BPF(4) BPF(4)
NAME
bpf - Berkeley Packet Filter
SYNOPSIS
pseudo-device bpfilter 16
DESCRIPTION
The Berkeley Packet Filter provides a raw interface to
data link layers in a protocol independent fashion. All
packets on the network, even those destined for other
hosts, are accessible through this mechanism.
The packet filter appears as a character special device,
/dev/bpf0, /dev/bpf1, etc. After opening the device, the
file descriptor must be bound to a specific network inter-
face with the BIOSETIF ioctl. A given interface can be
shared be multiple listeners, and the filter underlying
each descriptor will see an identical packet stream. The
total number of open files is limited to the value given
in the kernel configuration; the example given in the SYN-
OPSIS above sets the limit to 16.
A separate device file is required for each minor device.
If a file is in use, the open will fail and errno will be
set to EBUSY.
Associated with each open instance of a bpf file is a
user-settable packet filter. Whenever a packet is
received by an interface, all file descriptors listening
on that interface apply their filter. Each descriptor
that accepts the packet receives its own copy.
Reads from these files return the next group of packets
that have matched the filter. To improve performance, the
buffer passed to read must be the same size as the buffers
used internally by bpf. This size is returned by the
BIOCGBLEN ioctl (see below), and under BSD, can be set
with BIOCSBLEN. Note that an individual packet larger
than this size is necessarily truncated.
The packet filter will support any link level protocol
that has fixed length headers. Currently, only Ethernet,
SLIP and PPP drivers have been modified to interact with
bpf.
Since packet data is in network byte order, applications
should use the byteorder(3n) macros to extract multi-byte
values.
A packet can be sent out on the network by writing to a
bpf file descriptor. The writes are unbuffered, meaning
only one packet can be processed per write. Currently,
only writes to Ethernets and SLIP links are supported.
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BPF(4) BPF(4)
IOCTLS
The ioctl command codes below are defined in <net/bpf.h>.
All commands require these includes:
#include <sys/types.h>
#include <sys/time.h>
#include <sys/ioctl.h>
#include <net/bpf.h>
Additionally, BIOCGETIF and BIOCSETIF require <net/if.h>.
In addition to FIONREAD and SIOCGIFADDR, the following
commands may be applied to any open bpf file. The (third)
argument to the ioctl should be a pointer to the type
indicated.
BIOCGBLEN (uint)
Returns the required buffer length for reads on
bpf files.
BIOCSBLEN (uint)
Sets the buffer length for reads on bpf files.
The buffer must be set before the file is
attached to an interface with BIOCSETIF. If the
requested buffer size cannot be accomodated, the
closest allowable size will be set and returned
in the argument. A read call will result in EIO
if it is passed a buffer that is not this size.
BIOCGDLT (uint)
Returns the type of the data link layer under-
yling the attached interface. EINVAL is
returned if no interface has been specified.
The device types, prefixed with ``DLT_'', are
defined in <net/bpf.h>.
BIOCPROMISC
Forces the interface into promiscuous mode. All
packets, not just those destined for the local
host, are processed. Since more than one file
can be listening on a given interface, a lis-
tener that opened its interface non-
promiscuously may receive packets promiscuously.
This problem can be remedied with an appropriate
filter.
The interface remains in promiscuous mode until
all files listening promiscuously are closed.
BIOCFLUSH Flushes the buffer of incoming packets, and
resets the statistics that are returned by
BIOCGSTATS.
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BPF(4) BPF(4)
BIOCGETIF (struct ifreq)
Returns the name of the hardware interface that
the file is listening on. The name is returned
in the if_name field of ifr. All other fields
are undefined.
BIOCSETIF (struct ifreq)
Sets the hardware interface associate with the
file. This command must be performed before any
packets can be read. The device is indicated by
name using the ifname field of the ifreq.
Additionally, performs the actions of BIOCFLUSH.
BIOCSRTIMEOUT, BIOCGRTIMEOUT (struct timeval)
Set or get the read timeout parameter. The
timeval specifies the length of time to wait
before timing out on a read request. This
parameter is initialized to zero by open(2),
indicating no timeout.
BIOCGSTATS (struct bpfstat)
Returns the following structure of packet
statistics:
struct bpfstat {
uint bsrecv;
uint bsdrop;
};
The fields are:
bsrecv the number of packets received by
the descriptor since opened or
reset (including any buffered
since the last read call); and
bsdrop the number of packets which were
accepted by the filter but
dropped by the kernel because of
buffer overflows (i.e., the
application's reads aren't keep-
ing up with the packet traffic).
BIOCIMMEDIATE (uint)
Enable or disable ``immediate mode'', based on
the truth value of the argument. When immediate
mode is enabled, reads return immediately upon
packet reception. Otherwise, a read will block
until either the kernel buffer becomes full or a
timeout occurs. This is useful for programs
like rarpd(8c), which must respond to messages
in real time. The default for a new file is
off.
