Advertising Proxy for DNS-SD Service Registration Protocol
draft-sctl-advertising-proxy-02
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Stuart Cheshire
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Ted Lemon
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2021-07-28
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DNSSD S. Cheshire
Internet-Draft T. Lemon
Intended status: Standards Track Apple Inc.
Expires: 13 January 2022 12 July 2021
Advertising Proxy for DNS-SD Service Registration Protocol
draft-sctl-advertising-proxy-02
Abstract
An Advertising Proxy allows a device that accepts service
registrations using Service Registration Protocol (SRP) to make those
registrations visible to legacy clients that only implement Multicast
DNS.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 13 January 2022.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Simplified BSD License text
as described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Simplified BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions and Terminology Used in This Document . . . . 3
2. Advertising Proxy . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Name Conflicts . . . . . . . . . . . . . . . . . . . . . 3
2.1.1. Name Conflicts in Managed Namespaces . . . . . . . . 5
2.2. Data Translation . . . . . . . . . . . . . . . . . . . . 6
2.3. No Text-Encoding Translation . . . . . . . . . . . . . . 6
2.4. No Address Suppression . . . . . . . . . . . . . . . . . 6
2.5. No Support for Reconfirm . . . . . . . . . . . . . . . . 7
3. Security Considerations . . . . . . . . . . . . . . . . . . . 8
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
5. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
5.1. Normative References . . . . . . . . . . . . . . . . . . 8
5.2. Informative References . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
DNS-Based Service Discovery [RFC6763] [ROADMAP] was designed to
facilitate Zero Configuration IP Networking [RFC6760] [ZC].
When used with Multicast DNS [RFC6762] with ".local" domain names
[RFC6761] this works well on a single link (a single broadcast
domain).
There is also a desire to have DNS-Based Service Discovery work
between multiple links that aren't part of the same broadcast domain
[RFC7558]. Even within a single Wi-Fi broadcast domain it is
beneficial to reduce multicast traffic, because, in comparison to
Wi-Fi unicast traffic, Wi-Fi multicast is inefficient, slow, and
unreliable [MCAST].
There are three complementary ways that this move towards decreased
reliance on multicast is achieved.
One variant is pure end-to-end unicast, with services using unicast
Service Registration Protocol [SRP] to register with a service
registry, and clients using unicast DNS Push Notification
subscriptions [RFC8765] over DNS Stateful Operations [RFC8490] to
communicate with the service registry to discover and track changes
to those registered services.
A second variant is a hybrid approach that facilitates legacy devices
that only implement link-local Multicast DNS (like your ten-year-old
network laser printer) having their services discovered by remote
clients using a unicast DNS Push Notifications session to a Discovery
Proxy [RFC8766].
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The third variant, documented here, is a logical complement to the
second variant. It enables legacy clients (that only implement link-
local Multicast DNS) to discover services registered (using unicast)
with a service registry. The service registry accepts service
registrations using unicast Service Registration Protocol [SRP], and
makes those service registrations visible, both to remote clients
using unicast DNS Push Notifications [RFC8765] and, using the
Advertising Proxy mechanism documented here, to local clients using
Multicast DNS [RFC6762].
1.1. Conventions and Terminology Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. Advertising Proxy
An Advertising Proxy can be a component of any DNS authoritative
server, though it logically makes most sense as a component of a
service registry (a DNS authoritative server that implements Service
Registration Protocol [SRP]). A client can send registration
requests for any valid DNS records to a service registry, though in
practice the most common use is to register the PTR, SRV and TXT
records that describe a DNS-SD service [RFC6763], and the A and AAAA
records that give the IPv4 and IPv6 addresses of the target host
where that service can be reached.
When a service registry accepts a registration request for DNS
records, a service registry that implements an Advertising Proxy also
advertises equivalent records using Multicast DNS on one or more
configured local multicast-capable interfaces. An Advertising Proxy
could also advertise on one or more configured remote multicast-
capable interfaces using a Multicast DNS Relay [RELAY]. For the
purposes of this document, a local multicast-capable interface
directly attached to the host and a remote multicast-capable
interface connected via a relay are considered to be equivalent.
