|An Introduction to Using
Linux as a Multipurpose Firewall from the Linux Journal
Posted on Wednesday, March 01, 2000 by Jeff Regan
Feeling insecure? Here's a guide for getting
the protection you need.
High-speed Internet connections are becoming more readily available and popular for
home computer users. ADSL (Asymmetric Digital Subscriber Line), Nortel's 1MB modem and
cable modems all offer connection speeds many times faster than that of a standard 56K
POTS (plain old telephone service) modem that most of us know all too well. The other big
advantage of these new services is that they are always connected. That is, you don't need
to dial your service provider with your modem to start up your Internet connection. When
you turn on your computer, the connection is already there, and your operating system will
establish a link as it boots up.
Like the standard modem, these connections allow only one computer to connect to the
Internet at a time. In some cases, additional IP addresses can be assigned to additional
computers, but there is usually a monthly cost involved in providing this service.
By installing Linux on that old 486 you have sitting in the corner collecting dust, you
can create a firewall so all the computers on your local LAN can see the Internet, and at
the same time, transfer data back and forth between each other, (see Figure 1). You don't
even need a dedicated PC. A faster PC can simultaneously be used for other purposes while
acting as the firewall; however, there are two main drawbacks with this approach:
- Users on your LAN may experience a slower connection to the Internet.
- You could inadvertently open a security hole, allowing someone on the Internet to get in
and play havoc with your system or files.
Figure 1. Generic Diagram of Small LAN Configuration
I will be discussing two different types of Linux firewalls. The first type consists of
a 486 with 12MB of RAM, and a 200MB hard drive using either Red Hat 6.0 or Slackware 3.6.
The second, called the Linux Router Project (LRP), uses a 486, 12MB of RAM, a 1.44MB
floppy and no hard drive. Two Ethernet network interface cards (NICs) will be required,
regardless of which firewall configuration you install.
Someone is always watching, and people are always on the lookout for computers on the
Internet with poor security. Their motivation can be as simple as boredom, or more
seriously, a need to find a system to penetrate so they can use it to hide behind while
they continue breaking into other systems, leaving evidence that points to you.
If you are running a standard Windows installation, you probably don't have the means
to see who is trying to check out your machine. As long as ``File and Print Sharing'' is
turned off inside of Windows, for the most part, you are safe. However, it is possible
someone may find a new security hole in your PC and exploit it.
If you have Linux running, you can check out your system logs. Upon doing an informal
survey with friends who run Linux firewalls, I found on average five attempts by outsiders
each day to use TELNET or FTP to break into their Linux boxes. In the case of a firewall,
you can turn off or restrict most services. In general, the strength of your firewall
security decreases for each service you open up to the Internet, since each service is an
invitation for someone to try and sneak in to your system. For example, if you open
TELNET, someone can use it to break in. A safer alternative is to restrict TELNET to
certain incoming IP addresses, such as the IP addresses you might use to access your home
system from work. If you have no plans to TELNET or FTP into your firewall from the
Internet and all your traffic is originated inside your local LAN, you can lock your
firewall fairly tightly. It is always a good idea to stay caught up on new security holes
and the fixes for them. Check out http://www.cert.org/
for more information.
Theory of Operation
There are many reasons for having a firewall, some of which I have already mentioned.
- Ensuring that local traffic on your intranet does not spill out to the Internet.
- Allowing the full use of file and print sharing in your LAN without having to worry
about unwanted intrusions.
- Providing security for your LAN.
- Allowing yourself and authorized users access to your LAN to read e-mail, listen to MP3s
or access file backups.
LAN and Internet Traffic Routing
When you copy a file from another local PC on your LAN using Windows ``Network
Neighbourhood'', or when you FTP a file from another PC on your LAN, that traffic has no
reason to go to the Internet. If you had a high-speed modem directly connected to your
LAN, it would send out that data, because it has no way of knowing it should not be sent
there. By default, it sends all traffic it sees, and although it won't likely get past the
next router in the chain, it is sending out data that does not need to be there. This may
impact the overall speed of your local LAN. You probably don't want this particular
traffic to go out over the Internet, so the firewall prevents it.
