(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 .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 ifr_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 ifr_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 are not 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 .In net/bpf.h . An incompatible filter may result in undefined behavior (most likely, an error returned by ioctl or haphazard packet matching).
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 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.
BIOCSSEESENT BIOCGSEESENT
(u_int) 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.
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 is not 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 .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.
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_IMMA <- k
BPF_LD+BPF_MEMA <- 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_JApc += 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 does not 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.
