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
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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
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24 .\" THIS SOFTWARE IS PROVIDED BY WASABI SYSTEMS, INC. ``AS IS'' AND
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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 lines in
55 .Bd -literal -offset indent
62 driver creates a logical link between two or more IEEE 802 networks
64 .Dq "similar enough" )
66 For example, it is possible to bridge Ethernet and 802.11 networks together,
67 but it is not possible to bridge Ethernet and Token Ring together.
71 interface is created at runtime using interface cloning.
73 most easily done with the
83 interface randomly chooses a link (MAC) address in the range reserved for
84 locally administered addresses when it is created.
85 This address is guaranteed to be unique
89 interfaces on the local machine.
90 Thus you can theoretically have two bridges on the different machines with
91 the same link addresses.
92 The address can be changed by assigning the desired link address using
95 A bridge can be used to provide several services, such as a simple
96 802.11-to-Ethernet bridge for wireless hosts, and traffic isolation.
98 A bridge works like a hub, forwarding traffic from one interface
100 Multicast and broadcast packets are always forwarded to all
101 interfaces that are part of the bridge.
102 For unicast traffic, the bridge learns which MAC addresses are associated
103 with which interfaces and will forward the traffic selectively.
105 All the bridged member interfaces need to be up in order to pass network traffic.
106 These can be enabled using
109 .Va ifconfig_ Ns Ao Ar interface Ac Ns Li ="up"
113 The MTU of the first member interface to be added is used as the bridge MTU.
114 All additional members are required to have exactly the same value.
116 The TXCSUM capability is disabled for any interface added to the bridge, and it
117 is restored when the interface is removed again.
121 where the packets are discarded after
123 processing, and are not processed or forwarded further.
124 This can be used to multiplex the input of two or more interfaces into a single
127 This is useful for reconstructing the traffic for network taps
128 that transmit the RX/TX signals out through two separate interfaces.
132 driver implements the Rapid Spanning Tree Protocol (RSTP or 802.1w) with
133 backwards compatibility with the legacy Spanning Tree Protocol (STP).
134 Spanning Tree is used to detect and remove loops in a network topology.
136 RSTP provides faster spanning tree convergence than legacy STP, the protocol
137 will exchange information with neighbouring switches to quickly transition to
138 forwarding without creating loops.
140 The code will default to RSTP mode but will downgrade any port connected to a
141 legacy STP network so is fully backward compatible.
142 A bridge can be forced to operate in STP mode without rapid state transitions
148 The bridge can log STP port changes to
151 .Va net.link.bridge.log_stp
156 Packet filtering can be used with any firewall package that hooks in via the
159 When filtering is enabled, bridged packets will pass through the filter
160 inbound on the originating interface, on the bridge interface and outbound on
161 the appropriate interfaces.
162 Either stage can be disabled.
163 The filtering behaviour can be controlled using
165 .Bl -tag -width ".Va net.link.bridge.pfil_onlyip"
166 .It Va net.link.bridge.pfil_onlyip
167 Controls the handling of non-IP packets which are not passed to
171 to only allow IP packets to pass (subject to firewall rules), set to
173 to unconditionally pass all non-IP Ethernet frames.
174 .It Va net.link.bridge.pfil_member
177 to enable filtering on the incoming and outgoing member interfaces, set
181 .It Va net.link.bridge.pfil_bridge
184 to enable filtering on the bridge interface, set
188 .It Va net.link.bridge.ipfw
191 to enable layer2 filtering with
196 This needs to be enabled for
205 will be disabled so that IPFW
206 is not run twice; these can be re-enabled if desired.
207 .It Va net.link.bridge.ipfw_arp
210 to enable layer2 ARP filtering with
220 ARP and REVARP packets are forwarded without being filtered and others
221 that are not IP nor IPv6 packets are not forwarded when
224 IPFW can filter Ethernet types using
226 so all packets are passed to
227 the filter for processing.
229 The packets originating from the bridging host will be seen by
230 the filter on the interface that is looked up in the routing
233 The packets destined to the bridging host will be seen by the filter
234 on the interface with the MAC address equal to the packet's destination
236 There are situations when some of the bridge members are sharing
237 the same MAC address (for example the
239 interfaces: they are currenly sharing the
240 MAC address of the parent physical interface).
241 It is not possible to distinguish between these interfaces using
242 their MAC address, excluding the case when the packet's destination
243 MAC address is equal to the MAC address of the interface on which
244 the packet was entered to the system.
