DNS Push Notifications
draft-ietf-dnssd-push-10
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 8765.
|
|
|---|---|---|---|
| Authors | Tom Pusateri , Stuart Cheshire | ||
| Last updated | 2017-03-21 (Latest revision 2017-03-13) | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
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| Stream | WG state | In WG Last Call | |
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| IESG | IESG state | Became RFC 8765 (Proposed Standard) | |
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draft-ietf-dnssd-push-10
Internet Engineering Task Force T. Pusateri
Internet-Draft Seeking affiliation
Intended status: Standards Track S. Cheshire
Expires: September 14, 2017 Apple Inc.
March 13, 2017
DNS Push Notifications
draft-ietf-dnssd-push-10
Abstract
The Domain Name System (DNS) was designed to return matching records
efficiently for queries for data that is relatively static. When
those records change frequently, DNS is still efficient at returning
the updated results when polled. But there exists no mechanism
for a client to be asynchronously notified when these changes occur.
This document defines a mechanism for a client to be notified
of such changes to DNS records, called DNS Push Notifications.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 14, 2017.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
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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 . . . . . . . . . . . . . 12
6.2.1. SUBSCRIBE Request . . . . . . . . . . . . . . . . . . 13
6.2.2. SUBSCRIBE Response . . . . . . . . . . . . . . . . . 15
6.3. DNS Push Notification Updates . . . . . . . . . . . . . . 18
6.3.1. PUSH Message . . . . . . . . . . . . . . . . . . . . 19
6.3.2. PUSH Response . . . . . . . . . . . . . . . . . . . . 21
6.4. DNS Push Notification UNSUBSCRIBE . . . . . . . . . . . . 22
6.4.1. UNSUBSCRIBE Request . . . . . . . . . . . . . . . . . 23
6.4.2. UNSUBSCRIBE Response . . . . . . . . . . . . . . . . 24
6.5. DNS Push Notification RECONFIRM . . . . . . . . . . . . . 26
6.5.1. RECONFIRM Request . . . . . . . . . . . . . . . . . . 26
6.5.2. RECONFIRM Response . . . . . . . . . . . . . . . . . 28
6.6. Client-Initiated Termination . . . . . . . . . . . . . . 30
7. Security Considerations . . . . . . . . . . . . . . . . . . . 31
7.1. Security Services . . . . . . . . . . . . . . . . . . . . 31
7.2. TLS Name Authentication . . . . . . . . . . . . . . . . . 31
7.3. TLS Compression . . . . . . . . . . . . . . . . . . . . . 32
7.4. TLS Session Resumption . . . . . . . . . . . . . . . . . 32
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 32
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 33
10.1. Normative References . . . . . . . . . . . . . . . . . . 33
10.2. Informative References . . . . . . . . . . . . . . . . . 34
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 36
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1. Introduction
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" [RFC2119].
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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 to state changes
following the Observer design pattern [obs]. XMPP Publish-Subscribe
[XEP0060] and Atom [RFC4287] are examples. While DNS servers are
generally highly tuned and capable of a high rate of query/response
traffic, adding a publish/subscribe model for tracking changes to DNS
records can result in more timely notification of changes with
reduced CPU usage and lower network traffic.
Multicast DNS [RFC6762] implementations always listen on a well known
link-local IP multicast group, and new services and updates are sent
for all group members to receive. Therefore, Multicast DNS already
has asynchronous change notification capability. However, when DNS
Service Discovery [RFC6763] is used across a wide area network using
Unicast DNS (possibly facilitated via a 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) [I-D.sekar-dns-llq] mechanism is an
existing deployed solution to provide asynchronous change
notifications, used by Apple's Back to My Mac Service [RFC6281].
Back to My Mac was designed in an era when the data centre operations
staff asserted that it was impossible for a server to handle large
numbers of mostly-idle TCP connections, so LLQ had to 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
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 [SessSig]. Instead of inventing a new
vocabulary of messages to communicate DNS zone changes as LLQ did,
this specification adopts the 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 updates to these records, it unsubscribes.
