1 .\" Copyright (c) 2007 Seccuris Inc.
2 .\" All rights reserved.
4 .\" This software was developed by Robert N. M. Watson under contract to
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47 .\" This document is derived in part from the enet man page (enet.4)
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57 .Nd Berkeley Packet Filter
61 The Berkeley Packet Filter
62 provides a raw interface to data link layers in a protocol
64 All packets on the network, even those destined for other hosts,
65 are accessible through this mechanism.
67 The packet filter appears as a character special device,
69 After opening the device, the file descriptor must be bound to a
70 specific network interface with the
73 A given interface can be shared by multiple listeners, and the filter
74 underlying each descriptor will see an identical packet stream.
76 A separate device file is required for each minor device.
77 If a file is in use, the open will fail and
82 Associated with each open instance of a
84 file is a user-settable packet filter.
85 Whenever a packet is received by an interface,
86 all file descriptors listening on that interface apply their filter.
87 Each descriptor that accepts the packet receives its own copy.
89 The packet filter will support any link level protocol that has fixed length
91 Currently, only Ethernet,
95 drivers have been modified to interact with
98 Since packet data is in network byte order, applications should use the
100 macros to extract multi-byte values.
102 A packet can be sent out on the network by writing to a
105 The writes are unbuffered, meaning only one packet can be processed per write.
106 Currently, only writes to Ethernets and
111 devices deliver packet data to the application via memory buffers provided by
113 The buffer mode is set using the
115 ioctl, and read using the
118 .Ss Buffered read mode
121 devices operate in the
122 .Dv BPF_BUFMODE_BUFFER
123 mode, in which packet data is copied explicitly from kernel to user memory
127 The user process will declare a fixed buffer size that will be used both for
128 sizing internal buffers and for all
130 operations on the file.
131 This size is queried using the
133 ioctl, and is set using the
136 Note that an individual packet larger than the buffer size is necessarily
138 .Ss Zero-copy buffer mode
140 devices may also operate in the
141 .Dv BPF_BUFMODE_ZEROCOPY
142 mode, in which packet data is written directly into two user memory buffers
143 by the kernel, avoiding both system call and copying overhead.
144 Buffers are of fixed (and equal) size, page-aligned, and an even multiple of
146 The maximum zero-copy buffer size is returned by the
149 Note that an individual packet larger than the buffer size is necessarily
152 The user process registers two memory buffers using the
154 ioctl, which accepts a
156 pointer as an argument:
166 is a pointer to the userspace address of the first buffer that will be
169 is a pointer to the second buffer.
171 will then cycle between the two buffers as they fill and are acknowledged.
173 Each buffer begins with a fixed-length header to hold synchronization and
174 data length information for the buffer:
176 struct bpf_zbuf_header {
177 volatile u_int bzh_kernel_gen; /* Kernel generation number. */
178 volatile u_int bzh_kernel_len; /* Length of data in the buffer. */
179 volatile u_int bzh_user_gen; /* User generation number. */
180 /* ...padding for future use... */
184 The header structure of each buffer, including all padding, should be zeroed
185 before it is configured using
187 Remaining space in the buffer will be used by the kernel to store packet
188 data, laid out in the same format as with buffered read mode.
190 The kernel and the user process follow a simple acknowledgement protocol via
191 the buffer header to synchronize access to the buffer: when the header
196 hold the same value, the kernel owns the buffer, and when they differ,
197 userspace owns the buffer.
199 While the kernel owns the buffer, the contents are unstable and may change
200 asynchronously; while the user process owns the buffer, its contents are
201 stable and will not be changed until the buffer has been acknowledged.
203 Initializing the buffer headers to all 0's before registering the buffer has
204 the effect of assigning initial ownership of both buffers to the kernel.
205 The kernel signals that a buffer has been assigned to userspace by modifying
207 and userspace acknowledges the buffer and returns it to the kernel by setting
213 In order to avoid caching and memory re-ordering effects, the user process
214 must use atomic operations and memory barriers when checking for and
215 acknowledging buffers:
217 #include <machine/atomic.h>
220 * Return ownership of a buffer to the kernel for reuse.
