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 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 #include <sys/socket.h>
and
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
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 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.
BPF HEADER
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
#include <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
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_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 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.