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22 .TH PCAP 3 "3 January 2001"
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 u_char *pcap_next(pcap_t *p, struct pcap_pkthdr *h)
84 int pcap_datalink(pcap_t *p)
85 int pcap_list_datalinks(pcap_t *p, int **dlt_buf);
86 int pcap_set_datalink(pcap_t *p, int dlt);
87 int pcap_datalink_name_to_val(const char *name);
88 const char *pcap_datalink_val_to_name(int dlt);
89 int pcap_snapshot(pcap_t *p)
90 int pcap_is_swapped(pcap_t *p)
91 int pcap_major_version(pcap_t *p)
92 int pcap_minor_version(pcap_t *p)
93 int pcap_stats(pcap_t *p, struct pcap_stat *ps)
94 FILE *pcap_file(pcap_t *p)
95 int pcap_fileno(pcap_t *p)
96 void pcap_perror(pcap_t *p, char *prefix)
97 char *pcap_geterr(pcap_t *p)
98 char *pcap_strerror(int error)
102 void pcap_close(pcap_t *p)
103 void pcap_dump_flush(pcap_dumper_t *p)
104 void pcap_dump_close(pcap_dumper_t *p)
108 The Packet Capture library
109 provides a high level interface to packet capture systems. All packets
110 on the network, even those destined for other hosts, are accessible
111 through this mechanism.
117 .BR pcap_open_live() ,
118 .BR pcap_open_dead() ,
119 .BR pcap_open_offline() ,
120 .BR pcap_setnonblock() ,
121 .BR pcap_getnonblock() ,
122 .BR pcap_findalldevs() ,
123 .BR pcap_lookupdev() ,
126 is assumed to be able to hold at least
131 is used to obtain a packet capture descriptor to look
132 at packets on the network.
134 is a string that specifies the network device to open; on Linux systems
135 with 2.2 or later kernels, a
139 can be used to capture packets from all interfaces.
141 specifies the maximum number of bytes to capture. If this value is less
142 than the size of a packet that is captured, only the first
144 bytes of that packet will be captured and provided as packet data. A
145 value of 65535 should be sufficient, on most if not all networks, to
146 capture all the data available from the packet.
148 specifies if the interface is to be put into promiscuous mode.
149 (Note that even if this parameter is false, the interface
150 could well be in promiscuous mode for some other reason.) For now, this
151 doesn't work on the "any" device; if an argument of "any" or NULL is
156 specifies the read timeout in milliseconds. The read timeout is used to
157 arrange that the read not necessarily return immediately when a packet
158 is seen, but that it wait for some amount of time to allow more packets
159 to arrive and to read multiple packets from the OS kernel in one
160 operation. Not all platforms support a read timeout; on platforms that
161 don't, the read timeout is ignored. A zero value for
163 on platforms that support a read timeout,
164 will cause a read to wait forever to allow enough packets to
165 arrive, with no timeout.
167 is used to return error or warning text. It will be set to error text when
172 may also be set to warning text when
174 succeds; to detect this case the caller should store a zero-length string in
178 and display the warning to the user if
180 is no longer a zero-length string.
183 is used for creating a
185 structure to use when calling the other functions in libpcap. It is
186 typically used when just using libpcap for compiling BPF code.
188 .B pcap_open_offline()
189 is called to open a ``savefile'' for reading.
191 specifies the name of the file to open. The file has
192 the same format as those used by
196 The name "-" in a synonym for
199 is used to return error text and is only set when
200 .B pcap_open_offline()
205 is called to open a ``savefile'' for writing. The name "-" in a synonym
209 is returned on failure.
213 struct as returned by
214 .B pcap_open_offline()
216 .BR pcap_open_live() .
218 specifies the name of the file to open.
223 can be used to get the error text.
225 .B pcap_setnonblock()
226 puts a capture descriptor, opened with
227 .BR pcap_open_live() ,
228 into ``non-blocking'' mode, or takes it out of ``non-blocking'' mode,
229 depending on whether the
231 argument is non-zero or zero. It has no effect on ``savefiles''.
232 If there is an error, \-1 is returned and
234 is filled in with an appropriate error message; otherwise, 0 is
237 ``non-blocking'' mode, an attempt to read from the capture descriptor
240 will, if no packets are currently available to be read, return 0
241 immediately rather than blocking waiting for packets to arrive.
245 will not work in ``non-blocking'' mode.
