Updated: 2022/Sep/29

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BPF(4)                       Device Drivers Manual                      BPF(4)

     bpf - Berkeley Packet Filter raw network interface

     pseudo-device bpfilter

     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.

     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
     necessarily truncated.

     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 Ethernet-based (including
     Wi-Fi), SLIP and loopback links are supported.

     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 <net/if.h>.

     The (third) argument to the ioctl(2) should be a pointer to the type

     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
                   EINVAL 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>.

     BIOCGDLTLIST (struct bpf_dltlist)
                   Returns an array of the available types of the data link
                   layer underlying the attached interface:

                         struct bpf_dltlist {
                                 u_int bfl_len;
                                 u_int *bfl_list;

                   The available types are returned in the array pointed to by
                   the bfl_list field while their length in u_int is supplied
                   to the bfl_len field.  ENOMEM is returned if there is not
                   enough buffer space and EFAULT is returned if a bad address
                   is encountered.  The bfl_len field is modified on return to
                   indicate the actual length in u_int of the array returned.
                   If bfl_list is NULL, the bfl_len field is set to indicate
                   the required length of an array in u_int.

     BIOCSDLT (u_int)
                   Changes the type of the data link layer underlying the
                   attached interface.  EINVAL is returned if no interface has
                   been specified or the specified type is not available for
                   the interface.

     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.

                   The interface remains in promiscuous mode until all files
                   listening promiscuously are closed.

     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 ifreq.  All other fields are undefined.

     BIOCSETIF (struct ifreq)
                   Sets the hardware interface associated 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.  Additionally, performs the actions of

                   Sets or gets the "read timeout" parameter.  The timeval
                   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 {
                                 uint64_t bs_recv;
                                 uint64_t bs_drop;
                                 uint64_t bs_capt;
                                 uint64_t bs_padding[13];

                   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);

                         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 aren't
                                       keeping up with the packet traffic);

                         bs_capt       the number of packets accepted by the

     BIOCIMMEDIATE (u_int)
                   Enables or disables "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.

                   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.

     BIOCSETF (struct bpf_program)
                   Sets the filter program used by the kernel to discard
                   uninteresting packets.  An array of instructions and its
                   length are passed in using the following structure:

                         struct bpf_program {
                                 u_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

                   See section FILTER MACHINE for an explanation of the filter

     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

     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(2) or haphazard packet matching).

                   Sets or gets the receive signal.  This signal will be sent
                   to the process or process group specified by FIOSETOWN.  It
                   defaults to SIGIO.

                   Sets or gets 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

                   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.

                   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

                   Set (or get) "packet feedback mode".  This allows injected
                   packets to be fed back as input to the interface when
                   output via the interface is successful.  The first name is
                   meant for FreeBSD compatibility, the two others follow the
                   Get/Set convention.  Injected outgoing packets are not
                   returned by BPF to avoid duplication.  This flag is
                   initialized to zero by default.

     bpf supports several standard ioctl(2)'s which allow the user to do async
     and/or non-blocking I/O to an open bpf file descriptor.

     FIONREAD (int)
                   Returns the number of bytes that are immediately available
                   for reading.

     FIONBIO (int)
                   Set or clear non-blocking I/O.  If arg is non-zero, then
                   doing a read(2) when no data is available will return -1
                   and errno will be set to EAGAIN.  If arg is zero, non-
                   blocking I/O is disabled.  Note: setting this overrides the
                   timeout set by BIOCSRTIMEOUT.

     FIOASYNC (int)
                   Enable or disable async I/O.  When enabled (arg is non-
                   zero), the process or process group specified by FIOSETOWN
                   will start receiving SIGIO's when packets arrive.  Note
                   that you must do an FIOSETOWN in order for this to take
                   effect, as the system will not default this for you.  The
                   signal may be changed via BIOCSRSIG.

                   Set or get the process or process group (if negative) that
                   should receive SIGIO when packets are available.  The
                   signal may be changed using BIOCSRSIG (see above).

     The following structure is prepended to each packet returned by read(2):

           struct bpf_hdr {
                   struct bpf_timeval bh_tstamp;
                   uint32_t bh_caplen;
                   uint32_t bh_datalen;
                   uint16_t bh_hdrlen;

     The fields, whose values are stored in host order, are:

           bh_tstamp     The time at which the packet was processed by the
                         packet filter.  This structure differs from the
                         standard struct timeval in that both members are of
                         type long.

           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

           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 ensures 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 isn't 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.  malloc(3) will always return an
     aligned buffer.

