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

     bpf - Berkeley Packet Filter

     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/bpf0,
     /dev/bpf1, etc.  After opening the device, the file descriptor must be
     bound to a specific network interface with the BIOCSETIF ioctl(2).	 A
     given interface can be shared between 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.	 The number of
     open files can be increased by creating additional device nodes with the
     MAKEDEV(8) script.

     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(2) and can be set with BIOCSBLEN.	Note
     that an individual packet larger than this size is necessarily truncated.

     A packet can be sent out on the network by writing to a bpf file
     descriptor.  Each descriptor can also have a user-settable filter for
     controlling the writes.  Only packets matching the filter are sent out of
     the interface.  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(2).

     The ioctl(2) command codes below are defined in <net/bpf.h>.  All
     commands require these includes:

	   #include <sys/types.h>
	   #include <sys/time.h>
	   #include <sys/ioctl.h>
	   #include <net/bpf.h>

     Additionally, BIOCGETIF and BIOCSETIF require <sys/socket.h> and

     The (third) argument to the ioctl(2) call 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 the 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.

	     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.

	     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
	     rogue program to listen on any interface on the system, send
	     packets 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
	     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 affected 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
	     command 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.

     BIOCSRTIMEOUT struct timeval *
     BIOCGRTIMEOUT struct timeval *
	     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

     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
		      packet traffic).

     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.

     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 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

     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).

     BIOCSRSIG u_int *
     BIOCGRSIG u_int *
	     Sets or gets the receive signal.  This signal will be sent to the
	     process or process group specified by FIOSETOWN.  It defaults to

     BIOCSHDRCMPLT u_int *
     BIOCGHDRCMPLT u_int *
	     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 default.

     BIOCSFILDROP u_int *
     BIOCGFILDROP u_int *
	     Sets or gets the status of the ``filter drop'' flag.  If non-
	     zero, packets matching any filters will be reported to the
	     associated interface so that they can be dropped.

     BIOCSDIRFILT u_int *
     BIOCGDIRFILT u_int *
	     Sets or gets the status of the ``direction filter'' flag.	If
	     non-zero, packets matching the specified direction (either
	     BPF_DIRECTION_IN or BPF_DIRECTION_OUT) will be ignored.

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

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

     FIONBIO int *
	     Sets or clears non-blocking I/O.  If the argument is non-zero,
	     enable 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 *
	     Enables or disables 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.

     FIOSETOWN int *
     FIOGETOWN int *
	     Sets or gets 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
     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.

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

     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
     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
     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 defined bpf instruction.	We use the convention that A is the
     accumulator, X is the index register, P[] packet data, and M[] scratch
     memory store.  P[i:n] gives the data at byte offset ``i'' in the packet,
     interpreted as a word (n=4), unsigned halfword (n=2), or unsigned byte
     (n=1).  M[i] gives the i'th word in the scratch memory store, which is
     only addressed in word units.  The memory store is indexed from 0 to
     BPF_MEMWORDS-1.  k, jt, and jf are the corresponding fields in the
     instruction definition.  ``len'' refers to the length of the packet.

     BPF_LD  These instructions copy a value into the accumulator.  The type
	     of the source operand is specified by an ``addressing mode'' and
	     can be a constant (BPF_IMM), packet data at a fixed offset
	     (BPF_ABS), packet data at a variable offset (BPF_IND), the packet
	     length (BPF_LEN), or a word in the scratch memory store
	     (BPF_MEM).	 For BPF_IND and BPF_ABS, the data size must be
	     specified as a word (BPF_W), halfword (BPF_H), or byte (BPF_B).
	     The semantics of all 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) 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
	     packet should be ignored/dropped.	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

     The bpf interface provides the following macros to facilitate array

	   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_RET+BPF_K, sizeof(struct ether_arp) +
		       sizeof(struct ether_header)),

     This filter accepts only IP packets between host and

	   struct bpf_insn insns[] = {
		   BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x8003700f, 0, 2),
		   BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x80037023, 3, 4),
		   BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x80037023, 0, 3),
		   BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x8003700f, 0, 1),
		   BPF_STMT(BPF_RET+BPF_K, (u_int)-1),

     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_JUMP(BPF_JMP+BPF_JSET+BPF_K, 0x1fff, 6, 0),
		   BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 79, 2, 0),
		   BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 79, 0, 1),
		   BPF_STMT(BPF_RET+BPF_K, (u_int)-1),

     ioctl(2), read(2), select(2), signal(3), MAKEDEV(8), 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
     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.

OpenBSD 4.9			 April 9, 2010			   OpenBSD 4.9

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