idnits 2.17.1 draft-ietf-dnssd-srp-08.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: ---------------------------------------------------------------------------- == There is 1 instance of lines with non-ascii characters in the document. Checking nits according to https://www.ietf.org/id-info/checklist : ---------------------------------------------------------------------------- == There are 2 instances of lines with non-RFC2606-compliant FQDNs in the document. == There are 2 instances of lines with non-RFC3849-compliant IPv6 addresses in the document. If these are example addresses, they should be changed. ** The document seems to lack a both a reference to RFC 2119 and the recommended RFC 2119 boilerplate, even if it appears to use RFC 2119 keywords. RFC 2119 keyword, line 212: '...rvices using SRP MUST use the domain n...' RFC 2119 keyword, line 215: '...e than one domain name, it MUST NOT be...' RFC 2119 keyword, line 295: '...ce Instance Name MUST be referenced by...' RFC 2119 keyword, line 399: '.../private key pair. This key pair MUST...' RFC 2119 keyword, line 401: '..., the SRP client MUST be pre-configure...' (57 more instances...) Miscellaneous warnings: ---------------------------------------------------------------------------- == The copyright year in the IETF Trust and authors Copyright Line does not match the current year -- The document date (7 January 2021) is 1204 days in the past. Is this intentional? Checking references for intended status: Proposed Standard ---------------------------------------------------------------------------- (See RFCs 3967 and 4897 for information about using normative references to lower-maturity documents in RFCs) == Outdated reference: A later version (-03) exists of draft-sekar-dns-ul-02 -- Possible downref: Non-RFC (?) normative reference: ref. 'SUDN' -- Possible downref: Non-RFC (?) normative reference: ref. 'LSDZ' == Outdated reference: A later version (-02) exists of draft-sctl-advertising-proxy-00 Summary: 1 error (**), 0 flaws (~~), 6 warnings (==), 3 comments (--). Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Internet Engineering Task Force T. Lemon 3 Internet-Draft S. Cheshire 4 Intended status: Standards Track Apple Inc. 5 Expires: 11 July 2021 7 January 2021 7 Service Registration Protocol for DNS-Based Service Discovery 8 draft-ietf-dnssd-srp-08 10 Abstract 12 The Service Registration Protocol for DNS-Based Service Discovery 13 uses the standard DNS Update mechanism to enable DNS-Based Service 14 Discovery using only unicast packets. This makes it possible to 15 deploy DNS Service Discovery without multicast, which greatly 16 improves scalability and improves performance on networks where 17 multicast service is not an optimal choice, particularly 802.11 18 (Wi-Fi) and 802.15.4 (IoT) networks. DNS-SD Service registration 19 uses public keys and SIG(0) to allow services to defend their 20 registrations against attack. 22 Status of This Memo 24 This Internet-Draft is submitted in full conformance with the 25 provisions of BCP 78 and BCP 79. 27 Internet-Drafts are working documents of the Internet Engineering 28 Task Force (IETF). Note that other groups may also distribute 29 working documents as Internet-Drafts. The list of current Internet- 30 Drafts is at https://datatracker.ietf.org/drafts/current/. 32 Internet-Drafts are draft documents valid for a maximum of six months 33 and may be updated, replaced, or obsoleted by other documents at any 34 time. It is inappropriate to use Internet-Drafts as reference 35 material or to cite them other than as "work in progress." 37 This Internet-Draft will expire on 11 July 2021. 39 Copyright Notice 41 Copyright (c) 2021 IETF Trust and the persons identified as the 42 document authors. All rights reserved. 44 This document is subject to BCP 78 and the IETF Trust's Legal 45 Provisions Relating to IETF Documents (https://trustee.ietf.org/ 46 license-info) in effect on the date of publication of this document. 47 Please review these documents carefully, as they describe your rights 48 and restrictions with respect to this document. Code Components 49 extracted from this document must include Simplified BSD License text 50 as described in Section 4.e of the Trust Legal Provisions and are 51 provided without warranty as described in the Simplified BSD License. 53 Table of Contents 55 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 56 2. Service Registration Protocol . . . . . . . . . . . . . . . . 5 57 2.1. Protocol Variants . . . . . . . . . . . . . . . . . . . . 5 58 2.1.1. Full-featured Hosts . . . . . . . . . . . . . . . . . 5 59 2.1.2. Constrained Hosts . . . . . . . . . . . . . . . . . . 6 60 2.1.3. Why two variants? . . . . . . . . . . . . . . . . . . 6 61 2.2. Protocol Details . . . . . . . . . . . . . . . . . . . . 6 62 2.2.1. What to publish . . . . . . . . . . . . . . . . . . . 7 63 2.2.2. Where to publish it . . . . . . . . . . . . . . . . . 7 64 2.2.3. How to publish it . . . . . . . . . . . . . . . . . . 8 65 2.2.4. How to secure it . . . . . . . . . . . . . . . . . . 9 66 2.2.5. Service Behavior . . . . . . . . . . . . . . . . . . 9 67 2.3. SRP Server Behavior . . . . . . . . . . . . . . . . . . . 12 68 2.3.1. Validation of Adds and Deletes . . . . . . . . . . . 12 69 2.3.2. Valid SRP Update Requirements . . . . . . . . . . . . 14 70 2.3.3. FCFS Name And Signature Validation . . . . . . . . . 15 71 2.3.4. SRP Update response . . . . . . . . . . . . . . . . . 15 72 2.3.5. Optional Behavior . . . . . . . . . . . . . . . . . . 16 73 3. TTL Consistency . . . . . . . . . . . . . . . . . . . . . . . 16 74 4. Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . 17 75 4.1. Cleaning up stale data . . . . . . . . . . . . . . . . . 17 76 5. Sleep Proxy . . . . . . . . . . . . . . . . . . . . . . . . . 19 77 6. Security Considerations . . . . . . . . . . . . . . . . . . . 20 78 6.1. Source Validation . . . . . . . . . . . . . . . . . . . . 20 79 6.2. SRP Server Authentication . . . . . . . . . . . . . . . . 21 80 6.3. Required Signature Algorithm . . . . . . . . . . . . . . 21 81 7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 21 82 8. Delegation of 'service.arpa.' . . . . . . . . . . . . . . . . 21 83 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21 84 9.1. Registration and Delegation of 'service.arpa' as a 85 Special-Use Domain Name . . . . . . . . . . . . . . . . . 22 86 9.2. 'dnssd-srp' Service Name . . . . . . . . . . . . . . . . 22 87 9.3. 'dnssd-srp-tls' Service Name . . . . . . . . . . . . . . 22 88 9.4. Anycast Address . . . . . . . . . . . . . . . . . . . . . 23 89 10. Implementation Status . . . . . . . . . . . . . . . . . . . . 23 90 11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 24 91 12. Normative References . . . . . . . . . . . . . . . . . . . . 24 92 13. Informative References . . . . . . . . . . . . . . . . . . . 26 93 Appendix A. Testing using standard RFC2136-compliant servers . . 27 94 Appendix B. How to allow services to update standard 95 RFC2136-compliant servers . . . . . . . . . . . . . . . . 28 97 Appendix C. Sample BIND9 configuration for 98 default.service.arpa. . . . . . . . . . . . . . . . . . . 28 99 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29 101 1. Introduction 103 DNS-Based Service Discovery [RFC6763] is a component of Zero 104 Configuration Networking [RFC6760] [ZC] [I-D.cheshire-dnssd-roadmap]. 106 This document describes an enhancement to DNS-Based Service Discovery 107 [RFC6763] that allows services to register their services using the 108 DNS protocol rather than using Multicast DNS [RFC6762] (mDNS). There 109 is already a large installed base of DNS-SD clients that can discover 110 services using the DNS protocol. 112 This document is intended for three audiences: implementors of 113 software that provides services that should be advertised using 114 DNS-SD, implementors of DNS servers that will be used in contexts 115 where DNS-SD registration is needed, and administrators of networks 116 where DNS-SD service is required. The document is intended to 117 provide sufficient information to allow interoperable implementation 118 of the registration protocol. 120 DNS-Based Service Discovery (DNS-SD) allows services to advertise the 121 fact that they provide service, and to provide the information 122 required to access that service. DNS-SD clients can then discover 123 the set of services of a particular type that are available. They 124 can then select a service from among those that are available and 125 obtain the information required to use it. Although DNS-SD using the 126 DNS protocol (as opposed to mDNS) can be more efficient and 127 versatile, it is not common in practice, because of the difficulties 128 associated with updating authoritative DNS services with service 129 information. 