Internet Engineering Task Force T. Pusateri
Internet-Draft Unaffiliated
Intended status: Standards Track S. Cheshire
Expires: May 9, 2019 Apple Inc.
November 5, 2018
DNS Push Notifications
draft-ietf-dnssd-push-16
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 May 9, 2019.
Copyright Notice
Copyright (c) 2018 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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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
2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Transport . . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. State Considerations . . . . . . . . . . . . . . . . . . . . 8
6. Protocol Operation . . . . . . . . . . . . . . . . . . . . . 9
6.1. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 10
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 . . . . . . . . . . . . . . 19
6.3.1. PUSH Message . . . . . . . . . . . . . . . . . . . . 19
6.4. DNS Push Notification UNSUBSCRIBE . . . . . . . . . . . . 22
6.4.1. UNSUBSCRIBE Request . . . . . . . . . . . . . . . . . 22
6.5. DNS Push Notification RECONFIRM . . . . . . . . . . . . . 24
6.5.1. RECONFIRM Request . . . . . . . . . . . . . . . . . . 24
6.5.2. RECONFIRM Response . . . . . . . . . . . . . . . . . 26
6.6. DNS Stateful Operations TLV Context Summary . . . . . . . 28
6.7. Client-Initiated Termination . . . . . . . . . . . . . . 29
7. Security Considerations . . . . . . . . . . . . . . . . . . . 30
7.1. Security Services . . . . . . . . . . . . . . . . . . . . 30
7.2. TLS Name Authentication . . . . . . . . . . . . . . . . . 30
7.3. TLS Compression . . . . . . . . . . . . . . . . . . . . . 31
7.4. TLS Session Resumption . . . . . . . . . . . . . . . . . 31
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 32
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 33
10.1. Normative References . . . . . . . . . . . . . . . . . . 33
10.2. Informative References . . . . . . . . . . . . . . . . . 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 37
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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
"Key words for use in RFCs to Indicate Requirement Levels", when, and
only when, they appear in all capitals, as shown here [RFC2119]
[RFC8174].
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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, and record changes are sent to that
multicast group address for all group members to receive. Therefore,
Multicast DNS already has asynchronous change notification
capability. However, when DNS Service Discovery [RFC6763] is used
across a wide area network using Unicast DNS (possibly facilitated
via a Discovery Proxy [DisProx]) it would be beneficial to have an
equivalent capability for Unicast DNS, to allow clients to learn
about DNS record changes in a timely manner without polling.
The DNS Long-Lived Queries (LLQ) mechanism [LLQ] is an existing
deployed solution to provide asynchronous change notifications, used
by Apple's Back to My Mac Service [RFC6281] 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 poor imitations 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) [DSO] 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 resorting to excessive keepalive
traffic. Instead of inventing a new vocabulary of messages to
communicate DNS zone changes as LLQ did, this specification borrows
the established syntax and semantics of DNS Update messages
[RFC2136].
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3. Overview
The existing DNS Update protocol [RFC2136] provides a mechanism for
clients to add or delete individual resource records (RRs) or entire
resource record sets (RRSets) on the zone's server.
This specification adopts a simplified subset of these existing
syntax and semantics, and uses them for DNS Push Notification
messages going in the opposite direction, from server to client, to
communicate changes to a zone. The client subscribes for Push
Notifications by connecting to the server and sending DNS message(s)
indicating the RRSet(s) of interest. When the client loses interest
in receiving further updates to these records, it unsubscribes.
The DNS Push Notification server for a zone is any server capable of
generating the correct change notifications for a name. It may be a
primary, secondary, or stealth name server [RFC7719]. Consequently,
the "_dns-push-tls._tcp.<zone>" SRV record for a zone MAY reference
the same target host and port as that zone's
"_dns-update-tls._tcp.<zone>" SRV record. When the same target host
and port is offered for both DNS Updates and DNS Push Notifications,
a client MAY use a single TCP connection to that server for both DNS
Updates and DNS Push Notification Subscriptions.
Supporting DNS Updates and DNS Push Notifications on the same server
is OPTIONAL. A DNS Push Notification server does NOT also have to
support DNS Update.
