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PCAP(3)								       PCAP(3)

NAME
       pcap - Packet Capture library

SYNOPSIS
       #include	<pcap.h>

       char errbuf[PCAP_ERRBUF_SIZE];

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

       int pcap_setnonblock(pcap_t *p, int nonblock, char *errbuf);
       int pcap_getnonblock(pcap_t *p, char *errbuf);

       int pcap_findalldevs(pcap_if_t **alldevsp, char *errbuf)
       void pcap_freealldevs(pcap_if_t *alldevs)
       char *pcap_lookupdev(char *errbuf)
       int pcap_lookupnet(const	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 *);

       const u_char *pcap_next(pcap_t *p, struct pcap_pkthdr *h)
       int pcap_next_ex(pcap_t *p, struct pcap_pkthdr **pkt_header,
	       const u_char **pkt_data)

       void pcap_breakloop(pcap_t *)

       int pcap_datalink(pcap_t	*p)
       int pcap_list_datalinks(pcap_t *p, int **dlt_buf);
       int pcap_set_datalink(pcap_t *p,	int dlt);
       int pcap_datalink_name_to_val(const char	*name);
       const char *pcap_datalink_val_to_name(int dlt);
       const char *pcap_datalink_val_to_description(int	dlt);
       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)
       int pcap_get_selectable_fd(pcap_t *p)
       void pcap_perror(pcap_t *p, char	*prefix)
       char *pcap_geterr(pcap_t	*p)
       char *pcap_strerror(int error)
       const char *pcap_lib_version(void)

       void pcap_close(pcap_t *p)
       int pcap_dump_flush(pcap_dumper_t *p)
       FILE *pcap_dump_file(pcap_dumper_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_dead(),
       pcap_open_offline(), pcap_setnonblock(),	pcap_getnonblock(), pcap_find-
       alldevs(), pcap_lookupdev(), and	pcap_lookupnet() 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 net-
       work device to open; on Linux systems with  2.2	or  later  kernels,  a
       device  argument	 of  "any" or NULL can be used to capture packets from
       all interfaces.	snaplen	specifies the maximum number of	bytes to  cap-
       ture.   If  this	 value	is less	than the size of a packet that is cap-
       tured, only the first snaplen bytes of that packet will be captured and
       provided	 as  packet  data.   A value of	65535 should be	sufficient, on
       most if not all networks, to capture all	the data  available  from  the
       packet.	promisc	specifies if the interface is to be put	into promiscu-
       ous 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 milliseconds.	 The read timeout is used to arrange that  the
       read not	necessarily return immediately when a packet is	seen, but that
       it wait for some	amount of time to allow	more packets 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.  A zero value for to_ms, on	platforms that support
       a read timeout, will cause a read to wait forever to allow enough pack-
       ets  to	arrive,	 with  no  timeout.  errbuf is used to return error or
       warning text.  It will be set to	error text when	pcap_open_live() fails
       and  returns  NULL.   errbuf  may  also	be  set	 to  warning text when
       pcap_open_live()	succeds; to detect this	case the caller	should store a
       zero-length  string  in errbuf before calling pcap_open_live() and dis-
       play the	warning	to the user if	errbuf	is  no	longer	a  zero-length
       string.

       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.  errbuf 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_setnonblock()    puts    a	 capture   descriptor,	 opened	  with
       pcap_open_live(), into ``non-blocking'' mode, or	takes it out of	``non-
       blocking'' mode,	depending on whether the nonblock argument is non-zero
       or zero.	 It has	no effect on ``savefiles''.  If	there is an error,  -1
       is  returned and	errbuf is filled in with an appropriate	error message;
       otherwise, 0 is returned.  In ``non-blocking'' mode, an attempt to read
       from  the  capture  descriptor with pcap_dispatch() will, if no packets
       are currently available to be read, return 0  immediately  rather  than
       blocking	 waiting  for  packets to arrive.  pcap_loop() and pcap_next()
       will not	work in	``non-blocking'' mode.

       pcap_getnonblock() returns the current ``non-blocking''	state  of  the
       capture	descriptor; it always returns 0	on ``savefiles''.  If there is
       an error, -1 is returned	and errbuf is filled in	 with  an  appropriate
       error message.

