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
 
# 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
 
# BROOT FORCE protection on several ports
for PORT in 4369 5222 5223 5269 5280 8090 11194 49164; do
    iptables -A INPUT -p tcp --dport $PORT --syn -m limit --limit 1/m --limit-burst 5 -j ACCEPT
    iptables -A INPUT -p tcp --dport $PORT --syn -j DROP
done
 
#---------------------
#--- More examples ---
#---------------------
# Deny ICMP echo-requests
iptables -t filter -A INPUT -i $EXTIF -p icmp --icmp-type echo-request -j DROP
 
# Forward traffic from local net except the routers
ROUTER1_IP=192.168.1.333
ROUTER2_IP=192.168.1.555
iptables -A FORWARD -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
iptables -A FORWARD -s $ROUTER1_IP -m conntrack --ctstate NEW -j REJECT
iptables -A FORWARD -s $ROUTER2_IP -m conntrack --ctstate NEW -j REJECT
iptables -A FORWARD -s 192.168.0.0/16 ! -d 192.168.0.0/16 -m conntrack --ctstate NEW -j ACCEPT
iptables -A FORWARD -j REJECT
 
# Squid
iptables -t nat -A PREROUTING -s 192.168.1.0/24 -p tcp --dport 80 -j REDIRECT --to-port 3128
 
# SSH Guard
iptables -N sshguard
iptables -A INPUT -j sshguard
 
# Block Samba connections from Routers with old firmwares
iptables -A INPUT -i $EXTIF -p udp --dport 137 -s $ROUTER1_IP -j DROP
iptables -A INPUT -i $EXTIF -p udp --dport 138 -s $ROUTER1_IP -j DROP
iptables -A INPUT -i $EXTIF -p tcp --dport 139 -s $ROUTER1_IP -j REJECT
iptables -A INPUT -i $EXTIF -p tcp --dport 445 -s $ROUTER1_IP -j REJECT
iptables -A INPUT -i $EXTIF -p udp --dport 137 -s $ROUTER2_IP -j DROP
iptables -A INPUT -i $EXTIF -p udp --dport 138 -s $ROUTER2_IP -j DROP
iptables -A INPUT -i $EXTIF -p tcp --dport 139 -s $ROUTER2_IP -j REJECT
iptables -A INPUT -i $EXTIF -p tcp --dport 445 -s $ROUTER2_IP -j REJECT
 
# Drop all outbound UDP traffic except for the DNS, NTP
iptables -A INPUT -p udp --sport 53 -j ACCEPT
iptables -A INPUT -p udp --dport 53 -j ACCEPT
iptables -A OUTPUT -p udp --sport 53 -j ACCEPT
iptables -A OUTPUT -p udp --dport 53 -j ACCEPT
iptables -A INPUT -p udp --sport 123 -j ACCEPT
iptables -A INPUT -p udp --dport 123 -j ACCEPT
iptables -A OUTPUT -p udp --sport 123 -j ACCEPT
iptables -A OUTPUT -p udp --dport 123 -j ACCEPT
iptables -A INPUT -p udp -j DROP
iptables -A OUTPUT -p udp -j DROP
 
# Don't forward UPD traffic except for the DNS, NTP
iptables -A FORWARD -p udp --sport 53 -j ACCEPT
iptables -A FORWARD -p udp --dport 53 -j ACCEPT
iptables -A FORWARD -p udp --sport 123 -j ACCEPT
iptables -A FORWARD -p udp --dport 123 -j ACCEPT
iptables -A FORWARD -p udp -j DROP