1 .\" Copyright (c) 2007 Seccuris Inc.
2 .\" All rights reserved.
4 .\" This sofware was developed by Robert N. M. Watson under contract to
7 .\" Redistribution and use in source and binary forms, with or without
8 .\" modification, are permitted provided that the following conditions
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16 .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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24 .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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28 .\" Copyright (c) 1990 The Regents of the University of California.
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32 .\" modification, are permitted provided that: (1) source code distributions
33 .\" retain the above copyright notice and this paragraph in its entirety, (2)
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38 .\" ``This product includes software developed by the University of California,
39 .\" Lawrence Berkeley Laboratory and its contributors.'' Neither the name of
40 .\" the University nor the names of its contributors may be used to endorse
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43 .\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR IMPLIED
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47 .\" This document is derived in part from the enet man page (enet.4)
48 .\" distributed with 4.3BSD Unix.
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,
71 After opening the device, the file descriptor must be bound to a
72 specific network interface with the
75 A given interface can be shared by multiple listeners, and the filter
76 underlying each descriptor will see an identical packet stream.
78 A separate device file is required for each minor device.
79 If a file is in use, the open will fail and
84 Associated with each open instance of a
86 file is a user-settable packet filter.
87 Whenever a packet is received by an interface,
88 all file descriptors listening on that interface apply their filter.
89 Each descriptor that accepts the packet receives its own copy.
91 The packet filter will support any link level protocol that has fixed length
93 Currently, only Ethernet,
97 drivers have been modified to interact with
100 Since packet data is in network byte order, applications should use the
102 macros to extract multi-byte values.
104 A packet can be sent out on the network by writing to a
107 The writes are unbuffered, meaning only one packet can be processed per write.
108 Currently, only writes to Ethernets and
113 devices deliver packet data to the application via memory buffers provided by
115 The buffer mode is set using the
117 ioctl, and read using the
120 .Ss Buffered read mode
123 devices operate in the
124 .Dv BPF_BUFMODE_BUFFER
125 mode, in which packet data is copied explicitly from kernel to user memory
129 The user process will declare a fixed buffer size that will be used both for
130 sizing internal buffers and for all
132 operations on the file.
133 This size is queried using the
135 ioctl, and is set using the
138 Note that an individual packet larger than the buffer size is necessarily
140 .Ss Zero-copy buffer mode
142 devices may also operate in the
143 .Dv BPF_BUFMODE_ZEROCOPY
144 mode, in which packet data is written directly into two user memory buffers
145 by the kernel, avoiding both system call and copying overhead.
146 Buffers are of fixed (and equal) size, page-aligned, and an even multiple of
148 The maximum zero-copy buffer size is returned by the
151 Note that an individual packet larger than the buffer size is necessarily
154 The user process registers two memory buffers using the
156 ioctl, which accepts a
158 pointer as an argument:
168 is a pointer to the userspace address of the first buffer that will be
171 is a pointer to the second buffer.
173 will then cycle between the two buffers as they fill and are acknowledged.
175 Each buffer begins with a fixed-length header to hold synchronization and
176 data length information for the buffer:
178 struct bpf_zbuf_header {
179 volatile u_int bzh_kernel_gen; /* Kernel generation number. */
180 volatile u_int bzh_kernel_len; /* Length of data in the buffer. */
181 volatile u_int bzh_user_gen; /* User generation number. */
182 /* ...padding for future use... */
186 The header structure of each buffer, including all padding, should be zeroed
187 before it is configured using
189 Remaining space in the buffer will be used by the kernel to store packet
190 data, laid out in the same format as with buffered read mode.
192 The kernel and the user process follow a simple acknowledgement protocol via
193 the buffer header to synchronize access to the buffer: when the header
198 hold the same value, the kernel owns the buffer, and when they differ,
199 userspace owns the buffer.
201 While the kernel owns the buffer, the contents are unstable and may change
202 asynchronously; while the user process owns the buffer, its contents are
203 stable and will not be changed until the buffer has been acknowledged.
