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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Internet Engineering Task Force T. Pusateri 3 Internet-Draft Unaffiliated 4 Intended status: Standards Track S. Cheshire 5 Expires: January 22, 2020 Apple Inc. 6 July 21, 2019 8 DNS Push Notifications 9 draft-ietf-dnssd-push-23 11 Abstract 13 The Domain Name System (DNS) was designed to return matching records 14 efficiently for queries for data that are relatively static. When 15 those records change frequently, DNS is still efficient at returning 16 the updated results when polled, as long as the polling rate is not 17 too high. But there exists no mechanism for a client to be 18 asynchronously notified when these changes occur. This document 19 defines a mechanism for a client to be notified of such changes to 20 DNS records, called DNS Push Notifications. 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 January 22, 2020. 39 Copyright Notice 41 Copyright (c) 2019 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 46 (https://trustee.ietf.org/license-info) in effect on the date of 47 publication of this document. Please review these documents 48 carefully, as they describe your rights and restrictions with respect 49 to this document. Code Components extracted from this document must 50 include Simplified BSD License text as described in Section 4.e of 51 the Trust Legal Provisions and are provided without warranty as 52 described in the Simplified BSD License. 54 Table of Contents 56 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 57 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 58 1.2. Fatal Errors . . . . . . . . . . . . . . . . . . . . . . 3 59 2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 4 60 3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 5 61 4. State Considerations . . . . . . . . . . . . . . . . . . . . 6 62 5. Transport . . . . . . . . . . . . . . . . . . . . . . . . . . 7 63 6. Protocol Operation . . . . . . . . . . . . . . . . . . . . . 8 64 6.1. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 9 65 6.2. DNS Push Notification SUBSCRIBE . . . . . . . . . . . . . 13 66 6.2.1. SUBSCRIBE Request . . . . . . . . . . . . . . . . . . 13 67 6.2.2. SUBSCRIBE Response . . . . . . . . . . . . . . . . . 16 68 6.3. DNS Push Notification Updates . . . . . . . . . . . . . . 20 69 6.3.1. PUSH Message . . . . . . . . . . . . . . . . . . . . 20 70 6.4. DNS Push Notification UNSUBSCRIBE . . . . . . . . . . . . 25 71 6.4.1. UNSUBSCRIBE Message . . . . . . . . . . . . . . . . . 25 72 6.5. DNS Push Notification RECONFIRM . . . . . . . . . . . . . 27 73 6.5.1. RECONFIRM Message . . . . . . . . . . . . . . . . . . 28 74 6.6. DNS Stateful Operations TLV Context Summary . . . . . . . 30 75 6.7. Client-Initiated Termination . . . . . . . . . . . . . . 31 76 6.8. Client Fallback to Polling . . . . . . . . . . . . . . . 32 77 7. Security Considerations . . . . . . . . . . . . . . . . . . . 33 78 7.1. Security Services . . . . . . . . . . . . . . . . . . . . 33 79 7.2. TLS Name Authentication . . . . . . . . . . . . . . . . . 34 80 7.3. TLS Early Data . . . . . . . . . . . . . . . . . . . . . 34 81 7.4. TLS Session Resumption . . . . . . . . . . . . . . . . . 35 82 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 36 83 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 36 84 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 37 85 10.1. Normative References . . . . . . . . . . . . . . . . . . 37 86 10.2. Informative References . . . . . . . . . . . . . . . . . 38 87 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 41 89 1. Introduction 91 Domain Name System (DNS) records may be updated using DNS Update 92 [RFC2136]. Other mechanisms such as a Discovery Proxy [DisProx] can 93 also generate changes to a DNS zone. This document specifies a 94 protocol for DNS clients to subscribe to receive asynchronous 95 notifications of changes to RRsets of interest. It is immediately 96 relevant in the case of DNS Service Discovery [RFC6763] but is not 97 limited to that use case, and provides a general DNS mechanism for 98 DNS record change notifications. Familiarity with the DNS protocol 99 and DNS packet formats is assumed [RFC1034] [RFC1035] [RFC6895]. 101 1.1. Requirements Language 103 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 104 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 105 "OPTIONAL" in this document are to be interpreted as described in 106 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all 107 capitals, as shown here. These words may also appear in this 108 document in lower case as plain English words, absent their normative 109 meanings. 111 1.2. Fatal Errors 113 Certain invalid situations are described in this specification, like 114 a server sending a Push Notification subscription request to a 115 client, or a client sending a Push Notification response to a server. 116 These should never occur with a correctly implemented client and 117 server, and if they do occur then they indicate a serious 118 implementation error. In these extreme cases there is no reasonable 119 expectation of a graceful recovery, and the recipient detecting the 120 error should respond by unilaterally aborting the session without 121 regard for data loss. Such cases are addressed by having an engineer 122 investigate the cause of the failure and fixing the problem in the 123 software. 125 Where this specification says "forcibly abort", it means sending a 126 TCP RST to terminate the TCP connection, and the TLS session running 127 over that TCP connection. In the BSD Sockets API, this is achieved 128 by setting the SO_LINGER option to zero before closing the socket. 130 2. Motivation 132 As the domain name system continues to adapt to new uses and changes 133 in deployment, polling has the potential to burden DNS servers at 134 many levels throughout the network. Other network protocols have 135 successfully deployed a publish/subscribe model following the 136 Observer design pattern [obs]. XMPP Publish-Subscribe [XEP0060] and 137 Atom [RFC4287] are examples. While DNS servers are generally highly 138 tuned and capable of a high rate of query/response traffic, adding a 139 publish/subscribe model for tracking changes to DNS records can 140 deliver more timely notification of changes with reduced CPU usage 141 and lower network traffic. 143 Multicast DNS [RFC6762] implementations always listen on a well known 144 link-local IP multicast group address, and record changes are sent to 145 that multicast group address for all group members to receive. 146 Therefore, Multicast DNS already has asynchronous change notification 147 capability. However, when DNS Service Discovery [RFC6763] is used 148 across a wide area network using Unicast DNS (possibly facilitated 149 via a Discovery Proxy [DisProx]) it would be beneficial to have an 150 equivalent capability for Unicast DNS, to allow clients to learn 151 about DNS record changes in a timely manner without polling. 153 The DNS Long-Lived Queries (LLQ) mechanism [LLQ] is an existing 154 deployed solution to provide asynchronous change notifications, used 155 by Apple's Back to My Mac [RFC6281] service introduced in Mac OS X 156 10.5 Leopard in 2007. Back to My Mac was designed in an era when the 157 data center operations staff asserted that it was impossible for a 158 server to handle large numbers of mostly-idle TCP connections, so LLQ 159 was defined as a UDP-based protocol, effectively replicating much of 160 TCP's connection state management logic in user space, and creating 161 its own poor imitations of existing TCP features like the three-way 162 handshake, flow control, and reliability. 164 This document builds on experience gained with the LLQ protocol, with 165 an improved design. Instead of using UDP, this specification uses 166 DNS Stateful Operations (DSO) [RFC8490] running over TLS over TCP, 167 and therefore doesn't need to reinvent existing TCP functionality. 168 Using TCP also gives long-lived low-traffic connections better 169 longevity through NAT gateways without depending on the gateway to 170 support NAT Port Mapping Protocol (NAT-PMP) [RFC6886] or Port Control 171 Protocol (PCP) [RFC6887], or resorting to excessive keepalive 172 traffic. 174 3. Overview 176 A DNS Push Notification client subscribes for Push Notifications for 177 a particular RRset by connecting to the appropriate Push Notification 178 server for that RRset, and sending DSO message(s) indicating the 179 RRset(s) of interest. When the client loses interest in receiving 180 further updates to these records, it unsubscribes. 182 The DNS Push Notification server for a DNS zone is any server capable 183 of generating the correct change notifications for a name. It may be 184 a primary, secondary, or stealth name server [RFC7719]. 186 The "_dns-push-tls._tcp." SRV record for a zone MAY reference 187 the same target host and port as that zone's 188 "_dns-update-tls._tcp." SRV record. When the same target host 189 and port is offered for both DNS Updates and DNS Push Notifications, 190 a client MAY use a single DSO session to that server for both DNS 191 Updates and DNS Push Notification Subscriptions. DNS Updates and DNS 192 Push Notifications may be handled on different ports on the same 193 target host, in which case they are not considered to be the "same 194 server" for the purposes of this specification, and communications 195 with these two ports are handled independently. Supporting DNS 196 Updates and DNS Push Notifications on the same server is OPTIONAL. A 197 DNS Push Notification server is not required to support DNS Update. 199 Standard DNS Queries MAY be sent over a DNS Push Notification (i.e., 200 DSO) session. For any zone for which the server is authoritative, it 201 MUST respond authoritatively for queries for names falling within 202 that zone (e.g., the "_dns-push-tls._tcp." SRV record) both for 203 normal DNS queries and for DNS Push Notification subscriptions. For 204 names for which the server is acting as a recursive resolver, e.g., 205 when the server is the local recursive resolver, for any query for 206 which it supports DNS Push Notification subscriptions, it MUST also 207 support standard queries. 