BRIDGE(4) OpenBSD Programmer's Manual BRIDGE(4)NAME
bridge - Ethernet bridge interface
SYNOPSIS
pseudo-device bridge
#include <sys/types.h>
#include <net/if.h>
#include <netinet/in.h>
#include <netinet/if_ether.h>
#include <net/if_bridge.h>
DESCRIPTION
The bridge device creates a logical link between two or more Ethernet
interfaces or encapsulation interfaces (see gif(4)). 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
ip(4) datagrams.
A bridge interface can be created at runtime using the ifconfig bridgeN
create command or by setting up a hostname.if(5) configuration file for
netstart(8).
The bridges provided by this interface are learning bridges with
filtering; see pf(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.
The algorithm works as follows by default, but can be modified via
ioctl(2) or the utility ifconfig(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 ip(4) or ip6(4) datagram; if so, the datagram is run through the pf
interface so that it can be filtered. See the NOTES section for details.
SPANNING TREE
The bridge has support for 802.1D-2004 Spanning Tree Protocol (STP),
which can be used to detect and remove loops in a network topology.
Using the stp or -stp commands to bridge, STP can be enabled or disabled
on each port.
The bridge will use the Rapid Spanning Tree Protocol (RSTP) by default to
allow rapid transitions to the forwarding state. The proto command to
bridge can be used to force operation in the common Spanning Tree
Protocol without rapid state transitions. Note that RSTP will be
compatible with remote bridges running common STP.
STP will not work on gif(4) members because they lack a hardware MAC
address.
SPAN PORTS
The bridge can have interfaces added to it as span ports. Span ports
transmit a copy of every frame received by the bridge. This is most
useful for snooping a bridged network passively on another host connected
to one of the span ports of the bridge. Span ports cannot be bridge
members; instead, the addspan and delspan commands are used to add and
delete span ports to and from a bridge.
IOCTLS
A bridge interface responds to all of the ioctl(2) calls specific to
other 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 pointed to by ifbic_buf. On return it will contain the
length, in bytes, of the configuration list.
Alternatively, 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]; /* bridge ifs name */
char ifbr_ifsname[IFNAMSIZ];/* member ifs name */
u_int32_t ifbr_ifsflags; /* member ifs 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 */
};
/* ifbr_ifsflags flags about interfaces */
#define IFBIF_LEARNING 0x0001 /* ifs can learn */
#define IFBIF_DISCOVER 0x0002 /* sends packets w/unknown dst */
#define IFBIF_BLOCKNONIP 0x0004 /* ifs blocks non-IP/ARP in/out */
#define IFBIF_STP 0x0008 /* participate 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]; /* bridge ifs name */
u_int32_t ifbic_len; /* buffer size */
union {
caddr_t ifbicu_buf;
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, only ip(4), ip6(4), arp(4), and Reverse ARP packets will be
bridged from and to the interface.
SIOCBRDGGIFFLGS struct ifbreq *
Retrieve the bridge member interface flags for the interface
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 parameter. 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 configuration 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 will 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]; /* bridge name */
char ifba_ifsname[IFNAMSIZ];/* destination ifs */
u_int8_t ifba_age; /* address age */
u_int8_t ifba_flags; /* address flags */
struct ether_addr ifba_dst; /* destination addr */
};
#define IFBAF_TYPEMASK 0x03 /* address type mask */
#define IFBAF_DYNAMIC 0x00 /* dynamically learned */
#define IFBAF_STATIC 0x01 /* static address */
struct ifbaconf {
char ifbac_name[IFNAMSIZ]; /* bridge ifs name */
u_int32_t ifbac_len; /* buffer size */
union {
caddr_t ifbacu_buf; /* buffer */
struct ifbareq *ifbacu_req; /* request pointer */
} 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. Entries 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,
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.
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.
SIOCBRDGSCACHE struct ifbrparam *
Set the maximum address cache size for the bridge named in
ifbrp_name to ifbrp_csize entries.