205 Initializing the buffer headers to all 0's before registering the buffer has
206 the effect of assigning initial ownership of both buffers to the kernel.
207 The kernel signals that a buffer has been assigned to userspace by modifying
209 and userspace acknowledges the buffer and returns it to the kernel by setting
215 In order to avoid caching and memory re-ordering effects, the user process
216 must use atomic operations and memory barriers when checking for and
217 acknowledging buffers:
219 #include <machine/atomic.h>
222 * Return ownership of a buffer to the kernel for reuse.
225 buffer_acknowledge(struct bpf_zbuf_header *bzh)
228 atomic_store_rel_int(&bzh->bzh_user_gen, bzh->bzh_kernel_gen);
232 * Check whether a buffer has been assigned to userspace by the kernel.
233 * Return true if userspace owns the buffer, and false otherwise.
236 buffer_check(struct bpf_zbuf_header *bzh)
239 return (bzh->bzh_user_gen !=
240 atomic_load_acq_int(&bzh->bzh_kernel_gen));
244 The user process may force the assignment of the next buffer, if any data
245 is pending, to userspace using the
248 This allows the user process to retrieve data in a partially filled buffer
249 before the buffer is full, such as following a timeout; the process must
250 recheck for buffer ownership using the header generation numbers, as the
251 buffer will not be assigned to userspace if no data was present.
253 As in the buffered read mode,
258 may be used to sleep awaiting the availbility of a completed buffer.
259 They will return a readable file descriptor when ownership of the next buffer
260 is assigned to user space.
262 In the current implementation, the kernel may assign zero, one, or both
263 buffers to the user process; however, an earlier implementation maintained
264 the invariant that at most one buffer could be assigned to the user process
266 In order to both ensure progress and high performance, user processes should
267 acknowledge a completely processed buffer as quickly as possible, returning
268 it for reuse, and not block waiting on a second buffer while holding another
273 command codes below are defined in
278 #include <sys/types.h>
279 #include <sys/time.h>
280 #include <sys/ioctl.h>
297 the following commands may be applied to any open
300 The (third) argument to
302 should be a pointer to the type indicated.
303 .Bl -tag -width BIOCGETBUFMODE
306 Returns the required buffer length for reads on
311 Sets the buffer length for reads on
314 The buffer must be set before the file is attached to an interface
317 If the requested buffer size cannot be accommodated, the closest
318 allowable size will be set and returned in the argument.
319 A read call will result in
321 if it is passed a buffer that is not this size.
324 Returns the type of the data link layer underlying the attached interface.
326 is returned if no interface has been specified.
327 The device types, prefixed with
332 Forces the interface into promiscuous mode.
333 All packets, not just those destined for the local host, are processed.
334 Since more than one file can be listening on a given interface,
335 a listener that opened its interface non-promiscuously may receive
336 packets promiscuously.
337 This problem can be remedied with an appropriate filter.
339 Flushes the buffer of incoming packets,
340 and resets the statistics that are returned by BIOCGSTATS.
342 .Pq Li "struct ifreq"
343 Returns the name of the hardware interface that the file is listening on.
344 The name is returned in the ifr_name field of
348 All other fields are undefined.
350 .Pq Li "struct ifreq"
351 Sets the hardware interface associate with the file.
353 command must be performed before any packets can be read.
354 The device is indicated by name using the
359 Additionally, performs the actions of
363 .Pq Li "struct timeval"
364 Set or get the read timeout parameter.
366 specifies the length of time to wait before timing
367 out on a read request.
368 This parameter is initialized to zero by
370 indicating no timeout.
372 .Pq Li "struct bpf_stat"
373 Returns the following structure of packet statistics:
376 u_int bs_recv; /* number of packets received */
377 u_int bs_drop; /* number of packets dropped */
382 .Bl -hang -offset indent
384 the number of packets received by the descriptor since opened or reset
385 (including any buffered since the last read call);
388 the number of packets which were accepted by the filter but dropped by the
389 kernel because of buffer overflows
390 (i.e., the application's reads are not keeping up with the packet traffic).
