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Wessels 9 Verisign 10 22 April 2022 12 DNS Glue Requirements in Referral Responses 13 draft-ietf-dnsop-glue-is-not-optional-05 15 Abstract 17 The DNS uses glue records to allow iterative clients to find the 18 addresses of name servers that are contained within a delegated zone. 19 Authoritative Servers are expected to return all available in-domain 20 glue records in a referral response. If message size constraints 21 prevent the inclusion of all in-domain glue records, the server MUST 22 set the TC flag to inform the client that the response is incomplete, 23 and that the client SHOULD use another transport to retrieve the full 24 response. This document updates RFC 1034 to clarify correct server 25 behavior. 27 Status of This Memo 29 This Internet-Draft is submitted in full conformance with the 30 provisions of BCP 78 and BCP 79. 32 Internet-Drafts are working documents of the Internet Engineering 33 Task Force (IETF). Note that other groups may also distribute 34 working documents as Internet-Drafts. The list of current Internet- 35 Drafts is at https://datatracker.ietf.org/drafts/current/. 37 Internet-Drafts are draft documents valid for a maximum of six months 38 and may be updated, replaced, or obsoleted by other documents at any 39 time. It is inappropriate to use Internet-Drafts as reference 40 material or to cite them other than as "work in progress." 42 This Internet-Draft will expire on 24 October 2022. 44 Copyright Notice 46 Copyright (c) 2022 IETF Trust and the persons identified as the 47 document authors. All rights reserved. 49 This document is subject to BCP 78 and the IETF Trust's Legal 50 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 51 license-info) in effect on the date of publication of this document. 52 Please review these documents carefully, as they describe your rights 53 and restrictions with respect to this document. Code Components 54 extracted from this document must include Revised BSD License text as 55 described in Section 4.e of the Trust Legal Provisions and are 56 provided without warranty as described in the Revised BSD License. 58 Table of Contents 60 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 61 1.1. Reserved Words . . . . . . . . . . . . . . . . . . . . . 3 62 2. Types of Glue in Referral Responses . . . . . . . . . . . . . 3 63 2.1. Glue for In-Domain Name Servers . . . . . . . . . . . . . 3 64 2.2. Glue for Sibling Domain Name Servers . . . . . . . . . . 4 65 2.3. Glue for Cyclic Sibling Domain Name Servers . . . . . . . 5 66 2.4. Missing Glue . . . . . . . . . . . . . . . . . . . . . . 6 67 3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 7 68 3.1. Glue for In-Domain Name Servers . . . . . . . . . . . . . 7 69 3.2. Glue for Sibling Domain Name Servers . . . . . . . . . . 8 70 3.3. Updates to RFC 1034 . . . . . . . . . . . . . . . . . . . 8 71 4. Security Considerations . . . . . . . . . . . . . . . . . . . 8 72 5. Operational Considerations . . . . . . . . . . . . . . . . . 8 73 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 74 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9 75 8. Changes . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 76 9. Normative References . . . . . . . . . . . . . . . . . . . . 10 77 10. Informative References . . . . . . . . . . . . . . . . . . . 10 78 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11 80 1. Introduction 82 The Domain Name System (DNS) [RFC1034], [RFC1035] uses glue records 83 to allow iterative clients to find the addresses of name servers that 84 are contained within a delegated zone. Glue records are added to the 85 parent zone as part of the delegation process and returned in 86 referral responses, otherwise a resolver following the referral has 87 no way of finding these addresses. Authoritative servers are 88 expected to return all available in-domain glue records in a referral 89 response. If message size constraints prevent the inclusion of all 90 in-domain glue records over the chosen transport, the server MUST set 91 the TC (Truncated) flag to inform the client that the response is 92 incomplete, and that the client SHOULD use another transport retrieve 93 the full response. This document clarifies that expectation. 95 DNS responses sometimes contain optional data in the additional 96 section. In-domain glue records, however, are not optional. Several 97 other protocol extensions, when used, are also not optional. This 98 includes TSIG [RFC2845], OPT [RFC6891], and SIG(0) [RFC2931]. 