RFC 8765 PROPOSED STANDARD DNS · NOTIFICATIONS · TLS
DNS Push Notifications
Lets a DNS client subscribe to record changes over a long-lived TLS connection instead of polling for them.
RFC EDITOR
Pusateri, T. and S. Cheshire, "DNS Push Notifications", RFC 8765, DOI 10.17487/RFC8765, June 2020, <https://www.rfc-editor.org/info/rfc8765>.
BIBTEX
@misc{rfc8765,
series = {Request for Comments},
number = 8765,
howpublished = {RFC 8765},
publisher = {RFC Editor},
doi = {10.17487/RFC8765},
url = {https://www.rfc-editor.org/info/rfc8765},
author = {T. Pusateri and S. Cheshire},
title = {{DNS Push Notifications}},
pagetotal = 32,
year = 2020,
month = jun,
} RFC text
Contents
Internet Engineering Task Force (IETF) T. Pusateri
Request for Comments: 8765 Unaffiliated
Category: Standards Track S. Cheshire
ISSN: 2070-1721 Apple Inc.
June 2020
DNS Push Notifications
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
DNS records, called DNS Push Notifications.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8765.
Copyright Notice
Copyright (c) 2020 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
1.1. Requirements Language
1.2. Fatal Errors
2. Motivation
3. Overview
4. State Considerations
5. Transport
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 Updates
6.3.1. PUSH Message
6.4. DNS Push Notification UNSUBSCRIBE
6.4.1. UNSUBSCRIBE Message
6.5. DNS Push Notification RECONFIRM
6.5.1. RECONFIRM Message
6.6. DNS Stateful Operations TLV Context Summary
6.7. Client-Initiated Termination
6.8. Client Fallback to Polling
7. Security Considerations
7.1. Security Services
7.2. TLS Name Authentication
7.3. TLS Early Data
7.4. TLS Session Resumption
8. IANA Considerations
9. References
9.1. Normative References
9.2. Informative References
Acknowledgments
Authors' Addresses
1. Introduction
Domain Name System (DNS) records may be updated using DNS Update
[RFC2136]. Other mechanisms such as a Discovery Proxy [RFC8766] 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-based Service Discovery [RFC6763] but is
not limited to that use case; it 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.
1.2. Fatal Errors
Certain invalid situations are described in this specification, such
as 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]. Extensible Messaging and Presence
Protocol (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
notifications of changes with reduced CPU usage and lower network
traffic.
The guiding design principle of DNS Push Notifications is that
clients that choose to use DNS Push Notifications, instead of
repeated polling with DNS queries, will receive the same results as
they could via sufficiently rapid polling, except more efficiently.
This means that the rules for which records match a given DNS Push
Notification subscription are the same as the already established
rules used to determine which records match a given DNS query
[RFC1034]. For example, name comparisons are done in a case-
insensitive manner, and a record of type CNAME in a zone matches any
DNS TYPE in a query or subscription.
Multicast DNS [RFC6762] implementations always listen on a well-known
link-local IP multicast group address, and changes are sent to that
multicast group address for all group members to receive. Therefore,
Multicast DNS already has asynchronous change notification
capability. When DNS-based Service Discovery [RFC6763] is used
across a wide area network using Unicast DNS (possibly facilitated
via a Discovery Proxy [RFC8766]), it would be beneficial to have an
equivalent capability for Unicast DNS in order to allow clients to
learn about DNS record changes in a timely manner without polling.
The DNS Long-Lived Queries (LLQ) mechanism [RFC8764] is an existing
deployed solution to provide asynchronous change notifications; it
was 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 data center operations staff asserted that it was impossible
for a server to handle large numbers of TCP connections, even if
those connections carried very little traffic and spent most of their
time idle. Consequently, LLQ was defined as a UDP-based protocol,
effectively replicating much of TCP's connection state management
logic in user space and creating its own imitation of existing TCP
features like flow control, reliability, and the three-way handshake.
This document builds on experience gained with the LLQ protocol, with
an improved design. Instead of using UDP, this specification uses
DNS Stateful Operations (DSO) [RFC8490] running over TLS over TCP,
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
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 [RFC8499].
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 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 for names falling within
that zone (e.g., the "_dns-push-tls._tcp.<zone>" 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.
Therefore, 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 canceled 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 that, 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 on the display anyway) and
reinstate its subscriptions when reawakening 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.
Generally, as described in the DNS Stateful Operations specification
[RFC8490], a client must not keep a DSO session to a server open
indefinitely if it has no subscriptions (or other operations) active
on that session. A client should begin closing a DSO session
immediately after it becomes idle, and then, if needed in the future,
open a new session when required. Alternatively, a client may
speculatively keep an idle DSO 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].
Note that a DSO session that has an active DNS Push Notification
subscription is not considered idle, even if there is no traffic
flowing for an extended period of time. In this case, the DSO
inactivity timeout does not apply, because the session is not
inactive, but the keepalive interval does still apply, to ensure the
generation of sufficient messages to maintain state in middleboxes
(such at NAT gateways or firewalls) and for the client and server to
periodically verify that they still have connectivity to each other.
This is described in Section 6.2 of the DSO specification [RFC8490].
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 capacity, 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 DNS
over User Datagram Protocol (UDP) [RFC0768] or DNS over Transmission
Control Protocol (TCP) [RFC0793] as the transport protocol, provided
they follow the historical precedent that DNS queries must first be
sent using DNS over UDP and only switch to DNS over TCP if needed
[RFC1123]. This requirement to prefer 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
Stateful Operations [RFC8490] running over TLS over TCP [RFC7858].
Connection setup over TCP ensures return reachability and alleviates
concerns of state overload at the server, a potential problem with
connectionless protocols, which can be more vulnerable to being
exploited by attackers 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)
[RFC8684], TCP Fast Open (TFO) [RFC7413], the TCP RACK fast loss
detection algorithm [TCPRACK], and so on.
Transport Layer Security (TLS) [RFC8446] is well understood and is
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
Operations specification [RFC8490]. Those details are not repeated
here.
DNS Push Notification clients and servers MUST support DSO. A single
server can support DNS Queries, DNS Updates, and DNS Push
Notifications (using DSO) on the same TCP port.
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.
After making the TLS/TCP connection to the server, a typical DNS Push
Notification client will then immediately issue a DSO Keepalive
operation to establish the DSO session and request a session timeout
and/or keepalive interval longer than 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
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 canceling that subscription is simultaneously in flight from
client to server.
6.1. Discovery
The first step in establishing a DNS Push Notification subscription
is to discover an appropriate DNS server that supports DNS Push
Notifications for the desired zone.
The client begins by opening a DSO session to its normal configured
DNS recursive resolver and requesting a Push Notification
subscription. This connection is made to TCP port 853, the default
port for DNS over TLS [RFC7858]. If the request for a Push
Notification subscription is successful, and the recursive resolver
doesn't already have an active subscription for that name, type, and
class, then the recursive resolver will make a corresponding Push
Notification subscription on the client's behalf. Results received
are relayed to the client. This is closely analogous to how a client
sends a normal DNS query to its configured DNS recursive resolver,
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 Private DNS [RFC8499] can create some additional
complexity in the client software here; the techniques to handle VPN
tunnels and Private 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 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 establishment will fail [RFC8490].
