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20 .TH PCAP 3PCAP "20 January 2017"
22 pcap \- Packet Capture library
26 #include <pcap/pcap.h>
32 The Packet Capture library
33 provides a high level interface to packet capture systems. All packets
34 on the network, even those destined for other hosts, are accessible
35 through this mechanism.
36 It also supports saving captured packets to a ``savefile'', and reading
37 packets from a ``savefile''.
38 .SS Opening a capture handle for reading
39 To open a handle for a live capture, given the name of the network or
40 other interface on which the capture should be done, call
42 set the appropriate options on the handle, and then activate it with
45 To obtain a list of devices that can be opened for a live capture, call
46 .BR pcap_findalldevs ();
47 to free the list returned by
48 .BR pcap_findalldevs (),
50 .BR pcap_freealldevs ().
52 will return the first device on that list that is not a ``loopback``
55 To open a handle for a ``savefile'' from which to read packets, given the
56 pathname of the ``savefile'', call
57 .BR pcap_open_offline ();
58 to set up a handle for a ``savefile'', given a
60 referring to a file already opened for reading, call
61 .BR pcap_fopen_offline ().
63 In order to get a ``fake''
65 for use in routines that require a
67 as an argument, such as routines to open a ``savefile'' for writing and
68 to compile a filter expression, call
69 .BR pcap_open_dead ().
72 .BR pcap_open_offline (),
73 .BR pcap_fopen_offline (),
78 which is the handle used for reading packets from the capture stream or
79 the ``savefile'', and for finding out information about the capture
80 stream or ``savefile''.
81 To close a handle, use
84 The options that can be set on a capture handle include
86 If, when capturing, you capture the entire contents of the packet, that
87 requires more CPU time to copy the packet to your application, more disk
88 and possibly network bandwidth to write the packet data to a file, and
89 more disk space to save the packet. If you don't need the entire
90 contents of the packet - for example, if you are only interested in the
91 TCP headers of packets - you can set the "snapshot length" for the
92 capture to an appropriate value. If the snapshot length is set to
97 than the size of a packet that is captured, only the first
99 bytes of that packet will be captured and provided as packet data.
101 A snapshot length of 65535 should be sufficient, on most if not all
102 networks, to capture all the data available from the packet.
104 The snapshot length is set with
105 .BR pcap_set_snaplen ().
106 .IP "promiscuous mode"
107 On broadcast LANs such as Ethernet, if the network isn't switched, or if
108 the adapter is connected to a "mirror port" on a switch to which all
109 packets passing through the switch are sent, a network adapter receives
110 all packets on the LAN, including unicast or multicast packets not sent
111 to a network address that the network adapter isn't configured to
114 Normally, the adapter will discard those packets; however, many network
115 adapters support "promiscuous mode", which is a mode in which all
116 packets, even if they are not sent to an address that the adapter
117 recognizes, are provided to the host. This is useful for passively
118 capturing traffic between two or more other hosts for analysis.
120 Note that even if an application does not set promiscuous mode, the
121 adapter could well be in promiscuous mode for some other reason.
123 For now, this doesn't work on the "any" device; if an argument of "any"
124 or NULL is supplied, the setting of promiscuous mode is ignored.
126 Promiscuous mode is set with
127 .BR pcap_set_promisc ().
129 On IEEE 802.11 wireless LANs, even if an adapter is in promiscuous mode,
130 it will supply to the host only frames for the network with which it's
131 associated. It might also supply only data frames, not management or
132 control frames, and might not provide the 802.11 header or radio
133 information pseudo-header for those frames.
135 In "monitor mode", sometimes also called "rfmon mode" (for "Radio
136 Frequency MONitor"), the adapter will supply all frames that it
137 receives, with 802.11 headers, and might supply a pseudo-header with
138 radio information about the frame as well.
140 Note that in monitor mode the adapter might disassociate from the
141 network with which it's associated, so that you will not be able to use
142 any wireless networks with that adapter. This could prevent accessing
143 files on a network server, or resolving host names or network addresses,
144 if you are capturing in monitor mode and are not connected to another
145 network with another adapter.
