Listing 1
#!/bin/bash

IPTABLES=/sbin/iptables
IFCONFIG=/sbin/ifconfig
BRCTL=/usr/sbin/brctl
ROUTE=/sbin/route

MANAGEMENTIP=10.10.10.51
MANAGEMENTGATEWAY=10.10.10.1

# shut down our Ethernet devices
$IFCONFIG eth0 down
$IFCONFIG eth1 down

# bring the Ethernet devices back up with no IP addresses
$IFCONFIG eth0 up 0.0.0.0
$IFCONFIG eth1 up 0.0.0.0

# create our bridge device, and add our Ethernet devices
$BRCTL addbr br0
$BRCTL addif br0 eth0
$BRCTL addif br0 eth1

# add an IP address to the bridge device, this is for management purposes only
$IFCONFIG br0 $MANAGEMENTIP
$ROUTE add default gw $MANAGEMENTGATEWAY

# now for our firewall rules

# note that when bridging these rules are applied against 
# the FORWARD chain

#Allow already established connections and related packets to be forwarded
$IPTABLES -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT

#Stealthly drop SMB traffic - this is just an example of a protocol you could
#drop.

$IPTABLES -A FORWARD -p all -sport 135 -j DROP 
$IPTABLES -A FORWARD -p all -dport 135 -j DROP
$IPTABLES -A FORWARD -p all -sport 137 -j DROP
$IPTABLES -A FORWARD -p all -dport 137 -j DROP
$IPTABLES -A FORWARD -p all -sport 138 -j DROP
$IPTABLES -A FORWARD -p all -dport 138 -j DROP
$IPTABLES -A FORWARD -p all -sport 139 -j DROP
$IPTABLES -A FORWARD -p all -dport 139 -j DROP

#Allow all other traffic out from the 10.10.10.0/24 network
$IPTABLES -A FORWARD -i eth1 -o eth0 -s 10.10.10.0/24 -m state -state NEW -j ACCEPT

#Allow traffic in to the 10.10.10.0/24 networking
$IPTABLES -A FORWARD -i eth0 -o eth1 -d 10.10.10.0/24 -m state -state NEW -j ACCEPT