[Devel] Re: [patch 0/2][NETNS49][IPV4][IGMP] activate multicast per namespace

[Daniel Lezcano]

Denis V. Lunev wrote:
> Daniel Lezcano wrote:
>> Eric W. Biederman wrote:
>>> Daniel Lezcano <dlezcano at fr.ibm.com> writes:
>>>
>>>> The following patches activate the multicast sockets for
>>>> the namespaces. The results is a traffic going through differents 
>>>> namespaces. So if there are several applications
>>>> listenning to the same multicast group/port, running in
>>>> different namespaces, they will receive multicast packets.
>>>
>>> At a first glance this feels wrong.  I don't see any per
>>> namespace filtering of multicast traffic.  Unless the
>>> multicast traffic is routed/bridged between namespaces
>>> it should be possible to send multicast traffic in one
>>> namespace and listen for that same traffic in another
>>> namespace and not get it.
>>
>> The described behavior is the case were the namespaces are 
>> communicating via veth like:
>>
>> eth0
>>  |
>>  |        ------------- nsA
>> veth0 <--|--> veth1    |
>>  |        -------------
>>  |
>>  |        -------------nsB
>> veth2 <--|--> veth3    |
>>           -------------
>>
>>
>> If an application is listening in nsA and nsB. And if in nsA, an 
>> application sends multicast traffic, both will receive the packets 
>> because they are routed by the pair device.
>> As you said this is the correct behavior, if we have two machines 
>> hostA and hostB in the same network and both are listening on the 
>> multicast address and if an application on hostA send multicast 
>> packets, both should receive the multicast packets.
>> If the traffic is not routed, multicast will not pass through the 
>> namespaces.
>>
>> The description I gave in the patchset introduction was to describe 
>> such behavior which is, IMHO, important for inter-container 
>> communication.
>> Perhaps, I should have not gave this description which seems to sow 
>> confusion in mind, sorry for that.
>>
>> Anyway, I hope the patchset is ok :)
> 
> hmm, by the way, will this work with macvlan?

I will check that. At the first glance, IMO it will not work if the 
IP_MULTICAST_LOOP option is not set. Need to check ...

> also, I am dumb with multicasts :) who will clone the packet if there 
> are more than one namespace listen and there are some listeners behind 
> ethernet?

For local delivery, the function is:

	__udp4_lib_mcast_deliver

For packet emission and loopbacking packets to ourself, it is:

	ip_mc_output

For behind ethernet, the packet is multicasted to the network, so it is 
up the peers to manage the packet.

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