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IF_BRIDGE(4) | Device Drivers Manual | IF_BRIDGE(4) |
NAME¶
if_bridge
—
network bridge device
SYNOPSIS¶
To compile this driver into the kernel, place the following line in your kernel configuration file:device
if_bridge
if_bridge_load="YES" bridgestp_load="YES"
DESCRIPTION¶
Theif_bridge
driver creates a logical link
between two or more IEEE 802 networks that use the same (or “similar
enough”) framing format. For example, it is possible to bridge Ethernet
and 802.11 networks together, but it is not possible to bridge Ethernet and
Token Ring together.
Each if_bridge
interface is created at
runtime using interface cloning. This is most easily done with the
ifconfig(8)
create
command or using the
cloned_interfaces variable in
rc.conf(5).
The if_bridge
interface randomly chooses a
link (MAC) address in the range reserved for locally administered addresses
when it is created. This address is guaranteed to be unique
only across all
if_bridge
interfaces on the local machine.
Thus you can theoretically have two bridges on the different machines with the
same link addresses. The address can be changed by assigning the desired link
address using ifconfig(8).
If sysctl(8) node
net.link.bridge.inherit_mac has non-zero
value, newly created bridge will inherit MAC address from its first member
instead of choosing random link-level address. This will provide more
predictable bridge MAC without any additional configuration, but currently
this feature is known to break some L2 protocols, for example PPPoE that is
provided by ng_pppoe(4) and
ppp(8). Now this feature is considered as
experimental and is turned off by-default.
A bridge can be used to provide several services, such as a simple
802.11-to-Ethernet bridge for wireless hosts, and traffic isolation.
A bridge works like a switch, forwarding traffic from one interface to another.
Multicast and broadcast packets are always forwarded to all interfaces that
are part of the bridge. For unicast traffic, the bridge learns which MAC
addresses are associated with which interfaces and will forward the traffic
selectively.
All the bridged member interfaces need to be up in order to pass network
traffic. These can be enabled using ifconfig(8)
or
ifconfig_⟨interface⟩="up"
in rc.conf(5).
The MTU of the first member interface to be added is used as the bridge MTU. All
additional members are required to have exactly the same value.
The TXCSUM capability is disabled for any interface added to the bridge, and it
is restored when the interface is removed again.
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.
IPV6 SUPPORT¶
if_bridge
supports the
AF_INET6
address family on bridge interfaces. The
following rc.conf(5) variable configures an IPv6
link-local address on bridge0
interface:
ifconfig_bridge0_ipv6="up"
ifconfig_bridge0_ipv6="inet6 auto_linklocal"
AF_INET6
address family has a concept of
scope zone. Bridging multiple interfaces change the zone configuration because
multiple links are merged to each other and form a new single link while the
member interfaces still work individually. This means each member interface
still has a separate link-local scope zone and the
if_bridge
interface has another single,
aggregated link-local scope zone at the same time. This situation is clearly
against the description “zones of the same scope cannot overlap”
in Section 5, RFC 4007. Although it works in most cases, it can cause some
conterintuitive or undesirable behavior in some edge cases when both of the
if_bridge
interface and one of the member
interface have an IPv6 address and applications use both of them.
To prevent this situation, if_bridge
checks
whether a link-local scoped IPv6 address is configured on a member interface
to be added and the if_bridge
interface.
When the if_bridge
interface has IPv6
addresses, IPv6 addresses on the member interface will be automatically
removed before the interface is added.
This behavior can be disabled by setting sysctl(8)
variable net.link.bridge.allow_llz_overlap to
1
.
Note that ACCEPT_RTADV
and
AUTO_LINKLOCAL
interface flag are not enabled by
default on if_bridge
interface even when
net.inet6.ip6.accept_rtadv and/or
net.inet6.ip6.auto_linklocal is set to
1
.
SPANNING TREE¶
Theif_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 code will default to RSTP mode but will downgrade any port connected to a
legacy STP network so is fully backward compatible. A bridge can be forced to
operate in STP mode without rapid state transitions via the
proto command in
ifconfig(8).
The bridge can log STP port changes to syslog(3) by
enabling the net.link.bridge.log_stp variable
using sysctl(8).
PACKET FILTERING¶
Packet filtering can be used with any firewall package that hooks in via the pfil(9) framework. When 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. The filtering behaviour can be controlled using sysctl(8):- net.link.bridge.pfil_onlyip
- Controls the handling of non-IP packets which are not passed to
pfil(9). Set to
1
to only allow IP packets to pass (subject to firewall rules), set to0
to unconditionally pass all non-IP Ethernet frames. - net.link.bridge.pfil_member
- Set to
1
to enable filtering on the incoming and outgoing member interfaces, set to0
to disable it. - net.link.bridge.pfil_bridge
- Set to
1
to enable filtering on the bridge interface, set to0
to disable it. - net.link.bridge.pfil_local_phys
- Set to
1
to additionally filter on the physical interface for locally destined packets. Set to0
to disable this feature. - net.link.bridge.ipfw
- Set to
1
to enable layer2 filtering with ipfirewall(4), set to0
to disable it. This needs to be enabled for dummynet(4) support. When ipfw is enabled, pfil_bridge and pfil_member will be disabled so that IPFW is not run twice; these can be re-enabled if desired. - net.link.bridge.ipfw_arp
- Set to
1
to enable layer2 ARP filtering with ipfirewall(4), set to0
to disable it. Requires ipfw to be enabled.
mac-type
so all
packets are passed to the filter for processing.
