Shorewall and Bridged Firewalls without using physdev match
support
Tom
Eastep
2004
2005
2006
Thomas M. Eastep
Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation License, Version
1.2 or any later version published by the Free Software Foundation; with
no Invariant Sections, with no Front-Cover, and with no Back-Cover
Texts. A copy of the license is included in the section entitled
GNU Free Documentation
License
.
This article applies to Shorewall 3.3.3 and
later. If you are running a version of Shorewall earlier than Shorewall
3.3.3 then please see the documentation for that
release.
Background
Systems where Shorewall runs normally function as
routers. In the context of the Open System
Interconnect (OSI) reference model, a router operates at layer 3,
Shorewall may also be deployed on a GNU Linux System that acts as a
bridge. Bridges are layer 2 devices in the OSI
model (think of a bridge as an ethernet switch).
Some differences between routers and bridges are:
Routers determine packet destination based on the destination IP
address, while bridges route traffic based on the destination MAC
address in the ethernet frame.
As a consequence of the first difference, routers can be
connected to more than one IP network while a bridge may be part of
only a single network.
In most configurations, routers don't forward broadcast packets
while a bridges do.
Section 4 of RFC 1812 describes the conditions under which a
router may or must forward broadcasts.
The technique described in this article differs from that in Shorewall and Bridged Firewalls in that it
defines zones in terms of ip addresses (networks, hosts, and/or ranges)
accessed through the bridge device rather than in terms of ports on the
bridge. While using ports is more convenient, it requires a
fully-functional physdev match capability in your
kernel and iptables. Beginning with Linux kernel version 2.6.20, the
physdev match capability was reduced in function to the point where in can
no longer be used for Shorewall zone definition. To work around this
functional step backward, the technique described below can be
used.
To summarize the changes required required to move from a
Shorewall and Bridged Firewalls configuration to this
new type:
Set BRIDGING=No in shorewall.conf
Modify your /etc/shorewall/hosts file to
use IP addresses rather than bridge ports to define your zones.
Requirements
Note that if you need a bridge but do not need to restrict the
traffic through the bridge then any version of Shorewall will work. See
the Simple Bridge documentation for
details.
In order to use Shorewall as a bridging firewall:
Your kernel must contain bridge support (CONFIG_BRIDGE=m or
CONFIG_BRIDGE=y).
Your kernel must contain bridge/netfilter integration
(CONFIG_BRIDGE_NETFILTER=y).
You must have the bridge utilities (bridge-utils) package
installed.
Application
The following diagram shows a typical application of a
bridge/firewall. There is already an existing router in place whose
internal interface supports a network, and you want to insert a firewall
between the router, and the systems in the local network. In the example
shown, the network uses RFC 1918 addresses but that is not a requirement;
the bridge would work exactly the same if public IP addresses were used
(remember that the bridge doesn't deal with IP addresses).
![]()
There are a several key differences in this setup and a normal
Shorewall configuration:
The Shorewall system (the Bridge/Firewall) has only a single IP
address even though it has two ethernet interfaces! The IP address is
configured on the bridge itself, rather than on either of the network
cards.
The systems connected to the LAN are configured with the
router's IP address (192.168.1.254 in the above diagram) as their
default gateway.
traceroute doesn't detect the Bridge/Firewall
as an intermediate router.
If the router runs a DHCP server, the hosts connected to the LAN
can use that server without having dhcrelay running
on the Bridge/Firewall.
Inserting a bridge/firewall between a router and a set of local
hosts only works if those local hosts form a single IP network. In the
above diagram, all of the hosts in the loc zone are in the
192.168.1.0/24 network. If the router is routing between several local
networks through the same physical interface (there are multiple IP
networks sharing the same LAN), then inserting a bridge/firewall between
the router and the local LAN won't work.
There are other possibilities here -- there could be a hub or switch
between the router and the Bridge/Firewall and there could be other
systems connected to that switch. All of the systems on the local side of
the router would still be configured with
IP addresses in 192.168.1.0/24 as shown below.![]()
Configuring the Bridge
Configuring the bridge itself is quite simple and uses the
brctl utility from the bridge-utils package. Bridge
configuration information may be found at http://bridge.sf.net.
