BRCONFIG(8) BSD 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 interface-name
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,
even 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.
flushall
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
interface.
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
interface'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 IP multicast packets from being for-
warded by the bridge.
-link0 Clear the link0 flag on the bridge interface.
link1 Setting this flag stops all non-IP multicast packets from being
forwarded 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 as-
sociations (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 protect-
ing (i.e., they need to have the appropriate authentication and
authorization material, such as keys and certificates, to imper-
sonate 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 syntax
to those in pf.conf(5). Rules can be used to selectively block or
pass frames based on Ethernet MAC addresses. 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-
icy).
rulefile filename
Load a set of rules from the file filename.
rules interface
Display the active filtering rules in use on the given interface.
stp interface
Enable spanning tree protocol on interface.
-stp interface
Disable spanning tree protocol on interface. This is the default
for interfaces added to the bridge.
maxage time
Set the time (in seconds) that a spanning tree protocol confi-
guration 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.
Create a bridge pseudo network device:
# ifconfig bridge0 create
Add the Ethernet interfaces rl0 and xl0 to the bridge bridge0, and have
the bridge start forwarding packets:
# brconfig bridge0 add rl0 add xl0 up
Retrieve a list of interfaces that are members of bridge0, and the ad-
dresses learned by the bridge:
# brconfig bridge0
Stop bridge0 from forwarding packets:
# brconfig bridge0 down
Remove the interface xl0 from the bridge bridge0:
# brconfig bridge0 delete xl0
Flush all dynamically learned addresses from the address cache:
# brconfig bridge0 flush
Remove all addresses, including static addresses, from the address cache:
# brconfig bridge0 flushall
The following commands mark the xl0 interface so that it will not learn
addresses and add 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. Also, no
packets will be sent onto the xl0 segment by the bridge unless they are
broadcast packets or are destined for 8:0:20:1e:2f:2b.
# brconfig bridge0 -learn xl0 static xl0 8:0:20:1e:2f:2b
# brconfig bridge0 -discover xl0
The following commands will set up a filter so that 0:1:2:3:4:5 can send
frames through fxp0 only to 5:4:3:2:1:0, and 5:4:3:2:1:0 can return
frames through fxp0 only to 0:1:2:3:4:5. All other traffic trying to go
into or be sent from fxp0 will be blocked.
# 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 following 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:
# 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
An example pf.conf(5) rule using this tag is:
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 interfaces,
one or more gif(4), interfaces are added as members of the bridge. Ether-
net frames sent through the gif(4) interfaces are encapsulated inside
ip(4) datagrams and sent across the network to another bridge, which de-
capsulates 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
bridges.
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 1.2.3.4 on fxp0,
bridge2 on network2 has the external IP address 4.3.2.1 on fxp0, and both
bridges have fxp1 on their internal network (network1 and network2,
respectively), the following configuration can be used to bridge network1
and network2.
Add the encapsulation interface and internal Ethernet interface to the
bridge interface:
# brconfig bridge0 add gif0 add fxp1
Create and configure the gif0 interface:
(on bridge 1) # ifconfig gif0 create
(on bridge 1) # ifconfig gif0 tunnel 1.2.3.4 4.3.2.1
(on bridge 2) # ifconfig gif0 create
(on bridge 2) # ifconfig gif0 tunnel 4.3.2.1 1.2.3.4
Create Security Associations (SAs) between the external IP address of
each bridge:
# ipsecadm new esp -spi 4242 -dst 4.3.2.1 -src 1.2.3.4 -enc 3des \
-auth md5 -keyfile keyfile1 -authkeyfile authkeyfile1
# ipsecadm new esp -spi 4243 -dst 1.2.3.4 -src 4.3.2.1 -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 4.3.2.1 -out \
-transport etherip -require -addr 1.2.3.4/32 4.3.2.1/32
(on bridge2) # ipsecadm flow -dst 1.2.3.4 -out \
-transport etherip -require -addr 4.3.2.1/32 1.2.3.4/32
Bring up the internal interface (if not already up) and encapsulation in-
terface:
# ifconfig fxp1 up
# ifconfig gif0 up
Finally, bring the bridge interface up and allow it to start processing
frames:
# brconfig bridge0 up
The internal interface, i.e., fxp1, on each bridge need not have an IP
address; the bridge can function without it.
Note: It is possible to put the above commands in the hostname.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 address.
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 members;
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), pf(4), bridgename.if(5), pf.conf(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 <jason@thought.net> 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.
MirBSD #10-current February 26, 1999 5