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NAME
bpf — Berkeley Packet Filter
SYNOPSIS
device bpf
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/bpf. 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 by 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.
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.
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.
BUFFER MODES
bpf devices deliver packet data to the application via memory buffers provided by the application. The
buffer mode is set using the BIOCSETBUFMODE ioctl, and read using the BIOCGETBUFMODE ioctl.
Buffered read mode
By default, bpf devices operate in the BPF_BUFMODE_BUFFER mode, in which packet data is copied explicitly
from kernel to user memory using the read(2) system call. The user process will declare a fixed buffer
size that will be used both for sizing internal buffers and for all read(2) operations on the file. This
size is queried using the BIOCGBLEN ioctl, and is set using the BIOCSBLEN ioctl. Note that an individual
packet larger than the buffer size is necessarily truncated.
Zero-copy buffer mode
bpf devices may also operate in the BPF_BUFMODE_ZEROCOPY mode, in which packet data is written directly
into two user memory buffers by the kernel, avoiding both system call and copying overhead. Buffers are
of fixed (and equal) size, page-aligned, and an even multiple of the page size. The maximum zero-copy
buffer size is returned by the BIOCGETZMAX ioctl. Note that an individual packet larger than the buffer
size is necessarily truncated.
The user process registers two memory buffers using the BIOCSETZBUF ioctl, which accepts a struct
bpf_zbuf pointer as an argument:
struct bpf_zbuf {
void *bz_bufa;
void *bz_bufb;
size_t bz_buflen;
};
bz_bufa is a pointer to the userspace address of the first buffer that will be filled, and bz_bufb is a
pointer to the second buffer. bpf will then cycle between the two buffers as they fill and are
acknowledged.
Each buffer begins with a fixed-length header to hold synchronization and data length information for the
buffer:
struct bpf_zbuf_header {
volatile u_int bzh_kernel_gen; /* Kernel generation number. */
volatile u_int bzh_kernel_len; /* Length of data in the buffer. */
volatile u_int bzh_user_gen; /* User generation number. */
/* ...padding for future use... */
};
The header structure of each buffer, including all padding, should be zeroed before it is configured
using BIOCSETZBUF. Remaining space in the buffer will be used by the kernel to store packet data, laid
out in the same format as with buffered read mode.
The kernel and the user process follow a simple acknowledgement protocol via the buffer header to
synchronize access to the buffer: when the header generation numbers, bzh_kernel_gen and bzh_user_gen,
hold the same value, the kernel owns the buffer, and when they differ, userspace owns the buffer.
While the kernel owns the buffer, the contents are unstable and may change asynchronously; while the user
process owns the buffer, its contents are stable and will not be changed until the buffer has been
acknowledged.
Initializing the buffer headers to all 0's before registering the buffer has the effect of assigning
initial ownership of both buffers to the kernel. The kernel signals that a buffer has been assigned to
userspace by modifying bzh_kernel_gen, and userspace acknowledges the buffer and returns it to the kernel
by setting the value of bzh_user_gen to the value of bzh_kernel_gen.
In order to avoid caching and memory re-ordering effects, the user process must use atomic operations and
memory barriers when checking for and acknowledging buffers:
#include <machine/atomic.h>
/*
* Return ownership of a buffer to the kernel for reuse.
*/
static void
buffer_acknowledge(struct bpf_zbuf_header *bzh)
{
atomic_store_rel_int(&bzh->bzh_user_gen, bzh->bzh_kernel_gen);
}
/*
* Check whether a buffer has been assigned to userspace by the kernel.
* Return true if userspace owns the buffer, and false otherwise.
*/
static int
buffer_check(struct bpf_zbuf_header *bzh)
{
return (bzh->bzh_user_gen !=
atomic_load_acq_int(&bzh->bzh_kernel_gen));
}
The user process may force the assignment of the next buffer, if any data is pending, to userspace using
the BIOCROTZBUF ioctl. This allows the user process to retrieve data in a partially filled buffer before
the buffer is full, such as following a timeout; the process must recheck for buffer ownership using the
header generation numbers, as the buffer will not be assigned to userspace if no data was present.