223 buffer_acknowledge(struct bpf_zbuf_header *bzh)
226 atomic_store_rel_int(&bzh->bzh_user_gen, bzh->bzh_kernel_gen);
230 * Check whether a buffer has been assigned to userspace by the kernel.
231 * Return true if userspace owns the buffer, and false otherwise.
234 buffer_check(struct bpf_zbuf_header *bzh)
237 return (bzh->bzh_user_gen !=
238 atomic_load_acq_int(&bzh->bzh_kernel_gen));
242 The user process may force the assignment of the next buffer, if any data
243 is pending, to userspace using the
246 This allows the user process to retrieve data in a partially filled buffer
247 before the buffer is full, such as following a timeout; the process must
248 recheck for buffer ownership using the header generation numbers, as the
249 buffer will not be assigned to userspace if no data was present.
251 As in the buffered read mode,
256 may be used to sleep awaiting the availability of a completed buffer.
257 They will return a readable file descriptor when ownership of the next buffer
258 is assigned to user space.
260 In the current implementation, the kernel may assign zero, one, or both
261 buffers to the user process; however, an earlier implementation maintained
262 the invariant that at most one buffer could be assigned to the user process
264 In order to both ensure progress and high performance, user processes should
265 acknowledge a completely processed buffer as quickly as possible, returning
266 it for reuse, and not block waiting on a second buffer while holding another
271 command codes below are defined in
276 #include <sys/types.h>
277 #include <sys/time.h>
278 #include <sys/ioctl.h>
293 the following commands may be applied to any open
296 The (third) argument to
298 should be a pointer to the type indicated.
299 .Bl -tag -width BIOCGETBUFMODE
302 Returns the required buffer length for reads on
307 Sets the buffer length for reads on
310 The buffer must be set before the file is attached to an interface
313 If the requested buffer size cannot be accommodated, the closest
314 allowable size will be set and returned in the argument.
315 A read call will result in
317 if it is passed a buffer that is not this size.
320 Returns the type of the data link layer underlying the attached interface.
322 is returned if no interface has been specified.
323 The device types, prefixed with
328 Forces the interface into promiscuous mode.
329 All packets, not just those destined for the local host, are processed.
330 Since more than one file can be listening on a given interface,
331 a listener that opened its interface non-promiscuously may receive
332 packets promiscuously.
333 This problem can be remedied with an appropriate filter.
335 Flushes the buffer of incoming packets,
336 and resets the statistics that are returned by BIOCGSTATS.
338 .Pq Li "struct ifreq"
339 Returns the name of the hardware interface that the file is listening on.
340 The name is returned in the ifr_name field of
344 All other fields are undefined.
346 .Pq Li "struct ifreq"
347 Sets the hardware interface associate with the file.
349 command must be performed before any packets can be read.
350 The device is indicated by name using the
355 Additionally, performs the actions of
359 .Pq Li "struct timeval"
360 Set or get the read timeout parameter.
362 specifies the length of time to wait before timing
363 out on a read request.
364 This parameter is initialized to zero by
366 indicating no timeout.
368 .Pq Li "struct bpf_stat"
369 Returns the following structure of packet statistics:
372 u_int bs_recv; /* number of packets received */
373 u_int bs_drop; /* number of packets dropped */
378 .Bl -hang -offset indent
380 the number of packets received by the descriptor since opened or reset
381 (including any buffered since the last read call);
384 the number of packets which were accepted by the filter but dropped by the
385 kernel because of buffer overflows
386 (i.e., the application's reads are not keeping up with the packet traffic).
392 based on the truth value of the argument.
393 When immediate mode is enabled, reads return immediately upon packet
395 Otherwise, a read will block until either the kernel buffer
396 becomes full or a timeout occurs.
397 This is useful for programs like
399 which must respond to messages in real time.