DNS Updates and DNS Push Notifications may be handled on different
ports on the same target host, in which case they are not considered
to be the "same server" for the purposes of this specification, and
communications with these two ports are handled independently.
Standard DNS Queries MAY be sent over a DNS Push Notification
connection. For any zone for which the server is authoritative, it
MUST respond authoritatively for queries on names falling within that
zone (e.g., the <zone> in the "_dns-push-tls._tcp.<zone>" SRV record)
both for DNS Push Notification queries and for normal DNS queries.
For names for which the server is acting as a caching resolver, e.g.
when the server is the local resolver, for any query for which it
supports DNS Push Notifications, it MUST also support standard
queries.
DNS Push Notification clients are NOT required to implement DNS
Update Prerequisite processing. Prerequisites are used to perform
tentative atomic test-and-set type operations when a client updates
records on a server, and that concept has no applicability when it
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comes to an authoritative server unilaterally informing a client of
changes to DNS records.
This DNS Push Notification specification includes support for DNS
classes, for completeness. However, in practice, it is anticipated
that for the foreseeable future the only DNS class in use will be DNS
class "IN", as is the reality today with existing DNS servers and
clients. A DNS Push Notification server MAY choose to implement only
DNS class "IN". If messages are received for a class other than
"IN", and that class is not supported, an error with RCODE NOTIMPL
(Not Implemented) should be returned.
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). Implementations may want to
implement idle timeouts, so that if the user ceases interacting with
the device, the subscription is cancelled.
(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
[DSO], a client must not keep a session to a server open indefinitely
if it has no subscriptions (or other operations) active on that
session. A client MAY close a session as soon as it becomes idle,
and then if needed in the future, open a new session when required.
Alternatively, a client MAY speculatively keep an idle session open
for some time, subject to the constraint that it MUST NOT keep a
session open that has been idle for more than the session's idle
timeout (15 seconds by default).
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4. 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 (DSO) [DSO] running over TLS over TCP [RFC7858].
Connection setup over TCP ensures return reachability and alleviates
concerns of state overload at the server through anonymous
subscriptions. All subscribers are guaranteed to be reachable by the
server by virtue of the TCP three-way handshake. Flooding attacks
are possible with any protocol, and a benefit of TCP is that there
are already established industry best practices to guard against SYN
flooding and similar attacks [SYN] [RFC4953].
Use of TCP also allows DNS Push Notifications to take advantage of
current and future developments in TCP, such as Multipath TCP (MPTCP)
[RFC6824], TCP Fast Open (TFO) [RFC7413], Tail Loss Probe (TLP)
[I-D.dukkipati-tcpm-tcp-loss-probe], and so on.
Transport Layer Security (TLS) [RFC5246] is well understood and
deployed across many protocols running over TCP. It is designed to
prevent eavesdropping, tampering, and message forgery. TLS is
REQUIRED for every connection between a client subscriber and server
in this protocol specification. Additional security measures such as
client authentication during TLS negotiation MAY also be employed to
increase the trust relationship between client and server.
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5. State Considerations
Each DNS Push Notification server is capable of handling some finite
number of Push Notification subscriptions. This number will vary
from server to server and is based on physical machine
characteristics, network bandwidth, and operating system resource
allocation. After a client establishes a session to a DNS server,
each subscription is individually accepted or rejected. Servers may
employ various techniques to limit subscriptions to a manageable
level. Correspondingly, the client is free to establish simultaneous
sessions to alternate DNS servers that support DNS Push Notifications
for the zone and distribute subscriptions at the client's discretion.
In this way, both clients and servers can react to resource
constraints.
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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) [DSO].
For details of the DSO message format refer to the DNS Stateful
Operations specification [DSO]. 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 or keepalive
interval longer than the the 15-second default, 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. However, Push Notification
subscriptions can also be used to establish the 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 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 the default DNS-over-TLS
port as defined in DNS over TLS [RFC7858]. If this connection is
successful, then the recursive resolver will make appropriate Push
Notification subscriptions on the client's behalf, and the client
will receive appropriate results.
In many contexts, the local recursive resolver will be able to handle
push notifications for all zones that the client may need to follow.
In other cases, the client may require Push Notifications from more
than one zone, and those zones may be served by different servers.
