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25 .Dd January 16, 1996
26 .Dt BPF 4
27 .Os
28 .Sh NAME
29 .Nm bpf
30 .Nd Berkeley Packet Filter
31 .Sh SYNOPSIS
32 .Cd pseudo-device bpf
33 .Sh DESCRIPTION
34 The Berkeley Packet Filter
35 provides a raw interface to data link layers in a protocol
36 independent fashion.
37 All packets on the network, even those destined for other hosts,
38 are accessible through this mechanism.
39 .Pp
40 The packet filter appears as a character special device,
41 .Pa /dev/bpf0 ,
42 .Pa /dev/bpf1 ,
43 etc.
44 After opening the device, the file descriptor must be bound to a
45 specific network interface with the
46 .Dv BIOCSETIF
47 ioctl.
48 A given interface can be shared be multiple listeners, and the filter
49 underlying each descriptor will see an identical packet stream.
50 .Pp
51 A separate device file is required for each minor device.
52 If a file is in use, the open will fail and
53 .Va errno
54 will be set to
55 .Er EBUSY .
56 .Pp
57 Associated with each open instance of a
58 .Nm
59 file is a user-settable packet filter.
60 Whenever a packet is received by an interface,
61 all file descriptors listening on that interface apply their filter.
62 Each descriptor that accepts the packet receives its own copy.
63 .Pp
64 Reads from these files return the next group of packets
65 that have matched the filter.
66 To improve performance, the buffer passed to read must be
67 the same size as the buffers used internally by
68 .Nm .
69 This size is returned by the
70 .Dv BIOCGBLEN
71 ioctl (see below), and
72 can be set with
73 .Dv BIOCSBLEN .
74 Note that an individual packet larger than this size is necessarily
75 truncated.
76 .Pp
77 The packet filter will support any link level protocol that has fixed length
78 headers. Currently, only Ethernet,
79 .Tn SLIP ,
80 and
81 .Tn PPP
82 drivers have been modified to interact with
83 .Nm .
84 .Pp
85 Since packet data is in network byte order, applications should use the
86 .Xr byteorder 3
87 macros to extract multi-byte values.
88 .Pp
89 A packet can be sent out on the network by writing to a
90 .Nm
91 file descriptor. The writes are unbuffered, meaning only one
92 packet can be processed per write.
93 Currently, only writes to Ethernets and
94 .Tn SLIP
95 links are supported.
96 .Pp
97 When the last minor device is opened, an additional minor device is
98 created on demand. The maximum number of devices that can be created is
99 controlled by the sysctl debug.bpf_maxdevices.
100 .Sh IOCTLS
101 The
102 .Xr ioctl 2
103 command codes below are defined in
104 .Aq Pa net/bpf.h .
105 All commands require
106 these includes:
107 .Bd -literal
108 #include <sys/types.h>
109 #include <sys/time.h>
110 #include <sys/ioctl.h>
111 #include <net/bpf.h>
112 .Ed
113 .Pp
114 Additionally,
115 .Dv BIOCGETIF
116 and
117 .Dv BIOCSETIF
118 require
119 .Aq Pa sys/socket.h
120 and
121 .Aq Pa net/if.h .
122 .Pp
123 In addition to
124 .Dv FIONREAD
125 the following commands may be applied to any open
126 .Nm
127 file.
128 The (third) argument to
129 .Xr ioctl 2
130 should be a pointer to the type indicated.
131 .Bl -tag -width BIOCGRTIMEOUT
132 .It Dv BIOCGBLEN
133 .Pq Li u_int
134 Returns the required buffer length for reads on
135 .Nm
136 files.
137 .It Dv BIOCSBLEN
138 .Pq Li u_int
139 Sets the buffer length for reads on
140 .Nm
141 files. The buffer must be set before the file is attached to an interface
142 with
143 .Dv BIOCSETIF .
144 If the requested buffer size cannot be accommodated, the closest
145 allowable size will be set and returned in the argument.
146 A read call will result in
147 .Er EIO
148 if it is passed a buffer that is not this size.
149 .It Dv BIOCGDLT
150 .Pq Li u_int
151 Returns the type of the data link layer underlying the attached interface.
152 .Er EINVAL
153 is returned if no interface has been specified.
154 The device types, prefixed with
155 .Dq Li DLT_ ,
156 are defined in
157 .Aq Pa net/bpf.h .
