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22 .TH PCAP 3 "27 February 2004"
24 pcap \- Packet Capture library
33 char errbuf[PCAP_ERRBUF_SIZE];
37 pcap_t *pcap_open_live(const char *device, int snaplen,
39 int promisc, int to_ms, char *errbuf)
40 pcap_t *pcap_open_dead(int linktype, int snaplen)
41 pcap_t *pcap_open_offline(const char *fname, char *errbuf)
42 pcap_dumper_t *pcap_dump_open(pcap_t *p, const char *fname)
46 int pcap_setnonblock(pcap_t *p, int nonblock, char *errbuf);
47 int pcap_getnonblock(pcap_t *p, char *errbuf);
51 int pcap_findalldevs(pcap_if_t **alldevsp, char *errbuf)
52 void pcap_freealldevs(pcap_if_t *alldevs)
53 char *pcap_lookupdev(char *errbuf)
54 int pcap_lookupnet(const char *device, bpf_u_int32 *netp,
56 bpf_u_int32 *maskp, char *errbuf)
60 int pcap_dispatch(pcap_t *p, int cnt,
62 pcap_handler callback, u_char *user)
63 int pcap_loop(pcap_t *p, int cnt,
65 pcap_handler callback, u_char *user)
66 void pcap_dump(u_char *user, struct pcap_pkthdr *h,
72 int pcap_compile(pcap_t *p, struct bpf_program *fp,
74 char *str, int optimize, bpf_u_int32 netmask)
75 int pcap_setfilter(pcap_t *p, struct bpf_program *fp)
76 void pcap_freecode(struct bpf_program *);
80 const u_char *pcap_next(pcap_t *p, struct pcap_pkthdr *h)
81 int pcap_next_ex(pcap_t *p, struct pcap_pkthdr **pkt_header,
83 const u_char **pkt_data)
87 void pcap_breakloop(pcap_t *)
91 int pcap_datalink(pcap_t *p)
92 int pcap_list_datalinks(pcap_t *p, int **dlt_buf);
93 int pcap_set_datalink(pcap_t *p, int dlt);
94 int pcap_datalink_name_to_val(const char *name);
95 const char *pcap_datalink_val_to_name(int dlt);
96 const char *pcap_datalink_val_to_description(int dlt);
97 int pcap_snapshot(pcap_t *p)
98 int pcap_is_swapped(pcap_t *p)
99 int pcap_major_version(pcap_t *p)
100 int pcap_minor_version(pcap_t *p)
101 int pcap_stats(pcap_t *p, struct pcap_stat *ps)
102 FILE *pcap_file(pcap_t *p)
103 int pcap_fileno(pcap_t *p)
104 int pcap_get_selectable_fd(pcap_t *p);
105 void pcap_perror(pcap_t *p, char *prefix)
106 char *pcap_geterr(pcap_t *p)
107 char *pcap_strerror(int error)
108 const char *pcap_lib_version(void)
112 void pcap_close(pcap_t *p)
113 int pcap_dump_flush(pcap_dumper_t *p)
114 FILE *pcap_dump_file(pcap_dumper_t *p)
115 void pcap_dump_close(pcap_dumper_t *p)
119 The Packet Capture library
120 provides a high level interface to packet capture systems. All packets
121 on the network, even those destined for other hosts, are accessible
122 through this mechanism.
128 .BR pcap_open_live() ,
129 .BR pcap_open_dead() ,
130 .BR pcap_open_offline() ,
131 .BR pcap_setnonblock() ,
132 .BR pcap_getnonblock() ,
133 .BR pcap_findalldevs() ,
134 .BR pcap_lookupdev() ,
137 is assumed to be able to hold at least
142 is used to obtain a packet capture descriptor to look
143 at packets on the network.
145 is a string that specifies the network device to open; on Linux systems
146 with 2.2 or later kernels, a
150 can be used to capture packets from all interfaces.
152 specifies the maximum number of bytes to capture. If this value is less
153 than the size of a packet that is captured, only the first
155 bytes of that packet will be captured and provided as packet data. A
156 value of 65535 should be sufficient, on most if not all networks, to
157 capture all the data available from the packet.
159 specifies if the interface is to be put into promiscuous mode.
