path: root/share/man/man4/bridge.4

.\"	$OpenBSD: bridge.4,v 1.70 2013/07/16 16:05:48 schwarze Exp $
.\"
.\" Copyright (c) 1999-2001 Jason L. Wright (jason@thought.net)
.\" All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
.\" WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
.\" DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT,
.\" INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
.\" (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
.\" SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
.\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
.\" ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
.\" POSSIBILITY OF SUCH DAMAGE.
.\"
.Dd $Mdocdate: July 16 2013 $
.Dt BRIDGE 4
.Os
.Sh NAME
.Nm bridge
.Nd Ethernet bridge interface
.Sh SYNOPSIS
.Cd "pseudo-device bridge"
.Pp
.Fd #include <sys/types.h>
.Fd #include <net/if.h>
.Fd #include <netinet/in.h>
.Fd #include <netinet/if_ether.h>
.Fd #include <net/if_bridge.h>
.Sh DESCRIPTION
The
.Nm
device creates a logical link between two or more Ethernet interfaces or
encapsulation interfaces (see
.Xr gif 4 ) .
This link between the interfaces selectively forwards frames from
each interface on the bridge to every other interface on the bridge.
A bridge can serve several services, including isolation of traffic between
sets of machines so that traffic local to one set of machines is not
available on the wire of another set of machines, and it can act as
a transparent filter for
.Xr ip 4
datagrams.
.Pp
A
.Nm
interface can be created at runtime using the
.Ic ifconfig bridge Ns Ar N Ic create
command or by setting up a
.Xr hostname.if 5
configuration file for
.Xr netstart 8 .
.Pp
The bridges provided by this interface are learning bridges with
filtering; see
.Xr pf 4 .
In general a bridge works like a hub, forwarding traffic from one interface
to another.
It differs from a hub in that it will "learn" which machines
are on each of its attached segments by actively listening to
incoming traffic and examining the headers of each frame.
A table is built containing the MAC address and segment to which the
MAC address is attached.
This allows a bridge to be more selective about what it forwards,
which can be used to reduce traffic on a set of segments and also to provide
an IP firewall without changing the topology of the network.
.Pp
The algorithm works as follows by default, but can be modified via
.Xr ioctl 2
or the utility
.Xr ifconfig 8 .
When a frame comes in, the origin segment and the source address are
recorded.
If the bridge has no knowledge about where the destination is to be found,
the bridge will forward the frame to all attached segments.
If the destination is known to be on a different segment from its origin, the
bridge will forward the packet only to the destination segment.
If the destination is on the same segment as the origin segment, the bridge
will drop the packet because the receiver has already had a chance to see
the frame.
Before forwarding a frame, the bridge will check to see if the packet
contains an
.Xr ip 4
or
.Xr ip6 4
datagram; if so, the datagram is run through the
pf interface so that it can be filtered.
See the
.Sx NOTES
section for details.
.Sh SPANNING TREE
The bridge has support for 802.1D-2004 Spanning Tree Protocol (STP),
which can be used to detect and remove loops in a network topology.
Using the
.Cm stp
or
.Cm -stp
commands
to
.Nm ,
STP can be enabled or disabled on each port.
.Pp
The bridge will use the Rapid Spanning Tree Protocol (RSTP) by default
to allow rapid transitions to the forwarding state.
The
.Cm proto
command to
.Nm
can be used to force operation in the common Spanning Tree Protocol
without rapid state transitions.
Note that RSTP will be compatible with remote bridges running common STP.
.Pp
STP will not work on
.Xr gif 4
members because they lack a hardware MAC address.
.Sh SPAN PORTS
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 useful 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
.Cm addspan
and
.Cm delspan
commands are used to add and delete span ports to and from a bridge.
.Sh IOCTLS
A
.Nm
interface responds to all of the
.Xr ioctl 2
calls specific to other interfaces listed in
.Xr netintro 4 .
The following
.Xr ioctl 2
calls are specific to
.Nm
devices.
They are defined in
.Aq Pa sys/sockio.h .
