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/bpf.  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
     descriptor that accepts the packet receives its own copy.

     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
     descriptor.  The writes are unbuffered, meaning only one packet can be
     processed per write.  Currently, only writes to Ethernets and SLIP links
     are supported.

     bpf devices deliver packet data to the application via memory buffers
     provided by the application.  The buffer mode is set using the
     BIOCSETBUFMODE ioctl, and read using the BIOCGETBUFMODE ioctl.

   Buffered read mode
     By default, bpf devices operate in the BPF_BUFMODE_BUFFER mode, in which
     packet data is copied explicitly from kernel to user memory using the
     read(2) system call.  The user process will declare a fixed buffer size
     that will be used both for sizing internal buffers and for all read(2)
     operations on the file.  This size is queried using the BIOCGBLEN ioctl,
     and is set using the BIOCSBLEN ioctl.  Note that an individual packet
     larger than the buffer size is necessarily truncated.

   Zero-copy buffer mode
     bpf devices may also operate in the BPF_BUFMODE_ZEROCOPY mode, in which
     packet data is written directly into two user memory buffers by the kernel,
     avoiding both system call and copying overhead.  Buffers are of fixed (and
     equal) size, page-aligned, and an even multiple of the page size.  The
     maximum zero-copy buffer size is returned by the BIOCGETZMAX ioctl.  Note
     that an individual packet larger than the buffer size is necessarily

     The user process registers two memory buffers using the BIOCSETZBUF ioctl,
     which accepts a struct bpf_zbuf pointer as an argument:

     struct bpf_zbuf {
             void *bz_bufa;
             void *bz_bufb;
             size_t bz_buflen;

     bz_bufa is a pointer to the userspace address of the first buffer that will
     be filled, and bz_bufb is a pointer to the second buffer.  bpf will then
     cycle between the two buffers as they fill and are acknowledged.

     Each buffer begins with a fixed-length header to hold synchronization and
     data length information for the buffer:

     struct bpf_zbuf_header {
             volatile u_int  bzh_kernel_gen; /* Kernel generation number. */
             volatile u_int  bzh_kernel_len; /* Length of data in the buffer. */
             volatile u_int  bzh_user_gen;   /* User generation number. */
             /* ...padding for future use... */

     The header structure of each buffer, including all padding, should be
     zeroed before it is configured using BIOCSETZBUF.  Remaining space in the
     buffer will be used by the kernel to store packet data, laid out in the
     same format as with buffered read mode.

     The kernel and the user process follow a simple acknowledgement protocol
     via the buffer header to synchronize access to the buffer: when the header
     generation numbers, bzh_kernel_gen and bzh_user_gen, hold the same value,
     the kernel owns the buffer, and when they differ, userspace owns the

     While the kernel owns the buffer, the contents are unstable and may change
     asynchronously; while the user process owns the buffer, its contents are
     stable and will not be changed until the buffer has been acknowledged.

     Initializing the buffer headers to all 0's before registering the buffer
     has the effect of assigning initial ownership of both buffers to the
     kernel.  The kernel signals that a buffer has been assigned to userspace by
     modifying bzh_kernel_gen, and userspace acknowledges the buffer and returns
     it to the kernel by setting the value of bzh_user_gen to the value of

     In order to avoid caching and memory re-ordering effects, the user process
     must use atomic operations and memory barriers when checking for and
     acknowledging buffers:

     #include <machine/atomic.h>

      * Return ownership of a buffer to the kernel for reuse.
     static void
     buffer_acknowledge(struct bpf_zbuf_header *bzh)

             atomic_store_rel_int(&bzh->bzh_user_gen, bzh->bzh_kernel_gen);

      * Check whether a buffer has been assigned to userspace by the kernel.
      * Return true if userspace owns the buffer, and false otherwise.
     static int
     buffer_check(struct bpf_zbuf_header *bzh)

             return (bzh->bzh_user_gen !=

     The user process may force the assignment of the next buffer, if any data
     is pending, to userspace using the BIOCROTZBUF ioctl.  This allows the user
     process to retrieve data in a partially filled buffer before the buffer is
     full, such as following a timeout; the process must recheck for buffer
     ownership using the header generation numbers, as the buffer will not be
     assigned to userspace if no data was present.