396 based on the truth value of the argument.
397 When immediate mode is enabled, reads return immediately upon packet
399 Otherwise, a read will block until either the kernel buffer
400 becomes full or a timeout occurs.
401 This is useful for programs like
403 which must respond to messages in real time.
404 The default for a new file is off.
406 .Pq Li "struct bpf_program"
407 Sets the read filter program used by the kernel to discard uninteresting
409 An array of instructions and its length is passed in using
410 the following structure:
414 struct bpf_insn *bf_insns;
418 The filter program is pointed to by the
420 field while its length in units of
421 .Sq Li struct bpf_insn
430 for an explanation of the filter language.
432 .Pq Li "struct bpf_program"
433 Sets the write filter program used by the kernel to control what type of
434 packets can be written to the interface.
442 .Pq Li "struct bpf_version"
443 Returns the major and minor version numbers of the filter language currently
444 recognized by the kernel.
445 Before installing a filter, applications must check
446 that the current version is compatible with the running kernel.
447 Version numbers are compatible if the major numbers match and the application minor
448 is less than or equal to the kernel minor.
449 The kernel version number is returned in the following structure:
457 The current version numbers are given by
458 .Dv BPF_MAJOR_VERSION
460 .Dv BPF_MINOR_VERSION
463 An incompatible filter
464 may result in undefined behavior (most likely, an error returned by
466 or haphazard packet matching).
470 Set or get the status of the
473 Set to zero if the link level source address should be filled in automatically
474 by the interface output routine.
475 Set to one if the link level source
476 address will be written, as provided, to the wire.
477 This flag is initialized to zero by default.
481 These commands are obsolete but left for compatibility.
487 Set or get the flag determining whether locally generated packets on the
488 interface should be returned by BPF.
489 Set to zero to see only incoming packets on the interface.
490 Set to one to see packets originating locally and remotely on the interface.
491 This flag is initialized to one by default.
492 .It Dv BIOCSDIRECTION
493 .It Dv BIOCGDIRECTION
495 Set or get the setting determining whether incoming, outgoing, or all packets
496 on the interface should be returned by BPF.
499 to see only incoming packets on the interface.
502 to see packets originating locally and remotely on the interface.
505 to see only outgoing packets on the interface.
506 This setting is initialized to
511 Set packet feedback mode.
512 This allows injected packets to be fed back as input to the interface when
513 output via the interface is successful.
516 direction is set, injected outgoing packet is not returned by BPF to avoid
517 duplication. This flag is initialized to zero by default.
519 Set the locked flag on the
522 This prevents the execution of
523 ioctl commands which could change the underlying operating parameters of
525 .It Dv BIOCGETBUFMODE
526 .It Dv BIOCSETBUFMODE
528 Get or set the current
530 buffering mode; possible values are
531 .Dv BPF_BUFMODE_BUFFER ,
532 buffered read mode, and
533 .Dv BPF_BUFMODE_ZBUF ,
534 zero-copy buffer mode.
536 .Pq Li struct bpf_zbuf
537 Set the current zero-copy buffer locations; buffer locations may be
538 set only once zero-copy buffer mode has been selected, and prior to attaching
540 Buffers must be of identical size, page-aligned, and an integer multiple of
548 If buffers have already been set for this device, the ioctl will fail.
551 Get the largest individual zero-copy buffer size allowed.
552 As two buffers are used in zero-copy buffer mode, the limit (in practice) is
553 twice the returned size.
554 As zero-copy buffers consume kernel address space, conservative selection of
555 buffer size is suggested, especially when there are multiple
557 descriptors in use on 32-bit systems.
559 Force ownership of the next buffer to be assigned to userspace, if any data
560 present in the buffer.
561 If no data is present, the buffer will remain owned by the kernel.
562 This allows consumers of zero-copy buffering to implement timeouts and
563 retrieve partially filled buffers.