If the recursive resolver does support DSO on TCP port 853 but does
not support Push Notification subscriptions, then when the client
attempts to create a subscription, the server 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
server or establishing a connection but receiving a non-success
response code. In some cases, where the client has a pre-established
trust relationship with the owner of the zone (that is not handled
via the usual mechanisms for VPN software), the client may handle
these failures by contacting the zone's DNS Push Notification server
directly.
In any of the cases described above where the client fails to
establish a DNS Push Notification subscription via its configured
recursive resolver, the client should proceed to discover the
appropriate server for direct communication. The client MUST also
determine on which TCP port the server is listening for connections,
which need not be, and often is not, TCP port 53 (traditionally used
for conventional DNS) or TCP port 853 (traditionally used for DNS
over TLS).
The discovery algorithm described here is an iterative algorithm,
which starts with the full name of the record to which the client
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.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 that is 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
back a NOERROR/NODATA response or an NXDOMAIN/Name Error
response. In either case, the local resolver would normally
include the SOA record for the closest enclosing zone of the
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 two labels in it, then 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 name (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 as after a change in network
attachment.
5. Once the SOA is known (by virtue of being seen either 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.
6. If the zone in question is set up to offer DNS Push
Notifications, then this SRV record MUST exist. (If this SRV
record does not exist, then the zone is not correctly configured
for DNS Push Notifications as specified in this document.) 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 in Section 7.2 and in the specification for using
DNS-Based Authentication of Named Entities (DANE) TLSA Records
with SRV Records [RFC7673], if applicable.
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 willing to
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, it
SHOULD repeat the discovery process in order to determine the
preferred DNS server for that subscription at that time. If a client
already has a DSO session with that DNS server, the client SHOULD
reuse that existing DSO session for the new subscription; otherwise,
a new DSO session is established. The client MUST respect the DNS
TTL values on records it receives while performing the discovery
process and store them in its local cache with this lifetime (as it
will generally do anyway for all DNS queries it performs). This
means that, as long as the DNS TTL values on the authoritative
records are set to reasonable values, repeated application of the
discovery process can be completed practically 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 DSO Primary TLV for DNS Push Notification
SUBSCRIBE Requests (DSO Type Code 0x0040).
DSO messages with the SUBSCRIBE TLV as the Primary TLV are permitted
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 MUST forcibly abort the connection immediately.
Each SUBSCRIBE request generates exactly one SUBSCRIBE response from
the server. The entity that initiates a SUBSCRIBE response is by
definition the server. A client MUST NOT send a SUBSCRIBE response.
If a client does send a SUBSCRIBE response, this is a fatal error and
the server 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 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 either
the client has used it in a request for which it has not yet received
a response, or if the client has used it for a subscription that it
has not yet canceled using UNSUBSCRIBE. In the SUBSCRIBE response,
the server MUST echo back the MESSAGE ID value unchanged.
The other header fields MUST be set as described in the DSO
specification [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 SUBSCRIBE (0x0040).
The DSO-LENGTH is the length of the DSO-DATA that follows, which
specifies the name, type, and class of the record(s) being sought.
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) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| NSCOUNT (MUST BE ZERO) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| ARCOUNT (MUST BE ZERO) | /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ /
| DSO-TYPE = SUBSCRIBE (0x0040) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| DSO-LENGTH (number of octets in DSO-DATA) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \
\ NAME \ \
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| TYPE | > DSO-DATA
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| CLASS | /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ /
Figure 1: SUBSCRIBE Request
The DSO-DATA for a SUBSCRIBE request MUST contain exactly one NAME,
TYPE, and CLASS. Since SUBSCRIBE requests are sent over TCP,
multiple SUBSCRIBE DSO request messages can be concatenated in a
single TCP stream and packed efficiently into TCP segments.
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 [RFC0020] 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 forcibly
abort the connection immediately.
DNS wildcarding is not supported. That is, an asterisk character
("*") in a SUBSCRIBE message matches only a literal asterisk
character ("*") in a name and nothing else. Similarly, a CNAME in a
SUBSCRIBE message matches only a CNAME record with that name in the
zone and no other records with that 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
or both of TYPE or CLASS are ANY (255), then this subscription
matches all types and/or all classes as appropriate.
NOTE: A little-known quirk of DNS is that in DNS QUERY requests,
QTYPE and QCLASS 255 mean "ANY", not "ALL". They indicate that the
server should respond with ANY matching records of its choosing, not
necessarily ALL matching records. This can lead to some surprising
and unexpected results, where a query returns some valid answers, but
not all of them, and makes QTYPE = 255 (ANY) queries less useful than
people sometimes imagine.
When used in conjunction with SUBSCRIBE, TYPE 255 and CLASS 255
should be interpreted to mean "ALL", not "ANY". After accepting a
subscription where one or both of TYPE or CLASS are 255, the server
MUST send Push Notification Updates for ALL record changes that match
the subscription, not just some of them.
6.2.2. SUBSCRIBE Response
A SUBSCRIBE response begins with the standard DSO 12-byte header
[RFC8490]. The QR bit in the header is set indicating it is a
response. The header MAY be followed by one or more optional
Additional TLVs such as a Retry Delay Additional TLV. A SUBSCRIBE
response is illustrated in Figure 2.
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.
The other header fields MUST be set as described in the DSO
specification [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 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.
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) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| NSCOUNT (MUST BE ZERO) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| ARCOUNT (MUST BE ZERO) | /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ /
Figure 2: SUBSCRIBE Response
In the SUBSCRIBE response, the RCODE indicates whether or not the
subscription was accepted. Supported RCODEs are as follows:
+-----------+-------+-----------------------------------+
| Mnemonic | Value | Description |
+===========+=======+===================================+
| NOERROR | 0 | SUBSCRIBE successful. |
+-----------+-------+-----------------------------------+
| FORMERR | 1 | Server failed to process request |
| | | due to a malformed request. |
+-----------+-------+-----------------------------------+
| SERVFAIL | 2 | Server failed to process request |
| | | due to a problem with the server. |
+-----------+-------+-----------------------------------+
| NOTIMP | 4 | Server does not implement DSO. |
+-----------+-------+-----------------------------------+
| REFUSED | 5 | Server refuses to process request |
| | | for policy or security reasons. |
+-----------+-------+-----------------------------------+
| NOTAUTH | 9 | Server is not authoritative for |
| | | the requested name. |
+-----------+-------+-----------------------------------+
| DSOTYPENI | 11 | SUBSCRIBE operation not |
| | | supported. |
+-----------+-------+-----------------------------------+
Table 1: SUBSCRIBE Response Codes
This document specifies only these RCODE values for SUBSCRIBE
Responses. Servers sending SUBSCRIBE Responses SHOULD use one of
these values. Note that NXDOMAIN is not a valid RCODE in response to
a SUBSCRIBE Request. However, future circumstances may create
situations where other RCODE values are appropriate in SUBSCRIBE
Responses, so clients MUST be prepared to accept and handle SUBSCRIBE
Responses with any other nonzero RCODE error values.
