Internet Engineering Task Force T. Pusateri
Internet-Draft Seeking affiliation
Intended status: Standards Track S. Cheshire
Expires: January 9, 2017 Apple Inc.
July 8, 2016
DNS Push Notifications
draft-ietf-dnssd-push-08
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 January 9, 2017.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
Pusateri & Cheshire Expires January 9, 2017 [Page 1]
Internet-Draft DNS Push Notifications July 2016
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 . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Transport . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Client-Initiated Termination . . . . . . . . . . . . . . 7
4.2. Server-Initiated Termination . . . . . . . . . . . . . . 9
5. State Considerations . . . . . . . . . . . . . . . . . . . . 11
6. Protocol Operation . . . . . . . . . . . . . . . . . . . . . 12
6.1. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 13
6.2. DNS Push Notification SUBSCRIBE . . . . . . . . . . . . . 15
6.3. DNS Push Notification UNSUBSCRIBE . . . . . . . . . . . . 20
6.4. DNS Push Notification Update Messages . . . . . . . . . . 21
6.5. DNS RECONFIRM . . . . . . . . . . . . . . . . . . . . . . 24
6.6. DNS Push Notification Termination Message . . . . . . . . 25
7. Security Considerations . . . . . . . . . . . . . . . . . . . 28
7.1. Security Services . . . . . . . . . . . . . . . . . . . . 28
7.2. TLS Name Authentication . . . . . . . . . . . . . . . . . 28
7.3. TLS Compression . . . . . . . . . . . . . . . . . . . . . 29
7.4. TLS Session Resumption . . . . . . . . . . . . . . . . . 29
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 29
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 30
10.1. Normative References . . . . . . . . . . . . . . . . . . 30
10.2. Informative References . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 33
1. Introduction
IMPORTANT NOTE: This document currently references the EDNS(0) TCP
Keepalive option [RFC7828]. As a result of discussions about this
document, the community came to the realization that DNS needs
explicit session-level signaling, to complement the current EDNS(0)
per-message signaling. As a result, work on DNS Session Signaling
[I-D.bellis-dnsop-session-signal] is underway, and this document will
be updated shortly to make use of those new Session Signaling
mechanisms once they are agreed.
DNS records may be updated using DNS Update [RFC2136]. Other
mechanisms such as a Hybrid Proxy [I-D.ietf-dnssd-hybrid] can also
generate changes to a DNS zone. This document specifies a protocol
for Unicast DNS clients to subscribe to receive asynchronous
Pusateri & Cheshire Expires January 9, 2017 [Page 2]
Internet-Draft DNS Push Notifications July 2016
notifications of changes to RRSets of interest. It is immediately
relevant in the case of DNS Service Discovery [RFC6763] but is not
limited to that use case, and provides a general DNS mechanism for
DNS record change notifications. Familiarity with the DNS protocol
and DNS packet formats is assumed [RFC1034] [RFC1035] [RFC6895].
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
"Key words for use in RFCs to Indicate Requirement Levels" [RFC2119].
2. Motivation
As the domain name system continues to adapt to new uses and changes
in deployment, polling has the potential to burden DNS servers at
many levels throughout the network. Other network protocols have
successfully deployed a publish/subscribe model to state changes
following the Observer design pattern. XMPP Publish-Subscribe
[XEP0060] and Atom [RFC4287] are examples. While DNS servers are
generally highly tuned and capable of a high rate of query/response
traffic, adding a publish/subscribe model for tracking changes to DNS
records can result in more timely notification of changes with
reduced CPU usage and lower network traffic.
Multicast DNS [RFC6762] implementations always listen on a well known
link-local IP multicast group, and new services and updates are sent
for all group members to receive. Therefore, Multicast DNS already
has asynchronous change notification capability. However, when DNS
Service Discovery [RFC6763] is used across a wide area network using
Unicast DNS (possibly facilitated via a Hybrid Proxy
[I-D.ietf-dnssd-hybrid]) it would be beneficial to have an equivalent
capability for Unicast DNS, to allow clients to learn about DNS
record changes in a timely manner without polling.
DNS Long-Lived Queries (LLQ) [I-D.sekar-dns-llq] is an existing
deployed solution to provide asynchronous change notifications. Even
though it can be used over TCP, LLQ is defined primarily as a UDP-
based protocol, and as such it defines its own equivalents of
existing TCP features like the three-way handshake. This document
builds on experience gained with the LLQ protocol, with an improved
design that uses long-lived TCP connections instead of UDP (and
therefore doesn't need to duplicate existing TCP functionality), and
adopts the syntax and semantics of DNS Update messages [RFC2136]
instead of inventing a new vocabulary of messages to communicate DNS
zone changes.
Pusateri & Cheshire Expires January 9, 2017 [Page 3]
Internet-Draft DNS Push Notifications July 2016
Because DNS Push Notifications impose a certain load on the
responding server (though less load than rapid polling of that
server) DNS Push Notification clients SHOULD exercise restraint in
issuing DNS 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 of
that subscription 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 phone, and then leaves the phone showing the printer
discovery screen until the phone goes to sleep, then the printer
discovery screen should be automatically dismissed as the device goes
to sleep. If the user does still intend to print, this will require
them to press the "Print" button again when they wake their phone up.
