Internet Engineering Task Force T. Lemon
Internet-Draft 秦 良 (L. Qin)
Intended status: Standards Track Apple Inc.
Expires: 14 September 2023 13 March 2023
Multicast DNS conflict resolution using the Time Since Received (TSR) RR
draft-tllq-tsr-03
Abstract
This document specifies a new conflict resolution mechanism for DNS,
for use in cases where the advertisement is being proxied, rather
than advertised directly, e.g. when using a combined DNS-SD
Advertising Proxy and SRP registrar. A new DNS RR is defined that
communicates the time at which the set of resource records on a
particular DNS owner name was most recently updated.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 14 September 2023.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Current Behavior . . . . . . . . . . . . . . . . . . . . 3
1.2. Problem Statement . . . . . . . . . . . . . . . . . . . . 4
2. Time Since Received Resource Record . . . . . . . . . . . . . 5
3. mDNS Registrar Behavior . . . . . . . . . . . . . . . . . . . 5
3.1. When sending a probe . . . . . . . . . . . . . . . . . . 5
3.2. When processing a probe . . . . . . . . . . . . . . . . . 6
3.2.1. Processing multiple questions in a probe . . . . . . 7
3.2.2. The effect of network latency on time computations . 7
3.3. When sending a reply . . . . . . . . . . . . . . . . . . 7
3.4. When processing a reply . . . . . . . . . . . . . . . . . 7
4. Internal Handling of TSR records . . . . . . . . . . . . . . 7
5. Timeliness of Conflict Resolution . . . . . . . . . . . . . . 8
6. Legacy Registrars . . . . . . . . . . . . . . . . . . . . . . 8
7. When to Use TSR . . . . . . . . . . . . . . . . . . . . . . . 8
8. Registrant API considerations . . . . . . . . . . . . . . . . 8
9. Security Considerations . . . . . . . . . . . . . . . . . . . 9
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
11. Informative References . . . . . . . . . . . . . . . . . . . 9
12. Normative References . . . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
Unlike the Domain Name System [RFC1034], with its authority servers
and delegation of authority, Multicast DNS has no single source of
authority. Because of this, mDNS has a mechanism, conflict
resolution (Section 9 of [RFC6762]) for detecting and fixing
conflicts in mDNS advertisements.
The current goal of mDNS conflict resolution is to prevent a new
service being advertised from taking the place of an existing service
with the same name that is already being advertised. This goal,
however, assumes that the entity advertising an mDNS service is in
fact authoritative for that service. In the case of an Advertising
Proxy [I-D.sctl-advertising-proxy], this is not the case: the source
of truth for the service being advertised is an SRP
[I-D.ietf-dnssd-srp] client.
On a link with more than one SRP registrar, an SRP client may
register with one SRP registrar, and then subsequently update its
registration on a different SRP registrar. Both SRP registrars may
be acting as advertising proxies. If so, the original server may
still be advertising the old SRP registration using mDNS. If the
information in the new SRP registration is identical to that in the
old registration, this is not a problem. However if some information
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has changed (e.g., a new IP address has been added, or a TXT record
updated), then the new registration will be seen to be in conflict
with the old registration.
In the case of such a conflict, the current behavior of mDNS is for
the older (stale) registration to win, and the newer (current)
information to be discarded. This behavior, which is entirely
correct for services that are advertising on their own behalf, is
exactly wrong when a service registration is being proxied.
1.1. Current Behavior
When a new service is to be advertised, the server that wishes to
advertise its service typically registers the service with a central
mDNS registrar on the host on which it is running. This mDNS
registrar may have an internal database of services already
registered, and may detect a conflict with one of those services.
This can be true whether the conflicting database entry is data for
which the mDNS registrar is authoritative, or data it has received
via mDNS and cached.
In the case of such a conflict, no network transaction is required:
the mDNS registrar detects it locally. It addresses the conflict in
one of two ways. The first alternative is that the mDNS registrar
will report the conflict to the server as an error, which the server
must fix. Alternatively, the server may have indicated that the mDNS
mDNS registrar should automatically choose a new name for it, in
which case the mDNS registrar does so automatically, without
notifying the server.
