So, you want to configure a very very secure Linux Firewall using iptables? no probs. Here is how to do it;

#!/bin/sh
# My system IP/set ip address of server
SERVER_IP="2.2.2.2"

# Flushing all rules
iptables -F
iptables -X

# Setting default filter policy
iptables -P INPUT DROP
iptables -P OUTPUT DROP
iptables -P FORWARD DROP

# Allow ALL traffic on loopback
iptables -A INPUT -i lo -j ACCEPT
iptables -A OUTPUT -o lo -j ACCEPT
 
# Allow INCOMING CONNECTIONS ON SSH PORT 22
iptables -A INPUT -p tcp -s 0/0 -d $SERVER_IP --sport 513:65535 --dport 22 -m state --state NEW,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -p tcp -s $SERVER_IP -d 0/0 --sport 22 --dport 513:65535 -m state --state ESTABLISHED -j ACCEPT
# DROP ALL TRAFFIC COMING IN AND GOING OUT

iptables -A INPUT -j DROP
iptables -A OUTPUT -j DROP

This above configuration locks down everything apart from SSH.

But, most customers want to configure access from their client’s IP address only. I.e. they don’t want SSH to accept connections from any IP address. Only from the client on for example 1.1.1.1. Here’s how to do that:

# Allow incoming ssh only from IP 1.1.1.1
iptables -A INPUT -p tcp -s 1.1.1.1 -d $SERVER_IP --sport 513:65535 --dport 22 -m state --state NEW,ESTABLISHED -j ACCEPT
iptables -A OUTPUT -p tcp -s $SERVER_IP -d 1.1.1.1 --sport 22 --dport 513:65535 -m state --state ESTABLISHED -j ACCEPT

Credit goes to; http://www.cyberciti.biz/tips/linux-iptables-4-block-all-incoming-traffic-but-allow-ssh.html