A DNS Push Notification client MUST NOT routinely keep a DNS Push
Notification subscription active 24 hours a day 7 days a week just to
keep a list in memory up to date so that it will be really fast if
the user does choose to bring up an on-screen display of that data.
DNS Push Notifications are designed to be fast enough that there is
no need to pre-load a "warm" list in memory just in case it might be
needed later.
Generally, a client SHOULD NOT keep a connection to a server open
indefinitely if it has no active subscriptions on that connection.
After 30 seconds with no active subscriptions the client SHOULD close
the idle connection, and, if needed in the future, open a new
connection.
3. Overview
The existing DNS Update protocol [RFC2136] provides a mechanism for
clients to add or delete individual resource records (RRs) or entire
resource record sets (RRSets) on the zone's server.
This specification adopts a simplified subset of these existing
syntax and semantics, and uses them for DNS Push Notification
messages going in the opposite direction, from server to client, to
communicate changes to a zone. The client subscribes for Push
Notifications by connecting to the server and sending DNS message(s)
indicating the RRSet(s) of interest. When the client loses interest
in updates to these records, it unsubscribes.
The DNS Push Notification server for a zone is any server capable
Pusateri & Cheshire Expires January 9, 2017 [Page 4]
Internet-Draft DNS Push Notifications July 2016
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 is not REQUIRED to
support DNS Update.
DNS Updates and DNS Push Notifications may be handled on different
ports on the same target host, in which case they are not considered
to be the "same server" for the purposes of this specification, and
communications with these two ports are handled independently.
Standard DNS Queries MAY be sent over a DNS Push Notification
connection, provided that these are queries for names falling within
the server's zone (the <zone> in the "_dns-push-tls._tcp.<zone>" SRV
record). The RD (Recursion Desired) bit MUST be zero.
DNS Push Notification clients are NOT required to implement DNS
Update Prerequisite processing. Prerequisites are used to perform
tentative atomic test-and-set type operations when a client updates
records on a server, and that concept has no applicability when it
comes to an authoritative server informing a client of changes to DNS
records.
This DNS Push Notification specification includes support for DNS
classes, for completeness. However, in practice, it is anticipated
that for the foreseeable future the only DNS class in use will be DNS
class "IN", as it is today with existing DNS servers and clients. A
DNS Push Notification server MAY choose to implement only DNS class
"IN".
Pusateri & Cheshire Expires January 9, 2017 [Page 5]
Internet-Draft DNS Push Notifications July 2016
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. Because TCP SYN
flooding attacks are possible with any protocol over TCP,
implementers are encouraged to use industry best practices to guard
against such attacks [IPJ.9-4-TCPSYN] [RFC4953].
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.
A DNS Push Notification session begins with a client connecting to a
DNS Push Notification server. Over that connection the client then
issues DNS operation requests, such as SUBSCRIBE.
Pusateri & Cheshire Expires January 9, 2017 [Page 6]
Internet-Draft DNS Push Notifications July 2016
4.1. Client-Initiated Termination
An individual subscription is terminated by sending an UNSUBSCRIBE
message 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 continue to meet its keepalive obligations
[RFC7828] or the server is entitled to close the connection (see
below).
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.
In the first SUBSCRIBE response on a connection, the server MUST
include an explicit EDNS(0) TCP Keepalive option. If the first
SUBSCRIBE response does not include an explicit EDNS(0) TCP Keepalive
option this is an error and the client MUST immediately close the TCP
connection and not attempt any further DNS Push Notification requests
to that server until one hour has passed. This situation may occur
Pusateri & Cheshire Expires January 9, 2017 [Page 7]
Internet-Draft DNS Push Notifications July 2016
if a client connects to a server that doesn't implement DNS Push
Notifications at all, and it is important not to burden such servers
with continuous retries.
Upon receiving an error response from the server, a client SHOULD NOT
close the connection. An error relating to one particular operation
on a connection does not necessarily imply that all other operations
on that connection have also failed, or that future operations will
fail. The client should assume that the server will make its own
decision about whether or not to close the connection, based on the
server's determination of whether the error condition pertains to
this particular operation, or would also apply to any subsequent
operations. If the server does not close the connection then the
client SHOULD continue to use that connection for subsequent
operations.
Upon receiving a Termination Message from the server (see below), a
client MUST immediately close the connection.
Pusateri & Cheshire Expires January 9, 2017 [Page 8]
Internet-Draft DNS Push Notifications July 2016
4.2. Server-Initiated Termination
If a client makes a connection and then fails to send any DNS message
that uses EDNS(0) TCP Keepalive [RFC7828] (either SUBSCRIBE, where
Keepalive is implicit, or some other DNS message, with an explicit an
EDNS(0) TCP Keepalive option) then after 30 seconds of inactivity the
server SHOULD close the connection. If no data has been sent on the
connection the server MAY abort the connection with a TCP RST. If
data has been sent on the connection then the server SHOULD close the
connection gracefully with a TCP FIN so that the data is reliably
delivered.
