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BPF(4) | Device Drivers Manual | BPF(4) |
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 theBIOCSETIF
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; };
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... */ };
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)); }
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>
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 onbpf
files. BIOCSBLEN
- (
u_int
) Sets the buffer length for reads onbpf
files. The buffer must be set before the file is attached to an interface withBIOCSETIF
. 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 inEIO
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 theifreq
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 theifr_name
field of theifreq
structure. Additionally, performs the actions ofBIOCFLUSH
. 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 */ };
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; };
bf_insns
field while its length in units of ‘struct bpf_insn
’ is given by thebf_len
field. See section FILTER MACHINE for an explanation of the filter language. The only difference betweenBIOCSETF
andBIOCSETFNR
isBIOCSETF
performs the actions ofBIOCFLUSH
whileBIOCSETFNR
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 theBIOCSETF
command for more information on thebpf
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; };
BPF_MAJOR_VERSION
andBPF_MINOR_VERSION
from<net/bpf.h>
. An incompatible filter may result in undefined behavior (most likely, an error returned byioctl
() 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. UseBIOCSDIRECTION
andBIOCGDIRECTION
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 toBPF_D_IN
to see only incoming packets on the interface. Set toBPF_D_INOUT
to see packets originating locally and remotely on the interface. Set toBPF_D_OUT
to see only outgoing packets on the interface. This setting is initialized toBPF_D_INOUT
by default. BIOCSTSTAMP
BIOCGTSTAMP
- (
u_int
) Set or get format and resolution of the time stamps returned by BPF. Set toBPF_T_MICROTIME
,BPF_T_MICROTIME_FAST
,BPF_T_MICROTIME_MONOTONIC
, orBPF_T_MICROTIME_MONOTONIC_FAST
to get time stamps in 64-bit struct timeval format. Set toBPF_T_NANOTIME
,BPF_T_NANOTIME_FAST
,BPF_T_NANOTIME_MONOTONIC
, orBPF_T_NANOTIME_MONOTONIC_FAST
to get time stamps in 64-bit struct timespec format. Set toBPF_T_BINTIME
,BPF_T_BINTIME_FAST
,BPF_T_NANOTIME_MONOTONIC
, orBPF_T_BINTIME_MONOTONIC_FAST
to get time stamps in 64-bit struct bintime format. Set toBPF_T_NONE
to ignore time stamp. All 64-bit time stamp formats are wrapped in struct bpf_ts. TheBPF_T_MICROTIME_FAST
,BPF_T_NANOTIME_FAST
,BPF_T_BINTIME_FAST
,BPF_T_MICROTIME_MONOTONIC_FAST
,BPF_T_NANOTIME_MONOTONIC_FAST
, andBPF_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. TheBPF_T_MICROTIME_MONOTONIC
,BPF_T_NANOTIME_MONOTONIC
,BPF_T_BINTIME_MONOTONIC
,BPF_T_MICROTIME_MONOTONIC_FAST
,BPF_T_NANOTIME_MONOTONIC_FAST
, andBPF_T_BINTIME_MONOTONIC_FAST
store the time elapsed since kernel boot. This setting is initialized toBPF_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. WhenBPF_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 currentbpf
buffering mode; possible values areBPF_BUFMODE_BUFFER
, buffered read mode, andBPF_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 multiplebpf
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) */ };
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 tosizeof
(struct bpf_xhdr) orsizeof
(struct bpf_hdr).
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)
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; };
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
). ForBPF_IND
andBPF_ABS
, the data size must be specified as a word (BPF_W
), halfword (BPF_H
), or byte (BPF_B
). The semantics of all the recognizedBPF_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
orBPF_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 bitk
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
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 thebpf
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), };
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), };
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 theBIOCGBLEN
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.June 15, 2010 | Debian |