Firewalls serios con iptables

Que tiempos aquellos en que poníamos "echo 1 > /proc/sys/net/ipv4/ip_forward" para routear nuestras redes internas!
Hay muchas formas de construirse un buen firewall, una de ellas es copiando y pegando este script (la otra es leyéndose la documentación, pero seamos realistas...):

#!/bin/bash
SYSCTL="/sbin/sysctl -w"
IPT="/sbin/iptables"
INET_IFACE="eth0" #Interface conectado a Inet
INET_IP="192.168.0.2" #IP del iface conectado a Inet
LAN_IFACE="eth1" #Interface conectado a la Lan
LAN_IP="192.168.1.1" #IP del iface conectado a la Lan
LAN_NET="192.168.1.0/24"
LAN_BCAST="192.168.1.255"
LOCAL_IFACE="lo"
LOCAL_IP="127.0.0.1"

#Carga de módulos
modprobe ip_tables
modprobe ip_conntrack
modprobe ip_nat_ftp
modprobe ip_conntrack_ftp #Estos dos últimos por si tenemos un ftp no pasivo

#Parámetros del kernel
$SYSCTL net.ipv4.ip_forward="1"
$SYSCTL net.ipv4.tcp_syncookies="1"
$SYSCTL net.ipv4.conf.all.rp_filter="1"
$SYSCTL net.ipv4.icmp_echo_ignore_broadcasts="1"
$SYSCTL net.ipv4.conf.all.accept_source_route="0"
$SYSCTL net.ipv4.conf.all.secure_redirects="1"
$SYSCTL net.ipv4.conf.all.log_martians="1"

#Borrado de reglas establecidas
$IPT -P INPUT ACCEPT
$IPT -P FORWARD ACCEPT
$IPT -P OUTPUT ACCEPT
$IPT -t nat -P PREROUTING ACCEPT
$IPT -t nat -P POSTROUTING ACCEPT
$IPT -t nat -P PREROUTING ACCEPT
$IPT -t nat -P POSTROUTING ACCEPT
$IPT -t nat -P OUTPUT ACCEPT
$IPT -t mangle -P PREROUTING ACCEPT
$IPT -t mangle -P OUTPUT ACCEPT
$IPT -F
$IPT -t nat -F
$IPT -t mangle -F
$IPT -X
$IPT -t nat -X
$IPT -t mangle -X

#Política por defecto: DROP
$IPT -P INPUT DROP
$IPT -P OUTPUT DROP
$IPT -P FORWARD DROP
$IPT -N bad_packets
$IPT -N bad_tcp_packets
$IPT -N icmp_packets
$IPT -N udp_inbound
$IPT -N icmp_packets
$IPT -N udp_inbound