245 In this case the filter will see the incoming packet on this
247 In all other cases the interface seen by the packet filter is chosen
248 from the list of bridge members with the same MAC address and the
249 result strongly depends on the member addition sequence and the
250 actual implementation of
252 It is not recommended to rely on the order chosen by the current
254 implementation: it can be changed in the future.
256 The previous paragraph is best illustrated with the following
261 the MAC address of the incoming packet's destination is
262 .Nm nn:nn:nn:nn:nn:nn ,
264 the interface on which packet entered the system is
269 .Nm xx:xx:xx:xx:xx:xx ,
271 there are possibly other bridge members with the same MAC address
272 .Nm xx:xx:xx:xx:xx:xx ,
274 the bridge has more than one interface that are sharing the
276 .Nm yy:yy:yy:yy:yy:yy ;
283 Then if the MAC address
284 .Nm nn:nn:nn:nn:nn:nn
286 .Nm xx:xx:xx:xx:xx:xx
287 then the filter will see the packet on the interface
289 no matter if there are any other bridge members carrying the same
291 But if the MAC address
292 .Nm nn:nn:nn:nn:nn:nn
294 .Nm yy:yy:yy:yy:yy:yy
295 then the interface that will be seen by the filter is one of the
297 It is not possible to predict the name of the actual interface
298 without the knowledge of the system state and the
300 implementation details.
302 This problem arises for any bridge members that are sharing the same
303 MAC address, not only to the
305 ones: they we taken just as the example of such situation.
306 So if one wants the filter the locally destined packets based on
307 their interface name, one should be aware of this implication.
308 The described situation will appear at least on the filtering bridges
309 that are doing IP-forwarding; in some of such cases it is better
310 to assign the IP address only to the
312 interface and not to the bridge members.
313 But your mileage may vary.
315 The following when placed in the file
317 will cause a bridge called
319 to be created, and will add the interfaces
323 to the bridge, and then enable packet forwarding.
324 Such a configuration could be used to implement a simple
325 802.11-to-Ethernet bridge (assuming the 802.11 interface is
327 .Bd -literal -offset indent
328 cloned_interfaces="bridge0"
329 ifconfig_bridge0="addm ath0 addm fxp0 up"
332 For the bridge to forward packets all member interfaces and the bridge need
334 The above example would also require:
335 .Bd -literal -offset indent
336 ifconfig_ath0="up ssid my_ap mode 11g mediaopt hostap"
340 Consider a system with two 4-port Ethernet boards.
341 The following will cause a bridge consisting of all 8 ports with Rapid Spanning
342 Tree enabled to be created:
343 .Bd -literal -offset indent
344 ifconfig bridge0 create
346 addm fxp0 stp fxp0 \e
347 addm fxp1 stp fxp1 \e
348 addm fxp2 stp fxp2 \e
349 addm fxp3 stp fxp3 \e
350 addm fxp4 stp fxp4 \e
351 addm fxp5 stp fxp5 \e
352 addm fxp6 stp fxp6 \e
353 addm fxp7 stp fxp7 \e
357 The bridge can be used as a regular host interface at the same time as bridging
358 between its member ports.
359 In this example, the bridge connects em0 and em1, and will receive its IP
360 address through DHCP:
361 .Bd -literal -offset indent
362 cloned_interfaces="bridge0"
363 ifconfig_bridge0="addm em0 addm em1 DHCP"
368 The bridge can tunnel Ethernet across an IP internet using the EtherIP
370 This can be combined with
372 to provide an encrypted connection.
375 interface and set the local and remote IP addresses for the
376 tunnel, these are reversed on the remote bridge.
377 .Bd -literal -offset indent
379 ifconfig gif0 tunnel 1.2.3.4 5.6.7.8 up
380 ifconfig bridge0 create
381 ifconfig bridge0 addm fxp0 addm gif0 up
392 driver first appeared in
398 driver was originally written by
400 .Aq jason@thought.net
401 as part of an undergraduate independent study at the University of
402 North Carolina at Greensboro.
406 driver has been heavily modified from the original version by
408 .Aq thorpej@wasabisystems.com .
410 Rapid Spanning Tree Protocol (RSTP) support was added by
412 .Aq thompsa@FreeBSD.org .
416 driver currently supports only Ethernet and Ethernet-like (e.g., 802.11)
417 network devices, with exactly the same interface MTU size as the bridge device.
419 Only wireless interfaces in hostap mode can be bridged due to the 802.11
420 framing format, bridging a wireless client is not supported yet.