--- 1/draft-ietf-dnssd-push-22.txt 2019-07-21 20:13:10.537968026 -0700 +++ 2/draft-ietf-dnssd-push-23.txt 2019-07-21 20:13:10.617970055 -0700 @@ -1,19 +1,19 @@ Internet Engineering Task Force T. Pusateri Internet-Draft Unaffiliated Intended status: Standards Track S. Cheshire -Expires: January 9, 2020 Apple Inc. - July 8, 2019 +Expires: January 22, 2020 Apple Inc. + July 21, 2019 DNS Push Notifications - draft-ietf-dnssd-push-22 + draft-ietf-dnssd-push-23 Abstract The Domain Name System (DNS) was designed to return matching records efficiently for queries for data that are relatively static. When those records change frequently, DNS is still efficient at returning the updated results when polled, as long as the polling rate is not too high. But there exists no mechanism for a client to be asynchronously notified when these changes occur. This document defines a mechanism for a client to be notified of such changes to @@ -27,109 +27,130 @@ Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." - This Internet-Draft will expire on January 9, 2020. + This Internet-Draft will expire on January 22, 2020. Copyright Notice Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3 + 1.2. Fatal Errors . . . . . . . . . . . . . . . . . . . . . . 3 2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 5 - 4. Transport . . . . . . . . . . . . . . . . . . . . . . . . . . 6 - 5. State Considerations . . . . . . . . . . . . . . . . . . . . 7 + 4. State Considerations . . . . . . . . . . . . . . . . . . . . 6 + 5. Transport . . . . . . . . . . . . . . . . . . . . . . . . . . 7 6. Protocol Operation . . . . . . . . . . . . . . . . . . . . . 8 6.1. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 9 6.2. DNS Push Notification SUBSCRIBE . . . . . . . . . . . . . 13 6.2.1. SUBSCRIBE Request . . . . . . . . . . . . . . . . . . 13 6.2.2. SUBSCRIBE Response . . . . . . . . . . . . . . . . . 16 6.3. DNS Push Notification Updates . . . . . . . . . . . . . . 20 6.3.1. PUSH Message . . . . . . . . . . . . . . . . . . . . 20 6.4. DNS Push Notification UNSUBSCRIBE . . . . . . . . . . . . 25 6.4.1. UNSUBSCRIBE Message . . . . . . . . . . . . . . . . . 25 6.5. DNS Push Notification RECONFIRM . . . . . . . . . . . . . 27 6.5.1. RECONFIRM Message . . . . . . . . . . . . . . . . . . 28 6.6. DNS Stateful Operations TLV Context Summary . . . . . . . 30 6.7. Client-Initiated Termination . . . . . . . . . . . . . . 31 - 7. Security Considerations . . . . . . . . . . . . . . . . . . . 32 - 7.1. Security Services . . . . . . . . . . . . . . . . . . . . 32 - 7.2. TLS Name Authentication . . . . . . . . . . . . . . . . . 33 - 7.3. TLS Early Data . . . . . . . . . . . . . . . . . . . . . 33 - 7.4. TLS Session Resumption . . . . . . . . . . . . . . . . . 34 - 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34 - 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 35 - 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 35 - 10.1. Normative References . . . . . . . . . . . . . . . . . . 35 - 10.2. Informative References . . . . . . . . . . . . . . . . . 37 - Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 39 + 6.8. Client Fallback to Polling . . . . . . . . . . . . . . . 32 + 7. Security Considerations . . . . . . . . . . . . . . . . . . . 33 + 7.1. Security Services . . . . . . . . . . . . . . . . . . . . 33 + 7.2. TLS Name Authentication . . . . . . . . . . . . . . . . . 34 + 7.3. TLS Early Data . . . . . . . . . . . . . . . . . . . . . 34 + 7.4. TLS Session Resumption . . . . . . . . . . . . . . . . . 35 + 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 36 + 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 36 + 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 37 + 10.1. Normative References . . . . . . . . . . . . . . . . . . 37 + 10.2. Informative References . . . . . . . . . . . . . . . . . 38 + Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 41 1. Introduction Domain Name System (DNS) records may be updated using DNS Update [RFC2136]. Other mechanisms such as a Discovery Proxy [DisProx] can also generate changes to a DNS zone. This document specifies a protocol for DNS clients to subscribe to receive asynchronous - notifications of changes to RRSets of interest. It is immediately + notifications of changes to RRsets of interest. It is immediately relevant in the case of DNS Service Discovery [RFC6763] but is not limited to that use case, and provides a general DNS mechanism for DNS record change notifications. Familiarity with the DNS protocol and DNS packet formats is assumed [RFC1034] [RFC1035] [RFC6895]. 1.1. Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. These words may also appear in this document in lower case as plain English words, absent their normative meanings. +1.2. Fatal Errors + + Certain invalid situations are described in this specification, like + a server sending a Push Notification subscription request to a + client, or a client sending a Push Notification response to a server. + These should never occur with a correctly implemented client and + server, and if they do occur then they indicate a serious + implementation error. In these extreme cases there is no reasonable + expectation of a graceful recovery, and the recipient detecting the + error should respond by unilaterally aborting the session without + regard for data loss. Such cases are addressed by having an engineer + investigate the cause of the failure and fixing the problem in the + software. + + Where this specification says "forcibly abort", it means sending a + TCP RST to terminate the TCP connection, and the TLS session running + over that TCP connection. In the BSD Sockets API, this is achieved + by setting the SO_LINGER option to zero before closing the socket. + 2. Motivation As the domain name system continues to adapt to new uses and changes in deployment, polling has the potential to burden DNS servers at many levels throughout the network. Other network protocols have successfully deployed a publish/subscribe model following the Observer design pattern [obs]. XMPP Publish-Subscribe [XEP0060] and Atom [RFC4287] are examples. While DNS servers are generally highly tuned and capable of a high rate of query/response traffic, adding a publish/subscribe model for tracking changes to DNS records can deliver more timely notification of changes with reduced CPU usage and lower network traffic. Multicast DNS [RFC6762] implementations always listen on a well known - link-local IP multicast group, and record changes are sent to that - multicast group address for all group members to receive. Therefore, - Multicast DNS already has asynchronous change notification + link-local IP multicast group address, and record changes are sent to + that multicast group address for all group members to receive. + Therefore, Multicast DNS already has asynchronous change notification capability. However, when DNS Service Discovery [RFC6763] is used across a wide area network using Unicast DNS (possibly facilitated via a Discovery Proxy [DisProx]) it would be beneficial to have an equivalent capability for Unicast DNS, to allow clients to learn about DNS record changes in a timely manner without polling. The DNS Long-Lived Queries (LLQ) mechanism [LLQ] is an existing deployed solution to provide asynchronous change notifications, used by Apple's Back to My Mac [RFC6281] service introduced in Mac OS X 10.5 Leopard in 2007. Back to My Mac was designed in an era when the @@ -146,67 +167,65 @@ and therefore doesn't need to reinvent existing TCP functionality. Using TCP also gives long-lived low-traffic connections better longevity through NAT gateways without depending on the gateway to support NAT Port Mapping Protocol (NAT-PMP) [RFC6886] or Port Control Protocol (PCP) [RFC6887], or resorting to excessive keepalive traffic. 3. Overview A DNS Push Notification client subscribes for Push Notifications for - a particular RRSet by connecting to the appropriate Push Notification - server for that RRSet, and sending DSO message(s) indicating the - RRSet(s) of interest. When the client loses interest in receiving + a particular RRset by connecting to the appropriate Push Notification + server for that RRset, and sending DSO message(s) indicating the + RRset(s) of interest. When the client loses interest in receiving further updates to these records, it unsubscribes. The DNS Push Notification server for a DNS zone is any server capable of generating the correct change notifications for a name. It may be a primary, secondary, or stealth name server [RFC7719]. - Consequently, the "_dns-push-tls._tcp." SRV record for a zone - MAY reference the same target host and port as that zone's + + The "_dns-push-tls._tcp." SRV record for a zone MAY reference + the same target host and port as that zone's "_dns-update-tls._tcp." SRV record. When the same target host and port is offered for both DNS Updates and DNS Push Notifications, - a client MAY use a single TCP connection to that server for both DNS - Updates and DNS Push Notification Subscriptions. - - Supporting DNS Updates and DNS Push Notifications on the same server - is OPTIONAL. A DNS Push Notification server is not required to - support DNS Update. - - DNS Updates and DNS Push Notifications may be handled on different - ports on the same target host, in which case they are not considered - to be the "same server" for the purposes of this specification, and - communications with these two ports are handled independently. + a client MAY use a single DSO session to that server for both DNS + Updates and DNS Push Notification Subscriptions. DNS Updates and DNS + Push Notifications may be handled on different ports on the same + target host, in which case they are not considered to be the "same + server" for the purposes of this specification, and communications + with these two ports are handled independently. Supporting DNS + Updates and DNS Push Notifications on the same server is OPTIONAL. A + DNS Push Notification server is not required to support DNS Update. Standard DNS Queries MAY be sent over a DNS Push Notification (i.e., DSO) session. For any zone for which the server is authoritative, it - MUST respond authoritatively for queries on names falling within that - zone (e.g., the in the "_dns-push-tls._tcp." SRV record) - both for normal DNS queries and for DNS Push Notification - subscriptions. For names for which the server is acting as a - recursive resolver, e.g. when the server is the local recursive - resolver, for any query for which it supports DNS Push Notification - subscriptions, it MUST also support standard queries. + MUST respond authoritatively for queries for names falling within + that zone (e.g., the "_dns-push-tls._tcp." SRV record) both for + normal DNS queries and for DNS Push Notification subscriptions. For + names for which the server is acting as a recursive resolver, e.g., + when the server is the local recursive resolver, for any query for + which it supports DNS Push Notification subscriptions, it MUST also + support standard queries. DNS Push Notifications impose less load on the responding server than rapid polling would, but Push Notifications do still have a cost, so DNS Push Notification clients MUST NOT recklessly create an excessive number of Push Notification subscriptions. Specifically: (a) A subscription should only be active when there is a valid reason to need live data (for example, an on-screen display is currently showing the results to the user) and the subscription SHOULD be cancelled as soon as the need for that data ends (for example, when the user dismisses that display). In the case of a device like a smartphone which, after some period of inactivity, goes to sleep or otherwise darkens its screen, it should cancel its subscriptions when - darkening the screen (since the user cannot see any changes in the + darkening the screen (since the user cannot see any changes on the display anyway) and reinstate its subscriptions when re-awakening from display sleep. (b) A DNS Push Notification client SHOULD NOT routinely keep a DNS Push Notification subscription active 24 hours a day, 7 days a week, just to keep a list in memory up to date so that if the user does choose to bring up an on-screen display of that data, it can be displayed really fast. DNS Push Notifications are designed to be fast enough that there is no need to pre-load a "warm" list in memory just in case it might be needed later. @@ -214,21 +233,36 @@ Generally, as described in the DNS Stateful Operations specification [RFC8490], a client must not keep a session to a server open indefinitely if it has no subscriptions (or other operations) active on that session. A client MAY close a session as soon as it becomes idle, and then if needed in the future, open a new session