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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.

       packet buffer 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  "packet	buffer
	      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  packet  buffer  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 packet buffer	timeout; on  platforms
	      that  don't, the packet buffer timeout is	ignored.  A zero value
	      for the timeout, on platforms that support a packet buffer time-
	      out,  will  cause	a read to wait forever to allow	enough packets
	      to arrive, with no timeout.  A negative value  is	 invalid;  the
	      result  of  setting  the	timeout	 to a negative value is	unpre-

	      NOTE: the	packet buffer timeout cannot be	used  to  cause	 calls
	      that read	packets	to return within a limited period of time, be-
	      cause, on	some platforms,	the packet buffer timeout  isn't  sup-
	      ported,  and,  on	other platforms, the timer doesn't start until
	      at least one packet arrives.  This means that the	packet	buffer
	      timeout  should  NOT be used, for	example, in an interactive ap-
	      plication	to allow the packet capture 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 packet buffer	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(@MAN_MISC_INFO@) for  a  list
	      of time stamp types.

	      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 (@MAN_ADMIN_COM-
	      MANDS@) 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	macOS):
	      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 macOS), this	might  involve
	      more  than  just	having somebody	with super-user	access setting
	      the ownership or permissions on the BPF devices -	it  might  in-
	      volve  configuring devfs to set the ownership or permissions ev-
	      ery 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.

       Do NOT assume that the packets for a given capture or ``savefile`` will
       have any	given link-layer header	type, such as DLT_EN10MB for Ethernet.
       For example, the	"any" device on	Linux will have	 a  link-layer	header
       type of DLT_LINUX_SLL even if all devices on the	system at the time the
       "any" device is	opened	have  some  other  data	 link  type,  such  as
       DLT_EN10MB for Ethernet.

       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 pathname, and
		     specify the precision to provide for packet time stamps

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

		     open a pcap_t for a ``savefile'',	given  a  FILE *,  and
		     specify the precision to provide for packet time stamps

		     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  capture  protocol for	a not-yet-activated pcap_t for
		     live capture (Linux only)

		     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 packet	buffer timeout for a not-yet-activated	pcap_t
		     for live capture

		     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

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

		     get the time stamp	precision of a pcap_t for live capture

		     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).

       The callback is also supplied a	const  u_char  pointer	to  the	 first
       caplen  (as  given  in the struct pcap_pkthdr mentioned above) 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 snap-
       shot  length  specified	when  the capture was performed	will limit the
       amount of packet	data available.

       pcap_next() is passed an	argument that points to	a  struct  pcap_pkthdr
       structure,  and	fills  it in with the time stamp and length values for
       the packet.  It returns a const u_char to the first caplen bytes	of the
       packet on success, and NULL on error.

       pcap_next_ex()  is passed two pointer arguments,	one of which points to
       a structpcap_pkthdr* and	one of which points to a  const	 u_char*.   It
       sets  the first pointer to point	to a struct pcap_pkthdr	structure with
       the time	stamp and length values	for the	packet,	and  sets  the	second
       pointer to point	to the first caplen bytes of the packet.

       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 packet buf-
       fer timeout was specified, the wait will	 terminate  after  the	packet
       buffer timeout expires; 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  platforms.   Note that the wait might, or might not,
       terminate even if no packets are	available; applications	should be pre-
       pared for this to happen, but must not rely on it happening.

       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 any of a	set of
       descriptors to be ready to read.	 To obtain, for	a handle, a descriptor
       that can	be used	in those routines, call	pcap_get_selectable_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 de-
       tails.  Note that, just as an attempt to	read packets from a pcap_t may
       not  return  any	 packets  if  the packet buffer	timeout	expires, a se-
       lect(), poll(), or other	such call may, if the  packet  buffer  timeout
       expires,	 indicate that a descriptor is ready to	read even if there are
       no packets available to read.


		     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(@MAN_MISC_INFO@).  A	filter
       string is compiled 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_setfilter().  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_lookup-
       net()  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


		     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(@MAN_MISC_INFO@), pf-
       config(8), usermod(@MAN_ADMIN_COMMANDS@)

       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

       To report a security issue  please  send	 an  e-mail  to	 security@tcp-

       To  report  bugs	and other problems, contribute patches,	request	a fea-
       ture, provide generic feedback etc please see the file CONTRIBUTING  in
       the libpcap source tree root.

				20 January 2017			       PCAP(3)


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