The packets originating from the bridging host will be seen by the filter on the
interface that is looked up in the routing table.
The packets destined to the bridging host will be seen by the filter on the
interface with the MAC address equal to the packet's destination MAC. There
are situations when some of the bridge members are sharing the same MAC
address (for example the vlan(4) interfaces: they
are currently sharing the MAC address of the parent physical interface). It is
not possible to distinguish between these interfaces using their MAC address,
excluding the case when the packet's destination MAC address is equal to the
MAC address of the interface on which the packet was entered to the system. In
this case the filter will see the incoming packet on this interface. In all
other cases the interface seen by the packet filter is chosen from the list of
bridge members with the same MAC address and the result strongly depends on
the member addition sequence and the actual implementation of
if_bridge
. It is not recommended to rely on
the order chosen by the current if_bridge
implementation: it can be changed in the future.
The previous paragraph is best illustrated with the following pictures. Let
- the MAC address of the incoming packet's destination is
nn:nn:nn:nn:nn:nn
, - the interface on which packet entered the system is
ifX
, ifX
MAC address isxx:xx:xx:xx:xx:xx
,- there are possibly other bridge members with the same MAC address
xx:xx:xx:xx:xx:xx
, - the bridge has more than one interface that are sharing the same MAC
address
yy:yy:yy:yy:yy:yy
; we will call themvlanY1
,vlanY2
, etc.
nn:nn:nn:nn:nn:nn
is
equal to the xx:xx:xx:xx:xx:xx
then the
filter will see the packet on the interface
ifX
no matter if there are any other bridge
members carrying the same MAC address. But if the MAC address
nn:nn:nn:nn:nn:nn
is equal to the
yy:yy:yy:yy:yy:yy
then the interface that
will be seen by the filter is one of the
vlanYn
. It is not possible to predict the
name of the actual interface without the knowledge of the system state and the
if_bridge
implementation details.
This problem arises for any bridge members that are sharing the same MAC
address, not only to the vlan(4) ones: they we
taken just as the example of such situation. So if one wants the filter the
locally destined packets based on their interface name, one should be aware of
this implication. The described situation will appear at least on the
filtering bridges that are doing IP-forwarding; in some of such cases it is
better to assign the IP address only to the
if_bridge
interface and not to the bridge
members. Enabling
net.link.bridge.pfil_local_phys will let you
do the additional filtering on the physical interface.
EXAMPLES¶
The following when placed in the file /etc/rc.conf will cause a bridge called “bridge0
” to be created, and will add
the interfaces “wlan0
” and
“fxp0
” to the bridge, and then enable
packet forwarding. Such a configuration could be used to implement a simple
802.11-to-Ethernet bridge (assuming the 802.11 interface is in ad-hoc mode).
cloned_interfaces="bridge0" ifconfig_bridge0="addm wlan0 addm fxp0 up"
create_args_wlan0="wlanmode hostap" ifconfig_wlan0="up ssid my_ap mode 11g" ifconfig_fxp0="up"
ifconfig bridge0 create ifconfig bridge0 \ addm fxp0 stp fxp0 \ addm fxp1 stp fxp1 \ addm fxp2 stp fxp2 \ addm fxp3 stp fxp3 \ addm fxp4 stp fxp4 \ addm fxp5 stp fxp5 \ addm fxp6 stp fxp6 \ addm fxp7 stp fxp7 \ up
cloned_interfaces="bridge0" ifconfig_bridge0="addm em0 addm em1 DHCP" ifconfig_em0="up" ifconfig_em1="up"
ifconfig gif0 create ifconfig gif0 tunnel 1.2.3.4 5.6.7.8 up ifconfig bridge0 create ifconfig bridge0 addm fxp0 addm gif0 up
SEE ALSO¶
gif(4), ipf(4), ipfw(4), pf(4), ifconfig(8)HISTORY¶
Theif_bridge
driver first appeared in
FreeBSD 6.0.
AUTHORS¶
Thebridge
driver was originally written by
Jason L. Wright
⟨jason@thought.net⟩ as part of an undergraduate independent
study at the University of North Carolina at Greensboro.
This version of the if_bridge
driver has been
heavily modified from the original version by
Jason R. Thorpe
⟨thorpej@wasabisystems.com⟩.
Rapid Spanning Tree Protocol (RSTP) support was added by
Andrew Thompson
⟨thompsa@FreeBSD.org⟩.
BUGS¶
Theif_bridge
driver currently supports only
Ethernet and Ethernet-like (e.g., 802.11) network devices, with exactly the
same interface MTU size as the bridge device.July 27, 2013 | Debian |