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A firewall is a vital component in protecting a computer system, or network of computers from external attack (typically from an internet connection). Any computer connected directly to an internet connection must run a firewall to protect against malicious activity. Similarly, any internal network must have some form of firewall between it and an external internet connection.
Ubuntu Linux is supplied with powerful firewall technology known as ''iptables'' built-in. Entire books can, and indeed have, been written about configuring iptables. If you wouild would like to learn about ''iptables'' we recommend [http://www.linuxtopia.org/Linux_Firewall_iptables/index.html Linux Firewall Configuration - Packet Filtering and iptables].
Fortunately Ubuntu Linux also provides some tools which make firewall configuration for the average user easy. This chapter will cover the steps necessary to configure an Ubuntu Linux firewall using those tools.
The ''lokkit'' tool is not installed by default on Ubuntu Linux, so before we can begin to configure a firewall we first need to install the appropriate package. There are two ways to do this, either using the Linux command-line, or using the Synaptic Package Manager. We will begin by looking at the Synaptic Package Manager method.
To launch the Synaptic Package Manager , first click on the ''System'' menu on the desktop and then select "Synaptic Package Manager'' from the ''Administration'' sub-menu. Enter your password when prompted to do so. The Synaptic Package Manager will appear as follows:
[[Image:ubuntu_linux_synaptic_package_manager.jpg]]
Once the package manager has loaded the next step is to locate the package we need to install. Click on the ''Search'' button the the Synaptic toolbar, enter ''lokkit'' in the Search dialog and press ''Search''. The search should return two results, ''lokkit'' and ''gnome-lokkit''. ''lokkit'' is a text based version of the tool and ''gnome-lokkit'' is a graphical version. ''gnome-lokkit'' requires that ''lokkit'' also be installed so click on the check both next to ''gnome-lokkit'' and select ''Mark for installation'' from the drop down menu. A dialog appears stating that ''lokkit'' also needs to be installed. Click on the ''Mark'' button to also install ''lokkit''.
Now that we have defined what we want to install we now need to installed install it. Do To do so , simply click on the ''Apply'' button located in the Synaptic toolbar and click ''Apply'' in the Summary dialog. The package manager will now download and install the tweo two packages. Close the completion dialog and exit Synaptic once the installation is finished.
Alternatively the ''lokkit'' and ''gnome-lokkit'' packages can be installed from the Ubuntu Linux command-line using the ''apt-get'' tool. The first step is to launch a termial terminal window to gain access to the command-line. To do this , click on the desktop ''Applications'' menu and select ''Konsole'' from from ''System Tools'' menu. The resulting dialog provides a Linux command-line prompt. Type the following at the prompt:
<pre>
== Running the lokkit Firewall Tool ==
Unfortunately the last few releases of Ubuntu Linux contain a bug which prevent prevents gnome-lokkit from running. If gnone-lokkit fails for you you will need to use ''lokkit''. Fortunately, there is nothing in gnome-lokkit that cannot be achieved using ''lokkit''. To launch ''lokkit'' start a Linux console to access the command prompt (lick on the desktop ''Applications'' menu and select ''Konsole'' from from ''System Tools'' menu) and type the following at the command-line prompt:
<pre>
Notice that Ubuntu Linux defaults the highest level of security. This follows the general rule of security that a system should start with everything locked down and then lifted gradually to provide only the functionality needed, and nothing more.
The most useful ''lokkit'' screen the is the customization screen. TO To access this screen use the Tab key to navigate to the '''Customize'' button at the bottom of the screen. Once ''Customize'' is highlighted press the ''Enter'' or ''Space'' key to select it. The Customization screen should appear as follows:
[[Image:ubuntu_lokkit_firewall_customize.jpg]]
== Understanding the lokkit Firewall Customization Screen ==
The lokkit customization screen needs a little explanation. The first settings list the network devices present in the system and provides the option to set one or more of these as being trusted devices. Essentially data coming via a trusted device is not subject to any rules imposed by the firewall. This is useful if you have asystem a system where one network device is connected to the outside world via an internet connection and another device is connected to a trusted, internal network. Assuming you truly trust the internal network and all the systems on it, then you can mark that device as being trusted.
The second section of the screen controls access to a number of different services which may or may not be running on your Ubuntu Linux system. Descritions Descriptions of these are as follows:
'''DHCP''' - This refers to the Dynamic Host Configuration Protocol. This is a system by which devices are dynamically provided IP addresses instead of having static IP addresses manually assigned by users or system administrators. If you obtain your address from a DCHP server (such as one built into a wireless router or provided by your company) then you will want to enable DHCP on this page to enable your system to negotiate with the DHCP server to obtain an IP address.
'''SSH''' - The secure shell provides an encrypted mechanism for allowing password protected remote access to your system. With SSH you can remotely log into to your system, copy files to and from your system and anotehr another system and perform remote execution of programs. If you need remote access to your system you will need to activate this. If you do not need remote access leave this disabled. Note that the ssh server is not installed by default on Ubuntu Linux.
'''Telnet''' - Telnet provides remote terminal access to your system. It does not use encryption and use is strongly discouraged. Leave this disabled and use SSH instead fopr for remote access.
'''WWW (HTTP)''' - If you are hosting a web serv eron server on your Ubuntu Linux System you will need to enable HTTP traffic through the firewall to enable web page requests to reach the http server. If you do not plan to host a web server , leave this disabled. Note that the Apache web server is not installed by default on Ubuntu Linux.
'''mail (SMTP)''' - Specified whether the firewall block Simple Mail Transfer Protocol traffic. This is only necessary if you are hosting a mail server on your Ubuntu Linux system. If you only use a mail client to download email from a POP3 or IMAP server you can safely leave this disabled. Note that the SMTP server is not installed by default on Ubuntu Linux.