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PCAP(3) 										  PCAP(3)

NAME
       pcap - Packet Capture library

SYNOPSIS
       #include <pcap.h>

       pcap_t *pcap_open_live(char *device, int snaplen,
	       int promisc, int to_ms, char *ebuf)
       pcap_t *pcap_open_dead(int linktype, int snaplen)
       pcap_t *pcap_open_offline(char *fname, char *ebuf)
       pcap_dumper_t *pcap_dump_open(pcap_t *p, char *fname)

       char errbuf[PCAP_ERRBUF_SIZE];
       char *pcap_lookupdev(char *errbuf)
       int pcap_lookupnet(char *device, bpf_u_int32 *netp,
	       bpf_u_int32 *maskp, char *errbuf)

       int pcap_dispatch(pcap_t *p, int cnt,
	       pcap_handler callback, u_char *user)
       int pcap_loop(pcap_t *p, int cnt,
	       pcap_handler callback, u_char *user)
       void pcap_dump(u_char *user, struct pcap_pkthdr *h,
	       u_char *sp)

       int pcap_compile(pcap_t *p, struct bpf_program *fp,
	       char *str, int optimize, bpf_u_int32 netmask)
       int pcap_setfilter(pcap_t *p, struct bpf_program *fp)
       void pcap_freecode(struct bpf_program *);

       u_char *pcap_next(pcap_t *p, struct pcap_pkthdr *h)

       int pcap_datalink(pcap_t *p)
       int pcap_snapshot(pcap_t *p)
       int pcap_is_swapped(pcap_t *p)
       int pcap_major_version(pcap_t *p)
       int pcap_minor_version(pcap_t *p)
       int pcap_stats(pcap_t *p, struct pcap_stat *ps)
       FILE *pcap_file(pcap_t *p)
       int pcap_fileno(pcap_t *p)
       void pcap_perror(pcap_t *p, char *prefix)
       char *pcap_geterr(pcap_t *p)
       char *pcap_strerror(int error)

       void pcap_close(pcap_t *p)
       void pcap_dump_close(pcap_dumper_t *p)

DESCRIPTION
       The  Packet Capture library provides a high level interface to packet capture systems. All
       packets on the network, even those destined for other hosts, are accessible  through  this
       mechanism.

ROUTINES
       NOTE:  errbuf in pcap_open_live(), pcap_open_offline(), pcap_lookupdev(), and pcap_lookup-
       net() is assumed to be able to hold at least PCAP_ERRBUF_SIZE chars.

       pcap_open_live() is used to obtain a packet capture descriptor to look at packets  on  the
       network.   device  is a string that specifies the network device to open; on Linux systems
       with 2.2 or later kernels, a device argument of "any" or NULL can be used to capture pack-
       ets  from  all  interfaces.   snaplen  specifies  the  maximum number of bytes to capture.
       promisc specifies if the interface is to be put into promiscuous mode.  (Note that even if
       this  parameter	is  false, the interface could well be in promiscuous mode for some other
       reason.)  For now, this doesn't work on the "any" device; if an argument of "any" or  NULL
       is  supplied,  the promisc flag is ignored.  to_ms specifies the read timeout in millisec-
       onds.  The read timeout is used to arrange that the read not  necessarily  return  immedi-
       ately  when a packet is seen, but that it wait for some amount of time to allow more pack-
       ets to arrive and to read multiple packets from the OS kernel in one operation.	 Not  all
       platforms  support  a  read timeout; on platforms that don't, the read timeout is ignored.
       ebuf is used to return error text and is only set when pcap_open_live() fails and  returns
       NULL.

       pcap_open_dead()  is  used  for	creating a pcap_t structure to use when calling the other
       functions in libpcap.  It is typically used when just  using  libpcap  for  compiling  BPF
       code.

       pcap_open_offline()  is	called	to  open a ``savefile'' for reading.  fname specifies the
       name of the file to open. The file has the same format as those	used  by  tcpdump(1)  and
       tcpslice(1).   The name "-" in a synonym for stdin.  ebuf is used to return error text and
       is only set when pcap_open_offline() fails and returns NULL.

       pcap_dump_open() is called to open a ``savefile'' for writing. The name "-" in  a  synonym
       for  stdout.   NULL  is	returned  on  failure.	 p  is	a  pcap  struct  as  returned  by
       pcap_open_offline() or pcap_open_live().  fname specifies the name of the  file	to  open.
       If NULL is returned, pcap_geterr() can be used to get the error text.

       pcap_lookupdev()   returns   a	pointer  to  a	network  device  suitable  for	use  with
       pcap_open_live() and pcap_lookupnet().  If there is an error, NULL is returned and  errbuf
       is filled in with an appropriate error message.

       pcap_lookupnet() is used to determine the network number and mask associated with the net-
       work device device.  Both netp and maskp are bpf_u_int32 pointers.  A return of	-1  indi-
       cates an error in which case errbuf is filled in with an appropriate error message.

       pcap_dispatch()	is used to collect and process packets.  cnt specifies the maximum number
       of packets to process before returning.	This is not a minimum number; when reading a live
       capture, only one bufferful of packets is read at a time, so fewer than cnt packets may be
       processed. A cnt of -1 processes all the packets received in one  buffer  when  reading	a
       live capture, or all the packets in the file when reading a ``savefile''.  callback speci-
       fies a routine to be called with three arguments: a u_char pointer which is passed in from
       pcap_dispatch(),  a  pointer  to  the pcap_pkthdr struct (which precede the actual network
       headers and data), and a u_char pointer to the packet data.

