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Linux Bridge+Firewall Mini-HOWTO version 1.1.3

Peter Breuer (ptb@it.uc3m.es)
Dec. 10 1996, revised Jul. 10 1997, Nov. 3, Nov 19 1997

1. Introduction

You should look at the original Bridging mini-HOWTO
by Chris Cole
for a different perspective on this. He is chris@polymer.uakron.edu.
The version of his HOWTO that I have based this document on
(alternatively, ripped off) is 1.03 dated Aug 23 1996.

2. What and Why (and How?)

2.1. What

A bridge is an intelligent connecting wire betwen two network cards.
A firewall is an intelligent insulator.

2.2. Why

You might want a bridge if you have several computers:

1. to save the price of a new hub when you just happen to have an
extra ethernet card available.

2. to save the bother of learning how to do IP-forwarding and other
tricks when you _have_ two cards in your computer.

3. to avoid maintenance work in the future when things change around!

``Several computers'' might be as few as three if those are routing or
bridging or just moving around the room from time to time! You also
might want a bridge just for the fun of finding out what it does.
``2'' was what I wanted a bridge for.

If you are really interested in ``1'', you have to be one of the very
few. Check the NET-2-HOWTO
and the
for better tricks.

You want a firewall if

1. you are trying to protect your network from external accesses, or

2. you are trying to deny access to the world outside from your

Curiously, I needed ``2'' here too. Policy at my university presently
is that we should not act as internet service providers to

2.3. How?

I started out bridging the two network cards in a firewalling machine
and ended up firewalling without having cut the bridge. It seems to
work and is more flexible than either configuration alone. I can take
down the firewall and keep bridging or take down the bridge when I
want to be more circumspect.

I would guess that the bridge code lives just above the physical
device layer and the firewalling code lives one layer higher up, so
that the bridging and firewalling configurations effectively act as
though they are running connected together ``in sequence'' and not
``in parallel'' (ouch!). Diagram:

-> Bridge-in -> Firewall-in -> Kernel -> Firewall-out -> Bridge-out ->

There is no other way to explain how one machine can be a
``conductor'' and an ``insulator'' at the same time. Anyway, it all
seems to work together nicely. Here is what you do ...


3.1. Software

Get the bridge configuration utility
from Alan Cox's home
pages. This is the same reference as in Chris' document. I just didn't
realize that it was an ftp and not an http URL ...

3.2. Prior Reading.

Read the Multiple Ethernet HOWTO

for some advice on getting more than one network card recognized and

Yet more details of the kind of boot magic that you may need are in
the Boot Prompt HOWTO

You may be able to get away without the NET-2 HOWTO
. It is a good
long read and you will have to pick from it the details you need.

3.3. Boot configuration

The reading material above will tell you that you need to prepare the
kernel to recognize a second ethernet device at boot up by adding this
to your /etc/lilo.conf, and then re-run lilo:

append = 'ether=0,0,eth1'

Note the 'eth1'. 'eth0' is the first card. 'eth1' is the second card.
You can always add the boot parameters in your response to the line
that lilo offers you. This is for three cards:

linux ether=0,0,eth1 ether=0,0,eth2

I use loadlin to boot my kernel from DOS:

loadlin.exe c:\vmlinuz root=/dev/hda3 ro ether=0,0,eth1 ether=0,0,eth2

Note that this trick makes the kernel probe at bootup. That will not
happen if you load the ethernet drivers as modules (for safety since
the probe order can't be determined) so if you use modules you will
have to add the appropriate IRQ and port parameters for the driver in
your /etc/conf.modules. I have at least

alias eth0 3c509
alias eth1 de620
options 3c509 irq=5 io=0x210
options de620 irq=7 bnc=1

You can tell if you use modules by using ``ps -aux'' to see if kerneld
is running and checking that there are .o files in a subdirectory of
your /lib/modules directory. You want the directory named with what
uname -r tells you. If you have kerneld and/or you have a foo.o then
edit /etc/conf.modules and read the man page for depmod carefully.

Note also that until recently (kernel 2.0.25) the 3c509 driver could
not be used for more than one card if used as a module. I have seen a
patch floating around that fixes the oversight. It may be in the
kernel when you read this.

3.4. Kernel configuration

Recompile the kernel with bridging enabled.


I also compiled with firewalling and IP-forwarding and -masquerading
and the rest enabled. Only if you want firewalling too ...


Note added in formatting version 1.1.2 of this HOWTO: I have had
several messages from people with the new 2.0.31 kernel saying that
CONFIG_IP_FORWARD should not be enabled for that kernel because it
appears to cause packets to leak across a bridge even when the
firewall is up. This implies that you should not configure
IP_MASQUERADE either. I consider this a bug (in both 2.0.30preN and
2.0.31 as far as I can see) and I am staying at 2.0.25 until it is
fixed. I believe 2.0.29 should be OK too, but I have not tested. PTB

You don't need all of this. What you do need apart from this is the
standard net configuration:


and I do not think you need worry about any of the other networking
options. I have any options that I did not actually compile into the
kernel available through kernel modules that I can add in later.

