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draft-ietf-dnssd-push.txt
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Internet Engineering Task Force T. Pusateri
Internet-Draft Unaffiliated
Intended status: Standards Track S. Cheshire
Expires: April 15, 2020 Apple Inc.
October 13, 2019
DNS Push Notifications
draft-ietf-dnssd-push-25
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 Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on April 15, 2020.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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Internet-Draft DNS Push Notifications October 2019
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
1.2. Fatal Errors . . . . . . . . . . . . . . . . . . . . . . 3
2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. State Considerations . . . . . . . . . . . . . . . . . . . . 6
5. Transport . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. Protocol Operation . . . . . . . . . . . . . . . . . . . . . 8
6.1. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 9
6.2. DNS Push Notification SUBSCRIBE . . . . . . . . . . . . . 13
6.2.1. SUBSCRIBE Request . . . . . . . . . . . . . . . . . . 13
6.2.2. SUBSCRIBE Response . . . . . . . . . . . . . . . . . 16
6.3. DNS Push Notification Updates . . . . . . . . . . . . . . 20
6.3.1. PUSH Message . . . . . . . . . . . . . . . . . . . . 20
6.4. DNS Push Notification UNSUBSCRIBE . . . . . . . . . . . . 26
6.4.1. UNSUBSCRIBE Message . . . . . . . . . . . . . . . . . 26
6.5. DNS Push Notification RECONFIRM . . . . . . . . . . . . . 28
6.5.1. RECONFIRM Message . . . . . . . . . . . . . . . . . . 29
6.6. DNS Stateful Operations TLV Context Summary . . . . . . . 31
6.7. Client-Initiated Termination . . . . . . . . . . . . . . 32
6.8. Client Fallback to Polling . . . . . . . . . . . . . . . 33
7. Security Considerations . . . . . . . . . . . . . . . . . . . 34
7.1. Security Services . . . . . . . . . . . . . . . . . . . . 35
7.2. TLS Name Authentication . . . . . . . . . . . . . . . . . 35
7.3. TLS Early Data . . . . . . . . . . . . . . . . . . . . . 36
7.4. TLS Session Resumption . . . . . . . . . . . . . . . . . 36
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 37
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 37
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 38
10.1. Normative References . . . . . . . . . . . . . . . . . . 38
10.2. Informative References . . . . . . . . . . . . . . . . . 40
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 42
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1. Introduction
Domain Name System (DNS) records may be updated using DNS Update
[RFC2136]. Other mechanisms such as a Discovery Proxy [DisProx] can
also generate changes to a DNS zone. This document specifies a
protocol for DNS clients to subscribe to receive asynchronous
notifications of changes to RRsets of interest. It is immediately
relevant in the case of DNS Service Discovery [RFC6763] but is not
limited to that use case, and provides a general DNS mechanism for
DNS record change notifications. Familiarity with the DNS protocol
and DNS packet formats is assumed [RFC1034] [RFC1035] [RFC6895].
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. These words may also appear in this
document in lower case as plain English words, absent their normative
meanings.
1.2. Fatal Errors
Certain invalid situations are described in this specification, like
a server sending a Push Notification subscription request to a
client, or a client sending a Push Notification response to a server.
These should never occur with a correctly implemented client and
server, and if they do occur then they indicate a serious
implementation error. In these extreme cases there is no reasonable
expectation of a graceful recovery, and the recipient detecting the
error should respond by unilaterally aborting the session without
regard for data loss. Such cases are addressed by having an engineer
investigate the cause of the failure and fixing the problem in the
software.
Where this specification says "forcibly abort", it means sending a
TCP RST to terminate the TCP connection, and the TLS session running
over that TCP connection. In the BSD Sockets API, this is achieved
by setting the SO_LINGER option to zero before closing the socket.
