519 lines
15 KiB
Groff
519 lines
15 KiB
Groff
.\" $NetBSD: bridge.4,v 1.5 2004/01/31 20:14:11 jdc Exp $
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.\"
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.\" Copyright 2001 Wasabi Systems, Inc.
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.\" All rights reserved.
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.\"
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.\" Written by Jason R. Thorpe for Wasabi Systems, Inc.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed for the NetBSD Project by
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.\" Wasabi Systems, Inc.
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.\" 4. The name of Wasabi Systems, Inc. may not be used to endorse
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.\" or promote products derived from this software without specific prior
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.\" written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY WASABI SYSTEMS, INC. ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
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.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL WASABI SYSTEMS, INC
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.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
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.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
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.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
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.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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.\" POSSIBILITY OF SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd July 27, 2013
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.Dt IF_BRIDGE 4
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.Os
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.Sh NAME
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.Nm if_bridge
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.Nd network bridge device
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.Sh SYNOPSIS
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To compile this driver into the kernel,
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place the following line in your
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kernel configuration file:
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.Bd -ragged -offset indent
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.Cd "device if_bridge"
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.Ed
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.Pp
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Alternatively, to load the driver as a
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module at boot time, place the following lines in
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.Xr loader.conf 5 :
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.Bd -literal -offset indent
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if_bridge_load="YES"
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bridgestp_load="YES"
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.Ed
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.Sh DESCRIPTION
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The
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.Nm
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driver creates a logical link between two or more IEEE 802 networks
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that use the same (or
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.Dq "similar enough" )
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framing format.
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For example, it is possible to bridge Ethernet and 802.11 networks together,
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but it is not possible to bridge Ethernet and Token Ring together.
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.Pp
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Each
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.Nm
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interface is created at runtime using interface cloning.
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This is
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most easily done with the
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.Xr ifconfig 8
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.Cm create
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command or using the
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.Va cloned_interfaces
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variable in
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.Xr rc.conf 5 .
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.Pp
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The
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.Nm
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interface randomly chooses a link (MAC) address in the range reserved for
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locally administered addresses when it is created.
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This address is guaranteed to be unique
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.Em only
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across all
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.Nm
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interfaces on the local machine.
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Thus you can theoretically have two bridges on the different machines with
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the same link addresses.
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The address can be changed by assigning the desired link address using
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.Xr ifconfig 8 .
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.Pp
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If
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.Xr sysctl 8
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node
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.Va net.link.bridge.inherit_mac
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has non-zero value, newly created bridge will inherit MAC address
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from its first member instead of choosing random link-level address.
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This will provide more predictable bridge MAC without any
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additional configuration, but currently this feature is known
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to break some L2 protocols, for example PPPoE that is provided
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by
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.Xr ng_pppoe 4
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and
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.Xr ppp 8 .
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Now this feature is considered as experimental and is turned off
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by-default.
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.Pp
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A bridge can be used to provide several services, such as a simple
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802.11-to-Ethernet bridge for wireless hosts, and traffic isolation.
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.Pp
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A bridge works like a switch, forwarding traffic from one interface
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to another.
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Multicast and broadcast packets are always forwarded to all
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interfaces that are part of the bridge.
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For unicast traffic, the bridge learns which MAC addresses are associated
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with which interfaces and will forward the traffic selectively.
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.Pp
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All the bridged member interfaces need to be up in order to pass network traffic.
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These can be enabled using
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.Xr ifconfig 8
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or
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.Va ifconfig_ Ns Ao Ar interface Ac Ns Li ="up"
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in
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.Xr rc.conf 5 .
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.Pp
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The MTU of the first member interface to be added is used as the bridge MTU.
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All additional members are required to have exactly the same value.
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.Pp
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The TXCSUM capability is disabled for any interface added to the bridge, and it
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is restored when the interface is removed again.
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.Pp
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The bridge supports
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.Dq monitor mode ,
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where the packets are discarded after
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.Xr bpf 4
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processing, and are not processed or forwarded further.
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This can be used to multiplex the input of two or more interfaces into a single
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.Xr bpf 4
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stream.
