openbsd-manpages-bridge.sgml

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<SECT1 id="openbsd-manpages-bridge">
<TITLE>Bridging</TITLE>
<SCREEN>

NAME
     bridge - Ethernet bridge interface

SYNOPSIS
     pseudo-device bridge

DESCRIPTION
     The bridge device creates a logical link between two or more Ethernet 
     interfaces.  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 ip4 datagrams.

     The bridges provided by this interface are learning bridges with filter-
     ing.  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.

     The algorithm works as follows by default, but can be modified via ioctl.
     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.

IOCTLS
     A bridge interface responds to all of the ioctl calls specific to oth-
     er interfaces listed in <link linkend="openbsd-manpages-netintro">netintro</link>.  The following ioctl calls are
     specific to bridge devices.  They are defined in &lt;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 */
                                    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 */
                            };
                            /* SIOCBRDGIFFLGS, SIOCBRDGIFFLGS */
                            #define IFBIF_LEARNING    0x0001 /* ifs can learn */
                            #define IFBIF_DISCOVER    0x0002 /* ifs sends packets w/unknown dest */
                            #define IFBIF_BLOCKNONIP  0x0004 /* ifs blocks non-IP/ARP in/out */
                            #define IFBIF_STP         0x0008 /* ifs participates 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]; /* 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.

     SIOCBRDGADDS     (struct ifbreq) Add the interface named in ifbr_ifsname
                      as a span port to the bridge named in ifbr_name.

     SIOCBRDGDELS     (struct ifbreq) Delete the interface named in
                      ifbr_ifsname from the list of span ports of 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, and ifba_flags fields, respec-
                      tively.