Re: IP Multicast

[Eric Auge <eau%phear.org@localhost>]

Neel Sheyal wrote:
> Hi,

Hello,

>     I have two netbsd hosts both with two NICs.
>
> HOSTA: ex0: 192.168.21.2/24  and ex1: 172.16.18.2/16
> HOSTB: ex0: 192.168.22.2/24  and ex1: 172.18.20.2/16
>
> There is an application sending multicast traffic on hostA to
> 239.198.1.2. There is another application on hostB which is subsribing
> to this group.
>
> The application on hostA or hostB does not specific the outgoing
> interface for sending traffic or joining. I have also not send the
> routing table with the multicast group information.

I might be wrong but peeking into various manpages and specifically 
ip(4), you can find this :

[...]
     For hosts with multiple interfaces, each multicast transmission is 
sent from the primary network interface.  The IP_MULTICAST_IF option 
overrides the default for subsequent transmissions from a given socket:
[...]

and

[...]
     A host must become a member of a multicast group before it can 
receive datagrams sent to the group.  To join a multicast group, use the
IP_ADD_MEMBERSHIP option:
[...]

and

[...]
     imr_interface should be INADDR_ANY to choose the default multicast 
interface, or the IP address of a particular multicast-capable interface 
if the host is multihomed.  Membership is associated with a single 
interface; programs running on multihomed hosts may need to join the 
same group on more than one interface.  Up to IP_MAX_MEMBERSHIPS 
(currently 20) memberships may be added on a single socket.
[...]

> How does the OS know which interface to send the multicast traffic as in hostA?
>
> How does the OS know which interface to subscribe to for receiving
> multicast traffic as in hostB?

The program handling the multicast traffic (i.e. ospfd) need to make the 
socket join the group on specific interface(s), this is according to my 
understanding reading ip(4).

I have not tried it.

HTH,
Regards,
Eric.

> Thanks,
> ~~Neel