158 .It Dv BIOCPROMISC
159 Forces the interface into promiscuous mode.
160 All packets, not just those destined for the local host, are processed.
161 Since more than one file can be listening on a given interface,
162 a listener that opened its interface non-promiscuously may receive
163 packets promiscuously. This problem can be remedied with an
164 appropriate filter.
165 .It Dv BIOCFLUSH
166 Flushes the buffer of incoming packets,
167 and resets the statistics that are returned by BIOCGSTATS.
168 .It Dv BIOCGETIF
169 .Pq Li "struct ifreq"
170 Returns the name of the hardware interface that the file is listening on.
171 The name is returned in the ifr_name field of
172 the
173 .Li ifreq
174 structure.
175 All other fields are undefined.
176 .It Dv BIOCSETIF
177 .Pq Li "struct ifreq"
178 Sets the hardware interface associate with the file. This
179 command must be performed before any packets can be read.
180 The device is indicated by name using the
181 .Li ifr_name
182 field of the
183 .Li ifreq
184 structure.
185 Additionally, performs the actions of
186 .Dv BIOCFLUSH .
187 .It Dv BIOCSRTIMEOUT
188 .It Dv BIOCGRTIMEOUT
189 .Pq Li "struct timeval"
190 Set or get the read timeout parameter.
191 The argument
192 specifies the length of time to wait before timing
193 out on a read request.
194 This parameter is initialized to zero by
195 .Xr open 2 ,
196 indicating no timeout.
197 .It Dv BIOCGSTATS
198 .Pq Li "struct bpf_stat"
199 Returns the following structure of packet statistics:
200 .Bd -literal
201 struct bpf_stat {
202 u_int bs_recv; /* number of packets received */
203 u_int bs_drop; /* number of packets dropped */
204 };
205 .Ed
206 .Pp
207 The fields are:
208 .Bl -hang -offset indent
209 .It Li bs_recv
210 the number of packets received by the descriptor since opened or reset
211 (including any buffered since the last read call);
212 and
213 .It Li bs_drop
214 the number of packets which were accepted by the filter but dropped by the
215 kernel because of buffer overflows
216 (i.e., the application's reads aren't keeping up with the packet traffic).
217 .El
218 .It Dv BIOCIMMEDIATE
219 .Pq Li u_int
220 Enable or disable
221 .Dq immediate mode ,
222 based on the truth value of the argument.
223 When immediate mode is enabled, reads return immediately upon packet
224 reception. Otherwise, a read will block until either the kernel buffer
225 becomes full or a timeout occurs.
226 This is useful for programs like
227 .Xr rarpd 8
228 which must respond to messages in real time.
229 The default for a new file is off.
230 .It Dv BIOCSETF
231 .Pq Li "struct bpf_program"
232 Sets the filter program used by the kernel to discard uninteresting
233 packets. An array of instructions and its length is passed in using
234 the following structure:
235 .Bd -literal
236 struct bpf_program {
237 int bf_len;
238 struct bpf_insn *bf_insns;
239 };
240 .Ed
241 .Pp
242 The filter program is pointed to by the
243 .Li bf_insns
244 field while its length in units of
245 .Sq Li struct bpf_insn
246 is given by the
247 .Li bf_len
248 field.
249 Also, the actions of
250 .Dv BIOCFLUSH
251 are performed.
252 See section
253 .Sx "FILTER MACHINE"
254 for an explanation of the filter language.
255 .It Dv BIOCVERSION
256 .Pq Li "struct bpf_version"
257 Returns the major and minor version numbers of the filter language currently
258 recognized by the kernel. Before installing a filter, applications must check
259 that the current version is compatible with the running kernel. Version
260 numbers are compatible if the major numbers match and the application minor
261 is less than or equal to the kernel minor. The kernel version number is
262 returned in the following structure:
263 .Bd -literal
264 struct bpf_version {
265 u_short bv_major;
266 u_short bv_minor;
267 };
268 .Ed
269 .Pp
270 The current version numbers are given by
271 .Dv BPF_MAJOR_VERSION
272 and
273 .Dv BPF_MINOR_VERSION
274 from
275 .Aq Pa net/bpf.h .
276 An incompatible filter
277 may result in undefined behavior (most likely, an error returned by
278 .Fn ioctl
279 or haphazard packet matching).
280 .It Dv BIOCSHDRCMPLT
281 .It Dv BIOCGHDRCMPLT
282 .Pq Li u_int
283 Set or get the status of the
284 .Dq header complete
285 flag.
286 Set to zero if the link level source address should be filled in automatically
287 by the interface output routine. Set to one if the link level source
288 address will be written, as provided, to the wire. This flag is initialized
289 to zero by default.
