How do I build a Simple Linux Firewall for DSL/Dial-up connection?
If you’re new to Linux, here’s a simple firewall that can be setup in minutes. Especially those coming from a Windows background, often lost themselves while creating linux firewall.
This is the most common question asked by Linux newbies (noobs). How do I install a personal firewall on a standalone Desktop Linux computer. In other words “I wanna a simple firewall that allows or permits me to visit anything from my computer but it should block everything from outside world”.
Well that is pretty easy first remember INPUT means incoming and OUTPUT means outgoing connection/access. With following little script and discussion you should able to setup your own firewall.
Step # 1: Default Firewall policy
Set up default access policy to drop all incoming traffic but allow all outgoing traffic. This will allow you to make unlimited outgoing connections from any port but not incoming traffic/ports are allowed.
iptables -p INPUT DROP
iptables -p OUTPUT ACCEPT
Step # 2: Allow unlimited traffic from loopback (lo) device
iptables -A INPUT -i lo -j ACCEPT
iptables -A OUTPUT -i lo -j ACCEPT
Step # 3: Setup connection oriented access
Some protocol such as a FTP, DNS queries and UDP traffic needs an established connection access. In other words you need to allow all related connection using iptables state modules.
iptables -A INPUT -i eth0 -m state --state ESTABLISHED,RELATED -j ACCEPT
Step # 4: Drop everything else and log it
iptables -A INPUT -j LOG
iptables -A INPUT -j REJECT
But wait you cannot type all above commands at a shell command prompt. It is a good idea to create a script called fw.start as follows (copy and paste following script in fw.start file):
#!/bin/sh # A simple iptables -F iptables -X iptables -t nat -F iptables -t nat -X iptables -t mangle -F iptables -t mangle -X modprobe ip_conntrack modprobe ip_conntrack_ftp # Setting default filter policy iptables -P INPUT DROP iptables -P OUTPUT ACCEPT # Unlimited access to loop back iptables -A INPUT -i lo -j ACCEPT iptables -A OUTPUT -o lo -j ACCEPT # Allow UDP, DNS and Passive FTP iptables -A INPUT -i eth0 -m state --state ESTABLISHED,RELATED -j ACCEPT # DROP everything and Log it iptables -A INPUT -j LOG iptables -A INPUT -j DROP
You can enhance your tiny firewall with
- Create a script to stop a firewall
- This is optional, if you wish to start a firewall automatically as soon as Debian Linux boots up use the instruction outlined here
- Finally if you wanna open incoming ssh (port 22) or http (port 80) then insert following two rules before #DROP everything and Log it line in above script:
iptables -A INPUT -p tcp -i eth0 --dport 22 -m state --state NEW -j ACCEPT
iptables -A INPUT -p tcp -i eth0 --dport 80 -m state --state NEW -j ACCEPT
Easy to use Linux firewall programs/tools
- GUI tools – firestarter :: A graphical interfaced Open Source firewall for Linux. (highly recommended for Linux desktop users)
- IPCop Firewall and SmoothWall :: Setup a dedicated firewall box. (highly recommended for Linux server and LAN/WAN users)