• CONFIG_IP_NF_MATCH_STATE

• CONFIG_IP_NF_FILTER

• CONFIG_IP_NF_NAT

• CONFIG_IP_NF_TARGET_LOG

This script follows the golden rule to not trust anyone, not even our own employees. This is a sad fact, but a large part of the hacks and cracks that a company gets hit by are a matter of people from their own staff perpetrating the hit. This script will hopefully give you some clues as to what you can do with your firewall to strengthen it. It's not very different from the original rc.firewall.txt script, but it does give a few hints at what we would normally let through etc.

rc.test-iptables.txt

The rc.test-iptables.txt script can be used to test all the different chains, but it might need some tweaking depending on your configuration, such as turning on ip_forwarding, and setting up masquerading etc. It will work for most everyone who has all the basic set up and all the basic tables loaded into kernel. All it really does is set some LOG targets which will log ping replies and ping requests. This way, you will get information on which chain was traversed and in which order. For example, run this script and then do:

ping -c 1 host.on.the.internet

And tail -n 0 -f /var/log/messages while doing the first command. This should show you all the different chains used, and in which order, unless the log entries are swapped around for some reason.

Note This script was written for testing purposes only. In other words, it's not a good idea to have rules like this that log everything of one sort since your log partitions might get filled up quickly and it would be an effective Denial of Service attack against you and might lead to real attacks on you that would be unlogged after the initial Denial of Service attack.

rc.flush-iptables.txt

The rc.flush-iptables.txt script should not really be called a script in itself. The rc.flush-iptables.txt script will reset and flush all your tables and chains. The script starts by setting the default policies to ACCEPT on the INPUT, OUTPUT and FORWARD chains of the filter table. After this we reset the default policies of the PREROUTING, POSTROUTING and OUTPUT chains of the nat table. We do this first so we won't have to bother about closed connections and packets not getting through. This script is intended for actually setting up and troubleshooting your firewall, and hence we only care about opening the whole thing up and resetting it to default values.

After this we flush all chains first in the filter table and then in the NAT table. This way we know there are no redundant rules lying around anywhere. When all of this is done, we jump down to the next section where we erase all the user specified chains in the NAT and filter tables. When this step is done, we consider the script done. You may consider adding rules to flush your mangle table if you use it.

Note One final word on this issue. Certain people have mailed me asking me to put this script into the original rc.firewall script using Red Hat Linux syntax where you type something like rc.firewall start and the script starts. However, I will not do that since this is a tutorial and should be used as a place to fetch ideas mainly and it shouldn't be filled up with shell scripts and strange syntax. Adding shell script syntax and other things makes the script harder to read as far as I am concerned and the tutorial was written with readability in mind and will continue being so.

Limit-match.txt

The limit-match.txt script is a minor test script which will let you test the limit match and see how it works. Load the script up, and then send ping packets at different intervals to see which gets through, and how often they get through. All echo replies will be blocked until the threshold for the burst limit has again been reached.

Pid-owner.txt

The pid-owner.txt is a small example script that shows how we could use the PID owner match. It does nothing real, but you should be able to run the script, and then from the output of iptables -L -v be able to tell that the rule actually matches.

Recent-match.txt

The recent-match.txt script is a small example of how the recent match can be used. For a complete explanation of this script take a look at the Recent match section in the Iptables matches chapter.

Sid-owner.txt

The sid-owner.txt is a small example script that shows how we could use the SID owner match. It does nothing real, but you should be able to run the script, and then from the output of iptables -L -v be able to tell that the rule actually matches.

Ttl-inc.txt

A small example ttl-inc.txt script. This script shows how we could make the firewall/router invisible to traceroutes, which would otherwise reveal much information to possible attackers.

Iptables-save ruleset

A small example script used in the Saving and restoring large rule-sets chapter to illustrate how iptables-save may be used. This script is non-working, and should hence not be used for anything else than a reference.

What's next?

The chapter you have just read basically gave you a brief overlook of all the different scripts that are available with this tutorial and the basic idea that they are trying to bring across to you. Hopefully it has been able to explain something at the very least.

The next chapter will discuss some different graphical user interfaces that are available for iptables and netfilter. This is far from a complete listing of all the different interfaces available, but as you can see, there are quite a lot of othem. These interfaces mostly tries to simplify creating iptables scripts for you, and for simple setups they are more than enough most of the time. At other times, you may have higher and more complex needs and you must have to write your own script none the less.