247 .B pcap_getnonblock()
248 returns the current ``non-blocking'' state of the capture descriptor; it
249 always returns 0 on ``savefiles''.
250 If there is an error, \-1 is returned and
252 is filled in with an appropriate error message.
254 .B pcap_findalldevs()
255 constructs a list of network devices that can be opened with
256 .BR pcap_open_live() .
257 (Note that there may be network devices that cannot be opened with
261 .BR pcap_findalldevs() ,
262 because, for example, that process might not have sufficient privileges
263 to open them for capturing; if so, those devices will not appear on the
266 is set to point to the first element of the list; each element of the
269 and has the following members:
275 a pointer to the next element in the list;
277 for the last element of the list
280 a pointer to a string giving a name for the device to pass to
286 a pointer to a string giving a human-readable description of the device
289 a pointer to the first element of a list of addresses for the interface
296 set if the interface is a loopback interface
300 Each element of the list of addresses is of type
302 and has the following members:
308 a pointer to the next element in the list;
310 for the last element of the list
315 containing an address
322 that contains the netmask corresponding to the address pointed to by
330 that contains the broadcast address corresponding to the address pointed
333 may be null if the interface doesn't support broadcasts
340 that contains the destination address corresponding to the address pointed
343 may be null if the interface isn't a point-to-point interface
346 .B pcap_freealldevs()
347 is used to free a list allocated by
348 .BR pcap_findalldevs() .
351 returns a pointer to a network device suitable for use with
354 .BR pcap_lookupnet() .
355 If there is an error,
359 is filled in with an appropriate error message.
362 is used to determine the network number and mask
363 associated with the network device
372 A return of \-1 indicates an error in which case
374 is filled in with an appropriate error message.
377 is used to collect and process packets.
379 specifies the maximum number of packets to process before returning.
380 This is not a minimum number; when reading a live capture, only one
381 bufferful of packets is read at a time, so fewer than
383 packets may be processed. A
385 of \-1 processes all the packets received in one buffer when reading a
386 live capture, or all the packets in the file when reading a
389 specifies a routine to be called with three arguments:
392 pointer which is passed in from
393 .BR pcap_dispatch() ,
395 .I const struct pcap_pkthdr
396 pointer to a structure with the following members:
402 containing the time when the packet was captured
407 giving the number of bytes of the packet that are available from the
413 giving the length of the packet, in bytes (which might be more than the
414 number of bytes available from the capture, if the length of the packet
415 is larger than the maximum number of bytes to capture)
423 .I struct pcap_pkthdr
424 a pointer to which is passed to the callback routine)
425 bytes of data from the packet (which won't necessarily be the entire
426 packet; to capture the entire packet, you will have to provide a value
431 that is sufficiently large to get all of the packet's data - a value of
432 65535 should be sufficient on most if not all networks).
434 The number of packets read is returned.
435 0 is returned if no packets were read from a live capture (if, for
436 example, they were discarded because they didn't pass the packet filter,
437 or if, on platforms that support a read timeout that starts before any
438 packets arrive, the timeout expires before any packets arrive, or if the
439 file descriptor for the capture device is in non-blocking mode and no
440 packets were available to be read) or if no more packets are available
441 in a ``savefile.'' A return of \-1 indicates
442 an error in which case
446 may be used to display the error text.
449 when reading a live capture,
451 will not necessarily return when the read times out; on some platforms,
452 the read timeout isn't supported, and, on other platforms, the timer
453 doesn't start until at least one packet arrives. This means that the
456 be used in, for example, an interactive application, to allow the packet
457 capture loop to ``poll'' for user input periodically, as there's no
460 will return after the timeout expires.
465 except it keeps reading packets until
467 packets are processed or an error occurs.
470 return when live read timeouts occur.
471 Rather, specifying a non-zero read timeout to
475 allows the reception and processing of any packets that arrive when the
481 to loop forever (or at least until an error occurs). A negative number
482 is returned on an error; 0 is returned if
487 reads the next packet (by calling
493 pointer to the data in that packet. (The
495 struct for that packet is not supplied.)
497 is returned if an error occured, or if no packets were read from a live
498 capture (if, for example, they were discarded because they didn't pass
499 the packet filter, or if, on platforms that support a read timeout that
500 starts before any packets arrive, the timeout expires before any packets
501 arrive, or if the file descriptor for the capture device is in
502 non-blocking mode and no packets were available to be read), or if no
503 more packets are available in a ``savefile.'' Unfortunately, there is
504 no way to determine whether an error occured or not.
