Like most Linux users, I started out using a simple Bash script to configure the firewall policy on my Linux machines. Eventually, I got tired of writing the same code over and over again, so I decided to use a few program loops to try to factor out some of the redundancy. I also decided that I'd like to separate the actual policy from the rest of the program; this meant that the program would be reading external configuration files. As my Perl skills are vastly better than my Bash skills, I decided to write my firewall rules in Perl.

The program I'm outlining in this article could just as easily have been written in Bash or another scripting language (or even C++, for that matter). The language isn't important. The important thing to realize is that once you write a program like this and debug it, all you have to do to modify security rules is change a configuration file and rerun the script. The configuration files should have an intuitive format so that they are easy for humans to read, understand and modify.

Listing 1 shows the Perl script. I practice top-down programming, so the first several lines of code should give you a good idea of what the program does. Hopefully, you should be able to follow what the program does even if you aren't a Perl programmer.

As you can see, the program isn't very long and certainly isn't very complicated. However, the program is flexible enough to allow me to whitelist or blacklist either individual machines or entire networks completely. As we'll see later on, the build_chains() and add_rules() functions implement a rule-pruning algorithm that keeps the Linux kernel from having to evaluate irrelevant rules.

The set_ip_forwarding() function does what its name implies; it tells the Linux kernel either to forward IP packets, or not to forward packets. The function accepts a single parameter, either 0 or 1, which determines whether the kernel will forward. The script initially turns all forwarding off while it loads the firewall policy. Then, right before the script exits, the script turns forwarding back on. The reason for these extra steps is that we want the router to be in a safe state while we load the actual rules. It's better to block all traffic than it is to allow even one attack through.

The load interfaces() function reads in the names of the network interfaces and assigns mnemonic labels to them. These labels are then used in the rest of the configuration to refer to the actual interfaces. Being able to refer to an interface as lan or even vpn_to_work cuts down on misconfigurations. This also makes it easy to make changes in order to tailor the firewall for use by my friends. In many cases, I simply adjust the interfaces.conf file to reflect my friend's network and suddenly, my friend has a reasonable firewall configuration.

The script works via four configuration files: interfaces.conf, good_hosts.conf, bad_hosts.conf and ports.conf.

Listing 2 shows the contents of my interfaces.conf file. As you can see, I've got six network interfaces in my router. My Internet connection is on eth5. I've got a 10/100TX Ethernet for the house wiring. I've got a Gigabit Ethernet connecting my MythTV PVR to the router for file storage. I've got interfaces for the Wi-Fi and VoIP. Finally, I have a VPN connection to some of my friend's computers. It's much easier to remember that lan is my 10/100 copper network as opposed to trying to remember whether eth3 is my VoIP or Wi-Fi interface. The last thing you want to do is apply the right firewall rule to the wrong interface.

The init() function does the initial setup for the iptables environment. First, we flush, or remove, all of the rules and user chains. Then, we clear all of the rule counters. Later on, these counters will allow us to determine how many packets were caught by each rule in our firewall. Then, the script sets up the IP masquerading. I also put in a rule that allows traffic that is related to an already established connection to pass through the firewall without further evaluation. This keeps the entire rule set from having to be evaluated for each incoming packet. The rules apply only to each new connection request.

The set_default_policy() function configures how we treat network traffic in the absence of any other firewall rules. In this case, I'm interested only in policing incoming traffic; the policy accepts outbound traffic and traffic that the kernel has to route to its final destination. Trivial modifications to this script would allow you to configure policies for each direction. By default, my script denies traffic and requires the administrator to list explicitly all of the allowable traffic. This is the safest way to build a firewall, as opposed to a firewall that allows traffic by default and relies on the administrator to deny dangerous traffic specifically. You can never know in advance all of the dangerous traffic, so denying everything but well-understood traffic is a good idea.

The add_good_hosts() function creates firewall rules that allow all traffic from hosts or networks listed in the good_hosts.conf file. Note that I don't tie these rules to any particular interface. Traffic from whitelisted hosts or networks can come in on any interface. I usually put an entry in this file for my workstation at my home office as well as the network at work. This way, even if I make a silly mistake that would have kept me from logging in to my router remotely, I can still get in from work or my office workstation to undo the change. Of course, this also assumes that my workstation and the network at work haven't been compromised. Usually, the contents of this file are quite short indeed.

Conversely, the add_bad_hosts() function creates firewall rules that block all traffic from hosts or networks listed in the bad_hosts.conf file. This function works almost exactly like the add_good_hosts() function with one important difference. When traffic from a blacklisted host comes to the router, the router will not only log this fact, it will also include the comment from the bad_hosts.conf file in the log. This way, I can look at my log file and see why a particular host was blocked. A useful improvement to this function would be to have it place the bad host rules in a separate chain and have that chain called early on in the rule set. This would give you the ability to add and delete hosts conveniently from this chain from an external program, perhaps in response to entries in your server log files.

