slight modification after disq's post http://openwrt.org/forum/viewtopic.php?t=1227

SVN-Revision: 596

openwrt/target/default/target_skeleton/etc/init.d/S45firewall

@@ -24,11 +24,13 @@ fi
 # defaults
 
 $IPT -t filter -A INPUT -p icmp -j ACCEPT
-$IPT -t filter -A INPUT -p 47 -j ACCEPT
+$IPT -t filter -A INPUT -p 47 -j ACCEPT # allow GRE
+$IPT -t filter -A INPUT -i $WAN -p tcp --syn --tcp-option \! 2 -j  DROP
 $IPT -t filter -A INPUT -i $WAN -p tcp -j REJECT --reject-with tcp-reset 
 $IPT -t filter -A INPUT -i $WAN -j REJECT --reject-with icmp-port-unreachable 
 
-$IPT -t filter -A FORWARD -i $WAN -m state --state NEW,INVALID -j DROP
+$IPT -t filter -A FORWARD -i br0 -o br0 -j ACCEPT
+$IPT -t filter -A FORWARD -i $WAN -m state --state NEW -j DROP
 $IPT -t filter -A FORWARD -o $WAN -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu
 
 $IPT -t nat -A POSTROUTING -o $WAN -j MASQUERADE