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PCAP(3)			   Library Functions Manual		       PCAP(3)

       pcap - Packet Capture library

       #include	<pcap/pcap.h>

       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.  It also supports
       saving captured packets to a ``savefile'', and reading packets  from  a

   Opening a capture handle for	reading
       To  open	 a handle for a	live capture, given the	name of	the network or
       other interface on which	the capture should  be	done,  call  pcap_cre-
       ate(),  set the appropriate options on the handle, and then activate it
       with pcap_activate().

       To obtain a list	of devices that	can be opened for a live capture, call
       pcap_findalldevs();  to	free  the list returned	by pcap_findalldevs(),
       call pcap_freealldevs().	 pcap_lookupdev() will return the first	device
       on that list that is not	a ``loopback`` network interface.

       To  open	 a handle for a	``savefile'' from which	to read	packets, given
       the pathname of the ``savefile'', call pcap_open_offline(); to set up a
       handle  for  a ``savefile'', given a FILE * referring to	a file already
       opened for reading, call	pcap_fopen_offline().

       In order	to get a ``fake'' pcap_t for use in routines  that  require  a
       pcap_t  as  an  argument,  such	as routines to open a ``savefile'' for
       writing and to compile a	filter expression, call	pcap_open_dead().

       pcap_create(),	 pcap_open_offline(),	 pcap_fopen_offline(),	   and
       pcap_open_dead()	return a pointer to a pcap_t, which is the handle used
       for reading packets from	the capture stream or  the  ``savefile'',  and
       for  finding  out information about the capture stream or ``savefile''.
       To close	a handle, use pcap_close().

       The options that	can be set on a	capture	handle include

       snapshot	length
	      If, when capturing, you  capture	the  entire  contents  of  the
	      packet,  that  requires more CPU time to copy the	packet to your
	      application, more	disk and possibly network bandwidth  to	 write
	      the  packet  data	 to  a	file,  and more	disk space to save the
	      packet.  If you don't need the entire contents of	the  packet  -
	      for  example,  if	 you are only interested in the	TCP headers of
	      packets -	you can	set the	"snapshot length" for the  capture  to
	      an appropriate value.  If	the snapshot length is set to snaplen,
	      and snaplen is less than the size	of a packet that is  captured,
	      only the first snaplen bytes of that packet will be captured and
	      provided as packet data.

	      A	snapshot length	of 65535 should	be sufficient, on most if  not
	      all networks, to capture all the data available from the packet.

	      The snapshot length is set with pcap_set_snaplen().

       promiscuous mode
	      On  broadcast  LANs  such	 as  Ethernet,	if  the	 network isn't
	      switched,	or if the adapter is connected to a "mirror port" on a
	      switch to	which all packets passing through the switch are sent,
	      a	network	adapter	receives all packets  on  the  LAN,  including
	      unicast  or multicast packets not	sent to	a network address that
	      the network adapter isn't	configured to recognize.

	      Normally,	the adapter will discard those packets;	however,  many
	      network  adapters	support	"promiscuous mode", which is a mode in
	      which all	packets, even if they are not sent to an address  that
	      the  adapter recognizes, are provided to the host.  This is use-
	      ful for passively	capturing traffic between two  or  more	 other
	      hosts for	analysis.

	      Note  that even if an application	does not set promiscuous mode,
	      the adapter could	well be	in promiscuous	mode  for  some	 other

	      For  now,	 this doesn't work on the "any"	device;	if an argument
	      of "any" or NULL is supplied, the	setting	of promiscuous mode is

	      Promiscuous mode is set with pcap_set_promisc().

       monitor mode
	      On IEEE 802.11 wireless LANs, even if an adapter is in promiscu-
	      ous mode,	it will	supply to the host only	frames for the network
	      with  which  it's	 associated.   It  might also supply only data
	      frames, not management or	control	frames,	and might not  provide
	      the  802.11  header or radio information pseudo-header for those

	      In "monitor mode", sometimes also	called "rfmon mode" (for  "Ra-
	      dio Frequency MONitor"), the adapter will	supply all frames that
	      it receives, with	802.11 headers,	and  might  supply  a  pseudo-
	      header with radio	information about the frame as well.

