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<body>
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<pre>
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NAME
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bridge - Ethernet bridge interface
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SYNOPSIS
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pseudo-device bridge 2
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DESCRIPTION
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The bridge device creates a logical link between two or more Ethernet in
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terfaces. This link between the interfaces selectively forwards frames
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from each interface on the bridge to every other interface on the bridge.
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A bridge can serve several services, including, isolation of traffic be
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tween sets of machines so that traffic local to one set of machines is
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not available on the wire of another set of machines, and it can act as a
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transparent filter for ip(4) datagrams.
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The bridges provided by this interface are learning bridges with IP fil
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tering, see ipf(4). In general a bridge works like a hub, forwarding
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traffic from one interface to another. It differs from a hub in that it
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will "learn" which machines are on each of its attached segments by ac
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tively listening to incoming traffic and examining the headers of each
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frame. A table is built containing the MAC address and segment to which
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the MAC address is attached. This allows a bridge to be more selective
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about what it forwards, which can be used to reduce traffic on a set of
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segments and also to provide an IP firewall without changing the topology
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of the network.
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The algorithm works as follows by default, but can be modified via
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ioctl(2) or the utility brconfig(8). When a frame comes in, the origin
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segment and the source address are recorded. If the bridge has no knowl
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edge about where the destination is to be found, the bridge will forward
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the frame to all attached segments. If the destination is known to be on
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a different segment from its origin, the bridge will forward the packet
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only to the destination segment. If the destination is on the same seg
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ment as the origin segment, the bridge will drop the packet because the
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receiver has already had a chance to see the frame. Before forwarding a
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frame, the bridge will check to see if the packet contains an ip(4) data
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gram; if so, the datagram is run through the ipf(4) interface so that it
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can be filtered. Only the ipf(4) input rules for the source interface
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are checked with the datagram; output rules have no effect.
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IOCTLS
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A bridge interface responds to all of the ioctl(2) calls specific to oth
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er interfaces listed in netintro(4). The following ioctl(2) calls are
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specific to bridge devices. They are defined in <sys/sockio.h>.
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SIOCBRDGIFS (struct ifbifconf) Retrieve member interface list from a
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bridge. This request takes an ifbifconf structure (see
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below) as a value-result parameter. The ifbic_len field
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should be initially set to the size of the buffer point
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ed to by ifbic_buf. On return it will contain the
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length, in bytes, of the configuration list. Alterna
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tively, if the ifbic_len passed in is set to 0,
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SIOCBRDGIFS will set ifbic_len to the size that
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ifbic_buf needs to be to fit the entire configuration
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list, and will not fill in the other parameters. This
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is useful for determining the exact size that ifbic_buf
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needs to be in advance.
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The argument structure is defined as follows:
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struct ifbreq {
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char ifbr_name[IFNAMSIZ]; /* brdg nam */
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char ifbr_ifsname[IFNAMSIZ]; /* if name */
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u_int32_t ifbr_ifsflags; /* if flags */
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};
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#define IFBIF_LEARNING 0x1 /* learns addrs */
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#define IFBIF_DISCOVER 0x2 /* gets fwd'd pkts */
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struct ifbifconf {
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char ifbic_name[IFNAMSIZ]; /* brdg name */
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u_int32_t ifbic_len; /* buf size */
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union {
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caddr_t ifbicu_buf; /* buffer */
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struct ifbreq *ifbicu_req;
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} ifbic_ifbicu;
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#define ifbic_buf ifbic_ifbicu.ifbicu_buf
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#define ifbic_req ifbic_ifbicu.ifbicu_req
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};
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SIOCBRDGADD (struct ifbreq) Add the interface named in ifbr_ifsname
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to the bridge named in ifbr_name.
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SIOCBRDGDEL (struct ifbreq) Delete the interface named in
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ifbr_ifsname from the bridge named in ifbr_name.
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SIOCBRDGSIFFLGS (struct ifbreq) Set the bridge member interface flags
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for the interface named in ifbr_ifsname attached to the
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bridge ifbr_name. If the flag IFBIF_LEARNING is set on
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an interface, source addresses from frames received on
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the interface are recorded in the address cache. If the
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flag IFBIF_DISCOVER is set, the interface will receive
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packets destined for unknown destinations, otherwise a
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frame that has a destination not found in the address
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cache is not forwarded to this interface. The default
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for newly added interfaces has both flags set. If the
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flag IFBIF_BLOCKNONIP is set, packets that are one of
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ip(4), ip6(4), arp(4), or Reverse ARP, will not be
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bridged from and to the interface.
