726 lines
20 KiB
Groff
726 lines
20 KiB
Groff
.\" Copyright (c) 1990 The Regents of the University of California.
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.\" All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that: (1) source code distributions
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.\" retain the above copyright notice and this paragraph in its entirety, (2)
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.\" distributions including binary code include the above copyright notice and
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.\" this paragraph in its entirety in the documentation or other materials
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.\" provided with the distribution, and (3) all advertising materials mentioning
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.\" features or use of this software display the following acknowledgement:
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.\" ``This product includes software developed by the University of California,
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.\" Lawrence Berkeley Laboratory and its contributors.'' Neither the name of
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.\" the University nor the names of its contributors may be used to endorse
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.\" or promote products derived from this software without specific prior
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.\" written permission.
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.\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED
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.\" WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF
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.\" MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
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.\"
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.\" This document is derived in part from the enet man page (enet.4)
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.\" distributed with 4.3BSD Unix.
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.\"
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.\" $Id: bpf.4,v 1.14 1998/11/03 21:12:19 des Exp $
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.\"
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.Dd January 16, 1996
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.Dt BPF 4
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.Os BSD 4.4
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.Sh NAME
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.Nm bpf
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.Nd Berkeley Packet Filter
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.Sh SYNOPSIS
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.Cd pseudo-device bpfilter
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.Sh DESCRIPTION
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The Berkeley Packet Filter
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provides a raw interface to data link layers in a protocol
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independent fashion.
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All packets on the network, even those destined for other hosts,
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are accessible through this mechanism.
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.Pp
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The packet filter appears as a character special device,
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.Pa /dev/bpf0 ,
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.Pa /dev/bpf1 ,
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etc.
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After opening the device, the file descriptor must be bound to a
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specific network interface with the
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.Dv BIOCSETIF
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ioctl.
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A given interface can be shared be multiple listeners, and the filter
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underlying each descriptor will see an identical packet stream.
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The total number of open
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files is limited to the value given in the kernel configuration; the
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example given in the
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.Sx SYNOPSIS
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above sets the limit to 16.
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.Pp
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A separate device file is required for each minor device.
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If a file is in use, the open will fail and
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.Va errno
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will be set to
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.Er EBUSY .
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.Pp
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Associated with each open instance of a
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.Nm bpf
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file is a user-settable packet filter.
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Whenever a packet is received by an interface,
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all file descriptors listening on that interface apply their filter.
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Each descriptor that accepts the packet receives its own copy.
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.Pp
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Reads from these files return the next group of packets
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that have matched the filter.
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To improve performance, the buffer passed to read must be
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the same size as the buffers used internally by
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.Nm
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This size is returned by the
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.Dv BIOCGBLEN
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ioctl (see below), and
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can be set with
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.Dv BIOCSBLEN.
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Note that an individual packet larger than this size is necessarily
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truncated.
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.Pp
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The packet filter will support any link level protocol that has fixed length
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headers. Currently, only Ethernet,
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.Tn SLIP ,
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and
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.Tn PPP
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drivers have been modified to interact with
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.Nm bpf .
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.Pp
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Since packet data is in network byte order, applications should use the
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.Xr byteorder 3
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macros to extract multi-byte values.
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.Pp
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A packet can be sent out on the network by writing to a
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.Nm bpf
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file descriptor. The writes are unbuffered, meaning only one
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packet can be processed per write.
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Currently, only writes to Ethernets and
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.Tn SLIP
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links are supported.
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.Sh IOCTLS
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The
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.Xr ioctl 2
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command codes below are defined in <net/bpf.h>. All commands require
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these includes:
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.Bd -literal
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#include <sys/types.h>
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#include <sys/time.h>
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#include <sys/ioctl.h>
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#include <net/bpf.h>
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.Ed
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.Pp
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Additionally,
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.Dv BIOCGETIF
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and
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.Dv BIOCSETIF
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require
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.Aq Pa sys/socket.h
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and
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.Aq Pa net/if.h .
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In addition to
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.Dv FIONREAD
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and
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.Dv SIOCGIFADDR ,
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the following commands may be applied to any open
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.Nm
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file.
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The (third) argument to
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.Xr ioctl 2
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should be a pointer to the type indicated.
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.Bl -tag -width BIOCGRTIMEOUT
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.It Dv BIOCGBLEN
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.Pq Li u_int
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Returns the required buffer length for reads on
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.Nm
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files.