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This is useful for reconstructing the traffic for network taps
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that transmit the RX/TX signals out through two separate interfaces.
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.Sh IPV6 SUPPORT
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.Nm
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supports the
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.Li AF_INET6
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address family on bridge interfaces.
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The following
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.Xr rc.conf 5
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variable configures an IPv6 link-local address on
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.Li bridge0
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interface:
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.Bd -literal -offset indent
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ifconfig_bridge0_ipv6="up"
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.Ed
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.Pp
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or in a more explicit manner:
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.Bd -literal -offset indent
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ifconfig_bridge0_ipv6="inet6 auto_linklocal"
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.Ed
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.Pp
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However, the
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.Li AF_INET6
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address family has a concept of scope zone.
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Bridging multiple interfaces change the zone configuration because
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multiple links are merged to each other and form a new single link
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while the member interfaces still work individually.
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This means each member interface still has a separate link-local scope
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zone and the
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.Nm
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interface has another single,
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aggregated link-local scope zone at the same time.
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This situation is clearly against the description
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.Qq zones of the same scope cannot overlap
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in Section 5,
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RFC 4007.
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Although it works in most cases,
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it can cause some conterintuitive or undesirable behavior in some
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edge cases when both of the
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.Nm
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interface and one of the member interface have an IPv6 address
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and applications use both of them.
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.Pp
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To prevent this situation,
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.Nm
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checks whether a link-local scoped IPv6 address is configured on
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a member interface to be added and the
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.Nm
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interface.
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When the
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.Nm
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interface has IPv6 addresses,
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IPv6 addresses on the member interface will be automatically removed
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before the interface is added.
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.Pp
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This behavior can be disabled by setting
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.Xr sysctl 8
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variable
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.Va net.link.bridge.allow_llz_overlap
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to
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.Li 1 .
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.Pp
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Note that
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.Li ACCEPT_RTADV
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and
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.Li AUTO_LINKLOCAL
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interface flag are not enabled by default on
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.Nm
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interface even when
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.Va net.inet6.ip6.accept_rtadv
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and/or
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.Va net.inet6.ip6.auto_linklocal
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is set to
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.Li 1 .
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.Sh SPANNING TREE
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The
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.Nm
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driver implements the Rapid Spanning Tree Protocol (RSTP or 802.1w) with
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backwards compatibility with the legacy Spanning Tree Protocol (STP).
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Spanning Tree is used to detect and remove loops in a network topology.
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.Pp
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RSTP provides faster spanning tree convergence than legacy STP, the protocol
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will exchange information with neighbouring switches to quickly transition to
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forwarding without creating loops.
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.Pp
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The code will default to RSTP mode but will downgrade any port connected to a
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legacy STP network so is fully backward compatible.
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A bridge can be forced to operate in STP mode without rapid state transitions
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via the
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.Va proto
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command in
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.Xr ifconfig 8 .
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.Pp
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The bridge can log STP port changes to
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.Xr syslog 3
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by enabling the
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.Va net.link.bridge.log_stp
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variable using
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.Xr sysctl 8 .
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.Sh PACKET FILTERING
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Packet filtering can be used with any firewall package that hooks in via the
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.Xr pfil 9
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framework.
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When filtering is enabled, bridged packets will pass through the filter
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inbound on the originating interface, on the bridge interface and outbound on
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the appropriate interfaces.
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Either stage can be disabled.
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The filtering behaviour can be controlled using
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.Xr sysctl 8 :
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.Bl -tag -width ".Va net.link.bridge.pfil_onlyip"
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.It Va net.link.bridge.pfil_onlyip
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Controls the handling of non-IP packets which are not passed to
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.Xr pfil 9 .
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Set to
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.Li 1
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to only allow IP packets to pass (subject to firewall rules), set to
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.Li 0
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to unconditionally pass all non-IP Ethernet frames.
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.It Va net.link.bridge.pfil_member
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Set to
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.Li 1
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to enable filtering on the incoming and outgoing member interfaces, set
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to
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.Li 0
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to disable it.