564 In order to handle the case where no data is present in the buffer and
565 therefore ownership is not assigned, the user process must check
571 The following structure is prepended to each packet returned by
573 or via a zero-copy buffer:
576 struct timeval bh_tstamp; /* time stamp */
577 u_long bh_caplen; /* length of captured portion */
578 u_long bh_datalen; /* original length of packet */
579 u_short bh_hdrlen; /* length of bpf header (this struct
580 plus alignment padding */
584 The fields, whose values are stored in host order, and are:
586 .Bl -tag -compact -width bh_datalen
588 The time at which the packet was processed by the packet filter.
590 The length of the captured portion of the packet.
591 This is the minimum of
592 the truncation amount specified by the filter and the length of the packet.
594 The length of the packet off the wire.
595 This value is independent of the truncation amount specified by the filter.
599 header, which may not be equal to
600 .\" XXX - not really a function call
601 .Fn sizeof "struct bpf_hdr" .
606 field exists to account for
607 padding between the header and the link level protocol.
608 The purpose here is to guarantee proper alignment of the packet
609 data structures, which is required on alignment sensitive
610 architectures and improves performance on many other architectures.
611 The packet filter insures that the
613 and the network layer
614 header will be word aligned.
616 must be taken when accessing the link layer protocol fields on alignment
618 (This is not a problem on an Ethernet, since
619 the type field is a short falling on an even offset,
620 and the addresses are probably accessed in a bytewise fashion).
622 Additionally, individual packets are padded so that each starts
624 This requires that an application
625 has some knowledge of how to get from packet to packet.
632 It rounds up its argument to the nearest word aligned value (where a word is
638 points to the start of a packet, this expression
639 will advance it to the next packet:
640 .Dl p = (char *)p + BPF_WORDALIGN(p->bh_hdrlen + p->bh_caplen)
642 For the alignment mechanisms to work properly, the
645 must itself be word aligned.
649 will always return an aligned buffer.
651 A filter program is an array of instructions, with all branches forwardly
652 directed, terminated by a
655 Each instruction performs some action on the pseudo-machine state,
656 which consists of an accumulator, index register, scratch memory store,
657 and implicit program counter.
659 The following structure defines the instruction format:
671 field is used in different ways by different instructions,
676 fields are used as offsets
677 by the branch instructions.
678 The opcodes are encoded in a semi-hierarchical fashion.
679 There are eight classes of instructions:
689 Various other mode and
690 operator bits are or'd into the class to give the actual instructions.
691 The classes and modes are defined in
694 Below are the semantics for each defined
697 We use the convention that A is the accumulator, X is the index register,
698 P[] packet data, and M[] scratch memory store.
699 P[i:n] gives the data at byte offset
702 interpreted as a word (n=4),
703 unsigned halfword (n=2), or unsigned byte (n=1).
704 M[i] gives the i'th word in the scratch memory store, which is only
705 addressed in word units.
706 The memory store is indexed from 0 to
713 are the corresponding fields in the
714 instruction definition.
716 refers to the length of the packet.
718 .Bl -tag -width BPF_STXx
720 These instructions copy a value into the accumulator.
721 The type of the source operand is specified by an
723 and can be a constant
725 packet data at a fixed offset
727 packet data at a variable offset
731 or a word in the scratch memory store
737 the data size must be specified as a word
743 The semantics of all the recognized
748 BPF_LD+BPF_W+BPF_ABS A <- P[k:4]
749 BPF_LD+BPF_H+BPF_ABS A <- P[k:2]
750 BPF_LD+BPF_B+BPF_ABS A <- P[k:1]
751 BPF_LD+BPF_W+BPF_IND A <- P[X+k:4]
752 BPF_LD+BPF_H+BPF_IND A <- P[X+k:2]
753 BPF_LD+BPF_B+BPF_IND A <- P[X+k:1]
754 BPF_LD+BPF_W+BPF_LEN A <- len
755 BPF_LD+BPF_IMM A <- k
756 BPF_LD+BPF_MEM A <- M[k]
759 These instructions load a value into the index register.
761 the addressing modes are more restrictive than those of the accumulator loads,
764 a hack for efficiently loading the IP header length.