160 (Note that even if this parameter is false, the interface
161 could well be in promiscuous mode for some other reason.) For now, this
162 doesn't work on the "any" device; if an argument of "any" or NULL is
167 specifies the read timeout in milliseconds. The read timeout is used to
168 arrange that the read not necessarily return immediately when a packet
169 is seen, but that it wait for some amount of time to allow more packets
170 to arrive and to read multiple packets from the OS kernel in one
171 operation. Not all platforms support a read timeout; on platforms that
172 don't, the read timeout is ignored. A zero value for
174 on platforms that support a read timeout,
175 will cause a read to wait forever to allow enough packets to
176 arrive, with no timeout.
178 is used to return error or warning text. It will be set to error text when
183 may also be set to warning text when
185 succeds; to detect this case the caller should store a zero-length string in
189 and display the warning to the user if
191 is no longer a zero-length string.
194 is used for creating a
196 structure to use when calling the other functions in libpcap. It is
197 typically used when just using libpcap for compiling BPF code.
199 .B pcap_open_offline()
200 is called to open a ``savefile'' for reading.
202 specifies the name of the file to open. The file has
203 the same format as those used by
207 The name "-" in a synonym for
210 is used to return error text and is only set when
211 .B pcap_open_offline()
216 is called to open a ``savefile'' for writing. The name "-" in a synonym
220 is returned on failure.
224 struct as returned by
225 .B pcap_open_offline()
227 .BR pcap_open_live() .
229 specifies the name of the file to open.
234 can be used to get the error text.
236 .B pcap_setnonblock()
237 puts a capture descriptor, opened with
238 .BR pcap_open_live() ,
239 into ``non-blocking'' mode, or takes it out of ``non-blocking'' mode,
240 depending on whether the
242 argument is non-zero or zero. It has no effect on ``savefiles''.
243 If there is an error, \-1 is returned and
245 is filled in with an appropriate error message; otherwise, 0 is
248 ``non-blocking'' mode, an attempt to read from the capture descriptor
251 will, if no packets are currently available to be read, return 0
252 immediately rather than blocking waiting for packets to arrive.
256 will not work in ``non-blocking'' mode.
258 .B pcap_getnonblock()
259 returns the current ``non-blocking'' state of the capture descriptor; it
260 always returns 0 on ``savefiles''.
261 If there is an error, \-1 is returned and
263 is filled in with an appropriate error message.
265 .B pcap_findalldevs()
266 constructs a list of network devices that can be opened with
267 .BR pcap_open_live() .
268 (Note that there may be network devices that cannot be opened with
272 .BR pcap_findalldevs() ,
273 because, for example, that process might not have sufficient privileges
274 to open them for capturing; if so, those devices will not appear on the
277 is set to point to the first element of the list; each element of the
280 and has the following members:
286 a pointer to the next element in the list;
288 for the last element of the list
291 a pointer to a string giving a name for the device to pass to
297 a pointer to a string giving a human-readable description of the device
300 a pointer to the first element of a list of addresses for the interface
307 set if the interface is a loopback interface
311 Each element of the list of addresses is of type
313 and has the following members:
319 a pointer to the next element in the list;
321 for the last element of the list
326 containing an address
333 that contains the netmask corresponding to the address pointed to by
341 that contains the broadcast address corresponding to the address pointed
344 may be null if the interface doesn't support broadcasts
351 that contains the destination address corresponding to the address pointed
354 may be null if the interface isn't a point-to-point interface
358 is returned on failure, in which case
360 is filled in with an appropriate error message;
362 is returned on success.
364 .B pcap_freealldevs()
365 is used to free a list allocated by
366 .BR pcap_findalldevs() .
369 returns a pointer to a network device suitable for use with
372 .BR pcap_lookupnet() .
373 If there is an error,
377 is filled in with an appropriate error message.
380 is used to determine the network number and mask
381 associated with the network device
390 A return of \-1 indicates an error in which case
392 is filled in with an appropriate error message.
395 is used to collect and process packets.
397 specifies the maximum number of packets to process before returning.
