bpf(4)
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BPF(4) NetBSD Programmer's Manual BPF(4)
NAME
bpf - Berkeley Packet Filter raw network interface
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 interface 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 SYNOPSIS 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 pack-
et filter. Whenever a packet is received by an interface, all file de-
scriptors listening on that interface apply their filter. Each descrip-
tor 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 re-
turned by the BIOCGBLEN ioctl (see below), and under BSD, can be set with
BIOCSBLEN. Note that an individual packet larger than this size is nec-
essarily 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(3) macros to extract multi-byte values.
A packet can be sent out on the network by writing to a bpf file descrip-
tor. The writes are unbuffered, meaning only one packet can be processed
per write. Currently, only writes to Ethernets and SLIP links are sup-
ported.
IOCTLS
The ioctl(2) command codes below are defined in <net/bpf.h>. All com-
mands 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>.
The (third) argument to the ioctl(2) should be a pointer to the type in-
dicated.
BIOCGBLEN (u_int)
Returns the required buffer length for reads on bpf files.
BIOCSBLEN (u_int)
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
accommodated, 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 (u_int)
Returns the type of the data link layer underlying the at-
tached 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 in-
terface, a listener that opened its interface non-promis-
cuously 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.
BIOCGETIF (struct ifreq)
Returns the name of the hardware interface that the file
is listening on. The name is returned in the ifr_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 ifr_name field
of the ifreq. Additionally, performs the actions of
BIOCFLUSH.
BIOCSRTIMEOUT, BIOCGRTIMEOUT (struct timeval)
Set or get the read timeout parameter. The timeval speci-
fies 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 bpf_stat)
Returns the following structure of packet statistics:
struct bpf_stat {
u_int bs_recv;
u_int bs_drop;
};
The fields are:
bs_recv the number of packets received by the de-
scriptor since opened or reset (including
any buffered since the last read call); and
bs_drop the number of packets which were accepted
by the filter but dropped by the kernel be-
cause of buffer overflows (i.e., the appli-
cation's reads aren't keeping up with the
packet traffic).
BIOCIMMEDIATE (u_int)
Enable or disable ``immediate mode'', based on the truth
value of the argument. When immediate mode is enabled,
reads return immediately upon packet reception. Other-
wise, a read will block until either the kernel buffer be-
comes full or a timeout occurs. This is useful for pro-
grams like rarpd(8), which must respond to messages in re-
al time. The default for a new file is off.
BIOCSETF (struct bpf_program)
Sets the filter program used by the kernel to discard un-
interesting packets. An array of instructions and its
length is passed in using the following structure:
struct bpf_program {
int bf_len;
struct bpf_insn *bf_insns;
};
The filter program is pointed to by the bf_insns field
while its length in units of `struct bpf_insn' is given by
the bf_len field. Also, the actions of BIOCFLUSH are per-
formed.
See section FILTER MACHINE for an explanation of the fil-
ter language.
BIOCVERSION (struct bpf_version)
Returns the major and minor version numbers of the filter
language currently recognized by the kernel. Before in-
stalling a filter, applications must check that the cur-
rent version is compatible with the running kernel. Ver-
sion 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 fol-
lowing structure:
struct bpf_version {
u_short bv_major;
u_short bv_minor;
};
The current version numbers are given by BPF_MAJOR_VERSION
and BPF_MINOR_VERSION from <net/bpf.h>. An incompatible
filter may result in undefined behavior (most likely, an
error returned by ioctl(2) or haphazard packet matching).
BIOCSRSIG BIOCGRSIG (u_int signal)
Set or get the receive signal. This signal will be sent
to the process or process group specified by FIOSETOWN.
It defaults to SIGIO.
STANDARD IOCTLS
bpf now supports several standard ioctl(2)'s which allow the user to do
async and/or non-blocking I/O to an open file descriptor.
FIONREAD (int)
Returns the number of bytes that are immediately available
for reading.
SIOCGIFADDR (struct ifreq)
Returns the address associated with the interface.