131 Existing practice for updating DNS zones is to either manually enter 132 new data, or else use DNS Update [RFC2136]. Unfortunately DNS Update 133 requires either that the authoritative DNS server automatically trust 134 updates, or else that the DNS Update client have some kind of shared 135 secret or public key that is known to the DNS server and can be used 136 to authenticate the update. Furthermore, DNS Update can be a fairly 137 chatty process, requiring multiple round trips with different 138 conditional predicates to complete the update process. 140 The SRP protocol adds a set of default heuristics for processing DNS 141 updates that eliminates the need for DNS update conditional 142 predicates: instead, the SRP server has a set of default predicates 143 that are applied to the update, and the update either succeeds 144 entirely, or fails in a way that allows the registering service to 145 know what went wrong and construct a new update. 147 SRP also adds a feature called First-Come, First-Served Naming, which 148 allows the registering service to claim a name that is not yet in 149 use, and, using SIG(0) [RFC2931], to authenticate both the initial 150 claim and subsequent updates. This prevents name conflicts, since a 151 second SRP service attempting to claim the same name will not possess 152 the SIG(0) key used by the first service to claim it, and so its 153 claim will be rejected and the second service will have to choose a 154 new name. 156 Finally, SRP adds the concept of a 'lease,' similar to leases in 157 Dynamic Host Configuration Protocol [RFC8415]. The SRP registration 158 itself has a lease which may be on the order of an hour; if the 159 registering service does not renew the lease before it has elapsed, 160 the registration is removed. The claim on the name can have a longer 161 lease, so that another service cannot claim the name, even though the 162 registration has expired. 164 The Service Registration Protocol for DNS-SD (SRP), described in this 165 document, provides a reasonably secure mechanism for publishing this 166 information. Once published, these services can be readily 167 discovered by DNS-SD clients using standard DNS lookups. 169 The DNS-SD specification [RFC6763], Section 10 ("Populating the DNS 170 with Information"), briefly discusses ways that services can publish 171 their information in the DNS namespace. In the case of mDNS, it 172 allows services to publish their information on the local link, using 173 names in the ".local" namespace, which makes their services directly 174 discoverable by peers attached to that same local link. 176 RFC6763 also allows clients to discover services using the DNS 177 protocol [RFC1035]. This can be done by having a system 178 administrator manually configure service information in the DNS, but 179 manually populating DNS authoritative server databases is costly and 180 potentially error-prone, and requires a knowledgable network 181 administrator. Consequently, although all DNS-SD client 182 implementations of which we are aware support DNS-SD using DNS 183 queries, in practice it is used much less frequently than mDNS. 185 The Discovery Proxy [RFC8766] provides one way to automatically 186 populate the DNS namespace, but is only appropriate on networks where 187 services are easily advertised using mDNS. This document describes a 188 solution more suitable for networks where multicast is inefficient, 189 or where sleepy devices are common, by supporting both offering of 190 services, and discovery of services, using unicast. 192 2. Service Registration Protocol 194 Services that implement SRP use DNS Update [RFC2136] [RFC3007] to 195 publish service information in the DNS. Two variants exist, one for 196 full-featured hosts, and one for devices designed for "Constrained- 197 Node Networks" [RFC7228]. An SRP server is most likely an 198 authoritative DNS server, or else is updating an authoritative DNS 199 server. There is no requirement that the server that is receiving 200 SRP requests be the same server that is answering queries that return 201 records that have been registered. 203 2.1. Protocol Variants 205 2.1.1. Full-featured Hosts 207 Full-featured hosts are either configured manually with a 208 registration domain, or use the "dr._dns-sd._udp." query 209 ([RFC6763], Section 11) to learn the default registration domain from 210 the network. RFC6763 says to discover the registration domain using 211 either ".local" or a network-supplied domain name for . 212 Services using SRP MUST use the domain name received through the 213 DHCPv4 Domain Name option ([RFC2132], Section 3.17), if available, or 214 the Neighbor Discovery DNS Search List option [RFC8106]. If the DNS 215 Search List option contains more than one domain name, it MUST NOT be 216 used. If neither option is available, the Service Registration 217 protocol is not available on the local network. 219 Manual configuration of the registration domain can be done either by 220 querying the list of available registration zones ("r._dns-sd._udp") 221 and allowing the user to select one from the UI, or by any other 222 means appropriate to the particular use case being addressed. Full- 223 featured devices construct the names of the SRV, TXT, and PTR records 224 describing their service(s) as subdomains of the chosen service 225 registration domain. For these names they then discover the zone 226 apex of the closest enclosing DNS zone using SOA queries [RFC8765]. 227 Having discovered the enclosing DNS zone, they query for the 228 "_dnssd-srp._tcp." SRV record to discover the server to which 229 they should send DNS updates. Hosts that support SRP updates using 230 TLS use the "_dnssd-srp-tls._tcp." SRV record instead. 232 2.1.2. Constrained Hosts 234 For devices designed for Constrained-Node Networks [RFC7228] some 235 simplifications are available. Instead of being configured with (or 236 discovering) the service registration domain, the (proposed) special- 237 use domain name (see [RFC6761]) "default.service.arpa" is used. The 238 details of how SRP server(s) are discovered will be specific to the 239 constrained network, and therefore we do not suggest a specific 240 mechanism here. 242 SRP clients on constrained networks are expected to receive from the 243 network a list of SRP servers with which to register. It is the 244 responsibility of a Constrained-Node Network supporting SRP to 245 provide one or more SRP server addresses. It is the responsibility 246 of the SRP server supporting a Constrained-Node Network to handle the 247 updates appropriately. In some network environments, updates may be 248 accepted directly into a local "default.service.arpa" zone, which has 249 only local visibility. In other network environments, updates for 250 names ending in "default.service.arpa" may be rewritten internally to 251 names with broader visibility. 253 2.1.3. Why two variants? 255 The reason for these different assumptions is that low-power devices 256 that typically use Constrained-Node Networks may have very limited 257 battery power. The series of DNS lookups required to discover an SRP 258 server and then communicate with it will increase the power required 259 to advertise a service; for low-power devices, the additional 260 flexibility this provides does not justify the additional use of 261 power. It is also fairly typical of such networks that some network 262 service information is obtained as part of the process of joining the 263 network, and so this can be relied upon to provide nodes with the 264 information they need. 266 Networks that are not constrained networks can more complicated 267 topologies at the Internet layer. Nodes connected to such networks 268 can be assumed to be able to do DNSSD service registration domain 269 discovery. Such networks are generally able to provide registration 270 domain discovery and routing. By requiring the use of TCP, the 271 possibility of off-network spoofing is eliminated. 273 2.2. Protocol Details 275 We will discuss several parts to this process: how to know what to 276 publish, how to know where to publish it (under what name), how to 277 publish it, how to secure its publication, and how to maintain the 278 information once published. 