bpf
—
Berkeley Packet Filter
device bpf
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.
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.
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.
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 availability 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.
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
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.
BIOCSTSTAMP
-
BIOCGTSTAMP
- (
u_int
) Set or get format and resolution of the
time stamps returned by BPF. Set to
BPF_T_MICROTIME
,
BPF_T_MICROTIME_FAST
,
BPF_T_MICROTIME_MONOTONIC
, or
BPF_T_MICROTIME_MONOTONIC_FAST
to get
time stamps in 64-bit struct timeval
format. Set to BPF_T_NANOTIME
,
BPF_T_NANOTIME_FAST
,
BPF_T_NANOTIME_MONOTONIC
, or
BPF_T_NANOTIME_MONOTONIC_FAST
to get
time stamps in 64-bit struct timespec
format. Set to BPF_T_BINTIME
,
BPF_T_BINTIME_FAST
,
BPF_T_NANOTIME_MONOTONIC
, or
BPF_T_BINTIME_MONOTONIC_FAST
to get
time stamps in 64-bit struct bintime
format. Set to BPF_T_NONE
to ignore
time stamp. All 64-bit time stamp formats are wrapped in
struct bpf_ts. The
BPF_T_MICROTIME_FAST
,
BPF_T_NANOTIME_FAST
,
BPF_T_BINTIME_FAST
,
BPF_T_MICROTIME_MONOTONIC_FAST
,
BPF_T_NANOTIME_MONOTONIC_FAST
, and
BPF_T_BINTIME_MONOTONIC_FAST
are
analogs of corresponding formats without _FAST suffix but do not perform a
full time counter query, so their accuracy is one timer tick. The
BPF_T_MICROTIME_MONOTONIC
,
BPF_T_NANOTIME_MONOTONIC
,
BPF_T_BINTIME_MONOTONIC
,
BPF_T_MICROTIME_MONOTONIC_FAST
,
BPF_T_NANOTIME_MONOTONIC_FAST
, and
BPF_T_BINTIME_MONOTONIC_FAST
store the
time elapsed since kernel boot. This setting is initialized to
BPF_T_MICROTIME
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.
One of the following structures is prepended to each packet returned by
read(2)
or via a zero-copy buffer:
struct bpf_xhdr {
struct bpf_ts bh_tstamp; /* time stamp */
uint32_t bh_caplen; /* length of captured portion */
uint32_t bh_datalen; /* original length of packet */
u_short bh_hdrlen; /* length of bpf header (this struct
plus alignment padding) */
};
struct bpf_hdr {
struct timeval bh_tstamp; /* time stamp */
uint32_t bh_caplen; /* length of captured portion */
uint32_t 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_xhdr) or
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 ensures that the
bpf_xhdr,
bpf_hdr and the network layer header will be
word aligned. Currently,
bpf_hdr is used when
the time stamp is set to
BPF_T_MICROTIME
,
BPF_T_MICROTIME_FAST
,
BPF_T_MICROTIME_MONOTONIC
,
BPF_T_MICROTIME_MONOTONIC_FAST
, or
BPF_T_NONE
for backward compatibility
reasons. Otherwise,
bpf_xhdr is used.
However,
bpf_hdr may be deprecated in the
near future. 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.
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_MOD+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_XOR+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_MOD+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_XOR+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).
A set of
sysctl(8)
variables controls the behaviour of the
bpf
subsystem
- net.bpf.optimize_writers:
0
- Various programs use BPF to send (but not receive) raw packets (cdpd,
lldpd, dhcpd, dhcp relays, etc. are good examples of such programs). They
do not need incoming packets to be send to them. Turning this option on
makes new BPF users to be attached to write-only interface list until
program explicitly specifies read filter via
pcap_set_filter
(). This removes any
performance degradation for high-speed interfaces.
- net.bpf.stats:
- Binary interface for retrieving general statistics.
- net.bpf.zerocopy_enable:
0
- Permits zero-copy to be used with net BPF readers. Use with caution.
- net.bpf.maxinsns:
512
- Maximum number of instructions that BPF program can contain. Use
tcpdump(1)
-d
option to determine approximate
number of instruction for any filter.
- net.bpf.maxbufsize:
524288
- Maximum buffer size to allocate for packets buffer.
- net.bpf.bufsize:
4096
- Default buffer size to allocate for packets buffer.
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),
};
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.
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.
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.
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.