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BPF(4)			 BSD Kernel Interfaces Manual			BPF(4)

     bpf -- Berkeley Packet Filter

     device bpf

     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 by	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 de-
     scriptor 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	re-
     turned by the BIOCGBLEN ioctl (see	below),	and can	be set with BIOCSBLEN.
     Note that an individual packet larger than	this size is necessarily trun-

     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-

     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>

     Additionally, BIOCGETIF and BIOCSETIF require <sys/socket.h> and

     In	addition to FIONREAD and SIOCGIFADDR, the following commands may be
     applied to	any open bpf file.  The	(third)	argument to ioctl(2) 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 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 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 in-
		    terface, a listener	that opened its	interface non-promis-
		    cuously 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.


     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	de-
				  scriptor 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
				  of buffer overflows (i.e., the application's
				  reads	are not	keeping	up with	the packet

     BIOCIMMEDIATE  (u_int) Enable or disable "immediate mode",	based on the
		    truth value	of the argument.  When immediate mode is en-
		    abled, reads return	immediately upon packet	reception.
		    Otherwise, a read will block until either the kernel buf-
		    fer	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 in-
		    structions and its length is passed	in using the following

		    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).


     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.


     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 in-
		    terface.  Set to one to see	packets	originating locally
		    and	remotely on the	interface.  This flag is initialized
		    to one by default.

     The following structure is	prepended to each packet returned by read(2):

     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 */

     The fields, whose values are stored in host order,	and are:

     bh_tstamp	 The time at which the packet was processed by the packet fil-
     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
     is	not 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

     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.

     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 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
     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	in-
     dexed 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

     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 ef-
	       ficiently 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 re-
	       quired (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 conven-

	       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	does
	       not 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 accu-
	       mulator 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).

     /dev/bpfn	  the packet filter device

     The following filter is taken from	the Reverse ARP	Daemon.	 It accepts
     only Reverse ARP requests.

     struct bpf_insn insns[] = {
	     BPF_STMT(BPF_RET+BPF_K, sizeof(struct ether_arp) +
		      sizeof(struct ether_header)),

     This filter accepts only IP packets between host and

     struct bpf_insn insns[] = {
	     BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x8003700f, 0, 2),
	     BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x80037023, 3, 4),
	     BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x80037023, 0, 3),
	     BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x8003700f, 0, 1),
	     BPF_STMT(BPF_RET+BPF_K, (u_int)-1),

     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_JUMP(BPF_JMP+BPF_JSET+BPF_K, 0x1fff, 6, 0),
	     BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 79, 2, 0),
	     BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 79, 0, 1),
	     BPF_STMT(BPF_RET+BPF_K, (u_int)-1),

     tcpdump(1), ioctl(2), byteorder(3), ng_bpf(4), bpf(9)

     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.

     Steven McCanne, of	Lawrence Berkeley Laboratory, implemented BPF in Sum-
     mer 1990.	Much of	the design is due to Van Jacobson.

     The read buffer must be of	a fixed	size (returned by the BIOCGBLEN

     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 ad-
     ditional 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-

     The SEESENT flag has been observed	to work	incorrectly on some interface
     types, including those with hardware loopback rather than software	loop-
     back, and point-to-point interfaces.  It appears to function correctly on
     a broad range of Ethernet-style interfaces.

BSD			       January 16, 1996				   BSD


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