.Bl -tag -width Ds
.It Dv SIOCBRDGIFS Fa "struct ifbifconf *"
Retrieve member interface list from a bridge.
This request takes an
.Vt ifbifconf
structure (see below) as a value-result parameter.
The
.Va ifbic_len
field should be initially set to the size of the buffer
pointed to by
.Va ifbic_buf .
On return it will contain the length, in bytes, of the configuration
list.
.Pp
Alternatively, if the
.Va ifbic_len
passed in is set to 0,
.Dv SIOCBRDGIFS
will set
.Va ifbic_len
to the size that
.Va ifbic_buf
needs to be to fit the entire configuration list,
and will not fill in the other parameters.
This is useful for determining the exact size that
.Va ifbic_buf
needs to be in advance.
.Pp
The argument structure is defined as follows:
.Bd -literal
struct ifbreq {
	char	  ifbr_name[IFNAMSIZ];	 /* bridge ifs name */
	char	  ifbr_ifsname[IFNAMSIZ];/* member ifs name */
	u_int32_t ifbr_ifsflags;  /* member ifs flags */
	u_int8_t  ifbr_state;	  /* member stp state */
	u_int8_t  ifbr_priority;  /* member stp priority */
	u_int8_t  ifbr_portno;	  /* member port number */
	u_int32_t ifbr_path_cost; /* member stp path cost */
};

/* ifbr_ifsflags flags about interfaces */
#define	IFBIF_LEARNING	 0x0001 /* ifs can learn */
#define	IFBIF_DISCOVER	 0x0002 /* sends packets w/unknown dst */
#define	IFBIF_BLOCKNONIP 0x0004 /* ifs blocks non-IP/ARP in/out */
#define	IFBIF_STP	 0x0008 /* participate in spanning tree*/
#define	IFBIF_SPAN	 0x0100 /* ifs is a span port (ro) */
#define	IFBIF_RO_MASK	 0xff00 /* read only bits */

struct ifbifconf {
	char	  ifbic_name[IFNAMSIZ];	/* bridge ifs name */
	u_int32_t ifbic_len;		/* buffer size */
	union {
		caddr_t	ifbicu_buf;
		struct	ifbreq *ifbicu_req;
	} ifbic_ifbicu;
#define	ifbic_buf	ifbic_ifbicu.ifbicu_buf
#define	ifbic_req	ifbic_ifbicu.ifbicu_req
};
.Ed
.It Dv SIOCBRDGADD Fa "struct ifbreq *"
Add the interface named in
.Va ifbr_ifsname
to the bridge named in
.Va ifbr_name .
.It Dv SIOCBRDGDEL Fa "struct ifbreq *"
Delete the interface named in
.Va ifbr_ifsname
from the bridge named in
.Va ifbr_name .
.It Dv SIOCBRDGADDS Fa "struct ifbreq *"
Add the interface named in
.Va ifbr_ifsname
as a span port to the bridge named in
.Va ifbr_name .
.It Dv SIOCBRDGDELS Fa "struct ifbreq *"
Delete the interface named in
.Va ifbr_ifsname
from the list of span ports of the bridge named in
.Va ifbr_name .
.It Dv SIOCBRDGSIFFLGS Fa "struct ifbreq *"
Set the bridge member interface flags for the interface named in
.Va ifbr_ifsname
attached to the bridge
.Va ifbr_name .
If the flag
.Dv IFBIF_LEARNING
is set on an interface, source addresses from frames received on the
interface are recorded in the address cache.
If the flag
.Dv IFBIF_DISCOVER
is set, the interface will receive packets destined for unknown
destinations, otherwise a frame that has a destination not found
in the address cache is not forwarded to this interface.
The default for newly added interfaces has both flags set.
If the flag
.Dv IFBIF_BLOCKNONIP
is set, only
.Xr ip 4 ,
.Xr ip6 4 ,
.Xr arp 4 ,
and
Reverse ARP packets will be bridged from and to the interface.
