BPF(4) BSD Kernel Interfaces Manual BPF(4)NAMEbpf — Berkeley Packet Filter raw network interface
SYNOPSIS
pseudo-device bpfilter
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 net‐
work 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.
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 (see below), and can be set with
BIOCSBLEN. Note that an individual packet larger than this size is nec‐
essarily truncated.
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 descrip‐
tor. The writes are unbuffered, meaning only one packet can be processed
per write. Currently, only writes to Ethernets and SLIP links are sup‐
ported.
IOCTLS
The ioctl(2) command codes below are defined in <net/bpf.h>. All com‐
mands require these includes:
#include <sys/types.h>
#include <sys/time.h>
#include <sys/ioctl.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
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
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 the available types of the data link
layer underlying the attached interface:
struct bpf_dltlist {
u_int bfl_len;
u_int *bfl_list;
};
The available types are returned in the array pointed to by
the bfl_list field while their length in u_int is supplied
to the bfl_len field. ENOMEM is returned if there is not
enough buffer space and EFAULT is returned if a bad address
is encountered. 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 set to indicate
the required length of an array in u_int.
BIOCSDLT (u_int)
Changes 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 inter‐
face, a listener that opened its interface non-promiscu‐
ously may receive packets promiscuously. 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 sta‐
tistics 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 ifr. All other fields are undefined.
BIOCSETIF (struct ifreq)
Sets the hardware interface associated 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. Additionally, performs the actions of
BIOCFLUSH.
BIOCSRTIMEOUT, BIOCGRTIMEOUT (struct timeval)
Sets or gets the read timeout parameter. The timeval spec‐
ifies 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 the number of packets received by the
descriptor since opened or reset (including
any buffered since the last read call);
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 aren't keeping up with the packet
traffic); and
bs_capt the number of packets accepted by the fil‐
ter.
BIOCIMMEDIATE (u_int)
Enables or disables “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 (struct bpf_program)
Sets the filter program used by the kernel to discard unin‐
teresting packets. An array of instructions and its length
are 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. Also, the actions of BIOCFLUSH are per‐
formed.
See section FILTER MACHINE for an explanation 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 cur‐
rent version is compatible with the running kernel. Ver‐
sion 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 may result in undefined behavior (most likely, an
error returned by ioctl(2) or haphazard packet matching).
BIOCSRSIG BIOCGRSIG (u_int)
Sets or gets the receive signal. This signal will be sent
to the process or process group specified by FIOSETOWN. It
defaults to SIGIO.
BIOCGHDRCMPLT BIOCSHDRCMPLT (u_int)
Sets or gets 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.
BIOCGSEESENT BIOCSSEESENT (u_int)
Enable/disable or get the “see sent” flag status. If
enabled, packets sent by the host (not from bpf) will be
passed to the filter. By default, the flag is enabled
(value is 1).
BIOCFEEDBACK BIOCSFEEDBACK BIOCGFEEDBACK (u_int)
Set (or get) “packet feedback mode”. This allows injected
packets to be fed back as input to the interface when out‐
put via the interface is successful. The first name is
meant for FreeBSD compatibility, the two others follow the
Get/Set convention. Injected outgoing packets are not
returned by BPF to avoid duplication. This flag is ini‐
tialized to zero by default.
STANDARD IOCTLSbpf now supports several standard ioctl(2)'s which allow the user to do
async and/or non-blocking I/O to an open bpf file descriptor.
FIONREAD (int)
Returns the number of bytes that are immediately available
for reading.
FIONBIO (int)
Set or clear non-blocking I/O. If arg is non-zero, then
doing a read(2) when no data is available will return -1
and errno will be set to EAGAIN. If arg is zero, non-
blocking I/O is disabled. Note: setting this overrides 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
will start receiving SIGIO's when packets arrive. Note
that you must do an FIOSETOWN in order for this to take
effect, as the system will not default this for you. The
signal may be changed via BIOCSRSIG.
FIOSETOWN FIOGETOWN (int)
Set or get the process or process group (if negative) that
should receive SIGIO when packets are available. The sig‐
nal may be changed using BIOCSRSIG (see above).
BPF HEADER
The following structure is prepended to each packet returned by read(2):
struct bpf_hdr {
struct bpf_timeval bh_tstamp;
uint32_t bh_caplen;
uint32_t bh_datalen;
uint16_t bh_hdrlen;
};
The fields, whose values are stored in host order, are:
bh_tstamp The time at which the packet was processed by the
packet filter. This structure differs from the stan‐
dard struct timeval in that both members are of type
long.
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 align‐
ment of the packet data structures, which is required on alignment sensi‐
tive architectures and improves performance on many other architectures.
The packet filter ensures 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
isn't 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. malloc(3) 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 accumula‐
tor, 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;
uint32_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 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). Arithmetic overflow when calcu‐
lating a variable offset terminates the filter program and
the packet is ignored. 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 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_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 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 long‐
est 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‐
als 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
doesn't 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 ini‐
tializers:
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(1) 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(1) utility.
FILES
/dev/bpf
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 ALSOioctl(2), read(2), select(2), signal(3), tcpdump(8)
S. McCanne and V. Jacobson, "The BSD Packet Filter: A New Architecture
for User-level Packet Capture", Proceedings of the 1993 Winter USENIX.
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 Sum‐
mer 1990. The design was in collaboration with Van Jacobson, also of
Lawrence Berkeley Laboratory.
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 use a filter to reject foreign packets.
Under SunOS, if a BPF application reads more than 2^31 bytes of data,
read will fail in EINVAL. You can either fix the bug in SunOS, or lseek
to 0 when read fails for this reason.
“Immediate mode” and the “read timeout” are misguided features. This
functionality can be emulated with non-blocking mode and select(2).
BSD December 31, 2011 BSD
