Firewalls: Friend or Foe?

Terry Gray
August 2002

Minority Report

One of the first lessons IT managers learn about security is that when someone asks whether or not you have a firewall, the correct answer is "Of course we have a firewall... Whaddya think, we're idiots?" Indeed, the cornerstone of most enterprise computer security plans is the border firewall. Their efficacy has been accepted to the point where they have become "check-list" items on any security audit. Thus, the majority opinion --certainly in industry-- is that enterprise firewalls are at least necessary, if not sufficient, to protect computer systems, and those who ignore their obvious benefits are either uninformed or incompetent. Yet, a few organizations --even in industry-- are bucking the trend. Herewith is a sort of "Minority Report": an examination of why one might choose the road less-traveled, and what alternative approaches are available.

Our thesis is not that all firewalls are evil; rather, it is that all firewalls have significant disadvantages, often ignored, and that their advantages are often overstated. This is especially true of enterprise border firewalls, which are the focus of today's debate.

Can systems be made secure (network safe) without using external firewalls? Clearly yes. We have many examples of this. But that seems to be more the exception than the rule, both because most operating systems are not network-safe "out of the box", and because a large number of those systems are essentially unmanaged.

There is unanimous agreement that evil packets should not be permitted to reach a place where they can do harm, so the debate is not over whether to block, but rather where the blocking should be implemented, and how to deal with the fact that different people want different things blocked. ("One person's secure network is another's broken network.")

Key Questions

Several questions need to be asked more often:

  1. What are the downsides of firewalls?
  2. Where does it make sense to deploy firewalls?
  3. What alternatives are there to border firewalls?

In examining those questions, we conclude that a traditional enterprise border firewall is very often a mistake, that is, more foe than friend. In some situations it may even be true that an enterprise firewall reduces overall security.

Key Premise

One key premise behind this conclusion is that network-based security is maximized when the network protection perimeter is minimized, i.e. pushed as close to the end-system as possible. This is because of the "Perimeter Protection Paradox", namely: the expected value, or perceived effectiveness of a firewall is proportional to the number of systems behind it, but the actual effectiveness of a firewall is inversely proportional to the number of systems behind it. That is true for two reasons: first, as the population of machines in the trusted "vulnerability zone" behind the firewall grows, so does the probablility that at least one of them has been somehow compromised and can thus be used to launch an attack that circumvents the firewall; second, the number of "holes" that need to be poked through a firewall is usually a function of the population of systems behind it, representing diverse organizational units.

The Darkside

Problems with border firewalls include:
  1. Performance. Many current routers are seriously impacted when asked to do significant port blocking. This is an argument for viewing border blocking, in current generation routers, as an emergency response to a massive cyberattack. Alternatively, dedicated enterprise firewalls are both expensive and may also impact performance --especially at well-connected Internet2 sites.
  2. Policy consensus. Getting consensus on what to block at the border of a large heterogeneous enterprise is at best non-trivial. Even blocking the most obvious ports (e.g. SMB, NFS) is controversial. For example, some of our NFS users said "maybe in a year I could live with blocking NFS." Some SMB users said "why should I bear the expense of a VPN server for my remote SMB clients when I manage our PCs properly, and besides those who want border blocking can implement whatever perimeter they want via IP access lists on their hosts?"
  3. Mean Time To Repair. MTR for many system problems involving traffic across a perimeter defense line does go up. It's just the nature of these "network traffic disrupters". (Some of us have experienced this phenomenon even with simple home network firewalls!) And this ties in to firewall support costs...
  4. Management cost. The cost of managing firewall rules, especially if exceptions are permitted or they change often, is high. People with enterprise firewall experience in industry have told me that they needed nearly one FTE per firewall... not just to manage changes, but to track down allegations that the firewall was causing other problems.
  5. Organizational stress. Ideally, the "administrative distance" between policy decision makers and those impacted by their policies is small. This is an argument for smaller departmental or workgroup defensive perimeters, rather than enterprise perimeter blocking.
  6. Increased tunneling. Extensive border blocking tends to encourage (force?) people to tunnel more and more applications over port 80, the standard web port which is rarely blocked. We already see that happening.
  7. Irrelevance. A large percentage of current attacks are exploiting weaknesses in old versions of critical services. Thus they cannot realistically be blocked by a border firewall without creating a self-imposed denial-of-service attack. Examples include SSH, HTTP, and DNS. Moreover, no matter how restrictive border blocking rules might be, it stretches credulity to believe that sharing an inside-the-perimeter vulernability zone with 50,000 other computers, many of them unmanaged, would let anyone sleep easy at night if they were relying on that perimeter. (Easier? Maybe. But not easy.)
  8. Back door connections. Restrictive firewall rules can lead to heroic efforts by those who "just want to get their work done" to find or create back-door connections that bypass the enterprise firewall.
  9. False sense of security. Some system administrators take offense at the notion that border firewalls may provide a false sense of security. The observation is not directed at those system administrators who manage their computers as if there is no external line of defense. However, there are others who have candidly stated that they do not have time to properly manage all of their systems and want to rely on a border firewall for protection. The concern is that a large-perimeter (border) defense will necessarily be less restrictive, and therefore provide less security, than a smaller perimeter defense. This, combined with large vulnerabilty zones and attacks that cannot be blocked by firewalls may lead to scenarios where existence of a border firewall actually undermines security.

In short, a border firewall is clearly neither necessary nor sufficient, much less a panacea, so it's benefits have to be evaluated against the costs and the alternatives.

