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Firewalls are designed to protect systems on one side of the firewall from systems on the other side by analyzing packets that reach the firewall and determining whether the packet is allowed to pass through You will configure rules on the firewall that indicate to the firewall which traffic is to pass through and which is to be blocked For example, as a general rule you should configure the firewall to block all traffic, meaning that no traffic can pass through Once you have configured the default rule of blocking all traffic, you can configure exceptions to the rule, allowing selected traffic to pass through For example, if you have a web server that you want to expose out to the Internet, you would block all traffic except TCP port 80, the port on which web server traffic runs (as shown in Figure 12-15) Before we consider how to create these firewall rules, let s look at some firewall configurations that are currently used in networking environments
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figUre 12-15
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Rule: Allow-80-TCP Deny ALL Web server
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Firewalls allow selected traffic to pass through the firewall
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It is important to note that this chapter is focused on networkbased firewalls, which are firewall devices or systems that protect the entire network For the Network+ exam you need to be
familiar with network-based firewalls but also with host-based firewalls A host-based firewall only protects the one system and is personal firewall software installed on that one system
Dual-Homed Host firewalls
A dual-homed host firewall consists of a single computer with two physical network interfaces that acts as a gateway between the two networks The server s routing capability is disabled so that the firewall can handle all traffic management Either application-level proxy or circuit-level firewall software is run on this system to pass packets from one side of the dual-homed system to the other You must be careful not to enable routing within the network operating system that will be used as the dual-homed system or you will bypass your firewall software and simply be routing data Figure 12-16 shows a dual-homed host firewall configuration
screened-Host firewalls
Screened-host firewall configurations are considered by many to be more secure than the dual-homed firewall In this configuration, you place a screening router between the dual-homed host and the public network This enables you to provide packet filtering before the packets reach the dual-homed computer, thereby adding an extra layer of network security The dual-homed computer can then run a proxy to provide additional security to this configuration Figure 12-17 shows a screened-host configuration
Firewalls and Proxy Servers
figUre 12-16
A dual-homed system acting as a firewall has two network interfaces
Internet Firewall
Internal network
figUre 12-17
A screenedhost firewall configuration adds an extra layer of network security by adding a screening router to implement packet filtering
Internet Screen router Screen host firewall
Internal network
screened subnet firewalls
A screened subnet firewall configuration takes security to the next level by further isolating the internal network from the public network An additional screening router is placed between the internal network and the dual-homed firewall This provides two additional levels of security First, by adding a screening router internally, you can protect the dual-homed firewall host from an attack by an internal source Second, it makes an external attack much more difficult because the number of layers that an attacker must go through is increased Normally the outside screening router will be configured to pass any data that has passed the filter rule to the dual-homed firewall that will perform more tests on the incoming traffic Once the incoming traffic has passed the test performed by the dual-homed system, the traffic may then be sent to the internal screening router, where additional tests on the packet are performed The internal screening router is typically configured to accept only data from the dual-homed firewall, ensuring that hackers can t skip past the outside firewall layers Figure 12-18 shows the screened subnet firewall configuration
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