Re: Socket receives packet to multicast group to which it was not joined since kernel 3.13.10-1

["Yurij M. Plotnikov" <Yurij.Plotnikov@oktetlabs.ru>]

On 12/05/14 21:18, Eric Dumazet wrote:
> On Mon, 2014-05-12 at 20:38 +0400, Yurij M. Plotnikov wrote:
>> On kernel 3.13.10-1 I see that socket joined to one multicast group
>> receives packets to another multicast address. That can be reproduced by
>> the following example:
>>
>> 1. socket(DGRAM) -> 3
>> 2. bind(3, 0.0.0.0:12345) -> 0
>> 3. setsockopt(3, IP_MULTICAST_IF, {224.168.2.9, 7}) -> 0
>> // "7" is correct interface index
>>
>> 4. Send packet from peer host to 224.168.2.9:12345
>> 5. poll({3, POLLIN}) -> 1
>> 6. recv(3) -> <data_length>
>>
>> 5. Send packet from peer host to 225.168.2.9:12345
>> // Note that the address is not the same!
>> 6. poll({3, POLLIN}) -> 1
>> 7. recv(3) -> <data_length>
>>
>> I checked kernel 3.12.6-2, there is no such problem. I have placed
>> simple C-program in attachment to reproduce the behaviour. It should be
>> called:
>> ./mult_recv <mcast_address> <interface index> i.e. in example above:
>> ./mult_recv 224.168.2.9 7
>
> I suspect problem came with commit
> 421b3885bf6d56391297844f43fb7154a6396e12
> ("udp: ipv4: Add udp early demux")
>
> Is that better if you try :
>
> echo 0 >/proc/sys/net/ipv4/ip_early_demux
>
Yes, this fixes the problem.