280 2.2.1. What to publish 282 We refer to the DNS Update message sent by services using SRP as an 283 SRP update. Three types of updates appear in an SRP update: Service 284 Discovery records, Service Description records, and Host Description 285 records. 287 * Service Discovery records are one or more PTR RRs, mapping from 288 the generic service type (or subtype) to the specific Service 289 Instance Name. 290 * Service Description records are exactly one SRV RR, exactly one 291 KEY RR, and one or more TXT RRs, all with the same name, the 292 Service Instance Name ([RFC6763], Section 4.1). In principle 293 Service Description records can include other record types, with 294 the same Service Instance Name, though in practice they rarely do. 295 The Service Instance Name MUST be referenced by one or more 296 Service Discovery PTR records, unless it is a placeholder service 297 registration for an intentionally non-discoverable service name. 298 * The Host Description records for a service are a KEY RR, used to 299 claim exclusive ownership of the service registration, and one or 300 more RRs of type A or AAAA, giving the IPv4 or IPv6 address(es) of 301 the host where the service resides. 303 RFC 6763 describes the details of what each of these types of updates 304 contains and is the definitive source for information about what to 305 publish; the reason for summarizing this here is to provide the 306 reader with enough information about what will be published that the 307 service registration process can be understood at a high level 308 without first learning the full details of DNS-SD. Also, the 309 "Service Instance Name" is an important aspect of first-come, first- 310 serve naming, which we describe later on in this document. 312 2.2.2. Where to publish it 314 Multicast DNS uses a single namespace, ".local", which is valid on 315 the local link. This convenience is not available for DNS-SD using 316 the DNS protocol: services must exist in some specific unicast 317 namespace. 319 As described above, full-featured devices are responsible for knowing 320 in what domain they should register their services. Devices made for 321 Constrained-Node Networks register in the (proposed) special use 322 domain name [RFC6761] "default.service.arpa", and let the SRP server 323 handle rewriting that to a different domain if necessary. 325 2.2.3. How to publish it 327 It is possible to issue a DNS Update that does several things at 328 once; this means that it's possible to do all the work of adding a 329 PTR resource record to the PTR RRset on the Service Name, and 330 creating or updating the Service Instance Name and Host Description, 331 in a single transaction. 333 An SRP update takes advantage of this: it is implemented as a single 334 DNS Update message that contains a service's Service Discovery 335 records, Service Description records, and Host Description records. 337 Updates done according to this specification are somewhat different 338 than regular DNS Updates as defined in RFC2136. The RFC2136 update 339 process can involve many update attempts: you might first attempt to 340 add a name if it doesn't exist; if that fails, then in a second 341 message you might update the name if it does exist but matches 342 certain preconditions. Because the registration protocol uses a 343 single transaction, some of this adaptability is lost. 345 In order to allow updates to happen in a single transaction, SRP 346 updates do not include update prerequisites. The requirements 347 specified in Section 2.3 are implicit in the processing of SRP 348 updates, and so there is no need for the service sending the SRP 349 update to put in any explicit prerequisites. 351 2.2.3.1. How DNS-SD Service Registration differs from standard RFC2136 352 DNS Update 354 DNS-SD Service Registration is based on standard RFC2136 DNS Update, 355 with some differences: 357 * It implements first-come first-served name allocation, protected 358 using SIG(0) [RFC2931]. 359 * It enforces policy about what updates are allowed. 360 * It optionally performs rewriting of "default.service.arpa" to some 361 other domain. 362 * It optionally performs automatic population of the address-to-name 363 reverse mapping domains. 364 * An SRP server is not required to implement general DNS Update 365 prerequisite processing. 366 * SRP clients are allowed to send updates to the generic domain 367 "default.service.arpa" 369 2.2.4. How to secure it 371 Traditional DNS update is secured using the TSIG protocol, which uses 372 a secret key shared between the DNS Update client (which issues the 373 update) and the server (which authenticates it). This model does not 374 work for automatic service registration. 376 The goal of securing the DNS-SD Registration Protocol is to provide 377 the best possible security given the constraint that service 378 registration has to be automatic. It is possible to layer more 379 operational security on top of what we describe here, but what we 380 describe here is an improvement over the security of mDNS. The goal 381 is not to provide the level of security of a network managed by a 382 skilled operator. 384 2.2.4.1. First-Come First-Served Naming 386 First-Come First-Serve naming provides a limited degree of security: 387 a service that registers its service using DNS-SD Registration 388 protocol is given ownership of a name for an extended period of time 389 based on the key used to authenticate the DNS Update. As long as the 390 registration service remembers the name and the key used to register 391 that name, no other service can add or update the information 392 associated with that. FCFS naming is used to protect both the 393 Service Description and the Host Description. 395 2.2.5. Service Behavior 397 2.2.5.1. Public/Private key pair generation and storage 399 The service generates a public/private key pair. This key pair MUST 400 be stored in stable storage; if there is no writable stable storage 401 on the SRP client, the SRP client MUST be pre-configured with a 402 public/private key pair in read-only storage that can be used. This 403 key pair MUST be unique to the device. This key pair MUST be unique 404 to the device. A device with rewritable storage should retain this 405 key indefinitely. When the device changes ownership, it may be 406 appropriate to erase the old key and install a new one. Therefore, 407 the SRP client on the device SHOULD provide a mechanism to overwrite 408 the key, for example as the result of a "factory reset." 410 When sending DNS updates, the service includes a KEY record 411 containing the public portion of the key in each Host Description 412 update and each Service Description update. Each KEY record MUST 413 contain the same public key. The update is signed using SIG(0), 414 using the private key that corresponds to the public key in the KEY 415 record. The lifetimes of the records in the update is set using the 416 EDNS(0) Update Lease option [I-D.sekar-dns-ul]. 418 The KEY record in Service Description updates MAY be omitted for 419 brevity; if it is omitted, the SRP server MUST behave as if the same 420 KEY record that is given for the Host Description is also given for 421 each Service Description for which no KEY record is provided. 422 Omitted KEY records are not used when computing the SIG(0) signature. 424 2.2.5.2. Name Conflict Handling 426 Both Host Description records and Service Description Records can 427 have names that result in name conflicts. Service Discovery records 428 cannot have name conflicts. If any Host Description or Service 429 Description record is found by the server to have a conflict with an 430 existing name, the server will respond to the SRP Update with a 431 YXDOMAIN rcode. In this case, the Service MUST either abandon the 432 service registration attempt, or else choose a new name. 434 There is no specific requirement for how this is done; typically, 435 however, the service will append a number to the preferred name. 436 This number could be sequentially increasing, or could be chosen 437 randomly. One existing implementation attempts several sequential 438 numbers before choosing randomly. So for instance, it might try 439 host.service.arpa, then host-1.service.arpa, then host- 440 2.service.arpa, then host-31773.service.arpa. 442 2.2.5.3. Record Lifetimes 444 The lifetime of the DNS-SD PTR, SRV, A, AAAA and TXT records 445 [RFC6763] uses the LEASE field of the Update Lease option, and is 446 typically set to two hours. This means that if a device is 447 disconnected from the network, it does not appear in the user 448 interfaces of devices looking for services of that type for too long. 450 The lifetime of the KEY records is set using the KEY-LEASE field of 451 the Update Lease Option, and should be set to a much longer time, 452 typically 14 days. The result of this is that even though a device 453 may be temporarily unplugged, disappearing from the network for a few 454 days, it makes a claim on its name that lasts much longer. 