network 192.168.199.0
    netmask 255.255.255.0
    bridge_ports  none
    bridge_hw 52:54:00:7d:7f:fc
    post-up iptables -t nat -A POSTROUTING -s 192.168.9.0/24 -j MASQUERADE
    post-up  iptables -t mangle -A POSTROUTING -p udp  -j CHECKSUM --checksum-fill 

Note that if you dont' specify hw address for your bridge interface,
Note that if you don't specify hw address for your bridge interface,
windows guests would think they are connected to new network on each
start.

Last line is needed because some picky dhcp clients expect valid
IP checksum on dhcp replies, and linux kernel doesn't it by default
leaving it to network hardware (and we have no hardware on virtual
interface).