Here I present an abbreviated explanation of the process of creating firewall and cluster objects. More detailed step-by-step guides are available in sections “Firewall Object” and “Cluster Object” of the Firewall Builder Users Guide.
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This article continues mini-series started with the post Introduction to Firewall Builder 4.0. This article is also available as a section in the “Firewall Builder Cookbook” chapter of Firewall Builder Users Guide 4.0.
Firewall Builder 4.0 is currently in beta testing phase. If you find it interesting after reading this post, please download and try it out. Source code archives, binary deb and rpm packages for popular Linux distributions and commercially distributed Windows and Mac OS X packages are available for download here.
In this post I demonstrate how Firewall Builder can be used to generate firewall configuration for a clustered web server with multiple virtual IP addresses. The firewall is running on each web server in the cluster. This example assumes the cluster is built with heartbeat using “old” style configuration files, but which high availability software is used to build the cluster is not really essential. I start with the setup that consists of two identical servers running Linux but in the end of the article I am going to demonstrate how this configuration can be converted to OpenBSD with CARP.
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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
The Internet Control Message Protocol (ICMP) has many messages that are identified by a “type” field. You need to use 0 and 8 ICMP code types.
=> Zero (0) is for echo-reply
=> Eight (8) is for echo-request.
To enable ICMP ping incoming client request use following iptables rule (you need to add following rules to script).
My default firewall policy is blocking everything.
Task: Enable or allow ICMP ping incoming client request
Rule to enable ICMP ping incoming client request ( assuming that default iptables policy is to drop all INPUT and OUTPUT packets)
SERVER_IP="126.96.36.199" iptables -A INPUT -p icmp --icmp-type 8 -s 0/0 -d $SERVER_IP -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT iptables -A OUTPUT -p icmp --icmp-type 0 -s $SERVER_IP -d 0/0 -m state --state ESTABLISHED,RELATED -j ACCEPT
Task: Allow or enable outgoing ping request
To enable ICMP ping outgoing request use following iptables rule:
SERVER_IP="188.8.131.52" iptables -A OUTPUT -p icmp --icmp-type 8 -s $SERVER_IP -d 0/0 -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT iptables -A INPUT -p icmp --icmp-type 0 -s 0/0 -d $SERVER_IP -m state --state ESTABLISHED,RELATED -j ACCEPT
How do I disable outgoing ICMP request?
Use the following rules:
iptables -A OUTPUT -p icmp --icmp-type echo-request -j DROP
iptables -A OUTPUT -p icmp --icmp-type 8 -j DROP
ICMP echo-request type will be block by above rule.
See ICMP TYPE NUMBERS (type fields). You can also get list of ICMP types, just type following command at shell prompt:
# /sbin/iptables -p icmp -h