Therefore, it is assumed that the client may need to maintain
connections to more than one DNS Push server.
In some cases, the recursive resolver may not be able to get answers
for a particular zone. In this case, rather than returning SERVFAIL,
the resolver returns NOTAUTH. This signals the client that queries
for this zone can't be handled by the local caching resolver. For
that zone, the client SHOULD contact the zone's DNS Push server
itself, even if all other DNS Push queries can be handled by the
local resolver. This may be necessary in cases where the client is
connected to a VPN, for example, or where the client has a pre-
established trust relationship with the owner of the zone that allows
the client, but not the local resolver, to successfully get answers
for queries in that zone.
If the recursive resolver does not support Push Notification
subscriptions, then it will return an error code, DSONOTIMPL. This
occurs when the local resolver follows the procedure below and does
not find an SRV record indicating support for DNS Push Notifications.
In case of either failure, the client should proceed to discover the
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
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authoritative server is discovered. There is also an optimization to
enable the client to take a "short cut" directly to the SOA record of
the closest enclosing authoritative server in many cases.
1. The client begins the discovery by sending a DNS query to its
local resolver, with record type SOA [RFC1035] for the record
name to which it wishes to subscribe. As an example, suppose the
client wishes to subscribe to PTR records with the name
_ipp._tcp.foo.example.com (to discover Internet Printing Protocol
(IPP) printers [RFC8010] [RFC8011] being advertised at
"foo.example.com"). The client begins by sending an SOA query
for _ipp._tcp.foo.example.com to the local recursive resolver.
The goal is to determine the server authoritative for the name
_ipp._tcp.foo.example.com. The DNS zone containing the name
_ipp._tcp.foo.example.com could be example.com, or
foo.example.com, or _tcp.foo.example.com, or even
_ipp._tcp.foo.example.com. The client does not know in advance
where the closest enclosing zone cut occurs, which is why it uses
the 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 text is
not placing any new requirements on DNS recursive resolvers. It
is merely describing 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 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 text is not placing any new
requirements on DNS recursive resolvers. It is merely describing
the existing operation of the DNS protocol regarding negative
responses [RFC2308].)
4. If the client receives a response containing no SOA record, then
it proceeds with the iterative approach. The client strips the
leading label from the current query name and if the resulting
name has at least one label in it, the client sends a new SOA
query, and processing continues at step 2 above, repeating the
iterative search until either an SOA is received, or the query
name is empty. In the case of an empty name, 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.
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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.
Each time a client makes a new DNS Push Notification subscription
session, it SHOULD repeat the discovery process in order to determine
the preferred DNS server for subscriptions at that time. However,
the client device MUST respect the DNS TTL values on records it
receives, and store them in its local cache with this lifetime. This
means that, as long as the DNS TTL values on the authoritative
records were set to reasonable values, repeated application of this
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 over the established DSO
session to the server. A SUBSCRIBE request is encoded in a DSO [DSO]
message. This specification defines a primary DSO TLV for DNS Push
Notification SUBSCRIBE Requests (tentatively DSO Type Code 0x40).
The entity that initiates a SUBSCRIBE request is by definition the
client. A server MUST NOT send a SUBSCRIBE request over an existing
session from a client. If a server does send a SUBSCRIBE request
over a DSO session initiated by a client, this is a fatal error and
the client should immediately abort the connection with a TCP RST (or
equivalent for other protocols).
6.2.1. SUBSCRIBE Request
A SUBSCRIBE request begins with the standard DSO 12-byte header
[DSO], followed by the SUBSCRIBE TLV. A SUBSCRIBE request message is
illustrated in Figure 1.
The MESSAGE ID field MUST be set to a unique value, that the client
is not using for any other active operation on this 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
specification [DSO]. The DNS Opcode is the DSO Opcode. The four
count fields MUST be zero, and the corresponding four sections MUST
be empty (i.e., absent).
The DSO-TYPE is SUBSCRIBE. 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.
