29.5. Bridging

Written by Andrew Thompson.

29.5.1. Introduction

It is sometimes useful to divide one physical network (such as an Ethernet segment) into two separate network segments without having to create IP subnets and use a router to connect the segments together. A device that connects two networks together in this fashion is called a bridge. A FreeBSD system with two network interface cards can act as a bridge.

The bridge works by learning the MAC layer addresses (Ethernet addresses) of the devices on each of its network interfaces. It forwards traffic between two networks only when its source and destination are on different networks.

In many respects, a bridge is like an Ethernet switch with very few ports.

29.5.2. Situations Where Bridging Is Appropriate

There are many common situations in which a bridge is used today.

29.5.2.1. Connecting Networks

The basic operation of a bridge is to join two or more network segments together. There are many reasons to use a host based bridge over plain networking equipment such as cabling constraints, firewalling or connecting pseudo networks such as a Virtual Machine interface. A bridge can also connect a wireless interface running in hostap mode to a wired network and act as an access point.

29.5.2.2. Filtering/Traffic Shaping Firewall

A common situation is where firewall functionality is needed without routing or network address translation (NAT).

An example is a small company that is connected via DSL or ISDN to their ISP. They have a 13 globally-accessible IP addresses from their ISP and have 10 PCs on their network. In this situation, using a router-based firewall is difficult because of subnetting issues.

A bridge-based firewall can be configured and dropped into the path just downstream of their DSL/ISDN router without any IP numbering issues.

29.5.2.3. Network Tap

A bridge can join two network segments and be used to inspect all Ethernet frames that pass between them. This can either be from using bpf(4)/tcpdump(1) on the bridge interface or by sending a copy of all frames out an additional interface (span port).

29.5.2.4. Layer 2 VPN

Two Ethernet networks can be joined across an IP link by bridging the networks to an EtherIP tunnel or a tap(4) based solution such as OpenVPN.

29.5.2.5. Layer 2 Redundancy

A network can be connected together with multiple links and use the Spanning Tree Protocol to block redundant paths. For an Ethernet network to function properly only one active path can exist between two devices, Spanning Tree will detect loops and put the redundant links into a blocked state. Should one of the active links fail then the protocol will calculate a different tree and reenable one of the blocked paths to restore connectivity to all points in the network.

29.5.3. Kernel Configuration

This section covers if_bridge(4) bridge implementation, a netgraph bridging driver is also available, for more information see ng_bridge(4) manual page.

The bridge driver is a kernel module and will be automatically loaded by ifconfig(8) when creating a bridge interface. It is possible to compile the bridge in to the kernel by adding device if_bridge to your kernel configuration file.

Packet filtering can be used with any firewall package that hooks in via the pfil(9) framework. The firewall can be loaded as a module or compiled into the kernel.

The bridge can be used as a traffic shaper with altq(4) or dummynet(4).

29.5.4. Enabling the Bridge

The bridge is created using interface cloning. To create a bridge use ifconfig(8), if the bridge driver is not present in the kernel then it will be loaded automatically.

# ifconfig bridge create
bridge0
# ifconfig bridge0
bridge0: flags=8802<BROADCAST,SIMPLEX,MULTICAST> metric 0 mtu 1500
        ether 96:3d:4b:f1:79:7a
        id 00:00:00:00:00:00 priority 32768 hellotime 2 fwddelay 15
        maxage 20 holdcnt 6 proto rstp maxaddr 100 timeout 1200
        root id 00:00:00:00:00:00 priority 0 ifcost 0 port 0

A bridge interface is created and is automatically assigned a randomly generated Ethernet address. The maxaddr and timeout parameters control how many MAC addresses the bridge will keep in its forwarding table and how many seconds before each entry is removed after it is last seen. The other parameters control how Spanning Tree operates.

