686 lines
19 KiB
Groff
686 lines
19 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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.TH BPF 4 "23 May 1991"
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.SH NAME
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bpf \- Berkeley Packet Filter
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.SH SYNOPSIS
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.B "pseudo-device bpfilter 16"
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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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.I /dev/bpf0, /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 BIOSETIF 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 SYNOPSIS 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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.I errno
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will be set to EBUSY.
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.PP
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Associated with each open instance of a
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.I 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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.I bpf.
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This size is returned by the BIOCGBLEN ioctl (see below), and under
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BSD, can be set with 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, SLIP and PPP drivers have been
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modified to interact with
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.I 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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.I byteorder(3n)
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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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.I 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 SLIP links are supported.
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.SH IOCTLS
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The
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.I ioctl
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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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.ft B
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.nf
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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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.fi
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.ft R
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Additionally, BIOCGETIF and BIOCSETIF require \fB<net/if.h>\fR.
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In addition to FIONREAD and SIOCGIFADDR, the following commands
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may be applied to any open
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.I bpf
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file.
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The (third) argument to the
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.I ioctl
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should be a pointer to the type indicated.
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.TP 10
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.B BIOCGBLEN (u_int)
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Returns the required buffer length for reads on
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.I bpf
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files.
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.TP 10
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.B BIOCSBLEN (u_int)
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Sets the buffer length for reads on
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.I bpf
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files. The buffer must be set before the file is attached to an interface
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with BIOCSETIF.
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If the requested buffer size cannot be accomodated, 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 EIO if it is passed a buffer that is not this size.
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.TP 10
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.B BIOCGDLT (u_int)
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Returns the type of the data link layer underyling the attached interface.
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EINVAL is returned if no interface has been specified.
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The device types, prefixed with ``DLT_'', are defined in <net/bpf.h>.
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.TP 10
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.B 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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.IP
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The interface remains in promiscuous mode until all files listening
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promiscuously are closed.
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.TP 10
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.B 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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.TP 10
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.B BIOCGETIF (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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.I ifr.
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All other fields are undefined.
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.TP 10
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.B BIOCSETIF (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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.I if_name
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field of the
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.I ifreq.
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Additionally, performs the actions of BIOCFLUSH.
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.TP 10
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.B BIOCSRTIMEOUT, BIOCGRTIMEOUT (struct timeval)
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Set or get the read timeout parameter.
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The
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.I timeval
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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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.IR open(2),
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indicating no timeout.
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.TP 10
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.B BIOCGSTATS (struct bpf_stat)
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Returns the following structure of packet statistics:
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.ft B
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.nf
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struct bpf_stat {
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u_int bs_recv;
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u_int bs_drop;
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};
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.fi
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.ft R
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.IP
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The fields are:
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.RS
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.TP 15
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.I 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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.TP
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.I 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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.RE
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.TP 10
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.B BIOCIMMEDIATE (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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.I rarpd(8c),
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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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.TP 10
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.B BIOCSETF (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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.ft B
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.nf
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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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.fi
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.ft R
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.IP
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The filter program is pointed to by the
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.I bf_insns
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field while its length in units of `struct bpf_insn' is given by the
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.I bf_len
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field.
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Also, the actions of BIOCFLUSH are performed.
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.IP
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See section \fBFILTER MACHINE\fP for an explanation of the filter language.
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.TP 10
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.B BIOCVERSION (struct bpf_version)
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Returns the major and minor version numbers of the filter languange 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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.ft B
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.nf
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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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.fi
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.ft R
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.IP
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The current version numbers are given by
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.B BPF_MAJOR_VERSION
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and
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.B BPF_MINOR_VERSION
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from <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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.I 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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.I read(2):
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.in 15
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.ft B
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.nf
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struct bpf_hdr {
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struct timeval bh_tstamp;
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u_long bh_caplen;
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u_long bh_datalen;
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u_short bh_hdrlen;
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};
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.fi
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.ft R
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.in -15
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.PP
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The fields, whose values are stored in host order, and are:
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.TP 15
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.I bh_tstamp
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The time at which the packet was processed by the packet filter.
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.TP
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.I 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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.TP
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.I 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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.TP
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.I bh_hdrlen
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The length of the BPF header, which may not be equal to
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.I sizeof(struct bpf_hdr).
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.RE
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.PP
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The
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.I 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 and improves performance on many other architectures.
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The packet filter insures that the
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.I bpf_hdr
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and the \fInetwork layer\fR 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 BPF_WORDALIGN is defined in <net/bpf.h> 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 BPF_ALIGNMENT bytes wide).
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.PP
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For example, if `p' points to the start of a packet, this expression
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will advance it to the next packet:
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.sp
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.RS
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.ce 1
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.nf
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p = (char *)p + BPF_WORDALIGN(p->bh_hdrlen + p->bh_caplen)
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.fi
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.RE
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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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.I read(2)
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must itself be word aligned.