209 DNS Push Notifications impose less load on the responding server than 210 rapid polling would, but Push Notifications do still have a cost, so 211 DNS Push Notification clients MUST NOT recklessly create an excessive 212 number of Push Notification subscriptions. Specifically: 214 (a) A subscription should only be active when there is a valid reason 215 to need live data (for example, an on-screen display is currently 216 showing the results to the user) and the subscription SHOULD be 217 cancelled as soon as the need for that data ends (for example, when 218 the user dismisses that display). In the case of a device like a 219 smartphone which, after some period of inactivity, goes to sleep or 220 otherwise darkens its screen, it should cancel its subscriptions when 221 darkening the screen (since the user cannot see any changes on the 222 display anyway) and reinstate its subscriptions when re-awakening 223 from display sleep. 225 (b) A DNS Push Notification client SHOULD NOT routinely keep a DNS 226 Push Notification subscription active 24 hours a day, 7 days a week, 227 just to keep a list in memory up to date so that if the user does 228 choose to bring up an on-screen display of that data, it can be 229 displayed really fast. DNS Push Notifications are designed to be 230 fast enough that there is no need to pre-load a "warm" list in memory 231 just in case it might be needed later. 233 Generally, as described in the DNS Stateful Operations specification 234 [RFC8490], a client must not keep a session to a server open 235 indefinitely if it has no subscriptions (or other operations) active 236 on that session. A client MAY close a session as soon as it becomes 237 idle, and then if needed in the future, open a new session when 238 required. Alternatively, a client MAY speculatively keep an idle 239 session open for some time, subject to the constraint that it MUST 240 NOT keep a session open that has been idle for more than the 241 session's idle timeout (15 seconds by default) [RFC8490]. 243 4. State Considerations 245 Each DNS Push Notification server is capable of handling some finite 246 number of Push Notification subscriptions. This number will vary 247 from server to server and is based on physical machine 248 characteristics, network bandwidth, and operating system resource 249 allocation. After a client establishes a session to a DNS server, 250 each subscription is individually accepted or rejected. Servers may 251 employ various techniques to limit subscriptions to a manageable 252 level. Correspondingly, the client is free to establish simultaneous 253 sessions to alternate DNS servers that support DNS Push Notifications 254 for the zone and distribute subscriptions at the client's discretion. 255 In this way, both clients and servers can react to resource 256 constraints. 258 5. Transport 260 Other DNS operations like DNS Update [RFC2136] MAY use either User 261 Datagram Protocol (UDP) [RFC0768] or Transmission Control Protocol 262 (TCP) [RFC0793] as the transport protocol, in keeping with the 263 historical precedent that DNS queries must first be sent over UDP 264 [RFC1123]. This requirement to use UDP has subsequently been relaxed 265 [RFC7766]. 267 In keeping with the more recent precedent, DNS Push Notification is 268 defined only for TCP. DNS Push Notification clients MUST use DNS 269 Stateful Operations [RFC8490] running over TLS over TCP [RFC7858]. 271 Connection setup over TCP ensures return reachability and alleviates 272 concerns of state overload at the server which is a potential problem 273 with connectionless protocols using spoofed source addresses. All 274 subscribers are guaranteed to be reachable by the server by virtue of 275 the TCP three-way handshake. Flooding attacks are possible with any 276 protocol, and a benefit of TCP is that there are already established 277 industry best practices to guard against SYN flooding and similar 278 attacks [SYN] [RFC4953]. 280 Use of TCP also allows DNS Push Notifications to take advantage of 281 current and future developments in TCP, such as Multipath TCP (MPTCP) 282 [RFC6824], TCP Fast Open (TFO) [RFC7413], Tail Loss Probe (TLP) 283 [I-D.dukkipati-tcpm-tcp-loss-probe], and so on. 285 Transport Layer Security (TLS) [RFC8446] is well understood, and used 286 by many application-layer protocols running over TCP. TLS is 287 designed to prevent eavesdropping, tampering, and message forgery. 288 TLS is REQUIRED for every connection between a client subscriber and 289 server in this protocol specification. Additional security measures 290 such as client authentication during TLS negotiation MAY also be 291 employed to increase the trust relationship between client and 292 server. 294 6. Protocol Operation 296 The DNS Push Notification protocol is a session-oriented protocol, 297 and makes use of DNS Stateful Operations (DSO) [RFC8490]. 299 For details of the DSO message format refer to the DNS Stateful Oper- 300 ations specification [RFC8490]. Those details are not repeated here. 302 DNS Push Notification clients and servers MUST support DSO. A single 303 server can support DNS Queries, DNS Updates, and DNS Push 304 Notifications (using DSO) on the same TCP port. 306 A DNS Push Notification exchange begins with the client discovering 307 the appropriate server, using the procedure described in Section 6.1, 308 and then making a TLS/TCP connection to it. 310 A typical DNS Push Notification client will immediately issue a DSO 311 Keepalive operation to request a session timeout and/or keepalive 312 interval longer than the the 15-second default values, but this is 313 not required. A DNS Push Notification client MAY issue other 314 requests on the session first, and only issue a DSO Keepalive 315 operation later if it determines that to be necessary. Sending 316 either a DSO Keepalive operation or a Push Notification subscription 317 request over the TLS/TCP connection to the server signals the 318 client's support of DSO and serves to establish a DSO session. 320 In accordance with the current set of active subscriptions, the 321 server sends relevant asynchronous Push Notifications to the client. 322 Note that a client MUST be prepared to receive (and silently ignore) 323 Push Notifications for subscriptions it has previously removed, since 324 there is no way to prevent the situation where a Push Notification is 325 in flight from server to client while the client's UNSUBSCRIBE 326 message cancelling that subscription is simultaneously in flight from 327 client to server. 329 6.1. Discovery 331 The first step in establishing a DNS Push Notification subscription 332 is to discover an appropriate DNS server that supports DNS Push 333 Notifications for the desired zone. 335 The client begins by opening a DSO Session to its normal configured 336 DNS recursive resolver and requesting a Push Notification 337 subscription. This connection is made to TCP port 853, the default 338 port for DNS-over-TLS [RFC7858]. If the request for a Push 339 Notification subscription is successful, and the recursive resolver 340 doesn't already have an active subscription for that name, type, and 341 class, then the recursive resolver will make a corresponding Push 342 Notification subscription on the client's behalf. Results received 343 are relayed to the client. This is closely analogous to how a client 344 sends a normal DNS query to its configured DNS recursive resolver 345 which, if it doesn't already have appropriate answer(s) in its cache, 346 issues an upstream query to satisfy the request. 348 In many contexts, the recursive resolver will be able to handle Push 349 Notifications for all names that the client may need to follow. Use 350 of VPN tunnels and split-view DNS can create some additional 351 complexity in the client software here; the techniques to handle VPN 352 tunnels and split-view DNS for DNS Push Notifications are the same as 353 those already used to handle this for normal DNS queries. 355 If the recursive resolver does not support DNS over TLS, or supports 356 DNS over TLS but is not listening on TCP port 853, or supports DNS 357 over TLS on TCP port 853 but does not support DSO on that port, then 358 the DSO Session session establishment will fail [RFC8490]. 360 If the recursive resolver does support DSO but not Push Notification 361 subscriptions, then it will return the DSO error code, DSOTYPENI 362 (11). 364 In some cases, the recursive resolver may support DSO and Push 365 Notification subscriptions, but may not be able to subscribe for Push 366 Notifications for a particular name. In this case, the recursive 367 resolver should return SERVFAIL to the client. This includes being 368 unable to establish a connection to the zone's DNS Push Notification 369 server or establishing a connection but receiving a non success 370 response code. In some cases, where the client has a pre-established 371 trust relationship with the owner of the zone (that is not handled 372 via the usual mechanisms for VPN software) the client may handle 373 these failures by contacting the zone's DNS Push server directly. 375 In any of the cases described above where the client fails to 376 establish a DNS Push Notification subscription via its configured 377 recursive resolver, the client should proceed to discover the 378 appropriate server for direct communication. The client MUST also 379 determine which TCP port on the server is listening for connections, 380 which need not be (and often is not) the typical TCP port 53 used for 381 conventional DNS, or TCP port 853 used for DNS over TLS. 383 The discovery algorithm described here is an iterative algorithm, 384 which starts with the full name of the record to which the client 385 wishes to subscribe. Successive SOA queries are then issued, 386 trimming one label each time, until the closest enclosing 387 authoritative server is discovered. There is also an optimization to 388 enable the client to take a "short cut" directly to the SOA record of 389 the closest enclosing authoritative server in many cases. 