FIONBIO (int)
Set or clear non-blocking I/O. If arg is non-zero, then
doing a read(2) when no data is available will return -1
and errno will be set to EWOULDBLOCK. If arg is zero,
non-blocking I/O is disabled. Note: setting this over-
rides the timeout set by BIOCSRTIMEOUT.
FIOASYNC (int)
Enable or disable async I/O. When enabled (arg is non-ze-
ro), the process or process group specified by FIOSETOWN
will start receiving SIGIO's when packets arrive. Note
that you must do an FIOSETOWN in order for this to take
affect, as the system will not default this for you. The
signal may be changed via BIOCSRSIG.
FIOSETOWN FIOGETOWN (int)
Set or get the process or process group (if negative) that
should receive SIGIO when packets are available. The sig-
nal may be changed using BIOCSRSIG (see above).
The following structure is prepended to each packet returned by read(2):
struct bpf_hdr {
struct timeval bh_tstamp;
u_long bh_caplen;
u_long bh_datalen;
u_short bh_hdrlen;
};
The fields, whose values are stored in host order, and are:
bh_tstamp The time at which the packet was processed by the pack-
et filter.
bh_caplen The length of the captured portion of the packet. This
is the minimum of the truncation amount specified by
the filter and the length of the packet.
bh_datalen The length of the packet off the wire. This value is
independent of the truncation amount specified by the
filter.
bh_hdrlen The length of the BPF header, which may not be equal to
sizeof(struct bpf_hdr).
The bh_hdrlen field exists to account for padding between the header and
the link level protocol. The purpose here is to guarantee proper align-
ment of the packet data structures, which is required on alignment sensi-
tive architectures and and improves performance on many other architec-
tures. The packet filter insures that the bpf_hdr and the network layer
header will be word aligned. Suitable precautions must be taken when ac-
cessing the link layer protocol 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 application 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 accumula-
tor, index register, scratch memory store, and implicit program counter.
The following structure defines the instruction format:
struct bpf_insn {
u_short code;
u_char jt;
u_char jf;
long k;
};
The k field is used in different ways by different instructions, and the
jt and jf fields are used as offsets by the branch instructions. The op-
codes are encoded in a semi-hierarchical fashion. There are eight class-
es of instructions: 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 in-
to 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.
BPF_LD These instructions copy a value into the accumulator. The
type of the source operand is specified by an ``addressing
mode'' and can be a constant (BBPF_IMM), packet data at a
fixed offset (BPF_ABS), packet data at a variable offset
(BPF_IND), the packet length (BPF_LEN), or a word in the
scratch memory store (BPF_MEM). For BPF_IND and BPF_ABS,
the data size must be specified as a word (BPF_W), half-
word (BPF_H), or byte (BPF_B). The semantics of all the
recognized BPF_LD instructions follow.
BPF_LD+BPF_W+BPF_ABS A <- P[k:4]
BPF_LD+BPF_H+BPF_ABS A <- P[k:2]
BPF_LD+BPF_B+BPF_ABS A <- P[k:1]
BPF_LD+BPF_W+BPF_IND A <- P[X+k:4]
BPF_LD+BPF_H+BPF_IND A <- P[X+k:2]
BPF_LD+BPF_B+BPF_IND A <- P[X+k:1]
BPF_LD+BPF_W+BPF_LEN A <- len
BPF_LD+BPF_IMM A <- k
BPF_LD+BPF_MEM A <- M[k]
BPF_LDX These instructions load a value into the index register.
Note that the addressing modes are more restricted than
those of the accumulator loads, but they include BPF_MSH,
a hack for efficiently loading the IP header length.
BPF_LDX+BPF_W+BPF_IMM X <- k
BPF_LDX+BPF_W+BPF_MEM X <- M[k]
BPF_LDX+BPF_W+BPF_LEN X <- len
BPF_LDX+BPF_B+BPF_MSH X <- 4*(P[k:1]&0xf)
BPF_ST 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.
BPF_ST M[k] <- A
BPF_STX This instruction stores the index register in the scratch
memory store.
