Very restrictive set of firewall rules

Very restrictive set of firewall rules script is a sample firewall for ip_tables, the tool for doing firewalling and masquerading under the 2.3.x/2.4.x series of kernels.

Be warned, this is a very restrictive set of firewall rules (and they should be, for proper security). Anything that you do not _specifically_ allow is logged and dropped into /dev/null, so if you're wondering why something isn't working, check /var/log/messages.

This is about as close as you get to a 'secure' firewall. It's nasty, it's harsh, and it will make your machine nearly invisible to the rest of the internet world. Have fun.

To run this script you must 'chmod 700 iptables-script' and then execute it. To stop it from running, run 'iptables -F'

Sample:

#Point this to your copy of ip_tables
IPT="/usr/local/bin/iptables"

#Load the module.
modprobe ip_tables

#Flush old rules, delete the firewall chain if it exists
$IPT -F
$IPT -F -t nat
$IPT -X firewall

#Setup Masquerading. Change the IP to your internal network and uncomment
#this in order to enable it.
#$IPT -A POSTROUTING -t nat -s 192.168.1.0/24 -j MASQUERADE
#$IPT -P FORWARD ACCEPT
#echo 1 > /proc/sys/net/ipv4/ip_forward

#Set up the firewall chain
$IPT -N firewall
$IPT -A firewall -j LOG --log-level info --log-prefix "Firewall:"
$IPT -A firewall -j DROP

#Accept ourselves
$IPT -A INPUT -s 127.0.0.1/32 -d 127.0.0.1/32 -j ACCEPT
#If you're using IP Masquerading, change this IP to whatever your internl
#IP addres is and uncomment it
#$IPT -A INPUT -s 192.168.1.1/32 -d 0/0 -j ACCEPT

#Accept DNS, 'cause it's warm and friendly
$IPT -A INPUT -p udp --source-port 53 -j ACCEPT
$IPT -A INPUT -p tcp --source-port 113 -j ACCEPT
$IPT -A INPUT -p tcp --destination-port 113 -j ACCEPT

#Allow ftp to send data back and forth.
$IPT -A INPUT -p tcp ! --syn --source-port 20 --destination-port 1024:65535 -j ACCEPT

#Accept SSH. Duh.
#$IPT -A INPUT -p tcp --destination-port 22 -j ACCEPT

#Send everything else ot the firewall.
$IPT -A INPUT -p icmp -j firewall
$IPT -A INPUT -p tcp --syn -j firewall
$IPT -A INPUT -p udp -j firewall

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