391 1. The client begins the discovery by sending a DNS query to its 392 local resolver, with record type SOA [RFC1035] for the record 393 name to which it wishes to subscribe. As an example, suppose the 394 client wishes to subscribe to PTR records with the name 395 _ipp._tcp.headoffice.example.com (to discover Internet Printing 396 Protocol (IPP) printers [RFC8010] [RFC8011] being advertised in 397 the head office of Example Company.). The client begins by 398 sending an SOA query for _ipp._tcp.headoffice.example.com to the 399 local recursive resolver. The goal is to determine the server 400 authoritative for the name _ipp._tcp.headoffice.example.com. The 401 closest enclosing DNS zone containing the name 402 _ipp._tcp.headoffice.example.com could be example.com, or 403 headoffice.example.com, or _tcp.headoffice.example.com, or even 404 _ipp._tcp.headoffice.example.com. The client does not know in 405 advance where the closest enclosing zone cut occurs, which is why 406 it uses the iterative procedure described here to discover this 407 information. 409 2. If the requested SOA record exists, it will be returned in the 410 Answer section with a NOERROR response code, and the client has 411 succeeded in discovering the information it needs. 412 (This language is not placing any new requirements on DNS 413 recursive resolvers. This text merely describes the existing 414 operation of the DNS protocol [RFC1034] [RFC1035].) 416 3. If the requested SOA record does not exist, the client will get 417 back a NOERROR/NODATA response or an NXDOMAIN/Name Error 418 response. In either case, the local resolver would normally 419 include the SOA record for the closest enclosing zone of the 420 requested name in the Authority Section. If the SOA record is 421 received in the Authority Section, then the client has succeeded 422 in discovering the information it needs. 423 (This language is not placing any new requirements on DNS 424 recursive resolvers. This text merely describes the existing 425 operation of the DNS protocol regarding negative responses 426 [RFC2308].) 428 4. If the client receives a response containing no SOA record, then 429 it proceeds with the iterative approach. The client strips the 430 leading label from the current query name, and if the resulting 431 name has at least two labels in it, the client sends an SOA query 432 for that new name, and processing continues at step 2 above, 433 repeating the iterative search until either an SOA is received, 434 or the query name consists of a single label, i.e., a Top Level 435 Domain (TLD). In the case of a single-label (TLD), this is a 436 network configuration error, which should not happen, and the 437 client gives up. The client may retry the operation at a later 438 time, of the client's choosing, such after a change in network 439 attachment. 441 5. Once the SOA is known (either by virtue of being seen in the 442 Answer Section, or in the Authority Section), the client sends a 443 DNS query with type SRV [RFC2782] for the record name 444 "_dns-push-tls._tcp.", where is the owner name of 445 the discovered SOA record. 447 6. If the zone in question is set up to offer DNS Push Notifications 448 then this SRV record MUST exist. (If this SRV record does not 449 exist then the zone is not correctly configured for DNS Push 450 Notifications as specified in this document.) The SRV "target" 451 contains the name of the server providing DNS Push Notifications 452 for the zone. The port number on which to contact the server is 453 in the SRV record "port" field. The address(es) of the target 454 host MAY be included in the Additional Section, however, the 455 address records SHOULD be authenticated before use as described 456 below in Section 7.2 and in the specification for using DANE TLSA 457 Records with SRV Records [RFC7673], if applicable. 459 7. More than one SRV record may be returned. In this case, the 460 "priority" and "weight" values in the returned SRV records are 461 used to determine the order in which to contact the servers for 462 subscription requests. As described in the SRV specification 463 [RFC2782], the server with the lowest "priority" is first 464 contacted. If more than one server has the same "priority", the 465 "weight" indicates the weighted probability that the client 466 should contact that server. Higher weights have higher 467 probabilities of being selected. If a server is not willing to 468 accept a subscription request, or is not reachable within a 469 reasonable time, as determined by the client, then a subsequent 470 server is to be contacted. 472 Each time a client makes a new DNS Push Notification subscription 473 session, it SHOULD repeat the discovery process in order to determine 474 the preferred DNS server for subscriptions at that time. However, 475 the client device MUST respect the DNS TTL values on records it 476 receives, and store them in its local cache with this lifetime. This 477 means that, as long as the DNS TTL values on the authoritative 478 records are set to reasonable values, repeated application of this 479 discovery process can be completed nearly instantaneously by the 480 client, using only locally-stored cached data. 482 6.2. DNS Push Notification SUBSCRIBE 484 After connecting, and requesting a longer idle timeout and/or 485 keepalive interval if necessary, a DNS Push Notification client 486 then indicates its desire to receive DNS Push Notifications for 487 a given domain name by sending a SUBSCRIBE request to the server. 488 A SUBSCRIBE request is encoded in a DSO message [RFC8490]. 489 This specification defines a primary DSO TLV for DNS Push 490 Notification SUBSCRIBE Requests (tentatively DSO Type Code 0x40). 492 DSO messages with the SUBSCRIBE TLV as the Primary TLV are permitted 493 in TLS early data, provided that the precautions described in 494 Section 7.3 are followed. 496 The entity that initiates a SUBSCRIBE request is by definition the 497 client. A server MUST NOT send a SUBSCRIBE request over an existing 498 session from a client. If a server does send a SUBSCRIBE request 499 over a DSO session initiated by a client, this is a fatal error and 500 the client MUST forcibly abort the connection immediately. 502 6.2.1. SUBSCRIBE Request 504 A SUBSCRIBE request begins with the standard DSO 12-byte header 505 [RFC8490], followed by the SUBSCRIBE primary TLV. A SUBSCRIBE 506 request message is illustrated in Figure 1. 508 The MESSAGE ID field MUST be set to a unique value, that the client 509 is not using for any other active operation on this DSO session. For 510 the purposes here, a MESSAGE ID is in use on this session if the 511 client has used it in a request for which it has not yet received a 512 response, or if the client has used it for a subscription which it 513 has not yet cancelled using UNSUBSCRIBE. In the SUBSCRIBE response 514 the server MUST echo back the MESSAGE ID value unchanged. 516 The other header fields MUST be set as described in the DSO spec- 517 ification [RFC8490]. The DNS OPCODE field contains the OPCODE value 518 for DNS Stateful Operations (6). The four count fields MUST be zero, 519 and the corresponding four sections MUST be empty (i.e., absent). 521 The DSO-TYPE is SUBSCRIBE (tentatively 0x40). 523 The DSO-LENGTH is the length of the DSO-DATA that follows, which 524 specifies the name, type, and class of the record(s) being sought. 526 1 1 1 1 1 1 527 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 528 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 529 | MESSAGE ID | \ 530 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 531 |QR| OPCODE(6) | Z | RCODE | | 532 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 533 | QDCOUNT (MUST BE ZERO) | | 534 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER 535 | ANCOUNT (MUST BE ZERO) | | 536 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 537 | NSCOUNT (MUST BE ZERO) | | 538 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 539 | ARCOUNT (MUST BE ZERO) | / 540 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 541 | DSO-TYPE = SUBSCRIBE (tentatively 0x40) | 542 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 543 | DSO-LENGTH (number of octets in DSO-DATA) | 544 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 545 | | \ 546 \ NAME \ | 547 \ \ | 548 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > DSO-DATA 549 | TYPE | | 550 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 551 | CLASS | / 552 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 554 Figure 1: SUBSCRIBE Request 556 The DSO-DATA for a SUBSCRIBE request MUST contain exactly one NAME, 557 TYPE, and CLASS. Since SUBSCRIBE requests are sent over TCP, 558 multiple SUBSCRIBE DSO request messages can be concatenated in a 559 single TCP stream and packed efficiently into TCP segments. 561 If accepted, the subscription will stay in effect until the client 562 cancels the subscription using UNSUBSCRIBE or until the DSO session 563 between the client and the server is closed. 565 SUBSCRIBE requests on a given session MUST be unique. A client MUST 566 NOT send a SUBSCRIBE message that duplicates the NAME, TYPE and CLASS 567 of an existing active subscription on that DSO session. For the 568 purpose of this matching, the established DNS case-insensitivity for 569 US-ASCII letters applies (e.g., "example.com" and "Example.com" are 570 the same). If a server receives such a duplicate SUBSCRIBE message, 571 this is a fatal error and the server MUST forcibly abort the 572 connection immediately. 574 DNS wildcarding is not supported. That is, a wildcard ("*") in a 575 SUBSCRIBE message matches only a literal wildcard character ("*") in 576 the zone, and nothing else. 578 Aliasing is not supported. That is, a CNAME in a SUBSCRIBE message 579 matches only a literal CNAME record in the zone, and no other records 580 with the same owner name. 582 A client may SUBSCRIBE to records that are unknown to the server at 583 the time of the request (providing that the name falls within one of 584 the zone(s) the server is responsible for) and this is not an error. 585 The server MUST NOT return NXDOMAIN in this case. The server MUST 586 accept these requests and send Push Notifications if and when 587 matching records are found in the future. 589 If neither TYPE nor CLASS are ANY (255) then this is a specific 590 subscription to changes for the given NAME, TYPE and CLASS. If one 591 or both of TYPE or CLASS are ANY (255) then this subscription matches 592 any type and/or any class, as appropriate. 