The DNS Push Notification server for a zone is any server capable
of generating the correct change notifications for a name.
It may be a master, slave, or stealth name server [RFC1996].
Consequently, the "_dns-push-tls._tcp.<zone>" SRV record for a
zone MAY reference the same target host and port as that zone's
"_dns-update-tls._tcp.<zone>" SRV record. When the same target host
and port is offered for both DNS Updates and DNS Push Notifications,
a client MAY use a single TCP connection to that server for both DNS
Updates and DNS Push Notification Queries.
Supporting DNS Updates and DNS Push Notifications on the same server
is OPTIONAL. A DNS Push Notification server does NOT also have to
support DNS Update.
DNS Updates and DNS Push Notifications may be handled on different
ports on the same target host, in which case they are not considered
to be the "same server" for the purposes of this specification, and
communications with these two ports are handled independently.
Standard DNS Queries MAY be sent over a DNS Push Notification
connection, provided that these are queries for names falling within
the server's zone (the <zone> in the "_dns-push-tls._tcp.<zone>" SRV
record). The RD (Recursion Desired) bit MUST be zero.
DNS Push Notification clients are NOT required to implement DNS
Update Prerequisite processing. Prerequisites are used to perform
tentative atomic test-and-set type operations when a client updates
records on a server, and that concept has no applicability when it
comes to an authoritative server informing a client of changes to DNS
records.
This DNS Push Notification specification includes support for DNS
classes, for completeness. However, in practice, it is anticipated
that for the foreseeable future the only DNS class in use will be DNS
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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".
DNS Push Notifications impose less load on the responding server than
rapid polling would, but Push Notifications do still have a cost, so
DNS Push Notification clients MUST NOT recklessly create an excessive
number of Push Notification subscriptions. A subscription SHOULD
only be active when there is a valid reason to need live data (for
example, an on-screen display is currently showing the results to the
user) and the subscription SHOULD be cancelled as soon as the need
for that data ends (for example, when the user dismisses that
display). Implementations MAY want to implement idle timeouts, so
that if the user ceases interacting with the device, the display
showing the result of the DNS Push Notification subscription is
automatically dismissed after a certain period of inactivity. For
example, if a user presses the "Print" button on their smartphone,
and then leaves the phone showing the printer discovery screen until
the phone goes to sleep, then the printer discovery screen should be
automatically dismissed as the device goes to sleep. If the user
does still intend to print, this will require them to press the
"Print" button again when they wake their phone up.
A DNS Push Notification client MUST NOT routinely keep a DNS Push
Notification subscription active 24 hours a day, 7 days a week, just
to keep a list in memory up to date so that 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 Session Signaling specification
[SessSig], a client MUST NOT keep a connection to a server open
indefinitely if it has no subscriptions (or other operations) active
on that connection. A client MAY close a connection as soon as it
becomes idle, and then if needed in the future, open a new connection
when required. Alternatively, a client MAY speculatively keep an
idle connection open for some time, subject to the constraint that it
MUST NOT keep a connection open that has been idle for more than the
session's idle timeout (15 seconds by default).
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4. Transport
Implementations of DNS Update [RFC2136] MAY use either User Datagram
Protocol (UDP) [RFC0768] or Transmission Control Protocol (TCP)
[RFC0793] as the transport protocol, in keeping with the historical
precedent that DNS queries must first be sent over UDP [RFC1123].
This requirement to use UDP has subsequently been relaxed [RFC7766].
In keeping with the more recent precedent, DNS Push Notification is
defined only for TCP. DNS Push Notification clients MUST use TLS
over TCP.
Connection setup over TCP ensures return reachability and alleviates
concerns of state overload at the server through anonymous
subscriptions. All subscribers are guaranteed to be reachable by the
server by virtue of the TCP three-way handshake. Flooding attacks
are possible with any protocol, and a benefit of TCP is that there
are already established industry best practices to guard against SYN
flooding and similar attacks [IPJ.9-4-TCPSYN] [RFC4953].