       pcap_findalldevs()  constructs  a  list	of network devices that	can be
       opened with pcap_open_live().  (Note that there may be network  devices
       that  cannot  be	 opened	 with  pcap_open_live()	by the process calling
       pcap_findalldevs(), because, for	example, that process might  not  have
       sufficient  privileges to open them for capturing; if so, those devices
       will not	appear on the list.)  alldevsp is set to point	to  the	 first
       element of the list; each element of the	list is	of type	pcap_if_t, and
       has the following members:

	      next   if	not NULL, a pointer to the next	element	in  the	 list;
		     NULL for the last element of the list

	      name   a	pointer	 to  a	string giving a	name for the device to
		     pass to pcap_open_live()

	      description
		     if	not NULL, a pointer to a string	giving	a  human-read-
		     able description of the device

	      addresses
		     a pointer to the first element of a list of addresses for
		     the interface

	      flags  interface flags:

		     PCAP_IF_LOOPBACK
			    set	if the interface is a loopback interface

       Each element of the list	of addresses is	of type	pcap_addr_t,  and  has
       the following members:

	      next   if	 not  NULL, a pointer to the next element in the list;
		     NULL for the last element of the list

	      addr   a pointer to a struct sockaddr containing an address

	      netmask
		     if	not NULL, a pointer to a struct	sockaddr that contains
		     the  netmask  corresponding  to the address pointed to by
		     addr

	      broadaddr
		     if	not NULL, a pointer to a struct	sockaddr that contains
		     the   broadcast  address  corresponding  to  the  address
		     pointed to	by addr; may be	null if	the interface  doesn't
		     support broadcasts

	      dstaddr
		     if	not NULL, a pointer to a struct	sockaddr that contains
		     the destination  address  corresponding  to  the  address
		     pointed  to by addr; may be null if the interface isn't a
		     point-to-point interface

       -1 is returned on failure, in which case	errbuf is filled  in  with  an
       appropriate error message; 0 is returned	on success.

       pcap_freealldevs()  is  used  to	 free  a  list allocated by pcap_find-
       alldevs().

       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 asso-
       ciated with the	network	 device	 device.   Both	 netp  and  maskp  are
       bpf_u_int32  pointers.  A return	of -1 indicates	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 read-
       ing  a  live  capture,  or  all	the packets in the file	when reading a
       ``savefile''.  callback specifies a routine to  be  called  with	 three
       arguments:  a u_char pointer which is passed in from pcap_dispatch(), a
       const struct pcap_pkthdr	pointer	to a structure with the	following mem-
       bers:

	      ts     a	struct timeval containing the time when	the packet was
		     captured

	      caplen a bpf_u_int32 giving the number of	bytes  of  the	packet
		     that are available	from the capture

	      len    a	bpf_u_int32  giving the	length of the packet, in bytes
		     (which might be more than the number of  bytes  available
		     from  the	capture, if the	length of the packet is	larger
		     than the maximum number of	bytes to capture)

       and a const u_char pointer to the first caplen (as given	in the	struct
       pcap_pkthdr a pointer to	which is passed	to the callback	routine) bytes
       of data from the	packet (which won't necessarily	be the entire  packet;
       to  capture  the	 entire	 packet,  you will have	to provide a value for
       snaplen in your call to pcap_open_live()	that is	sufficiently large  to
       get all of the packet's data - a	value of 65535 should be sufficient on
       most if not all networks).

       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 time-
       out expires before any packets arrive, or if the	 file  descriptor  for
       the  capture  device is in non-blocking mode and	no packets were	avail-
       able 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.  A	return	of  -2
       indicates  that	the  loop terminated due to a call to pcap_breakloop()
       before  any  packets  were  processed.	If   your   application	  uses
       pcap_breakloop(),  make	sure  that you explicitly check	for -1 and -2,
       rather than just	checking for a return value < 0.

