Bridged FirewallsTomEastep20072009Thomas M. EastepPermission 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 4.4 and
later.BackgroundSystems 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/firewall may be
part of only a single network (see below).In most configurations, routers don't forward broadcast packets
while bridges do.Section 4 of RFC 1812 describes the conditions under which a
router may or must forward broadcasts.RequirementsNote 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).Your kernel must contain Netfilter physdev match support
(CONFIG_IP_NF_MATCH_PHYSDEV=m or CONFIG_IP_NF_MATCH_PHYSDEV=y).
Physdev match is standard in the 2.6 kernel series but must be patched
into the 2.4 kernels (see http://bridge.sf.net). Bering and
Bering uCLibc users must find and install ipt_physdev.o for their
distribution and add ipt_physdev to
/etc/modules.Your iptables must contain physdev match support and must
support multiple instances of '-m physdev' in a single rule. iptables
1.3.6 and later contain this support.You must have the bridge utilities (bridge-utils) package
installed.ApplicationThe 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 BridgeConfiguring 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
pre-down /usr/sbin/brctl delif br0 eth0
pre-down /sbin/ip link set eth0 down
pre-down /usr/sbin/brctl delif br0 eth1
pre-down /sbin/ip link set eth1 down
post-down /usr/sbin/brctl delbr br0
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:
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-br0BOOTPROTO='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: $0 {start|stop|restart}"
exit 1
esac
exit 0
Axel Westerhold has contributed this example of configuring a bridge
with a static IP address on a Fedora System (Core 1 and Core 2 Test 1).
Note that these files also configure the bridge itself, so there is no
need for a separate bridge config script.
Florin Grad at Mandriva provides this script
for configuring a bridge:
#!/bin/sh
# chkconfig: 2345 05 89
# description: Layer 2 Bridge
#
[ -f /etc/sysconfig/bridge ] && . /etc/sysconfig/bridge
PATH=$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 0The /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.bridge 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 ShorewallAs described above, Shorewall bridge support requires the
physdev match feature of Netfilter/iptables.
Physdev match allows rules to be triggered based on the bridge port that a
packet arrived on and/or the bridge port that a packet will be sent over.
The latter has proved to be problematic because it requires that the
evaluation of rules be deferred until the destination bridge port is
known. This deferral has the unfortunate side effect that it makes IPSEC
Netfilter filtration incompatible with bridges. To work around this
problem, in kernel version 2.6.20 the Netfilter developers decided to
remove the deferred processing in two cases:When a packet being sent through a bridge entered the firewall
on another interface and was being forwarded to the bridge.When a packet originating on the firewall itself is being sent
through a bridge.Notice that physdev match was only weakened with respect to the
destination bridge port -- it remains fully functional with respect to the
source bridge port.To deal with the asymmetric nature of the new physdev match,
Shorewall supports a new type of zone - a Bridge
Port (BP) zone. Bridge port zones have a number of
restrictions:BP zones may only be associated with bridge ports.All ports associated with a given BP zone must be on the same
bridge.Policies from a non-BP zone to a BP are disallowed.Rules where the SOURCE is a non-BP zone and the DEST is a BP
zone are disallowed.In /etc/shorewall/zones, BP zones are specified using the bport (or bport4)
keyword. If your version of shorewall.conf contains
the BRIDGING option, it must be set to
No.In the scenario pictured above, there would probably be two BP zones
defined -- one for the Internet and one for the local LAN so in
/etc/shorewall/zones:#ZONE TYPE OPTIONS
fw firewall
world ipv4
net:world bport
loc:world bport
