Skip site navigation (1)Skip section navigation (2)

FreeBSD Man Pages

Man Page or Keyword Search:
Man Architecture
Apropos Keyword Search (all sections) Output format
home | help
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_t *pcap_fopen_offline(FILE *fp, char *errbuf)
       pcap_dumper_t *pcap_dump_open(pcap_t *p,	const char *fname)
       pcap_dumper_t *pcap_dump_fopen(pcap_t *p, FILE *fp)

       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)

       typedef void (*pcap_handler)(u_char *user, const	struct pcap_pkthdr *h,
				   const u_char	*bytes);
       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 *)
       int pcap_setdirection(pcap_t *p,	pcap_direction_t d)

       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_inject(pcap_t *p, const	void *buf, size_t size)
       int pcap_sendpacket(pcap_t *p, const u_char *buf, int size)

       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)
       long pcap_dump_ftell(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_fopen_offline(),	   pcap_setnonblock(),
       pcap_getnonblock(),     pcap_findalldevs(),    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.  Alternatively, you may call pcap_fopen_offline() to
       read dumped data	from an	existing open stream fp.  Note	that  on  Win-
       dows,  that  stream should be opened in binary mode.  errbuf is used to
       return  error  text  and	 is  only  set	when  pcap_open_offline()   or
       pcap_fopen_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.	Alternatively,	you  may  call
       pcap_dump_fopen()  to  write  data to an	existing open stream fp.  Note
       that on Windows,	that stream should be opened in	binary mode.  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

       Note that not all the addresses in the list of addresses	are  necessar-
       ily IPv4	or IPv6	addresses - you	must check the sa_family member	of the
       struct sockaddr before interpreting the contents	of the address.

       -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_inject()  sends  a	raw  packet through the	network	interface; buf
       points to the data of the packet, including the link-layer header,  and
       size  is	 the  number of	bytes in the packet.  It returns the number of
       bytes written on	success.  A return of -1 indicates an error  in	 which
       case  pcap_perror()  or	pcap_geterr() may be used to display the error
       text.  Note that, even if you successfully open the network  interface,
       you  might  not	have permission	to send	packets	on it, or it might not
       support sending packets;	as pcap_open_live() doesn't  have  a  flag  to
       indicate	whether	to open	for capturing, sending,	or capturing and send-
       ing, you	cannot request an open that supports sending and  be  notified
       at  open	 time  whether	sending	will be	possible.  Note	also that some
       devices might not support sending packets.

       Note that, on some platforms,  the  link-layer  header  of  the	packet
       that's  sent  might  not	 be  the  same as the link-layer header	of the
       packet supplied to pcap_inject(), as the	source link-layer address,  if
       the header contains such	an address, might be changed to	be the address
       assigned	to the interface on which the packet it	sent, if the  platform
       doesn't	support	 sending  completely  raw and unchanged	packets.  Even
       worse, some drivers on some platforms might change the link-layer  type
       field to	whatever value libpcap used when attaching to the device, even
       on platforms that do  nominally	support	 sending  completely  raw  and
       unchanged packets.

       pcap_sendpacket()  is  like  pcap_inject(), but it returns 0 on success
       and -1 on  failure.   (pcap_inject()  comes  from  OpenBSD;  pcap_send-
       packet()	comes from WinPcap.  Both are provided for compatibility.)

       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_setdirection() is used to specify a	direction that packets will be
       captured.   pcap_direction_t  is	 one  of  the	constants   PCAP_D_IN,
       PCAP_D_OUT  or  PCAP_D_INOUT.   PCAP_D_IN  will	only  capture  packets
       received	by the device, PCAP_D_OUT will only capture  packets  sent  by
       the device and PCAP_D_INOUT will	capture	packets	received by or sent by
       the device.  PCAP_D_INOUT is the	default	setting	if  this  function  is
       not  called.   This  isn't necessarily supported	on all platforms; some
       platforms might return an error,	and some  other	 platforms  might  not
       support	PCAP_D_OUT.  This operation is not supported if	a ``savefile''
       is being	read.  -1 is returned on failure, 0 is returned	on success.

       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_ftell() returns the current file  position  for  the  ``save-
       file'',	representing  the  number of bytes written by pcap_dump_open()
       and pcap_dump().	 -1 is returned	on error.

       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

Want to link to this manual page? Use this URL:
<http://www.freebsd.org/cgi/man.cgi?query=pcap&sektion=3&manpath=FreeBSD+6.3-RELEASE>

home | help