.It Dv SIOCBRDGGIFFLGS Fa "struct ifbreq *"
Retrieve the bridge member interface flags for the interface named in
.Va ifbr_ifsname
attached to the bridge
.Va ifbr_name .
.It Dv SIOCBRDGRTS Fa "struct ifbaconf *"
Retrieve the address cache of the bridge named in
.Va ifbac_name .
This request takes an
.Vt ifbaconf
structure (see below) as a value-result parameter.
The
.Va ifbac_len
field should be initially set to the size of the buffer pointed to by
.Va ifbac_buf .
On return, it will contain the length, in bytes, of the configuration list.
.Pp
Alternatively, if the
.Va ifbac_len
passed in is set to 0,
.Dv SIOCBRDGRTS
will set it to the size that
.Va ifbac_buf
needs to be to fit the entire configuration list, and will not fill in the other
parameters.
As with
.Dv SIOCBRDGIFS ,
this is useful for determining the exact size that
.Va ifbac_buf
needs to be in advance.
.Pp
The argument structure is defined as follows:
.Bd -literal
struct ifbareq {
	char	 ifba_name[IFNAMSIZ];	/* bridge name */
	char	 ifba_ifsname[IFNAMSIZ];/* destination ifs */
	u_int8_t ifba_age;		/* address age */
	u_int8_t ifba_flags;		/* address flags */
	struct ether_addr ifba_dst;	/* destination addr */
};

#define	IFBAF_TYPEMASK	0x03		/* address type mask */
#define	IFBAF_DYNAMIC	0x00		/* dynamically learned */
#define	IFBAF_STATIC	0x01		/* static address */

struct ifbaconf {
	char	  ifbac_name[IFNAMSIZ];	/* bridge ifs name */
	u_int32_t ifbac_len;		/* buffer size */
	union {
		caddr_t	ifbacu_buf;	/* buffer */
		struct ifbareq *ifbacu_req; /* request pointer */
	} ifbac_ifbacu;
#define	ifbac_buf	ifbac_ifbacu.ifbacu_buf
#define	ifbac_req	ifbac_ifbacu.ifbacu_req
};
.Ed
.Pp
Address cache entries with the type set to
.Dv IFBAF_DYNAMIC
in
.Va ifba_flags
are entries learned by the bridge.
Entries with the type set to
.Dv IFBAF_STATIC
are manually added entries.
.It Dv SIOCBRDGSADDR Fa "struct ifbareq *"
Add an entry, manually, to the address cache for the bridge named in
.Va ifba_name .
The address and its associated interface and flags are set in the
.Va ifba_dst ,
.Va ifba_ifsname ,
and
.Va ifba_flags
fields, respectively.
.It Dv SIOCBRDGDADDR Fa "struct ifbareq *"
Delete an entry from the address cache of the bridge named in
.Va ifba_name .
Entries are deleted strictly based on the address field
.Va ifba_dst .
.It Dv SIOCBRDGFLUSH Fa "struct ifbreq *"
Flush addresses from the cache.
.Va ifbr_name
contains the name of the bridge device, and
.Va ifbr_ifsflags
should be set to
.Dv IFBF_FLUSHALL
to flush all addresses from the cache or
.Dv IFBF_FLUSHDYN
to flush only the dynamically learned addresses from the cache.
.It Dv SIOCBRDGSCACHE Fa "struct ifbrparam *"
Set the maximum address cache size for the bridge named in
.Va ifbrp_name
to
.Va ifbrp_csize
entries.