#LAST LINE - ADD YOUR ENTRIES ABOVE THIS ONE - DO NOT REMOVEThe world zone can be used when defining rules
whose source zone is the firewall itself (remember that fw-><BP
zone> rules are not allowed).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 REMOVEBridges use a special syntax in
/etc/shorewall/interfaces. Assuming that the router
is connected to eth0 and the
switch to eth1:#ZONE INTERFACE BROADCAST OPTIONS
world br0 detect bridge
net br0:eth0
loc br0:eth1
#LAST LINE -- ADD YOUR ENTRIES BEFORE THIS ONE -- DO NOT REMOVEThe world zone is associated with the bridge
itself which is defined with the bridge
option. Bridge port entries may not have any OPTIONS.When a bridge is configured without an IP address, the
option must also be specified.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 REMOVEThe /etc/shorewall/rules file from the
two-interface sample is a good place to start for defining a set of
firewall rules.Multiple Bridges with Wildcard PortsIt is sometimes required to configure multiple bridges on a single
firewall/gateway. The following seemingly valid configuration results in a
compile-time errorERROR: Duplicate Interface Name (p+)/etc/shorewall/zones: #ZONE TYPE
fw firewall
world ipv4
z1:world bport4
z2:world bport4/etc/shorewall/interfaces: #ZONE INTERFACE BROADCAST OPTIONS
world br0 - bridge
world br1 - bridge
z1 br0:p+
z2 br1:p+The reason is that the Shorewall implementation requires each bridge
port to have a unique name. The interface option
was added in Shorewall 4.4.4 to work around this problem. The above
configuration may be defined using the following in
/etc/shorewall/interfaces: #ZONE INTERFACE BROADCAST OPTIONS
world br0 - bridge
world br1 - bridge
z1 br0:x+ - physical=p+
z2 br1:y+ - physical=p+In this configuration, 'x+' is the logical name for ports p+ on
bridge br0 while 'y+' is the logical name for ports p+ on bridge
br1.If you need to refer to a particular port on br1 (for example
p1023), you write it as y1023; Shorewall will translate that name to p1023
when needed.Example from /etc/shorewall/rules: #ACTION SOURCE DEST PROTO DEST
# PORT(S)
REJECT z1:x1023 z1:x1024 tcp 1234Combination Router/BridgeA system running Shorewall doesn't have to be exclusively a bridge
or a router -- it can act as both, which is also know as a brouter. Here's
an example:This is basically the same setup as shown in the Shorewall Setup Guide with the
exception that the DMZ is bridged rather than using Proxy ARP. Changes in
the configuration shown in the Setup Guide are as follows:The /etc/shorewall/proxyarp file is empty
in this configuration.The /etc/shorewall/zones file is
modified:#ZONE TYPE OPTIONS
fw firewall
pub ipv4 #zone containing all public addresses
net:pub bport4
dmz:pub bport4
loc ipv4The /etc/shorewall/interfaces file is as
follows:#ZONE INTERFACE BROADCAST OPTIONS
pub br0 detect routefilter,bridge
net br0:eth0
dmz br0:eth2
loc eth1 detectThe DMZ systems need a route to the 192.168.201.0/24 network via
192.0.2.176 to enable them to communicate with the local
network.This configuration does not support separate fw->dmz and
fw->net policies/rules; similarly, it does not support separate
loc->dmz and loc->net rules. This will make it a bit trickier to
configure the rules. I suggest something like the following:/etc/shorewall/params:SERVERS=192.0.2.177,192.0.2.178 #IP Addresses of hosts in the DMZ
DMZ=pub:$SERVERS #Use in place of 'dmz' in rule DEST
NET=pub:!$SERVERS #Use in place of 'net' in rule DEST/etc/shorewall/policy:#SOURCE DEST POLICY LEVEL
loc pub ACCEPT
loc $FW REJECT info
loc all REJECT info
$FW pub REJECT info
$FW loc REJECT info
$FW all REJECT info
dmz net REJECT info
dmz $FW REJECT info
dmz loc REJECT info
dmz all REJECT info
net dmz DROP info
net $FW DROP info
net loc DROP info
net all DROP info
# THE FOLLOWING POLICY MUST BE LAST
all all REJECT info/etc/shorewall/rules:#ACTION SOURCE DEST PROTO DEST SOURCE
#
PORT(S) PORT(S)
ACCEPT all all icmp 8
ACCEPT loc $DMZ tcp 25,53,80,443,...
ACCEPT loc $DMZ udp 53
ACCEPT loc $NET
ACCEPT $FW $DMZ udp 53
ACCEPT $FW $DMZ tcp 53 LimitationsBridging doesn't work with some wireless cards — see http://bridge.sf.net.