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bpf - Berkeley Packet Filter



      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/bpf0,
      /dev/bpf1, etc.  After opening the device, the file descriptor must be
      bound to a specific network interface with the BIOCSETIF ioctl.  A given
      interface can be shared 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.
      Reads from these files return the next group of packets that have matched
      the filter.  To improve performance, the buffer passed to read must be
      the same size as the buffers used internally by bpf.  This size is
      returned by the BIOCGBLEN ioctl (see below), and can be set with
      BIOCSBLEN.  Note that an individual packet larger than this size is nec‐
      essarily truncated.
      The packet filter will support any link level protocol that has fixed
      length headers.  Currently, only Ethernet, SLIP, and PPP drivers have
      been modified to interact with bpf.
      Since packet data is in network byte order, applications should use the
      byteorder(3) macros to extract multi-byte values.
      A packet can be sent out on the network by writing to a bpf file descrip‐
      tor.  The writes are unbuffered, meaning only one packet can be processed
      per write.  Currently, only writes to Ethernets and SLIP links are sup‐


      The ioctl(2) command codes below are defined in 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 size.
      BIOCGDLT       (u_int) Returns the type of the data link layer underlying
                     the attached interface.  EINVAL is returned if no inter‐
                     face has been specified.  The device types, prefixed with
                     “DLT_”, are defined in
      BIOCPROMISC    Forces the interface into promiscuous mode.  All packets,
                     not just those destined for the local host, are processed.
                     Since more than one file can be listening on a given
                     interface, a listener that opened its interface non-
                     promiscuously may receive packets promiscuously.  This
                     problem can be remedied with an appropriate filter.
      BIOCFLUSH      Flushes the buffer of incoming packets, and resets the
                     statistics that are returned by BIOCGSTATS.
      BIOCGETIF      (struct ifreq) Returns the name of the hardware interface
                     that the file is listening on.  The name is returned in
                     the ifr_name field of the ifreq structure.  All other
                     fields are undefined.
      BIOCSETIF      (struct ifreq) Sets the hardware interface associate with
                     the file.  This command must be performed before any pack‐
                     ets can be read.  The device is indicated by name using
                     the ifr_name field of the ifreq structure.  Additionally,
                     performs the actions of BIOCFLUSH.
      BIOCGRTIMEOUT  (struct timeval) Set or get the read timeout parameter.
                     The argument specifies the length of time to wait before
                     timing out on a read request.  This parameter is initial‐
                     ized to zero by open(2), indicating no timeout.
      BIOCGSTATS     (struct bpf_stat) Returns the following structure of
                     packet statistics:
                     struct bpf_stat {
                             u_int bs_recv;    /* number of packets received */
                             u_int bs_drop;    /* number of packets dropped */
                     The fields are:
                           bs_recv the number of packets received by the
                                   descriptor since opened or reset (including
                                   any buffered since the last read call); and
                           bs_drop the number of packets which were accepted by
                                   the filter but dropped by the kernel because
                                   of buffer overflows (i.e., the application’s
                                   reads 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.
      BIOCSETF       (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 follow‐
                     ing structure:
                     struct bpf_program {
                             int bf_len;
                             struct bpf_insn *bf_insns;
                     The filter program is pointed to by the bf_insns field
                     while its length in units of ‘struct bpf_insn’ is given by
                     the bf_len field.  Also, the actions of BIOCFLUSH are per‐
                     formed.  See section FILTER MACHINE for an explanation of
                     the filter language.
      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 run‐
                     ning kernel.  Version numbers are compatible if the major
                     numbers match and the application minor is less than or
                     equal to the kernel minor.  The kernel version number is
                     returned in the following structure:
                     struct bpf_version {
                             u_short bv_major;
                             u_short bv_minor;
                     The current version numbers are given by BPF_MAJOR_VERSION
                     and BPF_MINOR_VERSION from An incompatible filter may
                     result in undefined behavior (most likely, an error
                     returned by ioctl() or haphazard packet matching).
      BIOCGHDRCMPLT  (u_int) Set or get the status of the “header complete”
                     flag.  Set to zero if the link level source address should
                     be filled in automatically by the interface output rou‐
                     tine.  Set to one if the link level source address will be
                     written, as provided, to the wire.  This flag is initial‐
                     ized to zero by default.
      BIOCGSEESENT   (u_int) 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
                     (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 pack‐
                     ets 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
      BIOCFEEDBACK   (u_int) Set packet feedback mode.  This allows injected
                     packets to be fed back as input to the interface when out‐
                     put 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.
