DNS Push Notifications
draft-ietf-dnssd-push-09

Abstract

The Domain Name System (DNS) was designed to return matching records efficiently for queries for data that is relatively static. When those records change frequently, DNS is still efficient at returning the updated results when polled. 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 DNS records, called DNS Push Notifications.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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 http://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 May 4, 2017.

Copyright Notice

Copyright (c) 2016 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 (http://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
• 1.1. Requirements Language
• 2. Motivation
• 3. Overview
• 4. Transport
• 5. State Considerations
• 6. Protocol Operation
• 6.1. Discovery
• 6.2. DNS Push Notification SUBSCRIBE
• 6.2.1. SUBSCRIBE Request
• 6.2.2. SUBSCRIBE Response
• 6.3. DNS Push Notification Update Messages
• 6.3.1. PUSH Message format
• 6.4. DNS Push Notification UNSUBSCRIBE
• 6.4.1. UNSUBSCRIBE Request
• 6.4.2. UNSUBSCRIBE Response
• 6.5. DNS Session Signaling Push Notification RECONFIRM
• 6.6. Client-Initiated Termination
• 7. Security Considerations
• 7.1. Security Services
• 7.2. TLS Name Authentication
• 7.3. TLS Compression
• 7.4. TLS Session Resumption
• 8. IANA Considerations
• 9. Acknowledgements
• 10. References
• 10.1. Normative References
• 10.2. Informative References
• Authors' Addresses

1. Introduction

DNS records may be updated using DNS Update [RFC2136]. Other mechanisms such as a Hybrid Proxy [I-D.ietf-dnssd-hybrid] 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 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 "Key words for use in RFCs to Indicate Requirement Levels" [RFC2119].

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 to state changes 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 result in 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 new services and updates are sent 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 Hybrid Proxy [I-D.ietf-dnssd-hybrid]) 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.

DNS Long-Lived Queries (LLQ) [I-D.sekar-dns-llq] is an existing deployed solution to provide asynchronous change notifications. Even though it can be used over TCP, LLQ is defined primarily as a UDP-based protocol, and as such it defines its own equivalents of existing TCP features like the three-way handshake, flow control, and reliability. This document builds on experience gained with the LLQ protocol, with an improved design. Instead of using UDP, this specification uses long-lived TCP connections [I-D.ietf-dnsop-session-signal], and therefore doesn't need to reinvent existing TCP functionality. Instead of inventing a new vocabulary of messages to communicate DNS zone changes, this specification adopts the syntax and semantics of DNS Update messages [RFC2136].

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. 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). Implementations MAY want to implement idle timeouts, so that if the user ceases interacting with the device, the display showing the result of the DNS Push Notification subscription is automatically dismissed after a certain period of inactivity. For example, if a user presses the "Print" button on their smartphone, and then leaves the phone showing the printer discovery screen until the phone goes to sleep, then the printer discovery screen should be automatically dismissed as the device goes to sleep. If the user does still intend to print, this will require them to press the "Print" button again when they wake their phone up.

A DNS Push Notification client MUST 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 it will be really fast if the user does choose to bring up an on-screen display of that data. 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.

Generally, a client SHOULD NOT keep a connection to a server open indefinitely if it has no active subscriptions on that connection. After 30 seconds with no active subscriptions the client SHOULD close the idle connection, and, if needed in the future, open a new connection.

3. Overview

The existing DNS Update protocol [RFC2136] provides a mechanism for clients to add or delete individual resource records (RRs) or entire resource record sets (RRSets) on the zone's server.

This specification adopts a simplified subset of these existing syntax and semantics, and uses them for DNS Push Notification messages going in the opposite direction, from server to client, to communicate changes to a zone. The client subscribes for Push Notifications by connecting to the server and sending DNS message(s) indicating the RRSet(s) of interest. When the client loses interest in updates to these records, it unsubscribes.

The DNS Push Notification server for a zone is any server capable
of generating the correct change notifications for a name.
It may be a master, slave, or stealth name server [RFC1996]. Consequently, the _dns‑push‑tls._tcp.<zone> SRV record for a
zone MAY reference the same target host and port as that zone's _dns‑update‑tls._tcp.<zone> 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 Queries.

Supporting DNS Updates and DNS Push Notifications on the same server is OPTIONAL. A DNS Push Notification server does NOT also have 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.

Standard DNS Queries MAY be sent over a DNS Push Notification connection, provided that these are queries for names falling within the server's zone (the <zone> in the _dns‑push‑tls._tcp.<zone> SRV record). The RD (Recursion Desired) bit MUST be zero.

DNS Push Notification clients are NOT required to implement DNS Update Prerequisite processing. Prerequisites are used to perform tentative atomic test-and-set type operations when a client updates records on a server, and that concept has no applicability when it comes to an authoritative server informing a client of changes to DNS records.

This DNS Push Notification specification includes support for DNS classes, for completeness. However, in practice, it is anticipated that for the foreseeable future the only DNS class in use will be DNS class "IN", as is the reality today with existing DNS servers and clients. A DNS Push Notification server MAY choose to implement only DNS class "IN".

4. Transport

Implementations of 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 TLS over TCP.

Connection setup over TCP ensures return reachability and alleviates concerns of state overload at the server through anonymous subscriptions. 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 [IPJ.9-4-TCPSYN] [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) [RFC5246] is well understood and deployed across many protocols running over TCP. It is designed to prevent eavesdropping, tampering, or 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.