400 The default for a new file is off.
403 .Pq Li "struct bpf_program"
404 Sets the read filter program used by the kernel to discard uninteresting
406 An array of instructions and its length is passed in using
407 the following structure:
411 struct bpf_insn *bf_insns;
415 The filter program is pointed to by the
417 field while its length in units of
418 .Sq Li struct bpf_insn
424 for an explanation of the filter language.
425 The only difference between
431 performs the actions of
437 .Pq Li "struct bpf_program"
438 Sets the write filter program used by the kernel to control what type of
439 packets can be written to the interface.
447 .Pq Li "struct bpf_version"
448 Returns the major and minor version numbers of the filter language currently
449 recognized by the kernel.
450 Before installing a filter, applications must check
451 that the current version is compatible with the running kernel.
452 Version numbers are compatible if the major numbers match and the application minor
453 is less than or equal to the kernel minor.
454 The kernel version number is returned in the following structure:
462 The current version numbers are given by
463 .Dv BPF_MAJOR_VERSION
465 .Dv BPF_MINOR_VERSION
468 An incompatible filter
469 may result in undefined behavior (most likely, an error returned by
471 or haphazard packet matching).
475 Set or get the status of the
478 Set to zero if the link level source address should be filled in automatically
479 by the interface output routine.
480 Set to one if the link level source
481 address will be written, as provided, to the wire.
482 This flag is initialized to zero by default.
486 These commands are obsolete but left for compatibility.
492 Set or get the flag determining whether locally generated packets on the
493 interface should be returned by BPF.
494 Set to zero to see only incoming packets on the interface.
495 Set to one to see packets originating locally and remotely on the interface.
496 This flag is initialized to one by default.
497 .It Dv BIOCSDIRECTION
498 .It Dv BIOCGDIRECTION
500 Set or get the setting determining whether incoming, outgoing, or all packets
501 on the interface should be returned by BPF.
504 to see only incoming packets on the interface.
507 to see packets originating locally and remotely on the interface.
510 to see only outgoing packets on the interface.
511 This setting is initialized to
517 Set or get format and resolution of the time stamps returned by BPF.
519 .Dv BPF_T_MICROTIME ,
520 .Dv BPF_T_MICROTIME_FAST ,
521 .Dv BPF_T_MICROTIME_MONOTONIC ,
523 .Dv BPF_T_MICROTIME_MONOTONIC_FAST
524 to get time stamps in 64-bit
529 .Dv BPF_T_NANOTIME_FAST ,
530 .Dv BPF_T_NANOTIME_MONOTONIC ,
532 .Dv BPF_T_NANOTIME_MONOTONIC_FAST
533 to get time stamps in 64-bit
538 .Dv BPF_T_BINTIME_FAST ,
539 .Dv BPF_T_NANOTIME_MONOTONIC ,
541 .Dv BPF_T_BINTIME_MONOTONIC_FAST
542 to get time stamps in 64-bit
547 to ignore time stamp.
548 All 64-bit time stamp formats are wrapped in
551 .Dv BPF_T_MICROTIME_FAST ,
552 .Dv BPF_T_NANOTIME_FAST ,
553 .Dv BPF_T_BINTIME_FAST ,
554 .Dv BPF_T_MICROTIME_MONOTONIC_FAST ,
555 .Dv BPF_T_NANOTIME_MONOTONIC_FAST ,
557 .Dv BPF_T_BINTIME_MONOTONIC_FAST
558 are analogs of corresponding formats without _FAST suffix but do not perform
559 a full time counter query, so their accuracy is one timer tick.
561 .Dv BPF_T_MICROTIME_MONOTONIC ,
562 .Dv BPF_T_NANOTIME_MONOTONIC ,
563 .Dv BPF_T_BINTIME_MONOTONIC ,
564 .Dv BPF_T_MICROTIME_MONOTONIC_FAST ,
565 .Dv BPF_T_NANOTIME_MONOTONIC_FAST ,
567 .Dv BPF_T_BINTIME_MONOTONIC_FAST
568 store the time elapsed since kernel boot.
