Obscure udp multicast issue/bug

[Gene Smith]

When I send to a multicast address with TTL set to 1 (default, restrict 
m/c to local subnet) and my gateway is set to NULL or to an invalid 
address, I get destination unreachable (errno = 118) on sendto() call. 
It fails in ip_output() before it even checks that the destination 
address is multicast.

So if I want to multicast on only my local subnet and there is no 
configured router/gateway to the outside world, I still have to specify 
a reachable host as the gateway to get multicast to work.

I don't know if this is expected behavior or something broken in the BSD 
stack.

-gene