290 .It Dv BIOCSSEESENT
291 .It Dv BIOCGSEESENT
292 .Pq Li u_int
293 Set or get the flag determining whether locally generated packets on the
294 interface should be returned by BPF. Set to zero to see only incoming
295 packets on the interface. Set to one to see packets originating
296 locally and remotely on the interface. This flag is initialized to one by
297 default.
298 .El
299 .Sh BPF HEADER
300 The following structure is prepended to each packet returned by
301 .Xr read 2 :
302 .Bd -literal
303 struct bpf_hdr {
304 struct timeval bh_tstamp; /* time stamp */
305 u_long bh_caplen; /* length of captured portion */
306 u_long bh_datalen; /* original length of packet */
307 u_short bh_hdrlen; /* length of bpf header (this struct
308 plus alignment padding */
309 };
310 .Ed
311 .Pp
312 The fields, whose values are stored in host order, and are:
313 .Pp
314 .Bl -tag -compact -width bh_datalen
315 .It Li bh_tstamp
316 The time at which the packet was processed by the packet filter.
317 .It Li bh_caplen
318 The length of the captured portion of the packet. This is the minimum of
319 the truncation amount specified by the filter and the length of the packet.
320 .It Li bh_datalen
321 The length of the packet off the wire.
322 This value is independent of the truncation amount specified by the filter.
323 .It Li bh_hdrlen
324 The length of the
325 .Nm
326 header, which may not be equal to
327 .\" XXX - not really a function call
328 .Fn sizeof "struct bpf_hdr" .
329 .El
330 .Pp
331 The
332 .Li bh_hdrlen
333 field exists to account for
334 padding between the header and the link level protocol.
335 The purpose here is to guarantee proper alignment of the packet
336 data structures, which is required on alignment sensitive
337 architectures and improves performance on many other architectures.
338 The packet filter insures that the
339 .Li bpf_hdr
340 and the network layer
341 header will be word aligned. Suitable precautions
342 must be taken when accessing the link layer protocol fields on alignment
343 restricted machines. (This isn't a problem on an Ethernet, since
344 the type field is a short falling on an even offset,
345 and the addresses are probably accessed in a bytewise fashion).
346 .Pp
347 Additionally, individual packets are padded so that each starts
348 on a word boundary. This requires that an application
349 has some knowledge of how to get from packet to packet.
350 The macro
351 .Dv BPF_WORDALIGN
352 is defined in
353 .Aq Pa net/bpf.h
354 to facilitate
355 this process. It rounds up its argument
356 to the nearest word aligned value (where a word is
357 .Dv BPF_ALIGNMENT
358 bytes wide).
359 .Pp
360 For example, if
361 .Sq Li p
362 points to the start of a packet, this expression
363 will advance it to the next packet:
364 .Dl p = (char *)p + BPF_WORDALIGN(p->bh_hdrlen + p->bh_caplen)
365 .Pp
366 For the alignment mechanisms to work properly, the
367 buffer passed to
368 .Xr read 2
369 must itself be word aligned.
370 The
371 .Xr malloc 3
372 function
373 will always return an aligned buffer.
374 .Sh FILTER MACHINE
375 A filter program is an array of instructions, with all branches forwardly
376 directed, terminated by a
377 .Em return
378 instruction.
379 Each instruction performs some action on the pseudo-machine state,
380 which consists of an accumulator, index register, scratch memory store,
381 and implicit program counter.
382 .Pp
383 The following structure defines the instruction format:
384 .Bd -literal
385 struct bpf_insn {
386 u_short code;
387 u_char jt;
388 u_char jf;
389 u_long k;
390 };
391 .Ed
392 .Pp
393 The
394 .Li k
395 field is used in different ways by different instructions,
396 and the
397 .Li jt
398 and
399 .Li jf
400 fields are used as offsets
401 by the branch instructions.
402 The opcodes are encoded in a semi-hierarchical fashion.
403 There are eight classes of instructions:
404 .Dv BPF_LD ,
405 .Dv BPF_LDX ,
406 .Dv BPF_ST ,
407 .Dv BPF_STX ,
408 .Dv BPF_ALU ,
409 .Dv BPF_JMP ,
410 .Dv BPF_RET ,
411 and
412 .Dv BPF_MISC .
413 Various other mode and
414 operator bits are or'd into the class to give the actual instructions.
415 The classes and modes are defined in
416 .Aq Pa net/bpf.h .