       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 platforms, 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 pack-
       ets until cnt packets 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_dispatch()
       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).  -1 is returned on	an error; 0 is
       returned	if cnt is exhausted; -2	is returned if the loop	terminated due
       to  a  call  to pcap_breakloop()	before any packets were	processed.  If
       your application	uses pcap_breakloop(), make sure that  you  explicitly
       check  for -1 and -2, rather than just checking for a return value < 0.

       pcap_next() reads the next packet (by calling  pcap_dispatch()  with  a
       cnt  of	1)  and	 returns  a u_char pointer to the data in that packet.
       (The pcap_pkthdr	struct for that	packet	is  not	 supplied.)   NULL  is
       returned	 if  an	 error occured,	or 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	 pack-
       ets 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.''  Unfortunately, there is no
       way to determine	whether	an error occured or not.

       pcap_next_ex() reads the	next  packet  and  returns  a  success/failure
       indication:

	      1	     the packet	was read without problems

	      0	     packets are being read from a live	capture, and the time-
		     out expired

	      -1     an	error occurred while reading the packet

	      -2     packets are being read from a ``savefile'', and there are
		     no	more packets to	read from the savefile.

       If  the packet was read without problems, the pointer pointed to	by the
       pkt_header argument is set to point to the pcap_pkthdr struct  for  the
       packet,	and  the pointer pointed to by the pkt_data argument is	set to
       point to	the data in the	packet.

       pcap_breakloop()	 sets  a  flag	that  will  force  pcap_dispatch()  or
       pcap_loop()  to return rather than looping; they	will return the	number
       of packets that have been processed so far, or -2 if  no	 packets  have
       been processed so far.

       This  routine  is safe to use inside a signal handler on	UNIX or	a con-
       sole control handler on Windows,	as it  merely  sets  a	flag  that  is
       checked within the loop.

       The  flag is checked in loops reading packets from the OS - a signal by
       itself will not necessarily terminate those loops - as well as in loops
       processing  a  set of packets returned by the OS.  Note that if you are
       catching	signals	on UNIX	systems	that support restarting	 system	 calls
       after a signal, and calling pcap_breakloop() in the signal handler, you
       must specify, when catching those signals, that system calls should NOT
       be  restarted  by  that signal.	Otherwise, if the signal interrupted a
       call reading packets in	a  live	 capture,  when	 your  signal  handler
       returns after calling pcap_breakloop(), the call	will be	restarted, and
       the loop	will not terminate until more packets arrive and the call com-
       pletes.

       Note  also  that,  in  a	 multi-threaded	 application, if one thread is
       blocked	  in	pcap_dispatch(),    pcap_loop(),    pcap_next(),    or
       pcap_next_ex(),	a  call	to pcap_breakloop() in a different thread will
       not unblock that	thread;	you will need to use whatever mechanism	the OS
       provides	 for  breaking	a  thread  out	of  blocking calls in order to
       unblock the thread, such	as thread cancellation in systems that support
       POSIX threads.

       Note  that  pcap_next()	will,  on some platforms, loop reading packets
       from the	OS; that loop will not necessarily be terminated by a  signal,
       so  pcap_breakloop() should be used to terminate	packet processing even
       if pcap_next() is being used.

       pcap_breakloop()	does not guarantee that	no  further  packets  will  be
       processed by pcap_dispatch() or pcap_loop() after it is called; at most
       one more	packet might be	processed.

       If -2 is	returned from pcap_dispatch()  or  pcap_loop(),	 the  flag  is
       cleared,	 so a subsequent call will resume reading packets.  If a posi-
       tive number is returned,	the flag is not	cleared, so a subsequent  call
       will return -2 and clear	the flag.

       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().	 If  called directly, the user
       parameter is of type pcap_dumper_t as returned by pcap_dump_open().

       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  result-
       ing  code is performed.	netmask	specifies the IPv4 netmask of the net-
       work on which packets are being captured; it is used only when checking
       for  IPv4 broadcast addresses in	the filter program.  If	the netmask of
       the network on which packets are	being captured isn't known to the pro-
       gram, or	if packets are being captured on the Linux "any" pseudo-inter-
       face that can capture on	more than one network, a value	of  0  can  be
       supplied;  tests	 for  IPv4 broadcast addreses won't be done correctly,
       but all other tests in the filter program will be OK.  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_com-
       pile_nopcap() is	a wrapper around pcap_open_dead(), pcap_compile(), and
       pcap_close(); the latter	three routines 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_com-
       pile().	 -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; link layer types	it can
       return include:

	    DLT_NULL
		 BSD loopback encapsulation; the link layer header is a	4-byte
		 field,	 in  host  byte	 order,	 containing  a	PF_ value from
		 socket.h for the network-layer	protocol of the	packet.