Once any locally-detectable conflicts have been resolved, the mDNS
registrar probes (see Section 8.1 of [RFC6762]) local network to see
if any other host has already registered a service the conflicts with
the proposed new service. If such a service is present on the
network, the mDNS registrar follows the same process previously
described, either reporting the error to the server or automatically
choosing a new name.
The effect of this approach is that generally whichever server first
registers a service under a particular name wins. If a server comes
along later and registers the same service with conflicting
information, the newcomer’s information is rejected.
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1.2. Problem Statement
The current behavior works well for services registering on their own
behalf. However, for example in the case of an SRP registrar, it
works poorly: an SRP registrar acting as an advertising proxy proxies
the contents of its registration dataset(s) using mDNS. The source
of truth for information in such datasets is whatever service has
registered with the SRP registrar, not the SRP registrar itself.
In the case of an advertising proxy proxying an SRP dataset, what we
want is not the oldest information, but the newest. When the SRP
client is able to continue registering with the same SRP registrar,
this works well: stale data is automatically removed and replaced
with current data. However, if more than one SRP registrar is
available, and for some reason the original SRP registrar with which
the registration was completed is still operating but no longer
reachable (e.g., in the case of a network partition), the SRP client
will wind up registering with a different SRP registrar. Similarly,
if the SRP service is being advertised using an anycast address,
there is no guarantee that the SRP renewal will be delivered to the
same SRP registrar.
When the SRP client registers with a different SRP registrar, the
behavior we get with the current conflict resolution approach is that
the SRP client will be given a new name, and both the old (stale)
advertisement (A) and the new (more recent) advertisement (A’) will
be discoverable as separate services.
This creates a new burden on consumers of such services: they need to
parse through the whole list of services of their type, using
metadata from the TXT record in the service instance data, if
possible, to determine that service A and service A’ are the same
service. If no such information is present in the TXT record, the
only way to determine that one of these two registrations is stale is
to attempt to use the advertised service, which may no longer be
reachable if, for example, the change that produced the conflict was
an IP address change. When the SRP lease for the stale service
expires, that service's advertisement will be removed, and the
service will no longer be discoverable under the original name, even
if the IP address hasn't changed.
This document proposes an enhancement to the current conflict
resolution algorithm for mDNS, which allows an mDNS proxy to report
the time at which it received the registration it is newly
advertising. This is done using a new Time Since Received RR, which
is attached to the name of the registration.
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2. Time Since Received Resource Record
The Time Since Received (TSR) RR is given the same owner name as the
RRset or RRsets for which it is asserting a received time. When a
service registration is successful, the mDNS registrar records the
wall clock time at which the registration request was received. This
may be the current time, or a time specified by a proxy service that
is doing the registration. This time is only recorded if the service
requesting the registration specifies it; otherwise, the time of
receipt is not recorded.
The TSR RR contains the difference, in seconds, between the the time
at which the TSR record is being generated and the time of receipt
for recorded for that owner name. If this difference is greater than
seven days (7 * 24 * 60 * 60), the mDNS registrar MUST use a value of
seven days rather than the larger value.
The resource record is formatted as described in Section 3.2.1 of
[RFC1035]. The RDATA consists of the time offset in the form of a
32-bit unsigned number in network byte order.
3. mDNS Registrar Behavior
3.1. When sending a probe
When probing, for each ANY RR in the question section of the probe
query, if a time of receipt has been recorded for the owner name of
that RR, the mDNS registrar generates a TSR record according to the
method described in Section 2. This TSR RR is then added to the
authority section of the query, along with the contents of all
conflict-producing RRsets it has recorded on that owner name.
A conflict-producing RRset is an RRset with an RRtype that can
produce a conflict. Most RRtypes can produce conflicts. RRtypes
that do not produce conflicts include the PTR RRtype and the TSR
RRtype.
This is new behavior: mDNS registrars that do not use the TSR record
typically do not include all such records in the authority section.
E.g., for a DNS-SD service instance, only the SRV record is included,
not the TXT record, in order to conserve space in the probe packet.
This doesn't work for TSR probes, because the mDNS registrar
receiving the probe needs to be able to determine if there is a
conflict by looking at the probe message alone.
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When constructing the authority section for a probe, all of the RRs
on any given name MUST be included in the same probe message. If
there isn't enough space to fit all the RRs for a name into the
authority section of a particular probe message, that name MUST NOT
appear in the question of that message.
