MirBSD manpage: bpf(4)

BPF(4)                     BSD Programmer's Manual                      BPF(4)


     bpf - Berkeley Packet Filter


     pseudo-device bpfilter [count]


     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 between multiple listeners, and the filter under-
     lying each descriptor will see an identical packet stream. The total
     number of open files is limited to the value given in the kernel confi-
     guration; the example given in the SYNOPSIS above sets the limit to 8.

     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 pack-
     et filter. Whenever a packet is received by an interface, all file
     descriptors listening on that interface apply their filter. Each descrip-
     tor 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 under BSD, can be set with
     BIOCSBLEN. Note that an individual packet larger than this size is neces-
     sarily 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. Each descriptor can also have a user-settable filter for controlling
     the writes. Only packets matching the filter are sent out of the inter-
     face. The writes are unbuffered, meaning only one packet can be processed
     per write.

     Once a descriptor is configured, further changes to the configuration can
     be prevented using the BIOCLOCK ioctl.


     The ioctl 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

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

     BIOCGBLEN (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 underlying the attached
             interface. EINVAL is returned if no interface has been specified.
             The device types, prefixed with "DLT_", are defined in

             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 prom-
             iscuously. This problem can be remedied with an appropriate

             The interface remains in promiscuous mode until all files listen-
             ing promiscuously are closed.

             Flushes the buffer of incoming packets and resets the statistics
             that are returned by BIOCGSTATS.

             This ioctl is designed to prevent the security issues associated
             with an open bpf descriptor in unprivileged programs. Even with
             dropped privileges, an open bpf descriptor can be abused by a ro-
             gue program to listen on any interface on the system, send pack-
             ets on these interfaces if the descriptor was opened read-write
             and send signals to arbitrary processes using the signaling
             mechanism of bpf. By allowing only "known safe" ioctls, the
             BIOCLOCK ioctl prevents this abuse. The allowable ioctls are
             BIOCGHDRCMPLT, TIOCGPGRP, and FIONREAD. Use of any other ioctl is
             denied with error EPERM. Once a descriptor is locked, it is not
             possible to unlock it. A process with root privileges is not af-
             fected by the lock.

             A privileged program can open a bpf device, drop privileges, set
             the interface, filters and modes on the descriptor, and lock it.
             Once the descriptor is locked, the system is safe from further
             abuse through the descriptor. Locking a descriptor does not
             prevent writes. If the application does not need to send packets
             through bpf, it can open the device read-only to prevent writing.
             If sending packets is necessary, a write-filter can be set before
             locking the descriptor to prevent arbitrary packets from being
             sent out.

     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
             struct ifreq. All other fields are undefined.

     BIOCSETIF (struct ifreq)
             Sets the hardware interface associated with the file. This com-
             mand must be performed before any packets can be read. The device
             is indicated by name using the ifr_name field of the struct
             ifreq. Additionally, performs the actions of BIOCFLUSH.

             Set or get 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

     BIOCGSTATS (struct bpf_stat)
             Returns the following structure of packet statistics:

                   struct bpf_stat {
                           u_int bs_recv;
                           u_int bs_drop;

             The fields are:

             bs_recv  Number of packets received by the descriptor since
                      opened or reset (including any buffered since the last
                      read call).

             bs_drop  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 pack-
                      et traffic).

     BIOCIMMEDIATE (u_int)
             Enable or disable "immediate mode", based on the truth value of
             the argument. When immediate mode is enabled, reads return im-
             mediately upon packet reception. Otherwise, a read will block un-
             til 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 filter program used by the kernel to discard uninterest-
             ing packets. An array of instructions and its length are 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.
             Also, the actions of BIOCFLUSH are performed.

             See section FILTER MACHINE for an explanation of the filter

     BIOCSETWF (struct bpf_program)
             Sets the filter program used by the kernel to filter the packets
             written to the descriptor before the packets are sent out on the
             network. See BIOCSETF for a description of the filter program.
             This ioctl also acts as BIOCFLUSH.

             Note that the filter operates on the packet data written to the
             descriptor. If the "header complete" flag is not set, the kernel
             sets the link-layer source address of the packet after filtering.

     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 com-
             patible 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 re-
             turned 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).