		 Note that ``host byte	order''	 is  the  byte	order  of  the
		 machine on which the packets are captured, and	the PF_	values
		 are for the OS	of the machine on which	the packets  are  cap-
		 tured;	 if  a live capture is being done, ``host byte order''
		 is the	byte order of the machine capturing the	 packets,  and
		 the  PF_  values are those of the OS of the machine capturing
		 the packets, but if a ``savefile'' is being  read,  the  byte
		 order and PF_ values are not necessarily those	of the machine
		 reading the capture file.

	    DLT_EN10MB
		 Ethernet (10Mb, 100Mb,	1000Mb,	and up)

	    DLT_IEEE802
		 IEEE 802.5 Token Ring

	    DLT_ARCNET
		 ARCNET

	    DLT_SLIP
		 SLIP; the link	layer header contains, in order:

		      a	1-byte flag, which is 0	for packets  received  by  the
		      machine and 1 for	packets	sent by	the machine;

		      a	 1-byte	 field,	the upper 4 bits of which indicate the
		      type of packet, as per RFC 1144:

			   0x40	an unmodified IP datagram (TYPE_IP);

			   0x70	an  uncompressed-TCP   IP   datagram   (UNCOM-
				PRESSED_TCP),  with  that byte being the first
				byte of	the raw	IP header on  the  wire,  con-
				taining	 the connection	number in the protocol
				field;

			   0x80	a compressed-TCP IP datagram (COMPRESSED_TCP),
				with  that  byte  being	 the first byte	of the
				compressed TCP/IP datagram header;

		      for  UNCOMPRESSED_TCP,  the  rest	 of  the  modified  IP
		      header,  and  for	 COMPRESSED_TCP, the compressed	TCP/IP
		      datagram header;

		 for a total of	16 bytes; the uncompressed IP datagram follows
		 the header.

	    DLT_PPP
		 PPP;  if  the	first  2  bytes	are 0xff and 0x03, it's	PPP in
		 HDLC-like framing, with the PPP header	 following  those  two
		 bytes,	 otherwise  it's  PPP  without framing,	and the	packet
		 begins	with the PPP header.

	    DLT_FDDI
		 FDDI

	    DLT_ATM_RFC1483
		 RFC 1483 LLC/SNAP-encapsulated	ATM; the packet	begins with an
		 IEEE 802.2 LLC	header.

	    DLT_RAW
		 raw IP; the packet begins with	an IP header.

	    DLT_PPP_SERIAL
		 PPP  in HDLC-like framing, as per RFC 1662, or	Cisco PPP with
		 HDLC framing, as per section 4.3.1 of	RFC  1547;  the	 first
		 byte  will  be	0xFF for PPP in	HDLC-like framing, and will be
		 0x0F or 0x8F for Cisco	PPP with HDLC framing.

	    DLT_PPP_ETHER
		 PPPoE;	the packet begins with a  PPPoE	 header,  as  per  RFC
		 2516.

	    DLT_C_HDLC
		 Cisco	PPP  with  HDLC	 framing,  as per section 4.3.1	of RFC
		 1547.

	    DLT_IEEE802_11
		 IEEE 802.11 wireless LAN

	    DLT_FRELAY
		 Frame Relay

	    DLT_LOOP
		 OpenBSD loopback encapsulation; the link layer	 header	 is  a
		 4-byte	 field,	 in network byte order,	containing a PF_ value
		 from OpenBSD's	socket.h for the network-layer protocol	of the
		 packet.

		 Note  that, if	a ``savefile'' is being	read, those PF_	values
		 are not necessarily those of the machine reading the  capture
		 file.