In the response to the first successful SUBSCRIBE, the included
EDNS(0) TCP Keepalive option specifies the idle timeout so that the
client knows the frequency of traffic it must generate to keep the
connection alive. If the idle timeout for that connection changes,
then the server communicates this by placing an updated EDNS(0) TCP
Keepalive option in a subsequent message to the client.
At both servers and clients, the generation or reception of any
complete request, response, update, or keepalive message resets the
keepalive timer for that connection.
In the absence of any requests, responses, or update messages on a
connection, a client MUST generate keepalive traffic before the idle
timeout expires, or the server is entitled to close the connection.
If a client disconnects from the network abruptly, without closing
its connection, the server learns of this after failing to receive
further traffic from that client. If no requests, responses, update
messages or keepalive traffic occurs on a connection for 1.5 times
the idle timeout, then this indicates that the client is probably no
longer on the network, and the server SHOULD abort the connection
with a TCP RST. The time before the server closes the connection is
intentionally 50% longer than the time before the client is required
to generate keepalive traffic, to allow for differences in clock rate
and network propagation delays.
[We need to discuss the nature of "the required keepalives". Are
they TCP-layer keepalives? DNS-layer keepalives? There is currently
no DNS-layer keepalive or 'no-op' operation defined. What would that
operation be? A DNS QUERY containing zero questions? A DNS
SUBSCRIBE containing zero questions? An "empty" DNS message over the
TCP connection (just a pair of zero bytes, signifying a zero-length
message)? One benefit of TCP-layer keepalives is that they transmit
fewer bytes, and involve less software overhead for processing those
bytes. Another benefit is that it is more feasible to implement
these in networking offload hardware, which can allow devices to meet
Pusateri & Cheshire Expires January 9, 2017 [Page 9]
Internet-Draft DNS Push Notifications July 2016
their TCP keepalive obligations while sleeping. This is particularly
important for battery-powered devices like mobile phones and tablets.
On the other hand, using TCP-layer keepalives requires an API for a
client to tell the networking stack at what frequency to perform TCP-
layer keepalives, and an API for a server to request the networking
stack to inform it when TCP-layer keepalives are not received by the
required deadline. TCP-layer keepalives also only verify liveness of
the remote networking stack, whereas DNS-layer keepalives provide
higher assurance of liveness of the remote server application
software -- though this a limited benefit, since there is no reason
to expect that DNS Push Notification server software will routinely
become wedged and unresponsive.]
After sending an error response to a client, the server MAY close the
connection with a TCP FIN, or may allow the connection to remain
open. For error conditions that only affect the single operation in
question, the server SHOULD return an error response to the client
and leave the connection open for further operations. For error
conditions that are likely to make all operations unsuccessful in the
immediate future, the server SHOULD return an error response to the
client and then close the connection with a TCP FIN.
If the server is overloaded and needs to shed load, it SHOULD send a
Termination Message to the client and close the connection with a TCP
FIN.
Apart from the cases described above, a server MUST NOT close a
connection with a DNS Push Notification client, except in
extraordinary error conditions. Closing the connection is the
client's responsibility, to be done at the client's discretion, when
it so chooses. A DNS Push Notification server only closes a DNS Push
Notification connection under exceptional circumstances, such as when
the server application software or underlying operating system is
restarting, the server application terminated unexpectedly (perhaps
due to a bug that makes it crash), or the server is undergoing
maintenance procedures. When possible, a DNS Push Notification
server SHOULD send a Termination Message (Section 6.6 ) informing the
client of the reason for the connection being closed.
After a connection is closed by the server, the client SHOULD try to
reconnect, to that server, or to another server supporting DNS Push
Notifications for the zone. If reconnecting to the same server, and
there was a Termination Message or error response containing a
EDNS(0) TCP Keepalive option, the client MUST respect the indicated
delay before attempting to reconnect.
Pusateri & Cheshire Expires January 9, 2017 [Page 10]
Internet-Draft DNS Push Notifications July 2016
5. State Considerations
Each DNS Push Notification server is capable of handling some finite
number of Push Notification subscriptions. This number will vary
from server to server and is based on physical machine
characteristics, network bandwidth, and operating system resource
allocation. After a client establishes a connection to a DNS server,
each record subscription is individually accepted or rejected.
Servers may employ various techniques to limit subscriptions to a
manageable level. Correspondingly, the client is free to establish
simultaneous connections to alternate DNS servers that support DNS
Push Notifications for the zone and distribute record subscriptions
at its discretion. In this way, both clients and servers can react
to resource constraints. Token bucket rate limiting schemes are also
effective in providing fairness by a server across numerous client
requests.
Pusateri & Cheshire Expires January 9, 2017 [Page 11]
Internet-Draft DNS Push Notifications July 2016
6. Protocol Operation
A DNS Push Notification exchange begins with the client discovering
the appropriate server, and then making a TLS/TCP connection to it.
The client may then add and remove Push Notification subscriptions
over this connection. 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.
A client SHOULD NOT make multiple TLS/TCP connections to the same DNS
Push Notification server. A client SHOULD share a single TLS/TCP
connection for all requests to the same DNS Push Notification server.
This shared connection should be used for all DNS Queries and DNS
Push Notification Queries queries to that server, and for DNS Update
requests too when the "_dns-update-tls._tcp.<zone>" SRV record
indicates that the same server also handles DNS Update requests.
