If a packet send failed because the carrier went down without a
netlink notification, then assume the hardware carrier was lost while
the machine was suspended (eg, ethernet cable pulled during suspend).
Simulate a netlink carrier down event and freeze the dhcp state
machine until a netlink carrier up event is received.
The ARP code is not yet handling this issue everywhere, but the
window of opportunity for it to happen there is much shorter.
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.
Without this change, it is possible for malicious UDP packets to
make the function read past the end of a buffer.
If this was ever a possibility in ndhc, the previous commit fixed
that issue, but there is no reason for udp_checksum() to have
such a subtle precondition to proper use. This change also makes
it easier to audit correctness.