BPF_STX M[k] <- X
BPF_ALU The alu instructions perform operations between the accu-
mulator and index register or constant, and store the re-
sult back in the accumulator. For binary operations, a
source mode is required (BPF_K or BPF_X).
BPF_ALU+BPF_ADD+BPF_K A <- A + k
BPF_ALU+BPF_SUB+BPF_K A <- A - k
BPF_ALU+BPF_MUL+BPF_K A <- A * k
BPF_ALU+BPF_DIV+BPF_K A <- A / k
BPF_ALU+BPF_AND+BPF_K A <- A & k
BPF_ALU+BPF_OR+BPF_K A <- A | k
BPF_ALU+BPF_LSH+BPF_K A <- A << k
BPF_ALU+BPF_RSH+BPF_K A <- A >> k
BPF_ALU+BPF_ADD+BPF_X A <- A + X
BPF_ALU+BPF_SUB+BPF_X A <- A - X
BPF_ALU+BPF_MUL+BPF_X A <- A * X
BPF_ALU+BPF_DIV+BPF_X A <- A / X
BPF_ALU+BPF_AND+BPF_X A <- A & X
BPF_ALU+BPF_OR+BPF_X A <- A | X
BPF_ALU+BPF_LSH+BPF_X A <- A << X
BPF_ALU+BPF_RSH+BPF_X A <- A >> X
BPF_ALU+BPF_NEG A <- -A
BPF_JMP The jump instructions alter flow of control. Conditional
jumps compare the accumulator against a constant (BPF_K)
or the index register (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 (BPF_JA) opcode uses the 32 bit k field as the off-
set, allowing arbitrarily distant destinations. All con-
ditionals use unsigned comparison conventions.
BPF_JMP+BPF_JA pc += k
BPF_JMP+BPF_JGT+BPF_K pc += (A > k) ? jt : jf
BPF_JMP+BPF_JGE+BPF_K pc += (A >= k) ? jt : jf
BPF_JMP+BPF_JEQ+BPF_K pc += (A == k) ? jt : jf
BPF_JMP+BPF_JSET+BPF_K pc += (A & k) ? jt : jf
BPF_JMP+BPF_JGT+BPF_X pc += (A > X) ? jt : jf
BPF_JMP+BPF_JGE+BPF_X pc += (A >= X) ? jt : jf
BPF_JMP+BPF_JEQ+BPF_X pc += (A == X) ? jt : jf
BPF_JMP+BPF_JSET+BPF_X pc += (A & X) ? jt : jf
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 (BPF_K) or the accumulator (BPF_A).
BPF_RET+BPF_A accept A bytes
BPF_RET+BPF_K accept k bytes
BPF_MISC
The miscellaneous category was created for anything that
doesn't fit into the above classes, and for any new in-
structions that might need to be added. Currently, these
are the register transfer instructions that copy the index
register to the accumulator or vice versa.
BPF_MISC+BPF_TAX X <- A
BPF_MISC+BPF_TXA A <- X
The BPF interface provides the following macros to facilitate array ini-
tializers:
BPF_STMT(opcode, operand)
BPF_JUMP(opcode, operand, true_offset, false_offset)
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),
};
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_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),
};
Finally, this filter returns only TCP finger packets. We must parse the
IP header to reach the TCP header. The BPF_JSET 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
ioctl(2), read(2), select(2), signal(3), tcpdump(8)
McCanne, S. and Jacobson V., "The BSD Packet Filter: A New Architecture
for User-level Packet Capture.", Proceedings of the 1993 Winter USENIX.
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. This could be fixed in the kernel with ad-
ditional 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.
Data link protocols with variable length headers are not currently sup-
ported.
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.
``Immediate mode'' and the ``read timeout'' are misguided features. This
functionality can be emulated with non-blocking mode and select(2).
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, implemented BPF in Sum-
mer 1990. The design was in collaboration with Van Jacobson, also of
Lawrence Berkeley Laboratory.
NetBSD 1.5.1 June 28, 1994 8
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