Install the new kernel in place, rerun lilo and reboot with the new
kernel. Nothing should have changed at this point!

Note added in formatting version 1.1.1 of this HOWTO: I have had
several messages from people with one (I don't know which) of the
2.0.30 kernel versions complaining that they can't stop bridging when
firewalling. I believe them, but some things are definitely wrong in
2.0.30 that may account for this. As far as I can gather, switching
the router to a different card and placing nets on each card sometimes
does the trick even on 2.0.30. I am still at 2.0.25 and staying there!
(and it works for me :-). PTB

3.5. Network addresses

Chris says that a bridge should not have an IP address but that is not
the setup to be described here.

You are going to want to use the machine for connecting to the net so
you need an address and you need to make sure that you have the
loopback device configured in the normal way so that your software can
talk to the places they expect to be able to talk to. If loopback is
down the name resolver or other net sevices might fail. See the
NET-2-HOWTO, but your standard configuration should already have done
this bit:

ifconfig lo route add -net

You will have to give addresses to both your network cards. I altered
the /etc/rc.d/rc.inet1 file in my slackware (3.x) to setup my two
cards and you should also essentially just look for your net
configuration file and double the number of instructions in it.
Suppose that you already have an address at

(that is the private net reserved address space, but never mind - it
won't hurt anybody if you use this address by mistake) then you
probably already have a line like

ifconfig eth0 netmask metric 1

in your configuration. The first thing you are going to probably want
to do is cut the address space reached by this card in half so that
you can eventually bridge the two halves. So add a line which reduces
the mask to address a smaller number of machines:

ifconfig eth0 netmask

Try it too. That restricts the card to at most the address space

Now you can set your second card up in the other half of the local
address space. Make sure that nobody already has the address. For
symmetry I set it at 228=128+100 here. Any address will do so long as
it is not in the other card's mask, and even then, well, maybe. Avoid
special addresses like .0, .1, .128 etc. unless you really know what
you are doing.

ifconfig eth1 netmask metric 1

That restricts the second card to addresses between .128 and .255.

3.6. Network routing

The above might be enough address configuration for a working bridge,
but I will be doing firewalling too and I want to control the physical
destination of some packets. Even then one might have to take
precautions against address spoofing.

I have the small net of machines attached to a hub hanging off eth0,
so I configure a net there:

route add -net netmask dev eth0

The 128 would be 0 if I had a full class C network there. The 'dev
eth0' is not necessary here because the cards address falls within
this net, but it may be necessary for you.

On the other card I have a line going straight through to a big router
that I trust.

client 129
__ | __
client 1 \ .0 .128 | / net 1
client 2 --- Hub - eth0 - Kernel - eth1 - Hub - Router --- net 2
client 3 __/ .100 .228 .2 | \__ net 3
client 254

I attach the address of the router to that card as a fixed ('static')
route because it would otherwise fall within the first cards netmask
and the kernel would be thinking wrongly about how to send packets to
the big router.

route add dev eth1

I don't need it, since I don't have any more machines in that half of
the address space, but I declare a net also on the second card

route add -net netmask dev eth1

I also need to send all non-local packets out to the world so I tell
the kernel to send them to the big router

route add default gw

3.7. Card configuration

So much was standard networking setup, but we are bridging so we also
have to listen on both (?) cards for packets that are not aimed at us.
The following should go into the network configuration file.

ifconfig promisc eth0 ifconfig promisc eth1

The man page says allmulti=promisc, but it didn't work for me.

3.8. Additional routing

One thing that I noticed was that I had to put at least the second
card into a mode where it would respond to the big router's questions
about which machines I was hiding in my local net.

ifconfig arp eth1

For good measure I did this to the other card too.

ifconfig arp eth0.

3.9. Bridge Configuration

Put bridging enabling on and into your configuration file:

brcfg -enable

You should have been trying this out in real time all along, of
course! The bridge configure will bring up some numbers. You can
experiment with turning on and off the ports one at a time

brcfg -port 0 -disable/-enable
brcfg -port 1 -disable/-enable

You get status reports anytime by just running


without any parameters. You will see that the bridge listens,learns,
and then does forwarding. (I don't understand why the code repeats the
same hardware addresses for both my cards, but never mind .. Chris'
howto say that is OK)

3.10. Try it out

If you are still up and running as things are, try out your
configuration script for real by taking down both cards and then
executing it:

ifconfig eth0 down ifconfig eth1 down /etc/rc.d/rc.inet1

With any luck the various subsystems (nfs, ypbind, etc.) won't
notice. Do not try this unless you are sitting at the keyboard!

If you want to be more careful than this, you should take down as many
daemons as possible beforehand, and unmount nfs directories. The worst
that can happen is that you have to reboot in single-user mode (the
'single' parameter to lilo or loadlin), and take out your changes
before rebooting with things the way they were before you started.

3.11. Checks

Verify that there is different traffic on each interface:

tcpdump -i eth0

tcpdump -i eth1

You should get used to using tcpdump to look for things that should
not be happening or that are happening and should not.

