  Bridge + Firewall + DSL Mini-HOWTO
  Derek Ney  derek@hipgraphics.com <mailto:derek@hipgraph­
  ics.com>

  Mar 24, 2000

  Configuring a Linux system to act as a firewall and bridge with a DSL
  network connection

  ______________________________________________________________________

  Table of Contents


  1. Introduction

     1.1 History
     1.2 New versions
        1.2.1 Version History
     1.3 Copyrights

  2. Bridging, Firewalls, and DSL connections

     2.1 The Problem
     2.2 The Solution
     2.3 Setup Overview
     2.4 References

  3. Procedure

     3.1 Example Setup
     3.2 Hardware Setup
     3.3 Bridge Config
     3.4 Kernel Configuration
     3.5 Putting It All Together
     3.6 Firewall Setup
     3.7 Local Machine Setup

  4. Quirks and Problems

     4.1 Odd message when using
     4.2 Shared Interrupts


  ______________________________________________________________________

  1.  Introduction

  1.1.  History

  This document was started on December 10, 1999 by Derek Ney
  derek@hipgraphics.com <mailto: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.

  1.2.  New versions

  The newest version may be found in different formats at the LDP
  homepage http://www.linuxdoc.org/ <http://www.linuxdoc.org/>.

  1.2.1.  Version History

  v0.03 (Mar 24, 2000)



  ·  Fixed up URL for BRCFG.tgz

  v0.02 (Dec 13, 1999)


  ·  Incorporate revisions from Leonard Dickens (thanks Leonard!)

  v0.01 (Dec 10, 1999)


  ·  Initial version

  1.3.  Copyrights

  (c) 1999,2000 Derek R. Ney

  This document may be distributed under the terms set forth in the LDP
  license at http://www.linuxdoc.org/COPYRIGHT.html
  <http://www.linuxdoc.org/COPYRIGHT.html>.

  2.  Bridging, Firewalls, and DSL connections

  Until recently, our local network was hooked into the global net via
  PPP over a modem. I had installed a firewall using IPChains
  (http://www.linuxdoc.org/HOWTO/IPCHAINS-HOWTO.html
  <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.

  I guess this is applicable to a cable modem setup, but YMMV as I know
  nothing about cable modem hookups.

  2.1.  The Problem

  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 IP
  Masquerade HOWTO <http://www.linuxdoc.org/HOWTO/IP-Masquerade-
  HOWTO.html>) which would otherwise probably be a simpler solution.
  This is not as simple as it seems.


  2.2.  The Solution

  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).

  Configuring the software really consists of two problems:


  ·  Route packets between the local and global net (bridging)

  ·  Filter the packets to stop some from traversing the firewall

  The Briding mini-Howto
  <http://www.linuxdoc.org/HOWTO/mini/Bridge.html> 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.

  It turns out you can assign your NIC's IP addresses and still use them
  as a bridge. Although the Briding mini-Howto
  <http://www.linuxdoc.org/HOWTO/mini/Bridge.html> 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 http://ac2i.tzo.com/bridge_filter/
  <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".

  2.3.  Setup Overview

  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.

  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.

  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.

  2.4.  References

  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.

  The Briding mini-Howto
  <http://www.linuxdoc.org/HOWTO/mini/Bridge.html> was instrumental in
  getting things up. Unfortunately using it alone does not implement a
  firewall.

  The Linux Bridge+Firewall mini-HOWTO
  <http://www.linuxdoc.org/HOWTO/mini/Bridge+Firewall.html> 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.

  The Bridge Filter Patch <http://ac2i.tzo.com/bridge_filter/> 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.

  The IPCHAINS HOWTO <http://www.linuxdoc.org/HOWTO/IPCHAINS-HOWTO.html>
  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.

  3.  Procedure

  The basic procedure is as follows:


  ·  Setup your hardware (and verify that it works)

  ·  Patch and configure the kernel

  ·  Configure your network (ifconfig, route, bridging)

  ·  Configure the firewall

  3.1.  Example Setup

  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.

  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.

  3.2.

  Hardware Setup

  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 Ethernet-
  HOWTO <http://www.linuxdoc.org/HOWTO/Ethernet-HOWTO.html> section
  Using More than one Ethernet Card per Machine
  <http://www.linuxdoc.org/HOWTO/Ethernet-HOWTO-3.html#ss3.2>) 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).

  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:
    ifconfig eth1 192.168.2.130 netmask 255.255.255.192 broadcast 192.168.2.191



  to configure the NIC. And then


    ifconfig eth0 down # just to make sure it does not interfere with things
    ping 192.168.2.129



  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:


    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



  At this point, you have verified that all the hardware is working.

  3.3.  Bridge Config

  You will need the bridge configuration utility
  <ftp://ftp.tux.org/people/alan-cox/BRCFG.tgz> from Alan Cox; this
  allows you to control the bridging in your kernel when CONFIG_BRIDGE
  is turned on. BRCFG 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:


  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"



  Apply the patch, recompile brcfg and install it somewhere appropriate
  (I chose /usr/sbin).


  3.4.  Kernel Configuration

  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):


    CONFIG_EXPERIMENTAL=y
    CONFIG_BRIDGE=y
    CONFIG_FIREWALL=y
    CONFIG_IP_FIREWALL=y



  You should grab the Bridge Filter Patch
  <http://ac2i.tzo.com/bridge_filter/> and apply it to your kernel.
  Recompile and install your kernel and then reboot.

  3.5.  Putting It All Together

  So you should have your two NIC's working, a newly configured kernel,
  and brcfg installed. Now you need to construct a startup script to put
  it all together. I did this using the RedHat type startup scripts
  (/etc/rc.d). I put specific network addresses and masks in
  /etc/sysconfig/network:


   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



  Next I created a script in /etc/rc.d/init.d/bridge to setup the
  bridge:



  #!/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



  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 /etc/rc.d:



   /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



  This makes it run right after the network start script. You should
  disable other configuration of eth0 (or eth1) such as done in the
  /etc/rc.d/init.d/network script (in RedHat by removing files ifcfg-
  eth? from /etc/sysconfig/network-scripts/).

  To try things out, I suggest rebooting in single user mode (specify
  "single" as an arg to the kernel, e.g. in lilo "lilo: linux single")
  and running the startup scripts in /etc/rc.d/rc3.d 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 "ping -n" for this
  to keep the nameserver out of the equation):


  ·  ping the DSL router

  ·  ping a local machine

  ·  ping a machine on the global net

  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 brcfg with
  no arguments.

  3.6.  Firewall Setup

  You still need to setup your firewall (assuming you want one) to
  prevent unauthorized access. The Bridge Filter Patch
  <http://ac2i.tzo.com/bridge_filter/> 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 IPCHAINS HOWTO <http://www.linuxdoc.org/HOWTO/IPCHAINS-
  HOWTO.html> for that.

  3.7.  Local Machine Setup

  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.

  4.  Quirks and Problems

  4.1.  Odd message when using ipchains -X

  The patch to add the bridgein built-in rule to ipchains makes the
  "delete all chains" command, ipchains -X, issue the following error:


   ipchains: Device or resource busy


  As far as I can tell this is harmless. I suspect that ipchains does
  not understand that the new bridgein rule is a builtin.

  4.2.  Shared Interrupts

  As I mentioned in ``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).



