Previously enabling the interface was the first action in the setup
steps. Linux theoretically do not forward the traffic until proper
IP address and route is added to the interface (depending on rp_filter
setting). But instead of relying on this opaque behavior better setup
anti-spoofing rules earlier. Also, add 'set -o pipefail' for more
reliable error handling.
Note the rules for actual VM traffic (qvm-firewall) are properly
enforced - until those rules are loaded, traffic from appropriate vif
interface is blocked. But this relies on proper source IP address,
anti-spoofing rules need to be setup race-free.
Reported-by: Demi M. Obenour <demiobenour@gmail.com>
68b61c2c6d