If the server sends a nonzero RCODE in the SUBSCRIBE response, that
means:
a. the client is (at least partially) misconfigured, or
b. the server resources are exhausted, or
c. there is some other unknown failure on the server.
In any case, the client shouldn't retry the subscription to this
server right away. If multiple SRV records were returned as
described in Section 6.1, Paragraph 9, Item 7, a subsequent server
MAY be tried immediately.
If the client has other successful subscriptions to this server,
these subscriptions remain even though additional subscriptions may
be refused. Neither the client nor the server is required to close
the connection, although either end may choose to do so.
If the server sends a nonzero RCODE, then it SHOULD append a Retry
Delay Additional TLV [RFC8490] to the response specifying a delay
before the client attempts this operation again. Recommended values
for the delay for different RCODE values are given below. These
recommended values apply both to the default values a server should
place in the Retry Delay Additional TLV and the default values a
client should assume if the server provides no Retry Delay Additional
TLV.
For RCODE = 1 (FORMERR), the delay may be any value selected by
the implementer. A value of five minutes is RECOMMENDED to reduce
the risk of high load from defective clients.
For RCODE = 2 (SERVFAIL), the delay should be chosen according to
the level of server overload and the anticipated duration of that
overload. By default, a value of one minute is RECOMMENDED. If a
more serious server failure occurs, the delay may be longer in
accordance with the specific problem encountered.
For RCODE = 4 (NOTIMP), which occurs on a server that doesn't
implement DNS Stateful Operations [RFC8490], it is unlikely that
the server will begin supporting DSO in the next few minutes, so
the retry delay SHOULD be one hour. Note that in such a case, a
server that doesn't implement DSO is unlikely to place a Retry
Delay Additional TLV in its response, so this recommended value in
particular applies to what a client should assume by default.
For RCODE = 5 (REFUSED), which occurs on a server that implements
DNS Push Notifications but is currently configured to disallow DNS
Push Notifications, the retry delay may be any value selected by
the implementer and/or configured by the operator.
If the server being queried is listed in a
"_dns-push-tls._tcp.<zone>" SRV record for the zone, then this is
a misconfiguration, since this server is being advertised as
supporting DNS Push Notifications for this zone, but the server
itself is not currently configured to perform that task. Since it
is possible that the misconfiguration may be repaired at any time,
the retry delay should not be set too high. By default, a value
of 5 minutes is RECOMMENDED.
For RCODE = 9 (NOTAUTH), which occurs on a server that implements
DNS Push Notifications but is not configured to be authoritative
for the requested name, the retry delay may be any value selected
by the implementer and/or configured by the operator.
If the server being queried is listed in a
"_dns-push-tls._tcp.<zone>" SRV record for the zone, then this is
a misconfiguration, since this server is being advertised as
supporting DNS Push Notifications for this zone, but the server
itself is not currently configured to perform that task. Since it
is possible that the misconfiguration may be repaired at any time,
the retry delay should not be set too high. By default, a value
of 5 minutes is RECOMMENDED.
For RCODE = 11 (DSOTYPENI), which occurs on a server that
implements DSO but doesn't implement DNS Push Notifications, it is
unlikely that the server will begin supporting DNS Push
Notifications in the next few minutes, so the retry delay SHOULD
be one hour.
For other RCODE values, the retry delay should be set by the
server as appropriate for that error condition. By default, a
value of 5 minutes is RECOMMENDED.
For RCODE = 9 (NOTAUTH), the time delay applies to requests for other
names falling within the same zone. Requests for names falling
within other zones are not subject to the delay. For all other
RCODEs, the time delay applies to all subsequent requests to this
server.
After sending an error response, the server MAY allow the session to
remain open, or MAY follow it with a DSO Retry Delay operation (using
the Retry Delay Primary TLV) instructing the client to close the
session as described in the DSO specification [RFC8490]. Clients
MUST correctly handle both cases. Note that the DSO Retry Delay
operation (using the Retry Delay Primary TLV) is different to the
Retry Delay Additional TLV mentioned above.
6.3. DNS Push Notification Updates
Once a subscription has been successfully established, the server
generates PUSH messages to send to the client as appropriate. In the
case that the answer set was already non-empty at the moment the
subscription was established, an initial PUSH message will be sent
immediately following the SUBSCRIBE Response. Subsequent changes to
the answer set are then communicated to the client in subsequent PUSH
messages.
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.
6.3.1. PUSH Message
A PUSH unidirectional message begins with the standard DSO 12-byte
header [RFC8490], followed by the PUSH Primary TLV. A PUSH message
is illustrated in Figure 3.
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
specification [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 PUSH (0x0041).
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 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 (0 to 2^(31) - 1, or
0x7FFFFFFF), then a new DNS Resource Record with the given name,
type, class, and RDATA is added. Type and class MUST NOT be 255
(ANY). If either type or class are 255 (ANY), this is a fatal error
and the client MUST forcibly abort the connection immediately. 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 canceled.
If the TTL has the value 0xFFFFFFFF, then the DNS Resource Record
with the given name, type, class, and RDATA is removed. Type and
class MUST NOT be 255 (ANY). If either type or class are 255 (ANY),
this is a fatal error and the client MUST forcibly abort the
connection immediately.
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 client MUST forcibly abort the
connection immediately.
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 removes all records of the specified type in the
specified class.
* If CLASS is not 255 (ANY) and TYPE is 255 (ANY), then for the
given name, this removes all records of all types in the specified
class.
* If CLASS is 255 (ANY), then for the given name, this removes 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:
* Remove all RRsets from a name in all classes:
TTL = 0xFFFFFFFE, RDLEN = 0, CLASS = 255 (ANY).
* Remove all RRsets from a name in given class:
TTL = 0xFFFFFFFE, RDLEN = 0, CLASS gives class, TYPE = 255 (ANY).
* Remove specified RRset from a name in given class:
TTL = 0xFFFFFFFE, RDLEN = 0,
CLASS and TYPE specify the RRset being removed.
* Remove an individual RR from a name:
TTL = 0xFFFFFFFF,
CLASS, TYPE, RDLEN, and RDATA specify the RR being removed.
* Add individual RR to a name:
TTL >= 0 and TTL <= 0x7FFFFFFF,
CLASS, TYPE, RDLEN, 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 in the range 0 to 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 removal specifically of that single record with the given
name, type, class, and empty RDATA.
If the TTL is any value other than 0xFFFFFFFF, 0xFFFFFFFE, or a value
in the range 0 to 0x7FFFFFFF, then the receiver SHOULD silently
ignore this particular change notification record. The connection is
not terminated and other valid change notification records within
this PUSH message are processed as usual.
In the case where a single change affects more than one active
subscription, only one PUSH message is sent. For example, a PUSH
message adding a given record may match both a SUBSCRIBE request with
the same TYPE and a different SUBSCRIBE request with TYPE = 255
(ANY). It is not the case that two PUSH messages are sent because
the new record matches two active subscriptions.