	      Note  that  in  monitor mode the adapter might disassociate from
	      the network with which it's associated, so that you will not  be
	      able to use any wireless networks	with that adapter.  This could
	      prevent accessing	files on a network server, or  resolving  host
	      names or network addresses, if you are capturing in monitor mode
	      and are not connected to another network with another adapter.

	      Monitor mode is set with pcap_set_rfmon(), and  pcap_can_set_rf-
	      mon()  can  be  used  to determine whether an adapter can	be put
	      into monitor mode.

       read timeout
	      If, when capturing, packets are delivered	as soon	 as  they  ar-
	      rive, the	application capturing the packets will be woken	up for
	      each packet as it	arrives, and might have	to make	 one  or  more
	      calls to the operating system to fetch each packet.

	      If,  instead,  packets are not delivered as soon as they arrive,
	      but are delivered	after a	short delay (called a "read timeout"),
	      more  than  one packet can be accumulated	before the packets are
	      delivered, so that a single wakeup would be  done	 for  multiple
	      packets,	and  each  set	of  calls made to the operating	system
	      would supply multiple packets,  rather  than  a  single  packet.
	      This reduces the per-packet CPU overhead if packets are arriving
	      at a high	rate, increasing the number of packets per second that
	      can be captured.

	      The  read	 timeout is required so	that an	application won't wait
	      for the operating	system's capture  buffer  to  fill  up	before
	      packets are delivered; if	packets	are arriving slowly, that wait
	      could take an arbitrarily	long period of time.

	      Not all platforms	support	a  read	 timeout;  on  platforms  that
	      don't,  the read timeout is ignored.  A zero value for the time-
	      out, on platforms	that support a read timeout, will cause	a read
	      to wait forever to allow enough packets to arrive, with no time-

	      NOTE: the	read timeout cannot be used to cause calls  that  read
	      packets  to  return within a limited period of time, because, on
	      some platforms, the read timeout isn't supported,	and, on	 other
	      platforms, the timer doesn't start until at least	one packet ar-
	      rives.  This means that the read timeout should NOT be used, for
	      example,	in an interactive application to allow the packet cap-
	      ture loop	to ``poll'' for	user input periodically, as there's no
	      guarantee	 that  a  call	reading	 packets will return after the
	      timeout expires even if no packets have arrived.

	      The read timeout is set with pcap_set_timeout().

       buffer size
	      Packets that arrive for a	capture	are stored  in	a  buffer,  so
	      that  they  do not have to be read by the	application as soon as
	      they arrive.  On some platforms, the buffer's size can be	set; a
	      size  that's  too	small could mean that, if too many packets are
	      being captured and the snapshot length doesn't limit the	amount
	      of  data that's buffered,	packets	could be dropped if the	buffer
	      fills up before the application can read packets from it,	 while
	      a	 size  that's  too large could use more	non-pageable operating
	      system memory than is necessary to prevent  packets  from	 being

	      The buffer size is set with pcap_set_buffer_size().

       timestamp type
	      On  some platforms, the time stamp given to packets on live cap-
	      tures can	come from different sources that  can  have  different
	      resolutions or that can have different relationships to the time
	      values for the current time supplied by routines on  the	native
	      operating	 system.   See pcap-tstamp(7) for a list of time stamp

	      The time stamp type is set with pcap_set_tstamp_type().

       Reading packets from a network interface	may require that you have spe-
       cial privileges:

       Under SunOS 3.x or 4.x with NIT or BPF:
	      You must have read access	to /dev/nit or /dev/bpf*.

       Under Solaris with DLPI:
	      You  must	 have  read/write access to the	network	pseudo device,
	      e.g.  /dev/le.  On at least some versions	of  Solaris,  however,
	      this  is not sufficient to allow tcpdump to capture in promiscu-
	      ous mode;	on those versions of Solaris, you must be root,	or the
	      application  capturing packets must be installed setuid to root,
	      in order to capture in promiscuous mode.	 Note  that,  on  many
	      (perhaps	all)  interfaces,  if you don't	capture	in promiscuous
	      mode, you	will not see any outgoing packets, so  a  capture  not
	      done in promiscuous mode may not be very useful.

	      In  newer	 versions  of  Solaris,	 you  must have	been given the
	      net_rawaccess privilege; this is both necessary  and  sufficient
	      to  give	you  access to the network pseudo-device - there is no
	      need to change the privileges on that device.   A	 user  can  be
	      given  that  privilege by, for example, adding that privilege to
	      the user's defaultpriv key with the usermod (1M) command.

