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

NAME
     bpf - Berkeley Packet Filter

SYNOPSIS
     pseudo-device bpfilter 8

DESCRIPTION
     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 BIOSETIF 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 configu-
     ration; 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 de-
     scriptors 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 re-
     turned 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.  The writes are unbuffered, meaning only one packet can be processed
     per write.  Currently, only writes to Ethernets and SLIP links are sup-
     ported.

   Ioctls
     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 <net/if.h>.

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

     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 argu-
             ment.  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 interface has been speci-
             fied.  The device types, prefixed with ``DLT_'', are defined in
             <net/bpf.h>.

     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 lis-
             tener that opened its interface non-promiscuously may receive
             packets promiscuously.  This problem can be remedied with an ap-
             propriate filter.

             The interface remains in promiscuous mode until all files listen-
             ing 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 lis-
             tening 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 de-
             vice is indicated by name using the ifr_name field of the struct
             ifreq. Additionally, performs the actions of BIOCFLUSH.

     BIOCSRTIMEOUT, BIOCGRTIMEOUT (struct timeval)
             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 time-
             out.

     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 imme-
             diately 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 is 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 lan-
             guage.

     BIOCVERSION (struct bpf_version)
             Returns the major and minor version numbers of the filter lan-
             guage 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).

     BIOCSRSIG, BIOCGRSIG (u_int)
             Set or get the receive signal.  This signal will be sent to the
             process or process group specified by FIOSETOWN. It defaults to
             SIGIO.

     BIOCSHDRCMPLT, 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 auto-
             matically by the 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


             reading.

     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, then
             doing a read when no data is available will return -1 and errno
             will be set to EWOULDBLOCK. If the argument is zero, non-blocking
             I/O is disabled.  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.

     FIOSETOWN, FIOGETOWN (int)
             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:

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

     bh_caplen
             Length of the captured portion of the packet.  This is the mini-
             mum of the truncation amount specified by the filter and the
             length of the packet.

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

     bh_hdrlen
             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 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 per-
     forms some action on the pseudo-machine state, which consists of an accu-
     mulator, 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 class-
     es 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 given 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 speci-
             fied 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]

     BPF_LDX
             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.

             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 possi-
             bility for the destination.

             BPF_ST                            M[k] <- A

     BPF_STX
             This instruction stores the index register in the scratch memory
             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 off-
             sets 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.

             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 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) of 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 doesn't
             fit into the above classes, and for any new instructions that
             might need to be added.  Currently, these are the register trans-
             fer 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 ini-
     tializers:

     BPF_STMT(opcode, operand)

     BPF_JUMP(opcode, operand, true_offset, false_offset)

EXAMPLES
     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_JUMP(BPF_JMP+BPF_JEQ+BPF_K, ETHERTYPE_REVARP, 0, 3),
                   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 128.3.112.15 and
     128.3.112.35.

           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),
           };

SEE ALSO
     ioctl(2), read(2), select(2), signal(3), tcpdump(8)

     McCanne, S., Jacobson V., An efficient, extensible, and portable network
     monitor.

FILES
     /dev/bpf[0-9]  BPF devices

AUTHORS
     Steve McCanne of Lawrence Berkeley Laboratory implemented BPF in Summer
     1990.  Much of the design is due to Van Jacobson.

HISTORY
     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.

BUGS
     The read buffer must be of a fixed size (returned by the BIOCGBLEN
     ioctl).

     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 ad-
     ditional 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-
     ported.

OpenBSD 3.0                      May 23, 1991                                8
