Firewalls and Network Access Control
Firewalls can be hardware, software, or a combination of both. They are usually located at the demarcation line between trusted and untrusted network elements. Firewalls play a critical role in the separation of important assets. An example of the placement of a typical firewall can be seen in Figure 4.1.
Firewall rules determine what type of traffic is inspected, what is allowed to pass, and what is blocked. The most basic way to configure firewall rules is by means of an access control list (ACL). An ACL is used for packet filtering and for selecting the types of traffic to be analyzed, forwarded, or influenced in some way by the firewall or device. ACLs are a basic example of data flow enforcement. Simple firewalls, and more specifically ACL configuration, may block traffic based on the source and destination address. However, more advanced configurations may deny traffic based on interface, port, protocol...