HOWTO Setup iptables

Kernel Configuration

   * NOTE This configuration is for basic firewalling only; we don't do NAT/packet-forwarding... so, if you're reading this, and wish to use NAT/forwarding, you will be missing a few key configuration items :-O
   * NOTE As of kernel 2.6.22 you must enable the following:

Networking  ---->
 Networking options  ---->
  Network packet filtering framework (Netfilter)--->
   Core Netfilter Configuration ---->
    ["enable"] Netfilter connection tracking support--->Layer 3 Independent Connection tracking
    ["enable"] Netfilter Xtables support (required for ip_tables)
    ["enable"] "NFLOG" target support
    ["enable"] "conntrack" connection tracking match support
    ["enable"] "state" match support 
   IP: Netfilter Configuration --->
    ["enable"] IPv4 connection tracking support (required for NAT) required by "Layer 3 Independent Connection tracking" above (caused many headaches)
    ["enable"] IP tables support (required for filtering/masq/NAT)
    ["enable"]   Packet Filtering
    ["enable"]     REJECT target support

Iptables Installation

emerge iptables
rc-update add iptables default

Scripting the Rules

Once iptables is up-and-running, simply execute the script below, to implement the policies:

sh /etc/iptables.back

#! /bin/sh
# /etc/iptables.bak

# Let's save typing & confusion with variables
IPTABLES=/sbin/iptables

# Flush active rules and custom tables
$IPTABLES --flush
$IPTABLES --delete-chain

# set the defaults so that by-default incoming packets are dropped, unless explicitly allowed;
# for a desktop workstation, we'll let lots of (unpredictable) outgoing packets go freely.
$IPTABLES -P INPUT DROP
$IPTABLES -P FORWARD DROP
$IPTABLES -P OUTPUT ACCEPT

# INBOUND POLICY
# ==============
# of course, accepting loopback is a good idea
$IPTABLES -A INPUT -i lo -j ACCEPT 

#   (Applies to packets entering our network interface from the network, 
#   and addressed to this host.)

$IPTABLES -A INPUT -m state --state INVALID -j DROP 
$IPTABLES -A INPUT -m state --state RELATED,ESTABLISHED -j ACCEPT 

# ftp incoming
$IPTABLES -A INPUT -p tcp -m state --state NEW --dport 20 -j ACCEPT
$IPTABLES -A INPUT -p tcp -m state --state NEW --dport 21 -j ACCEPT

# ssh incoming, including non-standard port (if needed)
$IPTABLES -A INPUT -p tcp -m state --state NEW --dport 22 -j ACCEPT 
#$IPTABLES -A INPUT -p tcp -m state --state NEW --dport 222 -j ACCEPT

# web serving, let's allow it!
$IPTABLES -A INPUT -p tcp -m state --state NEW --dport 80 -j ACCEPT

# nagios (5666); monitor time (123), allow snmp (161)
$IPTABLES -A INPUT -p tcp -m state --state NEW --dport 5666 -j ACCEPT
$IPTABLES -A INPUT -p udp -m state --state NEW --dport 123 -j ACCEPT
$IPTABLES -A INPUT -p udp -m state --state NEW --dport 161 -j ACCEPT


# OUTBOUND POLICY
# ===============
# of course, accepting loopback is a good idea
$IPTABLES -A OUTPUT -o lo -j ACCEPT

#   (Applies to packets sent to the network interface from local processes)

$IPTABLES -A OUTPUT -m state --state RELATED,ESTABLISHED -j ACCEPT

Save the configuration:

etc/init.d/iptables save

And then back up your working configuration in case you break something later you can quickly revert:

cp /var/lib/iptables/rules-save /var/lib/iptables/rules.working

Viewing/checking the active ruleset:

iptables -L