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.It Dv BIOCSBLEN
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.Pq Li u_int
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Sets the buffer length for reads on
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.Nm
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files. The buffer must be set before the file is attached to an interface
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with
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.Dv BIOCSETIF .
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If the requested buffer size cannot be accommodated, the closest
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allowable size will be set and returned in the argument.
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A read call will result in
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.Er EIO
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if it is passed a buffer that is not this size.
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.It Dv BIOCGDLT
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.Pq Li u_int
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Returns the type of the data link layer underlying the attached interface.
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.Er EINVAL
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is returned if no interface has been specified.
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The device types, prefixed with
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.Dq Li DLT_ ,
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are defined in
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.Aq Pa net/bpf.h .
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.It Dv BIOCPROMISC
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Forces the interface into promiscuous mode.
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All packets, not just those destined for the local host, are processed.
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Since more than one file can be listening on a given interface,
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a listener that opened its interface non-promiscuously may receive
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packets promiscuously. This problem can be remedied with an
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appropriate filter.
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.It Dv BIOCFLUSH
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Flushes the buffer of incoming packets,
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and resets the statistics that are returned by BIOCGSTATS.
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.It Dv BIOCGETIF
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.Pq Li "struct ifreq"
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Returns the name of the hardware interface that the file is listening on.
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The name is returned in the if_name field of
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the
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.Li ifreq
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structure.
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All other fields are undefined.
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.It Dv BIOCSETIF
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.Pq Li "struct ifreq"
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Sets the hardware interface associate with the file. This
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command must be performed before any packets can be read.
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The device is indicated by name using the
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.Li if_name
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field of the
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.Li ifreq
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structure.
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Additionally, performs the actions of
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.Dv BIOCFLUSH .
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.It Dv BIOCSRTIMEOUT
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.It Dv BIOCGRTIMEOUT
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.Pq Li "struct timeval"
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Set or get the read timeout parameter.
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The argument
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specifies the length of time to wait before timing
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out on a read request.
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This parameter is initialized to zero by
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.Xr open 2 ,
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indicating no timeout.
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.It Dv BIOCGSTATS
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.Pq Li "struct bpf_stat"
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Returns the following structure of packet statistics:
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.Bd -literal
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struct bpf_stat {
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u_int bs_recv; /* number of packets received */
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u_int bs_drop; /* number of packets dropped */
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};
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.Ed
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.Pp
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The fields are:
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.Bl -hang -offset indent
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.It Li bs_recv
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the number of packets received by the descriptor since opened or reset
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(including any buffered since the last read call);
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and
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.It Li bs_drop
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the number of packets which were accepted by the filter but dropped by the
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kernel because of buffer overflows
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(i.e., the application's reads aren't keeping up with the packet traffic).
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.El
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.It Dv BIOCIMMEDIATE
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.Pq Li u_int
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Enable or disable ``immediate mode'', based on the truth value of the argument.
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When immediate mode is enabled, reads return immediately upon packet
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reception. Otherwise, a read will block until either the kernel buffer
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becomes full or a timeout occurs.
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This is useful for programs like
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.Xr rarpd 8
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which must respond to messages in real time.
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The default for a new file is off.
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.It Dv BIOCSETF
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.Pq Li "struct bpf_program"
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Sets the filter program used by the kernel to discard uninteresting
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packets. An array of instructions and its length is passed in using
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the following structure:
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.Bd -literal
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struct bpf_program {
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int bf_len;
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struct bpf_insn *bf_insns;
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};
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.Ed
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The filter program is pointed to by the
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.Li bf_insns
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field while its length in units of
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.Sq Li struct bpf_insn
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is given by the
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.Li bf_len
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field.
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Also, the actions of
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.Dv BIOCFLUSH are performed.
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See section
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.Sx "FILTER MACHINE"
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for an explanation of the filter language.
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.It Dv BIOCVERSION
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.Pq Li "struct bpf_version"
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Returns the major and minor version numbers of the filter language currently
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recognized by the kernel. Before installing a filter, applications must check
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that the current version is compatible with the running kernel. Version
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numbers are compatible if the major numbers match and the application minor
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is less than or equal to the kernel minor. The kernel version number is
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returned in the following structure:
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.Bd -literal
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struct bpf_version {
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u_short bv_major;
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u_short bv_minor;
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};
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.Ed
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.Pp
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The current version numbers are given by
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.Dv BPF_MAJOR_VERSION
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and
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.Dv BPF_MINOR_VERSION
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from
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.Aq Pa net/bpf.h .