456 This means that even if a device is unplugged from the network for a 457 few days, and its services are not available for that time, no other 458 device can come along and claim its name the moment it disappears 459 from the network. In the event that a device is unplugged from the 460 network and permanently discarded, then its name is eventually 461 cleaned up and made available for re-use. 463 2.2.5.4. Compression in SRV records 465 Although [RFC2782] requires that the target name in the SRV record 466 not be compressed, an SRP client SHOULD compress the target in the 467 SRV record. The motivation for _not_ compressing in RFC2782 is not 468 stated, but is assumed to be because a caching resolver that does not 469 understand the format of the SRV record might store it as binary data 470 and thus return an invalid pointer in response to a query. This does 471 not apply in the case of SRP: an SRP server needs to understand SRV 472 records in order to validate the SRP update. Compression of the 473 target potentially saves substantial space in the SRP update. 475 2.2.5.5. Removing published services 477 2.2.5.5.1. Removing all published services 479 To remove all the services registered to a particular host, the SRP 480 client retransmits its most recent update with an Update Lease option 481 that has a LEASE value of zero. If the registration is to be 482 permanently removed, KEY-LEASE should also be zero. Otherwise, it 483 should have the same value it had previously; this holds the name in 484 reserve for when the SRP client is once again able to provide the 485 service. 487 SRP clients are normally expected to remove all service instances 488 when removing a host. However, in some cases a SRP client may not 489 have retained sufficient state to know that some service instance is 490 pointing to a host that it is removing. This method of removing 491 services is intended for the case where the client is going offline 492 and does not want its services advertised. Therefore, it is 493 sufficient for the client to send the Host Description Instruction 494 (Section 2.3.1.3). 496 To support this, when removing services based on the lease time being 497 zero, an SRP server MUST remove all service instances pointing to a 498 host when a host is removed, even if the SRP client doesn't list them 499 explicitly. If the key lease time is nonzero, the SRP server MUST 500 NOT delete the KEY records for these SRP clients. 502 2.2.5.5.2. Removing some published services 504 In some use cases a client may need to remove some specific service, 505 without removing its other services. This can be accomplished in one 506 of two ways. To simply remove a specific service, the client sends a 507 valid SRP update where the Service Discovery instruction 508 (Section 2.3.1.1) contains a single Delete an RR from an RRset 509 ([RFC2136], Section 2.5.4) update that deletes the PTR record whose 510 target is the service instance name. The Service Description 511 instruction (Section 2.3.1.2) in this case contains a single Delete 512 all RRsets from a Name ([RFC2136], Section 2.5.3) update to the 513 service instance name. 515 The second alternative is used when some service is being replaced by 516 a different service with a different service instance name. In this 517 case, the old service is deleted as in the first alternative. The 518 new service is added, just as it would be in an update that wasn't 519 deleting the old service. Because both the removal of the old 520 service and the add of the new service consist of a valid Service 521 Discovery instruction and a valid Service Description instruction, 522 the update as a whole is a valid SRP update, and will result in the 523 old service being removed and the new one added, or, to put it 524 differently, in the old service being replaced by the new service. 526 It is perhaps worth noting that if a service is being updated without 527 the service instance name changing, that will look very much like the 528 second alternative above. The difference is that because the target 529 for the PTR record in the Service Discovery instruction is the same 530 for both the Delete An RR From An RRset update and the Add To An 531 RRSet update, these will be seen as a single Service Description 532 instruction, not as two instructions. The same would be true of the 533 Service Description instruction. 535 Whichever of these two alternatives is used, the host lease will be 536 updated with the lease time provided in the SRP update. In neither 537 of these cases is it permissible to delete the host. All services 538 must point to a host. If a host is to be deleted, this must be done 539 using the method described in Section 2.2.5.5.1, which deletes the 540 host and all services that have that host as their target. 542 2.3. SRP Server Behavior 544 2.3.1. Validation of Adds and Deletes 546 The SRP server first validates that the DNS Update is a syntactically 547 and semantically valid DNS Update according to the rules specified in 548 RFC2136. 550 SRP Updates consist of a set of _instructions_ that together add or 551 remove one or more services. Each instruction consists some 552 combination of delete updates and add updates. When an instruction 553 contains a delete and an add, the delete MUST precede the add. 555 The SRP server checks each instruction in the SRP update to see that 556 it is either a Service Discovery update, a Service Description 557 update, or a Host Description update. Order matters in DNS updates. 558 Specifically, deletes must precede adds for records that the deletes 559 would affect; otherwise the add will have no effect. This is the 560 only ordering constraint; aside from this constraint, updates may 561 appear in whatever order is convenient when constructing the update. 563 Because the SRP update is a DNS update, it MUST contain a single 564 question that indicates the zone to be updated. Every delete and 565 update in an SRP update MUST be within the zone that is specified for 566 the SRP Update. 568 2.3.1.1. Service Discovery Instruction 570 An instruction is a Service Discovery Instruction if it contains 572 * exactly one "Add to an RRSet" or exactly one "Delete an RR from an 573 RRSet" ([RFC2136], Section 2.5.1) RR update, 574 * which updates a PTR RR, 575 * which points to a Service Instance Name 576 * for which a Service Description Instruction is present in the SRP 577 Update 578 * if the Service Discovery update is an "Add to an RRSet" 579 instruction, the Service Description Instruction does not match if 580 it does not contain an "Add to an RRset" update for the SRV RR 581 describing that service. 582 * if the Service Discovery Instruction is an "Delete an RR from an 583 RRSet" update, the Service Description Instruction does not match 584 if it contains an "Add to an RRset" update. 585 * Service Discovery Instructions do not contain any other add or 586 delete updates. 588 2.3.1.2. Service Description Instruction 590 An instruction is a Service Description Instruction if, for the 591 appropriate Service Instance Name, it contains 593 * exactly one "Delete all RRsets from a name" update for the service 594 instance name ([RFC2136], Section 2.5.3), 595 * zero or one "Add to an RRset" SRV RR, 596 * zero or one "Add to an RRset" KEY RR that, if present, contains 597 the public key corresponding to the private key that was used to 598 sign the message (if present, the KEY MUST match the KEY RR given 599 in the Host Description), 600 * zero or more "Add to an RRset" TXT RRs, 601 * If there is one "Add to an RRset" SRV update, there MUST be at 602 least one "Add to an RRset" TXT update. 603 * the target of the SRV RR Add, if present points to a hostname for 604 which there is a Host Description Instruction in the SRP Update, 605 or 606 * if there is no "Add to an RRset" SRV RR, then either 607 - the name to which the "Delete all RRsets from a name" applies 608 does not exist, or 610 - there is an existing KEY RR on that name, which matches the key 611 with which the SRP Update was signed. 612 * Service Descriptions Instructions do not modify any other RRs. 614 An SRP server MUST correctly handle compressed names in the SRV 615 target. 