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Linux Firewalls ...and their advantages over Windows

Linux Firewalls ...and their advantages over Windows

With all of this talk about firewalls, many people are asking "what are the advantages of Linux firewalls over firewalls running on Windows?" In this article, I hope to address this question from several different angles. First, is Linux more secure than Windows as a firewall platform? Second, what problems do Linux firewalls manage to avoid which plague Windows firewalls? Third, why do organizations use Windows firewalls at all? On the way, I'll introduce Bastille Linux, a popular security project that includes a fairly powerful free firewall.

The most obvious reason that you should put your firewall on a Linux or Unix platform is that the alternative, Windows, has shown itself to be rather insecure. MicroSoft doesn't appear to care one bit about security, as incident after incident has shown in the past few years. Between the Melissa, LoveLetter, Code Red and Nimda worms and a whole host of other programs that have easily preyed on vulnerabilities and security bugs in MicroSoft products, we've watched MicroSoft customers lose a huge amount of money and man-hours fixing unnecessary problems in their software.

The industry is even beginning to lose confidence in MicroSoft's ability to create secure applications. The Gartner Group recently recommended that organizations using MicroSoft's IIS Webserver switch immediately to another server application. In a world dominated by MicroSoft marketing, this was a significant deviation from the norm. The security community is currently discussing the fact that several insurance companies are now charging MicroSoft NT and IIS clients greater premiums, to account for the higher danger in running this operating system and server. MicroSoft doesn't appear capable of creating a sound Web server that these groups will trust. Now, when you take note of the fact that operating systems are far more complex than server applications, and that complexity tends to cause more security problems, you see why many of us agree with these insurance companies. We don't trust MicroSoft to create a secure operating system.

Many of us don't even trust MicroSoft to create a stable operating system, much less a secure one. This makes me wonder whether any program that must have a very, very low rate of failure should be running on a Windows system. As noted security writer Mike Rash puts it, "why would anyone run the single point of failure for your entire network on the king of reboot operating systems?!"

Now, Linux and Unix have had security problems as well, but none have caused the widespread damage seen from the high-profile Windows viruses and worms. Further, it's much easier to avoid these problems on a Linux/Unix operating system. See, the entire operating system can be tuned for security by a competent system administrator. While a Linux or Unix operating system usually ships as a general-purpose platform, it's designed to be tuned for the administrator's particular purposes, whether that be as a mail server, a firewall platform, or a workstation. Securing these systems is a well-understood process - there are a number of books, courses and articles on the topic. Windows, on the other hand, has historically been very difficult to secure. Firstly, the MicroSoft operating systems have either had less tunable security settings or they've had so many possibly conflicting settings that the unnecessary complexity buries the system administrator. Secondly, a number of MicroSoft products have been known to ignore their own security settings - even in the case where a system administrator was able to configure the right settings, the product's own bugs stopped those settings from taking effect and preventing a system compromise.

Linux and Unix are just the opposite here. The security settings are generally present by design. Further, because the operating systems are so comprehensible and open, it's a trivial task to remove unnecessary or vulnerable subsystems. This makes tuning for security fairly easy for an experienced system adminstrator. There are even a number of programs that can help an ordinary user or an inexperienced system administrator perform this task. Bastille Linux is one example - this program both tightens security on a machine and teaches the user about security. Bastille runs on Mandrake Linux, Red Hat Linux and HP-UX. It teaches the user about security so that he can make the best informed decisions about tightening security - it then uses the results of those decisions to carefully tune the security settings available. Titan is another such example. While it lacks the educational component of Bastille, it can tighten security on Solaris and FreeBSD as well.

By the way, due to the Open Source nature of Linux, it's even easier to secure than most of the Unix's. Part of this is due to the fact that a third party can modify the operating system itself, not just the settings, for greater security. For example, the United States NSA released SELinux (Security Enhanced Linux) which adds extreme security granularization to Linux, at the cost of some ease of administration. WireX also makes an entire host of solutions to do the same type of thing, including SubDomain, a product which allows a system administrator to define exactly which files that an application will have access to. A number of similar solutions exist as well. The wonderful thing about Linux's Open Source nature is that you're given the ability to not only inspect your entire operating system, but change it on a whim.