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BPF(4) BPF(4)
BIOCSETF (struct bpfprogram)
Sets the filter program used by the kernel to
discard uninteresting packets. An array of
instructions and its length is passed in using
the following structure:
struct bpfprogram {
int bflen;
struct bpfinsn *bfinsns;
};
The filter program is pointed to by the bfinsns
field while its length in units of `struct
bpf_insn' is given by the bflen field. Also,
the actions of BIOCFLUSH are performed.
See section FILTER MACHINE for an explanation of
the filter language.
BIOCVERSION (struct bpfversion)
Returns the major and minor version numbers of
the filter languange currently recognized by the
kernel. Before installing a filter, applica-
tions must check that the current version is
compatible with the running kernel. Version
numbers are compatible if the major numbers
match and the application minor is less than or
equal to the kernel minor. The kernel version
number is returned in the following structure:
struct bpfversion {
ushort bvmajor;
ushort bvminor;
};
The current version numbers are given by
BPFMAJORVERSION and BPFMINORVERSION from
<net/bpf.h>. An incompatible filter may result
in undefined behavior (most likely, an error
returned by ioctl() or haphazard packet match-
ing).
BPF HEADER
The following structure is prepended to each packet
returned by read(2):
struct bpfhdr {
struct timeval bhtstamp;
ulong bhcaplen;
ulong bhdatalen;
ushort bhhdrlen;
};
The fields, whose values are stored in host order, and
23 May 1991 4
BPF(4) BPF(4)
are:
bhtstamp The time at which the packet was processed
by the packet filter.
bhcaplen The length of the captured portion of the
packet. This is the minimum of the trunca-
tion amount specified by the filter and the
length of the packet.
bhdatalen The length of the packet off the wire.
This value is independent of the truncation
amount specified by the filter.
bhhdrlen The length of the BPF header, which may not
be equal to sizeof(struct bpfhdr).
The bhhdrlen field exists to account for padding between
the header and the link level protocol. The purpose here
is to guarantee proper alignment of the packet data struc-
tures, which is required on alignment sensitive architec-
tures and and improves performance on many other architec-
tures. The packet filter insures that the bpfhdr and the
network layer header will be word aligned. Suitable pre-
cautions must be taken when accessing the link layer pro-
tocol fields on alignment restricted machines. (This
isn't a problem on an Ethernet, since the type field is a
short falling on an even offset, and the addresses are
probably accessed in a bytewise fashion).
Additionally, individual packets are padded so that each
starts on a word boundary. This requires that an applica-
tion has some knowledge of how to get from packet to
packet. The macro BPF_WORDALIGN is defined in <net/bpf.h>
to facilitate this process. It rounds up its argument to
the nearest word aligned value (where a word is
BPF_ALIGNMENT bytes wide).
For example, if `p' points to the start of a packet, this
expression will advance it to the next packet:
p = (char *)p + BPF_WORDALIGN(p->bh_hdrlen + p->bh_caplen)
For the alignment mechanisms to work properly, the buffer
passed to read(2) must itself be word aligned. malloc(3)
will always return an aligned buffer.
FILTER MACHINE
A filter program is an array of instructions, with all
branches forwardly directed, terminated by a return
instruction. Each instruction performs some action on the
pseudo-machine state, which consists of an accumulator,
index register, scratch memory store, and implicit program
counter.
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BPF(4) BPF(4)
The following structure defines the instruction format:
struct bpfinsn {
ushort code;
uchar jt;
uchar jf;
long k;
};
The k field is used in differnet ways by different
insutructions, and the jt and jf fields are used as off-
sets by the branch intructions. The opcodes are encoded
in a semi-hierarchical fashion. There are eight classes
of intructions: BPF_LD, BPF_LDX, BPF_ST, BPF_STX, BPF_ALU,
BPF_JMP, BPF_RET, and BPF_MISC. Various other mode and
operator bits are or'd into the class to give the actual
instructions. The classes and modes are defined in
<net/bpf.h>.
Below are the semantics for each defined BPF instruction.
We use the convention that A is the accumulator, X is the
index register, P[] packet data, and M[] scratch memory
store. P[i:n] gives the data at byte offset ``i'' in the
packet, interpreted as a word (n=4), unsigned halfword
(n=2), or unsigned byte (n=1). M[i] gives the i'th word
in the scratch memory store, which is only addressed in
word units. The memory store is indexed from 0 to
BPF_MEMWORDS-1. k, jt, and jf are the corresponding
fields in the instruction definition. ``len'' refers to
the length of the packet.
BPFLD These instructions copy a value into the accumu-
lator. The type of the source operand is speci-
fied by an ``addressing mode'' and can be a con-
stant (BPFIMM), packet data at a fixed offset
(BPFABS), packet data at a variable offset
(BPFIND), the packet length (BPFLEN), or a
word in the scratch memory store (BPFMEM). For
BPFIND and BPFABS, the data size must be spec-
ified as a word (BPFW), halfword (BPFH), or
byte (BPFB). The semantics of all the recog-
nized BPF_LD instructions follow.