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An incompatible filter
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may result in undefined behavior (most likely, an error returned by
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.Fn ioctl
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or haphazard packet matching).
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.Sh BPF HEADER
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The following structure is prepended to each packet returned by
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.Xr read 2 :
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.Bd -literal
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struct bpf_hdr {
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struct timeval bh_tstamp; /* time stamp */
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u_long bh_caplen; /* length of captured portion */
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u_long bh_datalen; /* original length of packet */
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u_short bh_hdrlen; /* length of bpf header (this struct
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plus alignment padding */
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};
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.Ed
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.Pp
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The fields, whose values are stored in host order, and are:
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.Pp
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.Bl -tag -compact -width bh_datalen
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.It Li bh_tstamp
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The time at which the packet was processed by the packet filter.
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.It Li bh_caplen
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The length of the captured portion of the packet. This is the minimum of
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the truncation amount specified by the filter and the length of the packet.
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.It Li bh_datalen
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The length of the packet off the wire.
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This value is independent of the truncation amount specified by the filter.
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.It Li bh_hdrlen
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The length of the
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.Nm
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header, which may not be equal to
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.\" XXX - not really a function call
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.Fn sizeof "struct bpf_hdr" .
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.El
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.Pp
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The
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.Li bh_hdrlen
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field exists to account for
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padding between the header and the link level protocol.
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The purpose here is to guarantee proper alignment of the packet
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data structures, which is required on alignment sensitive
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architectures and improves performance on many other architectures.
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The packet filter insures that the
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.Li bpf_hdr
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and the network layer
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header will be word aligned. Suitable precautions
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must be taken when accessing the link layer protocol fields on alignment
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restricted machines. (This isn't a problem on an Ethernet, since
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the type field is a short falling on an even offset,
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and the addresses are probably accessed in a bytewise fashion).
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.Pp
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Additionally, individual packets are padded so that each starts
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on a word boundary. This requires that an application
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has some knowledge of how to get from packet to packet.
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The macro
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.Dv BPF_WORDALIGN
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is defined in
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.Aq Pa net/bpf.h
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to facilitate
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this process. It rounds up its argument
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to the nearest word aligned value (where a word is
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.Dv BPF_ALIGNMENT
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bytes wide).
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.Pp
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For example, if
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.Sq Li p
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points to the start of a packet, this expression
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will advance it to the next packet:
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.Dl p = (char *)p + BPF_WORDALIGN(p->bh_hdrlen + p->bh_caplen)
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.Pp
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For the alignment mechanisms to work properly, the
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buffer passed to
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.Xr read 2
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must itself be word aligned.
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The
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.Xr malloc 3
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function
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will always return an aligned buffer.
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.Sh FILTER MACHINE
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A filter program is an array of instructions, with all branches forwardly
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directed, terminated by a
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.Em return
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instruction.
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Each instruction performs some action on the pseudo-machine state,
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which consists of an accumulator, index register, scratch memory store,
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and implicit program counter.
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The following structure defines the instruction format:
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.Bd -literal
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struct bpf_insn {
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u_short code;
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u_char jt;
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u_char jf;
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u_long k;
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};
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.Ed
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The
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.Li k
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field is used in different ways by different instructions,
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and the
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.Li jt
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and
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.Li jf
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fields are used as offsets
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by the branch instructions.
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The opcodes are encoded in a semi-hierarchical fashion.
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There are eight classes of instructions:
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.Dv BPF_LD ,
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.Dv BPF_LDX ,
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.Dv BPF_ST ,
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.Dv BPF_STX ,
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.Dv BPF_ALU ,
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.Dv BPF_JMP ,
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.Dv BPF_RET ,
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and
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.Dv BPF_MISC .
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Various other mode and
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operator bits are or'd into the class to give the actual instructions.
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The classes and modes are defined in
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.Aq Pa net/bpf.h .
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Below are the semantics for each defined
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.Nm
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instruction.
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We use the convention that A is the accumulator, X is the index register,
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P[] packet data, and M[] scratch memory store.
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P[i:n] gives the data at byte offset ``i'' in the packet,
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interpreted as a word (n=4),
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unsigned halfword (n=2), or unsigned byte (n=1).
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M[i] gives the i'th word in the scratch memory store, which is only
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addressed in word units. The memory store is indexed from 0 to
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.Dv BPF_MEMWORDS
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- 1.
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.Li k ,
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.Li jt ,
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and
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.Li jf
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are the corresponding fields in the
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instruction definition. ``len'' refers to the length of the packet.