569 This setting is initialized to
574 Set packet feedback mode.
575 This allows injected packets to be fed back as input to the interface when
576 output via the interface is successful.
579 direction is set, injected outgoing packet is not returned by BPF to avoid
581 This flag is initialized to zero by default.
583 Set the locked flag on the
586 This prevents the execution of
587 ioctl commands which could change the underlying operating parameters of
589 .It Dv BIOCGETBUFMODE
590 .It Dv BIOCSETBUFMODE
592 Get or set the current
594 buffering mode; possible values are
595 .Dv BPF_BUFMODE_BUFFER ,
596 buffered read mode, and
597 .Dv BPF_BUFMODE_ZBUF ,
598 zero-copy buffer mode.
600 .Pq Li struct bpf_zbuf
601 Set the current zero-copy buffer locations; buffer locations may be
602 set only once zero-copy buffer mode has been selected, and prior to attaching
604 Buffers must be of identical size, page-aligned, and an integer multiple of
612 If buffers have already been set for this device, the ioctl will fail.
615 Get the largest individual zero-copy buffer size allowed.
616 As two buffers are used in zero-copy buffer mode, the limit (in practice) is
617 twice the returned size.
618 As zero-copy buffers consume kernel address space, conservative selection of
619 buffer size is suggested, especially when there are multiple
621 descriptors in use on 32-bit systems.
623 Force ownership of the next buffer to be assigned to userspace, if any data
624 present in the buffer.
625 If no data is present, the buffer will remain owned by the kernel.
626 This allows consumers of zero-copy buffering to implement timeouts and
627 retrieve partially filled buffers.
628 In order to handle the case where no data is present in the buffer and
629 therefore ownership is not assigned, the user process must check
635 One of the following structures is prepended to each packet returned by
637 or via a zero-copy buffer:
640 struct bpf_ts bh_tstamp; /* time stamp */
641 uint32_t bh_caplen; /* length of captured portion */
642 uint32_t bh_datalen; /* original length of packet */
643 u_short bh_hdrlen; /* length of bpf header (this struct
644 plus alignment padding) */
648 struct timeval bh_tstamp; /* time stamp */
649 uint32_t bh_caplen; /* length of captured portion */
650 uint32_t bh_datalen; /* original length of packet */
651 u_short bh_hdrlen; /* length of bpf header (this struct
652 plus alignment padding) */
656 The fields, whose values are stored in host order, and are:
658 .Bl -tag -compact -width bh_datalen
660 The time at which the packet was processed by the packet filter.
662 The length of the captured portion of the packet.
663 This is the minimum of
664 the truncation amount specified by the filter and the length of the packet.
666 The length of the packet off the wire.
667 This value is independent of the truncation amount specified by the filter.
671 header, which may not be equal to
672 .\" XXX - not really a function call
673 .Fn sizeof "struct bpf_xhdr"
675 .Fn sizeof "struct bpf_hdr" .
680 field exists to account for
681 padding between the header and the link level protocol.
682 The purpose here is to guarantee proper alignment of the packet
683 data structures, which is required on alignment sensitive
684 architectures and improves performance on many other architectures.
685 The packet filter ensures that the
688 and the network layer
689 header will be word aligned.
692 is used when the time stamp is set to
693 .Dv BPF_T_MICROTIME ,
694 .Dv BPF_T_MICROTIME_FAST ,
695 .Dv BPF_T_MICROTIME_MONOTONIC ,
696 .Dv BPF_T_MICROTIME_MONOTONIC_FAST ,
699 for backward compatibility reasons.
705 may be deprecated in the near future.
707 must be taken when accessing the link layer protocol fields on alignment
709 (This is not a problem on an Ethernet, since
710 the type field is a short falling on an even offset,
711 and the addresses are probably accessed in a bytewise fashion).