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.It Va net.link.bridge.pfil_bridge
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Set to
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.Li 1
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to enable filtering on the bridge interface, set
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to
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.Li 0
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to disable it.
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.It Va net.link.bridge.pfil_local_phys
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Set to
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.Li 1
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to additionally filter on the physical interface for locally destined packets.
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Set to
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.Li 0
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to disable this feature.
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.It Va net.link.bridge.ipfw
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Set to
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.Li 1
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to enable layer2 filtering with
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.Xr ipfirewall 4 ,
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set to
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.Li 0
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to disable it.
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This needs to be enabled for
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.Xr dummynet 4
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support.
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When
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.Va ipfw
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is enabled,
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.Va pfil_bridge
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and
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.Va pfil_member
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will be disabled so that IPFW
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is not run twice; these can be re-enabled if desired.
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.It Va net.link.bridge.ipfw_arp
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Set to
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.Li 1
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to enable layer2 ARP filtering with
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.Xr ipfirewall 4 ,
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set to
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.Li 0
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to disable it.
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Requires
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.Va ipfw
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to be enabled.
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.El
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.Pp
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ARP and REVARP packets are forwarded without being filtered and others
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that are not IP nor IPv6 packets are not forwarded when
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.Va pfil_onlyip
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is enabled.
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IPFW can filter Ethernet types using
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.Cm mac-type
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so all packets are passed to
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the filter for processing.
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.Pp
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The packets originating from the bridging host will be seen by
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the filter on the interface that is looked up in the routing
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table.
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.Pp
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The packets destined to the bridging host will be seen by the filter
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on the interface with the MAC address equal to the packet's destination
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MAC.
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There are situations when some of the bridge members are sharing
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the same MAC address (for example the
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.Xr vlan 4
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interfaces: they are currently sharing the
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MAC address of the parent physical interface).
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It is not possible to distinguish between these interfaces using
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their MAC address, excluding the case when the packet's destination
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MAC address is equal to the MAC address of the interface on which
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the packet was entered to the system.
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In this case the filter will see the incoming packet on this
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interface.
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In all other cases the interface seen by the packet filter is chosen
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from the list of bridge members with the same MAC address and the
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result strongly depends on the member addition sequence and the
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actual implementation of
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.Nm .
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It is not recommended to rely on the order chosen by the current
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.Nm
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implementation: it can be changed in the future.
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.Pp
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The previous paragraph is best illustrated with the following
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pictures.
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Let
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.Bl -bullet
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.It
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the MAC address of the incoming packet's destination is
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.Nm nn:nn:nn:nn:nn:nn ,
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.It
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the interface on which packet entered the system is
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.Nm ifX ,
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.It
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.Nm ifX
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MAC address is
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.Nm xx:xx:xx:xx:xx:xx ,
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.It
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there are possibly other bridge members with the same MAC address
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.Nm xx:xx:xx:xx:xx:xx ,
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.It
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the bridge has more than one interface that are sharing the
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same MAC address
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.Nm yy:yy:yy:yy:yy:yy ;
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we will call them
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.Nm vlanY1 ,
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.Nm vlanY2 ,
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etc.
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.El
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.Pp
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Then if the MAC address
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.Nm nn:nn:nn:nn:nn:nn
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is equal to the
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.Nm xx:xx:xx:xx:xx:xx
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then the filter will see the packet on the interface
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.Nm ifX
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no matter if there are any other bridge members carrying the same
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MAC address.
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But if the MAC address
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.Nm nn:nn:nn:nn:nn:nn
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is equal to the
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.Nm yy:yy:yy:yy:yy:yy
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then the interface that will be seen by the filter is one of the
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.Nm vlanYn .
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It is not possible to predict the name of the actual interface
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without the knowledge of the system state and the
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.Nm
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implementation details.
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.Pp
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This problem arises for any bridge members that are sharing the same
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MAC address, not only to the
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.Xr vlan 4
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ones: they we taken just as the example of such situation.
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So if one wants the filter the locally destined packets based on
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their interface name, one should be aware of this implication.
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The described situation will appear at least on the filtering bridges
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that are doing IP-forwarding; in some of such cases it is better
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to assign the IP address only to the
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.Nm
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interface and not to the bridge members.