One of the TCP/IP settings on our PCs, regardless of the operating system, is the
``default route''. When the destination IP address cannot be found on our local LAN (this
is determined by the subnet mask), then the default route is used. The default route in
this example will point to the IP address of the NIC on the local LAN side of the firewall
(Ethernet 0 in Figure 1). This IP address usually ends in 1. For example, if you have a
local LAN with a network address of 192.168.0.0 and a subnet mask of 255.255.255.0, you
have 192.168.0.1 to 192.168.0.254 available for local IP addresses (see Resources for more
information on the Linux NET-3-HOWTO ). In this case, 192.168.0.1 would normally be
assigned to the NIC on the firewall.
Any traffic intended for an IP address outside our local LAN will go into the firewall.
The firewall will replace (masquerade) the source address of the PC in the local LAN that
originated the packet with the firewall's IP address (assigned by your ISP), so that to
the Internet, the traffic looks as though it originates from the firewall and is coming
from a valid IP address. Any return packets relating to this originating packet will go
through the reverse transformation, so the traffic finds its way back to the originating
Rules can be set up to allow certain packets to make it through the firewall or to be
stopped dead. By default, nothing gets passed. A small set of rules are needed to support
features such as TELNET, HTTP, IMAP and POP3, and a few extra rules are needed to allow
other features such as RealAudio or on-line gaming to function. (Gaming can be a little
more difficult to set up, as each game is different.)
Designing the Network
Table 1. IP Addresses Reserved for Private Network Allocations
||# of Networks in class
||# of hosts supported
In Figure 1, you can see how a typical small LAN/firewall configuration
might look. You need to determine how many PCs will be in your network, and how many of
them will be connecting to the Internet. The IP addresses chosen for your internal network
will be determined by the size of the network. Table 1 shows which groups of IP addresses
have been reserved for private LANs, such as the one we are designing. For the most part,
a class C network address will be sufficient, as it will allow up to 253 hosts or PCs in
our LAN, leaving one IP for the firewall. Table 2 shows an example configuration.
Table 2. Example Internal LAN and External Internet Configuration
|Range of usable IP 192.168.0.1-
|Firewall Gateway IP:
The complete firewall will be built over several stages. These include
building and configuring the hardware, installing and configuring Linux, configuring the
network cards, building a new kernel, establishing routing between the networks, then
introducing security and locking down the PC and the local LAN.
Building Up the Hardware
First, you must decide what type of system you want to build. If you want to use your
firewall only for firewall/routing purposes, then once it is set up and running, it does
not need a keyboard or a monitor. In fact, many systems will run without a video card;
however, you might want to keep one handy in the event of a system failure. Software
changes can be done by either connecting to the firewall over your local LAN via TELNET,
or using a modem program on a laptop (such as Hyperterm) and connecting to the firewall
via the serial port. This type of configuration is well-suited to LRP. If you actually
want to have a few users on your machine reading things like e-mail (either locally or via
TELNET), you will need a hard drive and RAM sufficient to handle that. A 200MB hard drive
and 16MB of RAM will work for this if you don't load unneeded packages, such as the X
server or source code, and your users keep space constraints in mind. If you plan on the
LAN using your firewall PC for additional functions, you will need to upgrade it
appropriately in all respects: memory, hard drive size and processor speed.
You will need at least two network cards in your firewall. One card will face the
Internet and the other will face your local network (Figure 1). If you can't afford a hub
and you have only a few PCs to connect, you can put multiple cards in your firewall, one
for each PC, and wire an Ethernet cable as a ``turn-around'' cable. ISA network cards can
be found inexpensively in some markets these days, and may be less expensive than an
Ethernet hub. The use of more than two network cards in your firewall machine will require
more rules in your firewall, but that is easily handled.
You will need the DOS configuration disks for your network cards if they are jumperless
cards which use non-volatile RAM (NVRAM) to remember their settings (I/O address, IRQ,
etc.). The configuration software for most cards can be found on the Internet at the card
manufacturer's web page or at some of those helpful Windows driver repository sites.
Make a DOS floppy boot disk, and have the configuration program for each card handy on
Table 3. Configuring the NIC Cards: Standard IRQ Line Uses
||Cascades IRQ 8-15. Not a good choice unless
||you have no other option.
||Com. Port 2
||Com. Port 1
||Printer Port or Sound Card
||Printer Port or Sound Card
||Known on some cards as IRQ 2. (See IRQ 2)
||Some VGA cards
||PS/2 Mouse or Unassigned
||First IDE Hard disk controller (unless this
||is LRP, you probably need this)
||Second IDE Hard Disk controller or Unassigned
Install one network card at a time and boot your PC. Run the
configuration software for that card, and set the I/O address and IRQ settings. Make sure
you don't configure the card to a setting already in use by some other card. For I/O
addresses, the only item you may have trouble with is an old CD-ROM drive with a
proprietary controller (see Table 3). Once configured, remove the card, insert the next
network card and repeat the procedure. Once you have your network cards individually
configured, you can install them all in the firewall. In my firewall, my first network
card is set to an I/O address of 300 and an IRQ of 12, while my second network card is set
to an I/O address of 320 and an IRQ of 15.