The server SHOULD encode change notifications in the most efficient
manner possible. For example, when three AAAA records are removed
from a given name, and no other AAAA records exist for that name, the
server SHOULD send a "Remove specified RRset from a name in given
class" PUSH message, not three separate "Remove an individual RR from
a name" PUSH messages. Similarly, when both an SRV and a TXT record
are removed from a given name, and no other records of any kind exist
for that name in that class, the server SHOULD send a "Remove all
RRsets from a name in given class" PUSH message, not two separate
"Remove specified RRset from a name in given class" PUSH messages.
For efficiency, when generating a PUSH message, rather than sending
each change notification as a separate DSO message, a server SHOULD
include as many change notifications as it has immediately available
to send to that client, even if those change notifications apply to
different subscriptions from that client. Conceptually, a PUSH
message is a session-level mechanism, not a subscription-level
mechanism. Once it has exhausted the list of change notifications
immediately available to send to that client, a server SHOULD then
send the PUSH message immediately rather than waiting speculatively
to see if additional change notifications become available.
For efficiency, when generating a PUSH message a server SHOULD use
standard DNS name compression, with offsets relative to the beginning
of the DNS message [RFC1035]. When multiple change notifications in
a single PUSH message have the same owner name, this name compression
can yield significant savings. Name compression should be performed
as specified in Section 18.14 of the Multicast DNS specification
[RFC6762]; namely, owner names should always be compressed, and names
appearing within RDATA should be compressed for only the RR types
listed below:
NS, CNAME, PTR, DNAME, SOA, MX, AFSDB, RT, KX, RP, PX, SRV, NSEC
Servers may generate PUSH messages up to a maximum DNS message length
of 16,382 bytes, counting from the start of the DSO 12-byte header.
Including the two-byte length prefix that is used to frame DNS over a
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, 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
| ANCOUNT (MUST BE ZERO) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| NSCOUNT (MUST BE ZERO) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| ARCOUNT (MUST BE ZERO) | /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ /
| DSO-TYPE = PUSH (0x0041) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| DSO-LENGTH (number of octets in DSO-DATA) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \
\ NAME \ \
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| TYPE | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| CLASS | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| TTL | |
| (32-bit unsigned big-endian integer) | > DSO-DATA
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| 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
client MUST validate that the records being added or removed
correspond with at least one currently active subscription on that
session. Specifically, the record name MUST match the name given in
the SUBSCRIBE request, subject to the usual established DNS case-
insensitivity for US-ASCII letters. For individual additions and
removals, if the TYPE in the SUBSCRIBE request was not ANY (255),
then the TYPE of the record must either be CNAME or match the TYPE
given in the SUBSCRIBE request, and if the CLASS in the SUBSCRIBE
request was not ANY (255), then the CLASS of the record must match
the CLASS given in the SUBSCRIBE request. For collective removals,
at least one of the records being removed must match an active
subscription. If a matching active subscription on that session is
not found, then that particular addition/removal record is silently
ignored. The processing of other additions and removal records in
this message is not affected. The DSO session is not closed. This
is to allow for the unavoidable race condition where a client sends
an outbound UNSUBSCRIBE while inbound PUSH messages for that
subscription from the server are still in flight.
The TTL of an added record is stored by the client. While the
subscription is active the TTL is not decremented, because a change
to the TTL would produce a new update. For as long as a relevant
subscription remains active, the client SHOULD assume that when a
record goes away, the server will notify it of that fact.
Consequently, a client does not have to poll to verify that the
record is still there. Once a subscription is canceled
(individually, or as a result of the DSO session being closed),
record aging for records covered by the subscription resumes and
records are removed from the local cache when their TTL reaches zero.
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 entity that initiates an UNSUBSCRIBE message is by definition the
client. A server MUST NOT send an UNSUBSCRIBE message over an
existing session from a client. 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.
The other header fields MUST be set as described in the DSO
specification [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 UNSUBSCRIBE (0x0042).
The DSO-LENGTH field contains the value 2, the length of the 2-octet
MESSAGE ID contained in the DSO-DATA.
The DSO-DATA contains the value previously given in the MESSAGE ID
field of an active SUBSCRIBE request. This is how the server knows
which SUBSCRIBE request is being canceled. After receipt of the
UNSUBSCRIBE message, the SUBSCRIBE request is no longer active.
It is allowable for the client to issue an UNSUBSCRIBE message for a
previous SUBSCRIBE request for which the client has not yet received
a SUBSCRIBE response. This is to allow for the case where a client
starts and stops a subscription in less than the round-trip time to
the server. The client is NOT required to wait for the SUBSCRIBE
response before issuing the UNSUBSCRIBE message.
Consequently, it is possible for a server to receive an UNSUBSCRIBE
message that does not match any currently active subscription. This
can occur when a client sends a SUBSCRIBE request, which subsequently
fails and returns an error code, but the client sent an UNSUBSCRIBE
message before it became aware that the SUBSCRIBE request had failed.
Because of this, servers MUST silently ignore UNSUBSCRIBE messages
that do not match any currently active subscription.
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
| ANCOUNT (MUST BE ZERO) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| NSCOUNT (MUST BE ZERO) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| ARCOUNT (MUST BE ZERO) | /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ /
| DSO-TYPE = UNSUBSCRIBE (0x0042) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| DSO-LENGTH (2) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \
| SUBSCRIBE MESSAGE ID | > DSO-DATA
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ /
Figure 4: UNSUBSCRIBE Message
6.5. DNS Push Notification RECONFIRM
Sometimes, particularly when used with a Discovery Proxy [RFC8766], a
DNS Zone may contain stale data. When a client encounters data that
it believes may be stale (e.g., an SRV record referencing a target
host+port that is not responding to connection requests), the client
can send a RECONFIRM message to ask the server to re-verify that the
data is still valid. For a Discovery Proxy, this causes it to issue
new Multicast DNS queries to ascertain whether the target device is
still present. How the Discovery Proxy causes these new Multicast
DNS queries to be issued depends on the details of the underlying
Multicast DNS implementation being used. For example, a Discovery
Proxy built on Apple's dns_sd.h API [SD-API] responds to a DNS Push
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 it need not
cause any other 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
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.
The entity that initiates a RECONFIRM message is by definition the
client. A server MUST NOT send a RECONFIRM message over an existing
session from a client. 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.
The other header fields MUST be set as described in the DSO
specification [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 (0x0043).
The DSO-LENGTH is the length of the data that follows, which
specifies the name, type, class, and content of the record being
disputed.
A DNS Push Notifications RECONFIRM message contains exactly one
RECONFIRM Primary TLV. The DSO-DATA in a RECONFIRM Primary TLV MUST
contain exactly one record. The DSO-DATA in a RECONFIRM Primary TLV
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. Note that this means that DNS name compression cannot be
used between different RECONFIRM messages. However, when a client is
sending multiple RECONFIRM messages this indicates a situation with
serious network problems, and this is not expected to occur
frequently enough that optimizing efficiency in this case is
important.
