NAME¶
bpf —
Berkeley Packet Filter
SYNOPSIS¶
device bpf
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/bpf. After opening the device, the file descriptor must
be bound to a specific network interface with the
BIOCSETIF
ioctl. A given interface can be shared by
multiple listeners, and the filter underlying each descriptor will see an
identical packet stream.
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.
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 descriptor. The writes are unbuffered, meaning only one packet can be
processed per write. Currently, only writes to Ethernets and SLIP links are
supported.
BUFFER MODES¶
bpf devices deliver packet data to the application via memory
buffers provided by the application. The buffer mode is set using the
BIOCSETBUFMODE
ioctl, and read using the
BIOCGETBUFMODE
ioctl.
Buffered read mode¶
By default,
bpf devices operate in the
BPF_BUFMODE_BUFFER
mode, in which packet data is
copied explicitly from kernel to user memory using the
read(2) system call. The user process will declare a fixed
buffer size that will be used both for sizing internal buffers and for all
read(2) operations on the file. This size is queried using
the
BIOCGBLEN
ioctl, and is set using the
BIOCSBLEN
ioctl. Note that an individual packet larger
than the buffer size is necessarily truncated.
Zero-copy buffer mode¶
bpf devices may also operate in the
BPF_BUFMODE_ZEROCOPY
mode, in which packet data is
written directly into two user memory buffers by the kernel, avoiding both
system call and copying overhead. Buffers are of fixed (and equal) size,
page-aligned, and an even multiple of the page size. The maximum zero-copy
buffer size is returned by the
BIOCGETZMAX
ioctl. Note
that an individual packet larger than the buffer size is necessarily
truncated.
The user process registers two memory buffers using the
BIOCSETZBUF
ioctl, which accepts a
struct bpf_zbuf pointer as an argument:
struct bpf_zbuf {
void *bz_bufa;
void *bz_bufb;
size_t bz_buflen;
};
bz_bufa is a pointer to the userspace address of the first
buffer that will be filled, and
bz_bufb is a pointer to
the second buffer.
bpf will then cycle between the two
buffers as they fill and are acknowledged.
Each buffer begins with a fixed-length header to hold synchronization and data
length information for the buffer:
struct bpf_zbuf_header {
volatile u_int bzh_kernel_gen; /* Kernel generation number. */
volatile u_int bzh_kernel_len; /* Length of data in the buffer. */
volatile u_int bzh_user_gen; /* User generation number. */
/* ...padding for future use... */
};
The header structure of each buffer, including all padding, should be zeroed
before it is configured using
BIOCSETZBUF
. Remaining
space in the buffer will be used by the kernel to store packet data, laid out
in the same format as with buffered read mode.
The kernel and the user process follow a simple acknowledgement protocol via the
buffer header to synchronize access to the buffer: when the header generation
numbers,
bzh_kernel_gen and
bzh_user_gen, hold the same value, the kernel owns the
buffer, and when they differ, userspace owns the buffer.
While the kernel owns the buffer, the contents are unstable and may change
asynchronously; while the user process owns the buffer, its contents are
stable and will not be changed until the buffer has been acknowledged.
Initializing the buffer headers to all 0's before registering the buffer has the
effect of assigning initial ownership of both buffers to the kernel. The
kernel signals that a buffer has been assigned to userspace by modifying
bzh_kernel_gen, and userspace acknowledges the buffer
and returns it to the kernel by setting the value of
bzh_user_gen to the value of
bzh_kernel_gen.
In order to avoid caching and memory re-ordering effects, the user process must
use atomic operations and memory barriers when checking for and acknowledging
buffers:
#include <machine/atomic.h>
/*
* Return ownership of a buffer to the kernel for reuse.
*/
static void
buffer_acknowledge(struct bpf_zbuf_header *bzh)
{
atomic_store_rel_int(&bzh->bzh_user_gen, bzh->bzh_kernel_gen);
}
/*
* Check whether a buffer has been assigned to userspace by the kernel.