2.1. Name Conflicts
In the event that an SRP client attempts to register a record with a
name that was already created in that registry by a different SRP
client, or is otherwise disallowed by policy, a name conflict is
reported and the new client is required to choose a new name.
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Similarly, Multicast DNS implements first-come-first-served name
allocation. When a registered record is advertised using Multicast
DNS it may suffer a name conflict if a conflicting Multicast DNS
record with that name already exists on that link. In the case of
network failure and subsequent recovery, Multicast DNS can also
signal name conflicts at a later time during the life of a record
registration. For example, if the network link is partitioned at the
time of record registration, the name conflict may not be discovered
until later when the partition is healed.
Specifically, a name conflict can occur:
1. During the SRP validation process, because another SRP client has
already registered the same name.
2. Immediately while the Advertising Proxy is registering the name,
if the Multicast DNS uniqueness probes detect a conflicting
record.
3. After the name has been successfully registered, but before the
response has been sent to the client.
4. After the initial response has been sent to the client.
In the first three cases, the client can be notified of the conflict
at the time of registration, and is expected to choose a new name.
In the last case, SRP clients must be coded defensively to handle the
case where an apparently successful record registration is later
determined to be in conflict, just as existing Multicast DNS clients
have to be coded defensively to handle late conflicts gracefully.
With a sleepy SRP client there may be no way to notify it of the
conflict until it next re-registers. In the case of late conflicts,
the service registry with Advertising Proxy capability is responsible
for selecting a temporary new name to be used until the client
renews. When the client next renews, the service registry informs
the client of the new name the service registry selected on its
behalf. The client can choose to accept that new name, or select a
new name of its own choosing.
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The registration process has several steps. First the hostname
claimed by the SRP client must be registered. Once this has
succeeded, the Advertising Proxy can register all of the service
instances that point to that hostname. When all of these
registrations have succeeded, the service registry can finally send
its response to the SRP client. If any of them fail, they must all
be removed and the client notified of the failure. If the failure is
a result of a name conflict, the response code should be YXDOMAIN.
Other SRP failures are documented in the SRP specification. Any
other failures not contemplated in the SRP specification return
SERVFAIL.
2.1.1. Name Conflicts in Managed Namespaces
In some cases, the name conflict resolution behavior described above
is neither needed nor desirable. For instance, when the set of
expected SRP clients is known to include only clients added with some
kind of commissioning or on-boarding protocol that guarantees that
hostnames are unique, it may cause serious problems to rename such a
device.
In this situation, the Advertising Proxy behavior should be
different: it should be assumed that all names registered with SRP
that survive SRP's first-come, first-serve name conflict detection
are indeed as intended. Any conflict that may be discovered as a
result of advertising those names using mDNS can be assumed to either
be an error or an attack, and there is no benefit to renaming such a
device: it will not be usable under its new name.
In this case, the Advertising Proxy simply performs normal SRP first-
come, first-serve naming and then updates its local idea of the SRP
namespace. This update is then reflected in mDNS. If a conflict is
detected, the Advertising Proxy schedules a new attempt to claim the
name at some time in the future: long enough that these re-attempts
to not generate excessive multicast traffic, but short enough that an
accidental conflict is cured in a reasonable timeframe.
The downside to this approach is that if the device on the multicast
network persists in claiming the name, the SRP client that claimed it
will be unreachable. Networks that use Advertising Proxies
configured to behave in this way should provide a way to rename the
device that is suffering the conflict. However, if the failure is
the result of a malicious attack by a device on the multicast
network, that device will have to be identified and removed before
the attack can be eliminated.
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In order to address this problem, it may be advisable to provide with
a way for the advertising proxy to inform the mDNS service that it
should continue to advertise the name that is in conflict, rather
than ceasing to do so when the conflict is detected.
2.2. Data Translation
As with a Discovery Proxy [RFC8766] some translation needs to be
performed before the Advertising Proxy makes the registered unicast
data visible using Multicast DNS. Specifically, the unicast DNS
domain name suffix configured for Advertising Proxy use is stripped
off and replaced with the top-level label "local".
