Copyright (c) 1999 Jason L. Wright (jason@thought.net)
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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 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 .Xr ip 4 datagrams.
p The bridges provided by this interface are learning bridges with IP filtering, see .Xr 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 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.
p 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 datagram; if so, the datagram is run through the .Xr ipf 4 interface so that it can be filtered. .Sh IOCTLS A .Nm bridge 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 bridge devices. They are defined in .Aq Pa sys/sockio.h .
p l -tag -width SIOCBRDGGIFFLGS t Dv SIOCBRDGIFS
q Li "struct ifbifconf" Retrieve member interface list from a bridge. This request takes an .Ar ifbifconf structure (see below) as a value-result parameter. The .Ar ifbic_len field should be initially set to the size of the buffer pointed to by .Ar ifbic_buf . On return it will contain the length, in bytes, of the configuration list. Alternatively, if the .Ar ifbic_len passed in is set to 0, SIOCBRDGIFS will set .Ar ifbic_len to the size that .Ar 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 .Ar ifbic_buf needs to be in advance.
p The argument structure is defined as follows: d -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 t Dv SIOCBRDGADD
q Li "struct ifbreq" Add the interface named in .Ar ifbr_ifsname to the bridge named in .Ar ifbr_name . t Dv SIOCBRDGDEL
q Li "struct ifbreq" Delete the interface named in .Ar ifbr_ifsname from the bridge named in .Ar ifbr_name . t Dv SIOCBRDGSIFFLGS
q Li "struct ifbreq" Set the bridge member interface flags for the interface named in .Ar ifbr_ifsname attached to the bridge .Ar ifbr_name . If the flag .Ar IFBIF_LEARNING is set on an interface, source addresses from frames received on the interface are recorded in the address cache. If the flag .Ar 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. t Dv SIOCBRDGGIFFLGS Retrieve the bridge member interface flags for the interface named in .Ar ifbr_ifsname attached to the bridge .Ar ifbr_name . t Dv SIOCBRDGRTS
q Li "struct ifbaconf" Retrieve the address cache of the bridge named in .Ar ifbac_name . This request takes an .Ar ifbaconf structure (see below) as a value result parameter. The .Ar ifbac_len field should be initially set to the size of the buffer pointed to by .Ar ifbac_buf . On return, it will contain the length, in bytes, of the configuration list. Alternatively, if the .Ar ifbac_len passed in is set to 0, SIOCBRDGRTS will set it to the size that .Ar 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 .Ar ifbac_buf needs to be in advance.
p The argument structure is defined as follows: d -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 .Ar IFBAF_DYNAMIC in .Ar ifba_flags are entries learned by the bridge. Entries with the type set to .Ar IFBAF_STATIC are manually added entries. t Dv SIOCBRDGSADDR
q Li "struct ifbareq" Add an entry, manually, to the address cache for the bridge named in .Ar ifba_name . The address and its associated interface and flags are set in the .Ar ifba_dst , .Ar ifba_ifsname , .Ar ifba_flags fields, respectively. t Dv SIOCBRDGDADDR
q Li "struct ifbareq" Delete an entry from the address cache of the bridge named in .Ar ifba_name . Entries are deleted strictly based on the address field .Ar ifba_dst . t Dv SIOCBRDGSCACHE
q Li "struct ifbcachereq" Set the maximum address cache size for the bridge named in .Ar ifbc_name to .Ar ifbc_size entries.
p The argument structure is as follows: d -literal -offset indent struct ifbcachereq { char ifbc_name[IFNAMSIZ]; /* bridge */ u_int32_t ifbc_size; /* size */ }; .Ed t Dv SIOCBRDGGCACHE
q Li "struct ifbcachereq" Retrieve the maximum size of the address cache for the bridge .Ar ifbc_name . t Dv SIOCBRDGSTO
q 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: d -literal -offset indent struct ifbcachetoreq { char ifbct_name[IFNAMSIZ]; /* bridge */ u_int32_t ifbct_time; /* time */ }; .Ed t Dv SIOCBRDGGTO
q Li "struct ifbcachetoreq" Retrieve the address cache expiration time (see above). t Dv SIOCBRDGFLUSH
q Li "struct ifbreq" Flush addresses from the cache. .Ar ifbr_name contains the name of the bridge device, and .Ar ifbr_ifsflags should be set to .Ar IFBF_FLUSHALL to flush all addresses from the cache or .Ar IFBF_FLUSHDYN to flush only the dynamically learned addresses from the cache. .El .Sh ERRORS If the .Xr ioctl 2 call fails, .Xr errno 2 is set to one of the following values: l -tag -width Er t 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. t Bq Eq ENOMEM Memory could not be allocated for an interface or cache entry to be added to the bridge. t Bq Eq EEXIST The named interface is already a member of the bridge. t Bq Eq EBUSY The named interface is already a member of another bridge. t Bq Eq EINVAL The named interface is not an Ethernet interface or an invalid ioctl was performed on the bridge. t Bq Eq ENETDOWN Address cache operation (flush, add, delete) on a bridge that is in the down state. t Bq Eq EPERM Super-user privilege is required to add and delete interfaces to and from bridges and to set the bridge interface flags. t Bq Eq EFAULT The buffer used in a SIOCBRDGIFS or SIOCBRDGRTS request points outside of the process's allocated address space. t Bq Eq ESRCH No such member interface in the bridge. .El .Sh SEE ALSO .Xr errno 2 , .Xr ioctl 2 , .Xr ip 4 , .Xr ipf 4 , .Xr netintro 4 , .Xr brconfig 8 .Sh HISTORY The .Xr brconfig 8 command and the .Xr bridge 4 kernel interface first appeared in .Ox 2.5 . .Sh AUTHOR The .Xr brconfig 8 command and the .Xr bridge 4 kernel interface were written by Jason L. Wright <jason@thought.net> as part of an undergraduate independent study at the University of North Carolina at Greensboro. .Sh 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.