1 .\" $NetBSD: bridge.4,v 1.5 2004/01/31 20:14:11 jdc Exp $
3 .\" Copyright 2001 Wasabi Systems, Inc.
4 .\" All rights reserved.
6 .\" Written by Jason R. Thorpe for Wasabi Systems, Inc.
8 .\" Redistribution and use in source and binary forms, with or without
9 .\" modification, are permitted provided that the following conditions
11 .\" 1. Redistributions of source code must retain the above copyright
12 .\" notice, this list of conditions and the following disclaimer.
13 .\" 2. Redistributions in binary form must reproduce the above copyright
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15 .\" documentation and/or other materials provided with the distribution.
16 .\" 3. All advertising materials mentioning features or use of this software
17 .\" must display the following acknowledgement:
18 .\" This product includes software developed for the NetBSD Project by
19 .\" Wasabi Systems, Inc.
20 .\" 4. The name of Wasabi Systems, Inc. may not be used to endorse
21 .\" or promote products derived from this software without specific prior
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24 .\" THIS SOFTWARE IS PROVIDED BY WASABI SYSTEMS, INC. ``AS IS'' AND
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38 .Dd September 17, 2007
43 .Nd network bridge device
45 To compile this driver into the kernel,
46 place the following line in your
47 kernel configuration file:
48 .Bd -ragged -offset indent
49 .Cd "device if_bridge"
52 Alternatively, to load the driver as a
53 module at boot time, place the following line in
55 .Bd -literal -offset indent
61 driver creates a logical link between two or more IEEE 802 networks
63 .Dq "similar enough" )
65 For example, it is possible to bridge Ethernet and 802.11 networks together,
66 but it is not possible to bridge Ethernet and Token Ring together.
70 interface is created at runtime using interface cloning.
72 most easily done with the
82 interface randomly chooses a link (MAC) address in the range reserved for
83 locally administered addresses when it is created.
84 This address is guaranteed to be unique
88 interfaces on the local machine.
89 Thus you can theoretically have two bridges on the different machines with
90 the same link addresses.
91 The address can be changed by assigning the desired link address using
94 A bridge can be used to provide several services, such as a simple
95 802.11-to-Ethernet bridge for wireless hosts, and traffic isolation.
97 A bridge works like a hub, forwarding traffic from one interface
99 Multicast and broadcast packets are always forwarded to all
100 interfaces that are part of the bridge.
101 For unicast traffic, the bridge learns which MAC addresses are associated
102 with which interfaces and will forward the traffic selectively.
104 All the bridged member interfaces need to be up in order to pass network traffic.
105 These can be enabled using
108 .Va ifconfig_ Ns Ao Ar interface Ac Ns Li ="up"
112 The MTU of the first member interface to be added is used as the bridge MTU.
113 All additional members are required to have exactly the same value.
115 The TXCSUM capability is disabled for any interface added to the bridge, and it
116 is restored when the interface is removed again.
120 where the packets are discarded after
122 processing, and are not processed or forwarded further.
123 This can be used to multiplex the input of two or more interfaces into a single
126 This is useful for reconstructing the traffic for network taps
127 that transmit the RX/TX signals out through two separate interfaces.
131 driver implements the Rapid Spanning Tree Protocol (RSTP or 802.1w) with
132 backwards compatibility with the legacy Spanning Tree Protocol (STP).
133 Spanning Tree is used to detect and remove loops in a network topology.
135 RSTP provides faster spanning tree convergence than legacy STP, the protocol
136 will exchange information with neighbouring switches to quickly transition to
137 forwarding without creating loops.
139 The code will default to STP and can be set to operate in RSTP mode with
140 rapid state transitions via the
145 The bridge can log STP port changes to
148 .Va net.link.bridge.log_stp
153 Packet filtering can be used with any firewall package that hooks in via the
156 When filtering is enabled, bridged packets will pass through the filter
157 inbound on the originating interface, on the bridge interface and outbound on
158 the appropriate interfaces.
159 Either stage can be disabled.
160 The filtering behaviour can be controlled using
162 .Bl -tag -width ".Va net.link.bridge.pfil_onlyip"
163 .It Va net.link.bridge.pfil_onlyip
164 Controls the handling of non-IP packets which are not passed to
168 to only allow IP packets to pass (subject to firewall rules), set to
170 to unconditionally pass all non-IP Ethernet frames.
171 .It Va net.link.bridge.pfil_member
174 to enable filtering on the incoming and outgoing member interfaces, set
178 .It Va net.link.bridge.pfil_bridge
181 to enable filtering on the bridge interface, set
185 .It Va net.link.bridge.pfil_local_phys
188 to additionally filter on the physical interface for locally destined packets.
191 to disable this feature.
192 .It Va net.link.bridge.ipfw
195 to enable layer2 filtering with
200 This needs to be enabled for
209 will be disabled so that IPFW
210 is not run twice; these can be re-enabled if desired.
211 .It Va net.link.bridge.ipfw_arp
214 to enable layer2 ARP filtering with
224 ARP and REVARP packets are forwarded without being filtered and others
225 that are not IP nor IPv6 packets are not forwarded when
228 IPFW can filter Ethernet types using
230 so all packets are passed to
231 the filter for processing.
