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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(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(char *fname, char *errbuf)
42 pcap_dumper_t *pcap_dump_open(pcap_t *p, 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 *)
53 char *pcap_lookupdev(char *errbuf)
54 int pcap_lookupnet(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_close(pcap_dumper_t *p)
107 The Packet Capture library
108 provides a high level interface to packet capture systems. All packets
109 on the network, even those destined for other hosts, are accessible
110 through this mechanism.
116 .BR pcap_open_live() ,
117 .BR pcap_open_dead() ,
118 .BR pcap_open_offline() ,
119 .BR pcap_setnonblock() ,
120 .BR pcap_getnonblock() ,
121 .BR pcap_findalldevs() ,
122 .BR pcap_lookupdev() ,
125 is assumed to be able to hold at least
130 is used to obtain a packet capture descriptor to look
131 at packets on the network.
133 is a string that specifies the network device to open; on Linux systems
134 with 2.2 or later kernels, a
138 can be used to capture packets from all interfaces.
140 specifies the maximum number of bytes to capture. If this value is less
141 than the size of a packet that is captured, only the first
143 bytes of that packet will be captured and provided as packet data. A
144 value of 65535 should be sufficient, on most if not all networks, to
145 capture all the data available from the packet.
147 specifies if the interface is to be put into promiscuous mode.
148 (Note that even if this parameter is false, the interface
149 could well be in promiscuous mode for some other reason.) For now, this
150 doesn't work on the "any" device; if an argument of "any" or NULL is
155 specifies the read timeout in milliseconds. The read timeout is used to
156 arrange that the read not necessarily return immediately when a packet
157 is seen, but that it wait for some amount of time to allow more packets
158 to arrive and to read multiple packets from the OS kernel in one
159 operation. Not all platforms support a read timeout; on platforms that
160 don't, the read timeout is ignored. A zero value for
162 on platforms that support a read timeout,
163 will cause a read to wait forever to allow enough packets to
164 arrive, with no timeout.
166 is used to return error or warning text. It will be set to error text when
171 may also be set to warning text when
173 succeds; to detect this case the caller should store a zero-length string in
177 and display the warning to the user if
179 is no longer a zero-length string.
182 is used for creating a
184 structure to use when calling the other functions in libpcap. It is
185 typically used when just using libpcap for compiling BPF code.
187 .B pcap_open_offline()
188 is called to open a ``savefile'' for reading.
190 specifies the name of the file to open. The file has
191 the same format as those used by
195 The name "-" in a synonym for
198 is used to return error text and is only set when
199 .B pcap_open_offline()
204 is called to open a ``savefile'' for writing. The name "-" in a synonym
208 is returned on failure.
212 struct as returned by
213 .B pcap_open_offline()
215 .BR pcap_open_live() .
217 specifies the name of the file to open.
222 can be used to get the error text.
224 .B pcap_setnonblock()
225 puts a capture descriptor, opened with
226 .BR pcap_open_live() ,
227 into ``non-blocking'' mode, or takes it out of ``non-blocking'' mode,
228 depending on whether the
230 argument is non-zero or zero. It has no effect on ``savefiles''.
231 If there is an error, \-1 is returned and
233 is filled in with an appropriate error message; otherwise, 0 is
236 ``non-blocking'' mode, an attempt to read from the capture descriptor
239 will, if no packets are currently available to be read, return 0
240 immediately rather than blocking waiting for packets to arrive.
244 will not work in ``non-blocking'' mode.
246 .B pcap_getnonblock()
247 returns the current ``non-blocking'' state of the capture descriptor; it
248 always returns 0 on ``savefiles''.
249 If there is an error, \-1 is returned and
251 is filled in with an appropriate error message.
253 .B pcap_findalldevs()
254 constructs a list of network devices that can be opened with
255 .BR pcap_open_live() .