Add the member network interfaces to the bridge. For the bridge to forward packets all member interfaces and the bridge need to be up:

# ifconfig bridge0 addm fxp0 addm fxp1 up
# ifconfig fxp0 up
# ifconfig fxp1 up

The bridge is now forwarding Ethernet frames between fxp0 and fxp1. The equivalent configuration in /etc/rc.conf so the bridge is created at startup is:

cloned_interfaces="bridge0"
ifconfig_bridge0="addm fxp0 addm fxp1 up"
ifconfig_fxp0="up"
ifconfig_fxp1="up"

If the bridge host needs an IP address then the correct place to set this is on the bridge interface itself rather than one of the member interfaces. This can be set statically or via DHCP:

# ifconfig bridge0 inet 192.168.0.1/24

It is also possible to assign an IPv6 address to a bridge interface.

29.5.5. Firewalling

When packet filtering is enabled, bridged packets will pass through the filter inbound on the originating interface, on the bridge interface and outbound on the appropriate interfaces. Either stage can be disabled. When direction of the packet flow is important it is best to firewall on the member interfaces rather than the bridge itself.

The bridge has several configurable settings for passing non-IP and ARP packets, and layer2 firewalling with IPFW. See if_bridge(4) for more information.

29.5.6. Spanning Tree

The bridge driver implements the Rapid Spanning Tree Protocol (RSTP or 802.1w) with backwards compatibility with the legacy Spanning Tree Protocol (STP). Spanning Tree is used to detect and remove loops in a network topology. RSTP provides faster Spanning Tree convergence than legacy STP, the protocol will exchange information with neighbouring switches to quickly transition to forwarding without creating loops.

The following table shows the supported operating modes:

OS VersionSTP ModesDefault Mode
FreeBSD 5.4——FreeBSD 6.2STPSTP
FreeBSD 6.3+RSTP or STPSTP
FreeBSD 7.0+RSTP or STPRSTP

Spanning Tree can be enabled on member interfaces using the stp command. For a bridge with fxp0 and fxp1 as the current interfaces, enable STP with the following:

# ifconfig bridge0 stp fxp0 stp fxp1
bridge0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
        ether d6:cf:d5:a0:94:6d
        id 00:01:02:4b:d4:50 priority 32768 hellotime 2 fwddelay 15
        maxage 20 holdcnt 6 proto rstp maxaddr 100 timeout 1200
        root id 00:01:02:4b:d4:50 priority 32768 ifcost 0 port 0
        member: fxp0 flags=1c7<LEARNING,DISCOVER,STP,AUTOEDGE,PTP,AUTOPTP>
                port 3 priority 128 path cost 200000 proto rstp
                role designated state forwarding
        member: fxp1 flags=1c7<LEARNING,DISCOVER,STP,AUTOEDGE,PTP,AUTOPTP>
                port 4 priority 128 path cost 200000 proto rstp
                role designated state forwarding

This bridge has a spanning tree ID of 00:01:02:4b:d4:50 and a priority of 32768. As the root id is the same it indicates that this is the root bridge for the tree.

Another bridge on the network also has spanning tree enabled:

bridge0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> metric 0 mtu 1500
        ether 96:3d:4b:f1:79:7a
        id 00:13:d4:9a:06:7a priority 32768 hellotime 2 fwddelay 15
        maxage 20 holdcnt 6 proto rstp maxaddr 100 timeout 1200
        root id 00:01:02:4b:d4:50 priority 32768 ifcost 400000 port 4
        member: fxp0 flags=1c7<LEARNING,DISCOVER,STP,AUTOEDGE,PTP,AUTOPTP>
                port 4 priority 128 path cost 200000 proto rstp
                role root state forwarding
        member: fxp1 flags=1c7<LEARNING,DISCOVER,STP,AUTOEDGE,PTP,AUTOPTP>
                port 5 priority 128 path cost 200000 proto rstp
                role designated state forwarding

The line root id 00:01:02:4b:d4:50 priority 32768 ifcost 400000 port 4 shows that the root bridge is 00:01:02:4b:d4:50 as above and has a path cost of 400000 from this bridge, the path to the root bridge is via port 4 which is fxp0.