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1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \
| MESSAGE ID | \
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
|QR| Opcode | Z | RCODE | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| QDCOUNT (MUST BE ZERO) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER
| ANCOUNT (MUST BE ZERO) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| NSCOUNT (MUST BE ZERO) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| ARCOUNT (MUST BE ZERO) | /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ /
| DSO-TYPE = SUBSCRIBE |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| DSO-LENGTH (number of octets in DSO-DATA) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \
| | \
\ NAME \ |
\ \ |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > DSO-DATA
| TYPE | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| 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 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 applies (e.g., "example.com" and "Example.com" are
the same). If a server receives such a duplicate SUBSCRIBE message
this is an error and the server MUST immediately terminate the
connection with a TCP RST (or equivalent for other protocols).
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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 nothing else.
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 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=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
Each SUBSCRIBE request generates exactly one SUBSCRIBE response from
the server.
A SUBSCRIBE response message begins with the standard DSO 12-byte
header [DSO], possibly followed by one or more optional TLVs, such as
a Retry Delay TLV.
The MESSAGE ID field MUST echo the value given in the ID field of the
SUBSCRIBE request. This is how the client knows which request is
being responded to.
A SUBSCRIBE response message MUST NOT include a SUBSCRIBE TLV. If a
client receives a SUBSCRIBE response message containing a SUBSCRIBE
TLV then the response message is processed but the SUBSCRIBE TLV MUST
be silently ignored.
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.
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If the server sends a nonzero RCODE in the SUBSCRIBE response, that
means
a. the client is (at least partially) misconfigured,
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 discovery Section 6.1, Paragraph 7, a subsequent server
can be tried immediately.
If the client has other successful subscriptions to this server,
these subscriptions can 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 [DSO] 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 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 DSO [DSO], 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.
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If the server being queried is not the local resolver, this is a
misconfiguration, since this server is listed in a
"_dns-push-tls._tcp.<zone>" SRV record, but the server itself is
not currently configured to support DNS Push Notifications for
that zone. 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.
This is a misconfiguration, since this server is listed in a
"_dns-push-tls._tcp.<zone>" SRV record, but the server itself is
not currently configured to support DNS Push Notifications for
that zone. 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 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 send a DNS Push Notification Retry Delay
Operation TLV instructing the client to close the session, as
described in the DSO specification [DSO]. Clients MUST correctly
handle both cases.
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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 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.
6.3.1. PUSH Message
A PUSH message begins with the standard DSO 12-byte header [DSO],
followed by the PUSH TLV. A PUSH message is illustrated in Figure 2.
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 also be set as described in the DSO
specification [DSO]. The DNS Opcode is the DSO Opcode. The four
count fields MUST be zero, and the corresponding four sections MUST
be empty (i.e., absent).
The DSO-TYPE is PUSH (tentatively 0x41). The DSO-LENGTH is the
length of the DSO-DATA that follows, which specifies the changes
being communicated.
The DSO-DATA contains one or more Update records. A PUSH Message
MUST contain at least one Update record. If a PUSH Message is
received that contains zero Update records, this is a fatal error,
and the receiver MUST immediately terminate the connection with a TCP
RST (or equivalent for other protocols). The Update records are
formatted in the customary way for Resource Records in DNS messages.
Update records in a PUSH Message are interpreted according to the
same rules as for DNS Update [RFC2136] messages, namely:
Delete all RRsets from a name:
TTL=0, CLASS=ANY, RDLENGTH=0, TYPE=ANY.
Delete an RRset from a name:
TTL=0, CLASS=ANY, RDLENGTH=0;
TYPE specifies the RRset being deleted.
Delete an individual RR from a name:
TTL=0, CLASS=NONE;
TYPE, RDLENGTH and RDATA specifies the RR being deleted.
Add to an RRset:
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TTL, CLASS, TYPE, RDLENGTH and RDATA specifies the RR being added.