507 outputs a packet to the ``savefile'' opened with
508 .BR pcap_dump_open() .
509 Note that its calling arguments are suitable for use with
513 If called directly, the
518 .BR pcap_dump_open() .
521 is used to compile the string
523 into a filter program.
527 struct and is filled in by
530 controls whether optimization on the resulting code is performed.
532 specifies the IPv4 netmask of the network on which packets are being
533 captured; it is used only when checking for IPv4 broadcast addresses in
534 the filter program. If the netmask of the network on which packets are
535 being captured isn't known to the program, or if packets are being
536 captured on the Linux "any" pseudo-interface that can capture on more
537 than one network, a value of 0 can be supplied; tests for IPv4 broadcast
538 addreses won't be done correctly, but all other tests in the filter
539 program will be OK. A return of \-1 indicates an error in which case
541 may be used to display the error text.
543 .B pcap_compile_nopcap()
546 except that instead of passing a pcap structure, one passes the
547 snaplen and linktype explicitly. It is intended to be used for
548 compiling filters for direct BPF usage, without necessarily having
551 A return of \-1 indicates an error; the error text is unavailable.
552 .RB ( pcap_compile_nopcap()
554 .BR pcap_open_dead() ,
558 the latter three routines can be used directly in order to get the error
559 text for a compilation error.)
563 is used to specify a filter program.
567 struct, usually the result of a call to
570 is returned on failure, in which case
572 may be used to display the error text;
574 is returned on success.
577 is used to free up allocated memory pointed to by a
581 when that BPF program is no longer needed, for example after it
582 has been made the filter program for a pcap structure by a call to
583 .BR pcap_setfilter() .
586 returns the link layer type; link layer types it can return include:
591 BSD loopback encapsulation; the link layer header is a 4-byte field, in
593 byte order, containing a PF_ value from
595 for the network-layer protocol of the packet
597 Note that ``host byte order'' is the byte order of the machine on which
598 the packets are captured, and the PF_ values are for the OS of the
599 machine on which the packets are captured; if a live capture is being
600 done, ``host byte order'' is the byte order of the machine capturing the
601 packets, and the PF_ values are those of the OS of the machine capturing
602 the packets, but if a ``savefile'' is being read, the byte order and PF_
605 necessarily those of the machine reading the capture file.
608 Ethernet (10Mb, 100Mb, 1000Mb, and up)
611 IEEE 802.5 Token Ring
617 SLIP; the link layer header contains, in order:
620 a 1-byte flag, which is 0 for packets received by the machine and 1 for
621 packets sent by the machine;
623 a 1-byte field, the upper 4 bits of which indicate the type of packet,
628 an unmodified IP datagram (TYPE_IP);
631 an uncompressed-TCP IP datagram (UNCOMPRESSED_TCP), with that byte being
632 the first byte of the raw IP header on the wire, containing the
633 connection number in the protocol field;
636 a compressed-TCP IP datagram (COMPRESSED_TCP), with that byte being the
637 first byte of the compressed TCP/IP datagram header;
640 for UNCOMPRESSED_TCP, the rest of the modified IP header, and for
641 COMPRESSED_TCP, the compressed TCP/IP datagram header;
645 for a total of 16 bytes; the uncompressed IP datagram follows the header
649 PPP; if the first 2 bytes are 0xff and 0x03, it's PPP in HDLC-like
650 framing, with the PPP header following those two bytes, otherwise it's
651 PPP without framing, and the packet begins with the PPP header
657 RFC 1483 LLC/SNAP-encapsulated ATM; the packet begins with an IEEE 802.2
661 raw IP; the packet begins with an IP header
664 PPP in HDLC-like framing, as per RFC 1662, or Cisco PPP with HDLC
665 framing, as per section 4.3.1 of RFC 1547; the first byte will be 0xFF
666 for PPP in HDLC-like framing, and will be 0x0F or 0x8F for Cisco PPP
670 PPPoE; the packet begins with a PPPoE header, as per RFC 2516
673 Cisco PPP with HDLC framing, as per section 4.3.1 of RFC 1547
676 IEEE 802.11 wireless LAN
682 OpenBSD loopback encapsulation; the link layer header is a 4-byte field, in
684 byte order, containing a PF_ value from OpenBSD's
686 for the network-layer protocol of the packet
688 Note that, if a ``savefile'' is being read, those PF_ values are
690 necessarily those of the machine reading the capture file.
