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

     bpf -- Berkeley Packet Filter

     pseudo-device bpf

     The Berkeley Packet Filter provides a raw interface to data link layers in a protocol inde-
     pendent 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 necessarily trun-

     The packet filter will support any link level protocol that has fixed length headers.  Cur-
     rently, 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.  The writes are
     unbuffered, meaning only one packet can be processed per write.  Currently, only writes to
     Ethernets and SLIP links are supported.

     The ioctl(2) command codes below are defined in <net/bpf.h>.  All commands require these

	     #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>.

     In addition to FIONREAD 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 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 des-
		    tined 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 reme-
		    died 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 struc-
		    ture.  All other fields are undefined.

     BIOCSETIF	    (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 ifr_name field of the ifreq structure.	Addition-
		    ally, performs the actions of BIOCFLUSH.


     BIOCGRTIMEOUT  (struct timeval) Set or get the read timeout parameter.  The argument speci-
		    fies 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;    /* 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 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 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 instructions and its length is 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.

     BIOCVERSION    (struct bpf_version) Returns the major and minor version numbers of the fil-
		    ter 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 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


     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.

     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 filter.
     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 independent of the trunca-
		 tion 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 pro-
     tocol.  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 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 Ether-
     net, since the type field is a short falling on an even offset, and the addresses are proba-
     bly 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.

     A filter program is an array of instructions, with all branches forwardly directed, termi-
     nated 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_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-hierar-
     chical 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

     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 op-
	       erand 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 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 accu-
	       mulator 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).  A return value of
	       zero indicates that the packet should be ignored.  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.  Cur-
	       rently, 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) and BPF_JUMP(opcode, operand, true_offset, false_offset).

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

     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)

     McCanne, S.  and Jacobson V., An efficient, extensible, and portable network monitor.

     /dev/bpfn	  the packet filter device

     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.

     The Enet packet filter was created in 1980 by Mike Accetta and Rick Rashid at Carnegie-Mel-
     lon University.  Jeffrey Mogul, at Stanford, ported the code to BSD and continued its devel-
     opment 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 Summer 1990.  Much of
     the design is due to Van Jacobson.

BSD					 January 16, 1996				      BSD
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