.Pp
The argument structure is as follows:
.Bd -literal
struct ifbrparam {
	char		  ifbrp_name[IFNAMSIZ];
	union {
		u_int32_t ifbrpu_csize;	    /* cache size */
		int	  ifbrpu_ctime;	    /* cache time */
		u_int16_t ifbrpu_prio;	    /* bridge priority */
		u_int8_t  ifbrpu_hellotime; /* hello time */
		u_int8_t  ifbrpu_fwddelay;  /* fwd delay */
		u_int8_t  ifbrpu_maxage;    /* max age */
	} ifbrp_ifbrpu;
};
#define	ifbrp_csize	ifbrp_ifbrpu.ifbrpu_csize
#define	ifbrp_ctime	ifbrp_ifbrpu.ifbrpu_ctime
#define	ifbrp_prio	ifbrp_ifbrpu.ifbrpu_prio
#define	ifbrp_hellotime	ifbrp_ifbrpu.ifbrpu_hellotime
#define	ifbrp_fwddelay	ifbrp_ifbrpu.ifbrpu_fwddelay
#define	ifbrp_maxage	ifbrp_ifbrpu.ifbrpu_maxage
.Ed
.Pp
Note that the
.Va ifbrp_ctime , ifbrp_hellotime , ifbrp_fwddelay
and
.Va ifbrp_maxage
fields are in seconds.
.It Dv SIOCBRDGGCACHE Fa "struct ifbrparam *"
Retrieve the maximum size of the address cache for the bridge
.Va ifbrp_name .
.It Dv SIOCBRDGSTO Fa "struct ifbrparam *"
Set the time, in seconds, for how long addresses which have not been
seen on the network (i.e., have not transmitted a packet) will remain in
the cache to the value
.Va ifbrp_ctime .
If the time is set to zero, no aging is performed on the address cache.
.It Dv SIOCBRDGGTO Fa "struct ifbrparam *"
Retrieve the address cache expiration time (see above).
.It Dv SIOCBRDGARL Fa "struct ifbrlreq *"
Add an Ethernet address filtering rule to the bridge on a specific interface.
.Va ifbr_name
contains the name of the bridge device, and
.Va ifbr_ifsname
contains the name of the bridge member interface.
.Pp
Rules are applied in the order in which they were added to the bridge,
and the first matching rule's action parameter determines the fate of
the packet.
The
.Va ifbr_action
field is one of
.Dv BRL_ACTION_PASS
or
.Dv BRL_ACTION_BLOCK ,
to pass or block matching frames, respectively.
The
.Va ifbr_flags
field specifies whether the rule should match on input, output, or both
by using the flags
.Dv BRL_FLAG_IN
and
.Dv BRL_FLAG_OUT .
At least one of these flags must be set.
.Pp
The
.Va ifbr_flags
field
also specifies whether either (or both) of the source and destination
addresses should be matched by using the
.Dv BRL_FLAG_SRCVALID
and
.Dv BRL_FLAG_DSTVALID
flags.
The
.Va ifbr_src
field is the source address that triggers the rule (only considered if
.Va ifbr_flags
has the
.Dv BRL_FLAG_SRCVALID
bit set).
The
.Va ifbr_src
field is the destination address that triggers the rule (only considered if
.Va ifbr_flags
has the
.Dv BRL_FLAG_DSTVALID
bit set).
If neither bit is set, the rule matches all frames.
.Pp
The argument structure is as follows:
.Bd -literal
struct ifbrlreq {
	char	 ifbr_name[IFNAMSIZ];	 /* bridge ifs name */
	char	 ifbr_ifsname[IFNAMSIZ]; /* member ifs name */
	u_int8_t ifbr_action;		 /* disposition */
	u_int8_t ifbr_flags;		 /* flags */
	struct ether_addr ifbr_src;	 /* source mac */
	struct ether_addr ifbr_dst;	 /* destination mac */
	char	 ifbr_tagname[PF_TAG_NAME_SIZE]; /* pf tagname */
};
#define	BRL_ACTION_BLOCK	0x01	 /* block frame */
#define	BRL_ACTION_PASS		0x02	 /* pass frame */
#define	BRL_FLAG_IN		0x08	 /* input rule */
#define	BRL_FLAG_OUT		0x04	 /* output rule */
#define	BRL_FLAG_SRCVALID	0x02	 /* src valid */
#define	BRL_FLAG_DSTVALID	0x01	 /* dst valid */
.Ed
.It Dv SIOCBRDGFRL Fa "struct ifbrlreq *"
Remove all filtering rules from a bridge interface member.
