man/man4/bpf.4

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.Dd January 16, 1996
.Dt BPF 4
.Os
.Sh NAME
.Nm bpf
.Nd Berkeley Packet Filter
.Sh SYNOPSIS
.Cd device bpf
.Sh 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.
.Pp
The packet filter appears as a character special device,
.Pa /dev/bpf0 ,
.Pa /dev/bpf1 ,
etc.
After opening the device, the file descriptor must be bound to a
specific network interface with the
.Dv BIOCSETIF
ioctl.
A given interface can be shared by multiple listeners, and the filter
underlying each descriptor will see an identical packet stream.
.Pp
A separate device file is required for each minor device.
If a file is in use, the open will fail and
.Va errno
will be set to
.Er EBUSY .
.Pp
Associated with each open instance of a
.Nm
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.
.Pp
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
.Nm .
This size is returned by the
.Dv BIOCGBLEN
ioctl (see below), and
can be set with
.Dv BIOCSBLEN .
Note that an individual packet larger than this size is necessarily
truncated.
.Pp
The packet filter will support any link level protocol that has fixed length
headers.
Currently, only Ethernet,
.Tn SLIP ,
and
.Tn PPP
drivers have been modified to interact with
.Nm .
.Pp
Since packet data is in network byte order, applications should use the
.Xr byteorder 3
macros to extract multi-byte values.
.Pp
A packet can be sent out on the network by writing to a
.Nm
file descriptor.
The writes are unbuffered, meaning only one packet can be processed per write.
Currently, only writes to Ethernets and
.Tn SLIP
links are supported.
.Sh IOCTLS
The
.Xr ioctl 2
command codes below are defined in
.In net/bpf.h .
All commands require
these includes:
.Bd -literal
	#include <sys/types.h>
	#include <sys/time.h>
	#include <sys/ioctl.h>
	#include <net/bpf.h>
.Ed
.Pp
Additionally,
.Dv BIOCGETIF
and
.Dv BIOCSETIF
require
.In sys/socket.h
and
.In net/if.h .
.Pp
In addition to
.Dv FIONREAD
and
.Dv SIOCGIFADDR ,
the following commands may be applied to any open
.Nm
file.
The (third) argument to
.Xr ioctl 2
should be a pointer to the type indicated.
.Bl -tag -width BIOCGRTIMEOUT
.It Dv BIOCGBLEN
.Pq Li u_int
Returns the required buffer length for reads on
.Nm
files.
.It Dv BIOCSBLEN
.Pq Li u_int
Sets the buffer length for reads on
.Nm
files.
The buffer must be set before the file is attached to an interface
with
.Dv 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
.Er EIO
if it is passed a buffer that is not this size.
.It Dv BIOCGDLT
.Pq Li u_int
Returns the type of the data link layer underlying the attached interface.
.Er EINVAL
is returned if no interface has been specified.
The device types, prefixed with
.Dq Li DLT_ ,
are defined in
.In net/bpf.h .
.It Dv 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.
.It Dv BIOCFLUSH
Flushes the buffer of incoming packets,
and resets the statistics that are returned by BIOCGSTATS.
.It Dv BIOCGETIF
.Pq Li "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
.Li ifreq
structure.
All other fields are undefined.
.It Dv BIOCSETIF
.Pq Li "struct ifreq"
Sets the hardware interface associate with the file.  This
command must be performed before any packets can be read.
The device is indicated by name using the
.Li ifr_name
field of the
.Li ifreq
structure.
Additionally, performs the actions of
.Dv BIOCFLUSH .
.It Dv BIOCSRTIMEOUT
.It Dv BIOCGRTIMEOUT
.Pq Li "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 initialized to zero by
.Xr open 2 ,
indicating no timeout.
.It Dv BIOCGSTATS
.Pq Li "struct bpf_stat"
Returns the following structure of packet statistics:
.Bd -literal
struct bpf_stat {
	u_int bs_recv;    /* number of packets received */
	u_int bs_drop;    /* number of packets dropped */
};
.Ed
.Pp
The fields are:
.Bl -hang -offset indent
.It Li bs_recv
the number of packets received by the descriptor since opened or reset
(including any buffered since the last read call);
and
.It Li bs_drop
the 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).