398 This is not a minimum number; when reading a live capture, only one
399 bufferful of packets is read at a time, so fewer than
401 packets may be processed. A
403 of \-1 processes all the packets received in one buffer when reading a
404 live capture, or all the packets in the file when reading a
407 specifies a routine to be called with three arguments:
410 pointer which is passed in from
411 .BR pcap_dispatch() ,
413 .I const struct pcap_pkthdr
414 pointer to a structure with the following members:
420 containing the time when the packet was captured
425 giving the number of bytes of the packet that are available from the
431 giving the length of the packet, in bytes (which might be more than the
432 number of bytes available from the capture, if the length of the packet
433 is larger than the maximum number of bytes to capture)
441 .I struct pcap_pkthdr
442 a pointer to which is passed to the callback routine)
443 bytes of data from the packet (which won't necessarily be the entire
444 packet; to capture the entire packet, you will have to provide a value
449 that is sufficiently large to get all of the packet's data - a value of
450 65535 should be sufficient on most if not all networks).
452 The number of packets read is returned.
453 0 is returned if no packets were read from a live capture (if, for
454 example, they were discarded because they didn't pass the packet filter,
455 or if, on platforms that support a read timeout that starts before any
456 packets arrive, the timeout expires before any packets arrive, or if the
457 file descriptor for the capture device is in non-blocking mode and no
458 packets were available to be read) or if no more packets are available
459 in a ``savefile.'' A return of \-1 indicates
460 an error in which case
464 may be used to display the error text.
465 A return of \-2 indicates that the loop terminated due to a call to
467 before any packets were processed.
469 If your application uses pcap_breakloop(),
470 make sure that you explicitly check for \-1 and \-2, rather than just
471 checking for a return value < 0.
475 when reading a live capture,
477 will not necessarily return when the read times out; on some platforms,
478 the read timeout isn't supported, and, on other platforms, the timer
479 doesn't start until at least one packet arrives. This means that the
482 be used in, for example, an interactive application, to allow the packet
483 capture loop to ``poll'' for user input periodically, as there's no
486 will return after the timeout expires.
491 except it keeps reading packets until
493 packets are processed or an error occurs.
496 return when live read timeouts occur.
497 Rather, specifying a non-zero read timeout to
501 allows the reception and processing of any packets that arrive when the
507 to loop forever (or at least until an error occurs). \-1 is returned on
508 an error; 0 is returned if
510 is exhausted; \-2 is returned if the loop terminated due to a call to
512 before any packets were processed.
514 If your application uses pcap_breakloop(),
515 make sure that you explicitly check for \-1 and \-2, rather than just
516 checking for a return value < 0.
520 reads the next packet (by calling
526 pointer to the data in that packet. (The
528 struct for that packet is not supplied.)
530 is returned if an error occured, or if no packets were read from a live
531 capture (if, for example, they were discarded because they didn't pass
532 the packet filter, or if, on platforms that support a read timeout that
533 starts before any packets arrive, the timeout expires before any packets
534 arrive, or if the file descriptor for the capture device is in
535 non-blocking mode and no packets were available to be read), or if no
536 more packets are available in a ``savefile.'' Unfortunately, there is
537 no way to determine whether an error occured or not.
540 reads the next packet and returns a success/failure indication:
544 the packet was read without problems
547 packets are being read from a live capture, and the timeout expired
550 an error occurred while reading the packet
553 packets are being read from a ``savefile'', and there are no more
554 packets to read from the savefile.
557 If the packet was read without problems, the pointer pointed to by the
559 argument is set to point to the
561 struct for the packet, and the
562 pointer pointed to by the
564 argument is set to point to the data in the packet.
567 sets a flag that will force
571 to return rather than looping; they will return the number of packets
572 that have been processed so far, or \-2 if no packets have been
575 This routine is safe to use inside a signal handler on UNIX or a console
576 control handler on Windows, as it merely sets a flag that is checked
579 The flag is checked in loops reading packets from the OS - a signal by
580 itself will not necessarily terminate those loops - as well as in loops
581 processing a set of packets returned by the OS.
