Using Firewall Builder, Part I

Linux 2.4's Netfilter firewall code and its front end, iptables, deserve the praise and popularity they've garnered. They've brought Linux firewalls to the same level as commercial stateful packet-filtering firewalls, from the standpoints of functionality, intelligence and security.

Only one thing has been lacking from the Netfilter experience: user-friendliness. A good firewall GUI isn't merely a crutch to be used by nontechnical people. Even the most pointy-headed of us tend to work faster and make fewer mistakes in our firewall policies if we can construct rules with the aid of visual cues and reminders. There's little value in focusing on iptables' command syntax at the expense of the actual security policy your firewall needs to enforce.

Firewall Builder (Figure 1) is a good firewall GUI indeed. It lets you define host, network and service objects that can be used and reused in as many different firewall rulesets as you like; it displays your rules in an instinctive and clear way; and because it's intentionally OS-agnostic, you can use Firewall Builder to generate rulesets not only for Netfilter/iptables, but also for FreeBSD's ipfilter, OpenBSD's pf and even Cisco PIX firewalls.

Figure 1. Firewall Builder in Action

This issue and next I'll show you how to obtain and install Firewall Builder, and then I'll explain how to use it to build iptables policies of your own easily and instinctively. To begin, we focus on installing Firewall Builder and on populating its Objects database; next month we'll cover policy construction in-depth.

First, a few words on where to install and run Firewall Builder. I don't think it's a good idea to run Firewall Builder on an actual firewall or on any other hardened, publicly accessible host, called a bastion host. In short, I don't think you should run the X Window System on such hosts.

Instead, you should run Firewall Builder on your normal day-to-day workstation. Then, copy the firewall scripts you build to the host you actually wish to configure, using scp or some other secure means. Firewall Builder is designed to be used in this way.

On the other hand, if you intend to use Firewall Builder to create Netfilter scripts for local protection of one particular host, such as a Linux 2.4-based web server, perhaps it's okay to run Firewall Builder directly on the host on which its scripts will be installed. But, make sure X11 is installed on the host and the host itself is behind a proper firewall.

The important point is you don't need to run Firewall Builder on each host you want configured. Therefore, you shouldn't run it on any host on which you wouldn't otherwise run the X Window System. A single host running Firewall Builder can generate scripts for as many different hosts as you like. We'll see how this is possible shortly.

Naturally, the Firewall Builder Project has its own home page, where you can obtain the latest software releases and documents: www.fwbuilder.org. If anything I say here is different from something you read there, I defer to that site. Firewall Builder's on-line installation instructions are clear and accurate, and they may change between the time I wrote this article and the time it actually is printed.

I'll start with the easiest case. If you run Debian 3.0, you can install Firewall Builder directly from your Debian installation source; Debian has its own officially supported deb package, called fwbuilder. Among other things, this package depends on the Debian packages libfwbuilder0, fwbuilder-iptables, libgtk1.2, libgtkmm1.2, libxslt1, libxml2 and libsnmp4.2.

I'll skip the complete list of dependencies, though. If you use apt-get to install fwbuilder, apt-get will identify and install all required packages for you. I also recommend installing Debian's fwbuilder-doc package; it is optional (and therefore won't be installed automatically by apt-get in order to satisfy any dependencies) but contains comprehensive and useful documentation.

As of Red Hat 8.0 (the latest Red Hat release at the time of this writing), Firewall Builder isn't yet an official part of Red Hat Linux. However, the Firewall Builder team provides RPM files for several Red Hat releases; see the Firewall Builder download site at sourceforge.net/project/showfiles.php?group_id=5314.

You'll need the fwbuilder and libfwbuilder packages, plus at least one of fwbuilder-ipt, fwbuilder-ipf or fwbuilder-pf, depending on whether you create rulesets for Linux Netfilter/iptables, FreeBSD ipfilter or OpenBSD pf, respectively. You can install more than one of these last three if you wish. Because Firewall Builder's ultimate output is an ASCII script, using a Linux system to generate firewall rules for other platforms is not a problem.

Before installing the Firewall Builder packages, the following standard Red Hat packages must be present: bind-utils, gdk-pixbuf, glib, glibc, gtk+, gtkmm, libfwbuilder, libsigc++, libstdc++, libxml2, libxslt, openssl-0.9.6b, ucd-snmp and XFree86-libs.

In addition, you'll need gtkmm (the GIMP Tool Kit Minus Minus), which contains the C++ bindings for GTK+. This package is part of Ximian GNOME, but you also can download it from www.freshrpms.net.