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BRCONFIG(8)             OpenBSD System Manager's Manual            BRCONFIG(8)

     brconfig - manipulate bridge interfaces

     brconfig -a
     brconfig bridge-name [parameters]
     brconfig bridge-name rule { block | pass } { in | out | in/out } on
              interface-name [src address] [dst address] [tag tagname]

     The brconfig utility retrieves kernel state of bridge interfaces and al-
     lows user control of these bridges.  Bridge devices create a logical link
     between two or more Ethernet interfaces or encapsulation interfaces (see
     gif(4)), which will selectively forward frames from each interface on the
     bridge to every other interface on the bridge.  This can be used to iso-
     late traffic between sets of machines on the same segment and to provide
     a transparent filter for ip(4) datagrams.

     In the first synopsis, the -a flag will cause brconfig to list the status
     of all bridges in the system.  In the second, its command line consists
     of the name of a bridge and a set of operations to be performed on that
     bridge.  The commands are executed in the order they were specified.  If
     no command is specified in the second synopsis, the brconfig will display
     status information about the bridge.  With the third synopsis, rules for
     filtering Ethernet MAC addresses can be added to a bridge.

     The following parameters may be set with brconfig:

     up      Start the bridge forwarding packets.

     down    Stop the bridge from forwarding packets.

     addr    Display the addresses that have been learned by the bridge.

     add interface-name
             Add the interface named by interface-name as a member of the
             bridge.  The interface is put into promiscuous mode so that it
             can receive every packet sent on the network.  An interface can
             be a member of at most one bridge.

     delete interface-name
             Remove the interface named by interface-name from the bridge.
             Promiscuous mode is turned off for the interface when it is re-
             moved from the bridge.

     del     Alias for delete.

     addspan interface-name
             Add the interface named by interface-name as a span port on the
             bridge.  See the SPAN PORTS section for more details.

     delspan interface-name
             Delete the interface named by interface-name from the list of
             span ports of the bridge.

     maxaddr size
             Set the address cache size to size.  The default is 100 entries.

     timeout time
             Set the timeout, in seconds, for addresses in the cache to time.
             The default is 240 seconds.  If time is set to zero, then entries
             will not be expired.

     static interface-name address
             Add a static entry into the address cache pointing to interface-
             name.  Static entries are never aged out of the cache or replaced
             if the address is seen on a different interface.

     deladdr address
             Delete an address from the cache.

     flush   Remove all dynamically learned addresses from the cache.

             Remove all addresses from the cache including static addresses.

     blocknonip interface
             Mark an interface so that no non-IPv4, IPv6, ARP, or Reverse ARP
             packets are accepted from it or forwarded to it from other bridge
             member interfaces.

     -blocknonip interface
             Allow non-IPv4, IPv6, ARP, or Reverse ARP packets through the

     discover interface
             Mark an interface so that packets are sent out of the interface
             if the destination port of the packet is unknown.  If the bridge
             has no address cache entry for the destination of a packet, mean-
             ing that there is no static entry and no dynamically learned en-
             try for the destination, the bridge will forward the packet to
             all member interfaces that have this flag set.  This is the de-
             fault for interfaces added to the bridge.

     -discover interface
             Mark an interface so that packets are not sent out of the inter-
             face if the destination port of the packet is unknown.  Turning
             this flag off means that the bridge will not send packets out of
             this interface unless the packet is a broadcast packet, multicast
             packet, or a packet with a destination address found on the in-
             terface's segment.  This, in combination with static address
             cache entries, prevents potentially sensitive packets from being
             sent on segments that have no need to see the packet.

     learn interface
             Mark an interface so that the source address of packets received
             from interface are entered into the address cache.  This is the
             default for interfaces added to the bridge.

     -learn interface
             Mark an interface so that the source address of packets received
             from interface are not entered into the address cache.

     flushrule interface
             Remove all Ethernet MAC filtering rules from interface.

     link0   Setting this flag stops all non-IP multicast packets from being
             forwarded by the bridge.

     -link0  Clear the link0 flag on the bridge interface.

     link1   Setting this flags stops all IP multicast packets from being for-
             warded by the bridge.