This is to reduce unnecessary load on the DNS Push Notification
server.
For the purposes here, the determination of "same server" is made by
inspecting the target hostname and port, regardless of the name being
queried, or what zone if falls within. A given server may support
Push Notifications (and possibly DNS Updates too) for multiple DNS
zones. When a client discovers that the DNS Push Notification server
(and/or DNS Update server) for several different names (including
names that fall within different zones) is the same target hostname
and port, the client SHOULD use a single shared TCP connection for
all relevant operations on those names. A client SHOULD NOT open
multiple TCP connections to the same target host and port just
because the names being queried (or updated) happen to fall within
different zones.
Note that the "same server" determination described here is made
using the target hostname given in the SRV record, not the IP
address(es) that the hostname resolves to. If two different target
hostnames happen to resolve to the same IP address(es), then the
client SHOULD NOT recognize these as the "same server" for the
purposes of using a single shared connection to that server. If an
administrator wishes to use a single server for multiple zones and/or
multiple roles (e.g., both DNS Push Notifications and DNS Updates),
Pusateri & Cheshire Expires January 9, 2017 [Page 12]
Internet-Draft DNS Push Notifications July 2016
and wishes to have clients use a single shared connection for
operations on that server, then the administrator MUST use the same
target hostname in the appropriate SRV records.
However, server implementers and operators should be aware that this
connection sharing may not be possible in all cases. A single client
device may be home to multiple independent client software instances
that don't know about each other, so a DNS Push Notification server
MUST be prepared to accept multiple connections from the same client
IP address. This is undesirable from an efficiency standpoint, but
may be unavoidable in some situations, so a DNS Push Notification
server MUST be prepared to accept multiple connections from the same
client IP address.
Clients SHOULD silently ignore unrecognized messages (both requests
and responses) over the TLS/TCP connection. For example, UNSUBSCRIBE
and RECONFIRM currently generate no response, but if future versions
of this specification change that, existing clients SHOULD silently
ignore these unexpected responses. This allows for backwards
compatibility with future enhancements.
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 [I-D.ietf-dprive-dns-over-tls].
1. The client begins the discovery by sending a DNS query to the
local resolver with record type SOA [RFC1035] for the name of the
record it wishes to subscribe.
2. If the SOA record exists, it MUST be returned in the Answer
Section of the response. If not, the local resolver SHOULD
include the SOA record for the zone of the requested name in the
Authority Section.
3. If no SOA record is returned, the client then strips off the
leading label from the requested name. If the resulting name has
at least one label in it, the client sends a new SOA query and
processing continues at step 2 above. If the resulting name is
empty (the root label) then this is a network configuration error
and the client gives up. The client MAY retry the operation at a
later time.
Pusateri & Cheshire Expires January 9, 2017 [Page 13]
Internet-Draft DNS Push Notifications July 2016
4. Once the SOA is known (either by virtue of being seen in the
Answer Section, or in the Authority Section), the client sends a
DNS query with type SRV [RFC2782] for the record name
"_dns-push-tls._tcp.<zone>", where <zone> is the owner name of
the discovered SOA record.
5. If the zone in question does not offer DNS Push Notifications
then SRV record MUST NOT exist and the SRV query will return a
negative answer.
6. If the zone in question is set up to offer DNS Push Notifications
then this SRV record MUST exist. The SRV "target" contains the
name of the server providing DNS Push Notifications for the zone.
The port number on which to contact the server is in the SRV
record "port" field. The address(es) of the target host MAY be
included in the Additional Section, however, the address records
SHOULD be authenticated before use as described below in
Section 7.2 [RFC7673].
7. More than one SRV record may be returned. In this case, the
"priority" and "weight" values in the returned SRV records are
used to determine the order in which to contact the servers for
subscription requests. As described in the SRV specification
[RFC2782], the server with the lowest "priority" is first
contacted. If more than one server has the same "priority", the
"weight" indicates the weighted probability that the client
should contact that server. Higher weights have higher
probabilities of being selected. If a server is not reachable or
is not willing to accept a subscription request, then a
subsequent server is to be contacted.
Each time a client makes a new DNS Push Notification subscription
connection, it SHOULD repeat the discovery process in order to
determine the preferred DNS server for subscriptions at that time.
Pusateri & Cheshire Expires January 9, 2017 [Page 14]
Internet-Draft DNS Push Notifications July 2016
6.2. DNS Push Notification SUBSCRIBE
A DNS Push Notification client indicates its desire to receive DNS
Push Notifications for a given domain name by sending a SUBSCRIBE
request over the established TCP connection to the server. A
SUBSCRIBE request is formatted identically to a conventional DNS
QUERY request [RFC1035], except that the opcode is SUBSCRIBE (6)
instead of QUERY (0). If neither QTYPE nor QCLASS are ANY (255) then
this is a specific subscription to changes for the given name, type
and class. If one or both of QTYPE or QCLASS 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 DNS SUBSCRIBE, QTYPE and QCLASS 255
should be interpreted to mean "ALL", not "ANY". After accepting a
subscription where one or both of QTYPE or QCLASS are 255, the server
MUST send Push Notification Updates for ALL record changes that match
the subscription, not just some of them.