As in the buffered read mode, kqueue(2), poll(2), and select(2) may be used to sleep awaiting the
availability of a completed buffer. They will return a readable file descriptor when ownership of the
next buffer is assigned to user space.
In the current implementation, the kernel may assign zero, one, or both buffers to the user process;
however, an earlier implementation maintained the invariant that at most one buffer could be assigned to
the user process at a time. In order to both ensure progress and high performance, user processes should
acknowledge a completely processed buffer as quickly as possible, returning it for reuse, and not block
waiting on a second buffer while holding another buffer.
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 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 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
BIOCSETFNR (struct bpf_program) Sets the read 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. See section “FILTER MACHINE” for an
explanation of the filter language. The only difference between BIOCSETF and BIOCSETFNR
is BIOCSETF performs the actions of BIOCFLUSH while BIOCSETFNR does not.
BIOCSETWF (struct bpf_program) Sets the write filter program used by the kernel to control what
type of packets can be written to the interface. See the BIOCSETF command for more
information on the bpf filter program.
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).
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) These commands are obsolete but left for compatibility. Use BIOCSDIRECTION and
BIOCGDIRECTION instead. 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.
BIOCSDIRECTION
BIOCGDIRECTION (u_int) Set or get the setting determining whether incoming, outgoing, or all packets on
the interface should be returned by BPF. Set to BPF_D_IN to see only incoming packets on
the interface. Set to BPF_D_INOUT to see packets originating locally and remotely on the
interface. Set to BPF_D_OUT to see only outgoing packets on the interface. This setting
is initialized to BPF_D_INOUT by default.
BIOCSTSTAMP
BIOCGTSTAMP (u_int) Set or get format and resolution of the time stamps returned by BPF. Set to
BPF_T_MICROTIME, BPF_T_MICROTIME_FAST, BPF_T_MICROTIME_MONOTONIC, or
BPF_T_MICROTIME_MONOTONIC_FAST to get time stamps in 64-bit struct timeval format. Set
to BPF_T_NANOTIME, BPF_T_NANOTIME_FAST, BPF_T_NANOTIME_MONOTONIC, or
BPF_T_NANOTIME_MONOTONIC_FAST to get time stamps in 64-bit struct timespec format. Set
to BPF_T_BINTIME, BPF_T_BINTIME_FAST, BPF_T_NANOTIME_MONOTONIC, or
BPF_T_BINTIME_MONOTONIC_FAST to get time stamps in 64-bit struct bintime format. Set to
BPF_T_NONE to ignore time stamp. All 64-bit time stamp formats are wrapped in struct
bpf_ts. The BPF_T_MICROTIME_FAST, BPF_T_NANOTIME_FAST, BPF_T_BINTIME_FAST,
BPF_T_MICROTIME_MONOTONIC_FAST, BPF_T_NANOTIME_MONOTONIC_FAST, and
BPF_T_BINTIME_MONOTONIC_FAST are analogs of corresponding formats without _FAST suffix
but do not perform a full time counter query, so their accuracy is one timer tick. The
BPF_T_MICROTIME_MONOTONIC, BPF_T_NANOTIME_MONOTONIC, BPF_T_BINTIME_MONOTONIC,
BPF_T_MICROTIME_MONOTONIC_FAST, BPF_T_NANOTIME_MONOTONIC_FAST, and
BPF_T_BINTIME_MONOTONIC_FAST store the time elapsed since kernel boot. This setting is
initialized to BPF_T_MICROTIME by default.
BIOCFEEDBACK (u_int) Set packet feedback mode. This allows injected packets to be fed back as input
to the interface when output via the interface is successful. When BPF_D_INOUT direction
is set, injected outgoing packet is not returned by BPF to avoid duplication. This flag
is initialized to zero by default.
BIOCLOCK Set the locked flag on the bpf descriptor. This prevents the execution of ioctl commands
which could change the underlying operating parameters of the device.