.El
.It Dv BIOCIMMEDIATE
.Pq Li u_int
Enable or disable
.Dq 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
.Xr rarpd 8
which must respond to messages in real time.
The default for a new file is off.
.It Dv BIOCSETF
.Pq Li "struct bpf_program"
Sets the filter program used by the kernel to discard uninteresting
packets.
An array of instructions and its length is passed in using
the following structure:
.Bd -literal
struct bpf_program {
	int bf_len;
	struct bpf_insn *bf_insns;
};
.Ed
.Pp
The filter program is pointed to by the
.Li bf_insns
field while its length in units of
.Sq Li struct bpf_insn
is given by the
.Li bf_len
field.
Also, the actions of
.Dv BIOCFLUSH
are performed.
See section
.Sx "FILTER MACHINE"
for an explanation of the filter language.
.It Dv BIOCVERSION
.Pq Li "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:
.Bd -literal
struct bpf_version {
        u_short bv_major;
        u_short bv_minor;
};
.Ed
.Pp
The current version numbers are given by
.Dv BPF_MAJOR_VERSION
and
.Dv BPF_MINOR_VERSION
from
.In net/bpf.h .
An incompatible filter
may result in undefined behavior (most likely, an error returned by
.Fn ioctl
or haphazard packet matching).
.It Dv BIOCSHDRCMPLT
.It Dv BIOCGHDRCMPLT
.Pq Li u_int
Set or get the status of the
.Dq 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.
.It Dv BIOCSSEESENT
.It Dv BIOCGSEESENT
.Pq Li 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.
.El
.Sh BPF HEADER
The following structure is prepended to each packet returned by
.Xr read 2 :
.Bd -literal
struct bpf_hdr {
        struct timeval bh_tstamp;     /* time stamp */
        u_long bh_caplen;             /* length of captured portion */
        u_long bh_datalen;            /* original length of packet */
        u_short bh_hdrlen;            /* length of bpf header (this struct
					 plus alignment padding */
};
.Ed
.Pp
The fields, whose values are stored in host order, and are:
.Pp
.Bl -tag -compact -width bh_datalen
.It Li bh_tstamp
The time at which the packet was processed by the packet filter.
.It Li 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.
.It Li bh_datalen
The length of the packet off the wire.
This value is independent of the truncation amount specified by the filter.
.It Li bh_hdrlen
The length of the
.Nm
header, which may not be equal to
.\" XXX - not really a function call
.Fn sizeof "struct bpf_hdr" .
.El
.Pp
The
.Li 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 insures that the
.Li 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).
.Pp
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
.Dv BPF_WORDALIGN
is defined in
.In net/bpf.h
to facilitate
this process.
It rounds up its argument to the nearest word aligned value (where a word is
.Dv BPF_ALIGNMENT
bytes wide).
.Pp
For example, if
.Sq Li p
points to the start of a packet, this expression
will advance it to the next packet:
.Dl p = (char *)p + BPF_WORDALIGN(p->bh_hdrlen + p->bh_caplen)
.Pp
For the alignment mechanisms to work properly, the
buffer passed to
.Xr read 2
must itself be word aligned.
The
.Xr malloc 3
function
will always return an aligned buffer.
.Sh FILTER MACHINE
A filter program is an array of instructions, with all branches forwardly
directed, terminated by a
.Em 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.
.Pp
The following structure defines the instruction format:
.Bd -literal
struct bpf_insn {
	u_short	code;
	u_char 	jt;
	u_char 	jf;
	u_long k;
};
.Ed
.Pp
The
.Li k
field is used in different ways by different instructions,
and the
.Li jt
and
.Li 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:
.Dv BPF_LD ,
.Dv BPF_LDX ,
.Dv BPF_ST ,
.Dv BPF_STX ,
.Dv BPF_ALU ,
.Dv BPF_JMP ,
.Dv BPF_RET ,
and
.Dv BPF_MISC .
Various other mode and
operator bits are or'd into the class to give the actual instructions.
The classes and modes are defined in
.In net/bpf.h .
.Pp
Below are the semantics for each defined
.Nm
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
.Dq 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
.Dv BPF_MEMWORDS
- 1.
.Li k ,
.Li jt ,
and
.Li jf
are the corresponding fields in the
instruction definition.