when required. Alternatively, a client MAY speculatively keep an idle session open for some time, subject to the constraint that it MUST NOT keep a session open that has been idle for more than the session's idle timeout (15 seconds by default) [RFC8490]. -4. Transport +4. State Considerations + + Each DNS Push Notification server is capable of handling some finite + number of Push Notification subscriptions. This number will vary + from server to server and is based on physical machine + characteristics, network bandwidth, and operating system resource + allocation. After a client establishes a session to a DNS server, + each subscription is individually accepted or rejected. Servers may + employ various techniques to limit subscriptions to a manageable + level. Correspondingly, the client is free to establish simultaneous + sessions to alternate DNS servers that support DNS Push Notifications + for the zone and distribute subscriptions at the client's discretion. + In this way, both clients and servers can react to resource + constraints. + +5. Transport Other DNS operations like DNS Update [RFC2136] MAY use either User Datagram Protocol (UDP) [RFC0768] or Transmission Control Protocol (TCP) [RFC0793] as the transport protocol, in keeping with the historical precedent that DNS queries must first be sent over UDP [RFC1123]. This requirement to use UDP has subsequently been relaxed [RFC7766]. In keeping with the more recent precedent, DNS Push Notification is defined only for TCP. DNS Push Notification clients MUST use DNS @@ -238,46 +272,31 @@ concerns of state overload at the server which is a potential problem with connectionless protocols using spoofed source addresses. All subscribers are guaranteed to be reachable by the server by virtue of the TCP three-way handshake. Flooding attacks are possible with any protocol, and a benefit of TCP is that there are already established industry best practices to guard against SYN flooding and similar attacks [SYN] [RFC4953]. Use of TCP also allows DNS Push Notifications to take advantage of current and future developments in TCP, such as Multipath TCP (MPTCP) - [RFC6824], TCP Fast Open (TFO) [RFC7413], Tail Loss Probe (TLP) [I-D.dukkipati-tcpm-tcp-loss-probe], and so on. - Transport Layer Security (TLS) [RFC8446] is well understood and - deployed across many protocols running over TCP. It is designed to - prevent eavesdropping, tampering, and message forgery. TLS is - REQUIRED for every connection between a client subscriber and server - in this protocol specification. Additional security measures such as - client authentication during TLS negotiation MAY also be employed to - increase the trust relationship between client and server. - -5. State Considerations - - Each DNS Push Notification server is capable of handling some finite - number of Push Notification subscriptions. This number will vary - from server to server and is based on physical machine - characteristics, network bandwidth, and operating system resource - allocation. After a client establishes a session to a DNS server, - each subscription is individually accepted or rejected. Servers may - employ various techniques to limit subscriptions to a manageable - level. Correspondingly, the client is free to establish simultaneous - sessions to alternate DNS servers that support DNS Push Notifications - for the zone and distribute subscriptions at the client's discretion. - In this way, both clients and servers can react to resource - constraints. + Transport Layer Security (TLS) [RFC8446] is well understood, and used + by many application-layer protocols running over TCP. TLS is + designed to prevent eavesdropping, tampering, and message forgery. + TLS is REQUIRED for every connection between a client subscriber and + server in this protocol specification. Additional security measures + such as client authentication during TLS negotiation MAY also be + employed to increase the trust relationship between client and + server. 6. Protocol Operation The DNS Push Notification protocol is a session-oriented protocol, and makes use of DNS Stateful Operations (DSO) [RFC8490]. For details of the DSO message format refer to the DNS Stateful Oper- ations specification [RFC8490]. Those details are not repeated here. DNS Push Notification clients and servers MUST support DSO. A single @@ -288,22 +307,22 @@ the appropriate server, using the procedure described in Section 6.1, and then making a TLS/TCP connection to it. A typical DNS Push Notification client will immediately issue a DSO Keepalive operation to request a session timeout and/or keepalive interval longer than the the 15-second default values, but this is not required. A DNS Push Notification client MAY issue other requests on the session first, and only issue a DSO Keepalive operation later if it determines that to be necessary. Sending either a DSO Keepalive operation or a Push Notification subscription - over the TLS/TCP connection to the server signals the client's - support of DSO and serves to establish a DSO session. + request over the TLS/TCP connection to the server signals the + client's support of DSO and serves to establish a DSO session. In accordance with the current set of active subscriptions, the server sends relevant asynchronous Push Notifications to the client. Note that a client MUST be prepared to receive (and silently ignore) Push Notifications for subscriptions it has previously removed, since there is no way to prevent the situation where a Push Notification is in flight from server to client while the client's UNSUBSCRIBE message cancelling that subscription is simultaneously in flight from client to server. @@ -326,24 +345,24 @@ which, if it doesn't already have appropriate answer(s) in its cache, issues an upstream query to satisfy the request. In many contexts, the recursive resolver will be able to handle Push Notifications for all names that the client may need to follow. Use of VPN tunnels and split-view DNS can create some additional complexity in the client software here; the techniques to handle VPN tunnels and split-view DNS for DNS Push Notifications are the same as those already used to handle this for normal DNS queries. - If the recursive resolver does not support DNS over TLS, or does - support DNS over TLS but is not listening on TCP port 853, or does - support DNS over TLS on TCP port 853 but does