In a SUBSCRIBE request the DNS Header QR bit MUST be zero.
If the QR bit is not zero the message is not a SUBSCRIBE request.
The AA, TC, RD, RA, Z, AD, and CD bits, and the RCODE field, MUST be
zero on transmission, and MUST be silently ignored on reception.
The ID field may be set to any value of the client's choosing, and
the server MUST echo this value back in the response message. The
client is not required to select unique ID values; it is permissible
to use the same value (e.g., zero) for all operations. Since the
name, qtype, and qclass are sufficient to uniquely identify a
SUBSCRIBE operation on a connection, the name, qtype, and qclass in a
SUBSCRIBE response are sufficient to correlate a response with its
corresponding request. However, for convenience, the client may put
any value it chooses in the ID field of the SUBSCRIBE request, and
the server MUST echo that value back unchanged in the SUBSCRIBE
response. Note that the ID field of Push Notification Update
Messages is always zero, since a Push Notification Update Message
could potentially match more than one subscription, or could relate
to a subscription that the client has just cancelled with an
UNSUBSCRIBE message.
Pusateri & Cheshire Expires January 9, 2017 [Page 15]
Internet-Draft DNS Push Notifications July 2016
Like a DNS QUERY request, a SUBSCRIBE request MUST contain exactly
one question. Since SUBSCRIBE requests are sent over TCP, multiple
SUBSCRIBE requests can be concatenated in a single TCP stream and
packed efficiently into TCP segments, so the ability to pack multiple
SUBSCRIBE operations into a single DNS message within that TCP stream
would add extra complexity for little benefit.
ANCOUNT MUST be zero, and the Answer Section MUST be empty.
Any records in the Answer Section MUST be silently ignored.
NSCOUNT MUST be zero, and the Authority Section MUST be empty.
Any records in the Authority Section MUST be silently ignored.
ARCOUNT specifies the number of records in the Additional Data
Section. Typically this is zero, but it may be nonzero in some
cases, such as when the request includes an EDNS(0) OPT record.
If accepted, the subscription will stay in effect until the client
revokes the subscription 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
SUBSCRIBE message this is an error and the server MUST immediately
close the TCP connection.
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 matches are found in the future.
Since all SUBSCRIBE operations are implicitly long-lived operations,
the server MUST interpret a SUBSCRIBE request as if it contained an
EDNS(0) TCP Keepalive option [RFC7828]. A client MUST NOT include an
actual EDNS(0) TCP Keepalive option in the request, since it is
automatic, and implied by the semantics of SUBSCRIBE. If a server
Pusateri & Cheshire Expires January 9, 2017 [Page 16]
Internet-Draft DNS Push Notifications July 2016
receives a SUBSCRIBE request that does contain an actual EDNS(0) TCP
Keepalive option this is an error and the server MUST immediately
close the TCP connection.
A SUBSCRIBE operation MAY include an explicit EDNS(0) [RFC6891] OPT
record where necessary to carry additional EDNS(0) information other
than a TCP Keepalive option.
The presence of a SUBSCRIBE operation on a connection indicates to
the server that the client fully implements EDNS(0) [RFC6891], and
can correctly understand any response that conforms to that
specification. After receiving a SUBSCRIBE request, the server MAY
include OPT record in any of its responses, as needed.
Each SUBSCRIBE request generates exactly one SUBSCRIBE response from
the server.
In a SUBSCRIBE response the DNS Header QR bit MUST be one.
If the QR bit is not one the message is not a SUBSCRIBE response.
The AA, TC, RD, RA, Z, AD, and CD bits, MUST be zero on transmission,
and MUST be silently ignored on reception.
The ID field MUST echo the value given in the ID field of the
SUBSCRIBE request.
The Question Section MUST echo back the values provided by the client
in the SUBSCRIBE request that generated this SUBSCRIBE response.
ANCOUNT MUST be zero, and the Answer Section MUST be empty.
Any records in the Answer Section MUST be silently ignored.
If the subscription was accepted and there are positive answers for
the requested name, type and class, then these positive answers MUST
be communicated to the client in an immediately following Push
Notification Update, not in the Answer Section of the SUBSCRIBE
response. This simplifying requirement is made so that there is only
a single way that information is communicated to a DNS Push
Notification client. Since a DNS Push Notification client has to
parse information received via Push Notification Updates anyway, it
is simpler if it does not also have to parse information received via
the Answer Section of a SUBSCRIBE response.
NSCOUNT MUST be zero, and the Authority Section MUST be empty.
Any records in the Authority Section MUST be silently ignored.
ARCOUNT specifies the number of records in the Additional Data
Section, e.g., the EDNS(0) OPT record.
Pusateri & Cheshire Expires January 9, 2017 [Page 17]
Internet-Draft DNS Push Notifications July 2016
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 MUST NOT |
| | | return NXDOMAIN errors in response to |
| | | SUBSCRIBE requests. |
| NOTIMP | 4 | Server does not implement DNS Push |
| | | Notifications. |
| REFUSED | 5 | Server refuses to process request for policy |
| | | or security reasons. |
| NOTAUTH | 9 | Server is not authoritative for the requested |
| | | name. |
+----------+-------+------------------------------------------------+
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.
