BPF(4)

NAME
bpf - Berkeley Packet Filter

SYNOPSIS
pseudo-device bpfilter

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 BIOCSETIF ioctl. A given interface can be shared be multiple listeners, and the filter underlying each descriptor will see an identical packet stream. The total number of open files is limited to the value given in the kernel configuration; the example given in the SYNOPSIS above sets the limit to 16.

A separate device file is required for each minor device. If a file is in use, the open will fail and errno will be set to EBUSY.

Associated with each open instance of a bpf file is a user-settable packet filter. Whenever a packet is received by an interface, all file descriptors listening on that interface apply their filter. Each descriptor that accepts the packet receives its own copy.

Reads from these files return the next group of packets that have matched the filter. To improve performance, the buffer passed to read must be the same size as the buffers used internally by bpf This size is returned by the BIOCGBLEN ioctl (see below), and can be set with BIOCSBLEN. Note that an individual packet larger than this size is necessarily 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 descriptor. The writes are unbuffered, meaning only one packet can be processed per write. Currently, only writes to Ethernets and SLIP links are supported.

IOCTLS
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 <net/if.h>. In addition to FIONREAD and SIOCGIFADDR, the following commands may be applied to any open bpf file. The (third) argument to ioctl(2) should be a pointer to the type indicated.

BIOCGBLEN
    (u_int) Returns the required buffer length for reads on bpf files.

BIOCSBLEN
    (u_int) Sets the buffer length for reads on bpf files. The buffer must be set before the file is attached to an interface with BIOCSETIF. If the requested buffer size cannot be accommodated, the closest allowable size will be set and returned in the argument. A read call will result in EIO if it is passed a buffer that is not this size.

BIOCGDLT
    (u_int) Returns the type of the data link layer underlying the attached interface. EINVAL is returned if no interface has been specified. 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 listener that opened its interface non-promiscuously may receive packets promiscuously. This problem can be remedied with an appropriate filter.

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

BIOCGETIF
    (struct ifreq) Returns the name of the hardware interface that the file is listening on. The name is returned in the if_name field of the ifreq structure. All other fields are undefined.

BIOCSETIF
    (struct ifreq) Sets the hardware interface associate with the file. This command must be performed before any packets can be read. The device is indicated by name using the if_name field of the ifreq structure. Additionally, performs the actions of BIOCFLUSH.

BIOCSRTIMEOUT

BIOCGRTIMEOUT
    (struct timeval) Set or get the read timeout parameter. The argument specifies the length of time to wait before timing out on a read request. This parameter is initialized to zero by open(2), indicating no timeout.

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

        struct bpf_stat {
        	u_int bs_recv;    /* number of packets received */
        	u_int bs_drop;    /* number of packets dropped */
        };

    The fields are:

    bs_recv
        the number of packets received by the descriptor since opened or reset (including any buffered since the last read call); and

    bs_drop
        the number of packets which were accepted by the filter but dropped by the kernel because of buffer overflows (i.e., the application's reads aren't keeping up with the packet traffic).

BIOCIMMEDIATE
    (u_int) Enable or disable ``immediate mode'', based on the truth value of the argument. When immediate mode is enabled, reads return immediately upon packet reception. Otherwise, a read will block until either the kernel buffer becomes full or a timeout occurs. This is useful for programs like rarpd(8) which must respond to messages in real time. The default for a new file is off.

BIOCSETF
    (struct bpf_program) Sets the filter program used by the kernel to discard uninteresting 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 language.

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() or haphazard packet matching).

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

    struct bpf_hdr {
            struct timeval bh_tstamp;     /* time stamp */
            u_long bh_caplen;             /* length of captured portion */
            u_long bh_datalen;            /* original length of packet */
            u_short bh_hdrlen;            /* length of bpf header (this struct
    					 plus alignment padding */
    };

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

bh_tstamp
    The time at which the packet was processed by the packet filter.
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 filter.
bh_hdrlen
    The length of the bpf header, which may not be equal to sizeof(struct bpf_hdr).

The bh_hdrlen field exists to account for padding between the header and the link level protocol. The purpose here is to guarantee proper alignment of the packet data structures, which is required on alignment sensitive architectures and improves performance on many other architectures. The packet filter insures that the bpf_hdr and the network layer header will be word aligned. Suitable precautions must be taken when accessing the link layer protocol fields on alignment restricted machines. (This 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. The malloc(3) function 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 counter. The following structure defines the instruction format:

    struct bpf_insn {
    	u_short	code;
    	u_char 	jt;
    	u_char 	jf;
    	u_long k;
    };

The k field is used in different ways by different instructions, and the jt and jf fields are used as offsets by the branch instructions. The opcodes are encoded in a semi-hierarchical fashion. There are eight classes of instructions: BPF_LD, BPF_LDX, BPF_ST, BPF_STX, BPF_ALU, BPF_JMP, BPF_RET, and BPF_MISC. Various other mode and operator bits are or'd into the class to give the actual instructions. The classes and modes are defined in <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 the recognized BPF_LD instructions follow.

    BPF_LD+BPF_W+BPF_ABS
        A <- P[k:4]
    BPF_LD+BPF_H+BPF_ABS
        A <- P[k:2]
    BPF_LD+BPF_B+BPF_ABS
        A <- P[k:1]
    BPF_LD+BPF_W+BPF_IND
        A <- P[X+k:4]
    BPF_LD+BPF_H+BPF_IND
        A <- P[X+k:2]
    BPF_LD+BPF_B+BPF_IND
        A <- P[X+k:1]
    BPF_LD+BPF_W+BPF_LEN
        A <- len
    BPF_LD+BPF_IMM
        A <- k
    BPF_LD+BPF_MEM
        A <- M[k]
BPF_LDX
    These instructions load a value into the index register. Note that the addressing modes are more restrictive than those of the accumulator loads, but they include BPF_MSH, a hack for efficiently loading the IP header length.
    BPF_LDX+BPF_W+BPF_IMM
        X <- k
    BPF_LDX+BPF_W+BPF_MEM
        X <- M[k]
    BPF_LDX+BPF_W+BPF_LEN
        X <- len
    BPF_LDX+BPF_B+BPF_MSH
        X <- 4*(P[k:1]&0xf)
BPF_ST
    This instruction stores the accumulator into the scratch memory. We do not need an addressing mode since there is only one possibility for the destination.
    BPF_ST
        M[k] <- A
BPF_STX
    This instruction stores the index register in the scratch 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 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 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) or the accumulator (BPF_A).
    BPF_RET+BPF_A
        accept A bytes
    BPF_RET+BPF_K
        accept k bytes
BPF_MISC
    The miscellaneous category was created for anything that 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 initializers: BPF_STMT(opcode, operand) and 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_H+BPF_ABS, 26),
    	BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x8003700f, 0, 2),
    	BPF_STMT(BPF_LD+BPF_H+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_H+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
tcpdump(1), ioctl(2) McCanne, S., Jacobson V., An efficient, extensible, and portable network monitor.

FILES
/dev/bpfn
    the packet filter device

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 additional processing overhead. However, we favor the model where all files must assume that the interface is promiscuous, and if so desired, must utilize a filter to reject foreign packets.

Data link protocols with variable length headers are not currently supported.

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.

AUTHORS

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