Use of TCP also allows DNS Push Notifications to take advantage of
current and future developments in TCP, such as Multipath TCP (MPTCP)
[RFC6824], TCP Fast Open (TFO) [RFC7413], Tail Loss Probe (TLP)
[I-D.dukkipati-tcpm-tcp-loss-probe], and so on.
Transport Layer Security (TLS) [RFC5246] is well understood and
deployed across many protocols running over TCP. It is designed to
prevent eavesdropping, tampering, or message forgery. TLS is
REQUIRED for every connection between a client subscriber and server
in this protocol specification. Additional security measures such as
client authentication during TLS negotiation MAY also be employed to
increase the trust relationship between client and server.
Additional authentication of the SRV target using DNSSEC verification
and DANE TLSA records [RFC7673] is strongly encouraged. See below in
Section 7.2 for details.
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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 connection 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
connections to alternate DNS servers that support DNS Push
Notifications for the zone and distribute subscriptions at its
discretion. In this way, both clients and servers can react to
resource constraints. Token bucket rate limiting schemes are also
effective in providing fairness by a server across numerous client
requests.
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6. Protocol Operation
The DNS Push Notification protocol is a session-oriented protocol,
and makes use of DNS Session Signaling [SessSig].
For details of the DNS Session Signaling message format refer to the
DNS Session Signaling specification [SessSig]. Those details are not
repeated here.
DNS Push Notification clients and servers MUST support DNS Session
Signaling, but the server MUST NOT issue any DNS Session Signaling
operations until after the client has first initiated a DNS Session
Signaling operation of its own. A single server can support DNS
Queries, DNS Updates, and DNS Push Notifications (using DNS Session
Signaling) on the same TCP port, and until the client has sent at
least one DNS Session Signaling operation the server does not know
what kind of client has connected to it. Once the client has
indicated willingness to use DNS Session Signaling operations by
sending one of its own, either side of the connection may then
initiate further Session Signaling operations at any time.
A DNS Push Notification exchange begins with the client discovering
the appropriate server, using the procedure described in Section 6.1,
and then making a TLS/TCP connection to it.
A typical DNS Push Notification client will immediately issue a DNS
Session Signaling Keepalive operation to request a session timeout or
keepalive interval longer than the the 15-second defaults, but this
is NOT REQUIRED. A DNS Push Notification client MAY issue other
requests on the connection first, and only issue a DNS Session
Signaling Keepalive operation later if it determines that to be
necessary.
Once the connection is made, the client may then add and remove Push
Notification subscriptions. In accordance with the current set of
active subscriptions the server sends relevant asynchronous Push
Notifications to the client. Note that a client MUST be prepared to
receive (and silently ignore) Push Notifications for subscriptions it
has previously removed, since there is no way to prevent the
situation where a Push Notification is in flight from server to
client while the client's UNSUBSCRIBE message cancelling that
subscription is simultaneously in flight from client to server.
The exchange between client and server terminates when either end
closes the TCP connection with a TCP FIN or RST.
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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 MUST also determine which TCP port on
the server is listening for connections, which need not be (and often
is not) the typical TCP port 53 used for conventional DNS, or TCP
port 853 used for DNS over TLS [RFC7858].
1. The client begins the discovery by sending a DNS query to its
local resolver, with record type SOA [RFC1035], for the domain
name to which it wishes to subscribe.
2. If the SOA record exists, it MUST be returned in the Answer
Section of the response. If not, the local resolver SHOULD
include the SOA record for the zone of the requested name in the
Authority Section.
3. If no SOA record is returned, the client then strips off the
leading label from the requested name. If the resulting name has
at least one label in it, the client sends a new SOA query and
processing continues at step 2 above. If the resulting name is
empty (the root label) then this is a network configuration error
and the client gives up. The client MAY retry the operation at a
later time, of the client's choosing, such after a change in
network attachment.
4. Once the SOA is known (either by virtue of being seen in the
Answer Section, or in the Authority Section), the client sends a
DNS query with type SRV [RFC2782] for the record name
"_dns-push-tls._tcp.<zone>", where <zone> is the owner name of
the discovered SOA record.