In the first SUBSCRIBE response on a connection, the server MUST
include an explicit EDNS(0) TCP Keepalive option. If the first
SUBSCRIBE response does not include an explicit EDNS(0) TCP Keepalive
option this is an error and the client MUST immediately close the TCP
connection. In this case the client should act as if the response
contained an EDNS(0) TCP Keepalive option with a value of one hour,
and not attempt any further DNS Push Notification requests to that
server until one hour has passed. This situation may occur if a
client connects to a server that doesn't implement DNS Push
Notifications at all, and it is important not to burden such servers
with continuous retries.
The server MAY include EDNS(0) TCP Keepalive options in subsequent
messages, if the idle timeout changes. If the client receives
subsequent messages that do not contain an explicit EDNS(0) TCP
Keepalive option then the idle timeout for that connection remains
unchanged at that time.
Pusateri & Cheshire Expires January 9, 2017 [Page 18]
Internet-Draft DNS Push Notifications July 2016
In an error response, with nonzero RCODE, the server MUST contain an
EDNS(0) TCP Keepalive option specifying the delay before the client
submits further requests to this server:
For RCODE = 1 (FORMERR) the delay may be any value selected by the
implementer. A value of one minute is RECOMMENDED, to avoid 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 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. Note that a
server that doesn't implement DNS Push Notifications will most
likely not implement this retry delay mechanism using the EDNS(0)
TCP Keepalive option either, and in this case the client will fall
back to the case described above specifying how to handle
SUBSCRIBE responses that do not contain an EDNS(0) TCP Keepalive
option.
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.
Pusateri & Cheshire Expires January 9, 2017 [Page 19]
Internet-Draft DNS Push Notifications July 2016
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 close the TCP
connection with a FIN, or MAY allow it to remain open, depending on
the nature of the error. Clients MUST correctly handle both cases.
6.3. 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
formatted identically to the SUBSCRIBE message which created the
subscription, with the exact same name, type and class, except that
the opcode is UNSUBSCRIBE (7) instead of SUBSCRIBE (6).
A client MUST NOT send an UNSUBSCRIBE message that does not exactly
match the name, type and class of an existing active subscription on
that TLS/TCP connection. If a server receives such an UNSUBSCRIBE
message this is an error and the server MUST immediately close the
connection.
No response message is generated as a result of processing an
UNSUBSCRIBE message.
Having being successfully revoked with a correctly-formatted
UNSUBSCRIBE message, 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.
Pusateri & Cheshire Expires January 9, 2017 [Page 20]
Internet-Draft DNS Push Notifications July 2016
6.4. DNS Push Notification Update Messages
Once a subscription has been successfully established, the server
generates Push Notification Updates to send to the client as
appropriate. An initial Push Notification Update will be sent
immediately in the case that the answer set was non-empty at the
moment the subscription was established. Subsequent changes to the
answer set are then communicated to the client in subsequent Push
Notification Updates.
The format of Push Notification Updates borrows from the existing DNS
Update [RFC2136] protocol, with some simplifications.
The following figure shows the existing DNS Update header format:
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ID |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|QR| Opcode | Z | RCODE |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ZOCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| PRCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| UPCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ADCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
Figure 1
For DNS Push Notifications the following rules apply:
The QR bit MUST be zero, and the Opcode MUST be UPDATE (5).
Messages received where this is not true are not Push Notification
Update Messages and should be silently ignored for the purposes of
Push Notification Update Message handling.
ID, the Z bits, and RCODE MUST be zero on transmission,
and MUST be silently ignored on reception.
ZOCOUNT MUST be zero, and the Zone Section MUST be empty.
Any records in the Zone Section MUST be silently ignored.
PRCOUNT MUST be zero, and the Prerequisite Section MUST be empty.
Any records in the Prerequisite Section MUST be silently ignored.
Pusateri & Cheshire Expires January 9, 2017 [Page 21]
Internet-Draft DNS Push Notifications July 2016
UPCOUNT specifies the number of records in the Update Section.
ADCOUNT specifies the number of records in the Additional Data
Section. Typically this is zero, but it may be nonzero in some
cases, such as when the message includes an EDNS(0) OPT record.
The Update Section 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 Notification Update
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
QTYPE in the SUBSCRIBE request was not ANY (255) then the TYPE of the
record must match the QTYPE given in the SUBSCRIBE request. If the
QCLASS in the SUBSCRIBE request was not ANY (255) then the CLASS of
the record must match the QCLASS 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 race condition where a client sends an outbound UNSUBSCRIBE while
inbound Push Notification Updates 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 update is sent. For example, an update adding
a given record may match both a SUBSCRIBE request with the same QTYPE
and a different SUBSCRIBE request with QTYPE=ANY. It is not the case
that two updates are sent because the new record matches two active
subscriptions.
Pusateri & Cheshire Expires January 9, 2017 [Page 22]
Internet-Draft DNS Push Notifications July 2016
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" update, not three
separate "delete an individual RR from a name" updates. 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" update, not two separate
"delete an RRset from a name" updates.