For instance look for packets that have gone through the bridge to the
second card from the internal net. Here I am looking for packets from
the machine with address .22:

tcpdump -i eth1 -e host

Then send a ping from the .22 host to the router. You should see the
packet reported by tcpdump.

At this stage you should have a fully working bridge that also has two
network addreses. Test that you can ping them from outside and inside
your local net, and that you can telnet and ftp around between inside
and outside too.


4.1. Software and reading

You should read the Firewall-HOWTO

That will tell you where to get ipfwadm if you don't already have it.
There are other tools you can get but I made no progress until I tried
ipfwadm. It is nice and low level! You can see exactly what it is

4.2. Preliminary checks

You have compiled IP-forwarding and masquerading into the kernel so
you will want to check that the firewall is in its default (accepting)
state with

ipfwadm -I -l ipfwadm -O -l ipfwadm -F -l

That is respectively, 'display the rules affecting the ..' incoming
or outgoing or forwarding (masquerading) '.. sides of the firewall'.
The '-l' means 'list'.

You might have compiled in accounting too:

ipfwadm -A -l

You should see that there are no rules defined and that the default is
to accept every packet You can get back to this working state anytime

ipfwadm -I -f
ipfwadm -O -f
ipfwadm -F -f

The '-f' means 'flush'. You may need to use that.

4.3. Default rule

I want to cut the world off from my internal net and do nothing else,
so I will want to give as a last (default) rule that the firewall
should ignore any packets coming in from the internal net and directed
to outside. I put all the rules (in this order) into
/etc/rc.d/rc.firewall and execute it from /etc/rc.d/rc.local at

ipfwadm -I -a reject -S -D

The '-S' is the source address/mask. The '-D' is the destination

This format to is rather long-winded. Ipfwadm is intelligent about
network names and some common abbreviations. Check the man pages.

It is possibly more convenient to put some or all of these rules on
the outgoing half of the firewall by using '-O' instead of '-I', but
I'll state the rules here all formulated for the incoming half.

4.4. Holes per address

Before that default rule, I have to place some rules that serve as
exceptions to this general denial of external services to internal

I want to treat the firewall machines address on the internal net
specially. I will stop people logging in to the firewall machine
unless they have special permission, but once they are there they
should be allowed to talk to the world.

ipfwadm -I -i accept -S \

I also want the internal clients to be able to talk to the firewalling
machine. Maybe they can persuade it to let them get out!

ipfwadm -I -i accept -S \

Check at this point that you can get in to the clients from outside
the firewall via telnet, but that you cannot get out. That should mean
that you can just about make first contact, but the clients cannot
send you any prompts. You should be able to get all the way in if you
use the firewall machine as a staging post. Try rlogin and ping too,
with tcpdump running on one card or the other. You should be able to
make sense of what you see.

4.5. Holes per protocol

I went on to relax the rules protocol by protocol. I want to allow
pings from the outside to the inside to get an echo back, for
instance, so I inserted the rule:

ipfwadm -I -i accept -P icmp -S \

The '-P icmp' works the protocol-specific magic.

Until I get hold of an ftp proxy I am also allowing ftp calls out with
port-specific relaxations. This targets ports 20 21 and 115 on outside

ipfwadm -I -i accept -P tcp -S \
-D 20 21 115

I could not make sendmail between the local clients work without a
nameserver. Rather than set up a nameserver right then on the
firewall, I just lifted the firewall for tcp domain service queries
precisely aimed at the nearest existing nameserver and put its address
in the clients /etc/resolv.conf ('nameserver 123.456.789.31' on a
separate line).

ipfwadm -I -i accept -P tcp -S \
-D 123.456.789.31/ 54

You can find which port number and protocol a service requires with
tcpdump. Trigger the service with a an ftp or a telnet or whatever to
or from the internal machine and then watch for it on the input and
output ports of the firewall with tcpdump:

tcpdump -i eth1 -e host client04

for example. The /etc/services file is another important source of
clues. To let telnet and ftp IN to the firewall from outside, you have
to allow the local clients to call OUT on a specific port. I
understand why this is necessary for ftp - it's the server that
establishes the data stream in the end - but I am not sure why telnet
also needs this.

ipfwadm -I -i accept -P tcp -S ftp telnet \

There is a particular problem with some daemons that look up the
hostname of the firewalling machine in order to decide what is their
networking address. Rpc.yppasswdd is the one I had trouble with. It
insists on broadcasting information that says it is outside the
firewall (on the second card). That means the clients inside can't
contact it.

Rather than start IP aliasing or change the daemon code, I mapped the
name to the inside card address on the clients in their /etc/hosts.

4.6. Checks

You want to test that you can still telnet, rlogin and ping from the
outside. From the inside you should be able to ping out. You should
also be able to telnet to the firewall machine from the inside and the
latter should be able to do anything.

That is it. At this point you probably want to learn about rpc/Yellow
Pages and the interaction with the password file. The firewalled
network wants to run without its unprivileged users being able to log
on to the firewall - and thus get out. Some other HOWTO!

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