If multiple routes to the same network exist, Linux blindly picks
  the first interface (route) based on destination address, which might not be
  the one we're actually offering leases. Rather than relying on this,
  always set the interface for outgoing unicast DHCP packets.

The nasty code with static variable in retry_send() which
avoids looping forever needs to be called on success of the syscall,
to reset the static variable.