Basic IPtables Firewall

This is a very simple iptables firewall scripts to get you started. I use this all the time as a basis. It allows for access to your system via SSH (you can define a different port if you run SSH on a different port; you do, right? ;)), outbound DNS requests. I use this as a boilerplate for all containers in the virtual world.

#!/bin/bash
IPT="/sbin/iptables"
# Flush old rules, old custom tables
$IPT --flush
$IPT --delete-chain
# Set default policies for all three default chains
$IPT -P INPUT DROP
$IPT -P FORWARD DROP
$IPT -P OUTPUT DROP
# Enable free use of loopback interfaces
$IPT -A INPUT -i lo -j ACCEPT
$IPT -A OUTPUT -o lo -j ACCEPT
# All TCP sessions should begin with SYN
$IPT -A INPUT -p tcp ! --syn -m state --state NEW -s 0.0.0.0/0 -j DROP
# Accept inbound TCP packets
$IPT -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT
$IPT -A INPUT -p tcp --dport 22 -m state --state NEW -s 0.0.0.0/0 -j ACCEPT
# Accept inbound ICMP messages
$IPT -A INPUT -p ICMP --icmp-type 8 -s 0.0.0.0/0 -j ACCEPT
# Accept outbound packets
$IPT -I OUTPUT 1 -m state --state RELATED,ESTABLISHED -j ACCEPT
$IPT -A OUTPUT -p udp --dport 53 -m state --state NEW -j ACCEPT