2.3. No Text-Encoding Translation
As with a Discovery Proxy [RFC8766], an Advertising Proxy does no
translation between text encodings [RFC6055]. Specifically, an
Advertising Proxy does no translation between Punycode encoding
[RFC3492] and UTF-8 encoding [RFC3629], either in the owner name of
DNS records or anywhere in the RDATA of DNS records (such as the
RDATA of PTR records, SRV records, NS records, or other record types
like TXT, where it is ambiguous whether the RDATA may contain DNS
names). All bytes are treated as-is with no attempt at text-encoding
translation. A server implementing DNS-based Service Discovery
[RFC6763] will use UTF-8 encoding for its unicast DNS-based record
registrations, which the Advertising Proxy passes through without any
text-encoding translation to the Multicast DNS subsystem. Queries
from peers on the configured multicast-capable interface are answered
directly from the advertised data without any text-encoding
translation.
2.4. No Address Suppression
Unlike a Discovery Proxy [RFC8766], an Advertising Proxy does not
need to selectively suppress link-local [RFC3927] [RFC4862] or other
addresses. Since the clients of the service registry are registering
their records in a unicast DNS namespace, there is a presumption they
they will only register addresses with sufficient scope to be usable
by the anticipated clients. No further filtering or suppression by
the service registry is required. In most cases it is acceptable for
devices registering with a service registry to register all of their
available addresses, and a client implementing Happy Eyeballs
[RFC8305] connecting to that service will automatically select an
appropriate address to use.
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2.5. No Support for Reconfirm
For network efficiency, Multicast DNS [RFC6762] uses fairly long
record lifetimes (typically 75 minutes). When a client is unable to
reach a service that it discovered, Multicast DNS provides a
"reconfirm" mechanism that enables the client to signal to the
Multicast DNS subsystem that its cached data may be suspect, which
causes the Multicast DNS subsystem to reissue queries, and remove the
stale records if the queries are not answered.
Similarly, when using unicast service discovery with a Discovery
Proxy [RFC8766], the DNS Push Notifications [RFC8765] protocol
provides the RECONFIRM mechanism to signal that the Discovery Proxy
should perform a local Multicast DNS reconfirm operation to re-verify
the validity of the records.
When an Advertising Proxy is used, to support legacy clients that
only implement Multicast DNS, reconfirm operations have no effect.
If a device uses unicast Service Registration Protocol [SRP] to
register its services with a service registry with Advertising Proxy
capability, and the device then gets disconnected from the network,
the Advertising Proxy will continue to advertise those records until
the registrations expire. If a client discovers the service instance
using Multicast DNS and is unable to reach it, and uses a Multicast
DNS reconfirm operation to re-verify the validity of the records,
then the Advertising Proxy will continue to answer on behalf of the
departed device until the record registrations expire. The
Advertising Proxy has no reliable way to determine whether the
additional Multicast DNS queries are due to a reconfirm operation, or
due to other routine causes, like a client being rebooted, or
disconnecting and then reconnecting to the network. The service
registry has no reliable automatic way to determine whether a device
that registered records has failed or disconnected from the network.
Particularly with sleepy battery powered devices, the service
registry does not know what active duty cycle any given service is
expected to provide.
Consequently, reconfirm operations are not supported with an
Advertising Proxy. In cases where use of the reconfirm mechanism is
important, clients should be upgraded to use the unicast DNS Push
Notifications [RFC8765] protocol's RECONFIRM message. This RECONFIRM
message provides an unambiguous signal to the service registry that
it may be retaining stale records. (A future update to the Service
Registration Protocol document [SRP] will consider ways that this
unambiguous signal can be used to trigger expedited removal of stale
data.)
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3. Security Considerations
An Advertising Proxy may made data visible to eavesdroppers on the
configured multicast-capable link(s).
4. IANA Considerations
This document has no IANA actions.
5. References
5.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC6760] Cheshire, S. and M. Krochmal, "Requirements for a Protocol
to Replace the AppleTalk Name Binding Protocol (NBP)",
RFC 6760, DOI 10.17487/RFC6760, February 2013,
<https://www.rfc-editor.org/info/rfc6760>.
[RFC6761] Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
RFC 6761, DOI 10.17487/RFC6761, February 2013,
<https://www.rfc-editor.org/info/rfc6761>.