256 (Note that there may be network devices that cannot be opened with
260 .BR pcap_findalldevs() ,
261 because, for example, that process might not have sufficient privileges
262 to open them for capturing; if so, those devices will not appear on the
265 is set to point to the first element of the list; each element of the
268 and has the following members:
274 a pointer to the next element in the list;
276 for the last element of the list
279 a pointer to a string giving a name for the device to pass to
285 a pointer to a string giving a human-readable description of the device
288 a pointer to the first element of a list of addresses for the interface
295 set if the interface is a loopback interface
299 Each element of the list of addresses is of type
301 and has the following members:
307 a pointer to the next element in the list;
309 for the last element of the list
314 containing an address
321 that contains the netmask corresponding to the address pointed to by
329 that contains the broadcast address corresponding to the address pointed
332 may be null if the interface doesn't support broadcasts
339 that contains the destination address corresponding to the address pointed
342 may be null if the interface isn't a point-to-point interface
345 .B pcap_freealldevs()
346 is used to free a list allocated by
347 .BR pcap_findalldevs() .
350 returns a pointer to a network device suitable for use with
353 .BR pcap_lookupnet() .
354 If there is an error,
358 is filled in with an appropriate error message.
361 is used to determine the network number and mask
362 associated with the network device
371 A return of \-1 indicates an error in which case
373 is filled in with an appropriate error message.
376 is used to collect and process packets.
378 specifies the maximum number of packets to process before returning.
379 This is not a minimum number; when reading a live capture, only one
380 bufferful of packets is read at a time, so fewer than
382 packets may be processed. A
384 of \-1 processes all the packets received in one buffer when reading a
385 live capture, or all the packets in the file when reading a
388 specifies a routine to be called with three arguments:
391 pointer which is passed in from
392 .BR pcap_dispatch() ,
394 .I const struct pcap_pkthdr
395 pointer to a structure with the following members:
401 containing the time when the packet was captured
406 giving the number of bytes of the packet that are available from the
412 giving the length of the packet, in bytes (which might be more than the
413 number of bytes available from the capture, if the length of the packet
414 is larger than the maximum number of bytes to capture)
422 .I struct pcap_pkthdr
423 a pointer to which is passed to the callback routine)
424 bytes of data from the packet (which won't necessarily be the entire
425 packet; to capture the entire packet, you will have to provide a value
430 that is sufficiently large to get all of the packet's data - a value of
431 65535 should be sufficient on most if not all networks).
433 The number of packets read is returned.
434 0 is returned if no packets were read from a live capture (if, for
435 example, they were discarded because they didn't pass the packet filter,
436 or if, on platforms that support a read timeout that starts before any
437 packets arrive, the timeout expires before any packets arrive, or if the
438 file descriptor for the capture device is in non-blocking mode and no
439 packets were available to be read) or if no more packets are available
440 in a ``savefile.'' A return of \-1 indicates
441 an error in which case
445 may be used to display the error text.
448 when reading a live capture,
450 will not necessarily return when the read times out; on some platforms,
451 the read timeout isn't supported, and, on other platforms, the timer
452 doesn't start until at least one packet arrives. This means that the
455 be used in, for example, an interactive application, to allow the packet
456 capture loop to ``poll'' for user input periodically, as there's no
459 will return after the timeout expires.
464 except it keeps reading packets until
466 packets are processed or an error occurs.
469 return when live read timeouts occur.
470 Rather, specifying a non-zero read timeout to
474 allows the reception and processing of any packets that arrive when the
480 to loop forever (or at least until an error occurs). A negative number
481 is returned on an error; 0 is returned if
486 reads the next packet (by calling
492 pointer to the data in that packet. (The
494 struct for that packet is not supplied.)
496 is returned if an error occured, or if no packets were read from a live
497 capture (if, for example, they were discarded because they didn't pass
498 the packet filter, or if, on platforms that support a read timeout that
499 starts before any packets arrive, the timeout expires before any packets
500 arrive, or if the file descriptor for the capture device is in
501 non-blocking mode and no packets were available to be read), or if no
502 more packets are available in a ``savefile.'' Unfortunately, there is
503 no way to determine whether an error occured or not.
506 outputs a packet to the ``savefile'' opened with
507 .BR pcap_dump_open() .
508 Note that its calling arguments are suitable for use with
512 If called directly, the
517 .BR pcap_dump_open() .
520 is used to compile the string
522 into a filter program.