It is now time to install Linux. The sample configuration that follows is based on
Linux kernel 2.2.9. If you install a Linux distribution from the Net or from a recent CD,
you may find this kernel included. If not, you can get it from http://www.kernel.org/. The more recent
the distribution, the less likely it is that you will run into outdated libraries or
utilities. One of the utilities we will be using to control the firewall is called ipchains.
This program runs only on kernel version 2.2.x and higher. If you plan on using an
earlier version of the kernel, you will need to find ipfwadm. It is always best to
use a recent (but not necessarily the most recent) kernel version. Follow the instructions
provided with your distribution, and install the distribution. If the default kernel on
the CD is not of the 2.2.x variety, don't worry; you will need to build a new
kernel later anyway. If you are building a small system, you will want to install as
little of the distribution as possible. At a minimum, you will need to install the base
files and networking support.
Configuring the Network
At one point during the installation, you will be asked to configure the Ethernet
interfaces (ports). Generally, you will be able to configure only one of the interfaces
during the installation. The remaining interfaces can be configured by editing the
configuration files. Alternatively, Red Hat 6 offers a GUI-based application called netcfg;
however, it requires you to install the X server, something I don't recommend if you are
tight on hard drive space or don't plan on leaving a monitor connected to the PC. When you
do come across the configuration request for the first interface (generally called eth0),
you should enter the information for your local LAN. In our example as per Table 2, we
configure this interface as:
IP address: 192.168.0.1
Subnet Mask: 255.255.255.0
The default gateway of the firewall (not the PCs inside your LAN) is that of the
gateway provided by your ISP. If the subnet mask provided by your ISP ends in a .0, your
ISP gateway IP address will generally end in .1, for example 126.96.36.199. After the
setup and installation of the distribution is complete, you will have to add the
additional information on your second Ethernet interface (generally called eth1). We will
need to edit or create configuration files for both Slackware (Listing 1) and Red Hat 6.0
(Red Hat sidebar).
If your NIC cards are all of the same type or all use the same driver, you must tell
Linux to search for more than one card of that type at boot time. LILO provides a nice way
of doing this that works for most Ethernet drivers I have tried. Edit the file
/etc/lilo.conf and add the line
anywhere in the LILO global section near the top of the file. If you have more than two
Ethernet cards, you would add
You can also explicitly define all the cards instead of just telling the system to look
for additional cards by using the following on one line:
In the example of my configured cards above, I could then use
Don't forget to type lilo after you have finished editing the file so the new lilo
parameters are read and installed, but, more importantly at this point, so you know you
have not created any errors in the LILO configuration file.
Do not reboot yet, as we still need to build a kernel to support our various hardware
and firewalling needs.
Building the Kernel
A variety of configurations are required to make the kernel run. Listing 2 shows the
settings I have used in my system. If you have never built a kernel before, see ``Linux
Kernel Installation'' by David Bandel in the November 1997, issue 43 of Linux Journal.
A quick summary is as follows:
Look at the many screens, read the help and any other reference documentation it points
to. This will help you determine which options you need. After you have finished choosing
your options, save the kernel, then type:
The resulting files, the new kernel file called vmlinuz and a new System.map file, will
likely be located in the root directory /. You will need to copy the System.map file to
the root directory, like this:
cp /System.map root
Also make sure the file /etc/lilo.conf and the line inside it which reads image=IMAGENAME
(where IMAGENAME is the name and location of your kernel used at boot
time) is correct. If it does not point to the correct location, change it and re-execute
the lilo command to complete the process of setting up the new kernel.