	    DLT_LINUX_SLL
		 Linux	"cooked"  capture encapsulation; the link layer	header
		 contains, in order:

		      a	2-byte "packet type", in network byte order, which  is
		      one of:

			   0	packet was sent	to us by somebody else

			   1	packet was broadcast by	somebody else

			   2	packet	was  multicast,	 but not broadcast, by
				somebody else

			   3	packet was sent	by somebody else  to  somebody
				else

			   4	packet was sent	by us

		      a	 2-byte	 field,	 in  network  byte order, containing a
		      Linux ARPHRD_ value for the link layer device type;

		      a	2-byte field, in network byte  order,  containing  the
		      length  of  the  link layer address of the sender	of the
		      packet (which could be 0);

		      an 8-byte	field containing that number of	bytes  of  the
		      link  layer header (if there are more than 8 bytes, only
		      the first	8 are present);

		      a	2-byte field containing	an Ethernet protocol type,  in
		      network  byte  order,  or	 containing 1 for Novell 802.3
		      frames without an	802.2  LLC  header  or	4  for	frames
		      beginning	with an	802.2 LLC header.

	    DLT_LTALK
		 Apple	LocalTalk;  the	 packet	 begins	with an	AppleTalk LLAP
		 header.

	    DLT_PFLOG
		 OpenBSD pflog;	the link layer header contains,	in order:

		      a	1-byte header length, in host byte order;

		      a	4-byte PF_ value, in host byte order;

		      a	2-byte action code, in network byte  order,  which  is
		      one of:

			   0	passed

			   1	dropped

			   2	scrubbed

		      a	 2-byte	 reason	 code, in network byte order, which is
		      one of:

			   0	match

			   1	bad offset

			   2	fragment

			   3	short

			   4	normalize

			   5	memory

		      a	16-character interface name;

		      a	16-character ruleset name (only	meaningful if  subrule
		      is set);

		      a	4-byte rule number, in network byte order;

		      a	4-byte subrule number, in network byte order;

		      a	 1-byte	direction, in network byte order, which	is one
		      of:

			   0	incoming or outgoing

			   1	incoming

			   2	outgoing

	    DLT_PRISM_HEADER
		 Prism monitor mode information	followed by an 802.11  header.

	    DLT_IP_OVER_FC
		 RFC  2625  IP-over-Fibre  Channel, with the link-layer	header
		 being the Network_Header as described in that RFC.

	    DLT_SUNATM
		 SunATM	devices; the link layer	header contains, in order:

		      a	1-byte flag field, containing a	direction flag in  the
		      uppermost	 bit,  which is	set for	packets	transmitted by
		      the machine  and	clear  for  packets  received  by  the
		      machine,	and  a	4-byte traffic type in the low-order 4
		      bits, which is one of:

			   0	raw traffic

			   1	LANE traffic

			   2	LLC-encapsulated traffic

			   3	MARS traffic

			   4	IFMP traffic

			   5	ILMI traffic

			   6	Q.2931 traffic

		      a	1-byte VPI value;

		      a	2-byte VCI field, in network byte order.

	    DLT_IEEE802_11_RADIO
		 link-layer information	followed by an	802.11	header	-  see
		 http://www.shaftnet.org/~pizza/software/capturefrm.txt	 for a
		 description of	the link-layer information.

	    DLT_ARCNET_LINUX
		 ARCNET, with no exception frames, reassembled packets	rather
		 than raw frames, and an extra 16-bit offset field between the
		 destination host and type bytes.

	    DLT_LINUX_IRDA
		 Linux-IrDA packets, with a DLT_LINUX_SLL header  followed  by
		 the IrLAP header.

       pcap_list_datalinks()  is used to get a list of the supported data link
       types  of  the  interface  associated   with   the   pcap   descriptor.
       pcap_list_datalinks()  allocates	 an  array  to	hold the list and sets
       *dlt_buf.  The caller is	responsible for	 freeing  the  array.	-1  is
       returned	 on  failure;  otherwise, the number of	data link types	in the
       array is	returned.

       pcap_set_datalink() is used to set the current data link	 type  of  the
       pcap  descriptor	to the type specified by dlt.  -1 is returned on fail-
       ure.

       pcap_datalink_name_to_val() translates a	data link type name, which  is
       a  DLT_ name with the DLT_ removed, to the corresponding	data link type
       value.  The translation is case-insensitive.  -1	is returned  on	 fail-
       ure.

       pcap_datalink_val_to_name()  translates	a  data	link type value	to the
       corresponding data link type name.  NULL	is returned on failure.

