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 be 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.
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.
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. The writes are unbuffered, meaning only one packet can be processed
per write. Currently, only writes to Ethernets and SLIP links are sup-
ported.
When the last minor device is opened, an additional minor device is cre-
ated on demand. The maximum number of devices that can be created is con-
trolled by the sysctl debug.bpf_maxdevices.
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>
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 inter-
face 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.
BIOCFLUSH Flushes the buffer of incoming packets, and resets the
statistics 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 the ifreq structure. All other
fields are undefined.
BIOCSETIF (struct ifreq) Sets the hardware interface associate with
the file. This command must be performed before any pack-
ets can be read. The device is indicated by name using
the ifr_name field of the ifreq structure. Additionally,
performs the actions of BIOCFLUSH.
BIOCSRTIMEOUT
BIOCGRTIMEOUT (struct timeval) Set or get the read timeout parameter.
The argument specifies the length of time to wait before
timing out on a read request. This parameter is initial-
ized 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; /* number of packets received */
u_int bs_drop; /* number of packets dropped */
};
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); and
bs_drop the number of packets which were accepted by
the filter but dropped by the kernel because
instructions and its length is passed in using the follow-
ing 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 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 current version is compatible with the run-
ning 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() or haphazard packet matching).
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 rou-
tine. Set to one if the link level source address will be
written, as provided, to the wire. This flag is initial-
ized to zero by default.
BIOCSSEESENT
BIOCGSEESENT (u_int) Set or get the flag determining whether locally
generated packets on the interface should be returned by
BPF. Set to zero to see only incoming packets on the
interface. Set to one to see packets originating locally
and remotely on the interface. This flag is initialized
to one by default.
BPF HEADER
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 inde-
pendent 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 insures 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. The malloc(3) function 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_short code;
u_char jt;
u_char jf;
u_long 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_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 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 restrictive 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 mem-
ory. 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 mem-
ory 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
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 conven-
tions.
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 spec-
ify the amount of packet to accept (i.e., they return the trun-
cation amount). A return value of zero indicates that the
packet should be ignored. 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 instruc-
tions 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) and BPF_JUMP(opcode, operand,
true_offset, false_offset).
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),
};
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
tcpdump(1), ioctl(2), byteorder(3), ng_bpf(4)
McCanne, S. and Jacobson V., An efficient, extensible, and portable
network monitor.
FILES
/dev/bpfn the packet filter device
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.
Data link protocols with variable length headers are not currently sup-
ported.
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
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