TYPE MUST NOT be the value ANY (255) and CLASS MUST NOT be the value
ANY (255).
DNS wildcarding is not supported. That is, an asterisk character
("*") in a RECONFIRM message matches only a literal asterisk
character ("*") in a name and nothing else. Similarly, a CNAME in a
RECONFIRM message matches only a CNAME record with that name in the
zone and no other records with that 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.
Following the same rules as for PUSH messages, DNS name compression
SHOULD be used within the RDATA of the RECONFIRM message, with
offsets relative to the beginning of the DNS message [RFC1035].
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
| ANCOUNT (MUST BE ZERO) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| NSCOUNT (MUST BE ZERO) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| ARCOUNT (MUST BE ZERO) | /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ /
| DSO-TYPE = RECONFIRM (0x0043) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| DSO-LENGTH (number of octets in DSO-DATA) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \
\ NAME \ \
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| TYPE | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > DSO-DATA
| CLASS | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
\ RDATA \ /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ /
Figure 5: RECONFIRM Message
6.6. DNS Stateful Operations TLV Context Summary
This document defines four new DSO TLVs. As recommended 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
C-A: Client request or unidirectional message, Additional TLV
CRP: Response back to client, Primary TLV
CRA: Response back to client, Additional TLV
+-------------+-----+-----+-----+-----+-----+
| TLV Type | C-P | C-U | C-A | CRP | CRA |
+=============+=====+=====+=====+=====+=====+
| SUBSCRIBE | X | | | | |
+-------------+-----+-----+-----+-----+-----+
| PUSH | | | | | |
+-------------+-----+-----+-----+-----+-----+
| UNSUBSCRIBE | | X | | | |
+-------------+-----+-----+-----+-----+-----+
| RECONFIRM | | X | | | |
+-------------+-----+-----+-----+-----+-----+
Table 2: DSO TLV Client Context Summary
The server TLV contexts are:
S-P: Server request message, Primary TLV
S-U: Server Unidirectional message, primary TLV
S-A: Server request or unidirectional message, Additional TLV
SRP: Response back to server, Primary TLV
SRA: Response back to server, Additional TLV
+-------------+-----+-----+-----+-----+-----+
| TLV Type | S-P | S-U | S-A | SRP | SRA |
+=============+=====+=====+=====+=====+=====+
| SUBSCRIBE | | | | | |
+-------------+-----+-----+-----+-----+-----+
| PUSH | | X | | | |
+-------------+-----+-----+-----+-----+-----+
| UNSUBSCRIBE | | | | | |
+-------------+-----+-----+-----+-----+-----+
| RECONFIRM | | | | | |
+-------------+-----+-----+-----+-----+-----+
Table 3: DSO TLV Server Context Summary
6.7. Client-Initiated Termination
An individual subscription is terminated by sending an UNSUBSCRIBE
TLV for that specific subscription, or all subscriptions can be
canceled at once by the client closing the DSO session. When a
client terminates an individual subscription (via UNSUBSCRIBE) or all
subscriptions on that DSO session (by ending the session), it is
signaling to the server that it is no longer interested in receiving
those particular updates. It is informing the server that the server
may release any state information it has been keeping with regards to
these particular subscriptions.
After terminating its last subscription on a session via UNSUBSCRIBE,
a client MAY close the session immediately or it may keep it open if
it anticipates performing further operations on that session in the
future. If a client wishes to keep an idle session open, it MUST
respect the maximum idle time required by the server [RFC8490].
If a client plans to terminate one or more subscriptions on a session
and doesn't intend to keep that session open, then as an efficiency
optimization, it MAY instead choose to simply close the session,
which 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
canceled.
When closing a session, a client should perform an orderly close of
the TLS session. Typical APIs will provide a session close method
that will send a TLS close_notify alert as described in 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.
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.<zone>" SRV record, or the server
indicated in the SRV record is misconfigured, overloaded, 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 up to 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 receive a reply telling it that the answer
RRset has one second remaining; the client would then requery the
recursive resolver again one second later. If by this time the
answer RRset has actually expired from the recursive resolver's
cache, the recursive resolver would then issue a new query to fetch
fresh data from the authoritative server. Waiting until the answer
RRset has definitely expired from the cache on the client's
configured recursive resolver avoids this race condition and any
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 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 document Usage Profiles for DNS over
(D)TLS [RFC8310] for additional recommendations for various versions
of TLS usage.
As a consequence of requiring TLS, client certificate authentication
and verification may also be enforced by the server for stronger
client-server security or end-to-end security. However,
recommendations for security in particular deployment scenarios are
outside the scope of this document.
DNSSEC is RECOMMENDED for the authentication of DNS Push Notification
servers. TLS alone does not provide complete security. TLS
certificate verification can provide reasonable assurance that the
client is really talking to the server associated with the desired
host name, but since the desired host name is learned via a DNS SRV
query, if the SRV query is subverted, then the client may have a
secure connection to a rogue server. DNSSEC can provide added
confidence that the SRV query has not been subverted.
7.1. Security Services
It is the goal of using TLS to provide the following security
services:
Confidentiality: All application-layer communication is encrypted
with the goal that no party should be able to decrypt it except
the intended receiver.
Data integrity protection: Any changes made to the communication in
transit are detectable by the receiver.
Authentication: An endpoint of the TLS communication is
authenticated as the intended entity to communicate with.
Anti-replay protection: TLS provides for the detection of and
prevention against messages sent previously over a TLS connection
(such as DNS Push Notifications). If prior messages are re-sent
at a later time as a form of a man-in-the-middle attack, then the
receiver will detect this and reject the replayed messages.
Deployment recommendations on the appropriate key lengths and cipher
suites are beyond the scope of this document. Please refer to the
current TLS Recommendations [BCP195] for the best current practices.
Keep in mind that best practices only exist for a snapshot in time,
and recommendations will continue to change. Updated versions or
errata may exist for these recommendations.
7.2. TLS Name Authentication
As described in Section 6.1, the client discovers the DNS Push
Notification server using an SRV lookup for the record name
"_dns-push-tls._tcp.<zone>". The server connection endpoint SHOULD
then be authenticated using DANE TLSA records for the associated SRV
record. This associates the target's name and port number with a
trusted TLS certificate [RFC7673]. This procedure uses the TLS
Server Name Indication (SNI) extension [RFC6066] to inform the server
of the name the client has authenticated through the use of TLSA
records. Therefore, if the SRV record passes DNSSEC validation and a
TLSA record matching the target name is usable, an SNI extension 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 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 activated when the server detects an excessive number of
spurious subscription requests.
For further guidance on use of TLS early data, please see discussion
of zero round-trip data in Sections 2.3 and 8, and Appendix E.5, of
the TLS 1.3 specification [RFC8446].
7.4. TLS Session Resumption
TLS session resumption [RFC8446] is permissible on DNS Push
Notification servers. However, closing the TLS connection terminates
the DSO session. When the TLS session is resumed, the DNS Push
Notification server will not have any subscription state and will
proceed as with any other new DSO session. Use of TLS session
resumption may allow a TLS connection to be set up more quickly, but
the client will still have to recreate any desired subscriptions.
