Trying to understand multicast packet IP routing - why src IP address affects reception ??//

[Lev Olshvang]

Evening all,

I made some investigation in my Ubuntu Linux , kernel 2.6.30


Here the  case :
I have computer with 2 interfaces. It runs Ubuntu, kernel 2.6.30 and 
application that opens multicast socket and bind it to one of the
interfaces, 192.168.123.1
There are only multicast packets in this net.


Multicast Packets   with dest address 224.45.45.45 and source address 
192.168.123.23  are received by
interface defined in the same network 192.168.123.0 with no problems
and even delivered to application.

But if I change the interface IP address to  other network, these 
packets are ignored at IP layer  (data link layer got it since RX count 
is running according to ifconfig )
I have tcp ip forwarding set to 1 and application sets  multicast socket 
option  to join the multicast group.

I had an understanding that for multicast only  destination MAC address 
really rules  ( interface filters are set to accept specific multicast 
address as a result of muticats join by setsockopt()  )

So the question is -  why   IP SRC address is taken into consideration 
by IP layer and how to disable this feature ??


Thank you very much ..
Lev