617 2.3.1.3. Host Description Instruction 619 An instruction is a Host Description Instruction if, for the 620 appropriate hostname, it contains 622 * exactly one "Delete all RRsets from a name" RR, 623 * one or more "Add to an RRset" RRs of type A and/or AAAA, 624 * A and/or AAAA records must be of sufficient scope to be reachable 625 by all hosts that might query the DNS. If a link-scope address or 626 IPv4 autoconfiguration address is provided by the SRP client, the 627 SRP server MUST treat this as if no address records were received; 628 that is, the Host Description is not valid. 629 * exactly one "Add to an RRset" RR that adds a KEY RR that contains 630 the public key corresponding to the private key that was used to 631 sign the message, 632 * there is a Service Instance Name Instruction in the SRP update for 633 which the SRV RR that is added points to the hostname being 634 updated by this update. 635 * Host Description updates do not modify any other records. 637 2.3.2. Valid SRP Update Requirements 639 An SRP Update MUST include at zero or more Service Discovery 640 Instructions, the same number of Service Description Instructions, 641 and exactly one Host Description Instruction. A DNS Update that does 642 not is not an SRP update. A DNS Update that contains any other adds, 643 any other deletes, or any prerequisites, is not an SRP update. Such 644 messages should either be processed as regular RFC2136 updates, 645 including access control checks and constraint checks, if supported, 646 or else rejected with RCODE=REFUSED. 648 In addition, in order for an update to be a valid SRP update, the 649 target of every Service Discovery Instruction MUST be a Service 650 Description Instruction that is present in the SRP Update. There 651 MUST NOT be any Service Description Instruction to which no Service 652 Discovery Instruction points. The target of the SRV record in every 653 Service Description instruction MUST be the single Host Description 654 Instruction. 656 If the definitions of each of these instructions are followed 657 carefully and the update requirements are validated correctly, many 658 DNS Updates that look very much like SRP updates nevertheless will 659 fail to validate. For example, a DNS update that contains an RRset 660 Add to a Service Name and an RRset Add to a Service Instance Name, 661 where the Service Name does not reference the Service Instance Name, 662 is not a valid SRP update message, but may be a valid RFC2136 update. 664 2.3.3. FCFS Name And Signature Validation 666 Assuming that a DNS Update message has been validated with these 667 conditions and is a valid SRP Update, the server checks that the name 668 in the Host Description Instruction exists. If so, then the server 669 checks to see if the KEY record on that name is the same as the KEY 670 record in the Host Description Instruction. The server performs the 671 same check for the KEY records in any Service Description 672 Instructions. For KEY records that were omitted from Service 673 Description Instructions, the KEY from the Host Description 674 Instruction is used. If any existing KEY record corresponding to a 675 KEY record in the SRP Update does not match the KEY same record in 676 the SRP Update (whether provided or taken from the Host Description 677 Instruction), then the server MUST reject the SRP Update with the 678 YXDOMAIN RCODE. 680 Otherwise, the server validates the SRP Update using SIG(0) on the 681 public key in the KEY record of the Host Description update. If the 682 validation fails, the server MUST reject the SRP Update with the 683 REFUSED RCODE. Otherwise, the SRP update is considered valid and 684 authentic, and is processed according to the method described in 685 RFC2136. 687 KEY record updates omitted from Service Description update are 688 processed as if they had been explicitly present: every Service 689 Description that is updated MUST, after the update, have a KEY RR, 690 and it must be the same KEY RR that is present in the Host 691 Description to which the Service Description refers. 693 2.3.4. SRP Update response 695 The status that is returned depends on the result of processing the 696 update, and can be either SUCCESS or SERVFAIL: all other possible 697 outcomes should already have been accounted for when applying the 698 constraints that qualify the update as an SRP Update. 700 2.3.5. Optional Behavior 702 The server MAY add a Reverse Mapping that corresponds to the Host 703 Description. This is not required because the Reverse Mapping serves 704 no protocol function, but it may be useful for debugging, e.g. in 705 annotating network packet traces or logs. In order for the server to 706 add a reverse mapping update, it must be authoritative for the zone 707 or have credentials to do the update. The SRP client MAY also do a 708 reverse mapping update if it has credentials to do so. 710 The server MAY apply additional criteria when accepting updates. In 711 some networks, it may be possible to do out-of-band registration of 712 keys, and only accept updates from pre-registered keys. In this 713 case, an update for a key that has not been registered should be 714 rejected with the REFUSED RCODE. 716 There are at least two benefits to doing this rather than simply 717 using normal SIG(0) DNS updates. First, the same registration 718 protocol can be used in both cases, so both use cases can be 719 addressed by the same service implementation. Second, the 720 registration protocol includes maintenance functionality not present 721 with normal DNS updates. 723 Note that the semantics of using SRP in this way are different than 724 for typical RFC2136 implementations: the KEY used to sign the SRP 725 update only allows the SRP client to update records that refer to its 726 Host Description. RFC2136 implementations do not normally provide a 727 way to enforce a constraint of this type. 729 The server may also have a dictionary of names or name patterns that 730 are not permitted. If such a list is used, updates for Service 731 Instance Names that match entries in the dictionary are rejected with 732 YXDOMAIN. 734 3. TTL Consistency 736 All RRs within an RRset are required to have the same TTL 737 (Clarifications to the DNS Specification [RFC2181], Section 5.2). In 738 order to avoid inconsistencies, SRP places restrictions on TTLs sent 739 by services and requires that SRP servers enforce consistency. 741 Services sending SRP updates MUST use consistent TTLs in all RRs 742 within the SRP update. 744 SRP update servers MUST check that the TTLs for all RRs within the 745 SRP update are the same. If they are not, the SRP update MUST be 746 rejected with a REFUSED RCODE. 748 Additionally, when adding RRs to an RRset, for example when 749 processing Service Discovery records, the server MUST use the same 750 TTL on all RRs in the RRset. How this consistency is enforced is up 751 to the implementation. 753 TTLs sent in SRP updates are advisory: they indicate the SRP client's 754 guess as to what a good TTL would be. SRP servers may override these 755 TTLs. SRP servers SHOULD ensure that TTLs are reasonable: neither 756 too long nor too short. The TTL should never be longer than the 757 lease time (Section 4.1). Shorter TTLs will result in more frequent 758 data refreshes; this increases latency on the DNS-SD client side, 759 increases load on any caching resolvers and on the authoritative 760 server, and also increases network load, which may be an issue for 761 constrained networks. Longer TTLs will increase the likelihood that 762 data in caches will be stale. TTL minimums and maximums SHOULD be 763 configurable by the operator of the SRP server. 765 4. Maintenance 767 4.1. Cleaning up stale data 769 Because the DNS-SD registration protocol is automatic, and not 770 managed by humans, some additional bookkeeping is required. When an 771 update is constructed by the SRP client, it MUST include an EDNS(0) 772 Update Lease Option [I-D.sekar-dns-ul]. The Update Lease Option 773 contains two lease times: the Lease Time and the Key Lease Time. 775 These leases are promises, similar to DHCP leases [RFC2131], from the 776 SRP client that it will send a new update for the service 777 registration before the lease time expires. The Lease time is chosen 778 to represent the time after the update during which the registered 779 records other than the KEY record should be assumed to be valid. The 780 Key Lease time represents the time after the update during which the 781 KEY record should be assumed to be valid. 