The argument structure is as follows:
struct ifbrparam {
char ifbrp_name[IFNAMSIZ];
union {
u_int32_t ifbrpu_csize; /* cache size */
int ifbrpu_ctime; /* cache time */
u_int16_t ifbrpu_prio; /* bridge priority */
u_int8_t ifbrpu_hellotime; /* hello time */
u_int8_t ifbrpu_fwddelay; /* fwd delay */
u_int8_t ifbrpu_maxage; /* max age */
} ifbrp_ifbrpu;
};
#define ifbrp_csize ifbrp_ifbrpu.ifbrpu_csize
#define ifbrp_ctime ifbrp_ifbrpu.ifbrpu_ctime
#define ifbrp_prio ifbrp_ifbrpu.ifbrpu_prio
#define ifbrp_hellotime ifbrp_ifbrpu.ifbrpu_hellotime
#define ifbrp_fwddelay ifbrp_ifbrpu.ifbrpu_fwddelay
#define ifbrp_maxage ifbrp_ifbrpu.ifbrpu_maxage
Note that the ifbrp_ctime, ifbrp_hellotime, ifbrp_fwddelay and
ifbrp_maxage fields are in seconds.
SIOCBRDGGCACHE struct ifbrparam *
Retrieve the maximum size of the address cache for the bridge
ifbrp_name.
SIOCBRDGSTO struct ifbrparam *
Set the time, in seconds, for how long addresses which have not
been seen on the network (i.e., have not transmitted a packet)
will remain in the cache to the value ifbrp_ctime. If the time
is set to zero, no aging is performed on the address cache.
SIOCBRDGGTO struct ifbrparam *
Retrieve the address cache expiration time (see above).
SIOCBRDGARL struct ifbrlreq *
Add an Ethernet address filtering rule to the bridge on a
specific interface. ifbr_name contains the name of the bridge
device, and ifbr_ifsname contains the name of the bridge member
interface.
Rules are applied in the order in which they were added to the
bridge, and the first matching rule's action parameter determines
the fate of the packet. The ifbr_action field is one of
BRL_ACTION_PASS or BRL_ACTION_BLOCK, to pass or block matching
frames, respectively. The ifbr_flags field specifies whether the
rule should match on input, output, or both by using the flags
BRL_FLAG_IN and BRL_FLAG_OUT. At least one of these flags must
be set.
The ifbr_flags field also specifies whether either (or both) of
the source and destination addresses should be matched by using
the BRL_FLAG_SRCVALID and BRL_FLAG_DSTVALID flags. The ifbr_src
field is the source address that triggers the rule (only
considered if ifbr_flags has the BRL_FLAG_SRCVALID bit set). The
ifbr_src field is the destination address that triggers the rule
(only considered if ifbr_flags has the BRL_FLAG_DSTVALID bit
set). If neither bit is set, the rule matches all frames.
The argument structure is as follows:
struct ifbrlreq {
char ifbr_name[IFNAMSIZ]; /* bridge ifs name */
char ifbr_ifsname[IFNAMSIZ]; /* member ifs name */
u_int8_t ifbr_action; /* disposition */
u_int8_t ifbr_flags; /* flags */
struct ether_addr ifbr_src; /* source mac */
struct ether_addr ifbr_dst; /* destination mac */
char ifbr_tagname[PF_TAG_NAME_SIZE]; /* pf tagname */
};
#define BRL_ACTION_BLOCK 0x01 /* block frame */
#define BRL_ACTION_PASS 0x02 /* pass frame */
#define BRL_FLAG_IN 0x08 /* input rule */
#define BRL_FLAG_OUT 0x04 /* output rule */
#define BRL_FLAG_SRCVALID 0x02 /* src valid */
#define BRL_FLAG_DSTVALID 0x01 /* dst valid */
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 will 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]; /* bridge ifs name */
char ifbrl_ifsname[IFNAMSIZ]; /* member ifs name */
u_int32_t ifbrl_len; /* buffer size */
union {
caddr_t ifbrlu_buf;
struct ifbrlreq *ifbrlu_req;
} ifbrl_ifbrlu;
#define ifbrl_buf ifbrl_ifbrlu.ifbrlu_buf
#define ifbrl_req ifbrl_ifbrlu.ifbrlu_req
};
SIOCBRDGGPRI struct ifbrparam *
Retrieve the Spanning Tree Protocol (STP) priority parameter of
the bridge into the ifbrp_prio field.
SIOCBRDGSPRI struct ifbrparam *
Set the STP priority parameter of the bridge to the value in
ifbrp_prio.
SIOCBRDGGHT struct ifbrparam *
Retrieve the STP hello time parameter, in seconds, of the bridge
into the ifbrp_hellotime field.