8. IANA Considerations
This document defines a new service name, only applicable for the TCP
protocol, which has been recorded in the IANA "Service Name and
Transport Protocol Port Number Registry" [RFC6335] [SRVTYPE].
+-----------------------+------+----------------------+---------+
| Name | Port | Value | Section |
+=======================+======+======================+=========+
| DNS Push Notification | None | "_dns-push-tls._tcp" | 6.1 |
| Service Type | | | |
+-----------------------+------+----------------------+---------+
Table 4: IANA Service Type Assignments
This document defines four new DNS Stateful Operation TLV types,
which have been recorded in the IANA "DSO Type Codes" registry
[RFC8490] [DSOTYPE].
+-------------+--------+------------+-----------------+---------+
| Name | Value | Early Data | Status | Section |
+=============+========+============+=================+=========+
| SUBSCRIBE | 0x0040 | OK | Standards Track | 6.2 |
+-------------+--------+------------+-----------------+---------+
| PUSH | 0x0041 | NO | Standards Track | 6.3 |
+-------------+--------+------------+-----------------+---------+
| UNSUBSCRIBE | 0x0042 | NO | Standards Track | 6.4 |
+-------------+--------+------------+-----------------+---------+
| RECONFIRM | 0x0043 | NO | Standards Track | 6.5 |
+-------------+--------+------------+-----------------+---------+
Table 5: IANA DSO TLV Type Code Assignments
This document defines no new DNS OPCODEs or RCODEs.
9. References
9.1. Normative References
[DSOTYPE] IANA, "Domain Name System (DNS) Parameters",
<https://www.iana.org/assignments/dns-parameters/>.
[RFC0020] Cerf, V., "ASCII format for network interchange", STD 80,
RFC 20, DOI 10.17487/RFC0020, October 1969,
<https://www.rfc-editor.org/info/rfc20>.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980,
<https://www.rfc-editor.org/info/rfc768>.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/info/rfc1034>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[RFC1123] Braden, R., Ed., "Requirements for Internet Hosts -
Application and Support", STD 3, RFC 1123,
DOI 10.17487/RFC1123, October 1989,
<https://www.rfc-editor.org/info/rfc1123>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2136] Vixie, P., Ed., Thomson, S., Rekhter, Y., and J. Bound,
"Dynamic Updates in the Domain Name System (DNS UPDATE)",
RFC 2136, DOI 10.17487/RFC2136, April 1997,
<https://www.rfc-editor.org/info/rfc2136>.
[RFC2181] Elz, R. and R. Bush, "Clarifications to the DNS
Specification", RFC 2181, DOI 10.17487/RFC2181, July 1997,
<https://www.rfc-editor.org/info/rfc2181>.
[RFC2782] Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
specifying the location of services (DNS SRV)", RFC 2782,
DOI 10.17487/RFC2782, February 2000,
<https://www.rfc-editor.org/info/rfc2782>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<https://www.rfc-editor.org/info/rfc6066>.
[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", BCP 165,
RFC 6335, DOI 10.17487/RFC6335, August 2011,
<https://www.rfc-editor.org/info/rfc6335>.
[RFC6895] Eastlake 3rd, D., "Domain Name System (DNS) IANA
Considerations", BCP 42, RFC 6895, DOI 10.17487/RFC6895,
April 2013, <https://www.rfc-editor.org/info/rfc6895>.
[RFC7673] Finch, T., Miller, M., and P. Saint-Andre, "Using DNS-
Based Authentication of Named Entities (DANE) TLSA Records
with SRV Records", RFC 7673, DOI 10.17487/RFC7673, October
2015, <https://www.rfc-editor.org/info/rfc7673>.
[RFC7766] Dickinson, J., Dickinson, S., Bellis, R., Mankin, A., and
D. Wessels, "DNS Transport over TCP - Implementation
Requirements", RFC 7766, DOI 10.17487/RFC7766, March 2016,
<https://www.rfc-editor.org/info/rfc7766>.
[RFC7858] Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "Specification for DNS over Transport
Layer Security (TLS)", RFC 7858, DOI 10.17487/RFC7858, May
2016, <https://www.rfc-editor.org/info/rfc7858>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8310] Dickinson, S., Gillmor, D., and T. Reddy, "Usage Profiles
for DNS over TLS and DNS over DTLS", RFC 8310,
DOI 10.17487/RFC8310, March 2018,
<https://www.rfc-editor.org/info/rfc8310>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC8490] Bellis, R., Cheshire, S., Dickinson, J., Dickinson, S.,
Lemon, T., and T. Pusateri, "DNS Stateful Operations",
RFC 8490, DOI 10.17487/RFC8490, March 2019,
<https://www.rfc-editor.org/info/rfc8490>.
[SRVTYPE] IANA, "Service Name and Transport Protocol Port Number
Registry", <https://www.iana.org/assignments/service-
names-port-numbers/>.
9.2. Informative References
[BCP195] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, May 2015,
<https://www.rfc-editor.org/info/bcp195>.
[OBS] Wikipedia, "Observer pattern", February 2020,
<https://en.wikipedia.org/w/
index.php?title=Observer_pattern&oldid=939702131>.
[RFC2308] Andrews, M., "Negative Caching of DNS Queries (DNS
NCACHE)", RFC 2308, DOI 10.17487/RFC2308, March 1998,
<https://www.rfc-editor.org/info/rfc2308>.
[RFC3123] Koch, P., "A DNS RR Type for Lists of Address Prefixes
(APL RR)", RFC 3123, DOI 10.17487/RFC3123, June 2001,
<https://www.rfc-editor.org/info/rfc3123>.
[RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
Syndication Format", RFC 4287, DOI 10.17487/RFC4287,
December 2005, <https://www.rfc-editor.org/info/rfc4287>.
[RFC4953] Touch, J., "Defending TCP Against Spoofing Attacks",
RFC 4953, DOI 10.17487/RFC4953, July 2007,
<https://www.rfc-editor.org/info/rfc4953>.
[RFC6281] Cheshire, S., Zhu, Z., Wakikawa, R., and L. Zhang,
"Understanding Apple's Back to My Mac (BTMM) Service",
RFC 6281, DOI 10.17487/RFC6281, June 2011,
<https://www.rfc-editor.org/info/rfc6281>.
[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013,
<https://www.rfc-editor.org/info/rfc6762>.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
<https://www.rfc-editor.org/info/rfc6763>.
[RFC6886] Cheshire, S. and M. Krochmal, "NAT Port Mapping Protocol
(NAT-PMP)", RFC 6886, DOI 10.17487/RFC6886, April 2013,
<https://www.rfc-editor.org/info/rfc6886>.
[RFC6887] Wing, D., Ed., Cheshire, S., Boucadair, M., Penno, R., and
P. Selkirk, "Port Control Protocol (PCP)", RFC 6887,
DOI 10.17487/RFC6887, April 2013,
<https://www.rfc-editor.org/info/rfc6887>.