5. If the zone in question does not offer DNS Push Notifications
then SRV record MUST NOT exist and the SRV query will return a
negative answer.
6. If the zone in question is set up to offer DNS Push Notifications
then this SRV record MUST exist. The SRV "target" contains the
name of the server providing DNS Push Notifications for the zone.
The port number on which to contact the server is in the SRV
record "port" field. The address(es) of the target host MAY be
included in the Additional Section, however, the address records
SHOULD be authenticated before use as described below in
Section 7.2 [RFC7673].
7. More than one SRV record may be returned. In this case, the
"priority" and "weight" values in the returned SRV records are
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used to determine the order in which to contact the servers for
subscription requests. As described in the SRV specification
[RFC2782], the server with the lowest "priority" is first
contacted. If more than one server has the same "priority", the
"weight" indicates the weighted probability that the client
should contact that server. Higher weights have higher
probabilities of being selected. If a server is not reachable or
is not willing to accept a subscription request, then a
subsequent server is to be contacted.
Each time a client makes a new DNS Push Notification subscription
connection, it SHOULD repeat the discovery process in order to
determine the preferred DNS server for subscriptions at that time.
Note that this repeated discovery step is typically very fast and
typically results in no queries on the network. The client device
MUST respect the DNS TTL values on records it receives, and store
them in its local cache with this lifetime. This means that, as long
as the DNS TTL values on the authoritative records were set to
reasonable values, repeated application of this discovery process can
be completed nearly instantaneously by the client, using only
locally-stored 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 TLS
connection to the server. A SUBSCRIBE request is encoded in a DNS
Session Signaling [SessSig] message. This specification defines a
DNS Session Signaling TLV for DNS Push Notification SUBSCRIBE
Requests/Responses (tentatively Session Signaling Type Code 0x40).
A server MUST NOT initiate a SUBSCRIBE request.
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6.2.1. SUBSCRIBE Request
A SUBSCRIBE request message begins with the standard DNS Session
Signaling 12-byte header [SessSig], followed by the SUBSCRIBE TLV.
The SSOP-DATA for the the SUBSCRIBE TLV is as follows:
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| |
\ NAME \
\ \
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| TYPE |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| CLASS |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
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 connection. For
the purposes here, a MESSAGE ID is in use on this connection if the
client has used it in a request for which it has not yet received a
response, or if 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.
In the SUBSCRIBE TLV the SSOP-TYPE is SUBSCRIBE (tentatively 0x40).
The SSOP-LENGTH is the length of the SSOP-DATA that follows, which
specifies the name, type, and class of the record(s) being sought.
A SUBSCRIBE request MUST contain exactly one question. The SUBSCRIBE
TLV has no QDCOUNT field to specify more than one question. 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 connection
between the client and the server is closed.
SUBSCRIBE requests on a given connection 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 TLS/TCP connection.
For the purpose of this matching, the established DNS case-
insensitivity for US-ASCII letters applies (e.g., "foo.com" and
"Foo.com" are the same). If a server receives such a duplicate
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SUBSCRIBE message this is an error and the server MUST immediately
immediately terminate the connection with a TCP RST (or equivalent
for other protocols).
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, were 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 DNS Session
Signaling 12-byte header [SessSig], possibly followed by one or more
optional Modifier TLVs, such as a Retry Delay Modifier 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 contain a Session Signaling
Operation TLV. The Session Signaling Operation TLV is NOT copied
from the SUBSCRIBE request.
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 |
| | | resource exhaustion. |
| NXDOMAIN | 3 | NOT APPLICABLE. DNS Push Notification |
| | | servers MUST NOT return NXDOMAIN errors in |
| | | response to SUBSCRIBE requests. |
| NOTIMP | 4 | Server does not recognize DNS Session |
| | | Signaling Opcode. |
| REFUSED | 5 | Server refuses to process request for policy |
| | | or security reasons. |
| NOTAUTH | 9 | Server is not authoritative for the |
| | | requested name. |
| SSOPNOTIMP | 11 | SUBSCRIBE operation not supported. |
+------------+-------+----------------------------------------------+
SUBSCRIBE Response codes
This document specifies only these RCODE values for SUBSCRIBE
Responses. Servers sending SUBSCRIBE Responses SHOULD use one of
these values. 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 RCODE
value.