     A filter program is an array of instructions, with all branches forwardly
     directed, terminated by a return instruction.  Each instruction performs
     some action on the pseudo-machine state, which consists of an
     accumulator, index register, scratch memory store, and implicit program

     The following structure defines the instruction format:

           struct bpf_insn {
                   uint16_t code;
                   u_char  jt;
                   u_char  jf;
                   uint32_t 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

     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).
             Arithmetic overflow when calculating a variable offset terminates
             the filter program and the packet is ignored.  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]

             These instructions load a value into the index register.  Note
             that the addressing modes are more restricted 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

             This instruction stores the index register in the scratch memory

                   BPF_STX                          M[k] <- X

             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

             The jump instructions alter flow of control.  Conditional jumps
             compare the accumulator against a constant (BPF_K) or the index
             register (BPF_X).  If the result is true (or non-zero), the true
             branch is taken, otherwise the false branch is taken.  Jump
             offsets are encoded in 8 bits so the longest jump is 256
             instructions.  However, the jump always (BPF_JA) opcode uses the
             32 bit k field as the offset, allowing arbitrarily distant
             destinations.  All conditionals use unsigned comparison

                   BPF_JMP + BPF_JA                 pc += k
                   BPF_JMP + BPF_JGT + BPF_K        pc += (A > k) ? jt : jf
                   BPF_JMP + BPF_JGE + BPF_K        pc += (A >= k) ? jt : jf
                   BPF_JMP + BPF_JEQ + BPF_K        pc += (A == k) ? jt : jf
                   BPF_JMP + BPF_JSET + BPF_K       pc += (A & k) ? jt : jf
                   BPF_JMP + BPF_JGT + BPF_X        pc += (A > X) ? jt : jf
                   BPF_JMP + BPF_JGE + BPF_X        pc += (A >= X) ? jt : jf
                   BPF_JMP + BPF_JEQ + BPF_X        pc += (A == X) ? jt : jf
                   BPF_JMP + BPF_JSET + BPF_X       pc += (A & X) ? jt : jf

             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

             The miscellaneous category was created for anything that doesn't
             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

             Also, two instructions to call a "coprocessor" if initialized by
             the kernel component.  There is no coprocessor by default.

                   BPF_MISC + BPF_COP               A <- funcs[k](...)
                   BPF_MISC + BPF_COPX              A <- funcs[X](...)

             If the coprocessor is not set or the function index is out of
             range, these instructions will abort the program and return zero.

     The BPF interface provides the following macros to facilitate array

           BPF_STMT(opcode, operand)
           BPF_JUMP(opcode, operand, true_offset, false_offset)

     The following sysctls are available when bpf is enabled:

     net.bpf.maxbufsize      Sets the maximum buffer size available for bpf

     net.bpf.stats           Shows bpf statistics.  They can be retrieved with
                             the netstat(1) utility.

     net.bpf.peers           Shows the current bpf peers.  This is only
                             available to the super user and can also be
                             retrieved with the netstat(1) utility.

     On architectures with bpfjit(4) support, the additional sysctl is

     net.bpf.jit      Toggle just-in-time compilation of new filter programs.
                      In order to enable just-in-time compilation, the bpfjit
                      kernel module must be loaded.  Changing a value of this
                      sysctl doesn't affect existing filter programs.


     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_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 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.

           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),

     ioctl(2), read(2), select(2), signal(3), bpfjit(4), tcpdump(8)

     S. McCanne and V. Jacobson, "The BSD Packet Filter: A New Architecture
     for User-level Packet Capture", Proceedings of the 1993 Winter USENIX,
     Technical Conference, San Diego, CA.

     The Enet packet filter was created in 1980 by Mike Accetta and Rick
     Rashid at Carnegie-Mellon University.  Jeffrey Mogul, at Stanford, ported
     the code to BSD and continued its development from 1983 on.  Since then,
     it has evolved into the ULTRIX Packet Filter at DEC, a STREAMS NIT module
     under SunOS 4.1, and BPF.

     Steven McCanne, of Lawrence Berkeley Laboratory, implemented BPF in
     Summer 1990.  The design was in collaboration with Van Jacobson, also of
     Lawrence Berkeley Laboratory.

     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 use a filter to reject foreign packets.

     "Immediate mode" and the "read timeout" are misguided features.  This
     functionality can be emulated with non-blocking mode and select(2).

NetBSD 10.99                   November 30, 2022                  NetBSD 10.99