767 BPF_LDX+BPF_W+BPF_IMM X <- k
768 BPF_LDX+BPF_W+BPF_MEM X <- M[k]
769 BPF_LDX+BPF_W+BPF_LEN X <- len
770 BPF_LDX+BPF_B+BPF_MSH X <- 4*(P[k:1]&0xf)
773 This instruction stores the accumulator into the scratch memory.
774 We do not need an addressing mode since there is only one possibility
781 This instruction stores the index register in the scratch memory store.
787 The alu instructions perform operations between the accumulator and
788 index register or constant, and store the result back in the accumulator.
789 For binary operations, a source mode is required
795 BPF_ALU+BPF_ADD+BPF_K A <- A + k
796 BPF_ALU+BPF_SUB+BPF_K A <- A - k
797 BPF_ALU+BPF_MUL+BPF_K A <- A * k
798 BPF_ALU+BPF_DIV+BPF_K A <- A / k
799 BPF_ALU+BPF_AND+BPF_K A <- A & k
800 BPF_ALU+BPF_OR+BPF_K A <- A | k
801 BPF_ALU+BPF_LSH+BPF_K A <- A << k
802 BPF_ALU+BPF_RSH+BPF_K A <- A >> k
803 BPF_ALU+BPF_ADD+BPF_X A <- A + X
804 BPF_ALU+BPF_SUB+BPF_X A <- A - X
805 BPF_ALU+BPF_MUL+BPF_X A <- A * X
806 BPF_ALU+BPF_DIV+BPF_X A <- A / X
807 BPF_ALU+BPF_AND+BPF_X A <- A & X
808 BPF_ALU+BPF_OR+BPF_X A <- A | X
809 BPF_ALU+BPF_LSH+BPF_X A <- A << X
810 BPF_ALU+BPF_RSH+BPF_X A <- A >> X
811 BPF_ALU+BPF_NEG A <- -A
814 The jump instructions alter flow of control.
816 compare the accumulator against a constant
818 or the index register
820 If the result is true (or non-zero),
821 the true branch is taken, otherwise the false branch is taken.
822 Jump offsets are encoded in 8 bits so the longest jump is 256 instructions.
823 However, the jump always
825 opcode uses the 32 bit
827 field as the offset, allowing arbitrarily distant destinations.
828 All conditionals use unsigned comparison conventions.
831 BPF_JMP+BPF_JA pc += k
832 BPF_JMP+BPF_JGT+BPF_K pc += (A > k) ? jt : jf
833 BPF_JMP+BPF_JGE+BPF_K pc += (A >= k) ? jt : jf
834 BPF_JMP+BPF_JEQ+BPF_K pc += (A == k) ? jt : jf
835 BPF_JMP+BPF_JSET+BPF_K pc += (A & k) ? jt : jf
836 BPF_JMP+BPF_JGT+BPF_X pc += (A > X) ? jt : jf
837 BPF_JMP+BPF_JGE+BPF_X pc += (A >= X) ? jt : jf
838 BPF_JMP+BPF_JEQ+BPF_X pc += (A == X) ? jt : jf
839 BPF_JMP+BPF_JSET+BPF_X pc += (A & X) ? jt : jf
842 The return instructions terminate the filter program and specify the amount
843 of packet to accept (i.e., they return the truncation amount).
844 A return value of zero indicates that the packet should be ignored.
845 The return value is either a constant
851 BPF_RET+BPF_A accept A bytes
852 BPF_RET+BPF_K accept k bytes
855 The miscellaneous category was created for anything that does not
856 fit into the above classes, and for any new instructions that might need to
858 Currently, these are the register transfer instructions
859 that copy the index register to the accumulator or vice versa.