783 The reasoning behind the different lease times is discussed in the 784 section on first-come, first-served naming (Section 2.2.4.1). SRP 785 servers may be configured with limits for these values. A default 786 limit of two hours for the Lease and 14 days for the SIG(0) KEY are 787 currently thought to be good choices. Constrained devices with 788 limited battery that wake infrequently are likely to negotiate longer 789 leases. SRP clients that are going to continue to use names on which 790 they hold leases should update well before the lease ends, in case 791 the registration service is unavailable or under heavy load. 793 The lease time applies specifically to the host. All service 794 instances, and all service entries for such service instances, depend 795 on the host. When the lease on a host expires, the host and all 796 services that reference it MUST be removed at the same time-it is 797 never valid for a service instance to remain when the host it 798 references has been removed. If the KEY record for the host is to 799 remain, the KEY record for any services that reference it MUST also 800 remain. However, the service PTR record MUST be removed, since it 801 has no key associated with it, and since it is never valid to have a 802 service PTR record for which there is no service instance on the 803 target of the PTR record. 805 SRP Servers SHOULD also track a lease time per service instance. The 806 reason for doing this is that a client may re-register a host with a 807 different set of services, and not remember that some different 808 service instance had previously been registered. In this case, when 809 that service instance lease expires, the SRP server SHOULD remove the 810 service instance (although the KEY record for the service instance 811 SHOULD be retained until the key lease on that service expires). 812 This is beneficial because if the SRP client continues to renew the 813 host, but never mentions the stale service again, the stale service 814 will continue to be advertised. 816 The SRP server MUST include an EDNS(0) Update Lease option in the 817 response if the lease time proposed by the service has been shortened 818 or lengthened. The service MUST check for the EDNS(0) Update Lease 819 option in the response and MUST use the lease times from that option 820 in place of the options that it sent to the server when deciding when 821 to update its registration. The times may be shorter or longer than 822 those specified in the SRP update; the SRP client must honor them in 823 either case. 825 SRP clients should assume that each lease ends N seconds after the 826 update was first transmitted, where N is the lease duration. Servers 827 should assume that each lease ends N seconds after the update that 828 was successfully processed was received. Because the server will 829 always receive the update after the SRP client sent it, this avoids 830 the possibility of misunderstandings. 832 SRP servers MUST reject updates that do not include an EDNS(0) Update 833 Lease option. Dual-use servers MAY accept updates that don't include 834 leases, but SHOULD differentiate between SRP updates and other 835 updates, and MUST reject updates that would otherwise be SRP updates 836 if they do not include leases. 838 Lease times have a completely different function than TTLs. On an 839 authoritative DNS server, the TTL on a resource record is a constant: 840 whenever that RR is served in a DNS response, the TTL value sent in 841 the answer is the same. The lease time is never sent as a TTL; its 842 sole purpose is to determine when the authoritative DNS server will 843 delete stale records. It is not an error to send a DNS response with 844 a TTL of 'n' when the remaining time on the lease is less than 'n'. 846 5. Sleep Proxy 848 Another use of SRP is for devices that sleep to reduce power 849 consumption. 851 In this case, in addition to the DNS Update Lease option 852 [I-D.sekar-dns-ul] described above, the device includes an EDNS(0) 853 OWNER Option [I-D.cheshire-edns0-owner-option]. 855 The EDNS(0) Update Lease option constitutes a promise by the device 856 that it will wake up before this time elapses, to renew its 857 registration and thereby demonstrate that it is still attached to the 858 network. If it fails to renew the registration by this time, that 859 indicates that it is no longer attached to the network, and its 860 registration (except for the KEY in the Host Description) should be 861 deleted. 863 The EDNS(0) OWNER Option indicates that the device will be asleep, 864 and will not be receptive to normal network traffic. When a DNS 865 server receives a DNS Update with an EDNS(0) OWNER Option, that 866 signifies that the SRP server should set up a proxy for any IPv4 or 867 IPv6 address records in the DNS Update message. This proxy should 868 send ARP or ND messages claiming ownership of the IPv4 and/or IPv6 869 addresses in the records in question. In addition, the proxy should 870 answer future ARP or ND requests for those IPv4 and/or IPv6 871 addresses, claiming ownership of them. When the DNS server receives 872 a TCP SYN or UDP packet addressed to one of the IPv4 or IPv6 873 addresses for which it proxying, it should then wake up the sleeping 874 device using the information in the EDNS(0) OWNER Option. At present 875 version 0 of the OWNER Option specifies the "Wake-on-LAN Magic 876 Packet" that needs to be sent; future versions could be extended to 877 specify other wakeup mechanisms. 879 Note that although the authoritative DNS server that implements the 880 SRP function need not be on the same link as the sleeping host, the 881 Sleep Proxy must be on the same link. 883 It is not required that sleepy nodes on a Constrained-Node Network 884 support sleep proxy. Such devices may have different mechanisms for 885 dealing with sleep and wakeup. An SRP registration for such a device 886 will be useful regardless of the mechanism whereby messages are 887 delivered to the sleepy end device. For example, the message might 888 be held in a buffer for an extended period of time by an intermediate 889 device on a mesh network, and then delivered to the device when it 890 wakes up. The exact details of such behaviors are out of scope for 891 this document. 893 6. Security Considerations 895 6.1. Source Validation 897 SRP updates have no authorization semantics other than first-come, 898 first-served. This means that if an attacker from outside of the 899 administrative domain of the server knows the server's IP address, it 900 can in principle send updates to the server that will be processed 901 successfully. Servers should therefore be configured to reject 902 updates from source addresses outside of the administrative domain of 903 the server. 905 For updates sent to an anycast IP address of an SRP server, this 906 validation must be enforced by every router on the path from the 907 Constrained-Device Network to the unconstrained portion of the 908 network. For TCP updates, the initial SYN-SYN+ACK handshake prevents 909 updates being forged by an off-network attacker. In order to ensure 910 that this handshake happens, Service Discovery Protocol servers 911 relying on three-way-handshake validation MUST NOT accept TCP Fast 912 Open payloads. If the network infrastructure allows it, an SRP 913 server MAY accept TCP Fast Open payloads if all such packets are 914 validated along the path, and the network is able to reject this type 915 of spoofing at all ingress points. 917 Note that these rules only apply to the validation of SRP updates. A 918 server that accepts updates from SRP clients may also accept other 919 DNS updates, and those DNS updates may be validated using different 920 rules. However, in the case of a DNS service that accepts SRP 921 updates, the intersection of the SRP update rules and whatever other 922 update rules are present must be considered very carefully. 924 For example, a normal, authenticated DNS update to any RR that was 925 added using SRP, but that is authenticated using a different key, 926 could be used to override a promise made by the registration 927 protocol, by replacing all or part of the service registration 928 information with information provided by an SRP client. An 929 implementation that allows both kinds of updates should not allow DNS 930 Update clients that are using different authentication and 931 authorization credentialsto to update records added by SRP clients. 