BPFLD+BPFW+BPFABS A <- P[k:4]
BPFLD+BPFH+BPFABS A <- P[k:2]
BPFLD+BPFB+BPFABS A <- P[k:1]
BPFLD+BPFW+BPFIND A <- P[X+k:4]
BPFLD+BPFH+BPFIND A <- P[X+k:2]
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BPF(4) BPF(4)
BPFLD+BPFB+BPFIND A <- P[X+k:1]
BPFLD+BPFW+BPFLEN A <- len
BPFLD+BPFIMM A <- k
BPFLD+BPFMEM A <- M[k]
BPFLDX These instructions load a value into the index
register. Note that the addressing modes are
more retricted than those of the accumulator
loads, but they include BPFMSH, a hack for
efficiently loading the IP header length.
BPFLDX+BPFW+BPFIMM X <- k
BPFLDX+BPFW+BPFMEM X <- M[k]
BPFLDX+BPFW+BPFLEN X <- len
BPFLDX+BPFB+BPFMSH X <-
4*(P[k:1]&0xf)
BPFST This instruction stores the accumulator into the
scratch memory. We do not need an addressing
mode since there is only one possibility for the
destination.
BPFST M[k] <- A
BPFSTX This instruction stores the index register in
the scratch memory store.
BPFSTX M[k] <- X
BPFALU The alu instructions perform operations between
the accumulator and index register or constant,
and store the result back in the accumulator.
For binary operations, a source mode is required
(BPFK or BPFX).
BPFALU+BPFADD+BPFK A <- A + k
BPFALU+BPFSUB+BPFK A <- A - k
BPFALU+BPFMUL+BPFK A <- A * k
BPFALU+BPFDIV+BPFK A <- A / k
BPFALU+BPFAND+BPFK A <- A & k
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BPF(4) BPF(4)
BPFALU+BPFOR+BPFK A <- A | k
BPFALU+BPFLSH+BPFK A <- A << k
BPFALU+BPFRSH+BPFK A <- A >> k
BPFALU+BPFADD+BPFX A <- A + X
BPFALU+BPFSUB+BPFX A <- A - X
BPFALU+BPFMUL+BPFX A <- A * X
BPFALU+BPFDIV+BPFX A <- A / X
BPFALU+BPFAND+BPFX A <- A & X
BPFALU+BPFOR+BPFX A <- A | X
BPFALU+BPFLSH+BPFX A <- A << X
BPFALU+BPFRSH+BPFX A <- A >> X
BPFALU+BPFNEG A <- -A
BPFJMP The jump instructions alter flow of control.
Conditional jumps compare the accumulator
against a constant (BPFK) or the index register
(BPFX). 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 (BPFJA) opcode uses
the 32 bit k field as the offset, allowing arbi-
trarily distant destinations. All conditionals
use unsigned comparison conventions.
BPFJMP+BPFJA pc += k
BPFJMP+BPFJGT+BPFK pc += (A > k) ? jt
: jf
BPFJMP+BPFJGE+BPFK pc += (A >= k) ?
jt : jf
BPFJMP+BPFJEQ+BPFK pc += (A == k) ?
jt : jf
BPFJMP+BPFJSET+BPFK pc += (A & k) ? jt
: jf
BPFJMP+BPFJGT+BPFX pc += (A > X) ? jt
: jf
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BPF(4) BPF(4)
BPFJMP+BPFJGE+BPFX pc += (A >= X) ?
jt : jf
BPFJMP+BPFJEQ+BPFX pc += (A == X) ?
jt : jf
BPFJMP+BPFJSET+BPFX pc += (A & X) ? jt
: jf
BPFRET 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 (BPFK) or the accumulator
(BPFA).
BPFRET+BPFA accept A bytes
BPFRET+BPFK accept k bytes
BPFMISC The miscellaneous category was created for any-
thing 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 regis-
ter to the accumulator or vice versa.
BPFMISC+BPFTAX X <- A
BPFMISC+BPFTXA A <- X
The BPF interface provides the following macros to facili-
tate array initializers:
BPFSTMT(opcode, operand)
and
BPFJUMP(opcode, operand, trueoffset,
falseoffset)
EXAMPLES
The following filter is taken from the Reverse ARP Daemon.
It accepts only Reverse ARP requests.
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),
};
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BPF(4) BPF(4)
This filter accepts only IP packets between host
128.3.112.15 and 128.3.112.35.
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),
};
Finally, this filter returns only TCP finger packets. We
must parse the IP header to reach the TCP header. The
BPFJSET instruction checks that the IP fragment offset is
0 so we are sure that we have a TCP header.
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),
};
SEE ALSO
tcpdump(1)
McCanne, S., Jacobson V., `An efficient, extensible, and
portable network monitor'
FILES
/dev/bpf0, /dev/bpf1, ...
BUGS
The read buffer must be of a fixed size (returned by the
BIOCGBLEN ioctl).
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.
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BPF(4) BPF(4)
This could be fixed in the kernel with additional process-
ing 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.
Data link protocols with variable length headers are not
currently supported.
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.
HISTORY
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.
AUTHORS
Steven McCanne, of Lawrence Berkeley Laboratory, imple-
mented BPF in Summer 1990. Much of the design is due to
Van Jacobson.
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