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.Pp
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.Bl -tag -width BPF_STXx -compact
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.It Dv BPF_LD
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These instructions copy a value into the accumulator. The type of the
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source operand is specified by an ``addressing mode'' and can be
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a constant
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.Pq Dv BPF_IMM ,
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packet data at a fixed offset
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.Pq Dv BPF_ABS ,
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packet data at a variable offset
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.Pq Dv BPF_IND ,
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the packet length
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.Pq Dv BPF_LEN ,
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or a word in the scratch memory store
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.Pq Dv BPF_MEM .
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For
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.Dv BPF_IND
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and
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.Dv BPF_ABS,
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the data size must be specified as a word
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.Pq Dv BPF_W ,
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halfword
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.Pq Dv BPF_H ,
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or byte
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.Pq Dv BPF_B .
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The semantics of all the recognized
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.Dv BPF_LD
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instructions follow.
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.Pp
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.Bl -tag -width "BPF_LD+BPF_W+BPF_IND" -compact
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.It Li BPF_LD+BPF_W+BPF_ABS
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A <- P[k:4]
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.It Li BPF_LD+BPF_H+BPF_ABS
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A <- P[k:2]
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.It Li BPF_LD+BPF_B+BPF_ABS
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A <- P[k:1]
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.It Li BPF_LD+BPF_W+BPF_IND
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A <- P[X+k:4]
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.It Li BPF_LD+BPF_H+BPF_IND
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A <- P[X+k:2]
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.It Li BPF_LD+BPF_B+BPF_IND
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A <- P[X+k:1]
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.It Li BPF_LD+BPF_W+BPF_LEN
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A <- len
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.It Li BPF_LD+BPF_IMM
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A <- k
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.It Li BPF_LD+BPF_MEM
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A <- M[k]
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.El
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.It Dv BPF_LDX
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These instructions load a value into the index register. Note that
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the addressing modes are more restrictive than those of the accumulator loads,
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but they include
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.Dv BPF_MSH ,
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a hack for efficiently loading the IP header length.
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.Bl -tag -width "BPF_LDX+BPF_W+BPF_MEM" -compact
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.It Li BPF_LDX+BPF_W+BPF_IMM
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X <- k
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.It Li BPF_LDX+BPF_W+BPF_MEM
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X <- M[k]
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.It Li BPF_LDX+BPF_W+BPF_LEN
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X <- len
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.It Li BPF_LDX+BPF_B+BPF_MSH
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X <- 4*(P[k:1]&0xf)
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.El
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.It Dv BPF_ST
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This instruction stores the accumulator into the scratch memory.
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We do not need an addressing mode since there is only one possibility
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for the destination.
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.Bl -tag -width "BPF_ST" -compact
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.It Li BPF_ST
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M[k] <- A
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.El
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.It Dv BPF_STX
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This instruction stores the index register in the scratch memory store.
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.Bl -tag -width "BPF_STX" -compact
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.It Li BPF_STX
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M[k] <- X
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.El
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.It Dv BPF_ALU
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The alu instructions perform operations between the accumulator and
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index register or constant, and store the result back in the accumulator.
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For binary operations, a source mode is required
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.Po
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.Dv BPF_K
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or
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.Dv BPF_X
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.Pc .
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.Bl -tag -width "BPF_ALU+BPF_MUL+BPF_K" -compact
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.It Li BPF_ALU+BPF_ADD+BPF_K
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A <- A + k
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.It Li BPF_ALU+BPF_SUB+BPF_K
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A <- A - k
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.It Li BPF_ALU+BPF_MUL+BPF_K
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A <- A * k
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.It Li BPF_ALU+BPF_DIV+BPF_K
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A <- A / k
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.It Li BPF_ALU+BPF_AND+BPF_K
|
|
A <- A & k
|
|
.It Li BPF_ALU+BPF_OR+BPF_K
|
|
A <- A | k
|
|
.It Li BPF_ALU+BPF_LSH+BPF_K
|
|
A <- A << k
|
|
.It Li BPF_ALU+BPF_RSH+BPF_K
|
|
A <- A >> k
|
|
.It Li BPF_ALU+BPF_ADD+BPF_X
|
|
A <- A + X
|
|
.It Li BPF_ALU+BPF_SUB+BPF_X
|
|
A <- A - X
|
|
.It Li BPF_ALU+BPF_MUL+BPF_X
|
|
A <- A * X
|
|
.It Li BPF_ALU+BPF_DIV+BPF_X
|
|
A <- A / X
|
|
.It Li BPF_ALU+BPF_AND+BPF_X
|
|
A <- A & X
|
|
.It Li BPF_ALU+BPF_OR+BPF_X
|
|
A <- A | X
|
|
.It Li BPF_ALU+BPF_LSH+BPF_X
|
|
A <- A << X
|
|
.It Li BPF_ALU+BPF_RSH+BPF_X
|
|
A <- A >> X
|
|
.It Li BPF_ALU+BPF_NEG
|
|
A <- -A
|
|
.El
|
|
|
|
.It Dv BPF_JMP
|
|
The jump instructions alter flow of control. Conditional jumps
|
|
compare the accumulator against a constant
|
|
.Pq Dv BPF_K
|
|
or the index register
|
|
.Pq Dv BPF_X .