This will build and install the Linux kernel, update LILO to reflect the new kernel and
install a variety of modules, such as support for RealAudio which by default is blocked by
Listing 2 includes only those options required to make the firewall function. Other
options such as processor type are left out, since these are specific to the hardware you
are using for this project. As a rule, I put as little in the kernel as required, and I
minimize the use of modules. If you are not sure how an option I have shown in the table
is used, or where it shows up in the kernel-configuration program, you can match up the
item by clicking on help for the items in that section. You will find its configuration
file name at the top of the help page. Similarly, if it's not shown in Listing 2 and you
don't need it to make your hardware run or support some other feature, then it should be
set to off.
If you are building a bare-bones system and are going to compile the kernel elsewhere,
be sure to save your kernel on that machine first, and also save that PC's kernel
configuration in an alternate file (see the bottom of the kernel configuration program
menu). After it is built, you need to copy the files over to the firewall PC via sneaker
net or LAN. Don't forget to copy the modules installed in /lib/modules/2.2.9 as well.
It is now time to reboot the PC and cross your fingers. If everything works correctly,
both Ethernet cards will be recognized, and they will both be configured. When the system
is fully booted, log in as root and type ifconfig. It should show detailed
information about three interfaces:
- lo0: the local loop interface
- eth0: the Ethernet port pointing to your local LAN
- eth1: the Ethernet port pointing to the Internet
You can also type route and see what default routes are up. It should show a
default route to the Internet, as well as some information about your local LAN. At this
point, the firewall should be able to see both your local LAN and the Internet. If you
hook your high-speed modem to the eth1 port at this point, you should be able to ping
sites on the Internet (e.g., ping www.linuxjournal.com) and see an answer coming
back about once per second. Press CTRL-c to stop the pings. Some high-speed
modems need to learn your Ethernet card's MAC address, and only do so each time they are
turned on. Therefore, if you are connecting your modem to a different Ethernet card than
it was previously connected to, you will have to power off both the modem and your
firewall PC, power the modem on, wait a few seconds, then turn the PC on. If you don't do
this, you may find you can't see the Internet at all.
Locking Down the Firewall
We currently have a Linux PC, connected in the middle of two networks. It can see both,
and both can see it. The PC is also wide open with all the default ports turned on. We
want to restrict this as much as possible. People are always looking for new ways of
breaking into systems. The more we lock down this firewall to the outside, the less
vulnerable we are to attacks. Nothing is perfect, and the only true way to be sure people
are kept out is to unplug your Ethernet connection when you are not there. Since that's
undesirable for most of us, this is the next best thing.
What needs to be done now is disabling all services we don't need. If you are making
this a true firewall, you can disable almost everything except TELNET and FTP, and these
two will be limited to ports from only inside your LAN and trusted outside IP addresses.
The file /etc/inetd.conf, as shown in Listing 3, is where these ports are configured.
This file affects traffic terminating at the firewall, not passing through it. Disabling
something like POP3 or IMAP is acceptable, since when you go to get your mail from a PC
inside your network, this traffic will pass through the firewall (but not stop) on its way
to your ISP's POP3 or IMAP mail server.
Remember, the more ports and addresses you choose to leave open, the more closely you
will need to watch your firewall for break-in attempts. We have left TELNET and FTP open,
so we'll want to restrict the originating IP addresses on both networks to those we want
to let in.
This is done by editing the files /etc/hosts.deny and /etc/hosts.allow. By editing
these files, you can deny access to everyone except a few specific addresses or range of
addresses, or you can allow everyone in by default and disable problem IP addresses down
the road when you discover unwanted access from those points. If this is the case, be sure
to watch your system logs closely. See the ``Setting Services'' sidebar for more details.
In one sense, we could have left inetd.conf alone and restricted people from those ports
via the /etc/hosts.deny table; however, it is always best to lock down ports in multiple
By default, most UNIX systems do not allow root to log in from anywhere but the
console. If your system is not set up that way, it should be. You will at least want to
slow down someone who might want in your system. If they can't log in directly as root,
this is an additional security benefit. Check the file /etc/securetty. In Red Hat 6.0,
look for pty1, pty2, etc. entries in the table. In Slackware, look for ttyp0, ttyp1, etc.
entries in the table. If these entries are in place, root login is allowed on those TELNET
ttys; therefore, remove the entries. The other remaining entries in the table cover your
various consoles and serial ports.