Unfortunately, many Linux distributions don't have good bridge
configuration tools, and the network configuration GUIs don't detect the
presence of bridge devices. Here is an excerpt from a Debian
/etc/network/interfaces file for a two-port bridge
with a static IP address:
auto br0
iface br0 inet static
address 192.168.1.253
netmask 255.255.255.0
network 192.168.1.0
broadcast 192.168.1.255
pre-up /sbin/ip link set eth0 up
pre-up /sbin/ip link set eth1 up
pre-up /usr/sbin/brctl addbr br0
pre-up /usr/sbin/brctl addif br0 eth0
pre-up /usr/sbin/brctl addif br0 eth1
While it is not a requirement to give the bridge an IP address,
doing so allows the bridge/firewall to access other systems and allows the
bridge/firewall to be managed remotely. The bridge must also have an IP
address for REJECT rules and policies to work correctly — otherwise REJECT
behaves the same as DROP. It is also a requirement for bridges to have an
IP address if they are part of a bridge/router.
Get your bridge configuration working first, including bridge
startup at boot, before you configure and start Shorewall.
The bridge may have its IP address assigned via DHCP. Here's an
example of an /etc/sysconfig/network/ifcfg-br0 file from a
SUSE system:
BOOTPROTO='dhcp'
REMOTE_IPADDR=''
STARTMODE='onboot'
UNIQUE='3hqH.MjuOqWfSZ+C'
WIRELESS='no'
MTU=''
Here's an /etc/sysconfig/network-scripts/ifcfg-br0 file for a
Mandriva system:
DEVICE=br0
BOOTPROTO=dhcp
ONBOOT=yes
On both the SUSE and Mandriva systems, a
separate script is required to configure the bridge itself.
Here are scripts that I used on a SUSE 9.1
system.
/etc/sysconfig/network/ifcfg-br0
BOOTPROTO='dhcp'
REMOTE_IPADDR=''
STARTMODE='onboot'
UNIQUE='3hqH.MjuOqWfSZ+C'
WIRELESS='no'
MTU=''
/etc/init.d/bridge#!/bin/sh
################################################################################
# Script to create a bridge
#
# (c) 2004 - Tom Eastep (teastep@shorewall.net)
#
# Modify the following variables to match your configuration
#
#### BEGIN INIT INFO
# Provides: bridge
# Required-Start: coldplug
# Required-Stop:
# Default-Start: 2 3 5
# Default-Stop: 0 1 6
# Description: starts and stops a bridge
### END INIT INFO
#
# chkconfig: 2345 05 89
# description: GRE/IP Tunnel
#
################################################################################
PATH=$PATH:/sbin:/usr/sbin:/usr/local/sbin
INTERFACES="eth1 eth0"
BRIDGE="br0"
MODULES="tulip"
do_stop() {
echo "Stopping Bridge $BRIDGE"
brctl delbr $BRIDGE
for interface in $INTERFACES; do
ip link set $interface down
done
}
do_start() {
echo "Starting Bridge $BRIDGE"
for module in $MODULES; do
modprobe $module
done
sleep 5
for interface in $INTERFACES; do
ip link set $interface up
done
brctl addbr $BRIDGE
for interface in $INTERFACES; do
brctl addif $BRIDGE $interface
done
}
case "$1" in
start)
do_start
;;
stop)
do_stop
;;
restart)
do_stop
sleep 1
do_start
;;
*)
echo "Usage: $0 {start|stop|restart}"
exit 1
esac
exit 0
Axel Westerhold has contributed this example of configuring a bridge
with a static IP address on a Fedora System (Core 1 and Core 2 Test 1).
Note that these files also configure the bridge itself, so there is no
need for a separate bridge config script.
/etc/sysconfig/network-scripts/ifcfg-br0:
DEVICE=br0
TYPE=Bridge
IPADDR=192.168.50.14
NETMASK=255.255.255.0
ONBOOT=yes
/etc/sysconfig/network-scripts/ifcfg-eth0:DEVICE=eth0
TYPE=ETHER
BRIDGE=br0
ONBOOT=yes/etc/sysconfig/network-scripts/ifcfg-eth1:DEVICE=eth1
TYPE=ETHER
BRIDGE=br0
ONBOOT=yes
Florin Grad at Mandriva provides this script
for configuring a bridge:
#!/bin/sh
# chkconfig: 2345 05 89
# description: Layer 2 Bridge
#
[ -f /etc/sysconfig/bridge ] && . /etc/sysconfig/bridge
PATH=$PATH:/sbin:/usr/sbin:/usr/local/sbin
do_stop() {
echo "Stopping Bridge"
for i in $INTERFACES $BRIDGE_INTERFACE ; do
ip link set $i down
done
brctl delbr $BRIDGE_INTERFACE
}
do_start() {
echo "Starting Bridge"
for i in $INTERFACES ; do
ip link set $i up
done
brctl addbr br0
for i in $INTERFACES ; do
ip link set $i up
brctl addif br0 $i
done
ifup $BRIDGE_INTERFACE
}
case "$1" in
start)
do_start
;;
stop)
do_stop
;;
restart)
do_stop
sleep 1
do_start
;;
*)
echo "Usage: $0 {start|stop|restart}"
exit 1
esac
exit 0
The /etc/sysconfig/bridge file:
BRIDGE_INTERFACE=br0 #The name of your Bridge
INTERFACES="eth0 eth1" #The physical interfaces to be bridged
Andrzej Szelachowski contributed the following.
