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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.