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

[Aviv Greenberg]

Every incoming packet goes through "input routing". The result of that
routing is either to receive the packet locally, drop, or forward it.

There is a hidden "local" routing table, that is used to do routing
for incoming packets. It is populated automatically (e.g when u add an
new local interface)

You can display this table and it will help you understand the
behaviour of your system:
ip route list table local


On 04/08/2009, Lev Olshvang <levo at vboxcomm.com> wrote:
>
>  Shahar ,
>  Thanks ....
>
>  see below :
>  Shachar Shemesh wrote:
>  I'll just point out that my Multicast understanding is a bit tenuous.
>
>  Lev Olshvang wrote:
> Evening all,
>
>
>  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.
>
>  Did you change the IP address after the socket is up? Multicast needs to
> perform some handshaking before it can start sending data, and this
> handshake might be done only at the "start".
>
>
>     Since this is my network driver , I  reload it and set IP after reload,
> as well I put some printk in it  so I see data link accepts packages in
> either case.  tcpdump also shows these packets.
>
> Also, I would start the debugging by running a sniffer and seeing whether
> the packets actually leave the machine, and if so, with what destination MAC
> address.
>
>    I have no problem with MAC ( driver gets the packets), I put also some
> printk in net/ipv4/route.c and I see this packets are discarded at IP layer.
>
> Shachar
>  --
> Shachar Shemesh
> Lingnu Open Source Consulting Ltd.
> http://www.lingnu.com
>
>
>
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