In the case where the set of records on a particular name will not
fit into a single probe packet, the TSR option MUST NOT be used.
3.2. When processing a probe
A probe is an mDNS query that includes an authority section.
When an mDNS registrar receives a probe query, it processes it as
usual, unless it discovers a conflict. When data in the authority
section of the probe that matches a particular RR in the question
section, and that data conflicts with data in the mDNS registrar's
own database, as described in Section 8.1 of [RFC6762], this is
considered to be a probe conflict.
Note that TSR records should not exist in the authority database of
any mDNS registrar. This would not make sense, because the TSR
record encodes a relative time, and in order to generate a TSR
record, the mDNS registrar needs an absolute (wall clock) time. They
are also not conflict-producing records. So a TSR record can never
be the basis for detecting a conflict.
After a probe conflict has been detected, the registrar checks the
authority section of the probe query for a TSR record with the same
owner name as the name of the question for which a potential conflict
was detected. It also checks for a recorded time of receipt on that
owner name in its own authority database. If no TSR record is found,
or no recorded time of receipt is present, then the probe conflict is
processed normally as described in sections 8.1 (paragraph 8), 8.2
and 9.
If both the TSR record in the authority section and the recorded time
of receipt are found, then before doing normal processing of the
probe conflict, the two times are compared. This is done by
subtracting the value contained in the TSR record from the current
wall clock time to determine the time of receipt of the data in the
probe. If the time of receipt in the probe is more recent than
(greater then) the time of receipt in the registrar's authority
database, then the registrar sees this as losing the conflict and
removes its authority records.
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Otherwise, as when both times are not available for comparison, the
probe conflict is processed normally as described in sections 8.1 and
Section 8.2 of [RFC6762].
3.2.1. Processing multiple questions in a probe
Multicast DNS probes can contain more than one question. The
question and authoritative data for each owner name are handled
separately, as described above, so it may be that data on some owner
names is handled based on the TSR conflict detection process, while
data on other owner names is handled using the conflict detection
mechanism described in Section 9 of [RFC6762].
3.2.2. The effect of network latency on time computations
Because TSR computations are affected by network latency, comparisons
can’t be considered accurate. It is therefore necessary to tolerate
some amount of error. In practice, however, it should generally not
be the case that two advertising proxies receive SRP updates from the
same SRP client at nearly the same time. So it should always be the
case either that there is a clear ordering to the timestamps, or that
there is no conflict in the data. For example with anycast, a
retransmission could go to a different SRP registrar, but in this
case both servers would simultaneously receive identical data, so the
close ordering or even equality of the timestamps should not affect
the outcome.
3.3. When sending a reply
Records with the TSR RRtype MUST NOT be send in any section of an
mDNS reply.
3.4. When processing a reply
TSR records are not allowed in replies. An mDNS registrar receiving
an mDNS reply containing a TSR record MUST silently ignore the TSR
record.
4. Internal Handling of TSR records
The TSR record that is sent on the wire is expressed in seconds
relative to the time of receipt of the registration. In order to
derive a TSR record, the registrar must remember the time at which
the registration occurred. This time is recorded as an absolute
time, not a relative time. We refer to this as the time of receipt.
When sending a TSR RR, the registrar computes the difference between
the current time and the time of receipt, which must always be in the
past. This difference, which should be a positive integer, is
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converted to seconds, and that unsigned value is then used to
synthesize the TSR RR.
5. Timeliness of Conflict Resolution
It is expected that if a conflict exists, it will be recent, and will
be resolved quickly. Different hosts may be able to record shorter
or longer time differences. However, because of this expectation of
recentness, mDNS registrars should never need to report a TSR of
longer than seven days. It’s reasonable to expect that every mDNS
implementation should be able to remember time intervals of at least
seven days.
6. Legacy Registrars
An mDNS registrar that does not support TSR and receives a probe
containing a TSR record will treat the TSR record as part of the
authoritative data being probed, and will see that data as a
conflict. This will produce the correct behavior: the non-
implementing registrar will respond with its authoritative data, and
the probing registrar will either see a conflict (and treat the probe
as having failed) or will see no conflict, since the non-implementing
registrar will not have sent a TSR record.
mDNS registrars that support the TSR record MUST NOT send any TSR
record in an mDNS response. Consequently, non-implementing mDNS
registrars will never see a TSR record other than in a probe, which
we have shown will be handled correctly.