526 struct and is filled in by
529 controls whether optimization on the resulting code is performed.
531 specifies the IPv4 netmask of the network on which packets are being
532 captured; it is used only when checking for IPv4 broadcast addresses in
533 the filter program. If the netmask of the network on which packets are
534 being captured isn't known to the program, or if packets are being
535 captured on the Linux "any" pseudo-interface that can capture on more
536 than one network, a value of 0 can be supplied; tests for IPv4 broadcast
537 addreses won't be done correctly, but all other tests in the filter
538 program will be OK. A return of \-1 indicates an error in which case
540 may be used to display the error text.
542 .B pcap_compile_nopcap()
545 except that instead of passing a pcap structure, one passes the
546 snaplen and linktype explicitly. It is intended to be used for
547 compiling filters for direct BPF usage, without necessarily having
550 A return of \-1 indicates an error; the error text is unavailable.
551 .RB ( pcap_compile_nopcap()
553 .BR pcap_open_dead() ,
557 the latter three routines can be used directly in order to get the error
558 text for a compilation error.)
562 is used to specify a filter program.
566 struct, usually the result of a call to
569 is returned on failure, in which case
571 may be used to display the error text;
573 is returned on success.
576 is used to free up allocated memory pointed to by a
580 when that BPF program is no longer needed, for example after it
581 has been made the filter program for a pcap structure by a call to
582 .BR pcap_setfilter() .
585 returns the link layer type; link layer types it can return include:
590 BSD loopback encapsulation; the link layer header is a 4-byte field, in
592 byte order, containing a PF_ value from
594 for the network-layer protocol of the packet
596 Note that ``host byte order'' is the byte order of the machine on which
597 the packets are captured, and the PF_ values are for the OS of the
598 machine on which the packets are captured; if a live capture is being
599 done, ``host byte order'' is the byte order of the machine capturing the
600 packets, and the PF_ values are those of the OS of the machine capturing
601 the packets, but if a ``savefile'' is being read, the byte order and PF_
604 necessarily those of the machine reading the capture file.
607 Ethernet (10Mb, 100Mb, 1000Mb, and up)
610 IEEE 802.5 Token Ring
616 SLIP; the link layer header contains, in order:
619 a 1-byte flag, which is 0 for packets received by the machine and 1 for
620 packets sent by the machine;
622 a 1-byte field, the upper 4 bits of which indicate the type of packet,
627 an unmodified IP datagram (TYPE_IP);
630 an uncompressed-TCP IP datagram (UNCOMPRESSED_TCP), with that byte being
631 the first byte of the raw IP header on the wire, containing the
632 connection number in the protocol field;
635 a compressed-TCP IP datagram (COMPRESSED_TCP), with that byte being the
636 first byte of the compressed TCP/IP datagram header;
639 for UNCOMPRESSED_TCP, the rest of the modified IP header, and for
640 COMPRESSED_TCP, the compressed TCP/IP datagram header;
644 for a total of 16 bytes; the uncompressed IP datagram follows the header
648 PPP; if the first 2 bytes are 0xff and 0x03, it's PPP in HDLC-like
649 framing, with the PPP header following those two bytes, otherwise it's
650 PPP without framing, and the packet begins with the PPP header
656 RFC 1483 LLC/SNAP-encapsulated ATM; the packet begins with an IEEE 802.2
660 raw IP; the packet begins with an IP header
663 PPP in HDLC-like framing, as per RFC 1662, or Cisco PPP with HDLC
664 framing, as per section 4.3.1 of RFC 1547; the first byte will be 0xFF
665 for PPP in HDLC-like framing, and will be 0x0F or 0x8F for Cisco PPP
669 PPPoE; the packet begins with a PPPoE header, as per RFC 2516
672 Cisco PPP with HDLC framing, as per section 4.3.1 of RFC 1547
675 IEEE 802.11 wireless LAN
681 OpenBSD loopback encapsulation; the link layer header is a 4-byte field, in
683 byte order, containing a PF_ value from OpenBSD's
685 for the network-layer protocol of the packet
687 Note that, if a ``savefile'' is being read, those PF_ values are
689 necessarily those of the machine reading the capture file.