147 Monitor mode is set with
148 .BR pcap_set_rfmon (),
150 .BR pcap_can_set_rfmon ()
151 can be used to determine whether an adapter can be put into monitor
153 .IP "packet buffer timeout"
154 If, when capturing, packets are delivered as soon as they arrive, the
155 application capturing the packets will be woken up for each packet as it
156 arrives, and might have to make one or more calls to the operating
157 system to fetch each packet.
159 If, instead, packets are not delivered as soon as they arrive, but are
160 delivered after a short delay (called a "packet buffer timeout"), more
161 than one packet can be accumulated before the packets are delivered, so
162 that a single wakeup would be done for multiple packets, and each set of
163 calls made to the operating system would supply multiple packets, rather
164 than a single packet. This reduces the per-packet CPU overhead if
165 packets are arriving at a high rate, increasing the number of packets
166 per second that can be captured.
168 The packet buffer timeout is required so that an application won't wait
169 for the operating system's capture buffer to fill up before packets are
170 delivered; if packets are arriving slowly, that wait could take an
171 arbitrarily long period of time.
173 Not all platforms support a packet buffer timeout; on platforms that
174 don't, the packet buffer timeout is ignored. A zero value for the
175 timeout, on platforms that support a packet buffer timeout, will cause a
176 read to wait forever to allow enough packets to arrive, with no timeout.
179 the packet buffer timeout cannot be used to cause calls that read
180 packets to return within a limited period of time, because, on some
181 platforms, the packet buffer timeout isn't supported, and, on other
182 platforms, the timer doesn't start until at least one packet arrives.
183 This means that the packet buffer timeout should
185 be used, for example, in an interactive application to allow the packet
186 capture loop to ``poll'' for user input periodically, as there's no
187 guarantee that a call reading packets will return after the timeout
188 expires even if no packets have arrived.
190 The packet buffer timeout is set with
191 .BR pcap_set_timeout ().
193 Packets that arrive for a capture are stored in a buffer, so that they
194 do not have to be read by the application as soon as they arrive. On
195 some platforms, the buffer's size can be set; a size that's too small
196 could mean that, if too many packets are being captured and the snapshot
197 length doesn't limit the amount of data that's buffered, packets could
198 be dropped if the buffer fills up before the application can read
199 packets from it, while a size that's too large could use more
200 non-pageable operating system memory than is necessary to prevent
201 packets from being dropped.
203 The buffer size is set with
204 .BR pcap_set_buffer_size ().
206 On some platforms, the time stamp given to packets on live captures can
207 come from different sources that can have different resolutions or that
208 can have different relationships to the time values for the current time
209 supplied by routines on the native operating system. See
210 .BR pcap-tstamp (@MAN_MISC_INFO@)
211 for a list of time stamp types.
213 The time stamp type is set with
214 .BR pcap_set_tstamp_type ().
216 Reading packets from a network interface may require that you have
219 .B Under SunOS 3.x or 4.x with NIT or BPF:
220 You must have read access to
225 .B Under Solaris with DLPI:
226 You must have read/write access to the network pseudo device, e.g.
228 On at least some versions of Solaris, however, this is not sufficient to
231 to capture in promiscuous mode; on those versions of Solaris, you must
232 be root, or the application capturing packets
233 must be installed setuid to root, in order to capture in promiscuous
234 mode. Note that, on many (perhaps all) interfaces, if you don't capture
235 in promiscuous mode, you will not see any outgoing packets, so a capture
236 not done in promiscuous mode may not be very useful.