not support DSO on that - port, then the DSO Session session establishment will fail [RFC8490]. + If the recursive resolver does not support DNS over TLS, or supports + DNS over TLS but is not listening on TCP port 853, or supports DNS + over TLS on TCP port 853 but does not support DSO on that port, then + the DSO Session session establishment will fail [RFC8490]. If the recursive resolver does support DSO but not Push Notification subscriptions, then it will return the DSO error code, DSOTYPENI (11). In some cases, the recursive resolver may support DSO and Push Notification subscriptions, but may not be able to subscribe for Push Notifications for a particular name. In this case, the recursive resolver should return SERVFAIL to the client. This includes being unable to establish a connection to the zone's DNS Push Notification @@ -366,31 +385,32 @@ wishes to subscribe. Successive SOA queries are then issued, trimming one label each time, until the closest enclosing authoritative server is discovered. There is also an optimization to enable the client to take a "short cut" directly to the SOA record of the closest enclosing authoritative server in many cases. 1. The client begins the discovery by sending a DNS query to its local resolver, with record type SOA [RFC1035] for the record name to which it wishes to subscribe. As an example, suppose the client wishes to subscribe to PTR records with the name - _ipp._tcp.foo.example.com (to discover Internet Printing Protocol - (IPP) printers [RFC8010] [RFC8011] being advertised at - "foo.example.com"). The client begins by sending an SOA query - for _ipp._tcp.foo.example.com to the local recursive resolver. - The goal is to determine the server authoritative for the name - _ipp._tcp.foo.example.com. The closest enclosing DNS zone - containing the name _ipp._tcp.foo.example.com could be - example.com, or foo.example.com, or _tcp.foo.example.com, or even - _ipp._tcp.foo.example.com. The client does not know in advance - where the closest enclosing zone cut occurs, which is why it uses - the iterative procedure described here to discover this + _ipp._tcp.headoffice.example.com (to discover Internet Printing + Protocol (IPP) printers [RFC8010] [RFC8011] being advertised in + the head office of Example Company.). The client begins by + sending an SOA query for _ipp._tcp.headoffice.example.com to the + local recursive resolver. The goal is to determine the server + authoritative for the name _ipp._tcp.headoffice.example.com. The + closest enclosing DNS zone containing the name + _ipp._tcp.headoffice.example.com could be example.com, or + headoffice.example.com, or _tcp.headoffice.example.com, or even + _ipp._tcp.headoffice.example.com. The client does not know in + advance where the closest enclosing zone cut occurs, which is why + it uses the iterative procedure described here to discover this information. 2. If the requested SOA record exists, it will be returned in the Answer section with a NOERROR response code, and the client has succeeded in discovering the information it needs. (This language is not placing any new requirements on DNS recursive resolvers. This text merely describes the existing operation of the DNS protocol [RFC1034] [RFC1035].) 3. If the requested SOA record does not exist, the client will get @@ -400,27 +420,27 @@ requested name in the Authority Section. If the SOA record is received in the Authority Section, then the client has succeeded in discovering the information it needs. (This language is not placing any new requirements on DNS recursive resolvers. This text merely describes the existing operation of the DNS protocol regarding negative responses [RFC2308].) 4. If the client receives a response containing no SOA record, then it proceeds with the iterative approach. The client strips the - leading label from the current query name and if the resulting - name has at least one label in it, the client sends an SOA query + leading label from the current query name, and if the resulting + name has at least two labels in it, the client sends an SOA query for that new name, and processing continues at step 2 above, repeating the iterative search until either an SOA is received, or the query name consists of a single label, i.e., a Top Level - Domain (TLD). In the case of a single-label TLD, this is a - network configuration error which should not happen and the + Domain (TLD). In the case of a single-label (TLD), this is a + network configuration error, which should not happen, and the client gives up. The client may retry the operation at a later time, of the client's choosing, such after a change in network attachment. 5. Once the SOA is known (either by virtue of being seen in the Answer Section, or in the Authority Section), the client sends a DNS query with type SRV [RFC2782] for the record name "_dns-push-tls._tcp.", where is the owner name of the discovered SOA record. @@ -448,51 +468,43 @@ accept a subscription request, or is not reachable within a reasonable time, as determined by the client, then a subsequent server is to be contacted. Each time a client makes a new DNS Push Notification subscription session, it SHOULD repeat the discovery process in order to determine the preferred DNS server for subscriptions at that time. However, the client device MUST respect the DNS TTL values on records it receives, and store them in its local cache with this lifetime. This means that, as long as the DNS TTL values on the authoritative - records were set to reasonable values, repeated application of this + records are set to reasonable values, repeated application of this discovery process can be completed nearly instantaneously by the client, using only locally-stored cached data. 6.2. DNS Push Notification SUBSCRIBE After connecting, and requesting a longer idle timeout and/or keepalive interval if necessary, a DNS Push Notification client then indicates its desire to receive DNS Push Notifications for a given domain name by sending a SUBSCRIBE request to the server. A SUBSCRIBE request is encoded in a DSO message [RFC8490]. This specification defines a primary DSO TLV for DNS Push Notification SUBSCRIBE Requests (tentatively DSO Type Code 0x40). DSO messages with the SUBSCRIBE TLV as the Primary TLV are permitted - in early data, provided that the precautions described in Section 7.3 - are followed. + in TLS early data, provided