100 At the time of this writing, addresses (A or AAAA records) for a 101 delegation's authoritative name servers are the only type of glue 102 defined for the DNS. 104 Note that this document only clarifies requirements of name server 105 software implementations. It does not introduce or change any 106 requirements on data placed in DNS zones or registries. In other 107 words, this document only makes requirements on "available glue 108 records" (i.e., those given in a zone), but does not make 109 requirements regarding their presence in a zone. If some glue 110 records are absent from a given zone, an authoritative name server 111 may be unable to return a useful referral response for the 112 corresponding domain. The IETF may want to consider a separate 113 update to the requirements for including glue in zone data, beyond 114 those given in [RFC1034] and [RFC1035]. 116 1.1. Reserved Words 118 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 119 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 120 document are to be interpreted as described in [RFC2119]. 122 2. Types of Glue in Referral Responses 124 This section describes different types of glue that may be found in 125 DNS referral responses. Note that the type of glue depends on the 126 QNAME. A particular record can be in-domain glue for one response 127 and sibling glue for another. 129 2.1. Glue for In-Domain Name Servers 131 The following is a simple example of glue records present in the 132 delegating zone "test" for the child zone "foo.test". The name 133 servers for foo.test (ns1.foo.test and ns2.foo.test) are both below 134 the delegation point. They are configured as glue records in the 135 "test" zone: 137 foo.test. 86400 IN NS ns1.foo.test. 138 foo.test. 86400 IN NS ns2.foo.test. 139 ns1.foo.test. 86400 IN A 192.0.2.1 140 ns2.foo.test. 86400 IN AAAA 2001:db8::2:2 142 A referral response from "test" for "foo.test" with glue for in- 143 domain name servers looks like this: 145 ;; QUESTION SECTION: 146 ;www.foo.test. IN A 148 ;; AUTHORITY SECTION: 149 foo.test. 86400 IN NS ns1.foo.test. 150 foo.test. 86400 IN NS ns2.foo.test. 152 ;; ADDITIONAL SECTION: 153 ns1.foo.test. 86400 IN A 192.0.2.1 154 ns2.foo.test. 86400 IN AAAA 2001:db8::2:2 156 2.2. Glue for Sibling Domain Name Servers 158 Sibling domain name servers are NS records that are not contained in 159 the delegated zone itself, but in another zone delegated from the 160 same parent. In many cases, glue for sibling domain name servers are 161 not strictly required for resolution, since the resolver can make 162 follow-on queries to the sibling zone to resolve the name server 163 addresses (after following the referral to the sibling zone). 164 However, most name server implementations today provide them as an 165 optimization to obviate the need for extra traffic from iterative 166 resolvers. 168 Here the delegating zone "test" contains two delegations for the 169 child zones "bar.test" and "foo.test": 171 bar.test. 86400 IN NS ns1.bar.test. 172 bar.test. 86400 IN NS ns2.bar.test. 173 ns1.bar.test. 86400 IN A 192.0.2.1 174 ns2.bar.test. 86400 IN AAAA 2001:db8::2:2 176 foo.test. 86400 IN NS ns1.bar.test. 177 foo.test. 86400 IN NS ns2.bar.test. 179 A referral response from "test" for "foo.test" with glue for sibling 180 domain name servers looks like this: 182 ;; QUESTION SECTION: 183 ;www.foo.test. IN A 185 ;; AUTHORITY SECTION: 186 foo.test. 86400 IN NS ns1.bar.test. 187 foo.test. 86400 IN NS ns2.bar.test. 189 ;; ADDITIONAL SECTION: 190 ns1.bar.test. 86400 IN A 192.0.2.1 191 ns2.bar.test. 86400 IN AAAA 2001:db8::2:2 193 2.3. Glue for Cyclic Sibling Domain Name Servers 195 The use of sibling domain name servers can introduce cyclic 196 dependencies. This happens when one domain specifies name servers 197 from a sibling domain, and vice versa. This type of cyclic 198 dependency can only be broken when the delegating name server 199 includes glue for the sibling domain in a referral response. 201 Here the delegating zone "test" contains two delegations for the 202 child zones "bar.test" and "foo.test", and each use name servers 203 under the other: 205 bar.test. 86400 IN NS ns1.foo.test. 206 bar.test. 86400 IN NS ns2.foo.test. 207 ns1.bar.test. 86400 IN A 192.0.2.1 208 ns2.bar.test. 86400 IN AAAA 2001:db8::2:2 210 foo.test. 86400 IN NS ns1.bar.test. 211 foo.test. 86400 IN NS ns2.bar.test. 212 ns1.foo.test. 86400 IN A 192.0.2.3 213 ns2.foo.test. 86400 IN AAAA 2001:db8::2:4 215 A referral response from "test" for "bar.test" with glue for sibling 216 domain name servers looks like this: 218 ;; QUESTION SECTION: 219 ;www.bar.test. IN A 221 ;; AUTHORITY SECTION: 222 bar.test. 86400 IN NS ns1.foo.test. 223 bar.test. 86400 IN NS ns2.foo.test. 225 ;; ADDITIONAL SECTION: 226 ns1.foo.test. 86400 IN A 192.0.2.3 227 ns2.foo.test. 86400 IN AAAA 2001:db8::2:4 229 In late 2021 the authors analyzed zone file data available from 230 ICANN's Centralized Zone Data Service [CZDS] and found 222 out of 231 approximately 209,000,000 total delegations that had only sibling 232 domain NS RRs in a cyclic dependency as above. 