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.I malloc(3)
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will always return an aligned buffer.
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.ft R
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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 \fBreturn\fP 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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.RS
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.ft B
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.nf
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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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long k;
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};
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.fi
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.ft R
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.RE
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The \fIk\fP field is used in differnet ways by different insutructions,
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and the \fIjt\fP and \fIjf\fP fields are used as offsets
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by the branch intructions.
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The opcodes are encoded in a semi-hierarchical fashion.
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There are eight classes of intructions: BPF_LD, BPF_LDX, BPF_ST, BPF_STX,
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BPF_ALU, BPF_JMP, BPF_RET, and BPF_MISC. 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 <net/bpf.h>.
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Below are the semantics for each defined BPF 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 BPF_MEMWORDS-1.
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\fIk\fP, \fIjt\fP, and \fIjf\fP are the corresponding fields in the
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instruction definition. ``len'' refers to the length of the packet.
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.TP 10
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.B 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 (\fBBPF_IMM\fP), packet data at a fixed offset (\fBBPF_ABS\fP),
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packet data at a variable offset (\fBBPF_IND\fP), the packet length
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(\fBBPF_LEN\fP),
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or a word in the scratch memory store (\fBBPF_MEM\fP).
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For \fBBPF_IND\fP and \fBBPF_ABS\fP, the data size must be specified as a word
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(\fBBPF_W\fP), halfword (\fBBPF_H\fP), or byte (\fBBPF_B\fP).
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The semantics of all the recognized BPF_LD instructions follow.
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.RS
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.TP 30
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.B BPF_LD+BPF_W+BPF_ABS
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A <- P[k:4]
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.TP
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.B BPF_LD+BPF_H+BPF_ABS
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A <- P[k:2]
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.TP
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.B BPF_LD+BPF_B+BPF_ABS
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A <- P[k:1]
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.TP
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.B BPF_LD+BPF_W+BPF_IND
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A <- P[X+k:4]
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.TP
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.B BPF_LD+BPF_H+BPF_IND
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A <- P[X+k:2]
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.TP
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.B BPF_LD+BPF_B+BPF_IND
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A <- P[X+k:1]
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.TP
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.B BPF_LD+BPF_W+BPF_LEN
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A <- len
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.TP
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.B BPF_LD+BPF_IMM
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A <- k
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.TP
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.B BPF_LD+BPF_MEM
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A <- M[k]
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.RE
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.TP 10
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.B 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 retricted than those of the accumulator loads,
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but they include
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.B BPF_MSH,
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a hack for efficiently loading the IP header length.
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.RS
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.TP 30
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.B BPF_LDX+BPF_W+BPF_IMM
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X <- k
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.TP
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.B BPF_LDX+BPF_W+BPF_MEM
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X <- M[k]
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.TP
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.B BPF_LDX+BPF_W+BPF_LEN
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X <- len
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.TP
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.B BPF_LDX+BPF_B+BPF_MSH
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X <- 4*(P[k:1]&0xf)
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.RE
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.TP 10
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.B 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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.RS
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.TP 30
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.B BPF_ST
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M[k] <- A
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.RE
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.TP 10
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.B BPF_STX
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This instruction stores the index register in the scratch memory store.
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.RS
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.TP 30
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.B BPF_STX
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M[k] <- X
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.RE
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.TP 10
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.B 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 (\fBBPF_K\fP or
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\fBBPF_X\fP).
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.RS
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.TP 30
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.B BPF_ALU+BPF_ADD+BPF_K
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A <- A + k
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.TP
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.B BPF_ALU+BPF_SUB+BPF_K
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A <- A - k
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.TP
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.B BPF_ALU+BPF_MUL+BPF_K
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A <- A * k
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.TP
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.B BPF_ALU+BPF_DIV+BPF_K
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A <- A / k
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.TP
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.B BPF_ALU+BPF_AND+BPF_K
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A <- A & k
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.TP
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.B BPF_ALU+BPF_OR+BPF_K
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A <- A | k
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.TP
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.B BPF_ALU+BPF_LSH+BPF_K
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A <- A << k
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.TP
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.B BPF_ALU+BPF_RSH+BPF_K
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A <- A >> k
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.TP
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.B BPF_ALU+BPF_ADD+BPF_X
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A <- A + X
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.TP
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.B BPF_ALU+BPF_SUB+BPF_X
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A <- A - X
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.TP
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.B BPF_ALU+BPF_MUL+BPF_X
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A <- A * X
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.TP
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.B BPF_ALU+BPF_DIV+BPF_X
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A <- A / X
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.TP
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.B BPF_ALU+BPF_AND+BPF_X
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A <- A & X
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.TP
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.B BPF_ALU+BPF_OR+BPF_X
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A <- A | X
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.TP
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.B BPF_ALU+BPF_LSH+BPF_X
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A <- A << X
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.TP
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.B BPF_ALU+BPF_RSH+BPF_X
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A <- A >> X
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.TP
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.B BPF_ALU+BPF_NEG
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A <- -A
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.RE
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.TP 10
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.B BPF_JMP
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The jump instructions alter flow of control. Conditional jumps
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|
compare the accumulator against a constant (\fBBPF_K\fP) or
|
|
the index register (\fBBPF_X\fP). 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 (\fBBPF_JA\fP) opcode uses the 32 bit \fIk\fP
|
|
field as the offset, allowing arbitrarily distant destinations.
