Example webserver iptable ruleset

#! /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

Invoke and make these rules effective:

hostname ~ # sh /etc/iptables.bak

Resulting active rules:

hostname ~ # iptables -L

REMEMBER! If you like the ruleset, and want it to be in-effect the next time you start iptables (ie after a reboot), then you must:

hostname ~ # rc-update add iptables default
* iptables added to runlevel default

hostname ~ # /etc/init.d/iptables save
* Saving iptables state ...