594 NOTE: A little-known quirk of DNS is that in DNS QUERY requests, 595 QTYPE and QCLASS 255 mean "ANY" not "ALL". They indicate that the 596 server should respond with ANY matching records of its choosing, not 597 necessarily ALL matching records. This can lead to some surprising 598 and unexpected results, where a query returns some valid answers but 599 not all of them, and makes QTYPE=ANY queries less useful than people 600 sometimes imagine. 602 When used in conjunction with SUBSCRIBE, TYPE and CLASS 255 should be 603 interpreted to mean "ALL", not "ANY". After accepting a subscription 604 where one or both of TYPE or CLASS are 255, the server MUST send Push 605 Notification Updates for ALL record changes that match the 606 subscription, not just some of them. 608 6.2.2. SUBSCRIBE Response 610 Each SUBSCRIBE request generates exactly one SUBSCRIBE response from 611 the server. 613 A SUBSCRIBE response begins with the standard DSO 12-byte header 614 [RFC8490]. The QR bit in the header is set indicating it is a 615 response. The header MAY be followed by one or more optional TLVs, 616 such as a Retry Delay TLV. 618 The MESSAGE ID field MUST echo the value given in the MESSAGE ID 619 field of the SUBSCRIBE request. This is how the client knows which 620 request is being responded to. 622 A SUBSCRIBE response message MUST NOT include a SUBSCRIBE TLV. If a 623 client receives a SUBSCRIBE response message containing a SUBSCRIBE 624 TLV then the response message is processed but the SUBSCRIBE TLV MUST 625 be silently ignored. 627 A client MUST NOT send a SUBSCRIBE response. If a client does send a 628 SUBSCRIBE message, with the QR bit set indicating that it is a 629 response, this is a fatal error and the server MUST forcibly abort 630 the connection immediately. 632 1 1 1 1 1 1 633 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 634 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 635 | MESSAGE ID | \ 636 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 637 |QR| OPCODE(6) | Z | RCODE | | 638 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 639 | QDCOUNT (MUST BE ZERO) | | 640 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER 641 | ANCOUNT (MUST BE ZERO) | | 642 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 643 | NSCOUNT (MUST BE ZERO) | | 644 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 645 | ARCOUNT (MUST BE ZERO) | / 646 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 648 Figure 2: SUBSCRIBE Response Message 650 In the SUBSCRIBE response the RCODE indicates whether or not the 651 subscription was accepted. Supported RCODEs are as follows: 653 +-----------+-------+-----------------------------------------------+ 654 | Mnemonic | Value | Description | 655 +-----------+-------+-----------------------------------------------+ 656 | NOERROR | 0 | SUBSCRIBE successful. | 657 | FORMERR | 1 | Server failed to process request due to a | 658 | | | malformed request. | 659 | SERVFAIL | 2 | Server failed to process request due to a | 660 | | | problem with the server. | 661 | NOTIMP | 4 | Server does not implement DSO. | 662 | REFUSED | 5 | Server refuses to process request for policy | 663 | | | or security reasons. | 664 | NOTAUTH | 9 | Server is not authoritative for the requested | 665 | | | name. | 666 | DSOTYPENI | 11 | SUBSCRIBE operation not supported. | 667 +-----------+-------+-----------------------------------------------+ 669 Table 1: SUBSCRIBE Response codes 671 This document specifies only these RCODE values for SUBSCRIBE 672 Responses. Servers sending SUBSCRIBE Responses SHOULD use one of 673 these values. Note that NXDOMAIN is not a valid RCODE in response to 674 a SUBSCRIBE Request. However, future circumstances may create 675 situations where other RCODE values are appropriate in SUBSCRIBE 676 Responses, so clients MUST be prepared to accept SUBSCRIBE Responses 677 with any other RCODE value. 679 If the server sends a nonzero RCODE in the SUBSCRIBE response, that 680 means: 682 a. the client is (at least partially) misconfigured, 683 b. the server resources are exhausted, or 684 c. there is some other unknown failure on the server. 686 In any case, the client shouldn't retry the subscription to this 687 server right away. If multiple SRV records were returned as 688 described in Section 6.1, Paragraph 7, a subsequent server can be 689 tried immediately. 691 If the client has other successful subscriptions to this server, 692 these subscriptions remain even though additional subscriptions may 693 be refused. Neither the client nor the server are required to close 694 the connection, although, either end may choose to do so. 696 If the server sends a nonzero RCODE then it SHOULD append a Retry 697 Delay TLV [RFC8490] to the response specifying a delay before the 698 client attempts this operation again. Recommended values for the 699 delay for different RCODE values are given below. These recommended 700 values apply both to the default values a server should place in the 701 Retry Delay TLV, and the default values a client should assume if the 702 server provides no Retry Delay TLV. 704 For RCODE = 1 (FORMERR) the delay may be any value selected by the 705 implementer. A value of five minutes is RECOMMENDED, to reduce 706 the risk of high load from defective clients. 708 For RCODE = 2 (SERVFAIL) the delay should be chosen according to 709 the level of server overload and the anticipated duration of that 710 overload. By default, a value of one minute is RECOMMENDED. If a 711 more serious server failure occurs, the delay may be longer in 712 accordance with the specific problem encountered. 714 For RCODE = 4 (NOTIMP), which occurs on a server that doesn't 715 implement DNS Stateful Operations [RFC8490], it is unlikely that 716 the server will begin supporting DSO in the next few minutes, so 717 the retry delay SHOULD be one hour. Note that in such a case, a 718 server that doesn't implement DSO is unlikely to place a Retry 719 Delay TLV in its response, so this recommended value in particular 720 applies to what a client should assume by default. 722 For RCODE = 5 (REFUSED), which occurs on a server that implements 723 DNS Push Notifications, but is currently configured to disallow 724 DNS Push Notifications, the retry delay may be any value selected 725 by the implementer and/or configured by the operator. 727 If the server being queried is listed in a 728 "_dns-push-tls._tcp." SRV record for the zone, then this is 729 a misconfiguration, since this server is being advertised as 730 supporting DNS Push Notifications for this zone, but the server 731 itself is not currently configured to perform that task. Since it 732 is possible that the misconfiguration may be repaired at any time, 733 the retry delay should not be set too high. By default, a value 734 of 5 minutes is RECOMMENDED. 736 For RCODE = 9 (NOTAUTH), which occurs on a server that implements 737 DNS Push Notifications, but is not configured to be authoritative 738 for the requested name, the retry delay may be any value selected 739 by the implementer and/or configured by the operator. 741 If the server being queried is listed in a 742 "_dns-push-tls._tcp." SRV record for the zone, then this is 743 a misconfiguration, since this server is being advertised as 744 supporting DNS Push Notifications for this zone, but the server 745 itself is not currently configured to perform that task. Since it 746 is possible that the misconfiguration may be repaired at any time, 747 the retry delay should not be set too high. By default, a value 748 of 5 minutes is RECOMMENDED. 750 For RCODE = 11 (DSOTYPENI), which occurs on a server that 751 implements DSO but doesn't implement DNS Push Notifications, it is 752 unlikely that the server will begin supporting DNS Push 753 Notifications in the next few minutes, so the retry delay SHOULD 754 be one hour. 756 For other RCODE values, the retry delay should be set by the 757 server as appropriate for that error condition. By default, a 758 value of 5 minutes is RECOMMENDED. 760 For RCODE = 9 (NOTAUTH), the time delay applies to requests for other 761 names falling within the same zone. Requests for names falling 762 within other zones are not subject to the delay. For all other 763 RCODEs the time delay applies to all subsequent requests to this 764 server. 766 After sending an error response the server MAY allow the session to 767 remain open, or MAY send a DNS Push Notification Retry Delay 768 Operation TLV instructing the client to close the session, as 769 described in the DSO specification [RFC8490]. Clients MUST correctly 770 handle both cases. 772 6.3. DNS Push Notification Updates 774 Once a subscription has been successfully established, the server 775 generates PUSH messages to send to the client as appropriate. In the 776 case that the answer set was already non-empty at the moment the 777 subscription was established, an initial PUSH message will be sent 778 immediately following the SUBSCRIBE Response. Subsequent changes to 779 the answer set are then communicated to the client in subsequent PUSH 780 messages. 782 6.3.1. PUSH Message 784 A PUSH unidirectional message begins with the standard DSO 12-byte 785 header [RFC8490], followed by the PUSH primary TLV. A PUSH message 786 is illustrated in Figure 3. 788 In accordance with the definition of DSO unidirectional messages, the 789 MESSAGE ID field MUST be zero. There is no client response to a PUSH 790 message. 792 The other header fields MUST be set as described in the DSO spec- 793 ification [RFC8490]. The DNS OPCODE field contains the OPCODE value 794 for DNS Stateful Operations (6). The four count fields MUST be zero, 795 and the corresponding four sections MUST be empty (i.e., absent). 797 A client MUST NOT send a PUSH message. If a client does send a PUSH 798 message, or a PUSH message is sent with the QR bit set indicating 799 that it is a response, this is a fatal error and the receiver MUST 800 forcibly abort the connection immediately. 802 The DSO-TYPE is PUSH (tentatively 0x41). 804 The DSO-LENGTH is the length of the DSO-DATA that follows, which 805 specifies the changes being communicated. 807 The DSO-DATA contains one or more change notifications. A PUSH 808 Message MUST contain at least one change notification. If a PUSH 809 Message is received that contains no change notifications, this is a 810 fatal error, and the client MUST forcibly abort the connection 811 immediately. 