Re: kern/41005: kernel failed to select correct outgoing address for Multicast packets

[David Young <dyoung%pobox.com@localhost>]

On Mon, Mar 16, 2009 at 11:02:54AM +0100, Wolfgang Stukenbrock wrote:
> Hi, see below
>
> David Young wrote:
>
>> On Fri, Mar 13, 2009 at 01:50:00PM +0000, 
>> Wolfgang.Stukenbrock%nagler-company.com@localhost wrote:
>>
>>>> Number:         41005
>>>> Category:       kern
>>>> Synopsis:       kernel failed to select correct outgoing address for 
>>>> Multicast packets
>>>> Confidential:   no
>>>> Severity:       serious
>>>> Priority:       high
>>>> Responsible:    kern-bug-people
>>>> State:          open
>>>> Class:          sw-bug
>>>> Submitter-Id:   net
>>>> Arrival-Date:   Fri Mar 13 13:50:00 +0000 2009
>>>> Originator:     Wolfgang Stukenbrock
>>>> Release:        NetBSD 4.0
>>>> Organization:
>>>>
>>> Dr. Nagler & Company GmbH
>>>     
>>>
>>>> Environment:
>>>>
>>>     
>>>     
>>> System: NetBSD s012 4.0 NetBSD 4.0 (NSW-S012) #9: Fri Mar 13 12:31:52 CET 
>>> 2009 wgstuken@s012:/usr/src/sys/arch/amd64/compile/NSW-S012 amd64
>>> Architecture: x86_64
>>> Machine: amd64
>>>
>>>> Description:
>>>>
>>>     If alias addresses are defined on an interface, the kernel does not use 
>>> the
>>>     information passed to it by IP_MULTICAST_IF option. Instead it always 
>>> uses
>>>     the primary address of the interface.
>>>     This will break e.g. gated on such interfaces.
>>>     The problem is, that the information passed by IP_MULTICAST_IF is only 
>>> used
>>>     to select the interface itself in the call and to pass it back to user 
>>> level
>>>     on a call got getoption IP_MULTICAST_IF under some circumstances.
>>>     But is is ignored when sending packets with .e.g. sendto().
>>>
>>
>> What if you bind(2) the source address that you want?  Aren't the
>> multicast packets sent with the correct source address, then?
>>
>> Dave
>>
> I cannot answer your question at the moment, because the software uses  
> the (RAW) socket to send data to all interfaces and recieves data from  
> everywhere. (But I think it would use that address, because the  
> src-address of the socket is the first one used when the outgooing  
> address is determined by the kernel.)
> But for any software that uses RAW (or UDP) socket for all multicast  
> communication it is impossible to bind to any address without loosing  
> the socket for all other addresses. So bind(2) is no option.
>
> The program that failed was gated. It uses multicast and does some  
> checks on the packet contents and the check fail with the wrong src 
> address.
> E.g. our Solaris the systems uses the address set with IP_MULTICAST_IF  
> and everything works fine.
> The intent of IP_MULTICAST_IF looks to me, that it selects the interface  
> and the address to be used - if an address is specified.
> (remark : I haven't read the RFC's about Multicast, so I may be wrong -  
> even RFC's defines sometimes strange semantics.)
>
> I also haven't completly analysed the way netbsd selects addresses. I've  
> recognised that at the interface level multicast packets send over the  
> socket will ony come there without an address assigned, if there are  
> alias addresses defined on the interface. If there is only one address,  
> it is already set int the packet. Where this is done I haven't analysed.
>
> The whole problem is only related to multicast communication.
> If unicast pakets are send over the socket, the correct src address is  
> selected from the set of alias addresses.

It is my understanding of ip(4) that IP_MULTICAST_IF is only intended
to select the output interface.  I think that in applications where the
source address matters, you can and you should specify it with bind(2).

There is no strong notion of "correct source address" in the kernel,
unless you use 'options IPSELSRC'.

Dave

-- 
David Young             OJC Technologies
dyoung%ojctech.com@localhost      Urbana, IL * (217) 278-3933