417 .Pp
418 Below are the semantics for each defined
419 .Nm
420 instruction.
421 We use the convention that A is the accumulator, X is the index register,
422 P[] packet data, and M[] scratch memory store.
423 P[i:n] gives the data at byte offset
424 .Dq i
425 in the packet,
426 interpreted as a word (n=4),
427 unsigned halfword (n=2), or unsigned byte (n=1).
428 M[i] gives the i'th word in the scratch memory store, which is only
429 addressed in word units. The memory store is indexed from 0 to
430 .Dv BPF_MEMWORDS
431 - 1.
432 .Li k ,
433 .Li jt ,
434 and
435 .Li jf
436 are the corresponding fields in the
437 instruction definition.
438 .Dq len
439 refers to the length of the packet.
440 .Pp
441 .Bl -tag -width BPF_STXx
442 .It Dv BPF_LD
443 These instructions copy a value into the accumulator. The type of the
444 source operand is specified by an
445 .Dq addressing mode
446 and can be a constant
447 .Pq Dv BPF_IMM ,
448 packet data at a fixed offset
449 .Pq Dv BPF_ABS ,
450 packet data at a variable offset
451 .Pq Dv BPF_IND ,
452 the packet length
453 .Pq Dv BPF_LEN ,
454 or a word in the scratch memory store
455 .Pq Dv BPF_MEM .
456 For
457 .Dv BPF_IND
458 and
459 .Dv BPF_ABS ,
460 the data size must be specified as a word
461 .Pq Dv BPF_W ,
462 halfword
463 .Pq Dv BPF_H ,
464 or byte
465 .Pq Dv BPF_B .
466 The semantics of all the recognized
467 .Dv BPF_LD
468 instructions follow.
469 .Pp
470 .Bl -tag -width "BPF_LD+BPF_W+BPF_IND" -compact
471 .It Li BPF_LD+BPF_W+BPF_ABS
472 A <- P[k:4]
473 .It Li BPF_LD+BPF_H+BPF_ABS
474 A <- P[k:2]
475 .It Li BPF_LD+BPF_B+BPF_ABS
476 A <- P[k:1]
477 .It Li BPF_LD+BPF_W+BPF_IND
478 A <- P[X+k:4]
479 .It Li BPF_LD+BPF_H+BPF_IND
480 A <- P[X+k:2]
481 .It Li BPF_LD+BPF_B+BPF_IND
482 A <- P[X+k:1]
483 .It Li BPF_LD+BPF_W+BPF_LEN
484 A <- len
485 .It Li BPF_LD+BPF_IMM
486 A <- k
487 .It Li BPF_LD+BPF_MEM
488 A <- M[k]
489 .El
490 .It Dv BPF_LDX
491 These instructions load a value into the index register. Note that
492 the addressing modes are more restrictive than those of the accumulator loads,
493 but they include
494 .Dv BPF_MSH ,
495 a hack for efficiently loading the IP header length.
496 .Pp
497 .Bl -tag -width "BPF_LDX+BPF_W+BPF_MEM" -compact
498 .It Li BPF_LDX+BPF_W+BPF_IMM
499 X <- k
500 .It Li BPF_LDX+BPF_W+BPF_MEM
501 X <- M[k]
502 .It Li BPF_LDX+BPF_W+BPF_LEN
503 X <- len
504 .It Li BPF_LDX+BPF_B+BPF_MSH
505 X <- 4*(P[k:1]&0xf)
506 .El
507 .It Dv BPF_ST
508 This instruction stores the accumulator into the scratch memory.
509 We do not need an addressing mode since there is only one possibility
510 for the destination.
511 .Pp
512 .Bl -tag -width "BPF_ST" -compact
513 .It Li BPF_ST
514 M[k] <- A
515 .El
516 .It Dv BPF_STX
517 This instruction stores the index register in the scratch memory store.
518 .Pp
519 .Bl -tag -width "BPF_STX" -compact
520 .It Li BPF_STX
521 M[k] <- X
522 .El
523 .It Dv BPF_ALU
524 The alu instructions perform operations between the accumulator and
525 index register or constant, and store the result back in the accumulator.
526 For binary operations, a source mode is required
527 .Dv ( BPF_K
528 or
529 .Dv BPF_X ) .