       Under HP-UX with	DLPI:
	      You must be root or the application capturing  packets  must  be
	      installed	setuid to root.

       Under IRIX with snoop:
	      You  must	 be  root or the application capturing packets must be
	      installed	setuid to root.

       Under Linux:
	      You must be root or the application capturing  packets  must  be
	      installed	 setuid	to root	(unless	your distribution has a	kernel
	      that supports capability bits such as CAP_NET_RAW	 and  code  to
	      allow  those  capability bits to be given	to particular accounts
	      and to cause those bits to be set	on a user's initial  processes
	      when  they  log  in, in which case you  must have	CAP_NET_RAW in
	      order to capture and CAP_NET_ADMIN to enumerate network  devices
	      with, for	example, the -D	flag).

       Under ULTRIX and	Digital	UNIX/Tru64 UNIX:
	      Any  user	 may  capture  network traffic.	 However, no user (not
	      even the super-user) can capture in promiscuous mode on  an  in-
	      terface unless the super-user has	enabled	promiscuous-mode oper-
	      ation on that interface using pfconfig(8), and no	user (not even
	      the  super-user) can capture unicast traffic received by or sent
	      by the machine on	an interface unless the	super-user has enabled
	      copy-all-mode  operation	on  that  interface using pfconfig, so
	      useful packet capture on an interface probably requires that ei-
	      ther  promiscuous-mode or	copy-all-mode operation, or both modes
	      of operation, be enabled on that interface.

       Under BSD (this includes	Mac OS X):
	      You must have read access	to /dev/bpf*  on  systems  that	 don't
	      have  a  cloning	BPF device, or to /dev/bpf on systems that do.
	      On BSDs with a devfs (this includes Mac OS X),  this  might  in-
	      volve more than just having somebody with	super-user access set-
	      ting the ownership or permissions	on the BPF devices - it	 might
	      involve  configuring  devfs  to set the ownership	or permissions
	      every time the system is booted, if  the	system	even  supports
	      that;  if	 it  doesn't support that, you might have to find some
	      other way	to make	that happen at boot time.

       Reading a saved packet file doesn't require special privileges.

       The packets read	from the handle	may include a  ``pseudo-header''  con-
       taining	various	 forms	of  packet  meta-data, and probably includes a
       link-layer header whose contents	can differ for different  network  in-
       terfaces.   To determine	the format of the packets supplied by the han-
       dle, call pcap_datalink();	 lists
       the  values it returns and describes the	packet formats that correspond
       to those	values.

       To obtain the FILE * corresponding to a pcap_t  opened  for  a  ``save-
       file'', call pcap_file().


		     get a pcap_t for live capture

		     activate a	pcap_t for live	capture

		     get  a list of devices that can be	opened for a live cap-

		     free list of devices

		     get first non-loopback device on that list

		     open a pcap_t for a ``savefile'', given a pathname

		     open a pcap_t for a ``savefile'', given a FILE *

		     create a ``fake'' pcap_t

		     close a pcap_t

		     set the snapshot length for  a  not-yet-activated	pcap_t
		     for live capture

		     get the snapshot length for a pcap_t

		     set  promiscuous  mode for	a not-yet-activated pcap_t for
		     live capture

		     set monitor mode for a not-yet-activated pcap_t for  live

		     determine	whether	 monitor  mode can be set for a	pcap_t
		     for live capture

		     set read timeout for a not-yet-activated pcap_t for  live

		     set  buffer  size for a not-yet-activated pcap_t for live

		     set time stamp type for a	not-yet-activated  pcap_t  for
		     live capture

		     get  list of available time stamp types for a not-yet-ac-
		     tivated pcap_t for	live capture

		     free list of available time stamp types

		     get name for a time stamp type

		     get description for a time	stamp type

		     get time stamp type corresponding to a name

		     get link-layer header type	for a pcap_t

		     get the FILE * for	a pcap_t opened	for a ``savefile''

		     determine whether a ``savefile'' being read came  from  a
		     machine with the opposite byte order

		     get  the  major and minor version of the file format ver-
		     sion for a	``savefile''