       pcap_datalink_val_to_description() translates a data link type value to
       a  short	description of that data link type.  NULL is returned on fail-
       ure.

       pcap_list_datalinks() is	used to	get a list of the supported data  link
       types   of   the	  interface   associated  with	the  pcap  descriptor.
       pcap_list_datalinks() allocates an array	to  hold  the  list  and  sets
       *dlt_buf.   The	caller	is  responsible	 for freeing the array.	 -1 is
       returned	on failure; otherwise, the number of data link	types  in  the
       array is	returned.

       pcap_set_datalink()  is	used  to set the current data link type	of the
       pcap descriptor to the type specified by	dlt.  -1 is returned on	 fail-
       ure.

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

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

       pcap_major_version() returns the	major number of	the file format	of the
       savefile; pcap_minor_version() returns the minor	 number	 of  the  file
       format  of the savefile.	 The version number is stored in the header of
       the savefile.

       pcap_file() returns the standard	I/O stream of the ``savefile,''	 if  a
       ``savefile'' was	opened with pcap_open_offline(), or NULL, if a network
       device was opened with pcap_open_live().

       pcap_stats() returns 0 and fills	in a pcap_stat struct. The values rep-
       resent  packet  statistics from the start of the	run to the time	of the
       call. If	there is an error or the  underlying  packet  capture  doesn't
       support	packet	statistics,  -1	 is returned and the error text	can be
       obtained	with pcap_perror() or  pcap_geterr().	pcap_stats()  is  sup-
       ported  only  on	live captures, not on ``savefiles''; no	statistics are
       stored in ``savefiles'',	so no statistics are  available	 when  reading
       from a ``savefile''.

       pcap_fileno()  returns  the  file descriptor number from	which captured
       packets are read, if a network device was opened	with pcap_open_live(),
       or -1, if a ``savefile''	was opened with	pcap_open_offline().

       pcap_get_selectable_fd()	returns, on UNIX, a file descriptor number for
       a file descriptor on which one can do a select()	or poll() to wait  for
       it  to be possible to read packets without blocking, if such a descrip-
       tor exists, or -1, if no	such descriptor	exists.	 Some network  devices
       opened  with  pcap_open_live()  do  not support select()	or poll() (for
       example,	regular	network	devices	on FreeBSD 4.3 and 4.4,	and Endace DAG
       devices), so -1 is returned for those devices.

       Note  that  on most versions of most BSDs (including Mac	OS X) select()
       and   poll()    do    not    work    correctly	 on    BPF    devices;
       pcap_get_selectable_fd()	will return a file descriptor on most of those
       versions	(the exceptions	being FreeBSD 4.3 and 4.4), a simple  select()
       or   poll()   will  not	return	even  after  a	timeout	 specified  in
       pcap_open_live()	expires.  To work around  this,	 an  application  that
       uses  select()  or  poll()  to  wait for	packets	to arrive must put the
       pcap_t in non-blocking mode, and	must  arrange  that  the  select()  or
       poll()  have  a	timeout	less than or equal to the timeout specified in
       pcap_open_live(), and must try  to  read	 packets  after	 that  timeout
       expires,	 regardless  of	 whether select() or poll() indicated that the
       file descriptor for the pcap_t is ready to be read or not.  (That work-
       around  will not	work in	FreeBSD	4.3 and	later; however,	in FreeBSD 4.6
       and later, select() and poll() work correctly on	BPF  devices,  so  the
       workaround isn't	necessary, although it does no harm.)

       pcap_get_selectable_fd()	is not available on Windows.

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

       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_lib_version()  returns  a  pointer	to a string giving information
       about the version of the	libpcap	library	being used; note that it  con-
       tains more information than just	a version number.

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

       pcap_dump_file()	returns	the standard I/O stream	 of  the  ``savefile''
       opened by pcap_dump_open().

       pcap_dump_flush()  flushes  the	output	buffer to the ``savefile,'' so
       that any	packets	written	with pcap_dump() but not yet  written  to  the
       ``savefile'' will be written.  -1 is returned on	error, 0 on success.

       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 Laboratory, 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

			       27 February 2004			       PCAP(3)

NAME | SYNOPSIS | DESCRIPTION | ROUTINES | SEE ALSO | AUTHORS | BUGS

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