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3.2 What are the basic types of firewalls?

  Conceptually, there are two types of firewalls:
1.
Network layer
2.
Application layer

They are not as different as you might think, and latest technologies are blurring the distinction to the point where it's no longer clear if either one is ``better'' or ``worse.'' As always, you need to be careful to pick the type that meets your needs.

Which is which depends on what mechanisms the firewall uses to pass traffic from one security zone to another. The International Standards Organization (ISO) Open Systems Interconnect (OSI) model for networking defines seven layers, where each layer provides services that ``higher-level'' layers depend on. In order from the bottom, these layers are physical, data link, network, transport, session, presentation, application.

The important thing to recognize is that the lower-level the forwarding mechanism, the less examination the firewall can perform. Generally speaking, lower-level firewalls are faster, but are easier to fool into doing the wrong thing.

3.2.1 Network layer firewalls

These generally make their decisions based on the source, destination addresses and ports (see Appendix C for a more detailed discussion of ports) in individual IP packets. A simple router is the ``traditional'' network layer firewall, since it is not able to make particularly sophisticated decisions about what a packet is actually talking to or where it actually came from. Modern network layer firewalls have become increasingly sophisticated, and now maintain internal information about the state of connections passing through them, the contents of some of the data streams, and so on. One thing that's an important distinction about many network layer firewalls is that they route traffic directly though them, so to use one you either need to have a validly assigned IP address block or to use a ``private internet'' address block [3]. Network layer firewalls tend to be very fast and tend to be very transparent to users.


  
Figure 1: Screened Host Firewall
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In Figure 1, a network layer firewall called a ``screened host firewall'' is represented. In a screened host firewall, access to and from a single host is controlled by means of a router operating at a network layer. The single host is a bastion host; a highly-defended and secured strong-point that (hopefully) can resist attack.


  
Figure 2: Screened Subnet Firewall
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Example Network layer firewall : In figure 2, a network layer firewall called a ``screened subnet firewall'' is represented. In a screened subnet firewall, access to and from a whole network is controlled by means of a router operating at a network layer. It is similar to a screened host, except that it is, effectively, a network of screened hosts.

3.2.2 Application layer firewalls

These generally are hosts running proxy servers, which permit no traffic directly between networks, and which perform elaborate logging and auditing of traffic passing through them. Since the proxy applications are software components running on the firewall, it is a good place to do lots of logging and access control. Application layer firewalls can be used as network address translators, since traffic goes in one ``side'' and out the other, after having passed through an application that effectively masks the origin of the initiating connection. Having an application in the way in some cases may impact performance and may make the firewall less transparent. Early application layer firewalls such as those built using the TIS firewall toolkit, are not particularly transparent to end users and may require some training. Modern application layer firewalls are often fully transparent. Application layer firewalls tend to provide more detailed audit reports and tend to enforce more conservative security models than network layer firewalls.


  
Figure 3: Dual Homed Gateway
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Example Application layer firewall : In figure 3, an application layer firewall called a ``dual homed gateway'' is represented. A dual homed gateway is a highly secured host that runs proxy software. It has two network interfaces, one on each network, and blocks all traffic passing through it.

The Future of firewalls lies someplace between network layer firewalls and application layer firewalls. It is likely that network layer firewalls will become increasingly ``aware'' of the information going through them, and application layer firewalls will become increasingly ``low level'' and transparent. The end result will be a fast packet-screening system that logs and audits data as it passes through. Increasingly, firewalls (network and application layer) incorporate encryption so that they may protect traffic passing between them over the Internet. Firewalls with end-to-end encryption can be used by organizations with multiple points of Internet connectivity to use the Internet as a ``private backbone'' without worrying about their data or passwords being sniffed.

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