     -link1  Clear the link1 flag on the bridge interface.

     link2   Setting this flag causes all packets to be passed on to ipsec(4)
             for processing, based on the policies established by the adminis-
             trator using the ipsecadm(8) command.  If appropriate security
             associations (SAs) exist, they will be used to encrypt or decrypt
             the packets.  Otherwise, any key management daemons such as
             isakmpd(8) that are running on the bridge will be invoked to es-
             tablish the necessary SAs.  These daemons have to be configured
             as if they were running on the host whose traffic they are pro-
             tecting (i.e., they need to have the appropriate authentication
             and authorization material, such as keys and certificates, to im-
             personate the protected host(s).

     -link2  Clear the link2 flag on the bridge interface.

     rule [rulespec]
             Add a filtering rule to an interface.  Rules have a similar syn-
             tax to pf(4).  Rules can be used to selectively block or pass
             frames based on Ethernet MAC address.  They can also tag packets
             for pf(4) to filter on.  Rules are processed in the order in
             which they were added to the interface, and the first rule
             matched takes the action (block or pass) and, if given, the tag
             of the rule.  If no source or destination address is specified,
             the rule will match all frames (good for creating a catchall pol-

     rulefile filename
             Load a set of rules from the file filename.

     stp interface
             Enable spanning tree protocol on interface.

     -stp interface
             Disable spanning tree protocol on interface.

     maxage time
             Set the time (in seconds) that a spanning tree protocol configu-
             ration is valid.  Defaults to 20 seconds, minimum of 1, maximum
             of 255.

     fwddelay time
             Set the time (in seconds) before an interface begins forwarding
             packets.  Defaults to 15 seconds, minimum of 1, maximum of 255.

     hellotime time
             Set the time (in seconds) between broadcasting spanning tree pro-
             tocol configuration packets.  Defaults to 2 seconds, minimum of
             1, maximum of 255.

     priority num
             Set the spanning priority of this bridge to num.  Defaults to
             32768, minimum of 0, maximum of 65535.

     ifpriority interface num
             Set the spanning tree priority of interface to num.  Defaults to
             128, minimum of 0, maximum of 255.

     ifcost interface num
             Set the spanning tree path cost of interface to num.  Defaults to
             55, minimum of 1, maximum of 65535.

     # brconfig bridge0 add rl0 add xl0 up
               Add the Ethernet interfaces rl0 and xl0 to the bridge bridge0,
               and start the bridge forwarding packets.

     # brconfig bridge0
               Retrieve a list of interfaces that are members of bridge0, and
               the addresses learned by the bridge.

     # brconfig bridge0 down
               Stop bridge0 from forwarding packets.

     # brconfig bridge0 delete xl0
               Remove the interface xl0 from the bridge bridge0.

     # brconfig bridge0 flush
               Flush all dynamically learned addresses from the address cache.

     # brconfig bridge0 flushall
               Remove all addresses, including static addresses, from the ad-
               dress cache.

     # brconfig bridge0 -learn xl0 static xl0 8:0:20:1e:2f:2b

     # brconfig bridge0 -discover xl0
               The examples above mark the xl0 interface so that it will not
               learn addresses and adds a static entry for the host
               8:0:20:1e:2f:2b on the xl0 segment.  Finally, xl0 is marked so
               that it will not receive packets with destinations not found in
               the address cache of bridge0.  This setup is the most secure,
               and means that bogus MAC addresses seen by the xl0 side of the
               bridge will not be propagated to the rest of the network.  Al-
               so, no packets will be sent on xl0 segment by the bridge unless
               they are broadcast packets or are for 8:0:20:1e:2f:2b.

     # brconfig bridge0 rule pass in on fxp0 src 0:1:2:3:4:5 dst 5:4:3:2:1:0

     # brconfig bridge0 rule pass out on fxp0 src 5:4:3:2:1:0 dst 0:1:2:3:4:5

     # brconfig bridge0 rule block in on fxp0

     # brconfig bridge0 rule block out on fxp0
               The above commands will set up a filter so that 0:1:2:3:4:5 can
               send frames through fxp0 only to 5:4:3:2:1, and 5:4:3:2:1:0 can
               return frames through fxp0 to 0:1:2:3:4:5.  All other traffic
               trying to go into and be sent from fxp0 will be blocked.

