/etc/network/interfaces

auto vmbr2 
iface vmbr2 inet static
        address  10.99.0.254
        netmask  255.255.255.0
        bridge-ports none
        bridge-stp off
        bridge-fd 0
        post-up echo 1 > /proc/sys/net/ipv4/ip_forward
        post-up iptables -t nat -A POSTROUTING -s '10.99.0.0/24' -o vmbr0 -j MASQUERADE
        post-down iptables -t nat -D POSTROUTING -s '10.99.0.0/24' -o vmbr0 -j MASQUERADE
        post-up iptables -t nat -A PREROUTING -i vmbr0 -p tcp --dport 9999 -j DNAT --to 10.99.0.100:9999
        post-down iptables -t nat -D PREROUTING -i vmbr0 -p tcp --dport 9999 -j DNAT --to 10.99.0.100:9999
        post-up iptables -t nat -A PREROUTING -i vmbr0 -p udp --dport 11940 -j DNAT --to 10.99.0.100:11940
        post-down iptables -t nat -D PREROUTING -i vmbr0 -p udp --dport 11940 -j DNAT --to 10.99.0.100:11940
        post-up iptables -t nat -A PREROUTING -i vmbr0 -p tcp --dport 11940 -j DNAT --to 10.99.0.100:11940
        post-down iptables -t nat -D PREROUTING -i vmbr0 -p tcp --dport 11940 -j DNAT --to 10.99.0.100:11940
        post-up iptables -t nat -A PREROUTING -i vmbr0 -p tcp --dport 29900 -j DNAT --to 10.99.0.100:29900
        post-down iptables -t nat -D PREROUTING -i vmbr0 -p tcp --dport 29900 -j DNAT --to 10.99.0.100:29900
        post-up iptables -t nat -A PREROUTING -i vmbr0 -p udp --dport 29900 -j DNAT --to 10.99.0.100:29900
        post-down iptables -t nat -D PREROUTING -i vmbr0 -p udp --dport 29900 -j DNAT --to 10.99.0.100:29900