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Building Internet Firewalls

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5. Bastion Hosts

General Principles
Special Kinds of Bastion Hosts
Choosing a Machine
Choosing a Physical Location
Locating the Bastion Host on the Network
Selecting Services Provided by the Bastion Host
Don't Allow User Accounts on the Bastion Host
Building a Bastion Host
Operating the Bastion Host
Protecting the Machine and Backups

A bastion host is your public presence on the Internet. Think of it as the lobby of a building. Outsiders may not be able to go up the stairs and may not be able to get into the elevators, but they can walk freely into the lobby and ask for what they want. (Whether or not they will get what they ask for depends upon the building's security policy.) Like the lobby in your building, a bastion host is exposed to potentially hostile elements. The bastion host is the system that any outsiders - friends or possible foes - must ordinarily connect with to access a system or a service that's inside your firewall.

By design, a bastion host is highly exposed, because its existence is known to the Internet. For this reason, firewall builders and managers need to concentrate security efforts on the bastion host. You should pay special attention to the host's security during initial construction and ongoing operation. Because the bastion host is the most exposed host, it also needs to be the most fortified host.

Although we talk about a single bastion host in this chapter and elsewhere in this book, remember there may be multiple bastion hosts in a firewall configuration. The number depends on a site's particular requirements and resources, as discussed in Chapter 4, Firewall Design. Each is set up according to the same general principles, using the same general techniques.

Bastion hosts are used with many different firewall approaches and architectures; most of the information in this chapter should be relevant regardless of whether you're building a bastion host to use with a firewall based on packet filtering, proxying, or a hybrid approach. The principles and procedures for building a bastion host are extensions of those for securing any host. You want to use them, or variations of them, for any other host that's security critical, and possibly for hosts that are critical in other ways (e.g., major servers on your internal network).

5.1 General Principles

There are two basic principles for designing and building a bastion host: Keep it simple, and be prepared for the bastion host to be compromised.

Keep it simple

The simpler your bastion host is, the easier it is to secure.

Any service the bastion host offers could have software bugs or configuration errors in it, and any bugs or errors may lead to security problems. Therefore, you want the bastion host to do as little as possible. It should provide the smallest set of services with the least privileges it possibly can, while still fulfilling its role.

Be prepared for the bastion host to be compromised

Despite your best efforts to ensure the security of the bastion host, break-ins can occur. Don't be naive about it. Only by anticipating the worst, and planning for it, will you be most likely to avert it. Always keep the question, "What if the bastion host is compromised?" in the back of your mind as you go through the steps of securing the machine and the rest of the network.

Why do we emphasize this point? The reason is simple: the bastion host is the machine most likely to be attacked because it's the machine most accessible to the outside world. It's also the machine from which attacks against your internal systems are most likely to come because the outside world probably can't talk to your internal systems directly. Do your best to ensure that the bastion host won't get broken into, but keep in mind the question, "What if it does?"

In case the bastion host is broken into, you don't want that break-in to lead to a compromise of the entire firewall. You can prevent this by not letting internal machines trust the bastion host any more than is absolutely necessary for the bastion host to function. You will need to look carefully at each service the bastion host provides to internal machines, and determine, on a service-by-service basis, how much trust and privilege each service really needs to have.

Once you've made these decisions, you can use a number of mechanisms to enforce them. For example, you might install standard access control mechanisms (passwords, authentication devices, etc.) on the internal hosts, or you might set up packet filtering between the bastion host and the internal hosts.

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4.5 What the Future HoldsBook Index5.2 Special Kinds of Bastion Hosts