7. When to Use TSR
TSR is only relevant for mDNS proxies. Regular (non-proxy) mDNS
registrants are not expected to use it, since it will produce the
wrong behavior for this use case. An mDNS registrant that is a proxy
MUST explicitly request that a TSR be used for conflict resolution.
mDNS registrars MUST NOT record a time of receipt unless the
registrant has specifically requested it.
8. Registrant API considerations
When an mDNS proxy registers a service and requests the use of a time
of receipt, the proxy MUST specify when it received the registration.
In order to support this, the API is required not only to allow the
registrant to specify that TSR conflict resolution is wanted, but
must also provide a way for the proxy to specify an absolute time at
which the registration was received.
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This is important, for example, in the case of SRP Replication
[I-D.lemon-srp-replication], where an SRP registrar may receive a
registration from a peer during startup synchronization. This
registration will have occurred at some significant amount of time in
the past, and so it would be incorrect for the mDNS proxy receiving
the registration to use the time that the mDNS proxy registers the
service as the time of receipt.
9. Security Considerations
The TSR RR is an optimization: it ameliorates an edge case for mDNS
proxies. A malicious host on the same link could use the TSR RR to
win conflict resolution processes. However, because TSR is only used
by proxies, this technique will not work for normal mDNS service
registrations: in that case, normal mDNS conflict resolution is done,
and the attacker gains no benefit from using TSR.
Whether or not an mDNS registration has a recorded time of receipt,
an attacker can deny service by announcing its own conflicting data
and then answering the subsequent probe as described in Section 9 of
[RFC6762]. Because it does not include a TSR record in its authority
section, it can win the simultaneous conflict resolution process that
follows its bogus announcement.
So the TSR-based conflict resolution process creates no new
vulnerability. Addressing the existing vulnerability is out of scope
for this document. Protocols that rely on mDNS MUST NOT assume that
mDNS service is secure or private. If security (authentication,
authorization and/or secrecy) are needed, these must be provided at
the application layer, or by using DNSSEC rather than mDNS for
service discovery.
10. IANA Considerations
IANA is requested to allocate a new RR Type from the DNS Resource
Record (RR) TYPEs registry for the 'Time Since Received' Resource
Record. The type shall be 'TSR'. The value shall be allocated by
IANA. The meaning shall be 'Multicast DNS Time Since Received".
Reference shall refer to this document, once published. There is no
template specified, and IANA shall determine the registration date.
11. Informative References
12. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/info/rfc1034>.
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[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013,
<https://www.rfc-editor.org/info/rfc6762>.
[I-D.lemon-srp-replication]
Lemon, T., Keshavarzian, A., and J. Hui, "Automatic
Replication of DNS-SD Service Registration Protocol
Zones", Work in Progress, Internet-Draft, draft-lemon-srp-
replication-03, 22 February 2023,
<https://datatracker.ietf.org/doc/html/draft-lemon-srp-
replication-03>.
[I-D.sctl-advertising-proxy]
Cheshire, S. and T. Lemon, "Advertising Proxy for DNS-SD
Service Registration Protocol", Work in Progress,
Internet-Draft, draft-sctl-advertising-proxy-02, 12 July
2021, <https://datatracker.ietf.org/doc/html/draft-sctl-
advertising-proxy-02>.
[I-D.ietf-dnssd-srp]
Lemon, T. and S. Cheshire, "Service Registration Protocol
for DNS-Based Service Discovery", Work in Progress,
Internet-Draft, draft-ietf-dnssd-srp-18, 9 January 2023,
<https://datatracker.ietf.org/doc/html/draft-ietf-dnssd-
srp-18>.
Authors' Addresses
Ted Lemon
Apple Inc.
One Apple Park Way
Cupertino, California 95014
United States of America
Email: mellon@fugue.com
Liang Qin
Apple Inc.
One Apple Park Way
Cupertino, California 95014
United States of America
Email: Leonqin0101@gmail.com
Additional contact information:
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秦良
Apple Inc.
One Apple Park Way
Cupertino, California 95014
United States of America
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