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2. Motivation
As the domain name system continues to adapt to new uses and changes
in deployment, polling has the potential to burden DNS servers at
many levels throughout the network. Other network protocols have
successfully deployed a publish/subscribe model following the
Observer design pattern [obs]. XMPP Publish-Subscribe [XEP0060] and
Atom [RFC4287] are examples. While DNS servers are generally highly
tuned and capable of a high rate of query/response traffic, adding a
publish/subscribe model for tracking changes to DNS records can
deliver more timely notification of changes with reduced CPU usage
and lower network traffic.
Multicast DNS [RFC6762] implementations always listen on a well known
link-local IP multicast group 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 Service Discovery [RFC6763] is used across a
wide area network using Unicast DNS (possibly facilitated via a
Discovery Proxy [DisProx]) it would be beneficial to have an
equivalent capability for Unicast DNS, to allow clients to learn
about DNS record changes in a timely manner without polling.
The DNS Long-Lived Queries (LLQ) mechanism [LLQ] is an existing
deployed solution to provide asynchronous change notifications, used
by Apple's Back to My Mac [RFC6281] service introduced in Mac OS X
10.5 Leopard in 2007. Back to My Mac was designed in an era when the
data center operations staff asserted that it was impossible for a
server to handle large numbers of mostly-idle TCP connections, so 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 the three-way
handshake, flow control, and reliability.
This document builds on experience gained with the LLQ protocol, with
an improved design. Instead of using UDP, this specification uses
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.
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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 [RFC7719].
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, so
DNS Push Notification clients MUST NOT recklessly create an excessive
number of Push Notification subscriptions. Specifically:
(a) A subscription should only be active when there is a valid reason
to need live data (for example, an on-screen display is currently
showing the results to the user) and the subscription SHOULD be
cancelled as soon as the need for that data ends (for example, when
the user dismisses that display). In the case of a device like a
smartphone which, after some period of inactivity, goes to sleep or
otherwise darkens its screen, it should cancel its subscriptions when
darkening the screen (since the user cannot see any changes on the
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display anyway) and reinstate its subscriptions when re-awakening
from display sleep.
(b) A DNS Push Notification client SHOULD NOT routinely keep a DNS
Push Notification subscription active 24 hours a day, 7 days a week,
just to keep a list in memory up to date so that if the user does
choose to bring up an on-screen display of that data, it can be
displayed really fast. DNS Push Notifications are designed to be
fast enough that there is no need to pre-load a "warm" list in memory
just in case it might be needed later.
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 may close a DSO session immediately 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
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 bandwidth, and operating system resource
allocation. After a client establishes a session to a DNS server,
each subscription is individually accepted or rejected. Servers may
employ various techniques to limit subscriptions to a manageable
level. Correspondingly, the client is free to establish simultaneous
sessions to alternate DNS servers that support DNS Push Notifications
for the zone and distribute subscriptions at the client's discretion.
In this way, both clients and servers can react to resource
constraints.
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5. Transport
Other DNS operations like DNS Update [RFC2136] MAY use either User
Datagram Protocol (UDP) [RFC0768] or Transmission Control Protocol
(TCP) [RFC0793] as the transport protocol, in keeping with the
historical precedent that DNS queries must first be sent over UDP
[RFC1123]. This requirement to use UDP has subsequently been relaxed
[RFC7766].
In keeping with the more recent precedent, DNS Push Notification is
defined only for TCP. DNS Push Notification clients MUST use DNS
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, which is 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)
[RFC6824], TCP Fast Open (TFO) [RFC7413], the TCP RACK fast loss
detection algorithm [I-D.ietf-tcpm-rack], and so on.
Transport Layer Security (TLS) [RFC8446] is well understood, and used
by many application-layer protocols running over TCP. TLS is
designed to prevent eavesdropping, tampering, and message forgery.
TLS is REQUIRED for every connection between a client subscriber and
server in this protocol specification. Additional security measures
such as client authentication during TLS negotiation may also be
employed to increase the trust relationship between client and
server.