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Enabling
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.Va net.link.bridge.pfil_local_phys
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will let you do the additional filtering on the physical interface.
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.Sh EXAMPLES
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The following when placed in the file
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.Pa /etc/rc.conf
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will cause a bridge called
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.Dq Li bridge0
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to be created, and will add the interfaces
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.Dq Li wlan0
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and
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.Dq Li fxp0
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to the bridge, and then enable packet forwarding.
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Such a configuration could be used to implement a simple
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802.11-to-Ethernet bridge (assuming the 802.11 interface is
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in ad-hoc mode).
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.Bd -literal -offset indent
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cloned_interfaces="bridge0"
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ifconfig_bridge0="addm wlan0 addm fxp0 up"
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.Ed
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.Pp
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For the bridge to forward packets all member interfaces and the bridge need
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to be up.
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The above example would also require:
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.Bd -literal -offset indent
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create_args_wlan0="wlanmode hostap"
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ifconfig_wlan0="up ssid my_ap mode 11g"
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ifconfig_fxp0="up"
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.Ed
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.Pp
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Consider a system with two 4-port Ethernet boards.
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The following will cause a bridge consisting of all 8 ports with Rapid Spanning
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Tree enabled to be created:
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.Bd -literal -offset indent
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ifconfig bridge0 create
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ifconfig bridge0 \e
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addm fxp0 stp fxp0 \e
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addm fxp1 stp fxp1 \e
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addm fxp2 stp fxp2 \e
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addm fxp3 stp fxp3 \e
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addm fxp4 stp fxp4 \e
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addm fxp5 stp fxp5 \e
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addm fxp6 stp fxp6 \e
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addm fxp7 stp fxp7 \e
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up
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.Ed
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.Pp
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The bridge can be used as a regular host interface at the same time as bridging
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between its member ports.
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In this example, the bridge connects em0 and em1, and will receive its IP
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address through DHCP:
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.Bd -literal -offset indent
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cloned_interfaces="bridge0"
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ifconfig_bridge0="addm em0 addm em1 DHCP"
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ifconfig_em0="up"
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ifconfig_em1="up"
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.Ed
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.Pp
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The bridge can tunnel Ethernet across an IP internet using the EtherIP
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protocol.
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This can be combined with
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.Xr ipsec 4
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to provide an encrypted connection.
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Create a
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.Xr gif 4
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interface and set the local and remote IP addresses for the
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tunnel, these are reversed on the remote bridge.
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.Bd -literal -offset indent
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ifconfig gif0 create
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ifconfig gif0 tunnel 1.2.3.4 5.6.7.8 up
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ifconfig bridge0 create
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ifconfig bridge0 addm fxp0 addm gif0 up
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.Ed
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.Pp
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Note that
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.Fx
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6.1, 6.2, 6.3, 7.0, 7.1, and 7.2 have a bug in the EtherIP protocol.
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For more details and workaround, see
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.Xr gif 4
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manual page.
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.Sh SEE ALSO
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.Xr gif 4 ,
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.Xr ipf 4 ,
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.Xr ipfw 4 ,
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.Xr pf 4 ,
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.Xr ifconfig 8
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.Sh HISTORY
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The
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.Nm
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driver first appeared in
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.Fx 6.0 .
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.Sh AUTHORS
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.An -nosplit
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The
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.Nm bridge
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driver was originally written by
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.An Jason L. Wright Aq Mt jason@thought.net
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as part of an undergraduate independent study at the University of
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North Carolina at Greensboro.
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.Pp
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This version of the
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.Nm
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driver has been heavily modified from the original version by
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.An Jason R. Thorpe Aq Mt thorpej@wasabisystems.com .
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.Pp
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Rapid Spanning Tree Protocol (RSTP) support was added by
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.An Andrew Thompson Aq Mt thompsa@FreeBSD.org .
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.Sh BUGS
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The
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.Nm
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driver currently supports only Ethernet and Ethernet-like (e.g., 802.11)
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network devices, with exactly the same interface MTU size as the bridge device.
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