Why Do I Need a Firewall?
If it's not already clear, there are many reasons, but the two really big ones are that there are bad people in the world who want to get in to your networks and that your network depends on millions of lines of
If this doesn't sound like a problem for you, then you don't need a firewall. In practical terms, it's the very rare case when a firewall would not be helpful,
Thankfully, many Linux
Just remember, like everything else we've discussed so far, firewalls are not a silver bullet. Just because you installed one doesn't mean you're safe. A firewall is a compartmentalization tool with holes in it. If you let someone through to your web service port, it's not going to protect you from web attacks; if your browser has a flaw in it, it's not going to protect from spyware that
Do I Need More Than a Firewall?
Yes, always, without a doubt, you need more than a firewall. You will need tools to help you harden your system, to check the integrity of your files, binaries and drivers, intrusion detection and prevention tools, patch management technologies, and penetration testing tools. Sometimes you might even need military grade cryptography, security policies to describe what users can and cannot do, training for those users, and above all else, a certification program and even a risk management plan. A firewall is never enough by itself.
What Kinds of Firewalls Are There?
There are four types of firewalls, which are all available on Linux platforms. These are, in order of complexity and features, packet filtering, application proxies, stateful inspection, and hybrid. There are, as we will explain, a few very specialized firewalls for extremely high security environments, trusted
As stated previously, there are two other types of firewalls that we are not going to cover in this book. Their use is very specialized, and it's not practical to construct them with
. The other types fall into two categories, trusted guards and one-way firewalls. Trusted guards basically prevent data, in theory, from moving from one domain to another domain. Basically, they are designed to prevent "Top Secret" data, for instance, from moving into a domain that is only rated "Secret," and they accomplish this with specialized hardware and Multi Level Security (MLS) data labeling techniques. With commodity hardware, such as off-the-shelf PCs, it's not really possible to accomplish true compartmentalized security with trusted guards. You need some specialized hardware to protect the memory on the PC and even in some cases, specialized NIC cards. You can accomplish some less trusted attempts at MLS with commodity hardware if you want to tinker, but with that type of hardware the system will never really be "trusted" in the classical computer science sense of the
Aside from trusted guards, the other type of firewall we will not cover in this book is known as a "one way" firewall. This kind of firewall involves the use of specialized hardware that literally will only transmit data in one direction. This too is a highly specialized form of a firewall because it is designed to prevent a very unique form of attack through all modern firewalls called a "covert channel attack." Briefly, a "covert channel attack" is where someone behind a firewall is able to send data, or possibly to even construct a full data channel, through a firewall in spite of the firewall's policy. It's beyond the scope of this book to go into more detail on this type of attack, but it's a fascinating issue to consider as it may be relevant to your organization's risk management plan. Again, you can always check www.gotroot.com for information on this topic or to ask on our forums for more information.
Both of these types of firewalls are in response to risks that classic firewalls, all four of the types discussed at the first part of this section, cannot adequately protect against. We think it's good to point this out because your firewall can only protect against a finite range of risks. It alone cannot protect against all of the current known threats that networks face.