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NAME
bridge - Ethernet bridge interface
SYNOPSIS
pseudo-device bridge 2
DESCRIPTION
The bridge device creates a logical link between two or more Ethernet in
terfaces. 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 be
tween 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 ip(4) datagrams.
The bridges provided by this interface are learning bridges with IP fil
tering, see ipf(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 ac
tively 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.
The algorithm works as follows by default, but can be modified via
ioctl(2) or the utility brconfig(8). When a frame comes in, the origin
segment and the source address are recorded. If the bridge has no knowl
edge 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 seg
ment 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 ip(4) data
gram; if so, the datagram is run through the ipf(4) interface so that it
can be filtered. Only the ipf(4) input rules for the source interface
are checked with the datagram; output rules have no effect.
IOCTLS
A bridge interface responds to all of the ioctl(2) calls specific to oth
er interfaces listed in netintro(4). The following ioctl(2) calls are
specific to bridge devices. They are defined in <sys/sockio.h>.
SIOCBRDGIFS (struct ifbifconf) Retrieve member interface list from a
bridge. This request takes an ifbifconf structure (see
below) as a value-result parameter. The ifbic_len field
should be initially set to the size of the buffer point
ed to by ifbic_buf. On return it will contain the
length, in bytes, of the configuration list. Alterna
tively, if the ifbic_len passed in is set to 0,
SIOCBRDGIFS will set ifbic_len to the size that
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 ifbic_buf
needs to be in advance.
The argument structure is defined as follows:
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
};
SIOCBRDGADD (struct ifbreq) Add the interface named in ifbr_ifsname
to the bridge named in ifbr_name.
SIOCBRDGDEL (struct ifbreq) Delete the interface named in
ifbr_ifsname from the bridge named in ifbr_name.
SIOCBRDGSIFFLGS (struct ifbreq) Set the bridge member interface flags
for the interface named in ifbr_ifsname attached to the
bridge ifbr_name. If the flag IFBIF_LEARNING is set on
an interface, source addresses from frames received on
the interface are recorded in the address cache. If the
flag 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 IFBIF_BLOCKNONIP is set, packets that are one of
ip(4), ip6(4), arp(4), or Reverse ARP, will not be
bridged from and to the interface.
SIOCBRDGGIFFLGS Retrieve the bridge member interface flags for the in
terface named in ifbr_ifsname attached to the bridge
ifbr_name.
SIOCBRDGRTS (struct ifbaconf) Retrieve the address cache of the
bridge named in ifbac_name. This request takes an
ifbaconf structure (see below) as a value result parame
ter. The ifbac_len field should be initially set to the
size of the buffer pointed to by ifbac_buf. On return,
it will contain the length, in bytes, of the configura
tion list. Alternatively, if the ifbac_len passed in is
set to 0, SIOCBRDGRTS will set it to the size that
ifbac_buf needs to be to fit the entire configuration
list and not fill in the other parameters. As with
SIOCBRDGIFS, this is useful for determining the exact
size that ifbac_buf needs to be in advance.
The argument structure is defined as follows:
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
};
Address cache entries with the type set to IFBAF_DYNAMIC
in ifba_flags are entries learned by the bridge. En
tries with the type set to IFBAF_STATIC are manually
added entries.
SIOCBRDGSADDR (struct ifbareq) Add an entry, manually, to the address
cache for the bridge named in ifba_name. The address and
its associated interface and flags are set in the
ifba_dst, ifba_ifsname, ifba_flags fields, respectively.
SIOCBRDGDADDR (struct ifbareq) Delete an entry from the address cache
of the bridge named in ifba_name. Entries are deleted
strictly based on the address field ifba_dst.
SIOCBRDGSCACHE (struct ifbcachereq) Set the maximum address cache size
for the bridge named in ifbc_name to ifbc_size entries.
The argument structure is as follows:
struct ifbcachereq {
char ifbc_name[IFNAMSIZ]; /* bridge */
u_int32_t ifbc_size; /* size */
};
SIOCBRDGGCACHE (struct ifbcachereq) Retrieve the maximum size of the
address cache for the bridge ifbc_name.