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 | Z | RCODE | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| QDCOUNT (MUST BE ZERO) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER
| ANCOUNT (MUST BE ZERO) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| NSCOUNT (MUST BE ZERO) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| ARCOUNT (MUST BE ZERO) | /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ /
| DSO-TYPE = PUSH |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| DSO-LENGTH (number of octets in DSO-DATA) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \
\ NAME \ \
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| TYPE | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| CLASS | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| TTL | |
| (32 bits) | > DSO-DATA
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| RDLEN | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
\ RDATA \ |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
: NAME, TYPE, CLASS, TTL, RDLEN, RDATA : |
: Repeated As Necessary : /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ /
Figure 2: PUSH Message
When processing the records received in a PUSH Message, the receiving
client MUST validate that the records being added or deleted
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. If the TYPE in the SUBSCRIBE
request was not ANY (255) then the TYPE of the record must match the
TYPE given in the SUBSCRIBE request. If the CLASS in the SUBSCRIBE
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request was not ANY (255) then the CLASS of the record must match the
CLASS given in the SUBSCRIBE request. If a matching active
subscription on that session is not found, then that individual
record addition/deletion is silently ignored. Processing of other
additions and deletions 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.
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=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 deleted
from a given name, and no other AAAA records exist for that name, the
server SHOULD send a "delete an RRset from a name" PUSH message, not
three separate "delete an individual RR from a name" PUSH messages.
Similarly, when both an SRV and a TXT record are deleted from a given
name, and no other records of any kind exist for that name, the
server SHOULD send a "delete all RRsets from a name" PUSH message,
not two separate "delete an RRset from a name" PUSH messages.
A server SHOULD combine multiple change notifications in a single
PUSH message when possible, even if those change notifications apply
to different subscriptions. Conceptually, a PUSH message is a
session-level mechanism, not a subscription-level mechanism.
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 cancelled
(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.
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6.4. DNS Push Notification UNSUBSCRIBE
To cancel an individual subscription without closing the entire DSO
session, the client sends an UNSUBSCRIBE message over the established
DSO session to the server. The UNSUBSCRIBE message is encoded as a
DSO [DSO] unidirectional message. This specification defines a
primary unidirectional DSO TLV for DNS Push Notification UNSUBSCRIBE
Requests (tentatively DSO Type Code 0x42).
A server MUST NOT initiate an UNSUBSCRIBE request. If a server does
send an UNSUBSCRIBE request over a DSO session initiated by a client,
this is a fatal error and the client should immediately abort the
connection with a TCP RST (or equivalent for other protocols).
6.4.1. UNSUBSCRIBE Request
An UNSUBSCRIBE request begins with the standard DSO 12-byte header
[DSO], followed by the UNSUBSCRIBE TLV. An UNSUBSCRIBE request
message is illustrated in Figure 3.
The MESSAGE ID field MUST be zero. There is no server response to a
UNSUBSCRIBE message.
The other header fields MUST be set as described in the DSO
specification [DSO]. The DNS Opcode is the DSO Opcode. The four
count fields MUST be zero, and the corresponding four sections MUST
be empty (i.e., absent).
In the UNSUBSCRIBE TLV the DSO-TYPE is UNSUBSCRIBE. The DSO-LENGTH
is 2 octets.
The DSO-DATA contains the MESSAGE ID field of the value given in the
ID field of an active SUBSCRIBE request. This is how the server
knows which SUBSCRIBE request is being cancelled. After receipt of
the UNSUBSCRIBE request, the SUBSCRIBE request is no longer active.
It is allowable for the client to issue an UNSUBSCRIBE request 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 request.
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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 | Z | RCODE | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| QDCOUNT (MUST BE ZERO) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER
| ANCOUNT (MUST BE ZERO) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| NSCOUNT (MUST BE ZERO) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| ARCOUNT (MUST BE ZERO) | /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ /
| DSO-TYPE = UNSUBSCRIBE |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| DSO-LENGTH (2 octets) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \
| SUBSCRIBE MESSAGE ID | > DSO-DATA
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ /
Figure 3: UNSUBSCRIBE Request
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6.5. DNS Push Notification RECONFIRM
Sometimes, particularly when used with a Discovery Proxy [DisProx], a
DNS Zone may contain stale data. When a client encounters data that
it believe 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 request 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 requests to ascertain whether the target device is
still present. For other types of DNS server, the RECONFIRM
operation is currently undefined, and SHOULD result in a NOERROR
response, but otherwise need not cause any action to occur. Frequent
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 request, 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.
6.5.1. RECONFIRM Request
A RECONFIRM request begins with the standard DSO 12-byte header
[DSO], followed by the primary DSO RECONFIRM TLV. A RECONFIRM
request message is illustrated in Figure 4.