.Va ifbr_name
contains the name of the bridge device, and
.Va ifbr_ifsname
contains the name of the bridge member interface.
.It Dv SIOCBRDGGRL Fa "struct ifbrlconf *"
Retrieve all of the rules from the bridge,
.Va ifbrl_name ,
for the member interface,
.Va ifbrl_ifsname .
This request takes an
.Vt ifbrlconf
structure (see below) as a value-result parameter.
The
.Va ifbrl_len
field should be initially set to the size of the buffer pointed to by
.Va ifbrl_buf .
On return, it will contain the length, in bytes, of the configuration list.
.Pp
Alternatively, if the
.Va ifbrl_len
passed in is set to 0,
.Dv SIOCBRDGGRL
will set it to the size that
.Va ifbrl_buf
needs to be to fit the entire configuration list, and will not fill in the other
parameters.
As with
.Dv SIOCBRDGIFS ,
this is useful for determining the exact size that
.Va ifbrl_buf
needs to be in advance.
.Pp
The argument structure is defined as follows:
.Bd -literal
struct ifbrlconf {
	char	  ifbrl_name[IFNAMSIZ];	   /* bridge ifs name */
	char	  ifbrl_ifsname[IFNAMSIZ]; /* member ifs name */
	u_int32_t ifbrl_len;		   /* buffer size */
	union {
		caddr_t	ifbrlu_buf;
		struct	ifbrlreq *ifbrlu_req;
	} ifbrl_ifbrlu;
#define	ifbrl_buf ifbrl_ifbrlu.ifbrlu_buf
#define	ifbrl_req ifbrl_ifbrlu.ifbrlu_req
};
.Ed
.\" .It Dv SIOCBRDGGSIFS Fa "struct ifbreq *"
.It Dv SIOCBRDGGPRI Fa "struct ifbrparam *"
Retrieve the Spanning Tree Protocol (STP) priority parameter of the bridge into
the
.Va ifbrp_prio
field.
.It Dv SIOCBRDGSPRI Fa "struct ifbrparam *"
Set the STP priority parameter of the bridge to the value in
.Va ifbrp_prio .
.It Dv SIOCBRDGGHT Fa "struct ifbrparam *"
Retrieve the STP hello time parameter, in seconds, of the bridge into the
.Va ifbrp_hellotime
field.
.It Dv SIOCBRDGSHT Fa "struct ifbrparam *"
Set the STP hello time parameter, in seconds, of the bridge to the value in
.Va ifbrp_hellotime .
The value in
.Va ifbrp_hellotime
cannot be zero.
.It Dv SIOCBRDGGFD Fa "struct ifbrparam *"
Retrieve the STP forward delay parameter, in seconds, of the bridge into the
.Va ifbrp_fwddelay
field.
.It Dv SIOCBRDGSFD Fa "struct ifbrparam *"
Set the STP forward delay parameter, in seconds, of the bridge to the value in
.Va ifbrp_fwddelay .
The value in
.Va ifbrp_fwddelay
cannot be zero.
.It Dv SIOCBRDGGMA Fa "struct ifbrparam *"
Retrieve the STP maximum age parameter, in seconds, of the bridge into the
.Va ifbrp_maxage
field.
.It Dv SIOCBRDGSMA Fa "struct ifbrparam *"
Set the STP maximum age parameter, in seconds, of the bridge to the value in
.Va ifbrp_maxage .
The value in
.Va ifbrp_maxage
cannot be zero.
.It Dv SIOCBRDGSIFPRIO Fa "struct ifbreq *"
Set the STP priority parameter of the interface named in
.Va ifbr_ifsname
to the value in
.Va ifbr_priority .
.It Dv SIOCBRDGSIFCOST Fa "struct ifbreq *"
Set the STP cost parameter of the interface named in
.Va ifbr_ifsname
to the value in
.Va ifbr_path_cost .
The value in
.Va ifbr_path_cost
must be greater than or equal to one.
