Establishing a Security Perimeter

Last Updated on Tue, 02 May 2023 | Link Layer

When you define a network security policy, you must define procedures to safeguard your network and its contents and users against loss and damage. From this perspective, a network security policy plays a role in enforcing the overall security policy defined by an organization.

A critical part of an overall security solution is a network firewall, which monitors traffic crossing network perimeters and imposes restrictions according to security policy. Perimeter routers are found at any network boundary, such as between private networks, intranets, extranets, or the Internet. Firewalls most commonly separate internal (private) and external (public) networks.

A network security policy focuses on controlling the network traffic and usage. It identifies a network's resources and threats, defines network use and responsibilities, and details action plans for when the security policy is violated. When you deploy a network security policy, you want it to be strategically enforced at defensible boundaries within your network. These strategic boundaries are called perimeter networks.

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Perimeter Networks

To establish your collection of perimeter networks, you must designate the networks of computers that you wish to protect and define the network security mechanisms that protect them. To have a successful network security perimeter, the firewall server must be the gateway for all communications between trusted networks and untrusted and unknown networks.

Each network can contain multiple perimeter networks. When describing how perimeter networks are positioned relative to each other, three types of perimeter networks are present: the outermost perimeter, internal perimeters, and the innermost perimeter. Figure 47-1 depicts the relationships among the various perimeters. Note that the multiple internal perimeters are relative to a particular asset, such as the internal perimeter that is just inside the firewall server.

Figure 47-1: There are three types of perimeter networks: outermost, internal, and innermost.

The outermost perimeter network identifies the separation point between the assets that you control and the assets that you do not control---usually, this point is the router that you use to separate your network from your ISP's network. Internal perimeter networks represent additional boundaries where you have other security mechanisms in place, such as intranet firewalls and filtering routers.

Figure 47-2 depicts two perimeter networks (an outermost perimeter network and an internal perimeter network) defined by the placement of the internal and external routers and the firewall server.

Figure 47-2: The diagram is an example of a two-perimeter network security design.

Figure 47-1: There are three types of perimeter networks: outermost, internal, and innermost.

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Positioning your firewall between an internal and external router provides little additional protection from attacks on either side, but it greatly reduces the amount of traffic the firewall server must evaluate, which can increase the firewall's performance. From the perspective of users on an external network, the firewall server represents all accessible computers on the trusted network. It defines the point of focus, or choke point, through which all communications between the two networks must pass.

The outermost perimeter network is the most insecure area of your network infrastructure. Normally, this area is reserved for routers, firewall servers, and public Internet servers, such as HTTP, FTP, and Gopher servers. This area of the network is the easiest area to gain access to, and therefore, it is the most frequently attacked, usually in an attempt to gain access to the internal networks. Sensitive company information that is for internal use only should not be placed on the outermost perimeter network. Following this precaution helps avoid having your sensitive information stolen or damaged.

Developing Your Security Design

The design of the perimeter network and security policies requires the following subjects to be addressed. Knowing Your Enemy

Knowing your enemy refers to knowing attackers or intruders. Consider who might want to circumvent your security measures and identify their motivations. Determine what they might want to do and the damage that they could cause to your network.

Security measures can never make it impossible for a user to perform unauthorized tasks with a computer system; they can only make it harder. The goal is to make sure the network security controls are beyond the attacker's ability or motivation.

Counting the Cost

Security measures almost always reduce convenience, especially for sophisticated users. Security can delay work and create expensive administrative and educational overhead. Security can use significant computing resources and require dedicated hardware.

When you design your security measures, understand their costs and weigh those costs against the potential benefits. To do that, you must understand the costs of the measures themselves and the costs

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Identifying Your Assumptions

Every security system has underlying assumptions. For example, you might assume that your network is not tapped, or that attackers know less than you do, that they are using standard software, or that a locked room is safe. Be sure to examine and justify your assumptions. Any hidden assumption is a potential security hole.

Controlling Your Secrets

Most security is based on secrets. Passwords and encryption keys, for example, are secrets. Too often, though, the secrets are not really all that secret. The most important part of keeping secrets is knowing the areas you need to protect. What knowledge would enable someone to circumvent your system? You should jealously guard that knowledge and assume that everything else is known to your adversaries. The more secrets you have, the harder it will be to keep them all. Security systems should be designed so that only a limited number of secrets need to be kept.

Remembering Human Factors

Many security procedures fail because their designers do not consider how users will react to them. For example, because they can be difficult to remember, automatically generated "nonsense" passwords are often found written on the undersides of keyboards. For convenience, a "secure" door that leads to the system's only tape drive is sometimes propped open. For expediency, unauthorized modems are often connected to a network to avoid onerous dial-in security measures.

If your security measures interfere with essential use of the system, those measures will be resisted and perhaps circumvented. To get compliance, you must make sure that users can get their work done, and you must sell your security measures to users. Users must understand and accept the need for security.

Any user can compromise system security, at least to some degree. Passwords, for instance, can often be found simply by calling legitimate users on the telephone, claiming to be a system administrator, and asking for them. If your users understand security issues, and if they understand the reasons for your security measures, they are far less likely to make an intruder's life easier.

At a minimum, users should be taught never to release passwords or other secrets over unsecured telephone lines (especially cellular telephones) or e-mail. Users should be wary of people who call them on the telephone and ask questions. Some companies have implemented formalized network security training for their employees so that employees are not allowed access to the Internet until they have completed a formal training program.

Knowing Your Weaknesses

Every security system has vulnerabilities. You should understand your system's weak points and know how they could be exploited. You should also know the areas that present the greatest danger and prevent access to them immediately. Understanding the weak points is the first step toward turning them into secure areas.

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Limiting the Scope of Access

You should create appropriate barriers in your system so that if intruders access one part of the system, they do not automatically have access to the rest of the system. The security of a system is only as good as the weakest security level of any single host in the system.

Understanding Your Environment

Understanding how your system normally functions, knowing what is expected and what is unexpected, and being familiar with how devices are usually used will help you detect security problems. Noticing unusual events can help you catch intruders before they can damage the system. Auditing tools can help you detect those unusual events.

Limiting Your Trust

You should know exactly which software you rely on, and your security system should not have to rely on the assumption that all software is bug-free.

Remembering Physical Security

Physical access to a computer (or a router) usually gives a sufficiently sophisticated user total control over that computer. Physical access to a network link usually allows a person to tap that link, jam it, or inject traffic into it. It makes no sense to install complicated software security measures when access to the hardware is not controlled.

Making Security Pervasive

Almost any change you make in your system may have security effects. This is especially true when new services are created. Administrators, programmers, and users should consider the security implications of every change they make. Understanding the security implications of a change takes practice; it requires lateral thinking and a willingness to explore every way a service could potentially be manipulated.

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