that the precautions described in + Section 7.3 are followed. The entity that initiates a SUBSCRIBE request is by definition the client. A server MUST NOT send a SUBSCRIBE request over an existing session from a client. If a server does send a SUBSCRIBE request over a DSO session initiated by a client, this is a fatal error and - the client should immediately abort the connection with a TLS - close_notify alert (see Section 6.1 of the TLS 1.3 specification - [RFC8446]). After sending the TLS close_notify alert the client MUST - gracefully close the underlying connection using a TCP FIN, so that - the TLS close_notify is reliably delivered. The mechanisms for - gracefully closing a TCP connection with a TCP FIN vary depending on - the networking API. For example, in the BSD Sockets API, sending a - TCP FIN is achieved by calling "shutdown(s,SHUT_WR)" and keeping the - socket open until all remaining data has been read from it. + the client MUST forcibly abort the connection immediately. 6.2.1. SUBSCRIBE Request A SUBSCRIBE request begins with the standard DSO 12-byte header [RFC8490], followed by the SUBSCRIBE primary TLV. A SUBSCRIBE request message is illustrated in Figure 1. The MESSAGE ID field MUST be set to a unique value, that the client is not using for any other active operation on this DSO session. For the purposes here, a MESSAGE ID is in use on this session if the @@ -548,31 +560,31 @@ If accepted, the subscription will stay in effect until the client cancels the subscription using UNSUBSCRIBE or until the DSO session between the client and the server is closed. SUBSCRIBE requests on a given session MUST be unique. A client MUST NOT send a SUBSCRIBE message that duplicates the NAME, TYPE and CLASS of an existing active subscription on that DSO session. For the purpose of this matching, the established DNS case-insensitivity for US-ASCII letters applies (e.g., "example.com" and "Example.com" are - the same). If a server receives such a duplicate SUBSCRIBE message - this is a fatal error and the server MUST immediately terminate the - connection with a TLS close_notify alert followed by a TCP FIN. + the same). If a server receives such a duplicate SUBSCRIBE message, + this is a fatal error and the server MUST forcibly abort the + connection immediately. DNS wildcarding is not supported. That is, a wildcard ("*") in a SUBSCRIBE message matches only a literal wildcard character ("*") in the zone, and nothing else. Aliasing is not supported. That is, a CNAME in a SUBSCRIBE message - matches only a literal CNAME record in the zone, and not to any - records referenced by the owner name. + matches only a literal CNAME record in the zone, and no other records + with the same owner name. A client may SUBSCRIBE to records that are unknown to the server at the time of the request (providing that the name falls within one of the zone(s) the server is responsible for) and this is not an error. The server MUST NOT return NXDOMAIN in this case. The server MUST accept these requests and send Push Notifications if and when matching records are found in the future. If neither TYPE nor CLASS are ANY (255) then this is a specific subscription to changes for the given NAME, TYPE and CLASS. If one @@ -605,20 +617,25 @@ The MESSAGE ID field MUST echo the value given in the MESSAGE ID field of the SUBSCRIBE request. This is how the client knows which request is being responded to. A SUBSCRIBE response message MUST NOT include a SUBSCRIBE TLV. If a client receives a SUBSCRIBE response message containing a SUBSCRIBE TLV then the response message is processed but the SUBSCRIBE TLV MUST be silently ignored. + A client MUST NOT send a SUBSCRIBE response. If a client does send a + SUBSCRIBE message, with the QR bit set indicating that it is a + response, this is a fatal error and the server MUST forcibly abort + the connection immediately. + 1 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ | MESSAGE ID | \ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | |QR| OPCODE(6) | Z | RCODE | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | QDCOUNT (MUST BE ZERO) | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER | ANCOUNT (MUST BE ZERO) | | @@ -770,84 +787,90 @@ In accordance with the definition of DSO unidirectional messages, the MESSAGE ID field MUST be zero. There is no client response to a PUSH message. The other header fields MUST be set as described in the DSO spec- ification [RFC8490]. The DNS OPCODE field contains the OPCODE value for DNS Stateful Operations (6). The four count fields MUST be zero, and the corresponding four sections MUST be empty (i.e., absent). + A client MUST NOT send a PUSH message. If a client does send a PUSH + message, or a PUSH message is sent with the QR bit set indicating + that it is a response, this is a fatal error and the receiver MUST + forcibly abort the connection immediately. + The DSO-TYPE is PUSH (tentatively 0x41). The DSO-LENGTH is the length of the DSO-DATA that follows, which specifies the changes being communicated. The DSO-DATA contains one or more change notifications. A PUSH Message MUST contain at least one change notification. If a PUSH Message is received that contains no change notifications, this is a - fatal error, and the receiver MUST immediately terminate the - connection with a TLS close_notify alert followed by a TCP FIN. + fatal error, and the client MUST forcibly abort the connection + immediately. The change notification records are formatted similarly to how DNS Resource Records are conventionally expressed in DNS messages, as illustrated in Figure 3, and are interpreted as described below. The TTL field holds an unsigned 32-bit integer [RFC2181]. If the TTL is in the range 0 to 2,147,483,647 seconds (2^31 - 1, or 0x7FFFFFFF), then a new DNS Resource Record with the given name, type, class and RDATA is added. A TTL of 0 means that this record should be retained for as long as the subscription is active, and should be discarded immediately the moment the subscription is cancelled. If the TTL has the value 0xFFFFFFFF, then the DNS Resource Record with the given name, type, class and RDATA is removed. If the TTL has the value 0xFFFFFFFE, then this is a 'collective' remove notification. For collective remove notifications RDLEN MUST be zero and consequently the RDATA MUST be empty. If a change notification is received where TTL = 