SIOCBRDGSHT struct ifbrparam *
Set the STP hello time parameter, in seconds, of the bridge to
the value in ifbrp_hellotime. The value in ifbrp_hellotime
cannot be zero.
SIOCBRDGGFD struct ifbrparam *
Retrieve the STP forward delay parameter, in seconds, of the
bridge into the ifbrp_fwddelay field.
SIOCBRDGSFD struct ifbrparam *
Set the STP forward delay parameter, in seconds, of the bridge to
the value in ifbrp_fwddelay. The value in ifbrp_fwddelay cannot
be zero.
SIOCBRDGGMA struct ifbrparam *
Retrieve the STP maximum age parameter, in seconds, of the bridge
into the ifbrp_maxage field.
SIOCBRDGSMA struct ifbrparam *
Set the STP maximum age parameter, in seconds, of the bridge to
the value in ifbrp_maxage. The value in ifbrp_maxage cannot be
zero.
SIOCBRDGSIFPRIO struct ifbreq *
Set the STP priority parameter of the interface named in
ifbr_ifsname to the value in ifbr_priority.
SIOCBRDGSIFCOST struct ifbreq *
Set the STP cost parameter of the interface named in ifbr_ifsname
to the value in ifbr_path_cost. The value in ifbr_path_cost must
be greater than or equal to one.
ERRORS
If the ioctl(2) call fails, errno(2) is set to one of the following
values:
[ENOENT] For an add request, this means that the named interface is
not configured into the system. For a delete operation, it
means that the named interface is not a member of the
bridge. For an address cache deletion, the address was not
found in the table.
[ENOMEM] Memory could not be allocated for an interface or cache
entry 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
invalid ioctl was performed on the bridge.
[ENETDOWN] Address cache operation (flush, add, or delete) on a bridge
that is in the down state.
[EPERM] Super-user privilege is required to add and delete
interfaces 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.
NOTES
Bridged packets pass through pf(4) filters once as input on the receiving
interface and once as output on all interfaces on which they are
forwarded. In order to pass through the bridge packets must pass any in
rules on the input and any out rules on the output interface. Packets
may be blocked either entering or leaving the bridge.
Return packets generated by pf itself are not routed using the kernel
routing table. Instead, pf will send these replies back to the same
Ethernet address that the original packet came from. This applies to
rules with return, return-rst, return-icmp, return-icmp6, or synproxy
defined. At the moment, only return-rst on IPv4 is implemented and the
other packet generating rules are unsupported.
If an IP packet is too large for the outgoing interface, the bridge will
perform IP fragmentation. This can happen when bridge members have
different MTUs or when IP fragments are reassembled by pf. Non-IP
packets which are too large for the outgoing interface will be dropped.
If the IFF_LINK2 flag is set on the bridge interface, the bridge will
also perform transparent ipsec(4) processing on the packets (encrypt or
decrypt them), according to the policies set with the ipsecctl(8) command
by the administrator. If appropriate security associations (SAs) do not
exist, any key management daemons such as isakmpd(8) that are running on
the bridge will be invoked to establish the necessary SAs. These daemons
have to be configured as if they were running on the host whose traffic
they are protecting (i.e., they need to have the appropriate
authentication and authorization material, such as keys and certificates,
to impersonate the protected host(s)).
SEE ALSOerrno(2), ioctl(2), arp(4), gif(4), ip(4), ip6(4), ipsec(4), netintro(4),
pf(4), vether(4), hostname.if(5), ifconfig(8), ipsecctl(8), isakmpd(8),
netstart(8)HISTORY
The bridge kernel interface first appeared in OpenBSD 2.5.
AUTHORS
The bridge kernel interface was written by Jason L. Wright
<jason@thought.net> as part of an undergraduate independent study at the
University of North Carolina at Greensboro.
Support for rapid spanning tree reconfigurations (RSTP) was added by
Andrew Thompson <thompsa@freebsd.org> and ported to OpenBSD by
Reyk Floeter <reyk@openbsd.org>.
BUGS
There are some rather special network interface chipsets which will not
work in a bridge configuration. Some chipsets have serious flaws when
running in promiscuous mode, like the TI ThunderLAN (see tl(4)), which
receives its own transmissions (this renders the address learning cache
useless). Most other chipsets work fine though.
OpenBSD 4.9 December 11, 2009 OpenBSD 4.9