|
|
All conditionals use unsigned comparison conventions.
|
|
.RS
|
|
.TP 30
|
|
.B BPF_JMP+BPF_JA
|
|
pc += k
|
|
.TP
|
|
.B BPF_JMP+BPF_JGT+BPF_K
|
|
pc += (A > k) ? jt : jf
|
|
.TP
|
|
.B BPF_JMP+BPF_JGE+BPF_K
|
|
pc += (A >= k) ? jt : jf
|
|
.TP
|
|
.B BPF_JMP+BPF_JEQ+BPF_K
|
|
pc += (A == k) ? jt : jf
|
|
.TP
|
|
.B BPF_JMP+BPF_JSET+BPF_K
|
|
pc += (A & k) ? jt : jf
|
|
.TP
|
|
.B BPF_JMP+BPF_JGT+BPF_X
|
|
pc += (A > X) ? jt : jf
|
|
.TP
|
|
.B BPF_JMP+BPF_JGE+BPF_X
|
|
pc += (A >= X) ? jt : jf
|
|
.TP
|
|
.B BPF_JMP+BPF_JEQ+BPF_X
|
|
pc += (A == X) ? jt : jf
|
|
.TP
|
|
.B BPF_JMP+BPF_JSET+BPF_X
|
|
pc += (A & X) ? jt : jf
|
|
.RE
|
|
.TP 10
|
|
.B 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 (\fBBPF_K\fP) or the accumulator
|
|
(\fBBPF_A\fP).
|
|
.RS
|
|
.TP 30
|
|
.B BPF_RET+BPF_A
|
|
accept A bytes
|
|
.TP
|
|
.B BPF_RET+BPF_K
|
|
accept k bytes
|
|
.RE
|
|
.TP 10
|
|
.B 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 intructions
|
|
that copy the index register to the accumulator or vice versa.
|
|
.RS
|
|
.TP 30
|
|
.B BPF_MISC+BPF_TAX
|
|
X <- A
|
|
.TP
|
|
.B BPF_MISC+BPF_TXA
|
|
A <- X
|
|
.RE
|
|
.PP
|
|
The BPF interface provides the following macros to facilitate
|
|
array initializers:
|
|
.RS
|
|
\fBBPF_STMT\fI(opcode, operand)\fR
|
|
.br
|
|
and
|
|
.br
|
|
\fBBPF_JUMP\fI(opcode, operand, true_offset, false_offset)\fR
|
|
.RE
|
|
.PP
|
|
.SH EXAMPLES
|
|
The following filter is taken from the Reverse ARP Daemon. It accepts
|
|
only Reverse ARP requests.
|
|
.RS
|
|
.nf
|
|
|
|
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),
|
|
};
|
|
.fi
|
|
.RE
|
|
.PP
|
|
This filter accepts only IP packets between host 128.3.112.15 and
|
|
128.3.112.35.
|
|
.RS
|
|
.nf
|
|
|
|
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_H+BPF_ABS, 26),
|
|
BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x8003700f, 0, 2),
|
|
BPF_STMT(BPF_LD+BPF_H+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_H+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),
|
|
};
|
|
.fi
|
|
.RE
|
|
.PP
|
|
Finally, this filter returns only TCP finger packets. We must parse
|
|
the IP header to reach the TCP header. The \fBBPF_JSET\fP instruction
|
|
checks that the IP fragment offset is 0 so we are sure
|
|
that we have a TCP header.
|
|
.RS
|
|
.nf
|
|
|
|
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),
|
|
};
|
|
.fi
|
|
.RE
|
|
.SH SEE ALSO
|
|
tcpdump(8)
|
|
.LP
|
|
McCanne, S., Jacobson V.,
|
|
.RI ` "An efficient, extensible, and portable network monitor" '
|
|
.SH FILES
|
|
/dev/bpf0, /dev/bpf1, ...
|
|
.SH BUGS
|
|
The read buffer must be of a fixed size (returned by the 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.
|
|
.PP
|
|
Under SunOS, if a BPF application reads more than 2^31 bytes of
|
|
data, read will fail in EINVAL. You can either fix the bug in SunOS,
|
|
or lseek to 0 when read fails for this reason.
|
|
.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 DEC, a STREAMS NIT module under SunOS 4.1, and BPF.
|
|
.SH AUTHORS
|
|
.PP
|
|
Steven McCanne, of Lawrence Berkeley Laboratory, implemented BPF in
|
|
Summer 1990. Much of the design is due to Van Jacobson.
|