      The following structure is prepended to each packet returned by read(2):
      struct bpf_hdr {
              struct timeval bh_tstamp;     /* time stamp */
              u_long bh_caplen;             /* length of captured portion */
              u_long bh_datalen;            /* original length of packet */
              u_short bh_hdrlen;            /* length of bpf header (this struct
                                               plus alignment padding */
      The fields, whose values are stored in host order, and are:
      bh_tstamp   The time at which the packet was processed by the packet fil‐
      bh_caplen   The length of the captured portion of the packet.  This is
                  the minimum of the truncation amount specified by the filter
                  and the length of the packet.
      bh_datalen  The length of the packet off the wire.  This value is inde‐
                  pendent of the truncation amount specified by the filter.
      bh_hdrlen   The length of the bpf header, which may not be equal to
                  sizeof(struct bpf_hdr).
      The bh_hdrlen field exists to account for padding between the header and
      the link level protocol.  The purpose here is to guarantee proper align‐
      ment of the packet data structures, which is required on alignment sensi‐
      tive architectures and improves performance on many other architectures.
      The packet filter insures that the bpf_hdr and the network layer header
      will be word aligned.  Suitable precautions must be taken when accessing
      the link layer protocol fields on alignment restricted machines.  (This
      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
      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
      #include <net/bpf.h>
      to facilitate this process.  It rounds up its argument to the nearest
      word aligned value (where a word is BPF_ALIGNMENT bytes wide).
      For example, if ‘p’ points to the start of a packet, this expression will
      advance it to the next packet:
            p = (char *)p + BPF_WORDALIGN(p->bh_hdrlen + p->bh_caplen)
      For the alignment mechanisms to work properly, the buffer passed to
      read(2) must itself be word aligned.  The malloc(3) function will always
      return an aligned buffer.
      A filter program is an array of instructions, with all branches forwardly
      directed, terminated by a return instruction.  Each instruction performs
      some action on the pseudo-machine state, which consists of an accumula‐
      tor, index register, scratch memory store, and implicit program counter.
      The following structure defines the instruction format:
      struct bpf_insn {
              u_short code;
              u_char  jt;
              u_char  jf;
              u_long k;
      The k field is used in different ways by different instructions, and the
      jt and jf fields are used as offsets by the branch instructions.  The
      opcodes are encoded in a semi-hierarchical fashion.  There are eight
      classes of instructions: BPF_LD, BPF_LDX, BPF_ST, BPF_STX, BPF_ALU,
      BPF_JMP, BPF_RET, and BPF_MISC.  Various other mode and operator bits are
      or’d into the class to give the actual instructions.  The classes and
      modes are defined in
      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_LD+BPF_H+BPF_ABS    A <- P[k:2]
                BPF_LD+BPF_B+BPF_ABS    A <- P[k:1]
                BPF_LD+BPF_W+BPF_IND    A <- P[X+k:4]
                BPF_LD+BPF_H+BPF_IND    A <- P[X+k:2]
                BPF_LD+BPF_B+BPF_IND    A <- P[X+k:1]
                BPF_LD+BPF_W+BPF_LEN    A <- len
                BPF_LD+BPF_IMM          A <- k
                BPF_LD+BPF_MEM          A <- M[k]
      BPF_LDX   These instructions load a value into the index register.  Note
                that the addressing modes are more restrictive than those of
                the accumulator loads, but they include BPF_MSH, a hack for
                efficiently loading the IP header length.
                BPF_LDX+BPF_W+BPF_IMM   X <- k
                BPF_LDX+BPF_W+BPF_MEM   X <- M[k]
                BPF_LDX+BPF_W+BPF_LEN   X <- len
                BPF_LDX+BPF_B+BPF_MSH   X <- 4*(P[k:1]&0xf)
      BPF_ST    This instruction stores the accumulator into the scratch mem‐
                ory.  We do not need an addressing mode since there is only one
                possibility for the destination.
                BPF_ST                  M[k] <- A
      BPF_STX   This instruction stores the index register in the scratch mem‐
                ory store.
                BPF_STX                 M[k] <- X
      BPF_ALU   The alu instructions perform operations between the accumulator
                and index register or constant, and store the result back in
                the accumulator.  For binary operations, a source mode is
                required (BPF_K or BPF_X).