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 RECONFIRM response
the server MUST echo back the MESSAGE ID value unchanged.
The other header fields MUST be set as described in the DSO
specification [DSO]. The DNS Opcode is the DSO Opcode. The four
count fields MUST be zero, and the corresponding four sections MUST
be empty (i.e., absent).
The DSO-TYPE is RECONFIRM (tentatively 0x43). The DSO-LENGTH is the
length of the data that follows, which specifies the name, type,
class, and content of the record being disputed.
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1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \
| MESSAGE ID | \
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
|QR| Opcode | Z | RCODE | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| QDCOUNT (MUST BE ZERO) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > HEADER
| ANCOUNT (MUST BE ZERO) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| NSCOUNT (MUST BE ZERO) | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| ARCOUNT (MUST BE ZERO) | /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ /
| DSO-TYPE = RECONFIRM |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| DSO-LENGTH (number of octets in DSO-DATA) |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ \
\ NAME \ \
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
| TYPE | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ > DSO-DATA
| CLASS | |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ |
\ RDATA \ /
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ /
Figure 4: RECONFIRM Request
The DSO-DATA for a RECONFIRM request MUST contain exactly one record.
The DSO-DATA for a RECONFIRM request has no count field to specify
more than one record. Since RECONFIRM requests are sent over TCP,
multiple RECONFIRM request messages can be concatenated in a single
TCP stream and packed efficiently into TCP segments.
TYPE MUST NOT be the value ANY (255) and CLASS MUST NOT be the value
ANY (255).
DNS wildcarding is not supported. That is, a wildcard ("*") in a
RECONFIRM message matches only a literal wildcard character ("*") in
the zone, and nothing else.
Aliasing is not supported. That is, a CNAME in a RECONFIRM message
matches only a literal CNAME record in the zone, and nothing else.
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6.5.2. RECONFIRM Response
Each RECONFIRM request generates exactly one RECONFIRM response from
the server.
A RECONFIRM response message begins with the standard DSO 12-byte
header [DSO], possibly followed by one or more optional TLVs, such as
a Retry Delay TLV. For suggested values for the Retry Delay TLV, see
Section 6.2.2.
The MESSAGE ID field MUST echo the value given in the ID field of the
RECONFIRM request. This is how the client knows which request is
being responded to.
A RECONFIRM response message MUST NOT include a DSO RECONFIRM TLV.
If a client receives a RECONFIRM response message containing a
RECONFIRM TLV then the response message is processed but the
RECONFIRM TLV MUST be silently ignored.
In the RECONFIRM response the RCODE confirms receipt of the
reconfirmation request. Supported RCODEs are as follows:
+-----------+-------+-----------------------------------------------+
| Mnemonic | Value | Description |
+-----------+-------+-----------------------------------------------+
| NOERROR | 0 | RECONFIRM accepted. |
| 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 | RECONFIRM operation not supported. |
+-----------+-------+-----------------------------------------------+
Table 2: RECONFIRM Response codes
This document specifies only these RCODE values for RECONFIRM
Responses. Servers sending RECONFIRM Responses SHOULD use one of
these values. Note that NXDOMAIN is not a valid RCODE in response to
a RECONFIRM Request. However, future circumstances may create
situations where other RCODE values are appropriate in RECONFIRM
Responses, so clients MUST be prepared to accept RECONFIRM Responses
with any other RCODE value.
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Nonzero RCODE values signal some kind of error.
RCODE value FORMERR indicates a message format error, for example
TYPE or CLASS being ANY (255).
RCODE value SERVFAIL indicates that the server has exhausted its
resources or other serious problem occurred.
RCODE values NOTIMP indicates that the server does not support DSO,
and DSO is required for RECONFIRM requests.
RCODE value REFUSED indicates that the server supports RECONFIRM
requests but is currently not configured to accept them from this
client.
RCODE value NOTAUTH indicates that the server is not authoritative
for the requested name, and can do nothing to remedy the apparent
error. Note that there may be future cases in which a server is able
to pass on the RECONFIRM request to the ultimate source of the
information, and in these cases the server should return NOERROR.
RCODE value DSOTYPENI indicates that the server does not support
RECONFIRM requests.
