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/bpf0,
/dev/bpf1, etc. 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 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 usersettable 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 under BSD,
can be set with
BIOCSBLEN. Note that an individual packet larger than this
size is necessarily
truncated.
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.
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.
Ioctls [Toc] [Back]
The ioctl 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 (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.
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 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 readwrite
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
BIOCGBLEN, BIOCFLUSH, BIOCGDLT, BIOCGETIF, BIOCGRTIMEOUT,
BIOCSRTIMEOUT, BIOCIMMEDIATE, BIOCGSTATS, BIOCVERSION, BIOCGRSIG,
BIOCGHDRCMPLT, 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)
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 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 {
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)
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 (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 {
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)
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 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)
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(2) or haphazard packet matching).
BIOCSRSIG, BIOCGRSIG (u_int)
Set or get the receive signal. This signal will be
sent to the
process or process group specified by FIOSETOWN. It
defaults to
SIGIO.
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.
Standard ioctls [Toc] [Back]
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.
SIOCGIFADDR (struct ifreq)
Returns the address associated with the interface.
FIONBIO (int)
Set or clear non-blocking I/O. If the argument is
non-zero, enable
non-blocking I/O. If the argument is zero,
disable nonblocking
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 nonblocking 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 by BIOCSRTIMEOUT.
FIOASYNC (int)
Enable or disable asynchronous I/O. When enabled
(argument is
non-zero), the process or process group specified by
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, FIOGETOWN (int)
Set or get the process or process group (if negative) that should
receive SIGIO when packets are available. The signal may be
changed using BIOCSRSIG (see above).
BPF header [Toc] [Back]
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 [Toc] [Back]
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). 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_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 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 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)
/dev/bpf[0-9] BPF devices
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),
};
ioctl(2), read(2), select(2), signal(3), MAKEDEV(8), tcpdump(8)
McCanne, S. and Jacobson V., An efficient, extensible, and
portable
network monitor.
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.
Data link protocols with variable length headers are not
currently supported.
OpenBSD 3.6 May 23, 1991
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