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This is one that's on metalab as a miniHOWTO, but since it's in SGML, I've moved it to a full HOWTO. I'll try to send a message "saying" that I'm doing this to the ldp-discuss list soon, before I forget again, hopefully.

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<!doctype linuxdoc system>
<article>
<title>Bridge + Firewall + DSL Mini-HOWTO
</title>
<author>Derek Ney <tt><url url="mailto:derek@hipgraphics.com" name="derek@hipgraphics.com"></tt>
</author>
<date>Mar 24, 2000
</date>
<abstract>Configuring a Linux system to act as a firewall and bridge with a DSL network connection
</abstract>
<toc>
<sect>Introduction<label id="Introduction" >
<sect1>History<label id="History" >
<p>This document was started on December 10, 1999 by Derek Ney <url url="mailto:derek@hipgraphics.com" name="derek@hipgraphics.com"> after
three day's worth of frustration with bridging and firewalling after switching from a PPP network
link to a DSL link.
</p>
<sect1>New versions
<p>The newest version may be found in different formats at the LDP homepage <url url="http://www.linuxdoc.org/" name="http://www.linuxdoc.org/">.
</p>
<sect2>Version History
<p>v0.03 (Mar 24, 2000)</p>
<p><itemize>
<item>Fixed up URL for BRCFG.tgz
</itemize></p>
<p>v0.02 (Dec 13, 1999)</p>
<p><itemize>
<item>Incorporate revisions from Leonard Dickens (thanks Leonard!)
</itemize></p>
<p>v0.01 (Dec 10, 1999)</p>
<p><itemize>
<item>Initial version
</itemize></p>
<sect1>Copyrights
<p>(c) 1999,2000 Derek R. Ney
</p>
<p>This document may be distributed under the terms set forth in the LDP license
at <url url="http://www.linuxdoc.org/COPYRIGHT.html" name="http://www.linuxdoc.org/COPYRIGHT.html">.
</p>
<sect>Bridging, Firewalls, and DSL connections
<p>Until recently, our local network was hooked into the global net
via PPP over a modem. I had installed a firewall using IPChains
(<url url="http://www.linuxdoc.org/HOWTO/IPCHAINS-HOWTO.html" name="http://www.linuxdoc.org/HOWTO/IPCHAINS-HOWTO.html">)
with this setup and it worked nicely. We recently upgraded to a DSL connection.
I thought it would be trivial to simply switch my firewall to insulate me
from the larger net coming in via the DSL connection. I was wrong.
It took three days of work to finally get it up and running. I found a lot
of suspect information on the net that caused a good deal of confusion.
This mini-HOWTO was written because I suspected that our setup will be a quite
common configuration in the future when DSL becomes more widespread and I wanted to
help people avoid massive frustration.
</p>
<p> I guess this is applicable to a cable modem setup, but YMMV as I know nothing
about cable modem hookups.
</p>
<sect1>The Problem
<p>The problem I am trying to solve is to configure the system such
that the firewall code in the kernel (that is manipulated with
ipchains) can be used to filter the packets that travel back and
forth between the outside world and the local network. I also needed
some of the local machines to be "seen" on the global net (though always
filtered through the firewall). This ruled out IP masquerading (see
<url url="http://www.linuxdoc.org/HOWTO/IP-Masquerade-HOWTO.html"
name="IP Masquerade HOWTO">) which would otherwise probably be
a simpler solution. This is not as simple as it seems.
</p>
<p>
<sect1>The Solution
<p>To accomplish our goal of insulating a local net from the global
net (over DSL) by using our Linux box, we will use two ethernet (NIC) cards.
One card is hooked up to the local net and one to the global net. The only
machine that can directly talk to the outside world is the Linux box. All
other machines in our local net must go through the Linux box (firewall).
</p>
<p>Configuring the software really consists of two problems:
</p>
<p>
<itemize>
<item>Route packets between the local and global net (bridging)
<item>Filter the packets to stop some from traversing the firewall
</itemize></p>
<p>The <url url="http://www.linuxdoc.org/HOWTO/mini/Bridge.html" name="Briding mini-Howto">
gives detailed instructions that solves the first problem by routing packets
between the two sides of the network (local and global). This works by putting
both NIC's into "promiscuous" mode such that they sniff all the packets on
each NIC and transfer packets over when they belong on the other side.
This is done transparently; the other computers on the net do not even see
the bridge, because it does not even have an IP address. But this does not
totally solve the problem. I wanted the firewall to have an IP address
(for administration via the network, if nothing else) and more importantly,
the bridge code in the kernel intercepts and bridges packets BEFORE they get
to the firewall code, so the firewall will have no effect.