                BPF_ALU+BPF_ADD+BPF_K   A <- A + k
                BPF_ALU+BPF_SUB+BPF_K   A <- A - k
                BPF_ALU+BPF_MUL+BPF_K   A <- A * k
                BPF_ALU+BPF_DIV+BPF_K   A <- A / k
                BPF_ALU+BPF_AND+BPF_K   A <- A & k
                BPF_ALU+BPF_OR+BPF_K    A <- A | k
                BPF_ALU+BPF_LSH+BPF_K   A <- A << k
                BPF_ALU+BPF_RSH+BPF_K   A <- A >> k
                BPF_ALU+BPF_ADD+BPF_X   A <- A + X
                BPF_ALU+BPF_SUB+BPF_X   A <- A - X
                BPF_ALU+BPF_MUL+BPF_X   A <- A * X
                BPF_ALU+BPF_DIV+BPF_X   A <- A / X
                BPF_ALU+BPF_AND+BPF_X   A <- A & X
                BPF_ALU+BPF_OR+BPF_X    A <- A | X
                BPF_ALU+BPF_LSH+BPF_X   A <- A << X
                BPF_ALU+BPF_RSH+BPF_X   A <- A >> X
                BPF_ALU+BPF_NEG         A <- -A
      BPF_JMP   The jump instructions alter flow of control.  Conditional jumps
                compare the accumulator against a constant (BPF_K) or the index
                register (BPF_X).  If the result is true (or non-zero), the
                true branch is taken, otherwise the false branch is taken.
                Jump offsets are encoded in 8 bits so the longest jump is 256
                instructions.  However, the jump always (BPF_JA) opcode uses
                the 32 bit k field as the offset, allowing arbitrarily distant
                destinations.  All conditionals use unsigned comparison conven‐
                BPF_JMP+BPF_JA          pc += k
                BPF_JMP+BPF_JGT+BPF_K   pc += (A > k) ? jt : jf
                BPF_JMP+BPF_JGE+BPF_K   pc += (A >= k) ? jt : jf
                BPF_JMP+BPF_JEQ+BPF_K   pc += (A == k) ? jt : jf
                BPF_JMP+BPF_JSET+BPF_K  pc += (A & k) ? jt : jf
                BPF_JMP+BPF_JGT+BPF_X   pc += (A > X) ? jt : jf
                BPF_JMP+BPF_JGE+BPF_X   pc += (A >= X) ? jt : jf
                BPF_JMP+BPF_JEQ+BPF_X   pc += (A == X) ? jt : jf
                BPF_JMP+BPF_JSET+BPF_X  pc += (A & X) ? jt : jf
      BPF_RET   The return instructions terminate the filter program and spec‐
                ify the amount of packet to accept (i.e., they return the trun‐
                cation amount).  A return value of zero indicates that the
                packet should be ignored.  The return value is either a con‐
                stant (BPF_K) or the accumulator (BPF_A).
                BPF_RET+BPF_A           accept A bytes
                BPF_RET+BPF_K           accept k bytes
      BPF_MISC  The miscellaneous category was created for anything that 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/bpfn    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_STMT(BPF_LD+BPF_W+BPF_ABS, 26),
              BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x8003700f, 0, 2),
              BPF_STMT(BPF_LD+BPF_W+BPF_ABS, 30),
              BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x80037023, 3, 4),
              BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x80037023, 0, 3),
              BPF_STMT(BPF_LD+BPF_W+BPF_ABS, 30),
              BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x8003700f, 0, 1),
              BPF_STMT(BPF_RET+BPF_K, (u_int)-1),
              BPF_STMT(BPF_RET+BPF_K, 0),
      Finally, this filter returns only TCP finger packets.  We must parse the
      IP header to reach the TCP header.  The BPF_JSET instruction checks that
      the IP fragment offset is 0 so we are sure that we have a TCP header.
      struct bpf_insn insns[] = {
              BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 12),
              BPF_STMT(BPF_LD+BPF_B+BPF_ABS, 23),
              BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 20),
              BPF_JUMP(BPF_JMP+BPF_JSET+BPF_K, 0x1fff, 6, 0),
              BPF_STMT(BPF_LDX+BPF_B+BPF_MSH, 14),
              BPF_STMT(BPF_LD+BPF_H+BPF_IND, 14),
              BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 79, 2, 0),
              BPF_STMT(BPF_LD+BPF_H+BPF_IND, 16),
              BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 79, 0, 1),
              BPF_STMT(BPF_RET+BPF_K, (u_int)-1),
              BPF_STMT(BPF_RET+BPF_K, 0),
      tcpdump(1), ioctl(2), byteorder(3), ng_bpf(4), 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.


      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‐


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