* Return true if userspace owns the buffer, and false otherwise.
*/
static int
buffer_check(struct bpf_zbuf_header *bzh)
{
return (bzh->bzh_user_gen !=
atomic_load_acq_int(&bzh->bzh_kernel_gen));
}
The user process may force the assignment of the next buffer, if any data is
pending, to userspace using the
BIOCROTZBUF
ioctl.
This allows the user process to retrieve data in a partially filled buffer
before the buffer is full, such as following a timeout; the process must
recheck for buffer ownership using the header generation numbers, as the
buffer will not be assigned to userspace if no data was present.
As in the buffered read mode,
kqueue(2),
poll(2), and
select(2) may be used to
sleep awaiting the availbility of a completed buffer. They will return a
readable file descriptor when ownership of the next buffer is assigned to user
space.
In the current implementation, the kernel may assign zero, one, or both buffers
to the user process; however, an earlier implementation maintained the
invariant that at most one buffer could be assigned to the user process at a
time. In order to both ensure progress and high performance, user processes
should acknowledge a completely processed buffer as quickly as possible,
returning it for reuse, and not block waiting on a second buffer while holding
another buffer.
IOCTLS¶
The
ioctl(2) 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
<sys/socket.h> and
<net/if.h>.
In addition to
FIONREAD
and
SIOCGIFADDR
, the following commands may be applied to
any open
bpf file. The (third) argument to
ioctl(2) should be a pointer to the type indicated.
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 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.
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 the ifreq
structure. 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
structure. Additionally, performs the
actions of BIOCFLUSH
.
BIOCSRTIMEOUT
-
BIOCGRTIMEOUT
- (
struct timeval
) Set or get the
read timeout parameter. The argument 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; /* number of packets received */
u_int bs_drop; /* number of packets dropped */
};
The fields are:
bs_recv
- the number of packets received by the descriptor 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 are not 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. Otherwise, a read will block until either the kernel buffer
becomes full or a timeout occurs. This is useful for programs like
rarpd(8) which must respond to messages in real time.
The default for a new file is off.
BIOCSETF
-
BIOCSETFNR
- (
struct bpf_program
) Sets the read
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 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. See section FILTER MACHINE
for an explanation of the filter language. The only difference between
BIOCSETF
and BIOCSETFNR
is
BIOCSETF
performs the actions of
BIOCFLUSH
while BIOCSETFNR
does not.
BIOCSETWF
- (
struct bpf_program
) Sets the write
filter program used by the kernel to control what type of packets can be
written to the interface. See the BIOCSETF
command
for more information on the bpf filter program.
BIOCVERSION
- (
struct bpf_version
) Returns the
major and minor version numbers of the filter language currently
recognized by the kernel. Before installing a filter, applications 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 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() or haphazard packet matching).
BIOCSHDRCMPLT
-
BIOCGHDRCMPLT
- (
u_int
) Set or get the status of
the “header complete” flag. Set to zero if the link level
source address should be filled in automatically by the interface output
routine. Set to one if the link level source address will be written, as
provided, to the wire. This flag is initialized to zero by default.
BIOCSSEESENT
-
BIOCGSEESENT
- (
u_int
) These commands are obsolete
but left for compatibility. Use BIOCSDIRECTION
and
BIOCGDIRECTION
instead. Set or get the flag
determining whether locally generated packets on the interface should be
returned by BPF. Set to zero to see only incoming packets on the
interface. Set to one to see packets originating locally and remotely on
the interface. This flag is initialized to one by default.
BIOCSDIRECTION
-
BIOCGDIRECTION
- (
u_int
) Set or get the setting
determining whether incoming, outgoing, or all packets on the interface
should be returned by BPF. Set to BPF_D_IN
to see
only incoming packets on the interface. Set to
BPF_D_INOUT
to see packets originating locally and
remotely on the interface. Set to BPF_D_OUT
to see
only outgoing packets on the interface. This setting is initialized to
BPF_D_INOUT
by default.