693 Linux "cooked" capture encapsulation; the link layer header contains, in
697 a 2-byte "packet type", in network byte order, which is one of:
701 packet was sent to us by somebody else
704 packet was broadcast by somebody else
707 packet was multicast, but not broadcast, by somebody else
710 packet was sent by somebody else to somebody else
713 packet was sent by us
716 a 2-byte field, in network byte order, containing a Linux ARPHRD_ value
717 for the link layer device type;
719 a 2-byte field, in network byte order, containing the length of the
720 link layer address of the sender of the packet (which could be 0);
722 an 8-byte field containing that number of bytes of the link layer header
723 (if there are more than 8 bytes, only the first 8 are present);
725 a 2-byte field containing an Ethernet protocol type, in network byte
726 order, or containing 1 for Novell 802.3 frames without an 802.2 LLC
727 header or 4 for frames beginning with an 802.2 LLC header.
731 Apple LocalTalk; the packet begins with an AppleTalk LLAP header
734 SunATM devices; the link layer header contains, in order:
737 a 1-byte flag field, containing a direction flag in the uppermost bit,
738 which is set for packets transmitted by the machine and clear for
739 packets received by the machine, and a 4-byte traffic type in the
740 low-order 4 bits, which is one of:
750 LLC-encapsulated traffic
767 a 2-byte VCI field, in network byte order.
771 RFC 2625 IP-over-Fibre Channel, with the link-layer header being the
772 Network_Header as described in that RFC.
775 .B pcap_list_datalinks()
776 is used to get a list of the supported data link types of the interface
777 associated with the pcap descriptor.
778 .B pcap_list_datalinks()
779 allocates an array to hold the list and sets
781 The caller is responsible for freeing the array.
783 is returned on failure;
784 otherwise, the number of data link types in the array is returned.
786 .B pcap_set_datalink()
787 is used to set the current data link type of the pcap descriptor
788 to the type specified by
791 is returned on failure.
793 .B pcap_datalink_name_to_val()
794 translates a data link type name, which is a
798 removed, to the corresponding data link type value. The translation
800 is used to set the current data link type of the pcap descriptor
801 NULL is returned on failure.
803 .B pcap_datalink_val_to_name()
804 translates a data link type value to the corresponding data link type
807 is returned on failure.
810 returns the snapshot length specified when
815 returns true if the current ``savefile'' uses a different byte order
816 than the current system.
818 .B pcap_major_version()
819 returns the major number of the version of the pcap used to write the
822 .B pcap_minor_version()
823 returns the minor number of the version of the pcap used to write the
827 returns the standard I/O stream of the ``savefile,'' if a ``savefile''
829 .BR pcap_open_offline() ,
830 or NULL, if a network device was opened with
831 .BR pcap_open_live() .
834 returns 0 and fills in a
836 struct. The values represent packet statistics from the start of the
837 run to the time of the call. If there is an error or the underlying
838 packet capture doesn't support packet statistics, \-1 is returned and
839 the error text can be obtained with
844 is supported only on live captures, not on ``savefiles''; no statistics
845 are stored in ``savefiles'', so no statistics are available when reading
849 returns the file descriptor number from which captured packets are read,
850 if a network device was opened with
851 .BR pcap_open_live() ,
852 or \-1, if a ``savefile'' was opened with
853 .BR pcap_open_offline() .
856 prints the text of the last pcap library error on
862 returns the error text pertaining to the last pcap library error.
864 the pointer it returns will no longer point to a valid error message
867 passed to it is closed; you must use or copy the string before closing
877 closes the files associated with
879 and deallocates resources.
882 flushes the output buffer to the ``savefile,'' so that any packets
885 but not yet written to the ``savefile'' will be written.
888 closes the ``savefile.''
891 tcpdump(1), tcpslice(1)
893 The original authors are:
897 Steven McCanne, all of the
898 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
900 The current version is available from "The Tcpdump Group"'s Web site at
903 .I http://www.tcpdump.org/
906 Please send problems, bugs, questions, desirable enhancements, etc. to:
909 tcpdump-workers@tcpdump.org
912 Please send source code contributions, etc. to:
projects / libpcap / blob