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SIOCBRDGGIFFLGS Retrieve the bridge member interface flags for the in
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terface named in ifbr_ifsname attached to the bridge
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ifbr_name.
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SIOCBRDGRTS (struct ifbaconf) Retrieve the address cache of the
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bridge named in ifbac_name. This request takes an
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ifbaconf structure (see below) as a value result parame
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ter. The ifbac_len field should be initially set to the
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size of the buffer pointed to by ifbac_buf. On return,
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it will contain the length, in bytes, of the configura
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tion list. Alternatively, if the ifbac_len passed in is
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set to 0, SIOCBRDGRTS will set it to the size that
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ifbac_buf needs to be to fit the entire configuration
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list and not fill in the other parameters. As with
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SIOCBRDGIFS, this is useful for determining the exact
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size that ifbac_buf needs to be in advance.
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The argument structure is defined as follows:
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struct ifbareq {
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char ifba_name[IFNAMSIZ]; /* brdg nam */
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char ifba_ifsname[IFNAMSIZ];/* dest ifs */
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u_int8_t ifba_age; /* addr age */
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u_int8_t ifba_flags; /* addr flag */
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struct ether_addr ifba_dst; /* dst addr */
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};
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#define IFBAF_TYPEMASK 0x03 /* addr type mask */
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#define IFBAF_DYNAMIC 0x00 /* dynamic addr */
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#define IFBAF_STATIC 0x01 /* static address */
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struct ifbaconf {
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char ifbac_name[IFNAMSIZ]; /* brdg name */
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u_int32_t ifbac_len; /* buf size */
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union {
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caddr_t ifbacu_buf; /* buf */
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struct ifbareq *ifbacu_req;
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} ifbac_ifbacu;
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#define ifbac_buf ifbac_ifbacu.ifbacu_buf
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#define ifbac_req ifbac_ifbacu.ifbacu_req
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};
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Address cache entries with the type set to IFBAF_DYNAMIC
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in ifba_flags are entries learned by the bridge. En
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tries with the type set to IFBAF_STATIC are manually
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added entries.
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SIOCBRDGSADDR (struct ifbareq) Add an entry, manually, to the address
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cache for the bridge named in ifba_name. The address and
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its associated interface and flags are set in the
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ifba_dst, ifba_ifsname, ifba_flags fields, respectively.
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SIOCBRDGDADDR (struct ifbareq) Delete an entry from the address cache
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of the bridge named in ifba_name. Entries are deleted
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strictly based on the address field ifba_dst.
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SIOCBRDGSCACHE (struct ifbcachereq) Set the maximum address cache size
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for the bridge named in ifbc_name to ifbc_size entries.
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The argument structure is as follows:
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struct ifbcachereq {
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char ifbc_name[IFNAMSIZ]; /* bridge */
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u_int32_t ifbc_size; /* size */
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};
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SIOCBRDGGCACHE (struct ifbcachereq) Retrieve the maximum size of the
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address cache for the bridge ifbc_name.
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SIOCBRDGSTO (struct ifbcachetoreq) Set the time, in seconds, that
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addresses which have not been seen on the network
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(transmitted a packet) remain in the cache. If the time
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is set to zero, no aging is performed on the address
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cache. The argument structure is as follows:
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struct ifbcachetoreq {
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char ifbct_name[IFNAMSIZ]; /* bridge */
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u_int32_t ifbct_time; /* time */
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};
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SIOCBRDGGTO (struct ifbcachetoreq) Retrieve the address cache expi
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ration time (see above).
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SIOCBRDGFLUSH (struct ifbreq) Flush addresses from the cache.
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ifbr_name contains the name of the bridge device, and
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ifbr_ifsflags should be set to IFBF_FLUSHALL to flush
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all addresses from the cache or IFBF_FLUSHDYN to flush
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only the dynamically learned addresses from the cache.