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If the server sends a nonzero RCODE in the SUBSCRIBE response, either
the client is (at least partially) misconfigured or the server
resources are exhausted. In either case, the client shouldn't retry
the subscription right away. Either end can terminate the
connection, but the client may want to try this subscription again or
it may have other successful subscriptions that it doesn't want to
abandon. If the server sends a nonzero RCODE then it SHOULD append a
Retry Delay Modifier TLV [SessSig] 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:
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), which occurs due to resource exhaustion,
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.
For RCODE = 4 (NOTIMP), which occurs on a server that doesn't
implement DNS Session Signaling [SessSig], it is unlikely that the
server will begin supporting DNS Session Signaling in the next few
minutes, so the retry delay SHOULD be one hour.
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.
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. 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.
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For RCODE = 11 (DNS Push SUBSCRIBE operation not supported), 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 connection
to remain open, or MAY send a DNS Push Notification Retry Delay
Operation TLV and then close the TCP connection, as described in the
DNS Session Signaling specification [SessSig]. 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.
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6.3.1. PUSH Message
A PUSH message begins with the standard DNS Session Signaling 12-byte
header [SessSig], followed by the PUSH TLV.
The MESSAGE ID field MUST be set to a unique value, that the server
is not currently using for any other active outgoing request that it
has sent on this connection. The MESSAGE ID in the outgoing PUSH
message is selected by the server and has no relationship to the
MESSAGE ID in any of the client subscriptions it may relate to. In
the PUSH response the client MUST echo back the MESSAGE ID value
unchanged.
In the PUSH TLV the SSOP-TYPE is PUSH (tentatively 0x41). The SSOP-
LENGTH is the length of the SSOP-DATA that follows, which specifies
the changes being communicated.
The SSOP-DATA contains one or more Update records, in customary
Resource Record format, as used in DNS Update [RFC2136] messages. A
PUSH Message MUST contain at least one Update record. If a PUSH
Message is received that contains no 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 SSOP-DATA contains the relevant change information for the
client, formatted identically to a DNS Update [RFC2136]. To recap:
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:
TTL, CLASS, TYPE, RDLENGTH and RDATA specifies the RR being added.
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
connection. 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
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TYPE given in the SUBSCRIBE request. If the CLASS in the SUBSCRIBE
request was not ANY (255) then the CLASS of the record must match the
CLASS given in the SUBSCRIBE request. If a matching active
subscription on that connection 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 TCP
connection 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
connection-level mechanism, not a subscription-level mechanism.
Reception of a PUSH message by a client generates a PUSH response
back to the server.
The TTL of an added record is stored by the client and decremented as
time passes, with the caveat that for as long as a relevant
subscription is active, the TTL does not decrement below 1 second.
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 TCP connection being closed)
record ageing resumes and records are removed from the local cache
when their TTL reaches zero.
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6.3.2. PUSH Response
Each PUSH message generates exactly one PUSH response from the
receiver.
A PUSH response message begins with the standard DNS Session
Signaling 12-byte header [SessSig], possibly followed by one or more
optional Modifier TLVs, such as a Retry Delay Modifier TLV.
The MESSAGE ID field MUST echo the value given in the ID field of the
PUSH message.
A PUSH response message MUST NOT contain a Session Signaling
Operation TLV. The Session Signaling Operation TLV is NOT copied
from the PUSH message.
In a PUSH response the RCODE MUST be zero. Receiving a PUSH response
with a nonzero RCODE is a fatal error, and the receiver MUST
immediately terminate the connection with a TCP RST (or equivalent
for other protocols).