813 The change notification records are formatted similarly to how DNS 814 Resource Records are conventionally expressed in DNS messages, as 815 illustrated in Figure 3, and are interpreted as described below. 817 The TTL field holds an unsigned 32-bit integer [RFC2181]. If the TTL 818 is in the range 0 to 2,147,483,647 seconds (2^31 - 1, or 0x7FFFFFFF), 819 then a new DNS Resource Record with the given name, type, class and 820 RDATA is added. A TTL of 0 means that this record should be retained 821 for as long as the subscription is active, and should be discarded 822 immediately the moment the subscription is cancelled. 824 If the TTL has the value 0xFFFFFFFF, then the DNS Resource Record 825 with the given name, type, class and RDATA is removed. 827 If the TTL has the value 0xFFFFFFFE, then this is a 'collective' 828 remove notification. For collective remove notifications RDLEN MUST 829 be zero and consequently the RDATA MUST be empty. If a change 830 notification is received where TTL = 0xFFFFFFFE and RDLEN is not 831 zero, this is a fatal error, and the client MUST forcibly abort the 832 connection immediately. 834 There are three types of collective remove notification: 836 For collective remove notifications, if CLASS is not 255 (ANY) and 837 TYPE is not 255 (ANY) then for the given name this deletes all 838 records of the specified type in the specified class. 840 For collective remove notifications, if CLASS is not 255 (ANY) and 841 TYPE is 255 (ANY) then for the given name this deletes all records of 842 all types in the specified class. 844 For collective remove notifications, if CLASS is 255 (ANY), then for 845 the given name this deletes all records of all types in all classes. 846 In this case TYPE MUST be set to zero on transmission, and MUST be 847 silently ignored on reception. 849 Summary of change notification types: 851 Delete all RRsets from a name, in all classes 852 TTL=0xFFFFFFFE, RDLENGTH=0, CLASS=255 (ANY) 854 Delete all RRsets from a name, in given class: 855 TTL=0xFFFFFFFE, RDLENGTH=0, CLASS specifies class, TYPE=255 (ANY) 857 Delete specified RRset from a name, in given class: 858 TTL=0xFFFFFFFE, RDLENGTH=0 859 CLASS and TYPE specify the RRset being deleted 861 Delete an individual RR from a name: 862 TTL=0xFFFFFFFF 863 CLASS, TYPE, RDLENGTH and RDATA specify the RR being deleted. 865 Add individual RR to a name 866 TTLā©¾0 867 CLASS, TYPE, RDLENGTH, RDATA and TTL specify the RR being added. 869 Note that it is valid for the RDATA of an added or removed DNS 870 Resource Record to be empty (zero length). For example, an Address 871 Prefix List Resource Record [RFC3123] may have empty RDATA. 872 Therefore, a change notification with RDLEN=0 does not automatically 873 indicate a remove notification. If RDLEN=0 and TTL is the in the 874 range 0 - 0x7FFFFFFF, this change notification signals the addition 875 of a record with the given name, type, class, and empty RDATA. If 876 RDLEN=0 and TTL = 0xFFFFFFFF, this change notification signals the 877 removal specifically of that single record with the given name, type, 878 class, and empty RDATA. 880 If the TTL is any value other than 0xFFFFFFFF, 0xFFFFFFFE, or a value 881 in the range 0 - 0x7FFFFFFF, then the receiver SHOULD silently ignore 882 this particular change notification record. The connection is not 883 terminated and other valid change notification records within this 884 PUSH message are processed as usual. 886 For efficiency, when generating a PUSH message, a server SHOULD 887 include as many change notifications as it has immediately available 888 to send, rather than sending each change notification as a separate 889 DSO message. Once it has exhausted the list of change notifications 890 immediately available to send, a server SHOULD then send the PUSH 891 message immediately, rather than waiting to see if additional change 892 notifications become available. 894 For efficiency, when generating a PUSH message, a server SHOULD use 895 standard DNS name compression, with offsets relative to the beginning 896 of the DNS message [RFC1035]. When multiple change notifications in 897 a single PUSH message have the same owner name, this name compression 898 can yield significant savings. Name compression should be performed 899 as specified in Section 18.14 of the Multicast DNS specification 900 [RFC6762], namely, owner names should always be compressed, and names 901 appearing within RDATA should be compressed for only the RR types 902 listed below: 904 NS, CNAME, PTR, DNAME, SOA, MX, AFSDB, RT, KX, RP, PX, SRV, NSEC 906 Servers may generate PUSH messages up to a maximum DNS message length 907 of 16,382 bytes, counting from the start of the DSO 12-byte header. 908 Including the two-byte length prefix that is used to frame DNS over a 909 byte stream like TLS, this makes a total of 16,384 bytes. Servers 910 MUST NOT generate PUSH messages larger than this. Where the 911 immediately available change notifications are sufficient to exceed a 912 DNS message length of 16,382 bytes, the change notifications MUST be 913 communicated in separate PUSH messages of up to 16,382 bytes each. 914 DNS name compression becomes less effective for messages larger than 915 16,384 bytes, so little efficiency benefit is gained by sending 916 messages larger than this. 918 If a client receives a PUSH message with a DNS message length larger 919 than 16,382 bytes, this is a fatal error, and the client MUST 920 forcibly abort the connection immediately. 922 1 1 1 1 1 1 923 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 924 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 925 | MESSAGE ID (MUST BE ZERO) | \ 926 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 927 |QR| OPCODE(6) | Z | RCODE | | 928 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 929 | QDCOUNT (MUST BE ZERO) | | 930 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER 931 | ANCOUNT (MUST BE ZERO) | | 932 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 933 | NSCOUNT (MUST BE ZERO) | | 934 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 935 | ARCOUNT (MUST BE ZERO) | / 936 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 937 | DSO-TYPE = PUSH (tentatively 0x41) | 938 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 939 | DSO-LENGTH (number of octets in DSO-DATA) | 940 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 941 \ NAME \ \ 942 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 943 | TYPE | | 944 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 945 | CLASS | | 946 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 947 | TTL | | 948 | (32-bit unsigned big-endian integer) | > DSO-DATA 949 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 950 | RDLEN (16-bit unsigned big-endian integer) | | 951 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 952 \ RDATA (sized as necessary) \ | 953 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 954 : NAME, TYPE, CLASS, TTL, RDLEN, RDATA : | 955 : Repeated As Necessary : / 956 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 958 Figure 3: PUSH Message 960 When processing the records received in a PUSH Message, the receiving 961 client MUST validate that the records being added or deleted 962 correspond with at least one currently active subscription on that 963 session. Specifically, the record name MUST match the name given in 964 a SUBSCRIBE request, subject to the usual established DNS case- 965 insensitivity for US-ASCII letters. If the TYPE in the SUBSCRIBE 966 request was not ANY (255) then the TYPE of the record must match the 967 TYPE given in the SUBSCRIBE request. If the CLASS in the SUBSCRIBE 968 request was not ANY (255) then the CLASS of the record must match the 969 CLASS given in the SUBSCRIBE request. If a matching active 970 subscription on that session is not found, then that individual 971 record addition/deletion is silently ignored. Processing of other 972 additions and deletions in this message is not affected. The DSO 973 session is not closed. This is to allow for the unavoidable race 974 condition where a client sends an outbound UNSUBSCRIBE while inbound 975 PUSH messages for that subscription from the server are still in 976 flight. 978 In the case where a single change affects more than one active 979 subscription, only one PUSH message is sent. For example, a PUSH 980 message adding a given record may match both a SUBSCRIBE request with 981 the same TYPE and a different SUBSCRIBE request with TYPE=ANY (255). 982 It is not the case that two PUSH messages are sent because the new 983 record matches two active subscriptions. 985 The server SHOULD encode change notifications in the most efficient 986 manner possible. For example, when three AAAA records are deleted 987 from a given name, and no other AAAA records exist for that name, the 988 server SHOULD send a "delete an RRset from a name" PUSH message, not 989 three separate "delete an individual RR from a name" PUSH messages. 990 Similarly, when both an SRV and a TXT record are deleted from a given 991 name, and no other records of any kind exist for that name, the 992 server SHOULD send a "delete all RRsets from a name" PUSH message, 993 not two separate "delete an RRset from a name" PUSH messages. 995 A server SHOULD combine multiple change notifications in a single 996 PUSH message when possible, even if those change notifications apply 997 to different subscriptions. Conceptually, a PUSH message is a 998 session-level mechanism, not a subscription-level mechanism. 1000 The TTL of an added record is stored by the client. While the 1001 subscription is active, the TTL is not decremented, because a change 1002 to the TTL would produce a new update. For as long as a relevant 1003 subscription remains active, the client SHOULD assume that when a 1004 record goes away the server will notify it of that fact. 1005 Consequently, a client does not have to poll to verify that the 1006 record is still there. Once a subscription is cancelled 1007 (individually, or as a result of the DSO session being closed) record 1008 aging for records covered by the subscription resumes and records are 1009 removed from the local cache when their TTL reaches zero. 1011 6.4. DNS Push Notification UNSUBSCRIBE 1013 To cancel an individual subscription without closing the entire DSO 1014 session, the client sends an UNSUBSCRIBE message over the established 1015 DSO session to the server. The UNSUBSCRIBE message is encoded as a 1016 DSO unidirectional message [RFC8490]. This specification defines a 1017 primary unidirectional DSO TLV for DNS Push Notification UNSUBSCRIBE 1018 Messages (tentatively DSO Type Code 0x42). 