|
|
If the result is true (or non-zero),
|
|
the true branch is taken, otherwise the false branch is taken.
|
|
Jump offsets are encoded in 8 bits so the longest jump is 256 instructions.
|
|
However, the jump always
|
|
.Pq Dv BPF_JA
|
|
opcode uses the 32 bit
|
|
.Li k
|
|
field as the offset, allowing arbitrarily distant destinations.
|
|
All conditionals use unsigned comparison conventions.
|
|
|
|
.Bl -tag -width "BPF_JMP+BPF_KSET+BPF_X" -compact
|
|
.It Li BPF_JMP+BPF_JA
|
|
pc += k
|
|
.It Li BPF_JMP+BPF_JGT+BPF_K
|
|
pc += (A > k) ? jt : jf
|
|
.It Li BPF_JMP+BPF_JGE+BPF_K
|
|
pc += (A >= k) ? jt : jf
|
|
.It Li BPF_JMP+BPF_JEQ+BPF_K
|
|
pc += (A == k) ? jt : jf
|
|
.It Li BPF_JMP+BPF_JSET+BPF_K
|
|
pc += (A & k) ? jt : jf
|
|
.It Li BPF_JMP+BPF_JGT+BPF_X
|
|
pc += (A > X) ? jt : jf
|
|
.It Li BPF_JMP+BPF_JGE+BPF_X
|
|
pc += (A >= X) ? jt : jf
|
|
.It Li BPF_JMP+BPF_JEQ+BPF_X
|
|
pc += (A == X) ? jt : jf
|
|
.It Li BPF_JMP+BPF_JSET+BPF_X
|
|
pc += (A & X) ? jt : jf
|
|
.El
|
|
|
|
.It Dv BPF_RET
|
|
The return instructions terminate the filter program and specify the amount
|
|
of packet to accept (i.e., they return the truncation amount). A return
|
|
value of zero indicates that the packet should be ignored.
|
|
The return value is either a constant
|
|
.Pq Dv BPF_K
|
|
or the accumulator
|
|
.Pq Dv BPF_A .
|
|
|
|
.Bl -tag -width "BPF_RET+BPF_K" -compact
|
|
.It Li BPF_RET+BPF_A
|
|
accept A bytes
|
|
.It Li BPF_RET+BPF_K
|
|
accept k bytes
|
|
.El
|
|
|
|
.It Dv BPF_MISC
|
|
The miscellaneous category was created for anything that doesn't
|
|
fit into the above classes, and for any new instructions that might need to
|
|
be added. Currently, these are the register transfer instructions
|
|
that copy the index register to the accumulator or vice versa.
|
|
|
|
.Bl -tag -width "BPF_MISC+BPF_TAX" -compact
|
|
.It Li BPF_MISC+BPF_TAX
|
|
X <- A
|
|
.It Li BPF_MISC+BPF_TXA
|
|
A <- X
|
|
.El
|
|
.Pp
|
|
The
|
|
.Nm
|
|
interface provides the following macros to facilitate
|
|
array initializers:
|
|
.Fn BPF_STMT opcode operand
|
|
and
|
|
.Fn BPF_JUMP opcode operand true_offset false_offset .
|
|
.Pp
|
|
.Sh EXAMPLES
|
|
The following filter is taken from the Reverse ARP Daemon. It accepts
|
|
only Reverse ARP requests.
|
|
.Bd -literal
|
|
struct bpf_insn insns[] = {
|
|
BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 12),
|
|
BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, ETHERTYPE_REVARP, 0, 3),
|
|
BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 20),
|
|
BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, REVARP_REQUEST, 0, 1),
|
|
BPF_STMT(BPF_RET+BPF_K, sizeof(struct ether_arp) +
|
|
sizeof(struct ether_header)),
|
|
BPF_STMT(BPF_RET+BPF_K, 0),
|
|
};
|
|
.Ed
|
|
.Pp
|
|
This filter accepts only IP packets between host 128.3.112.15 and
|
|
128.3.112.35.