Since you can't log in remotely as root and you do not have a console with a monitor
and keyboard, it would be best to add a second user to the firewall to ensure you can ``su
to root'' to do work on the firewall.
useradd -g 100 -d /home/USER -s /bin/tcsh -c
'YOURNAME' USER passwd USER
The -g controls which group this user will belong to. In this example, 100 was
used, as this is the user's group in Red Hat 6.0. If this does not work for you, check out
/etc/group to find a suitable group. YOURNAME is whatever you want to put in the
Name field of the user account, and USER is the ID chosen for the user, i.e., I may
choose to use jeff as my ID.
Stopping Extra Processes
In a small system, the only processes we want running are ones that pertain to the
operation of the firewall. This means disabling processes: all but one or two consoles,
Sendmail and anything else you don't need. You can see what is running right now by
To keep Sendmail from starting next time, you will need to move or edit the file where
it starts. Linux usually starts up in runlevel 3. In Red Hat 6.0, you can check that by
looking at /etc/inittab and looking for the line that reads id:3:initdefault:.
The 3 indicates runlevel 3. Therefore, in /etc/rc.d/rc3.d, there is a file called
S80sendmail. Move this file to 80sendmail, as follows:
Some programs like elm require that sendmail be running to operate properly.
This opens up a potential hole to to the outside world since it also means port 25 will be
open to possible attacks and possibly even mail relaying--allowing others on the Internet
to use your firewall to send out spam mail. Turning off port 25 access is the easiest way
to prevent this problem. Other solutions can be found at http://www.sendmail.org/.
In Slackware, edit /etc/rc.d/rc.M and change the line:
/usr/sbin/sendmail -bd -q15m
/usr/sbin/sendmail -bm -q15m
In Red Hat 6.0, edit /etc/rc.d/rc3.d/S80sendmail and change the line:
daemon /usr/sbin/sendmail $([ "$DAEMON" = yes ] && echo -bd)
daemon /usr/sbin/sendmail $([ "$DAEMON" = yes ] && echo -bm)
Creating the Firewall
Currently, we have a reasonably secure PC quite incapable of passing the network
traffic from the local LAN to the Internet. It is now time to set up and configure the
rules that will make our firewall function. As mentioned earlier, these rules allow
acceptable packets to pass through the firewall, while still offering various levels of
security to unacceptable packets.
Download (with FTP) the ipchains package from http://www.rustcorp.com/ipchains/.
Follow the installation instructions you obtained with the package to install it on your
Listing 4 shows the /etc/rc.d/rc.local file which is used to start any process not
normally started as part of the distribution's installation. It is here where we set the
rules for our firewall. Since our firewall is fairly straightforward, all we need to do is
set up forwarding of masqueraded packets. To be able to use the full functionality of FTP,
RealAudio, IRC and others, we need to support their port requirements as well. Many of
these can be supported using the ipchains command above, but there are loadable modules
that will take care of this, such as those shown in the sample rc.local file in Listing 4.
See /lib/modules/2.2.9/ipv4 for a list of modules supported in your kernel. This directory
should have been created earlier when you built the kernel.
That should do it. You are now ready to test your network firewall. Set one of your PCs
inside your local LAN to one of the sample settings shown in Figure 1. For example, on
Windows 95, you will need to enter a local LAN IP (such as 192.168.0.10), a subnet mask of
255.255.255.0, a gateway IP of 192.168.0.1 and DNS entries given to you by the ISP. If the
high-speed modem was originally connected to this PC, the DNS entries in the PC should
already be set.
To test out your new firewall, try connecting to a web site with one of the PCs on your
internal LAN. Try using RealAudio, FTP and other functions you regularly use. If none of
these work, try using TELNET to get to the firewall PC. If you can do so, and you can ping
a site on the Internet (or get to it via TELNET) from the firewall PC, check your rules in
the /etc/rc.d/rc.local file, as you might not have turned on IP forwarding. If web access
works, but (say) IRC does not, check to see if you loaded the IRC module correctly. Use
the command lsmod to show which modules are loaded.
Building a Firewall Using the Linux Router Project
The configuration of LRP I will describe also uses the setup in Figure 1. It was set up
on a 486 with 12MB of memory, a 1.44MB floppy drive, two Western Digital ISA network cards
and no hard drive. For your system, install and configure the network cards in the same
way as for the full firewall build earlier in this article. LRP version 2.9.4 is based on
kernel version 2.0.36. This kernel is older than the 2.2.9 used above, and as a result,
does not offer some features you may require if you want an advanced firewall. By the time
you read this, there will likely be a new version available based on version 2.2.x
of the kernel. I will describe setting version 2.9.4, and if you need some of the 2.2.x
features, you have a foundation from which to work.