0xFFFFFFFE and RDLEN is not - zero, this is a fatal error, and the receiver MUST immediately - terminate the connection with a TLS close_notify alert followed by a - TCP FIN. There are three types of collective remove notification: + zero, this is a fatal error, and the client MUST forcibly abort the + connection immediately. - For collective remove notifications, if CLASS is 255 (ANY), then for - the given name this deletes all records of all types in all classes. - In this case TYPE MUST be set to zero on transmission, and MUST be - silently ignored on reception. + There are three types of collective remove notification: + + For collective remove notifications, if CLASS is not 255 (ANY) and + TYPE is not 255 (ANY) then for the given name this deletes all + records of the specified type in the specified class. For collective remove notifications, if CLASS is not 255 (ANY) and TYPE is 255 (ANY) then for the given name this deletes all records of all types in the specified class. - For collective remove notifications, if CLASS is not 255 (ANY) and - TYPE is not 255 (ANY) then for the given name this deletes all - records of the specified type in the specified class. + For collective remove notifications, if CLASS is 255 (ANY), then for + the given name this deletes all records of all types in all classes. + In this case TYPE MUST be set to zero on transmission, and MUST be + silently ignored on reception. Summary of change notification types: Delete all RRsets from a name, in all classes TTL=0xFFFFFFFE, RDLENGTH=0, CLASS=255 (ANY) Delete all RRsets from a name, in given class: TTL=0xFFFFFFFE, RDLENGTH=0, CLASS specifies class, TYPE=255 (ANY) Delete specified RRset from a name, in given class: TTL=0xFFFFFFFE, RDLENGTH=0 CLASS and TYPE specify the RRset being deleted Delete an individual RR from a name: TTL=0xFFFFFFFF CLASS, TYPE, RDLENGTH and RDATA specify the RR being deleted. Add individual RR to a name - TTL>=0 + TTLā©¾0 CLASS, TYPE, RDLENGTH, RDATA and TTL specify the RR being added. Note that it is valid for the RDATA of an added or removed DNS Resource Record to be empty (zero length). For example, an Address Prefix List Resource Record [RFC3123] may have empty RDATA. Therefore, a change notification with RDLEN=0 does not automatically indicate a remove notification. If RDLEN=0 and TTL is the in the range 0 - 0x7FFFFFFF, this change notification signals the addition of a record with the given name, type, class, and empty RDATA. If RDLEN=0 and TTL = 0xFFFFFFFF, this change notification signals the @@ -886,23 +909,22 @@ byte stream like TLS, this makes a total of 16,384 bytes. Servers MUST NOT generate PUSH messages larger than this. Where the immediately available change notifications are sufficient to exceed a DNS message length of 16,382 bytes, the change notifications MUST be communicated in separate PUSH messages of up to 16,382 bytes each. DNS name compression becomes less effective for messages larger than 16,384 bytes, so little efficiency benefit is gained by sending messages larger than this. If a client receives a PUSH message with a DNS message length larger - than 16,382 bytes, the this is a fatal error, and the receiver MUST - immediately terminate the connection with a TLS close_notify alert - followed by a TCP FIN. + than 16,382 bytes, this is a fatal error, and the client MUST + forcibly abort the connection immediately. 1 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ | MESSAGE ID (MUST BE ZERO) | \ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | |QR| OPCODE(6) | Z | RCODE | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | QDCOUNT (MUST BE ZERO) | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER @@ -918,21 +940,21 @@ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ \ NAME \ \ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | TYPE | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | CLASS | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | TTL | | | (32-bit unsigned big-endian integer) | > DSO-DATA +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | - | RDLEN | | + | RDLEN (16-bit unsigned big-endian integer) | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | \ RDATA (sized as necessary) \ | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | : NAME, TYPE, CLASS, TTL, RDLEN, RDATA : | : Repeated As Necessary : / +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / Figure 3: PUSH Message When processing the records received in a PUSH Message, the receiving @@ -988,24 +1010,25 @@ 6.4. DNS Push Notification UNSUBSCRIBE To cancel an individual subscription without closing the entire DSO session, the client sends an UNSUBSCRIBE message over the established DSO session to the server. The UNSUBSCRIBE message is encoded as a DSO unidirectional message [RFC8490]. This specification defines a primary unidirectional DSO TLV for DNS Push Notification UNSUBSCRIBE Messages (tentatively DSO Type Code 0x42). - A server MUST NOT initiate an UNSUBSCRIBE message. If a server does - send an UNSUBSCRIBE message over a DSO session initiated by a client, - this is a fatal error and the client should immediately abort the - connection with a TLS close_notify alert followed by a TCP FIN. + A server MUST NOT send an UNSUBSCRIBE message. If a server does send + an UNSUBSCRIBE message over a DSO session initiated by a client, or + an UNSUBSCRIBE message is sent with the QR bit set indicating that it + is a response, this is a fatal error and the receiver MUST forcibly + abort the connection immediately. 