bpf(7D) Devices bpf(7D)NAMEbpf - Berkeley Packet Filter raw network interface
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 spe‐
cific network interface with the BIOSETIF ioctl. A specific interface
can be shared by multiple listeners, and the filter underlying each
descriptor sees an identical packet stream.
Associated with each open instance of a bpf file is a user-settable
packet filter. Whenever a packet is received by an interface, all file
descriptors listening on that interface apply their filter. Each
descriptor that accepts the packet receives its own copy.
Reads from these files return the next group of packets that have
matched the filter. To improve performance, the buffer passed to read
must be the same size as the buffers used internally by bpf. This size
is returned by the BIOCGBLEN ioctl , and under BSD, can be set with
BIOCSBLEN. An individual packet larger than this size is necessarily
truncated.
The packet filter supports 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(3SOCKET) 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 that only one packet can
be processed per write. Currently, only writes to Ethernets and SLIP
links are supported.
IOCTLS
The ioctl(2) command codes in this section are defined in <net/bfp.h>.
All commands require these includes:
#include <sys/types.h>
#include <sys/time.h>
#include <sys/time.h>
#include <net/bpf.h>
Additionally, BIOCGETIF and BIOCSETIF require <net/if.h>.
The third argument to the ioctl(2)should be a pointer to the type indi‐
cated.
BIOCGBLEN (u_int)
Returns the required buffer length for reads on bpf files.
BIOCSBLEN (u_int)
Sets the buffer length for reads on bpf files. The buffer must be
set before the file is attached to an interface with BIOCSETIF. If
the requested buffer size cannot be accommodated, the closest
allowable size is set and returned in the argument. A read call
results in EINVAL 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>.
BIOCGDLTLIST (struct bpf_dltlist)
Returns an array of available type of the data link layer underly‐
ing the attached interface:
struct bpf_dltlist {
u_int bfl_len;
u_int *bfl_list;
};
The available type is returned to the array pointed to the bfl_list
field while its length in u_int is supplied to the bfl_len field.
NOMEM is returned if there is not enough buffer. The bfl_len field
is modified on return to indicate the actual length in u_int of the
array returned. If bfl_list is NULL, the bfl_len field is returned
to indicate the required length of an array in u_int.
BIOCSDLT (u_int)
Change the type of the data link layer underlying the attached
interface. EINVAL is returned if no interface has been specified or
the specified type is not available for the interface.
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 can receive packets promis‐
cuously. This problem can be remedied with an appropriate filter.
The interface remains in promiscuous mode until all files listening
promiscuously are closed.
BIOCFLUSH
Flushes the buffer of incoming packets, and resets the statistics
that are returned by BIOCGSTATS.
BIOCGETIF (struct ifreq)
Returns the name of the hardware interface that the file is listen‐
ing on. The name is returned in the ifr_name field of ifr. All
other fields are undefined.
BIOCSETIF (struct ifreq)
Sets the hardware interface associate with the file. This command
must be performed before any packets can be read. The device is
indicated by name using the ifr_name field of the ifreq. Addition‐
ally, performs the actions of BIOCFLUSH.
BIOCSRTIMEOUT, BIOCGRTIMEOUT (struct timeval)
Set or get the read timeout parameter. The timeval specifies the
length of time to wait before timing out on a read request. This
parameter is initialized to zero by open(2), indicating no timeout.
BIOCGSTATS (struct bpf_stat)
Returns the following structure of packet statistics:
struct bpf_stat {
uint64_t bs_recv;
uint64_t bs_drop;
uint64_t bs_capt;
uint64_t bs_padding[13];
};
The fields are:
bs_recv Number of packets received by the descriptor since
opened or reset (including any buffered since the last
read call.
bs_drop Number of packets which were accepted by the filter but
dropped by the kernel because of buffer overflows, that
is, the application's reads aren't keeping up with the
packet traffic.
bs_capt Number of packets accepted by the filter.
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 blocks until either the
kernel buffer becomes full or a timeout occurs. This is useful for
programs like rarpd(1M), which must respond to messages in real
time. The default for a new file is off.
BIOCSETF (struct bpf_program)
Sets the filter program used by the kernel to discard uninteresting
packets. An array of instructions and its length is passed in using
the following structure:
struct bpf_program {
u_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.
The actions of BIOCFLUSH are also performed.
See the FILTER MACHINE section of this manual page for an explana‐
tion of the filter language.
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 fol‐
lowing 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 can result in undefined behavior, most
likely, an error returned by ioctl(2) or haphazard packet matching.
BIOCGHDRCMPLT BIOCSHDRCMPLT (u_int)
Enable/disable or get the header complete flag status. If enabled,
packets written to the bpf file descriptor does not have network
layer headers rewritten in the interface output routine. By
default, the flag is disabled (value is 0).
BIOCGSEESENT BIOCSSEESENT (u_int)
Enable/disable or get the see sent flag status. If enabled, packets
sent is passed to the filter. By default, the flag is enabled
(value is 1).
Standard Ioctls
bpf supports several standard ioctl(2)'s that allow the user to do
async or non-blocking I/O to an open file descriptor.
FIONREAD (int) Returns the number of bytes that are
immediately available for reading.
SIOCGIFADDR (struct ifreq) Returns the address associated with the
interface.
FIONBIO (int) Set or clear non-blocking I/O. If arg is
non-zero, then doing a read(2) when no
data is available returns -1 and errno is
set to EAGAIN. If arg is zero, non-block‐
ing I/O is disabled. Setting this over‐
rides the timeout set by BIOCSRTIMEOUT.
