Iptables/Netfilter classical sample

Netfilter rules example

#!/bin/sh
 
# Interfaces
EXTIF1=eth0
EXTIF2=usb0
INTIF1=vboxnet0
INTIF2=wlan0
 
# Delete all existing rules
iptables -F
iptables -X
iptables -t nat -F
iptables -t mangle -F
 
# Always accept loopback traffic
iptables -A INPUT -i lo -j ACCEPT
 
# Allow established connections, and those not coming from the outside
iptables -A INPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
iptables -A FORWARD -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
iptables -A INPUT -m conntrack --ctstate NEW -i $INTIF1 -j ACCEPT
iptables -A INPUT -m conntrack --ctstate NEW -i $INTIF2 -j ACCEPT
 
# Allow outgoing connections from the LAN side
iptables -A FORWARD -i $INTIF1 -j ACCEPT
iptables -A FORWARD -i $INTIF2 -j ACCEPT
 
# Connect LANs
iptables -A FORWARD -i $INTIF -o $INTIF2 -j ACCEPT
iptables -A FORWARD -i $INTIF2 -o $INTIF -j ACCEPT
iptables -A FORWARD -i $INTIF -o $INTIF -j ACCEPT
iptables -A FORWARD -i $INTIF2 -o $INTIF2 -j ACCEPT
 
# Masquerade
iptables -t nat -A POSTROUTING -o $EXTIF1 -j MASQUERADE
iptables -t nat -A POSTROUTING -o $EXTIF2 -j MASQUERADE
 
# Don't forward from the outside to the inside
iptables -A FORWARD -i $EXTIF1 -j REJECT
iptables -A FORWARD -i $EXTIF2 -j REJECT
 
# MTU
iptables -I FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu
 
# Allow IP forward
echo 1 > /proc/sys/net/ipv4/ip_forward
 
# Switch On rp_filter
for f in /proc/sys/net/ipv4/conf/*/rp_filter ; do echo 1 > $f ; done
 
# Block all outgoing connections from $INTIF2 to all except several ips/subnets
iptables -I FORWARD -i $INTIF2 -j DROP
iptables -I FORWARD -i $INTIF2 -d 192.168.1.X -j ACCEPT
iptables -I FORWARD -i $INTIF2 -d 192.168.1.Y -j ACCEPT
iptables -I FORWARD -i $INTIF2 -d 192.168.1.Z -j ACCEPT
iptables -I FORWARD -i $INTIF2 ! -d 192.168.0.0/16 -j ACCEPT
iptables -I FORWARD -i $INTIF2 ! -s 192.168.3.0/24 -j DROP # bad wi-fi IPs