Criteria for Firewalling

Given the problems identified above, we generally relegate border firewalling to possible use during a cyberattack emergency, if such blocking is deemed to do more good than harm. However, blocking packets with spoofed source addresses is an example of a good thing to do at the border, and indeed at the subnet-level of the network as well. Are there other cases where blocking within the network makes sense? Our policy is that blocking packets within the network backbone should be implemented only when such blocking meets three tests:
  1. It protects against a threat that system managers cannot reasonably protect themselves against.
  2. It won't seriously impact performance of the network core.
  3. There is widespread consensus that such blocking is a Good Thing.

Each of those criteria is subjective, and in each case the answer may change over time. So far, the only thing that has clearly met all three tests is blocking packets with spoofed source addresses --and even that has had adverse network performance impact in a number of cases.

It is undeniable that the effectivness of a perimeter defense is inversely proportional to the size of the perimeter, so the desire to push the protection boundry as close to the network edge as possible is a consequence of wanting to see real security, not just check-list security. On the other hand, it is also undeniable, that managing a large number of devices, even centrally, is harder than managing a small number of devices. Yet, the indirect costs of border blocking are higher than is generally acknowledged, and the value of border blocking is lower than is generally acknowledged. However, the cost/benefit analysis may result in a different conclusion regarding departmental or subnet firewalls, and most certainly for host-based firewalls.

When Firewalls Make Sense

Firewalls can be implemented at different different places in the network topology. Having already addressed border firewalls, that leaves:
  1. Host-based firewalls.
  2. Lab/machine room firewalls; "secure sanctuaries".
  3. Subnet firewalls.

"Firewalls are such a great idea, every host should have one." Firewalls on end-systems, if centrally managed, can provide maximum flexibility in security policy, while still not having to touch each machine after installation. Unlike personal intrusion detection systems, which are notorious for false positives and corresponding support woes, centrally managed host firewalls have been successfully deployed without undo pain.

Contemporary operating systems can all emulate basic border firewall port blocking by using IP access lists, so that if you need to run insecure protocols locally, you can still implement "border blocking" at the hosts themselves. Even more sophisticated border firewall behavior can be emulated by host-based firewalls. Thus, the case for a larger-perimeter defense boils down to three things:

  1. Protecting older systems and devices that are vulnerable and cannot be configured, changed or upgraded to be net-safe.
  2. Easier management of one device vs. many.
  3. Defense in depth.

All of these are legitimate arguments, notwithstanding the implication of #2 that a perimeter defense means the many end devices don't need to be managed. And that leads us to subnet-firewall options. Their goal is to make it easy for units to establish whatever perimeter defense policy they want, without interfering with the management of the network utility. Whether this is easy or hard to accomplish depends on where service boundaries have been drawn within the institution. For example, at institutions where the central IT group is responsible for networking to the individual Ethernet outlet, there are special challenges in allowing departments to manage firewalls at subnet perimeters, as when firewall reconfigurations accidentally block access to the network devices behind them. Thus, having departmentally-managed firewalls in the middle of the network core is antithetical to high-availability network management

In such environments, providing for proper network management and at the same time allowing departments some level of autonomy on subnet firewalling requires unconventional techniques. We have identified two: first, the "logical firewall" (see RESOURCES), which plugs into any single network outlet and allows system administrators to protect hosts on an opt-in basis, and second a VLAN-based strategy where enforced firewalling is possible using a standard bridging firewall combined with network VLAN magic to make sure network management can be maintained on devices behind the firewall, regardless of the firewall configuration.

Another option is to interpose a standard firewall between the network core and a computer lab, machine room, or "server sanctuary" --a cluster of specially protected racks for critical hosts within a machine room. In the case of the academic computer lab, the firewall rules might be adjusted to protect the rest of the network from the student machines!

Multiple Perimeters

The key perimeter defense questions are: where to block and what to block? Those two questions are intertwined, since the closer to the end-system you are, the more restrictive your blocking policy can be, without risking limiting legitimate work. One set of options would be:





Defense In Depth

The doctrine of "defense in depth" calls for multiple perimeters, but it doesn't stop there. A comprehensive approach to the security problem involves all of the following strategies:
  1. Use of secure application protocols
  2. Host hardening
  3. Pro-active vulnerabilty probing
  4. Monitoring
  5. Requirement and accountability policies
  6. and, of course, prudent use of Firewalling

Role of Central IT Services

Even if enterprise/border firewalls are not the best way to achieve computer security, central IT organizations still have a major role in the solution. For example:

  1. Campus authentication infrastructure: design and operation (including Kerberos servers, a web authentication system, windows forest, etc).
  2. Wireless access control infrastructure.
  3. Email virus blocking within the messaging infrastructure.
  4. Developing departmental options, e.g. logical firewall concept.
  5. Incident response, forensic analysis.
  6. Proactive vulnerability probing project.
  7. Server sanctuaries (secure colo space).
  8. Site licensing of secure tools (e.g. commercial ssh).
  9. Documentation, web sites, training classes.
  10. Executive awareness and policy work (e.g. Privacy Assurance and System Security Council).
  11. Consulting services:


It's worth noting that a lot of the world's security problems would go away if Microsoft would implement a small number of very easy measures. Their new multimillion dollar intitiative to improve the security of all their code is a fine thing, but a few simple changes would make a dramatic improvement in security for customers who do not need to have server capabilities. For example:

Note that the default install of RedHat Linux gives you a system that only permits incoming DHCP packets and does not allow you to create accounts (much less root/administrator accounts) that do not have passwords. This serves as an existence proof for others. All of us should be pushing whoever will listen toward the goal of computers that are network-safe out-of-the-box.