BPF(4) | Device Drivers Manual | BPF(4) |
bpf
— Berkeley
Packet Filter
pseudo-device bpfilter
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(2). A given interface
can be shared between 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(2) and can be set with
BIOCSBLEN
. Note that an individual packet larger
than this size is necessarily truncated.
A packet can be sent out on the network by writing to a
bpf
file descriptor. Each descriptor can also have a
user-settable filter for controlling the writes. Only packets matching the
filter are sent out of the interface. The writes are unbuffered, meaning
only one packet can be processed per write.
Once a descriptor is configured, further changes to the
configuration can be prevented using the BIOCLOCK
ioctl(2).
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>
.
The (third) argument to the ioctl(2) call should be a pointer to the type indicated.
BIOCGBLEN
u_int *bpf
files.
BIOCSBLEN
u_int *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 *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 *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 the array in
u_int.
BIOCSDLT
u_int *EINVAL
is returned if no interface has been
specified or the specified type is not available for the interface.
BIOCPROMISC
The interface remains in promiscuous mode until all files listening promiscuously are closed.
BIOCFLUSH
BIOCGSTATS
.
BIOCLOCK
bpf
descriptor in unprivileged programs. Even
with dropped privileges, an open bpf
descriptor
can be abused by a rogue program to listen on any interface on the system,
send packets on these interfaces if the descriptor was opened read-write
and send signals to arbitrary processes using the signaling mechanism of
bpf
. By allowing only “known safe”
ioctls, the BIOCLOCK
ioctl prevents this abuse.
The allowable ioctls are BIOCFLUSH
,
BIOCGBLEN
, BIOCGDIRFILT
,
BIOCGDLT
, BIOCGDIRFILT
,
BIOCGDLTLIST
, BIOCGETIF
,
BIOCGHDRCMPLT
, BIOCGRSIG
,
BIOCGRTIMEOUT
, BIOCGSTATS
,
BIOCIMMEDIATE
, BIOCLOCK
,
BIOCSRTIMEOUT
,
BIOCVERSION
, TIOCGPGRP
,
and FIONREAD
. Use of any other ioctl is denied
with error EPERM
. Once a descriptor is locked, it
is not possible to unlock it. A process with root privileges is not
affected by the lock.
A privileged program can open a bpf
device, drop privileges, set the interface, filters and modes on the
descriptor, and lock it. Once the descriptor is locked, the system is
safe from further abuse through the descriptor. Locking a descriptor
does not prevent writes. If the application does not need to send
packets through bpf
, it can open the device
read-only to prevent writing. If sending packets is necessary, a
write-filter can be set before locking the descriptor to prevent
arbitrary packets from being sent out.
BIOCGETIF
struct ifreq *struct ifreq
. All other fields are undefined.
BIOCSETIF
struct ifreq *struct ifreq
. Additionally, performs the actions
of BIOCFLUSH
.
BIOCSRTIMEOUT
struct timeval *BIOCGRTIMEOUT
struct timeval *BIOCGSTATS
struct bpf_stat *struct bpf_stat { u_int bs_recv; u_int bs_drop; };
The fields are:
BIOCIMMEDIATE
u_int *BIOCSETF
struct bpf_program *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 performed.
See section FILTER MACHINE for an explanation of the filter language.
BIOCSETWF
struct bpf_program *BIOCSETF
for a description of the filter program.
This ioctl also acts as BIOCFLUSH
.
Note that the filter operates on the packet data written to the descriptor. If the “header complete” flag is not set, the kernel sets the link-layer source address of the packet after filtering.
BIOCVERSION
struct bpf_version *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
u_int *BIOCGRSIG
u_int *FIOSETOWN
. It
defaults to SIGIO
.
BIOCSHDRCMPLT
u_int *BIOCGHDRCMPLT
u_int *BIOCSFILDROP
u_int *BIOCGFILDROP
u_int *BIOCSDIRFILT
u_int *BIOCGDIRFILT
u_int *BPF_DIRECTION_IN
or
BPF_DIRECTION_OUT
) will be ignored.bpf
now supports several standard ioctls
which allow the user to do asynchronous and/or non-blocking I/O to an open
bpf
file descriptor.
FIONREAD
int *FIONBIO
int *EAGAIN
if no data is available. Note: setting this overrides the timeout set by
BIOCSRTIMEOUT
.
FIOASYNC
int *FIOSETOWN
will start receiving
SIGIO
signals when packets arrive. Note that you
must perform an FIOSETOWN
command in order for
this to take effect, as the system will not do it by default. The signal
may be changed via BIOCSRSIG
.
FIOSETOWN
int *FIOGETOWN
int *SIGIO
when packets are available. The
signal may be changed using BIOCSRSIG
(see
above).The following structure is prepended to each packet returned by read(2):
struct bpf_hdr { struct bpf_timeval bh_tstamp; u_int32_t bh_caplen; u_int32_t bh_datalen; u_int16_t bh_hdrlen; };
The fields, stored in host order, are as follows:
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_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.
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_int16_t code; u_char jt; u_char jf; u_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
logically 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
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 recognized
BPF_LD
instructions follow.
BPF_LDX
BPF_MSH
, a hack for efficiently
loading the IP header length.
BPF_ST
BPF_ST
BPF_STX
BPF_STX
BPF_ALU
BPF_K
or BPF_X
).
BPF_ALU
+BPF_ADD+BPF_K
BPF_ALU
+BPF_SUB+BPF_K
BPF_ALU
+BPF_MUL+BPF_K
BPF_ALU
+BPF_DIV+BPF_K
BPF_ALU
+BPF_AND+BPF_K
BPF_ALU
+BPF_OR+BPF_K
BPF_ALU
+BPF_LSH+BPF_K
BPF_ALU
+BPF_RSH+BPF_K
BPF_ALU
+BPF_ADD+BPF_X
BPF_ALU
+BPF_SUB+BPF_X
BPF_ALU
+BPF_MUL+BPF_X
BPF_ALU
+BPF_DIV+BPF_X
BPF_ALU
+BPF_AND+BPF_X
BPF_ALU
+BPF_OR+BPF_X
BPF_ALU
+BPF_LSH+BPF_X
BPF_ALU
+BPF_RSH+BPF_X
BPF_ALU
+BPF_NEGBPF_JMP
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_JABPF_JMP
+BPF_JGT+BPF_K
BPF_JMP
+BPF_JGE+BPF_K
BPF_JMP
+BPF_JEQ+BPF_K
BPF_JMP
+BPF_JSET+BPF_K
BPF_JMP
+BPF_JGT+BPF_X
BPF_JMP
+BPF_JGE+BPF_X
BPF_JMP
+BPF_JEQ+BPF_X
BPF_JMP
+BPF_JSET+BPF_X
BPF_RET
BPF_K
) or the
accumulator (BPF_A
).
BPF_MISC
The bpf
interface provides the following
macros to facilitate array initializers:
BPF_STMT
(opcode, operand)
BPF_JUMP
(opcode,
operand, true_offset,
false_offset)
bpf
deviceThe 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), };
ioctl(2), read(2), select(2), signal(3), MAKEDEV(8), tcpdump(8)
McCanne, S. and Jacobson, V., The BSD Packet Filter: A New Architecture for User-level Packet Capture, 1993 Winter USENIX Conference, January 1993.
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.
Steve McCanne of Lawrence Berkeley Laboratory implemented BPF in Summer 1990. Much of the design is due to Van Jacobson.
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.
April 28, 2016 | OpenBSD-6.1 |