.Dq len
refers to the length of the packet.
.Pp
.Bl -tag -width BPF_STXx
.It Dv BPF_LD
These instructions copy a value into the accumulator.
The type of the source operand is specified by an
.Dq addressing mode
and can be a constant
.Pq Dv BPF_IMM ,
packet data at a fixed offset
.Pq Dv BPF_ABS ,
packet data at a variable offset
.Pq Dv BPF_IND ,
the packet length
.Pq Dv BPF_LEN ,
or a word in the scratch memory store
.Pq Dv BPF_MEM .
For
.Dv BPF_IND
and
.Dv BPF_ABS ,
the data size must be specified as a word
.Pq Dv BPF_W ,
halfword
.Pq Dv BPF_H ,
or byte
.Pq Dv BPF_B .
The semantics of all the recognized
.Dv BPF_LD
instructions follow.
.Pp
.Bl -tag -width "BPF_LD+BPF_W+BPF_IND" -compact
.It Li BPF_LD+BPF_W+BPF_ABS
A <- P[k:4]
.It Li BPF_LD+BPF_H+BPF_ABS
A <- P[k:2]
.It Li BPF_LD+BPF_B+BPF_ABS
A <- P[k:1]
.It Li BPF_LD+BPF_W+BPF_IND
A <- P[X+k:4]
.It Li BPF_LD+BPF_H+BPF_IND
A <- P[X+k:2]
.It Li BPF_LD+BPF_B+BPF_IND
A <- P[X+k:1]
.It Li BPF_LD+BPF_W+BPF_LEN
A <- len
.It Li BPF_LD+BPF_IMM
A <- k
.It Li BPF_LD+BPF_MEM
A <- M[k]
.El
.It Dv 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
.Dv BPF_MSH ,
a hack for efficiently loading the IP header length.
.Pp
.Bl -tag -width "BPF_LDX+BPF_W+BPF_MEM" -compact
.It Li BPF_LDX+BPF_W+BPF_IMM
X <- k
.It Li BPF_LDX+BPF_W+BPF_MEM
X <- M[k]
.It Li BPF_LDX+BPF_W+BPF_LEN
X <- len
.It Li BPF_LDX+BPF_B+BPF_MSH
X <- 4*(P[k:1]&0xf)
.El
.It Dv 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.
.Pp
.Bl -tag -width "BPF_ST" -compact
.It Li BPF_ST
M[k] <- A
.El
.It Dv BPF_STX
This instruction stores the index register in the scratch memory store.
.Pp
.Bl -tag -width "BPF_STX" -compact
.It Li BPF_STX
M[k] <- X
.El
.It Dv 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
.Dv ( BPF_K
or
.Dv BPF_X ) .
.Pp
.Bl -tag -width "BPF_ALU+BPF_MUL+BPF_K" -compact
.It Li BPF_ALU+BPF_ADD+BPF_K
A <- A + k
.It Li BPF_ALU+BPF_SUB+BPF_K
A <- A - k
.It Li BPF_ALU+BPF_MUL+BPF_K
A <- A * k
.It Li BPF_ALU+BPF_DIV+BPF_K
A <- A / k
.It Li BPF_ALU+BPF_AND+BPF_K
A <- A & k
.It Li BPF_ALU+BPF_OR+BPF_K
A <- A | k
.It Li BPF_ALU+BPF_LSH+BPF_K
A <- A << k
.It Li BPF_ALU+BPF_RSH+BPF_K
A <- A >> k
.It Li BPF_ALU+BPF_ADD+BPF_X
A <- A + X
.It Li BPF_ALU+BPF_SUB+BPF_X
A <- A - X
.It Li BPF_ALU+BPF_MUL+BPF_X
A <- A * X
.It Li BPF_ALU+BPF_DIV+BPF_X
A <- A / X
.It Li BPF_ALU+BPF_AND+BPF_X
A <- A & X
.It Li BPF_ALU+BPF_OR+BPF_X
A <- A | X
.It Li BPF_ALU+BPF_LSH+BPF_X
A <- A << X
.It Li BPF_ALU+BPF_RSH+BPF_X
A <- A >> X
.It Li BPF_ALU+BPF_NEG
A <- -A
.El
.It Dv BPF_JMP
The jump instructions alter flow of control.