713 Additionally, individual packets are padded so that each starts
715 This requires that an application
716 has some knowledge of how to get from packet to packet.
723 It rounds up its argument to the nearest word aligned value (where a word is
729 points to the start of a packet, this expression
730 will advance it to the next packet:
731 .Dl p = (char *)p + BPF_WORDALIGN(p->bh_hdrlen + p->bh_caplen)
733 For the alignment mechanisms to work properly, the
736 must itself be word aligned.
740 will always return an aligned buffer.
742 A filter program is an array of instructions, with all branches forwardly
743 directed, terminated by a
746 Each instruction performs some action on the pseudo-machine state,
747 which consists of an accumulator, index register, scratch memory store,
748 and implicit program counter.
750 The following structure defines the instruction format:
762 field is used in different ways by different instructions,
767 fields are used as offsets
768 by the branch instructions.
769 The opcodes are encoded in a semi-hierarchical fashion.
770 There are eight classes of instructions:
780 Various other mode and
781 operator bits are or'd into the class to give the actual instructions.
782 The classes and modes are defined in
785 Below are the semantics for each defined
788 We use the convention that A is the accumulator, X is the index register,
789 P[] packet data, and M[] scratch memory store.
790 P[i:n] gives the data at byte offset
793 interpreted as a word (n=4),
794 unsigned halfword (n=2), or unsigned byte (n=1).
795 M[i] gives the i'th word in the scratch memory store, which is only
796 addressed in word units.
797 The memory store is indexed from 0 to
804 are the corresponding fields in the
805 instruction definition.
807 refers to the length of the packet.
808 .Bl -tag -width BPF_STXx
810 These instructions copy a value into the accumulator.
811 The type of the source operand is specified by an
813 and can be a constant
815 packet data at a fixed offset
817 packet data at a variable offset
821 or a word in the scratch memory store
827 the data size must be specified as a word
833 The semantics of all the recognized
837 BPF_LD+BPF_W+BPF_ABS A <- P[k:4]
838 BPF_LD+BPF_H+BPF_ABS A <- P[k:2]
839 BPF_LD+BPF_B+BPF_ABS A <- P[k:1]
840 BPF_LD+BPF_W+BPF_IND A <- P[X+k:4]
841 BPF_LD+BPF_H+BPF_IND A <- P[X+k:2]
842 BPF_LD+BPF_B+BPF_IND A <- P[X+k:1]
843 BPF_LD+BPF_W+BPF_LEN A <- len
844 BPF_LD+BPF_IMM A <- k
845 BPF_LD+BPF_MEM A <- M[k]
848 These instructions load a value into the index register.
850 the addressing modes are more restrictive than those of the accumulator loads,
853 a hack for efficiently loading the IP header length.
855 BPF_LDX+BPF_W+BPF_IMM X <- k
856 BPF_LDX+BPF_W+BPF_MEM X <- M[k]
857 BPF_LDX+BPF_W+BPF_LEN X <- len
858 BPF_LDX+BPF_B+BPF_MSH X <- 4*(P[k:1]&0xf)
861 This instruction stores the accumulator into the scratch memory.
862 We do not need an addressing mode since there is only one possibility
868 This instruction stores the index register in the scratch memory store.
873 The alu instructions perform operations between the accumulator and
874 index register or constant, and store the result back in the accumulator.