6.4.1. UNSUBSCRIBE Message An UNSUBSCRIBE unidirectional message begins with the standard DSO 12-byte header [RFC8490], followed by the UNSUBSCRIBE primary TLV. An UNSUBSCRIBE message is illustrated in Figure 4. In accordance with the definition of DSO unidirectional messages, the MESSAGE ID field MUST be zero. There is no server response to an UNSUBSCRIBE message. @@ -1080,31 +1103,37 @@ Notification RECONFIRM message by calling the underlying API's DNSServiceReconfirmRecord() routine. For other types of DNS server, the RECONFIRM operation is currently undefined, and SHOULD result in a NOERROR response, but otherwise need not cause any action to occur. Frequent use of RECONFIRM operations may be a sign of network unreliability, or some kind of misconfiguration, so RECONFIRM operations MAY be logged or otherwise communicated to a human - administrator to assist in detecting, and remedying, such network + administrator to assist in detecting and remedying such network problems. If, after receiving a valid RECONFIRM message, the server determines that the disputed records are in fact no longer valid, then subsequent DNS PUSH Messages will be generated to inform interested clients. Thus, one client discovering that a previously-advertised device (like a network printer) is no longer present has the side effect of informing all other interested clients that the device in question is now gone. + A server MUST NOT send a RECONFIRM message. If a server does send a + RECONFIRM message over a DSO session initiated by a client, or a + RECONFIRM message is sent with the QR bit set indicating that it is a + response, this is a fatal error and the receiver MUST forcibly abort + the connection immediately. + 6.5.1. RECONFIRM Message A RECONFIRM unidirectional message begins with the standard DSO 12-byte header [RFC8490], followed by the RECONFIRM primary TLV. A RECONFIRM message is illustrated in Figure 5. In accordance with the definition of DSO unidirectional messages, the MESSAGE ID field MUST be zero. There is no server response to a RECONFIRM message. @@ -1112,20 +1141,41 @@ ification [RFC8490]. The DNS OPCODE field contains the OPCODE value for DNS Stateful Operations (6). The four count fields MUST be zero, and the corresponding four sections MUST be empty (i.e., absent). The DSO-TYPE is RECONFIRM (tentatively 0x43). The DSO-LENGTH is the length of the data that follows, which specifies the name, type, class, and content of the record being disputed. + The DSO-DATA for a RECONFIRM message MUST contain exactly one record. + The DSO-DATA for a RECONFIRM message has no count field to specify + more than one record. Since RECONFIRM messages are sent over TCP, + multiple RECONFIRM messages can be concatenated in a single TCP + stream and packed efficiently into TCP segments. + + TYPE MUST NOT be the value ANY (255) and CLASS MUST NOT be the value + ANY (255). + + DNS wildcarding is not supported. That is, a wildcard ("*") in a + RECONFIRM message matches only a literal wildcard character ("*") in + the zone, and nothing else. + + Aliasing is not supported. That is, a CNAME in a RECONFIRM message + matches only a literal CNAME record in the zone, and no other records + with the same owner name. + + Note that there is no RDLEN field, since the length of the RDATA can + be inferred from DSO-LENGTH, so an additional RDLEN field would be + redundant. + 1 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \ | MESSAGE ID | \ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | |QR| OPCODE(6) | Z | RCODE | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | QDCOUNT (MUST BE ZERO) | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER | ANCOUNT (MUST BE ZERO) | | @@ -1142,36 +1192,20 @@ +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | | TYPE | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > DSO-DATA | CLASS | | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | \ RDATA \ / +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / Figure 5: RECONFIRM Message - The DSO-DATA for a RECONFIRM message MUST contain exactly one record. - The DSO-DATA for a RECONFIRM message has no count field to specify - more than one record. Since RECONFIRM messages are sent over TCP, - multiple RECONFIRM messages can be concatenated in a single TCP - stream and packed efficiently into TCP segments. - - TYPE MUST NOT be the value ANY (255) and CLASS MUST NOT be the value - ANY (255). - - DNS wildcarding is not supported. That is, a wildcard ("*") in a - RECONFIRM message matches only a literal wildcard character ("*") in - the zone, and nothing else. - - Aliasing is not supported. That is, a CNAME in a RECONFIRM message - matches only a literal CNAME record in the zone, and nothing else. - 6.6. DNS Stateful Operations TLV Context Summary This document defines four new DSO TLVs. As suggested in Section 8.2 of the DNS Stateful Operations specification [RFC8490], the valid contexts of these new TLV types are summarized below. The client TLV contexts are: C-P: Client request message, primary TLV C-U: Client unidirectional message, primary TLV @@ -1233,27 +1267,92 @@ implicitly terminates all subscriptions on that session. This may occur because the client computer is being shut down, is going to sleep, the application requiring the subscriptions has terminated, or simply because the last active subscription on that session has been cancelled. When closing a session, a client should perform an orderly close of the TLS session in order to allow for future TLS session resumption with the server (if available). See Section 7.4 below. Typical APIs will provide a session close method that will send a TLS close_notify - alert. This instructs the recipient that the sender will not send - any more data over the session. + alert (see Section 6.1 of the TLS 1.3 specification [RFC8446]). This + instructs the recipient that the sender will not send any more data + over the session. After sending the TLS close_notify alert the + client MUST gracefully close the underlying connection using a TCP + FIN, so that the TLS close_notify is reliably delivered. The + mechanisms for gracefully closing a TCP connection with a TCP FIN + vary depending on the networking API. For example, in the BSD + Sockets API, sending a TCP FIN is achieved by calling + "shutdown(s,SHUT_WR)" and keeping the socket open until all remaining + data has been read from it. If the session is forcibly closed at the TCP level by sending a RST from either end of the connection, data may be lost and TLS session resumption of this session will not be possible. +6.8. Client Fallback to Polling + + There are cases where a client may exhaust all avenues for + establishing a DNS Push Notification subscription without success. + This can happen if the client's configured recursive resolver does + not support DNS over TLS, or supports DNS over TLS but is not + listening on TCP port 853, or supports DNS over TLS on TCP port 853 + but does not support DSO on that port, or for some other reason is + unable to provide a DNS Push Notification subscription. In this case + the client will attempt to communicate directly with an appropriate + server, and it may be that the zone apex discovery fails, or there is + no "_dns-push-tls._tcp." SRV