Conditional jumps
compare the accumulator against a constant
.Pq Dv BPF_K
or the index register
.Pq Dv 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
.Pq Dv BPF_JA
opcode uses the 32 bit
.Li k
field as the offset, allowing arbitrarily distant destinations.
All conditionals use unsigned comparison conventions.
.Pp
.Bl -tag -width "BPF_JMP+BPF_KSET+BPF_X" -compact
.It Li BPF_JMP+BPF_JA
pc += k
.It Li BPF_JMP+BPF_JGT+BPF_K
pc += (A > k) ? jt : jf
.It Li BPF_JMP+BPF_JGE+BPF_K
pc += (A >= k) ? jt : jf
.It Li BPF_JMP+BPF_JEQ+BPF_K
pc += (A == k) ? jt : jf
.It Li BPF_JMP+BPF_JSET+BPF_K
pc += (A & k) ? jt : jf
.It Li BPF_JMP+BPF_JGT+BPF_X
pc += (A > X) ? jt : jf
.It Li BPF_JMP+BPF_JGE+BPF_X
pc += (A >= X) ? jt : jf
.It Li BPF_JMP+BPF_JEQ+BPF_X
pc += (A == X) ? jt : jf
.It Li BPF_JMP+BPF_JSET+BPF_X
pc += (A & X) ? jt : jf
.El
.It Dv BPF_RET
The return instructions terminate the filter program and specify the amount
of packet to accept (i.e., they return the truncation amount).
A return value of zero indicates that the packet should be ignored.
The return value is either a constant
.Pq Dv BPF_K
or the accumulator
.Pq Dv BPF_A .
.Pp
.Bl -tag -width "BPF_RET+BPF_K" -compact
.It Li BPF_RET+BPF_A
accept A bytes
.It Li BPF_RET+BPF_K
accept k bytes
.El
.It Dv 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.
.Pp
.Bl -tag -width "BPF_MISC+BPF_TAX" -compact
.It Li BPF_MISC+BPF_TAX
X <- A
.It Li BPF_MISC+BPF_TXA
A <- X
.El
.El
.Pp
The
.Nm
interface provides the following macros to facilitate
array initializers:
.Fn BPF_STMT opcode operand
and
.Fn BPF_JUMP opcode operand true_offset false_offset .
.Sh EXAMPLES
The following filter is taken from the Reverse ARP Daemon.
It accepts only Reverse ARP requests.
.Bd -literal
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),
};
.Ed
.Pp
This filter accepts only IP packets between host 128.3.112.15 and
128.3.112.35.
.Bd -literal
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),
};
.Ed
.Pp
Finally, this filter returns only TCP finger packets.
We must parse the IP header to reach the TCP header.
The
.Dv BPF_JSET
instruction
checks that the IP fragment offset is 0 so we are sure
that we have a TCP header.
.Bd -literal
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),
};
.Ed
.Sh SEE ALSO
.Xr tcpdump 1 ,
.Xr ioctl 2 ,
.Xr byteorder 3 ,
.Xr ng_bpf 4
.Rs
.%A McCanne, S.
.%A Jacobson V.
.%T "An efficient, extensible, and portable network monitor"
.Re
.Sh FILES
.Bl -tag -compact -width /dev/bpfXXX
.It Pa /dev/bpf Ns Sy n
the packet filter device
.El
.Sh BUGS
The read buffer must be of a fixed size (returned by the
.Dv BIOCGBLEN
ioctl).
.Pp
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.
.Pp
Data link protocols with variable length headers are not currently supported.
.Pp
The
.Dv SEESENT
flag has been observed to work incorrectly on some interface
types, including those with hardware loopback rather than software loopback,
and point-to-point interfaces.
It appears to function correctly on a
broad range of ethernet-style interfaces.
.Sh 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
.Bx
and continued its development from
1983 on.
Since then, it has evolved into the Ultrix Packet Filter at
.Tn DEC ,
a
.Tn STREAMS
.Tn NIT
module under
.Tn SunOS 4.1 ,
and
.Tn BPF .
.Sh AUTHORS
.An -nosplit
.An Steven McCanne ,
of Lawrence Berkeley Laboratory, implemented BPF in
Summer 1990.  Much of the design is due to
.An Van Jacobson .