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6. Protocol Operation
The DNS Push Notification protocol is a session-oriented protocol,
and makes use of DNS Stateful Operations (DSO) [RFC8490].
For details of the DSO message format refer to the DNS Stateful Oper-
ations specification [RFC8490]. Those details are not repeated here.
DNS Push Notification clients and servers MUST support DSO. A single
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.
A typical DNS Push Notification client will immediately issue a DSO
Keepalive operation to request a session timeout and/or keepalive
interval longer than the 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 cancelling that subscription is simultaneously in flight from
client to server.
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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 session establishment will fail [RFC8490].
If the recursive resolver does support DSO but not Push Notification
subscriptions, then it will return the DSO error code DSOTYPENI (11).
In some cases, the recursive resolver may support DSO and Push
Notification subscriptions, but may not be able to subscribe for Push
Notifications for a particular name. In this case, the recursive
resolver should return SERVFAIL to the client. This includes being
unable to establish a connection to the zone's DNS Push Notification
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 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
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appropriate server for direct communication. The client MUST also
determine which TCP port on the server is listening for connections,
which need not be (and often is not) the typical TCP port 53 used for
conventional DNS, or TCP port 853 used for DNS over TLS.
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
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
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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, 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 after a change in network
attachment.
5. Once the SOA is known (either by virtue of being seen in the
Answer Section, or in the Authority Section), the client sends a
DNS query with type SRV [RFC2782] for the record name
"_dns-push-tls._tcp.<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
below in Section 7.2 and in the specification for using 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.
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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 be 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 nearly instantaneously by the
client, using only locally-stored cached data.
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6.2. DNS Push Notification SUBSCRIBE
After connecting, and requesting a longer idle timeout and/or
keepalive interval if necessary, a DNS Push Notification client
then indicates its desire to receive DNS Push Notifications for
a given domain name by sending a SUBSCRIBE request to the server.
A SUBSCRIBE request is encoded in a DSO message [RFC8490].
This specification defines a primary DSO TLV for DNS Push
Notification SUBSCRIBE Requests (tentatively DSO Type Code 0x40).
DSO messages with the SUBSCRIBE TLV as the Primary TLV are permitted
in 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 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 which it
has not yet cancelled 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 spec-
ification [RFC8490]. The DNS OPCODE field contains the OPCODE value
for DNS Stateful Operations (6). The four count fields must be zero,
and the corresponding four sections must be empty (i.e., absent).
The DSO-TYPE is SUBSCRIBE (tentatively 0x40).
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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 (tentatively 0x40) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| 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.
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DNS wildcarding is not supported. That is, a wildcard ("*") in a
SUBSCRIBE message matches only a literal wildcard character ("*") in
the zone, and nothing else.
Aliasing is not supported. That is, a CNAME in a SUBSCRIBE message
matches only a literal CNAME record in the zone, and no other records
with the same owner name.
A client may SUBSCRIBE to records that are unknown to the server at
the time of the request (providing that the name falls within one of
the zone(s) the server is responsible for) and this is not an error.
The server MUST NOT return NXDOMAIN in this case. The server MUST
accept these requests and send Push Notifications if and when
matching records are found in the future.
If neither TYPE nor CLASS are ANY (255) then this is a specific
subscription to changes for the given NAME, TYPE and CLASS. If one
or both of TYPE or CLASS are ANY (255) then this subscription matches
any type and/or any class, 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 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.
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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 TLVs,
such as a Retry Delay 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 spec-
ification [RFC8490]. The DNS OPCODE field contains the OPCODE value
for DNS Stateful Operations (6). The four count fields must be zero,
and the corresponding four sections must be empty (i.e., absent).
A 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
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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 SUBSCRIBE Responses
with any other RCODE value.
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 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 are 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 TLV [RFC8490] to the response specifying a delay before the
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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 TLV, and the default values a client should assume if the
server provides no Retry Delay 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 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