583 Note that if you are catching signals on UNIX systems that support
584 restarting system calls after a signal, and calling pcap_breakloop()
585 in the signal handler, you must specify, when catching those signals,
586 that system calls should NOT be restarted by that signal. Otherwise,
587 if the signal interrupted a call reading packets in a live capture,
588 when your signal handler returns after calling pcap_breakloop(), the
589 call will be restarted, and the loop will not terminate until more
590 packets arrive and the call completes.
592 Note also that, in a multi-threaded application, if one thread is
594 .BR pcap_dispatch() ,
601 in a different thread will not unblock that thread; you will need to use
602 whatever mechanism the OS provides for breaking a thread out of blocking
603 calls in order to unblock the thread, such as thread cancellation in
604 systems that support POSIX threads.
609 will, on some platforms, loop reading packets from the OS; that loop
610 will not necessarily be terminated by a signal, so
612 should be used to terminate packet processing even if
617 does not guarantee that no further packets will be processed by
621 after it is called; at most one more packet might be processed.
623 If \-2 is returned from
627 the flag is cleared, so a subsequent call will resume reading packets.
628 If a positive number is returned, the flag is not cleared, so a
629 subsequent call will return \-2 and clear the flag.
632 outputs a packet to the ``savefile'' opened with
633 .BR pcap_dump_open() .
634 Note that its calling arguments are suitable for use with
638 If called directly, the
643 .BR pcap_dump_open() .
646 is used to compile the string
648 into a filter program.
652 struct and is filled in by
655 controls whether optimization on the resulting code is performed.
657 specifies the IPv4 netmask of the network on which packets are being
658 captured; it is used only when checking for IPv4 broadcast addresses in
659 the filter program. If the netmask of the network on which packets are
660 being captured isn't known to the program, or if packets are being
661 captured on the Linux "any" pseudo-interface that can capture on more
662 than one network, a value of 0 can be supplied; tests for IPv4 broadcast
663 addreses won't be done correctly, but all other tests in the filter
664 program will be OK. A return of \-1 indicates an error in which case
666 may be used to display the error text.
668 .B pcap_compile_nopcap()
671 except that instead of passing a pcap structure, one passes the
672 snaplen and linktype explicitly. It is intended to be used for
673 compiling filters for direct BPF usage, without necessarily having
676 A return of \-1 indicates an error; the error text is unavailable.
677 .RB ( pcap_compile_nopcap()
679 .BR pcap_open_dead() ,
683 the latter three routines can be used directly in order to get the error
684 text for a compilation error.)
688 is used to specify a filter program.
692 struct, usually the result of a call to
695 is returned on failure, in which case
697 may be used to display the error text;
699 is returned on success.
702 is used to free up allocated memory pointed to by a
706 when that BPF program is no longer needed, for example after it
707 has been made the filter program for a pcap structure by a call to
708 .BR pcap_setfilter() .
711 returns the link layer type; link layer types it can return include:
716 BSD loopback encapsulation; the link layer header is a 4-byte field, in
718 byte order, containing a PF_ value from
720 for the network-layer protocol of the packet.
722 Note that ``host byte order'' is the byte order of the machine on which
723 the packets are captured, and the PF_ values are for the OS of the
724 machine on which the packets are captured; if a live capture is being
725 done, ``host byte order'' is the byte order of the machine capturing the
726 packets, and the PF_ values are those of the OS of the machine capturing
727 the packets, but if a ``savefile'' is being read, the byte order and PF_
730 necessarily those of the machine reading the capture file.
733 Ethernet (10Mb, 100Mb, 1000Mb, and up)
736 IEEE 802.5 Token Ring
742 SLIP; the link layer header contains, in order:
745 a 1-byte flag, which is 0 for packets received by the machine and 1 for
746 packets sent by the machine;
748 a 1-byte field, the upper 4 bits of which indicate the type of packet,
753 an unmodified IP datagram (TYPE_IP);
756 an uncompressed-TCP IP datagram (UNCOMPRESSED_TCP), with that byte being
757 the first byte of the raw IP header on the wire, containing the
758 connection number in the protocol field;
761 a compressed-TCP IP datagram (COMPRESSED_TCP), with that byte being the
762 first byte of the compressed TCP/IP datagram header;
765 for UNCOMPRESSED_TCP, the rest of the modified IP header, and for
766 COMPRESSED_TCP, the compressed TCP/IP datagram header;
770 for a total of 16 bytes; the uncompressed IP datagram follows the header.
