Multicast and Gateway

I'm writing an application on Linux that sends UDP multicast packets.
My application fails to send the packets, and it cannot join a
multicast group if a default gateway is not set up. This makes no
sense as multicast doesn't even use gateway addressing. Does anyone
have any idea what might be the problem?

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On 22 Sep 2005 07:29:46 -0700, FLY135 <(E-Mail Removed)> wrote:
> I'm writing an application on Linux that sends UDP multicast packets.
> My application fails to send the packets, and it cannot join a
> multicast group if a default gateway is not set up. This makes no
> sense as multicast doesn't even use gateway addressing. Does anyone
> have any idea what might be the problem?
>

When your application tries to open/write/receive to/from a multicast
address, IGMP sends out "join" requests. These requests have to go
somewhere - your default route is the obvious place for IGMP to send
these requests. If you have no default route, then you could set up
a dummy ethernet device at, say, eth0:0, after which you could use the
route command to send multicast traffic to the dummy device e.g.
route add -net 224.0.0.0 netmask 240.0.0.0 deev eth0:0
(thats from the 'man route')

chris


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Chris Willing wrote:
> When your application tries to open/write/receive to/from a multicast
> address, IGMP sends out "join" requests. These requests have to go
> somewhere - your default route is the obvious place for IGMP to send
> these requests. If you have no default route, then you could set up
> a dummy ethernet device at, say, eth0:0, after which you could use the
> route command to send multicast traffic to the dummy device e.g.
> route add -net 224.0.0.0 netmask 240.0.0.0 deev eth0:0
> (thats from the 'man route')

Thanks Chris... After a bit of searching on the web I did find the
route command you described and that was the solution to the problem.
Athough I was able to use "dev eth0" instead of "dev eth0:0".

The reason why I was confused is that IGMP join requests are sent out
multicast, and multicast MAC addresses are constructed from a formula,
not the MAC address of any particular destination. So IGMP requests
are not sent anywhere in particular. In fact, I found that I could
assign a default gateway that didn't exist on the network and multicast
would work properly.