238 In newer versions of Solaris, you must have been given the
240 privilege; this is both necessary and sufficient to give you access to the
241 network pseudo-device - there is no need to change the privileges on
242 that device. A user can be given that privilege by, for example, adding
243 that privilege to the user's
246 .B usermod (@MAN_USERMOD_SECTION@)
249 .B Under HP-UX with DLPI:
250 You must be root or the application capturing packets must be installed
253 .B Under IRIX with snoop:
254 You must be root or the application capturing packets must be installed
258 You must be root or the application capturing packets must be installed
259 setuid to root (unless your distribution has a kernel
260 that supports capability bits such as CAP_NET_RAW and code to allow
261 those capability bits to be given to particular accounts and to cause
262 those bits to be set on a user's initial processes when they log in, in
263 which case you must have CAP_NET_RAW in order to capture and
264 CAP_NET_ADMIN to enumerate network devices with, for example, the
268 .B Under ULTRIX and Digital UNIX/Tru64 UNIX:
269 Any user may capture network traffic.
270 However, no user (not even the super-user) can capture in promiscuous
271 mode on an interface unless the super-user has enabled promiscuous-mode
272 operation on that interface using
274 and no user (not even the super-user) can capture unicast traffic
275 received by or sent by the machine on an interface unless the super-user
276 has enabled copy-all-mode operation on that interface using
280 packet capture on an interface probably requires that either
281 promiscuous-mode or copy-all-mode operation, or both modes of
282 operation, be enabled on that interface.
284 .B Under BSD (this includes Mac OS X):
285 You must have read access to
287 on systems that don't have a cloning BPF device, or to
290 On BSDs with a devfs (this includes Mac OS X), this might involve more
291 than just having somebody with super-user access setting the ownership
292 or permissions on the BPF devices - it might involve configuring devfs
293 to set the ownership or permissions every time the system is booted,
294 if the system even supports that; if it doesn't support that, you might
295 have to find some other way to make that happen at boot time.
297 Reading a saved packet file doesn't require special privileges.
299 The packets read from the handle may include a ``pseudo-header''
300 containing various forms of packet meta-data, and probably includes a
301 link-layer header whose contents can differ for different network
302 interfaces. To determine the format of the packets supplied by the
304 .BR pcap_datalink ();
305 .I http://www.tcpdump.org/linktypes.html
306 lists the values it returns and describes the packet formats that
307 correspond to those values.
311 assume that the packets for a given capture or ``savefile`` will have
312 any given link-layer header type, such as
314 for Ethernet. For example, the "any" device on Linux will have a
315 link-layer header type of
317 even if all devices on the system at the time the "any" device is opened
318 have some other data link type, such as
326 opened for a ``savefile'', call
332 .BR pcap_create (3PCAP)
337 .BR pcap_activate (3PCAP)
342 .BR pcap_findalldevs (3PCAP)
343 get a list of devices that can be opened for a live capture
345 .BR pcap_freealldevs (3PCAP)
348 .BR pcap_lookupdev (3PCAP)
349 get first non-loopback device on that list
351 .BR pcap_open_offline (3PCAP)
354 for a ``savefile'', given a pathname
356 .BR pcap_open_offline_with_tstamp_precision (3PCAP)
359 for a ``savefile'', given a pathname, and specify the precision to
360 provide for packet time stamps
362 .BR pcap_fopen_offline (3PCAP)
365 for a ``savefile'', given a
368 .BR pcap_fopen_offline_with_tstamp_precision (3PCAP)
371 for a ``savefile'', given a
373 and specify the precision to provide for packet time stamps
375 .BR pcap_open_dead (3PCAP)
379 .BR pcap_close (3PCAP)
383 .BR pcap_set_snaplen (3PCAP)
384 set the snapshot length for a not-yet-activated
388 .BR pcap_snapshot (3PCAP)
389 get the snapshot length for a
392 .BR pcap_set_promisc (3PCAP)