SIOCBRDGSTO (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:
struct ifbcachetoreq {
char ifbct_name[IFNAMSIZ]; /* bridge */
u_int32_t ifbct_time; /* time */
};
SIOCBRDGGTO (struct ifbcachetoreq) Retrieve the address cache expi
ration time (see above).
SIOCBRDGFLUSH (struct ifbreq) Flush addresses from the cache.
ifbr_name contains the name of the bridge device, and
ifbr_ifsflags should be set to IFBF_FLUSHALL to flush
all addresses from the cache or IFBF_FLUSHDYN to flush
only the dynamically learned addresses from the cache.
SIOCBRDGARL (struct ifbrlreq) Add a filtering rule to the bridge
named in ifbr_name on the interface named in
ifbr_ifsname. The argument structure is as follows:
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
Rules are applied in the order in which they were added
to the bridge, and the first matching rule's action pa
rameter determines the fate of the packet. The
ifbr_action parameter specifies whether a frame matching
the rule is to be blocked or passed.
If the BRL_FLAG_IN bit is set in ifbr_flags, then the
rule applies to frames received by the interface. If
the BRL_FLAG_OUT bit is set, then the rule applies to
frame transmitted by the interface. At least one of
BRL_FLAG_IN or BRL_FLAG_OUT must be set.
The source ethernet address in ifbr_src is checked if
the BRL_FLAG_SRCVALID bit is set in ifbr_flags. The des
tination address in ifbr_dst is check if the
BRL_FLAG_DSTVALID bit is set. If neither bit is set,
the rule is matches all frames.
SIOCBRDGFRL (struct ifbrlreq) Remove all filtering rules from a
bridge interface member. ifbr_name contains the name of
the bridge device, and ifbr_ifsname contains the name of
the bridge member interface.
SIOCBRDGGRL (struct ifbrlconf) Retrieve all of the rules from the
bridge, ifbrl_name, for the member interface,
ifbrl_ifsname.
This request takes an ifbrlconf structure (see below) as
a value result parameter. The ifbrl_len field should be
initially set to the size of the buffer pointed to by
ifbrl_buf. On return, it will contain the length, in
bytes, of the configuration list. Alternatively, if the
ifbrl_len passed in is set to 0, SIOCBRDGGRL will set it
to the size that ifbrl_buf needs to be to fit the entire
configuration list and not fill in the other parameters.
As with SIOCBRDGIFS, this is useful for determining the
exact size that ifbrl_buf needs to be in advance.
The argument structure is defined as follows:
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
};
ERRORS
If the ioctl(2) call fails, errno(2) is set to one of the following val
ues:
[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.
[ENOMEM] Memory could not be allocated for an interface or cache en
try to be added to the bridge.
[EEXIST] The named interface is already a member of the bridge.
[EBUSY] The named interface is already a member of another bridge.
[EINVAL] The named interface is not an Ethernet interface or an in
valid ioctl was performed on the bridge.
[ENETDOWN] Address cache operation (flush, add, delete) on a bridge
that is in the down state.
[EPERM] Super-user privilege is required to add and delete inter
faces to and from bridges and to set the bridge interface
flags.
[EFAULT] The buffer used in a SIOCBRDGIFS or SIOCBRDGRTS request
points outside of the process's allocated address space.
[ESRCH] No such member interface in the bridge.
SEE ALSO
errno(2), ioctl(2), ip(4), ipf(4), netintro(4), bridgename.if(5),
brconfig(8)
HISTORY
The brconfig(8) command and the bridge(4) kernel interface first appeared
in
AUTHOR
The brconfig(8) command and the bridge(4) kernel interface were written
by Jason L. Wright <jason@thought.net> as part of an undergraduate inde
pendent study at the University of North Carolina at Greensboro.
BUGS
There is currently no loop detection. Care must be taken to make sure
that loops are not created when a bridge is brought up.
Only ipf(4) input rules are checked with incoming packet; there is no
easy way to handle output rules.
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