[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<https://www.rfc-editor.org/info/rfc7413>.
[RFC8010] Sweet, M. and I. McDonald, "Internet Printing
Protocol/1.1: Encoding and Transport", STD 92, RFC 8010,
DOI 10.17487/RFC8010, January 2017,
<https://www.rfc-editor.org/info/rfc8010>.
[RFC8011] Sweet, M. and I. McDonald, "Internet Printing
Protocol/1.1: Model and Semantics", STD 92, RFC 8011,
DOI 10.17487/RFC8011, January 2017,
<https://www.rfc-editor.org/info/rfc8011>.
[RFC8499] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", BCP 219, RFC 8499, DOI 10.17487/RFC8499,
January 2019, <https://www.rfc-editor.org/info/rfc8499>.
[RFC8684] Ford, A., Raiciu, C., Handley, M., Bonaventure, O., and C.
Paasch, "TCP Extensions for Multipath Operation with
Multiple Addresses", RFC 8684, DOI 10.17487/RFC8684, March
2020, <https://www.rfc-editor.org/info/rfc8684>.
[RFC8764] Cheshire, S. and M. Krochmal, "Apple's DNS Long-Lived
Queries Protocol", RFC 8764, DOI 10.17487/RFC8764, June
2020, <https://www.rfc-editor.org/info/rfc8764>.
[RFC8766] Cheshire, S., "Discovery Proxy for Multicast DNS-Based
Service Discovery", RFC 8766, DOI 10.17487/RFC8766, June
2020, <https://www.rfc-editor.org/info/rfc8766>.
[SD-API] Apple Inc., "dns_sd.h",
<https://opensource.apple.com/source/mDNSResponder/
mDNSResponder-878.70.2/mDNSShared/dns_sd.h.auto.html>.
[SYN] Eddy, W., "Defenses Against TCP SYN Flooding Attacks", The
Internet Protocol Journal, Cisco Systems, Volume 9, Number
4, December 2006,
<https://www.cisco.com/web/about/ac123/ac147/
archived_issues/ipj_9-4/ipj_9-4.pdf>.
[TCPRACK] Cheng, Y., Cardwell, N., Dukkipati, N., and P. Jha, "RACK:
a time-based fast loss detection algorithm for TCP", Work
in Progress, Internet-Draft, draft-ietf-tcpm-rack-08, 9
March 2020,
<https://tools.ietf.org/html/draft-ietf-tcpm-rack-08>.
[XEP0060] Millard, P., Saint-Andre, P., and R. Meijer, "Publish-
Subscribe", XSF XEP 0060, October 2019,
<https://xmpp.org/extensions/xep-0060.html>.
Acknowledgments
The authors would like to thank Kiren Sekar and Marc Krochmal for
previous work completed in this field.
This document has been improved due to comments from Ran Atkinson,
Tim 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, Éric Vyncke, Christopher Wood,
Liang Xia, and Soraia Zlatkovic. Ted Lemon provided clarifying text
that was greatly appreciated.
Authors' Addresses
Tom Pusateri
Unaffiliated
Raleigh, NC 27608
United States of America
Phone: +1 919 867 1330
Email: pusateri@bangj.com
Stuart Cheshire
Apple Inc.
One Apple Park Way
Cupertino, CA 95014
United States of America
Phone: +1 (408) 996-1010
Email: cheshire@apple.com
AI ELI5
On this page
PROBLEMDNS is efficient for relatively static data, but when records change often the only option is polling, and there is no way for a client to be told asynchronously that something changed.
MECHANISMA client opens a DNS Stateful Operations session over TLS over TCP, sends SUBSCRIBE messages for a name, type and class, and the server sends PUSH messages that add or remove records, until the client sends UNSUBSCRIBE or closes the session.
WHY IT MATTERSClients get the same results as sufficiently rapid polling, more efficiently. If no subscription can be set up, they fall back to polling at a limited rate.
§2Why Push Instead of Poll
Polling can burden DNS servers at many levels of the network. A publish/subscribe model can deliver more timely notifications of changes with reduced CPU usage and lower network traffic.
The guiding principle is that a client using DNS Push Notifications receives the same results it could get via sufficiently rapid polling, only more efficiently. So the rules for which records match a subscription are the same as for a DNS query: names compare case-insensitively, and a CNAME matches any type.
Multicast DNS already announces changes to a link-local group. DNS-based Service Discovery (RFC 6763) across a wide area using Unicast DNS needs an equivalent. An earlier deployed solution, DNS Long-Lived Queries (RFC 8764), was UDP-based and reimplemented TCP-like state management in user space. This document instead uses DNS Stateful Operations (RFC 8490) over TLS over TCP, so it does not reinvent TCP features.
Asking 'did anything change?' over and over is tiring for everyone. It is better to say 'tell me when it does' once and wait.
§3Sessions, Transport and Subscription Limits
A client connects to the right Push Notification server for a record set and sends DSO messages naming what it cares about. The server for a zone is any server that can generate correct change notifications for a name: a primary, secondary or stealth name server. Supporting DNS Update and Push on the same server is OPTIONAL, and standard DNS queries MAY be sent over the same session.
Clients MUST use DSO running over TLS over TCP (RFC 7858). TCP gives return reachability and avoids state-overload by spoofed sources, and TLS is REQUIRED on every connection.
Push has a cost, so clients MUST NOT recklessly create too many subscriptions. A subscription should be active only while there is a valid reason, such as an on-screen display, and SHOULD be cancelled as soon as that need ends. A client SHOULD NOT routinely keep subscriptions open 24 hours a day just to keep a list warm. A session with an active subscription is not idle, but the DSO keepalive interval still applies. Servers can limit subscriptions, and clients may open sessions to alternate servers for the same zone.
You should only ask to be notified about things you are looking at right now, and stop asking when you look away.
§4Discovering the Push Server
The client first opens a DSO session to its normal recursive resolver on TCP port 853 and requests a subscription. If that works, the resolver subscribes upstream on the client's behalf and relays results. If the resolver lacks DSO support the session setup fails. If it has DSO but not Push, it returns the DSO error code DSOTYPENI (11). If it cannot subscribe for a name it should return SERVFAIL.
When that fails, the client discovers the server directly:
- Send an SOA query for the full name, then strip one leading label at a time until an SOA appears in the Answer or Authority section. A single-label name means a configuration error and the client gives up.
- Query the SRV record
_dns-push-tls._tcp.<zone>, where the zone is the owner name of the SOA. If the zone offers Push, this record MUST exist. Its target and port say where to connect. - With several SRV records, use
priority(lowest first) andweightto choose the order of servers.
Each new subscription SHOULD repeat discovery, reusing an existing session to the same server, and the client MUST respect DNS TTLs so repeated discovery is mostly answered from cache.
To find who rings the doorbell for your street, you ask up the chain: first your neighbor, then the block, then the city, until someone says it is them.