393 set promiscuous mode for a not-yet-activated
397 .BR pcap_set_rfmon (3PCAP)
398 set monitor mode for a not-yet-activated
402 .BR pcap_can_set_rfmon (3PCAP)
403 determine whether monitor mode can be set for a
407 .BR pcap_set_timeout (3PCAP)
408 set packet buffer timeout for a not-yet-activated
412 .BR pcap_set_buffer_size (3PCAP)
413 set buffer size for a not-yet-activated
417 .BR pcap_set_tstamp_type (3PCAP)
418 set time stamp type for a not-yet-activated
422 .BR pcap_list_tstamp_types (3PCAP)
423 get list of available time stamp types for a not-yet-activated
427 .BR pcap_free_tstamp_types (3PCAP)
428 free list of available time stamp types
430 .BR pcap_tstamp_type_val_to_name (3PCAP)
431 get name for a time stamp type
433 .BR pcap_tstamp_type_val_to_description (3PCAP)
434 get description for a time stamp type
436 .BR pcap_tstamp_type_name_to_val (3PCAP)
437 get time stamp type corresponding to a name
439 .BR pcap_set_tstamp_precision (3PCAP)
440 set time stamp precision for a not-yet-activated
444 .BR pcap_get_tstamp_precision (3PCAP)
445 get the time stamp precision of a
449 .BR pcap_datalink (3PCAP)
450 get link-layer header type for a
453 .BR pcap_file (3PCAP)
458 opened for a ``savefile''
460 .BR pcap_is_swapped (3PCAP)
461 determine whether a ``savefile'' being read came from a machine with the
464 .BR pcap_major_version (3PCAP)
467 .BR pcap_minor_version (3PCAP)
468 get the major and minor version of the file format version for a
472 .SS Selecting a link-layer header type for a live capture
473 Some devices may provide more than one link-layer header type. To
474 obtain a list of all link-layer header types provided by a device, call
475 .BR pcap_list_datalinks ()
479 To free a list of link-layer header types, call
480 .BR pcap_free_datalinks ().
481 To set the link-layer header type for a device, call
482 .BR pcap_set_datalink ().
483 This should be done after the device has been activated but before any
484 packets are read and before any filters are compiled or installed.
489 .BR pcap_list_datalinks (3PCAP)
490 get a list of link-layer header types for a device
492 .BR pcap_free_datalinks (3PCAP)
493 free list of link-layer header types
495 .BR pcap_set_datalink (3PCAP)
496 set link-layer header type for a device
498 .BR pcap_datalink_val_to_name (3PCAP)
499 get name for a link-layer header type
501 .BR pcap_datalink_val_to_description (3PCAP)
502 get description for a link-layer header type
504 .BR pcap_datalink_name_to_val (3PCAP)
505 get link-layer header type corresponding to a name
508 Packets are read with
512 which process one or more packets, calling a callback routine for each
517 which return the next packet.
522 is supplied a pointer to a
523 .IR "struct pcap_pkthdr" ,
524 which includes the following members:
530 containing the time when the packet was captured
535 giving the number of bytes of the packet that are available from the
541 giving the length of the packet, in bytes (which might be more than the
542 number of bytes available from the capture, if the length of the packet
543 is larger than the maximum number of bytes to capture).
546 The callback is also supplied a
551 .I struct pcap_pkthdr
553 bytes of data from the packet. This won't necessarily be the entire
554 packet; to capture the entire packet, you will have to provide a value
558 .BR pcap_set_snaplen ()
559 that is sufficiently large to get all of the packet's data - a value of
560 65535 should be sufficient on most if not all networks). When reading
561 from a ``savefile'', the snapshot length specified when the capture was
562 performed will limit the amount of packet data available.
565 is passed an argument that points to a
566 .I struct pcap_pkthdr
567 structure, and fills it in with the time stamp and length values for the
572 bytes of the packet on success, and NULL on error.
575 is passed two pointer arguments, one of which points to a
576 .IR struct pcap_pkthdr *
577 and one of which points to a
578 .IR "const u_char" *.
579 It sets the first pointer to point to a
580 .I struct pcap_pkthdr
581 structure with the time stamp and length values for the packet, and sets
582 the second pointer to point to the first
591 .BR pcap_breakloop ().
