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 interface with the BIOSETIF ioctl. A given
interface can be shared be multiple listeners, and the filter underly‐
ing each descriptor will see an identical packet stream. The total
number of open files is limited to the value given in the kernel con‐
figuration; 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
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.
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. (SIOCGIFADDR is obsolete under BSD sys‐
tems. SIOCGIFCONF should be used to query link-level addresses.) The
(third) argument to the ioctl should be a pointer to the type indi‐
cated.
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 accomo‐
dated, 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 underyling 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
listener 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 sta‐
tistics 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 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 if_name 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 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 descrip‐
tor 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 because
of buffer overflows (i.e., the application's
reads aren't keeping up with the packet traf‐
fic).
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. 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.
BIOCSETF (struct bpf_program)
Sets the filter program used by the kernel to discard unin‐
teresting 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 performed.
See section FILTER MACHINE for an explanation of the filter
language.
BIOCVERSION (struct bpf_version)
Returns the major and minor version numbers of the filter
languange currently recognized by the kernel. Before
installing a filter, applications must check that the current
version is compatible with the running kernel. Version num‐
bers are compatible if the major numbers match and the appli‐
cation minor is less than or equal to the kernel minor. The
kernel version number is returned in the following 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 fil‐
ter may result in undefined behavior (most likely, an error
returned by ioctl() or haphazard packet matching).
BPF HEADER
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 packet
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
alignment of the packet data structures, which is required on alignment
sensitive architectures and and improves performance on many other
architectures. The packet filter insures that the bpf_hdr and the net‐
work layer header will be word aligned. Suitable precautions must be
taken when accessing 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 for‐
wardly 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.
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 differnet ways by different insutructions, and
the jt and jf fields are used as offsets 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 opera‐
tor 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.
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 (BPF_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), halfword
(BPF_H), or byte (BPF_B). The semantics of all the recog‐
nized 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 retricted 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 mem‐
ory. 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 accumula‐
tor and index register or constant, and store the result 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 offset, allowing arbi‐
trarily distant destinations. All conditionals 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 instruc‐
tions that might need to be added. Currently, these are the
register transfer intructions 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
initializers:
BPF_STMT(opcode, operand)
and
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 ALSOtcpdump(1)
McCanne, S., Jacobson V., The BSD Packet Filter: A New Architecture for
User-level Packet Capture, Proceedings of the 1993 Winter USENIX Tech‐
nical Conference, pp 259-269.
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
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.
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.
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.
4.4 Berkeley Distribution April 25, 1995 BPF(4)