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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 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.
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 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.
BPF HEADER
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.
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_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_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).
SYSCTL VARIABLES
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.
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.