692 Linux "cooked" capture encapsulation; the link layer header contains, in
696 a 2-byte "packet type", in network byte order, which is one of:
700 packet was sent to us by somebody else
703 packet was broadcast by somebody else
706 packet was multicast, but not broadcast, by somebody else
709 packet was sent by somebody else to somebody else
712 packet was sent by us
715 a 2-byte field, in network byte order, containing a Linux ARPHRD_ value
716 for the link layer device type;
718 a 2-byte field, in network byte order, containing the length of the
719 link layer address of the sender of the packet (which could be 0);
721 an 8-byte field containing that number of bytes of the link layer header
722 (if there are more than 8 bytes, only the first 8 are present);
724 a 2-byte field containing an Ethernet protocol type, in network byte
725 order, or containing 1 for Novell 802.3 frames without an 802.2 LLC
726 header or 4 for frames beginning with an 802.2 LLC header.
730 Apple LocalTalk; the packet begins with an AppleTalk LLAP header
733 SunATM devices; the link layer header contains, in order:
736 a 1-byte flag field, containing a direction flag in the uppermost bit,
737 which is set for packets transmitted by the machine and clear for
738 packets received by the machine, and a 4-byte traffic type in the
739 low-order 4 bits, which is one of:
749 LLC-encapsulated traffic
766 a 2-byte VCI field, in network byte order.
770 RFC 2625 IP-over-Fibre Channel, with the link-layer header being the
771 Network_Header as described in that RFC.
774 .B pcap_list_datalinks()
775 is used to get a list of the supported data link types of the interface
776 associated with the pcap descriptor.
777 .B pcap_list_datalinks()
778 allocates an array to hold the list and sets
780 The caller is responsible for freeing the array.
782 is returned on failure;
783 otherwise, the number of data link types in the array is returned.
785 .B pcap_set_datalink()
786 is used to set the current data link type of the pcap descriptor
787 to the type specified by
790 is returned on failure.
792 .B pcap_datalink_name_to_val()
793 translates a data link type name, which is a
797 removed, to the corresponding data link type value. The translation
799 is used to set the current data link type of the pcap descriptor
800 NULL is returned on failure.
802 .B pcap_datalink_val_to_name()
803 translates a data link type value to the corresponding data link type
806 is returned on failure.
809 returns the snapshot length specified when
814 returns true if the current ``savefile'' uses a different byte order
815 than the current system.
817 .B pcap_major_version()
818 returns the major number of the version of the pcap used to write the
821 .B pcap_minor_version()
822 returns the minor number of the version of the pcap used to write the
826 returns the standard I/O stream of the ``savefile,'' if a ``savefile''
828 .BR pcap_open_offline() ,
829 or NULL, if a network device was opened with
830 .BR pcap_open_live() .
833 returns 0 and fills in a
835 struct. The values represent packet statistics from the start of the
836 run to the time of the call. If there is an error or the underlying
837 packet capture doesn't support packet statistics, \-1 is returned and
838 the error text can be obtained with
843 is supported only on live captures, not on ``savefiles''; no statistics
844 are stored in ``savefiles'', so no statistics are available when reading
848 returns the file descriptor number from which captured packets are read,
849 if a network device was opened with
850 .BR pcap_open_live() ,
851 or \-1, if a ``savefile'' was opened with
852 .BR pcap_open_offline() .
855 prints the text of the last pcap library error on
861 returns the error text pertaining to the last pcap library error.
863 the pointer it returns will no longer point to a valid error message
866 passed to it is closed; you must use or copy the string before closing
876 closes the files associated with
878 and deallocates resources.
881 closes the ``savefile.''
884 tcpdump(1), tcpslice(1)
886 The original authors are:
890 Steven McCanne, all of the
891 Lawrence Berkeley National Laboratory, University of California, Berkeley, CA.
893 The current version is available from "The Tcpdump Group"'s Web site at
896 .I http://www.tcpdump.org/
899 Please send problems, bugs, questions, desirable enhancements, etc. to:
902 tcpdump-workers@tcpdump.org
905 Please send source code contributions, etc. to:
projects / libpcap / blob