LRP uses a DOS-formatted floppy, either formatted as a standard 1.44MB disk or larger.
(A utility called 2m can squeeze additional, usable storage space out of a floppy.)
During boot time, a RAM disk is created, which is used as the live file system. Various
portions of the system are created from compressed archive files (tar) that end in .lrp
and are found on the floppy. In general, the floppy can run with write protect on. This
means if someone were to find a way in to your firewall, any changes they made would
disappear when the system is rebooted.
LRP is available in many forms. The hard way is to create a disk, make it bootable
using a program called syslinux, and install the kernel and various LRP files
required. However, at ftp://ftp.linuxrouter.org/linux-router/dists/2.9.4/, you will find in
the download section a file called idiot-image_1440KB_2.9.4. The name might not be
flattering, but it is the easiest way to start building an LRP disk. After you get the
file via FTP, copy it to the floppy in one of two ways. In DOS, use the rawrite
utility that came with your Linux distribution. In Linux, type:
cp idiot-image_1440KB_2.9.4 /dev/fd0
I have assumed /dev/fd0 is your 1.44MB floppy, but if it is not, change fd0 to the
correct device name.
Now go to http://www.linuxrouter.org/modmaker/ and make a kernel that includes hardware
support for our network cards and includes any modules required to support FTP, RealAudio,
etc. This web site is a very nice way to generate a kernel. Click on 2.0.36final and tick
off the modules you require. Unless you know you don't want support for one of the few
masquerading modules in this list (like IRC), tick off all options that start with ip_masq
such as ip_masq_irc and ip_masq_ftp. Then go down the list and find the drivers for your
hardware. You may have to do some research as to the driver your NIC cards require. If you
don't know which driver to pick, run make menuconfig on a working full Linux
system and look at the devices under Network Device Support. When you find your card, look
at the help and find out its module name. This module name is what you need to tick off on
the module maker screen you are looking at. If you want to support serial port access to
connect to your firewall using the serial port to do maintenance, select the serial module
Once you have made these choices, click ``Create modules.lrp file'' and download a copy
of modules.lrp and a copy of the kernel. Copy these files to your floppy using either the
DOS copy command or mcopy in Linux. When you copy your downloaded kernel to
floppy, name it linux. You will have to overwrite the existing modules.lrp and linux
You are now ready to boot the system. It should come up, but may or may not see one or
both of your network cards, as we might still have to add some options to the modules
setup as discussed below.
Log in as root. You will get a menu with various configuration options available.
Select 1 for Network Settings and 1 again for Network Configuration (auto). Listing 5
shows a sample of the file /etc/network.conf. Edit the file on your screen to meet your
needs, using mine as an example. I have placed as many details in the example as possible.
Save the file by pressing CTRL-W and go back to the menu by pressing CTRL-C.
Next, edit the /etc/hosts.allow and /etc/hosts.deny files, using menu options 4 and 5.
The same rules apply now as they did when the first firewall was built. Refer to the
sidebar ``Setting Services'' for more details.
Quit back to the main configuration menu, then go on to option 2 for the system
settings. If you wanted serial port access as mentioned earlier, you will need to edit the
/etc/inittab file by selecting option 2 from the menu. You will find the serial port
access commented out towards the end of the file. Uncomment the line to make it function.
The last step is configuring the modules to support the network cards, if they were not
automatically detected when you booted up. Quit back to the main configuration menu and
select 3 for Package Settings, 2 for modules and 1 for Kernel Modules. Find the line that
shows the module for your system. Listing 6 is an example of the /etc/modules file. For my
WD Ethernet cards, I use the configuration information to tell the WD driver exactly where
to find each card.
One last thing to avoid possible problems: delete the /etc/resolv.conf file. If you
have one lying around, it will be used even if you turn off DNS support in the
That's about it. Make sure you change the root password, and don't forget to back up
your changes (currently, they are changed only on your RAM disk) to floppy via the backup
option from the main menu. You should be able to boot up with a basic firewall, and it
should function similarly to the one we built above. It is less configurable only because
of the lack of a hard drive, but you can build on it. If you quit the menu program and get
to the shell prompt, but want to get back into the menu, type lrcfg and press ENTER.