|
|
.Bd -literal
|
|
struct bpf_insn insns[] = {
|
|
BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 12),
|
|
BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, ETHERTYPE_IP, 0, 8),
|
|
BPF_STMT(BPF_LD+BPF_W+BPF_ABS, 26),
|
|
BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x8003700f, 0, 2),
|
|
BPF_STMT(BPF_LD+BPF_W+BPF_ABS, 30),
|
|
BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x80037023, 3, 4),
|
|
BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x80037023, 0, 3),
|
|
BPF_STMT(BPF_LD+BPF_W+BPF_ABS, 30),
|
|
BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x8003700f, 0, 1),
|
|
BPF_STMT(BPF_RET+BPF_K, (u_int)-1),
|
|
BPF_STMT(BPF_RET+BPF_K, 0),
|
|
};
|
|
.Ed
|
|
.Pp
|
|
Finally, this filter returns only TCP finger packets. We must parse
|
|
the IP header to reach the TCP header. The
|
|
.Dv BPF_JSET
|
|
instruction
|
|
checks that the IP fragment offset is 0 so we are sure
|
|
that we have a TCP header.
|
|
.Bd -literal
|
|
struct bpf_insn insns[] = {
|
|
BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 12),
|
|
BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, ETHERTYPE_IP, 0, 10),
|
|
BPF_STMT(BPF_LD+BPF_B+BPF_ABS, 23),
|
|
BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, IPPROTO_TCP, 0, 8),
|
|
BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 20),
|
|
BPF_JUMP(BPF_JMP+BPF_JSET+BPF_K, 0x1fff, 6, 0),
|
|
BPF_STMT(BPF_LDX+BPF_B+BPF_MSH, 14),
|
|
BPF_STMT(BPF_LD+BPF_H+BPF_IND, 14),
|
|
BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 79, 2, 0),
|
|
BPF_STMT(BPF_LD+BPF_H+BPF_IND, 16),
|
|
BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 79, 0, 1),
|
|
BPF_STMT(BPF_RET+BPF_K, (u_int)-1),
|
|
BPF_STMT(BPF_RET+BPF_K, 0),
|
|
};
|
|
.Ed
|
|
.Sh SEE ALSO
|
|
.Xr tcpdump 1 ,
|
|
.Xr ioctl 2 ,
|
|
.Xr byteorder 3
|
|
.Rs
|
|
.%A McCanne, S.
|
|
.%A Jacobson V.
|
|
.%T "An efficient, extensible, and portable network monitor"
|
|
.Re
|
|
.Sh FILES
|
|
.Bl -tag -compact -width /dev/bpfXXX
|
|
.It Pa /dev/bpf Ns Sy n
|
|
the packet filter device
|
|
.El
|
|
.Sh BUGS
|
|
The read buffer must be of a fixed size (returned by the
|
|
.Dv BIOCGBLEN
|
|
ioctl).
|
|
.Pp
|
|
A file that does not request promiscuous mode may receive promiscuously
|
|
received packets as a side effect of another file requesting this
|
|
mode on the same hardware interface. This could be fixed in the kernel
|
|
with additional processing overhead. However, we favor the model where
|
|
all files must assume that the interface is promiscuous, and if
|
|
so desired, must utilize a filter to reject foreign packets.
|
|
.Pp
|
|
Data link protocols with variable length headers are not currently supported.
|
|
.Sh HISTORY
|
|
.Pp
|
|
The Enet packet filter was created in 1980 by Mike Accetta and
|
|
Rick Rashid at Carnegie-Mellon University. Jeffrey Mogul, at
|
|
Stanford, ported the code to BSD and continued its development from
|
|
1983 on. Since then, it has evolved into the Ultrix Packet Filter
|
|
at
|
|
.Tn DEC ,
|
|
a
|
|
.Tn STREAMS
|
|
.Tn NIT
|
|
module under
|
|
.Tn SunOS 4.1 ,
|
|
and
|
|
.Tn BPF .
|
|
.Sh AUTHORS
|
|
.An Steven McCanne ,
|
|
of Lawrence Berkeley Laboratory, implemented BPF in
|
|
Summer 1990. Much of the design is due to
|
|
.An Van Jacobson .
|