IN THIS CHAPTER
Network Address Translation
You have likely heard of a firewall before—your organization doubtlessly has one, and your asymmetric digital subscriber line (ADSL) router probably has one, too. Linux has had support for packet filtering (discussed later in the chapter) for quite a while now in some incarnation or another.
A firewall is a line of defense between two networks. It is used to explicitly allow network traffic to and from your networks, allowing you as an administrator to control what traffic can go where in your network. Many organizations not only use firewalls to protect their corporate network from the Internet, but also to protect one department from another. For example, is there any reason to allow the Sales department access to your Research and Development department? This also helps to slow down the spread of a malicious user or virus code through your network if your network is compromised because only explicit traffic or protocols can communicate with other networks.
Many companies provide firewall appliances that offer the user a graphical user interface to the firewall internals. With most asymmetric digital subscriber line (ADSL) routers, you also have the added bonus of a firewall. One of the most popular firewall appliances is the Cisco Pix. As with most things Cisco, it is aimed at the business end of the market. You need knowledge of firewalls ...