933 6.2. SRP Server Authentication 935 This specification does not provide a mechanism for validating 936 responses from DNS servers to SRP clients. In the case of 937 Constrained Network/Constrained Node clients, such validation isn't 938 practical because there's no way to establish trust. In principle, a 939 KEY RR could be used by a non-constrained SRP client to validate 940 responses from the server, but this is not required, nor do we 941 specify a mechanism for determining which key to use. 943 6.3. Required Signature Algorithm 945 For validation, SRP servers MUST implement the ECDSAP256SHA256 946 signature algorithm. SRP servers SHOULD implement the algorithms 947 specified in [RFC8624], Section 3.1, in the validation column of the 948 table, that are numbered 13 or higher and have a "MUST", 949 "RECOMMENDED", or "MAY" designation in the validation column of the 950 table. SRP clients MUST NOT assume that any algorithm numbered lower 951 than 13 is available for use in validating SIG(0) signatures. 953 7. Privacy Considerations 955 Because DNSSD SRP updates can be sent off-link, the privacy 956 implications of SRP are different than for multicast DNS responses. 957 Host implementations that are using TCP SHOULD also use TLS if 958 available. Server implementations MUST offer TLS support. The use 959 of TLS with DNS is described in [RFC7858] and [RFC8310]. 961 Hosts that implement TLS support SHOULD NOT fall back to TCP; since 962 servers are required to support TLS, it is entirely up to the host 963 implementation whether to use it. 965 Public keys can be used as identifiers to track hosts. SRP servers 966 MAY elect not to return KEY records for queries for SRP 967 registrations. 969 8. Delegation of 'service.arpa.' 971 In order to be fully functional, the owner of the 'arpa.' zone must 972 add a delegation of 'service.arpa.' in the '.arpa.' zone [RFC3172]. 973 This delegation should be set up as was done for 'home.arpa', as a 974 result of the specification in Section 7 of [RFC8375]. 976 9. IANA Considerations 977 9.1. Registration and Delegation of 'service.arpa' as a Special-Use 978 Domain Name 980 IANA is requested to record the domain name 'service.arpa.' in the 981 Special-Use Domain Names registry [SUDN]. IANA is requested, with 982 the approval of IAB, to implement the delegation requested in 983 Section 8. 985 IANA is further requested to add a new entry to the "Transport- 986 Independent Locally-Served Zones" subregistry of the the "Locally- 987 Served DNS Zones" registry [LSDZ]. The entry will be for the domain 988 'service.arpa.' with the description "DNS-SD Registration Protocol 989 Special-Use Domain", listing this document as the reference. 991 9.2. 'dnssd-srp' Service Name 993 IANA is also requested to add a new entry to the Service Names and 994 Port Numbers registry for dnssd-srp with a transport type of tcp. No 995 port number is to be assigned. The reference should be to this 996 document, and the Assignee and Contact information should reference 997 the authors of this document. The Description should be as follows: 999 Availability of DNS Service Discovery Service Registration Protocol 1000 Service for a given domain is advertised using the 1001 "_dnssd-srp._tcp.." SRV record gives the target host and 1002 port where DNSSD Service Registration Service is provided for the 1003 named domain. 1005 9.3. 'dnssd-srp-tls' Service Name 1007 IANA is also requested to add a new entry to the Service Names and 1008 Port Numbers registry for dnssd-srp with a transport type of tcp. No 1009 port number is to be assigned. The reference should be to this 1010 document, and the Assignee and Contact information should reference 1011 the authors of this document. The Description should be as follows: 1013 Availability of DNS Service Discovery Service Registration Protocol 1014 Service for a given domain over TLS is advertised using the 1015 "_dnssd-srp-tls._tcp.." SRV record gives the target host and 1016 port where DNSSD Service Registration Service is provided for the 1017 named domain. 1019 9.4. Anycast Address 1021 IANA is requested to allocate an IPv6 Anycast address from the IPv6 1022 Special-Purpose Address Registry, similar to the Port Control 1023 Protocol anycast address, 2001:1::1. The value TBD should be 1024 replaced with the actual allocation in the table that follows. The 1025 values for the registry are: 1027 +----------------------+-----------------------------+ 1028 | Attribute | value | 1029 +----------------------+-----------------------------+ 1030 | Address Block | 2001:1::TBD/128 | 1031 +----------------------+-----------------------------+ 1032 | Name | DNS-SD Service Registration | 1033 | | Protocol Anycast Address | 1034 +----------------------+-----------------------------+ 1035 | RFC | [this document] | 1036 +----------------------+-----------------------------+ 1037 | Allocation Date | [date of allocation] | 1038 +----------------------+-----------------------------+ 1039 | Termination Date | N/A | 1040 +----------------------+-----------------------------+ 1041 | Source | True | 1042 +----------------------+-----------------------------+ 1043 | Destination | True | 1044 +----------------------+-----------------------------+ 1045 | Forwardable | True | 1046 +----------------------+-----------------------------+ 1047 | Global | True | 1048 +----------------------+-----------------------------+ 1049 | Reserved-by-protocol | False | 1050 +----------------------+-----------------------------+ 1052 Table 1 1054 10. Implementation Status 1056 [Note to the RFC Editor: please remove this section prior to 1057 publication.] 1059 This section records the status of known implementations of the 1060 protocol defined by this specification at the time of posting of this 1061 Internet-Draft, and is based on a proposal described in RFC 7942. 1062 The description of implementations in this section is intended to 1063 assist the IETF in its decision processes in progressing drafts to 1064 RFCs. Please note that the listing of any individual implementation 1065 here does not imply endorsement by the IETF. Furthermore, no effort 1066 has been spent to verify the information presented here that was 1067 supplied by IETF contributors. This is not intended as, and must not 1068 be construed to be, a catalog of available implementations or their 1069 features. Readers are advised to note that other implementations may 1070 exist. 1072 According to RFC 7942, "this will allow reviewers and working groups 1073 to assign due consideration to documents that have the benefit of 1074 running code, which may serve as evidence of valuable experimentation 1075 and feedback that have made the implemented protocols more mature. 1076 It is up to the individual working groups to use this information as 1077 they see fit". 1079 There are two known independent implementations of SRP clients: 1081 * SRP Client for OpenThread: 1082 https://github.com/openthread/openthread/pull/6038 1084 * mDNSResponder open source project: https://github.com/Abhayakara/ 1085 mdnsresponder 1087 There are two related implementations of an SRP server. One acts as 1088 a DNS Update proxy, taking an SRP update and applying it to the 1089 specified DNS zone using DNS update. The other acts as an 1090 Advertising Proxy [I-D.sctl-advertising-proxy]. Both are included in 1091 the mDNSResponder open source project mentioned above. 1093 11. Acknowledgments 1095 Thanks to Toke Høiland-Jørgensen, Jonathan Hui, Esko Dijk, Kangping 1096 Dong and Abtin Keshavarzian for their thorough technical reviews. 1097 Thanks to Kangping and Abtin as well for testing the document by 1098 doing an independent implementation. Thanks to Tamara Kemper for 1099 doing a nice developmental edit, Tim Wattenberg for doing a SRP 1100 client proof-of-concept implementation at the Montreal Hackathon at 1101 IETF 102, and Tom Pusateri for reviewing during the hackathon and 1102 afterwards. 1104 12. Normative References 1106 [I-D.sekar-dns-ul] 1107 Cheshire, S. and T. Lemon, "Dynamic DNS Update Leases", 1108 Work in Progress, Internet-Draft, draft-sekar-dns-ul-02, 2 1109 August 2018, 1110 . 1112 [RFC2132] Alexander, S. and R. Droms, "DHCP Options and BOOTP Vendor 1113 Extensions", RFC 2132, DOI 10.17487/RFC2132, March 1997, 1114 . 1116 [RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound, 1117 "Dynamic Updates in the Domain Name System (DNS UPDATE)", 1118 RFC 2136, DOI 10.17487/RFC2136, April 1997, 1119 . 1121 [RFC2931] Eastlake 3rd, D., "DNS Request and Transaction Signatures 1122 ( SIG(0)s )", RFC 2931, DOI 10.17487/RFC2931, September 1123 2000, . 