A server SHOULD combine multiple change notifications in a single
Update Message when possible, even if those change notifications
apply to different subscriptions. Conceptually, a Push Notification
Update Message is a connection-level concept, not a subscription-
level concept.
Push Notification Update Messages MAY contain an EDNS(0) TCP
Keepalive option [RFC7828] if the idle timeout has changed since the
last time the server sent an EDNS(0) TCP Keepalive option on this
connection.
In the event that the server wishes to inform a client of a new idle
timeout for the connection, the server MAY combine that with the next
message it sends to the client, or the server MAY send an empty Push
Notification Update Message (zero records in the Update Section) to
carry the EDNS(0) TCP Keepalive option. Clients MUST correctly
receive and process the EDNS(0) TCP Keepalive option in both cases.
Reception of a Push Notification Update Message does not directly
generate a response back to the server. (Updates may indirectly
generate other operations; e.g., a Push Notification Update Message
declaring the appearance of a PTR record could lead to a query for
the SRV record named in the rdata of that PTR record[RFC6763].
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 aging resumes and records are removed from the local cache
when their TTL reaches zero.
Pusateri & Cheshire Expires January 9, 2017 [Page 23]
Internet-Draft DNS Push Notifications July 2016
6.5. DNS RECONFIRM
Sometimes, particularly when used with a Hybrid Proxy
[I-D.ietf-dnssd-hybrid], 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 sends a DNS RECONFIRM message to
request that the server re-verify that the data is still valid. For
a Hybrid Proxy, this causes it to issue new Multicast DNS requests to
ascertain whether the target device is still present. For other
kinds of DNS server the RECONFIRM operation is currently undefined
and SHOULD be silently ignored.
A RECONFIRM request is formatted similarly to a conventional DNS
QUERY request [RFC1035], except that the opcode is RECONFIRM (8)
instead of QUERY (0). QTYPE MUST NOT be the value ANY (255). QCLASS
MUST NOT be the value ANY (255).
In a RECONFIRM request the DNS Header QR bit MUST be zero.
If the QR bit is not zero the message is not a RECONFIRM request.
The AA, TC, RD, RA, Z, AD, and CD bits, the ID field, and the RCODE
field, MUST be zero on transmission, and MUST be silently ignored on
reception.
Like a DNS QUERY request, a RECONFIRM request MUST contain exactly
one question. Since RECONFIRM requests are sent over TCP, multiple
RECONFIRM requests can be concatenated in a single TCP stream and
packed efficiently into TCP segments, so the ability to pack multiple
RECONFIRM operations into a single DNS message within that TCP stream
would add extra complexity for little benefit.
ANCOUNT MUST be nonzero, and the Answer Section MUST contain the
rdata for the record(s) that the client believes to be in doubt.
NSCOUNT MUST be zero, and the Authority Section MUST be empty.
Any records in the Authority Section MUST be silently ignored.
ARCOUNT specifies the number of records in the Additional Data
Section. Typically this is zero, but it may be nonzero in some
cases, such as when the request includes an EDNS(0) OPT record.
DNS wildcarding is not supported. That is, a wildcard ("*") in a
SUBSCRIBE message matches only a wildcard ("*") in the zone, and
nothing else.
Aliasing is not supported. That is, a CNAME in a SUBSCRIBE message
matches only a CNAME in the zone, and nothing else.
Pusateri & Cheshire Expires January 9, 2017 [Page 24]
Internet-Draft DNS Push Notifications July 2016
No response message is generated as a result of processing a
RECONFIRM message.
If the server receiving the RECONFIRM request determines that the
records are in fact no longer valid, then subsequent DNS Push
Notification Update Messages will be generated to inform interested
clients. Thus, one client discovering that a previously-advertised
printer is no longer present has the side effect of informing all
other interested clients that the printer in question is now gone.
6.6. DNS Push Notification Termination Message
If a server is low on resources it MAY simply terminate a client
connection with a TCP RST. However, the likely behaviour of the
client may be simply to reconnect immediately, putting more burden on
the server. Therefore, a server SHOULD instead choose to shed client
load by (a) sending a DNS Push Notification Termination Message and
then (b) immediately closing the client connection with a TCP FIN
instead of RST, thereby facilitating reliable delivery of the
Termination Message. Upon successful reception of the Termination
Message the client is expected to close the connection. The server
SHOULD set a timer and, if the client has not closed the connection
within a reasonable time, the server SHOULD then terminate the TCP
connection with a TCP RST. The RECOMMENDED time the server should
wait before terminating the TCP connection with a TCP RST is ten
seconds.
The format of a Termination Message is similar to a Push Notification
Update.
Pusateri & Cheshire Expires January 9, 2017 [Page 25]
Internet-Draft DNS Push Notifications July 2016
The following figure shows the existing DNS Update header format:
1 1 1 1 1 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ID |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
|QR| Opcode | Z | RCODE |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ZOCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| PRCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| UPCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| ADCOUNT |
+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
Figure 2
For Termination Messages the following rules apply:
The QR bit MUST be zero, and the Opcode MUST be UPDATE (5).
Messages received where this is not true are not Termination Messages
and should be silently ignored.
ID and the Z bits MUST be zero on transmission,
and MUST be silently ignored on reception.