875 For binary operations, a source mode is required
880 BPF_ALU+BPF_ADD+BPF_K A <- A + k
881 BPF_ALU+BPF_SUB+BPF_K A <- A - k
882 BPF_ALU+BPF_MUL+BPF_K A <- A * k
883 BPF_ALU+BPF_DIV+BPF_K A <- A / k
884 BPF_ALU+BPF_MOD+BPF_K A <- A % k
885 BPF_ALU+BPF_AND+BPF_K A <- A & k
886 BPF_ALU+BPF_OR+BPF_K A <- A | k
887 BPF_ALU+BPF_XOR+BPF_K A <- A ^ k
888 BPF_ALU+BPF_LSH+BPF_K A <- A << k
889 BPF_ALU+BPF_RSH+BPF_K A <- A >> k
890 BPF_ALU+BPF_ADD+BPF_X A <- A + X
891 BPF_ALU+BPF_SUB+BPF_X A <- A - X
892 BPF_ALU+BPF_MUL+BPF_X A <- A * X
893 BPF_ALU+BPF_DIV+BPF_X A <- A / X
894 BPF_ALU+BPF_MOD+BPF_X A <- A % X
895 BPF_ALU+BPF_AND+BPF_X A <- A & X
896 BPF_ALU+BPF_OR+BPF_X A <- A | X
897 BPF_ALU+BPF_XOR+BPF_X A <- A ^ X
898 BPF_ALU+BPF_LSH+BPF_X A <- A << X
899 BPF_ALU+BPF_RSH+BPF_X A <- A >> X
900 BPF_ALU+BPF_NEG A <- -A
903 The jump instructions alter flow of control.
905 compare the accumulator against a constant
907 or the index register
909 If the result is true (or non-zero),
910 the true branch is taken, otherwise the false branch is taken.
911 Jump offsets are encoded in 8 bits so the longest jump is 256 instructions.
912 However, the jump always
914 opcode uses the 32 bit
916 field as the offset, allowing arbitrarily distant destinations.
917 All conditionals use unsigned comparison conventions.
919 BPF_JMP+BPF_JA pc += k
920 BPF_JMP+BPF_JGT+BPF_K pc += (A > k) ? jt : jf
921 BPF_JMP+BPF_JGE+BPF_K pc += (A >= k) ? jt : jf
922 BPF_JMP+BPF_JEQ+BPF_K pc += (A == k) ? jt : jf
923 BPF_JMP+BPF_JSET+BPF_K pc += (A & k) ? jt : jf
924 BPF_JMP+BPF_JGT+BPF_X pc += (A > X) ? jt : jf
925 BPF_JMP+BPF_JGE+BPF_X pc += (A >= X) ? jt : jf
926 BPF_JMP+BPF_JEQ+BPF_X pc += (A == X) ? jt : jf
927 BPF_JMP+BPF_JSET+BPF_X pc += (A & X) ? jt : jf
930 The return instructions terminate the filter program and specify the amount
931 of packet to accept (i.e., they return the truncation amount).
932 A return value of zero indicates that the packet should be ignored.
933 The return value is either a constant
938 BPF_RET+BPF_A accept A bytes
939 BPF_RET+BPF_K accept k bytes
942 The miscellaneous category was created for anything that does not
943 fit into the above classes, and for any new instructions that might need to
945 Currently, these are the register transfer instructions
946 that copy the index register to the accumulator or vice versa.
948 BPF_MISC+BPF_TAX X <- A
949 BPF_MISC+BPF_TXA A <- X
955 interface provides the following macros to facilitate
957 .Fn BPF_STMT opcode operand
959 .Fn BPF_JUMP opcode operand true_offset false_offset .
963 variables controls the behaviour of the
966 .Bl -tag -width indent
967 .It Va net.bpf.optimize_writers: No 0
968 Various programs use BPF to send (but not receive) raw packets
969 (cdpd, lldpd, dhcpd, dhcp relays, etc. are good examples of such programs).
970 They do not need incoming packets to be send to them.
971 Turning this option on
972 makes new BPF users to be attached to write-only interface list until program
973 explicitly specifies read filter via
974 .Fn pcap_set_filter .
975 This removes any performance degradation for high-speed interfaces.
976 .It Va net.bpf.stats:
977 Binary interface for retrieving general statistics.
978 .It Va net.bpf.zerocopy_enable: No 0
979 Permits zero-copy to be used with net BPF readers.
981 .It Va net.bpf.maxinsns: No 512
982 Maximum number of instructions that BPF program can contain.
986 option to determine approximate number of instruction for any filter.
