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Note This script might be a bit less secure than the rc.firewall.txt script. I would definitely advise you to use that script if at all possible since this script is more open to attacks from the outside.
Also, there is the possibility to add something like this to your scripts:
INET_IP=`ifconfig $INET_IFACE | grep inet | cut -d : -f 2 |
cut -d ' ' -f 1`
The above would automatically grab the IP address of the $INET_IFACE variable, grep the correct line which contains the IP address and then cuts it down to a manageable IP address. For a more elaborate way of doing this, you could apply the snippets of code available within the retreiveip.txt script, which will automatically grab your Internet IP address when you run the script. Do note that this may in turn lead to a little bit of "weird" behavior, such as stalling connections to and from the firewall on the internal side. The most common strange behaviors are described in the following list.
If the script is run from within a script which in turn is executed by, for example, the PPP daemon, it will hang all currently active connections due to the NEW not SYN rules (see the State NEW packets but no SYN bit set section). It is possible to get by, if you get rid of the NEW not SYN rules for example, but it is questionable.
If you got rules that are static and always want to be around, it is rather harsh to add and erase rules all the time, without hurting the already existing ones. For example, if you want to block hosts on your LAN to connect to the firewall, but at the same time operate a script from the PPP daemon, how would you do it without erasing your already active rules blocking the LAN?
It may get unnecessarily complicated, as seen above which, in turn, could lead to security compromises. If the script is kept simple, it is easier to spot problems, and to keep order in it.
rc.UTIN.firewall.txt
The rc.UTIN.firewall.txt script will in contrast to the other scripts block the LAN that is sitting behind us. In other words, we don't trust anyone on any networks we are connected to. We also disallow people on our LAN to do anything but specific tasks on the Internet. The only things we actually allow are POP3, HTTP and FTP access to the Internet. We also don't trust the internal users to access the firewall more than we trust users on the Internet.
The rc.UTIN.firewall.txt script requires the following options to be compiled statically to the kernel, or as modules. Without one or more of these, the script will become more or less flawed since parts of the script's required functionalities will be unusable. As you change the script you use, you could possibly need more options to be compiled into your kernel depending on what you want to use.
• CONFIG_NETFILTER
• CONFIG_IP_NF_CONNTRACK
• CONFIG_IP_NF_IPTABLES
• CONFIG_IP_NF_MATCH_LIMIT
• 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.
Chapter 15. Graphical User Interfaces for Iptables/netfilter
One side of iptables and netfilter that we haven't looked at very much yet, is the graphical user interfaces that are available for iptables and netfilter. One of the biggest problems with this is that netfilter is a very complex and flexible setup, that can perform the strangest of tasks. For this reason, it can become a very daunting task to create a GUI for netfilter.
Several persons and organisations have tried to create GUI's for netfilter and iptables, and some have succeeded better than others, while others have given up after some time. All have different reasoning behind their tries as well, so it isn't an easy task to show them all. However, this chapter is a small compilation of some of the GUI's for iptables and netfilter that may be worth looking at. Suggestions on others to add are always welcome.
fwbuilder
Firewall Builder, or simply fwbuilder, is an extremely versatile and powerful tool that can be used to build your own firewalls, or to maintain several firewalls for that matter. It can be used to create policies for several different types of firewalls, including iptables (Linux 2.4 and 2.6), ipfilter (freebsd, netbsd, etc), openbsd pf, and, a module that must be bought, Cisco PIX.
Fwbuilder has, as you can see, a very big audience and is well taken care of and continues to be developed. It is run on a separate host system, where you create the policy files, and then copy them over and run them on the target system. It is able to handle everything from very simple rulesets to large and rather complicated ones. It has extensive abilities to handle different versions and installations of iptables, by configuration of which targets/matches are available on each host system, etcetera. The end result may be saved in an xml file, or a system parsable configuration file (e.g., the real firewall scripts).
You can see the configuration of the "firewall" in the above example, and the main menus of the whole fwbuilder system. fwbuilder can be found at http://www.fwbuilder.org.
Turtle Firewall Project
Turtle Firewall is an excellent, yet simpler kind of user interface to iptables. It is integrated in something called webmin (a web administration interface). It is fairly basic, and neither as complex nor able to handle as complex changes as the fwbuilder package, but it is more than able to handle most simpler firewalls, as well as some more advanced ones as well.
One big advantage with Turtle Firewall is the fact that it is web-based, and hence can be remotely controlled in a totally different manner than with fwbuilder and most other tools. Of course, it also adds more of a security risk since webmin is a separate extra service running on the firewall itself.
The above screenshot shows the items page of the Turtle Firewall, where you can configure network interfaces and networks, and other items.
This final screenshot shows the turtlefirewalls main screen, and with the whole ruleset expanded at the bottom. The whole ruleset isn't showing, as you can see, but you get a good general idea of what it looks like in Turtle Firewall.
You can find the Turtle Firewall Project and more information over at http://www.turtlefirewall.com/.
Integrated Secure Communications System
The Integrated Secure Communications System, or shortly ISCS, is still undergoing development, and no public version has been released. However, this looks like it will become an extremely helpful tool once it is finished. The developer has very high standards, and this is the main reason that it has not been released yet. ISCS integrates several functionalities into a single suite of administration and management user interface. Basically this means that once this project is released, you will be able to fully configure all your firewalls from a centralized point using a single GUI, including VPN's, VLAN's, Tunnels, sysctl's, etcetera.
The main attack angle that the developer(s) of ISCS has, is to simplify management and administration and to remove tedious work for the administrators, so to save as much work hours as possible for the administrators. This is done by putting together policies, and then the programs creates the rulesets and "pushes" them out to the "enforcements points" (e.g., firewalls, proxies, etcetera). The administrator doesn't actually "write" or "click" together the rulesets, just simply put together policies that are then enforced by ISCS.
This tool isn't finished yet, as of writing this. However, I have been in touch with the main developer of this project before, and this is indeed a very large project. When it is finished, I believe this will be one of the best tools on the market. Of course, time can only tell, but it is well worth mentioning here. You can find the ISCS project over at http://iscs.sourceforge.net/.
Note The main developer, John Sullivan, of ISCS has specifically asked me to ask people to join his development efforts. The project is very big, and he would definitely like as much help with the project as possible. If you are able to help, you are, in other words, more than welcome.
IPMenu
IPMenu is a very able program, yet simple to operate and not too demanding on resources nor bandwidth. It is a console based program, so it works perfect over an SSH connection for example. It works perfectly on machines running over a simple and old modem as well.
As you can see from the screenshot, it is able to handle all iptables functionality, including filtering, mangling and nating. It is also able to handle routing tables and bandwidth shaping and to save and restore rulesets. You can add new rules directly into the currently running iptables script easily, and handle all of the different tables. Including adding and removing custom chains.
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