.\"	$OpenBSD: bridge.4,v 1.45 2002/09/18 07:35:13 deraadt Exp $
.\"
.\" Copyright (c) 1999-2001 Jason L. Wright (jason@thought.net)
.\" All rights reserved.
.\"
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.\"	This product includes software developed by Jason L. Wright
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.Dd February 26, 1999
.Dt BRIDGE 4
.Os
.Sh NAME
.Nm bridge
.Nd Ethernet bridge interface
.Sh SYNOPSIS
.Cd pseudo-device bridge 2
.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
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 brconfig 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
.Xr pf 4
interface so that it can be filtered.
.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 .
.Pp
.Bl -tag -width SIOCBRDGGIFFLGS
.It Dv SIOCBRDGIFS
.Pq Li "struct ifbifconf"
Retrieve member interface list from a bridge.
This request takes an
.Li ifbifconf
structure (see below) as a value-result parameter.
The
.Fa ifbic_len
field should be initially set to the size of the buffer
pointed to by
.Fa ifbic_buf .
On return it will contain the length, in bytes, of the configuration
list.
Alternatively, if the
.Fa ifbic_len
passed in is set to 0,
.Dv SIOCBRDGIFS
will set
.Fa ifbic_len
to the size that
.Fa 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
.Fa ifbic_buf
needs to be in advance.
.Pp
The argument structure is defined as follows:
.Bd -literal -offset indent
struct ifbreq {
	char ifbr_name[IFNAMSIZ];    /* brdg nam */
	char ifbr_ifsname[IFNAMSIZ]; /* if name */
	u_int32_t ifbr_ifsflags;     /* if flags */
};

#define IFBIF_LEARNING  0x1 /* learns addrs */
#define IFBIF_DISCOVER  0x2 /* gets fwd'd pkts */

struct ifbifconf {
	char ifbic_name[IFNAMSIZ]; /* brdg name */
	u_int32_t       ifbic_len; /* buf size */
	union {
		caddr_t ifbicu_buf; /* buffer */
		struct  ifbreq *ifbicu_req;
	} ifbic_ifbicu;
#define ifbic_buf       ifbic_ifbicu.ifbicu_buf
#define ifbic_req       ifbic_ifbicu.ifbicu_req
};
.Ed
.It Dv SIOCBRDGADD
.Pq Li "struct ifbreq"
Add the interface named in
.Fa ifbr_ifsname
to the bridge named in
.Fa ifbr_name .
.It Dv SIOCBRDGDEL
.Pq Li "struct ifbreq"
Delete the interface named in
.Fa ifbr_ifsname
from the bridge named in
.Fa ifbr_name .
.It Dv SIOCBRDGADDS
.Pq Li "struct ifbreq"
Add the interface named in
.Fa ifbr_ifsname
as a span port to the bridge named in
.Fa ifbr_name .
.It Dv SIOCBRDGDELS
.Pq Li "struct ifbreq"
Delete the interface named in
.Fa ifbr_ifsname
from the list of span ports of the bridge named in
.Fa ifbr_name .
.It Dv SIOCBRDGSIFFLGS
.Pq Li "struct ifbreq"
Set the bridge member interface flags for the interface named in
.Fa ifbr_ifsname
attached to the bridge
.Fa 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, packets that are one of
.Xr ip 4 ,
.Xr ip6 4 ,
.Xr arp 4 ,
or
Reverse ARP, will not be bridged from and to the interface.
.It Dv SIOCBRDGGIFFLGS
Retrieve the bridge member interface flags for the interface named in
.Fa ifbr_ifsname
attached to the bridge
.Fa ifbr_name .
.It Dv SIOCBRDGRTS
.Pq Li "struct ifbaconf"
Retrieve the address cache of the bridge named in
.Fa ifbac_name .
This request takes an
.Li ifbaconf
structure (see below) as a value result parameter.
The
.Fa ifbac_len
field should be initially set to the size of the buffer pointed to by
.Fa ifbac_buf .
On return, it will contain the length, in bytes, of the configuration list.
Alternatively, if the
.Fa ifbac_len
passed in is set to 0,
.Dv SIOCBRDGRTS
will set it to the size that
.Fa ifbac_buf
needs to be to fit the entire configuration list and not fill in the other
parameters.
As with
.Dv SIOCBRDGIFS ,
this is useful for determining the exact size that
.Fa ifbac_buf
needs to be in advance.
