Linux will quietly proceed as if the data were sent even if the carrier
is down and nothing actually happened. There is still a tiny race
condition where the carrier could drop between the check and the actual
write, but we really can't do anything about that and it is a very
small race.
Mostly reverts the previous commit and instead teaches ndhc to properly
handle the case when it is communicating with a DHCP relay agent on
its local segment rather than directly with a DHCP server.
different segment.
The network fingerprinting would never complete if the DHCP server was
on a different segment before this change, since it would be impossible
for the ARP messages sent by ndhc to ever reach the DHCP server
(and vice-versa).
Now just give up trying to find the hardware address after two tries
and assume that the DHCP server cannot be reached by ARP.
An alternative would be to fingerprint the relay agent instead, but
to do so would require a lot more work as the giaddr field is only
meaningful in the client->server message path, not in the
server->client path. Thus it would require gathering the source IP
for DHCP replies sent by unicast or broadcast and ferrying along
this information to the ARP checking code where it would be used
in place of the DHCP server address.
This is entirely possible to do, but is quite a bit more work.