§5SUBSCRIBE Request and Response
A SUBSCRIBE request is a DSO message with the standard 12-byte DSO header and the SUBSCRIBE Primary TLV (type 0x0040). The DNS OPCODE is 6, the four count fields are zero, and the data holds exactly one NAME, TYPE and CLASS. The MESSAGE ID must be unique among the client's active operations on the session, and the server echoes it in the response.
Subscriptions on a session MUST be unique; a duplicate is a fatal error and the server forcibly aborts the connection. Wildcards are not supported: an asterisk matches only a literal asterisk. Subscribing to records the server does not yet know is not an error, and the server MUST NOT return NXDOMAIN. When TYPE or CLASS is 255, it means ALL, so the server MUST send updates for all matching changes.
The response's RCODE says whether the subscription was accepted. On an error the server SHOULD add a Retry Delay Additional TLV, and the client should not retry against that server right away. Existing subscriptions on the session stay in effect.
Subscribing is like giving the mail carrier a note: 'ring me when anything arrives for this name.' The carrier writes back yes or explains why not.
ClientPush server
- SUBSCRIBEName, type and class of interest.
- SUBSCRIBE response (RCODE)Echoes the MESSAGE ID.
- PUSHAdds or removes records; MESSAGE ID is zero.
- UNSUBSCRIBECarries the original SUBSCRIBE MESSAGE ID.
- 0NOERROR — SUBSCRIBE successful.
- 1FORMERR — Malformed request.
- 2SERVFAIL — Problem with the server.
- 4NOTIMP — Server does not implement DSO.
- 5REFUSED — Refused for policy or security reasons.
- 9NOTAUTH — Server is not authoritative for the name.
- 11DSOTYPENI — SUBSCRIBE operation not supported.
§6PUSH Updates
After a subscription is accepted, the server sends PUSH messages (type 0x0041). If the answer set was already non-empty, an initial PUSH follows the SUBSCRIBE response immediately. PUSH is unidirectional: its MESSAGE ID is zero and the client does not reply. A client MUST NOT send PUSH.
Each PUSH holds one or more change notifications that look like resource records, and the TTL field tells the client what kind of change it is:
- TTL from 0 to 0x7FFFFFFF: add the record. A TTL of 0 means keep it while the subscription is active and discard it when the subscription ends.
- TTL 0xFFFFFFFF: remove that individual record.
- TTL 0xFFFFFFFE with RDLEN zero: a collective removal of a whole RRset, or all RRsets for a name in a class, or in all classes.
Any other TTL value is silently ignored. A change that affects several subscriptions produces just one PUSH. Servers SHOULD encode changes compactly, batch immediately available changes, and use DNS name compression. A PUSH MUST NOT exceed a DNS message length of 16,382 bytes; larger sets are split across messages.
While a subscription is active, stored TTLs are not decremented, and the client does not need to poll to confirm a record is still there. Clients silently ignore pushes for subscriptions they just removed, because UNSUBSCRIBE and PUSH can cross in flight.
The doorbell rings with a note: 'a new card arrived' or 'this old card was taken away.' You keep your list up to date without leaving your seat.
- 0 to 0x7FFFFFFFAdd record — Adds the record with this name, type, class and RDATA; TTL 0 keeps it only while subscribed.
- 0xFFFFFFFFRemove record — Removes the one record given by name, type, class and RDATA.
- 0xFFFFFFFECollective remove — RDLEN must be zero; removes an RRset, or all RRsets in a class, or in all classes.
§7UNSUBSCRIBE, RECONFIRM, Closing and Polling Fallback
To cancel one subscription, the client sends a unidirectional UNSUBSCRIBE (type 0x0042) whose data is the 2-byte MESSAGE ID of the original SUBSCRIBE. It may be sent before the SUBSCRIBE response arrives, and servers MUST silently ignore an UNSUBSCRIBE that matches nothing. Closing the session cancels all subscriptions. The client should do an orderly TLS close with a close_notify alert and then a TCP FIN, since a TCP RST can lose data.
A client that suspects stale data, such as an SRV record whose target does not respond, can send RECONFIRM (type 0x0043) naming one record. A Discovery Proxy then re-checks using Multicast DNS. On other servers the operation is undefined and SHOULD just get NOERROR. If the record is gone, normal PUSH messages inform all interested clients.
If a client cannot get a subscription at all, it may fall back to polling. The interval between queries for the same name, type and class SHOULD be at least the minimum of 900 seconds or two seconds more than the TTL of the answer RRset, and before each re-query the client should try to subscribe again.
You can cancel a notification at any time, or hang up the phone and cancel them all. If the doorbell is broken, you check the mailbox yourself, but not too often.
- 0x0040SUBSCRIBE — Client request; allowed in TLS early data.
- 0x0041PUSH — Server unidirectional message carrying changes.
- 0x0042UNSUBSCRIBE — Client unidirectional message cancelling a subscription.
- 0x0043RECONFIRM — Client unidirectional message disputing a record.
§8Security and TLS
The Strict Privacy profile for DNS over TLS (RFC 8310) is REQUIRED, and cleartext connections are not permissible. TLS aims to give confidentiality, integrity protection, authentication and anti-replay protection. Servers may also require client certificates.
TLS alone is not complete security: the server name is learned from an SRV query, so a subverted SRV answer could lead to a rogue server. DNSSEC is RECOMMENDED, and the server connection SHOULD be authenticated with DANE TLSA records (RFC 7673), using SNI for the authenticated target name.
SUBSCRIBE is permitted in TLS early data, which can save a round trip, but early data is not forward secret and gives no replay guarantees between connections. Spurious replayed subscriptions create only small, short-lived server state, and servers MAY rate-limit them. TLS session resumption is allowed, but the resumed session starts without subscription state, so the client must resubscribe.
All the notes travel in a locked box, and you double-check the doorbell is really yours, so nobody can fake a ring.
§8References
[RFC8490]RFC 8490Bellis, R., et al., "DNS Stateful Operations", RFC 8490, March 2019.March 2019[RFC7858]RFC 7858Hu, Z., et al., "Specification for DNS over Transport Layer Security (TLS)", RFC 7858, May 2016.May 2016[RFC8446]RFC 8446Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, August 2018.August 2018[RFC8764]RFC 8764Cheshire, S. and M. Krochmal, "Apple's DNS Long-Lived Queries Protocol", RFC 8764, June 2020.June 2020[RFC8766]RFC 8766Cheshire, S., "Discovery Proxy for Multicast DNS-Based Service Discovery", RFC 8766, June 2020.June 2020[RFC6763]RFC 6763Cheshire, S. and M. Krochmal, "DNS-Based Service Discovery", RFC 6763, February 2013.February 2013[RFC8310]RFC 8310Dickinson, S., et al., "Usage Profiles for DNS over TLS and DNS over DTLS", RFC 8310, March 2018.March 2018[RFC7673]RFC 7673Finch, T., et al., "Using DNS-Based Authentication of Named Entities (DANE) TLSA Records with SRV Records", RFC 7673, October 2015.October 2015[RFC2782]RFC 2782Gulbrandsen, A., et al., "A DNS RR for specifying the location of services (DNS SRV)", RFC 2782, February 2000.February 2000