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NAME

     bpf — Berkeley Packet Filter

SYNOPSIS

     device bpf

DESCRIPTION

     The Berkeley Packet Filter provides a raw interface to data link layers in a protocol
     independent fashion.  All packets on the network, even those destined for other hosts, are
     accessible through this mechanism.

     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.

BUFFER MODES

     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 truncated.

     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 buffer.

     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
     bzh_kernel_gen.

     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 !=
                 atomic_load_acq_int(&bzh->bzh_kernel_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 availbility 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.

IOCTLS

     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 <net/if.h>.

     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 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.

     BIOCSRTIMEOUT

     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 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 are not 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
                     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.

     BIOCSETF

     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).

     BIOCSHDRCMPLT

     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.

     BIOCSSEESENT

     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.

     BIOCSDIRECTION

     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 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.

     BIOCGETBUFMODE

     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.

BPF HEADER

     The following structure is prepended to each packet returned by read(2) or via a zero-copy
     buffer:

     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
                 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 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 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.

FILTER MACHINE

     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 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 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
               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 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 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).

FILES

     /dev/bpf  the packet filter device

EXAMPLES

     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_JUMP(BPF_JMP+BPF_JEQ+BPF_K, ETHERTYPE_REVARP, 0, 3),
             BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 20),
             BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, REVARP_REQUEST, 0, 1),
             BPF_STMT(BPF_RET+BPF_K, sizeof(struct ether_arp) +
                      sizeof(struct ether_header)),
             BPF_STMT(BPF_RET+BPF_K, 0),
     };

     This filter accepts only IP packets between host 128.3.112.15 and 128.3.112.35.

     struct bpf_insn insns[] = {
             BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 12),
             BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, ETHERTYPE_IP, 0, 8),
             BPF_STMT(BPF_LD+BPF_W+BPF_ABS, 26),
             BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x8003700f, 0, 2),
             BPF_STMT(BPF_LD+BPF_W+BPF_ABS, 30),
             BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x80037023, 3, 4),
             BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x80037023, 0, 3),
             BPF_STMT(BPF_LD+BPF_W+BPF_ABS, 30),
             BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x8003700f, 0, 1),
             BPF_STMT(BPF_RET+BPF_K, (u_int)-1),
             BPF_STMT(BPF_RET+BPF_K, 0),
     };

     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_JUMP(BPF_JMP+BPF_JEQ+BPF_K, ETHERTYPE_IP, 0, 10),
             BPF_STMT(BPF_LD+BPF_B+BPF_ABS, 23),
             BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, IPPROTO_TCP, 0, 8),
             BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 20),
             BPF_JUMP(BPF_JMP+BPF_JSET+BPF_K, 0x1fff, 6, 0),
             BPF_STMT(BPF_LDX+BPF_B+BPF_MSH, 14),
             BPF_STMT(BPF_LD+BPF_H+BPF_IND, 14),
             BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 79, 2, 0),
             BPF_STMT(BPF_LD+BPF_H+BPF_IND, 16),
             BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 79, 0, 1),
             BPF_STMT(BPF_RET+BPF_K, (u_int)-1),
             BPF_STMT(BPF_RET+BPF_K, 0),
     };

SEE ALSO

     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.

HISTORY

     The Enet packet filter was created in 1980 by Mike Accetta and Rick Rashid at Carnegie-
     Mellon University.  Jeffrey Mogul, at Stanford, ported the code to 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.

AUTHORS

     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.

BUGS

     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 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.