Nonzero RCODE values SERVFAIL, REFUSED and DSOTYPENI are benign from
the client's point of view. The client may log them to aid in
debugging, but otherwise they require no special action.
Nonzero RCODE values other than these three indicate a serious
problem with the client. After sending an error response other than
one of these three, the server SHOULD send a DSO Retry Delay TLV to
end the DSO session, as described in the DSO specification [DSO].
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6.6. DNS Stateful Operations TLV Context Summary
This document defines four new DSO TLVs. As suggested in [DSO],
Section 8.2, the valid contexts of these new TLV types are summarized
below.
The client TLV contexts are:
C-P: Client primary TLV
C-U: Client primary unidirectional TLV
C-A: Client additional TLV
CRP: Client response primary TLV
CRA: Client response additional TLV
+-------------+-----+-----+-----+-----+-----+
| TLV Type | C-P | C-U | C-A | CRP | CRA |
+-------------+-----+-----+-----+-----+-----+
| SUBSCRIBE | X | | | | |
| PUSH | | | | | |
| UNSUBSCRIBE | | X | | | |
| RECONFIRM | X | | | | |
+-------------+-----+-----+-----+-----+-----+
Table 3: DSO TLV Client Context Summary
The server TLV contexts are:
S-P: Server primary TLV
S-U: Server primary unidirectional TLV
S-A: Server additional TLV
SRP: Server response primary TLV
SRA: Server response additional TLV
+-------------+-----+-----+-----+-----+-----+
| TLV Type | S-P | S-U | S-A | SRP | SRA |
+-------------+-----+-----+-----+-----+-----+
| SUBSCRIBE | | | | | |
| PUSH | | X | | | |
| UNSUBSCRIBE | | | | | |
| RECONFIRM | | | | | |
+-------------+-----+-----+-----+-----+-----+
Table 4: DSO TLV Server Context Summary
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6.7. Client-Initiated Termination
An individual subscription is terminated by sending an UNSUBSCRIBE
TLV for that specific subscription, or all subscriptions can be
cancelled 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 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 [DSO].
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
cancelled.
When closing a session, a client will generally do an abortive
disconnect, sending a TCP RST. This immediately discards all
remaining inbound and outbound data, which is appropriate if the
client no longer has any interest in this data. In the BSD Sockets
API, sending a TCP RST is achieved by setting the SO_LINGER option
with a time of 0 seconds and then closing the socket.
If a client has performed operations on this session that it would
not want lost (like DNS updates) then the client SHOULD do an orderly
disconnect, sending a TLS close_notify followed by a TCP FIN. (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.)
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7. Security Considerations
The Strict Privacy Usage Profile for DNS over TLS is REQUIRED for DNS
Push Notifications as defined in "Usage Profiles for DNS over TLS and
DNS over DTLS" [RFC8310]. Cleartext connections for DNS Push
Notifications are not permissible. Since this is a new protocol,
transition mechanisms from the Opportunistic Privacy profile are
deemed unnecessary.
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 provided 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 end-point of the TLS communication is
authenticated as the intended entity to communicate with.
Deployment recommendations on the appropriate key lengths and cypher
suites are beyond the scope of this document. Please refer to TLS
Recommendations [RFC7525] 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 Sever
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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 useable, 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. Also note that a
DNS Push server is an authoritative server and a DNS Push client is a
standard DNS client. While the terminology in Usage Profiles for DNS
over TLS and DNS over DTLS [RFC8310] explicitly states it does not
apply to authoritative servers, it does in this case apply to DNS
Push Notification clients and servers.
7.3. TLS Compression
In order to reduce the chances of compression-related attacks, TLS-
level compression SHOULD be disabled when using TLS versions 1.2 and
earlier. In TLS 1.3 [RFC8446], TLS-level compression has been
removed completely.
7.4. TLS Session Resumption
TLS Session Resumption is permissible on DNS Push Notification
servers. The server may keep TLS state with Session IDs [RFC5246] or
operate in stateless mode by sending a Session Ticket [RFC5077] to
the client for it to store. 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.
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8. IANA Considerations
This document defines a new service name to be published in the IANA
Registry Service Types [RFC6335][ST] that is only applicable for the
TCP protocol.