       The number of packets read is returned.	0 is returned if no packets were read from a live
       capture	(if, for example, they were discarded because they didn't pass the packet filter,
       or if, on platforms that support a read timeout that starts before any packets arrive, the
       timeout	expires  before  any  packets  arrive,	or if the file descriptor for the capture
       device is in non-blocking mode and no packets were available to be read)  or  if  no  more
       packets	are  available in a ``savefile.'' A return of -1 indicates an error in which case
       pcap_perror() or pcap_geterr() may be used to display the error text.

       NOTE: when reading a live capture, pcap_dispatch() will not necessarily	return	when  the
       read  times  out; on some platforms, the read timeout isn't supported, and, on other plat-
       forms, the timer doesn't start until at least one packet arrives.   This  means	that  the
       read  timeout should NOT be used in, for example, an interactive application, to allow the
       packet capture loop to ``poll'' for user input periodically, as there's no guarantee  that
       pcap_dispatch() will return after the timeout expires.

       pcap_loop()  is similar to pcap_dispatch() except it keeps reading packets until cnt pack-
       ets are processed or an error occurs.  It does not return when live read  timeouts  occur.
       Rather,	specifying a non-zero read timeout to pcap_open_live() and then calling pcap_dis-
       patch() allows the reception and processing of any packets that arrive  when  the  timeout
       occurs.	 A  negative  cnt  causes pcap_loop() to loop forever (or at least until an error
       occurs).

       pcap_next() returns a u_char pointer to the next packet.

       pcap_dump() outputs a packet to the ``savefile'' opened with pcap_dump_open().  Note  that
       its calling arguments are suitable for use with pcap_dispatch() or pcap_loop().

       pcap_compile()  is  used  to  compile  the string str into a filter program.  program is a
       pointer to a bpf_program struct and is filled in  by  pcap_compile().   optimize  controls
       whether optimization on the resulting code is performed.  netmask specifies the netmask of
       the local net.  A return of -1 indicates an error in which case pcap_geterr() may be  used
       to display the error text.

       pcap_compile_nopcap()  is  similar to pcap_compile() except that instead of passing a pcap
       structure, one passes the snaplen and linktype explicitly.  It is intended to be used  for
       compiling  filters for direct BPF usage, without necessarily having called pcap_open().	A
       return of -1 indicates an error; the error text is unavailable.	(pcap_compile_nopcap() is
       a wrapper around pcap_open_dead(), pcap_compile(), and pcap_close(); the latter three rou-
       tines can be used directly in order to get the error text for a compilation error.)

       pcap_setfilter() is used to specify a filter program.  fp is a pointer  to  a  bpf_program
       struct,	usually  the  result  of a call to pcap_compile().  -1 is returned on failure, in
       which case pcap_geterr() may be used to display the error text; 0 is returned on success.

       pcap_freecode() is used to free up allocated memory pointed to  by  a  bpf_program  struct
       generated  by  pcap_compile() when that BPF program is no longer needed, for example after
       it has been made the filter program for a pcap structure by a call to pcap_setfilter().

       pcap_datalink() returns the link layer type, e.g.  DLT_EN10MB.

       pcap_snapshot() returns the snapshot length specified when pcap_open_live was called.

       pcap_is_swapped() returns true if the current ``savefile'' uses	a  different  byte  order
       than the current system.

       pcap_major_version() returns the major number of the version of the pcap used to write the
       savefile.

       pcap_minor_version() returns the minor number of the version of the pcap used to write the
       savefile.

       pcap_file() returns the name of the ``savefile.''

       int  pcap_stats()  returns  0 and fills in a pcap_stat struct. The values represent packet
       statistics from the start of the run to the time of the call. If there is an error or  the
       under lying packet capture doesn't support packet statistics, -1 is returned and the error
       text can be obtained with pcap_perror() or pcap_geterr().

       pcap_fileno() returns the file descriptor number of the ``savefile.''

       pcap_perror() prints the text of the last pcap library error on stderr, prefixed  by  pre-
       fix.

       pcap_geterr() returns the error text pertaining to the last pcap library error.	NOTE: the
       pointer it returns will no longer point to a valid error message string after  the  pcap_t
       passed to it is closed; you must use or copy the string before closing the pcap_t.

       pcap_strerror() is provided in case strerror(1) isn't available.

       pcap_close() closes the files associated with p and deallocates resources.

       pcap_dump_close() closes the ``savefile.''

SEE ALSO
       tcpdump(1), tcpslice(1)

AUTHORS
       The original authors are:

       Van  Jacobson, Craig Leres and Steven McCanne, all of the Lawrence Berkeley National Labo-
       ratory, University of California, Berkeley, CA.

       The current version is available from "The Tcpdump Group"'s Web site at

	      http://www.tcpdump.org/

BUGS
       Please send problems, bugs, questions, desirable enhancements, etc. to:

	      tcpdump-workers@tcpdump.org

       Please send source code contributions, etc. to:

	      patches@tcpdump.org

					  3 January 2001				  PCAP(3)
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