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6.4. DNS Push Notification UNSUBSCRIBE
To cancel an individual subscription without closing the entire
connection, the client sends an UNSUBSCRIBE message over the
established TCP connection to the server. The UNSUBSCRIBE message is
encoded in a DNS Session Signaling [SessSig] message. This
specification defines a DNS Session Signaling TLV for DNS Push
Notification UNSUBSCRIBE Requests/Responses (tentatively Session
Signaling Type Code 0x42).
A server MUST NOT initiate an UNSUBSCRIBE request.
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6.4.1. UNSUBSCRIBE Request
An UNSUBSCRIBE request message begins with the standard DNS Session
Signaling 12-byte header [SessSig], followed by the UNSUBSCRIBE TLV.
In the UNSUBSCRIBE TLV the SSOP-TYPE is UNSUBSCRIBE (tentatively
0x42). The SSOP-LENGTH is zero. There is no SSOP-DATA for
UNSUBSCRIBE.
The MESSAGE ID field MUST match 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. If a
server receives an UNSUBSCRIBE message where the MESSAGE ID does not
match the ID of an active SUBSCRIBE request the server MUST return a
response containing RCODE = 3 (NXDOMAIN).
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. A consequence of
this is that if the client issues an UNSUBSCRIBE request for an as-
yet unacknowledged SUBSCRIBE request, and the SUBSCRIBE request is
subsequently unsuccessful for some reason, then when the UNSUBSCRIBE
request is eventually processed it will be an UNSUBSCRIBE request for
a nonexistent subscription, which will result NXDOMAIN response.
Note that when the client issues an UNSUBSCRIBE request for an as-yet
unacknowledged SUBSCRIBE request, at that moment the client will have
two outstanding DNS Session Signaling operations with same MESSAGE
ID, a SUBSCRIBE request and an UNSUBSCRIBE request, which will both
receive responses, in that order. When the client has multiple
outstanding DNS Session Signaling operations with same MESSAGE ID,
care should be taken that when a DNS Session Signaling response
message is received for that MESSAGE ID, it is associated with the
*first* unacknowledged request.
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6.4.2. UNSUBSCRIBE Response
Each UNSUBSCRIBE request generates exactly one UNSUBSCRIBE response
from the server.
An UNSUBSCRIBE response message begins with the standard DNS Session
Signaling 12-byte header [SessSig], possibly followed by one or more
optional Modifier TLVs, such as a Retry Delay Modifier TLV.
The MESSAGE ID field MUST echo the value given in the ID field of the
UNSUBSCRIBE request. This is how the client knows which request is
being responded to.
An UNSUBSCRIBE response message MUST NOT contain a Session Signaling
Operation TLV. The Session Signaling Operation TLV is NOT copied
from the UNSUBSCRIBE request.
In the UNSUBSCRIBE response the RCODE indicates whether or not the
unsubscribe request was successful. Supported RCODEs are as follows:
+------------+-------+----------------------------------------------+
| Mnemonic | Value | Description |
+------------+-------+----------------------------------------------+
| NOERROR | 0 | UNSUBSCRIBE successful. |
| FORMERR | 1 | Server failed to process request due to a |
| | | malformed request. |
| NXDOMAIN | 3 | Specified subscription does not exist. |
| NOTIMP | 4 | Server does not recognize DNS Session |
| | | Signaling Opcode. |
| SSOPNOTIMP | 11 | UNSUBSCRIBE operation not supported. |
+------------+-------+----------------------------------------------+
UNSUBSCRIBE Response codes
This document specifies only these RCODE values for UNSUBSCRIBE
Responses. Servers sending UNSUBSCRIBE Responses SHOULD use one of
these values. However, future circumstances may create situations
where other RCODE values are appropriate in UNSUBSCRIBE Responses, so
clients MUST be prepared to accept UNSUBSCRIBE Responses with any
RCODE value.
Having being successfully revoked with a correctly-formatted
UNSUBSCRIBE message (resulting in a response with RCODE NOERROR) the
previously referenced subscription is no longer active and the server
MAY discard the state associated with it immediately, or later, at
the server's discretion.
Nonzero RCODE values signal some kind of error.
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RCODE value FORMERR indicates a message format error.