.Pp
The argument structure is defined as follows:
.Bd -literal -offset indent
struct ifbareq {
	char ifba_name[IFNAMSIZ];   /* brdg nam */
	char ifba_ifsname[IFNAMSIZ];/* dest ifs */
	u_int8_t ifba_age;          /* addr age */
	u_int8_t ifba_flags;        /* addr flag */
	struct ether_addr ifba_dst; /* dst addr */
};

#define IFBAF_TYPEMASK 0x03  /* addr type mask */
#define IFBAF_DYNAMIC  0x00  /* dynamic addr */
#define IFBAF_STATIC   0x01  /* static address */

struct ifbaconf {
	char ifbac_name[IFNAMSIZ]; /* brdg name */
	u_int32_t ifbac_len;       /* buf size */
	union {
		caddr_t ifbacu_buf;     /* buf */
		struct ifbareq *ifbacu_req;
	} ifbac_ifbacu;
#define	ifbac_buf       ifbac_ifbacu.ifbacu_buf
#define	ifbac_req       ifbac_ifbacu.ifbacu_req
};
.Ed
Address cache entries with the type set to
.Dv IFBAF_DYNAMIC
in
.Fa ifba_flags
are entries learned by the bridge.
Entries with the type set to
.Dv IFBAF_STATIC
are manually added entries.
.It Dv SIOCBRDGSADDR
.Pq Li "struct ifbareq"
Add an entry, manually, to the address cache for the bridge named in
.Fa ifba_name .
The address and its associated interface and flags are set in the
.Fa ifba_dst ,
.Fa ifba_ifsname ,
and
.Fa ifba_flags
fields, respectively.
.It Dv SIOCBRDGDADDR
.Pq Li "struct ifbareq"
Delete an entry from the address cache of the bridge named in
.Fa ifba_name .
Entries are deleted strictly based on the address field
.Fa ifba_dst .
.It Dv SIOCBRDGSCACHE
.Pq Li "struct ifbcachereq"
Set the maximum address cache size for the bridge named in
.Fa ifbc_name
to
.Fa ifbc_size
entries.
.Pp
The argument structure is as follows:
.Bd -literal -offset indent
struct ifbcachereq {
	char ifbc_name[IFNAMSIZ]; /* bridge */
	u_int32_t ifbc_size;      /* size */
};
.Ed
.It Dv SIOCBRDGGCACHE
.Pq Li "struct ifbcachereq"
Retrieve the maximum size of the address cache for the bridge
.Fa ifbc_name .
.It Dv SIOCBRDGSTO
.Pq Li "struct ifbcachetoreq"
Set the time, in seconds, that addresses which have not been
seen on the network (transmitted a packet) remain in the cache.
If the time is set to zero, no aging is performed on the address cache.
The argument structure is as follows:
.Bd -literal -offset indent
struct ifbcachetoreq {
	char ifbct_name[IFNAMSIZ]; /* bridge */
	u_int32_t ifbct_time;      /* time */
};
.Ed
.It Dv SIOCBRDGGTO
.Pq Li "struct ifbcachetoreq"
Retrieve the address cache expiration time (see above).
.It Dv SIOCBRDGFLUSH
.Pq Li "struct ifbreq"
Flush addresses from the cache.
.Fa ifbr_name
contains the name of the bridge device, and
.Fa 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 SIOCBRDGARL
.Pq Li "struct ifbrlreq"
Add a Ethernet address filtering rule to the bridge on a specific interface.
The argument structure is as follows:
.Bd -literal -offset indent
.Ed
.It Dv SIOCBRDGFRL
.Pq Li "struct ifbrlreq"
Remove all filtering rules from a bridge interface member.
.Fa ifbr_name
contains the name of the bridge device, and
.Fa ifbr_ifsname
contains the name of the bridge member interface.
.It Dv SIOCBRDGGRL
.Pq Li "struct ifbrlconf"
Retrieve all of the rules from the bridge,
.Fa ifbrl_name ,
for the member interface,
.Fa ifbrl_ifsname .
This request takes an
.Li ifbrlconf
structure (see below) as a value result parameter.
The
.Fa ifbrl_len
field should be initially set to the size of the buffer pointed to by
.Fa ifbrl_buf .
On return, it will contain the length, in bytes, of the configuration list.
Alternatively, if the
.Fa ifbrl_len
passed in is set to 0,
.Dv SIOCBRDGGRL
will set it to the size that
.Fa ifbrl_buf
needs to be to fit the entire configuration list and not fill in the other
parameters.
As with
.Dv SIOCBRDGIFS ,
this is useful for determining the exact size that
.Fa ifbrl_buf
needs to be in advance.
.Pp
The argument structure is defined as follows:
.Bd -literal -offset indent
struct ifbrlconf {
        char ifbrl_name[IFNAMSIZ];   /* brdg nam */
	char ifbrl_ifsname[IFNAMSIZ];/* ifs name */
        u_int32_t ifbr_len;         /* buf len */
	union {
                caddr_t ifbrlu_buf;
                struct ifbrlreq *ifbrlu_req;
        } ifrl_ifbrlu;
#define ifbrl_buf ifbrl_ifbrlu.ifbrlu_buf
#define ifbrl_req ifbrl_ifbrlu.ifbrlu_req
};
.Ed
.It Dv SIOCBRDGARL
.Pq Li "struct ifbrlreq"
Add a filtering rule to the bridge named in
.Fa ifbr_name
on the interface named in
.Fa ifbr_ifsname .