1020 A server MUST NOT send an UNSUBSCRIBE message. If a server does send 1021 an UNSUBSCRIBE message over a DSO session initiated by a client, or 1022 an UNSUBSCRIBE message is sent with the QR bit set indicating that it 1023 is a response, this is a fatal error and the receiver MUST forcibly 1024 abort the connection immediately. 1026 6.4.1. UNSUBSCRIBE Message 1028 An UNSUBSCRIBE unidirectional message begins with the standard DSO 1029 12-byte header [RFC8490], followed by the UNSUBSCRIBE primary TLV. 1030 An UNSUBSCRIBE message is illustrated in Figure 4. 1032 In accordance with the definition of DSO unidirectional messages, the 1033 MESSAGE ID field MUST be zero. There is no server response to an 1034 UNSUBSCRIBE message. 1036 The other header fields MUST be set as described in the DSO spec- 1037 ification [RFC8490]. The DNS OPCODE field contains the OPCODE value 1038 for DNS Stateful Operations (6). The four count fields MUST be zero, 1039 and the corresponding four sections MUST be empty (i.e., absent). 1041 The DSO-TYPE is UNSUBSCRIBE (tentatively 0x42). 1043 The DSO-LENGTH field contains the value 2, the length of the 2-octet 1044 MESSAGE ID contained in the DSO-DATA. 1046 The DSO-DATA contains the value given in the MESSAGE ID field of an 1047 active SUBSCRIBE request. This is how the server knows which 1048 SUBSCRIBE request is being cancelled. After receipt of the 1049 UNSUBSCRIBE message, the SUBSCRIBE request is no longer active. 1051 It is allowable for the client to issue an UNSUBSCRIBE message for a 1052 previous SUBSCRIBE request for which the client has not yet received 1053 a SUBSCRIBE response. This is to allow for the case where a client 1054 starts and stops a subscription in less than the round-trip time to 1055 the server. The client is NOT required to wait for the SUBSCRIBE 1056 response before issuing the UNSUBSCRIBE message. 1058 Consequently, it is possible for a server to receive an UNSUBSCRIBE 1059 message that does not match any currently active subscription. This 1060 can occur when a client sends a SUBSCRIBE request, which subsequently 1061 fails and returns an error code, but the client sent an UNSUBSCRIBE 1062 message before it became aware that the SUBSCRIBE request had failed. 1063 Because of this, servers MUST silently ignore UNSUBSCRIBE messages 1064 that do not match any currently active subscription. 1066 1 1 1 1 1 1 1067 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 1068 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 1069 | MESSAGE ID (MUST BE ZERO) | \ 1070 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1071 |QR| OPCODE(6) | Z | RCODE | | 1072 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1073 | QDCOUNT (MUST BE ZERO) | | 1074 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER 1075 | ANCOUNT (MUST BE ZERO) | | 1076 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1077 | NSCOUNT (MUST BE ZERO) | | 1078 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1079 | ARCOUNT (MUST BE ZERO) | / 1080 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 1081 | DSO-TYPE = UNSUBSCRIBE (tentatively 0x42) | 1082 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 1083 | DSO-LENGTH (2) | 1084 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 1085 | SUBSCRIBE MESSAGE ID | > DSO-DATA 1086 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 1088 Figure 4: UNSUBSCRIBE Message 1090 6.5. DNS Push Notification RECONFIRM 1092 Sometimes, particularly when used with a Discovery Proxy [DisProx], a 1093 DNS Zone may contain stale data. When a client encounters data that 1094 it believes may be stale (e.g., an SRV record referencing a target 1095 host+port that is not responding to connection requests) the client 1096 can send a RECONFIRM message to ask the server to re-verify that the 1097 data is still valid. For a Discovery Proxy, this causes it to issue 1098 new Multicast DNS queries to ascertain whether the target device is 1099 still present. How the Discovery Proxy causes these new Multicast 1100 DNS queries to be issued depends on the details of the underlying 1101 Multicast DNS implementation being used. For example, a Discovery 1102 Proxy built on Apple's dns_sd.h API responds to a DNS Push 1103 Notification RECONFIRM message by calling the underlying API's 1104 DNSServiceReconfirmRecord() routine. 1106 For other types of DNS server, the RECONFIRM operation is currently 1107 undefined, and SHOULD result in a NOERROR response, but otherwise 1108 need not cause any action to occur. 1110 Frequent use of RECONFIRM operations may be a sign of network 1111 unreliability, or some kind of misconfiguration, so RECONFIRM 1112 operations MAY be logged or otherwise communicated to a human 1113 administrator to assist in detecting and remedying such network 1114 problems. 1116 If, after receiving a valid RECONFIRM message, the server determines 1117 that the disputed records are in fact no longer valid, then 1118 subsequent DNS PUSH Messages will be generated to inform interested 1119 clients. Thus, one client discovering that a previously-advertised 1120 device (like a network printer) is no longer present has the side 1121 effect of informing all other interested clients that the device in 1122 question is now gone. 1124 A server MUST NOT send a RECONFIRM message. If a server does send a 1125 RECONFIRM message over a DSO session initiated by a client, or a 1126 RECONFIRM message is sent with the QR bit set indicating that it is a 1127 response, this is a fatal error and the receiver MUST forcibly abort 1128 the connection immediately. 1130 6.5.1. RECONFIRM Message 1132 A RECONFIRM unidirectional message begins with the standard DSO 1133 12-byte header [RFC8490], followed by the RECONFIRM primary TLV. 1134 A RECONFIRM message is illustrated in Figure 5. 1136 In accordance with the definition of DSO unidirectional messages, the 1137 MESSAGE ID field MUST be zero. There is no server response to a 1138 RECONFIRM message. 1140 The other header fields MUST be set as described in the DSO spec- 1141 ification [RFC8490]. The DNS OPCODE field contains the OPCODE value 1142 for DNS Stateful Operations (6). The four count fields MUST be zero, 1143 and the corresponding four sections MUST be empty (i.e., absent). 1145 The DSO-TYPE is RECONFIRM (tentatively 0x43). 1147 The DSO-LENGTH is the length of the data that follows, which 1148 specifies the name, type, class, and content of the record being 1149 disputed. 1151 The DSO-DATA for a RECONFIRM message MUST contain exactly one record. 1152 The DSO-DATA for a RECONFIRM message has no count field to specify 1153 more than one record. Since RECONFIRM messages are sent over TCP, 1154 multiple RECONFIRM messages can be concatenated in a single TCP 1155 stream and packed efficiently into TCP segments. 1157 TYPE MUST NOT be the value ANY (255) and CLASS MUST NOT be the value 1158 ANY (255). 1160 DNS wildcarding is not supported. That is, a wildcard ("*") in a 1161 RECONFIRM message matches only a literal wildcard character ("*") in 1162 the zone, and nothing else. 1164 Aliasing is not supported. That is, a CNAME in a RECONFIRM message 1165 matches only a literal CNAME record in the zone, and no other records 1166 with the same owner name. 1168 Note that there is no RDLEN field, since the length of the RDATA can 1169 be inferred from DSO-LENGTH, so an additional RDLEN field would be 1170 redundant. 1172 1 1 1 1 1 1 1173 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 1174 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 1175 | MESSAGE ID | \ 1176 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1177 |QR| OPCODE(6) | Z | RCODE | | 1178 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1179 | QDCOUNT (MUST BE ZERO) | | 1180 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER 1181 | ANCOUNT (MUST BE ZERO) | | 1182 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1183 | NSCOUNT (MUST BE ZERO) | | 1184 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1185 | ARCOUNT (MUST BE ZERO) | / 1186 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 1187 | DSO-TYPE = RECONFIRM (tentatively 0x43) | 1188 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ 1189 | DSO-LENGTH (number of octets in DSO-DATA) | 1190 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ 1191 \ NAME \ \ 1192 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1193 | TYPE | | 1194 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > DSO-DATA 1195 | CLASS | | 1196 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | 1197 \ RDATA \ / 1198 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / 1200 Figure 5: RECONFIRM Message 1202 6.6. DNS Stateful Operations TLV Context Summary 1204 This document defines four new DSO TLVs. As suggested in Section 8.2 1205 of the DNS Stateful Operations specification [RFC8490], the valid 1206 contexts of these new TLV types are summarized below. 1208 The client TLV contexts are: 1210 C-P: Client request message, primary TLV 1211 C-U: Client unidirectional message, primary TLV 1212 C-A: Client request or unidirectional message, additional TLV 1213 CRP: Response back to client, primary TLV 1214 CRA: Response back to client, additional TLV 1216 +-------------+-----+-----+-----+-----+-----+ 1217 | TLV Type | C-P | C-U | C-A | CRP | CRA | 1218 +-------------+-----+-----+-----+-----+-----+ 1219 | SUBSCRIBE | X | | | | | 1220 | PUSH | | | | | | 1221 | UNSUBSCRIBE | | X | | | | 1222 | RECONFIRM | | X | | | | 1223 +-------------+-----+-----+-----+-----+-----+ 1225 Table 2: DSO TLV Client Context Summary 1227 The server TLV contexts are: 1229 S-P: Server request message, primary TLV 1230 S-U: Server unidirectional message, primary TLV 1231 S-A: Server request or unidirectional message, additional TLV 1232 SRP: Response back to server, primary TLV 1233 SRA: Response back to server, additional TLV 1235 +-------------+-----+-----+-----+-----+-----+ 1236 | TLV Type | S-P | S-U | S-A | SRP | SRA | 1237 +-------------+-----+-----+-----+-----+-----+ 1238 | SUBSCRIBE | | | | | | 1239 | PUSH | | X | | | | 1240 | UNSUBSCRIBE | | | | | | 1241 | RECONFIRM | | | | | | 1242 +-------------+-----+-----+-----+-----+-----+ 1244 Table 3: DSO TLV Server Context Summary 1246 6.7. Client-Initiated Termination 1248 An individual subscription is terminated by sending an UNSUBSCRIBE 1249 TLV for that specific subscription, or all subscriptions can be 1250 cancelled at once by the client closing the DSO session. When a 1251 client terminates an individual subscription (via UNSUBSCRIBE) or all 1252 subscriptions on that DSO session (by ending the session) it is 1253 signaling to the server that it is longer interested in receiving 1254 those particular updates. It is informing the server that the server 1255 may release any state information it has been keeping with regards to 1256 these particular subscriptions. 