     # brconfig bridge0 rule pass in on fxp0 src 9:8:7:6:5:4 tag boss

     # brconfig bridge0 rule pass out on fxp0 dst 9:8:7:6:5:4 tag boss
               These commands will tag packets from and to 9:8:7:6:5:4 on fxp0
               so that pf(4) can refer to them using the tagged directive:

                     pass tagged boss keep state queue q_med

     The bridge can also be used to tunnel ethernet frames over IPv4 or IPv6
     by using the gif(4) interface.  In addition to adding Ethernet inter-
     faces, one or more gif(4), interfaces are added as members of the bridge.
     Ethernet frames sent through the gif(4) interfaces are encapsulated in-
     side ip(4) datagrams and sent across the network to another bridge, which
     decapsulates the datagram and then processes the resulting Ethernet frame
     as if it had originated on a normal Ethernet interface.  This effectively
     allows a layer-2 network to be extended from one point to another, possi-
     bly through the Internet.  This mechanism may be used in conjunction with
     IPsec, by specifying the appropriate IPsec flows between the two bridges.
     To only protect the bridge traffic between the two bridges, the transport
     protocol 97 (etherip) selector may be used in ipsecadm(8) or isakmpd(8).
     Otherwise, the ethernet frames will be sent in the clear between the two

     For example, given two physically separate Ethernet networks, the bridge
     can be used as follows to make them appear as the same local area net-
     work.  If bridge1 on network1 has the external IP address on
     fxp0, bridge2 on network2 has the external IP address on fxp0,
     and both bridges have fxp1 on their internal network (network1 and net-
     work2, respectively), the following configuration can be used to bridge
     network1 and network2.

     Add the encapsulation interface and internal ethernet interface to bridge

     # brconfig bridge0 add gif0 add fxp1

     Configure the gif0 interface:

     (on bridge 1) # ifconfig gif0 giftunnel
     (on bridge 2) # ifconfig gif0 giftunnel

     Create Security Associations (SAs) between the external IP address of
     each bridge:

     # ipsecadm new esp -spi 4242 -dst -src \
             -enc 3des -auth md5 -keyfile keyfile1 -authkeyfile authkeyfile1

     # ipsecadm new esp -spi 4243 -dst -src \
             -enc 3des -auth md5 -keyfile keyfile2 -authkeyfile authkeyfile2

     Set up ingress flows so that traffic is allowed between the two bridges
     for the above associations:

     (on bridge1) # ipsecadm flow -dst -out \
             -transport etherip -require \
     (on bridge2) # ipsecadm flow -dst -out \
             -transport etherip -require \

     Bring up the internal interface (if not already up) and encapsulation in-

     # ifconfig fxp1 up
     # ifconfig gif0 up

     Finally, bring the bridge interface up and allow it to start processing

     # brconfig bridge0 up

     The internal interface, ie. fxp1, on each bridge need not have an IP ad-
     dress; the bridge can function without it.

     Note:  It is possible to put all the following commands in the host-
     name.if(5) and bridgename.if(5) files, using the ! operator.

     The bridge has support for 802.1D Spanning Tree Protocol (STP), which can
     be used to detect and remove loops in a network topology.  Using the stp
     or -stp commands to brconfig STP can be enabled or disabled on each port.
     STP will not work on gif(4) members because they lack a hardware MAC ad-

     The bridge can have interfaces added to it as span ports.  Span ports
     transmit a copy of every frame received by the bridge.  This is most use-
     ful for snooping a bridged network passively on another host connected to
     one of the span ports of the bridge.  Span ports cannot be bridge mem-
     bers, instead the addspan and delspan commands are used to add and delete
     span ports to and from a bridge.

     bridge(4), gif(4), ip(4), ipsec(4), bridgename.if(5), ifconfig(8),
     ipsecadm(8), isakmpd(8)

     The brconfig command first appeared in OpenBSD 2.5.

     The brconfig command and the bridge(4) kernel interface were written by
     Jason L. Wright <> as part of an undergraduate indepen-
     dent study at the University of North Carolina at Greensboro.

     There are some rather special network interface chipsets which will not
     work in a bridge configuration.  Some chipsets have serious flaws when
     running in promiscuous mode, like the TI ThunderLAN (see tl(4)), which
     receives its own transmissions (this renders the address learning cache
     useless).  Most other chipsets work fine though.

OpenBSD 3.4                    February 26, 1999                             6


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