</p>
<p>It turns out you can assign your NIC's IP addresses and still use them as a
bridge. Although the <url url="http://www.linuxdoc.org/HOWTO/mini/Bridge.html" name="Briding mini-Howto">
does not do this (well actually, it uses loopback addresses), it works fine.
That solves one problem. For the firewall problem, we turn to a fine kernel patch
at <url url="http://ac2i.tzo.com/bridge_filter/" name="http://ac2i.tzo.com/bridge_filter/">
that causes the firewall rules to be invoked for packets that are being
bridged with a special new rule "bridgein".
</p>
<sect1>Setup Overview
<p>This mini-HOWTO is meant to handle the situation where you have a Linux
box configured as a gateway/firewall. The system has 2 NIC cards installed.
One of the NIC cards is connected to the outside world (in our case a DSL
modem) and nothing else. The other NIC is connected to our local network.
</p>
<p>Note that I have only had one experience with this and it was on my
i386 (ABIT BP6 MOBO, w/2 celery) box with RedHat 6.0 with the 2.2.13
kernel, and a DSL modem going to a router, and two Netgear FA310TX
NIC cards. Your mileage may vary.
</p>
<p>Also note that the steps here will leave your network open to potential
attack during setup (before the firewall is turned on). If you are very
paranoid you will want to take extra steps to avoid this.
</p>
<sect1>References
<p>I found a good deal of information on the net that I used to finally
get things working. Some of the information was useful, but inaccurate.
</p>
<p>The <url url="http://www.linuxdoc.org/HOWTO/mini/Bridge.html" name="Briding mini-Howto">
was instrumental in getting things up. Unfortunately using it alone does not
implement a firewall.
</p>
<p>The <url url="http://www.linuxdoc.org/HOWTO/mini/Bridge+Firewall.html" name="Linux Bridge+Firewall mini-HOWTO">
at first looked like just what I needed. However, it turns out that I think
it is inaccurate. I got things sort-of working with it, but in the end I
realized that it was not necessary to split your sub-net in two like it
directs and did not use that method. If you look at this document, take
it with a grain of salt.
</p>
<p>The <url url="http://ac2i.tzo.com/bridge_filter/" name="Bridge Filter Patch"> is
the key to getting the whole thing to work. Oddly enough, the information on
the web page directs you to the Bridge+Firewall mini-HOWTO. You do not need
to use the information in Bridge+Firewall mini-HOWTO to get things to work.
You will need this patch.
</p>
<p>The <url url="http://www.linuxdoc.org/HOWTO/IPCHAINS-HOWTO.html" name="IPCHAINS HOWTO">
is invaluable in setting up the firewall itself. I do not attempt to cover the details
of firewall setup in this document; only issues which are different because
of the bridging setup are mentioned here.
</p>
<sect>Procedure
<p>The basic procedure is as follows:
</p>
<p>
<itemize>
<item>Setup your hardware (and verify that it works)
<item>Patch and configure the kernel
<item>Configure your network (ifconfig, route, bridging)
<item>Configure the firewall
</itemize></p>
<sect1>Example Setup
<p>Throughout this procedure, I will assume a setup with two ethernet (NIC)
cards, an outside link via DSL (where a DSL modem connects to one of the NIC's),
and a local net that connects to the other NIC. I will arbitrarily call
the NIC to the DSL modem "eth1" and the local net NIC "eth0". The device naming
by the kernel of the NIC's depends on what slot they are in.
</p>
<p>I will assume that you have been assigned a subnet of IP addresses
at 192.168.2.128-191, i.e. a netmask of 255.255.255.192, and the router
provided by the DSL company is at 192.168.2.129. These are all arbitrary
fictional examples to illustrate the setup. I will use the address
192.168.2.130 for the firewall machine (both NIC's), though it turns
out you can also use distinct IP addresses for each NIC if you want.
</p>
<sect1>Hardware Setup
<label id="Hardware Setup">
<p>You will need two ethernet cards to make this work. The biggest problem
I had was that I randomly picked a slot in my motherboard for the second
NIC and it turned out that that slot (PCI) shared an interrupt with the
first NIC. I did not know that this was a problem (in fact there is little
information about this, and I thought it should work fine). It caused both
cards to shut down quietly (no error indication) and stop sending and
receiving packets. Naturally when you are doing all sort of configuration
changes, this is the last thing you need. I do not know if this is a problem
with all PCI NIC cards or just ours, but I would advise against sharing interrupts.