record, or server indicated in the + SRV record is misconfigured, or is unresponsive for some other + reason. + + Regardless of the reason for the failure, after being unable to + establish the desired DNS Push Notification subscription, it is + likely that the client will still wish to know the answer it seeks, + even if that answer cannot be obtained with the timely change + notifications provided by DNS Push Notifications. In such cases it + is likely that the client will obtain the answer it seeks via a + conventional DNS query instead, repeated at some interval to detect + when the answer RRset changes. + + In the case where a client responds to its failure to establish a DNS + Push Notification subscription by falling back to polling with + conventional DNS queries instead, the polling rate should be + controlled to avoid placing excessive burden on the server. The + interval between successive DNS queries for the same name, type and + class SHOULD be at least the minimum of: 900 seconds (15 minutes), or + two seconds more than the TTL of the answer RRset. + + The reason that for TTLs shorter than 898 seconds the query should + not be reissued until two seconds *after* the answer RRset has + expired is to ensure that the answer RRset has also expired from the + cache on the client's configured recursive resolver. Otherwise + (particularly if the clocks on the client and the recursive resolver + do not run at precisely the same rate) there's a risk of a race + condition where the client queries its configured recursive resolver + just as the answer RRset has one second remaining in the recursive + resolver's cache. The client would then receive a reply telling it + that the answer RRset has one second remaining, and then the client + would then re-query the recursive resolver again one second later + when the answer RRset actually expires, and only then would the + recursive resolver issue a new query to fetch new fresh data from the + authoritative server. Waiting until the answer RRset has definitely + expired from the the cache on the client's configured recursive + resolver avoids this race condition and unnecessary additional + queries it causes. + + Each time a client is about to reissue its query to discover changes + to the answer RRset, it should first make a new attempt to establish + a DNS Push Notification subscription, using previously cached DNS + answers as appropriate. After a temporary misconfiguration has been + remedied, this allows a client that is polling to return to using DNS + Push Notifications for asynchronous notification of changes. + 7. Security Considerations The Strict Privacy Usage Profile for DNS over TLS is REQUIRED for DNS Push Notifications [RFC8310]. Cleartext connections for DNS Push Notifications are not permissible. Since this is a new protocol, transition mechanisms from the Opportunistic Privacy profile are unnecessary. Also, see Section 9 of the DNS over (D)TLS Usage Profiles document [RFC8310] for additional recommendations for various versions of TLS @@ -1316,35 +1415,37 @@ must be used for the target name to ensure the client is connecting to the server it has authenticated. If the target name does not have a usable TLSA record, then the use of the SNI extension is optional. See Usage Profiles for DNS over TLS and DNS over DTLS [RFC8310] for more information on authenticating domain names. 7.3. TLS Early Data DSO messages with the SUBSCRIBE TLV as the Primary TLV are permitted in TLS early data. Using TLS early data can save one network round - trip, and can result in obtaining results faster. However, there are - some factors to consider before using TLS early data. + trip, and can result in the client obtaining results faster. + + However, there are some factors to consider before using TLS early + data. TLS Early Data is not forward secret. In cases where forward secrecy of DNS Push Notification subscriptions is required, the client should not use TLS Early Data. With TLS early data there are no guarantees of non-replay between connections. If packets are duplicated and delayed in the network, the later arrivals could be mistaken for new subscription requests. Generally this is not a major concern, since the amount of state generated on the server for these spurious subscriptions is small and short-lived, since the TCP connection will not complete the three-way handshake. Servers MAY choose to implement rate-limiting measures - that are imposed when the server detects excessive numbers of + that are activated when the server detects an excessive number of spurious subscription requests. For further guidance please see Section 2.3, Section 8, and Appendix E.5 of the TLS 1.3 specification [RFC8446]. 7.4. TLS Session Resumption TLS Session Resumption is permissible on DNS Push Notification servers. The server may keep TLS state with Session IDs [RFC8446] or operate in stateless mode by sending a Session Ticket [RFC5077] to @@ -1378,29 +1479,33 @@ | | | Data | | | +-------------+------------+--------+-----------------+-------------+ | SUBSCRIBE | TBA (0x40) | OK | Standards Track | Section 6.2 | | PUSH | TBA (0x41) | NO | Standards Track | Section 6.3 | | UNSUBSCRIBE | TBA (0x42) | NO | Standards Track | Section 6.4 | | RECONFIRM | TBA (0x43) | NO | Standards Track | Section 6.5 | +-------------+------------+--------+-----------------+-------------+ Table 5: IANA DSO TLV Type Code Assignments + This document defines no new DNS OPCODEs or RCODEs. + 9. Acknowledgements The authors would like to thank Kiren Sekar and Marc Krochmal for previous work completed in this field. This draft has been improved due to comments from Ran Atkinson, Tim - Chown, Mark Delany, Ralph Droms, Bernie Volz, Jan Komissar, Manju - Shankar Rao, Markus Stenberg, Dave Thaler, Soraia Zlatkovic, Sara - Dickinson, and Andrew Sullivan. Ted Lemon provided clarifying text + Chown, Sara Dickinson, Mark Delany, Ralph Droms, Jan Komissar, Eric + Rescorla, Michael Richardson, David Schinazi, Manju Shankar Rao, + Robert Sparks, Markus Stenberg, Andrew Sullivan, Michael Sweet, Dave + Thaler, Brian Trammell, Bernie Volz, Eric Vyncke, Christopher Wood, + Liang Xia, and Soraia Zlatkovic. Ted Lemon provided clarifying text that was greatly appreciated. 10. References 10.1. Normative References [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, DOI 10.17487/RFC0768, August 1980, .