Network Security, Firewalls, and VPNs, 3rd Edition

How Firewalls Work and What Firewalls Do

Firewalls work along a communication pathway. This can be the gateway point of a network, a chokepoint within a network, a point-of-zone transition, or on a host. In these locations, the firewall interrupts the traffic flow to inspect packets or sessions. If the traffic is authorized, it continues to its destination. If the traffic is not authorized, it is blocked and dropped.

When one thinks of a firewall, one generally pictures some sort of hardware, when in fact it is more accurate to think of the firewall as a function. There are really two broad types of firewall implementations: bump-in-the-wire and bump-in-the-stack. The bump-in-the-wire (FIGURE 5-4) is a separate hardware firewall implementation, ...