The build_chains() function builds a series of firewall rule chains. I build a separate chain for each combination of interface and protocol. For example, if I had a Linux router with eth0, eth1, eth2 and eth3, I would create chains for eth0-tcp, eth0-udp, eth1-tcp, eth1-udp and so on. Then, I build the rules necessary to send the decision-making process down the appropriate chain. What we end up with is a decision tree that determines what to do with each packet entering the router. Unlike a linear list of firewall rules, the decision tree prevents the kernel from having to evaluate obviously irrelevant rules. For example, a TCP packet coming in on the WAN interface will never be tested against the rules meant for UDP packets on the Wi-Fi interface.

I haven't done any objective testing to see if this tree-pruning actually makes a significant performance improvement. On the other hand, once the program is written and debugged, it costs me nothing to change the configuration files and have this decision tree generated automatically. So even if it improves performance by only a small amount, it adds so little to the program's complexity that I think it makes sense to do it.

The add_rules() function is where most of the work is done. This function reads the contents of ports.conf, which is shown in Listing 5. Before we discuss the add_rules() function in detail, we should discuss the format and content of the ports.conf file.

The ports.conf file contains one line for each firewall rule. Each line contains three columns and an optional comment preceded by the # character. The first column is the user-defined label for the interface that the rule will be applied to. The second column is the protocol, that is, tcp, udp or the special case, all. Using all for the protocol creates a rule that allows all traffic on the interface in question. Finally, we have the port number. For example, the first line of the file creates a rule that allows SSH traffic to come in on the wan interface.

You'll notice that I have a rule that allows all traffic on the lo, or loopback, interface. This is important, because without this rule, many programs break in ways that are hard to diagnose. You also may be asking why I go to the effort of creating so many firewall rules for my LAN interface, only to have an all at the end. I do this for several reasons. The primary reason is that until my kids grow up and get on the Internet, I trust the traffic on my local network. However, by having rules for each service I run, I'm able to extract statistics about how much traffic each service generates. Also, security is an iterative process. Over time, I'll add rules that will further tighten my firewall; eventually, I'll remove the final all from the policy.

Now, back to the add_rules() function. Even though this is the longest function in the entire program, it's still not too hard to understand. The sections of code that deal with tcp and udp rules simply create two rules for each rule in the ports.conf file. One rule is tied to the destination port number; the other rule is tied to the source port number. At first, this may seem odd, because we are policing only inbound traffic. What we're doing is making sure that both inbound and outbound connections are allowed to pass. For example, a packet coming in on the WAN interface with the destination port set to 80 corresponds to an inbound connection to my Web server. On the other hand, an incoming packet on my WAN interface with the source port set to 80 is coming from an outside Web server in response to a request that came from inside my network.

The code that handles the all rules is a special case. In this case, we create a rule on the given interface for each of the protocols. In hindsight, this might be overly complex, but it has an interesting side effect. If the router encounters a packet containing an unknown protocol, such as IPSec, the firewall will fall back to its default policy even though we've asked it to pass all traffic on this interface. So, in a sense, “all protocols” actually means “all known protocols”. I think this is a good thing.

For what it's worth, the script will put firewall rules into the kernel in roughly the same order that they appear in the ports.conf file. I say roughly, because the rules will be put into the appropriate chain depending on which interface and protocol they match against. But within each chain, the rules will be executed in order.

The set_default_action() function creates rules that determine what happens to packets that don't match any previous rules. This sounds very similar to the purpose of the set_default_policy() function, but there is a subtle difference. The set_default_policy() function configures the default firewall policy, and the set_default_action() function creates firewall rules that catch unmatched traffic before the kernel falls back to the default policy, essentially capping each chain. Once these rules match a packet, they create a log entry for the packet, and then they implement whatever policy we want, in this case, DROP. Once again, the log entries allow me to determine what traffic is being dropped and why.

I'm not trying to tell you that this program is perfect, nor will it do everything you want it to do. You might even find bugs in it. In fact, by the time you read this article, I'll probably have made several improvements to the script. As it is right now, the script isn't able to configure any firewall policy for the ICMP protocol. It would be nice to be able to allow outbound ping requests and deny incoming requests, for example. It would also be useful to be able to configure firewall policy for outgoing and routed traffic. And because I'm using VoIP, I'm thinking of changing my script to allow me to configure Quality of Service (QoS). If you come up with useful modifications to this script, I'd like to hear about them.

But there you have it, such as it is. In less than 200 lines of Perl code, I'm able to implement a quite flexible and efficient firewall policy containing potentially hundreds of individual rules. At the same time, making changes to my firewall policy is simple enough that even most beginning Linux users can make correct changes.

Mike Diehl works for SAIC at Sandia National Laboratories in Albuquerque, New Mexico, where he writes network management software. Mike lives with his wife and two small boys and can be reached via e-mail at mdiehl@diehlnet.com.