document Framework", 876 RFC 5890, DOI 10.17487/RFC5890, August 2010, 877 . 879 [RFC5936] Lewis, E. and A. Hoenes, Ed., "DNS Zone Transfer Protocol 880 (AXFR)", RFC 5936, DOI 10.17487/RFC5936, June 2010, 881 . 883 [RFC6055] Thaler, D., Klensin, J., and S. Cheshire, "IAB Thoughts on 884 Encodings for Internationalized Domain Names", RFC 6055, 885 DOI 10.17487/RFC6055, February 2011, 886 . 888 [RFC6761] Cheshire, S. and M. Krochmal, "Special-Use Domain Names", 889 RFC 6761, DOI 10.17487/RFC6761, February 2013, 890 . 892 [RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762, 893 DOI 10.17487/RFC6762, February 2013, 894 . 896 [RFC6943] Thaler, D., Ed., "Issues in Identifier Comparison for 897 Security Purposes", RFC 6943, DOI 10.17487/RFC6943, May 898 2013, . 900 [RFC7686] Appelbaum, J. and A. Muffett, "The ".onion" Special-Use 901 Domain Name", RFC 7686, DOI 10.17487/RFC7686, October 902 2015, . 904 [RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS 905 Terminology", RFC 7719, DOI 10.17487/RFC7719, December 906 2015, . 908 [SSAC064] ICANN Security and Stability Advisory Committee, "SSAC 909 Advisory on DNS "Search List" Processing", 2014, 910 . 913 [TONR15] Programming Languages and Systems - 24th European 914 Symposium on Programming, ESOP 2015, Held as Part of the 915 European Joint Conferences on Theory and Practice of 916 Software, ETAPS 2015, London, UK, April 11-18, 2015, 917 Proceedings. Lecture Notes in Computer Science, Springer, 918 April 2015., "A Theory of Name Resolution", last seen 919 2015, . 922 [TRAILDOT1] 923 "Trailing Dots in Domain Names", Undated, 924 . 926 [WIKIAR] Wikipedia, "Automated Reasoning", last edit 2016, 927 . 929 [WINSOCK] "getaddrinfo function", last seen 2017, 930 . 933 Author's Address 935 Edward Lewis 936 ICANN 938 Email: edward.lewis@icann.org