1125 [RFC3172] Huston, G., Ed., "Management Guidelines & Operational 1126 Requirements for the Address and Routing Parameter Area 1127 Domain ("arpa")", BCP 52, RFC 3172, DOI 10.17487/RFC3172, 1128 September 2001, . 1130 [RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service 1131 Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013, 1132 . 1134 [RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D., 1135 and P. Hoffman, "Specification for DNS over Transport 1136 Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May 1137 2016, . 1139 [RFC8106] Jeong, J., Park, S., Beloeil, L., and S. Madanapalli, 1140 "IPv6 Router Advertisement Options for DNS Configuration", 1141 RFC 8106, DOI 10.17487/RFC8106, March 2017, 1142 . 1144 [RFC8375] Pfister, P. and T. Lemon, "Special-Use Domain 1145 'home.arpa.'", RFC 8375, DOI 10.17487/RFC8375, May 2018, 1146 . 1148 [RFC8624] Wouters, P. and O. Sury, "Algorithm Implementation 1149 Requirements and Usage Guidance for DNSSEC", RFC 8624, 1150 DOI 10.17487/RFC8624, June 2019, 1151 . 1153 [RFC8765] Pusateri, T. and S. Cheshire, "DNS Push Notifications", 1154 RFC 8765, DOI 10.17487/RFC8765, June 2020, 1155 . 1157 [SUDN] "Special-Use Domain Names Registry", July 2012, 1158 . 1161 [LSDZ] "Locally-Served DNS Zones Registry", July 2011, 1162 . 1165 13. Informative References 1167 [RFC1035] Mockapetris, P., "Domain names - implementation and 1168 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 1169 November 1987, . 1171 [RFC2131] Droms, R., "Dynamic Host Configuration Protocol", 1172 RFC 2131, DOI 10.17487/RFC2131, March 1997, 1173 . 1175 [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS 1176 Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997, 1177 . 1179 [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for 1180 specifying the location of services (DNS SRV)", RFC 2782, 1181 DOI 10.17487/RFC2782, February 2000, 1182 . 1184 [RFC3007] Wellington, B., "Secure Domain Name System (DNS) Dynamic 1185 Update", RFC 3007, DOI 10.17487/RFC3007, November 2000, 1186 . 1188 [RFC6760] Cheshire, S. and M. Krochmal, "Requirements for a Protocol 1189 to Replace the AppleTalk Name Binding Protocol (NBP)", 1190 RFC 6760, DOI 10.17487/RFC6760, February 2013, 1191 . 1193 [RFC6761] Cheshire, S. and M. Krochmal, "Special-Use Domain Names", 1194 RFC 6761, DOI 10.17487/RFC6761, February 2013, 1195 . 1197 [RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762, 1198 DOI 10.17487/RFC6762, February 2013, 1199 . 1201 [RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for 1202 Constrained-Node Networks", RFC 7228, 1203 DOI 10.17487/RFC7228, May 2014, 1204 . 1206 [RFC8310] Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles 1207 for DNS over TLS and DNS over DTLS", RFC 8310, 1208 DOI 10.17487/RFC8310, March 2018, 1209 . 1211 [RFC8415] Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A., 1212 Richardson, M., Jiang, S., Lemon, T., and T. Winters, 1213 "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)", 1214 RFC 8415, DOI 10.17487/RFC8415, November 2018, 1215 . 1217 [RFC8766] Cheshire, S., "Discovery Proxy for Multicast DNS-Based 1218 Service Discovery", RFC 8766, DOI 10.17487/RFC8766, June 1219 2020, . 1221 [I-D.cheshire-dnssd-roadmap] 1222 Cheshire, S., "Service Discovery Road Map", Work in 1223 Progress, Internet-Draft, draft-cheshire-dnssd-roadmap-03, 1224 23 October 2018, . 1227 [I-D.cheshire-edns0-owner-option] 1228 Cheshire, S. and M. Krochmal, "EDNS0 OWNER Option", Work 1229 in Progress, Internet-Draft, draft-cheshire-edns0-owner- 1230 option-01, 3 July 2017, . 1233 [I-D.sctl-advertising-proxy] 1234 Cheshire, S. and T. Lemon, "Advertising Proxy for DNS-SD 1235 Service Registration Protocol", Work in Progress, 1236 Internet-Draft, draft-sctl-advertising-proxy-00, 13 July 1237 2020, . 1240 [ZC] Cheshire, S. and D.H. Steinberg, "Zero Configuration 1241 Networking: The Definitive Guide", O'Reilly Media, Inc. , 1242 ISBN 0-596-10100-7, December 2005. 1244 Appendix A. Testing using standard RFC2136-compliant servers 1246 It may be useful to set up a DNS server for testing that does not 1247 implement SRP. This can be done by configuring the server to listen 1248 on the anycast address, or advertising it in the 1249 _dnssd-srp._tcp. SRV and _dnssd-srp-tls._tcp. record. It 1250 must be configured to be authoritative for "default.service.arpa", 1251 and to accept updates from hosts on local networks for names under 1252 "default.service.arpa" without authentication, since such servers 1253 will not have support for FCFS authentication (Section 2.2.4.1). 1255 A server configured in this way will be able to successfully accept 1256 and process SRP updates from services that send SRP updates. 1257 However, no prerequisites will be applied, and this means that the 1258 test server will accept internally inconsistent SRP updates, and will 1259 not stop two SRP updates, sent by different services, that claim the 1260 same name(s), from overwriting each other. 1262 Since SRP updates are signed with keys, validation of the SIG(0) 1263 algorithm used by the client can be done by manually installing the 1264 client public key on the DNS server that will be receiving the 1265 updates. The key can then be used to authenticate the client, and 1266 can be used as a requirement for the update. An example 1267 configuration for testing SRP using BIND 9 is given in Appendix C. 1269 Appendix B. How to allow services to update standard RFC2136-compliant 1270 servers 1272 Ordinarily SRP updates will fail when sent to an RFC 2136-compliant 1273 server that does not implement SRP because the zone being updated is 1274 "default.service.arpa", and no DNS server that is not an SRP server 1275 should normally be configured to be authoritative for 1276 "default.service.arpa". Therefore, a service that sends an SRP 1277 update can tell that the receiving server does not support SRP, but 1278 does support RFC2136, because the RCODE will either be NOTZONE, 1279 NOTAUTH or REFUSED, or because there is no response to the update 1280 request (when using the anycast address) 1282 In this case a service MAY attempt to register itself using regular 1283 RFC2136 DNS updates. To do so, it must discover the default 1284 registration zone and the DNS server designated to receive updates 1285 for that zone, as described earlier, using the _dns-update._udp SRV 1286 record. It can then make the update using the port and host pointed 1287 to by the SRV record, and should use appropriate prerequisites to 1288 avoid overwriting competing records. Such updates are out of scope 1289 for SRP, and a service that implements SRP MUST first attempt to use 1290 SRP to register itself, and should only attempt to use RFC2136 1291 backwards compatibility if that fails. Although the owner name for 1292 the SRV record specifies the UDP protocol for updates, it is also 1293 possible to use TCP, and TCP should be required to prevent spoofing. 1295 Appendix C. Sample BIND9 configuration for default.service.arpa. 1297 zone "default.service.arpa." { 1298 type master; 1299 file "/etc/bind/master/service.db"; 1300 allow-update { key demo.default.service.arpa.; }; 1301 }; 1302 Figure 1: Zone Configuration in named.conf 1304 $ORIGIN . 1305 $TTL 57600 ; 16 hours 1306 default.service.arpa IN SOA ns3.default.service.arpa. 1307 postmaster.default.service.arpa. ( 1308 2951053287 ; serial 1309 3600 ; refresh (1 hour) 1310 1800 ; retry (30 minutes) 1311 604800 ; expire (1 week) 1312 3600 ; minimum (1 hour) 1313 ) 1314 NS ns3.default.service.arpa. 1315 SRV 0 0 53 ns3.default.service.arpa. 1316 $ORIGIN default.service.arpa. 1317 $TTL 3600 ; 1 hour 1318 _ipps._tcp PTR demo._ipps._tcp 1319 $ORIGIN _ipps._tcp.default.service.arpa. 1320 demo TXT "0" 1321 SRV 0 0 9992 demo.default.service.arpa. 1322 $ORIGIN _udp.default.service.arpa. 1323 $TTL 3600 ; 1 hour 1324 _dns-update PTR ns3.default.service.arpa. 1325 $ORIGIN _tcp.default.service.arpa. 1326 _dnssd-srp PTR ns3.default.service.arpa. 1327 $ORIGIN default.service.arpa. 1328 $TTL 300 ; 5 minutes 1329 ns3 AAAA 2001:db8:0:1::1 1330 $TTL 3600 ; 1 hour 1331 demo AAAA 2001:db8:0:2::1 1332 KEY 513 3 13 ( 1333 qweEmaaq0FAWok5//ftuQtZgiZoiFSUsm0srWREdywQU 1334 9dpvtOhrdKWUuPT3uEFF5TZU6B4q1z1I662GdaUwqg== 1335 ); alg = ECDSAP256SHA256 ; key id = 15008 1336 AAAA ::1 1338 Figure 2: Example Zone file 1340 Authors' Addresses 1342 Ted Lemon 1343 Apple Inc. 1344 One Apple Park Way 1345 Cupertino, California 95014 1346 United States of America 1348 Email: mellon@fugue.com 1349 Stuart Cheshire 1350 Apple Inc. 1351 One Apple Park Way 1352 Cupertino, California 95014 1353 United States of America 1355 Phone: +1 408 974 3207 1356 Email: cheshire@apple.com