BIOCGETBUFMODE
BIOCSETBUFMODE (u_int) Get or set the current bpf buffering mode; possible values are
BPF_BUFMODE_BUFFER, buffered read mode, and BPF_BUFMODE_ZBUF, zero-copy buffer mode.
BIOCSETZBUF (struct bpf_zbuf) Set the current zero-copy buffer locations; buffer locations may be set
only once zero-copy buffer mode has been selected, and prior to attaching to an
interface. Buffers must be of identical size, page-aligned, and an integer multiple of
pages in size. The three fields bz_bufa, bz_bufb, and bz_buflen must be filled out. If
buffers have already been set for this device, the ioctl will fail.
BIOCGETZMAX (size_t) Get the largest individual zero-copy buffer size allowed. As two buffers are
used in zero-copy buffer mode, the limit (in practice) is twice the returned size. As
zero-copy buffers consume kernel address space, conservative selection of buffer size is
suggested, especially when there are multiple bpf descriptors in use on 32-bit systems.
BIOCROTZBUF Force ownership of the next buffer to be assigned to userspace, if any data present in
the buffer. If no data is present, the buffer will remain owned by the kernel. This
allows consumers of zero-copy buffering to implement timeouts and retrieve partially
filled buffers. In order to handle the case where no data is present in the buffer and
therefore ownership is not assigned, the user process must check bzh_kernel_gen against
bzh_user_gen.
BPF HEADER
One of the following structures is prepended to each packet returned by read(2) or via a zero-copy
buffer:
struct bpf_xhdr {
struct bpf_ts bh_tstamp; /* time stamp */
uint32_t bh_caplen; /* length of captured portion */
uint32_t bh_datalen; /* original length of packet */
u_short bh_hdrlen; /* length of bpf header (this struct
plus alignment padding) */
};
struct bpf_hdr {
struct timeval bh_tstamp; /* time stamp */
uint32_t bh_caplen; /* length of captured portion */
uint32_t 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_xhdr) or
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 ensures that the bpf_xhdr, bpf_hdr and the network layer header will be word aligned. Currently,
bpf_hdr is used when the time stamp is set to BPF_T_MICROTIME, BPF_T_MICROTIME_FAST,
BPF_T_MICROTIME_MONOTONIC, BPF_T_MICROTIME_MONOTONIC_FAST, or BPF_T_NONE for backward compatibility
reasons. Otherwise, bpf_xhdr is used. However, bpf_hdr may be deprecated in the near future. 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 <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_MOD+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_XOR+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_MOD+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_XOR+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 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).
SYSCTL VARIABLES
A set of sysctl(8) variables controls the behaviour of the bpf subsystem
net.bpf.optimize_writers: 0
Various programs use BPF to send (but not receive) raw packets (cdpd, lldpd, dhcpd, dhcp relays,
etc. are good examples of such programs). They do not need incoming packets to be send to them.
Turning this option on makes new BPF users to be attached to write-only interface list until
program explicitly specifies read filter via pcap_set_filter(). This removes any performance
degradation for high-speed interfaces.
net.bpf.stats:
Binary interface for retrieving general statistics.
net.bpf.zerocopy_enable: 0
Permits zero-copy to be used with net BPF readers. Use with caution.
net.bpf.maxinsns: 512
Maximum number of instructions that BPF program can contain. Use tcpdump(1) -d option to
determine approximate number of instruction for any filter.
net.bpf.maxbufsize: 524288
Maximum buffer size to allocate for packets buffer.
net.bpf.bufsize: 4096
Default buffer size to allocate for packets buffer.
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
tcpdump(1), ioctl(2), kqueue(2), poll(2), select(2), byteorder(3), ng_bpf(4), bpf(9)
McCanne, S. and Jacobson V., An efficient, extensible, and portable network monitor.
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.
Support for zero-copy buffers was added by Robert N. M. Watson under contract to Seccuris Inc.
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.
The SEESENT, DIRECTION, and FEEDBACK settings have been observed to work incorrectly on some interface
types, including those with hardware loopback rather than software loopback, and point-to-point
interfaces. They appear to function correctly on a broad range of Ethernet-style interfaces.
Debian October 21, 2016 BPF(4)