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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 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-

     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 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
                    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.


     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
                                  of buffer overflows (i.e., the application's
                                  reads aren't 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
                    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 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).


     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
                    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 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
     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

     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
     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_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
     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 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
               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
               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).

     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_STMT(BPF_LD+BPF_H+BPF_ABS, 20),
             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 and

     struct bpf_insn insns[] = {
             BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 12),
             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_STMT(BPF_LD+BPF_B+BPF_ABS, 23),
             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),

     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

     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-

     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.

BSD                            January 16, 1996                            BSD