+-----------------------+------+----------------------+-------------+
| Name | Port | Value | Definition |
+-----------------------+------+----------------------+-------------+
| DNS Push Notification | None | "_dns-push-tls._tcp" | Section 6.1 |
| Service Type | | | |
+-----------------------+------+----------------------+-------------+
Table 5: IANA Service Type Assignments
This document also defines four new DNS Stateful Operation TLV types
to be recorded in the IANA DSO Type Code Registry.
+-------------+------------------------+---------------+
| Name | Value | Definition |
+-------------+------------------------+---------------+
| SUBSCRIBE | TBA (tentatively 0x40) | Section 6.2 |
| PUSH | TBA (tentatively 0x41) | Section 6.3.1 |
| UNSUBSCRIBE | TBA (tentatively 0x42) | Section 6.4 |
| RECONFIRM | TBA (tentatively 0x43) | Section 6.5.1 |
+-------------+------------------------+---------------+
Table 6: IANA DSO TLV Type Code Assignments
9. Acknowledgements
The authors would like to thank Kiren Sekar and Marc Krochmal for
previous work completed in this field.
This draft has been improved due to comments from Ran Atkinson, Tim
Chown, Mark Delany, Ralph Droms, Bernie Volz, Jan Komissar, Manju
Shankar Rao, Markus Stenberg, Dave Thaler, Soraia Zlatkovic, Sara
Dickinson, and Andrew Sullivan. Ted Lemon provided clarifying text
that was greatly appreciated.
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10. References
10.1. Normative References
[DSO] Bellis, R., Cheshire, S., Dickinson, J., Dickinson, S.,
Lemon, T., and T. Pusateri, "DNS Stateful Operations",
draft-ietf-dnsop-session-signal-18 (work in progress),
October 2018.
[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>.
[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>.
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[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.
[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>.
[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>.
[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>.
[ST] "Service Name and Transport Protocol Port Number
Registry", <http://www.iana.org/assignments/
service-names-port-numbers/>.
10.2. Informative References
[DisProx] Cheshire, S., "Discovery Proxy for Multicast DNS-Based
Service Discovery", draft-ietf-dnssd-hybrid-08 (work in
progress), March 2018.
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[I-D.dukkipati-tcpm-tcp-loss-probe]
Dukkipati, N., Cardwell, N., Cheng, Y., and M. Mathis,
"Tail Loss Probe (TLP): An Algorithm for Fast Recovery of
Tail Losses", draft-dukkipati-tcpm-tcp-loss-probe-01 (work
in progress), February 2013.
[LLQ] Sekar, K., "DNS Long-Lived Queries", draft-sekar-dns-
llq-01 (work in progress), August 2006.
[obs] "Observer Pattern",
<https://en.wikipedia.org/wiki/Observer_pattern>.
[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>.
[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>.
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
January 2008, <https://www.rfc-editor.org/info/rfc5077>.
[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>.
[RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
"TCP Extensions for Multipath Operation with Multiple
Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013,
<https://www.rfc-editor.org/info/rfc6824>.
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[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>.
[RFC7525] 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, DOI 10.17487/RFC7525, May
2015, <https://www.rfc-editor.org/info/rfc7525>.
[RFC7719] Hoffman, P., Sullivan, A., and K. Fujiwara, "DNS
Terminology", RFC 7719, DOI 10.17487/RFC7719, December
2015, <https://www.rfc-editor.org/info/rfc7719>.
[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>.
[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>.
[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>.
[SYN] Eddy, W., "Defenses Against TCP SYN Flooding Attacks", The
Internet Protocol Journal, Cisco Systems, Volume 9,
Number 4, December 2006.
[XEP0060] Millard, P., Saint-Andre, P., and R. Meijer, "Publish-
Subscribe", XSF XEP 0060, July 2010.
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Authors' Addresses
Tom Pusateri
Unaffiliated
Raleigh, NC 27608
USA
Phone: +1 919 867 1330
Email: pusateri@bangj.com
Stuart Cheshire
Apple Inc.
1 Infinite Loop
Cupertino, CA 95014
USA
Phone: +1 408 974 3207
Email: cheshire@apple.com
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