Design the firewall system.
Designing a Firewall requires that you understand and identify the boundaries between security domains in your network. A network security domain is a contiguous region of a network that operates under a single, uniform security policy. Wherever these domains intersect, there is a potential need for a policy conflict resolution mechanism at that boundary. This is where firewall technology can help.
The most common boundary where firewalls are applied today is between an organization???s internal networks and the Internet. When establishing an Internet firewall, the first thing you must decide is its basic architecture (assuming you have previously established your firewall requirements and the security policy it is intended to implement). In this context, architecture refers to the inventory of components (hardware and software), and the connectivity and distribution of functions among them. There are two classes of firewall architectures, which we refer to as the single layer and the multiple layer architectures.
In single layer architecture, one network host is allocated all firewall functions and is connected to each network for which it is to control access. This approach is usually chosen when containing cost is a primary factor or when there are only two networks to interconnect. It has the advantage that everything there is to know about the firewall resides on that one host. In cases where the policy to be implemented is simple and there are few networks being interconnected, this approach can also be very cost-effective to operate and maintain over time. The greatest disadvantage of the single layer approach is its susceptibility to implementation flaws or configuration errors ??? depending on the type, a single flaw or error might allow firewall penetration.
In multiple layer architecture, the firewall functions are distributed among a small number of hosts typically connected in series, with DMZ networks between them. This approach is more difficult to design and operate, but can provide substantially greater security by diversifying the defenses you are implementing. Although more costly, we advise using different technology in each of these firewall hosts. This reduces the risk that the same implementation flaws or configuration errors will exist in every layer. The most common design approach for this type of architecture is an Internet firewall composed of two hosts interconnected with one DMZ network.
Having chosen the basic architecture (i.e., the number of hosts, the method in which they are connected, the tasks that each will perform), the next step is to select the firewall functions to be implemented in these hosts. The two most basic categories of firewall function are packet filtering and application proxies. These functions can be used separately or jointly and can be implemented on the same or on different firewall hosts. Recently, packet filtering firewall products have gained some of the features of application proxies and are generally referred to as stateful inspection packet filters.
There are good reasons to use both packet filtering and application proxies. Certain services (e.g., SMTP, HTTP, or NTP) are usually safe to control via packet filters while others (e.g., DNS, FTP) may require the more complex features available only in proxies. Packet filtering is fast, while application proxies are generally slower. In cases where greater access control is required and the poorer performance of proxies cannot be tolerated, stateful inspection packet filters may be an acceptable compromise. In any case, one should plan to have as many of these different functions (i.e., packet filters, proxies, and stateful inspection) available as possible, applying each where appropriate.
Ideally, the design of your firewall architecture should precede firewall hardware and software selection. However, we recognize that in some organizations, some form of firewall may already be in place.
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