All in all, Linux just makes a much better platform to install a firewall on top of, rather than Windows. The Windows operating system has a bad track record for instability problems, bugs and security vulnerabilities. Linux in particular, and Unix in general, avoids these by either being built more solidly or having far greater tunability.

Let's take a look at a related issue: why do Linux machines tend to avoid many of the security problems that plague Windows? Windows machines appear designed for maximal ease of use, with many pieces automatically woven together to make the experience easier for the user. Unfortunately, this leads to a system that's far more complex than is safe - this creates security holes as different pieces interact with each other in ways that weren't originally intended. It also creates problems because MicroSoft often designs systems to make things easier on their users but doesn't realize the security ramifications of these decisions. One dangerous example of this was in MicroSoft's decision to bundle a scripting language (VBscript) into ther mail client, Outlook - because Outlook would run the VBscript enclosed with any mail message, a huge number of people were infected with a worm that continued to spread rapidly across the Internet. This simple judgement call cost the world a huge amount of time and money, as stopping and cleaning up after the worm was incredibly difficult.

MicroSoft continually makes these judgments in their operating system design, but the same kind of problems plague the treatment of their customers. Right now, a debate is raging concerning MicroSoft's stance on security vulnerability disclosure. MicroSoft is working hard to try to shut down communication about security holes in their software. Instead of trying to actually avoid creating these problems, MicroSoft is working to try to stop people from talking about them - they somehow hope that if none of their customers ever hear about the problems, no one will stop buying their products for fear of security holes. Unfortunately, whether or not a MicroSoft customer knows about a security hole, many people will discover the holes independently, communicate them through underground channels, and use them against the MicroSoft customers who don't even know that they're vulnerable!

On the other hand, the Linux companies don't really have the capability or the desire to hide security information from their customers. Actually, these companies have a proven track record of fixing security problems around four times faster than MicroSoft. In a SecurityPortal article two years back, Kurt Seifried examined the amount of time between when a vendor was alerted to a problem and when the vendor issued a "patch" to correct the problem. This time period, called the "Window of Vulnerability," is critical - during this time, every client running the affected program will be vulnerable. For this reason, it's important to minimize it as much as possible. Well, while Red Hat's average turnaround time, or window of vulnerability, was seven days, MicroSoft's average time was a full 30! With the immense resources of MicroSoft, it seems unlikely that they couldn't match, or beat, Red Hat's turnaround time - many of us thus draw the conclusion that they just don't care! Do you really want your firewall running on an operating system with MicroSoft's security history?

So, why do organizations use Windows firewalls at all? Well, the simplest reason that most of us can come up with is that Windows is very user-friendly. Since Linux is less user-friendly, it requires trained staff to administer it. Training your staff, or hiring staff with the necessary skills, definitely raises the cost of the technology, it is thought. Well, while it's certainly true that getting someone with the necessary skills and training is slightly more expensive, it can often be more expensive to recover from a successful break-in. See, while a less experienced person has a better chance of running a firewall on Windows, they probably don't have a better chance of running it well! Properly configuring a firewall for a medium or large size business is a challenging task that requires technical knowledge of networking and application protocols. Don't you want smart, trained people to run this critical subsystem?!

With that said, allow me to propose a solution. Purchase or build your firewall on a Linux or Unix system and take the time to properly "tighten" the system's configuration. You want a well-configured firewall running on a secure, tightened platform. Just don't do it on Windows...

More Stories By Jay Beale

Jay Beale is the lead developer of the Bastille Linux Project, which creates a hardening program for Linux. He also wrote the Center for Internet Security's system auditing tool for Solaris, Linux and HP-UX and the security benchmark document for Linux. Jay is the author of the upcoming book Locking Down Linux the Bastille Way, to be published by Addison-Wesley.

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