RCODE value NXDOMAIN indicates a MESSAGE ID that does not correspond
to any active subscription.
RCODE values NOTIMP and SSOPNOTIMP should not occur in practice.
A server would only generate NOTIMP if it did not support Session
Signaling, and if the server does not support Session Signaling then
it should not be possible for a client to have an active subscription
to cancel.
Similarly, a server would only generate SSOPNOTIMP if it did not
support Push Notifications, and if the server does not support Push
Notifications then it should not be possible for a client to have an
active subscription to cancel.
Nonzero RCODE values other than NXDOMAIN indicate a serious problem
with the client. After sending an error response other than
NXDOMAIN, the server SHOULD send a DNS Session Signaling Retry Delay
Operation TLV and then close the TCP connection, as described in the
DNS Session Signaling specification [SessSig].
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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 message begins with the standard DNS Session
Signaling 12-byte header [SessSig], followed by the RECONFIRM TLV.
The SSOP-DATA for the the RECONFIRM TLV is as follows:
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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
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| |
\ NAME \
\ \
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| TYPE |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| CLASS |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| RDLEN |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| |
\ RDATA \
\ \
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
Figure 2
The MESSAGE ID field MUST be set to a unique value, that the client
is not using for any other active operation on this connection. For
the purposes here, a MESSAGE ID is in use on this connection if the
client has used it in a request for which it has not yet received a
response, or if 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.
In the RECONFIRM TLV the SSOP-TYPE is RECONFIRM (tentatively 0x43).
The SSOP-LENGTH is the length of the data that follows, which
specifies the name, type, class, and content of the record being
disputed.
A RECONFIRM request MUST contain exactly one record. The RECONFIRM
TLV 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 DNS Session
Signaling 12-byte header [SessSig], possibly followed by one or more
optional Modifier TLVs, such as a Retry Delay Modifier TLV.
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 contain a Session Signaling
Operation TLV. The Session Signaling Operation TLV is NOT copied
from the RECONFIRM request.
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 |
| | | resource exhaustion. |
| NXDOMAIN | 3 | NOT APPLICABLE. DNS Push Notification |
| | | servers MUST NOT return NXDOMAIN errors in |
| | | response to RECONFIRM requests. |
| NOTIMP | 4 | Server does not recognize DNS Session |
| | | Signaling Opcode. |
| REFUSED | 5 | Server refuses to process request for policy |
| | | or security reasons. |
| NOTAUTH | 9 | Server is not authoritative for the |
| | | requested name. |
| SSOPNOTIMP | 11 | RECONFIRM operation not supported. |
+------------+-------+----------------------------------------------+
RECONFIRM Response codes
This document specifies only these RCODE values for RECONFIRM
Responses. Servers sending RECONFIRM Responses SHOULD use one of
these values. 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 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 is overloaded.
RCODE values NOTIMP indicates that the server does not support
Session Signaling, and Session Signaling 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 SSOPNOTIMP indicates that the server does not support
RECONFIRM requests.
Similarly, a server would only generate SSOPNOTIMP if it did not
support Push Notifications, and if the server does not support Push
Notifications then it should not be possible for a client to have an
active subscription to cancel.
Nonzero RCODE values SERVFAIL, REFUSED and SSOPNOTIMP 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 DNS Session Signaling
Retry Delay Operation TLV and then close the TCP connection, as
described in the DNS Session Signaling specification [SessSig].
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6.6. 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 connection. When a
client terminates an individual subscription (via UNSUBSCRIBE) or all
subscriptions on that connection (by closing the connection) 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 connection via
UNSUBSCRIBE, a client MAY close the connection immediately, or it may
keep it open if it anticipates performing further operations on that
connection in the future. If a client wishes to keep an idle
connection open, it MUST respect the maximum idle time required by
the server [SessSig].
If a client plans to terminate one or more subscriptions on a
connection and doesn't intend to keep that connection open, then as
an efficiency optimization it MAY instead choose to simply close the
connection, which implicitly terminates all subscriptions on that
connection. 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 connection has been cancelled.