1258 After terminating its last subscription on a session via UNSUBSCRIBE, 1259 a client MAY close the session immediately, or it may keep it open if 1260 it anticipates performing further operations on that session in the 1261 future. If a client wishes to keep an idle session open, it MUST 1262 respect the maximum idle time required by the server [RFC8490]. 1264 If a client plans to terminate one or more subscriptions on a session 1265 and doesn't intend to keep that session open, then as an efficiency 1266 optimization it MAY instead choose to simply close the session, which 1267 implicitly terminates all subscriptions on that session. This may 1268 occur because the client computer is being shut down, is going to 1269 sleep, the application requiring the subscriptions has terminated, or 1270 simply because the last active subscription on that session has been 1271 cancelled. 1273 When closing a session, a client should perform an orderly close of 1274 the TLS session in order to allow for future TLS session resumption 1275 with the server (if available). See Section 7.4 below. Typical APIs 1276 will provide a session close method that will send a TLS close_notify 1277 alert (see Section 6.1 of the TLS 1.3 specification [RFC8446]). This 1278 instructs the recipient that the sender will not send any more data 1279 over the session. After sending the TLS close_notify alert the 1280 client MUST gracefully close the underlying connection using a TCP 1281 FIN, so that the TLS close_notify is reliably delivered. The 1282 mechanisms for gracefully closing a TCP connection with a TCP FIN 1283 vary depending on the networking API. For example, in the BSD 1284 Sockets API, sending a TCP FIN is achieved by calling 1285 "shutdown(s,SHUT_WR)" and keeping the socket open until all remaining 1286 data has been read from it. 1288 If the session is forcibly closed at the TCP level by sending a RST 1289 from either end of the connection, data may be lost and TLS session 1290 resumption of this session will not be possible. 1292 6.8. Client Fallback to Polling 1294 There are cases where a client may exhaust all avenues for 1295 establishing a DNS Push Notification subscription without success. 1296 This can happen if the client's configured recursive resolver does 1297 not support DNS over TLS, or supports DNS over TLS but is not 1298 listening on TCP port 853, or supports DNS over TLS on TCP port 853 1299 but does not support DSO on that port, or for some other reason is 1300 unable to provide a DNS Push Notification subscription. In this case 1301 the client will attempt to communicate directly with an appropriate 1302 server, and it may be that the zone apex discovery fails, or there is 1303 no "_dns-push-tls._tcp." SRV record, or server indicated in the 1304 SRV record is misconfigured, or is unresponsive for some other 1305 reason. 1307 Regardless of the reason for the failure, after being unable to 1308 establish the desired DNS Push Notification subscription, it is 1309 likely that the client will still wish to know the answer it seeks, 1310 even if that answer cannot be obtained with the timely change 1311 notifications provided by DNS Push Notifications. In such cases it 1312 is likely that the client will obtain the answer it seeks via a 1313 conventional DNS query instead, repeated at some interval to detect 1314 when the answer RRset changes. 1316 In the case where a client responds to its failure to establish a DNS 1317 Push Notification subscription by falling back to polling with 1318 conventional DNS queries instead, the polling rate should be 1319 controlled to avoid placing excessive burden on the server. The 1320 interval between successive DNS queries for the same name, type and 1321 class SHOULD be at least the minimum of: 900 seconds (15 minutes), or 1322 two seconds more than the TTL of the answer RRset. 1324 The reason that for TTLs shorter than 898 seconds the query should 1325 not be reissued until two seconds *after* the answer RRset has 1326 expired is to ensure that the answer RRset has also expired from the 1327 cache on the client's configured recursive resolver. Otherwise 1328 (particularly if the clocks on the client and the recursive resolver 1329 do not run at precisely the same rate) there's a risk of a race 1330 condition where the client queries its configured recursive resolver 1331 just as the answer RRset has one second remaining in the recursive 1332 resolver's cache. The client would then receive a reply telling it 1333 that the answer RRset has one second remaining, and then the client 1334 would then re-query the recursive resolver again one second later 1335 when the answer RRset actually expires, and only then would the 1336 recursive resolver issue a new query to fetch new fresh data from the 1337 authoritative server. Waiting until the answer RRset has definitely 1338 expired from the the cache on the client's configured recursive 1339 resolver avoids this race condition and unnecessary additional 1340 queries it causes. 1342 Each time a client is about to reissue its query to discover changes 1343 to the answer RRset, it should first make a new attempt to establish 1344 a DNS Push Notification subscription, using previously cached DNS 1345 answers as appropriate. After a temporary misconfiguration has been 1346 remedied, this allows a client that is polling to return to using DNS 1347 Push Notifications for asynchronous notification of changes. 1349 7. Security Considerations 1351 The Strict Privacy Usage Profile for DNS over TLS is REQUIRED for DNS 1352 Push Notifications [RFC8310]. Cleartext connections for DNS Push 1353 Notifications are not permissible. Since this is a new protocol, 1354 transition mechanisms from the Opportunistic Privacy profile are 1355 unnecessary. 1357 Also, see Section 9 of the DNS over (D)TLS Usage Profiles document 1358 [RFC8310] for additional recommendations for various versions of TLS 1359 usage. 1361 As a consequence of requiring TLS, client certificate authentication 1362 and verification may also be enforced by the server for stronger 1363 client-server security or end-to-end security. However, 1364 recommendations for security in particular deployment scenarios are 1365 outside the scope of this document. 1367 DNSSEC is RECOMMENDED for the authentication of DNS Push Notification 1368 servers. TLS alone does not provide complete security. TLS 1369 certificate verification can provide reasonable assurance that the 1370 client is really talking to the server associated with the desired 1371 host name, but since the desired host name is learned via a DNS SRV 1372 query, if the SRV query is subverted then the client may have a 1373 secure connection to a rogue server. DNSSEC can provided added 1374 confidence that the SRV query has not been subverted. 1376 7.1. Security Services 1378 It is the goal of using TLS to provide the following security 1379 services: 1381 Confidentiality: All application-layer communication is encrypted 1382 with the goal that no party should be able to decrypt it except 1383 the intended receiver. 1385 Data integrity protection: Any changes made to the communication in 1386 transit are detectable by the receiver. 1388 Authentication: An end-point of the TLS communication is 1389 authenticated as the intended entity to communicate with. 1391 Anti-replay protection: TLS provides for the detection of and 1392 prevention against messages sent previously over a TLS connection 1393 (such as DNS Push Notifications). Prior messages cannot be re- 1394 sent at a later time as a form of a man-in-the-middle attack. 1396 Deployment recommendations on the appropriate key lengths and cypher 1397 suites are beyond the scope of this document. Please refer to TLS 1398 Recommendations [RFC7525] for the best current practices. Keep in 1399 mind that best practices only exist for a snapshot in time and 1400 recommendations will continue to change. Updated versions or errata 1401 may exist for these recommendations. 1403 7.2. TLS Name Authentication 1405 As described in Section 6.1, the client discovers the DNS Push 1406 Notification server using an SRV lookup for the record name 1407 "_dns-push-tls._tcp.". The server connection endpoint SHOULD 1408 then be authenticated using DANE TLSA records for the associated SRV 1409 record. This associates the target's name and port number with a 1410 trusted TLS certificate [RFC7673]. This procedure uses the TLS 1411 Server Name Indication (SNI) extension [RFC6066] to inform the server 1412 of the name the client has authenticated through the use of TLSA 1413 records. Therefore, if the SRV record passes DNSSEC validation and a 1414 TLSA record matching the target name is useable, an SNI extension 1415 must be used for the target name to ensure the client is connecting 1416 to the server it has authenticated. If the target name does not have 1417 a usable TLSA record, then the use of the SNI extension is optional. 1418 See Usage Profiles for DNS over TLS and DNS over DTLS [RFC8310] for 1419 more information on authenticating domain names. 1421 7.3. TLS Early Data 1423 DSO messages with the SUBSCRIBE TLV as the Primary TLV are permitted 1424 in TLS early data. Using TLS early data can save one network round 1425 trip, and can result in the client obtaining results faster. 1427 However, there are some factors to consider before using TLS early 1428 data. 1430 TLS Early Data is not forward secret. In cases where forward secrecy 1431 of DNS Push Notification subscriptions is required, the client should 1432 not use TLS Early Data. 1434 With TLS early data there are no guarantees of non-replay between 1435 connections. If packets are duplicated and delayed in the network, 1436 the later arrivals could be mistaken for new subscription requests. 