774 PPP; if the first 2 bytes are 0xff and 0x03, it's PPP in HDLC-like
775 framing, with the PPP header following those two bytes, otherwise it's
776 PPP without framing, and the packet begins with the PPP header.
782 RFC 1483 LLC/SNAP-encapsulated ATM; the packet begins with an IEEE 802.2
786 raw IP; the packet begins with an IP header.
789 PPP in HDLC-like framing, as per RFC 1662, or Cisco PPP with HDLC
790 framing, as per section 4.3.1 of RFC 1547; the first byte will be 0xFF
791 for PPP in HDLC-like framing, and will be 0x0F or 0x8F for Cisco PPP
795 PPPoE; the packet begins with a PPPoE header, as per RFC 2516.
798 Cisco PPP with HDLC framing, as per section 4.3.1 of RFC 1547.
801 IEEE 802.11 wireless LAN
807 OpenBSD loopback encapsulation; the link layer header is a 4-byte field, in
809 byte order, containing a PF_ value from OpenBSD's
811 for the network-layer protocol of the packet.
813 Note that, if a ``savefile'' is being read, those PF_ values are
815 necessarily those of the machine reading the capture file.
818 Linux "cooked" capture encapsulation; the link layer header contains, in
822 a 2-byte "packet type", in network byte order, which is one of:
826 packet was sent to us by somebody else
829 packet was broadcast by somebody else
832 packet was multicast, but not broadcast, by somebody else
835 packet was sent by somebody else to somebody else
838 packet was sent by us
841 a 2-byte field, in network byte order, containing a Linux ARPHRD_ value
842 for the link layer device type;
844 a 2-byte field, in network byte order, containing the length of the
845 link layer address of the sender of the packet (which could be 0);
847 an 8-byte field containing that number of bytes of the link layer header
848 (if there are more than 8 bytes, only the first 8 are present);
850 a 2-byte field containing an Ethernet protocol type, in network byte
851 order, or containing 1 for Novell 802.3 frames without an 802.2 LLC
852 header or 4 for frames beginning with an 802.2 LLC header.
856 Apple LocalTalk; the packet begins with an AppleTalk LLAP header.
859 OpenBSD pflog; the link layer header contains, in order:
862 a 4-byte PF_ value, in network byte order;
864 a 16-character interface name;
866 a 2-byte rule number, in network byte order;
868 a 2-byte reason code, in network byte order, which is one of:
889 a 2-byte action code, in network byte order, which is one of:
902 a 2-byte direction, in network byte order, which is one of:
917 Prism monitor mode information followed by an 802.11 header.
920 RFC 2625 IP-over-Fibre Channel, with the link-layer header being the
921 Network_Header as described in that RFC.
924 SunATM devices; the link layer header contains, in order:
927 a 1-byte flag field, containing a direction flag in the uppermost bit,
928 which is set for packets transmitted by the machine and clear for
929 packets received by the machine, and a 4-byte traffic type in the
930 low-order 4 bits, which is one of:
940 LLC-encapsulated traffic
957 a 2-byte VCI field, in network byte order.
960 .B DLT_IEEE802_11_RADIO
961 link-layer information followed by an 802.11 header - see
962 http://www.shaftnet.org/~pizza/software/capturefrm.txt for a description
963 of the link-layer information.
966 ARCNET, with no exception frames, reassembled packets rather than raw
967 frames, and an extra 16-bit offset field between the destination host
971 Linux-IrDA packets, with a
973 header followed by the IrLAP header.
976 .B pcap_list_datalinks()
977 is used to get a list of the supported data link types of the interface
978 associated with the pcap descriptor.
979 .B pcap_list_datalinks()
980 allocates an array to hold the list and sets
982 The caller is responsible for freeing the array.
984 is returned on failure;
985 otherwise, the number of data link types in the array is returned.
987 .B pcap_set_datalink()
988 is used to set the current data link type of the pcap descriptor
989 to the type specified by
992 is returned on failure.
994 .B pcap_datalink_name_to_val()
995 translates a data link type name, which is a
999 removed, to the corresponding data link type value. The translation
1000 is case-insensitive.