   Selecting a link-layer header type for a live capture
       Some devices may	provide	more than one link-layer header	type.  To  ob-
       tain  a	list of	all link-layer header types provided by	a device, call
       pcap_list_datalinks() on	an activated pcap_t for	the device.  To	free a
       list  of	 link-layer  header types, call	pcap_free_datalinks().	To set
       the link-layer header type  for	a  device,  call  pcap_set_datalink().
       This  should be done after the device has been activated	but before any
       packets are read	and before any filters are compiled or installed.


		     get a list	of link-layer header types for a device

		     free list of link-layer header types

		     set link-layer header type	for a device

		     get name for a link-layer header type

		     get description for a link-layer header type

		     get link-layer header type	corresponding to a name

   Reading packets
       Packets are read	with pcap_dispatch() or	pcap_loop(), which process one
       or  more	 packets,  calling a callback routine for each packet, or with
       pcap_next() or pcap_next_ex(), which return the next packet.  The call-
       back  for  pcap_dispatch()  and	pcap_loop() is supplied	a pointer to a
       struct pcap_pkthdr, which includes the following	members:

	      ts     a struct timeval containing the time when the packet  was

	      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).

       pcap_next_ex()  supplies	 that  pointer	through	 a  pointer  argument.
       pcap_next() is passed an	argument that points to	a  struct  pcap_pkthdr
       structure, and fills it in.

       The  callback  is  also	supplied  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.  This 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_set_snaplen() 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).  When reading from a ``savefile'', the  snapshot	length
       specified  when	the  capture  was  performed  will limit the amount of
       packet	data   available.    pcap_next()   returns    that    pointer;
       pcap_next_ex() supplies that pointer through a pointer argument.

       To  force the loop in pcap_dispatch() or	pcap_loop() to terminate, call

       By default, when	reading	packets	from an	interface opened  for  a  live
       capture,	 pcap_dispatch(),  pcap_next(),	and pcap_next_ex() will, if no
       packets are currently available to be read, block waiting  for  packets
       to  become available.  On some, but not all, platforms, if a read time-
       out was specified, the wait will	terminate after	the read  timeout  ex-
       pires;  applications should be prepared for this, as it happens on some
       platforms, but should not rely on it, as	it does	not  happen  on	 other

       A  handle can be	put into ``non-blocking	mode'',	so that	those routines
       will, rather than blocking, return an indication	that  no  packets  are
       available  to  read.  Call pcap_setnonblock() to	put a handle into non-
       blocking	mode or	to take	it out of non-blocking mode; call pcap_getnon-
       block()	to  determine  whether a handle	is in non-blocking mode.  Note
       that non-blocking mode does not work correctly in Mac OS	X 10.6.

       Non-blocking mode is often combined with	routines such as select(2)  or
       poll(2)	or other routines a platform offers to wait for	the availabil-
       ity of data on any of a set of descriptors.  To obtain, for a handle, a
       descriptor  that	 can  be  used	in  those  routines, call pcap_get_se-
       lectable_fd().  Not all	handles	 have  such  a	descriptor  available;
       pcap_get_selectable_fd()	 will  return -1 if no such descriptor exists.
       In addition, for	various	reasons, one or	more of	 those	routines  will
       not   work   properly   with  the  descriptor;  the  documentation  for
       pcap_get_selectable_fd()	gives details.


		     read a bufferful of packets from a	pcap_t open for	a live
		     capture or	the full set of	packets	from a pcap_t open for
		     a ``savefile''

		     read packets from a pcap_t	until an  interrupt  or	 error

		     read  the next packet from	a pcap_t without an indication
		     whether an	error occurred

		     read the next packet from a pcap_t	with an	error  indica-
		     tion on an	error

		     prematurely  terminate  the  loop	in  pcap_dispatch() or

		     set or clear non-blocking mode on a pcap_t

		     get the state of non-blocking mode	for a pcap_t

		     attempt to	get a descriptor for a pcap_t that can be used
		     in	calls such as select(2)	and poll(2)

       In  order  to  cause  only  certain packets to be returned when reading
       packets,	a filter can be	set on a handle.  For a	live capture, the fil-
       tering  will be performed in kernel mode, if possible, to avoid copying
       ``uninteresting'' packets from the kernel to user mode.