862 BPF_MISC+BPF_TAX X <- A
863 BPF_MISC+BPF_TXA A <- X
869 interface provides the following macros to facilitate
871 .Fn BPF_STMT opcode operand
873 .Fn BPF_JUMP opcode operand true_offset false_offset .
875 .Bl -tag -compact -width /dev/bpfXXX
876 .It Pa /dev/bpf Ns Sy n
877 the packet filter device
880 The following filter is taken from the Reverse ARP Daemon.
881 It accepts only Reverse ARP requests.
883 struct bpf_insn insns[] = {
884 BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 12),
885 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, ETHERTYPE_REVARP, 0, 3),
886 BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 20),
887 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, REVARP_REQUEST, 0, 1),
888 BPF_STMT(BPF_RET+BPF_K, sizeof(struct ether_arp) +
889 sizeof(struct ether_header)),
890 BPF_STMT(BPF_RET+BPF_K, 0),
894 This filter accepts only IP packets between host 128.3.112.15 and
897 struct bpf_insn insns[] = {
898 BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 12),
899 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, ETHERTYPE_IP, 0, 8),
900 BPF_STMT(BPF_LD+BPF_W+BPF_ABS, 26),
901 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x8003700f, 0, 2),
902 BPF_STMT(BPF_LD+BPF_W+BPF_ABS, 30),
903 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x80037023, 3, 4),
904 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x80037023, 0, 3),
905 BPF_STMT(BPF_LD+BPF_W+BPF_ABS, 30),
906 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 0x8003700f, 0, 1),
907 BPF_STMT(BPF_RET+BPF_K, (u_int)-1),
908 BPF_STMT(BPF_RET+BPF_K, 0),
912 Finally, this filter returns only TCP finger packets.
913 We must parse the IP header to reach the TCP header.
917 checks that the IP fragment offset is 0 so we are sure
918 that we have a TCP header.
920 struct bpf_insn insns[] = {
921 BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 12),
922 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, ETHERTYPE_IP, 0, 10),
923 BPF_STMT(BPF_LD+BPF_B+BPF_ABS, 23),
924 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, IPPROTO_TCP, 0, 8),
925 BPF_STMT(BPF_LD+BPF_H+BPF_ABS, 20),
926 BPF_JUMP(BPF_JMP+BPF_JSET+BPF_K, 0x1fff, 6, 0),
927 BPF_STMT(BPF_LDX+BPF_B+BPF_MSH, 14),
928 BPF_STMT(BPF_LD+BPF_H+BPF_IND, 14),
929 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 79, 2, 0),
930 BPF_STMT(BPF_LD+BPF_H+BPF_IND, 16),
931 BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, 79, 0, 1),
932 BPF_STMT(BPF_RET+BPF_K, (u_int)-1),
933 BPF_STMT(BPF_RET+BPF_K, 0),
948 .%T "An efficient, extensible, and portable network monitor"
951 The Enet packet filter was created in 1980 by Mike Accetta and
952 Rick Rashid at Carnegie-Mellon University.
954 Stanford, ported the code to
956 and continued its development from
958 Since then, it has evolved into the Ultrix Packet Filter at
970 of Lawrence Berkeley Laboratory, implemented BPF in
972 Much of the design is due to
975 Support for zero-copy buffers was added by
976 .An Robert N. M. Watson
977 under contract to Seccuris Inc.
979 The read buffer must be of a fixed size (returned by the
983 A file that does not request promiscuous mode may receive promiscuously
984 received packets as a side effect of another file requesting this
985 mode on the same hardware interface.
986 This could be fixed in the kernel with additional processing overhead.
987 However, we favor the model where
988 all files must assume that the interface is promiscuous, and if
989 so desired, must utilize a filter to reject foreign packets.
991 Data link protocols with variable length headers are not currently supported.
998 settings have been observed to work incorrectly on some interface
999 types, including those with hardware loopback rather than software loopback,
1000 and point-to-point interfaces.
1001 They appear to function correctly on a
1002 broad range of Ethernet-style interfaces.