When closing a connection, 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 connection that it would
not want lost (like DNS updates) then the client SHOULD do an orderly
disconnect, sending 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
TLS support is REQUIRED in DNS Push Notifications. There is no
provision for opportunistic encryption using a mechanism like
"STARTTLS".
DNSSEC is RECOMMENDED for DNS Push Notifications. 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
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
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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.
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 the draft version of TLS 1.3 [I-D.ietf-tls-tls13], 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, once the connection is closed,
any existing subscriptions will be dropped. 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 connection.
Use of TLS Session Resumption allows a new TLS connection to be set
up more quickly, but the client will still have to recreate any
desired subscriptions.
8. IANA Considerations
This document defines the service name: "_dns-push-tls._tcp".
It is only applicable for the TCP protocol.
This name is to be published in the IANA Service Name Registry
[RFC6335][SN].
This document defines three DNS Session Signaling TLV types:
SUBSCRIBE with (tentative) value 0x40 (64), PUSH with (tentative)
value 0x41 (65), UNSUBSCRIBE with (tentative) value 0x42 (66), and
RECONFIRM with (tentative) value 0x43 (67).
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, and Soraia Zlatkovic.
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10. References
10.1. Normative References
[I-D.ietf-tls-tls13]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-18 (work in progress),
October 2016.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980,
<http://www.rfc-editor.org/info/rfc768>.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<http://www.rfc-editor.org/info/rfc793>.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<http://www.rfc-editor.org/info/rfc1034>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://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,
<http://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,
<http://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,
<http://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,
<http://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,
<http://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,
<http://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,
<http://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, <http://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, <http://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,
<http://www.rfc-editor.org/info/rfc7766>.
[SessSig] Bellis, R., Cheshire, S., Dickinson, J., Dickinson, S.,
Mankin, A., and T. Pusateri, "DNS Session Signaling",
draft-ietf-dnsop-session-signal-02 (work in progress),
March 2017.
[SN] "Service Name and Transport Protocol Port Number
Registry", <http://www.iana.org/assignments/
service-names-port-numbers/>.
10.2. Informative References
[DisProx] Cheshire, S., "Hybrid Unicast/Multicast DNS-Based Service
Discovery", draft-ietf-dnssd-hybrid-06 (work in progress),
March 2017.
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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.
[I-D.sekar-dns-llq]
Sekar, K., "DNS Long-Lived Queries", draft-sekar-dns-
llq-01 (work in progress), August 2006.
[IPJ.9-4-TCPSYN]
Eddy, W., "Defenses Against TCP SYN Flooding Attacks", The
Internet Protocol Journal, Cisco Systems, Volume 9,
Number 4, December 2006.
[obs] "Observer Pattern", <https://en.wikipedia.org/wiki/
Observer_pattern>.
[RFC1996] Vixie, P., "A Mechanism for Prompt Notification of Zone
Changes (DNS NOTIFY)", RFC 1996, DOI 10.17487/RFC1996,
August 1996, <http://www.rfc-editor.org/info/rfc1996>.
[RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
Syndication Format", RFC 4287, DOI 10.17487/RFC4287,
December 2005, <http://www.rfc-editor.org/info/rfc4287>.
[RFC4953] Touch, J., "Defending TCP Against Spoofing Attacks",
RFC 4953, DOI 10.17487/RFC4953, July 2007,
<http://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, <http://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,
<http://www.rfc-editor.org/info/rfc6281>.
[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013,
<http://www.rfc-editor.org/info/rfc6762>.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
<http://www.rfc-editor.org/info/rfc6763>.
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[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,
<http://www.rfc-editor.org/info/rfc6824>.
[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<http://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, <http://www.rfc-editor.org/info/rfc7525>.
[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, <http://www.rfc-editor.org/info/rfc7858>.
[XEP0060] Millard, P., Saint-Andre, P., and R. Meijer, "Publish-
Subscribe", XSF XEP 0060, July 2010.
Authors' Addresses
Tom Pusateri
Seeking affiliation
Hilton Head Island, SC
USA
Phone: +1 843 473 7394
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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