     As in the buffered read mode, kqueue(2), poll(2), and select(2) may be used
     to sleep awaiting the availability of a completed buffer.  They will return
     a readable file descriptor when ownership of the next buffer is assigned to
     user space.

     In the current implementation, the kernel may assign zero, one, or both
     buffers to the user process; however, an earlier implementation maintained
     the invariant that at most one buffer could be assigned to the user process
     at a time.  In order to both ensure progress and high performance, user
     processes should acknowledge a completely processed buffer as quickly as
     possible, returning it for reuse, and not block waiting on a second buffer
     while holding another buffer.

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

     BIOCGBLEN       (u_int) Returns the required buffer length for reads on bpf

     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 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
                     packets 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
                     initialized to zero by open(2), indicating no timeout.

     BIOCGSTATS      (struct bpf_stat) Returns the following structure of packet

                     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 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
                     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 rarpd(8) which must respond to messages in
                     real time.  The default for a new file is off.


     BIOCSETFNR      (struct bpf_program) Sets the read 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.  See section FILTER MACHINE for an
                     explanation of the filter language.  The only difference
                     between BIOCSETF and BIOCSETFNR is BIOCSETF performs the
                     actions of BIOCFLUSH while BIOCSETFNR does not.

     BIOCSETWF       (struct bpf_program) Sets the write filter program used by
                     the kernel to control what type of packets can be written
                     to the interface.  See the BIOCSETF command for more
                     information on the bpf filter program.

     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
                     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:

                     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 default.


     BIOCGSEESENT    (u_int) These commands are obsolete but left for
                     compatibility.  Use BIOCSDIRECTION and BIOCGDIRECTION
                     instead.  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.


     BIOCGDIRECTION  (u_int) Set or get the setting determining whether
                     incoming, outgoing, or all packets on the interface should
                     be returned by BPF.  Set to BPF_D_IN to see only incoming
                     packets on the interface.  Set to BPF_D_INOUT to see
                     packets originating locally and remotely on the interface.
                     Set to BPF_D_OUT to see only outgoing packets on the
                     interface.  This setting is initialized to BPF_D_INOUT by


     BIOCGTSTAMP     (u_int) Set or get format and resolution of the time stamps
                     returned by BPF.  Set to BPF_T_MICROTIME,
                     BPF_T_MICROTIME_MONOTONIC_FAST to get time stamps in 64-bit
                     struct timeval format.  Set to BPF_T_NANOTIME,
                     BPF_T_NANOTIME_MONOTONIC_FAST to get time stamps in 64-bit
                     struct timespec format.  Set to BPF_T_BINTIME,
                     BPF_T_BINTIME_MONOTONIC_FAST to get time stamps in 64-bit
                     struct bintime format.  Set to BPF_T_NONE to ignore time
                     stamp.  All 64-bit time stamp formats are wrapped in struct
                     bpf_ts.  The BPF_T_MICROTIME_FAST, BPF_T_NANOTIME_FAST,
                     BPF_T_NANOTIME_MONOTONIC_FAST, and
                     BPF_T_BINTIME_MONOTONIC_FAST are analogs of corresponding
                     formats without _FAST suffix but do not perform a full time
                     counter query, so their accuracy is one timer tick.  The
                     BPF_T_NANOTIME_MONOTONIC_FAST, and
                     BPF_T_BINTIME_MONOTONIC_FAST store the time elapsed since
                     kernel boot.  This setting is initialized to
                     BPF_T_MICROTIME by default.

     BIOCFEEDBACK    (u_int) Set packet feedback mode.  This allows injected
                     packets to be fed back as input to the interface when
                     output via the interface is successful.  When BPF_D_INOUT
                     direction is set, injected outgoing packet is not returned
                     by BPF to avoid duplication.  This flag is initialized to
                     zero by default.

     BIOCLOCK        Set the locked flag on the bpf descriptor.  This prevents
                     the execution of ioctl commands which could change the
                     underlying operating parameters of the device.


     BIOCSETBUFMODE  (u_int) Get or set the current bpf buffering mode; possible
                     values are BPF_BUFMODE_BUFFER, buffered read mode, and
                     BPF_BUFMODE_ZBUF, zero-copy buffer mode.