BIOCFEEDBACK
- (
u_int
) Set packet feedback mode.
This allows injected packets to be fed back as input to the interface when
output via the interface is successful. When
BPF_D_INOUT
direction is set, injected outgoing
packet is not returned by BPF to avoid duplication. This flag is
initialized to zero by default.
BIOCLOCK
- Set the locked flag on the bpf
descriptor. This prevents the execution of ioctl commands which could
change the underlying operating parameters of the device.
BIOCGETBUFMODE
-
BIOCSETBUFMODE
- (
u_int
) Get or set the current
bpf buffering mode; possible values are
BPF_BUFMODE_BUFFER
, buffered read mode, and
BPF_BUFMODE_ZBUF
, zero-copy buffer mode.
BIOCSETZBUF
- (
struct bpf_zbuf
) Set the current
zero-copy buffer locations; buffer locations may be set only once
zero-copy buffer mode has been selected, and prior to attaching to an
interface. Buffers must be of identical size, page-aligned, and an integer
multiple of pages in size. The three fields bz_bufa,
bz_bufb, and bz_buflen must be
filled out. If buffers have already been set for this device, the ioctl
will fail.
BIOCGETZMAX
- (
size_t
) Get the largest individual
zero-copy buffer size allowed. As two buffers are used in zero-copy buffer
mode, the limit (in practice) is twice the returned size. As zero-copy
buffers consume kernel address space, conservative selection of buffer
size is suggested, especially when there are multiple
bpf descriptors in use on 32-bit systems.
BIOCROTZBUF
- Force ownership of the next buffer to be assigned to
userspace, if any data present in the buffer. If no data is present, the
buffer will remain owned by the kernel. This allows consumers of zero-copy
buffering to implement timeouts and retrieve partially filled buffers. In
order to handle the case where no data is present in the buffer and
therefore ownership is not assigned, the user process must check
bzh_kernel_gen against
bzh_user_gen.
The following structure is prepended to each packet returned by
read(2) or via a zero-copy buffer:
struct bpf_hdr {
struct timeval bh_tstamp; /* time stamp */
u_long bh_caplen; /* length of captured portion */
u_long bh_datalen; /* original length of packet */
u_short bh_hdrlen; /* length of bpf header (this struct
plus alignment padding */
};
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 improves performance on many other
architectures. The packet filter insures that the
bpf_hdr
and the network layer header will be word
aligned. Suitable precautions must be taken when accessing the link layer
protocol fields on alignment restricted machines. (This is not 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. The
malloc(3) function 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.
The following structure defines the instruction format:
struct bpf_insn {
u_short code;
u_char jt;
u_char jf;
u_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 opcodes are encoded in a semi-hierarchical fashion. There
are eight classes 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 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 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 restrictive 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_STX
- This instruction stores the index register in the scratch
memory store.
BPF_ALU
- 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
(
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 arbitrarily
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
does not fit into the above classes, and for any new instructions that
might need to be added. Currently, these are the register transfer
instructions that copy the index register to the accumulator or vice
versa.
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).
FILES¶
- /dev/bpf
- the packet filter device
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¶
tcpdump(1),
ioctl(2),
kqueue(2),
poll(2),
select(2),
byteorder(3),
ng_bpf(4),
bpf(9)
McCanne, S. and
Jacobson V., An efficient,
extensible, and portable network monitor.
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 Summer 1990. Much of the design is due to
Van Jacobson.
Support for zero-copy buffers was added by
Robert N. M.
Watson under contract to Seccuris Inc.
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 supported.
The
SEESENT
,
DIRECTION
, and
FEEDBACK
settings have been observed to work
incorrectly on some interface types, including those with hardware loopback
rather than software loopback, and point-to-point interfaces. They appear to
function correctly on a broad range of Ethernet-style interfaces.