233 The packets originating from the bridging host will be seen by
234 the filter on the interface that is looked up in the routing
237 The packets destined to the bridging host will be seen by the filter
238 on the interface with the MAC address equal to the packet's destination
240 There are situations when some of the bridge members are sharing
241 the same MAC address (for example the
243 interfaces: they are currenly sharing the
244 MAC address of the parent physical interface).
245 It is not possible to distinguish between these interfaces using
246 their MAC address, excluding the case when the packet's destination
247 MAC address is equal to the MAC address of the interface on which
248 the packet was entered to the system.
249 In this case the filter will see the incoming packet on this
251 In all other cases the interface seen by the packet filter is chosen
252 from the list of bridge members with the same MAC address and the
253 result strongly depends on the member addition sequence and the
254 actual implementation of
256 It is not recommended to rely on the order chosen by the current
258 implementation: it can be changed in the future.
260 The previous paragraph is best illustrated with the following
265 the MAC address of the incoming packet's destination is
266 .Nm nn:nn:nn:nn:nn:nn ,
268 the interface on which packet entered the system is
273 .Nm xx:xx:xx:xx:xx:xx ,
275 there are possibly other bridge members with the same MAC address
276 .Nm xx:xx:xx:xx:xx:xx ,
278 the bridge has more than one interface that are sharing the
280 .Nm yy:yy:yy:yy:yy:yy ;
287 Then if the MAC address
288 .Nm nn:nn:nn:nn:nn:nn
290 .Nm xx:xx:xx:xx:xx:xx
291 then the filter will see the packet on the interface
293 no matter if there are any other bridge members carrying the same
295 But if the MAC address
296 .Nm nn:nn:nn:nn:nn:nn
298 .Nm yy:yy:yy:yy:yy:yy
299 then the interface that will be seen by the filter is one of the
301 It is not possible to predict the name of the actual interface
302 without the knowledge of the system state and the
304 implementation details.
306 This problem arises for any bridge members that are sharing the same
307 MAC address, not only to the
309 ones: they we taken just as the example of such situation.
310 So if one wants the filter the locally destined packets based on
311 their interface name, one should be aware of this implication.
312 The described situation will appear at least on the filtering bridges
313 that are doing IP-forwarding; in some of such cases it is better
314 to assign the IP address only to the
316 interface and not to the bridge members.
318 .Va net.link.bridge.pfil_local_phys
319 will let you do the additional filtering on the physical interface.
321 The following when placed in the file
323 will cause a bridge called
325 to be created, and will add the interfaces
329 to the bridge, and then enable packet forwarding.
330 Such a configuration could be used to implement a simple
331 802.11-to-Ethernet bridge (assuming the 802.11 interface is
333 .Bd -literal -offset indent
334 cloned_interfaces="bridge0"
335 ifconfig_bridge0="addm ath0 addm fxp0 up"
338 For the bridge to forward packets all member interfaces and the bridge need
340 The above example would also require:
341 .Bd -literal -offset indent
342 ifconfig_ath0="up ssid my_ap mode 11g mediaopt hostap"
346 Consider a system with two 4-port Ethernet boards.
347 The following will cause a bridge consisting of all 8 ports with Rapid Spanning
348 Tree enabled to be created:
349 .Bd -literal -offset indent
350 ifconfig bridge0 create
351 ifconfig bridge0 proto rstp \e
352 addm fxp0 stp fxp0 \e
353 addm fxp1 stp fxp1 \e
354 addm fxp2 stp fxp2 \e
355 addm fxp3 stp fxp3 \e
356 addm fxp4 stp fxp4 \e
357 addm fxp5 stp fxp5 \e
358 addm fxp6 stp fxp6 \e
359 addm fxp7 stp fxp7 \e
363 The bridge can be used as a regular host interface at the same time as bridging
364 between its member ports.
365 In this example, the bridge connects em0 and em1, and will receive its IP
366 address through DHCP:
367 .Bd -literal -offset indent
368 cloned_interfaces="bridge0"
369 ifconfig_bridge0="addm em0 addm em1 DHCP"
374 The bridge can tunnel Ethernet across an IP internet using the EtherIP
376 This can be combined with
378 to provide an encrypted connection.
381 interface and set the local and remote IP addresses for the
382 tunnel, these are reversed on the remote bridge.
383 .Bd -literal -offset indent
385 ifconfig gif0 tunnel 1.2.3.4 5.6.7.8 up
386 ifconfig bridge0 create
387 ifconfig bridge0 addm fxp0 addm gif0 up
398 driver first appeared in
404 driver was originally written by
406 .Aq jason@thought.net
407 as part of an undergraduate independent study at the University of
408 North Carolina at Greensboro.
412 driver has been heavily modified from the original version by
414 .Aq thorpej@wasabisystems.com .
416 Rapid Spanning Tree Protocol (RSTP) support was added by
418 .Aq thompsa@FreeBSD.org .
422 driver currently supports only Ethernet and Ethernet-like (e.g., 802.11)
423 network devices, with exactly the same interface MTU size as the bridge device.
425 Only wireless interfaces in hostap mode can be bridged due to the 802.11
426 framing format, bridging a wireless client is not supported yet.