The tulip driver, which we use, reports the IRQ for each NIC in syslog when you boot.
There is a bunch of information out there
(see the <url url="http://www.linuxdoc.org/HOWTO/Ethernet-HOWTO.html" name="Ethernet-HOWTO">
section <url url="http://www.linuxdoc.org/HOWTO/Ethernet-HOWTO-3.html#ss3.2"
name="Using More than one Ethernet Card per Machine">) about making the kernel recognize two
ethernet cards using boot arguments; however, I did not need this (my kernel
recognized both cards with no arguments).
</p>
<p>Next, you need to hook the second NIC to the DSL modem
(or whatever links you to the outside world) and make sure that it is working.
You should be able to ifconfig the second ethernet card to a proper IP
address and ping the router on the other end of your outside link.
This verifies that you can send and receive packets over the DSL link.
For instance, for the sample net you would do:
</p>
<p><verb>
ifconfig eth1 192.168.2.130 netmask 255.255.255.192 broadcast 192.168.2.191
</verb></p>
<p>to configure the NIC. And then
</p>
<p><verb>
ifconfig eth0 down # just to make sure it does not interfere with things
ping 192.168.2.129
</verb></p>
<p>to test that you can get to the router. For good measure, you should also
test that you can get to the machines on your local network through the other
NIC:
</p>
<p><verb>
ifconfig eth1 down # just to make sure it does not interfere with things
ifconfig eth0 up
ping 192.168.2.x # where x is the address for a machine on your local net
</verb></p>
<p>At this point, you have verified that all the hardware is working.
</p>
<sect1>Bridge Config
<p> You will need the <url url="ftp://ftp.tux.org/people/alan-cox/BRCFG.tgz" name="bridge configuration utility">
from Alan Cox; this
allows you to control the bridging in your kernel when CONFIG_BRIDGE is turned on. <bf>BRCFG</bf> is distributed
as source with pre-compiled executables. I do not know what kernel the executable was compiled with, but I got
different results after I recompiled it with my kernel (2.2.13) include files. Unfortunately, to do this I had
to patch them slightly. Here are the patches:
</p>
<p><verb>
diff -C 3 -r /tmp/BRCFG/brcfg.c ./brcfg.c
*** /tmp/BRCFG/brcfg.c Wed Feb 21 19:11:59 1996
--- ./brcfg.c Wed Dec 8 12:52:23 1999
***************
*** 1,6 ****
! #include <sys/types.h>
! #include <sys/socket.h>
#include <skbuff.h>
#include "br.h"
--- 1,6 ----
! #include <types.h>
! #include <socket.h>
#include <skbuff.h>
#include "br.h"
</verb></p>
<p>Apply the patch, recompile <bf>brcfg</bf> and install it somewhere appropriate
(I chose <bf>/usr/sbin</bf>).
<sect1>Kernel Configuration
<p> You will need to patch and configure your kernel for bridging and the bridging filter
(as well as firewalling, networking, etc. if you do not already have it). The following
kernel configuration items will be needed (at least):
</p>
<p><verb>
CONFIG_EXPERIMENTAL=y
CONFIG_BRIDGE=y
CONFIG_FIREWALL=y
CONFIG_IP_FIREWALL=y
</verb></p>
<p> You should grab the <url url="http://ac2i.tzo.com/bridge_filter/" name="Bridge Filter Patch">
and apply it to your kernel. Recompile and install your kernel and then reboot.
</p>
<sect1>Putting It All Together
<p> So you should have your two NIC's working, a newly configured kernel, and <bf>brcfg</bf>
installed. Now you need to construct a startup script to put it all together. I did this using
the RedHat type startup scripts (<bf>/etc/rc.d</bf>). I put specific network addresses and
masks in <bf>/etc/sysconfig/network</bf>:
</p>
<p><verb>
GATEWAY=192.168.2.129 # the address of the DSL router
GATEWAYDEV=eth1 # the NIC that the router is connected to
ETH0_ADDR=192.168.2.130 # the IP address for the NIC on our LAN
ETH0_MASK=255.255.255.192 # the netmask of our LAN
ETH0_BROAD=192.168.2.191 # the broadcast address of our LAN
ETH1_ADDR=192.168.2.130 # the IP address for the NIC on the DSL side
# can be different from ETH0_ADDR if you want
ETH1_MASK=$ETH0_MASK # the DSL side netmask, should be the same as eth0
ETH1_BROAD=$ETH1_BROAD # ditto for the broadcast address
</verb></p>
<p> Next I created a script in <bf>/etc/rc.d/init.d/bridge</bf> to setup the bridge:
