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RFC2710 (MLDv1), section 3.7. says:
The length of a received MLD message is computed by taking the
IPv6 Payload Length value and subtracting the length of any IPv6
extension headers present between the IPv6 header and the MLD
message. If that length is greater than 24 octets, that indicates
that there are other fields present *beyond* the fields described
above, perhaps belonging to a *future backwards-compatible* version
of MLD. An implementation of the version of MLD specified in this
document *MUST NOT* send an MLD message longer than 24 octets and
MUST ignore anything past the first 24 octets of a received MLD
message.
RFC3810 (MLDv2), section 8.2.1. states for *listeners* regarding
presence of MLDv1 routers:
In order to be compatible with MLDv1 routers, MLDv2 hosts MUST
operate in version 1 compatibility mode. [...] When Host
Compatibility Mode is MLDv2, a host acts using the MLDv2 protocol
on that interface. When Host Compatibility Mode is MLDv1, a host
acts in MLDv1 compatibility mode, using *only* the MLDv1 protocol,
on that interface. [...]
While section 8.3.1. specifies *router* behaviour regarding presence
of MLDv1 routers:
MLDv2 routers may be placed on a network where there is at least
one MLDv1 router. The following requirements apply:
If an MLDv1 router is present on the link, the Querier MUST use
the *lowest* version of MLD present on the network. This must be
administratively assured. Routers that desire to be compatible
with MLDv1 MUST have a configuration option to act in MLDv1 mode;
if an MLDv1 router is present on the link, the system administrator
must explicitly configure all MLDv2 routers to act in MLDv1 mode.
When in MLDv1 mode, the Querier MUST send periodic General Queries
truncated at the Multicast Address field (i.e., 24 bytes long),
and SHOULD also warn about receiving an MLDv2 Query (such warnings
must be rate-limited). The Querier MUST also fill in the Maximum
Response Delay in the Maximum Response Code field, i.e., the
exponential algorithm described in section 5.1.3. is not used. [...]
That means that we should not get queries from different versions of
MLD. When there's a MLDv1 router present, MLDv2 enforces truncation
and MRC == MRD (both fields are overlapping within the 24 octet range).
Section 8.3.2. specifies behaviour in the presence of MLDv1 multicast
address *listeners*:
MLDv2 routers may be placed on a network where there are hosts
that have not yet been upgraded to MLDv2. In order to be compatible
with MLDv1 hosts, MLDv2 routers MUST operate in version 1 compatibility
mode. MLDv2 routers keep a compatibility mode per multicast address
record. The compatibility mode of a multicast address is determined
from the Multicast Address Compatibility Mode variable, which can be
in one of the two following states: MLDv1 or MLDv2.
The Multicast Address Compatibility Mode of a multicast address
record is set to MLDv1 whenever an MLDv1 Multicast Listener Report is
*received* for that multicast address. At the same time, the Older
Version Host Present timer for the multicast address is set to Older
Version Host Present Timeout seconds. The timer is re-set whenever a
new MLDv1 Report is received for that multicast address. If the Older
Version Host Present timer expires, the router switches back to
Multicast Address Compatibility Mode of MLDv2 for that multicast
address. [...]
That means, what can happen is the following scenario, that hosts can
act in MLDv1 compatibility mode when they previously have received an
MLDv1 query (or, simply operate in MLDv1 mode-only); and at the same
time, an MLDv2 router could start up and transmits MLDv2 startup query
messages while being unaware of the current operational mode.
Given RFC2710, section 3.7 we would need to answer to that with an MLDv1
listener report, so that the router according to RFC3810, section 8.3.2.
would receive that and internally switch to MLDv1 compatibility as well.
Right now, I believe since the initial implementation of MLDv2, Linux
hosts would just silently drop such MLDv2 queries instead of replying
with an MLDv1 listener report, which would prevent a MLDv2 router going
into fallback mode (until it receives other MLDv1 queries).
Since the mapping of MRC to MRD in exactly such cases can make use of
the exponential algorithm from 5.1.3, we cannot [strictly speaking] be
aware in MLDv1 of the encoding in MRC, it seems also not mentioned by
the RFC. Since encodings are the same up to 32767, assume in such a
situation this value as a hard upper limit we would clamp. We have asked
one of the RFC authors on that regard, and he mentioned that there seem
not to be any implementations that make use of that exponential algorithm
on startup messages. In any case, this patch fixes this MLD
interoperability issue.
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
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i) RFC3810, 9.2. Query Interval [QI] says:
The Query Interval variable denotes the interval between General
Queries sent by the Querier. Default value: 125 seconds. [...]
ii) RFC3810, 9.3. Query Response Interval [QRI] says:
The Maximum Response Delay used to calculate the Maximum Response
Code inserted into the periodic General Queries. Default value:
10000 (10 seconds) [...] The number of seconds represented by the
[Query Response Interval] must be less than the [Query Interval].
iii) RFC3810, 9.12. Older Version Querier Present Timeout [OVQPT] says:
The Older Version Querier Present Timeout is the time-out for
transitioning a host back to MLDv2 Host Compatibility Mode. When an
MLDv1 query is received, MLDv2 hosts set their Older Version Querier
Present Timer to [Older Version Querier Present Timeout].
This value MUST be ([Robustness Variable] times (the [Query Interval]
in the last Query received)) plus ([Query Response Interval]).
Hence, on *default* the timeout results in:
[RV] = 2, [QI] = 125sec, [QRI] = 10sec
[OVQPT] = [RV] * [QI] + [QRI] = 260sec
Having that said, we currently calculate [OVQPT] (here given as 'switchback'
variable) as ...
switchback = (idev->mc_qrv + 1) * max_delay
RFC3810, 9.12. says "the [Query Interval] in the last Query received". In
section "9.14. Configuring timers", it is said:
This section is meant to provide advice to network administrators on
how to tune these settings to their network. Ambitious router
implementations might tune these settings dynamically based upon
changing characteristics of the network. [...]
iv) RFC38010, 9.14.2. Query Interval:
The overall level of periodic MLD traffic is inversely proportional
to the Query Interval. A longer Query Interval results in a lower
overall level of MLD traffic. The value of the Query Interval MUST
be equal to or greater than the Maximum Response Delay used to
calculate the Maximum Response Code inserted in General Query
messages.
I assume that was why switchback is calculated as is (3 * max_delay), although
this setting seems to be meant for routers only to configure their [QI]
interval for non-default intervals. So usage here like this is clearly wrong.
Concluding, the current behaviour in IPv6's multicast code is not conform
to the RFC as switch back is calculated wrongly. That is, it has a too small
value, so MLDv2 hosts switch back again to MLDv2 way too early, i.e. ~30secs
instead of ~260secs on default.
Hence, introduce necessary helper functions and fix this up properly as it
should be.
Introduced in 06da92283 ("[IPV6]: Add MLDv2 support."). Credits to Hannes
Frederic Sowa who also had a hand in this as well. Also thanks to Hangbin Liu
who did initial testing.
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Cc: David Stevens <dlstevens@us.ibm.com>
Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
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