1437 Generally this is not a major concern, since the amount of state 1438 generated on the server for these spurious subscriptions is small and 1439 short-lived, since the TCP connection will not complete the three-way 1440 handshake. Servers MAY choose to implement rate-limiting measures 1441 that are activated when the server detects an excessive number of 1442 spurious subscription requests. 1444 For further guidance please see Section 2.3, Section 8, and 1445 Appendix E.5 of the TLS 1.3 specification [RFC8446]. 1447 7.4. TLS Session Resumption 1449 TLS Session Resumption is permissible on DNS Push Notification 1450 servers. The server may keep TLS state with Session IDs [RFC8446] or 1451 operate in stateless mode by sending a Session Ticket [RFC5077] to 1452 the client for it to store. However, closing the TLS connection 1453 terminates the DSO session. When the TLS session is resumed, the DNS 1454 Push Notification server will not have any subscription state and 1455 will proceed as with any other new DSO session. Use of TLS Session 1456 Resumption may allow a TLS connection to be set up more quickly, but 1457 the client will still have to recreate any desired subscriptions. 1459 8. IANA Considerations 1461 This document defines a new service name to be published in the IANA 1462 Registry Service Types [RFC6335][ST] that is only applicable for the 1463 TCP protocol. 1465 +-----------------------+------+----------------------+-------------+ 1466 | Name | Port | Value | Definition | 1467 +-----------------------+------+----------------------+-------------+ 1468 | DNS Push Notification | None | "_dns-push-tls._tcp" | Section 6.1 | 1469 | Service Type | | | | 1470 +-----------------------+------+----------------------+-------------+ 1472 Table 4: IANA Service Type Assignments 1474 This document also defines four new DNS Stateful Operation TLV types 1475 to be recorded in the IANA DSO Type Code Registry. 1477 +-------------+------------+--------+-----------------+-------------+ 1478 | Name | Value | Early | Status | Definition | 1479 | | | Data | | | 1480 +-------------+------------+--------+-----------------+-------------+ 1481 | SUBSCRIBE | TBA (0x40) | OK | Standards Track | Section 6.2 | 1482 | PUSH | TBA (0x41) | NO | Standards Track | Section 6.3 | 1483 | UNSUBSCRIBE | TBA (0x42) | NO | Standards Track | Section 6.4 | 1484 | RECONFIRM | TBA (0x43) | NO | Standards Track | Section 6.5 | 1485 +-------------+------------+--------+-----------------+-------------+ 1487 Table 5: IANA DSO TLV Type Code Assignments 1489 This document defines no new DNS OPCODEs or RCODEs. 1491 9. Acknowledgements 1493 The authors would like to thank Kiren Sekar and Marc Krochmal for 1494 previous work completed in this field. 1496 This draft has been improved due to comments from Ran Atkinson, Tim 1497 Chown, Sara Dickinson, Mark Delany, Ralph Droms, Jan Komissar, Eric 1498 Rescorla, Michael Richardson, David Schinazi, Manju Shankar Rao, 1499 Robert Sparks, Markus Stenberg, Andrew Sullivan, Michael Sweet, Dave 1500 Thaler, Brian Trammell, Bernie Volz, Eric Vyncke, Christopher Wood, 1501 Liang Xia, and Soraia Zlatkovic. Ted Lemon provided clarifying text 1502 that was greatly appreciated. 1504 10. References 1506 10.1. Normative References 1508 [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, 1509 DOI 10.17487/RFC0768, August 1980, 1510 . 1512 [RFC0793] Postel, J., "Transmission Control Protocol", STD 7, 1513 RFC 793, DOI 10.17487/RFC0793, September 1981, 1514 . 1516 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 1517 STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987, 1518 . 1520 [RFC1035] Mockapetris, P., "Domain names - implementation and 1521 specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, 1522 November 1987, . 1524 [RFC1123] Braden, R., Ed., "Requirements for Internet Hosts - 1525 Application and Support", STD 3, RFC 1123, 1526 DOI 10.17487/RFC1123, October 1989, 1527 . 1529 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 1530 Requirement Levels", BCP 14, RFC 2119, 1531 DOI 10.17487/RFC2119, March 1997, 1532 . 1534 [RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound, 1535 "Dynamic Updates in the Domain Name System (DNS UPDATE)", 1536 RFC 2136, DOI 10.17487/RFC2136, April 1997, 1537 . 1539 [RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS 1540 Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997, 1541 . 1543 [RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for 1544 specifying the location of services (DNS SRV)", RFC 2782, 1545 DOI 10.17487/RFC2782, February 2000, 1546 . 1548 [RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS) 1549 Extensions: Extension Definitions", RFC 6066, 1550 DOI 10.17487/RFC6066, January 2011, 1551 . 1553 [RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S. 1554 Cheshire, "Internet Assigned Numbers Authority (IANA) 1555 Procedures for the Management of the Service Name and 1556 Transport Protocol Port Number Registry", BCP 165, 1557 RFC 6335, DOI 10.17487/RFC6335, August 2011, 1558 . 1560 [RFC6895] Eastlake 3rd, D., "Domain Name System (DNS) IANA 1561 Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895, 1562 April 2013, . 1564 [RFC7673] Finch, T., Miller, M., and P. Saint-Andre, "Using DNS- 1565 Based Authentication of Named Entities (DANE) TLSA Records 1566 with SRV Records", RFC 7673, DOI 10.17487/RFC7673, October 1567 2015, . 1569 [RFC7766] Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and 1570 D. Wessels, "DNS Transport over TCP - Implementation 1571 Requirements", RFC 7766, DOI 10.17487/RFC7766, March 2016, 1572 . 1574 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 1575 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 1576 May 2017, . 1578 [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol 1579 Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, 1580 . 1582 [RFC8490] Bellis, R., Cheshire, S., Dickinson, J., Dickinson, S., 1583 Lemon, T., and T. Pusateri, "DNS Stateful Operations", 1584 RFC 8490, DOI 10.17487/RFC8490, March 2019, 1585 . 1587 [ST] "Service Name and Transport Protocol Port Number 1588 Registry", . 1591 10.2. Informative References 1593 [DisProx] Cheshire, S., "Discovery Proxy for Multicast DNS-Based 1594 Service Discovery", draft-ietf-dnssd-hybrid-10 (work in 1595 progress), March 2019. 1597 [I-D.dukkipati-tcpm-tcp-loss-probe] 1598 Dukkipati, N., Cardwell, N., Cheng, Y., and M. Mathis, 1599 "Tail Loss Probe (TLP): An Algorithm for Fast Recovery of 1600 Tail Losses", draft-dukkipati-tcpm-tcp-loss-probe-01 (work 1601 in progress), February 2013. 1603 [LLQ] Cheshire, S. and M. Krochmal, "DNS Long-Lived Queries", 1604 draft-sekar-dns-llq-03 (work in progress), March 2019. 1606 [obs] "Observer Pattern", 1607 . 1609 [RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS 1610 NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998, 1611 . 1613 [RFC3123] Koch, P., "A DNS RR Type for Lists of Address Prefixes 1614 (APL RR)", RFC 3123, DOI 10.17487/RFC3123, June 2001, 1615 . 1617 [RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom 1618 Syndication Format", RFC 4287, DOI 10.17487/RFC4287, 1619 December 2005, . 1621 [RFC4953] Touch, J., "Defending TCP Against Spoofing Attacks", 1622 RFC 4953, DOI 10.17487/RFC4953, July 2007, 1623 . 1625 [RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig, 1626 "Transport Layer Security (TLS) Session Resumption without 1627 Server-Side State", RFC 5077, DOI 10.17487/RFC5077, 1628 January 2008, . 1630 [RFC6281] Cheshire, S., Zhu, Z., Wakikawa, R., and L. Zhang, 1631 "Understanding Apple's Back to My Mac (BTMM) Service", 1632 RFC 6281, DOI 10.17487/RFC6281, June 2011, 1633 . 1635 [RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762, 1636 DOI 10.17487/RFC6762, February 2013, 1637 . 1639 [RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service 1640 Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013, 1641 . 1643 [RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure, 1644 "TCP Extensions for Multipath Operation with Multiple 1645 Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013, 1646 . 1648 [RFC6886] Cheshire, S. and M. Krochmal, "NAT Port Mapping Protocol 1649 (NAT-PMP)", RFC 6886, DOI 10.17487/RFC6886, April 2013, 1650 . 1652 [RFC6887] Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and 1653 P. Selkirk, "Port Control Protocol (PCP)", RFC 6887, 1654 DOI 10.17487/RFC6887, April 2013, 1655 . 1657 [RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP 1658 Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014, 1659 . 1661 [RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre, 1662 "Recommendations for Secure Use of Transport Layer 1663 Security (TLS) and Datagram Transport Layer Security 1664 (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May 1665 2015, . 1667 [RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS 1668 Terminology", RFC 7719, DOI 10.17487/RFC7719, December 1669 2015, . 1671 [RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D., 1672 and P. Hoffman, "Specification for DNS over Transport 1673 Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May 1674 2016, . 1676 [RFC8010] Sweet, M. and I. McDonald, "Internet Printing 1677 Protocol/1.1: Encoding and Transport", STD 92, RFC 8010, 1678 DOI 10.17487/RFC8010, January 2017, 1679 . 1681 [RFC8011] Sweet, M. and I. McDonald, "Internet Printing 1682 Protocol/1.1: Model and Semantics", STD 92, RFC 8011, 1683 DOI 10.17487/RFC8011, January 2017, 1684 . 1686 [RFC8310] Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles 1687 for DNS over TLS and DNS over DTLS", RFC 8310, 1688 DOI 10.17487/RFC8310, March 2018, 1689 . 1691 [SYN] Eddy, W., "Defenses Against TCP SYN Flooding Attacks", The 1692 Internet Protocol Journal, Cisco Systems, Volume 9, 1693 Number 4, December 2006. 1695 [XEP0060] Millard, P., Saint-Andre, P., and R. Meijer, "Publish- 1696 Subscribe", XSF XEP 0060, July 2010. 1698 Authors' Addresses 1700 Tom Pusateri 1701 Unaffiliated 1702 Raleigh, NC 27608 1703 USA 1705 Phone: +1 919 867 1330 1706 Email: pusateri@bangj.com 1708 Stuart Cheshire 1709 Apple Inc. 1710 One Apple Park Way 1711 Cupertino, CA 95014 1712 USA 1714 Phone: +1 (408) 996-1010 1715 Email: cheshire@apple.com