987 .It Va net.bpf.maxbufsize: No 524288
988 Maximum buffer size to allocate for packets buffer.
989 .It Va net.bpf.bufsize: No 4096
990 Default buffer size to allocate for packets buffer.
993 The following filter is taken from the Reverse ARP Daemon.
994 It accepts only Reverse ARP requests.
996 struct bpf_insn insns[] = {
997 BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 12),
998 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, ETHERTYPE_REVARP, 0, 3),
999 BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 20),
1000 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, REVARP_REQUEST, 0, 1),
1001 BPF_STMT(BPF_RET+BPF_K, sizeof(struct ether_arp) +
1002 sizeof(struct ether_header)),
1003 BPF_STMT(BPF_RET+BPF_K, 0),
1007 This filter accepts only IP packets between host 128.3.112.15 and
1010 struct bpf_insn insns[] = {
1011 BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 12),
1012 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, ETHERTYPE_IP, 0, 8),
1013 BPF_STMT(BPF_LD+BPF_W+BPF_ABS, 26),
1014 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x8003700f, 0, 2),
1015 BPF_STMT(BPF_LD+BPF_W+BPF_ABS, 30),
1016 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x80037023, 3, 4),
1017 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x80037023, 0, 3),
1018 BPF_STMT(BPF_LD+BPF_W+BPF_ABS, 30),
1019 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x8003700f, 0, 1),
1020 BPF_STMT(BPF_RET+BPF_K, (u_int)-1),
1021 BPF_STMT(BPF_RET+BPF_K, 0),
1025 Finally, this filter returns only TCP finger packets.
1026 We must parse the IP header to reach the TCP header.
1030 checks that the IP fragment offset is 0 so we are sure
1031 that we have a TCP header.
1033 struct bpf_insn insns[] = {
1034 BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 12),
1035 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, ETHERTYPE_IP, 0, 10),
1036 BPF_STMT(BPF_LD+BPF_B+BPF_ABS, 23),
1037 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, IPPROTO_TCP, 0, 8),
1038 BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 20),
1039 BPF_JUMP(BPF_JMP+BPF_JSET+BPF_K, 0x1fff, 6, 0),
1040 BPF_STMT(BPF_LDX+BPF_B+BPF_MSH, 14),
1041 BPF_STMT(BPF_LD+BPF_H+BPF_IND, 14),
1042 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 79, 2, 0),
1043 BPF_STMT(BPF_LD+BPF_H+BPF_IND, 16),
1044 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 79, 0, 1),
1045 BPF_STMT(BPF_RET+BPF_K, (u_int)-1),
1046 BPF_STMT(BPF_RET+BPF_K, 0),
1061 .%T "An efficient, extensible, and portable network monitor"
1064 The Enet packet filter was created in 1980 by Mike Accetta and
1065 Rick Rashid at Carnegie-Mellon University.
1067 Stanford, ported the code to
1069 and continued its development from
1071 Since then, it has evolved into the Ultrix Packet Filter at
1082 .An Steven McCanne ,
1083 of Lawrence Berkeley Laboratory, implemented BPF in
1085 Much of the design is due to
1088 Support for zero-copy buffers was added by
1089 .An Robert N. M. Watson
1090 under contract to Seccuris Inc.
1092 The read buffer must be of a fixed size (returned by the
1096 A file that does not request promiscuous mode may receive promiscuously
1097 received packets as a side effect of another file requesting this
1098 mode on the same hardware interface.
1099 This could be fixed in the kernel with additional processing overhead.
1100 However, we favor the model where
1101 all files must assume that the interface is promiscuous, and if
1102 so desired, must utilize a filter to reject foreign packets.
1104 Data link protocols with variable length headers are not currently supported.
1111 settings have been observed to work incorrectly on some interface
1112 types, including those with hardware loopback rather than software loopback,
1113 and point-to-point interfaces.
1114 They appear to function correctly on a
1115 broad range of Ethernet-style interfaces.