       A filter	can be specified as a text string; the syntax and semantics of
       the string are as described by pcap-filter(7).  A filter	string is com-
       piled into a program in a pseudo-machine-language by pcap_compile() and
       the  resulting program can be made a filter for a handle	with pcap_set-
       filter().  The result of	pcap_compile() can be freed  with  a  call  to
       pcap_freecode().	 pcap_compile()	may require a network mask for certain
       expressions in the filter string; pcap_lookupnet() can be used to  find
       the network address and network mask for	a given	capture	device.

       A  compiled  filter  can	 also be applied directly to a packet that has
       been read using pcap_offline_filter().


		     compile filter expression	to  a  pseudo-machine-language
		     code program

		     free a filter program

		     set filter	for a pcap_t

		     get network address and network mask for a	capture	device

		     apply a filter program to a packet

   Incoming and	outgoing packets
       By  default,  libpcap  will attempt to capture both packets sent	by the
       machine and packets received by the machine.  To	limit it to  capturing
       only packets received by	the machine or,	if possible, only packets sent
       by the machine, call pcap_setdirection().


		     specify whether to	 capture  incoming  packets,  outgoing
		     packets, or both

   Capture statistics
       To get statistics about packets received	and dropped in a live capture,
       call pcap_stats().


		     get capture statistics

   Opening a handle for	writing	captured packets
       To open a ``savefile`` to which to write	packets,  given	 the  pathname
       the ``savefile''	should have, call pcap_dump_open().  To	open a ``save-
       file`` to which to write	packets, given the pathname  the  ``savefile''
       should  have,  call  pcap_dump_open(); to set up	a handle for a ``save-
       file'', given a FILE * referring	to a file already opened for  writing,
       call  pcap_dump_fopen().	 They each return pointers to a	pcap_dumper_t,
       which is	the handle used	for writing packets to the  ``savefile''.   If
       it  succeeds,  it  will	have  created the file if it doesn't exist and
       truncated the file if it	does exist.  To	close  a  pcap_dumper_t,  call


		     open a pcap_dumper_t for a	``savefile``, given a pathname

		     open a pcap_dumper_t for a	``savefile``, given a FILE *

		     close a pcap_dumper_t

		     get  the  FILE * for a pcap_dumper_t opened for a ``save-

   Writing packets
       To write	a packet to a pcap_dumper_t, call pcap_dump().	Packets	 writ-
       ten  with  pcap_dump()  may  be buffered, rather	than being immediately
       written to the ``savefile''.  Closing the pcap_dumper_t will cause  all
       buffered-but-not-yet-written packets to be written to the ``savefile''.
       To force	all packets written to the pcap_dumper_t, and not yet  written
       to  the	``savefile'' because they're buffered by the pcap_dumper_t, to
       be written to the ``savefile'', without closing the pcap_dumper_t, call


		     write packet to a pcap_dumper_t

		     flush  buffered packets written to	a pcap_dumper_t	to the

		     get current file position for a pcap_dumper_t

   Injecting packets
       If you have the required	privileges, you	can inject packets onto	a net-
       work  with  a  pcap_t  for  a  live  capture,  using  pcap_inject()  or
       pcap_sendpacket().  (The	two routines exist for compatibility with both
       OpenBSD and WinPcap; they perform the same function, but	have different
       return values.)


		     transmit a	packet

   Reporting errors
       Some routines return error or warning status codes; to convert them  to
       a string, use pcap_statustostr().


		     get a string for an error or warning status code

   Getting library version information
       To get a	string giving version information about	libpcap, call pcap_li-


		     get library version string

       In versions of libpcap prior to 1.0, the	pcap.h header file was not  in
       a  pcap	directory on most platforms; if	you are	writing	an application
       that must work on versions of libpcap prior to 1.0,  include  <pcap.h>,
       which  will  include  <pcap/pcap.h>  for	 you,  rather  than  including

       pcap_create() and pcap_activate() were not  available  in  versions  of
       libpcap	prior to 1.0; if you are writing an application	that must work
       on versions of libpcap prior to 1.0, either use pcap_open_live()	to get
       a handle	for a live capture or, if you want to be able to use the addi-
       tional capabilities offered by using pcap_create() and pcap_activate(),
       use  an	autoconf(1) script or some other configuration script to check
       whether the libpcap 1.0 APIs are	available and use them	only  if  they

       autoconf(1),   tcpdump(1),  tcpslice(1),	 pcap-filter(7),  pfconfig(8),

       The original authors of libpcap 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

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

				 4 April 2008			       PCAP(3)


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