Using a Dynamic (DHCP) Address
Some service providers do not assign an IP address to your connection, but rather
assign you one each time you boot your PC, and at regular intervals afterwards. This is
known as DHCP, or Dynamic Host Configuration Protocol. Some providers give you a choice of
which you want to use. The decision to use one or the other depends on your situation.
DHCP means it would be more difficult for someone to find your PC and break into it over a
long period, because your IP address would change from time to time. However, if you wish
to use TELNET or FTP to log in from the Internet, you would have to find a way to notify
yourself of your firewall's new IP address each time it changed.
Red Hat 6.0 makes it easy to set up one interface for DHCP support. For the NIC facing
the Internet, edit the /etc/sysconfig/network-scripts/ifcfg-eth1 file and set it up for
DHCP use as per the Red Hat sidebar. If you have an X server available, you can also edit
the settings via netcfg.
For both Slackware and the LRP, it is not quite as easy. DHCPCD is a program that comes
installed on the latest Slackware, but may require configuration. For Slackware, you can
start dhcpcd INTERFACE, where INTERFACE in our example would be
eth1. Once that is in place, you would then create your routing tables. Therefore, start
DHCPCD by placing this line in the /etc/rc.d/rc.inet1 file as shown in Listing 1. You will
have no need to define the second card as we did in the static IP examples discussed
For LRP, you will have to use FTP to get DHCPCD and build it on your system (remember
that .lrp files are in fact groups of compressed (zip) files that make up the file system
in a tar archive). By the time you read this, there will hopefully be a DHCP.LRP file
available for LRP. For more information on DHCP setup, take a look at the mini-FAQ listed
In the past several months, some service providers have rolled out Point-to-Point over
Ethernet (PPPOE). As the name suggests, this is somewhat similar the Point-to-Point (PPP)
used for most dial-up connections. There are variety of reasons why this is being
introduced. Some reasons are the provision for access control as well as billing based on
the user's amount of use. Because it is similar to dial-up PPP, you will likely have a
different IP address assigned each time you boot your PC, as well as at regular intervals
if you never turn your PC off. I have seen IP addresses change anytime after a few days to
a week in the one installation I observed. For the service providers trying prevent their
customers from running a web page or FTP server on their high-speed connection, the
changing IP address makes it easy to discourage such uses.
To be able to communicate with a provider using PPPOE, you will require PPPOE
compatible software/drivers on your Firewall.X Some service providers will supply at least
the software and installation instructions, if not the full technical support for the
installation of the PPPOE software itself. If no software is available from your service
provider, look up PPPOE on the web, and you will should find enough information to get you
started. The basic firewall configuration does not change, only the configuration of the
Ethernet interface facing the ISP.
What I have presented to you is only a beginning. It may work very well for your
application, but you may also want to expand on its abilities. I have added a third
network card to my firewall so I can support a second local LAN segment. This works out
well because it saves me from having to purchase another hub, and it also means that while
I am using a lot of bandwidth, such as copying files on the local LAN, I am not chewing up
bandwidth on the other local LAN, so that user who happens to be the only user on the
second LAN segment can access the Internet without any difficulties. The firewall at that
level acts as a router, and allows the user to print to my printer or have access to files
on this segment, while preventing outsiders from doing so.
My firewall also acts as a DHCP server (using dhcpd) for my laptop, so that when
I am out on the road or at home, I can leave the PC set for DHCP in both cases.
Other uses you may consider include allowing a small number of outside users to read
their mail via a shell account on your firewall (they can IMAP or POP their mail and use
something like Elm or Pine to read it). You could also create a small web server for
either the internal LAN or the external LAN on your firewall. Shell accounts and Apache
require you to enable services that would otherwise have been disabled.
I have also had the opportunity to see some virtual private networking over a Linux
firewall. This could be used to allow you to access your internal LAN via the Internet and
add encryption and other features. This is handy if you are on the road a lot and might
normally call long distance to get into your LAN. Just dial up an ISP wherever you are
currently located, and off you go.
You can also increase the overall security by implementing additional rules to prevent
IP spoofing in the full firewall. These rules are already included in the LRP.
As you can see, you can do a lot with that old 486 sitting in the closet gathering
dust. It can make a wonderful firewall, and allows you some flexibility if you have more
than one PC you want to connect to your high-speed modem. Just remember to do what you can
to keep up with any new security holes that someone may have found.
Jeff Regan is an electrical engineer at a Canadian telecommunications provider. He
started using Linux in 1993 with kernel version 0.0.93. He can be reached at email@example.com.