Here is how I configured bridge in Slackware:
1) I had to compile bridge-utils (It's not in the standard distribution)
2) I've created rc.bridge in /etc/rc.d:
#########################
#! /bin/sh
ifconfig eth0 0.0.0.0
ifconfig eth1 0.0.0.0
#ifconfig lo 127.0.0.1 #this line should be uncommented if you don't use rc.inet1
brctl addbr most
brctl addif most eth0
brctl addif most eth1
ifconfig most 192.168.1.31 netmask 255.255.255.0 up
#route add default gw 192.168.1.1 metric 1 #this line should be uncommented if
#you don't use rc.inet1
#########################
3) I made rc.brige executable and added the following line to /etc/rc.d/rc.local
/etc/rc.d/rc.bridge
Joshua Schmidlkofer writes:
Bridge Setup for Gentoo
#install bridge-utils
emerge bridge-utils
## create a link for net.br0
cd /etc/init.d
ln -s net.eth0 net.br0
# Remove net.eth*, add net.br0 and bridge.
rc-update del net.eth0
rc-update del net.eth1
rc-update add net.br0 default
rc-update add bridge boot
/etc/conf.d/bridge:
#bridge contains the name of each bridge you want created.
bridge="br0"
# bridge_<bridge>_devices contains the devices to use at bridge startup.
bridge_br0_devices="eth0 eth1"
/etc/conf.d/net
iface_br0="10.0.0.1 broadcast 10.0.0.255 netmask 255.255.255.0"
#for dhcp:
#iface_br0="dhcp"
#comment this out if you use dhcp.
gateway="eth0/10.0.0.1"
Users who successfully configure bridges on other distributions,
with static or dynamic IP addresses, are encouraged to send me their configuration so I
can post it here.
Configuring Shorewall
To use this form of bridge support, you must turn off the BRIDGING
option in /etc/shorewall/shorewall.conf:
BRIDGING=No
In the scenario pictured above (where the hosts 192.168.1.10 and
192.168.1.11 are on the 'net' side of the bridge), there would probably be
two zones defined -- one for the internet, and one for the local LAN; so
in /etc/shorewall/zones:
#ZONE TYPE OPTIONS
fw firewall
net ipv4
loc:net ipv4
#LAST LINE - ADD YOUR ENTRIES ABOVE THIS ONE - DO NOT REMOVE
Note that the loc zone is defined
to be a sub-zone of the net zone.
A conventional two-zone policy file is appropriate here —
/etc/shorewall/policy:
#SOURCE DEST POLICY LOG LIMIT:BURST
loc net ACCEPT
net all DROP info
all all REJECT info
#LAST LINE - ADD YOUR ENTRIES ABOVE THIS ONE - DO NOT REMOVE
Only the bridge device itself is configured with an IP address, so
only that device is defined to Shorewall in
/etc/shorewall/interfaces:
#ZONE INTERFACE BROADCAST OPTIONS
net br0 192.168.1.255
#LAST LINE -- ADD YOUR ENTRIES BEFORE THIS ONE -- DO NOT REMOVE
The loc zone is defined using the
/etc/shorewall/hosts file. Assuming that the router
is connected to eth0 and the
switch to eth1:
#ZONE HOST(S) OPTIONS
loc br0:192.168.1.0/24!192.168.1.10/31,192.168.1.254
#LAST LINE -- ADD YOUR ENTRIES BEFORE THIS LINE -- DO NOT REMOVE
192.168.1.10/31 consists of the two local systems outside the
firewall; namely, 192.168.1.10 and 192.168.1.11. Those systems must be
excluded from the loc zone as must the
router (192.168.1.254).
When Shorewall is stopped, you want to allow only local traffic
through the bridge —
/etc/shorewall/routestopped:
#INTERFACE HOST(S) OPTIONS
br0 192.168.1.0/24 routeback
#LAST LINE -- ADD YOUR ENTRIES BEFORE THIS ONE -- DO NOT REMOVE
The /etc/shorewall/rules file from the
two-interface sample is a good place to start for defining a set of
firewall rules.
Limitations
Bridging doesn't work with some wireless cards — see http://bridge.sf.net.