iptables.sh

#!/bin/bash
IPT="/sbin/iptables"

# Server IP
SERVER_IP="$(ip addr show eth0 | grep 'inet ' | cut -f2 | awk '{ print $2}')"

# Your DNS servers you use: cat /etc/resolv.conf
DNS_SERVER="8.8.4.4 8.8.8.8"

# Allow connections to this package servers
PACKAGE_SERVER="ftp.us.debian.org security.debian.org"

echo "flush iptable rules"
$IPT -F
$IPT -X
$IPT -t nat -F
$IPT -t nat -X
$IPT -t mangle -F
$IPT -t mangle -X

echo "Set default policy to 'DROP'"
$IPT -P INPUT DROP
$IPT -P FORWARD DROP
$IPT -P OUTPUT DROP

## This should be one of the first rules.
## so dns lookups are already allowed for your other rules
for ip in $DNS_SERVER; do
    echo "Allowing DNS lookups (tcp, udp port 53) to server '$ip'"
    $IPT -A OUTPUT -p udp -d $ip --dport 53 -m state --state NEW,ESTABLISHED -j ACCEPT
    $IPT -A INPUT -p udp -s $ip --sport 53 -m state --state ESTABLISHED -j ACCEPT
    $IPT -A OUTPUT -p tcp -d $ip --dport 53 -m state --state NEW,ESTABLISHED -j ACCEPT
    $IPT -A INPUT -p tcp -s $ip --sport 53 -m state --state ESTABLISHED -j ACCEPT
done

echo "allow all and everything on localhost"
$IPT -A INPUT -i lo -j ACCEPT
$IPT -A OUTPUT -o lo -j ACCEPT

for ip in $PACKAGE_SERVER; do
    echo "Allow connection to '$ip' on port 21"
    $IPT -A OUTPUT -p tcp -d "$ip" --dport 21 -m state --state NEW,ESTABLISHED -j ACCEPT
    $IPT -A INPUT -p tcp -s "$ip" --sport 21 -m state --state ESTABLISHED -j ACCEPT

    echo "Allow connection to '$ip' on port 80"
    $IPT -A OUTPUT -p tcp -d "$ip" --dport 80 -m state --state NEW,ESTABLISHED -j ACCEPT
    $IPT -A INPUT -p tcp -s "$ip" --sport 80 -m state --state ESTABLISHED -j ACCEPT

    echo "Allow connection to '$ip' on port 443"
    $IPT -A OUTPUT -p tcp -d "$ip" --dport 443 -m state --state NEW,ESTABLISHED -j ACCEPT
    $IPT -A INPUT -p tcp -s "$ip" --sport 443 -m state --state ESTABLISHED -j ACCEPT
done

#######################################################################################################
## Global iptable rules. Not IP specific

echo "Allowing new and established incoming connections to port 21, 80, 443"
$IPT -A INPUT -p tcp -m multiport --dports 21,80,443 -m state --state NEW,ESTABLISHED -j ACCEPT
$IPT -A OUTPUT -p tcp -m multiport --sports 21,80,443 -m state --state ESTABLISHED -j ACCEPT

echo "Allow all outgoing connections to port 22"
$IPT -A OUTPUT -p tcp --dport 22 -m state --state NEW,ESTABLISHED -j ACCEPT
$IPT -A INPUT -p tcp --sport 22 -m state --state ESTABLISHED -j ACCEPT

echo "Allow outgoing icmp connections (pings,...)"
$IPT -A OUTPUT -p icmp -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT
$IPT -A INPUT -p icmp -m state --state ESTABLISHED,RELATED -j ACCEPT

echo "Allow outgoing connections to port 123 (ntp syncs)"
$IPT -A OUTPUT -p udp --dport 123 -m state --state NEW,ESTABLISHED -j ACCEPT
$IPT -A INPUT -p udp --sport 123 -m state --state ESTABLISHED -j ACCEPT

# Log before dropping
$IPT -A INPUT -j LOG -m limit --limit 12/min --log-level 4 --log-prefix 'IP INPUT drop: '
$IPT -A INPUT -j DROP

$IPT -A OUTPUT -j LOG -m limit --limit 12/min --log-level 4 --log-prefix 'IP OUTPUT drop: '
$IPT -A OUTPUT -j DROP

exit 0