     BIOCSETZBUF     (struct bpf_zbuf) Set the current zero-copy buffer
                     locations; buffer locations may be set only once zero-copy
                     buffer mode has been selected, and prior to attaching to an
                     interface.  Buffers must be of identical size, page-
                     aligned, and an integer multiple of pages in size.  The
                     three fields bz_bufa, bz_bufb, and bz_buflen must be filled
                     out.  If buffers have already been set for this device, the
                     ioctl will fail.

     BIOCGETZMAX     (size_t) Get the largest individual zero-copy buffer size
                     allowed.  As two buffers are used in zero-copy buffer mode,
                     the limit (in practice) is twice the returned size.  As
                     zero-copy buffers consume kernel address space,
                     conservative selection of buffer size is suggested,
                     especially when there are multiple bpf descriptors in use
                     on 32-bit systems.

     BIOCROTZBUF     Force ownership of the next buffer to be assigned to
                     userspace, if any data present in the buffer.  If no data
                     is present, the buffer will remain owned by the kernel.
                     This allows consumers of zero-copy buffering to implement
                     timeouts and retrieve partially filled buffers.  In order
                     to handle the case where no data is present in the buffer
                     and therefore ownership is not assigned, the user process
                     must check bzh_kernel_gen against bzh_user_gen.

     One of the following structures is prepended to each packet returned by
     read(2) or via a zero-copy buffer:

     struct bpf_xhdr {
             struct bpf_ts   bh_tstamp;     /* time stamp */
             uint32_t        bh_caplen;     /* length of captured portion */
             uint32_t        bh_datalen;    /* original length of packet */
             u_short         bh_hdrlen;     /* length of bpf header (this struct
                                               plus alignment padding) */

     struct bpf_hdr {
             struct timeval  bh_tstamp;     /* time stamp */
             uint32_t        bh_caplen;     /* length of captured portion */
             uint32_t        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
     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 truncation amount specified by the filter.
     bh_hdrlen   The length of the bpf header, which may not be equal to
                 sizeof(struct bpf_xhdr) or 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 alignment
     of the packet data structures, which is required on alignment sensitive
     architectures and improves performance on many other architectures.  The
     packet filter ensures that the bpf_xhdr, bpf_hdr and the network layer
     header will be word aligned.  Currently, bpf_hdr is used when the time
     for backward compatibility reasons.  Otherwise, bpf_xhdr is used.  However,
     bpf_hdr may be deprecated in the near future.  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 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, terminated 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-hierarchical fashion.  There are eight classes of
     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 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 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

               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_MOD+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_XOR+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_MOD+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_XOR+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

               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 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
               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).

     A set of sysctl(8) variables controls the behaviour of the bpf subsystem

     net.bpf.optimize_writers: 0
             Various programs use BPF to send (but not receive) raw packets
             (cdpd, lldpd, dhcpd, dhcp relays, etc. are good examples of such
             programs).  They do not need incoming packets to be send to them.
             Turning this option on makes new BPF users to be attached to write-
             only interface list until program explicitly specifies read filter
             via pcap_set_filter().  This removes any performance degradation
             for high-speed interfaces.

             Binary interface for retrieving general statistics.

     net.bpf.zerocopy_enable: 0
             Permits zero-copy to be used with net BPF readers.  Use with

     net.bpf.maxinsns: 512
             Maximum number of instructions that BPF program can contain.  Use
             tcpdump(1) -d option to determine approximate number of instruction
             for any filter.

     net.bpf.maxbufsize: 524288
             Maximum buffer size to allocate for packets buffer.

     net.bpf.bufsize: 4096
             Default buffer size to allocate for packets buffer.

     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), kqueue(2), poll(2), select(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 Summer
     1990.  Much of the design is due to Van Jacobson.

     Support for zero-copy buffers was added by Robert N. M. Watson under
     contract to Seccuris Inc.

     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

     The SEESENT, DIRECTION, and FEEDBACK settings have been observed to work
     incorrectly on some interface types, including those with hardware loopback
     rather than software loopback, and point-to-point interfaces.  They appear
     to function correctly on a broad range of Ethernet-style interfaces.

BSD                             October 21, 2016                             BSD