When the host is configured with very restrictive firewall (default policy
is DROP for all chains, including OUTPUT), the bridge driver for Linux
adds netfilter entries to allow DHCP and DNS requests to go from the VM
to the dnsmasq of the host.

The issue that this commit fixes is the fact that a DROP policy on the OUTPUT
chain blocks the DHCP replies from the host’s dnsmasq to the VM.
As DHCP replies are sent in UDP, they are not caught by any --ctstate ESTABLISHED
rule and so, need to be explicitly allowed.
Signed-off-by: NLénaïc Huard <lenaic@lhuard.fr.eu.org>

Packets sent by guests on virbrN, *or* by dnsmasq on the same, to
- 255.255.255.255/32 (netmask-independent local network broadcast
  address), or to
- 224.0.0.0/24 (local subnetwork multicast range)
are never forwarded, hence it is not necessary to masquerade them.

In fact we must not masquerade them: translating their source addresses or
source ports (where applicable) may confuse receivers on virbrN.

One example is the DHCP client in OVMF (= UEFI firmware for virtual
machines):

  http://thread.gmane.org/gmane.comp.bios.tianocore.devel/1506/focus=2640

It expects DHCP replies to arrive from remote source port 67. Even though
dnsmasq conforms to that, the destination address (255.255.255.255) and
the source address (eg. 192.168.122.1) in the reply allow the UDP
masquerading rule to match, which rewrites the source port to or above
1024. This prevents the DHCP client in OVMF from accepting the packet.

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=709418Signed-off-by: NLaszlo Ersek <lersek@redhat.com>

* Move platform specific things (e.g. firewalling and route
  collision checks) into bridge_driver_platform
* Create two platform specific implementations:
    - bridge_driver_linux: Linux implementation using iptables,
      it's actually the code moved from bridge_driver.c
    - bridge_driver_nop: dumb implementation that does nothing
Signed-off-by: NEric Blake <eblake@redhat.com>