$IPT -A bad_packets -p ALL -i $INET_IFACE -s $LAN_NET -j LOG --log-prefix "fp=bad_packets:2 a=DROP "
$IPT -A bad_packets -p ALL -i $INET_IFACE -s $LAN_NET -j DROP
$IPT -A bad_packets -p ALL -m state --state INVALID -j LOG --log-prefix "fp=bad_packets:1 a=DROP "
$IPT -A bad_packets -p ALL -m state --state INVALID -j DROP
$IPT -A bad_packets -p tcp -j bad_tcp_packets
$IPT -A bad_packets -p ALL -j RETURN
$IPT -A bad_tcp_packets -p tcp -i $LAN_IFACE -j RETURN
$IPT -A bad_tcp_packets -p tcp ! --syn -m state --state NEW -j LOG --log-prefix "fp=bad_packets:1 a=DROP "
$IPT -A bad_tcp_packets -p tcp ! --syn -m state --state NEW -j DROP
$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL NONE -j LOG --log-prefix "fp=bad_packets=2 a=DROP "
$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL NONE -j DROP
$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL ALL -j LOG --log-prefix "fp=bad_tcp_packets:3 a=DROP "
$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL ALL -j DROP
$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL FIN,URG,PSH -j LOG --log-prefix "fp=bad_tcp_packets:4 a=DROP "
$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL FIN,URG,PSH -j DROP
$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL SYN,RST,ACK,FIN,URG -j LOG --log-prefix "fp=bad_tcp_packets:5 a=DROP "
$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL SYN,RST,ACL,FIN,URG -j DROP
$IPT -A bad_tcp_packets -p tcp --tcp-flags SYN,RST SYN,RST -j LOG --log-prefix "fp=bad_tcp_packets:6 a=DROP "
$IPT -A bad_tcp_packets -p tcp --tcp-flags SYN,RST SYN,RST -j DROP
$IPT -A bad_tcp_packets -p tcp --tcp-flags SYN,FIN SYN,FIN -j LOG --log-prefix "fp=bad_tcp_packets:7 a=DROP "
$IPT -A bad_tcp_packets -p tcp --tcp-flags SYN,FIN SYN,FIN -j DROP
$IPT -A bad_tcp_packets -p tcp -j RETURN
$IPT -A icmp_packets --fragment -p ICMP -j LOG --log-prefix "fp=icmp_packets:1 a=DROP "
$IPT -A icmp_packets --fragment -p ICMP -j DROP
$IPT -A icmp_packets -p ICMP -s 0/0 --icmp-type 8 -j DROP
$IPT -A icmp_packets -p ICMP -s 0/0 --icmp-type 11 -j ACCEPT
$IPT -A icmp_packets -p ICMP -j RETURN
#Denegar el acceso a Netbios
$IPT -A udp_inbound -p UDP -s 0/0 --destination-port 137 -j DROP
$IPT -A udp_inbound -p UDP -s 0/0 --destination-port 130 -j DROP
##
$IPT -A udp_inbound -p UDP -j RETURN
$IPT -A udp_outbound -p UDP -s 0/0 -j ACCEPT
$IPT -A tcp_inbound -p TCP -s 0/0 --destination-port 22 -j ACCEPT #Aceptamos SSH
$IPT -A tcp_inbound -p TCP -j RETURN
$IPT -A tcp_outbound -p TCP -s 0/0 -j ACCEPT
$IPT -A INPUT -p ALL -i $LOCAL_IFACE -j ACCEPT
$IPT -A INPUT -p ALL -j bad_packets
$IPT -A INPUT -p ALL -d 224.0.0.1 -j DROP
$IPT -A INPUT -p ALL -i $LAN_IFACE -s $LAN_NET -j ACCEPT
$IPT -A INPUT -p ALL -i $LAN_IFACE -d $LAN_BCAST -j ACCEPT
$IPT -A INPUT -p ALL -i $INET_IFACE -m state --state ESTABLISHED,RELATED -j ACCEPT
$IPT -A INPUT -p TCP -i $INET_IFACE -j tcp_inbound
$IPT -A INPUT -p UDP -i $INET_IFACE -j udp_inbound
$IPT -A INPUT -p ICMP -i $INET_IFACE -j icmp_packets
$IPT -A INPUT -m pkttype --pkt-type broadcast -j DROP
$IPT -A INPUT -j LOG --log-prefix "fp=INPUT:99 a=DROP "
$IPT -A FORWARD -p ALL -j bad_packets
$IPT -A FORWARD -p tcp -i $LAN_IFACE -j tcp_outbound
$IPT -A FORWARD -p udp -i $LAN_IFACE -j udp_outbound
$IPT -A FORWARD -p ALL -i $LAN_IFACE -j ACCEPT
$IPT -A FORWARD -i $INET_IFACE -m state --state ESTABLISHED,RELATED -j ACCEPT

#Redireccionar un puerto
#$IPT -A FORWARD -p tcp -i $INET_IFACE --destination-port 80 # --destination 192.168.1.20 -j ACCEPT

$IPT -A FORWARD -j LOG --log-prefix "fp=FORWARD:99 a=DROP "
$IPT -A OUTPUT -m state -p icmp --state INVALID -j DROP
$IPT -A OUTPUT -p ALL -s $LOCAL_IP -j ACCEPT
$IPT -A OUTPUT -p ALL -o $LOCAL_IFACE -j ACCEPT
$IPT -A OUTPUT -p ALL -s $LAN_IP -j ACCEPT
$IPT -A OUTPUT -p ALL -o $LAN_IFACE -j ACCEPT
$IPT -A OUTPUT -p ALL -o $INET_IFACE -j ACCEPT
$IPT -A OUTPUT -j LOG --log-prefix "fp=OUTPUT:99 a=DROP "

$IPT -t nat -A POSTROUTING -o $INET_IFACE -j SNAT --to-source $INET_IP