234 2.4. Missing Glue 236 An example of missing glue is included here, even though it can not 237 be considered as a type of glue. While not common, real examples of 238 responses that lack required glue, and with TC=0, have been shown to 239 occur and cause resolution failures. 241 The example below is based on a response observed in June 2020. The 242 names have been altered to fall under documentation domains. It 243 shows a case where none of the glue records present in the zone fit 244 into the available space of the UDP response, and the TC flag was not 245 set. While this example shows a referral with DNSSEC records 246 [RFC4033], [RFC4034], [RFC4035], this behavior has been seen with 247 plain DNS responses as well. Some records have been truncated for 248 display purposes. Note that at the time of this writing, the servers 249 originally responsible for this example have been updated and now 250 correctly set the TC flag. 252 % dig +norec +dnssec +bufsize=512 +ignore @ns.example.net \ 253 rh202ns2.355.foo.example 255 ; <<>> DiG 9.15.4 <<>> +norec +dnssec +bufsize +ignore \ 256 @ns.example.net rh202ns2.355.foo.example 257 ; (2 servers found) 258 ;; global options: +cmd 259 ;; Got answer: 260 ;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 8798 261 ;; flags: qr; QUERY: 1, ANSWER: 0, AUTHORITY: 9, ADDITIONAL: 1 263 ;; OPT PSEUDOSECTION: 264 ; EDNS: version: 0, flags: do; udp: 4096 265 ;; QUESTION SECTION: 266 ;rh202ns2.355.foo.example. IN A 268 ;; AUTHORITY SECTION: 269 foo.example. 86400 IN NS rh120ns2.368.foo.example. 270 foo.example. 86400 IN NS rh202ns2.355.foo.example. 271 foo.example. 86400 IN NS rh120ns1.368.foo.example. 272 foo.example. 86400 IN NS rh202ns1.355.foo.example. 273 foo.example. 3600 IN DS 51937 8 1 ... 274 foo.example. 3600 IN DS 635 8 2 ... 275 foo.example. 3600 IN DS 51937 8 2 ... 276 foo.example. 3600 IN DS 635 8 1 ... 277 foo.example. 3600 IN RRSIG DS 8 2 3600 ... 279 3. Requirements 281 This section describes updated requirements for including glue in DNS 282 referral responses. 284 3.1. Glue for In-Domain Name Servers 286 This document clarifies that when a name server generates a referral 287 response, it MUST include all available glue records for in-domain 288 name servers in the additional section, or MUST set TC=1 if 289 constrained by message size. 291 At the time of writing, most iterative clients send initial queries 292 over UDP and retry over TCP upon receiving a response with the TC 293 flag set. UDP responses are generally limited to between 1232 and 294 4096 bytes, due to values commonly used for the EDNS0 UDP Message 295 Size field [RFC6891], [FLAGDAY2020]. TCP responses are limited to 296 65,535 bytes. 298 3.2. Glue for Sibling Domain Name Servers 300 This document clarifies that when a name server generates a referral 301 response, it SHOULD include all available glue records in the 302 additional section. If, after adding glue for all in-domain name 303 servers, the glue for all sibling domain name servers does not fit 304 due to message size constraints, the name server is NOT REQUIRED to 305 set TC=1. 307 Note that users may experience resolution failures for domains with 308 cyclically-dependent sibling name servers when the delegating name 309 server chooses to omit the corresponding glue in a referral response. 310 As described in Section 2.3, such domains are rare. 312 3.3. Updates to RFC 1034 314 Replace 316 "Copy the NS RRs for the subzone into the authority section of the 317 reply. Put whatever addresses are available into the additional 318 section, using glue RRs if the addresses are not available from 319 authoritative data or the cache. Go to step 4." 321 with 323 "Copy the NS RRs for the subzone into the authority section of the 324 reply. Put whatever NS addresses are available into the additional 325 section, using glue RRs if the addresses are not available from 326 authoritative data or the cache. If all glue RRs for in-domain name 327 servers do not fit, set TC=1 in the header. Go to step 4." 