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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.