593 By default, when reading packets from an interface opened for a live
595 .BR pcap_dispatch (),
599 will, if no packets are currently available to be read, block waiting
600 for packets to become available. On some, but
602 all, platforms, if a packet buffer timeout was specified, the wait will
603 terminate after the packet buffer timeout expires; applications should
604 be prepared for this, as it happens on some platforms, but should not
605 rely on it, as it does not happen on other platforms. Note that the
606 wait might, or might not, terminate even if no packets are available;
607 applications should be prepared for this to happen, but must not rely on
610 A handle can be put into ``non-blocking mode'', so that those routines
611 will, rather than blocking, return an indication that no packets are
612 available to read. Call
613 .BR pcap_setnonblock ()
614 to put a handle into non-blocking mode or to take it out of non-blocking
616 .BR pcap_getnonblock ()
617 to determine whether a handle is in non-blocking mode. Note that
618 non-blocking mode does not work correctly in Mac OS X 10.6.
620 Non-blocking mode is often combined with routines such as
624 or other routines a platform offers to wait for any of a set of
625 descriptors to be ready to read. To obtain, for a handle, a descriptor
626 that can be used in those routines, call
627 .BR pcap_get_selectable_fd ().
628 Not all handles have such a descriptor available;
629 .BR pcap_get_selectable_fd ()
630 will return \-1 if no such descriptor exists. In addition, for various
631 reasons, one or more of those routines will not work properly with the
632 descriptor; the documentation for
633 .BR pcap_get_selectable_fd ()
634 gives details. Note that, just as an attempt to read packets from a
636 may not return any packets if the packet buffer timeout expires, a
639 or other such call may, if the packet buffer timeout expires, indicate
640 that a descriptor is ready to read even if there are no packets
646 .BR pcap_dispatch (3PCAP)
647 read a bufferful of packets from a
649 open for a live capture or the full set of packets from a
651 open for a ``savefile''
653 .BR pcap_loop (3PCAP)
656 until an interrupt or error occurs
658 .BR pcap_next (3PCAP)
659 read the next packet from a
661 without an indication whether an error occurred
663 .BR pcap_next_ex (3PCAP)
664 read the next packet from a
666 with an error indication on an error
668 .BR pcap_breakloop (3PCAP)
669 prematurely terminate the loop in
674 .BR pcap_setnonblock (3PCAP)
675 set or clear non-blocking mode on a
678 .BR pcap_getnonblock (3PCAP)
679 get the state of non-blocking mode for a
682 .BR pcap_get_selectable_fd (3PCAP)
683 attempt to get a descriptor for a
685 that can be used in calls such as
691 In order to cause only certain packets to be returned when reading
692 packets, a filter can be set on a handle. For a live capture, the
693 filtering will be performed in kernel mode, if possible, to avoid
694 copying ``uninteresting'' packets from the kernel to user mode.
696 A filter can be specified as a text string; the syntax and semantics of
697 the string are as described by
698 .BR pcap-filter (@MAN_MISC_INFO@).
699 A filter string is compiled into a program in a pseudo-machine-language
702 and the resulting program can be made a filter for a handle with
703 .BR pcap_setfilter ().
706 can be freed with a call to
707 .BR pcap_freecode ().
709 may require a network mask for certain expressions in the filter string;
710 .BR pcap_lookupnet ()
711 can be used to find the network address and network mask for a given
714 A compiled filter can also be applied directly to a packet that has been
716 .BR pcap_offline_filter ().
721 .BR pcap_compile (3PCAP)
722 compile filter expression to a pseudo-machine-language code program
724 .BR pcap_freecode (3PCAP)
725 free a filter program
727 .BR pcap_setfilter (3PCAP)
731 .BR pcap_lookupnet (3PCAP)
732 get network address and network mask for a capture device
734 .BR pcap_offline_filter (3PCAP)
735 apply a filter program to a packet
737 .SS Incoming and outgoing packets
738 By default, libpcap will attempt to capture both packets sent by the
739 machine and packets received by the machine. To limit it to capturing
740 only packets received by the machine or, if possible, only packets sent
742 .BR pcap_setdirection ().