ZOCOUNT MUST be zero, and the Zone Section MUST be empty.
Any records in the Zone Section MUST be silently ignored.
PRCOUNT MUST be zero, and the Prerequisite Section MUST be empty.
Any records in the Prerequisite Section MUST be silently ignored.
UPCOUNT MUST be zero, and the Update Section MUST be empty.
Any records in the Update Section MUST be silently ignored.
ADCOUNT specifies the number of records in the Additional Data
Section, e.g., the EDNS(0) OPT record..
The RCODE MUST contain a nonzero code giving the reason for
termination, as indicated below:
Pusateri & Cheshire Expires January 9, 2017 [Page 26]
Internet-Draft DNS Push Notifications July 2016
+----------+-------+------------------------------------------------+
| Mnemonic | Value | Description |
+----------+-------+------------------------------------------------+
| SERVFAIL | 2 | The server is overloaded due to resource |
| | | exhaustion. |
| REFUSED | 5 | The server has been reconfigured and is no |
| | | longer accepting DNS Push Notification |
| | | requests for one or more of the currently |
| | | subscribed names. |
+----------+-------+------------------------------------------------+
Termination Response codes
This document specifies only these two RCODE values for Termination
Messages. Servers sending Termination Messages SHOULD use one of
these two values. However, future circumstances may create
situations where other RCODE values are appropriate in Termination
Messages, so clients MUST be prepared to accept Termination Messages
with any RCODE value. In particular, a Termination Message with
RCODE value zero (NOERROR) is still a Termination Message and should
be treated as such.
The Termination Message MUST contain an EDNS(0) TCP Keepalive option
[RFC7828]. The client MUST wait for the time indicated in the
EDNS(0) TCP Keepalive option's idle timeout before attempting any new
connections to this server. A client that receives a Termination
Message without an EDNS(0) TCP Keepalive option SHOULD treat it as
equivalent to a TCP Keepalive option with a zero timeout value.
In the case where the server is rejecting some, but not all, of the
existing subscriptions (perhaps because it has been reconfigured and
is no longer authoritative for those names) with a REFUSED (5) RCODE,
the EDNS(0) TCP Keepalive option's idle timeout MAY be zero,
indicating that the client SHOULD attempt to re-establish its
subscriptions immediately.
In the case where a server is terminating a large number of
connections at once (e.g., if the system is restarting) and the
server doesn't want to be inundated with a flood of simultaneous
retries, it SHOULD send different EDNS(0) TCP Keepalive values to
each client. These adjustments MAY be selected randomly,
pseudorandomly, or deterministically (e.g., incrementing the time
value by one tenth of a second for each successive client, yielding a
post-restart reconnection rate of ten clients per second).
Pusateri & Cheshire Expires January 9, 2017 [Page 27]
Internet-Draft DNS Push Notifications July 2016
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
Pusateri & Cheshire Expires January 9, 2017 [Page 28]
Internet-Draft DNS Push Notifications July 2016
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.
This document defines three DNS OpCodes: SUBSCRIBE with (tentative)
value 6, UNSUBSCRIBE with (tentative) value 7, and RECONFIRM with
(tentative) value 8.
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.
Pusateri & Cheshire Expires January 9, 2017 [Page 29]
Internet-Draft DNS Push Notifications July 2016
10. References
10.1. Normative References
[I-D.bellis-dnsop-session-signal]
Bellis, R., Cheshire, S., Marcon, J., Mankin, A., and T.
Pusateri, "DNS Session Signaling", draft-bellis-dnsop-
session-signal-00 (work in progress), July 2016.
[I-D.ietf-tls-tls13]
Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", draft-ietf-tls-tls13-13 (work in progress),
May 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>.
Pusateri & Cheshire Expires January 9, 2017 [Page 30]
Internet-Draft DNS Push Notifications July 2016
[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>.
[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>.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891, DOI 10.17487/
RFC6891, April 2013,
<http://www.rfc-editor.org/info/rfc6891>.
[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>.
[RFC7828] Wouters, P., Abley, J., Dickinson, S., and R. Bellis, "The
edns-tcp-keepalive EDNS0 Option", RFC 7828, DOI 10.17487/
RFC7828, April 2016,
<http://www.rfc-editor.org/info/rfc7828>.
10.2. Informative References
[I-D.ietf-dnssd-hybrid]
Cheshire, S., "Hybrid Unicast/Multicast DNS-Based Service
Discovery", draft-ietf-dnssd-hybrid-03 (work in progress),
November 2015.
Pusateri & Cheshire Expires January 9, 2017 [Page 31]
Internet-Draft DNS Push Notifications July 2016
[I-D.ietf-dprive-dns-over-tls]
Zi, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "Specification for DNS over TLS", draft-
ietf-dprive-dns-over-tls-09 (work in progress), March
2016.
[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.
[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>.
[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>.
[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>.
[XEP0060] Millard, P., Saint-Andre, P., and R. Meijer, "Publish-
Subscribe", XSF XEP 0060, July 2010.
Pusateri & Cheshire Expires January 9, 2017 [Page 32]
Internet-Draft DNS Push Notifications July 2016
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
Pusateri & Cheshire Expires January 9, 2017 [Page 33]