[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013,
<https://www.rfc-editor.org/info/rfc6762>.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
<https://www.rfc-editor.org/info/rfc6763>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8490] Bellis, R., Cheshire, S., Dickinson, J., Dickinson, S.,
Lemon, T., and T. Pusateri, "DNS Stateful Operations",
RFC 8490, DOI 10.17487/RFC8490, March 2019,
<https://www.rfc-editor.org/info/rfc8490>.
[RFC8765] Pusateri, T. and S. Cheshire, "DNS Push Notifications",
RFC 8765, DOI 10.17487/RFC8765, June 2020,
<https://www.rfc-editor.org/info/rfc8765>.
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[SRP] Lemon, T. and S. Cheshire, "Service Registration Protocol
for DNS-Based Service Discovery", Work in Progress,
Internet-Draft, draft-ietf-dnssd-srp-09, 11 January 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-dnssd-
srp-09>.
5.2. Informative References
[RFC3492] Costello, A., "Punycode: A Bootstring encoding of Unicode
for Internationalized Domain Names in Applications
(IDNA)", RFC 3492, DOI 10.17487/RFC3492, March 2003,
<https://www.rfc-editor.org/info/rfc3492>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/info/rfc3629>.
[RFC3927] Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
Configuration of IPv4 Link-Local Addresses", RFC 3927,
DOI 10.17487/RFC3927, May 2005,
<https://www.rfc-editor.org/info/rfc3927>.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862,
DOI 10.17487/RFC4862, September 2007,
<https://www.rfc-editor.org/info/rfc4862>.
[RFC6055] Thaler, D., Klensin, J., and S. Cheshire, "IAB Thoughts on
Encodings for Internationalized Domain Names", RFC 6055,
DOI 10.17487/RFC6055, February 2011,
<https://www.rfc-editor.org/info/rfc6055>.
[RFC7558] Lynn, K., Cheshire, S., Blanchet, M., and D. Migault,
"Requirements for Scalable DNS-Based Service Discovery
(DNS-SD) / Multicast DNS (mDNS) Extensions", RFC 7558,
DOI 10.17487/RFC7558, July 2015,
<https://www.rfc-editor.org/info/rfc7558>.
[RFC8305] Schinazi, D. and T. Pauly, "Happy Eyeballs Version 2:
Better Connectivity Using Concurrency", RFC 8305,
DOI 10.17487/RFC8305, December 2017,
<https://www.rfc-editor.org/info/rfc8305>.
[RFC8766] Cheshire, S., "Discovery Proxy for Multicast DNS-Based
Service Discovery", RFC 8766, DOI 10.17487/RFC8766, June
2020, <https://www.rfc-editor.org/info/rfc8766>.
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[MCAST] Perkins, C. E., McBride, M., Stanley, D., Kumari, W., and
J. C. Zuniga, "Multicast Considerations over IEEE 802
Wireless Media", Work in Progress, Internet-Draft, draft-
ietf-mboned-ieee802-mcast-problems-13, 4 February 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-mboned-
ieee802-mcast-problems-13>.
[RELAY] Lemon, T. and S. Cheshire, "Multicast DNS Discovery
Relay", Work in Progress, Internet-Draft, draft-ietf-
dnssd-mdns-relay-04, 22 February 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-dnssd-
mdns-relay-04>.
[ROADMAP] Cheshire, S., "Service Discovery Road Map", Work in
Progress, Internet-Draft, draft-cheshire-dnssd-roadmap-03,
23 October 2018, <https://datatracker.ietf.org/doc/html/
draft-cheshire-dnssd-roadmap-03>.
[ZC] Cheshire, S. and D. H. Steinberg, "Zero Configuration
Networking: The Definitive Guide", O'Reilly Media, Inc.,
ISBN 0-596-10100-7, December 2005.
Authors' Addresses
Stuart Cheshire
Apple Inc.
One Apple Park Way
Cupertino, California 95014
United States of America
Phone: +1 (408) 996-1010
Email: cheshire@apple.com
Ted Lemon
Apple Inc.
One Apple Park Way
Cupertino, California 95014
United States of America
Phone: +1 (408) 996-1010
Email: elemon@apple.com
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