29.5.7. Advanced Bridging

29.5.7.1. Reconstruct Traffic Flows

The bridge supports monitor mode, where the packets are discarded after bpf(4) processing, and are not processed or forwarded further. This can be used to multiplex the input of two or more interfaces into a single bpf(4) stream. This is useful for reconstructing the traffic for network taps that transmit the RX/TX signals out through two separate interfaces.

To read the input from four network interfaces as one stream:

# ifconfig bridge0 addm fxp0 addm fxp1 addm fxp2 addm fxp3 monitor up
# tcpdump -i bridge0

29.5.7.2. Span Ports

A copy of every Ethernet frame received by the bridge will be transmitted out a designated span port. The number of span ports configured on a bridge is unlimited, if an interface is designated as a span port then it may not also be used as a regular bridge port. This is most useful for snooping a bridged network passively on another host connected to one of the span ports of the bridge.

To send a copy of all frames out the interface named fxp4:

# ifconfig bridge0 span fxp4

29.5.7.3. Private Interfaces

A private interface does not forward any traffic to any other port that is also a private interface. The traffic is blocked unconditionally so no Ethernet frames will be forwarded, including ARP. If traffic needs to be selectively blocked then a firewall should be used instead.

29.5.7.4. Sticky Interfaces

If a bridge member interface is marked as sticky then dynamically learned address entries are treated at static once entered into the forwarding cache. Sticky entries are never aged out of the cache or replaced, even if the address is seen on a different interface. This gives the benefit of static address entries without the need to pre-populate the forwarding table, clients learnt on a particular segment of the bridge can not roam to another segment.

Another example of using sticky addresses would be to combine the bridge with VLANs to create a router where customer networks are isolated without wasting IP address space. Consider that CustomerA is on vlan100 and CustomerB is on vlan101. The bridge has the address 192.168.0.1 and is also an internet router.

# ifconfig bridge0 addm vlan100 sticky vlan100 addm vlan101 sticky vlan101
# ifconfig bridge0 inet 192.168.0.1/24

Both clients see 192.168.0.1 as their default gateway and since the bridge cache is sticky they can not spoof the MAC address of the other customer to intercept their traffic.

Any communication between the VLANs can be blocked using private interfaces (or a firewall):

# ifconfig bridge0 private vlan100 private vlan101

The customers are completely isolated from each other, the full /24 address range can be allocated without subnetting.

29.5.7.5. SNMP Monitoring

The bridge interface and STP parameters can be monitored via the SNMP daemon which is included in the FreeBSD base system. The exported bridge MIBs conform to the IETF standards so any SNMP client or monitoring package can be used to retrieve the data.

On the bridge machine uncomment the begemotSnmpdModulePath."bridge" = "/usr/lib/snmp_bridge.so" line from /etc/snmp.config and start the bsnmpd daemon. Other configuration such as community names and access lists may need to be modified. See bsnmpd(1) and snmp_bridge(3) for more information.

The following examples use the Net-SNMP software (net-mgmt/net-snmp) to query a bridge, the net-mgmt/bsnmptools port can also be used. From the SNMP client host add to $HOME/.snmp/snmp.conf the following lines to import the bridge MIB definitions in to Net-SNMP:

mibdirs +/usr/share/snmp/mibs
mibs +BRIDGE-MIB:RSTP-MIB:BEGEMOT-MIB:BEGEMOT-BRIDGE-MIB