329 4. Security Considerations 331 This document clarifies correct DNS server behavior and does not 332 introduce any changes or new security considerations. 334 5. Operational Considerations 336 At the time of this writing, the behavior of most DNS server 337 implementations is to set the TC flag only if none of the available 338 glue records fit in a response over UDP transport. The updated 339 requirements in this document might lead to an increase in the 340 fraction of UDP responses with the TC flag set, and consequently an 341 increase in the number of queries to over TCP transport. 343 6. IANA Considerations 345 There are no actions for IANA. 347 7. Acknowledgements 349 The authors wish to thank Joe Abley, David Blacka, Brian Dickson, 350 Kazunori Fujiwara, Paul Hoffman, Geoff Huston, Jared Mauch, George 351 Michaelson, Yasuhiro Orange Morishita, Benno Overeinder, John R 352 Levine, Hugo Salgado, Shinta Sato, Puneet Sood, Petr Spacek, Ralf 353 Weber, Tim Wicinski, Suzanne Woolf, and other members of the DNSOP 354 working group for their input. 356 8. Changes 358 RFC Editor: Please remove this section before publication. 360 This section lists substantial changes to the document as it is being 361 worked on. 363 From -01 to -02: 365 * Clarified that "servers" means "authoritative servers". 367 * Clarified that "available glue" means "all available glue". 369 * Updated examples and placed before RFC 1034 update. 371 From -02 to -03: 373 * Clarified scope to focus only on name server responses, and not 374 zone/registry data. 376 * Reorganized with section 2 as Types of Glue and section 3 as 377 Requirements. 379 * Removed any discussion of promoted / orphan glue. 381 * Use appropriate documentation addresses and domain names. 383 * Added Sibling Cyclic Glue example. 385 From -03 to -04: 387 * Use "referral glue" on the assumption that other types of glue may 388 be defined in the future. 390 * Added Operational Considerations section. 392 * Note many current implementations set TC=1 only when no glue RRs 393 fit. New requirements may lead to more truncation and TCP. 395 * Sibling glue can be optional. Only require TC=1 when all in- 396 domain glue RRs don't fit. 398 * Avoid talking about requirements for UDP/TCP specifically, and 399 talk more generically about message size constraints regardless of 400 transport. 402 From -04 to -05: 404 * Reverting the -04 change to use the phrase "referral glue". 406 * Rephrase "in-domain glue" as "glue for in-domain name servers". 408 * Rephrase "sibling glue" as "glue for sibling domain name servers". 410 * Expand paragraph noting this document does not make requirements 411 about presence of glue in zones. 413 9. Normative References 415 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 416 STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987, 417 . 419 [RFC1035] Mockapetris, P., "Domain names - implementation and 420 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 421 November 1987, . 423 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 424 Requirement Levels", BCP 14, RFC 2119, 425 DOI 10.17487/RFC2119, March 1997, 426 . 428 10. Informative References 430 [CZDS] ICANN, "Centralized Zone Data Service", January 2022, 431 . 433 [FLAGDAY2020] 434 Various DNS software and service providers, "DNS Flag Day 435 2020", October 2020, . 437 [RFC2845] Vixie, P., Gudmundsson, O., Eastlake 3rd, D., and B. 438 Wellington, "Secret Key Transaction Authentication for DNS 439 (TSIG)", RFC 2845, DOI 10.17487/RFC2845, May 2000, 440 . 442 [RFC2931] Eastlake 3rd, D., "DNS Request and Transaction Signatures 443 ( SIG(0)s )", RFC 2931, DOI 10.17487/RFC2931, September 444 2000, . 446 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 447 Rose, "DNS Security Introduction and Requirements", 448 RFC 4033, DOI 10.17487/RFC4033, March 2005, 449 . 451 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. 452 Rose, "Resource Records for the DNS Security Extensions", 453 RFC 4034, DOI 10.17487/RFC4034, March 2005, 454 . 456 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 457 Rose, "Protocol Modifications for the DNS Security 458 Extensions", RFC 4035, DOI 10.17487/RFC4035, March 2005, 459 . 461 [RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms 462 for DNS (EDNS(0))", STD 75, RFC 6891, 463 DOI 10.17487/RFC6891, April 2013, 464 . 466 Authors' Addresses 468 M. Andrews 469 ISC 470 Email: marka@isc.org 472 Shumon Huque 473 Salesforce 474 Email: shuque@gmail.com 476 Paul Wouters 477 Aiven 478 Email: paul.wouters@aiven.io 480 Duane Wessels 481 Verisign 482 Email: dwessels@verisign.com