NAME
bpf
—
Berkeley Packet Filter
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/bpf0, /dev/bpf1, etc.
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.
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
. The number of open files can be
increased by creating additional device nodes with the
MAKEDEV(8) script.
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).
IOCTL INTERFACE
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 *- 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 withBIOCSETIF
. 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 inEINVAL
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 andEFAULT
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 isNULL
, the bfl_len field is set to indicate the required length of the 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 interface, a listener that opened its interface
non-promiscuously 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 statistics that are
returned by
BIOCGSTATS
. BIOCLOCK
- This ioctl is designed to prevent the security issues associated with an
open
bpf
descriptor in unprivileged programs. Even with dropped privileges, an openbpf
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 ofbpf
. By allowing only “known safe” ioctls, theBIOCLOCK
ioctl prevents this abuse. The allowable ioctls areBIOCFLUSH
,BIOCGBLEN
,BIOCGDIRFILT
,BIOCGDLT
,BIOCGDLTLIST
,BIOCGETIF
,BIOCGHDRCMPLT
,BIOCGRSIG
,BIOCGRTIMEOUT
,BIOCGSTATS
,BIOCIMMEDIATE
,BIOCLOCK
,BIOCSRTIMEOUT
,BIOCVERSION
,TIOCGPGRP
, andFIONREAD
. Use of any other ioctl is denied with errorEPERM
. 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 throughbpf
, 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 *- Returns the name of the hardware interface that the file is listening on.
The name is returned in the ifr_name field of the
struct ifreq
. 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
struct ifreq
. Additionally, performs the actions ofBIOCFLUSH
. BIOCSRTIMEOUT
struct timeval *BIOCGRTIMEOUT
struct timeval *- Sets or gets 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 { u_int bs_recv; u_int bs_drop; };
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 (i.e., the application's reads aren't keeping up with the packet traffic).
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 uninteresting
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 ofBIOCFLUSH
are performed.See section FILTER MACHINE for an explanation of the filter language.
BIOCSETWF
struct bpf_program *- Sets the filter program used by the kernel to filter the packets written
to the descriptor before the packets are sent out on the network. See
BIOCSETF
for a description of the filter program. This ioctl also acts asBIOCFLUSH
.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 *- 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
andBPF_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 *- Sets or gets the receive signal. This signal will be sent to the process
or process group specified by
FIOSETOWN
. It defaults toSIGIO
. BIOCSHDRCMPLT
u_int *BIOCGHDRCMPLT
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.
BIOCSFILDROP
u_int *BIOCGFILDROP
u_int *- Sets or gets the status of the “filter drop” flag. If non-zero, packets matching any filters will be reported to the associated interface so that they can be dropped.
BIOCSDIRFILT
u_int *BIOCGDIRFILT
u_int *- Sets or gets the status of the “direction filter” flag. If
non-zero, packets matching the specified direction (either
BPF_DIRECTION_IN
orBPF_DIRECTION_OUT
) will be ignored.
Standard ioctls
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 *- Returns the number of bytes that are immediately available for reading.
FIONBIO
int *- Sets or clears non-blocking I/O. If the argument is non-zero, enable
non-blocking I/O. If the argument is zero, disable non-blocking I/O. If
non-blocking I/O is enabled, the return value of a read while no data is
available will be 0. The non-blocking read behavior is different from
performing non-blocking reads on other file descriptors, which will return
-1 and set errno to
EAGAIN
if no data is available. Note: setting this overrides the timeout set byBIOCSRTIMEOUT
. FIOASYNC
int *- Enables or disables asynchronous I/O. When enabled (argument is non-zero),
the process or process group specified by
FIOSETOWN
will start receivingSIGIO
signals when packets arrive. Note that you must perform anFIOSETOWN
command in order for this to take effect, as the system will not do it by default. The signal may be changed viaBIOCSRSIG
. FIOSETOWN
int *FIOGETOWN
int *- Sets or gets the process or process group (if negative) that should
receive
SIGIO
when packets are available. The signal may be changed usingBIOCSRSIG
(see above).
BPF header
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:
- bh_tstamp
- Time at which the packet was processed by the packet filter.
- bh_caplen
- 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
- Length of the packet off the wire. This value is independent of the truncation amount specified by the filter.
- bh_hdrlen
- 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 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 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
- 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
). ForBPF_IND
andBPF_ABS
, the data size must be specified as a word (BPF_W
), halfword (BPF_H
), or byte (BPF_B
). The semantics of all recognizedBPF_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 accumulator and index
register or constant, and store the result back in the accumulator. For
binary operations, a source mode is required
(
BPF_K
orBPF_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) or,
for the write filter, the maximum acceptable size for the packet (i.e.,
the packet is dropped if it is larger than the returned amount). A return
value of zero indicates that the packet should be ignored/dropped. 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 initializers:
BPF_STMT
(opcode, operand)
BPF_JUMP
(opcode,
operand, true_offset,
false_offset)
FILES
- /dev/bpf[0-9]
bpf
devices
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
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.
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
Steve McCanne of Lawrence Berkeley Laboratory implemented BPF in Summer 1990. Much of the design is due to Van Jacobson.
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.