530 .Pp
531 .Bl -tag -width "BPF_ALU+BPF_MUL+BPF_K" -compact
532 .It Li BPF_ALU+BPF_ADD+BPF_K
533 A <- A + k
534 .It Li BPF_ALU+BPF_SUB+BPF_K
535 A <- A - k
536 .It Li BPF_ALU+BPF_MUL+BPF_K
537 A <- A * k
538 .It Li BPF_ALU+BPF_DIV+BPF_K
539 A <- A / k
540 .It Li BPF_ALU+BPF_AND+BPF_K
541 A <- A & k
542 .It Li BPF_ALU+BPF_OR+BPF_K
543 A <- A | k
544 .It Li BPF_ALU+BPF_LSH+BPF_K
545 A <- A << k
546 .It Li BPF_ALU+BPF_RSH+BPF_K
547 A <- A >> k
548 .It Li BPF_ALU+BPF_ADD+BPF_X
549 A <- A + X
550 .It Li BPF_ALU+BPF_SUB+BPF_X
551 A <- A - X
552 .It Li BPF_ALU+BPF_MUL+BPF_X
553 A <- A * X
554 .It Li BPF_ALU+BPF_DIV+BPF_X
555 A <- A / X
556 .It Li BPF_ALU+BPF_AND+BPF_X
557 A <- A & X
558 .It Li BPF_ALU+BPF_OR+BPF_X
559 A <- A | X
560 .It Li BPF_ALU+BPF_LSH+BPF_X
561 A <- A << X
562 .It Li BPF_ALU+BPF_RSH+BPF_X
563 A <- A >> X
564 .It Li BPF_ALU+BPF_NEG
565 A <- -A
566 .El
567 .It Dv BPF_JMP
568 The jump instructions alter flow of control. Conditional jumps
569 compare the accumulator against a constant
570 .Pq Dv BPF_K
571 or the index register
572 .Pq Dv BPF_X .
573 If the result is true (or non-zero),
574 the true branch is taken, otherwise the false branch is taken.
575 Jump offsets are encoded in 8 bits so the longest jump is 256 instructions.
576 However, the jump always
577 .Pq Dv BPF_JA
578 opcode uses the 32 bit
579 .Li k
580 field as the offset, allowing arbitrarily distant destinations.
581 All conditionals use unsigned comparison conventions.
582 .Pp
583 .Bl -tag -width "BPF_JMP+BPF_KSET+BPF_X" -compact
584 .It Li BPF_JMP+BPF_JA
585 pc += k
586 .It Li BPF_JMP+BPF_JGT+BPF_K
587 pc += (A > k) ? jt : jf
588 .It Li BPF_JMP+BPF_JGE+BPF_K
589 pc += (A >= k) ? jt : jf
590 .It Li BPF_JMP+BPF_JEQ+BPF_K
591 pc += (A == k) ? jt : jf
592 .It Li BPF_JMP+BPF_JSET+BPF_K
593 pc += (A & k) ? jt : jf
594 .It Li BPF_JMP+BPF_JGT+BPF_X
595 pc += (A > X) ? jt : jf
596 .It Li BPF_JMP+BPF_JGE+BPF_X
597 pc += (A >= X) ? jt : jf
598 .It Li BPF_JMP+BPF_JEQ+BPF_X
599 pc += (A == X) ? jt : jf
600 .It Li BPF_JMP+BPF_JSET+BPF_X
601 pc += (A & X) ? jt : jf
602 .El
603 .It Dv BPF_RET
604 The return instructions terminate the filter program and specify the amount
605 of packet to accept (i.e., they return the truncation amount). A return
606 value of zero indicates that the packet should be ignored.
607 The return value is either a constant
608 .Pq Dv BPF_K
609 or the accumulator
610 .Pq Dv BPF_A .
611 .Pp
612 .Bl -tag -width "BPF_RET+BPF_K" -compact
613 .It Li BPF_RET+BPF_A
614 accept A bytes
615 .It Li BPF_RET+BPF_K
616 accept k bytes
617 .El
618 .It Dv BPF_MISC
619 The miscellaneous category was created for anything that doesn't
620 fit into the above classes, and for any new instructions that might need to
621 be added. Currently, these are the register transfer instructions
622 that copy the index register to the accumulator or vice versa.
623 .Pp
624 .Bl -tag -width "BPF_MISC+BPF_TAX" -compact
625 .It Li BPF_MISC+BPF_TAX
626 X <- A
627 .It Li BPF_MISC+BPF_TXA
628 A <- X
629 .El
630 .El
631 .Pp
632 The
633 .Nm
634 interface provides the following macros to facilitate
635 array initializers:
636 .Fn BPF_STMT opcode operand
637 and
638 .Fn BPF_JUMP opcode operand true_offset false_offset .
