BPF(4) BSD Programmer's Manual BPF(4)NAMEbpf - Berkeley Packet Filter
SYNOPSIS
pseudo-device bpfilter [count]
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. A given
interface can be shared between multiple listeners, and the filter under-
lying each descriptor will see an identical packet stream. The total
number of open files is limited to the value given in the kernel confi-
guration; the example given in the SYNOPSIS above sets the limit to 8.
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.
Associated with each open instance of a bpf file is a user-settable pack-
et filter. Whenever a packet is received by an interface, all file
descriptors listening on that interface apply their filter. Each descrip-
tor 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 neces-
sarily 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 descrip-
tor. Each descriptor can also have a user-settable filter for controlling
the writes. Only packets matching the filter are sent out of the inter-
face. 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
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 prom-
iscuously. This problem can be remedied with an appropriate
filter.
The interface remains in promiscuous mode until all files listen-
ing 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 ro-
gue program to listen on any interface on the system, send pack-
ets 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
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 af-
fected 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 com-
mand 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 pack-
et traffic).
BIOCIMMEDIATE (u_int)
Enable or disable "immediate mode", based on the truth value of
the argument. When immediate mode is enabled, reads return im-
mediately upon packet reception. Otherwise, a read will block un-
til 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 uninterest-
ing 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 com-
patible 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 re-
turned 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 au-
tomatically 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
bpf now supports several standard ioctls which allow the user to do asyn-
chronous 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, en-
able 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 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
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 align-
ment of the packet data structures, which is required on alignment-
sensitive architectures and improves performance on many other architec-
tures. The packet filter ensures that the bpf_hdr and the network layer
header will be word aligned. Suitable precautions must be taken when ac-
cessing 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 {
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 op-
codes 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 de-
fined 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 effi-
ciently 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 possi-
bility 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 instruc-
tions. 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 pack-
et should be ignored/dropped. The return value is either a con-
stant (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 accumu-
lator 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)
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 ALSOioctl(2), read(2), select(2), signal(3), tcpdump(8)
McCanne, S. and Jacobson V., An efficient, extensible, and portable
network monitor.
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 addi-
tional 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 sup-
ported.
MirOS BSD #10-current May 23, 1991 8
