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         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 net-
         work 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 descrip-
         tor.  The writes are unbuffered, meaning only one packet can be processed
         per write.  Currently, only writes to Ethernets and SLIP links are sup-


         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 ker-
         nel, 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
         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 con-
         tents are stable and will not be changed until the buffer has been
         Initializing the buffer headers to all 0's before registering the buffer
         has the effect of assigning initial ownership of both buffers to the ker-
         nel.  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 pro-
         cess 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);
         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 main-
         tained 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 per-
         formance, 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 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 #include <sys/socket.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
         BIOCGDLT        (u_int) Returns the type of the data link layer underly-
                         ing the attached interface.  EINVAL is returned if no
                         interface has been specified.  The device types, prefixed
                         with "DLT_", are defined in
         BIOCPROMISC     Forces the interface into promiscuous mode.  All packets,
                         not just those destined for the local host, are pro-
                         cessed.  Since more than one file can be listening on a
                         packets can be read.  The device is indicated by name
                         using the ifr_name field of the ifreq structure.  Addi-
                         tionally, performs the actions of BIOCFLUSH.
         BIOCGRTIMEOUT   (struct timeval) Set or get the read timeout parameter.
                         The argument specifies the length of time to wait before
                         timing out on a read request.  This parameter is initial-
                         ized to zero by open(2), indicating no timeout.
         BIOCGSTATS      (struct bpf_stat) Returns the following structure of
                         packet statistics:
                         struct bpf_stat {
                                 u_int bs_recv;    /* number of packets received */
                                 u_int bs_drop;    /* number of packets dropped */
                         The fields are:
                               bs_recv the number of packets received by the
                                       descriptor since opened or reset (including
                                       any buffered since the last read call); and
                               bs_drop the number of packets which were accepted
                                       by the filter but dropped by the kernel
                                       because of buffer overflows (i.e., the
                                       application's reads 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.
         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 fol-
                         lowing structure:
                         struct bpf_program {
                                 int bf_len;
                                 struct bpf_insn *bf_insns;
                         The filter program is pointed to by the bf_insns field
                         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 An incompat-
                         ible filter may result in undefined behavior (most
                         likely, an error returned by ioctl() or haphazard packet
         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 compati-
                         bility.  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 incom-
                         ing, 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 inter-
                         face.  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
                         tions; 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, con-
                         servative selection of buffer size is suggested, espe-
                         cially 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.


         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 fil-
         bh_caplen   The length of the captured portion of the packet.  This is
                     the minimum of the truncation amount specified by the filter
                     and the length of the packet.
         bh_datalen  The length of the packet off the wire.  This value is inde-
                     pendent of the truncation amount specified by the filter.
         bh_hdrlen   The length of the bpf header, which may not be equal to
                     sizeof(struct bpf_hdr).
         The bh_hdrlen field exists to account for padding between the header and
         the link level protocol.  The purpose here is to guarantee proper align-
         ment of the packet data structures, which is required on alignment sensi-
         For example, if 'p' points to the start of a packet, this expression will
         advance it to the next packet:
               p = (char *)p + BPF_WORDALIGN(p->bh_hdrlen + p->bh_caplen)
         For the alignment mechanisms to work properly, the buffer passed to
         read(2) must itself be word aligned.  The malloc(3) function will always
         return an aligned buffer.


