Firewall



Introduction
Traffic into or out of a computer is filtered through "ports," which are relatively
arbitrary designations appended to traffic packets destined for use by a
particular application.

By convention, some ports are routinely used for particular types of
applications. For example, port 80 is generally used for insecure web
browsing and port 443 is used for secure web browsing.

Traffic to particular applications can be allowed or blocked by "opening" or "closing" (i.e.
filtering) the ports designated for a particular type of traffic. If port 80
is "closed," for example, no (insecure) web browsing will be possible. The AntiVirus
page might also be of interest.

The Linux kernel includes the netfilter subsystem, which is used to
manipulate or decide the fate of network traffic headed into or through your
computer. All modern Linux firewall solutions use this system for packet
filtering.

The kernel's packet filtering system would be of little use to users or
administrators without a user interface with which to manage it. This is the
purpose of iptables. When a packet reaches your computer, it is handed off
to the netfilter subsystem for acceptance, manipulation, or rejection based
on the rules supplied to it via iptables. Thus, iptables is all you need to
manage your firewall (if you're familiar with it). Many front-ends are
available to simplify the task, however.

Users can therefore configure the firewall to allow certain types of network
traffic to pass into and out of a system (for instance SSH or web server
traffic). This is done by opening and closing TCP and UDP "ports" in the firewall.
Additionally, firewalls can be configured to allow or restrict access to
specific IP addresses (or IP address ranges).



Managing the Firewall



iptables
Iptables is the database of firewall rules and is the actual firewall used
in Linux systems. The traditional interface for configuring iptables in
Linux systems is the command-line interface terminal. The other utilities in
this section simplify the manipulation of the iptables database.



UFW
UFW (Uncomplicated Firewall) is a front-end for iptables and is particularly
well-suited for host-based firewalls. UFW was developed specifically for
Ubuntu (but is available in other distributions), and is also configured
from the terminal.



Gufw is a graphical front-end to UFW, and is recommended for beginners.

UFW was introduced in Ubuntu 8.04 LTS (Hardy Heron), and is available by
default in all Ubuntu installations after 8.04 LTS.



Guarddog
Guarddog is a front-end for iptables that functions in KDE-based desktops,
such as Kubuntu. It has a greater deal of complexity (and flexibility,
perhaps).



See Also

Security 


Other:

DynamicFirewall
firewall/ipkungfu
firewall/Linux_UPnP_Internet_Gateway_Device_(linux-idg)
Router/Firewall 




External Links
 http://en.wikipedia.org/wiki/Firewall
 http://www.netfilter.org/ - Netfilter and iptables homepage
 http://www.fs-security.com/ - Firestarter homepage
 UbuntuFirewall - Uncomplicated Firewall homepage