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SIOCBRDGARL (struct ifbrlreq) Add a filtering rule to the bridge
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named in ifbr_name on the interface named in
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ifbr_ifsname. The argument structure is as follows:
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struct ifbrlreq {
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char ifbr_name[IFNAMSIZ]; /* bridge */
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char ifbr_ifsname[IFNAMSIZ]; /* ifs */
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u_int8_t ifbr_action; /* handling */
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u_int8_t ifbr_flags; /* flags */
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struct ether_addr ifbr_src; /* src mac */
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struct ether_addr ifbr_dst; /* dst mac */
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};
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#define BRL_ACTION_BLOCK 0x01
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#define BRL_ACTION_PASS 0x02
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#define BRL_FLAG_IN 0x08
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#define BRL_FLAG_OUT 0x04
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#define BRL_FLAG_SRCVALID 0x02
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#define BRL_FLAG_DSTVALID 0x01
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Rules are applied in the order in which they were added
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to the bridge, and the first matching rule's action pa
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rameter determines the fate of the packet. The
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ifbr_action parameter specifies whether a frame matching
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the rule is to be blocked or passed.
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If the BRL_FLAG_IN bit is set in ifbr_flags, then the
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rule applies to frames received by the interface. If
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the BRL_FLAG_OUT bit is set, then the rule applies to
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frame transmitted by the interface. At least one of
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BRL_FLAG_IN or BRL_FLAG_OUT must be set.
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The source ethernet address in ifbr_src is checked if
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the BRL_FLAG_SRCVALID bit is set in ifbr_flags. The des
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tination address in ifbr_dst is check if the
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BRL_FLAG_DSTVALID bit is set. If neither bit is set,
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the rule is matches all frames.
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SIOCBRDGFRL (struct ifbrlreq) Remove all filtering rules from a
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bridge interface member. ifbr_name contains the name of
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the bridge device, and ifbr_ifsname contains the name of
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the bridge member interface.
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SIOCBRDGGRL (struct ifbrlconf) Retrieve all of the rules from the
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bridge, ifbrl_name, for the member interface,
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ifbrl_ifsname.
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This request takes an ifbrlconf structure (see below) as
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a value result parameter. The ifbrl_len field should be
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initially set to the size of the buffer pointed to by
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ifbrl_buf. On return, it will contain the length, in
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bytes, of the configuration list. Alternatively, if the
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ifbrl_len passed in is set to 0, SIOCBRDGGRL will set it
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to the size that ifbrl_buf needs to be to fit the entire
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configuration list and not fill in the other parameters.
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As with SIOCBRDGIFS, this is useful for determining the
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exact size that ifbrl_buf needs to be in advance.
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The argument structure is defined as follows:
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struct ifbrlconf {
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char ifbrl_name[IFNAMSIZ]; /* brdg nam */
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char ifbrl_ifsname[IFNAMSIZ];/* ifs name */
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u_int32_t ifbr_len; /* buf len */
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union {
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caddr_t ifbrlu_buf;
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struct ifbrlreq *ifbrlu_req;
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} ifrl_ifbrlu;
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#define ifbrl_buf ifbrl_ifbrlu.ifbrlu_buf
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#define ifbrl_req ifbrl_ifbrlu.ifbrlu_req
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};
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ERRORS
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If the ioctl(2) call fails, errno(2) is set to one of the following val
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ues:
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[ENOENT] For an add request, this means that the named interface is
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not configured into the system. For delete operation, it
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means that the named interface is not a member of the
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bridge. For a address cache deletion, the address was not
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found in the table.
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[ENOMEM] Memory could not be allocated for an interface or cache en
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try to be added to the bridge.
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[EEXIST] The named interface is already a member of the bridge.
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[EBUSY] The named interface is already a member of another bridge.
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[EINVAL] The named interface is not an Ethernet interface or an in
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valid ioctl was performed on the bridge.
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[ENETDOWN] Address cache operation (flush, add, delete) on a bridge
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that is in the down state.
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[EPERM] Super-user privilege is required to add and delete inter
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faces to and from bridges and to set the bridge interface
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flags.
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[EFAULT] The buffer used in a SIOCBRDGIFS or SIOCBRDGRTS request
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points outside of the process's allocated address space.
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[ESRCH] No such member interface in the bridge.
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SEE ALSO
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errno(2), ioctl(2), ip(4), ipf(4), netintro(4), bridgename.if(5),
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brconfig(8)
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HISTORY
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The brconfig(8) command and the bridge(4) kernel interface first appeared
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in
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AUTHOR
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The brconfig(8) command and the bridge(4) kernel interface were written
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by Jason L. Wright <jason@thought.net> as part of an undergraduate inde
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pendent study at the University of North Carolina at Greensboro.
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BUGS
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There is currently no loop detection. Care must be taken to make sure
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that loops are not created when a bridge is brought up.
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Only ipf(4) input rules are checked with incoming packet; there is no
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easy way to handle output rules.
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</pre>
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