         A filter program is an array of instructions, with all branches forwardly
         directed, terminated by a return instruction.  Each instruction performs
         some action on the pseudo-machine state, which consists of an accumula-
         tor, index register, scratch memory store, and implicit program counter.
         The following structure defines the instruction format:
         struct bpf_insn {
                 u_short code;
                 u_char  jt;
                 u_char  jf;
                 u_long k;
         The k field is used in different ways by different instructions, and the
         jt and jf fields are used as offsets by the branch instructions.  The
         opcodes are encoded in a semi-hierarchical fashion.  There are eight
         classes of instructions: BPF_LD, BPF_LDX, BPF_ST, BPF_STX, BPF_ALU,
         BPF_JMP, BPF_RET, and BPF_MISC.  Various other mode and operator bits are
         or'd into the class to give the actual instructions.  The classes and
         modes are defined in
         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
         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_LDX+BPF_W+BPF_IMM   X <- k
                   BPF_LDX+BPF_W+BPF_MEM   X <- M[k]
                   BPF_LDX+BPF_W+BPF_LEN   X <- len
                   BPF_LDX+BPF_B+BPF_MSH   X <- 4*(P[k:1]&0xf)
         BPF_ST    This instruction stores the accumulator into the scratch mem-
                   ory.  We do not need an addressing mode since there is only one
                   possibility for the destination.
                   BPF_ST                  M[k] <- A
         BPF_STX   This instruction stores the index register in the scratch mem-
                   ory store.
                   BPF_STX                 M[k] <- X
         BPF_ALU   The alu instructions perform operations between the accumulator
                   and index register or constant, and store the result back in
                   the accumulator.  For binary operations, a source mode is
                   required (BPF_K or BPF_X).
                   BPF_ALU+BPF_ADD+BPF_K   A <- A + k
                   BPF_ALU+BPF_SUB+BPF_K   A <- A - k
                   BPF_ALU+BPF_MUL+BPF_K   A <- A * k
                   BPF_ALU+BPF_DIV+BPF_K   A <- A / k
                   BPF_ALU+BPF_AND+BPF_K   A <- A & k
                   BPF_ALU+BPF_OR+BPF_K    A <- A | k
                   BPF_ALU+BPF_LSH+BPF_K   A <- A << k
                   BPF_ALU+BPF_RSH+BPF_K   A <- A >> k
                   BPF_ALU+BPF_ADD+BPF_X   A <- A + X
                   BPF_ALU+BPF_SUB+BPF_X   A <- A - X
                   BPF_ALU+BPF_MUL+BPF_X   A <- A * X
                   BPF_ALU+BPF_DIV+BPF_X   A <- A / X
                   BPF_ALU+BPF_AND+BPF_X   A <- A & X
                   BPF_ALU+BPF_OR+BPF_X    A <- A | X
                   BPF_ALU+BPF_LSH+BPF_X   A <- A << X
                   BPF_ALU+BPF_RSH+BPF_X   A <- A >> X
                   BPF_ALU+BPF_NEG         A <- -A
         BPF_JMP   The jump instructions alter flow of control.  Conditional jumps
                   compare the accumulator against a constant (BPF_K) or the index
                   register (BPF_X).  If the result is true (or non-zero), the
                   true branch is taken, otherwise the false branch is taken.
                   Jump offsets are encoded in 8 bits so the longest jump is 256
                   instructions.  However, the jump always (BPF_JA) opcode uses
                   the 32 bit k field as the offset, allowing arbitrarily distant
                   destinations.  All conditionals use unsigned comparison conven-
                   BPF_JMP+BPF_JA          pc += k
                   BPF_JMP+BPF_JGT+BPF_K   pc += (A > k) ? jt : jf
                   BPF_JMP+BPF_JGE+BPF_K   pc += (A >= k) ? jt : jf
                   BPF_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 accu-
                   mulator or vice versa.
                   BPF_MISC+BPF_TAX        X <- A
                   BPF_MISC+BPF_TXA        A <- X
         The bpf interface provides the following macros to facilitate array ini-
         tializers: BPF_STMT(opcode, operand) and BPF_JUMP(opcode, operand,
         true_offset, false_offset).


         /dev/bpf  the packet filter device


         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_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.
                 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 Sum-
         mer 1990.  Much of the design is due to Van Jacobson.
         Support for zero-copy buffers was added by Robert N. M. Watson under con-
         tract to Seccuris Inc.


         The read buffer must be of a fixed size (returned by the BIOCGBLEN
         A file that does not request promiscuous mode may receive promiscuously
         received packets as a side effect of another file requesting this mode on
         the same hardware interface.  This could be fixed in the kernel with
         additional processing overhead.  However, we favor the model where all
         files must assume that the interface is promiscuous, and if so desired,
         must utilize a filter to reject foreign packets.
         Data link protocols with variable length headers are not currently sup-
         The SEESENT, DIRECTION, and FEEDBACK settings have been observed to work
         incorrectly on some interface types, including those with hardware loop-
         back rather than software loopback, and point-to-point interfaces.  They
         appear to function correctly on a broad range of Ethernet-style inter-

    BSD February 26, 2007 BSD


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