The argument structure is as follows:
.Bd -literal -offset indent
struct ifbrlreq {
	char ifbr_name[IFNAMSIZ];    /* bridge */
	char ifbr_ifsname[IFNAMSIZ]; /* ifs */
	u_int8_t ifbr_action;        /* handling */
	u_int8_t ifbr_flags;         /* flags */
	struct ether_addr ifbr_src;  /* src mac */
	struct ether_addr ifbr_dst;  /* dst mac */
};
#define BRL_ACTION_BLOCK        0x01
#define BRL_ACTION_PASS         0x02
#define BRL_FLAG_IN             0x08
#define BRL_FLAG_OUT            0x04
#define BRL_FLAG_SRCVALID       0x02
#define BRL_FLAG_DSTVALID       0x01
.Ed
.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
.Fa ifbr_action
parameter specifies whether a frame matching the rule is to
be blocked or passed.
.Pp
If the
.Dv BRL_FLAG_IN
bit is set in
.Fa ifbr_flags ,
then the rule applies to frames received by the interface.
If the
.Dv BRL_FLAG_OUT
bit is set, then the rule applies to frame transmitted by the interface.
At least one of
.Dv BRL_FLAG_IN
or
.Dv BRL_FLAG_OUT
must be set.
.Pp
The source Ethernet address in
.Fa ifbr_src
is checked if the
.Dv BRL_FLAG_SRCVALID
bit is set in
.Fa ifbr_flags .
The destination address in
.Fa ifbr_dst
is check if the
.Dv BRL_FLAG_DSTVALID
bit is set.
If neither bit is set, the rule is matches all frames.
.It Dv SIOCBRDGFRL
.Pq Li "struct ifbrlreq"
Flush rules from the bridge
.Fa ifbr_name
on the interface
.Fa ifbr_ifsname .
.It Dv SIOCBRDGGRL
.Pq Li "struct ifbrlconf"
Retrieve an array of rules from the bridge for a particular interface.
This request takes an
.Li ifbrlconf
structure (see below) as a value-result parameter.
The
.Fa ifbrl_len
field should be initially set to the size of the buffer
pointed to by
.Fa ifbrl_buf .
On return it will contain the length, in bytes, of the rule list.
Alternatively, if the
.Fa ifbrl_len
passed in is set to 0,
.Dv SIOCBRDGGRL
will set
.Fa ifbrl_len
to the size that
.Fa ifbrl_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
.Fa ifbrl_buf
needs to be in advance.
.Pp
The argument structure is as follows:
.Bd -literal -offset indent
struct ifbrlconf {
	char ifbrl_name[IFNAMSIZ];   /* bridge */
	char ifbrl_ifsname[IFNAMSIZ];/* member */
	u_int32_t ifbrl_len;         /* buflen */
	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
.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 Eq ENOENT
For an add request, this means that the named interface is not configured
into the system.
For delete operation, it means that the named interface is not a member
of the bridge.
For a address cache deletion, the address was not found in the table.
.It Bq Eq ENOMEM
Memory could not be allocated for an interface or cache entry
to be added to the bridge.
.It Bq Eq EEXIST
The named interface is already a member of the bridge.
.It Bq Eq EBUSY
The named interface is already a member of another bridge.
.It Bq Eq EINVAL
The named interface is not an Ethernet interface or an invalid ioctl
was performed on the bridge.
.It Bq Eq ENETDOWN
Address cache operation (flush, add, delete) on a bridge that is
in the down state.
.It Bq Eq EPERM
Super-user privilege is required to add and delete interfaces to and from
bridges and to set the bridge interface flags.
.It Bq Eq EFAULT
The buffer used in a
.Dv SIOCBRDGIFS
or
.Dv SIOCBRDGRTS
request points outside of the process's allocated address space.
.It Bq Eq ESRCH
No such member interface in the bridge.
.El
.Sh NOTES
Bridged packets pass through
.Xr pf 4
twice.
They can be filtered on any interface, in both directions.
For stateful filtering, filtering on only one interface (using
.Sq keep state )
and passing all traffic on the other interfaces is recommended.
A state entry only permits outgoing packets from initial source to
destination and incoming packets from initial destination to source.
Since bridged packets pass through the filter twice with the source
and destination addresses reversed between interfaces, two state
entries (one for each direction) are required when all interfaces
are filtered statefully.
.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 mtu's or when IP fragments are reassembled by
.Xr pf 4 . 
Non-IP packets which are too large for the outgoing interface will be
dropped.
.Pp
If the LINK2 flag is set on the
.Xr bridge 4
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 ipsecadm 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 bridgename.if 5 ,
.Xr brconfig 8 ,
.Xr ipsecadm 8 ,
.Xr isakmpd 8
.Sh AUTHORS
The
.Xr brconfig 8
command and the
.Xr bridge 4
kernel interface were written by
.An Jason L. Wright Aq jason@thought.net
as part of an undergraduate independent study at the
University of North Carolina at Greensboro.
.Sh HISTORY
The
.Xr brconfig 8
command and the
.Xr bridge 4
kernel interface first appeared in
.Ox 2.5 .