Different behavior of ping'ing INADDR_BROADCAST?

[Bruce M. Simpson]

LI Xin wrote:
> Dear colleagues,
>
> I have a strange question about our way of handling INADDR_BROADCAST,
> the behavior looks different from all other operating systems I have
> tried, except Mac OS X ;-)
>
> By ping'ing 255.255.255.255 from FreeBSD (mostly RELENG_6 with some
> unrelated patches) or Mac OS X, I got response from another subnet (I
> guess that there is some configuration problem in the network, though),
> but no boxes (running various operating systems) on local network
> responds the ping.
>
> However, with OpenBSD, Linux and Windows, ping'ing 255.255.255.255 would
> get response from local network.
>
> Just curious why there is such difference.  Literally, I think
> INADDR_BROADCAST is supposed to reach local network nodes, no?
>   
With FreeBSD's stack, sending packets to the undirected broadcast 
address INADDR_BROADCAST will result in the first ifnet with IPv4 
configured and IFF_BROADCAST set being selected as the source ifnet. See 
ip_output.c for details.

In my local network, pinging 255.255.255.255 from my FreeBSD laptop 
(running 6.2-RC1) results in a single unicast ICMP reply from the edge 
router, with its source address on the same LAN.

The IP_SENDONES socket option may be used to select the source interface 
for undirected broadcasts, by sending to a directed broadcast address. 
The stack will munge the directed address to 255.255.255.255 before it 
goes on the wire. This came from BSD/OS; See ip(4) for details.

You might want to take a look at NetBSD's stack, which has recently had 
IPv4 source address selection logic added to it to support schemes such 
as link-local addressing (Zeroconf/Rendezvous).

It would be great if someone had time to look at this and perhaps port it.

Regards,
BMS