FIOASYNC (int) Enable or disable async I/O. When enabled
(arg is non-zero), the process or process
group specified by FIOSETOWN starts
receiving SIGIOs when packets arrive. You
must do an FIOSETOWN for this to take
effect, as the system does not default
this for you. The signal can be changed
using BIOCSRSIG.
FIOSETOWN FIOGETOWN (int) Set or get the process or process group
(if negative) that should receive SIGIO
when packets are available. The signal
can be changed using BIOCSRSIG.
bpf Header
The following structure is prepended to each packet returned by
read(2):
struct bpf_hdr {
struct timeval bh_tstamp;
uint32_t bh_caplen;
uint32_t bh_datalen;
uint16_t bh_hdrlen;
};
The fields, whose values are stored in host order, and are:
bh_tstamp The time at which the packet was processed by the packet
filter.
bh_caplen The length of the captured portion of the packet. This is
the minimum of the truncation amount specified by the
filter and the length of the packet.
bh_datalen The length of the packet off the wire. This value is
independent of the truncation amount specified by the
filter.
bh_hdrlen The length of the BPF header, which cannot 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 archi‐
tectures. The packet filter ensures that the bpf_hdr and the network
layer header is 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
advances it to the next packet:
p = (char *)p + BPF_WORDALIGN(p->bh_hdrlen + p->bh_caplen)
For the alignment mechanisms to work properly, the buffer passed to
read(2) must itself be word aligned. malloc(3C) always returns an
aligned buffer.
Filter Machine
A filter program is an array of instructions, with all branches for‐
wardly directed, terminated by a return instruction. Each instruction
performs some action on the pseudo-machine state, which consists of an
accumulator, index register, scratch memory store, and implicit program
counter.
The following structure defines the instruction format:
struct bpf_insn {
uint16_t code;
u_char jt;
u_char jf;
int32_t 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 corre‐
sponding 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 (BBPF_IMM), packet data at a
fixed offset (BPF_ABS), packet data at a variable offset
(BPF_IND), the packet length (BPF_LEN), or a word in the
scratch memory store (BPF_MEM). For BPF_IND and BPF_ABS,
the data size must be specified as a word (BPF_W), halfword
(BPF_H), or byte (BPF_B). The semantics of all the recog‐
nized BPF_LD instructions follow.
BPF_LD+BPF_W+BPF_ABS A <- P[k:4]
BPF_LD+BPF_H+BPF_ABS A <- P[k:2]
BPF_LD+BPF_B+BPF_ABS A <- P[k:1]
BPF_LD+BPF_W+BPF_IND A <- P[X+k:4]
BPF_LD+BPF_H+BPF_IND A <- P[X+k:2]
BPF_LD+BPF_B+BPF_IND A <- P[X+k:1]
BPF_LD+BPF_W+BPF_LEN A <- len
BPF_LD+BPF_IMM A <- k
BPF_LD+BPF_MEM A <- M[k]
BPF_LDX These instructions load a value into the index register.
The addressing modes are more restricted 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_ALU The alu instructions perform operations between the accumu‐
lator 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 condition also 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, that is, they
return the truncation amount. A return value of zero indi‐
cates 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 other classes in this section, and
for any new instructions that might need to be added. Cur‐
rently, 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)
BPF_JUMP (opcode, operand, true_offset, false_offset)
Sysctls
The following sysctls are available when bpf is enabled:
net.bpf.maxbufsize Sets the maximum buffer size available for bpf
peers.
net.bpf.stats Shows bpf statistics. They can be retrieved with
the netstat(1M) utility.
net.bpf.peers Shows the current bpf peers. This is only avail‐
able to the super user and can also be retrieved
with the netstat(1M) utility.
FILES
/dev/bpf
EXAMPLES
Example 1 Using bfp to Accept Only Reverse ARP Requests
The following example shows a filter 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),
};
Example 2 Using bfp to Accept IP Packets
The following example shows filter that 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),
};
Example 3 Using bfp to Return Only TCP Finger Packets
The following example shows a filter that returns only TCP finger pack‐
ets. The IP header must be parsed 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),
};
ATTRIBUTES
See attributes(5) for a description of the following attributes:
┌─────────────────────────────┬─────────────────────────────┐
│ATTRIBUTE TYPE │ATTRIBUTE VALUE │
├─────────────────────────────┼─────────────────────────────┤
│Architecture │Sparc, x86 │
├─────────────────────────────┼─────────────────────────────┤
│Interface Stability │Committed │
└─────────────────────────────┴─────────────────────────────┘
SEE ALSOnetstat(1M), rarpd(1M), lseek(2), ioctl(2), open(2), read(2), mal‐
loc(3C), select(3C), byteorder(3SOCKET), signal(3C), attributes(5)
S. McCanne and V. Jacobson, The BSD Packet Filter: A New Architecture
for User-level Packet Capture, Proceedings of the 1993 Winter USENIX.
BUGS
The read buffer must be of a fixed size returned by the BIOCGBLEN
ioctl.
A file that does not request promiscuous mode can receive promiscuous
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 use a filter to reject foreign packets.
Data link protocols with variable length headers are not currently sup‐
ported.
Under SunOS, if a BPF application reads more than 2^31 bytes of data,
read fails in EINVALsignal(3C). You can either fix the bug in SunOS, or
lseek(2) to 0 when read fails for this reason.
Immediate mode and the read timeout are misguided features. This func‐
tionality can be emulated with non-blocking mode and select(3C).
SunOS 5.11 22 Oct 2009 bpf(7D)