.El
.Sh ERRORS
If the
.Xr ioctl 2
call fails,
.Xr errno 2
is set to one of the following values:
.Bl -tag -width Er
.It Bq Er ENOENT
For an add request, this means that the named interface is not configured
into the system.
For a delete operation, it means that the named interface is not a member
of the bridge.
For an address cache deletion, the address was not found in the table.
.It Bq Er ENOMEM
Memory could not be allocated for an interface or cache entry
to be added to the bridge.
.It Bq Er EEXIST
The named interface is already a member of the bridge.
.It Bq Er EBUSY
The named interface is already a member of another bridge.
.It Bq Er EINVAL
The named interface is not an Ethernet interface, or an invalid ioctl
was performed on the bridge.
.It Bq Er ENETDOWN
Address cache operation (flush, add, or delete) on a bridge that is
in the down state.
.It Bq Er EPERM
Super-user privilege is required to add and delete interfaces to and from
bridges and to set the bridge interface flags.
.It Bq Er EFAULT
The buffer used in a
.Dv SIOCBRDGIFS
or
.Dv SIOCBRDGRTS
request points outside of the process's allocated address space.
.It Bq Er ESRCH
No such member interface in the bridge.
.El
.Sh NOTES
Bridged packets pass through
.Xr pf 4
filters once as input on the receiving interface and once
as output on all interfaces on which they are forwarded.
In order to pass through the bridge packets must pass
any
.Ar in
rules on the input and any
.Ar out
rules on the output interface.
Packets may be blocked either entering or leaving the bridge.
.Pp
Return packets generated by pf itself are not routed using the
kernel routing table.
Instead, pf will send these replies back to the same Ethernet
address that the original packet came from.
This applies to rules with
.Ic return ,
.Ic return-rst ,
.Ic return-icmp ,
.Ic return-icmp6 ,
or
.Ic synproxy
defined.
At the moment, only
.Ic return-rst
on IPv4 is implemented and the other packet generating rules
are unsupported.
.Pp
If an IP packet is too large for the outgoing interface, the bridge
will perform IP fragmentation.
This can happen when bridge members
have different MTUs or when IP fragments are reassembled by pf.
Non-IP packets which are too large for the outgoing interface will be
dropped.
.Pp
If the
.Dv IFF_LINK2
flag is set on the
.Nm
interface, the bridge will also perform transparent
.Xr ipsec 4
processing on the packets (encrypt or decrypt them), according to the
policies set with the
.Xr ipsecctl 8
command by the administrator.
If appropriate security associations (SAs) do not exist, any key
management daemons such as
.Xr isakmpd 8
that are running on the bridge will be invoked to establish the
necessary SAs.
These daemons have to be configured as if they were running on the
host whose traffic they are protecting (i.e., they need to have the
appropriate authentication and authorization material, such as keys
and certificates, to impersonate the protected host(s)).
.Sh SEE ALSO
.Xr errno 2 ,
.Xr ioctl 2 ,
.Xr arp 4 ,
.Xr gif 4 ,
.Xr ip 4 ,
.Xr ip6 4 ,
.Xr ipsec 4 ,
.Xr netintro 4 ,
.Xr pf 4 ,
.Xr vether 4 ,
.Xr hostname.if 5 ,
.Xr ifconfig 8 ,
.Xr ipsecctl 8 ,
.Xr isakmpd 8 ,
.Xr netstart 8
.Sh HISTORY
The
.Nm
kernel interface first appeared in
.Ox 2.5 .
.Sh AUTHORS
The
.Nm
kernel interface was written by
.An Jason L. Wright Aq Mt jason@thought.net
as part of an undergraduate independent study at the
University of North Carolina at Greensboro.
.Pp
Support for rapid spanning tree reconfigurations (RSTP) was added by
.An Andrew Thompson Aq Mt thompsa@freebsd.org
and ported to
.Ox
by
.An Reyk Floeter Aq Mt reyk@openbsd.org .
.Sh BUGS
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
.Xr tl 4 ) ,
which receives its own transmissions (this renders the address learning
cache useless).
Most other chipsets work fine though.