Syngress - Eleventh Hour Network+ Exam N10-004 Study Guide (11 ...

48 CHAPTER 3 Network Devices
■ Dual-homed host firewalls A dual-homed firewall consists of a single computer with
two physical network interfaces. This computer acts as a gateway between two
networks.
■ Screened host firewalls With screened host firewalls, you place a screening router
between the gateway host and the public network. This enables you to provide
packet filtering before the packets reach the host computer, as depicted in
Figure 3.1.
■ Screened subnet firewalls A screened subnet firewall configuration isolates the
internal network from the public network. An additional screening router is placed
between the internal network and the firewall proxy server. The internal router handles
local traffic while the external router handles inbound and outbound traffic to
the public network, as depicted in Figure 3.2.
Firewall Types
There are three basic categories of firewalls:
■
■
■
Packet level firewall This is a form of screening router that examines packets
based upon filters that are set up at the network and transport layers. You can
block incoming or outgoing transfers based on a TCP/IP address or other
rules. Rules can be based on source address, destination address, session
protocol type, and the source and destination port. It only functions at
Layer 3 rendering this a very basic form of protection.
Application level firewall This firewall understands the data at the application
level and functions at the application, presentation, and session
layers.
Circuit level firewall This circuit level firewall is similar to an application
proxy except that the security mechanisms are applied at the time the connection
is established. From then on, the packets flow between the hosts
without any further checking from the firewall. Circuit level firewalls operate
at the transport layer.
FIGURE 3.1
Screened host firewall
Public
Network
Screening Router
Screened Host Firewall
Internal
Network
FIGURE 3.2
Screened subnet firewall
Public
Network
Screening Router Screening Router
Screened Host Firewall
Internal
Network