639 .Sh EXAMPLES
640 The following filter is taken from the Reverse ARP Daemon. It accepts
641 only Reverse ARP requests.
642 .Bd -literal
643 struct bpf_insn insns[] = {
644 BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 12),
645 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, ETHERTYPE_REVARP, 0, 3),
646 BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 20),
647 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, REVARP_REQUEST, 0, 1),
648 BPF_STMT(BPF_RET+BPF_K, sizeof(struct ether_arp) +
649 sizeof(struct ether_header)),
650 BPF_STMT(BPF_RET+BPF_K, 0),
651 };
652 .Ed
653 .Pp
654 This filter accepts only IP packets between host 128.3.112.15 and
655 128.3.112.35.
656 .Bd -literal
657 struct bpf_insn insns[] = {
658 BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 12),
659 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, ETHERTYPE_IP, 0, 8),
660 BPF_STMT(BPF_LD+BPF_W+BPF_ABS, 26),
661 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x8003700f, 0, 2),
662 BPF_STMT(BPF_LD+BPF_W+BPF_ABS, 30),
663 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x80037023, 3, 4),
664 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x80037023, 0, 3),
665 BPF_STMT(BPF_LD+BPF_W+BPF_ABS, 30),
666 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x8003700f, 0, 1),
667 BPF_STMT(BPF_RET+BPF_K, (u_int)-1),
668 BPF_STMT(BPF_RET+BPF_K, 0),
669 };
670 .Ed
671 .Pp
672 Finally, this filter returns only TCP finger packets. We must parse
673 the IP header to reach the TCP header. The
674 .Dv BPF_JSET
675 instruction
676 checks that the IP fragment offset is 0 so we are sure
677 that we have a TCP header.
678 .Bd -literal
679 struct bpf_insn insns[] = {
680 BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 12),
681 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, ETHERTYPE_IP, 0, 10),
682 BPF_STMT(BPF_LD+BPF_B+BPF_ABS, 23),
683 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, IPPROTO_TCP, 0, 8),
684 BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 20),
685 BPF_JUMP(BPF_JMP+BPF_JSET+BPF_K, 0x1fff, 6, 0),
686 BPF_STMT(BPF_LDX+BPF_B+BPF_MSH, 14),
687 BPF_STMT(BPF_LD+BPF_H+BPF_IND, 14),
688 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 79, 2, 0),
689 BPF_STMT(BPF_LD+BPF_H+BPF_IND, 16),
690 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 79, 0, 1),
691 BPF_STMT(BPF_RET+BPF_K, (u_int)-1),
692 BPF_STMT(BPF_RET+BPF_K, 0),
693 };
694 .Ed
695 .Sh SEE ALSO
696 .Xr tcpdump 1 ,
697 .Xr ioctl 2 ,
698 .Xr byteorder 3 ,
699 .Xr ng_bpf 4
700 .Rs
701 .%A McCanne, S.
702 .%A Jacobson V.
703 .%T "An efficient, extensible, and portable network monitor"
704 .Re
705 .Sh FILES
706 .Bl -tag -compact -width /dev/bpfXXX
707 .It Pa /dev/bpf Ns Sy n
708 the packet filter device
709 .El
710 .Sh BUGS
711 The read buffer must be of a fixed size (returned by the
712 .Dv BIOCGBLEN
713 ioctl).
714 .Pp
715 A file that does not request promiscuous mode may receive promiscuously
716 received packets as a side effect of another file requesting this
717 mode on the same hardware interface. This could be fixed in the kernel
718 with additional processing overhead. However, we favor the model where
719 all files must assume that the interface is promiscuous, and if
720 so desired, must utilize a filter to reject foreign packets.
721 .Pp
722 Data link protocols with variable length headers are not currently supported.
723 .Sh HISTORY
724 The Enet packet filter was created in 1980 by Mike Accetta and
725 Rick Rashid at Carnegie-Mellon University. Jeffrey Mogul, at
726 Stanford, ported the code to
727 .Bx
728 and continued its development from
729 1983 on. Since then, it has evolved into the Ultrix Packet Filter
730 at
731 .Tn DEC ,
732 a
733 .Tn STREAMS
734 .Tn NIT
735 module under
736 .Tn SunOS 4.1 ,
737 and
738 .Tn BPF .
739 .Sh AUTHORS
740 .An -nosplit
741 .An Steven McCanne ,
742 of Lawrence Berkeley Laboratory, implemented BPF in
743 Summer 1990. Much of the design is due to
744 .An Van Jacobson .