Additional authentication of the SRV target using DNSSEC verification and DANE TLSA records [RFC7673] is strongly encouraged. See below in Section 7.2 for details.

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 connection to a DNS server, each record 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 connections to alternate DNS servers that support DNS Push Notifications for the zone and distribute record subscriptions at its discretion. In this way, both clients and servers can react to resource constraints. Token bucket rate limiting schemes are also effective in providing fairness by a server across numerous client requests.

6. Protocol Operation

The DNS Push Notification protocol is a session-oriented protocol, and makes use of DNS Session Signaling [I-D.ietf-dnsop-session-signal].

DNS Push Notification clients and servers MUST support DNS Session Signaling, but the server must not issue any DNS Session Signaling operations until after the client has first initiated a DNS Session Signaling operation of its own. A single server can support DNS Queries, DNS Updates, and DNS Push Notifications (using DNS Session Signaling) on the same TCP port, and until the client has sent at least one DNS Session Signaling operation the server does not know what kind of client has connected to it. Once the client has indicated willingness to use DNS Session Signaling operations by sending one of its own, either side of the connection may then initiate further Session Signaling operations at any time.

A DNS Push Notification exchange begins with the client discovering 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 DNS Session Signaling Idle Timeout operation to request a session timeout longer than the the 30-second default, but this is NOT REQUIRED. A DNS Push Notification client MAY issue other requests on the connection first, and only issue a DNS Session Signaling Idle Timeout operation later if it determines that to be necessary.

Once the connection is made, the client may then add and remove Push Notification subscriptions. 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.

The exchange between client and server terminates when either end closes the TCP connection with a TCP FIN or RST.

6.1. Discovery

The first step in DNS Push Notification subscription is to discover an appropriate DNS server that supports DNS Push Notifications for the desired zone. The client MUST also determine which TCP port on the server is listening for connections, which need not be (and often is not) the typical TCP port 53 used for conventional DNS, or TCP port 853 used for DNS over TLS [RFC7858].

1. The client begins the discovery by sending a DNS query to the local resolver with record type SOA [RFC1035] for the name of the record it wishes to subscribe.
2. If the SOA record exists, it MUST be returned in the Answer Section of the response. If not, the local resolver SHOULD include the SOA record for the zone of the requested name in the Authority Section.
3. If no SOA record is returned, the client then strips off the leading label from the requested name. If the resulting name has at least one label in it, the client sends a new SOA query and processing continues at step 2 above. If the resulting name is empty (the root label) then this is a network configuration error and the client gives up. The client MAY retry the operation at a later time.
4. 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.<zone>, where <zone> is the owner name of the discovered SOA record.
5. If the zone in question does not offer DNS Push Notifications then SRV record MUST NOT exist and the SRV query will return a negative answer.
6. If the zone in question is set up to offer DNS Push Notifications then this SRV record MUST exist. The SRV target contains the name of the server providing DNS Push Notifications for the zone. The port number on which to contact the server is in the SRV record port field. The address(es) of the target host MAY be included in the Additional Section, however, the address records SHOULD be authenticated before use as described below in Section 7.2 [RFC7673].
7. More than one SRV record may be returned. In this case, the priority and weight values in the returned SRV records are used to determine the order in which to contact the servers for subscription requests. As described in the SRV specification [RFC2782], the server with the lowest priority is first contacted. If more than one server has the same priority, the weight indicates the weighted probability that the client should contact that server. Higher weights have higher probabilities of being selected. If a server is not reachable or is not willing to accept a subscription request, then a subsequent server is to be contacted.

Each time a client makes a new DNS Push Notification subscription connection, it SHOULD repeat the discovery process in order to determine the preferred DNS server for subscriptions at that time.

Note that this repeated discovery step is typically very fast and typically results in no queries on the network. 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 discovery process can be completed nearly instantaneously by the client, using only locally-stored data.

6.2. DNS Push Notification SUBSCRIBE

After connecting, and requesting a longer idle timeout 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 over the established TLS connection to the server. A SUBSCRIBE request is encoded in a DNS Session Signaling [I-D.ietf-dnsop-session-signal] message. This specification defines a new DNS Session Signaling TLV for DNS Push Notification SUBSCRIBE Requests/Responses (tentatively Session Signaling Type Code 64).

A server may not initiate a SUBSCRIBE request.

6.2.1. SUBSCRIBE Request

A SUBSCRIBE request message begins with the standard DNS Session Signaling 4-byte header [I-D.ietf-dnsop-session-signal], followed by the SUBSCRIBE TLV.

                                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  |          Z         |   RCODE   |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|             SSOP-TYPE (SUBSCRIBE)             |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                  SSOP-LENGTH                  |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                                               |
\                     QNAME                     \
\                                               \
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                     QTYPE                     |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                    QCLASS                     |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

                                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  |          Z         |   RCODE   |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

                                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  |          Z         |   RCODE   |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|               SSOP-TYPE (PUSH)                |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                  SSOP-LENGTH                  |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                    UPCOUNT                    |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                                               |
\              Resource Records...              \
\                                               \
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

                                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  |          Z         |   RCODE   |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|            SSOP-TYPE (UNSUBSCRIBE)            |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|                SSOP-LENGTH (0)                |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

                                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  |          Z         |   RCODE   |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+