747 .BR pcap_setdirection (3PCAP)
748 specify whether to capture incoming packets, outgoing packets, or both
750 .SS Capture statistics
751 To get statistics about packets received and dropped in a live capture,
758 .BR pcap_stats (3PCAP)
759 get capture statistics
761 .SS Opening a handle for writing captured packets
762 To open a ``savefile`` to which to write packets, given the pathname the
763 ``savefile'' should have, call
764 .BR pcap_dump_open ().
765 To open a ``savefile`` to which to write packets, given the pathname the
766 ``savefile'' should have, call
767 .BR pcap_dump_open ();
768 to set up a handle for a ``savefile'', given a
770 referring to a file already opened for writing, call
771 .BR pcap_dump_fopen ().
772 They each return pointers to a
774 which is the handle used for writing packets to the ``savefile''. If it
775 succeeds, it will have created the file if it doesn't exist and
776 truncated the file if it does exist.
780 .BR pcap_dump_close ().
785 .BR pcap_dump_open (3PCAP)
788 for a ``savefile``, given a pathname
790 .BR pcap_dump_fopen (3PCAP)
793 for a ``savefile``, given a
796 .BR pcap_dump_close (3PCAP)
800 .BR pcap_dump_file (3PCAP)
805 opened for a ``savefile''
808 To write a packet to a
814 may be buffered, rather than being immediately written to the
815 ``savefile''. Closing the
817 will cause all buffered-but-not-yet-written packets to be written to the
819 To force all packets written to the
821 and not yet written to the ``savefile'' because they're buffered by the
823 to be written to the ``savefile'', without closing the
826 .BR pcap_dump_flush ().
831 .BR pcap_dump (3PCAP)
835 .BR pcap_dump_flush (3PCAP)
836 flush buffered packets written to a
840 .BR pcap_dump_ftell (3PCAP)
841 get current file position for a
844 .SS Injecting packets
845 If you have the required privileges, you can inject packets onto a
848 for a live capture, using
851 .BR pcap_sendpacket ().
852 (The two routines exist for compatibility with both OpenBSD and WinPcap;
853 they perform the same function, but have different return values.)
858 .BR pcap_inject (3PCAP)
861 .BR pcap_sendpacket (3PCAP)
866 Some routines return error or warning status codes; to convert them to a
868 .BR pcap_statustostr ().
873 .BR pcap_statustostr (3PCAP)
874 get a string for an error or warning status code
876 .SS Getting library version information
877 To get a string giving version information about libpcap, call
878 .BR pcap_lib_version ().
883 .BR pcap_lib_version (3PCAP)
884 get library version string
886 .SH BACKWARDS COMPATIBILITY
888 In versions of libpcap prior to 1.0, the
890 header file was not in a
892 directory on most platforms; if you are writing an application that must
893 work on versions of libpcap prior to 1.0, include
897 for you, rather than including
903 were not available in versions of libpcap prior to 1.0; if you are
904 writing an application that must work on versions of libpcap prior to
906 .BR pcap_open_live ()
907 to get a handle for a live capture or, if you want to be able to use the
908 additional capabilities offered by using
911 .BR pcap_activate (),
914 script or some other configuration script to check whether the libpcap
915 1.0 APIs are available and use them only if they are.
917 autoconf(1), tcpdump(1), tcpslice(1), pcap-filter(@MAN_MISC_INFO@), pfconfig(8),
918 usermod(@MAN_USERMOD_SECTION@)
920 The original authors of libpcap are:
924 Steven McCanne, all of the
925 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
927 The current version is available from "The Tcpdump Group"'s Web site at
930 .I http://www.tcpdump.org/
933 To report a security issue please send an e-mail to security@tcpdump.org.
935 To report bugs and other problems, contribute patches, request a
936 feature, provide generic feedback etc please see the file
938 in the libpcap source tree root.
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