To monitor a single bridge via the IETF BRIDGE-MIB (RFC4188) do

% snmpwalk -v 2c -c public bridge1.example.com mib-2.dot1dBridge
BRIDGE-MIB::dot1dBaseBridgeAddress.0 = STRING: 66:fb:9b:6e:5c:44
BRIDGE-MIB::dot1dBaseNumPorts.0 = INTEGER: 1 ports
BRIDGE-MIB::dot1dStpTimeSinceTopologyChange.0 = Timeticks: (189959) 0:31:39.59 centi-seconds
BRIDGE-MIB::dot1dStpTopChanges.0 = Counter32: 2
BRIDGE-MIB::dot1dStpDesignatedRoot.0 = Hex-STRING: 80 00 00 01 02 4B D4 50
...
BRIDGE-MIB::dot1dStpPortState.3 = INTEGER: forwarding(5)
BRIDGE-MIB::dot1dStpPortEnable.3 = INTEGER: enabled(1)
BRIDGE-MIB::dot1dStpPortPathCost.3 = INTEGER: 200000
BRIDGE-MIB::dot1dStpPortDesignatedRoot.3 = Hex-STRING: 80 00 00 01 02 4B D4 50
BRIDGE-MIB::dot1dStpPortDesignatedCost.3 = INTEGER: 0
BRIDGE-MIB::dot1dStpPortDesignatedBridge.3 = Hex-STRING: 80 00 00 01 02 4B D4 50
BRIDGE-MIB::dot1dStpPortDesignatedPort.3 = Hex-STRING: 03 80
BRIDGE-MIB::dot1dStpPortForwardTransitions.3 = Counter32: 1
RSTP-MIB::dot1dStpVersion.0 = INTEGER: rstp(2)

The dot1dStpTopChanges.0 value is two which means that the STP bridge topology has changed twice, a topology change means that one or more links in the network have changed or failed and a new tree has been calculated. The dot1dStpTimeSinceTopologyChange.0 value will show when this happened.

To monitor multiple bridge interfaces one may use the private BEGEMOT-BRIDGE-MIB:

% snmpwalk -v 2c -c public bridge1.example.com
enterprises.fokus.begemot.begemotBridge
BEGEMOT-BRIDGE-MIB::begemotBridgeBaseName."bridge0" = STRING: bridge0
BEGEMOT-BRIDGE-MIB::begemotBridgeBaseName."bridge2" = STRING: bridge2
BEGEMOT-BRIDGE-MIB::begemotBridgeBaseAddress."bridge0" = STRING: e:ce:3b:5a:9e:13
BEGEMOT-BRIDGE-MIB::begemotBridgeBaseAddress."bridge2" = STRING: 12:5e:4d:74:d:fc
BEGEMOT-BRIDGE-MIB::begemotBridgeBaseNumPorts."bridge0" = INTEGER: 1
BEGEMOT-BRIDGE-MIB::begemotBridgeBaseNumPorts."bridge2" = INTEGER: 1
...
BEGEMOT-BRIDGE-MIB::begemotBridgeStpTimeSinceTopologyChange."bridge0" = Timeticks: (116927) 0:19:29.27 centi-seconds
BEGEMOT-BRIDGE-MIB::begemotBridgeStpTimeSinceTopologyChange."bridge2" = Timeticks: (82773) 0:13:47.73 centi-seconds
BEGEMOT-BRIDGE-MIB::begemotBridgeStpTopChanges."bridge0" = Counter32: 1
BEGEMOT-BRIDGE-MIB::begemotBridgeStpTopChanges."bridge2" = Counter32: 1
BEGEMOT-BRIDGE-MIB::begemotBridgeStpDesignatedRoot."bridge0" = Hex-STRING: 80 00 00 40 95 30 5E 31
BEGEMOT-BRIDGE-MIB::begemotBridgeStpDesignatedRoot."bridge2" = Hex-STRING: 80 00 00 50 8B B8 C6 A9

To change the bridge interface being monitored via the mib-2.dot1dBridge subtree do:

% snmpset -v 2c -c private bridge1.example.com
BEGEMOT-BRIDGE-MIB::begemotBridgeDefaultBridgeIf.0 s bridge2

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