Re: Broadcasting UDP

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On 25/04/2016 16:18, ahlan@marriott.org wrote:

> This of course works but by using Broadcast_Inet_Addr
> (255.255.255.255) then we actually transmit more UDP messages than is
> strictly necessary.
>
> For example if I have two adapters one with 100.13.5.200 and the
> other  169.254.7.7 and I send a UDP broadcast on each adapter then four UDP
> messages are actually transmitted.
> 1) Src=100.13.5.200 Dest=255.255.255.255 on adapter 1
> 2) Src=100.13.5.200 Dest=255.255.255.255 on adapter 2
> 3) Src=169.254.7.7 Dest=255.255.255.255 on adapter 1
> 4) Src=169.254.7.7 Dest=255.255.255.255 on adapter 2
>
> This is because the destination address 255.255.255.255 is for all adapters.

But the socket is bound to the address of a concrete adapter. 
Send_Socket should not leak through another adapter. It does not look right.

> If I could find out the subnet masks I could derive the subnet
> directed broadcast addresses and use these as the destination addresses
> and thereby reduce the traffic.

AFAIK there is no means for that in the Berkeley sockets.

> Not by much I grant you but every little helps and besides it would
> be  a little more elegant than using the crude broadcast to all on all
> adapters approach.

Broadcasts with alien source address is not a big problem. I hope you 
are not doing anything but network discovery over broadcast.

> Any ideas?

Maybe, there is something with network settings (bridging etc). 
Broadcast packets should not cross segment boundaries.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de