             Set or get the receive signal. This signal will be sent to the
             process or process group specified by FIOSETOWN. It defaults to

             Set or get the status of the ``header complete'' flag. Set to
             zero if the link level source address should be filled in au-
             tomatically 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.

Standard ioctls

     bpf now supports several standard ioctls which allow the user to do asyn-
     chronous and/or non-blocking I/O to an open bpf file descriptor.

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

     SIOCGIFADDR (struct ifreq)
             Returns the address associated with the interface.

     FIONBIO (int)
             Set or clear non-blocking I/O. If the argument is non-zero, en-
             able non-blocking I/O. If the argument is zero, disable non-
             blocking I/O. If non-blocking I/O is enabled, the return value of
             a read while no data is available will be 0. The non-blocking
             read behavior is different from performing non-blocking reads on
             other file descriptors, which will return -1 and set errno to
             EAGAIN if no data is available. Note: setting this overrides the
             timeout set by BIOCSRTIMEOUT.

     FIOASYNC (int)
             Enable or disable asynchronous I/O. When enabled (argument is
             non-zero), the process or process group specified by FIOSETOWN
             will start receiving SIGIO signals when packets arrive. Note that
             you must perform an FIOSETOWN command in order for this to take
             effect, as the system will not do it by default. 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).

BPF header

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

           struct bpf_hdr {
                   struct bpf_timeval bh_tstamp;
                   u_int32_t       bh_caplen;
                   u_int32_t       bh_datalen;
                   u_int16_t       bh_hdrlen;

     The fields, stored in host order, are as follows:

             Time at which the packet was processed by the packet filter.

             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.

             Length of the packet off the wire. This value is independent of
             the truncation amount specified by the filter.

             Length of the BPF header, which may not be equal to sizeof(struct

     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-
     sensitive architectures and improves performance on many other architec-
     tures. The packet filter ensures that the bpf_hdr and the network layer
     header will be word aligned. Suitable precautions must be taken when ac-
     cessing 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.

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 accumula-
     tor, index register, scratch memory store, and implicit program counter.

     The following structure defines the instruction format:

           struct bpf_insn {
                   u_int16_t       code;
                   u_char          jt;
                   u_char          jf;
                   u_int32_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 op-
     codes 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 logically
     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 de-
     fined 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
             recognized BPF_LD instructions follow.

                  A <- P[k:4]
                  A <- P[k:2]
                  A <- P[k:1]
                  A <- P[X+k:4]
                  A <- P[X+k:2]
                  A <- P[X+k:1]
                  A <- len
                  A <- k
                  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 effi-
             ciently loading the IP header length.

                  X <- k
                  X <- M[k]
                  X <- len
                  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 possi-
             bility for the destination.

                  M[k] <- A

             This instruction stores the index register in the scratch memory

                  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).

                  A <- A + k
                  A <- A - k
                  A <- A * k
                  A <- A / k
                  A <- A & k
                  A <- A | k
                  A <- A << k
                  A <- A >> k
                  A <- A + X
                  A <- A - X
                  A <- A * X
                  A <- A / X
                  A <- A & X
                  A <- A | X
                  A <- A << X
                  A <- A >> X
                  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 instruc-
             tions. 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.

                  pc += k
                  pc += (A > k) ? jt : jf
                  pc += (A >= k) ? jt : jf
                  pc += (A == k) ? jt : jf
                  pc += (A & k) ? jt : jf
                  pc += (A > X) ? jt : jf
                  pc += (A >= X) ? jt : jf
                  pc += (A == X) ? jt : jf
                  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) or, for the write filter, the maximum acceptable size for
             the packet (i.e., the packet is dropped if it is larger than the
             returned amount). A return value of zero indicates that the pack-
             et should be ignored/dropped. 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.

             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 accumu-
             lator or vice versa.

                  X <- A
                  A <- X

     The bpf interface provides the following macros to facilitate array ini-

           BPF_STMT (opcode, operand)

           BPF_JUMP (opcode, operand, true_offset, false_offset)


     /dev/bpf[0-9]  BPF devices


     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), tcpdump(8)

     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.


     Steve McCanne of Lawrence Berkeley Laboratory implemented BPF in Summer
     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 addi-
     tional 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-

MirBSD #10-current               May 23, 1991                                9

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