1002 is returned on failure.
1004 .B pcap_datalink_val_to_name()
1005 translates a data link type value to the corresponding data link type
1006 name. NULL is returned on failure.
1008 .B pcap_datalink_val_to_description()
1009 translates a data link type value to a short description of that data
1010 link type. NULL is returned on failure.
1013 returns the snapshot length specified when
1017 .B pcap_is_swapped()
1018 returns true if the current ``savefile'' uses a different byte order
1019 than the current system.
1021 .B pcap_major_version()
1022 returns the major number of the file format of the savefile;
1023 .B pcap_minor_version()
1024 returns the minor number of the file format of the savefile. The
1025 version number is stored in the header of the savefile.
1028 returns the standard I/O stream of the ``savefile,'' if a ``savefile''
1030 .BR pcap_open_offline() ,
1031 or NULL, if a network device was opened with
1032 .BR pcap_open_live() .
1035 returns 0 and fills in a
1037 struct. The values represent packet statistics from the start of the
1038 run to the time of the call. If there is an error or the underlying
1039 packet capture doesn't support packet statistics, \-1 is returned and
1040 the error text can be obtained with
1045 is supported only on live captures, not on ``savefiles''; no statistics
1046 are stored in ``savefiles'', so no statistics are available when reading
1047 from a ``savefile''.
1050 returns the file descriptor number from which captured packets are read,
1051 if a network device was opened with
1052 .BR pcap_open_live() ,
1053 or \-1, if a ``savefile'' was opened with
1054 .BR pcap_open_offline() .
1056 .B pcap_get_selectable_fd()
1057 returns, on UNIX, a file descriptor number for a file descriptor on
1063 to wait for it to be possible to read packets without blocking, if such
1064 a descriptor exists, or \-1, if no such descriptor exists. Some network
1071 (for example, regular network devices on FreeBSD 4.3 and 4.4, and Endace
1072 DAG devices), so \-1 is returned for those devices.
1074 Note that on most versions of most BSDs (including Mac OS X)
1078 do not work correctly on BPF devices;
1079 .B pcap_get_selectable_fd()
1080 will return a file descriptor on most of those versions (the exceptions
1081 being FreeBSD 4.3 and 4.4), a simple
1085 will not return even after a timeout specified in
1087 expires. To work around this, an application that uses
1091 to wait for packets to arrive must put the
1093 in non-blocking mode, and must arrange that the
1097 have a timeout less than or equal to the timeout specified in
1098 .BR pcap_open_live() ,
1099 and must try to read packets after that timeout expires, regardless of
1104 indicated that the file descriptor for the
1106 is ready to be read or not. (That workaround will not work in FreeBSD
1107 4.3 and later; however, in FreeBSD 4.6 and later,
1111 work correctly on BPF devices, so the workaround isn't necessary,
1112 although it does no harm.)
1114 .B pcap_get_selectable_fd()
1115 is not available on Windows.
1118 prints the text of the last pcap library error on
1124 returns the error text pertaining to the last pcap library error.
1126 the pointer it returns will no longer point to a valid error message
1129 passed to it is closed; you must use or copy the string before closing
1138 .B pcap_lib_version()
1139 returns a pointer to a string giving information about the version of
1140 the libpcap library being used; note that it contains more information
1141 than just a version number.
1144 closes the files associated with
1146 and deallocates resources.
1149 returns the standard I/O stream of the ``savefile'' opened by
1150 .BR pcap_dump_open().
1152 .B pcap_dump_flush()
1153 flushes the output buffer to the ``savefile,'' so that any packets
1156 but not yet written to the ``savefile'' will be written.
1158 is returned on error, 0 on success.
1160 .B pcap_dump_close()
1161 closes the ``savefile.''
1164 tcpdump(1), tcpslice(1)
1166 The original authors are:
1170 Steven McCanne, all of the
1171 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
1173 The current version is available from "The Tcpdump Group"'s Web site at
1176 .I http://www.tcpdump.org/
1179 Please send problems, bugs, questions, desirable enhancements, etc. to:
1182 tcpdump-workers@tcpdump.org
1185 Please send source code contributions, etc. to:
projects / libpcap / blob