<p><verb>
#!/bin/sh
#
# bridge This shell script takes care of installing bridging for dsl
#
# description: Uses brcfg to start bridging and ifconfigs eths
# processname: bridge
# config:
# Source function library.
. /etc/rc.d/init.d/functions
# Source networking configuration.
. /etc/sysconfig/network
# See how we were called.
case "$1" in
start)
echo -n "Configuring bridge: "
ifconfig eth0 $ETH0_ADDR netmask $ETH0_MASK broadcast $ETH0_BROAD
ifconfig eth1 $ETH1_ADDR netmask $ETH1_MASK broadcast $ETH1_BROAD
route add $GATEWAY dev $GATEWAYDEV
route add default gw $GATEWAY dev $GATEWAYDEV
ifconfig eth0 promisc
ifconfig eth1 promisc
brcfg -enable
echo
;;
stop)
# Stop daemons.
brcfg -disable
ifconfig eth0 down
ifconfig eth1 down
;;
restart)
$0 stop
$0 start
;;
status)
ifconfig eth0
ifconfig eth1
brcfg
;;
*)
echo "Usage: bridge {start|stop|restart|status}"
exit 1
esac
exit 0
</verb></p>
<p> This script is run during bootup. It assigns addresses to each NIC, adds a default route
that goes to the DSL router, adds a specific route direct to the DSL router, puts each NIC
in "promiscuous" mode, and then enables bridging. I linked this script into the following
directories in <bf>/etc/rc.d</bf>:
<p><verb>
/etc/rc.d/rc0.d/K90bridge
/etc/rc.d/rc1.d/K90bridge
/etc/rc.d/rc2.d/S11bridge
/etc/rc.d/rc3.d/S11bridge
/etc/rc.d/rc4.d/S11bridge
/etc/rc.d/rc5.d/S11bridge
/etc/rc.d/rc6.d/K90bridge
</verb></p>
<p> This makes it run right after the network start script. You should disable
other configuration of eth0 (or eth1) such as done in the <bf>/etc/rc.d/init.d/network</bf> script
(in RedHat by removing files <bf>ifcfg-eth?</bf> from <bf>/etc/sysconfig/network-scripts/</bf>).
</p>
<p> To try things out, I suggest rebooting in single user mode (specify <bf>"single"</bf>
as an arg to the kernel, e.g. in lilo "lilo: linux single")
and running the startup scripts in <bf>/etc/rc.d/rc3.d</bf> one at a time
until you get to the bridge startup. Startup the bridge and then see if you can reach some
machines (you probably
want to use "<bf>ping -n</bf>" for this to keep the nameserver out of the equation):
</p>
<p><itemize>
<item>ping the DSL router
<item>ping a local machine
<item>ping a machine on the global net
</itemize></p>
<p> If you can ping all those places, there is a good chance that things are working.
Note that the bridge takes a few moments to startup. You can monitor the status of
the bridge by issuing the command <bf>brcfg</bf> with no arguments.
</p>
<sect1>Firewall Setup
<p>You still need to setup your firewall (assuming you want one) to prevent unauthorized
access. The <url url="http://ac2i.tzo.com/bridge_filter/" name="Bridge Filter Patch">
that you applied allows you to use a new built-in rule "bridgein" with ipchains. This rule
is used whenever a packet is going to be forwarded either from eth0 to eth1 or vice versa.
The bridgein rule is not used when a packet is destined for the firewall itself; you
will want to use the input rule for that. I will not attempt to delve into the firewall
setup in detail; please see the
<url url="http://www.linuxdoc.org/HOWTO/IPCHAINS-HOWTO.html" name="IPCHAINS HOWTO"> for that.
</p>
<sect1>Local Machine Setup
<p> For each of your local machines, you simply have to setup the proper IP address and netmask
and use the DSL router for the gateway (default route). The firewall/bridge will bridge the packets
to/from the DSL router.
</p>
<sect>Quirks and Problems
<sect1>Odd message when using <bf>ipchains -X</bf>
<p> The patch to add the bridgein built-in rule to ipchains makes the "delete all chains"
command, <bf>ipchains -X</bf>, issue the following error:
<p><verb>
ipchains: Device or resource busy
</verb></p>
<p> As far as I can tell this is harmless. I suspect that ipchains does not
understand that the new bridgein rule is a builtin.
</p>
<sect1> Shared Interrupts
<p>
As I mentioned in <ref id="Hardware Setup" name="Hardware Setup">, at least for PCI NIC's
you do not want to share interrupts between the two cards (or probably with any other device).
</p>
</article>